安徽女山麻粒岩包体:矿物学特征、下地壳地温曲线及其成因意义

The Nüshan granulite xenoliths from Anhui province,China:mineralogical characteristics,the lower crustal geotherm and their implications for genesis.

女山玄武岩中麻粒岩包体的主要矿物组合为 Opx+Cpx+Pl,均呈条带状构造 ,具有细粒粒状变晶结构或不等粒结构。女山麻粒岩包体的斜方辉石及单斜辉石的化学成分都非常相似于华北太古代麻粒岩地体 ,而明显区别于汉诺坝麻粒岩包体。通过对世界上含石榴石麻粒岩的检验 ,表明 Wells(1977) :二辉石温度计与 Mc Carthy & Pati～no Douce(1998)的 Cpx+Pl+Qz压力计的温压计组合是计算麻粒岩平衡温压的可行方法 ,并得到了女山麻粒岩包体平衡温度分布于 80 9～ 899℃之间 ,主要集中于 85 0± 2 0 C,平衡压力为 0 .6 0～ 0 .95 GPa。由麻粒岩包体的平衡温压所建立起的女山地区下地壳地温曲线与 Xuet a1.(1998)建立的上地幔地温曲线上延部分非常吻合。根据地温线推得的“岩石学”Moho面深度为 31km土 ,与“地震”Moho面深度相一致 ,表明女山地区不具有明显的壳幔过渡带。

The main mineral assemblage of granulite xenolith in Nüshan basalt is Opx+Cpx+Pl.The granulite xenoliths are commonly in banded structure,with fine-grained homeblastic texture or heteroblastic texture.The chemical compositions of orthopyroxene and clinopyroxene in Nüshan granulite xenoliths are very similar to those of North China Archean granulite terrain,while are distinctively different from those of Hannuoba grnaulite xenolith.Based on the test of the garnet-bearing granulite in the world,the authors confirm that the combination of the Wells(1977)geothermometer and the McCarthy&Pati^no Douee(1998)geobarometer is suitable for application to the granulite.The equilibrium T-P of Nüshan granulitexenolith vary in 809～899 C and 0.60～0.95 GPa,respectively.The lower crustal geotherm of Nüshan based on the equilibrium T-P of Nüshan granulite xenoliths is very similar to the extrapolated upper part of Xu’s geotherm(1998)basedon the garnet-bearing mantle-derived xenoliths.The petrological Moho depth based on the Nüshan geotherm is ca 31 km consistent with the seismic Moho depth,which shows that there is no obvious crust-mantle transition zone in the area.Theabsence of strong underplating and the mineralogical characteristics of the granulite xenoliths imply that the Nüshan granulite xenoliths should be the source from the basement of the North-China craton.