Chronology and Geochemistry of Gonjo Triassic Volcanic Rocks in Northern Part of Nujiang-Lancangjiang-Jinshajiang and Its Geological Significance

The Gonjo Triassic volcanic rocks in the northern area of the Nujiang-Lancangjiang-Jinshajiang are the products of subduction of the Jinshajiang Paleo-Tethyan Ocean to the west and subsequent arc-continent collision-orogeny effect. It is mainly composed of basalt, andesite fused tuff and andesite, etc.The study of Gonjo volcanic rocks is of great significance for the exploration of the Jinshajiang Paleo-Tethyan tectonic-magmatic evolution process. In this paper, it presents experiments on Gonjo volcanic rocks of LA-ICP-MS zircon U-Pb dating and master-trace element research. The LA-ICP-MS zircon U-Pb dating of representative samples of andesite fused tuff shows that the age of ²⁰⁶Pb/²³⁸U of 23 analysis points of zircon is 243-233 Ma. The LA-ICP-MS zircon U-Pb age of the representative sample of andesite fused tuff is 238.3±1.3 Ma, which indicates that the magmatic crystallization age is Middle-Late Triassic. Geochemical analysis shows that volcanic rocks contains low titanium (0.52%-0.91%), medium magnesium (3.38%-7.22%), rich sodium (3.63%-4.66%), depleted potassium (0.09%-4.05%), enriched light rare earth elements (LREE/HREE=4.09-13.27) and large ionic lithophilic elements (Th, U, La, Sm), deficient high field strength elements Nb and Ta (Nb=2.51×10^-6-24.5×10^-6, Ta=0.22×10^-6-1.89×10^-6), relatively high La/Nb (2.13-2.89) ratio and relatively low Nb/Th (1.12-1.62), which possesses the composition characteristics of main-trace elements similar to island arc or late collision orogenic volcanic rocks. It is concluded that the Middle-Late Triassic Gonjo volcanic rocks represent the magmatic response of the Jinshajiang during the evolution of Paleo-Tethys, which is probably the result of the interaction between the Jinshajiang oceanic crust material and the lithospheric mantle material remaining during the late collision.