Molybdenite Re–Os dating constrains gravitational collapse of the Sveconorwegian orogen, SW Scandinavia

Re–Os dating of molybdenite from small deposits is used to define crustal domains exhibiting ductile versus brittle behaviour during gravitational collapse of the Sveconorwegian orogen in SW Scandinavia. A 1019 ± 3 Ma planar quartz vein defines a minimum age for brittle behaviour in central Telemark. In Rogaland–Vest Agder, molybdenite associated with deformed quartz and pegmatite veins formed between 982 ± 3 and 947 ± 3 Ma in the amphibolite-facies domain (three deposits) and between 953 ± 3 and 931 ± 3 Ma west of the clinopyroxene-in isograd (two deposits) in the vicinity of the 0.93–0.92 Ga Rogaland anorthosite complex. The data constrain the last increment of ductile deformation to be younger than 0.95 and 0.93 Ga in these two metamorphic zones, respectively. Molybdenite is the product of an equilibrium between biotite, oxide and sulfide minerals and a fluid or hydrated melt phase, after the peak of 1.03–0.97 Ga regional metamorphism. Molybdenite precipitation is locally episodic. A model for gravitational collapse of the Sveconorwegian orogen controlled by lithospheric extension after 0.97 Ga is proposed. In the west of the orogen, the Rogaland–Vest Agder sector is interpreted as a large shallow gneiss dome, formed slowly in two stages in a warm and structurally weak crust. The first stage at 0.96–0.93 Ga was associated with intrusion of the post-collisional hornblende–biotite granite suite. The second stage at 0.93–0.92 Ga, restricted to the southwesternmost area, was associated with intrusion of the anorthosite–mangerite–charnockite suite. Most of the central part of the orogen was already situated in the brittle upper crust well before 0.97 Ga, and did not undergo significant exhumation during collapse. In the east of the orogen, situated against the colder cratonic foreland, exhumation of high-grade rocks of the Eastern Segment occurred between 0.97 and 0.95 Ga, and included preservation of high-pressure rocks but no plutonism.     

秦岭造山带秋树湾铜钼矿床辉钼矿Re-Os定年及其地质意义

秋树湾铜钼矿床是秦岭造山带东段最大的斑岩-夕卡岩型铜钼矿床,通过对矿石中6个辉钼矿样品的 Re-Os 同位素分析,得到了145.57±1.80～147.98±2.21Ma 的模式年龄(平均为146.42±1.77Ma)及一个相关性很好的等时线年龄147±4Ma,表明秋树湾铜钼矿床形成于晚侏罗世,与其以北的华北克拉通南缘的主要斑岩型钼矿床及位于扬子克拉通北缘的长江中下游铁铜矿床的大规模成矿时间一致,是中国东部中生代第2期大规模成矿作用的产物。秋树湾铜钼矿床辉钼矿的含铼量平均达151.8×10~(-6),明显高于华北克拉通南缘钼矿带同期形成的钼矿床(16.13×10~(-6)～28.09×10~(-6)),认为主要是由于两者成矿元素 Cu/Mo 比值的不同造成的;结合矿石硫同位素特征,认为矿床的物质主要来源于下地壳。     

青藏高原与大陆动力学——地体拼合、碰撞造山及高原隆升的深部驱动力

运用地体和地体活动论观点,提出青藏高原结构划分的新方案;强调青藏高原的形成经历了新元古代以来长期活动的过程,青藏高原是一个“非原地”诸多地体会聚、拼合以及经历复合碰撞造山的“造山的高原”;大型走滑断裂在青藏高原形成中起着地体相对位移、侧向挤出、移置及使高原几何形态扭曲的作用。提出青藏高原隆升的“南缘超深俯冲(>600km)、北缘陆内俯冲、腹地深部热结构及岩石圈范围内的向NE右旋隆升”的多元驱动力机制。     

赣东北蛇绿混杂岩带和变质岩系中"放射虫硅质岩"的再研究

赣东北蛇绿混杂岩和变质岩中的硅质岩时代问题已成为研究华南区域构造和古地理的焦点之一，长期以来被地质学界认为属于江南古陆元古代“板溪群”的范畴。近年来，关于该构造岩系中存在晚古生代放射虫动物群的报道，导致一些学者怀疑该地区传统的构造古地理格局需要重新解释。对此，许多地学工作者提出异议。为了验证上述放射虫动物群的报道的准确性，从古生物学、构造地质学和区域地质调查等多领域进行了野外调查和样品的多次重复系统采集；同时，对已发表的资料进行了再分析。研究显示，赣东北蛇绿混杂岩和变质岩中的硅质岩和扳岩的岩石薄片和微体古生物分析样品中未产出放射虫化石，但是在部分硅质岩和板岩样品中却发现了中-新元古代的疑源类化石。有关硅质岩的主量元素、稀上元素中Ce异常值、（La/Ce）x比值和微量元素等地球化学特征显示其沉积作用与陆源物质有关，沉积环境接近大陆边缘、远离深海远洋环境。     

Ore Geology, Fluid Geochemistry and Genesis of the Shanggong Gold Deposit, Eastern Qinling Orogen, China

Abstract. The Shanggong Au deposit in the Xiong'er Terrane, East Qinling, has reserves of about 30 t Au, making it one of the largest orogenic‐type Au deposits hosted in volcanic rocks in China. The deposit is hosted in the andesitic assemblage of the Xiong'er Group of 1.85?1.4 Ga. Three stages of hydrothermal ore‐forming processes are recognized, Early (E), Middle (M) and Late (L), characterised by quartz‐pyrite, polymetallic sulfides and carbonate‐quartz, respectively. Homogenization temperatures of fluid inclusions are between 380‐320d?C for the E‐stage, 300‐220d?C for the M‐stage and 200‐120d?C for the L‐stage. The composition of fluid inclusions changed from CO₂‐rich in the E‐stage to CO₂‐poor L‐stage. The M‐stage fluid has the highest contents of cations and anions (e.g., SO₄^2‐, Cl¹, K+), the highest (K+Na)/(Mg+Ca) and lowest CO₂/H₂O ratios, which probably resulted from CO₂ phase separation. This, together with the alkaline and reducing conditions, as indicated by highest pH and lowest Eh values, is most conducive to the deposition of polymetallic sulfides and native elements such as Au, Ag and Te. H‐O isotope systematics indicate that ore fluids evolved from deep‐sourced through to shallow‐sourced, with the M‐stage being a mixing phase of these two fluid‐systems. Nineteen δ¹⁸O_W values, from 4.2 to 13.4 %o, averaging 8.1 %o, suggest that the E‐stage fluids derived from metamorphic devolatilization of sedimentary rocks at depth. Comparison of the H‐O isotope systematics between the Shanggong deposit and the main lithologies in the Xiong'er Terrane, shows that neither these nor the underlying lower crust and mantle, or combinations thereof, could be considered as the source of ore fluids and metals for the Shanggong Au deposit. Instead, a source which meets the isotopic constraints, is a carbonaceous carbonate‐sandstone‐shale‐chert (CSC) sequence, which is present in the Guandaokou and Luanchuan Groups in the south of the Xiong'er Terrane. This conclusion is supported by thirteen high δ¹⁸O values of the Meso‐Neoproterozoic strata south of the Machaoying fault, and the high δ¹⁸O_W values calculated for their possibly metamorphic fluids. It can be also supported by previous observation that the Guandaokou and Luanchuan Groups were underthrust beneath the Xiong'er Terrane, during the Mesozoic collision between the Yangtze and Sinokorean continents. Available isotope ages, together with geological field data, constrain the timing of the Au metallogenesis between 250?110 Ma. This metallogenesis and associated granitic magmatism, can be related to the Yangtze‐Sinokorean continental collision that resulted in the formation of the Qinling Orogen. This collision event progressed from early compression (Triassic to Early Jurassic), through middle compression‐to‐extension transition (Late Jurassic to Early Cretaceous), to late extension (Cretaceous). These three stages in the evolution of the Qinling Orogen form the basis of an ore genesis model that combines collisional orogeny, metallogeny and fluid flow (CMF model). These three evolutionary stages correspond to the three‐stages of ore‐forming fluids of the Shanggong Au deposit. We conclude that the formation of the Shanggong Au deposit is a result of the Mesozoic northward intracontinental A‐type subduction along the Machaoying fault during Yangtze‐Sinokorean continental collision, which led to the metamorphic devolatilization of the CSC sequence, thereby providing both fluids and metals.     

广西大容山S-型花岗岩套

大容山S-型花岗岩套位于华南海西地槽褶皱带的钦州地槽区。岩套包括侵入岩和火山岩两个建造和三个岩相:侵入岩相——堇青石黑云母花岗岩;次火山岩相——自碎斑状紫苏辉石花岗斑岩;火山岩相——辉石英安岩和英安流纹岩。它们是在麻粒岩相条件下,由过铝成质源岩重熔产生的S-型花岗岩浆派生而来。主体形成于海西晚期。从东向西岩套具有规律的演化趋势。     

山东玲珑和郭家岭岩体的同位素年龄及其地质意义

本文采用~(40)K-Ar/~(39)Ar快中子活化和K-Ar稀释法定年技术对玲珑岩体和郭家岭岩体进行了年龄测定。~(40)Ar/~(39)Ar阶段加热年龄谱表明,玲珑岩体和郭家岭岩体的坪年龄分别为164.2±0.7Ma和134.8±1.7Ma,其形成时代同属燕山期,为同一成因不同阶段的产物。同位素年龄提供的数据支持了两岩体为交代混合岩化的成因观点。~(40)Ar/~(39)At和K-Ar年龄结果还表明:角闪石和黑云母均是~(40)Ar/~(39)Ar理想定年矿物,对K-Ar法而言,角闪石最理想,黑云母次