Muscovite Melting Research Articles

Evolution of the rare-metal mineralization system associated with collision-related pegmatites in the western Altyn Tagh Orogen, Tugeman, NW China

On a regional scale, rare-metal pegmatite groups (pegmatites of common origin) display a zonal distribution with different chemical compositions. Whether the regional distribution of such pegmatites dike in a convergent zone is related to dehydration melting of different micas remains unclear. In recent years, the Altyn Tagh Orogen (ATO) in the northwest China has gained attention due to economically significant rare-metal mineralization. Our study focuses on the geochronology of columbite-group minerals (CGMs) and geochemistry of tourmaline from the Be-mineralized pegmatites in the Tugeman and Ayage Be deposits. CGM dating shows that these Be-mineralized pegmatites formed between 485 and 484 Ma. Subsequent hydrothermal activity, occurring around 453–450 Ma, is related to the development of a ductile shear zone. Evidence from tourmaline mineralogy and major elements reveals that the evolution of the Be-mineralized pegmatite is influenced by crystallization differentiation and subsequent fluid exsolution. Tourmalines from these pegmatites exhibit B isotope compositions ranging from –12.2 ‰ to –14.8 ‰, indicating a crustal metasediment source. Based on these findings, we propose a model for the rare-metal mineralization system in the Tugeman area: the sequential formation of Be to Li-mineralized pegmatites likely results from dehydration melting of muscovite and biotite during prolonged subduction and collision between the South Altyn Subduction–Collision Belt (SAB) and Central Altyn Block (CAB).

Li-mineralized pegmatite of anatectic origin: Constraints from Li[sbnd]Hf isotopes and geochemical modeling

Pegmatites are important reservoirs of lithium resources. There is an ongoing debate as to whether Li-mineralized pegmatites were generated from evolved granitic systems via extreme differentiation or by direct melting of metasedimentary rocks. The Chinese Altay is one of the largest pegmatite provinces in the world, and includes many rare-metal pegmatites (e.g., Koktokay, Kelumute, Kaluan and Azubai). However, they display significant differences in formation ages and isotopic compositions compared with the neighbouring or potential parental granites. In this contribution, we present a case study of the Kaluan Li ore deposit in the Chinese Altay, elucidating the origins of pegmatites and related Li mineralization. The Kaluan Li-mineralized pegmatites are characterized by high SiO2, K2O, Na2O, Li, Cs, Ta, and Rb contents; low Fe2O3T, MgO, Ba, and Sr contents; and peraluminous signatures. They have homogeneous HfLi isotopic compositions (εHf (t) = −0.6 to +2.5; δ7Li = +1.6 to +5.8‰) that are within the range of HfLi isotopic compositions of the Kulumuti Group schists (εHf (t) = −2.8 to +7.5; δ7Li = −8.6 to +10.8‰). In addition, the chemical compositions of the melt derived from the partial melting of the Kulumuti Group by phase equilibrium modeling are compatible with our analyses of the Kaluan Li-mineralized pegmatites. These similarities suggest that the Kaluan Li-mineralized pegmatites were derived from the partial melting of the Kulumuti Group schists at depths during the Triassic, in a post-orogenic and extensional setting. The generation of these pegmatites was mostly controlled by biotite dehydration melting at relatively high temperatures, rather than by the low-temperature melting of muscovite. Trace elements modeling results show that approximately 33–36% partial melting of the average Kulumuti Group schist may produce melts with maximum Li concentrations ranging from 664 to 751 ppm. Subsequently, a high degree of fractionation exceeding 85%, was required for the Li concentration of the residual melt to reach concentrations suitable for crystallization into albite-spodumene pegmatites. Based on our research, an anatectic origin model may have general applicability in explaining the genesis and mineralization of rare-metal pegmatites in the Chinese Altay.

Fluid-fluxed melting in the Himalayan orogenic belt: Implications for the initiation of E-W extension in southern Tibet

The geochemistry of granite is largely controlled by physical and chemical parameters that are closely linked to tectonic processes in evolving orogenic belts. Therefore, temporal changes in the geochemical compositions of granites could be used to infer critical shifts in tectonic processes. The Himalayan leucogranites are crustal anatexis products, providing a case to formulate petrogenetic models for granites and test tectonic models. From west to east, in the High Himalaya and the Tethyan Himalaya, two groups of leucogranites are derived from fluid-absent melting (Group A) and fluid-fluxed melting of muscovite (Group B), respectively. In the Cona and Mount Everest areas, Group B granites crystallized at 26−10 Ma, and Group A granites formed at 19−13 Ma. Group B granites have higher CaO, Sr, Ba, Zr, Hf, Th, Sr/Y, Zr/Hf, and Th/U, and lower Rb, Nb, Ta, U, Rb/Sr, and 87Sr/86Sr than those in Group A granites. These geochemical differences highlight the role of deep-origin fluids and the dissolution control of the accessory phases on the geochemical compositions in silicic magma systems. Field and microstructural observations show that E-W extension occurred synchronously with the granite intrusion derived from fluid-fluxed melting. Elevated heat flow accompanying the E-W extension could dehydrate hydrous minerals and release fluids from deep-seated crust (e.g., Lesser Himalayan Sequence). Such fluids could flux and melt the metasedimentary rocks within the High Himalaya and produce Group B granites. Together with literature data, from the Lhasa terrane to the Himalayan belt, E-W extensions in Tibet may have initiated as early as 26 Ma.

The origin and compositions of melt inclusions in an Al2SiO5‐free paragneiss from the Namche Barwa Complex in the Eastern Himalayan Syntaxis

AbstractMelt inclusions (MIs) in high‐temperature metamorphic rocks provide a unique window into crustal anatexis in collisional orogenic belts and have been widely used to characterize compositions of anatectic melts as well as melting mechanisms. In this study, MIs hosted by peritectic garnet were for the first time identified in an Al2SiO5‐free graywacke‐type paragneiss from the Namche Barwa Complex, the Eastern Himalaya, Southeast Tibet. These MIs occur as nanogranites in the rims of porphyroblastic garnet, exhibit negative crystal shapes with an average diameter of ~12 μm and consist of a mineral assemblage of biotite + quartz + plagioclase + K‐feldspar ± muscovite. Re‐homogenization experiments of these nanogranites were conducted at a pressure of 1.5 GPa and temperatures of 800°C, 850°C and 900°C and produced homogeneous glasses at 850°C. The homogenized glasses are strongly peraluminous and calc‐alkalic in composition, with 66.43–71.31 wt.% SiO2, 12.64–15.06 wt.% Al2O3, high alkaline (5.41–7.22 wt.%) and low ferromagnesian (2.72–4.46 wt.%) contents. They are lower in silica and CaO but higher in K2O compared with MI produced by fluid‐present melting of metasedimentary rocks, thus indicating fluid‐absent melting. These glasses are also characterized by enrichment of large ion lithophile elements (particularly Cs and Rb), depletion of Ba and Sr, low contents of light rare earth elements (3.6 to 33.7 ppm), high Rb/Sr ratios (6.19–37.3) and low Nb/Ta ratios (2.55–18.7). In combination with phase equilibrium modelling, these compositional features suggest that a sequential dehydration melting of muscovite and biotite was responsible for the production of MI during prograde metamorphism of the studied paragneiss. By compiling MI data published in the literature, we show that dehydration melting of metasedimentary rocks from the Himalayan orogen can produce initial melts with various peraluminous and granitic compositions.

Multiple Sources of Indosinian Granites and Constraints on the Tectonic Evolution of the Paleo-Tethys Ocean in East Kunlun Orogen

Numerous Indosinian granitoids occur in the East Kunlun Orogen (EKO). The Indosinian was a key transitional period associated with the evolution of the Paleo-Tethys Ocean. Here, we study the relationship between the petrogenesis of the granitoids and the regional tectonic setting based on a comprehensive analysis of the petrology, geochronology, and geochemistry of typical granitoids in the eastern part of the EKO. The Indosinian granitoid compositions are dominated by quartz diorites, granodiorites, monzogranites, porphyritic monzogranites, and syenogranites. Early Indosinian granitoids are large, granitic batholiths, while the middle and late Indosinian granitoids are smaller in size. From the early Indosinian to late Indosinian, the granitoids show a transition from a medium-K calc-alkaline to high-K calc-alkaline composition. They are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) and depleted in high-field-strength elements (HFSEs), especially for the Helegangxilikete and the Kekeealong plutons. The late Indosinian granitoids have relatively low Y and Yb contents, high Sr contents, and high La/Yb and Sr/Y ratios, which suggests adakitic affinity. The zircon saturation temperatures of the early Indosinian syenogranite and the Keri syenogranite are above 800 °C. The zircon saturation temperatures of other Indosinian granites (average 749 °C) are lower than those of the biotite and amphibole partial melting experiment. In the early Indosinian (255–240 Ma), numerous granitoids were the products of the partial melting of the juvenile lower crust by mafic magma underplating. This underplating is geodynamically related to the continuous subduction of a branch of Paleo-Tethys Ocean, with slab break-off, rapid upwelling, and mantle decompression. In the middle Indosinian (240–230 Ma), the compression that accompanied the continent–continent collision was not conducive to fluid activity, and hence, the formation of magma could be attributed to dehydration partial melting of muscovite, biotite, or amphibole. In the late Indosinian (230–200 Ma), the delamination of thickened crust would provide heat and channels for fluid migration, leading to a flare-up of the magmas. The composition and petrogenesis of the Indosinian granitoids in the eastern EKO are the result of processes associated with the subduction, collisional, and post-collisional stages, during the evolution of the Paleo-Tethys Ocean.

Tectonic discontinuity, partial melting and exhumation in the Garhwal Himalaya (Northwest India): Constrains from spatial and temporal pressure-temperature conditions along the Bhagirathi valley

The pattern and timing of tectonic discontinuity and partial melting in the Garhwal Himalaya, resulting from the India–Asia collision, are crucial factors for understanding the evolution of the Himalayan orogen, but these parameters remain poorly constrained. Middle- to high-grade metamorphic rocks from the Main Central Thrust (MCT) zone and the High Himalaya Crystalline Sequence (HHCS) along the Bhagirathi valley in northwest India have been investigated in this study. The primary mineral assemblage of the gneisses and migmatites in the HHCS is garnet + biotite + white mica + quartz + kyanite + rutile ± ilmenite ± sillimanite. The metamorphic grade at thermal peak rapidly increases upwards across the MCT from garnet schists (515–550 °C, 8–9 kbar) in the footwall to garnet-bearing migmatite (705–735 °C, 10–13 kbar) in the hanging wall. The P–T condition decreased upwards in kyanite gneiss of the lower HHCS (690 °C, 8.3 kbar), yielding a high field pressure gradient near the MCT. The prominent metamorphic discontinuity could have been caused by the southward extrusion of the HHCS along with the MCT. The base of the HHCS experienced high-grade metamorphism at 38–25 Ma, mainly at 31.9 ± 0.5 Ma. The new data, combined with previous studies, suggest that the prominent metamorphic discontinuity is characterized by the different periods of peak metamorphism and the overprint in the hanging wall (Eocene–Oligocene metamorphism and Early Miocene overprint) and footwall (Early Miocene metamorphism and Late Miocene to Pliocene overprint) of the MCT. The second metamorphic discontinuity occurred near the migmatite zone in the lower-middle HHCS, which experienced the anatexis (690–740 °C, 7–9 kbar) at 37–25 Ma, mainly at 33.9 ± 1.2 Ma. The Bhagirathi HHCS experienced a regional Barrovian-type Eohimalayan metamorphism associated with fluid-present melting of muscovite + plagioclase + quartz + H2O → melt ± kyanite at 34–32 Ma without a strong Miocene overprint. The period of partial melting is significantly older than the intrusion of ca. 22 Ma Early Miocene Gangotri leucogranites at the top of the HHCS. Protracted periods of crystallization (38–22 Ma) indicate a long duration of more than ca. 15 Myr of anatexis, from initiation of migmatization to pluton formation in the Bhagirathi HHCS. The protracted fluid-present melting and subsequent rapid exhumation in the Bhagirathi HHCS could be associated with heterogeneous extrusion forming the tectonic discontinuity in the lower-middle HHCS. The metamorphic grade decreased towards the upper HHCS (620 °C, 6.5 kbar) across the Jhala Normal Fault where occurs in the right-way-up sequence. On the other hand, in the central Himalaya, the thrusting occurred between upper HHCS and lower-middle HHCS in the inverted metamorphic sequence. The Early Miocene migmatites in the footwall of the significant tectonic discontinuity are dominant, and diachronous extrusion accompanied with the fluid-absent melting reactions is the common mechanism to exhume the HHCS, unlike the heterogeneous extrusion in the northwest Himalaya.

Late Triassic post-collisional high-K two-mica granites in Peninsular Thailand, SE Asia: Petrogenesis and Sn mineralization potential

Triassic magmatism in Thailand provides the critical geological record for understanding the evolution of the Paleo-Tethys Ocean. This paper presents zircon U-Pb dating, zircon trace element compositions, elemental geochemistry and Sr-Nd-Hf isotopes for the newly identified Late Triassic granite intrusions in the Upper Peninsula of Thailand where voluminous Late Cretaceous-Paleogene granite intrusions occur. Zircon U-Pb dating results indicate that the two-mica granites were emplaced at ca. 217–216 Ma. The unaltered two-mica granite samples are high silicic (SiO2 = 74.03–75.06 wt%) and display strongly peraluminous characteristics, with A/CNK ratios of 1.21–1.33 and CIPW normative corundum contents of 2.86–3.88 wt%. They have fractionated REE patterns ((La/Yb)N = 5.64–7.34) and notable negative Eu anomalies (Eu/Eu* = 0.35–0.49), with enrichment of Rb, Th, U, Ta and Pb and depletion of Ba, Nb, Sr and Ti. The two-mica granites can be assigned as the highly fractionated S-type granites. They have negative whole-rock εNd(t) (−13.2 to −12.3) and zircon εHf(t) (−11.5 to −7.4) values with corresponding two-stage Nd and Hf model ages ranging from 1.99 to 2.06 Ga and from 1.72 to 1.98 Ga, respectively, indicating a Paleoproterozoic crustal source. The combined geochemical and isotopic data reveal that these rocks were derived by fluid-absent muscovite melting of metapelite at the middle-upper crustal depths and modified by fractional crystallization. In combination of the results in this study with regional geological data, it can be concluded that the two-mica granites were more likely formed during the post-collisional stage of the Sibumasu-Indochina collision, induced by slab break-off of the Paleo-Tethys Ocean. Based on the integrated consideration of a set of factors, including temporal and spatial proximity, source lithology and source enrichment, redox state, and fractional crystallization coupled with melt-fluid interaction, it is suggested that the ca. 216 Ma two-mica granites exhibit high Sn mineralization potential and could be helpful for exploring the primary Sn deposit of Late Triassic age nearby.

Crustal thickening of the western Tibetan Plateau during the Early Cretaceous: Evidence from adakitic granodiorites, leucogranites, and monzogranites in the Songxi area, western Qiangtang Terrane

Felsic igneous rocks in the Songxi area are a key part of the Qiangtang Terrane and India–Asia collision zone, and can constrain the evolution of the Tibetan Plateau. This paper presents new geochronological, geochemical, and isotopic data for Early Cretaceous granodiorites (119 Ma), leucogranites (111 Ma), and monzogranites (107–102 Ma) from the Songxi area in the western Qiangtang Terrane. The granodiorites are characterized by enriched Sr–Nd–Pb–Hf isotopic compositions (87Sr/86Sri = 0.70721–0.70925, εNd(t) = −7.6 to −5.6, 206Pb/204Pbi = 18.907–18.964, 207Pb/204Pbi = 15.782–15.825, and εHf(t) = −24.0 to −2.0) and are adakitic with low Mg# values (39–44) and high K2O contents (2.6–3.9 wt%). The leucogranites have low Sr concentrations (80–100 ppm), high Rb/Sr (2.83–3.53) and low CaO/Na2O (0.19–0.22) ratios, high initial 87Sr/86Sr ratios (0.71653–0.71721), and low εNd(t) (−12.4 to −11.8) and εHf(t) (−18.8 to −6.8) values. The monzogranites are I-type granitoids and have enriched Sr–Nd–Pb–Hf isotopic compositions (87Sr/86Sri = 0.70956–0.70966, εNd(t) = −8.5 to −8.3, 206Pb/204Pbi = 18.740–18.749, 207Pb/204Pbi = 15.759–15.778, and εHf(t) = −16.9 to −1.4). Our data suggest that these three types of granitoids were likely generated by partial melting of a thickened lower crust, metapelite-dominated source by fluid-absent muscovite melting, and normal lower crust, respectively. Integration of our new and existing data allows us to conclude that the Songxi granitoids formed in a continental margin arc setting. This further suggests that some sediment-derived leucogranites form in a continental arc setting, and are unrelated to continental collision. Tectonic compression and crustal thickening of the western Tibetan Plateau during the Early Cretaceous occurred when the Bangong–Nujiang oceanic lithosphere was subducted northward at a low angle beneath the western Qiangtang Terrane before ca. 119 Ma.

Fluid-present and fluid-absent melting of muscovite in migmatites in the Himalayan orogen: Constraints from major and trace element zoning and phase equilibrium relationships

Partial melting of the continental crust may proceed through either influx of free water (hydration, fluid-present melting) or breakdown of hydrous minerals (dehydration, fluid-absent melting). The two types of physicochemical mechanism would be operated for crustal anatexis in nature, and their effects on the composition of anatectic products were closely examined by experimental approaches as well as the study of natural leucogranites from the Himalayan orogen. However, the leucogranites usually experienced several stages of evolution during magmatism, leading to difficulties in retrieving the composition of original melts. This issue is now addressed by studying Himalayan migmatitic metapelites because they generally share similar source rocks and P–T conditions with the leucogranites. In combined with field observations, an integrated study of whole-rock geochemistry, mineral chemistry, zircon geochronology, and phase equilibrium modelling was conducted for migmatitic metapelites from the Nyalam Valley in the Greater Himalayan Sequence. The results show that the migmatitic metapelites share similar clockwise P–T paths. They experienced an early prograde metamorphism through compressional heating to reach peak pressures of 7.3–9.0 kbar at ~700 °C before ca. 31 Ma. Afterwards, the migmatitic metapelites underwent decompressional heating to ~725 °C at 6.0 kbar for upper-amphibolite facies metamorphism at 26–21 Ma. Subsequently, they experienced decompressional cooling to a temperature below the solidus at ca. 18 Ma. The crustal anatexis was dominated by muscovite melting reactions during the decompressional heating from ca. 26.4 to 18 Ma. Despite the similar P–T paths, the migmatitic metapelites experienced different types of melting reactions involving muscovite (i.e., dehydration and hydration). Whereas the hydration melting resulted in a marked increase in An contents (~65 to 75 mol%) and the absence of K-feldspar, the dehydration melting led to relatively low-An (<30 mol%) plagioclase and abundant K-feldspar. The two types of muscovite melting reaction may be common in the Himalayan orogen in the Late Cenozoic. While the fluid-present melting would take place at a shallower depth, the fluid-absent melting would occur at a greater depth. This coupled dehydration-hydration relationship has important implications for the geodynamic mechanism of crustal anatexis in the continental collision zone.

Behavior of apatite in granitic melts derived from partial melting of muscovite in metasedimentary sources

Fluid-absent and fluid-fluxed melting of muscovite in metasedimentary sources are two types of crustal anatexis to produce the Himalaya Cenozoic leucogranites. Apatite grains separated from melts derived from the two types of parting melting have different geochemical compositions. The leucogranites derived from fluid-fluxed melting have relict apatite grains and magmatic crystallized apatite grains, by contrast, there are only crystallized apatite grains in the leucogranites derived from fluid-absent melting. Moreover, apatite grains crystallized from fluid-fluxed melting of muscovite contain higher Sr, but lower Th and LREE than those from fluid-absent melting of muscovite, which could be controlled by the distribution of partitioning coefficient (<i>D</i>_Ap/Melt) between apatite and leucogranite. <i>D</i>_Ap/Melt in granites derived from fluid-absent melting is higher than those from fluid-fluxed melting. So, not only SiO₂ and A/CNK, but also types of crustal anatexis are sensitive to trace element partition coefficients for apatite. In addition, due to being not susceptible to alteration, apatite has a high potential to yield information about petrogenetic processes that are invisible at the whole-rock scale and thus is a useful tool as a petrogenetic indicator.

Early Paleozoic granitic rocks of the South Qiangtang Terrane, northern Tibetan Plateau: Implications for subduction of the Proto- (Paleo-) Tethys Ocean

Knowledge of the early Paleozoic tectonic framework of the northern margin of Gondwana continent is critical for reconstructions of the Gondwana supercontinent. Early Paleozoic leucogranite and gneissic granite units within the South Qiangtang Terrane, northern Tibetan Plateau, can constrain the timing and geological processes that occurred along the northern margin of Gondwana continent at this time. Here, we present new field, petrological, zircon geochronological, geochemical, and Nd–Hf isotopic data for this suite of granitic rocks, and use these data to provide insights into the evolution of this Gondwanan margin. Zircon U–Pb dating yielded two groups of ages at 469–453 and 486–473 Ma. All of the granitic rocks in the Gemuri area are high-K calc-alkaline and peraluminous, and record the fractionation of variable amounts of plagioclase, K-feldspar, and biotite. These leucogranites formed from magmas generated by the dehydration melting of muscovite within Paleoproterozoic–Mesoproterozoic metapelites under relatively low-temperature conditions (TZr = 616 °C–695 °C). In contrast, the protoliths of the Duguer gneissic granite formed from magmas generated by the partial melting of Paleoproterozoic–Mesoproterozoic metapelites contaminated by mantle-derived material under relatively high-temperature conditions (TZr = 730 °C–884 °C). The coeval nature and linear distribution of the igneous rocks within the northern Gondwanan margin, and the fact that the granites in the Gemuri area are enriched in large-ion lithophile elements (LILEs; e.g., Rb and K) and depleted in high-field-strength elements (HFSEs; e.g., Zr, Nb, Ta, and Ti), suggest that this magmatism occurred in an active continental margin setting involving the formation of arc-type rocks. The presence of an early Paleozoic suprasubduction zone-type ophiolite within the South Qiangtang Terrane provides evidence of the southward subduction in the Paleo-Tethyan Ocean at this time, suggesting that the two stages (i.e., the Gemuri leucogranites and Duguer gneissic granites) of arc-type magmatism in the Gemuri area may both be related to this subduction event.

Muscovite Dehydration Melting in Silica-Undersaturated Systems: A Case Study from Corundum-Bearing Anatectic Rocks in the Dabie Orogen

Corundum-bearing anatectic aluminous rocks are exposed in the deeply subducted North Dabie complex zone (NDZ), of Central China. The rocks consist of corundum, biotite, K-feldspar and plagioclase, and show clear macro- and micro-structural evidence of anatexis by dehydration melting of muscovite in the absence of quartz. Mineral textures and chemical data integrated with phase equilibria modeling, indicate that coarse-grained corundum in leucosome domains is a peritectic phase, reflecting dehydration melting of muscovite through the reaction: Muscovite = Corundum + K-feldspar + Melt. Aggregates of fine-grained, oriented, corundum grains intergrown with alkali feldspar in the mesosome domains are, instead, formed by the dehydration melting of muscovite with aluminosilicate, through the reaction: Muscovite + Al-silicate = Corundum + K-feldspar + Melt. P-T pseudosections modeling in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2 system constrains peak pressure-temperature (P-T) conditions at 900–950 °C, 9–14 kbar. The formation of peritectic corundum in the studied rocks is a robust petrographic evidence of white mica decompression melting that has occurred during the near-isothermal exhumation of the NDZ. Combined with P-T estimates for the other metamorphic rocks in the area, these new results further confirm that the NDZ experienced a long-lived high-T evolution with a near-isothermal decompression path from mantle depths to lower-crustal levels. Furthermore, our new data suggest that white mica decompression melting during exhumation of the NDZ was a long-lasting process occurring on a depth interval of more than 30 km.

Oligocene initiation of the South Tibetan Detachment System: Constraints from syn-tectonic leucogranites in the Kampa Dome, Northern Himalaya

Himalayan leucogranites provide a robust record of the thermal and tectonic processes that occurred during convergence of the Eurasian and Indian tectonic plates. We present structural, petrological, mineralogical, whole-rock geochemical, SrNd isotope, and geochronological results for Cenozoic leucogranites from the Kampa Dome, Northern Himalaya. The Kampa leucogranites are typical syn-tectonic granites and occur as a series of sills, dikes, and puddings in the Kampa Shear Zone, which is considered to be the exhumed portion of the South Tibetan Detachment System (STDS) in the Northern Himalaya. The Kampa syn-tectonic leucogranites provide information about the timing and nature of regional kinematics, and record thermal conditions and the initiation of the STDS. Whole-rock geochemistry and SrNd isotope analyses show that the Kampa leucogranites are characterized by the following: (1) they are peraluminous and have high SiO2, Al2O3, and A/CNK, low TiO2, MgO, P2O5, and TFe2O3, and variable CaO, Na2O, K2O, and total alkalis; (2) they are enriched in large-ion lithophile elements (e.g., K and Rb) and depleted in high-field-strength elements (e.g., Zr and Ti), with negative Eu anomalies; and (3) they display variable initial 87Sr/86Sr compositions (0.750758-0.826065), and their εNd(i) values range between −7.6 and −12.2. Anatexis of graywackes and metapelites in the Great Himalayan Crystalline Series played an important role in the formation of the Kampa leucogranites. Anatexis was controlled by fluid-absent partial melting of muscovite. Mineralogical and petrological analyses show that zircons belong to an early crystallization phase, whereas monazites are remnants of the final crystallization phase of the granitic melt. Zircon UPb dating reveals that anatexis in the deep Himalayan crust occurred at ca. 32–23 Ma, during the Oligocene. Monazites yield homogeneous 208Pb/232Th ages of ca. 23.5 Ma and record rapid emplacement in the Kampa Shear Zone. These results indicate that movement on the STDS was initiated during the early Oligocene and became well-developed during the late Oligocene in the Kampa region. The Oligocene was a unique period of counterbalance between continued plate convergence and intense extension in the Himalayan Orogen.

Muscovite dehydration melting: Reaction mechanisms, microstructures, and implications for anatexis

AbstractDehydration melting of muscovite in metasedimentary sequences is the initially dominant mechanism of granitic melt generation in orogenic hinterlands. In dry (vapour‐absent) crust, muscovite reacts with quartz to produce K‐feldspar, sillimanite, and monzogranitic melt. When water vapour is present in excess, sillimanite and melt are the primary products of muscovite breakdown, and any K‐feldspar produced is due to melt crystallization. Here we document the reaction mechanisms that control nucleation and growth of K‐feldspar, sillimanite, and silicate melt in the metamorphic core of the Himalaya, and outline the microstructural criteria used to distinguish peritectic K‐feldspar from K‐feldspar grains formed during melt crystallization. We have characterized four stages of microstructural evolution in selected psammitic and pelitic samples from the Langtang and Everest regions: (a) K‐feldspar nucleates epitaxially on plagioclase while intergrowths of fibrolitic sillimanite and the remaining hydrous melt components replace muscovite. (b) In quartzofeldspathic domains, K‐feldspar replaces plagioclase by K+–Na+ cation exchange, while melt and intergrowths of sillimanite+quartz form in the aluminous domains. (c) At 7–8 vol.% melt generation, the system evolves from a closed to open system and all phases coarsen by up to two orders of magnitude, resulting in large K‐feldspar porphyroblasts. (d) Preferential crystallization of residual melt on K‐feldspar porphyroblasts and coarsened quartz forms an augen gneiss texture with a monzogranitic‐tonalitic matrix that contains intergrowths of sillimanite+tourmaline+muscovite+apatite. Initial poikiloblasts of peritectic K‐feldspar trap fine‐grained inclusions of quartz and biotite by replacement growth of matrix plagioclase. During subsequent coarsening, peritectic K‐feldspar grains overgrow and trap fabric‐aligned biotite, resulting in a core to rim coarsening of inclusion size. These microstructural criteria enable a mass balance of peritectic K‐feldspar and sillimanite to constrain the amount of free H2O present during muscovite dehydration. The resulting modal proportion of K‐feldspar in the Himalayan metamorphic core requires vapour‐absent conditions during muscovite dehydration melting and leucogranite formation, indicating that the generation of large volumes of granitic melts in orogenic belts is not necessarily contingent on an external source of fluids.

Identifying the leucogranites in the Ailaoshan-Red River shear zone: Constraints on the timing of the southeastward expansion of the Tibetan Plateau

The uplift of the Tibetan Plateau significantly affected the global climate system. However, the timing of its uplift and the formation of its vast expanse are poorly understood. The occurrence of two types of leucogranites (the two-mica leucogranites and garnet-bearing leucogranites) identified in the Ailaoshan-Red River (ASRR) shear zone suggests an extension event in the southeastern Tibetan Plateau. The age of these leucogranites could be used to constrain the timing of uplift and southeastward expansion of the plateau. Petrography, geochronology and geochemistry investigations, including Sr–Nd isotope analysis, were conducted on the two-mica leucogranites and garnet-bearing leucogranites from the ASRR shear zone. LA-ICP-MS zircon U–Pb dating indicates that these rocks were emplaced at ~27 ​Ma, implying that the Tibetan Plateau had already achieved maximum uplift prior to the late Oligocene. It subsequently started to expand southeastward as a result of crustal flow. Compared to classic metapelite-derived leucogranites from Himalaya, the two-mica leucogranites show high K2O/Na2O (1.31–1.92), low Rb/Sr, CaO, lower 87Sr/86Sr ratios (0.7089–0.7164) and higher εNd(t) (−8.83 to −3.10). This whole-rock geochemical characteristics likely indicates a mixing source origin, composed predominantly of amphibolite with subordinated metapelite, which is also evidenced by 87Sr/86Sr vs. εNd(t) diagram. However, The garnet-bearing leucogranites with high SiO2 contents (72.25–74.12 ​wt.%) have high initial 87Sr/86Sr ratios (0.7332–0.7535) and low εNd(t) (−16.36 to −18.98), indicating that they are derived from the source comprised of metapelite and results of fluexed muscovite melting under lower crustal level, which is also evidenced by the Rb–Sr–Ba systematics. These leucogranites formed from partial melting of the thickened lower crust, which resulted in the formation of granitic melt that weakened the crust. The weakened crust aided the left-lateral strike-slip movement of the ASRR shear zone, triggering the escape of the Indochina terrane in the southeastern Tibetan Plateau during the late Oligocene.

Different melting conditions and petrogenesis of peraluminous granites in western Qinling, China, and tectonic implications

The Qinling orogen records the prolonged amalgamation history between the North China Block (NCB) and the South China Block (SCB) accompanied by major granitoid intrusions during the Triassic. However, the petrogenesis of these granitoids, especially that of the peraluminous granites, remains a matter of debate. Detailed study of these peraluminous granites, which are the products of crustal melting, will give insights into the melting process and petrogenesis of peraluminous magmas and will further constrain the regional geological evolution. In this study, Xiahe (XH) tourmaline-bearing two-mica granite and Baijiazhuang (BJZ) two-mica granite were selected for systematic analysis of their mineralogy, petrology, zircon U-Pb geochronology, and geochemistry. The zircon U-Pb dating showed that the XH and BJZ granites were emplaced at approximately 242 Ma and approximately 214 Ma, respectively. The presence of muscovite and tourmaline, high peraluminosity at >1.1, low oxygen fugacity (ƒO2) conditions below the fayalite–magnetite–quartz (FMQ) buffer, and low zircon saturation temperatures indicate sedimentary protolith (S-type) granitic affinity. The low CaO/Na2O ratios <0.4 suggest that the granites formed by partial melting of a pelitic source. The low heavy rare earth element (HREE) concentration and weak negative Eu anomaly in the XH granite implies garnet residue in the magma source, whereas an obvious Eu anomaly in the BJZ granite indicates no residual garnet. The Rb/Sr ratios remained constant in the XH granite with decreasing Sr and Ba concentrations; however, these ratios increased in the BJZ granite. This observation reflects that the XH (BJZ) granite was the product of fluid-present (fluid-absent) muscovite melting. This finding is in accord with the different geochemical features and the estimates of zircon and monazite saturation temperature between the XH and BJZ granites. In western Qinling, low-Sr/high-REE (group A) and high-Sr/low-REE (group B) peraluminous granites have been identified. We propose that the former were generated during fluid-absent muscovite melting, whereas the latter were derived from fluid-present muscovite melting. This hypothesis was further confirmed by higher zircon and monazite saturation temperature estimates in addition to higher REE, Th, Nb, and Ta concentrations in the group A granitoids. From regional geological considerations, it can be concluded that the Middle Triassic granitoids in western Qinling formed during subduction of the A'nimaqing–Mianlue oceanic crust, whereas the Late Triassic granitoids are related to syn- or post-collisional processes.

Contrasting geochemical signatures of fluid-absent versus fluid-fluxed melting of muscovite in metasedimentary sources: The Himalayan leucogranites

Abstract Most of the Himalayan Cenozoic leucogranites are products of partial melting of metapelite sources. In the Malashan-Gyirong area (southern Tibet), the geochemical compositions of leucogranites define two groups with distinct whole-rock major elements, large ion lithophile elements, rare earth elements, high field strength elements, and Sr and Hf isotope ratios. Based on published experimental results that define generalized melting reactions of metapelitic sources, we infer that these leucogranites are the products of two different types of crustal anatexis: fluid-fluxed melting and fluid-absent melting of muscovite in metasedimentary sources. As compared to the leucogranites derived from fluid-absent melting, those from fluid-fluxed melting have relatively higher Ca, Sr, Ba, Zr, Hf, Th, and light rare earth element concentrations, and Zr/Hf, Eu/Eu*, and Nd/Nd*, but lower Rb, Nb, Ta, and U concentrations, Rb/Sr and 87Sr/86Sr ratios, and εHf(t). The geochemical differences can be explained by melting behaviors of major (muscovite, feldspar) and accessory minerals (zircon and monazite) during different modes of crustal anatexis. The systematic elemental and isotopic signatures of different types of crustal anatexis and, in particular, the coupling of major and trace elements that results from common influences on rock-forming and accessory mineral behaviors provide tools with which to refine our understanding of the nature of crustal anatexis.

Multiple migmatite events and cooling from granulite facies metamorphism within the Famatina arc margin of northwest Argentina

The Famatina margin records an orogenic cycle of convergence, metamorphism, magmatism, and extension related to the accretion of the allochthonous Precordillera terrane. New structural, petrologic, and geochronologic data from the Loma de Las Chacras region demonstrate two distinct episodes of lower crustal migmatization. The first event preserves a counterclockwise pressure‐temperature path in kyanite‐K‐feldspar pelitic migmatites that resulted in lower crustal migmatization via muscovite dehydration melting at ∼12 kbar and 868°C at 461 ±1.7 Ma. The shape of the pressure temperature path and timing of metamorphism are similar to those of regional midcrustal granulites and suggest pervasive Ordovician migmatization throughout the Famatina margin. One‐dimensional thermal modeling coupled with regional isotopic data suggests Ordovician melts remained at temperatures above their solidus for 20–30 Ma following peak granulite facies metamorphism, throughout a time period marked by regional oblique convergence. The onset of synconvergent extension occurred only after regional migmatites cooled beneath their solidus and was synchronous with the cessation of Precordillera terrane accretion at ∼436 Ma. The second migmatite event was regionally localized and occurred at ∼700°C and 12 kbar between 411 and 407 Ma via vapor saturated melting of muscovite. Migmatization was synchronous with extension, exhumation, and strike‐slip deformation that likely resulted from a change in the plate boundary configuration related to the convergence and collision of the Chilenia terrane.

Lead contents of S-type granites and their petrogenetic significance

An evaluation of Pb and Ba contents in S-type granites can provide important information on the processes of crustal partial melting. Primary low-T S-type granites, which form mainly by fluid-absent muscovite melting, may acquire a significant enrichment in Pb when compared to higher-T S-type granites for a given Ba content. We consider the following factors are responsible for this enrichment: Muscovite is a major carrier of Pb in amphibolite facies metapelites, and thus large quantities of Pb can be liberated upon its breakdown. The typical restite assemblage of Qz + Bt + Sil ± Pl ± Grt ± Kfsp that forms during low-T, fluid-absent muscovite melting can take up only minor amounts of this Pb. This is because the crystal/melt Pb distribution coefficients for these restite minerals are low to very low. Only K-feldspar is moderately compatible for Pb, with a crystal/melt distribution coefficient of ~3, but its modal content in restites is usually low. At the same time, the restite assemblage will retain much Ba owing to the very high Ba uptake in both biotite and K-feldspar, which is an order of magnitude higher than for Pb. Thus, during a low-T anatectic event involving a low degree of crustal melting, Pb (as an incompatible element) can become strongly enriched in the partial melt relative to Ba and also relative to source rock values. In the case of higher-T anatexis and larger partial melt amounts, the Pb becomes less enriched and the Ba less depleted or even enriched relative to source rock values. During fractional crystallization of a S-type granite magma, Ba behaves strongly compatibly and Pb weakly compatibly. The concentrations of both elements decrease along the liquid line of decent. Owing to this sympathetic fractionation behavior, the primary, source-related Pb–Ba fingerprint (with weak or strong Pb enrichment) remains in evolved S-type granites. This facilitates a distinction between primary low-T S-type granites, which are related to muscovite melting, and secondary low-T S-type granites that evolve through fractional crystallization from a higher-T parental magma. We show in this paper that a simple logarithmic Pb versus Ba diagram can be a valuable aid for interpreting the petrogenesis of S-type granite suites.

Miocene andalusite leucogranite in central-east Himalaya (Everest–Masang Kang area): Low-pressure melting during heating

The studied Miocene andalusite-bearing leucogranites intrude the upper part of the High Himalayan Crystallines (HHC) and the north Himalayan domes and outcrop in an area stretching from Mt. Everest to the Kula Khangri massif (Bhutan) towards the east.The leucogranites constitute both dykes as well as sills and parts of larger andalusite-free leucogranite plutons (e.g., Makalu). They represent mainly of two-mica (muscovite+biotite±tourmaline±cordierite±andalusite±sillimanite±dumortierite) leucogranite, and tourmaline (muscovite+tourmaline±biotite±andalusite±sillimanite±garnet±kyanite±spinel±corundum) leucogranites. Microstructures reveal several generations of andalusite (from residual/peritectic early magmatic to cotectic late magmatic), even in the same sample. The occurrence of residual and/or peritectic andalusite, together with inclusions of sillimanite+biotite in cordierite, indicates that melts formed by dehydration melting of biotite at T=660–700°C during prograde heating at low-pressure conditions (P<about 400MPa).According to current models, leucogranites are produced by dehydration melting of muscovite and/or biotite during exhumation of the HHC. In this case, micas are consumed in the sillimanite stability field. As a consequence, these models cannot explain the occurrence of residual and/or peritectic magmatic andalusite. Conditions for anatexis in the andalusite field may have been achieved by heat transfer within the exhuming (extruding) HHC, from structurally lower and hotter rocks towards upper and colder fertile lithologies.