Geochemical Modelling of Magma Evolution Process in Bodie … process of magma evolution: from the generation of magma to the eruption on the Earth’s Please cite this article as:

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Vol. 11 (II), April, 2018, pp. 99 - 121

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Geochemical Modelling of Magma Evolution Process

in Bodie Hills Volcanic Field, Nevada, USA

Liubomyr Gavryliv 1, Gogolev Konstantin 2 and Aleksieienko Anton 1

1Institute of Geology, Taras Shevchenko National University of Kyiv, 03022 Vasylkivska st 90, Kyiv, Ukraine

2 Department of Precambrian Geology and Chronostratigraphy, Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of NAS

of Ukraine, 34 Palladina prospect, Kyiv, 03680, Ukraine Email: [email protected]

ABSTRACT

The middle to late Miocene Bodie Hills Volcanic Field (BHVF) represents one of the largest volcanic complexes of the ancestral Cascades magmatic arc. Though, the former volcanoes are known to contain geochemical imprints of tectonic regime, deep structure evolution and lithosphere characteristics, there are still virtually no links or correlations made between various volcanic suites of BHVF, which accounts for the necessity of their thorough analysis. The paper aims to improve petrologic and geochemical knowledge of magma evolution of four volcanic centers: Mount Biedeman, Aurora, Masonic and West Brawley Peak. There are used published whole-rock XRF (X-ray fluorescence), ICP-AES (Inductively Coupled Plasma Atomic Emission Spectroscopy) and ICP-MS (Inductively Coupled Plasma Mass Spectrometry) chemical analyses performed in analytical laboratories of SGS Minerals, Toronto, Canada. Many of the rocks show the effects of magma mixing, interaction with crustal rocks and open system crystallization. The results indicate that BHVF rocks contain a significant portion, at least 10% by mass, of granodiorite and volcanic sediment of Sierra Nevada. BHVF magmas rising from mid to shallow crustal levels and initiating assimilation processes in shallower reservoirs apparently had >1500 ppm Sr and 87Sr/86Sr <0.7040. The chemical composition of trace elements in BHVF indicate fractional crystallization with the accumulation of plagioclase, olivine, augite and magnetite. Conclusively, geochemical studies of BHVF rocks point out that magmas were variably contaminated, and that much of the rocks were recycled in a MASH (Melting, Assimilation, Storage, Homogenization) zone.

Keywords: Magma Evolution; Trace Elements; Fractional Crystallization with

Assimilation; MASH zone; Geochemical Modelling; Bodie Hills Volcanic Field.

INTRODUCTION

The key issue of volcanoes investigations at convergent plate boundaries,

associated with island arcs and active continental margins, is reconstruction of the whole process of magma evolution: from the generation of magma to the eruption on the Earth’s

Please cite this article as: Gavryliv, Liubomyr, Konstantin, Gogolev and Anton, Aleksieienko (2018) Geochemical modelling of magma evolution process in Bodie Hills Volcanic Field, Nevada, USA. e-Journal Earth Science India, v. 11, pp. 99-121.

Geochemical Modelling of Magma Evolution Process in Bodie Hills Volcanic Field, Nevada, USA: Gavryliv et al.

surface. Recently, significant efforts are made to solve this problem by application of petrologic and geochemical methods. However, many questions remain undiscussed. The depth of melt generation as well as differentiation conditions make up key issues common to real magmatic complexes (Portnyagin et al., 2007). These issues are topical for Bodie Hills Volcanic Field, California - Nevada, USA.

A number of papers have been published which focus on volcanic (Chesterman, 1968; Chesterman and Boylan, 1975), mineralogical (Herrera et al., 1991, 1993; Osborne 1991; Silberman and Chesterman, 1991; Vikre and Henry, 2011; Vikre et al., 2015) and geological (Lange et al., 1993; John et al., 2012, 2015) studies of the Bodie Hills. General geochemical and petrologic characteristics of BHVF are discussed in du Bray et al. (2016). The authors concluded a limited plagioclase fractionation, clinopyroxene, hornblende, biotite, and lesser Fe-Ti oxide and olivine crystallization and fractionation. The trace element geochemical signature is represented by high large ion lithophile elements (LILE; e.g. Ba, Pb, K, Sr) contents relative to high field strength elements (HFSE; e.g. Ta, Nb, Zr, Hf, Ti) (Kingdon, 2016). Most BHVF rocks remain hydrothermally unaltered. Therefore, K2O increase with increasing SiO2 content is more likely to record primary magmatic processes. The potassic character of eruptive products indicate lithospheric delamination and subsequent partial melting of a K2O-enriched lower-crust or upper-mantle source (du Bray et al., 2016). Alternatively, elevated K2O abundances may result from low-degree partial mantle melting and subsequent fractional crystallization (Farmer et al., 2002; Putirka and Busby, 2007). The elevated Sr abundances and high Sr/Y ratios point out that magmas represented by the BHVF rocks reflect partial melting in the mantle beneath thick continental crust at pressures greater than about 20 kbar (garnet stable – plagioclase unstable partial melting) (du Bray et al., 2016). Consequently, BHVF magmas are claimed to be derived from an enriched mantle source with 87Sr/86Sr ~0.7055 and εNd ~3 with trace element abundance variations to be principally controlled by MASH (melting, assimilation, storage, and homogenization) zone processes (Hildreth and Moorbath, 1988). The observed positive correlation between 87Sr/86Sr and 1/Sr, Rb/Sr, and Pb/Ce values for intermediate BHVF rocks indicates the interaction between mantle-derived magmas and Rb- and Pb-rich crustal rocks. Nevertheless, BHVF REE (Rare-Earth Elements) abundance variations are nonsystematic with respect to extent of magma evolution (SiO2 content) or age, which reflects nonsystematic petrogenetic evolution of the entire BHVF in time and space. Although Sr and Nd isotope ratios are negatively correlated neither Sr nor Nd isotope ratios correlate with Pb isotope ratios. Moreover, neither age nor whole rock compositions of BHVF rocks are well correlated with isotopic variations. Diverse textural and chemical features of BHVF phenocrysts indicate open-system behavior, including complex crystallization histories, magma reservoir recharge events, mixing/mingling processes and degassing. However, the composition of the contaminants and the degree of mixing remains virtually unstudied. Moreover, reconstruction of magma evolution of BHVF individual eruptive centers is almost a research gap for this volcanic complex. The paper takes attempts to use geochemical and petrologic tools and methods that were not mentioned in the previous researches but remain crucial in geochemical analysis. Major and trace element abundance in the rocks, that could provide information about different stages of magma evolution and variations in melt composition, forms the subject of the research. The research proposed focuses on geochemical modelling of Bodie Hills magmatic system to identify the principal mechanisms of magma evolution for Mt Biedeman, Aurora, Masonic and West Brawley Peak volcanoes.

GEOLOGICAL SETTING

Being well-described in database (see “Analytical Data”), four volcanoes of the Bodie Hills Volcanic Field, which covers the area of more than 700 km2 on the border



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between California and Nevada, to the north from Mono Lake and nearly 20 km to the east from Central Sierra Nevada, are selected as objects of research (du Bray et al., 2013). They are: Mt Biedeman, Aurora, Masonic and West Brawley Peak.

The middle to late Miocene BHVF is a large, long-lived (≈9 million years), but

episodic, eruptive center in the southern segment of the ancestral Cascades arc north of Mono Lake, California. The field is near the west side of the Walker Lane and the northwest end of the Mina deflection where structures related to these tectonic features might have localized BHVF magmatism. The Walker Lane belt of eastern California and western Nevada is the northernmost extension of the Gulf of California transtensional rift. Tectonic activity in the Walker Lane shear zone was initiated ~13-12 Ma (Faulds and Henry, 2008; Busby, 2013). The BHVF includes at least 32 volcanic rock units.

Rock composition vary from basalts to rhyolites; dacites and andesites are more

common (Fig. 1). Formation of the BHVF spanned the transition between subduction of the Pacific plate beneath the west coast of North America and the establishment of a transform plate margin at about 10 million years ago (Fig. 3). Rock compositions form a high-potassium (K) calc-alkaline series (Fig. 2) with pronounced negative Ti-P-Nb-Ta anomalies and high Ba/Nb, Ba/Ta and Lа/Nb (elements) typical of subduction-related continental margin arcs (Irvine and Baragar, 1971; John et al., 2012). Most BHVF rocks are metaluminous to peraluminous; calc-alkalic to alkali-calcic; magnesian (calc-alkaline) to weakly ferroan (tholeiitic) (du Bray et al., 2016).

Fig. 1: TAS classification, after (Le Maitre, 2002), of rocks from BHVF (geochemical data published in du Bray et al., 2013 is used).

The area is mostly underlain by the partly overlapping, middle to late Miocene

BHVF and Pliocene to late Pleistocene Aurora volcanic field. Upper Miocene to Pliocene sedimentary deposits, mostly basin-filling sediments, gravel deposits, and fanglomerates, lap onto the west, north, and east sides of the Bodie Hills, where they cover older Miocene volcanic rocks. Quaternary surficial deposits, including extensive colluvial, fluvial, glacial, and lacustrine deposits, locally cover all older rocks. Miocene and younger rocks are tilted


≤30° in variable directions. These rocks are cut by several sets of high-angle faults that exhibit a temporal change from conjugate northeast-striking left-lateral and north-striking right-lateral oblique-slip faults in rocks older than about 9 Ma to north- and northwest-striking dip-slip faults in late Miocene rocks (John et al., 2012).

Numerous hydrothermal systems were operative in the Bodie Hills during the

Miocene. Structurally focused hydrothermal systems formed large epithermal gold-silver vein deposits in the Bodie and Aurora mining districts (Vikre et al., 2015).

Fig. 2: SiO2 vs. K2O diagram, after (Peccerillo and Taylor, 1976) of rocks from BHVF (geochemical data published in du Bray et al., 2013 is used).

Fig. 3: Schematic cross-section of California - Andean type continental margin, with

magmatic generation model (during Neogene time) after (Ingersoll, 1979; Richards, 2003).



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Petrographic characteristics

The petrography of BHVF rocks is well-described by du Bray et al. (2016). Most rocks remain unaffected by hydrothermal alteration, however some of samples are weakly weathered. Basically all BHVF rocks are porphyritic. Other common textures are hyalophitic, aphanitic, seriate, intersertal, intergranular, pilotaxitic, flow laminated and hypidiomorphic. The phenocrysts of trachyandesite of West Brawley Peak are represented by coarse (>5 mm) plagioclase and the trachyandesite of Aurora contains coarse (1-2 cm) hornblende phenocrysts. The relative abundance of plagioclase phenocrysts for Mount Biedeman, West Brawley Peak, Aurora and Masonic is 21, 12, 15, 19 percent respectively. Almost all plagioclase phenocrysts are oscillatory zoned, contain glass inclusions and distinctive reaction rims. Black opacite reaction rims of brown to green pleochroic hornblende is stated to reflect shallow-level magmatic degassing (Rutherford and Hill 1993). Clinopyroxene is a common mineral in many BHVF rocks and it forms pale tan to pale green subhedral to euhedral crystals. Orthopyroxene is less common and remains colorless to rosy tan and forms euhedral to subhedral crystals. Biotite is subhedral, tan to deep red brown and is entirely altered to dark brown to black in samples affected by shallow degassing. Fe-Ti oxides are represented by magnetite and ilmenite being variably exsolved or altered.

The groundmass of BHVF rocks contains microphenocrysts (0.05 to 0.2 mm) assemblages, dominated by plagioclase, a small amount of Fe-Ti oxide minerals, clinopyroxene. Accessory minerals include ubiquitous apatite and zircon. Less common is titanite and allanite.

METHODOLOGY

Analytical data

Whole-rock chemical analyses (504 samples) performed in analytical laboratories of SGS Minerals, Toronto, Canada published in du Bray et al. (2013) are used in the research. Major oxide abundances (recalculated to 100 percent, volatile-free) have been analysed by wavelength dispersive X-ray fluorescence spectrometry. A 55-element method that uses a combination of inductively coupled plasma–atomic emission spectrometry and inductively coupled plasma–mass spectrometry has been used to determine trace element abundances. This chemical data is also archived in the National Geochemical Database.

Although every effort by authors has been made to collect only unaltered samples, a review of the geochemical data indicates that a small subset of the analyzed samples is affected by post-magmatic hydrothermal alteration. Samples with any of the following characteristics are considered to be altered: SiO2 abundance greater than 78 percent, volatile (loss on ignition) content greater than 4 percent (excluding hydrated vitrophyres), Na2O abundance less than 1 percent, K2O abundance greater than 7.5 percent, or Na2O/K2O less than 0.5 (du Bray et al., 2009). Methods

Crystallization of magma proceeds by two main processes: equilibrium and fractional (Rayleigh) crystallization (FC). The solid phases remain in the melt and stay in chemical equilibrium with the liquid phase during equilibrium crystallization. In FC, early formed phases are continuously removed from the liquid during cooling and crystallization


of the magma (Ersoy and Helvaci, 2010). The latter process is described by the next equation (Rayleigh 1896)

𝐶𝑙

𝐹𝐶 = 𝐶0𝑓(𝐷−1) (1)

where 𝐶𝑙𝐹𝐶 – concentration of an element in remaining melt during FC. 𝐶0 – concentration

of an element in parental liquid (starting composition). f – fraction wt of melt remaining during crystallization (in the range of 0 to 1). D – the bulk partition coefficient of the elements for fractionating phases. In the case of the presence of more than one mineral phase, D is calculated by

𝐷 = ∑ 𝑥𝑖𝐾𝑑𝑖 = 𝑥𝑎𝐾𝑑min 𝑎 + 𝑥𝑏𝐾𝑑

min 𝑏 + ⋯ + 𝑥𝑛𝐾𝑑min 𝑛

𝑛

𝑖 (2)

where x is the weight fraction of the mineral phase i, and 𝐾𝑑 is the partition coefficient of any element in mineral phase i.

Magma differentiation is often accompanied by a range of petrological processes such as contamination of the magma via assimilation of the wall rocks (assimilation), and mixing of magmas of different compositions. FC and assimilation (AFC) are generally combined and they may occur simultaneously (DePaolo, 1981; Taylor, 1980; Grove et al., 1988, 1982; Wyers and Barton, 1986).

When the compositions of the endmembers are known or can be estimated, mixing/differentiation model may allow trace element and isotopic data to be used to place constraints on the rates and extent of assimilation and the liquidus phases involved (DePaolo 1981a). The AFC process is expressed by

𝐶𝑙𝐴𝐹𝐶 = 𝐶0 [𝑓−𝑧 + (

𝑟

𝑟 − 1)

𝐶𝑎

𝑧𝐶0

(1 − 𝑓−𝑧)] (3)

where 𝐶𝑙𝐴𝐹𝐶, 𝐶0 and 𝐶𝑎 are the concentration of an element in the resulting magma, parental

magma, and assimilating material (wall rock), respectively. r describes the relative ratio of assimilated material to crystallized material and is calculated by

𝑟 =𝑚𝑎

𝑚𝑐 (4)

where 𝑚𝑎 is the amount of assimilated material and 𝑚𝑐 is the amount of crystallized material. z value in AFC equation is expressed by

𝑧 =𝑟 + 𝐷 − 1

𝑟 − 1

(5)

If 𝑚𝑎 = 𝑚𝑐, this case is described as zone refining. If 𝑚𝑐 = 0, then this process represents

simple mixing between two phases (i.e., no crystallization). If 𝑚𝑎 = 0, the equation represents pure FC.

In some magmatic systems assimilation and crystallization are not strictly related and the mass assimilated is decoupled from, and therefore varies independently of, the mass crystallized (Cribb and Barton, 1996). Defant and Nielsen (1990); Grove et al. (1988); and O’Hara and Mathews (1981) pointed out that the amount of assimilation and crystallization may be decoupled in chambers in which magma replenishment occurs. Assuming that there is more than sufficient heat for resorption of previously formed



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phenocryst phases, the hybrid magmas can assimilate crustal material without undergoing crystallization (Cribb and Barton 1996). The concentration of any element or oxide during FCA (decoupled fractional crystallization and assimilation) is given by

𝐶𝑙𝐹𝐶𝐴 =

𝐶0𝑅𝑀𝑐 + 𝐶𝑓(1 − 𝑀𝑐)

𝑓

(6)

where R is the ratio of assimilated material (𝑀𝑎) to crystallized material (𝑀𝑐). 𝐶0 and 𝐶𝑓 are

the concentrations of the element on the original magma and resulting from FC, respectively. Concentration of an element in a magma resulted from mixing of two different magmas (Powell, 1984) is expressed by

𝐶𝑚 = 𝑋(𝐶𝑎 − 𝐶𝑏) + 𝐶𝑏 (7) where 𝐶𝑎, 𝐶𝑏 and 𝐶𝑚 are the concentration of an element in magma a, magma b, and in mixed magma resulting from mixing of magmas a and b, respectively. X is the degree of mixing.

A suite of magmas affected by an open-system processes (mixing, AFC, FCA, metasomatism etc.), will generally display regular geochemical patterns. Trace element modelling of crystallization involves a balance between solid phases and melt assessed by the partitioning concept. The key parameter for the trace elements is the ratio of their concentrations in the solid phases and the liquid (Shaw, 1970, 2006). Methodology of magmatic and magmatic hydrothermal systems modelling is based on principles pointed up in works of Albarède (2009); Greenland (1970); O’hara (1977); Spera and Bohrson (2004); Zou (2007).

The model in the paper is simulated from the viewpoint of take into account the

conclusions of du Bray et al. (2016) that BHVF associated reservoirs experienced significant open-system behavior, including complex crystallization histories, reservoir recharge events, mixing/mingling processes, and degassing. However, preliminary results of the data examination indicate that the influence of FC on BHVF rocks genesis is considerable (Gavryliv et al., 2016).

GENERAL GEOCHEMICAL CHARACTERISTICS OF BHVF

The majority of geochemical features resemble those observed in Neogene–

Quaternary volcanic rocks in the Carpathian–Pannonian region, where calc-alkaline magmatism was closely related to subduction, rollback, collision and extension (Seghedi et al., 2004).

Magnesium-numbers (Mg#; 100MgO/(MgO+FeO*), where FeO* as total FeO) in Mt Biedeman, Aurora, West Brawley Peak and Masonic indicate a large fractionation of mafic minerals: 42-25, 44-16, 40-13, 54-9 respectively. As it is already mentioned by du Bray et al. (2016) systematic decreases in the abundances of TiO2, FeO*, MgO, and CaO with increasing SiO2 content are consistent with clinopyroxene, hornblende, biotite, and lesser Fe-Ti oxide and olivine crystallization and fractionation.


There are differences between four volcanoes: average TiO2, P2O5, Ce are the highest in Biedeman, while West Brawley Peak shows the lowest values. Aurora shows the lowest average Th/Y, Nb/Y, Nb/Zr and higher K/Ba ratios. On the other hand, Biedeman has the highest Nb/Y, Nb/Zr and K/Rb values.

The Th/Y ratio can be used to observe the transfer of slab-derived components to the mantle wedge, because Th is non-conservative during subduction, and Nb/Y can be used to monitor differences between mantle sources due to Nb conservative behavior during subduction (Pearce, 1996). Nb/Y is also less sensitive to fractional crystallization or alteration, as it incorporates the two immobile elements. Here variable Nb/Y enrichment may be associated with mantle wedge enrichment by an OIB component, possibly for trachyandesite of Mt Biedeman (Fig. 4). The data of Masonic clearly demonstrates the reduction of subduction-components (increase of Nb/Y) in the lithospheric-mantle with time in favor of less affected asthenospheric mantle. Mixing of magmas derived from lithosphere and asthenosphere could explain sudden increase of Nb/Y (Seghedi and Downes, 2011). Another option is mafic recharge, which is also recognized by du Bray et al. (2016) through the presence of reversely zoned clinopyroxene grains from the oldest part of the trachyandesite of Masonic.

It should be noted that similar behavior of these ratios is observed in volcanic rocks of Western Carpathians, Pannonian basin and Transcarpathian basin. In the former the high Nb/Y “transitional” andesites generated at 11-8 Ma show a different Nb/Y trend and fractionation. Seghedi and Downes (2011) suggested that magmatism at Transcarpathian basin was generated at the mantle-lithosphere/crust level as a result of major extension via counter-clock rotation of the easternmost part of ALCAPA that caused core-complex exhumation of lower-middle crustal units.

High Rb/Zr values indicate crustal contribution through assimilation or partial melting, which normally would follow the trend of AFC for the majority of rocks (Fig. 5) (Seghedi et al., 2004). Variable Rb/Zr ratios at constant K2O/Na2O ratios, could be associated with mixing event. A derivation from a crustal source is known to be marked by strong enrichment of K2O over Na2O, as well as LILE over HFSE, such as Rb over Zr. It is pointed out that during the final stages of differentiation magmas significantly interacted with crustal rocks.

Ti is an incompatible element during basalt fractionation, but the addition of the

oxide as a crystallizing phase then causes Ti to become compatible. The extent of fractionation should therefore be indicated by the extent of the ratio:

X/Ti

where X is an incompatible element during fractionation

By this way, the most commonly used ratio is Zr/Ti (Pearce 1996). This ratio is less affected by alkalinity and calc-alkalinity than Nb, Th or Ce and less affected by garnet than Y (Winchester and Floyd, 1977). The predominant crystallizing phases in basic magma (plagioclase, olivine, pyroxene), do not significantly affect the Zr/Ti ratio of the melt. However, the incoming magnetite or hornblende can cause a depletion in Ti while Zr, as well as SiO2 continue to be enriched (Fig. 6). The dominant fractionation vector observed is POAM (plagioclase + olivine + augite + magnetite). A sudden increase of Nb/Y for Mt Biedeman is accompanied by virtually constant Zr/Ti. This feature can be attributed to a mixing event, as it is shown on Fig. 4.



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Fig. 4: lg Nb/Y vs.lg Th/Y diagram for BHVF volcanic rocks. OIB after (Sun and McDonough, 1989).

Fig. 5: Rb/Zr vs. K2O/Na2O diagram for BHVF volcanic rocks. Symbols and data as in Fig. 4.


On a Zr-Ni plot (Fig. 7) a significance of open system fractional crystallization is observed, since it produces steeper trajectories shown in Fig. 6 and rapidly increases Zr in residual liquids. These rocks also show rapid increases in incompatible elements at relatively constant FeO*/MgO (Fig. 8, 9), a feature also characteristic of open system fractional crystallization (Condie, 2014).

Fig. 6: lg Nb/Y vs. lg Zr/Ti diagram for BHVF volcanic rocks. Symbols and data as in Fig. 4. For the fractionation vectors, POA = plagioclase + olivine + augite; POAM = plagioclase + olivine + augite + magnetite; POAHM = plagioclase + olivine + augite + hornblende + magnetite.

To sum up, geochemical features of BHVF rocks show notable crustal and mixing influence. Major oxide variations for the BHVF as a whole are consistent with crystallization and fractionation involving the observed phenocrysts. BHVF isotopic compositions (initial 87Sr/86Sr range from 0.70399 to 0.70602 and εNd from +2.26 to -4.17) are consistent with the derivation from enriched sources (i.e., crustal components) and they correlate positively with 1/Sr, Rb/Sr and Pb/Ce values. However, a few samples have isotopic compositions that resemble a depleted source (εNd > 2). du Bray et al. (2016) concluded that the observed positive correlation between Pb/Ce values and SiO2 content probably reflects the more evolved magmas having assimilated larger amounts of crustal components. Bodie Hills rhyolites high Rb/Zr ratios (~1-3.5) support the hypothesis that the evolved magmas assimilated larger amounts of the crust.



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Fig. 7: Ni vs. Zr diagram for BHVF volcanic rocks. Symbols and data as in Fig. 4.

Fig. 8: lg Th vs. FeO*/MgO diagram for BHVF volcanic rocks. Symbols and data as in Fig. 4.


Fig. 9: Rb vs. FeO*/MgO diagram for BHVF volcanic rocks. Symbols and data as in Fig. 4.

A mixing with newly intruded magma, immediately prior to the eruption is also

proposed by du Bray et al. (2016): high Mg rims of apatite are consistent with apatite growth in a higher temperature regime than that indicated by lower Mg apatite core compositions, which are consistent with lower temperatures. Another evidence is the presence of compositionally distinct (higher Al and Fe) hornblende overgrowths on pyroxenes in the magma blob of Mt Biedeman which likely indicate interaction with felsic magma (Rutherford and Hill 1993). The presence of multiple size populations of hornblende and plagioclase, oscillatory zoned, partly resorbed and sieve textured plagioclase favor the assumption that the magma reservoir evolved by open-system behavior with a periodic recharge. Nevertheless, fractional crystallization with some degree of assimilation is likely to be the leading mechanism of magma differentiation.

GEOCHEMICAL MODELLING OF MAGMA EVOLUTION

Interaction between magma and crustal rocks may occur during magma ascent and storage in magma chambers, where FC, AFC and mixing can take place before magmas reach the surface. When mantle-derived basaltic magmas induce local melting of crustal rocks, the magma becomes subject to selective contamination processes. These effects are governed by the relative diffusivities of the various melt components and by the principle of transient two-liquid equilibrium. Selective contamination by rare earth elements and other trace cations of high charge is likely (Hildreth, 1981; Watson, 1982; Hildreth and Moorbath, 1988).

Composition of the contaminants

AFC, FC and FCA curves can be modelled using the most primitive compositions of each of the trachyandesite units and various crustal contaminants characteristic of the region (Taylor and McLennan, 1995). Because of the large isotopic diversity anticipated for



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the continental crust beneath BHVF, all the mixing calculations are carried out employing 16 different isotopic compositions of Nd and Sr and and the concentrations of K, Rb, Sr, Sm, and Nd referring to metasedimentary, metamorphic and granitic rocks representative of Sierra Nevada (Table-1; DePaolo, 1981b; Hurley et al., 1965; Wollenberg and Smith, 1968). Crystallizing minerals proportions are taken from the published data on petrography of BHVF rocks (du Bray et al., 2013). Mixing models

Sm and Nd are weakly susceptible to fractionation due to petrological processes controlled by crystal-melt equilibrium. Their limited mobility during rock alteration allows them to retain their primary concentrations during weathering, hydrothermal alteration, and low-grade metamorphism (Rollinson, 1993; Faure and Mensing, 2005). Therefore, the Sm-Nd isotope pair will maintain their source isotopic signature, but they will not be sensitive to small proportions of crustal mixing. In contrast, Rb and Sr belong to different chemical groups (alkali metal and alkaline earth groups, respectively), causing differences in their crystal-melt affinities, and a high degree of mobility in crustal processes. Sr has an affinity to plagioclase, while Rb prefers to remain in the melt phase, and Sr will remain immobile during hydrothermal alteration while Rb is fluid mobile. This isotope pair is very sensitive to crustal mixing and hydrothermal alteration (Faure and Mensing, 2005).

For the mixing models (Fig. 10), we retained the compositions 00BA8 (West Brawley

Peak), 203133 (Mt Biedeman), 10810A (Aurora), and MAS1073 (Masonic) as that of the initial magma because they are the least radiogenic in regard to Sr and the most radiogenic for Nd (Table-1). Thus, these compositions appear parental and little modified by crustal contamination.

Using the contaminant SNB-73-5 the mixing model of Mt Biedeman fits very well

the trachyandesites 09BA19, 13AU15 indicating between 25 and 30% of assimilated crustal materials. However, a little degree of mixing (<10%) between 09BA19 and Bodie Hills Rhyolite 00BA31 is likely to account for Sr isotopic shift of Mt Biedeman sample 13AU15 to the right.

The composition of the trachyandesite MAS1076 and 203128 of Masonic is

modelled by the assimilation of less than 10% of one of the contaminants SNB-73-3, SNB-73-5, SNB-73-9, SNB-73-15, SNB-73-26, or SNB-73-29 with MAS1073. Further assimilation of less than 20% of SN-RRPV-1 with MAS1076 can produce the same isotopic composition as the Masonic sample 09BA13. However, the latter composition can be simulated also by mixing of less than 20% of MAS1073 with SN-RRPV-1. This suggests that the assimilation with at least two different sources took place: with Sierra Nevada plutonic rocks, and volcanic and metasedimentary rocks from the central western Sierra Nevada (DePaolo, 1981b).

The mixing model fails to account for the isotopic trend of the West Brawley Peak

and Aurora samples despite the large range of crustal compositions. Trachyandesite 00BA8 of West Brawley Peak has the lowest initial 87Sr/86Sr (~0.7042) among all BHVF rocks, which probably reflect major mantle wedge source contributions. Isotope ratio trends suggest an assimilation of less than 10% with a different source with initial 87Sr/86Sr ~0.7060. The crustal assimilation and fractional crystallization (AFC) model can provide


significantly different results than end member mixing models. However, it is a more accurate and more complex contamination model.

Fig. 10: 87Sr/86Sr vs 143Nd/144Nd for BHVF volcanic rocks and Sierra Nevada crustal contaminants. Ticks on curves are in 10% increments of wt% contaminant.

AFC and FC The AFC model involves heat balance and incorporates the combined influence of

concurrent assimilation and fractional crystallization; the latent heat of crystallization during fractional crystallization drives assimilation of country rock, which further drives crystallization of the host magma (Taylor, 1980; DePaolo, 1981a).

The large ion lithophile elements (LILE), K and Rb, are useful indicators of crust-magma interactions since they are concentrated in early crustal anatectic melts and because the ratio of K to Rb is relatively insensitive to crystal fractionation. K and Rb are very similar and show close geochemical coherence: deviations from this coherence are therefore potentially useful petrogenetic indicators (Shaw 1968; Davidson et al., 1987). K/Rb values decline with progressive Rb enrichment. Crustal contamination should produce a negative correlation of K/Rb with differentiation (Orozco-Esquivel et al., 2002). By contrast, fractional crystallization processes do not significantly alter this ratio. K/Rb values range between ~200 and 600 in BHVF suites and vary sharply at almost the same silica content.

Using any of the contaminants SNB-73-3, SNB-73-5, SNB-73-9, SNB-73-14, SNB-73-15, SNB-73-25, SNB-73-26, SNB-73-29, SN-FD-39, SN-FD-46, SN-RRPV-1, or SN-PC-30a, the AFC models fits well the majority of trachyandesite units indicating between 20 and 70% of assimilated crustal materials and between 20 and 50% of AFC if the parental liquid chosen is 09BA39 of Masonic, 01BA22 of Aurora, 203381A of Mt Biedeman and 08BA2 of West Brawley Peak (Fig. 11).



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Fig. 11: Rb content (ppm) vs K/Rb for BHVF volcanic rocks and Sierra Nevada crustal contaminants. AFC curves reflect the following parameters: K and Rb content (ppm) of the contaminant, DK, DRb, and r = mass assimilation rate/crystallization rate. Curve A1 (14112.2, 48.5, 0.171, 0.3, r=0.1) models assimilation of Sierra Nevada Batholith granodiorite SNB-73-29. Curves B1 (18594.9, 125.5, 0.171, 0.3, r=0.5), C3 (r=0.7) and D1 (r=0.1) model assimilation of an Sierra Nevada Batholith granodiorite SNB-73-5. Curve A2 (25817, 151.3, 0.171, 0.3, r=0.2), A3 (r=0.6), B2 (r=0.7), C1 (r=0.2), C2 (r=0.7), D2 (r=0.2), D3 (r=0.7) model assimilation of the composites of volcanic and sedimentary rocks from the central western Sierra Nevada SN-RRPV-1. Ticks on curves are in 2%; 3%; 4%; 5%; 6% increments of fraction of the initial magma remaining for A3, C2, C3; D3; A1, B2; A2, B1; C1, D1, D2 respectively.

Decoupled fractional crystallization and assimilation du Bray et al. (2016) concluded that trace element abundance variations among

BHVF rocks appear to be principally controlled by MASH zone processes and the superimposed effects of crystallization and fractionation. Decoupled fractional crystallization and assimilation (FCA) is known to be an important process, especially in MASH zones located near the base of the crust, where ambient temperatures are relatively high and the heat released by crystallization of multiple batches of mafic magma should cause crustal anatexis (Hildreth and Moorbath, 1988; Cribb and Barton, 1996). FCA yields magmas that are richer in compatible elements than magmas produced by AFC. In such cases, assimilation approximates a simple mixing process. Magmas produced by FCA have significantly higher 87Sr/86Sr and lower 143Nd/144Nd ratios than magmas produced by AFC.


Using any of the contaminants SNB-73-3, SNB-73-5, SNB-73-9, SNB-73-14, SNB-73-15, SNB-73-25, SNB-73-26, SN-FD-39, SN-FD-46, SN-RRPV-1, or SN-PC-30a, the FAC models fits well all of the trachyandesite units indicating between 10 and 30% of assimilated crustal materials and 5-54% of fractionation if the parental liquid chosen is 09BA39 of Masonic, 01BA22 of Aurora, 203381A of Mt Biedeman and 08BA2 of West Brawley Peak (Fig. 12).

Fig. 12: Rb content (ppm) vs K/Rb for BHVF volcanic rocks and Sierra Nevada crustal contaminants. FAC curves reflect the following parameters: K and Rb content (ppm) of the contaminant, DK, DRb, and r = mass assimilation rate/crystallization rate. Curve A1 (14112.2, 48.5, 0.171, 0.3, r=0.1) models assimilation of Sierra Nevada Batholith granodiorite SNB-73-29. Curves B1 (18594.9, 125.5, 0.171, 0.3, r=0.3), C3 (r=0.3) and D1 (r=0.2) model assimilation of Sierra Nevada Batholith granodiorite SNB-73-5. Curve A2 (25817, 151.3, 0.171, 0.3, r=0.2), A3 (r=0.3), B2 (r=0.4), C1 (r=0.2), C2 (r=0.4), D2 (r=0.2), D3 (r=0.3) model assimilation of the composites of volcanic and sedimentary rocks from the central western Sierra Nevada SN-RRPV-1. Ticks on curves are in 5%; 6%; 7% increments of fraction of the initial magma remaining for A3; A1, A2, B1, B2, C2, C3; C1. D1, D2, D3 respectively.

DISCUSSION

Trace element abundance of West Brawley Peak, Mt Biedeman, Aurora and

Masonic basically fit fractional crystallization model of magma evolution (Gavryliv et al., 2016). However, mixing models presented in this paper indicate that isotopic contents evolved by different mechanisms such as magma mixing and AFC. During the research the principal mechanism of magma evolution is revealed to be fractional crystallization with assimilation.



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The extent to which isotope compositions of contaminated magmas are shifted toward those of assimilants depends on the element concentrations in the mixing end members and the process (es) whereby contamination occurs (Davidson et al., 1987). The effects of magma mixing are likely to be superimposed on those of assimilation and fractional crystallization, yielding eruptive products that show complex geochemical variations. It is apparent that the large diversity of the BHVF rocks cannot be explained only by mixing with one single crustal composition, whatever the choice of the initial composition. The research revealed that the assimilation (at least 10% by mass) with at least two different sources took place: with Sierra Nevada plutonic rocks (granodiorites), and volcanic and metasedimentary rocks from the central western Sierra Nevada Such a large amount of assimilation requires a high heat flow beneath BHVF during the Miocene. An upwelling of hot asthenosphere as a result of the thinning of the lithosphere and the intrusion of mantle-derived magmas could explain the latter phenomenon.

Mixing modelling indicate a little degree of mixing (<10%) between Mt Biedeman and Bodie Hills Rhyolite. Furthermore, geochronologic data indicate that the trachyandesite of Mt Biedeman and rhyolite of Bodie Hills were, at least in part, coeval (from 9.95 to 8.90 Ma and from 9.89 to 9.74 Ma respectively) and field relations demonstrate that the corresponding magmas locally mixed (John et al., 2012; du Bray et al., 2016). These products of mixing consist of rounded and locally flattened, partly disaggregated hornblende pyroxene trachyandesite in matrix of quartz-rich rhyolite of Bodie Hills. Mingled magmas are exposed in at least four localities surrounding Mt Biedeman (John et al., 2012). The lack of higher-Fe plagioclase in the magma blob of Mt Biedeman confirms that magma represented by the trachyandesite of Mt Biedeman locally mingled and reacted with hot rhyolitic magma.

Intrasuite trends of Mt Biedeman and the Rhyolite of Bodie Hills show no systematic

increase of 87Sr/86Sr with rising SiO2 and declining Sr. The latter phenomenon can be attributed to a number of processes: (1) closed-system fractionation, which is not likely at least for Mt Biedeman; (2) deep-crustal MASH prior to intrasuite fractionation; (3) AFC processes that involve assimilants having little isotopic contrast with the magma (Hildreth and Moorbath, 1988). The good inverse correlation between Sr abundance and 87Sr/86Sr values of BHVF rocks probably indicates concurrent AFC processes during magma evolution.

Both Bodie Hills rhyolites and trachyandesites of Mt Biedeman show close 87Sr/86Sr values ~0.7058 suggesting derivation of the rhyolites from FC process. Although, closed system fractionation yields satisfactorily results for generating the range of Mt Biedeman compositions with the similar 87Sr/86Sr level, a closed system process is geologically unlikely, as interaction with the rhyolite magma and the crustal components is recognized in the field by several researchers (John et al., 2012; du Bray et al., 2016).

In spite the fact that K/Rb values decline with progressive Rb enrichment in all suites, they tend to have steep trajectories up to about 70 ppm Rb. These sharp drops in K/Rb are likely to reflect various crustal processes (addition of crustal partial melts, AFC, blending of variably evolved batches). Shaw, (1968) interpreted an enrichment of Rb relative to K as a result of three primary processes: biotite accumulation, metasomatism after separation of aqueous fluids, and crystallization and removal of hornblende. Petrographic descriptions (du Bray et al., 2013) suggest that crystallization of hornblende


is more likely to explain the negative correlation of K/Rb and Rb. Microscope-based estimate of abundance of hornblende relative to the whole rock, in volume percent is 1- 8% for Aurora, 3- 15% for Masonic, 2- 10% for Mt Biedeman, and 4- 20% for West Brawley Peak. The abundance of hornblende at the Rhyolite of Bodie Hills is significantly lower (0.5- 3%). The latter unit is the most enriched in K and Rb relative to the other volcanics. The more coherent trachyandesite-rhyolite trends probably result from subsequent fractionation of plagioclase and hornblende. However, the early steep drops of K/Rb are likely to reflect crustal interactions that affect mafic magmas stalled in the lower crust or percolating upward through heterogeneous crust (Hildreth and Moorbath, 1988).

The distribution of the elements observed is generally consistent with AFC and FCA model. However, the amounts of assimilation in some AFC models are unrealistic (>70%). Such levels of contamination achieved by AFC modelling generate many problems on terms of thermal budget, such that the magma will “freeze”, given the cooling effects of large amounts of assimilation (Riley et al., 2001). Assimilation driven by simple contact melting is not energetically efficient (Barboza and Bergantz, 1996, 2000). Hildreth and Moorbath (1988) proposed an alternative mechanism of MASH zones where the domain magma mixes with melts of the upper-middle crust. Mixing in MASH zone would not present the thermal problems that the high levels of crustal contamination creates.

BHVF magmas rising from mid to shallow crustal levels (≤10-15 km; du Bray et al., 2016) and initiating assimilation processes in shallower reservoirs apparently had >1500 ppm Sr and 87Sr/86Sr <0.7040. Field, petrologic, geochemical and isotopic studies of BHVF indicate that magmas were variably contaminated, and that much of the rocks were recycled in a MASH zone. In the MASH zone, conditions were favorable for high volume dehydration partial melting of pre-existing crustal rocks, probably of granitic composition, because high-Rb/Sr contributions tend to raise 87Sr/86Sr above the base-level values. These felsic crustal melts mixed with the evolving mantle-derived melts to form hybrid intermediate-composition rocks of BHVF. MASH processing is known to enhance the fertility of magmas by further concentrating volatiles and incompatible elements in the evolved melts. Some metallic components could be added to the magma from assimilated crustal materials (Richards, 2005). The results indicate an acceptable correlation with the results obtained by the other researchers of the area (du Bray et al., 2016; Kingdon, 2016).

Acknowledgments: We thank Alex Deutsch for a thoughtful review that improved the manuscript in form and substance. Modelling procedure is performed using R language and GCDkit. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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Table- 1: Isotopic composition and Sr, Nd, Sm, K, Rb content of the samples chosen as initial liquid (du Bray et al. 2013) and of the crustal contaminants (DePaolo 1981b) used in the mixing, AFC and FCA calculations.

(Received: 21.01.2018; Accepted: 05.04.2018)

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