甜水海岔路口地区早石炭世地层化石的发现和帕斯群的建立

在1∶25万岔路口幅区域地质填图时,在原划中泥盆统落石沟组中采到了大量腕足、珊瑚类化石,经鉴定其成时代为早石炭世中期。该套地层可与西邻区域地层进行对比,对此新建下石炭统帕斯群。     

Geochronology and geochemistry of Grenville-aged (1063 ± 16 Ma) metabasalts in the Shennongjia district,Yangtze block: implications for tectonic evolution of the South China Craton

To better understand the evolution of the South China Craton (SCC), we have determined the geochronological and geochemical compositions of newly recognized Grenville-aged metabasalts in the Shennongjia region of the northern Yangtze block. LA-ICP-MS U-Pb dating of zircons indicates that the metabasalts formed at 1063 ± 16 Ma. The rocks are calc-alkaline, are characterized by SiO₂ contents (50.50 to 55.62 wt.%), and have moderate-to-high MgO contents (7.25–9.60 wt.%). They display light rare earth element enrichment ((La/Yb)_N = 7.0–9.8) with slightly negative Eu anomalies (Eu/Eu* = 0.82–0.90) and have pronounced depletion in high-field strength elements as well as positive Pb anomalies in the primitive mantle-normalized trace element pattern. They possess high initial Sr isotopic ratios of 0.7092–0.7107, large negative ε_Nd(t) values of ?12.1 to ?11.0, and a relatively narrow range of initial Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb = 16.503–17.019, ²⁰⁷Pb/²⁰⁴Pb = 15.259–15.452, and ²⁰⁸Pb/²⁰⁴Pb = 36.169–36.994). These isotopic characteristics are typical of basalts derived from an EM2 source region and suggest a subcontinental lithospheric mantle source that was metasomatized by subducted components (fluids and melts). Integrating our new data with documented igneous and metamorphic events during late Mesoproterozoic to early Neoproterozoic time in the region and western segments of the Yangtze block, we suggest that the Shennongjia area might be a microcontinent that was independent of the continental nucleus of the Yangtze block and was accreted to the Yangtze block at the end of Mesoproterozoic time. This study thus argues against the traditional view that the SCC was formed simply by Yangtze–Cathaysia collision and supports a hypothesis in which the Yangtze block was a collage of microcontinents accreted during the Grenvillian period accompanying the assembly of the Rodinia supercontinent.     

塔里木古大陆东缘的微大陆块体群

塔里木古大陆东缘至少存在4个微大陆，自北而南分别是阿拉善、祁连、欧龙布鲁克和柴达木微大陆，它们有相近但又有一定差异的早前寒武纪变质基底和中元古代变质沉积地层。每一个微大陆边缘均有复杂的生成历史，特别是新元古代早期的热-构造事件十分发育，并在柴达木微大陆北缘形成一条长700km的花岗片麻岩带，而在多数微大陆边缘则叠加了寒武纪至奥陶纪火山弧或蛇绿混杂岩。在微大陆之间自北而南分别发育了北祁蛇绿岩混杂岩带、南祁连蛇绿岩混杂岩带、沙柳河-鱼卡河高压-超高压变质带和昆仑中部清水泉蛇绿混杂岩，它们代表了微大陆这间的结合带，其形成时代集中在寒武纪至奥陶纪。这几个微大陆总体表现出亲塔里木古大陆的特征，特别是自新元古代以来具有相似的地质演化特征。     

Palaeozoic accretion of the microcontinent Chilenia,North Patagonian Andes: high-pressure metamorphism and subsequent thermal relaxation

Basement rocks of the Colohuincul Complex (CC) crop out in the eastern foothills of the North Patagonian Andes (latitude 41°S). We studied the chemical composition of mineral phases in a mica-schist and a migmatite of this complex and constructed P–T pseudosections contoured by various chemical parameters of minerals. The P–T metamorphic path of the mica-schist is characterized by a high-pressure, low-temperature event (1.8 GPa and 440°C) indicated by a spessartine-rich core in prograde-zoned garnet and phengite relicts with high Si contents (3.40 pfu). The increase of X_pyrope (from 0.02 to 0.08) towards the garnet rim and the decrease of Si (to 3.16) in phengite reflect decompression accompanied by heating to 580°C (1.1 GPa), followed by cooling to 570°C (0.9 GPa). In contrast, the migmatitic paragneiss underwent partial melting and subsequent P–T conditions of 610°C and 0.5 GPa. Thermal relaxation after crustal thickening deduced from the mica-schist is interpreted to be the result of collision as the microcontinent Chilenia was thrust under the western South American part of Gondwana. Mid-upper crustal P–T conditions of the migmatite reflect its location within the Gondwanan crust. Two populations of monazite Th–U–Pb ages in migmatites and schists of the Colohuincul Complex with weighted average peaks at 391.7 ± 4.0 Ma (2σ) and 350.4 ± 5.8 Ma (2σ) are ascribed to the collisional and a later retrograde event.