Two Types of Granites in the Western Yangtze Block and Their Implications for Regional Tectonic Evolution: Constraints from Geochemistry and Isotopic Data
Authors: MABI Awei , YANG Zhengxi , ZHANG Mingchun , WEN Dengkui , LI Yanlong , LIU Xuyang 
Published: February 2018

In the western Yangtze Block, widespread Mesoproterozoic to Neoproterozoic rocks are the key to understanding the Precambrian tectonic‐magmatic evolution of the region. However, their petrogenesis and tectonic setting are still controversial. In this paper, zircon U‐Pb ages, Sm‐Nd isotopic and whole‐rock geochemical data are reported from selected fresh samples in the southern Dechang county, southwestern China, in order to constrain their emplacement age and magma source, as well as their petrogenesis and tectonic setting. They are mainly composed of biotite monzogranite, monzonitic granite, biotite granodiorites, and quartz diorite. Two ages of 1055 ± 43 Ma and 837.6 ± 3.8 Ma were obtained through zircon U‐Pb dating by LA‐ICP‐MS and LA‐MC‐ICP‐MS, respectively. According to their major element compositions, the Grenville‐age granites are peraluminous calc‐alkaline series calcic S‐type granite. In contrast, the mid‐Neoproterozoic granites are metaluminous calc‐alkaline series alkalic I‐type granite. Furthermore, the S‐type granites are enriched in LREEs relative to HREEs with (La/Yb)N ratios of 3.85–18.56 and underwent major fractionation with strongly negative Eu anomalies (Eu/Eu* = 0.38–0.66). In the MORB‐normalized trace element variation diagram, all the samples are enriched in Ce and large ion lithophile elements such as Rb, Th, and K, and depleted in high field strength elements such as Nb, and Ti, with negative Sr and Ti anomalies. The I‐type granites are enriched in LREEs with slight negative Eu anomalies (Eu/Eu* = 0.83–0.93). They are characterized by the enrichment of highly incompatible elements (such as K, Rb, Ba, Th) and LREEs, relative to MORB. Neodymium isotopic data show that the S‐type granites display 143Nd/144Nd values of 0.51241–0.51256, and have εNd (t = 1055 Ma) values of (−3.29) to (−3.81). Calculated tDM ages yield values from 1.87 to 1.91 Ga with the tDM.2stg ages of 1.86 to 1.9 Ga. The I‐type granites have 143Nd/144Nd ratios between 0.51192 and 0.51195, corresponding to initial εNd (t = 837 Ma) values of 1.22 to 5.63. Calculated tDM ages yield values from 1.0 to 1.38 Ga and the tDM.2stg ages yield values from 0.99 to 1.06 Ga. The S‐type granites are distinguished as syn‐collision granite, whereas the I‐type granites were formed as arc magmas according to the Rb‐(Yb+Ta) and R1‐R2 tectonic discrimination diagrams. To conclude, there are two types of spatially associated granite, the Mesoproterozoic S‐type granite which were derived from re‐melting of upper crustal mudstone and/or clastics and resulted from the convergence of two continental plates, and the mid‐Neoproterozoic I‐type granite which formed in continental arc and resulted from mantle‐derived magma mixed crust material, in the western Yangtze Block. Furthermore, we suggest that collision between the Yangtze and Cathaysia blocks occurred at about 1055 Ma, and caused the S‐type granite. The I‐type granite related to the subduction of oceanic lithosphere eastward underneath the Yangtze Block in the mid‐Neoproterozoic.