©2016 Society of Economic Geologists, Inc.Special Publication 19, pp. 301–327

Chapter 12

Tectonic Processes and Metallogeny along the Tethyan Mountain Ranges of the Middle East and South Asia

(Oman, Himalaya, Karakoram, Tibet, Myanmar, Thailand, Malaysia)

Michael P. Searle,† Laurence J. Robb, and Nicholas J. Gardiner*

Department of Earth Sciences, Oxford University, South Parks Road, Oxford OX1 3AN, United Kingdom

AbstractThe genesis of mineral deposits has been widely linked to specific tectonic settings, but has less frequently been linked to tectonic processes. Understanding processes of oceanic and continental collision tectonics is crucial to understanding key factors leading to the genesis of magmatic-, metamorphic-, hydrothermal-, and sedimentary-related mineral deposits. Geologic studies of most ore deposits typically focus on the final stages of concentration and emplacement. The ultimate source (mantle, lower crust, upper crust) of mineral deposits in many cases remains more cryptic. Uniquely, along the Tethyan collision zones of Asia, every stage of the conver-gence process can be studied from the initial oceanic settings where ophiolite complexes were formed, through subduction zone and island-arc settings with ultrahigh- to high-pressure metamorphism, to the continental col-lision settings of the Himalaya, and advanced, long-lived collisional settings such as Afghanistan, the Karakoram Ranges, and the Tibetan plateau. The India-Asia collision closed the intervening Neotethys ocean at ~50 Ma and resulted in the formation of the Himalayan mountain ranges, and increased crustal thickening, metamor-phism, deformation, and uplift of the Karakoram-Hindu Kush ranges, Tibetan plateau, and older collision zones across central Asia. Metallogenesis in oceanic crust (hydrothermal Cu-Au; Fe, Mn nodules) and mantle (Cr, Ni, Pt) can be deduced from ophiolite complexes preserved around the Arabia/India-Asia collision (Oman, Ladakh, South Tibet, Myanmar, Andaman Islands). Tectonic-metallogenic processes in island arcs and ancient subduc-tion complexes (VMS Cu-Zn-Pb) can be deduced from studies in the Dras-Kohistan arc (Pakistan) and the various arc complexes along the Myanmar-Andaman segment of the collision zone. Metallogenesis of Andean-type margins (Cu-Au-Mo porphyry; epithermal Au-Ag) can be seen along the Jurassic-Eocene Transhimalayan ranges of Pakistan, Ladakh, South Tibet, and Myanmar. Large porphyry Cu deposits in Tibet are related to both precollisional calc-alkaline granites and postcollisional alkaline adakite-like intrusions. Metallogenesis of continent-continent collision zones is prominent along the Myanmar-Thailand-Malaysia Sn-W granite belts, but less common along the Himalaya. The Mogok metamorphic belt of Myanmar is known for its gemstones associated with regional high-temperature metamorphism (ruby, spinel, sapphire, etc). In Myanmar it is likely that extensive alkaline magmatism has contributed extra heat during the formation of high-temperature meta-morphism. This paper attempts to link metallogeny of the Himalaya-Karakoram-Tibet and Myanmar collision zone to tectonic processes derived from multidisciplinary geologic studies.

IntroductionMineral deposits have traditionally been linked to specific tec-tonic settings (e.g., Barley and Groves, 1992; Kerrich et al., 2005; Groves and Bierlein, 2007; Bierlein et al., 2009; Hou and Cook, 2009; Richards, 2015) but the origins and concen-tration processes of the metals contained within them are less well known. In many cases this is because the ore-forming processes that are associated with many mineral deposits take place at the magmatic-hydrothermal stage, are typically emplaced at high structural levels, and thus are commonly wholly or partly eroded. Other deposits are entirely epigen-etic and their relationship to metal source, heat production, and fluid flow may be less clear. It is only rare occurrences of deep magmatic intrusions (e.g., Bushveld, Skaergaard) or obducted ophiolite complexes (e.g., Oman) where the origi-nal mantle or crustal processes may be directly deduced from

the preserved geology. Classic porphyry Cu-Au-Mo deposits, and also granite-hosted Sn-W deposits of Andean-type oro-genic belts are relatively well understood in terms of their formational processes, but their metal specificity is not always obvious. A range of other mineral deposit types, including sediment-hosted base and precious metal ores, and “oro-genic” Au deposits, are broadly synchronous with orogenic events, but their detailed link to tectonic processes is likewise poorly understood. In this paper we review the broad tectonic evolution and setting of the multifacetted Tethyan orogenic belt and provide a brief overview of the metallogeny of the belt in terms of tectonic processes.

Rifting of the southern Gondwana supercontinent (Africa, Arabia, India) from the northern Laurasia supercontinent (Europe, northern Asia) resulted in opening of the Tethyan ocean, an east-west seaway that spanned the entire globe (Scotese, 2004). The northern tract, termed Paleo-Tethys, rifted during the Devonian and closed diachronously during the Triassic and Jurassic (the Indosinian orogeny in SE Asia). A prominent Permo-Triassic island-arc terrane, the Yidun arc, extends across Tibet for over 1,000 km (Deng et al., 2014).

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† Corresponding author: e-mail, mike.searle@earth.ox.ac.uk*Present address: Centre for Exploration Targeting–Curtin Node, Depart-

ment of Applied Geology, Western Australian School of Mines, Curtin University, Perth, WA 6102, Australia.

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The Paleo-Tethyan suture zone runs across central Tibet (e.g., the Bangong-Nujiang suture; Kapp et al., 2007; Deng et al., 2014), around the East Himalayan syntaxis, and extends south through Myanmar and Thailand to Malayasia (e.g., the Ben-tong-Raub suture; Metcalfe, 2000, 2011; Sone and Metcalfe, 2008). Several distinct continental terranes south of the main Paleo-Tethyan suture are termed the Cimmerian continents (Sengör et al., 1993).

The southern tract of Tethys, termed Neotethys, marks the most recent collision between Gondwana-derived microcon-tinents and Eurasia. Along the Alpine-Zagros-Oman segment, Neotethys rifted during the Early Permian with establishment of a stable carbonate-dominated passive continental margin (the host rocks to the vast Middle Eastern oil resources in Permian to Cenomanian reservoirs), and closed diachronously during the Cenozoic era. Early stages of the collision occurred along the Zagros Mountains in southwestern Iran with large-scale folding showing around 100 km of crustal shortening (Blanc et al., 2003). Two localities along the Alpine-Himala-yan belt show remnant pieces of Neotethys where continental collision has yet to occur (NE Mediterranean and the Gulf of Oman).

In this paper we review the tectonic processes involved in the formation and evolution of the Tethyan margin around the Arabian and Indian plates (Fig. 1) along the mountain ranges of Oman, Pakistan, India, Nepal, South Tibet, Myan-mar, Thailand, and Malaysia, and provide an initial attempt to incorporate a metallogenic framework into the convo-luted history of the region. Previous reviews by Groves and Bierlein (2007), Hou and Cook (2009) and Richards (2015) are extended here by focusing on tectonic processes rather than only tectonic settings. The review begins by defining the age and nature of the India-Asia collision. We then dis-cuss the distinct mineralization seen in ophiolites (thrust slices of oceanic crust and upper mantle emplaced onto con-tinental margins) and processes associated with progressive subduction and island-arc formation, followed by Andean-type settings where oceanic subduction zones dip beneath an active granitic-volcanic arc. Finally, we review processes occurring along continent-continent collision zones typi-fied by the Himalaya-Tibet region extending into southeast Asia. A particularly intriguing aspect of this review is that in some portions of the orogenic belt, such as Afghanistan and Myanmar, there is a rich metallogenic endowment, whereas

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in others, such as along the main Himalayan range, there is a distinct lack of mineralization.

India-Asia Collision and the Closing of NeotethysThe age of collision of India with Asia has been disputed, with ages ranging from 65 Ma (Ding et al., 2005; Cai et al., 2011) to as young as 34 Ma (Aitchison et al., 2007). Paleo-magnetic data indicate initial contact between India and Asia during the early Eocene with overlapping paleolatitudes for the Indian plate Tethyan Himalaya and the Asian plate Lhasa block occurring at 48.6 ± 6.2 Ma (Najman et al. 2010; van Hinsbergen et al., 2011a, b). The timing of the India-Asia col-lision is geologically well constrained at early Eocene based on a number of lines of evidence. The stratigraphic record and the geologic record clearly show a marine to continen-tal transition both along the Indus suture zone and along the northern Indian shelf margin at ~50.5 Ma (Garzanti et al., 1987; Beck et al., 1995, 1996; Rowley, 1996, 1998; Searle et al., 1997b; Zhu et al., 2005; Green et al., 2008; Najman et al., 2010). This body of work provides unequivocal evidence for the nature of the collision, with the timing at 50.5 Ma based on planktonic biostratigraphy. Similar ages obtained for mark-ers of ocean-continent transition recorded all along the Indus suture from Pakistan to Tibet suggest that the collision was essentially coeval.

Structural and geochronological work along the Greater Himalaya has shown that the Lesser, Greater, and Tethyan Himalaya were all part of one contiguous Indian plate (Searle et al., 2003, 2006). The three zones are now bounded by major Cenozoic thrust faults (e.g. ~25–11 Ma Main Central thrust; 10–0 Ma Main Boundary thrust) and low-angle normal fault (~25–11 Ma South Tibetan detachment), and restoration has shown that there was no Mesozoic-Cenozoic ocean between these zones (Fig. 2, Model A). The closure of the Neotethys ocean resulted in the formation of the Himalaya and Indo-Myanmar ranges (Indian plate), as well as enhanced crustal thickening of the Asian plate margin from the Karakoram and Hindu Kush ranges in northwestern Pakistan, along the Lhasa and Qiangtang terranes of south Tibet, and around the East Himalayan (Namche Barwa) syntaxis into Myanmar and Thailand (Searle et al., 2011, 2016; Searle and Morley, 2011). Prior to the closing of Neotethys and the India-Asia collision, ophiolite thrust sheets were obducted onto the previously passive continental margin of India and the Burmese plate. Following the India-Asia collision and closing of Neotethys, crustal shortening and thickening processes resulted in uplift of the Himalaya, while Tibet was underthrust by the Indian lower crust and lithospheric mantle, resulting in enhanced uplift and thickening.

An alternative model of the India-Asia collision was pre-sented by van Hinsbergen et al. (2011a, b, 2012) showing an early “soft collision” of a Tethyan Himalaya microplate with Asia at ~50 Ma and a later “hard collision” of a contigu-ous Greater India (Lesser Himalaya and India) between 25 to 20 Ma (Fig. 2, Model B). Their reconstruction shows a Tibetan (Tethys) Himalaya microplate with an ocean (their “Greater Indian basin”) approximately 1,000 km wide along the Greater Himalaya during the period 50 to 25 Ma (van Hinsbergen et al., 2012, fig. 3). Many decades of study along the Greater Himalaya sequence have shown that these rocks

are all Neoproterozoic to late Mesozoic protoliths, metamor-phosed to kyanite and sillimanite grade during the Oligocene-Miocene, and the southern boundary is a continental ductile shear zone and thrust fault, the Main central thrust. There are no oceanic rocks or ophiolites anywhere along the Greater Himalaya sequence or the Main Central thrust zone, and so this model lacks any geologic credence.

Thus, we contend that the Tethyan Himalaya, Greater Himalaya, and Lesser Himalaya were all conjoined as one Indian plate prior to the collision with Asia and the closing of Neotethys at 50.5 Ma. After this collision the leading edge of India was subducted to ultrahigh-pressure depths, resulting in eclogites preserved along the leading margin in north Paki-stan and Ladakh. Postcollision crustal thickening resulted in folding, thrusting, shortening, and regional kyanite-silliman-ite-grade metamorphism and partial melting during the latest Eocene-Oligocene-early Miocene (e.g., Hodges, 2000; Searle, 2015).

Comparative OrogenesisMountain belts can be broadly divided according to a succes-sion of stages and processes as follows:

1. Those formed during ophiolite obduction where a thrust sheet of oceanic lithosphere has been emplaced onto a previously passive continental margin (e.g., Oman). Typi-cal mineral deposits include magmatic concentrations of Cr, Pt, and Ni in ultramafic mantle rocks; exhalative hydro-thermal Cu-Zn-(Au) deposits in volcanic rocks; and concre-tionary Mn-Fe nodules associated with pelagic sediment.

2. Island-arc processes above oceanic subduction zones (e.g., Andaman Islands; Dras-Kohistan island arc). Typical min-eral deposits include: porphyry deposits and exhalative hydrothermal Cu-Zn-Pb deposits hosted in volcanic rocks.

3. Continental magmatic arc formed where an oceanic plate subducts beneath an active continental margin (e.g., Ladakh-Gangdese ranges, south Tibet; Wuntho-Popa arc, Myanmar). Typical mineral deposits include magmatic-hydrothermal porphyry Cu-Mo-(Au) and epithermal Au-Ag deposits associated with I-type granite magmas; and magmatic-hydrothermal Sn-W deposits associated with S-type granite magmas from the continental side of the arc.

4. Early-stage continental collision (e.g., Zagros Mountains, Iran). These mountain belts reflect thickened crust and high-grade metamorphic rocks, and have large porphyry deposits (Shafiei et al., 2009; Richards and Sholeh, 2016).

5. Late-stage continental collision where two continental plates have collided (e.g., Himalaya; Mogok belt, Myan-mar; West Malaysia). These mountain belts reflect thick-ened crust, regional high-grade metamorphic rocks, and anatectic leucogranitic and pegmatitic melts, and also generally lack large-scale mineral deposits. They do, how-ever, contain small deposits associated with enrichments of U-Th-REE-Sn in leucogranites as well as gem miner-als (ruby, sapphire, emerald) hosted in metamorphic rocks and pegmatites.

6. Long-lived plateau-type mountains with both pre- and postcollision history (e.g., Tibet) with a long history of crustal thickening, metamorphism, melting, and miner-alization. These mountain belts can include a variety of

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mineral deposits linked to previous stages of the orogenic cycle (Deng et al., 2014; Richards, 2015). Some porphyry Cu-Mo deposits in Tibet are postcollisional (Miocene) within more evolved adakite-like intrusive rocks.

7. Long-lived mountains formed by double-vergent conti-nental subduction systems (e.g., Karakoram, Pamir; Searle et al., 2010a; Stearns et al., 2013). As with continental col-lision belts these terranes show long-lived crustal thicken-ing, metamorphism, and melting histories, but typically lack major mineral deposit types.

8. Old mountain ranges reactivated and uplifted by younger tectonics (e.g., Afghanistan, Hindu Kush, Tien Shan, Kun

Lun ranges). Major mineral deposits formed at upper crustal levels would have been eroded away, but deeper sourced deposits, such as orogenic gold, may be preserved and reflect processes active in past orogenic cycles.

Each type of mountain range has a distinctive geologic fingerprint, distinct igneous and metamorphic assemblages, and distinct structural style. Specific mineral deposits can be linked to each type of mountain belt and its tectonic setting, in both space and time. The combination of geologic and geochemical fingerprinting can be linked to specific mineral deposits in order to gain understanding of processes required

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Fig. 2. Models for the India-Asia collision. Model A shows a single contiguous Indian plate (conjoined Lesser-Greater-Tethyan Himalaya), with closure of Neotethys at 50 Ma. Model B, after van Hinsbergen et al. (2012), shows a separated Tibetan Himalaya microplate with an intervening ocean the Greater Indian basin (GIB) separating this from the main Indian plate This model involves a “soft collision” at 50 Ma and a “hard collision” at 25 to 20 Ma.

TECTONIC PROCESSES & METALLOGENY, TETHYAN MOUNTAIN RANGES, MIDDLE EAST AND SOUTH ASIA 305

to generate and preserve each mineral province. Examples of each type of mountain belt listed above occur along the Tethyan mountain ranges of the Middle East and Asia.

Himalaya-Karakoram-Tibet Orogenic CycleThe India-Asia collision zone lies along the Indus-Tsangpo suture zone that divides the Indian plate Himalaya to the south from the Asian plate Lhasa and Qiangtang terranes of the Tibetan plateau to the north (Fig. 3). The Himalayan mountain range stretches in an arc from northwestern Paki-stan eastward across Ladakh (NW India), southern Tibet to Yunnan, and the East Himalayan syntaxis region. The Hima-layan orogenic cycle can be temporally divided into five stages (Fig. 4): (1) Late Cretaceous-Paleocene precollision ophiolite obduction stage (~ 65 Ma; Fig. 4a); (2) crustal subduction and formation of ultrahigh-pressure eclogite facies metamorphism (~57–47 Ma; Fig. 4b); (3) crustal thickening along the Hima-laya and peak kyanite-grade metamorphism (~35–30  Ma; Fig. 4c); (4) decompression melting, formation of migma-tites and leucogranites, and south-directed extrusion of the ductile midcrust by channel flow (~24–15 Ma; Fig. 4d); and

(5) southward-propagating thrusting along the Lesser Hima-laya forming a subcritical wedge with active underthusting of India beneath the Himalaya and south Tibet.

This tectonic evolution is well constrained by structural mapping combined with extensive thermobarometric and U-Pb zircon and monazite dating (e.g., Searle and Rex, 1989; Grujic et al., 2002; Searle et al., 1999, 2010a, b; Streule et al., 2010). Timing of mineralization can be linked both spatially and temporally to this basic framework.

For the Asian side of the India-Asia collision zone in the Karakoram and Pamir ranges as well as across the Lhasa and Qiangtang blocks of Tibet, stratigraphic and structural data combined with geochronology suggest the following broad tec-tonic evolution phases: (1) Triassic-Early Jurassic crustal thick-ening and regional metamorphism, the Indosinian orogeny (e.g., Weller et al., 2013); (2) pre-India-Asia collision crustal thickening in an Andean-type setting over a period spanning Early Jurassic-early Eocene, along the Kohistan-Ladakh-Gangdese batholiths (~198–49 Ma; Chung et al., 2005; Chu et al., 2006); (3) postcollisional crustal thickening resulting in areas of kyanite- and sillimanite-grade metamorphism formed

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during the Cenozoic (Palin et al., 2013); (4) postcollisional, within-plate alkaline (potassic, sodic) magmatism formed by lower crustal melting to produce adakite-like intrusions, and mantle-derived melting to produce shoshonites (Chung et al., 2005, 2009; Lee et al., 2009, Wang et al., 2010); and (5) Oligo-cene-Miocene crustal thickening and regional metamorphism along the Karakoram (Searle et al., 2010a) and Pamir (Stearns et al., 2013).

In Myanmar various Tibetan terranes have been affected by clockwise rotation around the East Himalayan syntaxis, later transpression along continental-scale strike-slip faults (e.g., the Sagaing fault), arc formation (Wuntho-Popa arc), and deep subduction seen along the Burma seismic zone (Searle and Morley, 2011).

OphiolitesOphiolite complexes are recorded in four major tectonic set-tings along the Tethyan mountain ranges of the Middle East and Asia: (1) large thrust sheets obducted onto previously passive continental margins (e.g., Oman ophiolite; Fig. 5), (2) ophiolites trapped along suture zones (e.g., Indus suture zone ophiolites), (3) high-pressure ophiolites exhumed from subduction zones (e.g., Jade belt, Myanmar), and (4) ophi-olitic rocks exposed in accretionary prism complexes above active subduction zones (e.g., Andaman island ophiolite). The mantle portion of ophiolite complexes (harzburgites, lherzo-lites, dunites) is dominated by magmatic concentrations of chromium, vanadium, platinum, copper, and nickel, whereas crustal sequences of ophiolites preserve concentrations of chalcophile metals (copper-zinc) associated with ocean-water circulation and venting. The uppermost ocean floor levels of ophiolites commonly have Fe- and Mn-rich umbers and nod-ules, similar to those dredged from deep ocean basins.

Large ophiolite thrust sheets

The Oman (Semail) ophiolite in eastern Arabia is the largest and best exposed example of a relatively intact thrust sheet of oceanic crust and upper mantle emplaced onto a previously passive continental margin anywhere in the world. In recon-structed sections the ophiolite includes a 6- to 7-km-thick sequence of crustal rocks and over 15 km of upper mantle peridotites, including depleted harzburgites, lherzolites, and dunites (Fig. 5). Mantle peridotites host podiform chromites and concentrations of vanadium and platinum group metals. These metals clearly originate from the mantle and may be concentrated to ore grades by processes such as fractional crystallization, sulfide-silicate liquid immiscibility. The recent discovery of microdiamonds as well as a range of highly reduced minerals (Ni-Mn-Co alloys, Fe-Si and Fe-C phases, stishovite, moissanite) as inclusions in chromitites and peri-dotites, notably in the Luobusa ophiolite, south Tibet, are consistent with the suggestion that some mineral inclusions in

ophiolite mantle sequences may have a deeper origin, possi-bly near the upper-lower mantle transition zone between the 410- to 660-km discontinuities (Yang et al., 2014). Diamonds formed in situ in ophiolites, for example, reflect depths of 150 to 300 km, far deeper than the origin of suprasubduction zone ophiolites (Yang et al., 2007). These authors proposed a model involving a deep mantle plume rising beneath a spread-ing ridge bringing deep mantle minerals upward, followed by normal ophiolite obduction processes to emplace these oce-anic thrust sheets onto continental margins.

The upper sections of the ophiolite crustal section are composed of a series of pillow lavas, including depleted arc tholeiites and boninites (high Mg andesites) overlying ocean ridge basalts, with interbedded radiolarian cherts at higher structural levels. Geochemical compositions of the lavas in the Oman ophiolite suggest that all units formed in a suprasu-bduction zone environment (Pearce et al., 1981; MacLeod et al., 2013). Immobile elements (Ti, Y, Nb, V, etc.) can poten-tially distinguish between MORB and suprasubduction zone settings and have been used as proxies to interpret fraction-ation processes, alkalinity, and temperature as well as tec-tonic setting (Pearce et al., 1981). In many ophiolites such as Oman, Troodos (Cyprus), and the Bay of Islands (Newfound-land), there appears to be no obvious island arc preserved in the ophiolite even though the lavas are clearly tholeiitic and boninitic in composition. For this reason, these ophiolites are commonly referred to as suprasubduction zone ophiolites. The basalts are fed by a series of sheeted dikes pointing to 100% crustal extension at a ridge axis. In Oman individual doleritic dikes can be mapped out as feeder dikes to the lower Geotimes series and the later Lasail arc-related boninites and arc tholeiites. Volcanogenic massive sulfide (VMS) deposits are known to occur throughout the lava series and have been mined at Lasail in northern Oman, as well as extensively in Cyprus.

The sheeted dikes feed magma up from a magma cham-ber that is represented by homogeneous gabbros that become progressively more layered toward their base. The ophiol-ite lower crust rocks represent a dynamic magma chamber beneath a spreading ridge, continually replenished by man-tle-derived melts from below and continually feeding magma up to the sheeted dike and basaltic pillow lavas above. More primitive magmas are represented by gabbro norites, whereas late-stage wehrlites (olivine + clinopyroxene) that cut the lay-ered gabbros are thought to be plutonic equivalents of later arc magmatism.

Processes involved in the thrusting of ophiolites onto passive continental margins can be deduced from studies of the meta-morphic sole rocks. These are typically granulite, amphibolite, and greenschist facies rocks, showing a narrow, inverted, and highly condensed P-T gradient and intense mylonite fabrics (e.g., Searle and Cox, 2002; Cowan et al., 2014). U-Pb dating

Fig. 4. Model for the evolution of the western Himalaya showing four stages of (a) precollision ophiolite obduction stage, (b) deep crustal subduction of the leading margin of India to coesite eclogite ultrahigh-pressure depths, (c) the major crustal thickening event resulting in kyanite grade metamorphism, and (d) peak sillimanite grade metamorphism with widespread partial melting and generation of migmatites and leucogranites. This stage resulted in ductile extrusion of the partially molten middle crust (channel flow, shown in pink; after Searle et al., 1997a). Abbreviations: Mz = Mesozoic, NH = North Himalaya, Pz = Paleozoic, TM = Tso Morari eclogites.

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Fig. 5. Tectono-stratigraphic column for the Semail ophiolite complex in Oman, after Searle (2007).

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of zircons in the sole amphibolites and the more fractionated gabbro-tonalite-trondhjemite ophiolite rocks shows that sub-duction zone sole rocks formed at precisely the same time as the ophiolite crustal sequence formation (Rioux et al., 2013). Possible processes involved include: (1) slab pull, required to get more buoyant continental crust deep into the mantle; (2) slab break-off, required to release the buoyant slice of ultrahigh-pressure eclogitic material; or (3) rapid exhumation of the ultrahigh-pressure slice back up the same subduction zone, driven by buoyancy contrasts.

Ophiolites along suture zones

Several examples are known of ophiolites trapped along the Indus-Yarlung Tsangpo suture zone that demarcates the India-Asia collision (Fig. 4). These include the ophiolitic mélanges and Nidar ophiolite complex in the Ladakh segment and the Kiogar-Amlang-la ophiolites of southwestern Tibet. Many Tethyan ophiolites in southern Tibet (e.g., the Luo-busa, Zedang, Xigase, Purang, and Dongbo massifs) are either trapped within the Yarlung-Tsangpo suture zone or occur along the northern margin of the Indian plate. These ophiol-ites are commonly structurally broken up and fault-bounded, and in some cases occur within a giant mélange (e.g., Kiogar, Amlang-la ophiolites). It is not possible to accurately deter-mine processes involved in their emplacement due to later subsequent structural overprinting. Several of these ophiolite complexes, notably the Luobusa ophiolite, show the range of deep mantle ultrahigh-pressure mineral inclusions in chro-mitites, including coesite the high-pressure polymorph of quartz, and microdiamond (Yang et al., 2014).

Ophiolite complexes also mark the line of older suture zones across the Tibetan plateau, particularly along the Shyok and Bangong-Nujiang sutures and their extensions farther southeast into Myanmar (Fig. 6). In Ladakh the Nidar ophi-olite is a suture zone ophiolite preserved along the Indus-Tsangpo suture zone in Ladakh. In Myanmar the Myitkyina ophiolite also represents a suture zone ophiolite, although the exact trace of the main India-Asia suture, thought be along the Mount Victoria belt in the eastern Indo-Myanmar ranges, remains unknown.

High-pressure ophiolites

The Jade mines ophiolites in the Hpakant region of Kachin State, northwestern Myanmar, are examples of ophiolitic rocks that have been exhumed from subduction zones and preserved their high-pressure mineral assemblages (Fig. 6). The Jade mines belt is composed dominantly of ophiolitic mantle-derived rocks that have been subjected to high-pres-sure metamorphism and direct crystallization from Na-rich fluids during the serpentinization process. A possible source for the fluid may be seawater that is drawn down the sub-duction zone and ultimately linked to the process of serpen-tinization of peridotite, and also perhaps with the formation of rodingites (Ca-metasomatized gabbro or plagiogranite), or pyroxenite (Wang et al., 2012), both during subseafloor hydro-thermal metamorphism and subsequently in the subduction channel.

Two main types of “jade” occur, a monomineralic pyroxene jadeite (NaAlSi2O6) and a lower pressure amphibole jade or nephrite, comprising tremolite-actinolite (Ca2(Mg,Fe)5Si8O22

(OH)2). The emerald green color in some Burmese jade (Imperial Jade) results from chromium (Cr3+) enrichment, particularly associated with the Cr-rich pyroxene kosmochlor (NaCrSi2O6) in the variety known as Maw-sit-sit (Gübe-lin, 1965a, b). Pale mauve varieties result from manganese (Mn2+) enrichment, and blue-green varieties from iron (Fe2+ and Fe3+) enrichment. Although jadeitites are high-pressure rocks their P-T conditions lie in the blueschist-eclogite facies transition (Sorensen and Harlow, 1999; Harlow and Sorensen, 2001). They require devolatilization of fluids derived from serpentinized ultramafic rocks, perhaps from above the sub-ducting oceanic slab.

The Jade mines belt is dominated by serpentinite and peri-dotite, but owing to thick laterite and jungle cover is very poorly exposed. The majority of mined jade and analyzed material comes from rounded boulders apparently exposed in young alluvial deposits, particularly along the Uru River. The Uru Conglomerate, described by Chhibber (1934), is the host to most of the jadeite extraction although at least one primary jadeite occurrence is found at the PNO mine (Nant Maw), which is a lozenge-shaped body encapsulated in a shear zone (Douglas Kirwin, pers. commun., 2016). However, given the size of the jade boulders and classic serpentinite weather-ing pattern of ultramafic rocks it is suggested that some of the “boulders” are actually remnant serpentinite weathering of large in situ ophiolitic peridotite sheets, and are not allu-vial. In addition to the abundant ultramafic clasts, Goffé et al. (2002) reported a variety of jade rock assemblages, including pure jadeitite, amphibole-jadeite, omphacite-jadeite-zoisite-kyanite, and kosmochlor with chromite, as well as less com-mon eclogite, amphibolite, and blueschist.

There is consensus that the jade rocks formed at high pres-sure and low temperature, although P-T conditions are not precisely constrained, owing to the predominance of high-vari-ance mineral assemblages, with estimates of peak conditions falling in the broad range of 10 to 15 kbars, 300° to 500°C (Shi et al., 2012). A more complex metamorphic history is implied by the suite of rocks studied by Goffé et al. (2002), where the sequence of overprinting assemblages seen in eclogite, jade veins, amphibolite, and blueschist implies a four-stage evolution from (1) an eclogitic stage at P ≥14 kbars, 550° to 600°C; (2) overprinting by amphibole-epidote-albite during decompression to ~8 kbars, 500° to 550°C; (3) blueschist-facies conditions at P ≥14 kbars, 400° to 450°C, with jadeitite vein formation at this stage; and (4) cooling and decompres-sion represented by pumpellyite and albite-nepheline partial replacements.

It is suggested that the protolith of the Burmese Hpakan jadeitites may have been older components of a late Meso-zoic ophiolitic suite obducted onto the Myanmar plate, and that the high-pressure metamorphism was Late Cretaceous in age (Searle et al., 2016). The slice of high-pressure peridotite-jadeitite could have been exhumed by obduction from the subduction channel and then offset by late Cenozoic dextral transpressional shearing along the Sagaing fault (Searle et al., 2016).

Accretionary prism ophiolites

Accretionary prism ophiolites are tectonically dismem-bered slices of oceanic crust that occur in the hanging wall

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of a subduction zone. In some cases (e.g., Franciscan high-pressure mélanges, California) they are associated with high-pressure blueschists and eclogites recording some of the subducted crustal rocks. In other cases (e.g., Andaman Islands ophiolite) they are not associated with high- or ultrahigh-pres-sure rocks and form structural slices in the upper plate. The Andaman ophiolites are presently exposed within an accre-tionary prism that lies above the active Andaman-Nicobar-Sumatra subduction zone. The Andaman ophiolites form the basement of the Andaman Islands, part of the outer forearc to the Sumatra volcanic arc. Upper mantle harzburgite and dunite are overlain by a cumulate peridotite-gabbro complex, high-level intrusive rocks, and both tholeiitic and calc-alkaline volcanic rocks. The upper crust of the South Andaman ophio-lite shows a prominent trondhjemite-diorite-andesite volcanic suite, suggesting that arc volcanism was built on oceanic crust. Zircon U-Pb dating of a trondhjemitic rock from Chiriya Tapu in South Andaman Island at 94.6 ± 1.3 Ma (Pedersen et al., 2001) is remarkably similar to U-Pb ages of the Troodos ophi-olite, Cyprus, and the Semail ophiolite, Oman.

Island Arcs and Subduction Zones

Kohistan island arc

In the western Himalaya a large-scale Late Cretaceous to Paleocene-Eocene island-arc complex, the Kohistan-Dras island arc, crops out within the Tethyan suture zone between India and Asia, bounded by two sutures, the Shyok suture to the north and the Indus suture (Main mantle thrust) to the south (Jan and Howie, 1981; Khan et al., 1993; Pettersen and Treloar, 2004; Dhuime et al., 2007; Garrido et al., 2007; Jag-outz et al., 2007; Jagoutz and Schmidt, 2012). The Kohistan arc comprises a complete crustal section through an island arc together with a slice of upper mantle peridotite (Fig. 7). The mantle component includes the ultramafic lower part of the Jijal complex (layered dunite, wehrlite, Cr-rich pyroxenites overlain by websterites and pyroxenites; Dhuime et al., 2007; Garrido et al., 2007), and the Sapat ultramafic (dunite, harz-burgite) thrust slices. The latter includes unique gem-qual-ity peridot (olivine) in the dunites. The crustal component includes the upper part of the Jijal complex, garnet granulites, and basal amphibolites of MORB affinity (Kiru and Kamila complexes; Jagoutz and Schmidt, 2012). The Chilas com-plex is dominantly composed of gabbronorites and diorites with some ultramafic components at lower structural levels (dunite, lherzolite, pyroxenite). It is possible that the base of the Chilas complex is a thrust contact bringing deeper mantle and lower crust over shallower parts of the midcrust amphibo-lites (Kamila amphibolites). Overlying the Chilas complex is a series of andesitic-dacitic volcanic complexes (Dir, Utror, Shamran, and Chalt volcanic suites). All these rocks have been intruded by extensive biotite- and hornblende-bearing mon-zogranites, granodiorites, and tonalites of the Kohistan batho-lith. The eastward extension of this batholith in Ladakh and Gangdese, south Tibet, has U-Pb zircon ages ranging from ca. 198 to 49 Ma (Chung et al., 2003, 2005; Wen et al., 2008a, b).

The Kohistan arc was an intraoceanic island arc formed above a N-dipping subduction zone and obducted onto the northern margin of India (Khan et al., 1993; Searle et al., 1999; Jagoutz and Schmidt, 2012). A second long-lived subduction

zone dipping northward beneath the Asian margin lasted from at least the late Jurassic through the Cretaceous to the early Eocene (Chiu et al., 2009). Subduction-related I-type granite magmatism characterizes the magmatic evolution of both northern Kohistan and the Karakoram terrane to the north (Searle et al., 1999). Much of the succession of crustal sedimentary rocks of the north Indian plate margin (Tethyan Himalaya), comprising up to 5 km of stratigraphic thickness in Ladakh, has been removed by tectonics and erosion in Paki-stan, such that the Kohistan arc lies above high-grade regional metamorphic rocks of the Greater Himalayan sequence.

Mineralization in island arcs is dominated by VMS-type deposits (Cu-Zn-Pb ± Au-Ag) related to exhalative hydrother-mal fluid circulation on the ocean floor and hosted in inter-mediate-felsic volcanic rocks. The Dras-Kohistan arc contains some indications of massive sulfide mineralization but none of these has proven economically viable—erosion may also have destroyed the bigger deposits. The Himalayan arc systems in particular have been subjected to subsequent collision-related regional Barrovian metamorphism and extreme struc-tural shortening, which accentuated the uplift and erosion of higher levels of the arc system.

Andean-Type Continental Margins in Tibet and Myanmar

Ladakh-Gangdese granite batholith

Along the southern margin of the Lhasa terrane (south Asian margin) a 2,500-km-long batholith composed mainly of I-type hornblende- and biotite-bearing granites, granodiorites, and diorites crops out from northern Kohistan (Pakistan) across Ladakh and southern Tibet along the southern margin of the Asian plate (Fig. 3). These Ladakh-Gangdese granites are related to the northward subduction of Tethyan oceanic lithosphere beneath the south Asian continental margin. The granites have an extensive calc-alkaline volcanic superstruc-ture comprising andesites, rhyolites, and ignimbrite flows (Linzizong volcanic rocks; Ding et al., 2005; Kapp et al., 2007; Mo et al., 2007, 2008; Wen et al., 2008a, b; Chiu et al., 2009) of similar composition and areal extent to the central Andean volcanic province (Pitcher, 1987). Zircon U-Pb ages of Gang-dese granites range from Early Jurassic to early Eocene (198–49 Ma), suggesting long-lasting I-type magmatism (Chung et al., 2005; Chu et al., 2006). Zircon U-Pb ages from the Gang-dese granites and 40Ar-39Ar ages from the Linzizong volcanic sequence show two distinct peaks of magmatism, a widespread Cretaceous stage (~133–110 Ma), and an intense magmatic “flare-up” in the Paleocene (66–57 Ma) when compositions varied from low K tholeiite through calc-alkaline andesite to shoshonitic suites (Chiu et al., 2009; Lee et al., 2009).

Both Ladakh-Gangdese granitoids and Linzizong-type andesite-ignimbrite volcanic rocks ceased around the time of Indian plate collision as oceanic subduction beneath the south Asian margin ended. Final magmatism along the Kohistan-Ladakh part of the batholith comprised a series of peralumi-nous garnet-bearing leucogranitic dikes (Indus confluence dikes in Kohistan; Chumatang dikes in Ladakh) formed by extreme differentiation of the initial calc-alkaline batho-lith or melting of a predominantly sedimentary protolith in the source. After 47 Ma no subduction-related calc-alkaline

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magmatism is recorded. Plutons substantially younger than ca. 45 to 40 Ma are distinct and have an alkaline chemical compo-sition and are related to slab break-off and lower crustal melt-ing to produce postcollisional adakite-like magmas (Chung et al., 2005).

Seven major porphyry Cu-Mo ± Au deposits with ages spanning 120 to 12 Ma (Jiama, Qulong, Lakang, Nanmu, Tinggong, Chongjiang, Dongga) are all hosted either within the Gangdese granite batholith (Tafti et al., 2001; Yang et al., 2009; Wang et al., 2014a, b) and their extrusive equivalents (the ~65–43 Ma Linzizong andesites), or in Miocene ada-kites (Qu et al., 2009; Wang et al., 2014; Richards, 2015; Fig. 8). Some porphyry deposits including the Yulong (46.5 Ma) and Malasongduo (42.5 Ma) deposits were formed immedi-ately after the India-Asia collision (Hou et al., 2003; Rich-ards, 2015). These deposits are related to fluids driven off the subducting slab and are generally hosted in evolved I-type granites that have been affected by metasomatism. Copper and other metals are transported by hot saline fluids and pre-cipitated in fractures within the high-level granites (Richards, 2015). Others, such as the giant Qulong porphyry Cu-Mo deposit (16.4 ± 0.5 Ma; Yang et al., 2005) are younger. Zhu et al. (2009) described Early Cretaceous adakite-like rocks (Si-rich, high Sr/Y and La/Yb) with zircon SHRIMP ages of ~136  Ma, which are clearly part of the subduction-related Gangdese batholith. Some precollisional Gangdese zircons

have younger rim ages spanning 26.0 to 17.7 Ma (Qu et al., 2009) but it is unclear precisely what these ages reflect.

Gangdese magmatism is thought to be the product of con-tinuous subduction of Tethyan oceanic lithosphere beneath Asia from Late Jurassic until the early Eocene Indian plate collision. The precollision adakites are interpreted as derived from partial melting of subducted Neo-Tethyan slab (MORB + sediment + fluid) subsequently having been hybridized by peridotite in the mantle wedge (Zhu et al., 2009). Following collision, oceanic subduction beneath Tibet ceased, calc-alka-line magmatism ended, and magmatism evolved to more alka-line adakite-like compositions, as the crust thickened.

Adakite-like intrusions in south Tibet

Adakite-like plutonic and volcanic rocks formed both during the arc thickening phase prior to continental collision and dur-ing the postcollisional phase across southern Tibet. Adakites (sensu stricto) are high silica, low Y, and heavy REE, and high Sr/Y and La/Yb igneous rocks formed by fractional crystalliza-tion of mafic magmas that must have had garnet, hornblende, or clinopyroxene in an eclogite or amphibolite lower crustal source (Martin, 1988; Defant and Drummond, 1990; Castillo, 2012). Adakites were originally described from melting of thickening lower crust of island arcs such as the Aleutian arc (Kay and Kay, 2002), but they have also been described from areas of thickened crust such as Tibet (Chung et al., 2005).

Gangdese batholith (~120 - 40 Ma) Porphyry Copper deposits

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In southern Tibet a distinct change in magmatism occurred soon after the time of the India-Asia collision (~50.5 Ma; Zhu et al., 2005; Green et al., 2008). Small-volume, alkaline gran-itoids with minor volcanic flows were formed between ~30 to 9 Ma (Chung et al., 2003, 2005; Hou et al., 2003). Crustal thickening resulted in high K and high Na adakite-like mag-mas formed from melting a garnet-bearing lower crust source (Chung et al., 2005). Along the Gangdese belt of south Tibet an important suite of Oligocene-Miocene porphyry Cu depos-its are related to syn- to postcollisional adakite-like intrusions, based on young U-Pb zircon ages (Wang et al., 2014; Rich-ards, 2015). Hou et al. (2009, 2011), Yang et al. (2009), and Wang et al. (2014) reported Miocene U-Pb (SHRIMP) ages and Re-Os molybdenite ages (22.2–15.3 Ma) for some of the Gangdese porphyry Cu deposits. In the northern Lhasa block some younger postcollisional porphyry Mo ± Cu deposits associated with significant vein-hosted Pb-Zn-W mineraliza-tion appear to be hosted in the high K adakite-like felsic intru-sions (Hou et al., 2009; Wang et al., 2014). Mantle-derived shoshonites were also intruded from a deeper source during this time (Chung et al., 2005; Searle et al., 2011). Porphyry Cu-Mo ± Au mineralization appears to be related to both pre-collisional calc-alkaline and postcollisional adakite-like stages of magmatism.

Paired granite-mineralization belts of Myanmar

Myanmar contains two major north-south magmatic belts attributed to the subduction and subsequent closure of Neo-tethys, and which exhibit contrasting metallogenic character (Mitchell, 1977; Gardiner et al., 2015a). The westerly Wun-tho-Popa arc is a discontinuous continental magmatic arc, comprising Late Cretaceous and Eocene-Miocene grano-diorites-diorites (Fig. 6; Barley et al., 2003; Mitchell et al., 2012; Gardiner et al., 2016). It hosts porphyry-type Cu-Au deposits with associated volcanic rock-hosted epithermal deposits (United Nations, 1978; Mitchell, 1993). The easterly Mogok-Mandalay-Mergui belt is marked by a series of Late Cretaceous-Eocene S-type crustal melt granites (Searle et al., 2007, 2016). Where these granites intrude the Slate belt, a low-grade metasedimentary sequence, significant Sn-W min-eralization is located (Hutchison and Taylor, 1978; Khin Zaw, 1990; Gardiner et al., 2015a).

These belts parallel the Neotethys subduction zone. U-Pb magmatic and detrital zircon and monazite geochronology imply that magmatism occurred at intervals from the Late Cretaceous-Miocene in the Wuntho-Popa arc, and during the Paleogene in the Mogok-Mandalay-Mergui belt (Barley et al., 2003; Mitchell et al., 2012; Gardiner et al., 2016). The spa-tial and temporal relationship between these belts, and their distinct but consistent metallic endowment over several hun-dreds of kilometers, has invoked comparison with the South American Cordillera (Peru/Bolivia; Gardiner et al., 2015a; Searle et al., 2016). Within the Central Andean margin, proxi-mal I-type magmatism exhibits Cu-Au-Mo type metallogeny, while inboard S-type belts host the tin porphyry deposits of Bolivia and Peru. The early model of Sillitoe (1972), as well as substantial more recent work, suggests that the petrogenetic and metallogenic properties of these Andean belts reflect the influence or otherwise of the mantle, primarily controlled by distance from the subducting slab, and an increase in the

crustal component of the magmatic source toward the east (Fig. 9).

The Central Andes has experienced a prolonged tectonic history, with multiple episodes of magmatism (Pitcher, 1987). Myanmar, however, represents a geologically simpler environ-ment that operated over a shorter period of time, resulting in differences in the timing of styles of mineralization (Gardiner et al., 2015a, 2016). However, the broad pattern of magmatic style and age, and of mineralization and geochronology leads to a simplified petrogenetic and tectonic model, namely that of an Andean-type setting, a continental magmatic arc sited above an eastward-dipping subduction zone on the margin of Neotethys. The following are the principal elements of Myan-mar metallogeny and their relationship to Neo-Tethyan ocean closure.

Wuntho-Popa arc, Myanmar (porphyry Cu-Au and epithermal deposits)

An arcuate belt of Pleistocene calc-alkaline volcanoes defines the trend of the 100-km-long Wuntho-Popa arc in western Myanmar. Interpreted as a continental magmatic arc (Mitch-ell and McKerrow, 1975), it is sited above the Burma seismic zone, an E-dipping active subduction zone with earthquakes recorded down to at least 230 km (e.g., Searle and Morley, 2011). At least two major earlier phases of magmatism have been recorded. Late Cretaceous granodiorite and dacite intru-sions were followed by Eocene and then Miocene intrusive and extrusive volcanic rocks (e.g., Barley et al., 2003; Mitch-ell et al., 2011, 2012; Gardiner et al., 2016). These intrude a Paleozoic amphibolite-gneissic basement overlain by lime-stones and pelagic sediments.

Mineralization in the arc is largely confined to the Banmauk-Wuntho batholith in the north and the Monywa-Mount Popa region to the south. Close to Wuntho, porphyry-type Cu-Au deposits, currently uneconomic, have been reported at Shan-galon, as well as other Au-bearing quartz veining related to the magmatism. The porphyry Cu-Au deposit at Shangalon has been dated through zircon U-Pb geochronology to 40 Ma (Gar-diner et al., 2016). Mineralization at the high-sulfidation Cu deposit at Monywa, however, is proposed to be of mid-Miocene in age (13.5 ± 0.2 Ma; Mitchell et al., 2011), which is confirmed by a U-Pb zircon age from an andesite porphyry at Leptadaung of 19.9 Ma (Knight and Zaw, 2015; no error provided).

Another well-known gold deposit in Myanmar, and the country’s largest gold mine, is located at Kyaukpahto, Kawlin Township, Sagaing Division. Here, Au mineralization is asso-ciated with stockwork-style quartz veins hosted in silicified sandstones. Veins comprising pyrite, chalcopyrite, and arse-nopyrite are best developed in competent silicified sandstone locally extending into the adjacent mudstones of the lower mid-Eocene Male Formation (Mitchell et al., 1999; Ye Myint Swe et al., 2004). These host rocks have undergone intense hydrothermal alteration and silicification, which appears to be critical for the genesis of the veining, the latter mainly con-fined to the silicified sandstone. Fluid circulation and vein for-mation has been linked to movement on the Sagaing fault in that NNE-trending extensional faults formed by a component of dextral strike-slip movement host the stockwork epithermal Au mineralization in extensional structures (Ye Myint Swe et al., 2004).

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Despite anecdotal reports linking mineralization to the Late Cretaceous magmatism in the Wuntho region (e.g., United Nations, 1978), no mineralization has been definitively dated as relating to this period of magmatic activity. It is neverthe-less apparent that mineralization (and magmatic activity) along the Wuntho-Popa arc has been relatively long-lived, extending from the Paleocene to the Miocene, and the belt is relatively well endowed with respect to porphyry Cu-Au and high- to low-sulfidation epithermal styles of mineraliza-tion. It is also evident that many other examples of Miocene-aged magmatic-hydrothermal mineralization formed along the Neo-Tethyan belt, including major deposits such as Sar Cheshmeh (13.6 Ma) in Iran, Reko Dig (13 Ma) and Saindak (22.3 Ma) in Pakistan, and Qulong (16 Ma) and Jiama (13 Ma) in Tibet (Richards, 2015). In the light of the longevity of magmatism along the Wuntho-Popa arc, the suggestion that Miocene porphyry and epithermal styles of mineralization in

Tibet and southeast Asia are linked to postcollisional, exten-sion-related alkaline magmatism (Hou et al., 2009; Shafiei et al., 2009; Wang et al., 2014; Richards, 2015) requires further evaluation.

Himalayan-Type Continental Collision Zones

Greater Himalayan sequence

The stratigraphy, structure, and metamorphic-magmatic evolution of the Himalaya is reasonably well-constrained, as a result of field studies, combined with detailed structural, metamorphic, thermobarometric, and U-Pb geochronologi-cal studies (see reviews by Hodges, 2000; Yin and Harrison, 2000; Searle et al., 2010b). Figure 10 shows a profile, though approximately 70 km of Himalayan crust, with folded and thrusted upper crustal sedimentary rocks of the North Indian margin underlain by the regional Barrovian metamorphic

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Fig. 9. Tectonic profile across an Andean-type margin, comparing the structural positions of the paired granite belts of the Andes and Myanmar. Abbreviations: bt = biotite, grt = garnet, hbl = hornblende, ms = muscovite, tur = tourmaline

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rocks of the Greater Himalayan middle crust. The lower crust is the underthrust basement of the Indian shield. A major episode of precollision deformation was synchronous with a Late Cretaceous-Paleocene phase of obduction of large ophi-olitic thrust sheets onto the Indian passive margin sequence. Only a few remnant ophiolites are preserved at the highest structural levels of the Tethyan zone, the best example being the Spontang ophiolite in Ladakh (Fig. 3; Corfield and Searle, 2000; Corfield et al., 2001). The northern part of the Indian plate mid-lower crust exposed in the Tso Morari dome in Ladakh and along the Kaghan valley, north Pakistan (Fig. 3) shows an early ultrahigh-pressure metamorphism recorded by coesite-bearing eclogites (e.g., O’Brien et al., 2001; St-Onge

et al., 2013). These ultrahigh-pressure assemblages have been overprinted by later kyanite- and sillimanite-grade metamor-phism during the main phase of crustal thickening in the late Eocene-early Miocene.

Deformation of the Indian plate resulted in folding and thickening of upper crustal sedimentary units (the Tethyan Himalaya), whereas middle and lower crustal lithologies along the Greater Himalaya were buried and heated as a result of crustal thickening (Fig. 4). The metamorphic core of the Himalayan range, the Greater Himalayan sequence, experienced a regional Barrovian-type metamorphism up to kyanite-grade P-T conditions (560°–630°C; 10–12 kbars, 35- to 40-km depth of burial) between ca. 35 to 30 Ma, which was

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followed by decompression and heating leading to regional sillimanite- and cordierite-grade metamorphism, migmatiza-tion, and formation of leucogranite melts between ca. 24 to 19 Ma (620°–700°C; 4–7 kbars, 15- to 20-km depth of burial; Searle et al., 2010b). Melting along the Greater Himalaya sequence began probably around 35 m.y. ago, resulting in the earliest kyanite-bearing migmatites at ca 700°C and 7 kbars, followed by later sillimanite- and cordierite-bearing melts recording higher temperatures but lower pressures. These extensively developed melt phases, commonly accompanied by pegmatites emanating from the top of leucogranite intru-sions, are large-ion lithophile element (LILE) enriched, but typically devoid of mineral deposits, with the exception of gem-quality tourmaline with garnet and muscovite.

This structural and thermal evolution is consistent along the 2,000-km length of the central Himalaya (e.g., Searle et al., 1999, 2006; Walker et al., 2001; Godin et al., 2006). How-ever, in the orogenic syntaxes of Nanga Parbat (NW syntaxis; Fig. 3) and Namche Barwa (NE syntaxis), the rocks have been through the same Cenozoic thermal history as the rest of the Himalaya, but show evidence of an additional, younger Plio-cene-Quaternary high-grade metamorphism and migmatiza-tion event (Zeitler et al., 2001; Booth et al., 2008; Crowley et al., 2009). The youngest metamorphic and structural episodes dated in the syntaxes (Pliocene-Quaternary) are character-ized by extremely rapid exhumation rates, and may reflect processes operating at depth beneath the northern Himalaya, Karakoram, and south Tibet today.

Mogok metamorphic belt, Myanmar

The Mogok metamorphic belt extends for over 1,500 km from the east Himalayan syntaxis to the Andaman Sea. It comprises a middle and lower crustal section exhumed by compres-sional deformation prior to dextral strike-slip faulting along the Sagaing fault. The belt includes precollision I-type gran-ite magmatism, including hornblende-bearing granodiorites dated by U-Pb zircon geochronology as Jurassic-Cretaceous (Barley et al., 2003; Mitchell et al., 2012). The Mogok belt also includes postcollisional regional metamorphic rocks up to sil-limanite grade. U-Pb monazite dating suggests two phases of metamorphism, one prior to 59.4 Ma, the age of crosscutting biotite granite dikes, and the other between 37 to 29 Ma (Searle et al., 2007). Localized partial melting resulted in the formation of tourmaline + garnet leucogranitic melt pods at 24.5 ± 0.7 Ma (Searle et al., 2007), whereas the sizeable Kabaing granite that intrudes the marbles has been dated at 17 Ma (Gardiner et al., 2016). Around the town of Mogok, high-temperature marbles have been intruded by alkali syenite intrusions, and contain abundant rubies and sapphire gemstones (Searle et al., 2016).

Tethyan Mineral Zones

Tin-tungsten belts (Malaysia-Thailand-Myanmar)

The granite belts of southeast Asia collectively comprise one of the world’s greatest metallotects of Sn and major, albeit localized, W. The region has been the dominant global Sn producer, accounting for some 54% of historic production (Schwartz et al., 1995). Three principal belts are identified (Fig. 11): the Western province, or Mogok-Mandalay-Mergui belt (central Myanmar and Thailand; Fig. 6); the West Malaya

Main Range plutons (eastern Myanmar, western Thailand, and western Malaysia peninsula); and the eastern Malay plu-tons (eastern Myanmar, central Thailand, and eastern Malay peninsula; Hutchison, 1977; Cobbing et al., 1986, 1992; Khin Zaw 1990; Sone and Metcalfe, 2008; Searle et al., 2012; Ng et al., 2015a, b; Gardiner et al., 2015b). These granite belts are the magmatic expressions of the closure and suturing of Paleo-Tethys and Neotethys, and were thus emplaced during multiple time periods including the Early Triassic (Eastern province, Malaysia), Late Triassic (western Main Range gran-ites, Malaysia; Ng et al., 2015a, b), Late Cretaceous-Eocene (SW Myanmar; Gardiner et al., 2016), and Paleogene (Phuket, SW Thailand; Searle et al., 2012).

The Paleo-Tethyan suture is represented by the Bentong-Raub suture zone, which separates the Main Range tin gran-ites of the Western province from the dominantly subduction related I-type granites of the Eastern province of Malaysia (Fig. 11). In Malaysia the most prolific tin mineralization is associated with the western Main Range province granites with U-Pb zircon ages spanning 227 to 201 Ma, although lesser but nevertheless still significant Sn mineralization also occurs in the subduction-related I-type granites of the East-ern province, which have U-Pb zircon ages spanning 289 to 220 Ma (Ng et al., 2015b). Thus, tin granites crop out across the Malay peninsula on both sides of the suture. In Thailand and the Malay peninsula, tin is principally mined from placer deposits both onshore and offshore, reflecting the high degree of erosion of these granites.

The Mogok-Mandalay-Mergui belt in eastern Myanmar is related to crustal thickening following the closure of Neo-tethys (Searle et al., 2007, 2016; Gardiner et al., 2015a). It comprises a mixture of I- and S-type granites (Cobbing et al., 1986; Khin Zaw, 1990; Barley et al., 2003; Mitchell et al., 2012), although magma compositions tend to evolve toward more peraluminous S-type granites with time. Magmatic ages range from Late Cretaceous to Eocene (Barley et al., 2003; Searle et al., 2007, 2016), with many of the tin deposits related to granites crystallizing at 70 to 50 Ma (Aung Zaw Myint et al., 2016; Gardiner et al., 2016), which we interpret as rep-resenting the timing of mineralization. These S-type granites are likely the product of melting of a crustal protolith. Tin granites thus crystallized prior to the main phases of regional metamorphism along the Mogok belt, which occurred during the Late Cretaceous-Paleocene and again in the Eocene-early Miocene (Searle et al., 2007, 2016). The tin granites in Myan-mar are significantly younger (Cretaceous to Paleogene) than those of the Malaysian belts and are substantially less eroded. Accordingly, there are many more primary deposits found in the region, including the historic mines focused around the port town of Dawei (Fig. 6). The region is also substantially more tungsten specific than Malaysia, and hosts the famous W-Sn Mawchi mine, 250 km northeast of Yangon (Fig. 6).

Tin mineralization in southern Myanmar is focused where the Cretaceous-Eocene S-type granites intrude the Slate belt, a predominantly late Paleozoic succession of low-grade metasedimentary units, pebbly mudstones with occasional limestones, collectively defined as the Mergui Group (Mitch-ell, 1992). Primary tin mineralization is found as cassiterite-hosting quartz veins and as pegmatites, either intruding the country rock, or within the upper parts of the granite bodies.

318 SEARLE ET AL.

Tungsten is spatially associated with the tin mineralization, commonly as wolframite, and more rarely as scheelite.

Origin of tin granites

Although most Sn-W deposits are hosted in peraluminous S-type granites, in the Tethyan belts and elsewhere, fraction-ated I-type granites may represent the preferred host rocks to LILE mineralization (Groves and Bierlein, 2007; Ng et al.,

2015a). Although significantly enriched, the highly peralumi-nous Himalayan leucogranites are not associated with signifi-cant Sn, or LILE mineralization, whereas the I-type granites of the Malaysian Eastern province are relatively Sn rich. In a recent review, Romer and Kroner (2016) proposed that the formation of Sn and/or W mineralization was the result of three main processes: source enrichment, magma accumu-lation, and subsequent concentration of metals by fractional

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TECTONIC PROCESSES & METALLOGENY, TETHYAN MOUNTAIN RANGES, MIDDLE EAST AND SOUTH ASIA 319

crystallization and hydrothermal processes. The model calls upon an inherited metal endowment in the source region(s) a factor that is then used to explain the diachronous nature of Sn deposits that occur at multiple times during orogenic events and also across different terranes—such is very much the case in the Tethyan belts of southeast Asia.

Skarn Au-base metal deposits (Shante, Myanmar, and Mengapur, Malaysia)

The Shante gold district of Myanmar lies 50 km south of Mogok (Fig. 6), within the high-temperature marbles of the Mogok metamorphic belt. Skarn-type Pb-Zn ± Au mineral-ization is quartz-vein hosted within the marbles. The Kwin-thonze mine, near Thabeikkyin, is a marble-hosted Au-base metal sulfide deposit, interpreted as skarn type, and through spatial association interpreted as related to the intrusion of the Kabaing Granite (Tin Aung Myint et al., 2014). The Kabaing Granite has been dated through zircon U-Pb geochronology to ca. 17 Ma (Gardiner et al., 2016), thereby providing some age constraints on this mineralization. It is thus interpreted as related to the late stages of Mogok metamorphism.

Skarn-hosted Cu-Au mineralization is also know from east-central Malaysia. This deposit is hosted by Permian calcareous sediments that have been intruded by an undated, but likely Triassic-aged, adamellite body that is also linked to extrusive rhyolite and tuff (Snowden Report, 2012). Both garnet- and pyroxene-rich skarns host vein sulfide mineralization that is dominated by pyrrhotite and chalcopyrite. Fluid inclusion studies suggest that sulfide mineralization is related to retro-grade processes involving a component of low-salinity mete-oric fluids possibly implicated in the precipitation of metals (Heng et al., 2003). The Mengapur Cu-Ag system occurs within the so-called Central belt of Malaysia, occupying a posi-tion intermediate between the Sn-W-dominant, S-type Main Range and the I-type Eastern province. The Central belt in Malaysia contains numerous gold and base metal deposits, but its metallogenic significance and relationship to the surround-ing, better defined belts is not well understood.

Orogenic gold (Mogok-Mandalay-Mergui belt, Myanmar, and peninsular Malaysia)

In central Myanmar, prospective Au mineralization is found as quartz vein and pyrite stringers within the low-grade metasedi-mentary Slate belt (Mitchell et al., 2004). Mineralization is found extending in a belt for over 100 km around Mandalay. The style of mineralization is typical of turbidite-hosted orogenic-style Au; however, neither its age nor genesis is well constrained. Reported age determinations from studies of the Modi-Taung-Nankwe and the Meyon deposits range from the Jurassic to Paleogene (Mitchell et al., 2004; Zaw Naing Oo and Khin Zaw, 2009), and reflect the polyorogenic history of the region and the difficulty in ascribing gold deposits to a particular event.

Orogenic-style gold is typically associated with the wan-ing stage of orogeny. Mineralization is focused along major deep-penetrating shear zones, along which aqueous-carbonic fluids derived from midcrustal metamorphism during crustal thickening circulate to precipitate gold ores at crustal levels that broadly equate to greenschist-amphibolite grades (e.g., Groves et al., 1998). The genesis of the Slate belt Au is pos-sibly related to the Himalayan orogeny (closure of Neotethys)

in the Eocene, although some workers (Mitchell et al., 2004) believe that older metamorphic overprints were related to the Late Triassic-Jurassic Indosinian orogeny (closure of Paleo-Tethys).

Orogenic gold deposits also occur in peninsular Malaysia where mineralization is almost certainly related to the older orogenic cycle. Significant gold deposits at Raub, Penjom, and Selinsing all occur close to the Bentong-Raub suture in the Central belt of Malaysia. With many similarities to the Slate belt gold deposits of Myanmar, but located in an entirely dif-ferent magmatic arc, the existence of widespread orogenic gold styles of mineralization point to a pervasive interplay between the tectonic and magmatic evolution of the region and circulation of auriferous metamorphogenic fluids.

Intraplate Alkaline MagmatismAlkaline magmatism is common during the early stages of continental rifting (e.g., East Africa, Red Sea) and the later stages of orogenesis (e.g., lamprophyre dikes; adakite-like and shoshonitic dikes in Tibet), or even long after mountain build-ing (e.g., kimberlites). Lamproites and lamprophyres are syn- to late-collisional alkaline intrusions that commonly intrude mountain belts during the later tectonic history.

Syenites, alkali peridotites

In the Mogok metamorphic belt of Myanmar several syenite bodies have been intruded into the regional metamorphic rocks that are dominated by thick marbles, some of which are rich in rubies and sapphires. Pressure-temperature condi-tions of metamorphism were around 600° to 680°C and 4 to 6 kbars in the rare pelites in the Mogok belt (Searle et al., 2007). There is little evidence of large-scale folding and thrusting as seen elsewhere along the Himalaya, so it appears that crustal thickening was not an obvious process involved in regional metamorphism. An additional source of heat is required to obtain the high temperatures needed for metamorphism in the Mogok belt, which may have been from mantle-sourced syenite intrusions (Searle et al., 2016). Whereas rubies are present throughout some marble bands, sapphires appear to be mainly located along syenite-marble contact zones, sug-gesting a link between syenite-derived heat source and mar-ble host rock. In a few localities alkali peridotites represent the deepest part of the syenite intrusion, suggesting internal differentiation and a magma origin related to highly enriched subcontinental mantle lithosphere.

Mantle-derived lamproites, shoshonites

In addition to the lower crust-derived adakite-like intrusions, the Tibetan plateau hosts numerous mantle-derived shosho-nite dikes (Chung et al., 2003, 2005). Potassium-rich shosho-nites and subordinate sodium-rich lavas were erupted across the Tibetan plateau between 50 and 10 Ma (Chung et al., 2003, 2005; Fig. 12). These shoshonitic magmas require a hot, hydrous (phlogopite-bearing) and weak mantle lithosphere source. Felsic and mafic granulite and pyroxenite xenoliths entrained in 17 to 14 Ma ultrapotassic dikes suggest very high pressures and temperatures (1,330°–1,130°C; 26–22 kbars; Chan et al., 2009). Using both surface geology and xenoliths in the dikes, Searle et al. (2011) proposed a profile through the 75-km Tibetan crust (Fig. 13). The youngest magmatic rocks

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Fig. 12. (a). Simplified geologic map of the Tibetan plateau, showing the distribution in space and time of postcollisional adakite and shoshonite intrusions, after Chung et al. (2005). Abbreviations: BNS = Bangong-Nujiang suture zone, ITS = Indus-Tsangpo suture zone, MCT = Main Central thrust, STDS = south Tibetan detachment sytem. (b). Space-time diagram showing variation of Tibetan magmatism across the Tibetan plateau. See Chung et al. (2005) for sources of data.

TECTONIC PROCESSES & METALLOGENY, TETHYAN MOUNTAIN RANGES, MIDDLE EAST AND SOUTH ASIA 321

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in Tibet are a swath of potassium-rich shoshonitic volcanic rocks along the KunLun mountain range in the far north of Tibet. The spatial and temporal distribution of volcanic rocks across the Tibetan plateau suggest the progressive northward shift of a hot mantle source, as cold Indian lithosphere pro-gressively underthrust Tibet from south to north, from 50 Ma to the present day (Searle, 2015).

Placer DepositsThe largest gold deposits in the world are derived from secondary concentrations of detrital gold into sedimentary basins accumulating erosional debris from adjacent island-arc terranes or collisional mountain belts (e.g., Witwatersrand Basin). Detrital fluviatile gold deposits along the upper Indus valley are almost certainly derived from the Ladakh island arc and batholith to the north. Gold is presently mined from recent basins and river systems along the Shan Scarp in Myan-mar. These are likely derived from erosion of the Slate belt, where in situ Au occurs in orogenic vein systems such as at Modi Taung (Fig. 6; Mitchell et al., 2004). Some gemstones, notably rubies, spinel, sapphire, and rare diamond, are mined from placer deposits along the Mogok metamorphic belt.

Far more widespread than gold or gem placers in southeast Asia, however, are the vast accumulations of placer tin miner-alization that have formed in a variety of fluvial, fluvio-deltaic, and shallow-marine settings. The southeast Asian tin belt has been almost entirely mined from placer deposits in alluvial and fluvial basins across the Malay peninsula as well as off-shore southwestern Thailand in the Phuket area. The tin was derived from erosion of the Triassic Main Range granites in the western Malay peninsula (Ng et al., 2015a, b) and S-type granites from the Phuket western Thailand belt that may be as young as Paleocene in age (Searle et al., 2012).

ConclusionsThe record of mineralization formed during the closure of Paleo-Tethys (the Indosinian orogeny) is generally not well preserved. By contrast, Neo-Tethyan Himalayan orogenesis is better preserved and understood than earlier cycles and, importantly, the diversity and preservation of mineralization during closure of Neotethys has resulted in the formation of very significant metallogenic belts. We summarize Tethyan orogenesis on a regional basis where well-defined and chrono-logically constrained magmatic events can be identified (Fig. 14). The broad metallogenic framework, however, is markedly different on either side of the collision zone and is presented on this basis below.

Metallogeny south of the Neo-Tethyan suture

The Himalayan orogenic cycle and its metallogenic affinities south of the Neo-Tethyan suture can be summarized in terms of well-defined orogenic stages, including precollisional ophi-olite obduction, collisional crustal thickening, metamorphism, and partial melting. The precollisional stage is marked by Late Cretaceous-Paleocene ophiolite formation and obduction. The mantle portions of ophiolite complexes are dominated by magmatic concentrations of Cr, Ni, and PGM, whereas shal-lower, crustal sequences preserve concentrations of chalco-philic metals (Cu-Zn) associated with ocean-water circulation and venting. In the western Himalaya a large-scale island arc,

the Kohistan island arc, has been obducted southward onto the northern margin of the Indian plate approximately at the same time as ophiolite complexes preserved in the Ladakh Himalaya (Searle at al., 1997b; Corfield and Searle, 2000). Subduction of the leading edge of the downgoing Indian con-tinental crust led to ultrahigh-pressure eclogite facies meta-morphism along the northern margin of the Indian plate at Kaghan (Pakistan) and Tso Morari (Ladakh; ~57–47 Ma). Few known mineral deposits of any significance characterize this phase.

Crustal thickening and shortening along the Himalaya resulted in kyanite-grade metamorphism (~40–30 Ma) and partial melting, and is likewise characterized by few known mineral deposits of any significance. Sillimanite-grade metamorphism was accompanied by decompression melt-ing, formation of migmatites and leucogranites, and south-directed extrusion of the ductile midcrust by channel flow (~24–15 Ma). This stage is characterized by the formation of anatectic, peraluminous granite melts significantly enriched in granitophile elements, but is of little significance as a metal-logenic province. Southward-propagating thrusting along the Lesser Himalaya formed a subcritical wedge with active underthusting of India beneath the Himalaya. The downgoing slab in the Himalayan context (Indian plate) is also featured by a lack of processes (magmatism, focused hydrothermal fluid circulation, etc.) that typically give rise to significant mineral deposits.

Metallogeny north of the Neo-Tethyan suture

For the Asian side of the India-Asia collision zone in the Karakoram and Pamir ranges, as well as across the Lhasa and Qiangtang blocks of Tibet (i.e., the upper, overriding plate), the orogenic and metallogenic framework is very different and can be summarized as follows. A phase of Triassic-Early Jurassic subduction, crustal thickening, and regional meta-morphism resulted in the Indosinian orogeny that spanned central Asia from Afghanistan and the Pamir region across central Tibet to eastern Myanmar-Thailand and into Malaysia. The tin granite belts of Malaysia and southwestern Thailand, which young progressively from east to west, are associated with Permian to Late Triassic granites that formed during clo-sure of Paleo-Tethys along both sides of the Bentong-Raub suture (Searle et al., 2012; Ng et al., 2015a, b). This repre-sents a major metallogenic province dominated by Sn-W min-eralization (now significantly eroded and reconcentrated into substantial paleoplacer deposits, both onshore and offshore of Malaysia-Thailand).

Pre-India-Asia collision crustal thickening in an Andean-type setting over a period spanning the Early Jurassic–early Eocene, occurred along the Kohistan-Ladakh-Gangdese batholith (~198–49 Ma; Chung et al., 2005; Chu et al., 2006). This major magmatic event across the entire collision zone, spanning some 2,500 km, does have some significant porphyry Cu-Au-Mo deposits hosted in typical calc-alkaline granitoids ranging from Jurassic to Eocene in age (Tafti et al., 2001; Qu et al., 2009). However, work by Wang et al. (2014a, b) and Rich-ards (2015) indicates that factors such as magma water con-tent and magmatic oxidation state, which play a role in terms of metal concentration and ore-forming processes, were not optimal for the formation of porphyry and epithermal styles of

TECTONIC PROCESSES & METALLOGENY, TETHYAN MOUNTAIN RANGES, MIDDLE EAST AND SOUTH ASIA 323

mineralization along the central portion of the batholith. Else-where, toward the east in Tibet, younger Miocene adakite-like magmas were wetter, more oxidized, and more conducive to the formation of significant magmatic-hydrothermal styles of mineralization. Beyond the eastern syntaxis, magmatism

during the Cretaceous and Paleogene was more peralumi-nous, giving rise to significant granite-hosted Sn-W mineral-ization along the Mogok-Mandalay-Mergui arc of Myanmar. Calc-alkaline magmatism occurred closer to the leading edge of subduction along the Wuntho-Popa arc in Myanmar, giving

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Ma

150

200

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QIANGTANGEAST

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OU

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EN

OZ

OIC

PALE

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AS

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MID

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PP

ER

UP

PE

RPA

LO

LIG

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CE

NE

MIOC

ENE

LLO

WE

RLO

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R

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ECLOGITE

Jade

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SUMDA SUTUREBENTONG-RAUB SUTURE

Danba

Danba

Gan

gdes

eG

angd

ese

Gang

dese

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shon

ites

Takena Fm.

INDIA - ASIA SUTURE

Met

am Met

amor

phis

m

Met

amor

phis

morph

ism

Mt. Popavolcano

PayangazuSedawgyi

Mae Klang

Lansang

Klong Marui /RanongThabsilaKlong MaruiBhumipol

lakeDoi Inthanon

RanongPhuketzir rims

LansangKlong Lan

Tioman Is.

Klong Lhan

Doi Inthanonprotolith

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PeraluminousI-type granites

East Malaya

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KyausheMandalay

MECSedo

KyanikanBelin dyke

Nattaung

Mokpalin

Monywa-Salingyi

Kyaushe

Yeboksondiorite

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Belin

Linziz

ong

Fig. 14. Mesozoic-Cenozoic time chart showing age ranges of metamorphic, magmatic, and mineralization processes. Blue line represents youngest marine sedimentary rocks and timing of suture zone closure; orange colors represent I-type (calc-alkaline) magmatism; yellow colors represent periods of regional Barrovian-type metamorphism; pink lines represent S-type (more peraluminous) granites associated with postcollisional crustal thickening; green line represents age range of mantle-derived shoshonites.

324 SEARLE ET AL.

rise to significant porphyry and epithermal mineral deposits (Gardiner et al., 2015a).

Postcollisional crustal thickening in Tibet resulted in lower crust kyanite- and sillimanite-grade metamorphism during the Cenozoic, exhumed in selected areas of southeast Tibet (Palin et al., 2013). This event was not characterized by any known mineral deposits of significance. In Myanmar, however, this stage of mainly Paleogene metamorphism hosted many gem-stones along the Mogok belt, including world-class ruby and sapphire-bearing marbles. Searle et al. (2007) used U-Th-Pb dating to define two main phases of high-temperature meta-morphism: a pre-59 Ma event and a later sillimanite + mus-covite event at least from 37, possibly from 47 Ma to 29 Ma.

Postcollisional lower crustal melting resulted in adakite-like rocks, and mantle melting-generated shoshonites (Chung et al., 2005, 2009; Lee et al., 2009; Wang, Q. et al., 2010; Wang, R. et al., 2014a, b). This event has assumed increasing metal-logenic significance as mineral deposits and their host rocks throughout the region are more accurately dated by Re-Os and U-Pb zircon techniques. It is increasingly evident that the period between ca. 22 and 12 Ma (peaking at around 15 Ma) is characterized by concentration of very significant por-phyry Cu-Au deposits mainly along the eastern portions of the Lhasa block, but less so in the western portions. Magmatism during this part of the cycle has been attributed to slab break-off and melting of a fertile protolith (Chung et al., 2005) to form numerous, world-class porphyry and epithermal depos-its (Hou et al., 2009; Wang et al., 2014a, b; Richards, 2015).

Oligocene-Miocene crustal thickening and regional meta-morphism along the Karakoram (Searle et al., 2010a) and Pamir (Stearns et al., 2013) is characterized by few known mineral deposits of any significance. Gem-quality tourmaline and aquamarine (beryl) in the Karakoram Range of north Pak-istan are associated with young (<5 Ma) pegmatite dikes in sillimanite-grade gneiss domes. It is likely that extreme crustal thickening and high-grade amphibolite, granulite, and eclog-ite facies regional metamorphism in the west (Karakoram, Pamir) and east (SE Tibet) was not conducive to preservation of major mineral deposits, such as seen along the Gangdese belt of central southern Tibet.

Similar to the Andes, the Tethyan orogenic belts represent a vast region of plate subduction, accretion, magmatism, and associated tectonism that ultimately consumed a major por-tion of oceanic crust and gave rise to the creation of continen-tal crust from the late Paleozoic to the present. These events yielded the conditions that favored the formation of many different styles of ore deposits, providing these regions with a substantial endowment of mineral wealth. Although the overall pattern of events that link crustal evolution to metal-logeny are well understood, the details of where and why the biggest mineral deposits occur, and the controls of metal fer-tility and specificity in magmas, are not. The Tethyan belt is a metallogenic province that offers considerable promise for the discovery of new deposits, as well as answers to the many questions that remain.

AcknowledgmentsWe appreciate the insights gained from numerous discussions and field trips with Andrew Mitchell and Chris Morley, and drafting of diagrams by Dave Sansom. The manuscript was

substantially improved by the detailed reviews provided by Doug Kirwin, Graham Begg, and editors Jeremy Richards and Yongjun Lu.

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