Early-Middle Paleozoic Tethyan Evolution In SW Yunnan, China And Northern Thailand

Tethys is a global giant orogenic belt from the Alps through Afghanistan to southwestern China and Peninsular Malaysia. Present-day Southwest Yunnan and Southeast Asia which is located in the junction of two tectonopaleogeographic domains involving Cathaysian and Gondwana province blocks, is an important component of Proto-Tethyan and Paleo-Tethyan tectonic zone. It preserves abundant records of the Tethyan evolution and is a critical area for studying Proto-, Paleo-, and Neotethyan tectonic patterns. In the dissertation, three examples from SW Yunnan, China and northern Thailand are studied to find out the rock type and spatial distribution, analyze the magma source/provenance, unravel tectonic setting of the northern margin of East Gondwana, and reveal the history of Gondwana breakup and the tectonic evolution of Tethyan oceans. The studied three examples are the Huimin metavolcanic rocks, the Nanguang formation turbidite in the southern Lancangjiang zone, SW Yunnan and detrital zircons of the Silurian-Devonian in the Sukhothai terrane, northern Thailand and the Upper Devonian in Changning-Menglian Belt, SW Yunnan.The Huimin metavolcanic rocks, consisting mainly of basalt, andesite, and dacite, are exposed predominantly in the Huimin area of the southern Lancangjiang zone, SW Yunnan. Three metavolcanic rock samples yielded zircon U-Pb ages of 456±3 Ma, 456±7 Ma and 459±14 Ma, suggesting the eruption occurred in the Late Ordovician. These metavolcanics are characterized by sub-alkaline rocks, with high Al2O3 (12.45-17.68 wt%) and low TiO2 (0.60-0.96 wt%). The rocks are enriched in LREEs and LILEs with weakly negative Eu anomalies (Eu/Eu*=0.37-0.99) and depleted in HFSEs, which is geochemically similar to typical subduction-related arc volcanic rocks. Zircons with Late Ordovician magmatic ages (-456 Ma) have homogeneous Lu-Hf isotopic compositions with in-situ εHf (t) values of -4.36-3.30 and Hf model ages of 1.51-1.55 Ga, indicating that the Huimin metavolcanic rocks might have originated from a metasomatic mantle source. Our data provide reliable evidence for the presence of Ordovician magmatism in the Sanjiang area, represent the magmatic record of Prototethyan evolution in SW Yunnan, and identify a hitherto unknown early subduction event at the northeastern Gondwana margin.The Devonian and the southern Lancangjiang zone, western Yunnan is a critical period and key region for studying the transformation between the "Proto-Tethyan" and Paleo-Tethyan oceanic systems. New geochemical data and LA-ICPMS U-Pb zircon ages of the Late Devonian volcanogenic sediments from the southern Lancangjiang zone in western Yunnan, SW China, are presented. The studied sedimentary rocks of the Nanguang Formation are volcaniclastic rocks with high volcanic lithic content (55-65%, mostly andesite, dacite, with some rhyolite and tuffs). Whole rock geochemistry, zircon trace elements and detrital modal analyses indicate derivation from a subduction-related magmatic arc. Three tuff samples yield Late Devonian weighted mean 206Pb/238U ages of 378±4 Ma,366±5 Ma and 382±8 Ma, suggesting a Late Devonian depositional age. 104 zircon U-Pb analyses on 104 zircon grains from two sandstone samples present extremely tight age clusters, mostly ranging from 380Ma to 360Ma. This indicates a single Late Devonian igneous source. A short transport distance and a high rate of denudation and deposition on the flank of the Jinghong back-arc basin are considered likely for the tuffs and volcaniclastic rocks in this study. This implies the presence of an as yet unidentified Late Devonian magmatic arc in the southern Lancangjiang zone. The cryptic Late Devonian arc is likely to result from the initial subduction of the main Paleo-Tethyan ocean, and indicates that subduction of the Changning-Menglian ocean beneath the Simao/Indochina Block occurred in the Late Devonian.The Sukhothai Terrane and Baoshan Block are on the two side of the main Paleo-Tethyan suture zone (Changning-Menglian-Chiangmai Belt). The Silurian-Devonian in the Sukhothai, northern Thailand and the Upper Devonian in the Changning-Menglian Belt, SW Yunnan, contain important information on the provenance of the Sukhothai Terrane and Baoshan Block. Detrital zircons geochronological results are presented in this study to analyze the affinities of Sukhothai terrane and Baoshan block, reveal the palaeogeographic framework of the northern margin of East Gondwana in the Early-Middle Paleozoic, and discuss the tectonic evolution of Tethys oceans in SW Yunnan, China and northern Thailand.This dissertation concluded the following understanding:(1) The Sukhothai Terrane has the same continental basement with the Indochina block. It is a part of the greater Indochina Block in the Early Paleozoic. Our detrial zircon geochronological data provide reliable evidence to support Metcalfe’s assumption on the history of the Sukhothai Terrane. (2) The detrital zircon age distribution of the Silurian-Devonian in the Sukhothai is consistent with the proposed ages of Ordovician and older tectonic events of the Greater Himalayan. This implies that the Indochina Block (including the Sukhothai terrane) is likely to be near the Greater Indida margin of East Gondwana continent in the Early Paleozoic. (3) The detrital zircon age distribution of the Upper Devonian in Changning-Menglian Belt is similar to those of the South Qiangtang, Sibumasu and Tethyan Himalayan, but quite different from the Lhasa Block and western Australia. This signature indicates that the Baoshan, South Qiangtang and Sibumasu are located on the Greater India margin of East Gondwana before the Permian. (4) Both the Silurian-Devonian, Sukhothai and the Upper Devonian in Changning-Menglian Belt have a remarkable age peak on 460-420Ma. And these magmatic zircons are also existed on the Indochina Block, South China Block and Sibumasu, etc. Combined with records of the contemporaneous magmatic-metamorphic events in the northern Tibet and SW Yunnan, these Ordovician-Silurian detrital zircons are proposed to be related to the subduction and close of the Proto-Tethyan ocean in the Silurian. They are probably the products of magmatic and metamorphic activities in the process of the Cathysian province blocks (Indochina, South China, etc.) and Gondwana continent collisionBased on this dissertation and combined with former achievements, we propose a hypothetical tectonic model for the Early-Middle Paleozoic evolution of the northern margin of Indo-Australian Gondwana. According to our model, the Early-Paleozoic Proto-Tethyan ocean (the Lancangjiang Ocean) between Cathysian province blocks (Simao-Indochina, South China, etc.) and East Gondwana subducted beneath the Greater India margin of East Gondwana from Ordovician to Silurian, resulted in the subsequent close of Proto-Tethyan ocean in the Late Silurian. That is to say, these Cathysian province blocks collided with the northern margin of Gondwana in the Late Silurian. Renewed subduction of Proto-Tethys ocean crust and subsequent Silurian collision triggered slab break-off, leading to the rapidly opening of the Paleo-Tethyan ocean developing from the back-arc basin of Proto-Tethyan ocean. The Paleo-Tethyan ocean opened rapidly in the Early Devonian in response to the detachment of Cathysian province blocks from the northern margin of Gondwana. The Paleo-Tethyan ocean started to subduct beneath the Cathysian province blocks (Simao-Indochina, South China, etc) in the Late Deovnian.