Late Triassic Tectono-magmatic Records In The Southwest Yangtze Block, SW China:Implications For Indosinian Orogenesis

The Indosinian orogeny in response of the closure of the eastern beanch of the Paleo-Tethys Ocean is recorded by widely occurring Triassic unconformities, granitoids and deformations in South China. This thesis focuses on geochronology and geochemistry of the Late Triassic granitoids and tuffaceous rocks from the central Xuefengshan belt, and structural and apatite fission-track constraints of the Meso-Cenozoic tectonic belt in southwest Yangtze Block. These results provide constraints on the Late Triassic tectonomagmatic record of late stage of Indosinian orogeny in the southwestern Yangtze Block. First-order conclusions are as follows:(1) The Longtan pluton in the Xuefengshan belt is composed of high-K and calc-alkaline granodiorites and granites, emplaced at 218 Ma; The granodiorites were derived from partial melting of interlayed Proterozoic metabasaltic to metatonalitic source rocks, whereas the granites were derived from a mixture of Proterozoic metagraywackes and metaigneous source rocks; Geochemical evidence indicates that partial melting and fractional crystallization were dominant mechanism of the granitoid evolution; The early and late Indosinian magmatic episodes with peak ages of-238 Ma and-218 Ma in the SCB, formed in a similar manner as Longtan granitoids, were derived from decompressional partial melting which triggered by syncollisional extension. (2) The tuffaceous rocks belong to altered rhyolite tuff and andesitic or basaltic tuff, and yield a youngest age population of 212 Ma; The REE patterns, HFSE geochemistry, Sr-Nd isotopes, Hf isotopic compositions and inherited major zircon age populations of the tuffaceous rocks suggest that the parental magma were derived from mantle with crustal contamination during emplacement; The tuffaceous rocks were probably derived in a tectonic setting of syncollisional extension/rifting of the Indosinian Orogeny in the central Yangtze Block, indicating a Late Triassic volcanic eruption; (3) The-230-210 Ma Indosinian deformation (D1) and subsequent overprinting of the-125-100 Ma deformation (D2) and-60-0 Ma (D3) results are consistent with three stages of uplifting-denudational history in southwest Yangtze Block. The Late Triassic deformation (D1) represented by thrust-related structures is simultaneous with a consistent turning point from quickly descent to stable or slowly uplift of the southwest Yangtze Block, also supported by a stratigraphic gap since Middle Triassic in the Youjiang Basin and the late Late Triassic unconformity in the Qianzhong Tectonic Unit. The Late Triassic D1 deformation and subsequent denudation recorded the collision of Indochina Block and the SCB during the Indosinian orogeny belongs to the Tethyan tectonic domain. Overall, we propose a syncollisional extension model that Indosinian magmatism were generated by decompressional partial melting of crustal materials triggered by two progressive extensions during collision of the Indochina and South China blocks, which is also supported by the records of the Late Triassic deformation and thermal history. Thus, the Longtan granitoids and tuffaceous rocks in the northwesternmost part of the orogenic belt and Indosinian deformation in the southwest Yangtze Block is the tectonomagmatic records of late stage of Indosinian orogenesis.