北山地区古元古界北山岩群中长英质脉内锆石U- Pb年龄及其地质意义

深熔作用是大陆地壳分异、元素迁移富集的重要地质过程。位于中亚造山带南缘的北山地区,笔者等对前寒武系北山岩群进行了研究,结果表明该岩群普遍经历了角闪岩相变质及深熔作用,长英质浅色脉体分布广泛,却鲜有文献报道。绵山地区出露的古元古代北山岩群斜长角闪岩及相关的长英质浅色脉体的野外地质特征、相互关系共同指示了北山岩群变质地层经历了部分熔融,熔体在原地分凝聚积。锆石LA- ICP- MS U- Pb 测定结果表明,顺北山岩群变质地层片理产出的长英质浅色脉体锆石n(206Pb)/n(238U)加权平均年龄约为370 Ma,表明深熔作用发生时间为晚泥盆世; 一测点的n(207Pb)/n(206Pb)年龄为1623±12 Ma,与前人报道的北山岩群测年结果高度相近,虽此年龄值不足为证,但结合产出特征,可能指示长英质浅色脉体的源岩为北山岩群。此外,北山岩群中深熔作用普遍发育,露头可见大量的石榴子石、电气石、云母类矿物,指示区内部分稀有元素成矿潜力巨大。     

The Tectonic Implications of the Hongliuhe-Xichangjing Ophiolitic Mélanges Belt in the Central Region of the Beishan Orogen, NW China — Constrained by the U-Pb Ages of Detrital Zircons of the Metasandstones

The tectonic attributes of different blocks within orogenic belts are of great significance for the study of accretionary processes and the evolution of Earth. The Hongliuhe-Niujianzi-Baiyunshan-Xichangjing ophiolitic mélange belt(HXOMB) is distributed in the heart of the Beishan Orogen, the Shuangyingshan and Minshui-Hanshan blocks being distributed in the south and north of the HXOMB respectively, and a large number of Early Paleozoic geological units are exposed on the blocks. According to the zircon age populations of the metasandstones in the Baiyunshan area recovered in this paper, when compared with the zircon age populations of the Paleozoic metasandstones reported in the Niujuanzi and Hanshan areas, we found that the metasandstones of the Shuangyingshan Block have age peaks at c. 598 Ma, 742 Ma, 828 Ma, 941 Ma, 990 Ma, 1168 Ma, 1636 Ma, 2497 Ma with non-significant age populations of 1500–1300 Ma, showing a possible affinity with the Tarim Craton; the metasandstones of the Minshui-Hanshan Block have age peaks at c. 606 Ma, 758 Ma, 914 Ma, 1102 Ma, 1194 Ma, 1304 Ma, 1672 Ma with significant age populations of 1500-1300 Ma, showing a possible affinity with the Chinese Central Tianshan Block. Therefore, the HXOMB of the Beishan Orogen is of great significance in plate segmentation, which separates the Tarim Craton in the south and the Chinese Central Tianshan Block in the north. Based on the evolutionary process of the Hongliuhe-Xichangjing ocean in the Beishan Orogen, we believe that break-up and convergence can be recognized as having occurred twice between the Chinese Central Tianshan Block and the Tarim Craton since the Mesoproterozoic in the Beishan area. This was related firstly to the break-up of the Columbia Supercontinent and the convergence of the Rodinia Supercontinent, mainly during the Middle Mesoproterozoic to Early Neoproterozoic, and secondly to the opening and closing of the Hongliuhe-Xichangjing ocean, mainly during the Early Paleozoic.     

Revisiting the ca. 845–820-Ma S-type granitic magmatism in the Jiangnan Orogen: new insights on the Neoproterozoic tectono-magmatic evolution of South China

ABSTRACT

The Neoproterozoic tectonic evolution of the Jiangnan Orogen is controversial, with one of the issues being whether the ca. 850–820-Ma granitoids were generated by mantle plumes or the collision between the Yangtze and Cathaysia blocks. This paper tackles this problem by examining the age and petrogenesis of one of the granitoids, the Getengling pluton in the central Jiangnan Orogen, and through comparison with a regional geochronological–geochemical database compiled from previous studies. The Getengling pluton is characterized by high A/CNK values (~1.5), slight negative whole-rock ε_Nd(t) values (?2.8 to ?3.4), and positive zircon ε_Hf(t) values (0.7 ± 1.1), suggesting S-type granite affinities with juvenile contributions. Rb/Sr, Rb/Ba, and high CaO/Na₂O ratios indicate psammitic sources with both clay-rich and clay-poor characters. These geochemical characteristics are distinct from those of the granitoids (typically of A type) associated with mantle plumes. The zircon laser ablation-inductively coupled plasma-mass spectrometry U–Pb age of 845 ± 4 Ma obtained in this study, together with other ca. 835–820 Ma ages of S-type granites in the Jiangnan Orogen, indicates that the felsic magmatism in the Jiangnan Orogen lasted for ca. 25 Ma, which is longer than typical plume-related felsic magmatism. In addition, the mafic rocks in the Jiangnan Orogen and elsewhere in the South China Block are geochemically distinct from the coeval mantle plume-related ones in Australia and west Laurentia. In geochemical diagrams diagnostic of tectonic settings, the Getengling pluton and other ca. 850–820 Ma intrusions plot in the syn- and post-collisional fields, whereas the pre-850 and post-820-Ma igneous rocks plot in the arc and within-plate settings, respectively. This sequential tectonic evolution from plate subduction through collision to within-plate environments further supports the hypothesis that the ca. 850–820-Ma granitoids in the Jiangnan Orogen resulted from the Yangtze–Cathaysia collision rather than from mantle pluming.     

新疆铁木尔特铅锌铜矿床锆石U-Pb和黑云母40Ar/39Ar年代学及其矿床成因意义

新疆铁木尔特铅锌铜矿床位于阿尔泰造山带南缘克兰盆地内,矿体呈脉状产于康布铁堡组火山岩地层中.为准确厘定其成岩成矿时代,作者分别对矿区赋矿火山岩和含矿石英脉中的云母进行了年龄测定,获得2件火山岩样品的锆石LA-ICP-MS U-Pb年龄分别为396±5Ma和405±5Ma,2件黑云母样品的40 Ar/39 Ar坪年龄分别为240±2Ma和235±2Ma,相应的39Ar/36Ar-40Ar/36Ar等时线年龄分别为238±3Ma和233±3Ma,与坪年龄在误差范围内一致.据此,认为矿区内康布铁堡组火山岩形成于396～405 Ma,成矿作用发生于235～240Ma;成岩年龄早于成矿年龄约165Ma.因此,铁木尔特铅锌铜矿为典型的后生矿床,而不可能是同生VMS型矿床.考虑到成矿年龄稍晚于区域大规模变质作用(约250Ma),推测成矿作用与阿尔泰造山带碰撞造山作用有关.结合矿床地质特征和流体包裹体特征,认为铁木尔特铅锌铜矿为典型的陆陆碰撞体制下形成的造山型矿床.     

新疆康西瓦伟晶岩的成矿潜力判断: 来自锆石U-Pb年代学、地球化学与Hf同位素证据

稀有金属作为战略性关键金属矿产对我国国家安全和经济健康可持续发展具有重要的意义。它们是不可再生的,并具有独特的物理化学性质。新疆西昆仑地区通过多年的基础地质调查和找矿勘查,在大红柳滩-白龙山一带的稀有金属找矿取得重大突破。本文对大红柳滩西的康西瓦铅银矿区中的无矿伟晶岩进行了岩石学、元素和同位素地球化学及同位素年代学综合研究。无矿伟晶岩富SiO₂,贫Al₂O₃、TiO₂、MgO、P₂O₅,主体属于高钾钙碱型系列岩石;无矿伟晶岩中的锆石是岩浆锆石,LA-ICPMS U-Pb年龄为208±5.5Ma~210.3±2.3Ma。稀土总量低（∑REE 4.21×10^-6~26.98×10^-6）,亏损重稀土元素,Eu负异常明显（δEu为0.04~0.61）,富集Rb、U和K,亏损Nb、La和Zr,严重亏损Ba、Ti和Sr。研究表明,康西瓦无矿伟晶岩与其东侧的大红柳滩岩体形成于同一时期,Hf同位素特征相似。根据大红柳滩稀有金属矿床成矿特征,预测康西瓦含绿柱石白云母伟晶岩脉带以北喀拉喀什河第四系覆盖区及康西瓦一带海拔更高的地段是下一步锂矿找矿取得突破的有利区段。

     

Structural and timing constraints on molybdenum and tungsten mineralisation at Yea,Victoria

The molybdenite and scheelite mineralisation in the Native Dog Pluton at Monkey Gully near Yea is hosted within an I-type (post-orogenic) pluton, which shows extensive fractionation and magma mixing, and was emplaced in an extensional environment. The pluton comprises four principal rock types: tonalite, granodiorite, dacite and leucogranite. Emplacement of the pluton was in an extensional northwest to southeast paleostress field. Early extensional quartz veins, related to cooling, are overprinted by both dacitic dykes and late-stage quartz sheeted veins. The late-stage veins host the molybdenum and tungsten mineralisation in the deposit. ²⁰⁶Pb/²³⁸U zircon ages of 356 ±14 Ma and 375 ± 22 Ma place pluton formation and mineralisation at the onset of the Kanimblan Orogeny and later than other major molybdenum deposits in Victoria. Key factors governing the source for the granite and its associated mineralisation are: (1) the presence of a highly fractionated and sulfur-rich leucogranite; and (2) the pluton's location in a regional jog overlying the Selwyn basement block. Finally, a model is developed to explain the differences between this Melbourne Zone molybdenum and tungsten deposit compared with other metallogenic porphyry deposits.     

河南银洞沟银金钼矿床铼-锇同位素定年和加里东期造山-成矿事件

河南内乡银洞沟银金钼矿床是夏馆银多金属矿田新发现的造山型矿床,赋矿地层为新元古代-早古生代弧后盆地堆积的二郎坪群火山-沉积岩系,二郎坪群在志留纪或加里东运动中遭受区域变形和变质.矿床5件辉钼矿Re-Os同位素模式年龄介于423.4±4.4～432.2±3.4Ma,加权平均年龄为429.3±3.9Ma(MSWD=0.73),指示秦岭地区存在显著的加里东期造山-成矿事件.     

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.     

南秦岭山阳-柞水矿集区构造-岩浆-成矿作用

山阳-柞水矿集区是南秦岭成矿带主要矿集区之一。从新元古代到晚侏罗世-早白垩世,山阳-柞水地区经历了洋壳俯冲和陆陆碰撞的构造演化,在演化的不同阶段形成了不同矿化类型、矿化组合的Cu、PbZn、Ag、Fe、Mo、Au矿床。新元古代时期,由于洋壳的俯冲作用,山阳-柞水地区形成了与基性岩有关的钛磁铁矿;泥盆纪-二叠纪时期,古秦岭洋消减形成山阳-柞水弧前盆地的同时,也形成一系列沉积型Fe、Ag、PbZn矿床;三叠纪时期,华北、扬子板块全面碰撞并形成了与碰撞作用密切相关的晚三叠世斑岩型Mo矿和造山型Au矿;晚侏罗世-早白垩世时期,则形成了碰撞后伸展环境下的矽卡岩-斑岩型CuMo-Au矿床。山阳-柞水矿集区内的各种时代和类型的矿床是秦岭造山带不同阶段的构造-岩浆活动的产物,对真实、客观的理解秦岭地区的构造演化具有重要意义。     

江南造山带西南缘石英脉型金矿中毒砂Re-Os同位素定年研究

江南造山带西南缘的贵州东南部地区石英脉型金矿产于低绿片岩相的板溪群浅变质岩中,具有典型的造山型金矿特征。与江南造山带其他造山型金矿一样,其成矿时代具有较大的争议,金矿的形成与造山运动的关系还不清楚,制约了对本区金矿成矿作用的深入研究。本文采集了平秋和八克两个石英脉型金矿中与金成矿作用密切相关的含金石英脉中毒砂样品进行高精度的Re-Os同位素分析,分别得到235±3.4 Ma和410±52 Ma两个Re-Os同位素等时线年龄,八克金矿毒砂样品的年龄误差较大的原因是毒砂中Re含量较低所致,但其中低含量高射性样品计算得到的Re-Os同位素模式年龄仍然与其等时线年龄一致。两个矿床的毒砂Re-Os同位素年龄揭示了江南造山带石英脉型金矿与加里东期和印支期两次陆内造山运动有关,是华南显生宙陆内造山运动的产物。