西秦岭白依沟群的同位素地质年代学研究

西秦岭白依沟群火山岩的全岩Rb-Sr及锆石U-Pb同位素定年结果表明,白依沟群应当归属下震旦统,其同位素地质年龄为734±63Ma。1008.9Ma的晋宁造山期花斑岩砾石的锆石U-Pb年龄及535.5±11.4Ma的上覆地层年龄同样佐证了上述结论是合理的。白依沟群火山-沉积岩系的地质、地球化学及同位素地质年龄特征与同时期其他地区的有关地层(如川西乌斯大桥群)完全可以对比,它们可能都是夭折大陆裂谷环境的产物。此外,还首次发现了该区在泥盆纪早期(400Ma±)有一次明显的构造-岩桨-变质作用。     

打古寨——罗布里两种类型花岗岩的同位素地质研究

本文研究的打古寨和罗布里岩体是桃山复式花岗岩体内部两种不同成因类型的岩体,成岩时代均为燕山早期。根据Sr,Pb,O和S同位素资料,结合地质特征,笔者认为打古寨岩体是由上部地壳重熔改造形成,属陆壳改造型花岗岩;罗布里岩体是由壳幔混合岩浆形成,属同熔型花岗岩。     

南天山库勒湖蛇绿岩锆石年龄及其地质意义

目的准确测定南天山库勒湖蛇绿岩的形成时代,探讨其地质意义。方法采用激光剥蚀等离子质谱(LA-ICP-MS)法,对库勒湖蛇绿岩中辉长岩锆石进行U-Pb同位素定年。结果辉长岩形成年龄(或结晶年龄)为418.2 Ma±2.6 Ma(MSWD=0.80)。结论南天山库勒湖蛇绿岩的形成时代为末志留世—早泥盆世早期,与古南天山洋盆的俯冲-消减时代相当,它的形成很有可能与古南天山洋盆的俯冲-消减作用有关。     

Porphyry Cu-Mo deposits in the eastern Xing’an-Mongolian Orogenic Belt: Mineralization ages and their geodynamic implications

The eastern Xing’an-Mongolian (Xing-Meng) Orogenic Belt (XMOB) is one of the important areas of porphyry copper (Cu)-molybdenum (Mo) deposits in China. However, studies on the exact ages of mineralization and their geodynamic significance are very limited. In this study, granodioritic rocks from the Duobaoshan Cu deposit and Daheishan Mo deposit were selected to make zircon SHRIMP U-Pb analyses in order to constrain their mineralization ages. Geochronological data indicate that two episodes of mineralization took place in the Duobaoshan Cu deposits. The granodiorite related to the Duobaoshan porphyry Cu deposit was formed in the Early Paleozoic with zircon U-Pb age of 485±8 Ma, whereas the granodiorites related to the Sankuanggou skarn-type Cu deposit were emplaced in the Jurassic with zircon U-Pb ages of 176±3 and 177±3 Ma. In the Daheishan area of Jilin Province, the emplacement age of the granodiorite porphyry related to the porphyry Mo deposit was dated at 170±3 Ma, and the unmineralized monzogranite at 178±3 Ma. Therefore, two episodes of Cu-Mo mineralization were developed in the eastern XMOB, at ~485 Ma and ~175 Ma, respectively. Based on the geological history and spatial-temporal distribution of the granitoids in northeastern (NE) China, it is proposed that the Duobaoshan Cu deposit was related to the collision of the Xing’an and Erguna blocks in the Early Paleozoic, and the Sankuanggou Cu and Daheishan Mo deposits were related to subduction of the Paleo-Pacific plate during the Jurassic.     

U-Pb zircon age for the Daohugou Biota at Ningcheng of Inner Mongolia and comments on related issues

SHRIMP U-Pb zircon dating was carried out for the Daohugou Biota near Ningcheng of Inner Mongolia and for lavas overlying or underlying sala-mander-bearing strata at Reshuitang in Lingyuan of West Liaoning. The results suggest that the Dao- hugou Biota occurred at an interval from 168 Ma to 164―152 Ma. Both the Daohugou Biota and the salamander-bearing fossil assemblage are the same biota and thus developed from 168 to 152 Ma, i.e. from late Middle Jurassic to the early Late Jurassic. The Daohugou Biota-bearing rocks, resting on the Jiulongshan Formation in disconformity and being overlain in unconformity by Late Jurassic Tuchengzi Formation and Early Cretaceous rocks containing the Jehol Biota, are mainly composed of volcanic-sedi- mentary rocks in a normal sequence. It is recom- mended that the Daohugou Biota and the related stratigraphy should be correlated with the Tiaojishan Formation (Lanqi Formation in West Liaoning) or its synchronous rocks. It is suggested that the Dao- hugou Biota and the Jehol Biota would be neither taken into one biota nor considered as the earliest elements of the Jehol Biota. The Daohugou Biota and the related rocks and the Yixian Formation were respectively formed in different periods of volcanic-sedimentary tectonics.     

Geochronologic and petrochemical evidence for the genetic link between the Maomaogou nepheline syenites and the Emeishan large igneous province

The Maomaogou nepheline syenite is located at the inner zone of the Emeishan large igneous province and exhibits intrusive contact with the Emeishan basalts. SHRIMP U-Pb dating on zircons from this syenite yields an age of 261.6±4.4 Ma, in agreement with the age of the Panzhihua layered intrusion and the eruption age of the Emeishan basalts as constrained by stratigraphic data. Geochemical data fur-ther suggest that the Maomaogou syenite has a source analogue to the Emeishan basalt, and may have been formed by partial melting of gabbroic cumulates underplated in the lower crust. As s result, tem-poral and spatial relationships and petrogenetic constraints provide evidence for the genetic link be-tween basalts, mafic/ultramafic and intermediate/acidic intrusives in the Panxi area.     

Cosmogenic ^10Be dating of Guxiang and Baiyu Glaciations

Guxiang and Baiyu Glaciations are two previously recognized local glaciations of the Tibetan Plateau. They have been widely used as the reference standard for classifying Late Quaternary glaciations on the Tibetan Plateau and its surrounding mountains. However, the numerical chronologies of both glaciations have been lacking. In this study, cosmogenic They dating was undertaken to define the timing of these two glaciations. The surface boulders deposited by the glaciers of the Guxiang and Baiyu Glaciations have exposure ages of 112.9±16.7-136.5±15.8 ka BP and 11.1±1.9-18.5±2.2 ka BP, respectively. It is likely that the Guxiang and Baiyu Glaciations correspond to marine isotope stages 6 and 2, respectively.