Columbia超大陆裂解:来自塔里木克拉通东南缘大红山A型花岗岩的证据

摘要

Lithological geochemical and geochrological study was carried out for the Dahongshan granite in the southeastern margin of the Tarim Craton.LA-ICP-MS dating for the zircons from the Dahongshan granite yields a weighted 207pb/206Pb age of 1732.3±7.4Ma,revealing late Paleoproterozoic granites in the Tarim craton.δ18OvsMow values of whole rocks range from 7.6‰ ～ 8.6‰.The granite is rich in Si (SiO2 =71.14％～ 75.85％) and alkali (Na2O+ K2O =8.32％ 9.94％).REE distribution diagram shows an obvious right-dipping seagull-shaped distribution,and strong Eu negative anomaly.Trace element analysis shows that the granite has high contents in Rb (80.9×10-6～129×10-6),Th (17.3×10-6～28.6× 10-6),Y (62×10-6～220×10-6),Ga (28.1×10-6～44.6×10-6) and Nb (52.6×10-6～145×10-6),and low in Sr (12.2×10-6～79.3× 10-6),with high Zr+Nb+Ce+Y and 104 ×Ga/A1,suggesting that the granite has the geochemical feature of A-type granite.In combination with Sc/Nb-Y/Nb diagram,the Dahongshan granite (which belongs to A1-type granite) formed in the low pressure setting.Zircon saturated temperature TZr,calculated from 18 granite samples,ranges from 732.45℃ to 1128.33℃ (averaging 916.3℃),suggesting a high temperature.The Dahongshan granite was the product of partial melting of the lower crustal material due to asthenosphere upwelling or the mantle-derived magma underplating at high temperature and low pressure.The Dahongshan granite,which records not only the whole transformation process from compression to extension but also high thermal event in the Tarim craton after aggregation of Columbia supercontinents,the tracer of the initial stage of large-scale break-up of the Tarim craton.%笔者对塔里木克拉通东南缘大红山花岗岩体进行了岩石学、地球化学和年代学研究.通过大红山花岗岩锆石LA-ICP-MS测年,获得207pb/206Pb加权平均年龄为1732.3±7.4Ma,全岩δ18OV-SMOW为7.6‰～8.6‰.岩体富硅(SiO2=71.14％～75.85％)、富碱(Na2O+K2O=8.32％～9.94％);稀土配分曲线呈显略右倾的“海鸥型”分布特征,显示强烈的Eu负异常(δEu=0.08～0.30);微量元素特征显示具有较高Rb (80.9×10-6～129×10-6)、Th(17.3×10-6～28.6×10 6)、Y(62×10-6～220×10-6)、Ga(28.1×10-6～44.6×10 6)和Nb (52.6×10-6～145×10-6)含量,较低的Sr (12.2×10-6～79.3×10-6)含量,Zr+Nb+Ce+Y和104×Ga/Al值均较高,具有A型花岗岩的地球化学特征.结合Sc/Nb-Y/Nb图解,大红山花岗岩为A1型花岗岩,是低压条件下形成.通过计算18个花岗岩样品的锆石饱和温度,TZr为732.45～1128.33℃,平均为916.3℃,属高温花岗岩.大红山花岗岩是由挤压体制向伸展转变过程的地质记录,是伸展构造背景下软流圈上涌或幔源岩浆底侵导致下地壳在高温低压条件下发生部分熔融形成,记录了塔里木克拉通在Columbia超大陆聚合后的高热事件,是克拉通大规模裂解初期阶段的痕迹.