Objectives:The study area is located in the south of late Paleozoic to early Mesozoic rift zone in Zongwulong Mountain—Qinghai Nanshan Mountain (southern Qinghai Mountain),and its tectonic position lies in the third grade tectonic unit of Oulongbuluke Block in Qinling—Qilian—Kunlun orogenic belts.Due to the influence of multi-period tectonic movement and magmatic activity,various type of intrusive rocks and volcanic rocks were formed.The molasses of the MaoniushanFormation is considered as a sign of the end of the early Paleozoic orogeny.Up to now,alittle study on petrology and geochemistry of volcanic rocks in Shidiquanarea in Delingha city has been reported,and the formation age of mafic rocks is still unknown as well.Therefore,the authors of this paper intends to take volcanic rocks that crop out in Shidiquanarea as research objects.Through detailed field observation as well as petrological analysis,geochemical analysis and U-Pb isotopic dating method,we will discuss the problems of petrological and geochemical characteristics,formation time and magma origin in the study area in depth.Methods:A gray altered andesite sample,which weighs about 40 ～ 55kg,was collected from the Maoniushan Formation in Shidiquan area for isotopic dating,with its geographical coordinate of E96 ° 12′55.7″ 、N37 °30′ 14.8″.Zircon separation was carried out using conventional methods,and zircon grains were then hand-picked under a binocular microscope.Afterwards,zircon grains with fine crystalline shape and transparency were mounted in epoxy mounts and next been polished to section the crystals in half for further analysis.These steps were undertaken at the Langfang Rock and Mineral Testing Technology,Co.Ltd,Hebei Province,China.Later,Then zircon grains were taken cathodoluminescence (CL) images by using scanning electron microscope to reveal their internal structures.Zircon U-Pb isotopic ages were analyzed using the laser ablation-multicollector inductively coupled-plasma mass spectrometry (LA-MC-ICP-MS) at the State Key Laboratory of Continental Dynamics,Northwest University,China.Analytical instruments include Agilent7500a type four stage rod mass spectrometer,Geolas200M laser ablation system and excimer laser with wavelength of 193nm.The diameter of laser beam was 30 μm and the depth of it is 20 ～ 40 μm.Results:LA-ICP-MS zircon U-Pb dating of the Andesite from Upper part of the Maoniushan Formation shows an age of 395.7 ± 2.7 Ma,which confirm that the age is Early Devonian.The analyses of major and trace elements show that the Shidiquan gabbro has characteristic of rich in K,Na,alkali and poor in Fe,Mg.These geochemiscal characteristics indicate that the neutral (acidic) rocks were formed in volcanics arc area.Conclusions:Comprehensive analyses suggest that the volcanic rocks from the MaoniushanFormation is the product of the active continental margin environment.It is concluded that the Maoniushan Formation is closely related to the subduction of the plate and formed in the Late Devonian.This study provides a new time constraint for the Early Paleozoic orogeny in Northern Qaidam.%本文对德令哈市石底泉地区牦牛山组火山岩进行了年代学、岩石学和地球化学研究,主量元素和微量元素分析结果显示岩石具有高钾高钠,贫铁镁的特征,属中酸性岩类,具有岛弧火山岩的地球化学特征.牦牛山组上部火山岩段的安山岩LA-ICP-MS锆石U-Pb年龄为395.7±2.7Ma,确定其形成时代不晚于早泥盆世,而不是传统认为的晚泥盆世,为柴达木盆地北缘早古生代的构造演化提供新的年代学依据.综合分析表明,石底泉地区牦牛山组火山岩为活动大陆边缘构造环境的产物,其形成与板块俯冲作用密切相关.
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