The Jinshajiang-Ailaoshan Suture Zone, China: tectonostratigraphy, age andevolution

The Jinshajiang Suture Zone is important for enhancing our understanding of the evolution of the Paleo-Tethys and its age, tectonic setting and relati onship to the Ailaoshan Suture Zone have long been controversial. Based on integrated tectonic, biostratigraphic, chemostratigraphic and isotope geoch ronological studies, four tectono-stratigraphic units can be recognized in the Jinshajiang Suture Zone: the Eaqing Complex, the Jinshajiang Ophiolitic Melange, the Gajinxueshan "Group" and the Zhongxinrong "Group". Isotope ge ochronology indicates that the redefined Eaqing Complex, composed of high-g rade-metamorphic rocks, might represent the metamorphic basement of the Jin shajiang area or a remnant micro-continental fragment. Eaqing Complex proto lith rocks are pre-Devonian and probably of Early-Middle Proterozoic age an d are correlated with those of the Ailaoshan Complex. Two zircon U-Pb ages of 340 +/- 3 and 294 +/- 3 Ma, separately dated from the Shusong and Xuitui plagiogranites within the ophiolitic assemblage, indicate that the Jinshaj iang oceanic lithosphere formed in latest Devonian to earliest Carboniferou s times. The oceanic lithosphere was formed in association with the opening and spreading of the Jinshajiang oceanic basin, and was contiguous and equ ivalent to the Ailaoshan oceanic lithosphere preserved in the Shuanggou Oph iolitic Melange in the Ailaoshan Suture Zone; the latter yielded a U-Pb age of 362 +/- 41 Ma from plagiogranite. The re-defined Gajinxueshan and Zhong xinrong "groups" are dated as Carboniferous to Permian, and latest Permian to Middle Triassic respectively, on the basis of fossils and U-Pb dating of basic volcanic interbeds. The Gajinxueshan "Group" formed in bathyal slope to neritic shelf environments, and the Zhongxinrong "Group" as bathyal to abyssal turbidites in the Jinshajiang-Ailaoshan back-are basin. Latest Perm ian-earliest Middle Triassic synorogenic granitoids, with ages of 238 +/- 1 8 and 227 +/- 5-255 +/- 8 Ma, respectively, and an Upper Triassic overlap m olasse sequence, indicate a Middle Triassic age for the Jinshajiang-Ailaosh an Suture, formed by collision of the Changdu-Simao Block with South China. (C) 2000 Elsevier Science Ltd. All rights reserved.