Muscovite Dehydration 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).

Genesis of the Jiada pegmatite lithium deposit in the Ke’eryin ore field, western Sichuan, China: Evidence from whole-rock trace element and Li isotope

The origin and petrogenesis of pegmatite-type lithium deposits within the Songpan-Garze Fold Belt (SGFB) of western China remain subjects of debate. To provide further insight on this subject, this study investigated the whole-rock trace elements and whole-rock Li isotopic compositions of the two-mica monzogranite (TMG) from the Ke’eryin composite pluton, the Jiada albite pegmatite (AP) and albite-spodumene pegmatite (ASP) and their metasedimentary wall rocks within the Ke’eryin ore field in the eastern SGFB. Modeling of muscovite-dehydration melting based on the whole-rock trace elements (e.g., Li, Rb, and Sr) and Rayleigh dehydration modeling related to the whole-rock Li isotopes suggest that the Jiada pegmatites are unlikely to have originated from the low-degree partial melting of metasandstone (MS) of the Zhuwo Formation. Variations in the whole-rock trace elements and Li isotope between the TMG and the Jiada pegmatites are consistent with the evolution trend of magmatic fractionation. However, the whole-rock Li isotopic compositions of the AP and ASP display inverse variation characteristics with the muscovite Li isotopic compositions, with the AP exhibiting lower Li concentrations and higher whole-rock δ7Li values than ASP. Furthermore, the variation patterns of whole-rock trace elements do not align perfectly with the Rayleigh-type fractional crystallization processes, implying other geological processes would be also involved in the formation of the Jiada pegmatites. Collectively considering the Li isotope behavior during various geological processes, the observed variations in whole-rock Li isotopic compositions between AP and ASP can be explained by aqueous fluid enrichment during the pegmatitic melt evolution. This suggests that fractional crystallization and fluid exsolution jointly contributed to pegmatite formation and Li mineralization in the Jiada deposit. Moreover, the whole-rock δ7Li values of the wall rock of ASP (MS2) are lower than those of MS and the wall rock of ASP (MS1), primarily due to metasomatism by 6Li-enriched fluids expelled from ASP and the kinetic diffusion of Li isotopes. The high concentrations of highly mobile elements, especially Li and Cs, combined with the low whole-rock δ7Li value of the metasandstone, could serve as effective tracers for locating concealed orebodies and evaluating the metallogenic potential of Li mineralization.

Partial melting mechanisms of peraluminous felsic magmatism in a collisional orogen: An example from the Khondalite belt, North China craton

AbstractSedimentary‐derived (S‐type) granites are an important product of orogenic metamorphism, and a range of subtypes can be recognized by differences in field occurrence, mineralogy and geochemistry. These subtypes can reflect variations of initial protolith composition, partial melting reactions, pressure and temperature of anatexis, or magmatic processes that occur during ascent through the crust (e.g. mineral fractional crystallization or crustal assimilation). Together, these diverse factors complicate geological interpretation of the history of peraluminous felsic melt fractions in orogenic settings. To assess the influence of these factors, we performed integrated field investigation, petrology, geochemistry, geochronology and phase equilibrium modelling on a series of leucosomes within migmatite associated with different S‐type granites within the Khondalite belt, North China craton (NCC), which is an archetypal collisional orogen. Three types of leucosome are recognized in the east Khondalite belt: leucogranitic leucosome, K‐feldspar (Kfs)‐rich granitic leucosome and garnet (Grt)‐rich granitic leucosome. Phase equilibrium modelling of partial melting and fractional crystallization processes indicate that the leucogranitic leucosomes were mostly produced through fluid‐present melting, Kfs‐rich granitic leucosomes are produced through muscovite dehydration melting with 3 vol.% garnet fractional crystallization, and Grt‐rich granitic leucosomes are produced through biotite dehydration melting with 20–40 vol.% K‐feldspar fractional crystallization and up to 20 vol.% peritectic garnet entrainment. Mineral fractional crystallization and peritectic mineral entrainment occur in the source during melting, and play equally important roles in partial melting mechanisms in terms of affecting the geochemical compositions of granitic melts. Thus, we suggest that peraluminous felsic magmas preserved in collisional orogens are dominantly produced by fluid‐absent melting in the middle to deep continental crust, although extraction of low‐volume melt fractions from an anatectic source region at shallower depths during fluid‐present melting can also generate small amounts of S‐type granites that subsequently crystallize at high structural levels in the crust.

Geochemical Evidence for Genesis of Nb–Ta–Be Rare Metal Mineralization in Highly Fractionated Leucogranites at the Lalong Dome, Tethyan Himalaya, China

Leucogranites in the Lalong Dome are composed of two-mica granite, muscovite granite, albite granite, and pegmatite from core to rim. Albite granite-type Be–Nb–Ta rare metal ore bodies are hosted by albite granite and pegmatite. Based on field and petrographic observations and whole-rock geochemical data, highly differentiated leucogranites have been identified in the Lalong Dome. Two-mica granites, albite granites, and pegmatites yielded monazite ages of 23.6 Ma, 21.9 Ma, and 20.6 Ma, respectively. The timing of rare metal mineralization is 20.9 Ma using U–Pb columbite dating. Leucogranites have the following characteristics: high SiO2 content (>73 wt.%); peraluminosity with high Al2O3 content (13.6–15.2 wt.%) and A/CNK (mostly > 1.1); low TiO2, CaO, and MgO content; enrichment of Rb, Th, and U; depletion of Ba, Nb, Zr, Sr, and Ti; strong negative Eu anomalies; low εNd(t) values ranging from −12.7 to −9.77. These features show that the leucogranites are crust-derived high-potassium calc-alkaline and peraluminous S-type granites derived from muscovite dehydration melting under the water-absent condition, which possibly resulted from structural decompression responding to the activity of the South Tibetan detachment system (STDS). Geochemical data imply a continuous magma fractional crystallization process from two-mica granites through muscovite granites to albite granites and pegmatites. The differentiation index (Di) gradually strengthens from two-mica granite, muscovite granite, and albite granite to pegmatite, in which albite granite and pegmatite are highest (Di = 94). The Nb/Ta and Zr/Hf ratios of albite granite and pegmatite were less than 5 and 18, respectively, which suggests that albite granite and pegmatite belong to rare metal granites and have excellent potential for rare metal mineralization.

Two episodes of Mesozoic granitic magmatism in East Qinling, central China: Constraints from geochemistry and isotope geochronology of the Huangbeiling pluton

The East Qinling Orogen as an important segment of central China that records a complex tectonic evolution history. The Huangbeiling pluton is a rare S-type granite in this orogen and has vital significance in understanding the tectonic evolution of the Qinling Orogen during the Late Mesozoic. The biotite monzogranite of this pluton is the major ore-hosting rocks of the Yumugou Mo-W deposit. In this study, we newly reported a muscovite monzogranite and conducted detailed petrological, geochemical (whole-rock, biotite, muscovite major and trace elements), and zircon U-Pb dating investigations on both the biotite and muscovite monzogranites from the Huangbeiling pluton, with a view to reveal their physico-chemical conditions of formation, petrogenesis and tectonic evolution. Zircon U-Pb dating yielded a weighted mean 206Pb/238U age of 126.8 ± 0.6 Ma for muscovite monzogranite, integrated with published zircon U-Pb age (156–132 Ma) of biotite monzogranite, indicating that the Huangbeiling pluton was produced by two episodes of magmatism during the Late Mesozoic. The pluton shows formation temperatures in the range of 670–772°C, and is classified as cold granites that formed in a subduction-related setting. The emplacement depth of Huangbeiling pluton is constrained at ca. 0.3–4.6 km, with an average depth of 2.0 km. The biotite monzogranite has higher oxygen and fluorine fugacities than the muscovite monzogranite, suggesting high potential for Mo mineralization. Geochemically, the muscovite monzogranite is identified as a S-type granite sourced from crust-derived clay-rich mudstone, and underwent muscovite dehydration melting and subsequent fractional crystallization of K-feldspar and plagioclase during magma ascend. In combination with the syn- to post-collision geochemical affinities of the Huangbeiling pluton and the Late Mesozoic tectonic evolution of the Qinling Orogen, we propose two episodes of magmatism in the Huangbeiling pluton during the tectonic transition (150–125 Ma) from compression to extension in the Qinling Orogen. The first episode (156–132 Ma) of magmatism is correlated with the delamination of lithospheric mantle and intra-continental subduction of the North China Craton under compressional tectonics, whereas the second episode (∼126 Ma) of magmatism occurred through muscovite dehydration melting during pressure decrease.

Fingerprinting crustal anatexis with apatite trace element, halogen, and Sr isotope data

The geochemistry of apatite is a valuable tool for granite petrogenetic study. Here we explore the capability of apatite for fingerprinting anatectic reactions during crustal melting. We conducted a detailed petrographic, trace element, volatile, and Sr isotopic study of the apatites from four groups of leucosomes and leucogranites that formed by amphibole dehydration melting (group I), hydration melting (group II), muscovite dehydration melting (group III), and feldspar accumulation (Group IV) from the central Himalayan orogen. The coexistence of primary CO2-rich fluid and multiphase solid inclusions in apatite indicates fluid–melt immiscibility and suggests that C–O–H fluids were present during hydration melting. We observe marked differences in major and trace element, as well as halogen (F and Cl) contents in apatite from different groups, corresponding to different melting reactions. Apatite REE, Y, Mn, F, and F/Cl exhibit a decrease, whereas Sr/Y and Cl increase, from muscovite dehydration melting to hydration melting, and to amphibole dehydration melting. Amphibole dehydration melting results in similar CaO, Sr, and Eu/Eu* in apatite to hydration melting, but the latter leads to lower (La/Yb)N in apatite. These geochemical differences in apatite can be explained by different minerals participating in the melting reactions and varying melt polymerization resulting from the different melting reactions. The degree of melt polymerization decreases in the sequence from muscovite dehydration melting to hydration melting, and to amphibole dehydration melting. A higher degree of melt polymerization results in higher values for trace element partition coefficients between apatite and melt, and therefore the enhanced incorporation of REE, Y and Mn in apatite. Higher Sr and Eu/Eu* in apatite reflect the consumption of a larger amount of plagioclase in the melting reaction. The F and Cl contents in partial melts were calculated based on apatite composition, which exhibit an increasing trend from hydration melting to muscovite hydration melting, and to amphibole hydration melting. Such a trend can be explained by different proportions of hydrous minerals (muscovite and amphibole) participating in the melting reactions, with amphibole responsible for the high Cl content in melt and apatite. In addition, the apatite Sr isotopic compositions record the Sr isotopic disequilibrium during partial melting and exhibit distinct evolutional trends resulting from different anatectic reactions. Muscovite dehydration melting forms melt with higher 87Sr/86Sr than the restite due to radiogenic Sr released by muscovite. Apatite therefore records progressively decreasing 87Sr/86Sr during melt evolution before the isotopic homogenization between the partial melt and the restite. In contrast, apatites crystallized from the melt of hydration melting show an evolutional trend of increasing 87Sr/86Sr. In summary, the elemental and isotopic compositions of apatite can be related to different types of melting reactions, and can therefore be used as an indicator of the mechanisms of melt formation during crustal anatexis. We demonstrate such an application through the case study of group IV apatite from feldspar accumulation, and also explore its utility in granite petrogenetic studies.

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.

Contemporaneous crust-derived I- and S-type granite magmatism and normal faulting on Tinos, Delos, and Naxos, Greece: Constraints on Aegean orogenic collapse

Granitoids of varying mineralogy are exposed on the Cycladic islands of Greece; they include both hornblende-bearing I-type granites and garnet ± muscovite−bearing S-type granites, suggesting heterogeneous magma sources. In this contribution, we present new field observations, major- and trace-element geochemistry, Sr-Nd isotopes, and U-Pb geochronology of granitoids from Tinos, Delos, and Naxos that provide insight into these magma sources, along with the timing of adjacent extensional structures. I-type (biotite and hornblende-biotite) granites have initial 87Sr/86Sr = 0.70956−0.71065 and εNd(t) = −6.3 to −9.3, and S-type (garnet ± tourmaline-muscovite) leucogranites have overlapping initial εNd(t) = −7.5 to −10.1, with initial 87Sr/86Sr values overlapping as well as extending to higher values (0.70621−0.73180). These isotope signatures are comparable to those of the Variscan-age Cycladic basement, but not the Hellenic arc. We suggest that both I- and S-type granites were derived via crustal anatexis of variable sources, dominantly metaigneous and metasedimentary, respectively, during the climax of Barrovian metamorphism between ca. 17 and 12 Ma, and critically, they are not related to the Hellenic subduction zone. I-type granitoids were likely derived from dehydration melting of igneous Variscan- or Cadomian-aged basement protoliths, whereas S-type leucogranites formed by muscovite dehydration melting of sedimentary protoliths. Top-to-the-(N)NE shear zones on Naxos and Tinos were active from ca. 20 to 15 Ma and are folded and cut by later low- and high-angle normal faults. S-type leucogranites at Livada Bay, Tinos, dated at ca. 14 Ma, are cut by domino-style normal faults, placing a maximum age on the timing of extension. This is similar to ca. 15−14 Ma dates from NNE-SSW horizontally boudinaged S-type granites on Naxos. We propose that the concurrent intrusion of both I- and S-type granitoids with the onset of normal faulting marked the transition from an overall compressional to an extensional stress field associated with orogenic collapse at ca. 15 Ma.

Identification of Jurassic pure sediment-derived granites in the Central Lhasa Terrane, Tibetan Plateau: Implications for continental crustal reworking during Mesozoic Tethyan subduction

A bstract Pure sediment-derived granite, a recently recognized crustal geochemical end-member, is relatively rare but provide primary access to probe the process of continental crustal reworking. This paper presents geochemical and geochronological data for granites emplaced in the Asuo area within the central Lhasa Terrane of the Tibetan Plateau at ca. 158 Ma (zircon U − Pb isotopic dating). These granites are two mica-bearing and contain high SiO 2 (75.40–79.11 wt%), high A/CNK (1.06–1.33), heterogeneous ( 87 Sr/ 86 Sr) i (0.705519–0.710891), and relatively uniform and low ε Nd (t) (−13.18 to −14.81) values, which are analogous to typical S-type granites. The low ε Hf (t) (−13.98 to −12.08) and high δ 18 O (8.01‰–8.85‰) values of magmatic zircons, the similar of ε Nd (t) values between the Asuo granites and regional Jurassic slates, and the absence of mafic enclaves, verify that these granites were derived from a pure sedimentary source. The trondhjemitic features, low CaO/Na 2 O (<0.1) and molar CaO/(MgO + FeO T ) (0.06–0.58), relatively constant and low Rb/Sr (0.04–0.32), as well as the heterogeneous ( 87 Sr/ 86 Sr) i ratios, indicate that the Asuo granites formed via fluid-present muscovite-dehydration melting of metapelitic protoliths, with mica as the main residue. Combined with a synthesis of existing tectonomagmatic data on the Lhasa Terrane, we propose that the Jurassic Asuo granites were formed in a continental extensional setting associated with the northward subduction of the Neo-Tethyan lithosphere. The present study highlights that the inboard region of an active subduction zone is probably an important site of crustal reworking, and provides a geochemical end-member of pure sediment-derived melts for the central Lhasa Terrane. • Jurassic two-mica granites are discovered in the Asuo Area of central Lhasa Terrane; • The Asuo granites were derived from a pure sediment protolith of pelite; • The rear-arc region of a subduction zone is an important site for crustal reworking.

P-T-fluid evolution of migmatites from the Leo Pargil gneissic dome, India: Insights into partial melting and exhumation processes in North Himalayan Domes

• Water-fluxed melting leads to the development of amphibole + biotite migmatite . • Muscovite dehydration melting leads to formation of sillimanite-bearing migmatite. • Peak P–T conditions of 770–790 °C, 9.3–10.5 kbar for the amphibole + biotite migmatite. • Necking, hook, and stretching inclusions developed during isothermal decompression. • Selective leakage of the H 2 O phase from the H 2 O-CO 2 inclusions. Migmatites exposed within the North Himalayan Domes result from the reworking and partial melting of different structural levels of the Indian continental crust during a major Himalayan tectonic event. Although their petrologic investigation is potentially a powerful method to unravel the anatectic and tectono-thermal processes related to the Himalayan orogeny, petrological studies of the migmatites from North Himalayan Domes are virtually lacking. This paper presents the results of a petrological and fluid inclusion study in order to understand the partial melting processes, the P–T conditions of migmatitization, and the fluid evolution of the migmatites from one of the North Himalayan Domes, the Leo Pargil Dome, exposed in central Himalaya. Phase equilibria calculated in the MnO–Na 2 O–CaO–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –TiO 2 –H 2 O (MnNCKFMASTH) system suggest peak P–T conditions of 770–790 °C, 9.3–10.5 kbar for the amphibole + biotite migmatite and of 780–820 °C, 9.3–9.8 kbar for the sillimanite-bearing migmatite, followed by an early nearly isothermal decompression. The formation of these migmatites was related to water-fluxed melting and muscovite dehydration melting. At peak metamorphic conditions, aqueous-carbonic (H 2 O-CO 2 ) fluids were present in the system. Subsequently, the H 2 O phase was preferentially leached out from the biphase aqueous-carbonic inclusions to form primary CO 2 inclusions and directly went into the anatectic melt during migmatization. The exhumation of LPD followed an early nearly isothermal decompression path and a later isochoric cooling driven by the Kaurik-Chango normal fault.

Early Cambrian S-type granites in the Sakarya Zone, NE Turkey: A record for transition from subduction to post-collisional extension deduced from U Pb zircon age and Nd Hf isotopes

Cadomian realms have recently received much attention in the Alpine–Mediterranean orogenic belts, with discontinuous outcrops extending from the Alps to the İstanbul Zone, Menderes Massif, Taurides in Turkey and Iran as well. However, they have not been identified in the Sakarya Zone, northern Turkey to date. Here, we present, for the first time, zircon UPb geochronology, zircon Hf-isotopes and trace elements as well as whole-rock geochemistry and Nd-isotopes from the Karameşe metagranitoid in the Yusufeli (Artvin) area, NE Turkey. The zircon UPb age data demonstrate that the Karameşe metagranitoid intruded into the Precambrian basement units in the Early Cambrian (ca. 534–530 Ma). Zircon overgrowths in the Karameşe metagranitoid interpreted as a result of metamorphic overprint yielded ages of 328 Ma. This age indicates that the Sakarya Zone experienced the Variscan metamorphic events at 328 Ma. The Early Cambrian metagranitoid shows that some rocks in the basement of the Sakarya Zone date back to Precambrian in age. The Karameşe metagranitoid is an S-type granite, with high modal content of muscovite (~10%). The samples show highly peraluminous [A/CNK = molar Al2O3/(CaO + K2O + Na2O) = 1.34–1.58] and medium-K calc-alkaline geochemical affinities. The Karameşe metagranitoid displays low REE concentrations and a slight positive Eu anomaly and show slightly light REEs and LILEs enrichments, and HFSEs depletions. The samples have negative εNd(t) of −4.76 to −2.90 and εHf (t) of −4.75 to −1.08 values. These geochemical-isotopic characteristics reveal that the Karamaşe metagranitoid originated through fluid-absent muscovite dehydration melting of heterogeneous metasedimentary source rocks (mostly greywacke), with a minor input of juvenile mafic melt. We argue that the Early Paleozoic metagranitoids along the eastern Sakarya Zone is an expression of crustal thinning in the Cadomian orogenic belt in northern Gondwana, with the tectonic turnover from convergent margin to crustal extension possibly induced by break-off of Tornquist oceanic lithosphere which is a branch of Iapetus Ocean during Early Cambrian.

Fluorine behavior during experimental muscovite dehydration melting and natural partitioning between micas: Implications for the petrogenesis of peraluminous leucogranites and pegmatites

Abstract Fluorine behavior during the partial melting of two mica-bearing protoliths has been experimentally investigated at 700 to 930 °C and 0.4 and 0.6 GPa. Muscovite dehydration and H2O-HF fluid-assisted partial-melting experiments were carried out using both a natural and synthetic two-mica schist made of natural micas. The mineral composition of the experiments was assessed by BSE imaging and EDS analyses. The F, Cl, and major elements contents of the glass and micas were determined by EPMA. The muscovite dehydration melting reaction is muscovite + quartz + plagioclase = peraluminous melt + biotite + sillimanite + potassic feldspar ± hercynite. The starting biotite stays largely stable, showing only minor melt + ilmenite and trace magnetite formation in the cleavages. The newly formed biotite shows similar F contents and a slightly higher XSid component when compared to the starting biotite. HF-added experiments yield F-rich newly formed biotite. The experimentally produced melts were of a peraluminous leucogranitic composition with F contents increasing with F-rich protoliths. The bulk partition coefficient DFschist/melt increases from 0.5 to 3.0 when the F content of the protolith rises from 0.05 to 1.2 wt%. The partition coefficient, DFBt/melt, increases from 2.0 to 6.0 where the biotite MgO content increases from 5 to 18 wt%. The natural partition coefficient DFBt/Ms, measured for a set of rocks with a varied lithology from the Seridó Belt, northeastern Brazil, was 2.7 ± 0.5. The F partition coefficients measured in this study, along with published F partition coefficients between biotite and melt, biotite and muscovite, and fluid and melt, allow for the modeling of F behavior during muscovite dehydration and fluid-present melting. F-rich, two-mica protoliths will increase F partitioning in favor of the micaceous anatectic residue compared to the peraluminous melt. Furthermore, the model indicates that the more Fe-rich the schist and its residual biotite are, the higher the F content of the melt and the fluid. Fluorine-rich peraluminous leucogranites and related fluids may be generated by the anatexis of F- and Fe-rich, two-mica protoliths. As F can be a complexing ligand for Li, Be, Cs, Nb, Ta, W, Sn, and U, muscovite dehydration could potentially be associated with metallic occurrences associated with peraluminous melts.

Neoproterozoic–Early Cambrian igneous and sedimentary sequences in the Songliao Block, NE China: Records of Rodinia supercontinent evolution in eastern Central Asian orogenic Belt

As the core of NE China blocks (i.e., Songliao, Jiamusi-Khanka, Erguna, Xing’an), the Songliao Block plays an important role in understanding the Precambrian evolutionary history of the eastern Central Asian Orogenic Belt (CAOB), as well as the linkage with reconstructions of Rodinia supercontinent. In this work, we present sedimentological, whole-rock geochemical, and zircon U–Pb–Hf isotopic data for the Neoproterozoic-early Cambrian igneous and sedimentary rocks in eastern Songliao Block. The newly identified 954 ± 2 Ma gneissic syenogranite pluton，916 ± 5 Ma meta-basaltic andesite, and the reported 929–841 Ma intrusive rocks formed the Precambrian crystalline basement of the eastern Songliao Block, which was overlain by the late early Neoproterozoic to early Cambrian stable sedimentary cover. The early Neoproterozoic (954 Ma) gneissic syenogranite was likely the product of muscovite dehydration melting of sedimentary rocks in deep crust during the post-collisional setting, probably associated with the collision between the Songliao and Jiamusi-Khanka blocks. The late Neoproterozoic to early Cambrian Chenming Formation displays characteristics of passive margin deposition, which formed after the opening of Paleo-Mudanjiang Ocean at 777 Ma. Integration of the spatial and temporal distributions of Neoproterozoic magmatism and sedimentation in the Songliao, Erguna, Xing'an and Jiamusi-Khanka blocks, as well as the geochemical variations from 0.96 to 0.73 Ga, we suggest that the blocks in NE China have experienced a similar two-stage Neoproterozoic tectonic evolutionary history associated with the Rodinia assembly and breakup. An early advancing subduction along peripheral Rodinia led to the collision and collapse extension between the Songliao and Jiamusi-Khanka blocks before 954 Ma, and the generation of 929–880 Ma syenites and granitoids in compressional continental arc setting. Then the retreating subduction formed the 850–737 Ma mafic and felsic rocks in Songliao, Erguna and Jiamusi-Khanka blocks, and caused breakup of the joint continent and opening of the Paleo-Mudanjiang and Paleo-Xinlin oceans, two branches of Paleo-Asian Ocean, since 777 Ma. In response to the oceanic spreading, the passive continental margin sedimentation developed in these blocks during 750–520 Ma.

Miocene Crustal Anatexis of Paleozoic Orthogneiss in the Zhada Area, Western Himalaya

AbstractGranitic gneiss (orthogneiss) and Himalayan leucogranite are widely distributed in the Himalayan orogen, but whether or not the granitic gneiss made a contribution to the Himalayan leucogranite remains unclear. In this study, we present the petrological, geochronological and geochemical results for orthogneisses and leucogranites from the Zhada area, Western Himalayas. Zhada orthogneiss is composed mainly of quartz, plagioclase, K‐feldspar, biotite and muscovite, with accessory zircon and apatite. Orthogneiss zircon cathodoluminescence (CL) images show that most grains contain a core with oscillatory zoning, which indicates an igneous origin. Sensitive high‐resolution ion microprobe (SHRIMP) U‐Pb dating of the zircon cores in the orthogneiss shows a weighted 206Pb/238U age of 515 ± 4 Ma (early Paleozoic), with spongelike zircon rims of 17.9 ± 0.5 Ma (Miocene). Zhada leucogranite shows 206Pb/238U ages ranging from 19.0 ± 0.4 Ma to 12.4 ± 0.2 Ma, the weighted average age being 16.2 ± 0.4 Ma. The leucogranites have a low Ca content (&lt;1 wt%), FeOt content (&lt;1 wt%), Rb content (67.0–402 ppm), Sr content (&lt;56.6 ppm), Ba content (3.35–238 ppm) and Rb/Sr ratio (0.5–14.7), which are similar to the geochemical characteristics of the Himalayan leucogranite derived from muscovite dehydration partial melting of metasediments and representative of most Himalayan leucogranites. The highly variable Na2O + K2O (4.33 wt%–9.13 wt%), Al2O3 (8.44 wt%–13.51 wt%), ∑REE (40.2–191.0 ppm), Rb (67.0–402 ppm) and Nb (8.23–26.4 ppm) contents, 87Sr/86Sr(t) ratios (0.7445–0.8605) and εNd(t) values (–3.6 to –8.2) indicate that the leucogranite is derived from a heterogenetic source. The nonradiogenic Nd isotope values of the studied Zhada leucogranite and orthogneiss range from –8.2 to –3.6 and from –8.7 to –4.1, respectively. Therefore, the general mixing equation was used to perform the Sr and Nd isotope mixing calculations. The results indicate that the heterogenetic source was the Tethyan Himalayan Sequence (THS)/Higher Himalayan Crystalline (HHC) metasediments and Zhada orthogneiss. The Zhada area experienced crustal anatexis during the Miocene and the heterogenetic source of the orthogneiss and metasediment may have experienced crustal anatexis controlled by muscovite dehydration. The Zhada leucogranite inherited not only the geochemical characteristics of the Himalayan metasediment (muscovite dehydration melting), but also the trace elements and Sr‐Nd isotopic characteristics of the Zhada orthogneiss. These results indicate that the Paleozoic Zhada orthogneiss was involved in crustal anatexis at 17.9 ± 0.5 Ma (Miocene) and that the muscovite dehydration of the metasediments in the heterogenetic source produced fluid, which may have caused the orthogneiss solidus lines to decline, triggering a partial melting of the Zhada orthogneiss. It is therefore proposed that Himalayan leucogranite is a crust‐derived granite rather than a S‐type granite, as previously hypothesized.

Geochronological and metamorphic constraints on the exhumation of the Ama Drime Massif: Implications for the mid-Miocene evolution of Himalayan extensional structures

The east-west (E-W) trending South Tibetan Detachment System (STDS) and the north-south (N-S) trending normal faults and detachments have played significant roles in the Himalayan tectonic evolution since the Miocene. The Ama Drime Massif (ADM), which is bounded by the STDS in the north as well as the Dinggye and Kharta N-S trending detachments in the east and west, respectively, is a rare place where N-S and E-W extensions interacted and thus contains key information to detect the evolution of Himalayan extensional structures. Aiming at fully reconstructing the exhumation process of the ADM and figuring out the mid-Miocene evolution of Himalayan extensional structures, this contribution provides detailed field investigations and geochronological, petrological, as well as mineral compositional studies to constrain the activity of the detachments surrounding the ADM, the metamorphic process, and the thermal state of the ADM. The geochronological results suggest that both the STDS and the Dinggye detachment were active at 13 Ma. Unlike the Dinggye detachment, whose ductile deformation terminated at ~10 Ma, the Kharta detachment maintained its ductile deformation until at least 8.5 Ma. Geochronological, petrological, and mineral compositional analyses show that the ADM paragneiss underwent muscovite dehydration melting and garnet breakdown from at least 20 Ma to 12.5 Ma, indicating a long-term exhumation of the ADM. Importantly, biotite dehydration melting and garnet growth in the ADM orthogneiss suggest a heating event in the ADM at 15–13 Ma. Based on the temporal relationship of different extensional structures, the exhumation of the ADM was dominated by the STDS before 15 Ma, and subsequently by the E-W extension of the Dinggye and Kharta detachments since 15–13 Ma. The E-W extension led to significant uplift and melting of the ADM gneisses, which made the STDS reactivate as a passive stretching detachment after its ~15 Ma cessation. In the mid-Miocene E-W extensional regime, the ADM had exhumed asymmetrically with decreasing time and amount of the exhumation from its eastern side to the western side. Together with the heating event, the asymmetric exhumation indicates the lateral delamination of the subducted Indian lithosphere beneath the ADM in the middle Miocene. From a wider perspective, the mid-Miocene E-W extensions in the Himalayan Orogen were intrinsically controlled by the deep geodynamic process of the subducted Indian lithosphere and played a important role in the diachronous cessation of the STDS.

Sr-Nd-Hf Isotopic Disequilibrium During the Partial Melting of Metasediments: Insight From Himalayan Leucosome

Radiogenic isotopes of granitoids are widely applied to fingerprint the source of granitoids and study the magma mingling and assimilation processes, aiming to decipher the planetary differentiation. This weapon is based on the assumption that crustal melts inherit the radiogenic isotopes of protoliths. However, complicated melting processes in the crust would drive the radiogenic isotopes of melt away from the source, thus calling for a clear understanding of the behaviors of radiogenic isotopes during crustal melting. This study investigated the joint behavior of Rb-Sr, Sm-Nd, and Lu-Hf isotope systems during the melting of metasediments. Leucosome, schist, and leucogranite samples were collected from the Nyalam, South Tibet, where the leucosome was produced by muscovite dehydration melting of metapelites. Results show that the leucosome has a broad range of Sr-Nd-Hf isotopes (87Sr/86Sri: 0.763,48–0.875321, εNd(t): 14.6–11.6, εHf(t): 15.0–4.4) that deviate from the metasediments. We attributed it to the non-modal or disequilibrium melting of metapelites. The variation of Sr isotopes of leucosome is formed due to preferential entry into the melt of muscovite relative to plagioclase during melting. The changing 87Sr/86Sri and 87Rb/86Sr of leucosome define an errorchron at ∼ 500 Ma indicating an early Paleozoic tectono-thermal event supported by the U-Pb age of zircon cores (∼460 Ma) in the leucosome. The Nd isotopes of leucosome are mainly controlled by the preferential dissolution of apatite relative to monazite. The inadequate dissolution of zircon caused more depleted initial Hf isotopes compositions in the leucosome than the source. However, the leucosome with a higher crystallization temperature has radiogenic isotopic composition closer to the source. It indicates that the melting temperature greatly affects the isotope equilibrium between source and melt.

Temporal Separation of W and Sn Mineralization by Temperature-Controlled Incongruent Melting of a Single Protolith: Evidence from the Wangxianling Area, Nanling Region, South China

Abstract Tungsten and Sn display similar behavior during magmatic processes and are commonly associated spatially and genetically with highly evolved granites. Nonetheless, they typically form separate deposits, even if their associated granites have the same protolith. This separation may be due to the fractionation of the metals at the magmatic-hydrothermal transition or their differential mobility during partial melting of the metasedimentary protolith. If this separation occurred at the magmatic-hydrothermal transition, the ages of the W and Sn deposits would be very similar, whereas if it occurred during partial melting, the deposits are likely to have different ages because of the concentration of the metals in different magma batches and, in extreme cases, during different magmatic events. New age data from the Wangxianling ore field in the western part of the world-class Nanling W-Sn metallogenic province demonstrate that the W and Sn mineralization took place at different times. The W mineralization (219.5 ± 3.4 Ma) is related to Triassic granites (224.9–217.8 Ma), whereas the Sn mineralization is related to granites of Late Jurassic age (154.7 ± 1.1 Ma). This difference in ages rules out fractionation at the magmatic-hydrothermal transition as an explanation for the spatial separation of the W and Sn deposits and implies that the separation was due to differences in the mobility of W and Sn during partial melting. Both suites of granite originated from the partial melting of the same metasedimentary rocks, and both are reduced and highly evolved. The W granites, however, have a lower zircon saturation temperature (~750°C) than the Sn granites (~800°C), which indicates that the magma forming the W granites was mainly the product of muscovite-dehydration melting, whereas that forming the Sn granites was largely the result of biotite-dehydration melting. The different melting paths indicate that W released during muscovite breakdown dissolved in the magma, whereas Sn was sequestered by restite biotite. At the higher melting temperature, the residual W and Sn, released during the subsequent breakdown of biotite, dissolved in the magma. Thus, the magma that generated at low temperature was enriched in W, leading to subsequent W mineralization, whereas the magma that generated at high temperature was enriched in Sn and produced an Sn-mineralized granite. The whole-rock Sr-Nd isotope data for the Triassic W granites plot in the compositional field of the regional basement rocks and are consistent with partial melting of an orogenically thickened crust by internal heating in a collisional setting. In contrast, the Sr-Nd isotope data for the Late Jurassic Sn(-W) granites are displaced toward a mantle composition, likely reflecting contributions from mantle-derived material. Given the emplacement of many of the Late Jurassic Sn(-W) granites close to the Chenzhou-Linwu fault, we propose that this structure was the focus of decompression melting of the mantle and the injection of mantle-derived melts into the crust during the Late Jurassic, which supplied the additional heat for the melting at higher temperature needed to generate magmas enriched in Sn. This model, which is based on differences in the behavior of Sn and W during crustal melting, is potentially applicable to other Sn-W metallogenic provinces where Sn and W deposits are temporally separated.

Diverse Anatexis in the Main Central Thrust Zone, Eastern Nepal: Implications for Melt Evolution and Exhumation Process of the Himalaya

Abstract Sitting between the Greater Himalayan sequence (GHS) and Lesser Himalayan sequence (LHS), the Main Central Thrust zone (MCTZ) has experienced multiple episodes of anatexis, which presents an opportunity to explore the nature of partial melting and its response to Himalayan orogenic processes. A series of deformed rocks, including migmatites, gneisses, and leucosomes were collected across the MCT at Arun Valley, eastern Nepal. We investigated the bulk rock major and trace elements, Sr-Nd isotopes, mineral chemistry, zircon geochronology and Hf isotopes, and conducted phase equilibria modeling. The protolith boundary between the GHS and LHS is recognized on the basis of Sr–Nd isotopes with εNd(0) of −16.7 to −8.0 for the GHS and −31.2 to −23.9 for the LHS. Samples from both the GHS and LHS have undergone partial melting, as revealed by in situ leucosomes at outcrops and melt inclusions at thin-section scale. Leucosomes separated from their host rocks are divided into four groups: those derived from hydration melting, muscovite dehydration melting, amphibole dehydration melting, and feldspar accumulation. Phase equilibria modeling results for the GHS migmatite show isothermal decompression from peak P–T conditions of 11 kbar and 795°C, accompanied by muscovite dehydration melting evolving into biotite dehydration melting. In contrast, rocks from the LHS are modeled to have undergone hydration melting at P–T conditions of 9 kbar and 685°C. Zircon U–Pb geochronology suggests that long-lived partial melting (35–13 Ma) occurred in the MCTZ. Moreover, anatectic zircon Hf isotopes show that the protoliths for partial melting changed from the GHS to the LHS with εHf(t) of −19.4 to −5.7 during the early Miocene, and lower values of −42.5 to −16.7 during the middle to late Miocene. These zircon geochemical results indicate that hydrous metasediments from the LHS were progressively accreted to the base of the GHS, resulting in hydration melting of both the GHS and LHS assisted by MCT. The timing of activity of the MCT is constrained to 25–13 Ma, coeval with movement of the South Tibetan detachment system. Integration of petrogenetic modeling, the chronology of partial melting, and metamorphic P–T paths allows us to propose that thickened Himalayan crust was heated from the middle to late Eocene, and widespread anatexis occurred during the Oligocene to middle Miocene, forming a large-scale melt channel. The hot GHS channel flow moved upward in association with the synchronous activity of the MCT system, triggered intense dehydration of LHS metasediments, resulting in fluid-present melting in both the GHS and LHS during middle to late Miocene, and the formation of leucogranite with mixture features of GHS and LHS. Furthermore, with the cooling of the melt channel, duplexing has gradually operated since the middle to late Miocene in the shallow crust.

Crustal Melting Evidence in Migmatites of Higher Himalayan Crystallines (HHC) along Bhagirathi, Dhauliganga Valleys, and Sikkim Himalaya, India

Abstract An attempt has been made to evaluate crustal melting evidence through textural studies within a narrow zone of migmatite present in NW Himalaya along Bhagirathi and Dhauliganga valleys and two zones close to MCT and throughout within in the Sikkim Himalayas, NE Himalaya. It appears that partial melting was initiated by muscovite dehydration melting with a positive volume change driving melt segregation and discontinuous crystallization of peritectic biotite in the leucosome. Further, during retrogression due to cooling, a certain amount of melt was consumed. The occurrence of isolated pseudomorphosed melt pockets and lack of euhedral magmatic flow textured feldspar further indicates that the melt fraction was low for the alignment of crystals. The processes of melt segregation and migration could have been limited. The migmatite leucosomes and a small volume of in situ tourmaline-bearing leucogranite along extensional crenulation cleavages and melt pods indicate water-saturated melting of pelitic metasedimentary rocks. The formation of migmatites happened at around 46 Ma, corresponding to a peak metamorphic event due to collisional tectonics of the Himalayan orogeny. The presence of feeder dikes for main tourmaline-bearing leucogranite indicates that the source for the main body could be migmatite which is also supported by the similarity in REE patterns of the main body and in situ tourmaline-bearing leucogranite.

The role of phyllosilicate partial melting in segregating tungsten and tin deposits in W-Sn metallogenic provinces

AbstractMost tungsten (W) and tin (Sn) deposits are associated with highly evolved granites derived from the anatexis of metasedimentary rocks. They are commonly separated in both space and time, and in the rare cases where the W and Sn mineralization are part of a single deposit, the two metals are temporally separate. The factors controlling this behavior, however, are not well understood. Our compilation of whole-rock geochemical data for W- and Sn-related granites in major W-Sn metallogenic belts shows that the Sn-related granites are generally the products of higher-temperature partial melting (~800 °C) than the W-related granites (~750 °C). Thermodynamic modeling of partial melting and metal partitioning shows that W is incorporated into the magma formed during low-temperature muscovite-dehydration melting, whereas most of the Sn is released into the magma at a higher temperature during biotite-dehydration melting; the Sn of the magma may be increased significantly if melt is extracted prior to biotite melting. At the same degree of partial melting, the concentrations of the two metals in the partial melt are controlled by their concentration in the protolith. Thus, the nature of the protolith and the melting temperature and subsequent evolution of the magma all influence the metallogenic potential of a magma and, in combination, helped control the spatial and temporal segregation of W and Sn deposits in all major W-Sn metallogenic belts.