安徽西冲钼矿床细粒花岗岩的岩石定年、岩石化学及与成矿的关系研究

西冲钼矿是大别成矿带东段近年新发现的斑岩型钼-多金属矿床,其矿床学和矿床地球化学研究少见报导。该研究成果丰富了我国斑岩-矽卡岩型钼钨-多金属矿床研究成果,同时对指导区域找矿具有一定意义。本文在详细的野外地质调研基础上,对大别成矿带西冲钼矿区出露的主要岩石单元、矿化和蚀变特征、细粒花岗岩的岩石学、岩石化学和成岩年代学进行了系统的研究。结果表明,矿区出露的主要侵入岩单元由老至新依次为:石英二长岩、细粒花岗岩(130.8±1.1Ma)及双峰式脉岩组合(131.9±1.4Ma和131.3±1.6Ma),其均为燕山期岩浆活动产物,产于华北与扬子陆块拼合后的陆内环境。矿化类型以脉状、网脉状和浸染状为主,为一斑岩型钼-多金属矿床。矿区蚀变发育,包括黑云母-磁铁矿化、钾长石化、硅化、石英-绢云母化、石英-绿帘石化、绿泥石-碳酸盐化和泥化,且空间上具有一定的分带规律。矿区已知矿化、蚀变及Cu-Mo化探异常的空间展布与细粒花岗岩关系密切,且细粒花岗岩中具有高的Mo含量,表明该矿床的形成主要与细粒花岗岩有关。岩石化学结果表明,细粒花岗岩属过铝质-准铝质、高钾钙碱性-钾玄质过渡岩石,锆石U-Pb年龄与双峰式脉岩的在误差范围内一致,表明其均形成于大别造山带大规模伸展背景。

苏皖地区幔源橄榄岩捕虏体的锇同位素组成、模式年龄及其意义

苏皖地区发育的新生代玄武岩中富含地幔橄榄岩捕虏体 ,测定了 2 5个橄榄岩全岩的锇同位素组成。结果表明大部分样品呈亏损特征 ,1 87Os/ 1 88Os=0 .119～ 0 .12 9. Os同位素比值与主量成分之间有显著的相关性 .采用 1 87Os/ 1 88Os- Al2 O3代用等时线方法 ,由截距得到初始值 (1 87Os/ 1 88Os) 0 =0 .10 9,在对流地幔的 1 87Os/ 1 88Os比值演化线上获得模式年龄 t=2 .5 (±0 .1) Ga,为晚太古 -早元古代。用样品组中最低的锇同位素组成 ,即 1 87Os/ 1 88Os=0 .119,计算 Re亏损模式年龄 ,t=1.2 Ga,为中元古代。表明苏皖地区新生代玄武岩中的地幔橄榄岩捕虏体具有古老的形成年龄 ,它们是经过显生宙减薄作用后的残余地幔的碎片。大陆岩石圈地幔的古老形成年龄与上覆地壳克拉通的古老稳定年龄相耦合。

Geochemical characteristics and petrogenesis of volcanic rocks from Baiyingaolao Formation in northeastern Hailar Basin

The volcanic rocks from Baiyingaolao Formation in the northeastern Hailar Basin are mainly composed of rhyolite and trachydacite. U-Pb dating of zircon shows these volcanic rocks formed in Early Cretaceous( 128--124 Ma). Geochemical data indicate that they are sub-alkaline series in composition and rich in alkali and potassium. All samples have similar rare earth element patterns characterized by high total rare earth elements contents( ∑REE = 113. 96 × 10- 6-204. 33 × 10- 6),significant fractionation of heavy and light rare earth elements( ∑LREE / ∑HREE = 3. 10-4. 52) with middle negative Eu anomalies( δEu = 0. 46-0. 76).The trace elements are characterized by enrichment in large-ion lithophile elements such as K,Rb,LREE and depletion in high field strength elements e. g. Nb,Ta,HREE,P and Ti,while enriched in Th and U. Rhyolite and trachydacite contain low initial87 Sr /86 Sr ratios( 0. 704 9--0. 7 053) and positive εNd( t) values( ca. 4.15). These data suggest that the magma of rhyolite and trachydacite were derived from mafic lower crust newly accreted from mantle,with the evolutional trend of comagmatic fractional crystallization.

Neoproterozoic basic magmatism in the southeast margin of North China Craton:Evidence from whole-rock geochemistry,U-Pb and Hf isotopic study of zircons from diabase swarms in the Xuzhou-Huaibei area of China

This paper presents a synthetic U-Pb and Hf isotopic study of zircons and a whole-rock geochemical study on diabase swarms that were emplaced into the Proterozoic formations in the Xuzhou-Huaibei area,the southeast margin of North China Craton.Zircons from the diabase dykes display weak oscillatory zoning in the cathodoluminescence(CL) images and show high Th/U ratios(0.91-6.90),both of which are typical of a magmatic origin.The SHRIMP and Q-ICP-MS U-Pb zircon dating results indicate that the diabase swarms were emplaced at ca.890 Ma.Hf isotopic analysis on these zircons gives Hf(t) values ranging from 2.26 to 14.74 and Hf model age t DM1 and t DM2 ranging from 843 to 1321 Ma and from 808 to 1779 Ma,respectively.Geochemically,the diabase swarms are characterized by a relative enrichment in LREE and LILE(Rb,Ba and La),a comparative enrichment in Cr and Ni,but a slight depletement in HFSE(Th,Nb,Ta,Zr and Hf).They are plotted in the within-plate tholeiite series on the tectonic discrimination diagrams.Taken together,these geochemical and isotopic data suggest that the primary magmas to form the diabase dykes in the Xuzhou-Huaibei area could have been derived from a transitional mantle in the Mesoproterozoic,and were most probably emplaced under a continental margin extensional setting at ca.890 Ma.The results of this study demonstrate that the Neoproterozoic(800-900 Ma)magmatism was not restricted to the Yangtze Block,but also occurred at the southeast margin of the North China Craton.

Petrogenesis of Early Cretaceous adakitic granodiorite: Implication for a crust thickening event within the Cathaysia Block, South China

Adakitic rocks in continental settings are commonly considered to be formed by partial melting of thickened or delaminated lower crust. Investigations on this kind of rocks can provide important information about crustal evolution complementary to information from other rocks. This paper reports adakitic granodiorite of the Lingxi pluton in the interior of the Cathayisa Block. LA-ICP-MS zircon U-Pb dating shows that it was formed in the late Early Cretaceous(100±1 Ma). The granodiorite has geochemical features of adakitic rocks derived from partial melting of the thickened lower crust, e.g., high SiO2(mainly ranging from 64.4 to 68.9 wt.%) and Sr(624–894 ppm) contents, Sr/Y(49.9–60.8) and La/Yb(23.4–42.8) values, low Y(10.3–17.1 ppm), Ni(5.62–11.8 ppm) and MgO(mostly from 0.86 wt.% to 1.57 wt.%) contents and weak Eu anomaly. It has initial 87Sr/86 Sr ratios of 0.7086–0.7091, εNd(t) values of.6.2 to.5.9 and zircon εHf(t) values mostly of.10.1 to.7.6. Based on the geochemical characteristics and simple modelling, it is suggested that the most likely generation mechanism of the Lingxi granodiorite is partial melting of a thickened Proterozoic lower continental crust at a pressure ≥12 kbar(or crust thickness ≥40km), leaving a garnet-bearing amphibolite residue. Combining our results and previous studies of the tectonic evolution of the Cathaysia Block, we propose that the crust was thickened to over 40 km by a compressive event occurring during the late Early Cretaceous, which is supported by the observation that there is an angular unconformity between the Upper Cretaceous Series and the early Lower Cretaceous or the Jurassic rocks. After this event, the Cathaysia Block experienced a lithospheric extension and thinning probably driven by the high-angle paleo-Pacific subduction. With the attenuation of lithosphere, the lower crust was heated to partial melting by upwelling asthenospheric materials, resulting in generation of the Lingxi granodiorite and other coeval granitoids in the Cathaysia Block. This study provides new information on the crustal evolution of the Cathaysia Block during the Early Cretaceous.

Petrogenetic simulation of the Archean trondhjemite from Eastern Hebei, China

It is generally believed that trondhjemitic rock, an important component of TTG rocks, is the anatectic product of mafic rocks. However, in many TTG gneiss terranes, for instance, the granulite facies terrane in Eastern Hebei, trondhjemites occur as small dikes, intrusions or leucosomes in tonalitic gneisses, suggesting their origin of in-situ partial melting. Based on the petrological analysis of a tonalitic gneiss sample from Eastern Hebei, in combination with zircon U-Pb dating, we investigated the petrogenesis of trondhjemite through simulating anatectic reactions and the major and trace element characteristics of the product melt at different pressures(0.7, 1.0 and 2.0 GPa). The results indicate that hornblende dehydration melting in a tonalitic gneiss at 0.9–1.1 GPa and 800–850°C, corresponding to the high-T granulite facies, with melting degrees of 5–10wt.% and a residual assemblage containing 5–10wt.% garnet, can produce felsic melts with a great similarity, for instance of high La/Yb ratios and low Yb contents to the trondhjemitic rocks from Eastern Hebei. However, the modelled melts exhibit relatively higher K2 O, and lower CaO and Mg~# than those in the trondhjemitic dikes and leucosomes from Eastern Hebei, suggesting that the leucosomes may not only contain some residual minerals but also be subjected to the effect of crystal fractionation. The zircon U-Pb dating for the tonalitic and trondhjemitic rocks in the Eastern Hebei yields a protolith age of 2518±12 Ma and a metamorphic age of 2505±19 Ma for the tonalitic gneiss. The latter age is consistent with a crystallization age of 2506±6 Ma for the trondhjemitic rock, confirming a close petrogenetic relation between them.

Emplacement and deformation of Shigujian syntectonic granite in central part of the Dabie orogen:Implications for tectonic regime transformation

The Shigujian pluton is a gneissic quartz monzonite located in Tiantangzhai area in central part of the Dabie orogen.Anisotropy of magnetic susceptibility(AMS) data show that most magnetic foliations dip steeply to southeast.About 85% of sampling points dip from 40° to 90°.Magnetic foliations are generally parallel to the foliations measured in the field.The pluton has NWW-SEE trending lineations in the southeast and NE-SW trending lineations in central part and north,but the lineations plunge to SW in central part and to NE in the north.All plunges are moderate.The anisotropy degree(P) is between 1.065 and 1.532 and the shape parameter(T) is between 0.005 and 0.694.A Flinn diagram of the magnetic fabrics shows that the value of K is less than 1.The analysis of AMS suggests that the pluton was emplaced and deformed under a SE-NW compressional stress regime.The analysis of quartz C-axis fabrics indicates that the pluton was deformed under compressional stress and deformation temperatures range from 400 to 500℃.Microstructures indicate that the pluton is deformed in near solidus conditions and the pluton is a synkinematic intrusion.The emplacement of the Shigujian granite is inferred to have taken place syntectonically.The zircon U-Pb dating of the granite suggests that the pluton was intruded at 141±2.3 Ma.By synthesizing all data,it seems that the Shigujian pluton was emplaced in a compressional environment and the transformation time of the Dabie orogen from compression to extension took place after 141 Ma.The structural evolution of the Dabie orogen was controlled by the Pacific tectonic domain when the Shigujian pluton was emplaced,whereas the adjacent Tiantangzhai complex massif is the result of an extensional environment.