U–Pb zircon ages, geochemical and isotopic compositions of granitoids in Songpan-Garze fold belt, eastern Tibetan Plateau: constraints on petrogenesis and tectonic evolution of the basement

The Songpan-Garze fold belt, located in the eastern part of the Tibetan Plateau, covers a huge triangular area bounded by the Yangtze (South China), the North China and the Tibetan Plateau blocks. In the northeastern part of the Songpan-Garze fold belt, the Yanggon and Maoergai granitoids provide insights into regional tectono-magmatic events, basement nature and tectonic evolution. U–Pb zircon SHRIMP dating shows that the Yanggon and Maoergai granitoids have magmatic crystallization ages of 221 ± 3.8 Ma and 216 ± 5.7 Ma, respectively. Both the granitoids display adakitic geochemical signatures, suggesting that their magma was derived from partial melting of thickened lower crust. Pb–Sr–Nd isotopic compositions for granitoids reveal that there is an unexposed Proterozoic basement in the Songpan-Garze belt, which has an affinity with the Yangtze block. During development of the Paleo-Tethys ocean, the basement of the Songpan-Garze belt would be a peninsula approaching the Paleo-Tethys ocean from the Yangtze block.     

A-type granite and adakitic magmatism association in Songpan–Garze fold belt, eastern Tibetan Plateau: Implication for lithospheric delamination

The Songpan–Garze fold belt covers a huge triangular area (> 200,000 km²), confined by the South China (Yangtze), North China and Tibetan Plateau continental blocks. In the Songpan–Garze fold belt, Triassic adakitic granitoids have been identified. However, whether there are Triassic A-type granites is unclear. Here, we report our first finding of an A-type granite (Nianbaoyeche), which occurs in the central part of the Songpan–Garze fold belt. The Nianbaoyeche granite ( 820 km²) is characterized by arfvedsonite in its mineral assemblage. Using both LA-ICPMS and TIMS U–Pb zircon dating methods, we obtain a magma crystallization age of 211 ± 1 Ma, which is slightly younger than Triassic adakitic granitoids (216–221 Ma) in the Songpan–Garze fold belt. The Nianbaoyeche granite is enriched in Si, K, Na, Rb, REE, HFSE (Nb, Ta, Zr, Hf), with elevated FeO^tot/(FeO^tot + MgO) and Ga/Al ratios, but is depleted in Al, Mg, Ca, Ba and Sr. The REE compositions show moderately fractionated patterns with (La/Yb)_N = 2.67–7.54 and Eu/Eu = 0.09–0.34. These geochemical characteristics indicate that the Nianbaoyeche granite has an A-type affinity. Geochemical data and U–Pb zircon age, combined with regional studies, show that the Nianbaoyeche granite formed in a post-collisional tectonic setting. Sr–Nd isotopic data for the granite exhibit I_Sr = 0.7090–0.7123 and ε_Nd(t) = − 2.72 to − 4.26 with T_DM = 1.15–1.51 Ga, suggesting that the magma has a dominantly crustal source, though a minor contribution from the mantle cannot be ruled out. Melting to produce an A-type granite may have resulted from Triassic lithospheric delamination after Triassic crustal thickening of the Songpan–Garze fold belt due to convergence between the Yangtze, North China and North Tibet continental blocks. The lithospheric delamination model also helps to explain the Triassic adakitic magma generation in the Songpan–Garze belt. We conclude that association of A-type granite and adakitic granitoids in post-collisional environment could be a useful indicator of lithospheric delamination.     

松潘造山带马尔康强过铝质花岗岩的成因及其构造意义

松潘造山带广泛出露印支期后碰撞型花岗岩类, 其中包括埃达克质花岗岩类、A型花岗岩和I型花岗岩, 但目前人们对该区印支期强过铝质花岗岩尚未有深入的研究.松潘造山带马尔康花岗岩属于强过铝质花岗岩(A/CNK=1.10~1.20), 其岩石类型主要为中粒二云母花岗岩和中细粒二云母花岗岩.利用LA-ICP-MS锆石U-Pb定年方法, 获得中粒二云母花岗岩的岩浆结晶年龄为208±2Ma, 中细粒二云母花岗岩的岩浆结晶年龄为200±2Ma.马尔康强过铝质花岗岩K₂O/Na₂O=1.13~1.75, 富Rb、Th和U, 贫Sr、Ba、Co和Ni等元素; 稀土元素组成上显示存在强到中等的负Eu异常(Eu/Eu*=0.15~0.65);全岩初始87Sr/86Sr比值(ISr) 为0.70712~0.71137, εNd (t) =-10.36~-8.43, 锆石εHf (t) =-11.8~-1.1.地球化学和Sr-Nd-Hf同位素组成一致表明, 它们的岩浆来自于地壳物质的部分熔融, 其中中粒二云母花岗岩的源岩类型主要为地壳中的泥质岩类, 而中细粒二云母花岗岩的源岩主要为地壳中的杂砂岩类.结合松潘带的地质背景、区域构造-岩浆事件及其岩浆岩的组合分析, 印支期岩石圈拆沉作用可以用来解释马尔康强过铝质花岗岩的形成机制.在松潘带, 印支期岩石圈拆沉作用导致软流圈物质上涌, 这不仅促使了加厚下地壳物质发生部分熔融, 如松潘带印支期埃达克质和I型花岗岩浆的形成, 而且还诱发了中地壳物质的部分熔融, 如马尔康强过铝质花岗岩的形成.这表明松潘带印支期岩石圈拆沉作用已使地壳不同层次发生部分熔融作用.      

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.     

Two periods of mineralization in Xiaoxinancha Au–Cu deposit,NE China: evidences from the geology and geochronology

The Xiaoxinancha Au–Cu deposit is located at the eastern segment of the Tianshan–Xingmeng orogenic belt in northeast China. The deposit includes porphyry Au–Cu orebodies, veined Au–Cu orebodies and veined Mo mineralizations. All of them occur within the diorite intrusion. The Late Permian diorite, Late Triassic granodiorite, Early Cretaceous granite and granite porphyry are developed in the ore area. The studies on geological features show that the porphyry Au–Cu mineralization is related to the Late Permian diorite intrusion. New geochronologic data for the Xiaoxinancha porphyry Au–Cu deposit yield Permian crystallization zircon U–Pb age of 257 ± 3 Ma for the diorite that hosts the Au–Cu mineralization. Six molybdenite samples from quartz + molybdenite veins imposed on the porphyry Au–Cu orebodies yield an isochron age of 110.3 ± 1.5 Ma. The ages of the molybdenites coeval to zircon ages of the granite within the errors suggest that the Mo mineralization was genetically related to the Early Cretaceous granite intrusion. The formation of the diorite and the related Au–Cu mineralization were caused by the partial melting of the subduction slab during the Late Palaeozoic palaeo‐Asia Ocean tectonic stage. The Re contents and Re–Os isotopic data indicate that the crustal resource is dominated for the Mo mineralization during the Cretaceous extensional setting caused by the roll‐back of the palaeo‐Pacific plate. Copyright © 2014 John Wiley & Sons, Ltd.     

滇西北贡山地块始新世花岗岩的成因及其构造意义

滇西三江地区发育古近纪花岗岩,记录了印度－欧亚大陆碰撞的岩浆活动信息。对贡山地块福贡花岗岩开展岩石地球化学及锆石U Pb Hf系统研究,结果表明,该花岗岩为钙碱性、过铝质特征的I S型花岗岩。锆石U Pb同位素分析表明,福贡马吉花岗岩侵位于55 Ma,并含有252~77 Ma的继承锆石。锆石Hf同位素分析表明,该区岩浆锆石具有与青藏高原及东其南缘同时代长英质侵入体相似的Hf同位素组成,暗示其相似的岩浆起源。微量元素和同位素组成模拟计算结果表明,马吉花岗岩的原生岩浆是由53%的新生地壳组分和47%古老地壳基底物质混合而成的原岩经 5%~15%(F=005~015)的部分熔融而成。贡山地块福贡马吉花岗岩与冈底斯地块和腾冲地块早始新世岩浆岩(约55 Ma)具相似的年龄及地球化学特征,暗示它们之间可能存在类似的成因机制,均为新特提斯洋俯冲板片断离引起的壳内减压熔融的产物。     

云南保山东缘早古生代高Si花岗岩的成因及构造意义

对保山地块东缘高Si花岗岩开展矿物化学、岩石地球化学及锆石U Pb Hf系统研究,结果表明该高Si花岗岩为具钙碱性、强过铝质特征的S型花岗岩。锆石U Pb同位素分析表明,高Si花岗岩侵位于454 Ma,并含有800~1 100 Ma的继承锆石。锆石Hf同位素分析表明其岩浆锆石具有与青藏高原及东南缘同时代长英质侵入体相似的Hf同位素组成,暗示其相似的岩浆起源。矿物化学、同位素组成及Melts模拟计算结果表明,保山东缘高Si花岗岩为一系列复杂作用的结果：高硅花岗岩母岩浆起源于该区沉积岩部分熔融;熔体形成后经高度分异演化,在侵位过程中同化混染围岩;岩浆冷凝至固相线下部分矿物再平衡。保山东缘高Si花岗岩体与平河花岗岩体具相似年龄和地球化学特征,暗示它们之间可能存在类似的成因机制。     

Zircon U–Pb age and Hf isotopic compositions of Mesozoic granitoids in southern Qiangtang,Tibet: Implications for the subduction of the Bangong–Nujiang Tethyan Ocean

The southern Qiangtang magmatic belt was formed by the north-dipping subduction of the Bangong–Nujiang Tethyan Ocean during Mesozoic. To better understand the petrogenesis, time–space distribution along the length of this belt, 21 samples of several granitoid bodies, from west to east, in the Bangong Co, Gaize, Dongqiao and Amdo areas were selected for in-situ zircon U–Pb dating, Hf isotopic and whole-rock chemical analyses. The results suggest a prolonged period of magmatic activity (185–84 Ma) with two major stages during the Jurassic (185–150 Ma) and the Early Cretaceous (126–100 Ma). Both the Jurassic and Cretaceous granitoids are high-K calc-alkaline I-type rocks, except the Cretaceous two-mica granite from Amdo in the east, which belongs to S-type. The granitoids are generated from different source materials as indicated by zircon Hf isotopic compositions. The Bangong Co and Dongqiao granitoids show high zircon ε_Hf(t) values of − 1.3–13.6 with younger T_DM^C ages of 293–1263 Ma, suggesting a relatively juvenile source; whereas the Gaize and Amdo granitoids have low ε_Hf(t) values of − 16.1–2.9 with older T_DM^C ages of 999–2024 Ma, indicating an old crustal contribution. These source rocks melt at different P–T conditions as suggested by Sr/Y ratio and T_Zr. The Sr/Y ratio of both stage granitoids increases with decreasing age. However, the T_Zr of the Jurassic granitoids decreases, whereas the T_Zr of the Cretaceous granitoids increases with decreasing age. The contrasting geochemical signatures of these granitoids may be controlled by the varying contribution of slab-derived fluids involved in the generation of the Jurassic and Cretaceous granitic magmas; i.e. increasing amount of fluids in the Jurassic, whereas decreasing amount of fluids in the Cretaceous. Therefore, it is proposed that the Jurassic and Cretaceous magmatism may be related to subduction and closure of the Bangong–Nujiang Tethyan Ocean, respectively. The age pattern of the Jurassic and Cretaceous granitoids suggests an oblique subduction of the Bangong–Nujiang Tethyan Ocean and a diachronous collision between the Lhasa and Qiangtang blocks.     

Early Cretaceous Adakitic Rocks in the Northern Great Xing’an Range, NE China: Implications for the Final Closure of Mongol-Okhotsk Ocean and Regional Extensional Setting

A large amount of igneous rocks in NE China formed in an extensional setting during Late Mesozoic. However, there is still controversy about how the Mongol-Okhotsk Ocean and the Paleo-Pacific Ocean effected the lithosphere in NE China. In this paper, we carried out a comprehensive study for andesites from the Keyihe area using LA-ICP-MS zircon UPb dating and geochemical and Hf isotopic analysis to investigate the petrogenesis and tectonic setting of these andesites. The U-Pb dating yields an Early Cretaceous crystallization age of 128.3±0.4 Ma. Geochemically, the andesites contain high Sr(686–930 ppm) and HREE contents, low Y(11.9–19.8 ppm) and Yb(1.08–1.52 ppm) contents, and they therefore have high Sr/Y(42–63) and La/Yb(24–36) ratios, showing the characteristics of adakitic rocks. Moreover, they exhibit high K_2O/Na_2O ratios(0.57–0.81), low Mg O contents(0.77–3.06 wt%), low Mg# value(17–49) and negative εHf(t) values(-1.7 to-8.5) with no negative Eu anomalies, indicating that they are not related to the oceanic plate subduction. Based on the geochemical and isotopic data provided in this paper and regional geological data, it can be concluded that the Keyihe adakitic rocks were affected by the Mongol-Okhotsk tectonic regime, forming in a transition setting from crustal thickening to regional extension thinning. They were derived from the partial melting of the thickened lower crust. The closure of the Mongol-Okhotsk Ocean may finish in early Early Cretaceous, followed by the collisional orogenic process. The southern part region of its suture belt was in a post-orogenic extensional setting in the late Early Cretaceous.     

Timing of Magma Mixing in the Gangdisě Magmatic Belt during the India-Asia Collision： Zircon SHRIMP U-Pb Dating

Abstract  Abundant mafic microgranular enclaves (MMEs) extensively distribute in granitoids in the Gangdisê giant magmatic belt, within which the Qüxü batholith is the most typical MME‐bearing pluton. Systematic sampling for granodioritic host rock, mafic microgranular enclaves and gabbro nearby at two locations in the Qüxü batholith, and subsequent zircon SHRIMP II U‐Pb dating have been conducted. Two sets of isotopic ages for granodioritic host rock, mafic microgranular enclaves and gabbro are 50.4±1.3 Ma, 51.2±1.1 Ma, 47.0±1 Ma and 49.3±1.7 Ma, 48.9±1.1 Ma, 49.9±1.7 Ma, respectively. It thus rules out the possibilities of mafic microgranular enclaves being refractory residues after partial melting of magma source region, or being xenoliths of country rocks or later intrusions. Therefore, it is believed that the three types of rocks mentioned above likely formed in the same magmatic event, i.e., they formed by magma mixing in the Eocene (c. 50 Ma). Compositionally, granitoid host rocks incline towards acidic end member involved in magma mixing, gabbros are akin to basic end member and mafic microgranular enclaves are the incompletely mixed basic magma clots trapped in acidic magma. The isotopic dating also suggested that huge‐scale magma mixing in the Gangdisê belt took place 15–20 million years after the initiation of the India‐Asia continental collision, genetically related to the underplating of subduction‐collision‐induced basic magma at the base of the continental crust. Underplating and magma mixing were likely the main process of mass‐energy exchange between the mantle and the crust during the continental collision, and greatly contributed to the accretion of the continental crust, the evolution of the lithosphere and related mineralization beneath the portion of the Tibetan Plateau to the north of the collision zone.     

Zircon LA-ICP-MS dating and geochemical characteristics of I-type granitoids from the Yanhu area,west segment of the Bangongco-Nujiang suture (western Tibet): Petrogenesis and implications for the southward subduction of the Tethyan Ocean

The Yanhu granitoids are located in the west segment of the Bangongco-Nujiang suture in the western Tibetan Plateau. The main rock types of the granitoids are diorite porphyry, quartz diorite, granodiorite, granite and granite porphyry. Here, their zircon LA-ICP-MS U-Pb ages and petrogeochemical data are reported. Three groups of magmatic events can be distinguished from the Yanhu area: group 1 includes samples AK01 and ZK01 of diorite porphyry, and sample D3658 of quartz diorite that yield mean zircon U-Pb ages of 121.0 ± 2.7 Ma, 116.6 ± 2.0 Ma and 116.0 ± 3.9 Ma, respectively; group 2 includes sample D0050 of diorite porphyry, samples D1393 and D3660 of granodiorite and sample D3065 of granite porphyry that yield mean zircon U-Pb ages of 104.9 ± 2.0 Ma, 105.4 ± 3.8 Ma, 104.2 ± 1.9 Ma and 104.2 ± 1.9 Ma, respectively; group 3 includes sample D3093 of granite that yields mean zircon U-Pb ages of 93.6 ± 1.5 Ma. The zircon LA-ICP-MS U-Pb ages suggest that the Yanhu granitoids were emplaced at 121.0–93.6 Ma, representing Cretaceous magmatism in the west segment of the Bangongco-Nujiang suture. The granitoids are composed of SiO₂ (56.57 to 76.98 wt.%), Al₂O₃ (12.20 to 17.90 wt.%), Na₂O (3.61 to 4.98 wt.%), K₂O (2.06 to 4.71 wt.%) and CaO (0.27 to 5.74 wt.%). The Yanhu granitoids exhibit enrichment in LREE (light REE) and LILE (large ion lithophile elements) such as Rb, Th, U, Pb and K and depletion of HREE (heavy REE), P, Ti, Nb, Ta and Zr. Their A/CNK ratios of 0.85-1.06 are <1.1, implying that they are high-K, metaluminous-weakly peraluminous I-type granites. TheYanhu granitoids were generated mainly by partial melts of the meta-igneous lower crust and some arc-related materials. The Yanhu granitoids probably formed in VAG and syn-COLG tectonic settings related to the southward subduction of the Tethyan Ocean. Diorite porphyry and quartz diorite magmatism from 121.0 Ma to 116.0 Ma may be associated with the southward Bangongco–Nujiang Tethys oceanic crust subduction. Diorite porphyry, granodiorite, and granite porphyry magmatism from 105.4 Ma to 104.2 Ma may be associated with the rising asthenosphere induced by the slab breakoff. Granite magmatism from 93.6 Ma may be related to the crustal thickening induced by the final amalgamation of the Lhasa Terrane and the Qiangtang Terrane.     

Late Jurassic granitoids in the Xilamulun Mo belt,Northeastern China: geochronology,geochemistry, and tectonic implications

ABSTRACT

The Xilamulun Mo belt of Northeastern China, located in the southeastern segment of the Central Asia Orogenic Belt (CAOB), is composed of large deposits of porphyry Mo and quartz-vein-type Mo, which are related to Mesozoic granitoids. Previous studies led to the conclusion that all granitoids in the region formed during the Cretaceous and Triassic, but our new laser ablation inductively coupled plasma mass spectrometry U–Pb zircon dating of magmatic zircons from five samples of four mineralized plutons (Nailingou, Longtoushan, and Hashitu granites and Erbadi and Hashitu granite porphyries) reveals that these range in age from 143.8 ± 1.2 to 149.5 ± 1.0 Ma. These granites show post-collisional (A-type) geochemical characteristics (e.g. enrichment in total alkali, LILE, and LREE and depletion in Eu, Ba, P, and Nb). The Erbadi, Longtoushan, Hashitu, and Longtoushan granitoids exhibit moderately positive Hf isotopic compositions (ε_Hf(t) = ?0.3 to 10.2), indicating that granitic magmas may reflect mixtures of mantle melts and continental crust. These mineralized granites were all emplaced along a major fault over a time span of ~6 million years during the Late Jurassic. We conclude that igneous activity and mineralization resulted from the rollback of the subducted Palaeo-Pacific plate beneath Eurasia. Confirming that the Late Jurassic granitic intrusives are related to the Mo mineralization is useful for understanding the Mesozoic tectonic evolution of the Xilamulun Mo belt and also has significant implications for the regional exploration of ores.     

湘东北金鸡金矿床岩体成岩机制

江南造山带湖南段中早古生代花岗质岩石对于研究早古生代构造演化以及金成矿作用具有重要的意义。位于该区中段的金鸡金矿床钻孔中新发现有两类花岗质岩石,分别为花岗岩和花岗闪长岩。对两类岩体样品进行了锆石LA-ICP-MS U-Pb测年,获得的年龄分别为(425.2±1.5)Ma和(430.6±1.5)Ma。岩石地球化学数据表明,花岗岩属I型花岗岩,其来源于地壳中变泥质岩石的部分融熔;花岗闪长岩属埃达克岩,其起源于地壳中变砂质岩石的部分融熔。Sr-Nd同位素分析显示,金鸡花岗闪长岩具有较高的(87Sr/86Sr)i(0.722369~0.722488)、较低的(143Nd/144Nd)i(0.511941~0.511990)以及εNd(t)值较低(–8.2~–7.2),并且金鸡花岗闪长岩的二阶段Nd模式年龄值为1.75~1.84 Ga,与江南造山带变质基底的二阶段模式年龄(1.65~2.14 Ga)一致。金鸡金矿床花岗岩和花岗闪长岩的岩石地球化学、年代学以及Sr-Nd同位素特征表明二者是华南早古生代陆内造山事件的产物,岩体成因及地球动力学背景的研究将有助于揭示湘东北地区金矿形成的地球动力学机制。     

西秦岭党川地区花岗岩的成因及其构造意义

对西秦岭造山带党川地区的党川花岗岩和石门花岗岩进行了LA-ICP-MS锆石U-Pb定年、元素地球化学和Sr-Nd-Pb同位素组成的研究.结果表明, 党川花岗岩和石门花岗岩的岩浆结晶年龄分别为438±3Ma和220±2Ma.在岩石地球化学特征上, 党川花岗岩类似于C-型埃达克质岩石, 岩浆产生于增厚地壳物质的部分熔融, 而石门花岗岩类似于普通的地壳深熔型花岗岩.党川花岗岩的I_Sr=0.70660~0.70929, ε_Nd (t) =-2.24~-4.48;石门花岗岩的I_Sr=0.70581~0.70804, ε_Nd (t) =-3.73~-4.72.Sr-Nd同位素组成进一步指示它们的岩浆派生于地壳物质.然而, 在Pb同位素组成上, 党川花岗岩和石门花岗岩存在着明显的差异.党川花岗岩以相对富放射成因Pb同位素组成为特征, 初始Pb同位素比值为: 206Pb/204Pb=18.288~18.484, 207Pb/204Pb=15.677~15.693, 208Pb/204Pb=38.182~38.283;而石门花岗岩以相对低的放射成因Pb同位素组成为特征, 初始Pb同位素比值为: 206Pb/204Pb=17.989~18.189, 207Pb/204Pb=15.560~15.567, 208Pb/204Pb=37.982~38.000.这表明党川花岗岩和石门花岗岩的岩浆来自于不同地壳物质的部分熔融.区域分析表明, 西秦岭党川地区中古生代和早中生代的岩浆事件、岩石成因机制及岩浆源区均可与东秦岭地区北秦岭构造单元相对比, 由此说明西秦岭党川地区是东秦岭地区北秦岭构造单元的西延, 并且东秦岭地区早中生代南秦岭块体向北秦岭块体的大陆俯冲作用向西一直延至到西秦岭地区.      

Zircon U‐Pb Dating on Granitoids from the Northern South China Sea and its Geotectonic Relevance

Zircon U-Pb ages of 163.8–100.4 Ma and 146.6–134.5?Ma are obtained for the granitoids from the Pearl River mouth basin, and from southern Guangdong Province, respectively. These new dating data accord well with the crystallization ages of Yanshanian granitoids broadly in the Nanling. The active continental margin of South China, as revealed by a combination of zircon U-Pb data, underwent a key granitoid-dominated magmatism in 165–100?Ma. Its evolution varied temporally, and spatially, registering under control of the paleo-Pacific slab subduction. The granitoids that occurred in 165–150?Ma broadly from the South China Sea to the Nanling are preferably related to two settings from volcanic-arc to back-arc extension, respectively. The activities of Cretaceous granitoids migrated from the southeastern Guangdong (148–130?Ma) to the Pearl River Mouth basin (127–112?Ma), corresponding to the model of a retreating subduction. The subduction-related granitoid magmatism in South China continued until 108–97?Ma. A tectonic transformation from slab-subduction to extension should occur at ~100?Ma.     

Zircon U–Pb and Molybdenite Re–Os Ages of the Lakange Porphyry Cu–Mo Deposit,Gangdese Porphyry Copper Belt,Southern Tibet,China

The Lakange porphyry Cu–Mo deposit within the Gangdese metallogenic belt of Tibet is located in the southern–central part of the eastern Lhasa block, in the Tibetan Tethyan tectonic domain. This deposit is one of the largest identified by a joint Qinghai–Tibetan Plateau geological survey project undertaken in recent years. Here, we present the results of the systematic logging of drillholes and provide new petrological, zircon U–Pb age, and molybdenite Re–Os age data for the deposit. The ore‐bearing porphyritic granodiorite contains elevated concentrations of silica and alkali elements but low concentrations of MgO and CaO. It is metaluminous to weakly peraluminous and has A/CNK values of 0.90–1.01. The samples contain low total REE concentrations and show light REE/heavy REE (LREE/HREE) ratios of 17.51–19.77 and (La/Yb)_N values of 29.65–41.05. The intrusion is enriched in the large‐ion lithophile elements (LILE) and depleted in the HREE and high field‐strength elements (HFSE). The ore‐bearing porphyritic granodiorite yielded a Miocene zircon U–Pb crystallization age of 13.58 ± 0.42 Ma, whereas the mineralization within the Lakange deposit yielded Miocene molybdenite Re–Os ages of 13.20 ± 0.20 and 13.64 ± 0.21, with a weighted mean of 13.38 ± 0.15 Ma and an isochron age of 13.12 ± 0.44 Ma. This indicates that the crystallization and mineralization of the Lakange porphyry were contemporaneous. The ore‐bearing porphyritic granodiorite yielded zircon εHf(t) values between ?3.99 and 4.49 (mean, ?0.14) and two‐stage model ages between 1349 and 808 Myr (mean, 1103 Myr). The molybdenite within the deposit contains 343.6–835.7 ppm Re (mean, 557.8 ppm). These data indicate that the mineralized porphyritic granodiorite within the Lakange deposit is adakitic and formed from parental magmas derived mainly from juvenile crustal material that partly mixed with older continental crust during the evolution of the magmas. The Lakange porphyry Cu–Mo deposit and numerous associated porphyry–skarn deposits in the eastern Gangdese porphyry copper belt (17–13 Ma) formed in an extensional tectonic setting during the India–Asia continental collision.     

阿拉善北部亚干地区花岗岩锆石年代学、地球化学特征及其构造意义

由于缺少可靠的年代学资料和系统研究,阿拉善北部亚干地区的基底时代和性质尚不清楚,制约了对本区构造属性及造山带结构的进一步认识.利用亚干地区广泛出露的花岗岩锆石U-Pb年代学和Hf同位素研究,揭示源区深部物质组成特征,对探讨该地区的基底性质具有重要意义.LA-ICP-MS锆石U-Pb分析结果表明,切刀黑云母二长花岗岩体侵位于380±1 Ma,亚东花岗闪长岩体侵位于271±2 Ma,同时,原北山群中识别出大量的三叠纪花岗岩（亚干片麻状花岗岩,228±2 Ma;都热糜棱岩化花岗岩,214±2 Ma）.地球化学分析表明,切刀花岗岩和都热糜棱岩化花岗岩为准铝质-弱过铝质的A型花岗岩,亚东花岗岩为钙碱性I型花岗岩.锆石Hf同位素分析显示主要的花岗岩体ε_Hf（t）值为-2.8~+4.3,地壳模式年龄为1.0~1.5 Ga,表明源区可能以中元古代地壳物质为主.结合前人获取的前寒武纪岩石年龄,亚干地区花岗岩Hf同位素特征,以及花岗岩出现中-新元古代继承锆石等证据,亚干地区深部应具有中-新元古代基底,南戈壁微陆块范围可以延伸到阿拉善北部边境地区.      

Geochronology and Ore‐Forming Fluids of the Baizhangyan W–Mo Deposit in the Chizhou Area,Middle‐Lower Yangtze Valley,SE‐China

The Baizhangyan skarn‐porphyry type W–Mo deposit is located in a newly defined Mo–W–Pb–Zn metallogenic belt, which is in the south of Middle‐Lower Yangtze Valley Cu–Fe–Au polymetallic metallogenic belt in SE China. The W–Mo orebodies occur mainly within the contact zone between fine‐grained granite and Sinian limestone strata. There are two types of W–Mo mineralization: major skarn W–Mo mineralization and minor granite‐hosted disseminated Mo mineralization which was traced by drilling at depth. Eight molybdenite samples from Mo‐bearing ores yield Re–Os dates that overlap within analytical error, with a weighted average age of 134.1 ± 2.2 Ma. These dates are in close agreement with SIMS U–Pb concordant zircon age for fine‐grained granite at 133.3 ± 1.3 Ma, indicating that crystallization of the granite and hydrothermal molybdenite formation were coeval and likely cogenetic. The Baizhangyan W–Mo deposit formed in the Early Cretaceous extensional tectonic setting at the Middle‐Lower Yangtze Valley metallogenic belt and the Jaingnan Ancient Continent. Based on mineral compositions and crosscutting relationships of veinlets, hydrothermal alteration and mineralization, the ore mineral paragenesis of the Baizhangyan deposit is divided into four stages: skarn stage (I), oxide stage (II), sulfide stage (III), and carbonate stage (IV). Fluid inclusions in garnet, scheelite, quartz and calcite from W–Mo ores are mainly aqueous‐rich (L + V) type inclusions. Following garnet deposition at stage I, the high‐temperature fluids gave way to progressively cooler, more dilute fluids associated with tungsten–molybdenite–base metal sulfide deposition (stage II and stage III) (162–360°C, 2.7–13.2 wt % NaCl equivalent) and carbonate deposition (stage IV) (137–190°C, 0.9–5 wt % NaCl equiv.). Hydrogen‐oxygen isotope data from minerals of different stages suggest that the ore‐forming fluids consisted of magmatic water, mixed in various proportions with meteoric water. From stage I to stage IV, there is a systematic decrease in the homogenization temperature of the fluid‐inclusion fluids and calculated δ¹⁸O values of the fluids. These suggest that increasing involvement of formation water or meteoric water during the fluid ascent resulted in successive deposition of scheelite and molybdenite at Baizhangyan.     

Geochemistry, Zircon U–Pb Dating and Hf Isotopic Characteristics of Neoproterozoic Granitoids in the Yaganbuyang Area, Altyn Tagh, NW China

The South Altyn continental block is an important geological unit of the Altyn Tagh orogenic belt, in which numerous Neoproterozoic granitoids crop out. Granitoids are mainly located in the Paxialayidang–Yaganbuyang area and can provide indispensable information on the dynamics of Rodinia supercontinent aggregation during the Neoproterozoic. Therefore, the study of granitoids can help us understand the formation and evolutionary history of the Altyn Tagh orogenic belt. In this work, we investigated the Yaganbuyang granitic pluton through petrography, geochemistry, zircon U–Pb chronology, and Hf isotope approaches. We obtained the following conclusions:(1) Yaganbuyang granitoids mainly consist of two-mica granite and granodiorite. Geochemical data suggested that these granitoids are peraluminous calc–alkaline or high-K calc–alkaline granite types. Zircon U–Pb data yielded ages of 939±7.1 Ma for granodiorite and ~954 Ma for granitoids, respectively.(2) The εHf(t) values of two–mica granite and granodiorite are in the range of-3.93 to +5.30 and-8.64 to +5.19, respectively. The Hf model ages(TDM2) of two-mica granite and granodiorite range from 1.59–.05 Ga and 1.62–2.35 Ga, respectively, indicating that the parental magma of these materials is derived from ancient crust with a portion of juvenile crust.(3) Granitoids formed in a collisional orogen setting, which may be a response to Rodinia supercontinent convergence during the Neoproterozoic.     

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.