Prolonged Variscan to Alpine history of an active Eurasian margin (Georgia, Armenia) revealed by Ar/Ar dating

Variscan to Alpine magmatic activity on the North Tethys active Eurasian margin in the Caucasus region is revealed by ⁴⁰Ar/³⁹Ar ages from rocks sampled in the Georgian Crystalline basement and exotic blocs in the Armenian foreland basin. These ages provide insights into the long duration of magmatic activity and related metamorphic history of the margin, with: (1) a phase of transpression with little crustal thickening during the Variscan cycle, evidenced by HT-LP metamorphism at 329–337 Ma; (2) a phase of intense bimodal magmatism at the end of the Variscan cycle, between 303 and 269 Ma, which is interpreted as an ongoing active margin during this period; (3) further evolution of the active margin evidenced by migmatites formed at ca. 183 Ma in a transpressive setting; (4) paroxysmal arc plutonic activity during the Jurassic (although the active magmatic arc was located farther south than the studied crystalline basements) with metamorphic rocks of the Eurasian basement sampled in the Armenian foreland basin dated at 166 Ma; (5) rapid cooling suggested by similar within-error ages of amphibole and muscovite sampled from the same exotic block in the Armenian fore-arc basin, ascribed to rapid exhumation related to extensional tectonics in the arc; and finally (6) cessation of ‘Andean’-type magmatic arc history in the Upper Cretaceous. Remnants of magmatic activity in the Early Cretaceous are found in the Georgian crystalline basement at c. 114 Ma, which is ascribed to flat slab subduction of relatively hot oceanic crust. This event corresponds to the emplacement of an oceanic seamount above the N Armenian ophiolite at 117 Ma. The activity of a hot spot between the active Eurasian margin and the South Armenian Block is thought to have heated and thickened the Neo-Tethys oceanic crust. Finally, the South Eurasian margin was uplifted and transported over this hot oceanic crust, resulting in the cessation of subduction and the erosion of the southern edge of the margin in Upper Cretaceous times. Emplacement of Eocene volcanics stitches all main collisional structures.     

基于P波三重震相的下扬子克拉通地幔转换带顶部低速层初探

本文基于中国地震观测台网记录到的震中距为10°~23°之间琉球俯冲区一个中深源地震的P波三重震相信息,研究了下扬子克拉通转换带顶部P波速度结构.通过射线追踪和理论地震图与观测地震波形的对比,发现下扬子克拉通下方的410 km间断面为一厚度20 km的梯度带,其上存在一由西南向东北变厚的低速层,厚度变化40~57 km,P波速度减低2.7%~4.5%.该低速层可以被认为是由于地幔橄榄岩部分熔融引起的.     

Morphogenesis of Congo submarine canyon and valley: implications about the theories of the canyons formation

The bottom and edges of the Congo submarine canyon are made of deposits which show morphologies of superposed terraces associated with lense side slope. Using 3D seismic, 3.5 kHz acoustic profiles and core datas, these geometries are interpreted as being mainly the result of the polyphase processes due to the activity of turbidity currents. All the turbidite deposits located below the canyon floor and the actual Congo fan belong to a unique depositional sequence which begun at the early Pliocene. Probably, this sequence has been initiated by an important uplift of the West Africa margin.     

REE-HFSE distribution/partitioning between garnetiferous restites and TTG from Nademavinapura area,Western Dharwar craton

The major part of the Peninsular Gneiss in Dharwar craton is made up of Trondjhemite-Tonalite-Granodiorite (TTG) emplaced at different periods ranging from 3.60 to 2.50 Ga. The sodic-silicic magma precursors of these rocks have geochemical features characteristic of partial melting of hydrated basalt. In these TTGs, enclaves of amphibolites (± garnet) are abundant. These restites are considered to be the residue of a basaltic crust after its partial melting. A detailed study of these (residue) enclaves reveals textures formed due to the process of partial melting. Major, trace and REE analysis of these residue enclaves and the melt TTGs and microprobe analysis of the coexisting minerals show partitioning of REE and HFSE between the precursor melt of TTGs and the upper amphibolite facies residues. Formation of garnetiferous amphibolites with biotite, Cpx and plagioclase consequent to melting, has squeezed the original MORB type of basaltic crust and given rise to the TTGs, depleted in Y, Yb, K₂O, MgO, FeO, TiO₂ and enriched in La, Th, U, Zr and Hf. Coevally during the process of melting, the hydrated basalt was depleted in Na₂O, Al₂O₃, LREE, Th, U and enriched in K₂O, MgO, Nb, Ti, Yb, Y, Sc, Ni, Cr and Co. Mineral chemistry of co-existing garnet-biotite and amphibole-plagioclase in these amphibolitic (restite) enclaves indicates an average temperature of 700 ± 50° C and pressure of 5 ± 1 Kbar. These data are inferred to indicate that during the garnet stability field metamorphism, effective fractionation of HREE and HFSE has taken place between the restites having Fe-Mg silicates, ilmenites and the extracted melt generated from the MORB type of hydrated basalt. These results are strongly substantiated by the reported melting experiments on hydrated basalts.     

Late Cenomanian–Early Turonian ammonites of the southern Tethys margin from Morocco to Oman: Biostratigraphy,paleobiogeography and morphology

The Cenomanian–Turonian ammonite biostratigraphical framework for the southern Tethys margin (North Africa, Middle East and the Arabian Peninsula) is becoming better understood. A first attempt at a synthetic range chart is presented, with 85 taxa and precise correlations for ammonites along a west–east transect from Morocco to Oman, inclusive of the Trans-Saharan Seaway as far south as northern Nigeria. On the basis of a critical review of ammonite taxonomy, 13 bioevents can be identified in the interval from the Late Cenomanian to the Early Turonian (c. 3.5 myr) with each bioevent corresponding to a time interval of approximately 270,000 years, on average. They are consistent throughout several regions along the southern Tethys margin, though some gaps remain, at least at the stage boundary. These bioevents are correlated with the zonation defined for the stratotype (GSSP) of the base of the Turonian in the Western Interior (USA). The paleobiogeographic distribution of ammonites reveals some endemism but the predominant picture is that of a homogeneous fauna throughout the area, even though distinct Boreal and Western Tethys (Atlantic domain) marine influences are evident. An interpretation of the evolution of conch morphology and ornamentation through the zones of the Late Cenomanian–Early Turonian is proposed.     

内蒙古哈达门沟金矿床地质特征及成矿作用探讨

哈达门沟金矿床地处华北克拉通北缘阴山隆起带南缘乌拉山复背斜南翼,金矿体主要产于新太古界乌拉山群变质岩中,主要矿石类型包括含金石英脉、含金石英-钾长石脉、含金钾化蚀变岩等。成矿流体主要为中低温(160~300℃)、中低盐度(5%~15%NaCleq),气相成分以H2O和CO2为主,液相组分阴离子以Cl-和SO2-4为主,阳离子以Na+、K+和Ca2+为主。矿石δ34S变化于-21.7‰~5.4‰,极差为27.1‰,说明成矿物质来源的复杂性,平均值为-10.6‰,表现出亏损重硫的特点,硫继承了太古代地层中硫的同位素特点,并混有深部含矿流体的硫。矿石铅同位素组成、单阶段模式年龄、Th/U比值、μ值变化范围较大,表现出多源特点,在铅构造模式图上,投点比较分散,表明哈达门沟矿石铅来源的复杂性。氢-氧同位素表明,成矿流体来源于岩浆水和部分变质热液,后期有天水的混入。矿石辉钼矿Re-Os同位素加权平均年龄为386.4±2.7Ma,等时线年龄为386.6±6.1Ma,金钼矿床主要成矿时代为早泥盆世,后期有多期热液活动叠加。矿床形成于弧-陆碰撞后的伸展构造背景。     

滇东北下田坝花岗岩年代学、地球化学及其构造意义

滇东北下田坝花岗岩的SiO2质量分数在71.1%～78.15%之间,Al2O3值在11.55%～14.90%之间,A/CNK=0.94～1.25,属于弱过铝质花岗岩。稀土元素总量(∑REE)为259.71×10-6～804.60×10-6,∑LREE/∑HREE为4.12～10.28,说明轻稀土较重稀土富集。岩石具有明显的负Eu异常(δEu=0.04～0.07),大离子亲石元素Rb、Ba、Pb、U等相对富集,高场强元素Nb、Ta、Zr、Ti等明显亏损,这些特征说明下田坝花岗岩是由壳源物质部分熔融形成的,具有A型花岗岩的特征。对下田坝花岗岩进行的激光探针等离子体质谱(LA-ICP-MS)锆石U-Pb同位素年代学研究获得了206Pb/238 U加权年龄年龄769±4.4Ma(MSWD=0.45),可代表花岗岩的形成年龄,下田坝花岗岩并非原来认为的早古生代花岗岩。结合对区域地质资料的分析认为,下田坝花岗岩的形成年龄与扬子地块西部Rodinia超大陆裂解事件在时间上一致,该花岗岩是在Rodinia超大陆裂解过程中由地幔柱活动引起的大陆裂谷环境中形成的,这也证实了扬子地块西南缘新元古代的裂谷环境已影响到滇东北的东川地区。     

柴北缘牦牛山地区牦牛山组沉积相组合特征

柴达木盆地北缘牦牛山地区出露的牦牛山组是一套由冲积扇和扇三角洲相共同构成的陆相沉积组合,冲积扇相砾岩-粗砂岩组合主要分布于研究区SE侧,扇三角洲相砂岩-泥岩组合主要分布于研究区NW侧。古水流分析表明牦牛山组沉积物主要来自其SE侧古隆起,但后期扇三角洲相包含少量来自NW和NE向的沉积物。该套沉积组合序列特征与区域上分布在牦牛山西侧同时期形成的湖泊相、滨浅海相沉积共同表明,柴达木盆地北缘在晚志留-早泥盆世时期存在一NW向倾斜的古斜坡,且晚期北侧发生抬升。砾岩和砂岩碎屑组成与区域岩石组合类型对比表明,牦牛山组沉积碎屑物主要来自于滩间山群。沉积组合序列特征、碎屑组成和区域构造背景综合研究表明,牦牛山组可能为柴达木板块向北俯冲过程中形成的局部断陷盆地的充填物。     

柴北缘绿梁山地区早古生代弧后盆地型蛇绿岩的年代学、地球化学及大地构造意义

岩石学、地球化学、年代学及Lu-Hf同位素综合研究表明在柴北缘西段绿梁山大平沟地区出露一套弧后盆地型蛇绿岩,岩石类型主要包括变质橄榄岩、变火山岩、变辉长岩及斜长花岗岩。其中变火山岩具有LREE亏损,类似N-MORB的稀土配分模式,同时又具有富集大离子亲石元素,亏损Nb、Ta等高场强元素的岛弧火山岩的地球化学特征,应形成在弧后盆地环境。斜长花岗岩为低钾准铝质花岗岩,具有LREE略微富集,HREE平坦的稀土配分型式,显示强烈Eu正异常,其εHf(t)值介于13.7～15.3之间,为变辉长岩部分熔融的产物,熔融温压条件可能为P=0.8～0.9GPa和T=～800℃。年代学研究结果表明变辉长岩的形成时代为535±2Ma,斜长花岗岩的形成时代为493±3Ma,指示本地区弧后盆地拉张时限至少介于493～535Ma之间,而柴北缘地区古大洋俯冲消减作用应早于535Ma。     

柴北缘都兰地区含石榴石、蓝晶石片麻岩的变质演化历史

在柴北缘超高压变质带东段,新识别出一个高压麻粒岩单元,其主要的岩石组合包括基性（长英质）高压麻粒岩、花岗质片麻岩、富铝质片麻岩（片岩）、石榴角闪岩和英云闪长岩。岩相学和变质反应序列、矿物化学和温压估算结果表明,蓝晶-石榴-黑云-二长片麻岩共经历了4阶段的变质演化：Ⅰ早期进变质阶段,以石榴石核部发育的黑云母、白云母、斜长石和石英等矿物包裹体为特征;Ⅱ峰期高压麻粒岩相阶段,矿物组合为石榴石+蓝晶石+条纹长石+斜长石+石英,金红石Zr温度计和GASP压力计限定其峰期温压条件为：t=800～840℃和p=1.4～1.6GPa;Ⅲ高角闪岩相退变质阶段,矿物组合为石榴石（边部）+黑云母+长石+石英;Ⅳ晚期低角闪岩相-绿片岩相退变质阶段,以蓝晶石周围出现的Ms+Pl±Zo和Mrg+Qtz±Ms±Pl后成合晶为特征。锆石LA-ICP-MSU-Pb定年结果指示高压麻粒岩相变质时代为431Ma。蓝晶-石榴-黑云-二长片麻岩具有顺时针的pt演化轨迹,与基性高压麻粒岩形成于相同的动力学过程。