Abstract:

The Kangding complex is a S-N trending zone in the western margin of the Yangtze Block. It is composed of basaltic, intermediate and felsic rocks, with the essential components of intermediate and felsic rocks. It contains meta-gabbro，dioritic-gneiss, quartz-dioritic gneiss and granitic gneiss,The complex was traditionally considered as the Archean basement of the Yangtze Block,but recent isotopic dating results indicate that it was not formed in Archean. The samples in this paper are collected from the Kangding-Luding region, two representative ones(71704.2,71501.1)are taken for chemical analyses and SHRIMP U-Pb dating. The results show that the samples are of calc-alkaline series, the primitive mantle normalized trace element patterns are characterized by relative enrichment in large ion lithophile elements(K,Rb etc.)and depletion in high field strength elements(Nb,Ta,Ti,P etc.). REE patterns are of slightly right inclined type with unobvious Eu anomaly. These are typical features of subduction-related island-arc igneous rocks. SHRIMP U-Pb zircon analyses give crystallization age of zircons from the complex ranging from 765 to 771 Ma，whose Th/U ratios are greater than 0.1, indicating its igneous genesis. There are a few younger zircon grains, such as the grain 12.1 of 71704.2 with the age of(430±8)Ma, interpreted as the age of post event when it was newly formed in the post tectonic, magmatic and metamorphic activities. The above data indicate that the Kangding complex was formed at the Late Neoproterozoic. Above mentioned geochemical characteristics suggest that the Kangding complex was probably formed in island-arc setting. The magma source originated from subducted oceanic crust. Late Neoproterozoic igneous activity was related to dispersion of the Rodinia supercontinent in the western Yangtze Block.

Key words: Kangding complex, zircon SHRIMP U-Pb dating, Neoproterozoic, Rodinia supercontinent