A LATE JURASSIC COLLISION EVENT IN CENTRAL TIBET IDENTIFIED BY DETRITAL ZIRCONS FROM THE MUGAGANGRI MéLANGE IN THE BANGONG-NUJIANG SUTURE ZONE

The E-W-trending Bangong-Nujiang suture zone in central Tibet results from the Mesozoic collision between the Lhasa Terrane to the south and the Qiangtang Terrane to the north. The closure of the Bangong-Nujiang Ocean remains enigmatic and the depositional age of subduction-related mélange could provide an older limit to collision. We present field and petrographic observations and zircon U-Pb geochronological data for six sandstone samples in the ophiolitic mélange (Mugagangri Group) in the Jiang Tso–Amdo region.

The Mugagangri Group is a greenschist-facies schistose unit characterized by blocks of sandstone, limestone, radiolarite, and ophiolitic fragments in shale and fine sandstone matrix and is interpreted as an accretionary prism. Three samples of sandstone blocks and three samples of fine sandstone matrix were collected from three regions and 650 zircons were analyzed by LA-ICP-MS; ages with <10% discordant are used (n=575). The samples of blocks and matrix reported here and matrix samples from Gerze (Li et al., 2017; Tectonophysics) show similar age spectra. They define a distinctive age population between 250–200 Ma with peak at ca. 220 Ma. Triassic magmatic and metamorphic rocks are widespread along the central Qiangtang belt (246–200 Ma) but are rare in the Lhasa Terrane. Additional age groups at 2600–2400 Ma, 2000–1700 Ma, 900–700 Ma, 500–400 Ma, and 280–250 Ma are very similar to those of pre-Cretaceous sedimentary rocks of the Qiangtang Terrane. Thus, the Mugagangri Group was most likely sourced from the central Qiangtang belt. The youngest age for a matrix zircon is 169 ± 2 Ma; 7 zircons have Jurassic ages (190–169 Ma) for matrix samples (n=271). In Gerze, only 3 zircons with Jurassic ages (177 Ma, 160 Ma, and 154 Ma) have been found in 26 matrix samples (n= 2342). We conclude on this basis that sedimentation to form the Mugagangri Group may have continued until Late Jurassic time.

Our results support that a collision event may have occurred during the Late Jurassic. Considering the results of paleomagnetic data, the presence of Early Cretaceous subduction-related magmatic rocks, and the development of thrusting during the Late Jurassic to Early Cretaceous, we propose that this collision may have occurred between a microcontinent, ocean plateau, or mid-ocean ridge and Qiangtang Terrane.