藏东同卡石榴辉石岩和拉萨地体松多蓝片岩

Tongka garnet pyroxenite in eastern Tibet and Songduo blue schist in Lhasa block

元古宇卡穷岩群位于班公湖—怒江缝合带内,岩性为黑云斜长片麻岩、黑云斜长变粒岩,花岗片麻岩、大理岩及斜长角闪岩。黑云斜长片麻岩中具铁铝榴石、矽线石、蓝晶石组合。锆石U-Pb同位素年龄为1334Ma,花岗片麻岩锆石U-Pb年龄(507±10)Ma(SHRIMP),后者可能属稍晚期变形变质的花岗岩侵入体。卡穷岩群属冈瓦纳大陆北缘裂离出来的微陆块。黑云斜长片麻岩中首次发现石榴辉石岩、榴闪岩包体。较新鲜的石榴辉石岩由角闪石、透辉石、石榴石及少量石英、金红石组成,无长石。包体岩石化学成分中SiO248.65%～51.18%,K2O<1%,稀土配分形式为平坦型,无明显的Eu异常。微量元素Rb、Ba、Th富集,低Sr,具洋脊玄武岩特征。石榴石端元组分中铁铝榴石51.44%、镁铝榴石26.73%、钙铝榴石17.29%。透辉石端元组分中,硬玉分子平均为6.40%。镜下常见石榴石、辉石退变为角闪石和斜长石,包体岩石应为榴辉岩退变的产物。

The Proterozoic Kaqiong Group is distributed in northern Basu County of east Tibet,with an area of 80 km2.It is located in the Bandonghu-Neijiang juncture zone.The group consists lithologically of biotite-plagioclase gneiss,black mica-plagioclase granulite,granite gneiss,marble and amphibolite.The biotite-plagioclase gneiss is composed of alamandine,sillimanite and kyanite.The U-Pb zircon age is 1334 Ma,and the U-Pb zircon age of granite-gneiss is（507±10）Ma（SHRIMP）.The granite-gneiss might be a intrusive granite which underwent the process of deformation-metamorphism in later periods.The Kaqiong Group is a micro-continent separated from the northern margin of Gondwana continent.Garnet pyroxenite and garnet amphibolite inclusions were found for the first time in the biotite-plagioclase geniss.The fresh garnet amphibolites are composed of hornblende,diopside,garnet and a small amount of quartz and rutile,without feldspar.As for chemical constituents of the inclusions,SiO2 is 48.65%～51.18%,k2O〈1%.The REE pattern is a flat pattern,with no obvious En anomaly.Trace elements Rb,Ba,Th are rich,and Sr is low,showing characteristics of MORB.The end member molecules for garnets in eclogite are pyrope 51.44%,almondite 26.73%,and grossular 17.29%.The end member molecules for diopside are jadeite 6.40%.Grt-Cpx is commonly overprinted by Hb-Pl and can be observed under microscope.Inclusions should be the products of eclogite degradation.