柴达木盆地北缘鱼卡河含柯石英榴辉岩的确定及其意义

DETERMINATION AND SIGNIFICANCE OF THE COESITE ECLOGITE ON THE YUQIA RIVER ON THE NORTH MARGIN OF THE QAIDAM BASIN

在柴达木盆地北缘鱼卡河边达肯大坂杂岩之花岗质片麻岩中发现典型的含柯石英榴辉岩。榴辉岩主要由石榴石、绿辉石和少量（多硅）白云母、柯石英和石英、角闪石、金红石等矿物组成。石榴石中铁铝、镁铝和钙铝榴石分子含量分别为５１％～５９％、２６％～３１％和１３％～１９％；绿辉石中硬玉分子含量为４５％～４８％；岩石中残留有ｂ０值极大的高压矿物多硅白云母（ｂ０＝９０７５×１０－１ｎｍ）；角闪石亦为高压类型的冻蓝闪石；最为重要的是确定了柯石英的存在。榴辉岩原始特征保存完好，仅遭受轻微的退变质作用和叠加变质作用。组成矿物可分为３个世代：（１）峰期矿物组合：石榴石＋绿辉石＋（多硅）白云母＋（柯）石英＋金红石；（２）退变交生组合：冻蓝闪石＋蠕虫状石英；（３）后期叠加变质矿物白云母。榴辉岩相变质作用发生在压力大于２８ＧＰａ的超高压至大约７３０℃、１７０ＧＰａ的高压环境，表明柴达木盆地北缘是一条重要的古板块汇聚边界。

A typical coesite eclogite outcrop has been discovered recently on the Yuqia River on the north margin of the Qaidam Basin. The eclogite occurs lenticularly in the granitic gneiss of the Dakendaban Complex. The eclogite consists predominantly of garnet and omphacite with minor quartz/coesite, muscovite/phengite, homblende and rutile. The garnet is composed of 51%～59% almandine, 26%～31% pyrope and 13%～19% grossular respectively. The omphacite contains 45%～48% jadeite. A little amount of phengite, with abnormal high b 0 value of 9 075×10 -1 nm, remains in the eclogite. The hornblende is the ultra high pressure type, the barroisite. The most important thing is the determination of the coesite in the eclogite. The eclogite remains well its original characteristics, which has only suffered slight retrogressive metamorphism. There are three generations of minerals in the eclogite:(1)the peak metamorphism stage mineral assemblage(the earliest generation): garnet+omphacite+muscovite/phengite+quartz/coesite+rutile,(2)the retrogressive metamorphism mineral assemblage: barroisite+worm like quartz and(3)the later stage metamorphism mineral muscovite. The peak stage metamorphism mineral assemblage is dominant in amount. The eclogite facies metamorphism occurred under the condition from ultra high pressure(>2 8GPa)to high pressure of about 1 7GPa/730℃. It might be concluded that the north margin of the Qaidam Basin once was a significant convergent zone of ancient continental plates.