藏东同卡石榴辉石岩和拉萨地体松多蓝片岩

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1334 Ma,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SiO_248.65%～51.18%,K_2O＜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.

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

The Proterozoic Kaqiong Group is distributed in northern Basu County of east Tibet, with an area of 80 km~2. 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, SiO_2 is 48.65%～51.18% ,k_2O＜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-P1 and can be observed under microscope. Inclusions should be the products of eclogite degradation.