Indosinian magmatism and tectonic setting of Xiahe-Hezuo area, western Qinling Mountains——implications from the Dewulu quartz diorite and Laodou quartz dioritic porphyry

Abstract: The tectonic setting of western Qinling orogenic belt was studied through the research on the petrogeochemical characteristics and U-Pb age of zircon from Dawulu quartz-diorite and Laodou granitic-diorite using LA ICP-MS. These two kinds of rocks belong to peraluminous calc-alkaline middle rocks. Trace element studies suggest that the rocks are predominately characterized by enrichment of LILE elements such as Rb, Th and U and other light rare-earth elements, depletion of Ba ,K, Nb, Sr, P, and low Sr and Y, suggesting typical characteristics of Himalaya-type granite. REE data indicate that the light rare earth elements were abundant, and heavy rare-earth elements are poor in these rocks. The light and heavy rare -earth fractionation is obvious. δEu has a weak negative anomaly. U-Pb age of zircon indicates that the weighted average ages of both Dewulu quartz-diorite and Laodou granitic diorite are (245.8±1.7) Ma and (243.4±1.9) Ma, (241.4±2.1) Ma, (38.2±1.7)Ma and (241.4±1.6) Ma respectively, which suggests that these two intrusions formed at the earlier stage of Indosinian. It is pointed out that Laodou granitic diorite and Dewulu quartz diorite were derived from the different sources which came from the different depths, and the emplacement and accumulation were from multiple origins. The Laodou granitic diorite and Dewulu quartz diorite are projected into the range of arc magma. The tectonic setting research suggests that the western Qinling lies in the continental-to-continental collision in the earlier Indosinian period, and hence this region is favorable in search for porphyry Cu-Au deposits, skarn Cu-Au deposits and epithermal Cu-Au-Sb deposits.