大别山港河浅变质火山碎屑岩及其中含柯石英榴辉岩的差异变质及地球动力学意义

A wedge-shaped coesite-bearing eclogite at Ganghe in Dabieshan is enclosed in a low-grade metamorphic volcanic breccia. Some workers consider they both subducted to a great depth but the metavolcanic rock was not subjected to high-pressure metamorphism due to the lack of fluid. Some others interpret that the eclogite experienced deep subduction while the country rock was located in an accretionary wedge, they were juxtaposed during the exhumation of the eclogite. Our preliminary observations show that the eclogite consists of cataclastic high-pressure minerals, some of which display abrupt compositional changes; the metavolcanic breccia contains epidote spherulites and radial euhedral crystal aggregates, skeletal and poikilitic garnet, and acicular phengite. These data imply that both the eclogite and the country rock formed in a short-lived process. Based on detailed field, petrographic and geochemical work, the present project is designed to model the P-T and fluid variations during the metamorphic processes by phase diagrams; retrieve the timescale of the processes by diffusion modeling of the mineral compositional variations; exploit the kinetics significance of the quench textures, in order to develop a model that is able to accommodate all the data gathered by different groups.

大别山港河一个楔状含柯石英榴辉岩被夹在变质程度很低的火山碎屑岩中。有观点认为二者一同经历了深俯冲和折返过程，由于围岩缺乏流体而没有发生高压变质作用。另有观点认为榴辉岩经历了深俯冲而围岩处于加积楔部位，在榴辉岩折返时构造并置在一起。申请人小组的初步结果显示，榴辉岩由碎裂高压矿物组成，矿物成分局部发生突变；火山碎屑岩中出现绿帘石球状结构和放射状自形晶集合体，表明局部有流体渗入并导致其发生了较低温压的变质作用。石榴石呈骨架状和筛状，多硅白云母呈针状。矿物的成分突变和雏晶结构均指示二者形成于快速冷却过程。本项目拟在野外、岩石学和地球化学工作基础上，通过相图模拟追溯其变质过程的温压和流体含量变化；对矿物成分变化进行扩散模拟，以获得其变质过程的时间尺度；揭示雏晶结构所包含的反应动力学信息；提出一个能够解释所有观测结果的成因模型。

本项目工作显示，港河含柯石英榴辉岩的围岩属于绿片岩相变质火山角砾岩，没有任何证据表明其经历过超高压变质作用。热力学计算模拟结果表明其变质温压在435~515 ºC和0.5~0.7 GPa，接近水饱和。根据含柯石英榴辉岩呈薄板状尖灭于其中判断它是在发生了超高压变质作用之后经构造作用插入围岩的，而不是前人所说的作为脉状侵入岩与围岩一同经历了超高压变质作用。在围岩中发现了变质成因的绿帘石球状结构和放射状自形晶集合体。它们是典型的非平衡过饱和或者过冷却结晶的产物。前者指示其经历了与岩浆或者地震假熔岩类似的快速结晶过程，估算其时间尺度为<320年；后者指示的结晶过程相对缓慢，估算≤3万年。因此火山碎屑岩的变质作用对应于一个温度压力和流体的脉冲，很可能是伴随着超高压榴辉岩插入围岩的地震过程。港河榴辉岩中的高压矿物形态不规则并且有碎裂现象。矿物内部化学组成分布不均匀，不具有生长环带，矿物颗粒内部因生长较快而俘获的包裹体分布也不规则。因此港河榴辉岩在形成之后发生了较大的变形。不同组成的石榴石共生指示它们在形成之后成分并没有发生扩散调整。初步扩散模拟计算结果表明，港河榴辉岩所经历的超高压变质作用时间<50万年，没有伴随缓慢的深俯冲折返过程，而更可能是地壳内发生的地震冲击作用所致。根据榴辉岩中的灰闪石与石榴石和绿辉石的结构关系判断其为榴辉岩峰变质矿物退变质的产物。相图模拟结果限定灰闪石稳定性为<1.2 GPa和<500 ºC，表明灰闪石并非如前人工作所说是一个高压甚至超高压角闪石，而是在榴辉岩退变质晚期形成的中低压角闪石。