摘要
Harzburgite-dunite associations are the dominant type of ultramafic heterogeneities in the exhumed peridotites.Although the dunite network are usually small in volume of exposed mantle rocks,their mineral structures and chemical variations are widely observed in various tectonic environments and record complex mantle evolution.The Yushigou peridotite complex displays a 25 km^2 mantle section belonging to a partly dismembered ophiolite in North Qilian suture zone,northwest China.It is mostly composed of depleted spinel harzburgite reequilibrated at temperatures of 900℃,contain dunite veins and minor podiform chromite at its Moho level.Here we report the petrological observations and mineral chemical analyses of harzburgite-dunite associations in the Yushigou peridotite complex.Based on the data we classified high Cr#(51-74)and low Cr#(19-34)dunite veins in this Yushigou complex.The podiform chromite are mainly high Cr#affinity and are production of melt-harzburgite interaction.There are three main microstructures recorded the episodic melt-rock interaction process:(1)complex spinel mineral inclusions both in harzburgite and two kind of dunite record widespread melt percolating.The observed primary inclusions(not related to later low temperature fluid activities)including hydrous and anhydrous silicates and minor sulphides,such as PGE and Fe-Cu-Ni related sulfidies,olivine,orthopyroxene,clinopyroxene,amphibole and phlogopite.(2)In all dunite samples,clinopyroxene dominated sieve-textured rims around spinel are more abundant and more thicken than the harzburgite;implying stained/pulsated melt supply after the chromite crystallization.(3)Millimeter scale veined reaction patches in some dunite samples consist of clinopyroxene,amphibole,phlogopite,Fe-Ni sulfidies and carbonates.Such metasomatic minerals assemblages may response for the later fluid infiltration after chromite mineralization.All these petrological observations and mineral chemical compositions(major and trace element contents)thus provide a wealth of information on the pathways of migrating melts in the mantle wedge,and provide constrain on nature and evolution of the uprising magma.Strong enrichment of incompatible trace elements in dunite clinopyroxene(LREE/HREE>10)possible reflect a slab origin of the percolating melt which is not observed elsewhere participating in chromite forming.
基金
granted by the NSFC(41520104003)
作者简介
first author:ZHOU Xiang,male,born in 1987 in Hohhot,Inner Mongolia,doctor,graduated from China University of Geosciences,Wuhan,research assistant of China University of Geosciences,Wuhan.He is now interested in the composition and evolution of Tibet Plateau lithosphere and surrounding orogenic belt.Email:zhouxianggeology@hotmail.com,phone:15827607857;corresponding author:ZHENG Jianping,male,born in 1964,Professor of China University of Geosciences,Wuhan.His fields of research include Lithosphere Evolution and Earth Dynamics.Email:jpzheng@cug.edu.cn,Tel/fax:027-67883873.
