甘肃玉门昌马地区的蓝闪片岩

THE GLAUCOPHANE SCHISTS OF CHANGMA AREA OF YUMEN IN NORTH QILIAN MOUNTAINS CHINA

蓝闪片岩产于一套含放射虫硅质岩、硅质大理岩、板岩、变质基性火山岩中,与此相伴的超基性岩块,以一系列向南逆冲的断片产出。蓝闪片岩中的蓝闪石往往和黑硬绿泥石、钠长石、绿泥石以及钠钙闪石(蓝透闪石、冻蓝闪石等)、钙闪石(阳起石等)共存。据钠角闪石、绿泥石计算的生成压力是0.65GPa至0.8GPa,略大于美国佛兰西斯科的瓦尔德溪蓝片岩中该矿物对生成的压力。稀土元素分析证明蓝片岩的原岩是典型洋底(中脊)玄武岩和岛弧玄武岩,这是蓝片岩沟-弧俯冲-碰撞的证据之一。

The glaucophane schists of glaucophane bearing and other highpressure and low-temperature minerals are recently collected at Changma area of Yumen in Gansu Province.This is the first discovery in west part of North Qilian Mountains. In this area, glaucophane schists occur in an assemblage of which is consisted of chert, siliceous marble, slate and metabasic volcanic rock and associated with some ultrabasic blocks and a group of thrust-southwards faults.This assemblage, as a typical subduction complex, reflect the evolutional processes, first, to produce alkali basalt by strecthing of inner plate, then,to further produce plate-inner tholeiite and ocean-floor tholeiite by continuous strecthing of the plate, finally, forming island-arc basalt,which are proved by chemical analytic resul of pyroxene remained in basic lava.The REE analysis have demonstrated that the mother rocks of the blueschists are typical ocean-floor (midridge) basalt and island arc basalt. Alkali amphiboles in glaucophane schist are glaucophanes in this area Glaucophane is commonly coexisted with sodic-calci amphibole(winchite and barroisite) , calcic amphibole (actinolite etc. ), stilpnomelane, albite and chlorite in the blueschist.The forming pressure of blueschist based on calculation of mineral pair of coexistee chlorite and sodic amphibole is about 6.5-8.0kbar, which just a little higher than the same pair of blueschist in ward Creek, Franciscan. Glaucophane schist in this area might be linked up with Qinghai glaucophane schist metamorphosed from mid Cambrian volcanic rocks in its east part.They represent the locality of ancient subduction zone in North Qilian Mountains.