Petrogenesis Of The Heigou Ultramafic Rocks And Evolution Of Crust-Mantle Transition Zone In Yanchiwan Area,Western Center Qilian Belt

The crust mantle transition zone at the arc roots is the key zone of primitive magmatic evolution,initial crust differentiation,mantle-lower crust chemical heterogeneity evolution and material circulation in the earth.The ultramafic rocks in the subduction zone are the petrological "probe" to study the crust mantle interaction in the subduction zone.By revealing the petrogenesis of the ultramafic rocks in the crust mantle transition zone and the later melt /fluid metasomatism in the subduction zone,it is of great significance for the evolution of crust mantle transition zone in subduction zone.In this paper,the Heigou ultramafic rocks(Peridotite,Pyroxenite,Pyroxene hornblendite)in Yanchiwan area in the western center Qilian orogen are selected as the research object.Based on the relevant work of field geological survey,mineralogy,petrology,geochemistry,Sr-Nd-Hf isotope geochemistry,zircon and sphene chronology,the petrogenesis of the Heigou ultramafic rocks and the melt /fluid metasomatism process was discussed,Reveal the evolution history of the crust mantle transition zone at the arc roots of the the western center Qilian belt.This paper obtains the following main understandings:(1)Through field geological survey,it is revealed that the Heigou ultramafic rocks are mainly composed of mantle Peridotite(harzburgite,lherzolite)and Pyroxenite(clinopyroxenite,hornblende pyroxenite,pyroxene hornblendite),which record the petrological information of the crust mantle transition zone at the arc roots.(2)Zircon / Sphene U-Pb chronology shows that the formation age of the Heigou ultramafic rock is 523 ± 2Ma,and the time of late melt / fluid metasomatism is 485 ± 5Ma.(3)Heigou mantle peridotite is derived from the spinel phase lherzolite mantle source area and is a residual harzburgite formed by 10-20% separate partial melting and melting of the primitive mantle at 2-3GPa.(4)The petrological and geochemical characteristics of Heigou mantle peridotites show that they underwent partial melting and then underwent melt / fluid metasomatism,resulting in peridotite melt reaction.The melt-rock reaction consumes orthopyroxene to produce olivine and clinopyroxene,and generates new metasomatic minerals phlogopite and amphibole,which leads to the enrichment of light rare earth elements(LREE)and fluid active elements(such as CS,Rb,Ba,K and U)in mantle peridotite.(5)Heigou pyroxenite records the petrological and geochemical characteristics of different stages.In the early stage(Ca.523Ma),pyroxenite represents the cumulates of initial water bearing basaltic magma at the arc roots,In the later stage(Ca.485Ma),it suffered from the metasomatism reaction of melt/fluid with water-rich to form phlogopite-rich pyroxenite and pyroxene hornblendite.(6)Heigou pyroxenite has arc-type trace element characteristics and enriched whole rock Sr-Nd-Hf isotopic characteristics,indicating that its source area may be formed by the mixing of mantle peridotite and aqueous felsic melt produced by partial melting of subducted sediments in the mantle source area.