Geochronology And Geochemistry Of Neoproterozoic Magmatism In Diancangshan,western Yunnan,and Its Geological Significances

The Diancangshan-Ailaoshan tectonic belt in the western Yunnan is located on the southwestern margin of the Yangtze block.The tectonic belt records numerous Proterozoic strata and Precambrian magmatic rocks that provide an ideal window to reveal the evolution of the Diancangshan-Ailaoshan tectonic belt and to understand the crustal evolution and dynamic process of the Yangtze block.This paper focuses on the Neoproterozoic magmatic rocks in the Diancangshan area on the southwestern margin of the Yangtze block.Detailed field geological surveys,zircon U-Pb dating,zircon Hf isotope analysis,petrology,mineralogy,and element geochemistry are presented in this paper in order to(i)clarify the age and geochemical characteristics of the Neoproterozoic magmatic rocks;(ii)discuss the petrogenesis of the Neoproterozoic magmatic rocks;and(iii)reveal the crustal evolution and dynamic process of the Yangtze block.The following main results have been achieved:(1)The geochronological data show that two metabasite samples yield zircon U-Pb ages of764±6 Ma and 779±9 Ma;one gneissic granite sample yields zircon U-Pb ages of 799±8 Ma.This indicates that Neoproterozoic magmatism was recorded in the Diancangshan area.Combined with previous studies,it is believed that there is extensive Neoproterozoic magmatism in the Diancangshan-Ailaoshan tectonic belt,and the age of the magmatism is mainly limited in the range of early Neoproterozoic(840?740 Ma).(2)The whole-rock elemental analysis results show that the metabasite samples can be divided into two groups based on their Nb/La ratios.Group 1 has Nb/La ratios of 0.75?0.76,resembling those of Nb-enriched basalts,while Group 2 has Nb/La ratios of 0.20?0.21,and shows obvious negative Nb and Ta anomalies,and thus are similar to those of arc basalt.The group 1samples have Si O₂,Mg O and Fe Ot contents of 52.38?52.56 wt%,4.12?4.18 wt%,and 7.60?7.68wt%,respectively.The group 2 samples are characterized by lower Si O₂ contents(47.36?47.48wt%),and higher Mg O(5.39?5.45 wt%)and Fe Ot(8.56?8.64 wt%)contents.Both of the two group samples show slightly high degree of fractionation.The group 1 samples have(La/Yb)cn ratios of 3.0?3.2,and(Gd/Yb)cn ratios of 1.42?1.51,and show negative Eu anomalies(?Eu=0.75?0.76).The group 2 samples have(La/Yb)cn ratios of 14.0?14.3,and(Gd/Yb)cn ratios of1.85?1.93,and show unobvious Eu anomalies(?Eu=0.99?1.13).The gneissic granites show higher Si O₂(average=76.83%)and Na₂O+K₂O(average=7.77%)contents,and lower Al₂O₃(average=13.31%)and Ca O(average=0.12%)contents relative to the metabasites.They are characterized by low total amount of REE(17?22 ppm,average=19 ppm)and depletion of Nb.(3)The zircon Hf isotopic results show that most of the metabasites have?_Hf(t)of-3.41 to+1.79 with corresponding two-stage model age(T_DMHf₂)of 1405?1665 Ma.The?_Hf(t)values of the gneissic granite range from-0.65 to+1.39 with T_DMHf₂ of 1287?1392 Ma.These data indicate that the metabasites originated from a mantle source of spinel lherzolite and their source was modified by subducted sediments.In addition,fractional crystallization of plagioclase,ilmenite and mafic minerals played a role during the magma evolution.The gneissic granites were produced by partial melting of mafic low crustal materials.(4)These magmatic rocks likely formed in a back-arc basin.The Yangtze block was likely an isolated continent and intermittent oceanic crustal subduction took place around it in the Neoproterozoic period.This is the reason why a long period of magmatism(?100 Ma)occurred synchronously around different margins of the Yangtze block.