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特提斯洋对流上地幔Os同位素组成估算的新方法及初步运用
2013年
对流上地幔Os同位素组成的准确估算是运用Re-Os同位素体系探讨地幔演化的基础。前人研究主要是以地幔橄榄岩为研究对象,由于地幔橄榄岩Os同位素存在明显的不均一性,因而直接影响估算值的准确性。对流上地幔中包含的不同亏损程度的难熔组分在部分熔融过程中难以熔融,对形成的熔体相的Os同位素组成贡献很少或者没有。因此,与对流上地幔具有相同的Os同位素组成初始值的早期分离结晶岩石(如堆晶岩),结合堆晶岩中锆石的准确定年,可以用来估算对流上地幔Os同位素组成。本文根据这一方法测试了那曲地区弧后盆地堆晶岩的Os同位素组成和锆石U-Pb年龄,推测那曲地区新特提斯洋对流上地幔Os同位素组成为碳质球粒陨石型的。根据这一模型,对比了罗布莎和东巧铬铁矿岩、含矿围岩以及不含矿围岩的Os同位素特征,揭示出矿石及围岩均具有古老大陆岩石圈地幔信息,而不含矿围岩(泽当岩体)的Os同位素组成为碳质球粒陨石型的,无古老大陆岩石圈地幔信息。
黄启帅刘德亮龚小晗张晓冉史仁灯
Os isotopic evidence for a carbonaceous chondritic mantle source for the Nagqu ophiolite from Tibet and its implications被引量:3
2013年
Early-crystallizing chromian spinel(Cr-spinel) in the Nagqu ophiolite has high Os and low Re contents,and it is resistant to alteration during serpentinization,weathering and metamorphism.The chemical composition of primitive magma is preserved in Cr-spinel,which makes it suitable for determining the initial Os-isotope composition of the mantle source.This study presents Cr-spinel Os isotopes and zircon U-Pb ages for cumulate dunite and gabbro,respectively,in the same cumulate section of the ophiolite at Nagqu in Tibet.The results shed light on the formation and evolution of lithospheric mantle.The Nagqu ophiolite is located in the central part of the Bangong-Nujiang suture zone.It is a remnant of the Neotethyan oceanic crust,and contains cumulate dunite and gabbro.Zircon from the gabbro yielded a weighted mean 206 Pb/238 U age of 183.7±1 Ma.Cr-spinel exhibits Os values of 0.2 to 0.3,suggesting that the mantle source for the dunite is similar to that of carbonaceous chondrites.Thus,the Tibetan lithosphere is primarily a relic of Tethyan oceanic lithosphere,which has formed by the transformation of the normal asthenospheric mantle in the Mesozoic.This is the first study to combine the spinel Os isotopes with accurate zircon U-Pb ages to constrain the geochemical characteristics of the mantle source for the ophiolite.
HUANG QiShuaiSHI RenDengLIU DeLiangZHANG XiaoRanFAN ShuaiQuanDING Lin
关键词:同位素组成碳质球粒陨石蛇绿岩OS
Finding of high-pressure mafic granulites in the Amdo basement,central Tibet被引量:12
2010年
High-pressure mafic granulites with a peak mineral assemblage of garnet + clinopyroxene + rutile + quartz were found in the Amdo basement,central Tibet. Two kinds of symplectites were identified that are composed of orthopyroxene + plagioclase ± spinel and hornblende + plagioclase around garnet,which were interpreted to develop during the retrogressing stages in the granulites. P-T estimates suggested that peak metamorphic conditions were about 860-920°C and 1.46-1.56 GPa,which retrogressed from post-peak phase at 820-890°C and 0.88-1.15 GPa to amphibolite facies at 550-670°C and 0.52-0.65 GPa. These three stages define a clockwise P-T path with near-isothermal decompression and cooling following the peak high-pressure metamorphism. This suggests that the Amdo granulites underwent an initial subduction to a deep crustal level of ~50 km and then were rapidly exhumed to a shallow crustal level (~20 km). The formation of Amdo granulites is considered to result from the arc-continent collision between the Amdo basement and the Qiangtang terrane in the middle Jurassic,which is a crucial step to the tectonic evolution of the Tibetan Plateau.
ZHANG XiaoRanSHI RenDengHUANG QiShuaiLIU DeLiangCIDAN SuoLangYANG JingSuiDING Lin
关键词:高压基性麻粒岩高压变质作用变质条件角闪岩相PT轨迹
碎屑锆石对班公湖地区晚侏罗世—早白垩世沉积物源的制约被引量:6
2011年
本文对班公湖地区中生代沙木罗组(J_3—K_1s)和日松组(J_3r)地层的碎屑锆石进行了形态学及U-Pb年代学的研究。结果表明:锆石颗粒粒径约为100~150μm,内部结构清晰,晶体为长柱状,自形程度较高,多数锆石不含暗色包体及浑圆形内核;有些锆石颗粒有扇型分带结构。Th/U比值较大,多数大于0.1,均值约为0.86,说明岩浆的成因以锆石为主体,部分颗粒或晶体可能为变质成因;锆石年龄主要分布在6个区间范围内:1)180~100 Ma,2)350~180 Ma,3)600~450 Ma,4)1100~600 Ma,5)1800~1400 Ma,6)2200~1800 Ma。锆石U-Pb年龄谱对应了羌塘地块经历的几次构造热事件,验证了晚侏罗世—早白垩世班公湖地区的物源主要来自其北部的羌塘地块。
樊帅权史仁灯丁林张国凯
关键词:班公湖碎屑锆石羌塘地块
云南大理苦橄岩的Re-Os同位素特征:对峨眉山大火成岩省成因的制约被引量:9
2011年
云南大理苦橄岩产出于峨眉山大火成岩省内带,位于峨眉山玄武岩系底部。岩石具斑状结构,斑晶占20%~40%左右,由自形-半自形的橄榄石和单斜辉石组成;基质约占60%~80%,主要由长条状斜长石和颗粒状单斜辉石组成,辉绿结构;含少量尖晶石。绝大部分样品全岩SiO2低于47%,为45.94%~46.37%(1个样品达47.35%),MgO大于18%,介于19.01%~23.77%之间,Na2O+K2O低于2%,介于1.52%~1.97%之间,具典型苦橄岩的岩相学和岩石化学特征。全岩Re含量变化范围较小,介于0.349×10-9~0.424×10-9之间;Os含量变化范围较大,介于0.889×10-9~4.276×10-9之间;187Re/188Os=0.437±0.012~2.708±0.025,187Os/188Os=0.1283±0.0002~0.1354±0.0004;从中分选出的橄榄石的Re、Os含量分别为0.030×10-9~0.049×10-9、0.625×10-9~0.757×10-9,187Re/188Os=0.191±0.038~0.377±0.062,187Os/188Os=0.1254±0.0005~0.1268±0.0005,均低于全岩;尖晶石的Os含量最高,为80.5×10-9,187Os/188Os最低,为0.1252±0.0003。经质量平衡计算,基质的187Os/188Os比值介于0.1380~0.1415之间,与原始上地幔相比,基质的Re-Os同位素组成具有壳层熔岩的特点,而橄榄石和尖晶石具有熔融残留相的特点,基质的γOs大于0,介于+2.0~+3.28之间,矿物的γOs均小于0,介于-2.01~-2.59之间,显示明显的亏损特征,无核-幔边界源区信息,而全岩的介于-1.11~-3.24之间,为基质和斑晶及尖晶石的混合结果,推测峨眉山大火成岩省是壳-幔相互作用的产物。
郝艳丽黄启帅张晓冉史仁灯
关键词:苦橄岩RE-OS同位素峨眉山大火成岩省
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