东海超高压榴辉岩中绿帘石、褐帘石、磷灰石和钍石集合体的电子探针成分和化学定年研究

Electron-microprobe compositions and chemical dating of composite grains of epidote, allanite, apatite and Th-silicate from the Sulu UHP eclogites (CCSD main hole, Donghai, eastern China).

中国大陆超深钻(CCSD)主孔从100米到3000米切穿含柯石英榴辉岩,为系统研究大陆深俯冲过程中含稀土元素副矿物提供了机会。本文报道了该钻孔榴辉岩存在发现的绿帘石、褐帘石、磷灰石和钍石复合颗粒。它们以同心环带结构为特征,从边部向中心依次为绿帘石、褐帘石和磷灰石;钍石既可包裹再磷灰石中,也可见于褐帘石中。电子探针分析显示,褐帘石不仅含有较高的轻稀土元素(LREE2O3=23．36wt％),而且可含有2．6wt％ThO2。根据绿帘石和褐帘石的电子探针高分辨U、Th、Pb成分计算得到绿帘石边部的形成年龄为738±88Ma,该年龄与大别-苏鲁地体榴辉岩的元古代源岩的年龄相当,因此,绿帘石可能为残留相。相反,褐帘石的年龄为220±41Ma,与大别-苏鲁超高压变质年龄相近。因此,褐帘石与磷灰石和钍石一起应该为超高压变质事件的产物。根据绿帘石、褐帘石及其共生的磷灰石和钍石的结构、成分特征和化学定年结果,我们推测绿帘石与可能已完全消耗的独居石很可能为变质副矿物组合的先驱矿物。独居石-绿帘石复合颗粒在榴辉岩相高压／超高压变质过程中反应形成磷灰石+钍石+褐帘石组合。

Chinese Continental Scientific Drilling （CCSD） main hole intersected coesite-bearing eclogites from a depth of about 100 to 3000m in Donghai region, which lies as eastern part of the Dabie-Sulu ultrahigh-pressure （UHP） metamorphic terrain, China. It provides us an opportunity to make a systematic study on distribution of REE-bearing accessory minerals in continental subduction zone. This paper reported composite grains of epidote, allanite, apatite and Th-silicate （ considered as thorite） in eclogite taken from this deep drillhole. They are always characterized by concentric zoning texture, from the rim inward, consisting ofepidote, allanite and apatite; micro-inclusions of thorite are found within both apatite and allanite. Electron-microprobe analyses reveal that allanite incorporates not only great amounts of LREE2 03 （23. 36wt% ）, but also as high as 2.6wt% ThO：. High-resolution spot U, Th, Pb analyses of epidote and allanite with electron microprobe yielded an age of 738 ± 88Ma for the epidote rim, which is comparable with proterozoic protholith ages of UHP rocks in the Dabie-Sulu terrain, and suggests that epidote is believed to be a relict phase. In contrast, an age of 220 ±41Ma for the allanite mantle was obtained, and is interestingly similar to the Dabie-Sulu UHP metamorphic age. Thus, allanite, associated with apatite and thorite, is product of the UHP metamorphic event. Based on textural, compositional features and chemical dating of epidote and allanite and associated apatite and thorite, we inferred that epidote, along with ＂completely destroyed＂ monazite, favors to be precursor phase. Composite monazite-epidote grains have reacted to form apatite ＋ thorite ＋ allanite during eclogite-facies HP/UHP metamorphism .