西秦岭宕昌地区晚三叠世酸性火山岩的锆石U-Pb年代学、地球化学及其地质意义  

Zircon U-Pb chronology,geochemistry of the Late Triassic acid volcanic rocks in Tanchang area,West Qinling and their geological significance

作　　者：黄雄飞[1] 莫宣学[1] 喻学惠[1] 李小伟[1] 丁一[1,2] 韦萍[1,3] 和文言[1] 

机构地区：[1]中国地质大学地质过程与矿产资源国家重点实验室,地球科学与资源学院,北京100083 [2]江苏省地质矿产局第四地质大队,苏州215129 [3]广西省地质矿产局第四地质队,南宁530007

出　　处：《岩石学报》2013年第11期3968-3980,共13页Acta Petrologica Sinica

基　　金：中国地质调查局工作项目(1212011121260、E90139);国家973项目(2011CB403102、2009CB421002)联合资助

摘　　要：西秦岭造山带在早中生代发育了以印支早期(245~234Ma)和印支晚期(225~205Ma)两期高钾钙碱性花岗岩占绝对优势,并伴有少量火山岩的强烈岩浆活动。本文报道的西秦岭宕昌火山岩总体上是一套以流纹岩为主,并含粗面岩和英安岩的酸性火山岩组合。流纹岩锆石U-Pb测年结果为229Ma,限定其喷发时代为晚三叠世早期。宕昌火山岩属于过铝质的高钾钙碱性-钾玄岩系列,以富集Rb、Ba、Th、U、K并相对亏损Nb、Ta、Ti、P为特征,(87Sr/86Sr)i=0.7048~0.7068,εNd(t)=-4.3^-4.0,亏损地幔模式年龄(t DM)为1.20~1.24Ga。宕昌火山岩起源于下地壳富钾变基性岩的部分熔融,部分岩浆还可能受到了少量地幔或上地壳物质的混染,源区深度约40km。西秦岭地块存在扬子型的中新元古代基底,但不同地段的基底特征存在较大差异。华北板块与扬子板块在西秦岭地区的初始碰撞可能接近229Ma,宕昌火山岩则形成于以整体挤压为主、局部剪切伸展为辅的地壳逐渐加厚的动力学环境。Strong magxnatic activities dominated by the Early Indosinian and Late Indosinian high potassic calc-alkaline granitoids （245 - 234Ma and 225 -205Ma, respectively） and a small amount of volcanic rocks were developed in western Qinling orogenic belt in Early Mesozoic. The volcanic rocks from Tanchang area of West Qinling, as reported in this paper, are acid rocks which contain mainly rhyolite with minor trachyte and dacite. These volcanic rocks erupted in the early Late Triassic limited by the zircon U-Pb age of 229Ma of the rhyolite. Tanchang volcanic rocks belonging to the peraluminous, shoshonitic to high- K calc-a/kaline series are characterized by enrichment in Rb, Ba, Th, U, K, relative depletion in Nb, Ta, Ti, P, （S7Sr/S6Sr） i = 0. 7048 -0. 7068, εNd （t） = - 4. 3 - - 4. 0 and depleted mantle model ages ranging from 1.20Ga to 1.24Ga. Tanchang volcanic rocks were generated by partial melting of the K-rich meta-mafic protolith in lower crust while some of them might also be related with the contamination of a very small amount of mantle-derived or upper crustal materials, and the depth of the source was ca. 40km. The West Qinling terrane has Mesoproterozoic-Neoproterozoic crustal basement with a relatively large heterogeneity in different regions showing an affinity to Yangtze block. The time of the initial collision between North China block and Yangtze block in West Qinling might be close to 229Ma. Tanchang volcanic rocks were formed in the crustal thickening geodynamic environment mainly characterized by compression on the whole and shearing-extension in local part.

关 键 词：印支期 西秦岭 宕昌 火山岩 花岗岩 大陆碰撞 

分 类 号：P588.141[天文地球—岩石学] P597.3[天文地球—地质学]