Petrogenesis of keratophyes in the Pingshui Group, Zhejiang: Constraints from zircon U-Pb ages and Hf isotopes

Zircon LA-ICP-MS U-Pb ages and Hf isotopic as well as whole-rock geochemical data are reported for keratophyes in the Pingshui Group, Zhejiang. The results are used to discuss their petrogenesis and geological significance. The keratophyes were dated at 904±8 to 906±10 Ma. These intermediate-felsic rocks are characterized by high LREE contents and depletion of HREE and HFSE (e.g., Nb, Ta, Ti, P), resembling arc-derived rocks. The keratophyes exhibit positive εHf(t) values of 8.6 to 15.4, consistent with the...     

一个造山后花岗岩基：石耳山花岗岩的形成时代和成因

根据研究所得新资料，报道了江南造山带东段造山后花岗岩的一个实例：石耳山花岗岩．它的单颗粒锆石Ｕ－Ｐｂ同位素年龄值为０．７７～０．８３Ｇａ，因此该花岗岩形成于晚元古代江南造山带造山运动趋于结束或结束以后（造山运动结束时限被认为是０．８Ｇａ前后［１３］）．它为含绿帘石的碱性长石花岗岩，具高硅、富碱、过铝和较低的Ｚｒ、Ｈｆ、ＨＲＥＥ、Ｌａ／Ｙｂ、Ｒｂ／Ｍ（Ｍ＝ＨＲＥＥ、Ｙ、Ｚｒ、Ｈｆ等），以及趋近于零的εＮｄ（Ｔ）值等特点，起源于有当时年青的含基性岩组分参与的不成熟地壳物质．     

Petrogenesis and significance of the Hongshan syenitic pluton,South Taihang: zircon SHRIMP U-Pb age, chemical compositions and Sr-Nd isotopes

The Hongshan syenitic pluton (South Taihang) is mainly made up of hornblende syenite and minor granite. SHRIMP zircon dating yields a U-Pb age of 135±2.7 Ma for the emplacement of the pluton, while biotite Rb-Sr isotopic data give an isochron of 120. 3±2.4 Ma. Hongshan syenites show low silica, high alkalis and LILE such as Sr, and exhibit negligible Eu anomalies or slightly positive anomalies in the REE patterns. The syenites show quite enriched isotopic compositions with Isr from 0.7052 to 0. 7102 and εNd(t) from -7.5 to - 11.1. Petrogenesis of the pluton can be that partial melting of an enriched lithospheric mantle gave birth to an alkali basaltic magma, which subsequently underplated in the lower crust and experienced a coupled fractionation of ferromagnesian phases like pyroxene and hornblende and minor (<10 %) contamination of lower continental crust, producing the Hongshan syenites that partially e-volved into granite through combined fractionation of hornblende and feldspar. Our data are not in agreement with a previous model that the syenites originated from melting of lower crust in a thickened crust circumstance, and thus do not support the speculation of "East China Plateau" in the Mesozoic.     

湖南宁乡钾镁煌斑岩及相关岩石重砂锆石地球化学特征、U-Pb年龄及其地质意义

 对湖南宁乡等地多个含有金刚石的钾镁煌斑岩及相关岩石的重砂矿物进行了分离,利用电子探针、阴极发光图像及LA-ICPMS等对其中的锆石进行了主微量元素成分、内部结构及U-Pb同位素年龄的分析。结果表明,不同地点钾镁煌斑岩重砂锆石的地球化学特征具有一定相似性;与钾镁煌斑岩有关锆石的U-Pb年龄显示研究区的岩浆活动具有多期次的特点,其中宁乡Ⅰ号及Ⅲ号岩管钾镁煌斑岩4颗重砂锆石的²⁰⁶Pb/²³⁸U加权平均年龄为(101.6 ± 5.1)Ma,显示出较为年轻的特点,但石门上五通地区白垩系红层中也发现具有²⁰⁷Pb/ ²⁰⁶Pb 谐和年龄为(2 008±29)Ma古老岩浆结晶锆石的存在,证实该地和金刚石有关火山物质的来源具有复杂性。     

Discovery of ∼4.0 Ga detrital zircons in the Changdu Block,North Qiangtang,Tibetan Plateau

Using in situ zircon dating by LA-ICP-MS and MC-ICP-MS, detrital zircon of 3981±9 Ma age was found in metamorphic rocks of the Ningduo Rock Group, Changdu Block of Northern Qiangtang. This is the oldest age record that has been found in the Qiangtang area. This finding also constitutes the third zircon locality in China with an age older than 3.9 Ga. Thus, the discovery provides new information for the study of Hadean crust. In addition, we found 3.51–3.13 Ga, ∼2440 Ma, ∼1532 Ma, ∼982 Ma and ∼618 Ma age peaks from 100 test spots. The younger ages of ∼982 Ma and ∼618 Ma correspond to the formation of the Rodinian super-continent and the Pan-African event, respectively. These findings suggest a close relationship between these zircons and the Gondwanan super-continent. The age of ∼618 Ma defines the lower limit on the deposit time of the protolith for the garnet-mica-quartz schist in the Ningduo Rock Group. Zircons with an age of ∼982 Ma generally display a negative ɛ_Hf(t) and a two-stage Hf model with concentrated ages around 1933–2553 Ma. This pattern indicates that the source area of the Ningduo Rock Group underwent a significant separation of depleted mantle into the crust during the Paleoproterozoic Era. However, zircons with ages of 2854–3505 Ma also show a negative ɛ_Hf(t) and a two-stage Hf model with a concentration of ages around 3784–4316 Ma. These results demonstrate that the source area of the Ningduo Rock Group contains a residual amount of ancient (Hadean) crustal materials. This paper provides new information on the relationship between the basement of the Qiangtang area and the Paleoproterozoic basements of the Gangdese and Himalayan regions, which constrains the northern boundary of Gondwana.     

西藏朗县蛇绿混杂岩中变辉绿岩和变玄武岩的年代学和地球化学

对西藏雅鲁藏布缝合带东段朗县混杂岩中的辉绿岩和玄武岩进行了岩石学、地球化学和SHRIMP锆石年代学研究.在早侏罗世(大约191 Ma B.P.)和侏罗纪最晚期到白垩纪最早期(大约146～148 Ma B.P.)分别有2幕岩浆活动;侵入于191.4±3.7 MaB.P.的变辉绿岩,是迄今为止在雅鲁藏布蛇绿混杂岩带发现的最...     

江西乐平塔前钼(钨)矿床成岩成矿时代及意义

探讨江西省乐平市塔前钼(钨)矿床成矿岩体的形成时代。通过对塔前钼(钨)矿床进行野外地质调查和SIMS锆石U-Pb测龄,结果表明其成矿母岩为花岗闪长斑岩,其加权平均年龄为160.9±2.5 Ma(MSWD=0.69),代表矿区花岗闪长斑岩的结晶时间。这与矿区矽卡岩矿石中辉钼矿Re-Os同位素年龄(162±2 Ma)在误差范围内一致,说明此矿床形成于中侏罗世晚期。将其与钦杭成矿带东段塔前-赋春成矿带与乐华—德兴成矿带中同类矿床进行对比发现,前者成矿年龄集中于161 Ma±,控岩控矿构造皆以北东向为主;而后者成矿年龄集中于170 Ma±,控岩控矿构造以北西向为主,预示钦杭成矿带东段2个成矿阶段(170~161 Ma B.P.)区域构造应力方向可能发生了重大转变。     

In situ simultaneous determination of trace elements,U-Pb and Lu-Hf isotopes in zircon and baddeleyite

This paper describes a combined method of simultaneously measuring U-Pb and Lu-Hf isotopes as well as trace elements in Phalaborwa baddeleyite and 91500, GJ-1, TEMORA-1 and SK10-2 zircons by means of Neptune MC-ICPMS and Agilent Q-ICPMS connected to a 193 nm excimer laser ablation system. Material ablated by laser was carried in different proportions into Q-ICPMS for U-Pb isotopic and trace elemental and MC-ICPMS for Lu-Hf isotopic compositions. Experiments indicate that different proportions of ablated material for the Q-ICPMS and MC-ICPMS (6:4, 5:5 and 4:6 respectively) do not show any bias for the zircon/baddeleyite U-Pb age, Lu-Hf isotope and trace elemental compositions within ana-lytical errors. Using 40―60 μm spot size, the obtained U-Pb ages of Phalaborwa baddeleyite, 91500, GJ-1, TEMORA and SK10-2 zircons are 2065±15 (2σ, n=20), 1063±6 (2σ, n=19), 613±6 (2σ, n=20), 416±5 (2σ, n=20) and 32.6±0.5 (2σ, n=20) Ma, respectively. The 176Hf/177Hf ratios are 0.281231±24 (2SD, n=20), 0.282310±35 (2SD, n=19), 0.282028±34 (2SD, n=20), 0.282687±34 (2SD, n=20) and 0.282752±53 (2SD, n=20), respectively. The obtained trace elemental compositions are identical to the reference values. Therefore, this kind of technique makes it possible to simultaneously obtain the U-Pb age, Lu-Hf iso-topes and trace elemental compositions of zircon and baddeleyite, which could be an important tool in solving problems in earth sciences.     

207Pb/206Pb determination of zircons by laser probe-inductively coupled plasma mass spectrometry

Laser probe_inductively coupled plasma mass spectrometry (LP_ICP_MS) was used to determine 207 Pb/ 206 Pb ratios in single zircon grains. A Nd∶YAG laser operating at 1 064 nm and a Fisons_VG PQII Turbo ICP_MS were employed. For the known zircons, the 207 Pb/ 206 Pb results of LP_ICP_MS are close to the thermal ionization mass spectrometry (TIMS) results. The spatial resolution of this method is 40-50 μm and the sensitivities for lead isotopes are about 300-400 cps/ppm. The precision of 207 Pb/ 206 Pb ratio is 0.3%-4% and depends on the lead concentration of the sampling pits. LP_ICP_MS provides a simple, rapid and economic method for zircon 207 Pb/ 206 Pb determination.     

湘西南-桂北交界地区加里东期基性岩的发现及其地质意义

湘西南-桂北交界地区属于江南造山带(或称之为钦杭结合带)西南段的重要组成部分,该区与Rodinia超大陆聚合与裂解有关的岩浆活动十分强烈,但至今还没有加里东期基性岩存在的报道。采用LA-ICP-MS锆石U-Pb法对湘西南—桂北交界地区首次发现的加里东期变辉长辉绿岩进行了年代学研究。结果表明,变辉长辉绿岩形成于(421.9±7.8)Ma,为加里东期岩浆活动的产物。该基性岩的发现及其年龄的确定;并结合区域上的研究结果表明,扬子板块和华夏板块结合带(钦杭结合带)西南段可能存在有从新元古代至早古生代的双向洋陆俯冲-碰撞过程。