Underplating of basaltic magmas and crustal growth in a continental arc: Evidence from Late Mesozoic intermediate–felsic intrusive rocks in southern Qiangtang, central Tibet
Underplating of basaltic magmas and crustal growth in a continental arc: Evidence from Late Mesozoic intermediate–felsic intrusive rocks in southern Qiangtang, central Tibet
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玄武质岩浆底侵与大陆弧地壳生长：来自西藏中部羌塘南部晚中生代中长英质侵入岩的证据
DOI：
10.1016/j.lithos.2015.09.015
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发表时间：
2016-02
期刊：
影响因子：
3.5
通讯作者：
Jie Li
中科院分区：
文献类型：
--
作者：
Fu-Yuan Wu;Jin-Hui Yang;Xiao-Ping Long;Jie Li
陆壳显生宙生长一直被广泛认为是一种重要的地质现象，主要发生在弧形背景下。然而，地壳生长模式(地幔玄武岩底侵或岛弧杂岩或洋弧杂岩或大洋高原的增生)一直存在争议。本文给出了藏中羌塘南部Rena Co地区晚中生代中长英质侵入岩的新的锆石LA-ICPMS U-Pb年龄、全岩主量元素和微量元素、Sr-ND和锆石Hf同位素数据。对2个花岗闪长岩、3个闪长岩样品和1个花岗闪长斑岩样品进行了La-ICPMS锆石U-Pb定年，测得年龄分别为150 Ma和112 Ma，表明它们形成于晚侏罗世-早白垩世。岩石成分均为亚碱性，属高钾钙碱性系列。~(150)Ma闪长岩(SiO_2=57.9~61.2%)具有相对较高的氧化镁(3.13~3.88%)、铬(52.4~282 ppm)和镁#(47~51)值，与镁闪长岩相似。它们的地球化学特征是均一低的εNd(T)(−=5.5~−=5.2)，高的(87Sr/86Sr)I(0.7071~0.7078)和Th/La(0.22~0.32)，以及可变的锆石εHf(T)(−=8.7~+34.8)。它们可能是在班公-怒江大洋岩石圈向北俯冲过程中，大洋沉积物底辟熔融并与周围地幔相互作用而产生的。~150 Ma花岗闪长岩和~112 Ma花岗闪长斑岩具有低的镁含量和镁含量，高的Al_2O_3含量和高的La/Yb比值，高的Al_2O_3含量和高的La/Yb比值，与典型的埃达克岩相似。花岗闪长岩具有低εNd(T)(−7.6~−3.7)和锆石εHf(T)(−9.8~+80.2)和高(87Sr/86Sr)I(0.7069~0.7086)值，可能是增厚的、不均匀的古陆壳部分熔融的产物。花岗闪长斑岩的相对贫化同位素组成[(87Sr/86Sr)i=0.7054-0.7065；εNd(T)−=0.61~+0.25；锆石εHf(T)=0.61~+9.7]表明，它们很可能是新底侵和加厚的玄武岩下地壳部分熔融的结果。结合班公-怒江缝合带的蛇绿岩和羌塘亚块南部晚中生代的岩浆岩，我们认为该地区处于大陆弧环境。此外，晚侏罗世至早白垩世，南羌塘地块的古下地壳逐渐被地幔来源的年轻物质取代。地壳演化表明，在大陆弧中，玄武岩岩浆底侵作用对地壳垂直生长起着关键作用。
Phanerozoic growth of continental crust has widely been considered as an important geological phenomenon and mainly occurs in an arc setting. However, the crustal growth models (mantle-derived basalt underplating or accretion of island or intra-oceanic arc complexes or oceanic plateau) have been disputed. Here we present new zircon LA–ICPMS U–Pb age, whole-rock major and trace element, Sr–Nd and zircon Hf isotopic data for Late Mesozoic intermediate–felsic intrusive rocks in the Rena Co area in southern Qiangtang, central Tibet. LA–ICP–MS zircon U–Pb dating for two granodiorite and three diorite samples and one granodiorite porphyry sample gives ages of ca. 150 Ma, ca. 112 Ma, respectively, indicating they were generated in the Late Jurassic–Early Cretaceous. All rocks are sub-alkaline in composition and belong to the high-K cal-alkaline series. The ~ 150 Ma diorites (SiO2= 57.9–61.2 wt.%) exhibit relatively high MgO (3.13–3.88 wt.%) and Cr (52.4–282 ppm) contents and Mg#(47–51) values, similar to magnesian diorites. They are geochemically characterized by uniformly low εNd(t) (− 5.5 to − 5.2), high (87Sr/86Sr)i(0.7071 to 0.7078) and Th/La (0.22–0.32), and variable zircon εHf(t) (− 8.7 to + 4.8) values. They were probably generated by melting of oceanic sediment diapirs, followed by interaction with the surrounding mantle during the northward subduction of Bangong–Nujiang Oceanic lithosphere. The ~ 150 Ma granodiorites and ~ 112 Ma granodiorite porphyries are characterized by low MgO (< 3 wt.%) contents and Mg#(< 45) values, high Al2O3(> 15% wt.%) and Sr (> 400 ppm) and low Y (< 18 ppm) and Yb (< 1.9 ppm) contents, and high Sr/Y and La/Yb ratios, which are similar to those of typical adakites. The granodiorites have low εNd(t) (− 7.6 to − 3.7) and zircon εHf(t) (− 9.8 to + 0.2) and high (87Sr/86Sr)i(0.7069 to 0.7086) values, and were likely produced by partial melting of a thickened and heterogeneous ancient lower continental crust. The relatively depleted isotope compositions [(87Sr/86Sr)i= 0.7054–0.7065; εNd(t) = − 0.61 to + 0.25; zircon εHf(t) = + 4.7 to + 9.7] of the granodiorite porphyries indicate that they were most probably generated by partial melting of newly underplated and thickened basaltic lower crust. Taking into account ophiolites in the Bangong–Nujiang Suture and Late Mesozoic magmatic rocks in the southern Qiangtang sub-block, we suggest that this area was located in a continental arc setting. Moreover, from the Late Jurassic to Early Cretaceous, the ancient lower crust in the southern Qiangtang sub-block was gradually replaced by mantle-derived juvenile materials. The crustal evolution indicates that, in a continental arc, basaltic magma underplating plays a key role in vertical crustal growth.
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影响因子：
3.5
作者：
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通讯作者：
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