东天山三条高压变质带地质特征和流体作用

The geologic characteristics and fluid evolution in the three high-pressure metamorphic belts of eastern Tianshan.

微板块活动边缘常常产生高压变质作用,我们在东天山地区发现了三条高压变质带,分别出露在南天山北缘、中天山北缘和北天山南缘的早古生代至晚古生代地层中,它们是天山微板块多期次俯冲-碰撞-拼贴作用的产物。(南天山北缘铜花山高压变质带中有不同变异的蓝闪石类高压矿物,蓝闪石Ar^(40)/Ar^(39)同位素测定为360Ma年龄,而在同一带的西部的榆树沟有C型和B型的榴辉岩分布。中天山北缘乌斯特沟-米什沟高压变质带中发现了青铝闪石,同一阶段生成的多硅白云母为345Ma的年龄;北天山南缘后峡高压变质带中以出露在石炭世地层中钠闪石和多硅白云母为特征,三条高压变质带均经历过三个变质-变形作用阶段:①高压变质;②退变质;③韧性-脆性变质变形作用。) 流体活动与微板块构造作用密切相关,不同构造阶段有不同的变质-变形作用和矿物-流体反应:①高压变质作用阶段:微板块碰撞阶段产生不同的进变质作用,主要发生脱挥发组分的矿物反应,含水矿物转化成无水或少水矿物。南天山北缘有两次微板块与洋壳碰撞-俯冲作用,第一次碰撞-俯冲的热力学条件是:中-高温、高压和埋藏深度较大(540～720℃,0.92～1.29GPa,35～>50km),导致榴辉岩的生成,除了大量挥发组分逸出以外,另有不少熔体产生。

It is well known that the active margin of micro-plates is usually associated with the high-pressure metamorphism. The three high-pressure metamorphic belts had been identified, which had been respectively exposed in the Palaeozoic stratigraphic succession in the north margin of Southern Tianshan and the Middle Tianshan, as well as in the south margin of Northern Tianshan, and had been supposed as the results of multi-periods subduction, collision and collage of micropalaeoslabs. The studies demonstrated that the high-pressure minerals, such as glaucophane with varied chemical composition and the (ArP)-Ar-40/(39) Ar 360Ma isotopic spectrum age are distributed in the high-pressure metamorphic belt in north margin of Southern Tianshan and ecologits are discerned along Yushugou, which lies to the west of the same belt, and crossites are confirmed in Wusitegou - Mishigou high-pressure metamorphic belt of the northern margin of the Middle Tianshan, and coeval phengite have the 345Ma age, while outcropping riebeckites and phenggites in stratum of Caboniferous System are developed in Houxia high-pressure metamorphic belts in the southern margin of Northern Tianshan. furthermore, the above three high-pressure belts had undergone the three-stage of deformation and metamorphic transformation, i.e. (1) the high pressure metamorphism; (2) the retreated metamorphism; (3)the ductile-brittle metamorphism and deformation. A wealth of evidence indicates that the fluid activities has close relationship with the tectonic movement of micro-palaeoslabs, and the different metamorphism and deformation and its related fluid-mineral reactions are genetically distinct, i.e. during the high pressure metamorphic transformation, the varied degree of progressive metamorphism is of characteristic of collisional of micropalaeoslabs, and the devolatilization in the fluid-mineral reactions are frequently occurred, and this render the hydrates into either the non-hydrates or less-hydrated molecule minerals. The studies also shows that there are two collisional and subduction events in the north margin of Southern Tianshan; and the thermodynamic condition in the first collisional and subduction event is of type the middle - high temperature, high pressure (540similar to720degreesC, 0.92similar to1.29GPa) and rather high burial depth (35 similar to> 50 km), the eclogite is the distinctive mineral, In addition, except form escaping of a great deal of volatile components, the large quantity of melt took place 9 the metamorphic minerals is of characteristic of predominant melt and a few gas and liquid inclusions in which contain the CO2-fluid in company with decarbonation of carbonate, and water are usually rich in the condensed quartz-veins; In the second collisional and subduction event, the glaucophane and blueschist consists of the major mineral assemblage under low temperature and high pressure condition(350similar to460degreesC, 0.5similar to0.72GPa, 19similar to27km); and the high density of immiscible fluid with abundant of CO2 and H2O content is one of the distinguishable featuresq herein, CO2 may be origined either from the mantle or the CO2 + H2O fluid, which had released from reaction between fluid and rock. And the H, 0 component may be combined into the glaucophane, which the remained CO2 component entered into either the cleavages of glaucophane or grains and intermediate space of sub-grains of quartz by the way of shearing and cracking of rock, and part of CO2 and CO2 + H2O in fluid in the form of OH- and H+ entrapped in the crystalline lattice of phengites, part of them had been entrapped in the quartz, as well the high-pressure metamorphic belts of Wusitegou --Mishigou and Houxia also shows that low-to-middle temperature, high-to-middle pressure (300similar to456degreesC, 0.45similar to0. 71GPa and 375similar to480degreesC, 0.42similar to0.75 GPa) and rather high-to-middle burial depth circumstances ( 17 similar to 27 km, and 16 similar to 29 km), (2) in the retreat metamorphic period, due to the rapid uplift of high-pressure terrain during the collision and rebound of palaeoslabs, the pressure decreased quickly, however, the temperature can be kept on at the primary value because of its temperature postpone effect, and the great quantity of flux had intruded into the structure fractures, simultaneously, the thermal energy had been brought by the cycled-fluid, and this leads to a series of water-bearing reactions of minerals in the metamorphic belts, and gave rise to the mineral assemblage of amphibole-greenschist face with the result of pyroxene retreated into amphibole, and the ion Ca2+, Mg2+ and Cl-, F- released into the fluid, and the salinity of water solution increased, meanwhile, the component of fluid is of characteristic of CaCl2 (MgCl2)-NaCl-H2O and CO2-H2O and salt-water inclusions are frequently entrapped in minerals of the assemblage of amphibole greenschist. there shows that middle temperature, middle pressure (350similar to510degreesC, 0.22similar to0.43GPa; 345similar to500degreesC, 0.26similar to0.47GPa and 340similar to570degreesC, 0.28similar to0.52GPa) and middle burial depth circumstances (8.5similar to16 km; 10similar to18 km and 10.8similar to20 km). ( 3) in the ductile-fragile metamorphism and deformation stage, the metamorphic terrain had been uplifted to the shallow position of crust under the thrusting-up faults, and the middle-low temperature and middle pressure thermodynamic condition can be defined as the following parameters with variation of temperature (125similar to380degreesC; 150similar to410degreesC and 175similar to385degreesC), pressure (0.15similar to0.4 GPa; 0.08similar to0.33 GPa and 0.11similar to0.35 GPa) and the burial depth (5.8similar to15 km; 3similar to12.7 km and 4.2similar to13.5 km). The fractures induced by ductile shearing had provided a favorable conductor for cycle of hear-surface fluid and the CO2-H2O immiscible fluid formed and pressed into grains or interval of sub-grains of quartz, and CO2-CH4 and CO2-H2O high density inclusions had been produced, by the way due to the high strain condition of rocks in the ductile shear zone, the devolatilization reaction are developed. In the fragile deformation stage, due to low temperature and pressure condition of near-surface tectonic strain, no evidence of rock-fluid reaction can be found, the fragile deformation with the developed of a great deal of tension fractures, shear fractures and tension-shear fractures and the related divided microfractures I and this fractures become a principal place for accumulation and movement of fluid, the immiscible fluid with richness of CO, and low-salinity salt water had intruded the secondary macro-fractures and produced closure or seal effect, in addition, the fluid inclusion plane (FIP) took place in this self-seal formation, and the NaCl- H2O and CO2 immiscible fluid inclusions had been trapped. The paths for the three high-pressure metamorphic belts illustrated that the belts had experienced a clockwise variation process from increasing of temperature and pressure to rapid decreasing of pressure and slowly decreasing of temperature, and its tectonic evolution history from rapid declining of the burial depth to rapid uplift for the metamorphic belts in the collision margin of micro-plates. It is proved that the micro-plates in Eastern Tianshan had the characteristics of 'Accordion' like movement and more cycles according to fluid action and formation thermodynamics condition.