Identification of Neoproterozoic Gabbro from Diancangshan in West Yunnan and Its Geotectonic Implication
The Diancangshan-Ailaoshan tectonic belt in the West Yunnan has developed numerous Precambrian rocks and thus provides an ideal window for revealing the Precambrian tectonic history of the southwestern margin of the Yangtze block.In this study,elemental geochemistry and isotopic geochronology of gabbro in the Diancangshan area are presented.The results show that the intrusion age of the gabbro is 771-769 Ma,indicating that Neoproterozoic mafic magmatic activity developed in the Diancangshan area.The gabbro of the Diancangshan area exhibits a variation in total alkali content(K2O+Na2O)of 4.40%-4.49%,with low Na2O/K2O ratios(2.30-2.34),and thus belongs to the calc-alkaline series.The samples have relatively high contents of Fe2O3t(7.36%-7.50%),MgO(7.15%-7.30%),and Mg#(69.46-69.52).They are characterized by enrichment in large ion lithophile elements(LILE)such as Rb,Ba,and Sr,and depletion in high field strength elements(HFSE)such as Nb,Ta,and Ti.The samples display a relative enrichment of light rare earth elements with a pronounced fractionation between light and heavy rare earth elements,and have slightly positive anomalies of Eu(Eu/Eu*=1.17-1.29),They have depleted zircon Hf isotopic compositions with positive εHf(t)values of 6.72-10.84.Comprehensive data show that the gabbro formed in back-arc basin environment and is the product of partial melting of mantle wedge peridotite that was metasomated by subducted fluid.During the Neoproterozoic,arc-basin system was developed at the southwestern Yangtze Block,and the South China located at the margin of the Rodinia Supercontinent during this period.
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