Geochemistry and Sr–Nd–Pb–Hf isotopic composition of the Donggou Mo-bearing granite porphyry,Qinling orogenic belt,central China

The eastern Qinling belt is characterized by widespread Mesozoic post-orogenic magmatism and abundant Mo–(Au–Ag) polymetallic mineralization. Most Mo deposits in this belt are genetically related to Mesozoic granitoids. The tectonic context of this close spatial and temporal relationship is still debated. This study reports U–Pb ages and Hf isotopic composition of zircons, major and trace element and Sr–Nd–Pb isotopic composition of the Donggou granite porphyry, host rock to one of the important Mesozoic Mo deposits in this orogen. Based on geochemical results, the Donggou granite porphyry is a silica-supersaturated, high-K metaluminous A-type granite showing enrichment in light REEs, depletion in middle REEs and significant negative Eu, Ba, Nb, Sr, P, and Ti anomalies. Negative initial ?_Nd values of??17.0 to??13.2 for whole-rock and negative initial ?_Hf values of??19.9 to??7.8 for zircon suggest that the magma was derived from a mixture of Archaean/Proterozoic crustal rocks and mantle-derived or newly added crust. Its Pb isotopic composition is similar to the lower crust of the North China block, but different from superjacent country rocks (Xiong'er and Taihua Groups). Zircon U–Pb dating yields a late Mesozoic emplacement age of 118–117 Ma, identical with the third episode of Mo mineralization in the eastern Qinling–Dabie belt. We postulate that the Donggou Mo-related porphyry granite formed by reworking of North China lower crust with significant input of juvenile material. The magmas formed in an extensional tectonic setting, induced by lithospheric thinning and asthenospheric upwelling beneath eastern China during Cretaceous time.     

Late Permian–Triassic siliciclastic provenance,palaeogeography, and crustal growth of the Songpan terrane,eastern Tibetan Plateau: evidence from U–Pb ages,trace elements,and Hf isotopes of detrital zircons

In order to constrain the detrital provenance of the siliciclastic rocks, palaeogeographic variations, and crustal growth history of central China, we carried out simultaneously in situ U–Pb dating and trace element and Hf isotope analyses on 368 detrital zircons obtained from upper Permian–Triassic sandstones of the Songpan terrane, eastern Tibetan Plateau. Two groups of detrital zircons, i.e. magmatic and metamorphic in origin, have been identified based on cathodoluminescence images, zircon Ti-temperatures, and Th/U ratios. Our data suggest that the derivation of siliciclastic rocks in the Songpan terrane was mainly from the Qinling, Qilian, and Kunlun orogens, whereas the Yangtze and North China Cratons served as minor source areas during late Permian–Triassic times. The detrital zircons from Middle–Late Triassic siliciclastic rocks exhibit wide age spectra with two dominant populations of 230–600 Ma and >1600 Ma, peaking at ~1.8–1.9 Ga and ~2.4–2.5 Ga, suggestive of a derivation from the Qinling, Qilian, and Kunlun orogens and the Yangtze Craton being the minor source area. The proportions of detrital zircon populations from the northern Qinling, Qilian, and Kunlun orogens distinctly decreased during Middle–Late Triassic time, demonstrating that the initial uplift of the western Qinling occurred then and it could have blocked most of the detritus from the Qilian–northern Qinling orogens and North China Cratons into the main Songpan–Ganzi depositional basin. The relatively detrital zircon proportions of the Yangtze Craton source decreased during Early-Middle Late Triassic time, indicating that the Longmenshan orogen was probably being elevated, since the early Late Triassic and gradually formed a barrier between the Yangtze Craton and the Songpan terrane. In addition, our Lu–Hf isotopic results also reveal that the Phanerozoic magmatic rocks in central China had been the primary products of crustal reworking with insignificant formation of a juvenile crust.     

胶东地区东部晚侏罗世花岗岩锆石U-Pb定年、Hf同位素特征及其对金成矿构造背景的限定SCIEI北大核心CSCD

胶东地区是目前我国最重要的金矿产地,已累积探明黄金储量接近5000t。金矿在胶东地区分布非常不均,主要集中在胶北地体,而胶东东部地区金矿产出较少。晚侏罗世的玲珑花岗岩是胶北金矿区最重要的赋矿围岩之一。虽然胶东东部同样有晚侏罗世花岗岩发育,但赋存金矿较少,因此前人对东部这些花岗岩体的研究相对较少。本文选取胶东东部的文登岩体和垛崮山岩体进行了锆石U-Pb定年和Hf同位素分析,同时收集整理了胶北地体的玲珑岩体数据,通过开展胶东东部及西北部同一时期岩体的差异性研究,为该区金成矿作用的研究提供重要线索。分析结果显示,文登岩体和垛崮山岩体的形成时代为晚侏罗世(~160Ma),与玲珑岩体时代一致。三个岩体均含有较多的三叠纪(~230Ma)和新元古代(~780Ma)的继承锆石,表明晚侏罗世花岗岩的岩浆源区主要为经历了超高压变质作用的扬子板块。不同的是,玲珑岩体中有更多的华北板块物源的年龄记录(~1400Ma、~1700Ma、~2500Ma),垛崮山岩体中这些年龄的继承锆石少于玲珑岩体,而文登岩体更少。文登岩体、垛崮山岩体的新生岩浆锆石ε;(t)值范围分别为-30.7~-20.3和-27.5~-17.9,玲珑岩体的新生岩浆锆石ε;(t)值的变化范围更广,在-29.3~-9.3之间,而且呈现出从文登岩体到垛崮山岩体到玲珑岩体,ε;(160Ma)值逐渐增高的现象。其原因可能是扬子板块与华北板块碰撞后,幔源岩浆底侵并诱发加厚的地壳发生部分熔融,形成花岗岩。在此过程中胶北地体深部受到幔源物质的影响,从而对金及成矿相关元素产生一定的预富集作用,而同时期胶东东部地区未受到明显的地幔物质影响,可能是该区金矿产出较少的原因之一。     

Geochemical and isotopic constraints on the age and origin of mafic dikes from eastern Shandong Province,eastern North China Craton

Post-orogenic mafic dikes are widespread across eastern Shandong Province, North China Craton, eastern China. We here report new U–Pb zircon ages and bulk-rock geochemical and Sr–Nd–Pb isotopic data for representative samples of these rocks. LA-ICP-MS U–Pb zircon analysis of two mafic dike samples yields consistent ages of 118.7 ± 0.25 million years and 122.4 ± 0.21 million years. These Mesozoic mafic dikes are characterized by high (⁸⁷Sr/⁸⁶Sr) _i ranging from 0.7082 to 0.7087, low ?_Nd(t) values from??17.0 to??17.5, ²⁰⁶Pb/²⁰⁴Pb from 17.14 to 17.18, ²⁰⁷Pb/²⁰⁴Pb from 15.44 to 15.55, and ²⁰⁸Pb/²⁰⁴Pb from 37.47 to 38.20. Our results suggest that the parental magmas of these dikes were derived from an ancient, enriched lithospheric mantle source that was metasomatized by foundered lower crustal eclogitic materials prior to magma generation. The mafic dikes underwent minor fractionation during ascent and negligible crustal contamination. Combined with previous studies, these findings provide additional evidence that intense lithospheric thinning beneath eastern Shandong occurred at ～120 Ma, and that this condition was caused by the removal/foundering of the lithospheric mantle and lower crust.     

华北东南部中生代岩石圈地幔性质、组成、富集过程及其形成机理

本文通过对我国华北东南部中生代幔源岩浆活动的时空分布规律及其地球化学特征的系统总结来进一步厘定该地区中生代岩石圈地幔的性质和组成,并通过与华北内部如鲁中地区中生代岩石圈地幔的对比研究探讨华北东部岩石圈的时空演化规律、富集过程及其形成机理。幔源岩石的 Sr-Nd-Ph 同位素特征表明华北东部中生代岩石圈地幔存在明显的时空不均匀性,其中心部位如鲁中地区以弱富集地幔为主体;而东南部如鲁西南和胶东地区则为类似 EM2型地幔(~(87)Sr/~(86)Sr_i 可高达0.7114)。华北东南部中生代岩石圈地幔随时间的演化特征也很明显。这些幔源岩石的地球化学特征和玄武岩中地幔岩捕虏体(橄榄岩和辉石岩)和捕虏晶(橄榄石和辉石)的组成和结构特征皆证明华北东南部中生代岩石圈地幔曾受到过富硅熔体的强烈改造。橄榄岩-熔体的相互反应是该区岩石圈改造和组成转变的重要方式,从而造成古生代高镁橄榄岩转变为晚中生代低镁橄榄岩和辉石岩。进入岩石圈地幔的熔体具下/中地壳物质重熔的特征,从而导致该区晚中生代岩石圈地幔的快速富集。有关华北东部中生代岩石圈减薄和改造的时限、过程和机制等问题也进行较详细的讨论。     

Some progress on understanding the Phanerozoic granitoids in China

There are large volumes of the Phanerozoic granitoid rocks in China and neighboring areas. In recent years, numerous new and precise U-Pb zircon ages have been published for these granitoids, and define many important magmatic events, such as ca. 500 Ma granitoid events in the West Junggar, Altai orogens in the NW China, and Qinling orogen in the central China. These ages accurately constrain the time of important Early Paleozoic, Late Paleozoic, Early Mesozoic and Late Mesozoic magmatic events of the northern, central, western, southern and eastern orogenic Mountains in China. There occur various types of granitoids in China, such as calc-alkaline granite, alkali granite, highly-fractionated granite, leucogranite, adakite, and rapakivi granite. Rapakivi granites are not only typical Proterozoic as in the North China Craton, but were also emplaced during Paleozoic and Mesozoic in the Kunlun-Qinling orogen, a part of the China Central Orogenic Belt (CCOB). Nd-Hf isotopic tracing and mapping show that granitoids in the southern Central Asian Orogenic Belt (CAOB) in China (or the Northern China Orogenic Belt) are characterized predominantly by juvenile sources. The juvenile crust in this orogenic domain accounts for over 50% by area, distinguishing it from other orogenic belts in the world, and those in central (e.g., Qinling), southwestern and eastern China. Based on a large amount of new age data, a preliminary granitoid and granitoid-tectonic maps of China have been preliminarily compiled, and an evolutionary framework of Phanerozoic granitoids in China and neighboring areas has been established from the view of assembly and breakup of continental blocks. Research ideas on granitoid tectonics has also been proposed and discussed.     

Sedimentary provenance of the Hengyang and Mayang basins,SE China,and implications for the Mesozoic topographic change in South China Craton: Evidence from detrital zircon geochronology

Detrital zircon U–Pb data from sedimentary rocks in the Hengyang and Mayang basins, SE China reveal a change in basin provenance during or after Early Cretaceous. The results imply a provenance of the sediment from the North China Craton and Dabie Orogen for the Upper Triassic to Middle Jurassic sandstones and from the Indosinian granitic plutons in the South China Craton for the Lower Cretaceous sandstones. The 90–120 Ma age group in the Upper Cretaceous sandstones in the Hengyang Basin is correlated with Cretaceous volcanism along the southeastern margin of South China, suggesting a coastal mountain belt have existed during the Late Cretaceous. The sediment provenance of the basins and topographic evolution revealed by the geochronological data in this study are consistent with a Mesozoic tectonic setting from Early Mesozoic intra-continental compression through late Mesozoic Pacific Plate subduction in SE China.     

河南卢氏八宝山花岗斑岩LA-ICP-MS锆石U-Pb年龄和Hf同位素组成特征

河南省卢氏县八宝山岩体位于华北克拉通南缘东秦岭西段,岩体呈筒状,可能为古火山机构岩颈相的超浅成侵入体,成矿组合上为独特的以铁为主的多金属矿化。八宝山岩体边缘相为钾长花岗斑岩、中心相为黑云母二长花岗斑岩。二者的LA-ICP-MS锆石U-Pb年龄非常一致,分别为146.6±1.6Ma和145.9±1.9Ma,说明他们可能是同期岩浆侵入作用分异的产物。钾长花岗斑岩和黑云母二长花岗斑岩锆石Hf同位素组成特征也非常相似,εHf(t)值分别为-27.55～-20.71和-27.30～-21.90,tDM2值分别为1.80～2.93Ga和2.03～2.92Ga,表明该岩体的源区物质以壳源物质为主。综合分析表明,八宝山岩体可能是扬子俯冲陆壳部分熔融的作物,并可能混入少量的太华群和熊耳群的物质,其形成的地球动力学背景可能为俯冲碰撞后的伸展环境。     

Lithospheric mantle signatures as revealed by zircon Hf isotopes of Late Triassic post‐collisional plutons from the central Korean peninsula,and their tectonic implications

The Mesozoic tectonic architecture of the Korean peninsula is largely governed by the continental collision between the North and South China blocks. Zircon Hf isotopic compositions presented in this study and whole‐rock geochemical and Sr‐Nd isotope data in the literature collectively suggest that the lithophile‐elements‐enriched signature of the Late Triassic post‐collisional plutons from the Gyeonggi massif in central Korea is a primary feature inherited from the metasomatized mantle lithosphere. Highly negative zircon ε_Hf (t) values (?23 to ?19) of plutons from the middle and eastern parts of the massif indicate an ancient metasomatism of their mantle source. Distinctly higher zircon ε_Hf (t) values (?15 to ?12) from the southwestern plutons are ascribed to a contribution from an accreted component of the South China‐like block. The involvement of asthenospheric mantle is not recognized in zircons from either group. The implications of these isotopic features are discussed in the context of Mesozoic collisional tectonics.     

Zircon ages of late Palaeoproterozoic (ca. 1.72–1.70 Ga) extension-related granitoids in NE Rajasthan,India: Regional and tectonic significance

The Khetri region forms a late Palaeoproterozoic igneous–metamorphic complex in NE Rajasthan, India. Seven granitoid plutons of the Khetri complex have been studied for zircon U–Pb and Pb–Pb dating along with whole-rock and Nd–Sr isotope geochemistry to provide new constraints on the Palaeoproterozoic magmatic activity in the Aravalli orogen of northwestern India. Most intrusives show evidence of moderate to extreme albitisation forming microcline–albite granite and albite granite, respectively. The rocks are metaluminous to weakly peraluminous, largely ferroan and intraplate A-type granites. The U–Pb zircon ages for four plutons cover a time span of 1732–1682 Ma, whereas Pb–Pb zircon evaporation data for three intrusives indicate minimum emplacement ages between 1671 and 1537 Ma. The Nd–Sr isotopic systematics suggest the involvement of Neoarchaean to Palaeoproterozoic crustal components in the petrogenesis of these granitoids. A regional survey of late Palaeoproterozoic ages in the Aravalli orogen provides evidence for a geographically widespread extension-related event in the northwestern Indian shield about 1720–1700 Ma ago. The record of comparable ages and the magmatic history reported in parts of North America and the North China Craton may indicate the significance of this event for the rift tectonics of the hypothetical supercontinent Columbia.     

Late Mesozoic magmatism in the East Qinling Orogen,China and its tectonic implications

The Qinling Orogen in Central China records the history of a complex geological evolution and tectonic transition from compression to extension during the Late Mesozoic,with concomitant voluminous granitoids formation.In this study,we present results from petrological,geochemical,zircon U-Pb-Lu-Hf isotopic studies on the Lengshui felsic dykes from Luanchuan region in the East Qinling Orogen.We also compile published geochronological,geochemical,and Hf isotopic data from Luanchuan region and present zircon Hf isotopic contour maps.The newly obtained age data yield two group of ages at～145 Ma and 140 Ma for two granite porphyries from the Lengshui felsic dykes,with the ～145 Ma interpreted as response to the peak of magmatism in the region,and the ～140 Ma as the timing of formation of the felsic dykes.The corresponding Hf isotopic data of the granite porphyries display negativeeHit)values of-16.67 to-4.61,and Hf crustal model ages(T_(DM~C_)of 2255-1490 Ma,indicating magma sourced from the melting of Paleo-to Mesoproterozoic crustal materials.The compiled age data display two major magmatic pulses at 160-130 Ma and 111-108 Ma with magmatic quiescence in between,and the zircon Hf isotopic data display/ε_(Hf)(t)values ranging from-41.9 to 2.1 and T_(DM)~c values of3387-1033 Ma,suggesting mixed crustal and mantle-derived components in the magma source,and correspond to multiple tectonic events during the Late Mesozoic.The Luanchuan granitoids are identified as 1-type granites and most of these are highly fractionated granites,involving magma mixing and mingling and crystal fractionation.The tectonic setting in the region transformed from the Late Jurassic syn-collision setting to Early Cretaceous within-plate setting,with E-W extension in the Early Cretaceous.This extension is correlated with the N-S trending post-collisional extension between the North China Craton and Yangtze Craton as well as the E-W trending back-arc extension triggered by the westward Paleo-Pacific Plate subduction,eventually leading to lithospheric thinning,asthenospheric upwelling,mafic magma underplating,and crustal melting in the East Qinling Orogen.     

微区-微量样品Rb-Sr同位素分析技术及其应用前景

利用微钻取样技术和微量样品Rb-Sr同位素分析方法,本文对出露在东秦岭造山带的中生代合峪花岗岩的自形钾长石巨晶进行微区-微量样品Rb-Sr同位素组成分析。分析结果表明,钾长石斑晶具有显著的Rb/Sr比值和Sr同位素组成变化,斑晶和基质钾长石均构成年龄为132~133Ma的Rb-Sr等时线,代表岩浆的后期冷却时代。钾长石晶体的初始⁸⁷Sr/⁸⁶Sr比值由边缘相到中心相没有明显的变化,代表花岗质岩浆结晶阶段的Sr同位素组成,暗示合峪花岗岩的钾长石巨晶为原生成因。以高空间分辨率为特征,微区取样技术已经广泛地应用在变质岩和深成岩浆岩的同位素年代学和成因研究。结合微量样品同位素分析技术,微区-微量样品Rb-Sr同位素方法有望在火山岩的成因和年代学方面得到应用。     

内蒙古白云鄂博地区晚古生代闪长质-花岗质岩石年代学框架及其地质意义

华北克拉通北缘的内蒙古白云鄂博地区分布着由辉长岩、辉长闪长岩、花岗闪长岩、二长花岗岩、黑云母花岗岩及钾长花岗岩等基性-中酸性-碱性花岗岩组成的闪长质-花岗质杂岩体,它们曾被认为是泥盆纪至侏罗纪多期次侵位的产物,而新的年代学资料揭示这些侵入体都是在晚古生代形成的.白云鄂博地区15件不同岩性的锆石LA-ICP-MS U-Pb测年表明,其侵位年龄具有很好的一致性,仅发生在263～281Ma狭窄时间段内,峰期年龄269Ma.这表明白云鄂博矿床的形成与这些闪长质-花岗质岩石的侵入活动没有直接的成因联系,也为重新认识华北克拉通北缘晚古生代构造环境及演化史提供了新的年代学资料.     

Petrogenesis of Early Cretaceous low-Mg adakitic rocks along the southernmost margin of the North China Craton: implications for late Mesozoic crustal evolution

This article reports systematic zircon U–Pb dating, whole-rock geochemistry, and Sr–Nd isotopic data for the Early Cretaceous Jialou granitoids along the southernmost margin of the North China Craton (NCC), adjacent to the Tongbai Orogen. These results will provide significant constrains on the crustal evolution of the southern margin of the NCC. Zircon U–Pb analyses, using laser ablation–multicollector–inductively coupled plasma–mass spectrometry, indicate that the Jialou granitoids were emplaced at ~130 Ma. The granitoids have high SiO₂, K₂O, Al₂O₃, Sr, and Ba contents, high Sr/Y and (La/Yb)N ratios, and low concentrations of MgO, Y, and heavy rare earth elements, indicating a low-Mg adakitic affinity. They have relatively high initial ⁸⁷Sr/⁸⁶Sr ratios (0.707464–0.708190) and negative εNd(t) values (–11.8 to –15.2), similar to those of the Palaeoproterozoic lower crust in the NCC. These geochemical and isotopic features indicate that the Jialou low-Mg adakitic rocks were derived by partial melting of mafic Palaeoproterozoic lower crust of the NCC at >50 km depth, leaving behind a garnet amphibolite residue. The petrogenesis of the Jialou low-Mg adakitic rocks, plus the petrogenesis of Mesozoic granitoids and lower crustal xenoliths entrained in the Late Jurassic Xinyang volcaniclastic diatreme, suggests that the continental crust along the southern margin of the NCC was thickened during the Middle Jurassic to Early Cretaceous, but thinned after 130 Ma. We propose that crustal thickening was caused by a late Middle Jurassic to Early Cretaceous intra-continental orogeny, rather than continent–continent collision between the NCC and the Yangtze Craton. We also suggest that crustal thinning and Early Cretaceous magmatism were related to subduction of the palaeo-Pacific plate, rather than post-orogenic collapse of the Qinling–Tongbai–Dabie Orogen.     

内蒙古沙麦钨矿花岗岩岩石地球化学、锆石U-Pb年代学及其地质意义

沙麦钨矿床位于内蒙古东乌旗地区,是该区目前已探明的中型岩浆热液型钨矿床。矿体主要赋存在黑云母二长花岗岩和黑云母二长花岗斑岩中,对这两种花岗质岩石的岩相学、岩石地球化学和LA-ICP-MS锆石U-Pb年代学进行了研究。结果表明,黑云母二长花岗岩锆石U-Pb年龄为135.6±1.6 Ma和136.3±1.8 Ma,黑云母二长花岗斑岩锆石U-Pb年龄为138.6±1.1 Ma,二者侵位时间均为早白垩世。两种花岗质岩体具有富SiO₂(73.73%～78.23%)、高钾钠(Na₂O+K₂O)(7.56%～8.89%)、贫MgO(0.09%～0.20%)、贫CaO(0.51%～0.89%)、贫TiO₂(0.03%～0.12%)的特征,属于过铝质-高钾钙碱性系列;微量元素富集Rb、K、Th和U,相对亏损Sr、Ba、Nb、P和Ti元素,具有强烈的Eu负异常,具有较高的FeO^T含量,较高的FeO^T/MgO和FeO^T/(FeO^T+M...     

Early Cretaceous porphyry copper mineralization in Northeast China: the Changfagou example

The Changfagou Cu deposit is a newly discovered porphyry deposit located in the southern Jilin Province of Northeastern China, on the northeastern margin of the North China Craton. To better understand the formation of the Cu deposit, we report the zircon U–Pb and molybdenite Re–Os dating, and Sr-, Nd-, and Hf- isotopic data of the granite porphyry. LA-ICP-MS dating of zircon grains from two mineral zones in the granite porphyry yield ages of 115.7 ± 0.8 and 115.3 ± 0.6 Ma, which is interpreted as the emplacement age of the granite porphyry. The molybdenite Re–Os model ages of 112.5 to 113.8 Ma, an isochron age of 113.3 ± 1.3 Ma, and a weighted mean model age of 113.0 ± 0.7 Ma, which represents the age of the Cu mineralization quite well. The Changfagou granite porphyry samples lack amphibole and muscovite, and are compositionally characterized by high SiO₂, high Na₂O+K₂O, and low P₂O₅, enriched in some Rb, Th, U, and Pb, and depleted in Nb, Ta, Ti, P, and Eu. Mineralogical and geochemical features suggest that the Changfagou granite porphyry samples are slightly peraluminous and are of highly fractionated I-type granitoids. The granitic rocks also have relatively high (⁸⁷Sr/⁸⁶Sr)_i (0.71199 to 0. 71422), and both low εNd(t) (?14.56 to ?13.19) and εHf(t) values (?14.916 to ?8.644), which suggest that Changfagou granite porphyry are derived from mixed sources of crustal and mantle, and diagenesis and mineralization were possibly related to the switch in subduction direction of the Palaeo-Pacific Plate in the late phase of Early Cretaceous.     

太行山和华北其它地区中生代岩浆作用的锆石U-Pb年代学和地球化学特征及其岩浆成因和地球动力学意义

华北克拉通东部中生代岩浆岩的主要特征是岩石类型复杂(从辉长质到二长花岗质),显示高钾钙碱性、高Sr-Ba、高Sr/Y和La/Yb比值和高度富集的Sr-Nd同位素成分。锆石SHRIMP定年表明,太行山地区岩浆作用发生在138-127Ma之间。该年代结果与东亚其它地区已经发表的锆石年代数据揭示了中生代岩浆作用的发展具有从日本岛(和朝鲜半岛;210Ma),到胶辽半岛(180Ma),再到大别山-太行山(138Ma)的年轻化趋势。这暗示华北中生代岩浆作用可能与古太平洋板块的俯冲有关,但华北中生代岩浆岩似乎没有明显的向内陆方向的成分变化极性,可能与古太平洋板块在地幔过渡带的水平俯冲有关。地球化学数据表明,华北中生代岩浆岩可能主要形成于壳幔岩浆混合作用和随后的分离结晶过程,而不是形成于基性下地壳部分熔融作用。     

Petrogenesis and tectonic implications of ultrapotassic microgranitoid enclaves in Late Triassic arc granitoids,Qinling orogen,central China

The origin of microgranitoid enclaves in granitic plutons has long been debated (hybrid magma blobs vs. refractory restites or cognate fragments). This article presents detailed petrography, SHRIMP zircon U–Pb chronology, bulk-rock major and trace element analyses, and Sr–Nd isotope and in situ zircon Hf isotopic geochemistry for microgranitoid enclaves within two Late Triassic granitic plutons in the Qinling orogen. Zircon U–Pb dating shows that the enclaves formed during the Carnian (222.5 ± 2.1 to 220.7 ± 1.9 Ma) coeval with their host granitoids (220.0 ± 2.0 to 218.7 ± 2.4 Ma). Field and petrological observations (e.g. double enclaves, xenocrysts, acicular apatite, and poikilitic K-feldspar or quartz) suggest that the enclaves are globules of a mantle-derived more mafic magma that was injected into and mingled with the host magma. The enclaves are mainly ultrapotassic, distinct from the host granitoids that have high-K calc-alkaline bulk-rock compositions. Although the enclaves have closely similar bulk-rock Sr–Nd isotope [initial ⁸⁷Sr/⁸⁶Sr?=?0.7046–0.7056, ?_Nd (T)?=?–0.3 to –5.0] and in situ zircon Hf isotope [?_Hf (T)?=?–1.5 to?+2.9] ratios as the granitoids [initial ⁸⁷Sr/⁸⁶Sr?=?0.7042–0.7059, ?_Nd (T)?=?–0.6 to –6.3, ?_Hf (T)?=?–2.2 to?+1.6], chemical relationships including very different bulk-rock compositions at a given SiO₂ content lead us to interpret the isotopic similarities as reflecting similar but separate isotopic source rocks. Detailed elemental and isotopic data suggest that the enclaves and the host granitoids were emplaced in a continental arc environment coupled with northward subduction of the Palaeo-Tethyan oceanic crust. Partial melting of subducted sediments triggered by dehydration of the underlying igneous oceanic crust, with melts interacting with the overlying mantle wedge, formed high-K calc-alkaline granitic magmas, whereas partial melting of diapiric phlogopite-pyroxenites, solidified products of the same subducting sediment-derived melts, generated ultrapotassic magmas of the microgranitoid enclaves. Our new data further confirm that in the Late Triassic time the Qinling terrane was an active continental margin rather than a post-collisional regime, giving new insights into the tectonic evolution of this orogen.     

Origin of Middle Cambrian and Late Silurian potassic granitoids from the western Kunlun orogen,northwest China: a magmatic response to the Proto-Tethys evolution

This paper presents new SHRIMP zircon U–Pb chronology, major and trace element, and Sr–Nd–Hf isotopic data of two Early Paleozoic granitic plutons (Yierba and North Kudi) from the western Kunlun orogen, in attempt to further constrain the Proto-Tethys evolution. SHRIMP zircon U–Pb dating shows that the Yierba pluton was emplaced in the Middle Cambrian (513?±?7 Ma) and the North Kudi pluton was emplaced in the Late Silurian (420.6?±?6.3 Ma). The Yierba pluton consists of quartz monzodiorite, quartz monzonite and granodiorite. These granitoids are metaluminous and potassic, with initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7072–0.7096, ε_Nd (T) of ?0.2 to ?1.6 and ε_Hf (T) (in-situ zircon) of ?1.2. Elemental and isotopic data suggest that they were formed by partial melting of subducted sediments, with subsequent melts interacting with the overlying mantle wedge in an oceanic island arc setting in response to the intra-oceanic subduction of Proto-Tethys. The North Kudi pluton consists of syenogranite and alkali-feldspar granite. These granites are metaluminous to weakly peraluminous and potassic. They show an affinity of A₁ subtype granite, with initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7077–0.7101, ε_Nd (T) of ?3.5 to ?4.0 and ε_Hf (T) (in-situ zircon) of ?3.9. Elemental and isotopic data suggest that they were formed by partial melting of the Precambrian metamorphic basement at a shallow depth (<30 km) during the post-orogenic regime caused by Proto-Tethyan oceanic slab break-off. Our new data suggest that the subduction of the Proto-Tethyan oceanic crust was as early as Middle Cambrian (～513 Ma) and the final closure of Proto-Tethys was not later than Late Silurian (～421 Ma), most probably in Middle Silurian.     

Links between Mesozoic intermediate–felsic igneous rocks and lower crustal granulite xenoliths,North China Craton: O and Pb isotopic evidence

We present and compare whole-rock and zircon O and Pb isotopic compositions for the Hannuoba granulite xenoliths and Mesozoic intermediate-to-felsic igneous rocks from the Zhangjiakou region, northern margin of the North China Craton, northeast China. The xenoliths have an overall Pb isotopic range similar to rocks from the regionally exposed Neoarchaean granulite terrain. Mesozoic zircons from different types of granulite xenoliths have a narrow range of δ¹⁸O values (6.0–7.7‰) higher than normal mantle δ¹⁸O values (～5.7‰). Mesozoic intermediate–felsic igneous rocks have O and Pb isotopic compositions indistinguishable from the Hannuoba intermediate–mafic granulite xenoliths. Our new data suggest that the Mesozoic igneous rocks and granulite xenoliths are genetically linked and that both were derived from the late Neoarchaean lower crust. This argues against previous proposals that the granulite xenoliths are either products of Mesozoic basaltic underplating or formed by mixing between mantle-derived and pre-existing crustal magmas.