Episodic Melt Percolating and Chromite Mineralization in Yushigou Peridotite, North Qilian Suture Zone

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.