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Photomicrographs of mafic granulites showing granoblastic growth of garnet in contact with clinopyroxene. Black specks in Fig. 2a are oxide phases. Brown specks in the large clinopyroxene in Fig. 2b are ilmenite.
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Analyses of trace elements in the mineral phases of granulites provide important information about the trace element distribution in the lower crust. Since granulites are often considered residues of partial melting processes, trace element characteristics of their mineral phases may record mineral/melt equilibria thus giving an opportunity to unde...
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The paper provides new U–Pb, Sm–Nd, and Nd–Sr isotope-geochronological data on rocks of the Paleoproterozoic Kandalaksha-Kolvitsa gabbro-anorthosite complex. Rare earth element (REE) contents in zircons from basic rock varieties of the Kandalaksha-Kolvitsa area were analyzed in situ using laser ablation inductively coupled plasma mass spectrometry...
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... At higher temperature conditions, V might be preferentially incorporated into other phases (e.g. ilmenite, clinopyroxene, amphibole; Nehring et al., 2010), and, as a consequence, rutile will contain less V and proportionally more Nb, for which it is the main carrier (Zack et al., 2002). This seems to be the case of HT rutile grains that are plotted in Figure 17. ...
Rutile, titanite, and ilmenite are the most common Ti-bearing minerals in metamorphic rocks. Experimental constraints have shown that titanite is stable at low-grade metamorphic conditions, rutile at HP, and ilmenite at HT-LP conditions. Yet, petrological evidence suggests that titanite can also be stable at LT-HP. This implies that both titanite and rutile can be used to develop proxies to track HP metamorphism, which can have interesting applications.
In this study, we have investigated the natural occurrence and chemistry of Ti-bearing minerals in gabbroic rocks that record different degrees of metamorphism, including LP amphibole-bearing gabbros from the ocean floor (Mid-Atlantic and Indian ridge IODP LEGs) and from an obducted ophiolite (Chenaillet) and HP Alpine metagabbros including blueschist and eclogite facies rocks from the Western Alps and Corsica. We have performed detailed petrography and Raman spectroscopy and analysed major and trace elements mineral chemistry using EPMA and LA-ICP-MS.
We found that rutile is stable at low pressure (< 2 kbar) in ocean-floor amphibole-bearing gabbros, lower than experimental constraints had previously suggested. Rutile is also found in eclogitic metagabbros from the Western Alps and can be chemically distinguished from LP rutile. Blueschist metagabbros from the Western Alps and eclogitic metabasalts from Corsica have titanite stable instead of rutile. While the titanite to rutile transition is pressure and temperature dependent, we demonstrate how small variations in bulk-rock Ti/Ca and Ca/Al values within the NCKFMASHTO chemical system may shift their stabilities. High-pressure titanite from these metamafic rocks exhibits La depletion and low La/SmN values in comparison to titanite from amphibolite-facies mafic rocks. La/SmN or Nb together with Yb and V can be used to distinguish HP titanite from titanite formed under other P-T settings. These new systematics can be useful in studies using detrital Ti-bearing minerals to probe the HP metamorphic record through time.
... Direct analyses of amphibole and biotite in granites show that they have similar Ni isotopic compositions (Wu et al., 2022), which is consistent with that amphibole has a Ni-O bonding length (2.07 Å) (Della Ventura et al., 1997) similar to that in biotite (2.06 Å) (Redhammer & Roth, 2002). Garnet and plagioclase have low D Ni (0.03-0.66 and <0.09, respectively), orders of magnitude lower than other ferromagnesian silicate minerals (Bougault & Hekinian, 1974;Gill, 1974;Laubier et al., 2014;Nehring et al., 2009). Therefore, whether or not garnet/plagioclase has joined the fractionation assemblages during magmatic differentiation will not significantly impact the Ni isotopic evolution of melts. ...
The behavior of nickel (Ni) isotopes during magmatic differentiation is not adequately explored. Here, we find that tholeiitic rocks in the Kīlauea Iki (KI) lava lake, Hawai'i, show progressively lighter Ni isotopic compositions with increasing magmatic differentiation, whereas calc‐alkaline rocks from the thick Kamchatka arc (30–45 km), located at the convergent boundary of the Eurasian and Pacific plates show increasing Ni isotope values as MgO and Ni decrease. Forty‐three global intermediate‐felsic continental igneous rocks analyzed in this study display large Ni isotopic variations, with the Eoarchean samples having light Ni isotopic compositions that fall in the trend defined by the KI lavas, and the post‐Eoarchean samples showing systematically heavier Ni isotopic compositions overlapping those of Kamchatka arc rocks. The isotopic dichotomy results from the crystallization of isotopically heavy magnetite during low‐pressure differentiation of KI lavas, whereas the participation of sulfide separation that removes isotopically light Ni during high‐pressure differentiation of magmas traversing thick continental crust. Combined with Rhyolite‐MELTS and sulfur concentration at sulfide saturation simulations, we demonstrate that the Ni isotope fractionation during magmatic differentiation is mainly controlled by the tempo of magnetite crystallization and sulfide formation, which is a function of pressure, oxygen fugacity, and water activity. High‐pressure calc‐alkaline differentiation usually suppresses magnetite crystallization while stabilizing sulfide, leading to heavy Ni isotopic compositions for the evolved magmas, significantly deviating from the low‐pressure fractionation trend seen in the KI lavas. Ni isotopes can be used in the future as a tracer of magmatic differentiation and processes of continent formation and differentiation.
... In order to assess the possibility that mineral separates contain more than one mineral component, the trace element compositions of the separates were compared with two datasets: 1) LA-ICPMS from hornblende, chlorite, biotite and epidote, thin sections of Wallundry Suite samples (Iles et al., 2015), and 2) a compilation of literature values for hornblende, biotite, titanite, ilmenite, zircon and epidote in other rocks (Luhr and Carmichael, 1980;Bea et al., 1994;Bea, 1996;Bingen et al., 1996;Pan and Fleet, 1996;Ayres and Harris, 1997;Belousova et al., 2002;Hermann, 2002;Jang and Naslund, 2003;Villaseca et al., 2003;Storkey et al., 2005;Tiepolo and Tribuzio, 2005;Bea et al., 2006;Gregory et al., 2009;Acosta-Vigil et al., 2010;Driouch et al., 2010;Nehring et al., 2010;Colombini et al., 2011;Gregory et al., 2012;Starijaš Mayer et al., 2014;Xing et al., 2014). Based on these comparisons (particularly examining REE, Hf and Zr), the hornblende separates from sample J are essentially monomineralic (i.e., minimal contribution from inclusions of other phases), but the pseudomorphs after hornblende from WG are a mixture dominated by chlorite and biotite with a possible minor component similar to the titanite-epidote separate. ...
Open-system magmatic processes are expected to impart various sorts of isotopic heterogeneity upon the igneous rocks they produce. The range of processes under the “open-system” umbrella (e.g., simple two-component mixing, magma mingling, assimilation with fractional crystallization) cannot usually be uniquely identified using data from a single isotope system. The use of bulk-rock, mineral separate and in situ techniques and multiple isotope systems allows the characterization of isotopic variability at different sampling scales, illuminating details of the petrogenesis of a magmatic system. This approach has been applied to granitoids of the Wallundry Suite in the Lachlan Fold Belt, Australia. The Wallundry Suite exhibits variations in mineral assemblage, mineral composition and trends in bulk-rock major and trace element compositions consistent with the involvement of liquid-crystal sorting processes such as fractional crystallization. In situ paired O-Hf isotope data from zircon in six samples show an array indicating the isotopic evolution of the melt phase. Similarly, bulk-rock Sr-Nd-Hf isotope arrays support open-system magma evolution. These data combined with the petrographic observations and major and trace element geochemical variations suggest some form of assimilation-fractional crystallization process in the petrogenesis of the Wallundry Suite. Added complexity is revealed by two observations: 1) the isotopic variations are only weakly coupled to the lithology and major element compositions of the samples; and 2) there are distinguishable differences between the Hf isotope compositions of bulk-rock samples and those of the magmatic zircons they host. To varying degrees the rocks consistently show negative ΔεHfbulk-zrc values (i.e., the bulk-rock compositions have less radiogenic Hf isotope values than their coexisting zircons). The preservation of distinctly low Nd and Hf isotope ratios in an Fe-Ti oxide mineral separate suggests that the bulk-rock vs. zircon discrepancy is caused by the presence of unmelted components derived from a contaminant of continental origin (i.e., a rock with low Sm/Nd and Lu/Hf and thus unradiogenic Nd and Hf). Evidently, a complex interplay of assimilation, crystallization and melt segregation is required to account for the data. This investigation demonstrates that such complexity can, nevertheless, be disentangled through comparison of complementary isotope data at multiple sampling scales.
... This has limited our understanding of the mechanism for the differentiation of these elements. Another case is the limited knowledge of D Nb ilmenite/melt and D Ta ilmenite/melt in felsic systems (e.g., Ewart and Griffin, 1994;Mahood and Hildreth, 1983), which has hindered the quantitative constraint on the behaviors of Nb and Ta during partial melting as ilmenite is a typical product of biotite-dehydration melting and always uptakes abundant Nb and Ta (e.g., Nehring et al., 2010;Patiño Douce and Beard, 1995;Stepanov and Hermann, 2013). In addition, the partitioning behaviors of first-row transition elements (FRTE; such as Sc, Co, Ni, V, Cr) were only partially constrained in felsic granulites (e.g., Ewart and Griffin, 1994;Mahood and Hildreth, 1983;Nash and Crecraft, 1985), impeding systematic evaluation of their behaviors during partial melting. ...
... Ultra-HT granulites are products of anatectic metamorphism at supersolidus temperatures of >900 • C (Harley, 1998), which make the partitioning of trace elements between anatectic melt and minerals (residual and peritectic phases) susceptible to achievement of thermodynamic equilibrium (e.g., Acosta-Vigil et al., 2012;Nehring et al., 2010). In addition, such high temperatures generally exceed the stability of biotite (e.g., Patiño Douce and Beard, 1995), excluding the shield effect of major minerals on the dissolution of accessory minerals. ...
... As a typical residual phase after partial melting, a large number of studies have investigated the transference and distribution of trace elements among minerals in amphibolite-to granulite-facies rocks (e.g., Acosta-Vigil et al., 2010Bea, 1996a;Bea and Montero, 1999;Nehring et al., 2010). Comparably, despite the significance of UHT metamorphism for crustal differentiation (e.g., Harley, 1998;Kelsey and Hand, 2015), seldom studies dealt with trace element behaviors in UHT granulites (e.g., Huang et al., 2021), and it is still debated which one of major and accessory minerals controls the LREE in UHT granulites (e.g., Bingen et al., 1996;Villaseca et al., 2003;Villaseca et al., 2007). ...
It is known that partial melting and melt extraction of crustal rocks result in chemical differentiation of the continental crust. But it is unknown how these two processes have affected the composition of granites due to a limited knowledge of the trace element behaviors during crustal anatexis. In order to quantify this issue, a combined study of whole-rock and mineral trace elements and mineral modal proportions was conducted for high-temperature (HT) to ultrahigh-temperature (UHT) felsic granulites from the Tongbai orogen in central China. Reconstruction of LREE budgets suggests that LREE mainly reside in monazite in both HT and UHT granulites. As monazite is predicted not stable at UHT condition in the modelling of previous studies, the presence of monazite in the Tongbai UHT granulites suggest that it was not sufficiently dissolved into melt. Considering monazite mainly occurs as interstitial grains, fast melt-residue separation may be the dominating factor for impeding its dissolution. Calculation of mineral/mineral trace element ratios indicates that K-feldspar has a higher capacity to accommodate Ba and Sr than biotite and plagioclase, with DBaK-feldspar/biotite of 2.5–3.0 and DSrK-feldspar/plagioclase values of 1.4–1.9, and ilmenite can preserve more Nb and Ta than biotite with DNbbiotite/ilmenite values of 0.07–0.1 and DTabiotite/ilmenite values of 0.04–0.05. In addition, partitioning of first row transition elements (FRTE) was constrained in the felsic granulites, with preference of Sc into garnet, and Co, V, Ni, and Cr into biotite. Modelling based on the presently constrained partition coefficients indicates that trace elements show consistent differentiation trends during anatexis of various felsic rocks, with enrichment of Sr, Ba, Nb, Ta and FRTE but depletion of Rb in anatectic restites. The high DNb/Ta values of biotite and ilmenite cause the residues produced by biotite dehydration melting to be commonly of higher Nb/Ta ratios than protoliths, indicating that the residues are potentially high Nb/Ta reservoirs. In contrast, the change of peritectic minerals due to various protolith compositions and melting conditions results in differential responses of Sr, Ba, and FRTE to partial melting, suggesting that these elements may be useful indicators to trace the petrogenetic process of granites. The correlation between Ba and Sc contents or between Sr/Ba and Sc/Co ratios is demonstrated to be a valid index in discriminating melts from metagreywackes and metapelites due to different modes of garnet, orthopyroxene, and K-feldspar in their anatectic residues. Moreover, as the proportions of garnet and orthopyroxene in the residues are significantly influenced by melting pressures, the Sc/Co ratio and Sc content of granites can be used to trace their anatectic depth.
... J o u r n a l P r e -p r o o f Journal Pre-proof ºC, with the amount of melt drastically increasing around 830-850 ºC due to biotite and hornblende breakdown (Fig. 8B). Although during the amphibolite-granulite facies transition pyroxene is formed in addition to a melt phase, pyroxene and hornblende both have low partition coefficients for Zr with melt (Nehring et al., 2010) such that the melt produced during hornblende and biotite breakdown could become saturated in Zr and precipitate zircon. ...
The Winnipeg River terrane is one of three plutonic-gneiss terranes that contains the oldest rocks in the Archean Superior Province and is integral to understanding the evolution of Earth's largest Archean craton. We evaluate the evolution of the Winnipeg River terrane using whole-rock Sr-Nd-Pb isotope data for a suite of 17 samples of the Cedar Lake gneiss, along with U-Pb-Lu-Hf isotopes and trace elements in zircon grains from one sample. Most whole-rock Sr-Nd-Pb isotope data trend along 3.25 Ga reference isochrons, which overlap with the dominant population of ca. 3.25 Ga zircon, and are interpreted to mark the igneous crystallization of the protolith to the Cedar Lake gneiss. An older population of zircon aged ca. 3.5–3.4 Ga indicate either Hadean or Eoarchean crust was reworked at ca. 3.25 Ga. At ca. 2.7 Ga, new zircon grew with limited dissolution of older grains resulting in near-chondritic time-integrated Hf isotope signatures and low Th/U ratios relative to the older grains. As a result of the overlap between whole-rock reference isochrons at 3.25 Ga and the dominant zircon population ca. 3.25 Ga, the radiogenic Hf isotope signature of the ca. 2.7 Ga zircon grains imply Hf was recycled from non-zircon constituents within the rock rather than from external input. Additional support is drawn from elevated Nb and Ta concentrations in ca. 2.7 Ga zircon, hornblende and biotite. Our data highlights a complexity in the LuHf isotope systematics of zircon, where metamorphic zircon inherited radiogenic Hf isotope compositions as a result of internal redistribution of Hf from non-zircon phases in the rock rather than juvenile input.
... The trace elements modelled (except Sr and Sc) behave incompatibly during fractionation of the magma. The common cause of divergence between the FC and AFC models is (2000), Marks et al. (2004) and Nehring et al. (2010) 2 Constrained from: Bea et al. (1994) and Nash and Crecraft (1985) 3 Constrained from: Hill et al. (2011), Kleine et al. (2000) and Klemme et al. (2002) 4 Constrained from: Adam and Green (2006), Dwarzski et al. (2006), Kleine et al. (2000), Klemme et al. (2002), Koepke et al. (2003), McKenzie and O'Nions (1991), Philpotts and Schnetzler (1970), Rubatto and Hermann (2007), Shimizu (1980), Sweeney et al. (1992) and Tuff and Gibson (2007) 5 Constrained from: Acosta-Vigil et al. (2012), Klemme et al. (2006), Nash and Crecraft (1985) and van Kan Parker et al. (2011) 6 Constrained from: Fabbrizio et al. (2008), Minissale et al. (2019) and Wood and Trigila (2001) 7 Constrained from: Bacon and Druitt (1988), Ewart and Griffin (1994), Luhr and Carmichael (1980), Nash and Crecraft (1985) and Nielson et al. (1992) 8 Constrained from: Bédard (2007) 9 Constrained from: Bédard (2006) and Ren et al. (2003) that these elements are mostly also incompatible during melting of the WR, which leads to their enrichment in the WR melt that is assimilated in the early stages of the AFC process, creating much stronger enrichment of the elements in the magma than FC alone would cause. For the LREE and Th, the over-enrichment of the magma (Fig. 13f, g and i) is potentially due to monazite not being handled by MELTS in the WR. ...
Understanding the origins of major and trace element variations and the isotopic character of granite samples in terms of sources and magmatic processes is, arguably, the core of granite petrology. It is central to attempts to place these rocks in the context of broader geologic processes and continent evolution. For the granites of the Lachlan and New England Fold Belts (LFB and NEFB) of Australia there has been great debate between competing petrogenetic models. The open-system view is that the isotopic variability and within-suite compositional trends can be accounted for by magma mixing, assimilation and fractional crystallisation. In contrast, the restite unmixing model views the isotope compositions of diverse granites as a feature inherited from individual protoliths that underwent partial melting to produce magmas entraining varying proportions of residual material in a felsic melt. Reconciling all aspects of the geochemical data in a mixing model is contingent on a plausible fractionation regime to produce the observed consistently linear (or near-linear) trends on Harker diagrams; however, published fractional crystallisation models lack phase equilibria constraints on the liquidus assemblage and do not account for the likely changes in trace element partitioning across the modelled compositional range.
The Magma Chamber Simulator (MCS) can be used to model fractional crystallisation alone or with assimilation (AFC), constraining phase equilibria and accounting for the thermal budget. Here, this tool was used to conduct a case study of the I-type Jindabyne Suite of granites from the LFB, testing whether thermodynamically feasible geochemical trends matching the observed linear variations can arise through fractional crystallisation (with or without assimilation of supracrustal material). The results of 112 MCS models show (1) that major element liquid lines of descent (LLDs) may be sensibly linear over limited compositional ranges, (2) that the involvement of assimilation extends the range in which trends are relatively simple and near-linear, and (3) that, despite these observations, neither fractional crystallisation nor AFC are able to correctly reproduce the geochemical evolution of the I-type Jindabyne Suite granitoids as an LLD (contrary to existing models) – instead, these processes persistently produce curved and kinked trends. The output of these simulations were further refined to explore models in which: (a) crystal-bearing magmas evolve via fractional crystallisation or AFC (with chemical isolation assumed to be achieved through crystal zoning) and undergo varying degrees of melt-crystal segregation at different stages to produce the sample compositions; and (b) in situ crystallisation occurs via fractional crystallisation within the crystallisation zone, driving the evolution of a liquid resident magma, which the samples represent. These models are able to reproduce the Jindabyne Suite trends reasonably well. The modelling implies that fractional crystallisation, or some variant thereof, is a viable explanation for the linear trends in Jindabyne; however, tendency for grossly non-linear LLDs highlights that it should not be assumed that fractional crystallisation can generally explain linear trends in granites without careful modelling such as shown here.
... Experimental studies have indicated that peraluminous granites are generated from melting of biotite and muscovite bearing metapelitic rocks (Gardien et al. 1995;Patino Douce and Harris 1998). Li, Rb, Cs, Nb and Ta elements are hosted in biotite in metapelites (Stepanov et al. 2014) and Nb and Ta are also hosted in ilmenites in granulite facie rocks (Nehring et al. 2010). Biotites and muscovites are also potential source of H 2 O, F and Cl in metapelites (Shaw et al. 2016), these elements behave incompatibly during melting of pelitic rocks and partition into the melt (Icenhower and London 1995). ...
The pegmatites in southern Akwanga occur within the reactivated belt of the basement complex of Nigeria. The pegmatites consist of dominantly albite–muscovite pegmatites (rare-metal), southern parts of the map areas and biotite-microcline pegmatites (barren) central parts of the map. The pegmatites intruded gneiss-migmatitic complex consist of metasedimentry rocks; granitic gneisses and biotite gneisses and rarely meta-igneous, amphibolites. The rare metal pegmatites are composed of quartz, albite and muscovites and tourmaline. Garnets, ilmenites and minor tin–columbite–tantalite mineralization constitute accessory minerals in contrast to the biotite-microcline pegmatites. The host rocks are composed of quartz, plagioclase (An5–21; albites–oligoclase), microcline and muscovite. Minor constituents include biotites, cordierites and hornblendes. Ilmenite occurs as opaques. The pegmatites and their host rocks are corundum and hypersthene normative, highly peraluminous, exhibiting similar geochemical signatures; however, the rare metal pegmatites are more fractionated than the host rocks and the biotite-microcline pegmatites. The rare metal pegmatites are relatively enriched in Rb, Li, Cs, B, Be, Nb and Ta, low in K/Rb and Al/Ga ratios than the biotite–microcline–pegmatites and their host rocks. The pegmatites are products of crustal anatexis of sedimentary origin. This indicates that the rare metal pegmatites are source rock controlled (product of post-collision activities) rather than fractional crystallization. Rare metal pegmatites occur in Southern Akwanga, north central NigeriaHighly peraluminous and LCT pegmatitesSn–Nb–Ta mineralization potentials in the pegmatiteThey are related to the migmatitic-gneiss complex by anatexis Rare metal pegmatites occur in Southern Akwanga, north central Nigeria Highly peraluminous and LCT pegmatites Sn–Nb–Ta mineralization potentials in the pegmatite They are related to the migmatitic-gneiss complex by anatexis
... Amp 1 in garnet amphibolites generally exhibit variable and weaker negative Eu anomalies than Amp 2 (Fig. 8), consistent with their growth under granulite-facies conditions. The REE partition between Amp 1 and Cpx 2 in sample 15NQ302 falls in the range of equilibrium partitioning under granulite-facies conditions (Fig. 8d, Nehring et al., 2010), confirming their concurrent growth under granulite-facies conditions. The occurrence of amphibole at late granulite-facies stage is also consistent with the phase equilibrium modelling of granulitized eclogite (e.g., Li et al., 2020). ...
A combined study of garnet and zircon from the same growth stages can provide effective constraints on the pressure-temperature-time path of metamorphic processes. This is illustrated by distinguishing different episodes of garnet and zircon in metabasites from North Qaidam. Two different garnet domains grown under eclogite- and granulite-facies conditions were recognized to have different trace elements and inclusions. The variable trace elements are ascribed to the coexisting MREE-rich minerals and whole-rock compositions under eclogite-facies conditions, and to variable growth of feldspar and consumption of precursor garnet at the continuous granulite facies metamorphism. The majority of zircon cores with ages of 430-441 Ma and zircon rims with ages of 419-424 Ma record metamorphism at eclogite-facies and granulite-facies, respectively. They exhibit different trace elements and Hf-O isotopes, indicating zircon growth via different mechanisms with differential involvement of precursor zircon and other minerals as well as anatectic melts under eclogite- and granulite-facies conditions. The metabasites experienced an increase of metamorphic thermal gradients with time and thus the tectonic transition from deeply continental subduction for HP to UHP eclogite-facies metamorphism at 2.6-3.1 GPa and 827-864°C through decompressional exhumation to the lower crust for HP granulite-facies overprinting at 1.05-1.70 GPa and 725-872°C, eventually to extensional exhumation to the mid-crustal level for low-pressure amphibolite-facies overprinting at 0.34-0.56 GPa and 528-667°C in the collisional orogen. While HREE disequilibrium between the two minerals is common, the equilibrium HREE partitioning may be achieved between zircon and garnet rims during crustal anatexis at the late HP granulite-facies metamorphism.
... The present study shows that the zircon mantle and rim domains mostly have lower REE and Y contents, Th/U ratios than those of the zircon cores ( Figs. 9 and 10b, d), and a significant decrease in HREE and Y contents, and a significant increase in Gd+Tb contents with decreasing ages (Fig. 10a-c). These changes are consistent with the growth of garnet and breakdown of amphibole during the prograde metamorphism and associated partial melting (Fig. 12) due to the garnet being as the main host of HREE and Y, and amphibole as the main host of Gd+Tb in mafic granulites (Nehring et al., 2010). In this case, we consider that the prograde metamorphism and partial melting of the mafic granulite probably began at ~42 and lasted to ~20 Ma (Fig. 11). ...
The Himalayan orogen, resulting from the Early Cenozoic collision of the Indian and Asian plates, is an ideal vehicle to study active orogenic processes and test geodynamic models of how the crust responds to collisional orogeny. This paper focused on migmatitic high-pressure (HP) mafic granulite and associated leucosome from the Greater Himalayan Crystallines (GHC) in the Eastern Himalayan Syntaxis (EHS) in order to understand the conditions and timescales over which high-grade rocks and partial melts were produced during the Himalayan orogeny. Combining with previous study results from the Western and Central Himalayas and Trans-Himalayan magmatic arc, we obtained the following conclusions: (1) The mafic granulites from the EHS underwent HP and high-temperature (HT) granulite facies metamorphism and partial melting, with peak metamorphic conditions of 15–17 kbar and 820–880 °C. The GHC, at least its western part of the EHS, underwent coherent HP granulite-facies metamorphism. (2) The HP mafic granulites experienced long-lived dehydration melting of amphibole from ∼40 Ma to ∼20 Ma during prograde metamorphism and generated up to ∼16 vol.% partial melt. The variable degrees of dehydration melting of the HP mafic, pelitic and felsic granulites in the EHS generated voluminous granitic melts with distinct compositions, and provided the source for the Himalayan granites. (3) Peak metamorphic pressure of the GHC gradually decreases, whereas the metamorphic temperature progressively increases from the Western to Eastern Himalayas. This indicates that the Indian continental crust deeply subducted into the mantle in the Western Himalaya after the Indo-Asia collision, whereas the Indian crust underthrusted or relaminated beneath the Asian continental crust, and formed the thickened lower crust in the Central and Eastern Himalayas and Gangdese arc. (4) The melts derived from the underthrusted Indian crust probably resulted in isotopic compositional enrichment of the Early Cenozoic mantle- and juvenile crust-derived magmatic rocks of the Gangdese arc.
... Although leucosome compositions usually do not match a typical melt composition, formation of leucosomes gives evidence for partial melting and the presence of a melt phase at peak metamorphic conditions (e.g. Nehring et al. 2009Nehring et al. , 2010. ...
Geological investigations of a part of the crystalline basement in the Baltic Sea have been performed on a drill core collected from the depth of 1092–1093 m beneath the Phanerozoic sedimentary cover offshore the Latvian/Lithuanian border. The sample was analyzed for geochemistry and dated with the SIMS U–Pb zircon method. Inherited zircon cores from this migmatized granodioritic orthogneiss have an age of 1854 ± 15 Ma. Its chemical composition and age are correlated with the oldest generation of granitoids of the Transscandinavian Igneous Belt (TIB), which occur along the southwestern margin of the Svecofennian Domain in the Fennoscandian Shield and beneath the Phanerozoic sedimentary cover on southern Gotland and in northwestern Lithuania. It is suggested that the southwestern border of the Svecofennian Domain is located at a short distance to the SW of the investigated drill site. The majority of the zircon population shows that migmatization occurred at 1812 ± 5 Ma, with possible evidence of disturbance during the Sveconorwegian orogeny.
