苏鲁地体超高压和非超高压花岗质片麻岩的判别标志——来自锆石中矿物包裹体的证据

Distinguishable Signature of UHP and Non-UHP Granitic Gneisses in the Sulu Terrane: Evidence from Mineral Inclusions in Zircons

中国大陆科学钻探工程预先导孔CCSD-PP1中存在两类不同性质的花岗质片麻岩,它们的岩相学性质、地球化学特征、锆石中的包体矿物组合和分布规律以及阴极发光图像均存在明显的差别。其中第一类花岗质片麻岩中的锆石呈完好的自形晶,不含任何超高压矿物包体,自晶体中心至边缘均具有典型的岩浆结晶生长环带,表明该类花岗质片麻岩未经历超高压变质作用;第二类花岗质片麻岩中的锆石呈半自形晶—它形晶,晶体核部保存典型的岩浆结晶生长环带,且无超高压矿物包体,具有继承性锆石的特点。而在晶体的过渡带及其与边缘的交界部位,普遍存在以柯石英为代表的包体矿物,表明第二类花岗质片麻岩曾经历了超高压变质作用,且锆石在此变质过程进一步结晶生长,形成新的生长环带。该项成果对于深入探讨苏鲁超高压变质带的俯冲-折返机制及其与岩浆作用的相互关系以及对中国大陆科学钻探工程的选址和实施有着重要的科学意义。

The pre-pilot drillhole CCSD-PP1 of the Chinese Continental Scientific Drilling Project, with a depth of 432. 08 m, is located in the Donghai area in the southeastern Sulu terrane. The core samples are comprised mainly of paragneiss, granitic gneiss and ultramafic rock with minor intercalated layers of phengite-bearing kyanite quartzite and eclogite. All the analyzed paragneiss and granitic gneiss samples experienced amphibolite-facies retrogressive metamorphism. According to the whole-rock analyses, mineral inclusions in zircons and cathodoluminescence images, the granitic gneisses may be divided into two types. In the zircons in the first type of granitic gneiss samples form pre-pilot drillhole CCSD-PP1, many non-UHP mineral inclusions, comprised of Qtz + Phe + Ap, were identified from the core (C) through the mantle (M) to the rim (R) of zircon. All analyzed zircons show magmatogenetic features and are characterized by euhedral crystal shapes. The cathodoluminescence images show typical crystallization growth zoning from the core to the rim. These features indicate that the first type of granitic gneisses in pre-pilot drillhole CCSD-PP1 has not experienced UHP metamorphism. However, in the second type of granitic gneiss, a low-pressure mineral inclusion assemblage, mainly consisting of Qtz + Phe + Ab + Ksp + Ap, was identified in the core (C) of a zircon, while coesite or coesite-bearing UHP mineral inclusions were identified in the mantle (M) and rim (R) of the same zircon. These features suggest that the second type of granitic gneiss may have formed prior to UHP metamorphism and then together with paragneisses, phengite-bearing kyanite quartzite and eclogite experienced UHP metamorphism. This research is of great significance for an in-depth study of the subduction-exhumation mechanism and magmatism of Sulu UHP metamorphic belt and the selection of the drilling site for the Chinese Continental Scientific Drilling Project.