Magmatic and Sedimentary Constraints on the Evolution of the Triassic Yidun ARC, Eastern Tibet

This dissertation, "Magmatic and Sedimentary Constraints on the Evolution of the Triassic Yidun Arc, Eastern Tibet" by Baiqiu, Wang, 王伯秋, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: The Yidun Terrane in the eastern Tibetan plateau is separated from the Songpan-Ganzi Terrane by the Ganzi-Litang suture zone to the east and Qiangtang Block by the Jinsha suture zone to the west. Both suture zones are marked by eastern Paleo-Tethyan ophiolites. The western part of the Yidun Terrane, the Zhongza Massif, is dominated by Paleozoic sedimentary sequences. In the eastern part, Triassic subduction-related plutonic rocks and volcanic-flysch successions of the Yidun Group are important elements for understanding the evolution of eastern Paleo-Tethys and amalgamation of East Asia. The Yidun Group includes the Lieyi, Qugasi, Tumugou and Lanashan Formations from base upwards. Two major depocenters for the Yidun Group can be recognized. Sedimentary detritus supplying for the northern depocenter were dominantly sourced from the Zhongza Massif and received recycling components in a passive margin setting. In the southern depocenter, the Qugasi Formation accumulated materials from the Zhongza Massif, whereas the Tumugou Formation received additional materials from locally distributed Triassic arc rocks and crystalline basement rocks, which indicates transition from a passive margin to a magmatic arc setting. In the southern Yidun Terrane, (quartz-) dioritic hypabyssal intrusions are spatially associated with andesites and dacites and have zircon U-Pb ages from 230 to 215 Ma. They have adakitic geochemical features and are divided into the 230-215 Ma high silica (HSA) and 215 Ma low silica (LSA) adakitic rocks. The HSA formed from subducted slab melts with limited interaction with the overlying mantle wedge, whereas the LSA were generated from slab melts with more extensive interaction with mantle due to slab break-off at 215 Ma. In the northern Yidun Terrane, granitic plutons and volcanic rocks occur in two parallel N-S belts. The 228 Ma volcanic rocks in the Xiangcheng region are adakites generated from slab melts, whereas the 231-230 Ma volcanic rocks in the Changtai region, including basalts, andesites, dacites and rhyolites, formed in a back-arc setting. The Changtai basalts were produced by low degrees of partial melting of an OIB-like mantle source with minor involvement of subducted slab components. The Changtai andesites/dacites represent evolved members from the basaltic magmas through an AFC process, whereas the rhyolites formed from anatexis of a garnet-bearing crustal source. These volcanic rocks are 4-6 mys older than arc granitic rocks in the northern Yidun Terrane. The spatio-temporal framework of all the subdution-related igneous rocks suggests initiation of subduction of the Ganzi-Litang oceanic lithophere under the southern Yidun Terrane at 230 Ma, resulting in the adakitic magmatism in the Shangri-La and Xiangcheng regions and the back-arc magmatism in the Changtai region. Subsequently at 224 Ma, the subduction extended to the northern Yidun Terrane, leading to the formation of the arc granitic plutons. From south to north, the Yidun Terrane was sequentially amalgamated with the Songpan-Ganzi Terrane during the Late Triassic. DOI: 10.5353/th_b5060571 Subjects: Geology - Tibet, Plateau of