塔里木西克尔新生代碧玄岩中辉石岩捕虏体的发现及其地质意义

Discovery of pyroxenite xenoliths in the Cenozoic basalt from Xikeer,Tarim,Northwest China and its geological implications

首次在塔里木西克尔地区新生代碧玄岩中发现了辉石岩捕虏体。其主要矿物为ol+opx+cpx+pl+sp,含少量次生amp和phl。辉石岩中单斜辉石和斜方辉石普遍发育出溶结构。尖晶石明显分为晶形和产状不同的两个世代。变质成因的斜长石包裹大量高度自形的微小尖晶石颗粒。通过结构分析,出溶成份恢复,矿物温压计投图和P-T视剖面图分析确定:辉石岩捕虏体最初可能为无斜长石的尖晶石二辉岩,在降压至～1.5GPa,温度为1100～1200℃的过程中新生变质斜长石包裹了第二世代的自形尖晶石微晶;进一步的降温降压过程导致单斜和斜方辉石出溶和斜长石、橄榄石的生长,最终斜方辉石所记录的压力为～0.9GPa,温度为800～900℃。该变质作用过程说明辉石岩从尖晶石二辉岩稳定深度(>45km,A'点)抬升至斜长石二辉岩稳定深度(～28km,B点)并发生冷却,该过程可能代表了塔里木板块的壳幔过渡带曾发生了较大幅度的抬升,可能与地幔上涌造成的大陆岩石圈伸展、减薄有关。

A suite of newly discovered pyroxenite xenoliths in the Cenozoic basanite from Xikeer,Tarim Block have the primary mineral assemblage of ol＋opx＋cpx＋pl＋sp and the secondary mineral assemblage of amp＋phl,Exsolved texture is common in clinopyroxenes and orthopyroxenes,Two generations of spinels occur,among which,the later euhedral small spinel grains are enclosed in metamorphic plagioclases,After recalling the exsolved components,the primary plagioclase-bearing mineral assemblage record temperature and pressure of 1100～1200℃ and 1.5GPa,which possibly follow an earlier plagioclase-free spinel websterite stage at higher pressure;Whereas the exsolved orthopyroxenes record a lower temperature and pressure of 800～900℃ and 0.9GPa,This indicates extensive uplifting and cooling of the protolith of the pyroxenite from the depth of over 45km to ～28km,during which original spinel websterite transform into plagioclase websterite,accompanied with growth of plagioclase and olivine,This process may imply that the crust-mantle transition zone of the Tarim Block have once been greatly uplifted,due to mantle upwelling and consequent stretch of the subcontinental lithospheric mantle.