石榴石辉石岩的单斜辉石中韭闪石和钛铁矿出溶结构及地质意义  

Exsolution Textures of Pargasites and Ⅱ menites in Clinopyroxenes of Garnet Pyroxenite from the Sulu UHP Terrane and Their Geological Implications

作　　者：陈晶[1] 许志琴[2] 李旭平[3] 陈志忠[2] 李天福[2] 陈方远[2] 

机构地区：[1]北京大学物理学院电子显微镜实验室,100871 [2]中国地质科学院地质研究所,北京100037 [3]北京大学地球与空问科学学院,100871

出　　处：《地质学报》2003年第4期556-565,共10页Acta Geologica Sinica

基　　金：科技部基础研究重大项目"中国大陆科学钻探工程科学研究前期研究专项"(编号 2001CCB00900);国土资源部中国大陆科学钻探工程年度(2002)科学研究项目(编号 2002207)的资助

摘　　要：本文首次报道了在苏鲁超高压变质带胡家岭地区石榴石辉石岩中发现的韭闪石出溶结构。该石榴石辉石岩是由石榴子石、单斜辉石以及镁铝尖晶石、钛铁矿和磁铁矿组成。在石榴子石巨斑晶中有单斜辉石包体,单斜辉石包体中又有石榴子石包体以及大量韭闪石出溶片晶和钛铁矿(+磁铁矿)出溶棒。单斜辉石中韭闪石出溶片晶呈平行条带状,其宽度在1～10μm之间。透射电子显微镜(TEM)观察韭闪石的(010)面平行于主晶透辉石的(010)面,且两者的界面在纳米尺度上是连续过渡的,证明韭闪石是出溶形成的。大量钛铁矿出溶棒的定向排列也显示出它们是自然出溶形成的。从单斜辉石出溶的矿物组合推测其原始单斜辉石成分中应富含H_2O、Na和Ti,属高压型单斜辉石。该石榴石辉石岩经历了超高压变质作用后,在抬升减压过程中形成这些出溶结构。大量钛铁矿出溶棒均斜切韭闪石出溶片晶,表明该超高压岩石在减压过程中至少经历了两期以上导致温压明显变化的地质事件。We here report the first case of sodic-amphibole exsolution that was discovered from garnet pyroxenites of Hujialing in the Sulu UHP terrane. These rocks were exhumed from the deep mantle together with coesite and eclogites in the Sulu UHP metamorphic belt. The garnet pyroxenite consists mainly of garnet and clinopyroxene with minor amounts of Mg-Al spinel, magnetite, tremolite, orthopyroxene, and olivine. Clinopyroxene contains garnet inclusions as well as the exsolved lamellae of pargasite and exsolved rods of ilmenite ( + magnetite). Exsolution lamellae of pargasite in the clinopyroxene are between 1 ~ 10μm. HRTEM image indicates that planes of pargasite (010) are parallel to planes of clinopyroxene (010). The interface of these two minerals shows a continuous transformation in the range of nanometer. All the evidences suggest that pargasite is the exsolution from clinopyroxene. The orientation distribution of ilmenite rods also suggests their natural exsolution. The mineral assemblage of the exsolution from clinopyroxene implies that the primary clinopyroxene in Hujialing might be a high-pressure clinopyroxene, and rich in H2O, Na and Ti components. The garnet pyroxenites have experienced subduction related UHP metamorphism. The exsolution textures resulted from decompression during the exhumation of the UHP terrane. The pervasive exsolution of pargasite and ilmenite suggests at least two stages of exsolution processes from different geological events, which implies a complicated exhumation history of the Hujialing garnet-pyroxenite.

关 键 词：出溶结构 俯冲岩片 折返过程 石榴辉石岩 单斜辉石 韭闪石 

分 类 号：P578[天文地球—矿物学]