DETRITAL ZIRCON PROVENANCE OF THE MESOZOIC QAMDO BASIN, EASTERN TIBET AND ITS IMPLICATION ON REGIONAL TECTONICS

The Qamdo basin, located in eastern Tibetan Plateau, is a large basin covering an area of approximately 12,000 km² and contains ~7000 m of well-exposed Upper Triassic to Cretaceous strata that rest on top of the Qiangtang (QT) terrane. The basin is bound by two major Tethyan suture zones: the Jinsha suture to the northeast and the Bangong-Nujiang suture to the southwest.

Detrital zircon provenance study reveals that the Upper Triassic marine strata contain two major zircon age populations at 1.8-2.0 and 2.4-2.5 Ga, along with three minor age groups at 235-260, 310-330, and 420-460 Ma. This age signature indicates that the Upper Triassic deposits were either sourced primarily from earliest inversion and erosion of the Songpan-Ganzi Turbidite (SGT) system, or they represent a continuation of SGT deposystem. A significant transition of sediment sources occurred at the beginning of Jurassic indicated by a spatial diversification in detrital zircon age signatures. Three minor depocenters developed in the Qamdo basin, characterized by dominant zircon age groups at 250-500 Ma, 1800-1900 Ma, and 220-250 Ma. The age signatures indicate the major sediment sources from the SGT metasandstone, the metamorphic basement of Zhongza block, and the Yidun arc volcanic rocks, respectively. The Middle to Upper Jurassic strata contain a dominant zircon age population at 170-195 Ma, indicating they were sourced primarily from the Bangong arc system existed in the Meso-Tethys realm. The Cretaceous strata show dominant age peaks at Triassic (225-240 Ma). Along with westward paleocurrent directions, this age signature suggests the Cretaceous sediments were derived primarily from granitic plutonic rocks of the exhumed Yidun arc system.

One significance of this study is that it reveals the QT terrane and the Jinsha suture mélange (JSSM) were not contributing sediments into the Qamdo deposystem during Mesozoic time, as indicated by the significant 500-1000 Ma zircon age population in the QT and the JSSM documented by other workers which is missing in Qamdo basin strata. The result indicates that the Mesozoic accretionary and convergent tectonics of eastern Tibet did not lead to significant upper crustal deformation and exhumation of QT terrane rocks and adjacent suture zone, as might be expected for the early stage of a foreland basin.