Dr Xiao Liang

Tectonic framework and Geological evolution of Tethys Ocean in central Qiangtang of North Tibet

It is a heated debate whether the >500 km long and nearly 100 km wide E–W-trending eclogite/bleuschist-bearing central Qiangtang metamorphic belt (CQMB) represents a Permo–Triassic suture zone of Tethys Ocean.
After geological mappings along a N–S-trending corridor across the CQMB, a late Permian–Triassic Andean-type accretionary orogen was unraveled as a result of northward subduction of the Tethys Ocean, which includes late Triassic intermediate-acid sadi Kangri volcanics of slab break-off origin, late Permian-Triassic Raggyorcaka fore-arc basin, central Qiangtang accretionary complex, Yibug Caka foreland thrust belt, respectively from north to south.
Generally similar age probability peaks and ranges were discerned between the North Qiangtang, the Central Qiangtang metamorphic belt (CQMB) and the Tethyan Himalaya, which constrain the northern boundary of Gondwana realm to the north. HP rocks were exhumed in the later stage of the oceanic subduction and probably formed as a trans-extensional MCC.
The roll-back of the oceanic slab coupled with a strong fore-arc rifting could probably explain the exhumation.

Exhumation process and mechanism of the HP-LT rocks in Central Qiangtang

The well-exposed Lanling HP–LT rock complex in Central Qiangtang metamorphic belt (CQMB) was chosen to decipher the mode and process by which the complex was exhumed.
After a detailed petrological and structural mapping, three distinct N–S-trending mélange-like metamorphic domains were distinguished, which contact with each other by ductile shear zones. The core domain rocks experienced a heating plus depressurization trajectory after peak eclogitic metamorphism, which is evident in syn-tectonic vein-form porphyroblastic garnets with a clear retrograde path in terms of the altering chemical rings and mineralogical zonings. Blueschists of the mantle domain lacking the growth of porphyroblastic garnets underwent temperature-increasing hydrated retrogression showed by pervasive epidote/zoisite swarms/veins.
The compilation of radiometric results of high-pressure mineral separates in Lanling and Central Qiantang, and reassessments on the newly acquired or published phengite datasets of Lanling using Arrhenius plots allows a two-step exhumation model to be formulated. Eclogitic rocks were brought onto blueschist-facies level starting at 244–230 Ma, with exhumation continuing to 227–223.4 Ma.
Subsequently, they were exhumed together starting at 223–220 Ma, reaching lower greenschist facies conditions after 222–217 Ma which perhaps continued to ~211 Ma.

Manuscripts submitted in ANU

[1] Stepwise exhumation of the Triassic Lanling high-pressure metamorphic belt in Central Qiangtang, Tibet: insights from a coupled study of metamorphism, deformation and geochronology

[2] Late Permian–Triassic Raggyorcaka forearc basin in North Tibet: Records of the accretionary orogeny between the Tethys Ocean and the North Qiangtang terrane