The Geological Significance Of Indosinian Orogenesis Occurred In Lhasa Terrane

In this study, the orogenetic process, regional geological background, development and evolution, and the exhumation machanism of high-ultrahigh pressure metamorphic rocks are analysed to the Sumdo eclogite belt. Based on the studies of the deformation characteristics and the ductile shear zones in Sumdo Group, the quartz fabric by EBSD, the data of muscovite or amphibole ⁴⁰Ar-³⁹Ar geochronology to the muscovite quartz schist, eclogite and retrograde eclogite, and the zircon SHRIMP U-Pb chronology of granites in Sumdo region, Lhasa terrane is thought to have experienced an important Indosinian orogenic event. The main advancements and conclusions are as followings:1 From north to south, the working area is divided into two structural-lithological units, such as the northern Lhasa Paleo-Tethys belt and Gangdise tectonic magmatic accretion zone. Sumdo Group is thought to experienced three periods of tectonic deformation. In the main deforming stage, several thrusting ductile shear zones were formed. In certain area, the characteristics of clockwise right-hand slipping is the extruding transform deformation. Retrograded eclogite lenses, with the long axes paralleled to the tectonic line, were found in some ductile shear zones. The flow deformations of garnets show that the retrograded eclogites experienced deformational transformation by the ductile shear zones.2 The muscovite ⁴⁰Ar-³⁹Ar chronological studies to the muscovite quartz schist of Sumdo Group, the eclogite and retrograde eclogite show that there has been a Indosinian orogenesis at 220 -240 Ma in Lhasa terrane, which caused the closure of the Paleo- Tethys ocean and the collision between northern part and southern part of Lhasa terrane. The consistence of the muscovite or amphibole ⁴⁰Ar-³⁹Ar age for the the eclogite, retrograde eclogite and muscovite quartz schist indicates that the exhumation of eclogites and the Indosinian Orogenesis Occurred at the same time, which reveals that the regional ductile extrusion should be the main mechanism for the eclogite exhumation.3 The zircon SHRIMP U-Pb chronology of 190Ma for biotite adamellite, with the regional geological data about granites in Lhasa terrane, indicates that there is a granite belt of late indochina epoch along Dangxiong -Gongbujiangda -Bomi county. The Indosinian granites consist of not only basic granodiorites, but also acidic theleucogranites, with the High precision single zircon U-Pb ages of 210Ma - 190Ma, which are lower than the collision orogenic age of Sumdo Group and the timing of the exhumation of eclogites. Based on the formation age and discrimination diagrams, these granites should be syn-collisional or late-collisional granites relate to Indosinian orogenesis occurred in inner Lhasa terrane.4 Based on the evidences of regional sedimentary, volcano-magmatic activity and ophiolite melange, the Indosinian orogenic belt in Lhasa terrane is thought at least starts from Coqen county, and goes eastward through the northern part of Nanmulin county, Mozhugongka county, Gongbujiangda county, and at last goes around the eastern Himalayan syntaxis into the Bomi county. The discovery of Indosinian orogenic belt in Lhasa terrane expanses the southern boundary of Indosinian orogenic belt in Qinghai-Tibet Plateau to Lhasa terrane from Qiangtang terrane, which changes the understanding about the distribution of Indosinian orogenic belt in Qinghai-Tibet Plateau and extends the "T" type Indosinian orogenic belt in China.5 Through the detailed studies, The temporal framework of geological evolution from early Palaeozoic to early Mesozoic for Lhasa terrane is built. Based on the evolutionary process, the geological development of Lhasa terrane from early Palaeozoic to early Mesozoic can be divided into seven stages. All of the seven stages make up a whole Wilson circle and reveal a perfect evolutionary process of the Paleo- Tethys ocean between northern part and southern part of Lhasa terrane...