Discovery of the early Jurassic Gajia m,lange in the Bangong-Nujiang suture zone: Southward subduction of the Bangong-Nujiang Ocean?

摘要

M,lange records a series of geological processes associated with oceanic subduction and continental collision. This paper reports for the first time the presence of Early Jurassic m,lange from NW Nagqu in the southern margin of the Bangong-Nujiang suture zone, termed as the Gajia m,lange. It shows typically blocks-in-matrix structure with matrix of black shale and siliceous mudstone, and several centimeters to several meters sized blocks of sandstone, silicalite, limestone and basalt. The sandstone blocks consist of homologous sandstone and two types of exotic sandstone, with different modal compositions. The Group 1 of exotic sandstone blocks consists of mainly of feldspar and quartz, whereas the Group 2 is rich in volcanic detritus. The Group 3 of homologous sandstone blocks is rich in feldspar and volcanic detritus with rare occurrence of quartz. U-Pb age data and in situ Hf isotopic compositions of detrital zircons from sandstone blocks are similar to those from the Lhasa terrane, suggesting that the sandstone blocks in the Gajia m,lange most probably came from the Lhasa terrane. The YC1 sigma(2+) age of homologous sandstone blocks is 177 +/- 2.4 Ma, suggesting an Early Jurassic depositional age for the sandstones within the Gajia m,lange. The Gajia m,lange likely records the southward subduction of the Bangong-Nujiang Ocean during the Early Jurassic.