Genesis Of Ultramafic Rocks And Banded Chromitites In Songshugou,North Qinling Orogenic Belt

The Songshugou ultramafic massif is located to the north of Shang-Dan faults,North Qinling orogenic belt of central China,and it stretching in NW-SE direction,is about 20 km long and 2 km wide and covers an area of ca.21km².This ultramafic massif represents fragments of the Neoproterozoic fossil oceanic lithospheric mantle that emplaced in to Proterozoic,Qinling Group due to the Early Paleozoic subduction processes.The Songshugou ultramafic massif are mainly composed of fine-grained dunite,coarse-grained dunite,harzburgite and minor clinopyroxenite,and part of the coarse-grained dunite host banded chromitite.In this study,based on detailed microstructure observation,combined with major and trace element compositions reported in literatures,we present mineral chemical research of olivine,orthopyroxexe,clinopyroxene,chromite and Cr-chlorite,by EMPA and LA-ICP-MS,and discuss the peridotite petrogenesis,mantle metasomatism,parental magma of chromite,Cr saturated mechanism and tectonic setting.The primary conclusions are as follows:?1?According to characteristics of major and trace element,and mineral chemical,we divide the Shongshugou ultramafic rock into two groups.Group 1 consist of fine-grained dunite and fine-grained harzburgite.The chondrite-normalized REE patterns of it exhibit a gentle U-type curve,and primitive mantle-normalized trace elements patterns display positive anomaly of Ta,Sr,Y and Hf,which represent fossil oceanic lithospheric mantle that experienced mantle metasomatism by silicious melt.Group 2 consist of coarse-grained dunite and coarse-grained huazburgite.It enrich LREE and primitive mantle-normalized trace elements patterns display negative anomaly of Nb.?2?Chromite contain abundant primary inclusions,such as high-Cr olivine?with Cr₂O₃content range from 0.08%to 0.71%?,orthopyroxene,clinopyroxene,tremolite and dolomite.Occurrence of high-Cr olivine suggests the formation of chromitite begin in deep mantle.?3?Mg-Fe exchange between chromite and olivine,and alteration/metamorphism during emplacement change the major and trace elements composition of chromite,result in high Cr and Fe,low Mg and Al,and the Cr#value and Mg#value vary from 67 to 91,and 24 to 39,respectively.Cr-chlorite produced by reaction of olivine and chromite with exits of water in the middle tempereature.?4?Chromite major composition in massive ore and chromite with zoned structure slightly influenced by alteration/metamorphism due to the low silicon/chromite ratio of chromitite and zoned structure,calculated parent magma composition based them show a good overlapping with boninite field melts,with average Al₂O₃ and TiO₂ content is 13.1%,0.23%and 15.4%,0.11%,respectively.?5?The Songshugou banded chromitite formed in SSZ setting,melts mingling with different composition is the key to Cr saturated in parental magma.