Origin of Late Oligocene adakitic intrusives in the southeastern Lhasa terrane: Evidence from in situ zircon U-Pb dating, Hf-O isotopes, and whole-rock geochemistry
Origin of Late Oligocene adakitic intrusives in the southeastern Lhasa terrane: Evidence from in situ zircon U-Pb dating, Hf-O isotopes, and whole-rock geochemistry
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拉萨地体东南部渐新世晚期埃达克岩侵入岩的成因：来自原位锆石U-Pb定年、Hf-O同位素和全岩地球化学的证据
DOI：
10.1016/j.lithos.2012.05.026
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发表时间：
2012-09
期刊：
影响因子：
3.5
通讯作者：
Huang, Ke-Xian
中科院分区：
文献类型：
--
作者：
Zheng, Yuan-Chuan;Hou, Zeng-Qian;Li, Qiu-Yun;Sun, Qing-Zhong;Liang, Wei;Fu, Qiang;Li, Wei;Huang, Ke-Xian
本文描述了拉萨东南部林芝二云母花岗岩类的全岩地球化学、黑云母Ar-Ar和锆石U-Pb年代学及原位Hf-O同位素组成。花岗岩类成分为钙碱性-高钾钙碱性，与埃达克岩具有地球化学亲缘关系。全岩地球化学特征为高SiO_2、Al_2O_3，低MgO、Cr、Ni，低Mg#值，负εNd（t）（-6.2至-2.9），以及低87 Sr/86 Sr（i）（0.7059-0.7074），以及原位锆石Hf-O同位素数据（εHf（t）=−2.7 - 2.7; δ 18 O =7.2‰-8.8‰），表明该花岗岩类主要来自拉萨火山口下再生的下地壳部分熔融。下地壳主要由古老的地壳物质组成，少量的年轻地幔输入（<55%）。两个样品的西姆斯锆石U-Pb分析表明，林芝花岗岩类的结晶作用发生在26 Ma左右，表明在埃达克质岩石侵位之前，拉萨地区的地壳已经增厚了50 km。黑云母的40 Ar/39 Ar年龄和岩浆绿帘石的存在表明，该花岗岩类是在中地壳深度侵入的。表明拉萨东部在26 ~ 22 Ma期间经历了一次脉冲式快速隆升。林芝埃达克质岩石的地球化学特征与喜马拉雅带北部始新世埃达克质岩石的地球化学特征明显不同，这表明可以排除印度下地壳撞击造成拉萨东部地壳增厚的可能性。碰撞后（26- 10 Ma）埃达克质岩石产于拉萨造山带中与雅鲁藏布江缝合线平行的东西向岩浆带中。这些岩石的地球化学特征沿拉萨造山带走向沿着有明显的变化，表明沿造山带沿着由西向东下地壳具有同位素不均匀性。这些地球化学变化可能是由于不同程度的输入，沿着走向，年轻的幔源熔体到一个老的下地壳。拉萨盆地中部是一个年轻的下地壳，其中含有大量的幔源熔体，在新特提斯洋壳俯冲过程中底侵。相比之下，东、西部的下地壳则相对古老。因此，地幔岩浆底侵作用导致的地壳增厚可能是造成中拉古陆中段地壳增厚的重要原因，而印度板块和亚洲板块碰撞导致的地壳挤压缩短可能是造成中拉古陆西、东段地壳增厚的主要原因。
We describe the whole-rock geochemistry, biotite Ar–Ar and zircon U–Pb geochronology, and in situ Hf–O isotopic compositions of the Linzhi two-mica granitoid in the southeastern Lhasa terrane. The granitoids are calc-alkaline to high-K calc-alkaline in composition, and they have geochemical affinities with adakites. The whole-rock geochemistry, with high SiO2and Al2O3contents, low MgO, Cr, and Ni contents, low Mg# values, negative εNd(t)(−6.2 to −2.9), and low87Sr/86Sr(i)(0.7059–0.7074), together with the in situ zircon Hf–O isotopic data (εHf(t)=−2.7 to 2.7; δ18O=7.2‰–8.8‰), indicates that the granitoid was derived mainly from the partial melting of a rejuvenated lower crust underneath the Lhasa terrane. This lower crust was dominated by ancient crustal material with the minor involvement of juvenile mantle inputs (<55%). SIMS zircon U–Pb analyses of two samples indicate that the crystallization of the Linzhi granitoid took place around 26Ma, implying that the crust of the Lhasa terrane was already thickened up to 50km prior to emplacement of the adakitic rocks.40Ar/39Ar ages for biotites, and the presence of magmatic epidote, suggest the granitoid was intruded at mid-crustal depths, indicating that the eastern Lhasa terrane experienced a pulse of rapid uplift in the interval from 26 to 22Ma. The geochemical features of the Linzhi adakitic rocks are significantly different from those of Eocene adakitic rocks from the north Himalayan belt, and this suggests that we can rule out the possibility of crustal thickening in the eastern part of Lhasa terrane, caused by impingement of the Indian lower crust. Post-collisional (26–10Ma) adakitic rocks occur within an E–W-trending magmatic belt that runs parallel to the Yarlung–Tsangpo suture in the Lhasa terrane. These rocks have distinctive variations in geochemistry along the strike of the Lhasa terrane, indicating that the lower crust is isotopically heterogeneous from west to east along the terrane. These geochemical variations probably result from variable degrees of input, along strike, of juvenile mantle-derived melts into an old lower crust. The middle part of the Lhasa terrane is marked by a juvenile lower crust that contains large amounts of mantle-derived melt, underplated during Neo-Tethyan oceanic crust subduction. In contrast, the lower crust below the western and eastern parts of the terrane is relatively ancient. Therefore, crustal thickening caused by underplating of mantle-derived magmas could play an important role in the middle segment of the terrane, whereas crustal compression and shortening as a result of the collision of the Indian and Asian plates could be the major cause for crustal thickening in the western and eastern segments.
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