新疆西天山西段低温榴辉岩相平衡岩石学研究及地质意义

Based on the mineral phase equilibrium princeple, the low temperature eclogites exposed in Western Tianshan, China, have been studied in details. According to the characteristics of their field occurrences, three types eclogites have been recognized, i.e., lenticular eclogites interlayered with mafic blueschists, eclogites with pillow lava structure and eclogites lenses within marbles. Inclusions of coesite pseudomorphs in garnets have been discovered first in type I eclogites and carbonate eclogites. Quartz exsolution lamellae in omphacites have been recognized in type II eclogites with pillow structure and magnesites of ultrahigh pressure metamorphic (UHPM) origin were discovered in type III eclogites in western Tianshan, China. Based on the mineralogy and petrology, the metamorphic evolution of western Tianshan eclogites can be divided into three stages, 乸re-peak ultrahigh-pressure (UHP) eclogite-facies stage (356-443℃, 8-10kb), ?UHP eclogite-facies stage (496-598℃, 25.72-26.66±1 kbar) and ?retrograde epidote blueschist-facies stage (500-530℃, 10-12kbar). Using THERMOCALC (v2.75) and the updated internally consistent mineral thermodynamic database of Holland and Powell (1998), we have calculated the P-T grid involving glaucophane (gl), grossular (gr), jadeite (jd), magnesite (mag), dolomite (dol), clinozoisite (cz), coesite (coe), CO2 and H2O in a 7-component system of NCMASCH (Na2O-CaO-MgO-Al2O3- SiO2-CO2-H2O). UHP eclogites from western Tianshan is the second location of oceanic rocks in the world, which is very significance for to study the geodynamics of UHPM and explore the interaction beween the mantle and crust.

该项目拟以西天山西段低温榴辉岩相变质岩石为例，利用矿物内部一致性热力学数据，确立NC FM ASH体系中低温榴辉岩相平衡及其与蓝片岩相、绿片岩相和可能的绿帘角闪岩相的转叵怠T条件及其影响因素。结合同位素定年，确立西天山榴辉岩PTt轨迹及其地质意义。该项研究无论在高压变质相平衡理论方面，还是对于探讨天山造山带形成与演化都具有重要意义。