西准噶尔蛇绿混杂岩中的白云石大理岩和石榴角闪岩:早古生代残余洋壳深俯冲的证据  

Dolomite marble and garnet amphibolite in the ophiolitic mélange in western Junggar:Relics of the Early Paleozoic oceanic crust and its deep subduction.

作　　者：朱永峰[1] 徐新 陈博[1] 薛云兴[1] 

机构地区：[1]北京大学地球与空间科学学院,造山带与地壳演化教育部重点实验室,北京100871 [2]新疆资源环境中心,乌鲁木齐830000

出　　处：《岩石学报》2008年第12期2767-2777,共11页Acta Petrologica Sinica

基　　金：国家科技支撑计划重点项目(2006BAB07B08)资助

摘　　要：本文在描述达拉布特和克拉玛依蛇绿岩中四个典型剖面(达拉布特、萨尔托海、白碱滩和百口泉)的基础上,通过研究变白云石大理岩和石榴角闪岩的形成和演化过程,展示西准噶尔蛇绿混杂岩的复杂性。白碱滩白云石大理岩是尖晶石二辉橄榄岩变质反应的产物。变质反应导致单斜辉石转变为白云石+石英+磁铁矿,橄榄石和斜方辉石先转变为蛇纹石+磁铁矿,蛇纹石再转变为白云石+石英+磁铁矿。透辉石+CO_2=白云石+石英+磁铁矿以及蛇纹石+CaCO_3=白云石+石英+磁铁矿+H_2O这两个变质反应基本同时发生,他们以及石英—柯石英相变关系所限定的 P-T 空间(660～670℃,26.5～27.5 kbar)代表白云石大理岩的形成温度和压力。白云石大理岩与尖晶石二辉橄榄岩在一个不大的区域共存,说明循环到地球深部的尖晶石二辉橄榄岩(即:白云石大理岩的原岩)折返到地壳浅部后又与蛇绿岩中未俯冲的尖晶石二辉橄榄岩混杂在一起。因此,克拉玛依蛇绿混杂岩代表古俯冲带的位置。变辉长岩和尖晶石二辉橄榄岩中的单斜辉石均发育出溶结构(出溶尖晶石和斜方辉石)。这些出溶结构是蛇绿岩快速上升就位时由于温度和压力突然降低辉石发生分解反应的产物。因此,西准噶尔蛇绿混杂岩中的岩石组合记录着蛇绿岩形成时期、以及之后部分岩块通过俯冲带循环到地幔深处(最大深度～70km)并折返到地壳浅部的全过程。西准噶尔蛇绿混杂岩带是一个多时期、多阶段和多成因岩石的混杂体。This paper systematically described four outcrops （ Darbut, Saertuohai, Baijiantan, and Baikouquan） of two ophiolitic belts （Darbut and Keramay） in western Junggar （Xinjiang, NW China）, and especially made detailed studies on petrology of the spinel-beating dolomite marble and meta-gabbro in these mélanges. The results showed that the dolomite marble was formed by prograde metamorphism of spinel pefidotite in subduetion zone, during which clinopyroxene was transformed to dolomite ＋ quartz ＋ magnetite assemblage, orthopyroxene and olivine were transformed to serpentine first, and serpentine was replaced by dolomite ＋ quartz ＋ magnetite assemblage finally. Three reactions： 1 ） diopside ＋ CO2 = dolomite ＋ quartz, 2） serpentine ＋ CaCO3 = dolomite ＋ quartz ＋ magnetite ＋ H2O, and 3） the coesite/quartz transformation reaction constrain the P-T values of the metamorphism for the Bajiantan marble to be 660 - 670℃ and 26. 5 - 27. 5 kbar. These P-T values suggested that the Early Paleozoie ophiolitie mélange was subducted （to depth of -70km） and exhumated to surface finally. Therefore, the Keramay ophiolite mélange represents the subduction zone formed at Late Paleozoic. Clinopyroxene in both meta-gabbro and spinel peridotites display exsolution lamellaes of spinel and orthopyroxene formed pre-subduetion, and thus record the emplacement process of the ophiolitie mélanges. The studied opbiolitic mélanges in western Junggar not only represent the P-T conditions for the formation of the ophiolitic mélanges, but also record the P-T conditions for the subduction process of at least some rock units from these ophiolitic mélanges as well as their exhumation process. The ophiolitic mélanges in western Junggar really are a complex combination of very different rock units including primary ophiolitic units as well as the exhumated units from the subduction zone.

关 键 词：蛇绿岩 大理岩 俯冲带 达拉布特 克拉玛依 准噶尔 新疆 

分 类 号：P588.313[天文地球—岩石学] P588.125[天文地球—地质学]