北秦岭二郎坪杂岩变沉积岩碎屑锆石年代学及其构造地质意义

碎屑锆石为沉积岩中最稳定的矿物,其年龄谱系不仅可限定沉积物的最大沉积年龄与物源性质,而且能够为探讨其形成构造环境提供关键约束。作为北秦岭构造带主要构造岩石地层单元之一,二郎坪杂岩由北、中、南三个构造岩片——北部碎屑岩片、中部蛇绿岩片和南部变碎屑岩片组成。本文对二郎坪杂岩南部变碎屑岩片中的碎屑锆石进行了LA-ICP-MS U-Pb年龄测试研究,获得样品锆石谐和年龄值分布在500±7Ma~3894±5Ma之间,1个年龄高频集中区为0.9~1.0Ga,6个次要年龄集中区分别为500~600Ma、750~850Ma、1.35~1.48Ga、1.6~1.75Ga、2.6~2.7Ga和3.0~3.4Ga。研究结果表明:1)最小峰值年龄限定二郎坪杂岩南部碎屑岩片原岩的最大沉积时代为500Ma,结合西庄河花岗闪长岩侵入南部岩片的地质事实,南部岩片的形成时代应为早古生代寒武-奥陶纪(500~475Ma),明显早于中部蛇绿岩片中的火山岩的形成时代(463~475Ma);2)通过与邻区地质事件年龄谱峰及其锆石阴极发光图像特征的对比,二郎坪杂岩变碎屑岩原岩的物源分别主要来自南部秦岭杂岩中的早新元古代花岗岩和北部华北南缘熊耳群中元古代火山岩,其物源具有双源性,暗示其形成于与伸展作用相关的裂谷或弧后盆地构造背景,而二郎坪蛇绿岩片所代表的古洋盆可能正是在此基础上发育产生的;3)结合区域地质背景资料分析,二郎坪杂岩中的沉积碎屑岩片可能形成于商丹洋向北俯冲期间所产生的弧后伸展盆地构造环境;4)通过与宽坪岩群沉积岩中的碎屑锆石年龄数据的对比,揭示二者碎屑锆石具有相似的年龄谱峰,结合宽坪群变沉积岩中发现大量疑源类、几丁虫和虫颚等早-中奥陶世化石组合以及二郎坪杂岩蛇绿岩片中的火山岩夹层硅质岩中发现早-中奥陶世牙形石和放射虫的研究,分析认为二者沉积物的沉积时代相近,沉积物源几乎完全一致,暗示它们形成的构造环境可能具有一致性;5)二郎坪蛇绿岩片中的典型的与洋壳俯冲有成因联系的弧火山岩的形成时代(463~475Ma)明显迟后于区内高压-超高压岩石的峰期变质时代(514~484Ma)约20~30Myr,清楚地表明二郎坪洋壳拖曳秦岭杂岩发生陆壳俯冲-深俯冲作用的可能性不大;6)二郎坪杂岩南部碎屑岩片和宽坪岩群碎屑沉积物中碎屑锆石中最主要的年龄集中区(0.9~1.0Ga)的物源来自秦岭杂岩中的早新元古代花岗质岩石,而缺少秦岭杂岩中的早古生代岩浆岩和HP/UHP变质岩石锆石的年龄信息,明确指示二郎坪与宽坪盆地沉积时,秦岭杂岩重要组成的早新元古代花岗质岩石已出露于地表接受剥蚀,而秦岭杂岩中出露的HP/UHP岩石和早古生代岩浆岩未抬升出露地表,即秦岭杂岩现今出露的前早古生代陆壳物质不是整体而是部分经历了陆壳俯冲-深俯冲作用。

Geochronology of detrital zircons from metaclastic of Erlangping complex in the North Qinling belt and its tectonic implication

Detrital zircon is the most stable minerals in the sedimentary rocks. The detrital zircon age spectra are widely used to determine the maximum age of deposition, to identify the provenance in sedimentary rocks and to study the evolution history on their source region. Erlangping complex, one of the main tectono-stratigraphic units of the North Qinling belt, consists of three tectonic slices: the northern clastic slice, the middle ophiolite slice and the southern metaclastic slice. This paper carried out the detrital zircons from the southern metaclastic slice by LA-ICP-MS U-Pb dating. The concordant zircon ages from the metaclastic sample range from 500±7Ma to 3894±5Ma. The age spectrum displays one dominant peak at 0.9~1.0Ga and six sub-dominant peaks at 500~600Ma, 750~850Ma, ca.1.35~1.48Ga, ca.1.6~1.75Ga, ca.2.6~2.7Ga and ca.3.0~3.4Ga, which indicate: 1) the youngest detrital zircon age of 500Ma from Erlangping complex demonstrated the maximum depositional age. Xizhuanghe granodiorite intruded into the southern metaclastic slice, which gave an intrusion age of 475Ma and can be used to constrain the minimum depositional age. Therefore, the formation age of the southern slice should be constrained from Paleozoic Cambrian to Ordovician (500~475Ma), which was obviously older than the formation age of volcanic rocks (463~475Ma) in the middle ophiolite slice; 2) Compared with age spectrum and cathode luminescence image of the detrital zircons from the adjacent regions, the protolith of the metaclastic in the Erlangping complex sourced both from Early Neoproterozoic granite in the Qinling complex and Mesoproterozoic volcanic rocks in Xiong''er Group of southern margin of the North China Block. This result suggests a bidirectional source, indicating Erlangping complex formed in a rifting or back-arc basin setting related to extension, and the ancient Erlangping ocean basin represented by the ophiolite slice possibly developed exactly on this setting; 3) Integrated with regional geological setting, the northward subduction of the Shangdan ocean led to the formation of back-arc extensional basin in which the clastic slice in the Erlangping complex deposited; 4) Metaclastic rocks in the Erlangping complex and the Kuanping Group show essentially similar spectrum. In addition, Early-Middle Ordovician fossils were discovered both from the meta-sedimentary rocks of the Kuanping Group and intercalations of chert in the volcanic rocks of the Erlangping complex. Therefore, the consistency of the depositional age and the provenance imply that they formed in a similar setting; 5) The formation age (463~475Ma) of arc-volcanic rock related with the subduction of oceanic crust from ophiolite slice in the Erlangping complex was obviously younger (20~30Myr) than the peak metamorphic age (~500Ma) of HP/UHP rocks in the Qingling complex, indicating that the Qinling complex impossibly was underthrust/dragged by the downgoing Erlangping oceanic lithosphere to the mantle depths and underwent HP/UHP metamorphism; 6) The detrital zircon age spectrum with contributions from the Early Neoproterozoic granitoids (ca.0.9~1.0Ga) whereas missing contributions from the Early Paleozoic igneous and HP/UHP rocks (ca.0.4~0.5Ga) in the Qinling complex, may imply that the Early Neoproterozoic granitoids was uplifted, eroded and delivered to Erlangping and Kuanping basin and did not undergo HP/UHP metamorphism (484~514Ma). Therefore, we conclude that the Qinling complex did not undergo continental deep subduction on a whole before the Early Pelaozoic.