马里亚纳弧前区锥形海山岩石学及成因

Petrology and Origin of the Conical Seamount in the Mariana Fore-arc Area

锥形海山是马里亚纳弧前区非火山成因的海山。它是由俯冲的太平洋板块对仰冲板块的上推作用,地壳下的上地幔物质底辟侵入的结果。海山顶部的岩石是由蛇纹石化超镁铁质方辉橄榄岩组成。其主要矿物成分为斜方辉石、橄榄石、尖晶石、磁铁矿、蛇纹石和碳酸盐。手标本呈显晶质块状构造,薄片中呈粒状、残留状、筛网状及斑状变晶结构。岩石化学特征表现为:低Si和Al,高M/F值,含水,亏损了所有过渡金属。岩石的成因属超基性岩浆直接结晶,后经海水的参入,在热液作用下普遍发生蛇纹石化和碳酸盐化。

The Conical Seamount is a non-volcanic one in the Mariana Fore Arc. It was formed by diapir of the upper mantle material, due to the uppusing of the thrusting plate caused by the subduction of the Pacific Plate.The rocks at the top of the seamount consist of serpentinized ultramafic harzburgite. The main mineral components of the rock are enstenite, olivine spinel, magnetite and some altered minerals such as serpentine and carbonates. The harzburgite shows massive structure in hand-specimens, and granular, diablastic and porphyroblastic textures in thin-sections. The rocks are chemically characterized by lower in Si and A1, higher in M/F, containing water and lacking transitional metals. They were products directly crystallized from ultrabasic magma, and were generally serpentinized and carbonated later through hydrothermalism with sea water.