The Geochemistry And Geochronology Study Of Aoyougou Volcanic Rock In Sunan County, Gansu Province

The Aoyougou volcanic rock located in the Sunan County, Gansu Province has longbeen a focus for geoscientists because of the the forming age, tectonic environment and therelationship between Aoyougou ophiolite. The research objects in this article are theAoyougou ophiolite mélange. We have discussed the formation age, geochemistrycharacteristic, the tectonic environment and the petrogenesis from the rock assemblage,petrography and geochemistry. Through the route geology surveying, we had figured outthe relationship of the Aoyougou ophiolite and Aoyougou volcanic rocks. According to theLA-ICP-MS U-Pb dating, we had defined the formation age of the two series, discussed thepetrogenesis of the ophiolite and volcanic rocks and its tectonic setting.This research hasdeepen the evolutionary history of North Qilian ocean basin and defined the formation ageof the Zhulongguan Group for guiding the region mineral prospecting.Specific progress and understandings are as follows:1. There is a classical ophiolite assemblage in the south of Aoyougou ophiolitemélange. The rock assemblage is ultrabasics, gabbro, basalt, silica rock, dolomiticmarmorize and so on which consist of the relatively complete ophiolite.There is classicalvolcanic rock series above the oceanic crust. It is mainly basalt with amygdaloidal structureand pillow lava, with layers of carbonate.2. According to LA-ICP-MS zircon U-Pb dating, the formation age of the volcanicrock series in the north of Aoyougou is middle Ordovician (470±11Ma), and the analysis ofgeochemistry reveals it formed in the ocean within-plate environment. It is typical oceanicisland basalt. The age of gabbro in the Aoyougou ophiolite assemblage is522±12Ma, wethink the formation age of the Aoyougou ophiolite is middle-late Cambrian; The analysis ofgeochemistry reveals it formed in the mid-ocean ridge environment, and the basite igneousrock can represent the middle Cambrian North Qilian ocean crust.3. The source region of the volcanic rock series in north Aoyougou is mantle plumewhich is similar to the OIB source region, and it have different source region componentfrom the Aoyougou ophiolite. The original magma have been experienced a due measure ofcrystal fractionation in the chamber or on the way of raising up to the surface. Aoyougou volcanic rock series should be scraped when the North Qilian ocean basin subducted inearly paleozoic era,and it is not the component of the ophiolite.5. We have done the zircon LA-ICP-MS U-Pb dating of the phyllite in the hanging sideand lyling side of Jingtieshan iron mine and the Longkong volcanic rock which areclassified into Zhulongguan Group before. And combining the age (508±4.6Ma)of theChaidanuoer granite mass which intrude into the congenial host rock of Shalong iron minereported by previous studies, we consider Zhulongguan Group volcanic rock, theferric(cupric) series of the west part of the North Qilian mountain, should form later than1000Ma, and early than508Ma, is767.3±7.4Ma. It is the product of the Neoproterozoiccontinental rift volcanogenic sedimentation.