Partial Melting Of Metasedimentary Rocks Research Articles

Hf-Nd isotope decoupling in S-type granitoids caused by disequilibrium melting of metasedimentary rocks: Evidence from the North Qinling Terrane, Central China

The intrinsic assumption of radiogenic isotope tracing is that isotope systems maintain equilibrium between melt and source. However, this assumption is not naturally correct, particularly for S-type granites, which are formed by partial melting of metasedimentary rocks. The S-type metagranites from the North Qinling Terrane show various degrees of HfNd isotope decoupling and discrepancies between whole-rock and zircon Hf isotopes. Zircon UPb dating yields emplacement ages from 946 to 890 Ma. Their high δ18O values, (87Sr/86Sr)i ratios, and negative εNd(t) values overlapped with metasedimentary rocks in the Qinling Group imply that they are derived from partial melting of the Qinling Group. The low Rb/Ba, Rb/Sr, Al2O3/TiO2, and high CaO/Na2O ratios indicate that their sources are metagraywackes. The negative correlation between decoupling degrees of HfNd isotopes and Zr concentrations suggests that the incomplete dissolution of zircon during partial melting results in the decoupling. The discrepancy between whole-rock and zircon Hf isotopes can be attributed to the combined effect of incomplete dissolution of zircons and the release of various Hf isotope compositions from partly dissolved inherited zircons to local melts. The compiled literature data suggest that whole-rock HfNd isotope decoupling of most metagraywacke-derived S-type granites is caused by incomplete melting of zircons, whereas those of metapelite-derived S-type granites may be controlled by both incomplete melting of zircons and source inheritance.

Peraluminous leucogranites generation during different stages of the Brasiliano Orogeny in the Borborema Province, NE Brazil

The Mamanguape pluton comprises muscovite, biotite syeno-to monzogranite intruded in metasedimentary rocks. Geochemically, the studied samples of the Mamanguape pluton peraluminous granites are ferroan and alkali-calcic. Furthermore, trace element distribution diagrams are characterized by troughs at Ba, Nb, Sr, P and Ti. The REE patterns are fractionated showing (Ce/Yb)N ratios ranging from 12 to 15, and deep negative Eu anomalies, with Eu/Eu* ratios varying between 0.18 and 0.25. The geochemistry of the two-mica granites of the Mamanguape pluton shows similarities with peraluminous leucogranites from the Borborema Province with crystallization ages related to the contractional (640–600 Ma) and transcurrent (590–545 Ma) stages of the Brasiliano/Pan-African Orogeny. Peraluminous, two-mica and biotite-leucogranites were emplaced through all Borborema Province with ages within 640–600 Ma and 590–545 Ma intervals. Their generation is interpreted as the result of partial melting of metasedimentary rocks involving muscovite dehydration. The crystallization age (574 ± 2 Ma) of the Mamanguape two-mica granites suggest intrusion related to the transcurrent stage of the Brasiliano Orogeny. Zircon grains extracted from two samples of a two-mica leucogranites of the Jurema and Jupi Plutons, Pernambuco-Alagoas Domain, show a large variation of εHf(t) values (−45 to −5) and Hf TDM model age between 1.2 and 3.9 Ga. Archean Hf TDM model ages were recorded in Cryogenian/Ediacaran zircon grains as well as Paleoproterozoic grains, suggesting contribution of an Archean component in the source of the granites. One sample of the Mamanguape Pluton, Transversal subprovince, have εHf(t) values ranging between −13.7 and + 1.4 and Hf TDM model age between 1.4 and 2.2 Ga, suggesting that the magma was generated by partial melting of metasedimentary rocks that received a large contribution of Neoproterozoic and Paleoproterozoic igneous sources. Small amount of mantle melt contributing to the source of the two-mica granites is supported by the presence of mafic enclaves and positive εHf(t) values.

Europium anomalies in zircon: A signal of crustal depth?

Trace element concentrations and ratios in zircon provide important indicators of the petrological processes that operate in igneous and metamorphic systems. In granitoids, the compositions of zircon have been linked to the behaviour of garnet and plagioclase—pressure-sensitive minerals—in the source during partial melting. This has led to the proposal that Europium anomalies in detrital zircon are linked to the depth of crustal melting or magmatic differentiation and are a proxy for average crustal thickness. In addition to the mineral assemblage present during partial melting, Eu anomalies in zircon are also sensitive to redox conditions as well as magma evolution during extraction, ascent, and emplacement. Here we quantitatively model how rock type, mineral assemblages, redox changes, and reaction sequences influence Eu anomalies of zircon in equilibrium with silicate melt. Partial melting of metasedimentary rocks and metabasites yields felsic to intermediate melts with a large range of Eu anomalies, which do not correlate simply with pressure (i.e. depth) of melting. Europium anomalies of zircon associated with partial melting of metasedimentary rocks are most sensitive to temperature whereas Eu anomalies associated with metabasite melting are controlled by plagioclase proportion—a function of pressure, temperature, and rock composition—as well as changes in oxygen fugacity. Furthermore, magmatic crystallization of granitoids can increase or decrease Eu anomalies in zircon from those of the initial (anatectic) melt. Therefore, Eu anomalies in zircon should not be used as a proxy for the crustal thickness or depth of melting but can be used to track the complex processes of metamorphism, partial melting, and magmatic differentiation in modern and ancient systems. Secular changes of Eu/Eu* from the zircon archive may reflect a change in thermal gradients of crustal melting or an increase in the reworking of sedimentary rocks over time.

Petrogenesis of Late Cretaceous Muscovite-Bearing Peraluminous Granites in the Youjiang Basin, South China Block: Implications for Tin Mineralization

Most primary Sn deposits worldwide are associated with muscovite-bearing peraluminous granites, commonly believed to originate from the partial melting of metasedimentary rocks. We studied the whole-rock geochemistry and Sm–Nd isotopes of Late Cretaceous (~90 Ma) Laojunshan muscovite-bearing peraluminous granites in the Youjiang Basin, South China Block. The globally significant Dulong tin mineralization was co-genetic with the Laojunshan muscovite-bearing monzogranites. The Laojunshan granites exhibit slightly higher εNd(t) values than the Precambrian basement, indicating a hybrid crustal source comprising both Precambrian rock and juvenile components. Characterized by weakly peraluminous compositions, these granites display highly evolved geochemical features: notably low levels of Ca, P, Mg, Fe, and Ti contents, elevated Si content, a high FeOT/MgO ratio, and a low Zr/Hf ratio. These distinctive geochemical features can be attributed to the differentiation of plagioclase, biotite, and zircons, with the remarkably low Nb/Ta and K/Rb ratios further suggesting a fluid exsolution process. The geochemical data propose that tin-enriched Laojunshan granites originate from mineral differentiation and fluid exsolution of crust-derived melts during magmatic evolution. By integrating these novel findings with existing data on coeval muscovite-bearing granites co-genetic with tin mineralization in the Youjiang Basin, it is deduced that these granites share a unified origin. Their genesis can be attributed to mineral differentiation and fluid exsolution of crust-derived melts rather than a direct melting of metasedimentary rocks.

Petrogenesis of Paleozoic–Early Mesozoic Granites and Pegmatites in the Yuechengling Pluton of South China

Abstract Although pegmatites are volumetrically minor in the upper continental crust, these rocks host abundant rare metal deposits (e.g., Li, Be, Rb, Ta, and Nb). Pegmatites can be formed either by extensive fractional crystallization of granitic magmas or by low-degree partial melting of metasedimentary rocks. The Mao’ershan–Yuechengling composite batholith in the Nanling Range in the South China Block (SCB) is of early Paleozoic–Triassic age (440–381 and 236–204 Ma, respectively). Recently, hundreds of pegmatites associated with Nb, Ta, Be, Rb, and Li mineralization have been identified in this batholith. These pegmatites are hosted by granitic wall rocks. However, the relationships between the pegmatites and granitic wall rocks are not well constrained. To address this, we investigated the Mao’antang (MAT) and Tongzuo (TZ) pegmatites and their biotite granite wall rocks in the middle part of the Yuechengling pluton. Laser ablation inductively coupled plasma mass spectrometry zircon U–Pb ages revealed that the MAT pegmatites formed during the Permian (269 Ma) and Triassic (231 Ma) and that the MAT biotite granite wall rock records two stages of magmatic activity (271 and 231 Ma) that are coeval with the pegmatites. The TZ pegmatites probably formed during the Triassic (235 Ma), and the TZ biotite granite wall rock formed during the Silurian (435 Ma). The MAT biotite granite and pegmatites (εNdt=−12.0 to −10.6; εHft=−8.0 to −1.0), TZ pegmatites (εNdt=−10.4 to −6.1), and TZ biotite granites (εNdt=−9.1 to −8.7; εHft=−7.7 to −4.1) have enriched whole-rock Nd and zircon Hf isotopic compositions that are similar to those of early Paleozoic and Triassic S-type granites in the SCB. In addition, the whole-rock Pb isotopic compositions of the MAT and TZ pegmatites and granites are distributed along the upper crust evolution line. We suggest that the MAT and TZ biotite granites were mainly derived from Paleoproterozoic metasedimentary rocks in the middle crust. The MAT pegmatites are fractional crystallization products of the MAT biotite granites, whereas the TZ pegmatites were formed by fractional crystallization of hidden parental S-type granites. We propose that the MAT and TZ pegmatites have potential for rare metal (Nb, Ta, Be, and Li) mineralization, as they record high degrees of fractional crystallization. The MAT and TZ areas in the middle of the Yuechengling pluton are promising targets for rare metal exploration.

Late Cretaceous Nature of SW Borneo and Paleo-Pacific Subduction: New Insights from the Granitoids in the Schwaner Mountains

Abstract Cretaceous igneous rocks in SW Borneo are significant for investigating the Paleo-Pacific subduction and regional correlation. This paper presents new zircon ages and Sr–Nd–Pb–Hf–O isotopic and whole-rock geochemical data for the Cretaceous granitoids in the Schwaner Mountains. These granitoid samples yield formation ages of 92–80 Ma and can be geochemically subdivided into Group 1 biotite granite and granodiorite and Group 2 monzogranite. Group 1 has higher εHf t values of +1.2–+14.5 and lower δ18O values of 5.2–6.6‰ than Group 2 with εHf t and δ18O values of −1.5–+5.9 and 6.1–6.6‰, respectively. Group 1 samples show variable εNd t values of −1.9–+1.7, (206Pb/204Pb)i ratios of 18.63–19.29, (207Pb/204Pb)i ratios of 15.62–15.69, and (208Pb/204Pb)i ratios of 38.67–39.49 and are the products of partial melting of newly underplated mafic rocks with a component of crustal rocks. Group 2 with slightly negative εNd t values of −1.1–−1.5 was originated from partial melting of metasedimentary rocks with a juvenile mafic component. These granitoids were formed in a continental arc setting related to the Paleo-Pacific subduction during the Late Cretaceous and can compare with those contemporaneous I-type granites along the Kuching zone in NW Borneo. Those Late Cretaceous granitoids with A-type affinities from the easternmost Peninsular Malaysia and Singapore to the east of Sundaland might be formed in a back-arc extensional setting after the Paleo-Pacific subduction.

Contrasting styles of peraluminous S-type and I-type granitic magmatism: Identification and implications for the accretionary history of the Chinese South Tianshan

Peraluminous granitoids have aluminum saturation indices (A/CNK) higher than 1.0, which overlap to some extent between S- and I-type granitoids. However, their source and petrogenesis are still disputed. For example, whole-rock compositions alone are not always a valid way to discriminate the sources of peraluminous granitoids. To identify the geochemical affinities, source and petrogenesis of the peraluminous granitoids, we present new geochemical data, <i>in situ</i> zircon U-Pb ages and Hf-O isotopic data, and whole-rock Sr-Nd isotopic data for the peraluminous granitoids in the South Tianshan Orogen Belt (STOB), Northwesten China. Zircon U-Pb ages suggest that these peraluminous granitoids were emplaced in the latest Carboniferous (<i>ca.</i> 299 Ma). They contain the diagnostic mineral muscovite and have high δ¹⁸O_Zrn values (&gt;8.0 ‰) demonstrating a close affinity with S-type granitoids. Their low εNd(t) values (−5.3 to −7.6), combined with variable zircon εHf(t) values (−0.35 to −10.18), indicate that these S-type granitoids were likely derived from partial melting of metasedimentary rocks. In addition, inherited zircon cores from the S-type granitoids have variable δ¹⁸O values (6.34–10.5 ‰) and zircon εHf(t) values (−4.3 to +6.3), with age populations (<i>ca</i>. 400 to 500 Ma) similar to those of detrital zircons from late Carboniferous metasedimentary rocks in the region. These data show that the S-type granitoids were dominantly derived from late Carboniferous metasedimentary rocks rather than Precambrian crustal materials. The studied granitoids have a transitional composition between I- and S-type granitoids, which could be related to low compositional maturity of the late Carboniferous metasedimentary source. According to the spatial and temporal distribution and petrogenesis of the Carboniferous intrusive rocks in the STOB, we propose that a slab roll-back model can account for the generation of late Carboniferous S-type granitoids in the STOB.

Mesoproterozoic magmatism redefines the tectonics and paleogeography of the SW Yangtze Block, China

The geodynamic setting of the Yangtze Block and its paleogeography in the reconstructed Rodinia supercontinent during the late Mesoproterozoic is loosely constrained due to the limited geological information reported. In this contribution, we identify a new suite of amphibolites in the SW Yangtze with an 1175 ± 5 Ma emplacement age. Geochemical modeling demonstrates they were derived from low-degree (3.5%–6.5%) partial melting of spinel lherzolite mantle. Additionally, three suites of late Mesoproterozoic granites in the SW Yangtze Block are revisited by providing new zircon U-Pb-Hf-O isotopes and whole-rock geochemical data. The granites have been dated to emplace during 1060–1042 Ma and have Zr + Nb + Y + Ce and Tzr values mostly at 304–576 ppm and 831–886 ℃, respectively, similar to A-type granites. Zircon Hf-O isotopic data indicate two subgroups of granites, in which one group has suprachondritic εHf(t) values of + 2.4 to + 11.4 with δ18O values of 6.5‰ to 7.4‰ while the other group has εHf (t) values of −3.9 to +1.5 with δ18O values of 10.0‰–11.6‰. The first group of granites was mainly derived from partial melting of mafic tholeiitic rocks while the second group of granites was mainly formed by partial melting of metasedimentary rocks. Integrated with published work, we propose that an intra-continental rift basin was developed in the SW Yangtze Block during ∼ 1.2–1.04 Ga, arguing against a late Mesoproterozoic orogenic event in the SW Yangtze Block. Statistical analyses based on compiled zircon U-Pb-Hf isotopes from Laurentia, Yangzte, and Amazonia show that the Yangtze has an isotopic affinity with west Nuna. These continents likely remained united during the transition from Nuna/Columbia to Rodina along the western periphery of the Rodinia supercontinent.

Preferential dissolution of uranium-rich zircon can bias the hafnium isotope compositions of granites

Abstract Hafnium (Hf) isotopes in zircon are important tracers of granite petrogenesis and continental crust evolution. However, zircon in granites generally shows large Hf isotope variations, and the reasons for this are debated. We applied U-Pb geochronology, trace-element, and Hf isotope analyses of zircon from the Miocene Himalayan granites to address this issue. Autocrystic zircon had εHf values (at 20 Ma) of–12.0 to–4.3 (median =–9). Inherited zircon yielded εHf values (at 20 Ma) of–34.8 to +0.3 (median =–13); the majority of εHf values were lower than those of autocrystic zircon. The εHf values of inherited zircon with high U concentrations resembled those of autocrystic zircon. Geochemical data indicates that the granites were generated during relatively low-temperature (&amp;lt;800 °C) partial melting of metasedimentary rocks, which, coupled with kinetic hindrance, may have led to the preferential dissolution of high-U zircon that could dissolve more efficiently into anatectic melt due to higher amounts of radiation damage. Consequently, Hf values of autocrystic zircon can be biased toward the values of U-rich zircon in the source. By contrast, literature data indicate that granites generated at high temperatures (&amp;gt;820–850 °C) generally contain autocrystic and inherited zircons with comparable Hf isotope values. During higher-temperature melting, indiscriminate dissolution of source zircon until saturation is reached will result in near-complete inheritance of Hf isotope ratios from the source. Our results impose an extra layer of complexity to interpretation of the zircon Hf isotope archive that is not currently considered.

Petrogenetic and tectonic interpretation of strongly peraluminous granitic rocks and their significance in the Archean rock record

Abstract Strongly peraluminous granitic rocks (SPG), defined by an aluminum saturation index ≥1.1, become abundant in the rock record in the Neoarchean. This study identifies three different varieties of Neoarchean SPG in the Archean Wyoming Province, U.S.A. These include calcic SPG, represented by the Webb Canyon Gneiss and Bitch Creek Gneiss of the Teton Range; calc-alkalic to alkali-calcic suites composed entirely of SPG, including the Rocky Ridge garnet granite gneiss of the northern Laramie Mountains and the Bear Mountain granite in the Black Hills; and calc-alkalic to alkali-calcic suites that include both weakly and strongly peraluminous granitic rocks, such as the Mount Owen batholith, Wyoming batholith, and Bears Ears granite. Although the petrogenesis of all the SPG suites involves partial melting of crustal sources, the composition of those sources, the melting conditions, and the tectonic settings vary. The calcic suites originate by dehydration melting or water excess melting of hornblende-plagioclase rocks at relatively high temperature. The suites composed entirely of SPG formed by partial melting of metasedimentary rocks by reactions involving muscovite at lower temperatures. Suites with both weakly and strongly peraluminous granite may form by partial melting of metasedimentary rocks by reactions involving biotite or by assimilation of aluminous melts of felsic crust by differentiated calc-alkalic magma. Most of the Wyoming SPG appear to have formed in collisional orogens, but SPG of the Wyoming batholith and Bears Ears granite are associated with continental arc magmatism. The appearance of SPG in the Neoarchean rock record marks the time when subduction enabled the formation of strong, thick, increasingly felsic continental crust, which in turn allowed the development of a mature, clastic sedimentary cover. Lateral movement of crustal blocks led to collisional orogeny, SPG magma genesis, and the formation of the first supercontinents.

Unraveling the link between mantle upwelling and formation of Sn-bearing granitic rocks in the world-class Dachang tin district, South China

AbstractIncreasing evidence shows that the mantle contributes (directly or indirectly) to Sn-bearing granites worldwide. However, the specific role of mantle in the formation of tin granites and related mineralization remains poorly understood. In the world-class Dachang district, South China, tin mineralization is related to the Longxianggai equigranular/porphyritic biotite granites and tin orebodies are cut by granite porphyry dykes hosting mafic microgranular enclaves (MMEs). A combination of zircon U-Pb dating and Hf-O isotopes, mineral chemistry, and whole-rock elemental and Sr-Nd isotopic compositions—for granitic rocks and MMEs, is employed to constrain the petrogenesis and to unravel the link between tin fertility and mantle upwelling. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon U-Pb dating indicates that the biotite granites were emplaced at ca. 93 Ma, and the granite porphyry dykes and MMEs were formed at ca. 86 Ma. The biotite granites are silica- and alkali-enriched with A/CNK ratios of 1.04–1.36, and exhibit elevated concentrations of Li, F, P, Rb, Cs, Ta, Sn, W, and U, showing affinities with highly fractionated S-type granites. Whole-rock geochemical and Nd isotopic (εNd(t) = −10.0 to −7.8) data, and in situ zircon Hf-O (εHf(t) = −9.9 to −3.9, δ18O = 6.2–8.9‰) isotopes indicate that the biotite granites were formed by partial melting of metasedimentary rocks at relatively high temperatures (≥782 °C), possibly with minor input of mantle material. Likewise, the post-ore granite porphyry dykes have similar chemical and mineralogical characteristics as fractionated S-type granites. Zircon Hf-O isotopes (εHf(t) = −9.0 to −4.9, δ18O = 6.5–8.2‰) and whole-rock geochemical data suggest they were derived from a similar source as the biotite granites, whereas elevated εNd(t) values of −5.0 to −3.3 for granite porphyry dykes relative to biotite granites reveal an increasing mantle input. Distinct εNd(t) (−0.4 and −0.3) and zircon Hf-O (εHf(t) = 1.5–5.0, δ18O = 6.5–7.2‰) isotopes of the MMEs, suggest that the mafic melt could be sourced from the asthenospheric mantle, contaminated by subcontinental lithospheric mantle/continental crust during magma ascent, and hybridized by felsic melt at emplacement-level. The magmatic sequence in the Dachang district is indicative of an extensional tectonic setting where mantle-derived magmas are predicted to migrate to shallower crustal levels as the crust progressively becomes thinner and hotter. High-temperature partial melting of mature metasedimentary crust triggered by heat input from the upwelled mantle, may contribute to biotite breakdown, which is important for concentrating tin in melts. Fractional crystallization of initially Sn-rich felsic melts under reduced conditions makes further tin enrichment and produces Sn-bearing granites (the Longxianggai pluton). Prolonged mantle upwelling results in distinct magma mixing and the formation of granite porphyry dykes and MMEs. These dykes are highly fractionated with elevated Sn and W contents, which show great potential to form hydrothermal Sn-W mineralization.

Geochronology, geochemistry, and isotope compositions of “Grenvillian” S-type granites in the North Qinling unit, central China: Petrogenesis and tectonic significance

An integrated study of whole-rock elements, Sr–Nd isotopes, and zircon U–Pb ages and Hf-O isotopes was carried out for the “Grenvillian” granites from the North Qinling unit, central China. The results not only place clear constraints on the petrogenesis of these granites, but also give insights into the geodynamic evolution of the Qinling orogen during the assemblage of supercontinent Rodinia. LA-ICPMS zircon U–Pb dating of three representative granites from the Dehe, Fangzhuang, and Zhaigen outcrops yielded formation ages of ca. 940–900 Ma. These muscovite-bearing granites have high silica contents (72–79 wt%) and are weak to strong peraluminous (ASI = 1.01–1.15). They exhibit arc-like trace element distribution patterns with enrichment of LREE and LILE (e.g., Rb, K, Pb) but depletion in HFSE (e.g., Nb, Ta, Ti). Besides, they show variable initial 87Sr/86Sr ratios of 0.704 to 0.729 and relatively low εNd(t) values of −3.2 to −6.1, as well as variable zircon εHf(t) and δ18O values (−8.1–+2.6, 7.9‰–12.0‰). These results suggest their derivation from partial melting of a heterogeneous source that dominated by supracrustal components. Furthermore, a few inherited grains give negative εHf(t) values of −9.4 to −18.4 with two-stage Hf model ages up to 2.9 Ga, suggesting the involvement of minor Archean crustal components. Along with their peraluminous features, high zircon δ18O values, and enriched Sr–Nd isotope compositions, these granites were unambiguously derived from partial melting of metasedimentary rocks and thus are of S-type affinity. Geothermometric and compositional approaches suggest these S-type granites were likely generated by dehydration melting of metagraywacke- and/or metapelite-dominated sources. They might be formed in an accretionary orogen associated with the Rodinia-assembly during the early Neoproterozoic, through which the outboard North Qinling terrane was eventually adjacent to the South China Block.

Genesis of highly fractionated granite and associated W-Sn-Be mineralization in the Xuebaoding area, Sichuan Province, China

The Pankou and Pukouling granites are alkali, peraluminous (A/CNK value >1.1) and contribute to the Xuebaoding W-Sn-Be deposit. Muscovite , quartz and albite as the main rock-forming minerals, Zr/Hf and Nb/Ta ratios of both granites indicate that both granites are extremely fractionated. Together with Sr-Nd-Pb-B isotope values of both granite and tourmaline in both granites and the deposit suggest Pankou and Pukouling are S-type granites. • Petrographic studies and geochemical data suggest Xuebaoding granites are S-type granites. • The Pankou and Pukouling granites experienced a high degree of differentiation. • The Pankou and Pukouling highly fractionated granites contribute to the Xuebaoding W-Sn-Be deposit. The Xuebaoding W–Sn–Be deposit in the Songpan–Garzê Orogenic Belt of western China contains coarse-grained W–Sn–Be–F–P-bearing minerals, and has been the subject of several previous geochronological, petrographic, and ore-forming fluid studies. However, the genesis of the deposit, including the Pankou and Pukouling granites and associated W–Sn–Be mineralization has remained unclear. In this study, geochemical and Sr–Nd–Pb isotope analyses of both Pankou and Pukouling granites were performed revealing high A/CNK values (>1.09) and alkali contents (8.16–13.01 wt%), and are enriched in W, Sn, Be, B, F and volatiles. Sr–Nd–Pb isotope values of 87 Sr/ 86 Sr (i) = 0.67–0.73, ε Nd (t) = − 11.49 to − 23.19, 206 Pb/ 204 Pb = 18.710–19.908, 207 Pb/ 204 Pb = 15.647–15.708, and 208 Pb/ 204 Pb = 38.434–38.929 were determined. The samples contain 30–35 vol% muscovite and no hornblende. In addition, the granites have high Be (6.73–1681 ppm), Sn (28.9–200 ppm), and W (2.07–54.1 ppm) contents, and the host marble has low and variable Be (0.05–0.21 ppm), Sn (0.23–0.82 ppm), and W (0.20–2.37 ppm) contents. Further, three types of tourmaline were identified as associated with the magmatic–hydrothermal stage of the granites, with B isotope values of − 11.21‰ to − 13.65‰, also indicating a metasedimentary origin for the granites. All these results indicate that the Pankou and Pukouling granites show highly fractionated S-type characters, derived by partial melting of metasedimentary rocks. Thus, the W–Sn–Be mineralization was closely related to the evolution of such highly fractionated granites in Xuebaoding.

Early Cenozoic partial melting of meta-sedimentary rocks of the eastern Gangdese arc, southern Tibet, and its contribution to syn-collisional magmatism

AbstractContinental magmatic arcs are characterized by the accretion of voluminous mantle-derived magmatic rocks and the growth of juvenile crust. However, significant volumes of meta-sedimentary rocks occur in the middle and lower arc crust, and the contributions of these rocks to the evolution of arc crust remain unclear. In this paper, we conduct a systematic study of petrology, geochronology, and geochemistry of migmatitic paragneisses from the eastern Gangdese magmatic arc, southern Tibet. The results show that the paragneisses were derived from late Carboniferous greywacke, and underwent an early Cenozoic (69–41 Ma) upper amphibolite-facies metamorphism and partial melting at pressure-temperature conditions of ~11 kbar and ~740 °C, and generated granitic melts with enriched Hf isotopic compositions (anatectic zircon εHf(t) = −10.57 to +0.78). Combined with the existing results, we conclude that the widely distributed meta-sedimentary rocks in the eastern Gangdese arc deep crust have the same protolith ages of late Carboniferous, and record northwestward-decreasing metamorphic conditions. We consider that the deeply buried sedimentary rocks resulted in the compositional change of juvenile lower crust from mafic to felsic and the formation of syn-collisional S-type granitoids. The mixing of melts derived from mantle, juvenile lower crust, and ancient crustal materials resulted in the isotopic enrichment of the syn-collisional arc-type magmatic rocks of the Gangdese arc. We suggest that crustal shortening and underthrusting, and the accretion of mantle-derived magma during the Indo-Asian collision transported the supracrustal rocks to the deep crust of the Gangdese arc.

Reconstructing the metamorphic evolution of the Araçuaí orogen (SE Brazil) using in situ U–Pb garnet dating and P–T modelling

AbstractThe Araçuaí orogen of SE Brazil consists of a suite of deformed, metamorphosed, and partly migmatitic sedimentary rocks and granitoid batholiths that predominantly formed during collision of the São Francisco and Congo cratons during West Gondwana assembly in the late Neoproterozoic. While widespread anatexis and a prolonged magmatic record are well‐established, insufficient information exists about the extensive exposure of metamorphic rocks along the Araçuaí orogen. Combining information from a wide range of these metamorphic rocks is essential to reconstruct mountain‐building processes and heat sources that operate in convergent tectonic settings. New petrographic observations, mineral chemistry, and phase equilibria modelling were used to constrain peak metamorphic conditions of amphibolite facies metasedimentary rocks from the central domain of the orogen at 580–620°C and 7–9 kbar. We applied in situ low‐U (&lt;1 µg/g) garnet U–Pb geochronology, which yielded ages in the range of 590–565 Ma, interpreted as the timing of bulk garnet growth during prograde metamorphism, and consistent with previously established monazite ages for the peak metamorphic event. Metamorphic zircon ages of c. 630 Ma are related to a cryptic terrane accretion event prior to the orogeny, whereas garnet U–Pb ages record the main orogenic event as revealed by disequilibrium rare earth element partitioning between the two minerals. A compilation of our results with previously reported metamorphic conditions defines a clockwise P–T evolution for the Araçuaí orogeny, characterized by slow burial to depths of 26–30 km, followed by nearly isothermal decompression from ~10 to 6 kbar. Substantial differences of peak temperatures in various domains of the orogen are consistent with the presence of different protoliths in the crustal sections. The higher concentration of heat‐producing elements (HPEs) in lithologies from the anatectic domains may have triggered partial melting of metasedimentary rocks. This suggests that HPEs were a significant heat source for crustal reworking during West Gondwana convergent tectonics.

Petrogenesis of the Middle–Late Triassic S- and I-type granitoids in the eastern Pamir and implications for the Tanymas–Jinshajiang Paleo-Tethys Ocean

A series of terranes in northwestern Tibet were progressively accreted to the southern margin of the Asian plate during the late Paleozoic to early Cenozoic. Numerous igneous rocks generated in the South Kunlun and Karakul–Mazar terranes help to elucidate the evolution of the northern active continental margin of the Tanymas–Jinshajiang Paleo-Tethys Ocean. In this study, zircon SHRIMP U–Pb data, zircon trace elements, hornblende major and trace elements, whole-rock major and trace elements, whole-rock Sr–Nd isotopes, and zircon Hf isotopes are analyzed to study the age, petrogenesis, and tectonic setting of the Qiate and Bulunkou plutons emplaced in the Karakul–Mazar terrane. Zircon U–Pb dating shows that the Qiate pluton was emplaced at ~ 240 Ma and the Bulunkou pluton at 221–219 Ma. Elemental and isotopic data indicate that the Qiate S-type leucogranite was generated by the partial melting of metasedimentary rocks in the thickened lower continental crust (~ 40–50 km). The Bulunkou I-type granodiorite was likely generated through the reworking of metasedimentary rocks by oxidized mantle-derived mafic magmas in the normal lower continental crust (< 40 km). Because of the injection of the oxidized mantle-derived magmas, the Bulunkou granodiorite is more oxidized than the pure crust-derived Qiate leucogranite. Integrated with previous findings, this study provides new data and interpretations to support that during the Middle–Late Triassic, in response to the northward subduction of the Tanymas–Jinshajiang Paleo-Tethys oceanic slab, the South Kunlun and Karakul–Mazar terranes developed into an active continental margin undergoing crustal thickening and lithospheric delamination.

Sana Granite, a post-collisional S-type magmatic suite of the Ribeira Belt (Rio de Janeiro, SE Brazil)

The postcollisional magmatism in the Ribeira Belt, a collisional Orogen developed through several episodes during the convergence of the Pan-African/Brasiliano Orogeny, was marked in the Eastern Terrane (Rio de Janeiro, SE Brazil) by intense granitic magmatism of Cambro-Ordovician age. This magmatism was previously divided into the Suruí and Nova Friburgo suites, both interpreted as I-type granites generated by the interactions of magmas with crustal and mantle origins. This paper presents a new contribution to the understanding of the Sana Granite. It is based on the analysis of field work, mineralogical and geochemical (elemental concentrations by Inductively Coupled Plasma Emission Spectrometer, ICP-ES, and Inductively Coupled Plasma Mass Spectrometry, ICP-MS) data, and U–Pb dating and Lu–Hf isotope analyses, by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) from the main body and one satellite of the Sana Granite. The results of this work show that the Sana Granite, previously grouped with the Nova Friburgo suite, presents petrographic, geochemical, and isotopic characteristics that do not match its initial petrogenetic interpretation. Data from this work show that the Sana Granite is composed of alkali feldspar granites, syenogranites and monzogranites. All these rocks are hololeucocratic to leucocratic, with small grains ranging from fine to coarse. They are silica-supersaturated, peraluminous, and mostly alkali-calcic rocks and plot at the limit between the ferrous and magnesian fields. Crystallization ages of 480 ± 6 million years (Ma) and 495 ± 4 Ma are obtained in the main body of the Sana Granite, while the ages obtained in the satellite body are 506 ± 10 Ma and 508 ± 5 Ma. The Hf isotope data indicate crustal sources, with depleted mantle model ages (TDM) varying between 2.22 and 1.69 Ga and εHf values ranging between −15.54 and − 6.54. The data set from this work suggests that Sana is an S-type granite formed by the partial melting of metasedimentary rocks from the dehydration of hydrous minerals, such as biotite and muscovite. This massif is composed of muscovite-bearing peraluminous granitoids (MPGs) associated with high-pressure collisional Orogens. Considering that the characteristics of the Sana Granite are incompatible with those of the Nova Friburgo suite, it is proposed to use the Sana suite to characterize S-type granites generated in the postcollisional stage of the Ribeira Belt.

Source of the Zhawulong granitic pegmatite-type lithium deposit in the Songpan-Ganzê orogenic belt, Western Sichuan, China: Constrants from Sr-Nd-Hf isotopes and petrochemistry

The Zhawulong ore field hosts lots of rare-element pegmatite dikes and is an important producing area for lithium resources in the Songpan–Ganzê orogenic belt (SGOB), east Tibet. This paper aims to evaluate the sources and ore-forming processes for granite and pegmatite in the Zhawulong ore field, as well as the petrogenetic links between these rocks, using petrography, whole-rock major and trace-element analysis, and Sr–Nd–Hf isotope analysis. In this ore field, pegmatites are common as dike swarms that emanate from the external and/or internal contact zone of the Zhawulong granitic pluton and strata of the Triassic Xikang group, showing close spatial relationships. Measured values of K/Rb vs. Zr, Hf, and Cs contents indicate geochemical affinities between granite and pegmatite, which suggests that the pegmatite was generated by processes involving high fractional crystallization from the residual parental magma of the host granite. Additionally, the granite have high (87Sr/86Sr)i values (0.711064 to 0.716105), negative εNd(t) values (−9.6 to −9.1) and εHf(t) values (−32.6 to −2.0), and ancient Nd and Hf model ages (1.2–3.1 Ga), which indicate a continental crustal source. Metasedimentary rocks of the Xikang group are rich in Li (~95 ppm), Rb (~124 ppm), Nb (~15 ppm), and Cs (~17 ppm), and their rare-earth element (REE) patterns show moderate enrichment in light REEs and a negative Eu anomaly (Eu/Eu* = ~0.64), revealing characteristics similar to those of granite. The coeval nature and close spatial relationships between the Zhawulong granite and other syn-orogenic S-type granites indicate that the formation of this granite was related to crustal melting. The petrology and geochemical signatures of the Zhawulong rare-metal-bearing granite–pegmatite imply that it was generated by the partial melting of metasedimentary rocks during the Late Triassic orogeny assembly and was further enriched in Li through processes involving high fractional crystallization.

Early Paleozoic intracontinental granites in the Guangzhou region of South China: Partial melting of a metasediment-dominated crustal source

Whether the formation of early Paleozoic granites in the South China Block was related to intracontinental orogenic processes or oceanic subduction continues to be debated. Here, we present whole-rock major element, trace element, and SrNd isotopic data, as well as secondary ion mass spectrometry and laser ablation–inductively coupled plasma–mass spectrometry zircon UPb geochronological and HfO isotopic data, from biotite, two-mica, and muscovite granites in the Baiyunshan and Maofengshan areas of Guangzhou City, South China. Zircon UPb ages reveal that these rocks were formed during the early Paleozoic (446–438 Ma). These muscovite- and biotite-bearing granites have high SiO2 (69.0–80.4 wt%) and K2O (3.19–5.32 wt%) contents, and low MgO (0.46–1.17 wt%) contents. The granites are enriched in Rb, Th, U, and Pb, and depleted in Ba, Sr, Nb, and Ti. The Baiyunshan muscovite granites and Maofengshan biotite granites are slightly to moderately enriched in light rare-earth elements [LREEs; (La/Yb)CN = 2.27–15.1] with slight to pronounced negative Eu anomalies (Eu/Eu* = 0.27–0.72), whereas the Baiyunshan two-mica granites are significantly enriched in LREEs [(La/Yb)CN = 19.6–32.1] with moderate negative Eu anomalies (Eu/Eu* = 0.42–0.58). These muscovite- and biotite-bearing granites have enriched whole-rock Nd and zircon Hf isotopic compositions (εNd(t) = −12.1 to −7.6; εHf(t) = −11.4 to −0.8), and high zircon δ18O values (8.5‰–10.0‰). We suggest that the Baiyunshan granites were formed by partial melting of metasedimentary rocks, as they contain primary muscovite and have high zircon δ18O values. The Maofengshan granites were formed by partial melting of a hybridized crustal source containing metasedimentary rocks with subordinate amphibolites, as they have high zircon δ18O values, variable whole-rock SiO2 contents, and biotite compositions similar to those crystallized in I- and S-type magmas. These granites, along with the widespread early Paleozoic S-type granites in the Wuyi–Yunkai orogen, are distributed across a broad area rather than in a narrow belt. In addition, early Paleozoic S-type granites in the South China Block generally do not show younging trends from east to west or south to north. Moreover, the Baiyunshan–Maofengshan granites show geochemical characteristics similar to syn- or post-collisional granites and different from volcanic arc granites. We therefore propose that these muscovite- and biotite-bearing granites were formed in an intracontinental orogenic setting.

Oxygen isotope ratios in zircon and garnet: A record of assimilation and fractional crystallization in the Dinkey Dome peraluminous granite, Sierra Nevada, California

Abstract The 119 Ma Dinkey Dome pluton in the central Sierra Nevada Batholith is a peraluminous granite and contains magmatic garnet and zircon that are complexly zoned with respect to oxygen isotope ratios. Intracrystalline SIMS analysis tests the relative importance of magmatic differentiation processes vs. partial melting of metasedimentary rocks. Whereas δ18O values of bulk zircon concentrates are uniform across the entire pluton (7.7‰ VSMOW), zircon crystals are zoned in δ18O by up to 1.8‰, and when compared to late garnet, show evidence of changing magma chemistry during multiple interactions of the magma with wall rock during crustal transit. The evolution from an early high-δ18O magma [δ18O(WR) = 9.8‰] toward lower values is shown by high-δ18O zircon cores (7.8‰) and lower δ 18O rims (6.8‰). Garnets from the northwest side of the pluton show a final increase in δ18O with rims reaching 8.1‰. In situ REE measurements show zircon is magmatic and grew before garnets. Additionally, δ18O in garnets from the western side of the pluton are consistently higher (avg = 7.3‰) relative to the west (avg = 5.9‰). These δ18O variations in zircon and garnet record different stages of assimilation and fractional crystallization whereby an initially high-δ18O magma partially melted low-δ18O wallrock and was subsequently contaminated near the current level of emplacement by higher δ18O melts. Collectively, the comparison of δ18O zoning in garnet and zircon shows how a peraluminous pluton can be constructed from multiple batches of variably contaminated melts, especially in early stages of arc magmatism where magmas encounter significant heterogeneity of wall-rock assemblages. Collectively, peraluminous magmas in the Sierran arc are limited to small &amp;lt;100 km2 plutons that are intimately associated with metasedimentary wall rocks and often surrounded by later and larger metaluminous tonalite and granodiorite plutons. The general associations suggest that early-stage arc magmas sample crustal heterogeneities in small melt batches, but that with progressive invigoration of the arc, such compositions are more effectively blended with mantle melts in source regions. Thus, peraluminous magmas provide important details of the nascent Sierran arc and pre-batholithic crustal structure.