Magma Minling Tectonic and Geodynamic Implications

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ISSN 0016-8521, Geotectonics, 2006, Vol. 40, No. 2, pp. 120134. Pleiades Publishing, Inc., 2006.Original Russian Text E.V. Sklyarov, V.S. Fedorovskii, 2006, published in Geotektonika, 2006, No. 2, pp. 4764.

INTRODUCTION

Magma mingling and mixing1 are widespread pro-cesses in many geodynamic settings. It is a paradoxicalfact that the idea of mixing of magmas differing in com-position was put forward for the first time in the midstof the 19th century by a specialist in analytical chemis-try rather than in geology, Robert Bunsen [42]. Havingsampled volcanic rocks in an area with geysers in Ice-land, Bunsen revealed substantial compositional differ-ences in the sampled basalts and suggested that a layerof basaltic magma was overlapped by a layer of rhy-olitic melt and the entire range of igneous rocks wasformed owing to various degrees of basaltrhyolite

mixing. It came as no surprise that this idea was imme-diately heavily criticized by the geologists who studiedIceland and afterwards was adopted but with a geolog-ical priority. Towards the turn of the 19th century, addi-tional evidence for the mingling of magmas contrastingin composition was provided [42] concerning primarilycomposite dikes and sills and granitic plutons withnumerous mafic inclusions. The effects of minglinghave been described in other volcanic provinces inaddition to Iceland. The idea of contemporaneousemplacement and crystallization of magmas of con-trasting compositions became popular. However, in thebeginning of the 20th century, this idea was displacedby the concept of crystal fractionation stated compre-

hensively by N. Bowen in his book The Evolution of Igneous Rocks

[24], which became a guidebook for

1

The term magma mingling

widely used in English language pub-lications implies a mechanical interaction between the coexistingmelts of different compositions that does not result in their com-plete homogenization (see [42] for the historical review) and is incontrast to the term magma mixing

that accentuates the formationof hybrid melts of intermediate composition; both English termsare commonly translated by the same Russian word. To avoidconfusion, the transliterated English terms are often given follow-ing the Russian word in the Russian version of this paper.

several generations of igneous geologists. This univer-sally accepted concept is still developing, and itsappearance would not threaten the concept of minglingif crystal fractionation were not proclaimed as a singlemechanism that provides the diversity of igneous rocks.As always happens during the emergence of new para-digms and panaceas, the other ideas supported by reli-able data are pushed aside. Furthermore, the model ofmingling was deemed unrealistic for a long timebecause of the substantial difference in viscosity andtemperature of felsic and mafic magmas. However, alarge body of new information obtained in the mid-20thcentury provided decisive evidence for the coexistenceof contrasting magmas and the idea of their minglinginspired a renewed interest and gave impetus to numer-ous publications (reviews in [12, 26, 27, 42]), theamount of which continues to increase.

It should be noted that the concept of mingling con-siderably expanded and fell outside the scope of inter-action between melts. Piperites

have been recognized andcharacterized as products of interaction of erupting lavaswith water-saturated unconsolidated sediments [35]. Theterm metamorphic mingling

has been introduced fordescribing the mechanical mixing of ductile metamor-phic rocks and mafic melts in collisional zones [18]. Inthis paper, we focus our attention on classic magmamingling as emphasized in the title. Moreover, becauseonly petrologic problems related to the interactionbetween magmas differing in composition are dis-cussed in the overwhelming majority of the publica-tions and only in a few works is the considerationextended beyond specific bodies of igneous rocks, itseems expedient to center our attention on tectonic andgeodynamic implications of mingling. The paper pre-sented is based on the data obtained by us and otherresearchers in the western Baikal and Transbaikalregions.

Magma Mingling: Tectonic and Geodynamic Implications

E. V. Sklyarov

a

and V. S. Fedorovskii

b

a

Institute of the Earths Crust, Siberian Division, Russian Academy of Sciences, ul. Lermontova 128, Irkutsk, 664033 Russia

b

Geological Institute, Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017 Russia

e-mail: [email protected]

Received February 7, 2005

Abstract

An attempt is made to consider the tectonic and geodynamic implications of the mingling of maficand felsic magmas, particularly, the relationships between mafic and felsic igneous rocks in composite dikes andplutons. Magma mingling develops in suprasubduction, intraplate, and collisional settings. The attributes typicalof each type of mingling are discussed with special emphasis on the magma mingling of the collisional type, whichis related to synmetamorphic shearing and may be regarded as a direct indicator of synorogenic collapse of colli-sional structural features. This phenomenon is exemplified in the Olkhon collisional system in Siberia.

DOI: 10.1134/S001685210602004X


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MAGMA MINGLING: TECTONIC AND GEODYNAMIC IMPLICATIONS 121

TYPICAL ATTRIBUTES OF MAGMA MINGLING

In terms of tectonics, it is most important that themingling of the mantle-derived mafic and crustal felsicmagmas that coexist in the liquid state eventuallyoccurs in the middle and upper crust or at the day sur-face. The mingling of products of magma fractionationwithin a single magma chamber is also known. How-ever, such occurrences are insignificant in size and notabundant.

According to a popular concept, the considerablemasses of granitoids are generated in the lower crustaffected by ascending mantle-derived mafic melts, asfollows from intimate spatial association of mafic andgranitic rocks. The most pictorial qualitative modeldeveloped by Huppert and Sparks [30] demonstratesthe consecutive emplacement of mafic melt into thecrust and the accompanying generation of graniticmagma (Fig. 1). At the initial stage of rifting, basaltserupt on the surface of the cold and brittle crust(Fig. 1a) and at the same time intrude the upper andmiddle crust, while forming sills and relatively small

magma chambers. The conductive heating of the crusteventually leads to the generation of granitoid magma(Fig. 1b), and the partially melted crust serves as a nat-ural barrier that becomes impermeable to new portionsof basaltic magma and thus promotes further heating ofthe crust; particular batches of granitic magma canerupt on the surface. The advanced heating of the crust(Fig. 1c) provides the formation of large plutons, whileignimbrites erupt at the surface. The late basaltic erup-tions are confined to the margins of igneous provinces.

The mafic and felsic rocks occur in composite plu-tons as complexes of contemporaneous dikes, includingring dikes, minor intrusions, schlieren and autoliths(dialiths, after Popov [12]) in granites [25, 26, 34] orsyenites [10]. In the fields of felsic volcanics, basaltsoccur as discrete lava flows, cinder cones, inclusionsand interlayers within mafic rocks [21, 32]. Compositedikes and sills are noteworthy. They may be shallow-seated conduits of volcanic edifices [36, 37, 39], sepa-rate intrusive phases [811, 25, 26, 43], or componentsof high-temperature collisional metamorphic com-plexes [6, 14, 22].

(a) (b)

(c)

Mafic intrusions

Mafic volcanics

Granitic plutons

Felsic volcanics

Flank cinder cones

Fig. 1.

Evolution of the granitic magmatic system controlled by emplacement of mafic magma into the crust: a conceptual scheme,after [30]. (a) The early stage of emplacement of basalts in the cold crust when they can erupt on the surface; (b) the stage of thecrusts heating and the onset of generation of felsic crustal magma that erupts on the surface or crystallizes under hypabyssal con-ditions; and (c) the stage of the mass melting of the crust characterized by eruption of ignimbrites, caldera collapse, and emplace-ment of large granitic plutons. Basalts can penetrate into the upper crust only at the periphery of an igneous province.


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SKLYAROV, FEDOROVSKII

Thus, the lava flows, composite dikes, and magmachambers are three main modes of occurrence typicalof mingling.

The magma mingling in lava flows is indicated notonly by mafic inclusions and schlieren in felsic volca-nics but also by xenocrysts of mafic minerals and high-Caplagioclase in silicic lavas, whereas quartz and alkalifeldspar occur in basic and intermediate lavas. As arule, the volcanics with these attributes do not bearsigns of superimposed deformation.

The composite dikes are diverse in morphology andmay be linear, curved, branching, or ring-shaped.Through relationships between igneous rocks of differ-ent compositions, two main types of composite dikesare distinguished: (1) those with a felsic core rimmedby mafic rocks and (2) those with a mafic core com-bined with felsic margins [41]. The composite dikes ofthe first type (Fig. 2), which were described byA. Harker in his classic monograph [28], are formed asa result of consecutive injections of mafic and felsicmelts; the latter were emplaced into the axial, incom-pletely solidified zone of the preceding mafic dike. Thesecond, prevalent type is formed during contemporane-ous injection of two magmas contrasting in their com-positions and is characterized by more diverse relation-ships between the mafic and felsic materials. The pil-low and breccia structures are typical; however,pseudolayered varieties are also noted. The dikes with

absolute predominance of mafic rocks cut by numerousfelsic veinlets (net-veined complexes) are rather abun-dant. The examples of such relationships in granitoidplutons of the Transbaikal region and in metamorphicrocks of the Olkhon region will be discussed below.The entire range of composite dikes, from those com-posed of undeformed mafic and felsic rocks via partlytectonized rocks of different compositions to the com-pletely metamorphosed mafic and felsic rocks, isobservable.

The development of mingling in magma chambersis especially diverse and controlled by many factors, themost important of which is the degree of crystallizationof granitic magma. The two extreme states correspondto (i) the liquid state of granitic magma with variablecontents of phenocrysts and (ii) the solid crystallinestate, occasionally with insignificant amounts of resid-ual melt. Let us dwell in more detail on the character-

ization of mingling for both variants.The crucial indications of mafic melt injections into

felsic magma (the first variant) were set forth by Litvi-novskii et al. [10]:

(1) The grain size of mafic rocks at the contact withthe host felsic material diminishes not only in largeintrusive bodies but also in particular nodules as a resultof the fast solidification of basaltic magma at theboundary with felsic melt that had a lower temperaturethan the basalt solidus.

(2) As a rule, the mafic rocks are fine-grained(medium-grained in exceptional cases), including thebodies reaching 2030 m across. This is also evidence

for their fast crystallization.(3) The grain size of granitic rocks at the contact

with mafic rocks does not decrease, even if the maficbodies are rather large; thin granitic injections intomafic rocks also do not reveal a decrease in grain size.At the same time, the chilled margins of syenitic bodiesare present always at the contacts with country meta-morphic rocks.

(4) The basic inclusions have festoon-lobate out-lines in cross section owing to a great number of smallglobules. Such a shape of contacts is typical of theboundaries between two liquids having different vis-cosities [27].

(5) Large mafic bodies are constantly surrounded byspherical and oval inclusions, and the oblong bodies areaccompanied by trains of such inclusions. This is addi-tional evidence for dispersion of a more viscous melt ina less viscous liquid.

(6) In some places, the inclusions have the appear-ance of bent strips and ribbons that bear no indicationsof cataclasis and translation of rock-forming minerals.Such inclusions may be regarded as a product of ductiledeformation of mafic material in the felsic magma.

(7) Ductile deformation and oriented mafic inclu-sions are especially evident in those places where thegranitic melt moved along the contact with solid gabbro

and injected gabbro along fractures. The mafic inclu-sions occur here as elongated, often curved lenses ori-ented parallel to the contact of gabbroic blocks (xeno-liths). The orientation of tabular plagioclase and pris-matic grains of dark-colored minerals is clearly seenunder a microscope both in mafic rocks and to a lesserextent in host granitoids.

(8) The tonalitic and biotiteplagioclase selvages atcontacts of felsic rocks with mafic inclusions are con-clusive evidence for the coexistence and partial mixing

Fig. 2.

Section of a composite dike with mafic rocks at itsmargins, after [28].


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of contrasting magmas. As shown in [38], these sel-vages testify, first, to the diffusion exchange betweentwo magmas and, second, to additional heating of sye-nitic melt at the contact with mafic magma.

Although the aforementioned features were estab-lished for the particular Romanovsky pluton in the Trans-baikal region, they may be regarded as typical structuraland petrographic attributes of magma mingling.

The elongated swarms or chains of melanocraticinclusions, synplutonic dikes, and related linear chainsof globular and droplike mafic bodies serve as indica-tors of the emplacement of mafic dikes into incom-

pletely consolidated granites that contained as much as30% of the residual melt (the second variant of min-gling) (see Fig. 3). In this case, the mafic rocks do notundergo boudinage and their morphology results onlyfrom crystallization. The brittle failure of granitic rockswith the formation of linear fractures filled with maficmagma indicates that the granitic rocks are crystallizedsignificantly by the moment of mafic melt emplace-ment. The felsic melt that contains 3570 vol % of crys-tals has the rheological properties of Bingham liquid[31] and provides development of linear shear zones.Such zones control the injections of mafic melts. Thetransition from swarms and chains of inclusions to syn-plutonic dikes cut by injections of residual melts derived

from host granitoids depends on the proportions betweenthe crystals and the liquid phase retained in some loca-tion. Additional heating of wall rock that results in melt-ing within the contact zone is also possible.

MINGLING IN VARIOUS GEODYNAMICSETTINGS

The spatiotemporal association of felsic and maficmagmas is typical of suprasubduction orogenic belts;

specific regions of intracontinental extension, e.g., theBasin and Range Province; intracontinental and oce-anic provinces of intraplate magmatism; and collisionalzones. Let us consider the development of magma min-gling in various geodynamic settings and combinethose of them that exhibit common magmatic and tec-tonic features. In particular, the suprasubduction settingcomprises the island-arc systems and active continentalmargins of the Californian and Andean types, while theintraplate settings are represented by both the rift sys-tems and mantle plumes. We realize that comprehen-sive characterization of mingling in various settings isnot possible at the current state of knowledge primarilybecause of insufficient published data. Therefore, wepresent the most typical and best studied cases, whiledeliberately omitting the examples of mingling in thenear-surface environment (lava flows, dikes as lavaconduits) and dwelling on deeper levels withoutrepeated reference to composite dikes and lava flows.

Suprasubduction Setting

Effects of magma mingling are extremely diverse inmature island arcs and continental magmatic marginsand have been described in most detail at the western

margin of North America, where the igneous events aredated back to the Late Cretaceous and Cenozoic. TheChelan migmatite complex is one of the best studiedcases of magma mingling [29]. The complex consists ofLate Cretaceous migmatized metatonalite. Amphibo-lite, hornblendite, metagabbro, and metadiorite are lessabundant but important constituents of the complex.Basic dikes deformed to various extents are numerous.The conceptual scheme illustrating relationships betweendifferent igneous rocks is shown in Fig. 4.

Fig. 3.

System of synplutonic dolerite dikes, after [8]. Thin dikes are chains of spherical, oval, and oblong fragments commonlywith signs of ductile deformation.


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Five phases of mafic and ultramafic magmatism arerecognized [29]: (1) older metagabbroic rocks tecton-ized and transformed into agmatites, (2) migmatizedmafic rocks that flow around the agmatitic blocks

together with the associated leucosome and make upthick layers (2M) or small conformable dikes dismem-bered into boudines (2S), (3) older synplutonic dikesand larger bodies that cut through the agmatites and mig-matized dikes and experience viscousductile deforma-tion together with country granitoids, (4) younger syn-plutonic dikes that underwent only slight deformation,and (5) postplutonic lamprophyre dikes with character-istic chilled margins.

The preanatectic mafic intrusions were emplaced asdiscrete small bodies into the consolidated tonalite. Inparticular bodies, the crystal fractionation resulted inthe formation of mafic and ultramafic cumulates and

residual diorites as small chambers. No signs of min-gling and hybridization of mafic magma are noted atthis stage. Subsequently, the mafic rocks underwentbrittle failure and were cut by granitic veins. The belt ofthe older mafic rocks extends for 13 km. The late brittlefailure and rotation of particular blocks led to the localdevelopment of typical agmatitic structures.

The synanatectic magmatic intrusions are character-ized by banding and sharply distinct in this respect frommafic rocks of the first phase. The structural patterns

indicate that the mafic melts were emplaced into theplastic, partially melted country felsic rocks and min-gled with leucocratic injections. The latter, togetherwith the batches of crystallizing mafic magma and seg-

regated felsic melts, acquired a gneissic appearance.The older synplutonic mafic dikes are widespread in

metatonalites and migmatites. They are discordant rel-ative to the migmatite banding but, at the same time,have experienced rather intense deformation at the latestages of crystallization and after its completion. Theproducts of remelting of felsic rocks affected by maficmagma have an insignificant volume and occur as thinveinlets of irregular shape hosted in mafic rocks.

The younger synplutonic dikes are similar in theircomposition and structure to the older synplutonicdikes but distinguished from the latter by a lowerdegree of postcrystallization deformation and by cross-

cutting relations with the felsic veins that accompanied thepreceding mafic dikes. Both the older and the youngersynplutonic dikes are rather widespread in the fields ofmetatonalites and migmatites belonging to the ChelanComplex and are extremely rare in the country rocks.

The youngest lamprophyre dikes crosscut the entirecomplex of the above-listed igneous rocks and do notreveal signs of interaction with felsic melts.

The entire Chelan Complex may be regarded as anexample of metamingling with complex spatiotem-

12

3

5

4M

1

3A

4

3

5

2S

2M

3M

Fig. 4.

Relationships between intrusive phases and the structure of the Chelan migmatite complex: a conceptual scheme, after [29].Numerals denote five types of mafic and ultramafic rocks. See text for explanation.


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poral interrelations between magmas of different com-positions. The high degree of syncrystallization, late-crystallization, and postcrystallization deformations ofmafic rocks related to the secondfourth phases hasattracted attention.

Intraplate Setting

The effects of mingling have been described in detailfor the Late Paleozoic and Early Mesozoic (after [3])granitoid plutons of the MongolianTransbaikal alkaligranitesyenite province and for the granitoid plutonsthat preceded formation of this province. The intraplategeodynamic setting of these plutons was established byYarmolyuk et al. [19].

The mingling of felsic and mafic magmas wasdeveloped at various stages of emplacement of theRomanovsky [8, 10], Ust-Khilok [9, 15], Khari-tonovsky [43], and Shalutinsky [11] plutons. Theeffects of mingling are expressed extremely diversely

and embrace the entire range of attributes set forth byLitvinovskii et al. [10]. Melanocratic inclusions inmonzonite, syenite, and granite, as well as swarms ofsynplutonic dikes and various composite dikes, arewidespread. The most typical features of mingling areshown in Fig. 5. In general, it may be noted thatemplacement and evolution of the Late Paleozoic gra-nitic and syenitic plutons were characterized by a mul-tifold supply of mafic magmas into plutonic chambers;the mafic magmas were contaminated to some extentwith felsic materials. The depth of pluton formation isestimated at 67 km [15]. While on the subject of thisdepth, it should be kept in mind that we are discussingonly a level with distinctly expressed mingling. At thesame time, the magmas were generated in the entiresection from the upper mantle (mafic melts) to theupper crust. The diversity of magmas, their generationand evolution in the intermediate chambers at variousdepths were considered comprehensively for the Ust-Khilok pluton, where three consecutive intrusiverhythmstwo syenitic and the youngest graniticarerecognized [9]. The effects of mingling are related to allof these intrusive phases, and the final basaltic injec-tions correspond to the complete consolidation of thepluton. No less than six pulses of mafic magma injec-tions into the large Ust-Khilok pluton are documentedby (1) blocks-xenoliths of gabbro in monzosyenite of

the first phase, (2) mesocratic monzosyenite withnumerous melanocratic inclusions and large microgab-bro inclusions in monzosyenite, (3) synplutonic micro-gabbro dikes in monzosyenite, (4) leucosyenite of thesecond phase with sporadic mafic inclusions, (5) syen-itegabbro composite dikes, (6) monzonitegabbrocomposite dikes, and (7) aplitegabbro composite dikes[11]. The relationships of mafic dikes with felsic rocksare shown in Figs. 5a and 5b. Detailed geological andpetrologic studies showed that the entire variety of

igneous rocks in this pluton (mafic rocks, monzonite,monzosyenite, syenite, quartz syenite, and aplite) is aresult of interaction of mafic and syenitic magmas andfractionation of hybrid melts during their crystalliza-tion in transitional chambers [9]. The syenitic magmawas generated and mixed in the lower crust. The inter-mediate magma chambers that yielded a wide range offelsic melts could have been located at different levelsof the lower and middle crust, and finally, the plutonproper was formed in the upper crust. In other words,the interaction of mafic and felsic magmas with variouseffects of magma mingling and mixing embraced theentire Earths crust.

The specific feature of mingling in the MongolianTransbaikal province consists in the complete absenceof syncrystallization, late-crystallization, and postcrys-tallization deformations. More precisely, such defor-mations are pointed out, but only as they relate to lateralpropagation of felsic magmas [9].

0.2 m0.1 m

(a) (b)

3 m

30 cm

30 cm

10cm

(c)

Fig. 5.

Typical attributes of magma mingling in granitoidplutons of the Transbaikal region, after [9, 10]. (a) Festoonboundaries of mafic rocks and chilled margins at the contactwith aplite in a composite dike, (b) flamelike boundary of alarge synplutonic mafic inclusion in monzosyenite, (c) com-posite dike that serves as a conduit of a large combined sillin syenite and (d) close-up of its offset, (e) contact of a largemafic body with host syenite and (f) close-up of this contact.

(a, b)the Ust-Khilok pluton and (c)(f)the Shalutinskypluton.

(e)

(f)

(d)


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Collisional Setting

The effects of mingling in collisional complexeshave not been described in the same detail as in thesuprasubduction and intraplate settings. The paper byBerthelsen [22] that deals with the so-called globulites,i.e., specific occurrences of mafic material in metamor-phic rocks, hardly exhausts the problem. The compositemingling-dikes of the second type have been describedin the Early Paleozoic collisional complex of Sangilen

[4]; however, these dikes occur within the granitic plu-ton, and only publication [5] that considers relation-ships between igneous, metamorphic, and tectonic pro-cesses made it possible to understand that emplacementof mingling-dikes was related to the late stage of colli-sion. The Olkhon collisional system in the westernBaikal region may be offered as a tectonotype ofmagma mingling in collisional systems at the level ofthe middle and lower crust.

The Early Paleozoic Olkhon collisional system inthe western Baikal region is a complex of various igne-ous and metamorphic rocks [17]. Several tectonicstages of nappe and dome formation and shearing wereaccompanied by high-temperature metamorphism anddiverse, largely granitoid magmatism [16, 17]. Thelithotectonic complexes characterized in detail innumerous publications were formed in the Early Paleo-zoic as a result of microcontinentisland arc and micro-continentcontinent collision [1, 13]. The structures ofmagma mingling that are mainly represented by com-posite dikes have been revealed here recently [6, 14].These dikes have much in common and at the sametime are characterized by substantial differences. Wedivide them into two types.

The first type is represented in the Olkhon regionby sporadic dikes composed of granites and low-K andmedium-Ti tholeiitic dolerites metamorphosed to someextent [14]. By the degree of metamorphism, the com-plete series may be traced from unaltered subvolcanicbodies to amphibolites in metamorphosed near-concor-dant dikes. The fresh and metamorphosed doleritesmay be observed at one exposure near the Oval dome(Fig. 6). The intrusive body exposed here is not a lineardike but a structural feature that was deformed in the

process of shearing. Dolerites are hosted in migmatizedgneisses as chains of boudines and globular bodiesextending as far as 10 m. The boudines are incorporatedinto a shell of pegmatoid granite 0.31.0 m in thick-ness. As has been mentioned above, both mafic rocksand pegmatoid granites are devoid of signs of ductiledeformation at the mesoscopic and microscopic levels.A unit of medium-grained amphibolite, the shape ofwhich shows that the unit was involved in shearing, islocated to the northwest of the above composite dike.Dolerite and amphibolite are identical in chemicalcomposition and fit the low-K and medium-Ti tholeiiticbasalt [14]. It may be suggested that the abundance of

synmetamorphic mafic dikes in the Olkhon regionremains strongly underestimated, because when theyare completely metamorphosed, it is impossible to dis-tinguish them from amphibolites of the country granite-gneiss complex.

The youngest composite dikes cut the metamorphicrocks obliquely and do not bear indications of the sub-sequent deformation. The dike located near the KrestPeninsula in the Olkhonskie Vorota Strait and tracedfor more than a kilometer is the most indicative in this

20 m

Amphibolite

Granitic pegmatiteGneiss and migmatite

DoleriteViscous shear

80

3030

45

30

6040

7070

7050

30

60

50

30

80 70 40

3075353570

65

5065 75

30

40

5065

50 7070

6040

40

Fig. 6.

Structure of the composite dike in the Oval dome, after [14].


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respect (Fig. 7). The dike varies from 23 to 10 m inthickness (the latter value is related to bulges). Themain body of the dike is composed of medium-grainedgranite with dolerite lenses therein. Segments that aredolerite-free reach lengths of 300350 m. Dolerites do

not make direct contact with the country graniticgneisses that alternate with amphibolites and amphib-ole gneisses and are armored everywhere by pegmatoidgranites 0.53.0 m thick. The structure of dolerite is notreworked. The grain size in marginal zones of doleritedikes occasionally decreases, thus testifying to theeffect of chilling.

Thus, the following series may be outlined by meta-morphic grade: crosscutting undeformed dikes, deformednear-concordant dikes without structural changes, and-completely metamorphosed dikes (amphibolites). Thecompositional similarity of mafic rocks in the compos-ite dikes provides evidence for a single episode of

injection of mantle-derived tholeiitic magma at the latestage of metamorphism and structural transformationof the collisional complex.

The second type of composite dikes occurs in thenorth of Olkhon Island [6]. Numerous composite andgranitic dikes cut through the metamorphic sequencewith complex, tectonized intercalation of crystallineschists and marbles; gneisses are not very abundant.The composite dikes reside within a tract about 7 kmlong. Only gently dipping and near-horizontal granitic

dikes are known to the north and to the south of thistract; they crosscut the gneissic banding of countryrocks. The composite dikes, conversely, are conform-able with metamorphic structure. Ladder dikes arenoted occasionally. The granitic material makes up nar-

row near-contact zones in most dikes and thin veinletsof irregular shape in their cores. The pillow structure ofmafic rocks in contact with granites is observed quiteoften (Fig. 8a). The volumetric proportion of mafic andgranitic rocks varies from 30 : 1 (Fig. 8b) to 1 : 1(Fig. 8c). Fragments of mafic rocks incorporated intothe granitic matrix are in most cases elongated andgneissic in appearance (Fig. 8c); however, angular andirregular fragments do occur (Fig. 8d). The geologicrelationships clearly demonstrate in all cases that gra-nitic rocks solidified later than mafic rocks, as expectedfrom the different solidus temperatures of mafic andgranitic melts. The degree of metamorphism superim-posed on granites and mafic rocks in the dikes is vari-able. The massive granites almost completely lackingsigns of the subsequent deformation grade into the typ-ically metamorphic gneisses without relics of primaryigneous structure and texture. The gneissic structure incrosscutting dikes is parallel to their strike. The maficrocks with clinopyroxene, amphibole, and biotite areoften massive; however, the banded varieties, whichbecome indistinguishable from country crystallineschists both by the naked eye and under microscope, arealso not rare.

1

2

3

4

5

6

7

8

L.

Baikal

N

250 m

Fig. 7.

Geologic map of the Krest Peninsula. (

1

) Marble; (

2

) amphibolite; (

3

) gneiss, migmatite, and granite gneiss; (

4

) synmeta-morphic granite; (

5

) synmetamorphic dolerite; (

6

) synmetamorphic shear (blastomylonite); (

7

) structural lines in gneiss andamphibolite; (

8

) orientation of gneissic banding and foliation.


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The dikes where neither mafic rocks nor granitesunderwent ductile deformation (Fig. 8d) pass into thebodies where both igneous rocks are completely meta-morphosed (Fig. 8c). The transitional varieties are rep-

resented commonly by composite dikes with massiveor slightly gneissic mafic rocks and granitic gneisses inmarginal zones. The involvement of composite dikes infolding (Fig. 9) is the most important argument in favor

()

(b) (c)

(d)

25 cm

50 cm 50 cm

50 cm

Fig. 8.

Relationships between mafic rocks and granites in composite dikes on Olkhon Island in the western Baikal region. (a) Pillowstructure of mafic rock at the contact with granite, (b) composite dike with granite at the contact and prevalent mafic rock (net-veinedcomplex), (c) strongly deformed composite dike, (d) undeformed composite dike.


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of their relation to the final stage of collisional tectoge-nesis. The morphology and attitude of the dikes testifyto their synkinematic character.

The stocklike bodies, 300400 m across, that con-sist of a contrasting magmatic mixture of angular orlenticular mafic fragments incorporated into the gra-nitic matrix (Fig. 10) are observed together with com-posite dikes. The granitic material occupies no morethan 30% of the total volume. These bodies areregarded as magmatic chambers formed by simulta-neous emplacement of mafic and felsic melts.

The mafic rocks in composite dikes and stocks of thesecond group correspond in chemical composition tothe low-Mg alkali basalts [6] typical of the intraplatesetting.

Thus, two types of mantle-derived mafic melts,tholeiitic and alkaline, participated in the mingling withcrustal melts. They are not spatially juxtaposed,although they are localized at a short distance from

each other. Their chronological relationships remainambiguous. The published data on the age of metamor-phic and magmatic events [1, 2, 7] and unpublishedauthors data allow us to suggest two episodes of con-temporaneous mafic and felsic magmatism and high-temperature metamorphism: ~500 and 470480 Ma. Ifthis actually is the case, the suggested events are in linewith the Olkhon Terranes tectonic evolution [13, 17]that assumes collision of a microcontinent with an islandarc and the subsequent collision of the amalgamated ter-

rane with the Siberian Craton. Moreover, this scheme isapplicable not only to the Olkhon Terrane but also to allCaledonides in the Central Asian Foldbelt [20]. Accord-ing to the concept developed by V.V. Yarmolyuk and hiscoauthors, the Caledonides in this sector of the Earthslithosphere were formed in the Vendian and Cambrianunder effect of the North Asian hot field (superplume),thereby providing extensive alkaline and subalkalinemagmatism. The latter is represented by rather largeplutons of alkali gabbroids, e.g., the Birkhin and Kre-

() (b)3 m

Fig. 9.

(a) Photograph and (b) position in the fold structure of the thickest composite dike on Olkhon Island.

1 m

Mafic rock

Granite

Ice and snow

Fig. 10.

Relationship of mafic rock and granite in an intrusive body in the north Olkhon Island.


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stovsky plutons, and by alkali basalts in compositedikes of the second type. The second episode of tholei-itic mantle magmatism, related to the partial melting ofthe depleted lithospheric mantle, occurred during colli-sion of the terrane with the Siberian Craton.

We would like to emphasize once again two impor-tant points: (1) the relation of magma mingling toshearing and (2) the synmetamorphic character of both

processes. Because metamorphism is an indispensablecomponent of collisional geodynamics, it becomesclear that the magma mingling in the Olkhon region isalso a syncollisional event. At first glance, this state-ment is internally controversial because compressionand the respective synmetamorphic deformation in thecollisional setting hamper the mantle-derived magmaspenetration through the crust. However, it is well-known that uneven surfaces of strike-slip faults giverise to the development of numerous local extensionzones (pull-apart structures) and series of systemati-cally arranged tension cracks (Riedel structures) disin-tegrate the displaced sheets. Apparently, these condi-tions are sufficient for draining of mantle magmas andtheir ascent through the thick but gravitationally insta-ble collisional crust. It may be suggested that the colli-sional system is doomed to breakdown or collapse assoon as shearing becomes an active participant of thecollisional scenario. The synmetamorphic magma min-gling, one of the earliest indicators of this process,implies the continuation of the general compressionand the development of tectonic flow and folding,although the local extension zones are already arising atthat time. The mantle-derived magmas are injected intothe softened crustal rocks involved in metamorphismand subject to partial melting. The interaction of themantle and crustal melts results in the formation of

structural features typical of magma mingling. The col-lapse of collisional systems proceeds in the extensionregime, and the magma mingling directly testifies to theearly stage of collapse.

DISCUSSION AND CONCLUSIONS

In order to understand the character of relationshipsbetween granites and mafic rocks and the cause ofincomplete miscibility, or alternatively, complete immis-cibility, of melts contrasting in composition, three pointsshould be stressed.

(1) The marked difference in temperature of graniticand mafic melts: 650800

C against 11001300

C.First, this implies that the felsic melt behaves as a cool-ant for hot or even overheated mafic melts. Second, ifthe temperature of the country granitic or felsic meta-morphic rocks is close to eutectic, the energetic capac-ity of even small bodies of mafic melt is sufficient toinduce the partial melting of the country rocks. Theprogress in such melting is controlled by the volume ofthe supplied mantle-derived magma. Finally, the thirdimplication of the temperature difference consists in thelate crystallization of felsic melts, as is confirmed by

structural relationships between coexisting mafic andfelsic melts.

(2) The sharp difference in viscosity and rheologicalproperties of fluid-saturated granitic melt and anhy-drous basaltic melt. The incomplete miscibility ofmelts, various reaction structures, and pillow structureof mafic rocks in composite dikes are explained pre-cisely by this difference. The modeling of interaction

between liquids with different viscosity [33] has shownthat the interface between such liquids becomes moreintricate as the difference in viscosity increases(Fig. 11E). The morphological features observed inexperiments are consistent with their counterparts innature (Figs. 11A11D).

(3) The sharply distinct density and, accordingly,buoyancy of mafic and granitic melts. The partiallymelted continental crust is an effective barrier to theascent of the basaltic magma [30], which havingreached this barrier starts to spread aside and crystallizeas sheetlike bodies. The same mechanism may workwithin magmatic chambers filled with granitoid mate-

rial. The morphology and spatial position of sheetlikemafic bodies in granitoid plutons [40] indicate the mul-tifold emplacement of mafic melts into the magmachamber (Fig. 12). The stratification of the magmachamber caused by settling of crystals in the lower partof the chamber plays an important role in this process.

Zones of plastic high-grade metamorphic rocks areefficient barriers to mafic magmas. The mafic magmasthat have reached the base of such zones induce themelting of gneisses and provide magma mingling andascent of the magma mixture to the upper brittleduc-tile level where the linear fractures are filled with thismixture. An additional conductive heating of rocks may

change the style of deformation that provides emplace-ment of dikes along shear zones [14] or their subse-quent involvement in the folding. The viscousductilestyle of deformation at the lower level results in devel-opment of metamorphic mingling, as is observed in theOlkhon collisional orogen [18].

In general, the character of mingling at deep levelsof suprasubduction and intraplate settings is highlysimilar. The conspicuous attributes of mingling developin the uppermost 57 km of the crust. The magmachambers filled with granitoid crustal melts serve astraps for mantle-derived magmas. The interaction ofcrustal and mantle magmas in the collisional setting atthe late stage of collision occurs at a depth of 720 kmand is distinguished by some specific features (seeabove). The main difference in the mingling that occursin specific geodynamic settings consists in differentdegree of syncrystallization, late-crystallization andpostcrystallization deformations. Under the extensionconditions of the intraplate setting, the deformationexpressed in the gneissic texture of felsic rocks and inthe flattening of mafic fragments is related to thespreading within the magma chamber or to the flow ofmagma along feeding conduits (dikes). The structures


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related to the deformation during the late stage of crys-tallization (gneissic banding, boudinage, structures ofrupture and fragmentation of competent mafic bodies)

are typical of the suprasubduction setting. The entirerange of deformations, from no deformation to com-plete metamorphic reworking of composite dikes, isobserved in the collisional setting. Moreover, the mor-phology of mafic bodies implies that some compositedikes are controlled by shear zones (Fig. 6). The geo-logic examples considered above confirm the tendencyof increasing intensity of ductile deformation of theigneous rocks that participate in mingling; however,this tendency is far from unequivocal. Besides our point

of view stated above, another explanation unrelated tothe specific features of different tectonic settings is pos-sible. All examples of mingling in different geody-

namic settings correspond to different levels of theEarths crust. The granitoid plutons in the Transbaikalregion crystallized at a depth of 57 km [15]; the mainstages of the evolution of the Chelan Complex corre-spond to the middle crust [29]; and finally, the level ofmingling in the Olkhon collisional system fits the mid-dle and lower crust. Thus, the different style of defor-mation may be controlled by variation in the depth.However, we emphasize once again the role of shearingin the penetration of mantle magmas into the lower and

20 m

V

R

= 2.69

V

R

= 12.9

V

R

= 130.6

V

R

= 1403

B

C D

E

Fig. 11.

(AD) Relationships of granite and mafic rock in the Terra Nova intrusive complex, Antarctica and (E) models simulatingrelationships between liquids with different viscosities, after [33].


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() Injection of mafic

melt as a lopolith

Feeding dike (1)Mingling in the marginal partof the lopolith

(b) Cooling and sinking of mafic melt

Formation of mafic inclusionsby convection

(c)

Ongoing sinking and tectonizationof the mafic body

(d)

Final stage of the first pulse

Deformation of mafic inclusionsby lateral spreading of

granitic melt

(e)

The second pulse of emplacement of mafic melt

1 2 3

4 5

Feeding dike (2)

Mafic inclusionsand rafts

Flamelikeoffsets

Dike remnants

Intricate configuration

of the lower contact of mafic body

Flat upper contactof the mafic body

Fig. 12.

A model of consecutive emplacement of mafic melt into granites with formation of mafic intrusive sheets, after [40].(

1

) Gabbrodiorite, (

2

) granitic magma, (3) cumulative layer enriched in crystals, (4) direction of flow in local convective cells,(5) mafic inclusions.


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middle crustal units, which is not realized in other geo-dynamic settings.

The petrology of mantle and crustal magmas partic-ipating in mingling were deliberately omitted becausediscussion of these topics would greatly lengthen thepaper. Only the most general features are noted. Thealkaline and ultra-alkaline mantle melts are typical ofthe intraplate setting. The calc-alkaline mafic rocks are

predominant in the suprasubduction setting, althoughthe mafic rocks of the OIB-type also are not uncom-mon. As has been exemplified by the Olkhon system,both alkali basalts and primitive tholeiites are formed inthe collisional setting. The composition of mantle andcrustal melts is controlled by numerous factors, includ-ing the composition of the upper mantle and the geom-etry of a subducted plate [23].

ACKNOWLEDGMENTS

We thank D.P. Gladkochub, T.V. Donskaya, A.B. Ko-tov, and A.M. Mazukabzov for their long-standing col-laboration in field studies of the Olkhon region and fordiscussion of the subjects touched on in this paper. Weare grateful to B.A. Litvinovskii for reading the manu-script, participating in discussion, and making helpfulcomments.

This study was supported by the Russian Founda-tion for Basic Research (project nos. 05-05-64761 and05-05-64016) and fulfilled under the Integration Pro-gram Geodynamic Evolution of the Lithosphere in theCentral Asian Mobile Belt: from Ocean to Continentof the Russian Academy of Sciences and the SiberianDivision of the Russian Academy of Sciences.

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Reviewers: V.V. Yarmolyuk and V.I. Kovalenko

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Magma Minling Tectonic and Geodynamic Implications