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

Geochemistry of nine Neoproterozoic granitoid bodies in the northwestern ma rgin of the Yangtze Block (YB) has been studied in this paper. Their ages r ange from 876 Ma to 786 Md based on U-Pb zircon dating. These granitoids ca n 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 27 4 to 122 ppm. Group III includes migmatized granites characterized by low R EE 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 tw o groups (EREE = 105 to 212 ppm, Eu/Eu* = 0.73 to 0.79). Granitoids of Grou p II and III belong to calc-alkaline series and I-type, which were formed d uring the Jinning Orogeny before 820 Ma. related to subduction or collision between the Yangtze Block and oceanic Qinghai-Yunnan-Tibet Plate. The Grou p I granites were formed after 805 Ma in the late stage of or post the Jinn ing Orogeny. The Neoproterozoic granitoids have epsilon (Nd)(T) values rang ing 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 marg ins 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 0.7 05 to 0.708. The difference in geochemistry of the three groups was due to difference in their sources. The Neoproterozoic granitoids of this study we re formed by magmas probably derived from sources with different proportion s of juvenile crust and Meso- to Paleo-Proterozoic crust. The granites of G roup III were derived probably from the lower crust. The crust sources for Group I granitoids probably contain less amounts of juvenile crust componen t and have higher maturity when compared with those for Group H. (C) 2001 E lsevier Science Ltd. All rights reserved.