THE BLUESCHITS-BEARING QIANGTANG METAMORPHIC BELT (NORTHERN TIBET, CHINA) AS AN IN SITU SUTURE ZONE: EVIDENCE FROM GEOCHEMICAL COMPARISON WITH THE JINSA SUTURE

Abstract

Metasiliciclastic rocks and metabasalts from the blueschist-bearing Qiangtang metamorphic belt and the Jinsa suture zone were analyzed for major and trace elements in an attempt to evaluate the affinities of these two tectonic entities. Tholeiitic mid-oceanic-ridge basalts (MORBs) from the Jinsa suture can be distinguished from metabasalts of Qiangtang, which have alkalic compositions and exhibit a range of characteristics typical of many within-plate oceanic islands. The Qiangtang metasiliciclastic rocks were derived from a passive continental margin source, whereas those from the Jinsa suture zone were sourced from a continental island arc or an active continental margin source. The geochemical distinction of metasiliciclastic rocks and metabasalts of the Qiangtang metamorphic belt from their counterparts within the Jinsa suture indicates that there is no affinity between these two tectonic entities and that the Qiangtang metamorphic belt could not have been underthrust from the Jinsa suture. It most likely represents an exhumed accretionary complex composed of sediments derived from a passive continental margin and fragments of seamount sequences, and marks an in situ suture zone that separates northern and southern Qiangtang terranes. © 2006 Geological Society of America.