Geochronological and geochemical constraints on the petrogenesis of late Mesoproterozoic mafic and granitic rocks in the southwestern Yangtze Block

Late Mesoproterozoic igneous rocks in the SW Yangtze Block are important for understanding the role of it in reconstruction of the Rodinia supercontinent.In the present study,we report new geochronological,geochemical,and Nd-Hf isotopic data for the Cuoke plagioclase amphibolites and granites in the SW Yangtze Block.Geochronological results show that the plagioclase amphibolites and granites have similar late Mesoproterozoic zircon U-Pb ages of 1168-1162 Ma,constituting a bimodal igneous assemblage.The plagioclase amphibolites have high and variable TiO2 contents(1.15-4.30 wt.%)and Mg#(34-66)values,similar to the tholeiitic series.They are characterized by enrichment in LREEs and LILEs,and have OIB-like affinities with positive Nb and Ta anomalies.The plagioclase amphibolites have positive whole-rockεNd(t)(+3.2 to+4.3)and zirconεHf(t)(+4.3 to+10.7)values,indicating that they were derived from an OIB-like asthenospheric mantle source.The granites belong to the reduced peralkaline A-type series and have negativeεNd(t)value of-6.0 andεHf(t)values of-5.8 to-13.8,indicating a derivation from the partial melting of ancient mafic lower crust.In combination with the~1.05-1.02 Ga bimodal igneous assemblage in the SW Yangtze Block,we propose that the Cuoke 1168-1162 Ma igneous rocks were likely formed in a continental rift basin and argue against the existance of Grenvillian Orogen in the SW Yangtze Block during the late Mesoproterozoic.

Late Mesoproterozoic to Early Neoproterozoic Tectonic Evolution of the SW Yangtze Block, South China: Evidence from U-Pb Geochronology and Lu-Hf Isotopes of Detrital Zircons from Sedimentary Rocks

In situ zircon U-Pb geochronological and Lu-Hf isotope studies of detrital zircons from Late Mesoproterozoic to Early Neoproterozoic sedimentary units on the southwestern margin of the Yangtze Block have important implications for the tectonic evolution of the Yangtze Block.The Huili Group contains zircons whose ages are mainly Late Archean to Mesoproterozoic(2650-2450,2100-1800,and 1350-1150 Ma).The Dengxiangying Group has one major age population of 1900-1600 Ma,and two subordinate age populations of 1350-1100 and 2300-2000 Ma.Yanbian Group sedimentary rocks have a zircon age population mainly in the range of 970-850 Ma,contemporaneous with the ages of widespread arc-related magmatism in the western Yangtze Block.Combining these results with previous work,the Huili and Dengxiangying groups were most likely deposited during ca.1160 to 1000 Ma in an intra-continental rift basin setting,while the Yanbian Group accumulated during>920 to 782 Ma in a back-arc basin setting at the southwestern margin of the Yangtze Block.In addition,all these results further suggest a tectonic transition from a continental rift basin to a convergent environment at the southwestern margin of the Yangtze Block at 1000-970 Ma.

A review of the～1600 Ma sedimentation,volcanism,and tectono-thermal events in the Singhbhum craton,Eastern India

The Palaeoproterozoic-Mesoproterozoic transition （ -1600 Ma） is a significant event in the Earth history as a global thermal perturbation affected the pre-1600 Ma landmasses. Like other cratonic blocks of the world, lithospheric thinning, sedimentation, magmatism, metamorphism and crustal melting/anatexis are associated with this significant geological event in the Singhbhum cratonic province of India. This paper is a review of sedimentological, magmatic and tectono-thermal events in the Singhbhum craton at 1600 Ma. The Palaeo-Mesoproterozoic sedimentation and volcanism in the Singhbhum craton took place in a terrestrial intracontinental rift setting. The available geochronological data are indicative of late Palaeoproterozoic to Neoproterozoic tectono-thermal events in the Chhotanagpur Granite Gneissic Complex （CGGC）, an east-west trending arcuate belt of granite gneisses, migmatites and metasedi- mentary rocks. A detailed multidisciplinary geo-scientific investigation of the Dalma volcanic belt and the area to its north （Chandil Formation） and further north in CGGC will enable us to constrain the extant surface processes and crust-mantle interactions, the collision events between the North and South Indian cratonic blocks, and the position of India in the Columbia supercontinent.

Zircon SHRIMP U-Pb Ages,Geochemistry and Nd-Hf Isotopes of~1.0 Ga A-Type Felsic Rocks in the Southwestern Yangtze Block,South China:Petrogenesis and Tectonic Implications

The petrogenesis and tectonic affinity of Late Mesoproterozoic igneous rocks in the Yangtze Block are important to understand its tectonic evolution within the context of the Rodinia supercontinent’s reconstruction.Here,we report the SHRIMP zircon U-Pb ages,geochemistry,and Nd-Hf isotopic data for dacite from the Zegu Formation of the Dengxiangying Group in southwestern Yangtze Block.The crystallization age of the Zegu dacite is 1037 Ma.These rocks have relatively high Ga,Zr,Hf,and HREE contents,as well as high(10000×Ga)/Al(2.7-2.9)and FeOt/MgO(3.8-10.5)ratios,showing a geochemical affinity of A-type granitoids.Moreover,they are characterized by negativeεNd(t)(-3.6 to-7.5)and zirconεHf(t)values(-0.3 to-14.4),indicating that they likely generated via partial melting of an ancient continental crust source.The tectonic discriminant diagrams showed that the Zegu dacites have high Y and Nb contents(plotted in the‘within-plate’magmatic rocks field).In conclusion,the geochemistry and isotopic data indicated that the 1037 Ma felsic volcanic rock in the Dengxiangying Group generated during an intra-plate rift basin along a passive margin.Our work thus argues against a Grenvillian orogen’s existence in the Yangtze Block.In view of the subsequent Neoproterozoic subduction-related magmatism in the region,we suggest that the Yangtze Block might have been along the periphery of the Rodinia.