Dehydration Melting Research Articles

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.

Response of trace elements to partial melting of felsic crust at high to ultrahigh temperatures: Implications for granite geochemistry

It is known that partial melting and melt extraction of crustal rocks result in chemical differentiation of the continental crust. But it is unknown how these two processes have affected the composition of granites due to a limited knowledge of trace element behaviors during crustal anatexis. In order to quantify this issue, a combined study of whole-rock and mineral trace elements and mineral modal proportions was conducted for high-temperature (HT) to ultrahigh-temperature (UHT) felsic granulites from the Tongbai orogen in central China. Reconstruction of LREE budgets suggests that LREE mainly reside in monazite in both HT and UHT granulites. As monazite is predicted to be not stable at UHT conditions in the modelling of previous studies, the presence of monazite in the Tongbai UHT granulites suggest that it was not sufficiently dissolved into anatectic melts. Considering monazite mainly occurs as interstitial grains, fast melt-residue separation may be the dominant factor for impeding its dissolution. Calculation of mineral/mineral trace element ratios indicates that K-feldspar has a higher capacity to accommodate Ba and Sr than biotite and plagioclase, with DBaK-feldspar/biotite of 2.5–3.0 and DSrK-feldspar/plagioclase values of 1.4–1.9, and ilmenite can preserve more Nb and Ta than biotite with DNbbiotite/ilmenite values of 0.07–0.1 and DTabiotite/ilmenite values of 0.04–0.05. In addition, partitioning of first row transition elements (FRTE) was constrained in the felsic granulites, with preference of Sc into garnet, and Co, V, Ni, and Cr into biotite. Modelling based on the presently constrained partition coefficients indicates that trace elements show consistent differentiation trends during anatexis of various felsic rocks, with enrichment of Sr, Ba, Nb, Ta and FRTE but depletion of Rb in anatectic restites. The high DNb/Ta values of biotite and ilmenite cause the residues produced by biotite dehydration melting to be commonly of higher Nb/Ta ratios than protoliths, indicating that the residues are potentially high Nb/Ta reservoirs. In contrast, the change of peritectic minerals due to various protolith compositions and melting conditions results in differential responses of Sr, Ba, and FRTE to partial melting, suggesting that these elements may be useful indicators to trace the petrogenetic process of granites. The correlation between Ba and Sc contents or between Sr/Ba and Sc/Co ratios is demonstrated to be a valid index in discriminating melts from metagreywackes and metapelites due to different modes of garnet, orthopyroxene, and K-feldspar in their anatectic residues. Moreover, as the proportions of garnet and orthopyroxene in the residues are significantly influenced by melting pressures, the Sc/Co ratio and Sc content of granites can be used to trace their anatectic depth.

The Role of Fluids in Melting the Continental Crust and Generating Granitoids: An Overview

Granite is a distinctive constituent part of the continental crust on Earth, the formation and evolution of which have long been hot research topics. In this paper, we reviewed the partial melting processes of crustal rocks without or with fluid assistance and summarized the role of fluids and volatiles involved in the formation of granitic melts. As a conventional model, granitoids were thought to be derived from the dehydration melting of hydrous minerals in crustal basement metamorphic rocks in the absence of external fluids. However, the external-fluid-assisted melting of crustal metamorphic rocks has recently been proposed to produce granitoids as extensive fluids could be active in the deep continental crust, especially in the subduction zones. It has been demonstrated experimentally that H2O plays a crucial role in the partial melting of crustal rocks, in which H2O can (1) significantly lower the solidus temperature of the melted rocks to facilitate partial melting; (2) affect the melting reaction process, mineral stability, and the composition of melt; and (3) help the melt to separate more easily from the source area and aggregate to form a large-scale magma chamber. More importantly, dissolved volatiles and salts in the crustal fluids could also lower the solidus temperature of rocks, affect the partitioning behaviors of trace elements between minerals and melts, and facilitate the formation of some distinctive granitoids (e.g., B-rich, F-rich, and high-K granitoids). Furthermore, various volatiles dissolved in fluids could result in elemental or isotopic fractionation as well as the diversity of mineralization during fluid-assisted melting. In-depth studies regarding the fluid-assisted partial melting of crustal rocks will facilitate a more comprehensive understanding of melting of the Earth’s crust, thus providing strong theoretical constraints on the genesis and mineralization of granitoids as well as the formation and evolution of the continental crust.

Petrogenesis and Tectonic Implications of the Neoproterozoic Peraluminous Granitic Rocks from the Tianshui Area, Western Margin of the North Qinling Terrane, China: Evidence from Whole-Rock Geochemistry and Zircon U–Pb–Hf–O Isotopes

The source and petrogenesis of peraluminous granitic rocks in orogenic belts can provide insights into the evolution, architecture, and composition of continental crust. Neoproterozoic peraluminous granitic rocks are sporadically exposed in the Tianshui area of the western margin of the North Qinling Terrane (NQT), China. However, the source, petrogenesis, and tectonic setting of these rocks still remain unclear, which limits our understanding of the Precambrian tectonic and crustal evolution of the Qinling Orogenic Belt (QOB). Here, we determined the whole-rock geochemical compositions and in situ zircon U–Pb ages, trace-element contents, and Hf–O isotopic compositions of a series of peraluminous granitic mylonites and granitic gneisses in the Tianshui area at the west end of North Qinling. Zircon U–Pb dating revealed that the protoliths of the studied granitic mylonites and granitic gneisses crystallized at 936–921 Ma. The granitic rocks displayed high A/CNK values (1.12–1.34) and were enriched in large-ion lithophile elements (e.g., Rb, Ba, Th, U, and K) and light rare earth elements, and they were depleted of high-field-strength elements (e.g., Nb, Ta, and Ti). These rocks showed variable zircon εHf(t) (−12.2 / 9.7) and δ18O (3.56‰ / 11.07‰) values, suggesting that they were derived from heterogeneous crustal sources comprising predominantly supracrustal sedimentary rocks and subordinate igneous rocks. In addition, the U–Pb–Hf isotopic compositions from the core domains of inherited zircons were similar to those of detrital zircons from the Qinling Group, suggesting that the Qinling Group was an important crustal source for the granitic rocks. The lithological and geochemical features of these granitic rocks indicate that they were generated by biotite dehydration melting of heterogeneous sources at lower crustal depths. Combining our results with those of previous studies, we suggest that the NQT underwent a tectonic transition from syn-collision to post-collision at 936–874 Ma in response to the assembly and breakup of the Rodinia supercontinent.

Tectonic control on the spatial distribution of Sn mineralization in the Gejiu Sn district, China

Late Cretaceous granitic rocks occur in the Gejiu ore district to the east and west of the N-S striking Gejiu Fault, whereas major Sn deposits are only known to occur to the east of the Gejiu Fault. Comparison of the whole-rock chemistry, the apatite trace-element chemistry, and zircon Hf and O isotope data of the various granites demonstrates that fertile granites occur to both sides of the Gejiu fault. The results demonstrate that the c. 83 Ma old granitic intrusions (i) have similar magma sources, which are dominated by metasedimentary rocks that had experienced intense chemical weathering, resulting in reduced melts, (ii) had similar melting conditions, i.e., high temperature biotite dehydration melting, and (iii) in part had experienced large extents of fractional crystallization. The most evolved granites to both sides of the Gejiu Fault have the characteristics typical of tin granites. Therefore, the absence of major deposits to the west of the Gejiu Fault is not due to the absence of fertile granites. The areas to the east and the west of the Gejiu Fault, however, have fundamentally different fault pattern, which indicates different orientation of the stress field to both sides of the Gejiu Fault at the time of the emplacement of the Cretaceous granites. Late Cretaceous dextral movement along the Ailaoshan Fault Zone resulted in a (trans)tensional setting in areas to the east of the Gejiu Fault and in a (trans)pressional setting to the west of that fault. We speculate that the tectonic setting influences the potential for mineralization because the Sn bearing fluids need efficient pathways to transporting metals from the roof zone of the batholith into the wall rocks. In contrast to regional compression, hydraulic fracturing in an overall extensional setting has the potential to develop efficient fluid pathways and, thus, may lead to major ore deposits. To the west of the Gejiu Fault, however, granite intrusions in an overall compressional setting are likely to develop no or only small mineralization.

The effect of crystal fractionation on the geochemical composition of syn-exhumation magmas: Implication for the formation of high δ56Fe granites in collisional orogens

Syn-exhumation magmatism in collisional orogens is an important process for crustal differentiation and crust-mantle interaction at convergent plate boundaries. It is intriguing to elucidate which factors control the geochemical composition of such magmatic products. To answer this question, a combined study of whole-rock major-trace elements and Nd-Fe isotopes, mineral O isotopes, and zircon U–Pb ages and trace elements was carried out for syn-exhumation granites from the Sulu ultrahigh-pressure (UHP) metamorphic belt in east-central China. Several granitic plutons of Triassic age were newly found in this typical collisional orogen, with a total exposure area of ∼10 km2, suggesting large-scale partial melting of the deeply subducted continental crust in the terminal stage of continental collision. Field observations indicate that the massive granites show little deformation. The granites are high-K calc-alkaline and exhibit enrichment in LILE and LREE but depletion in HFSE relative to HREE in trace element distribution patterns. Zircons in these granites contain relict magmatic cores characterized by steep HREE patterns with strong negative Eu anomalies. These cores show middle Neoproterozoic U-Pb ages of 724–779 Ma, consistent with the protolith age of UHP metaigneous rocks in the Dabie-Sulu orogenic belt. Zircon rims exhibit oscillatory zoning, and are characterized by Triassic U-Pb ages of 210–216 Ma, low LREE contents and steep HREE patterns with significant negative Eu anomalies, suggesting their growth from granitic magmas during exhumation of the deeply subducted continental crust. The granites have low δ18O values of 2.0–6.1‰ for quartz, 1.1–5.2‰ for whole-rock, and 0.9–3.2‰ for zircon cores. The low δ18O values and Neoproterozoic U-Pb ages of zircon cores in the granites are characteristic feature of the deeply subducted continental crust in the northern margin of the South China Block. One coesite inclusion was found in the anatectic zircon rim, pointing to the deep origin of anatectic melts at a subarc depth of >80 km. Whole-rock SiO2 contents are correlated with major and trace elements, which are ascribed to crystal fractionation of mainly biotite and plagioclase during magma evolution. In particular, these granites exhibit highly variable δ56Fe values from 0.05 to 0.30‰, which are correlated with Fe3+/ΣFe, SiO2, Al2O3, Nb/Ta and Eu/Eu*. This suggests that the crystal fractionation of biotite would have controlled the Fe isotope variation in the granites. Therefore, the syn-exhumation granites experienced significant geochemical differentiation mainly through the fractional crystallization during the magma evolution. A compilation of syn-exhumation granites in the typical collisional orogens of the world shows that such granites were mainly formed through dehydration melting of hydrous minerals during decompressional exhumation. They fall into the magnesian group with relatively low REE and HFSE (like Ti, Nb and Zr) contents, distinct from the geochemical compositions of A-type granites. Therefore, syn-exhumation granites record crustal differentiation in the terminal stage of continental collision.

Whole-rock geochemistry and zircon O-Hf isotope compositions of ca. 2.35 Ga strongly peraluminous granites: Implications for increase in zircon δ18O values during the Paleoproterozoic

Zircon oxygen isotope ratios have been used to trace the incorporation of sedimentary rocks into magmas. The dramatic increase in maximum zircon δ18O values in the Paleoproterozoic observed in global databases coincides with changes in surface environments (e.g., the rise of subaerial and oxidative weathering), implying a connection between elevated zircon δ18O and these changes. Zircon δ18O between 2.5 and 2.2 Ga, however, is relatively under-constrained owing to limited available data in this age range. To augment data from this critical time period and understand potential causes for the elevated zircon δ18O values, we report U-Pb zircon ages and δ18O values of zircon, as well as, whole-rock major and trace element geochemistry of Paleoproterozoic strongly peraluminous granites (SPGs) from the southwestern margin of the Yangtze Block (China). Our geochronological data demonstrate that these SPGs crystallized at ∼2.35 Ga and that inherited zircon with ages of 2428–2721 Ma are present in these granites, indicating the source rocks of these granites were deposited, subsequently metamorphosed, and partially melted between 2.43 and 2.35 Ga. Synmagmatic zircon from samples dated in this study have εHf(t) values of −6.4 to −0.9 and high δ18O values of 7.6–9.9‰, elevated above the maximum value observed in Archean zircon (∼7‰). These granites can be divided into two groups based on whole-rock geochemistry. Both Group 1 and Group 2 granites were derived from a similar high δ18O, metapelitic source, but were generated by dehydration melting and hydrous melting, respectively. Our results demonstrate that the fine-grained sedimentary rocks from which the SPGs were derived had relatively high δ18O (as compared to older sedimentary rocks) by 2.43–2.35 Ga. The depositional time interval of the high-δ18O sedimentary sources for SPGs studied here coincides with the emergence of continental crust above sea level and the Great Oxidation Event. Supporting the findings of previous studies, the contemporaneity of our dataset with these changes in Earth’s surface environments suggests that subaerial and potentially oxidative weathering contributed (at least partially) to the elevation of δ18O of fine-grained sedimentary rocks. Recycling of these high-δ18O sedimentary rocks into magmas contributed to the dramatic change in δ18O of magmatic zircon in the earliest Paleoproterozoic. In addition, although this study is focused on a single locality, our results suggest that the abrupt shift observed in global zircon δ18O data sets likely occurred by 2.35 Ga. Last, a literature compilation of zircon δ18O data from SPGs suggested that zircon δ18O values may have also experienced a stepwise increase in the Neoproterozoic to Phanerozoic from 12 to 14‰. The coincidence of these increases in zircon δ18O values with global oxygenation events suggests that atmospheric oxygenation may have contributed to the increase in δ18O of sedimentary rocks.

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 (<1 wt%), FeOt content (<1 wt%), Rb content (67.0–402 ppm), Sr content (<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.

Nb and Ta intracrustal differentiation during granulite-facies metamorphism: Evidence from geochemical data of natural rocks and thermodynamic modeling

Abstract Both continental crust and depleted mantle are characterized by subchondritic Nb/Ta, leading to a mass imbalance when compared to the bulk Earth. Even though several potential high-Nb/Ta reservoirs in Earth’s core and undepleted mantle have been proposed, little attention has been given to those in the crust. Here we present bulk-rock and rutile geochemical data for samples from a lower crustal pelitic granulite, North China Craton, which exhibit systematic variation in their Nb and Ta contents. High-temperature (HT) and ultrahigh-temperature (UHT) granulite residues exhibit Nb/Ta ratios that are close to chondritic and subchondritic, respectively, whereas leucosomes from UHT granulite mostly have suprachondritic Nb/Ta. These variations are best explained via competition for Nb and Ta between biotite and rutile during metamorphism, although initial bulk-rock Nb/Ta values also have an effect. As biotite preferentially incorporates Nb over Ta, the early stages of biotite dehydration melting produce a high-Nb/Ta residue and a low-Nb/Ta melt; however, geochemical modeling suggests that once biotite is depleted, the Nb/Ta ratio of the system is instead controlled by rutile growth, which promotes the formation of a lower Nb/Ta residue and a higher Nb/Ta melt, even though the volume of melt produced at this stage may be small. We propose that in situ and in-source leucosomes and leucocractic veins in UHT terranes may retain a high-Nb/Ta geochemical signature. However, residual crustal-derived A2-type granites that experience significant fractionation of Nb- or Ta-bearing minerals during crystallization or contamination from other low-Nb/Ta sources cannot retain this high-Nb/Ta ratio, even though these ratios are generally higher than that of S-type granites. Anhydrous partial melting of metapelite can generate Nb-rich melts, such that high-temperature leucosomes, in addition to related A2-type granites, may represent significant Nb deposits.

Peritectic and phenocrystic garnet accumulation and the origin of strongly peraluminous granitic rocks: The Flagstaff Lake Igneous Complex, Maine

The Flagstaff Lake Igneous Complex of Maine contains a metaluminous phase that ranges from diorite to granite, and three phases of strongly peraluminous, garnet-bearing rocks with silica contents that range between 43 and 59 wt%. These peraluminous rocks have increasing amounts of normative corundum with increasing maficity, indicating that they do not represent liquid compositions, rather garnet accumulation accounts for their bulk-rock A/CNK compositions. Garnet in these rocks displays a variety of textures, inclusion patterns, and trace element compositions. The Loon Lake phase is characterized by garnet with abundant, randomly distributed apatite inclusions. Some grains have spiral inclusion patterns that are peritictic grains formed during partial melting of source rocks while they were undergoing differential stress. In contrast, the Quarry Phases A and B contain inclusion-free garnet with euhedral grain boundaries. These grains have trace element profiles indicative of crystal fractionation processes and are interpreted to be phenocrysts. Some of these phenocrysts have inclusion poor, resorbed cores that are mantled by inclusion-free overgrowths. The cores have lower concentrations of compatible trace elements and higher concentrations of incompatible elements than the mantles. These cores are resorbed phenocrysts from an evolved magma that were mingled in a more primitive, garnet crystallizing magma. Also present in Quarry Phase B rocks are a minor population of inclusion-rich garnets with random inclusion distributions. These are also interpreted as peritectic garnet, entrained from the source, or from dehydration melting of country rock biotite. They are mantled by inclusion-free garnet with the same composition as the phenocrysts and are phenocrystic overgrowths. Thus, the original Flagstaff Lake anatectic melts were mildly peraluminous with the current bulk-rock compositions of the garnet tonalites dominated by the accumulation of variable portions of phenocrystic garnet or the entrainment of peritictic garnet, both of which generated the increasing peraluminosity with increasing maficity of the complex. Magmatic, mafic enclaves present in the peraluminous rocks are similar to the metaluminous, main phase rocks of the complex, suggesting that the coeval basaltic magmas provided the heat for anataxis to generate the peraluminous magmas.

Polyphase growth history of peritectic garnet from a granite: Trace-element zonation, Lu-Hf ages and their significance for the duration of granite-forming processes

Determining the ages and duration of high-grade metamorphic episodes during orogenesis and concomitant melt extraction events is pivotal in reconstructing and quantifying the chemical and petrological evolution of the Earth's continental crust over time. Major and trace elements combined with multichronometric garnet data from the Pan-African Damara belt of Namibia place absolute time constraints on high-grade regional metamorphism and melting in the lower crust that led to the production of widespread granites emplaced in the upper crust. Some of these granites contain inherited peritectic garnet with inferred igneous rims that record their genesis and evolution from formation by partially melting in the deep crust to the emplacement of their host rock at shallower levels. Particular characteristics of these garnets are textural and chemical disequilibrium features such as resorption, local development of partly euhedral crystal faces and occurrence of quartz-feldspar stringers in the rim of the garnet and trace element abundances with sharp concentration gradients. These features document a previous melting event followed by garnet crystallization in an ascending melt. With the exception of the outermost rim, which records late-stage diffusional features, the garnet crystals have constant Mg and Mn concentrations. In contrast, Ca shows pronounced chemical zoning. Trace elements (HREE, LREE, Zr, Sr, Y) are depleted in the core and enriched in the rim region indicating disequilibrium conditions during garnet growth. A pseudosection calculated with the garnet rim composition and the composition of the host granite yields P-T conditions of 0.62 GPa and 760 °C for the formation of the rim. This result is consistent with a P-T estimate of 0.68 GPa and 760 °C derived from the conventional garnet-biotite thermobarometer. Regional metamorphic conditions were above the cordierite-andalusite stability field with estimated conditions of ~550-600 °C and 0.4–0.5 GPa. This P-T regime indicates that the garnet originally crystallized at deep crustal levels. Diffusion modelling suggests that the observed concentration gradient of Sm in garnet is consistent with a maximum duration of ca. 0.5–1.5 Ma for the inferred melting interval during peak metamorphism in the lower crust. High-precision multi-point Lu-Hf garnet-whole rock data yield dates of 522.0 ± 1.0 and 522.3 ± 0.9 Ma for two samples. Because of presumed short intervals between melting in the lower crust and intrusion of the granite, these dates are interpreted as the age of the melting event during high-grade metamorphism and are temporally indistinguishable from the crystallization interval. Thus, dehydration melting in the deeper crust during peak metamorphism produced peritectic garnet that was incorporated into the granitic melt followed by subsequent new growth of igneous garnet around peritectic cores during intrusion. Previously obtained Sm-Nd garnet whole rock ages are 20–25 Ma younger and likely represent cooling ages. This study shows that granites enriched in mafic components such as garnet and biotite may intrude as mixtures of melt and the peritectic assemblage produced by incongruent melting involving dehydration-melting of biotite in the lower crust. Melting episodes represent short-lived processes relative to the absolute duration of orogenic cycles.

Boron isotopes of tourmalines from the central Himalaya: Implications for fluid activity and anatexis in the Himalayan orogen

The relationship between fluid activity and anatexis in the Himalaya is ambiguous in regard to whether it is dominated by fluid-saturated melting triggered by Lesser Himalayan Sequence (LHS) fluids or dehydration melting of Greater Himalayan Sequence (GHS) metasedimentary rocks. Boron isotopes are a sensitive probe for tracing the source of fluids and their role during partial melting. However, a lack of constraints on LHS fluid activity has limited our understanding of anatectic processes in the Himalayan orogen. Here we report the results of in situ tourmaline and muscovite B isotope analyses of a suite of samples from the LHS and GHS, including chlorite schist from the LHS, quartz–tourmaline rock from the Main Central Thrust Zone, and leucogranites, leucosomes, mica schist, and garnet amphibolite from the GHS. Tourmaline B isotopic compositions fall in the range of δ11B = −18.3‰ to −8.2‰, with tourmalines from leucogranites showing δ11B values of −17.2‰ to −8.2‰, from leucosomes of −11.8‰ to −10.7‰, from schists of −17.8‰ to −13.9‰, from quartz–tourmaline rock of −18.3‰ to −12.8‰, and from garnet amphibolite of −16.9‰ to −13.6‰. Muscovites from leucogranite and mica schist yield relatively low B isotopes of −23.3‰ to −17.2‰ and −22.4‰ to −15.4‰, respectively. Different source compositions, prograde devolatilization, tourmaline fractionation, fluid exsolution, and external fluid metasomatism are inferred to be the main mechanisms responsible for the varied B isotopes in this study. Our data suggest that the LHS is characterized by depleted 11B, with LHS-derived dehydration fluids calculated to have δ11B values of −15.2‰ to −12.0‰ at 600 °C, whereas metamorphic fluids in the GHS have δ11B values of −11.8‰ to −4.8‰. Thus, we propose that dehydration of LHS hydrous minerals produced relatively light-B fluids. Consequently, leucogranites derived from hydration melting triggered by LHS fluids exhibit light B isotopic compositions compared with those derived from dehydration melting in the GHS. To investigate the source of heavy B in GHS leucogranites, we modeled B isotopic fractionation during muscovite prograde devolatilization, with results showing that substantial fractionation (∆11B > 7‰) would occur during muscovite dehydration. Hence, B lost from micas could be a source of heavy B in the leucogranites. On the basis of these results, we propose a model whereby LHS hydrous metasediments underwent intense dehydration, with the resultant fluids migrating upward and triggering hydration melting on the edge of melt channel in the GHS. Hence, tourmalines that crystallized from the hydrated magmas inherited the low δ11B characteristic of the LHS fluids, whereas those derived from dehydration melting of micas in the interior of melt channel acquired higher δ11B values.

Data for paper entitled "Origin of the high conductivity anomalies in the mid-lower crust of the Tibetan Plateau: Dehydration melting of garnet amphibolites"

Data for paper entitled "Origin of the high conductivity anomalies in the mid-lower crust of the Tibetan Plateau: Dehydration melting of garnet amphibolites"

Barium isotope fractionation during dehydration melting of the subducting oceanic crust: Geochemical evidence from OIB-like continental basalts

Continental basalts commonly show ocean island basalts (OIB)-like geochemical compositions, and their origin is geochemically linked to metasomatism of the big mantle wedge by the deeply subducting oceanic slab at postarc depths of >200 km. This linkage is tested by a combined study of Ba isotopes with major-trace elements and Sr–Nd isotopes in Cenozoic continental basalts from eastern China. These basalts show OIB-like trace element distribution patterns and depleted to weakly enriched Sr–Nd isotope compositions, suggesting incorporation of the dehydrated slab component into their mantle sources. Their δ138Ba values vary from −0.05‰ to +0.06‰, which are lower than the depleted mid-ocean ridge basalts (MORB) mantle but similar to island arc basalts, OIB and OIB-like continental flood basalts. The variation of δ138Ba values cannot be ascribed to fractional crystallization and crustal contamination of mantle-derived magmas, but it results from mixing of the depleted mantle with the recycled crustal material of low δ138Ba values. In other words, the geochemical deviations from the depleted MORB mantle are attributable to the source mixing at mantle depths rather than the crustal contamination at crustal depths. Although the δ138Ba values of the dehydrated oceanic crust would be lower than the initially subducting crust based on the lower coordination number of Ba in liquid phases relative to solid phases in the crust, the low δ138Ba recycled crustal material including igneous oceanic crust (IOC) and seafloor sediment is still required to produce the low δ138Ba values for the studied basalts. The similarity in δ138Ba to island arc basalts not only indicates the similarly low δ138Ba values for the recycled crustal components in these basalts, but also suggests the limited Ba isotope fractionation during dehydration melting of the subducting oceanic crust at postarc depths.

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.

High‐pressure granulite facies re‐equilibration and zoisite–biotite dehydration melting during decompression of an ultrahigh‐pressure garnet clinopyroxenite from the island of Fjørtoft, Norway

AbstractA garnet clinopyroxenite from the island of Fjørtoft, Western Gneiss Region (WGR) in Norway, has experienced a complex metamorphic evolution demonstrated by studies involving petrographic observations, mineral chemistry, phase equilibria modelling and geothermobarometry. This rock, originally an eclogite, documents ultrahigh‐pressure (UHP) conditions higher than 3.5 GPa (Stage 1), witnessed by Ca‐rich garnet inclusions in kyanite. The UHP assemblage was pervasively re‐equilibrated at high‐pressure (HP) granulite facies conditions around 1.75 GPa and 870°C, during which and/or somewhat before Ca‐poor garnet, sodian diopside, zoisite and biotite formed (Stage 2a). Dehydration melting of zoisite and biotite (Stage 2b) took place at similar pressure–temperature conditions. The produced melt was poor in Si and rich in alkalis and crystallized mainly to analcime + plagioclase + K‐feldspar. Oriented composite inclusions of quartz + amphibole + phengite ± garnet, found in clinopyroxene, are interpreted to have resulted from interaction between a fluid (hydrous melt and/or aqueous fluid) and clinopyroxene at the early retrogression Stage 3. Stage 1 resulted from deep subduction of the rock. Stage 2 followed after early exhumation probably due to stagnation at a hot and thickened orogenic root. Consequently, other HP granulite facies metabasites, which are similar to the studied garnet clinopyroxenite and widespread in the northwestern WGR, could have been transformed from former eclogite at this stage, too. In addition, this study provides a direct link between the generation of peralkaline melts and crustal anatexis in collision zones and new insights into the retrograde formation of oriented mineral inclusions in HP‐UHP minerals.

Sanbe volcano: Long‐term evolution of an arc magmatic system

AbstractA suite of samples was studied that represents the major explosive eruptions of Sanbe volcano, SW Japan. We demonstrate how rate of magma flux into the Trans‐Crustal Magmatic System (TCMS) presents a major control on the type and style of the subsequently forthcoming eruptions. Erupted products can be separated into two distinct groups. An older group is characterized by highly evolved, high‐K, LILE‐rich rhyolitic magmas, showing a supressed adakitic trace element signature (otherwise characteristic for the young stratovolcanoes in the SW Japan arc) with low Ca, Sr concentrations and a negative Eu anomaly. In contrast, the younger group (dominantly of andesitic—dacitic composition) displays a strong adakitic trace element signature with characteristic steep REE profiles and high Sr concentrations. An Eu anomaly is generally lacking here. The two groups are also distinct in their petrographic features, with the early group being almost aphyric showing simple log linear crystal size distributions and homogeneous, uniform mineral chemistries. In contrast, products of the younger group show complex crystal size distributions with diverse mineral compositions and abundant disequilibrium features. Our study shows that an initial high melt‐production rate allowed dehydration melting of lower crustal rocks leading to the formation of highly evolved K‐rich magmas. These magmas intruded into the shallow crust and produced two large Plinian rhyolitic, caldera forming eruptions. Subsequently the primary magma production rate decreased and the lower crust became too refractory for additional dehydration melting by these lower volume magma batches, causing the conventional adakitic magmatism to produced several additional eruptions of smaller magnitude, mainly of Sub‐Plinian or Pelean styles.

Extremely fractionated magmas linked with W mineralization: Evidence from the LyangarW‐Modeposit, South Tianshan Orogenic Belt

AbstractTungsten (W) deposits are commonly related to the exsolution of magmatic‐hydrothermal fluids from high‐Si granites (SiO2 > 70%). However, whether the W‐related high‐Si granitic magma is produced via partial melting of metasedimentary source rocks or by high degree of fractional crystallization remains controversial. Here we present new geochronological and geochemical data on the intrusions associated with the Lyangar W‐Mo skarn deposit in the Southern Tianshan Orogenic Belt, Uzbekistan. Our new U–Pb zircon age data show that the major intrusion exposed in the region are ca. 280 Ma biotite gabbroic diorite and biotite granite and about 260 Ma porphyritic granite and muscovite porphyritic granite. The molybdenite grains in the skarn rocks and orebodies show weighted Re‐Os ages of 261.4 ± 7.8 Ma and 261.1 ± 3.8 Ma, respectively. In combination with the field contact, we confirm that the muscovite porphyritic granite is genetically related to the W mineralization. The gradual transition from the porphyritic granite to muscovite porphyritic granite, similar mineral assemblages and geochemical variations indicate that they are co‐magmatic, and that the porphyritic granite represents less evolved member. Rhyolite‐MELTS modeling further reinforces that the muscovite porphyritic granites can be produced by high degree of fractional crystallization (~33%, including ~1.2% biotite, ~27% plagioclase, ~2% alkali‐feldspar, ~0.21% Fe‐Ti oxides, and ~2.7% amphibole) of the porphyritic granite magma. On the basis of the positive ƐHf(t) values (+3.03 to +6.02), high‐SiO2contents and CIPW characters, the porphyritic granite is considered to have formed from dehydration melting at low pH2O of juvenile basaltic source rocks around 16 kbar and 850–1000°C. Our study demonstrates that extreme fractional crystallization of granitic magma plays a significant role in W enrichment in the granitic melt.