新疆西南天山哈布腾苏一带榴辉岩的岩石学特征及变质作用P-T轨迹

Petrology and metamorphic P-T path of eclogites from Habutengsu, southwestern Tianshan, Xinjiang.

西南天山哈布腾苏河沿岸的含石墨的石榴石多硅白云母石英片岩中出露一套若干大小不等的布丁状变基性岩块,产状与区域面理一致。本文对其中保存完好的榴辉岩体进行了较为细致的岩石学研究和温压演化条件计算。根据主要矿物的含量,将该套榴辉岩大致分为两类—角闪榴辉岩和钠云母榴辉岩,二者的主要矿物均为 Grt+Omp+Na-Ca-Amp+Pg+Dol/Cal+Rt±Qtz。石榴石变斑晶两阶段生长明显,从核部到边部 X_(Mn)和X_(Fe)降低,X_(Mg)和 X_(Ca)升高,指示了升温降压的变质过程。根据石榴石核部和边部的包体组合特征,确立了两期榴辉岩相变质作用:前一阶段经历了高压但较低温的硬柱石-硬绿泥石(仅见假象)榴辉岩相,变质温度为400～500℃,压力不低于1.8～1.9GPa,表明早期经历了快速俯冲过程;后一阶段的变质温度为570±30℃,压力为2.0～2.5GPa。在退变质绿帘角闪岩相阶段,形成低压脉体(矿物组合为 Ab+Di+Na-Ca-Amp+Ep/Czo+Cal)和一系列退变质反应结构,如 Dol 的 Cal 增生边,Omp 的 Di+Ab 后成合晶结构。利用 Dol-Cal 分溶温度计和 Di的 Jd 分子含量得到该阶段的温度约500～530℃,压力小于0.9～1.1GPa,表明其退变质经历的是降温降压过程。这与利用Thermocalc 3.1在 NCFMASH 体系下计算的 PT 视剖面图是一致的。

Some boudin-shaped metabasites of variable sizes are exposed in graphite-bearing garnet-phengite-quartz schists along Habutengsu River, southwestern Tianshan. In this paper, petrology and P-T evolution of the well-preserved eclogites were studied in detail. According to the modal content of major minerals, Grt ＋ Omp ＋ Na-Ca-Amp ＋ Pg ＋ DoL/Cal ＋ Rt Qtz, these eclogites can be divided into two kinds-hornblende eclogites and paragonite eclogites. The porphyroblastic garnet shows two distinct growth stages, with XMn and XFe deceasing, XMg and XCa increasing outwards, which indicates a process of increasing temperature and decreasing pressure. According to the assemblages of inclusions in the core and rim of garnet, two stages of eclogite facies metamorphism were recognized. The former stage experienced lawsonite-chloritoid （ only pseudomorphs left） eclogite facies of high pressure and lower temperature, with the conditions of 400 -500℃ and 〉 1.8 -1.9GPa, indicating the fast subduction. The latter stage＇s conditions were estimated to be 570± 30℃ and 2.0 -2.5GPa. During the retrogression of epidote amphibolite facies stage, the low pressure veins with the assemblage of Ab ＋ Di ＋ Na-Ca-Amp ＋ Ep/Czo ＋ Cal and retrograde reaction texture developed well, such as dolomite rimmed by calcite and omphacite with symplectite of Di ＋ Ab. Using Dol-Cal solvus thermometry and the Jd content of Di, the conditions of 500 - 530℃ and less than 0.9 -1.1GPa were estimated, indicating the cooling during decompression. The evolution conditions calculated above are in accordance with the pseudosection using Thermocalc 3.1 in the system NCFMASH.