Zircon U–Pb dating and geochemistry of intrusive rocks in the Shangdan suture zone of the Qinling Orogenic Belt: petrogenesis and tectonic implications

Abstract

We studied the zircon U–Pb ages, whole-rock geochemistry and Sr–Nd isotopic compositions of a Paleozoic gabbro–diabase–granite association in the Shangdan suture zone to constrain the petrogenesis of these intrusive rocks and identify their tectonic implications. These rocks have distinctive crystallization ages: the Guchenggou gabbro yielded zircon U–Pb ages of 424–423 Ma, the average age of the Yueritan diabase was 411 Ma and the Danfeng granites yielded zircon U–Pb ages of 414–411 Ma. The Guchenggou gabbro was formed by partial melting of a phlogopite-bearing garnet lherzolite modified by subduction fluids in a continental arc setting. The Yueritan diabase was generated by mixing a phlogopite-bearing garnet lherzolite modified by subduction melts with continental crustal rocks in a post-collision setting. The Danfeng granites were weakly peraluminous I-type granites formed by mixing juvenile crust with metagreywackes at high temperatures and pressures in a tectonic environment transitioning from compression to extension. We suggest that the subduction of the Shangdan Ocean induced the formation of the gabbro. The diabases and granites are related to relamination of the continental crust in response to the collision of the North Qinling Belt with the South Qinling Belt, suggesting that closure of the Shangdan Ocean may have occurred at c. 410 Ma.