Geochemical characteristics and genesis of Neoproterozoic granitoids in the northwestern margin of the Yangtze Block

Geochemistry of nine Neoproterozoic granitoid bodies in the northwestern margin of the Yangtze Block (YB) has been studied in this paper. Their ages range from 876 Ma to 786 Md based on U-Pb zircon dating. These granitoids can be divided into three groups in terms of major and trace elements. Rocks of Group I are alkaline series. granites characterized by evident negative Eu anomaly (Eu/Eu* = 0.31 to 0.41) with total REE concentrations between 274 to 122 ppm. Group III includes migmatized granites characterized by low REE concentrations (14 to 45 ppm) and positive Eu. anomaly (Eu/Eu* = 1.1 to 2.5). Group II comprises tonalite and diorite with REE between the above two groups (EREE = 105 to 212 ppm, Eu/Eu* = 0.73 to 0.79). Granitoids of Group II and III belong to calc-alkaline series and I-type, which were formed during the Jinning Orogeny before 820 Ma. related to subduction or collision between the Yangtze Block and oceanic Qinghai-Yunnan-Tibet Plate. The Group I granites were formed after 805 Ma in the late stage of or post the Jinning Orogeny. The Neoproterozoic granitoids have epsilon (Nd)(T) values ranging from -4.3 to +4.5 and initial Sr-87/Sr-86 ratios less than or equal to 0.705, similar to those of the Neoproterozoic granitoids. in the other margins of the YB, but different from those of coeval granitoids. within the YB which have epsilon (Nd)(T) of -8.1 to -14.2 and initial Sr-87/Sr-86 of 0.705 to 0.708. The difference in geochemistry of the three groups was due to difference in their sources. The Neoproterozoic granitoids of this study were formed by magmas probably derived from sources with different proportions of juvenile crust and Meso- to Paleo-Proterozoic crust. The granites of Group III were derived probably from the lower crust. The crust sources for Group I granitoids probably contain less amounts of juvenile crust component and have higher maturity when compared with those for Group H. (C) 2001 Elsevier Science Ltd. All rights reserved.