Petrogenesis Of Granites Research Articles

An ƐHf and ẟ18O isotopic study of zircon of the Mount Osceola and Conway Granites, White Mountain Batholith, New Hampshire: Deciphering the petrogenesis of A-type granites

An ƐHf and ẟ18O isotopic study of zircon of the Mount Osceola and Conway Granites, White Mountain Batholith, New Hampshire: Deciphering the petrogenesis of A-type granites

Petrogenesis and tectonic settings of epithermal mineralization-related granites in the Xinchenggou area, NE China

The Xinchenggou area is located in the northeast part of the eastern segment of the Xingmeng orogenic belt (EXOB), NE China and has been demonstrated to be a promising exploration target for epithermal deposits. Although previous studies have shown that syenogranite and monzogranite occurring in the Xinchenggou area are promising in forming epithermal mineralization, the petrogenesis and geodynamic settings in which these granites were emplaced are still unclear. To address these problems, in this study detailed whole-rock major and trace element analyses for these granites were conducted. Combined with previously published data, we show that both syenogranite and monzogranite in the Xinchenggou area are high-K calc-alkaline and peraluminous with high SiO2. Their rare earth element concentrations are low (ΣREE = 72.35 × 10−6–217.64 × 10−6) and show obvious differentiation between LREE and HREE (LaN/YbN=2.74–11.37), with apparent Eu negative anomalies (δEu = 0.14–0.83) and indistinctive Ce anomalies (δCe = 0.96–1.11). Combined with petrographical observations, it is suggested that both syenogranite and monzogranite are (slightly fractionated) I-type granite. Nb/Ta ratios of syenogranite and monzogranite range from 6.18 to 26.33, indicating that the granitic magma was derived from the upper mantle or the lower crust. Both syenogranite and monzogranite were emplaced in a continental arc setting, which was related to the subduction of the Paleo-Pacific Plate beneath the Eurasian Plate during the Late Triassic to Early Jurassic.

Cover Image

The cover image is based on the Original Article Significance of selective crystal entrainment and differential crystal-melt separation in petrogenesis of granites from the Tongbai orogen by Qiang-Qiang Zhang et al., https://doi.org/10.1111/jmg.12691

Petrogenesis of high heat production granite in eastern Hebei Province, China: Constraints from geochronology, geochemistry and Sr-Nd-Hf-O isotopes

Petrogenesis of high heat production granite in eastern Hebei Province, China: Constraints from geochronology, geochemistry and Sr-Nd-Hf-O isotopes

Petrogenesis and tectonic implications of A-type granites in the Kattasay region, Middle Tianshan, Uzbekistan

Petrogenesis and tectonic implications of A-type granites in the Kattasay region, Middle Tianshan, Uzbekistan

Petrogenesis of Early Cretaceous Laoshan A-type granites and the implications for the tectonic evolution of Jiaodong Peninsula

早白垩世崂山花岗岩位于苏鲁超高压变质带中部,是中国东部典型的A型花岗岩之一。其岩浆源区和岩石成因目前还存在一定的争议,而且其成岩过程中的物理化学条件及相关研究也比较缺乏。本文在详细的野外工作和岩相学研究的基础上,采用角闪石、黑云母的成分估算了成岩的温度、压力和氧逸度条件,并开展了岩石地球化学、Sr-Nd同位素分析,探讨岩石的源区和成因。结果显示,崂山花岗岩的岩浆形成于高温(>850℃)和高氧逸度条件。岩浆在地壳浅部压力较低的条件下结晶,岩浆固结温度在700℃左右,随后经历了快速降温的过程。岩石地球化学方面,崂山花岗岩富SiO₂、富碱,贫Ca、Mg;高场强元素Th、Zr、Nb、Y和大离子亲石元素Rb、K含量高,而Ba、Sr含量低;稀土元素含量高,且具有显著的Eu负异常,呈右倾"海鸥型"分配特征。同位素方面,所有样品均显示出富集的Sr、Nd同位素特征,(⁸⁷Sr/⁸⁶Sr)_i=0.706447~0.707876,<i>ε</i>_Nd(117Ma)值为-14.8~-17.4,全岩Nd两阶段模式年龄(<i>t</i>_DM2)为2112~2325Ma。结合胶东地区前人的研究成果,本文认为崂山A型花岗岩是由地壳中富K的碱性岩重熔而成,其岩浆源区很可能与该区三叠纪碱性岩浆活动有关。崂山A型花岗岩的形成标志着碰撞造山带底部的扬子板块已经垮塌完毕,同时岩石圈减薄达到高峰。

Petrogenesis of Ⅰ-type garnet-bearing granite in the Yangbajing area within the Gangdese Batholith, Tibet

Petrogenesis of Ⅰ-type garnet-bearing granite in the Yangbajing area within the Gangdese Batholith, Tibet

Petrogenesis and geodynamic implications of Oligocene A-type granite in the Guadalcazar area, San Luis Potosi, central Mexico

The Guadalcazar is located in the Mesa Central (MC) province, which is mainly composed of granitic rocks and is known for its metallogenetic. The granitic rocks contain complex Sn-Hg-Ag-F mineralization and were emplaced during Eocene to Oligocene. However, the source, origin, and evolution of magma and the tectonic setting of this magmatic area have never been explained. In this study, we have conducted petrology, whole-rock geochemistry, and U–Pb zircon geochronology on granitic rocks from the Guadalcazar to constrain the petrogenesis and tectonic environment. LA-ICP-SF-MS zircon U–Pb dating shows that the Guadalcazar granite was emplaced ca. 31 Ma. These rocks are characterized by high (SiO2) contents (64–75 wt%), low CaO (0.28–1.78 wt%), with relatively high (FeOt)adj/(FeOt + MgO) values ranging from 0.90 to 0.98. The geochemical diagrams of SiO2 vs [(FeOt)/(FeOt + MgO)] and SiO2 vs [(Na2O + K2O) − CaO] show the ferroan and mostly alkali-calcic nature of these rocks. The granite shows an A2-type affinity and is strongly peraluminous, with ASI (molar Al2O3/[CaO + Na2O + K2O]) values of 1.13 to 2.60. These granitic rocks are characterized by enrichments in rare earth elements (REE) and high field strength elements (HFSE), and depletion in Ba, Nb, Sr, Ti, and Eu. These features suggest that these A-type granites were derived from the metasedimentary rocks and evolved through extensive fractional crystallization. The multidimensional discrimination diagrams showed a continental rift or within-plate setting. By combining previous and new data, we proposed a new magmatic evolution model that supports an extension during ca. 34–28 Ma in the Guadalcazar, central Mexico.

Multiple Palaeozoic–Mesozoic orogenic events in the SE Yangtze Block, China: evidence from the petrogenesis and deformation of gneissic granites from the Nanwenhe metamorphic dome and regional correlation analysis

ABSTRACT The Nanwenhe metamorphic dome contains a rare Silurian granitic pluton that developed in the SE Yangtze Block during early Palaeozoic orogenesis, and this pluton comprises the Nanwenhe gneissic granites. The petrogenetic and structural deformation histories of the gneissic granites recorded the multi-stage interaction of the SE Yangtze Block with other blocks during the Palaeozoic and Mesozoic, including the Cathaysia, Indochina, and Paleo-Pacific blocks. Zircon U–Pb and Hf isotopes and geochemical data indicate that the Early Palaeozoic granites (433–420 Ma) are S-type granitic rocks, and they were derived from the partial melting of ancient continental crust with no contribution from mantle material. And the large variations and positive values of εHf(t) (−12.25 to +10.69) of the gneissic granites are ascribed to a heterogeneous source and disequilibrium melting. Here we compare early Palaeozoic granitoids in the SE Yangtze Block (foreland belt) with those in the orogenic core, and suggest that the SE Yangtze Block underwent limited syn-collisional crustal thickening and metamorphism, and following which post-collisional granitoids were formed through partial melting of continental crust. Structural analysis of the Nanwenhe gneissic granites allows two stages of deformation to be identified: northward detachment (D1) and northwestward thrusting (D2). 40Ar–39Ar isotope dating of muscovites from the gneissic granites yields a well-defined plateau age of 229.61 Ma, which is interpreted as the timing of Late Triassic structural overprinting associated with the formation of the Nanwenhe metamorphic dome. Integration of our new results with previous research findings from the dome and South China allows us to conclude that the detachment deformation (D1) represents Late Triassic post-collisional extension, and D2 represented Jurassic intracontinental orogeny associated with subduction of the Paleo-Pacific Block.

Petrogenesis of Late Devonian I- and A-type granitoids, and associated mafic microgranular enclaves in the northwestern North Qaidam Orogenic Belt, China: Implications for continental crust growth during the post-collisional stage

Post-collisional magmatic intrusions and their mafic microgranular enclaves (MMEs) can provide significant insights into granite petrogenesis, crust-mantle interaction, and the reworking and growth of the continental crust. A combined petrological and geochemical study was carried out on MMEs and their host Niubiziliang I-type felsic composite pluton (granite and granodiorite) and aluminous A 2 -type Datonggou biotite granite from the Niubiziliang batholith in the northwestern segment of the North Qaidam Orogenic Belt (NQOB), China. This study presents whole-rock major- and trace elements and Sr Nd isotopic compositions, electron probe microanalyses of plagioclase, hornblende, and biotite, and zircon U Pb dating and Hf isotopes. Zircon U Pb dating demonstrates that the studied rocks formed during the Late Devonian (ca. 382 Ma). The Niubiziliang composite felsic pluton samples yield a metaluminous calc-alkaline to high-K calc-alkaline I-type granitic composition. The MMEs from Niubiziliang composite felsic pluton have low SiO 2 (52–64 wt%) and high MgO (2.8–4.9 wt%) concentrations. Both MMEs and host granites are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and are depleted in P and high field strength elements (HFSEs). All MMEs and host granitoids samples from Niubiziliang have indistinguishable Sr-Nd-Hf isotopic compositions characterized by initial Sr isotope ratios of 0.70545–0.70633 (MMEs), 0.70555–0.70742 (host rock), whole-rock ε Nd ( 382 Ma ) values of −2.95 to +1.6, −0.25 to +3.5, and zircon ε Hf( 382Ma ) values of +2.2 to +10.2, +1.3 to +10.4, respectively. The Niubiziliang composite felsic pluton is a product of mixing juvenile crust-derived and depleted mantle-derived melts in a 40:60 proportion. MMEs crystallized from the same source as the host granite, and they were formed by self-mixing with the earlier crystallized cumulates when they were affected by the emplacement of subsequent hot magmas. The Datonggou biotite granite has high TFe 2 O 3 /(TFe 2 O 3 + MgO), TiO 2 /MgO, Ga/Al, and Y/Nb ratios, high zircon saturation temperatures (802–846 °C), high HFSE concentrations (e.g., Zr, Nb, and Y), moderate (Na 2 O + K 2 O) concentrations, and low Eu and Sr concentrations. Interstitial biotite has high Fe 2 O 3 but low MgO concentrations, similar to those of aluminous A-type granite. The above-mentioned characteristics imply that the Datonggou biotite granite is an aluminous A 2 -type granite. Considering their high initial Sr isotopic composition (0.72218–0.72652), low ε Nd ( 382 Ma ) (−6.73 to −6.31), and comparatively old two-stage Nd model ages (1640–1674 Ma), we suggest the biotite granite was generated by partial melting of Ordovician-Silurian granitoids with tonalitic compositions at H T -L P conditions. The formation of coeval I- and aluminous A 2 -type granitoids in this region was probably related to the lithospheric mantle delamination and associated asthenospheric upwelling at ca. 390 Ma. The I-type granitoids and MMEs from the Niubiziliang area represent a net addition of juvenile mantle to the crust and, given the wide distribution of contemporaneous I-type granitoids, the post-collisional magmatism has significantly contributed to the growth of the continental crust in the NQOB. • The Niubiziliang granitoids, MMEs, and biotite granite were emplaced at 382 Ma. • Lithospheric delamination occurred at ca. 390 Ma in the North Qaidam belt. • The post-collisional granitoids contribute to the growth of the continental crust.

Lithium isotopic constraints on the petrogenesis of the Jiajika two-mica granites and associated Li mineralization

Petrogenesis of two-mica granites and the associated Li mineralization from the Jiajika super-large Li deposit in Sichuan Province, southwest China, remains equivocal. This study analyzed Li contents and isotopic compositions of metasedimentary rocks (schists and slates) and two-mica granites in this deposit to investigate the issue. The metasedimentary rocks show large variations in Li (71.7 ppm to 1792 ppm) and δ7Li (–14.1 ‰ to +7.9 ‰), while the two-mica granites have Li contents from 60.2 ppm to 388 ppm and δ7Li values from –3.1 ‰ to +1.9 ‰. Integrated with bulk geochemistry and available Nd–Hf isotopes from this deposit, the two-mica granites probably originated from partial melting of the middle-upper crustal pelitic metasediments. The lower average δ7Li value of metasedimentary rocks (–2.9 ‰) than that of two-mica granites (–0.4 ‰) is possibly attributed to 6Li-rich garnets entering residuum during incongruent melting. We speculate that Li isotopic fractionation might have occurred during crustal anatexis. The partial melting that contributed to the transfer of volatiles and rare-metal elements into the granitic melts played an important role in Li mineralization. Li isotopes can potentially guide Li deposit prospecting as Li-bearing veins are likely to occur in regions with light 6Li.

Petrogenesis of the Triassic Sihaiping granite in the South Qinling orogen, central China: Implications for Mo-W mineralization

Late Triassic granites and related magmatic-hydrothermal Mo-W mineralization are common in the South Qinling orogen of central China, for which the Sihaiping granite pluton is a typical example. However, the petrogenesis of the Sihaiping granite remains enigmatic. Unraveling its petrogenesis can aid in our understanding of the Mo-W metallogeny in the South Qinling orogen. In this study, we present in situ zircon U-Pb ages, whole-rock geochemistry, and Sr-Nd-Hf isotopes of the Sihaiping granite. Zircon U-Pb dating by laser ablation inductively coupled plasma mass spectroscopy yielded Late Triassic ages of 206.6 ± 0.94, 206.5 ± 0.96, and 205.9 ± 0.75 Ma. The granite displayed high SiO2, K2O, and Rb contents, but low MgO, P2O5, TiO2, Sr, Ba, and Eu contents, indicating highly fractionated I-type granite affinity. The magma was interpreted to have been derived from Proterozoic crystalline basement in the South Qinling given its relatively low (87Sr/86Sr)i (0.704006 to 0.705039), negative εNd(t) (–4.9 to –3.5), and zircon εHf(t) (–6.2 to +1.6) values, with Nd model ages ranging from 1.39 to 1.27 Ga and two-stage Hf model ages ranging from 1.36 to 1.04 Ga. The strong fractional crystallization and suitable oxygen fugacity (ΔFMQ values mainly concentrated between +2 to +8) in the magmatic system played a major role in the Mo-W mineralization, indicating that the pluton has significant Mo(-W) metallogenic potential. Overall, the findings suggest that the Triassic Sihaiping granite and its related Mo-W mineralization likely formed in the transitional stage from syn- to post-collision between the South China Block and South Qinling along the Mianlue suture.

Geochronological and geochemical constraints on the petrogenesis of alkali granites from the Makrohar Granulite Belt: Evidence for Mesoproterozoic extensional regime in the eastern Central Indian Shield

The Chotanagpur Gneissic Complex (CGC) is characterized by the Makrohar Granulite Belt (MGB) at its north‐western margin parallel to Son‐Narmada lineaments. The MGB exhibits a typical association of mantle‐derived mafic tholeiitic and alkaline rocks along with the felsic units. The zircon U–Pb data on the intrusive granite gneiss from this association define a crystallization age of 1,498 ± 38 Ma, representing a widespread Mesoproterozoic within‐plate magmatic event in CGC synchronous with the breakup of the Columbia Supercontinent. These granite gneisses have an alkaline affinity and are metaluminous in character. They exhibit many of the geochemical characteristics of anorogenic (A‐type) magmas, including enrichment of incompatible trace elements, like Zr, Hf, Nb, Y, Ga, and rare earth elements (REE), but low in Ba and Sr as well as high FeOt/(FeOt + MgO) (~0.98) values and Ga/Al (~3.92) ratios. They show less fractionated REE patterns, characterized by relatively enriched LREE and flat HREE with negative Eu anomaly, and have high Zr‐saturation temperatures (~955°C). Considering their field characteristics and all the geochemical and isotopic features, we conclude that they were derived from melting the lower crust metasomatized by mantle‐derived alkali‐rich fluids due to asthenospheric upwelling.

A Comparison of Granite Genesis in the Adelaide Fold Belt and Glenelg River Complex Using U–Pb, Hf and O Isotopes in Zircon

Abstract We present new U–Pb ages and Hf and O isotope data for zircon from I-, S- and A-type granites from both the western and eastern edges of the Delamerian Orogen in southeastern Australia. The I-type Tanunda Creek Gneiss contains zircon populations of 507 ± 4 and 492 ± 6 Ma inferred to reflect igneous and metamorphic ages, respectively. The I-type Palmer Granite yielded an age of 509 ± 3 Ma, and the Port Elliot S-type Granite has a magmatic age of 508 ± 7 Ma. Inherited zircon in these granites range from 1092 to 3343 Ma, probably derived from assimilation of Adelaide Group sediments. The Murray Bridge A-type Granite is 490 ± 2 Ma in age and lacks inherited zircon. In the Glenelg River Complex, an S-type migmatite from near Harrow contains a complex zircon population. It is most likely ~500 Ma in age and has inherited zircon of 550–700, 1000–1100 and 2437 Ma, hence matching those from the Kanmantoo Group. From this and detrital zircon ages, we infer that only the Kanmantoo Group extends across the Murray Basin into the Glenelg River Complex. The Wando Tonalite and Loftus Creek I-type granites yielded ages of 501 ± 2 and 486 ± 3 Ma, respectively. Zircon from the Dergholm Granite has suffered Pb loss, and the best age estimate for this granite is 488 ± 5 Ma. Combining all the granite data together, εHft and δ18O in the magmatic zircon range from 5.6 to −10.3 and from 5.8 to 8.1, respectively, and are well correlated. The zircon indicates the same temporal and compositional evolution of granitic petrogenesis across ~300 km of strike, reaffirming the notion that these terranes form part of the same orogen. Westward-directed subduction caused orogenic thickening, heating and increasing amounts of crustal contribution. This was followed by convective thinning of the thickened mantle lithosphere and a return to more primitive magmas lacking significant crustal contributions. It contrasts significantly with inferred granite petrogenesis and tectonic style in the younger Lachlan and New England Fold Belts further east that were not built upon extended cratonic lithosphere.

Does the Mantle Contribute to Granite Petrogenesis?

Does the Mantle Contribute to Granite Petrogenesis?

Petrogenesis and tectonic implication of the alkaline ferroan granites from Ropp complex, north‐central Nigeria: Clues from zircon chemistry, U–Pb dating, and Lu–Hf isotope

Zircon holds crucial information for deciphering the origin of granitic rocks, enabling robust scrutiny of their magmatic evolution. Herein, we report an integrated whole‐rock and zircon U–Pb and Lu–Hf isotope chemistry of Ropp alkaline granites in north‐central Nigeria. The rock units include riebeckite‐biotite granite porphyries (161.1 ± 1.7 Ma), aegirine‐biotite granites (162.4 ± 1.6 Ma), biotite granites (162.3 ± 5.1 Ma), and albite riebeckite granites. They show diagnostic characteristics of ferroan alkaline granites such as high FeOt/(FeOt + MgO), low Y/Nb (&lt;1.2), and high Nb/Y (&gt;1.0). Their zircon Hf isotopic compositions (εHf(t) = −8.08 to −4.38; 176Hf/177Hf = 0.282492–0.282543), major and trace element compositions such as high SiO2 &gt;70% and incompatible trace elements (e.g., Rb, Ba, and La), low ratios of whole rock Th/U, Hf/Ta. Ni/Co and Th/Ta = 2.82, and zircon U/Yb, Gd/Yb, and Th/Yb ratios as well as oxygen fugacity (logfO2 = −26.09 to −12.32) reflect protracted fractional crystallization of a highly reduced mantle‐derived oceanic island basalt (OIB) magma from EM2 with considerable crustal involvement. Magma evolution was fuelled by fractionation of calcic‐pyroxene, calcic‐amphibole, Mg‐olivines, feldspars, titanite, and apatite, as corroborated by the marked depletions of CaO, MgO, TiO2, and P2O5. Intense crustal assimilation, particularly in the biotite granite masked their enriched mantle (EM) ancestry. This is attested by the presence of inherited Pan‐African (ca. 582 Ma) zircon, elevated crustal‐like zircon U/Yb, low zircon Gd/Yb, and Th/Yb ratios as well as high whole‐rock Rb/Nb ratios and relatively low estimated temperatures, which mimic crustal‐derived granites. Reactivation of pre‐existing megashear zones of weaknesses and opening of related transcurrent faults following periodic plate movements generated intense stress field which invoked the release of pressure and hot metasomatic fluids that partially melted the lower EM, giving rise to the Ropp OIB magmas.

Significance of selective crystal entrainment and differential crystal‐melt separation in petrogenesis of granites from the Tongbai orogen

AbstractPartial melting has been shown to be an important mechanism for intracrustal differentiation and granite petrogenesis. However, a series of compositional differences between granitic melt from experiments and natural granites indicate that the processes of crustal differentiation are complex. To shed light on factors that control the processes of crustal differentiation, and then the compositions of granitic magma, a combined study of petrology and geochemistry was carried out for granites (in the forms of granitic veins and parautochthonous granite) from a granulite terrane in the Tongbai orogen, China. These granites are characterized by high SiO2 (&gt;72 wt%) and low FeO and MgO (&lt;4 wt%) with low Na2O/K2O ratios (&lt;0.7). Minerals in these granites show variable microstructures and compositions. Phase equilibrium modelling using P–T pseudosections shows that neither anatectic melts nor fractionated melts match the compositions of the target granites, challenging the conventional paradigm that granites are the crystallized product of pure granitic melts. Based on the microstructural features of minerals in the granites, and a comparison of their compositions with crystallized minerals from anatectic melts and minerals in granulites, the minerals in these granitoids are considered to have three origins. The first is entrained garnets, which show comparable compositions with those in host granulites. The second is early crystallized mineral from melts, which include large plagioclase and K‐feldspar (with high Ca contents) crystals as well as a part of biotite whose compositions can be reproduced by crystallization of the anatectic melts. The compositions of other minerals such as small grained plagioclase, K‐feldspar and anorthoclase in the granites with low Ca contents are not well reconstructed, so they are considered as the third origin of crystallized products of fractionated melts. The results of mass balance calculation show that the compositions of these granites can be produced by mixing between different proportions of crystallized minerals and fractionated melts with variable amounts of entrained minerals. However, the calculated modal proportions of different crystallized minerals (plagioclase, K‐feldspar, biotite and quartz) in the granites are significantly different from those predicted by melt crystallization modelling. Specifically, some rocks have lower modes of biotite and plagioclase, whereas others show lower K‐feldspar modes than those produced by melt crystallization. This indicates that the crystallized minerals would be differentially separated from the primary magmas to form the evolved magmas that produce these granites. Therefore, the crystal entrainment and differential melt‐crystal separation make important contributions to the composition of the target granites. Compared with leucogranites worldwide, the target granites show comparable compositions. As such, the leucogranites may form through the crystal fractionation of primary granitic magmas at different extents in addition to variable degrees of partial melting.

Karakteristik Dan Petrogenesa Batuan Beku Granit Di Daerah Sibayu, Kecamatan Balaesang, Kabupaten Donggala, Provinsi Sulawesi Tengah

The tectonic history of Sulawesi is closely related to the activities of the three major world plates that surround it. This results in the complexity of geological conditions on the island of Sulawesi, including the diversity of rock types. One of the most common types of rock is igneous granite, which is widespread in the Sibayu area, Donggala Regency, Central Sulawesi Province. This study aims to determine the characteristics and petrogenesis of igneous granite using megascopic and microscopic or petrographic observation methods. The results showed that the rock has a megascopic appearance in the form of a grayish-white fresh color and a brownish weathered color, a holocrystalline texture, a massive structure, and a mineral composition consisting of orthoclase, amphibole, and quartz. Its microscopic appearance shows a brownish-transparent color on parallel nikol, on brown cross-linked nikol, light gray to dark gray, mineral size 0.15-0.05 mm, the average mineral composition is orthoclase (41.25% ), quartz (43.75%), biotite (11.25%) and plagioclase (3.75%). Based on these data, it can be concluded that the rocks are Late Miocene to Late Pliocene, which proves that the Palu-Koro fault was still active during the Late Miocene to Late Pliocene. The presence of biotite in granitic rocks characterizes S-Type granite which is formed deep below the earth's surface which is characterized by euhedral-subhedral forms.

Homogenization of zircon Hf isotopes during late-stage granite crystallization

Zircon Hf isotope compositions provide valuable constraints on granite petrogenesis and the evolution of continental crust. However, the causes of variations in zircon Hf isotope ratios during magma evolution are poorly understood. Here we compiled zircon UPb ages and LuHf isotope compositions of the Permian-Triassic granites in South China to demonstrate that Hf isotope variability is common in the early stages of granitic magma evolution, but Hf isotope ratios are homogenized during the final stages of granite crystallization. Late-grown zircons have relatively high concentrations of U and have εHf(t) values approximating the median values of multiple analyses for the low-U zircon in the same samples. Hafnium isotope variations measured in low-U zircon could be related to incremental assembly of multiple isotopically distinct magma batches derived from partial melting of heterogeneous crustal rocks; these discrete magma batches may have remained isolated from each other during segregation and extraction in the source and during ascent through the crust. After emplacement, these magma batches may have experienced variable degrees of mixing resulting in homogenization of Hf isotopes in late crystallized zircon. Coupled with whole-rock geochemical data of the Permian-Triassic granites, we argue that the Hf isotope homogenization is related to crystallization of zircon from late-stage magmas or precipitation of hydrothermal zircon from exsolved hydrothermal fluids during the final stages of magma solidification. Hafnium isotope homogenization in late-stage melts/fluids may have been driven by an increased concentration of fluorine and the efficient diffusion of hafnium-fluoride complexes through silicic melt. These results provide a deeper understanding of the mechanisms that contribute to Hf isotope heterogeneity in zircon during granite crystallization.

Petrogenesis of Eocene high-silica granites in the Maliaoshan area, northern Tibet: Implications for the Eocene magmatic flare-up in the Northern Qiangtang Block

The Eocene magmatic flare-up in the Northern Qiangtiang Block is a key event that was genetically related to the uplift of the Tibetan Plateau. This uplift has been considered to have an important impact on the global cooling and oceanic isotopic compositions. Here, we report zircon ages and bulk rock elemental and isotopic compositions for high-silica porphyritic granites in the Maliaoshan area of the Northern Qiangtang Block. Zircon U–Pb dating shows that the porphyritic granites were generated at ca. 41.02 Ma. The porphyritic granites are characterized by uniformly high SiO2 contents (74.3–75.0 wt%), high contents of total alkalis, metaluminous compositions, and low contents of TiO2, Fe2O3T, MgO, CaO, and P2O5. Moreover, the Maliaoshan porphyritic granites have Nd–Hf isotopic compositions similar to those of coeval metaluminous magmatic rocks in the Northern Qiangtang Block. These data together with geochemical modeling results suggest that the Maliaoshan porphyritic granites were formed by the fractional crystallization of 43.06% major minerals and 0.76% accessory minerals from the assumed starting composition (average Eocene metaluminous igneous rock). Based on our new data and Eocene magmatic flare-up event in the Northern Qiangtiang Block, we propose a new geodynamic model to explain such a magmatic flare-up event. The stress produced by the collision of India and Eurasia gradually transferred northward, prompting the Songpan–Garzê Block to subduct southward beneath the Northern Qiangtang Block at ca. 55 Ma, accompanied by minor magmatic activity. Subsequently, the break-off of the Songpan–Garzê Block at ca. 46 Ma resulted in the Eocene magmatic flare-up.