Geochronology,Geochemistry And Geological Significance Of Mafic-ultramafic Rocks In The Western Part Of The Jiangnan Orogenic Belt

The Jiangnan Orogenic Belt?JOB?,locating in Southern China,is traditionally considered as a collisional suture zone between the Yangtze and Cathaysia blocks.Neoproterozoic igneous rocks in the western JOB,including mafic-ultramafic complex,provide important evidence to understand the Precambrian tectonic evolution of South China Craton.Here,we report whole rock geochemistry,mineral electron probe micro-analysis?EPMA?,Re-Os isotopic analysis and zircon U-Pb age data for two representative mafic-ultramafic rocks exposed in the Huaqiao and Shuanglang?Longsheng?areas along the western JOB.The peridotite samples have relatively high Mg^#?0.78-0.81?,Cr?2185-3087 ppm?and Ni?1238-1561 ppm?contents,which show geochemical features of mantle peridotite.Their negative correlations between MgO and SiO₂,AlO₃ and TiO₂ and positive correlation between Ni and Cr indicate that the magmas have experienced obvious accumulating processes of olivine,clinopyroxene and chromite.The geochemical characteristics of relatively low REE?15.79-22.27 ppm?,slight enriched LREE pattern,absence of obvious Nb,Ta,Ti depletions and enriched large ion lithophile suggest that the petrogenesis of the peridotites is different from the typical island arc magmatic rocks and the abyssal peridotites,which are characterized by LREE depletion.Thus,we infer that the peridotites were likely formed in an intracontinental environment and then altered by later metasomatism.The Mg^#variations of clinopyroxene in the peridotites?0.91-0.93,0.83-0.86?were dominated by the change of mineral composition during the process of peridotite cumulation.Besides,Re-Os isotopic analysises of the peridotites show high Re?116-633 ppt?and Os?837-2468 ppt?contents,similar ¹⁸⁷Os/¹⁸⁸Os?0.1496-0.1896?and¹⁸⁷Re/¹⁸⁸Os?0.5939-2.2937?isotope ratios,homogeneous?Os?6.4-26.9?values,a good linear relationship between ¹⁸⁷Os/¹⁸⁸Os and 1/¹⁹²Os ratios,indicating the heterogeneity of initial Os isotopes in magmatic system.In combination with the shallow depth recealed by spinel peridotite facies,we infer that the ¹⁸⁷Os/¹⁸⁸Os ratios in our peridotite samples??0.1296?are higher than those of chondrites.This may be related to the fact that the continental lithospheric mantle has undergone many high-proportion fluid/melt permeation reactions.Their high T_MA model ages indicate an ancient continental lithospheric mantle existed from the Paleoproterozoic to the Archean at least in the Longsheng area.Compared with the peridotites,the gabbro samples have lower Mg^#?0.41-0.50?,Cr?34-85 ppm?and Ni?27-67 ppm?,and show negatively correlations between MgO and SiO₂,CaO,Al₂O₃,TiO₂,indicating a fractional crystallization process during the upwelling of mafic magmas.These samples have relatively high REE?35.77-53.20 ppm?,more flat REE patterns and slight enriched LREE,showing geochemical features of E-MORB obviously diffirent from the typical arc magmatic rocks and OIB.This indicates that the gabbros were possiblely formed in an intracontinental environment,with the geochemical characteristics of trace elements influenced by later metasomatism.Amphibole minerals of the gabbros have lower Mg^#?0.43-0.61?,TiO₂,Na₂O and Al₂O₃ contents inherited from the composition characteristics of parental pyroxene,which indicate an origin from metasomatizd pyroxene by Si-rich fluids.Plagioclase minerals have relatively homogeneous compositions of low contents of CaO?0.08-0.31 wt.%?and K₂O?0.03-0.07wt.%?,high contents of Na₂O?10.7-11.9 wt.%?and Al₂O₃?15.6-19.0 wt.%?,indicating that a typical product of fractional crystallization of mafic magmas.Besides,their Re-Os isotopic data of gabbros show relatively low Re?199-452 ppt?and Os?7.6-30.8 ppt?contents,large ¹⁸⁷Re/¹⁸⁸Os?37.37-4528.49?and ¹⁸⁷Os/¹⁸⁸Os?1.19-444.67?ratios and display a good linear relationship between ¹⁸⁷Os/¹⁸⁸Os and 1/¹⁹²Os ratios,indicating the heterogeneity of initial Os isotopes in the magmatic system.Together with the obvious relationship between ¹⁸⁷Re/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os,we consider the gabbro as a result of fractional crystallization of mafic magmas and suggest the rocks may have undergone Si-rich fluid metasomatism in the later stage.The results of LA-ICP-MS zircon U-Pb dating show a weighted average age of 810.5±3.9 Ma,which is interpreted as the crystallization age of the gabbros and also represents the intrusive age of the Longsheng mafic complex.Based on the previous research results,we conclude that the Longsheng peridotites show characteristics of cumulated rocks and may be originated from an ancient continental lithospheric mantle.The gabbros may undergo fractional crystallization during during its ascension of mafic magmas.In combination with the published geochemical data of mafic-ultramafic rocks in this region,we infer that the mafic-ultramafic rocks in the Longsheng area are more likely to be formed in an intraplate rifting environment rather than mantle plume or island arc settings.