大兴安岭南段海西期花岗岩类锆石U-Pb年龄、元素和Sr-Nd-Pb同位素地球化学:岩石成因及构造意义

Zircon U-Pb ages,elemental and Sr-Nd-Pb isotopic geochemistry of the Hercynian granitoids from the southern segment of the Da Hinggan Mts.:Petrogenesis and tectonic implications

大兴安岭南段位于华北板块与西伯利亚板块之间的缝合-造山带(中亚造山带)的东段,该区广泛分布有海西期花岗岩类。已有的年代学研究表明这些花岗岩类的侵位时代集中于320~240Ma,但对该区海西期花岗岩类岩石成因及成岩背景研究的一些关键问题的看法仍不一致且存在争论。本文选取大兴安岭南段拜仁达坝等10个典型地区的海西期花岗岩类为研究对象,对其进行系统的岩石学、锆石U-Pb年代学、元素和Sr-Nd-Pb同位素地球化学研究,以探讨岩浆作用的时间、岩石成因以及构造意义。锆石U-Pb定年表明,这些花岗岩类的侵位时间为323~240Ma。据地球化学特征,该区花岗岩类可分为两类:Ⅰ类花岗岩的SiO_2>62.00%,Al_2O_3>15.00%,MgO含量较低(均<2.40%);其具有较高的Sr含量(最高达700×10^(-6))、低的Y和Yb含量(分别为≤18.1×10^(-6)和<1.70×10^(-6))和较高的Sr/Y比值(均>20),LREE相对HREE强烈富集,显示弱的Eu异常,与经典的埃达克岩的特征相似;Ⅱ类花岗岩的SiO_2>57.00%,MgO和Al2O3含量范围较大(分别为0.25%~4.12%和10.55%~16.58%),绝大多数样品显示高钾钙碱性特征,Sr含量相对较低(12.2×10^(-6)~334×10^(-6)),Y和Yb含量相对较高(分别为14.1×10^(-6)~40.1×10^(-6)和1.27×10^(-6)~4.58×10^(-6)),Eu异常明显,相对富集Rb、Th、K等大离子亲石元素,亏损Nb、Ti、P等高场强元素,可归为弧岩浆岩。Sr-Nd-Pb同位素结果表明,弧岩浆岩可能来源于交代(富集)地幔楔,而埃达克岩则来源于下地壳的熔融。综合前人研究认为,在晚泥盆世-中二叠世古亚洲洋洋内俯冲作用形成了区域上广泛发育的弧岩浆岩(弧下加厚地壳熔融形成~320Ma埃达克岩);晚二叠世-早三叠世,华北板块与西伯利亚板块碰撞缝合;之后至早-中三叠世,大量后碰撞花岗岩侵位,同时下地壳拆沉并熔融形成了本区250~240Ma埃达克岩。中亚造山带东段(至少大兴安岭地区)海西期造山及岩浆作用可能开始于晚泥盆世,结束于中三叠世,时间持续约140Myr,其造山作用显示独特性。

The southern Da Hinggan Range is located in the eastern part of the Central Asian Orogenic Belt（ CAOB）,which marks the junction of the Siberia and North China blocks. Hercynian granitoids are widespread in the area. Existing geochronological data show that these granitoids were emplaced concentrated in the period of 320 ~ 240 Ma,however,there exist hot debates on the petrogenesis and tectonic settings for these granitoids. In this paper, an integrated geochemical, zircon LA-ICP-MS U-Pb geochronological and Sr-Nd-Pb isotopic study has been conducted on 10 typical granitoids（ e. g.,the Bairendaba granodiorite） in southern Da Hinggan Range,aiming at elucidating their emplacement ages,petrogenesis and tectonic implications. Zircon U-Pb dating establishes Early Carboniferous to Middle Triassic ages（ 323 ~ 240Ma） for these granitoids. According to geochemical characteristics,they can be divided into two groups as follows： Group Ⅰ have relatively high SiO_2（ 62. 00%） and Al2O3（ 15. 00%） and low MgO（ 2. 40%） content. They also contain high Sr（ up to 700 × 10^（-6）） and low Y（ ≤18. 1 × 10^（-6）） and Yb（ 1. 70 × 10^（-6））content,with relatively high Sr / Y ratios（ 20）. They are enriched in LREE over HREE and show weak Eu anormaly. Thus they show the special feature of classic adakite. Whereas Group Ⅱ have SiO_2 of more than 57. 00% and wide range of Mg O（ 0. 25% ~ 4. 12%） and Al2O3（ 10. 55% ~ 16. 58%）,and most of them belong to high potassium calcic-alkaline series. They possess relatively low Sr（ 12. 2× 10-6~ 334 × 10-6） and high Y（ 14. 1 × 10^（-6）~ 40. 1 × 10^（-6）） and Yb（ 1. 27 × 10^（-6）~ 4. 58 × 10^（-6）） content,exhibit significant negative Eu anormaly and are enriched in Rb,Th and K and depleted in Nb,Ti and P,characteristics typical for arc magmatic rocks.The Sr-Nd-Pb results show that the aforementioned arc magmatic rocks were likely derived from metasomatic enriched mantle,while the adakites from lower crust. Compilation of existing geological,geochronological and geochemical data suggests that,during Late Devonian to Middle Permian,an intra-oceanic subduction environment existed in the Paleo-Asian Ocean and thus gave rise to widespread arc magmatic rocks in southern Da Hinggan Range. At the same time,melting of sub-arc thickening crust produced the~ 320 Ma adakites of this study. Then during Late Permian to Early Triassic,the Siberia and North China blocks began to collision and finally sutured. To Early-Middle Triassic,numerous post-collisional granites occurred and melting of delaminated lower crust produced the 250 ~ 240 Ma adakites. The Hercynian orogeny in CAOB（ at least in southern Da Hinggan Range） was initiated no later than370 Ma and ended shortly after 240 Ma,lasting for ~ 140 Myr.