Geochronology, petrogenesis and tectonic significance of peraluminous granites from the Chinese Altai, NW China
Geochronology, petrogenesis and tectonic significance of peraluminous granites from the Chinese Altai, NW China
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DOI：
10.1016/j.lithos.2011.09.001
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发表时间：
2011-11
期刊：
影响因子：
3.5
通讯作者：
Keda Cai;M. Sun;C. Yuan;Guochun Zhao;W. Xiao;X. Long;Fu-Yuan Wu
中科院分区：
文献类型：
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作者：
Keda Cai;M. Sun;C. Yuan;Guochun Zhao;W. Xiao;X. Long;Fu-Yuan Wu
中国阿尔泰古生代花岗岩对研究中亚造山带构造演化和地壳生长具有重要意义。选取4个具有代表性的过铝花岗岩侵入体，系统研究了锆石U-Pb和Hf同位素组成，并进行了全岩地球化学和Nd-Sr同位素分析。这些岩石具有较高的ASI（氧化铝饱和指数，Al2O3/(CaO+Na2O+K2O)=1.01 ~ 1.46分子比），重量为0.6 ~ 5.6wt。Eu/Eu*=0.38 ~ 0.98)为中等负异常，Ba、Nb、Sr元素的损耗较强。这些侵入岩的侵位时间为419 ~ 393Ma，与中国阿尔泰地区岩浆活动和高温变质作用强烈的时期一致。原位锆石Hf同位素分析显示，这些花岗岩的εHf(t)值主要为正（+0.8 ~ +12.8），少数继承锆石的εHf(t)值为负（- 12.5 ~ - 1.53）。这些继承锆石的U-Pb年龄和Hf同位素数据与广泛分布的变质沉积岩相似。此外，过铝质花岗岩的εNd(t)值接近于零或为负值（−3.3 ~−0.5），初始87sr /86Sr比值较高（0.7079 ~ 0.7266），与研究区i型花岗岩不同，但与早古生代哈坝河沉积物相似。这些同位素组成表明，哈巴河群新增生的变质岩可能是过铝花岗岩的主要烃源岩。地球化学成分表明其前体岩浆来源于较浅的地壳水平（P≤5kbar），锆石饱和温度表明其侵位在672 ~ 861℃。过铝质花岗岩岩浆是由新增加的物质脱水熔融形成的，这些物质可能是在活动边缘通过与俯冲有关的作用至少带到地壳中部深度，随后在古生代与脊俯冲相关的热软流圈上涌引起的极高环境温度下熔融而成。
The Paleozoic granites in the Chinese Altai are important for the study of tectonic evolution and crustal growth in the Central Asian Orogenic Belt (CAOB). Four representative peraluminous granitic intrusions were selected for systematic studies of zircon U–Pb and Hf isotopic compositions and whole-rock geochemical and Nd–Sr isotopic analyses. These rocks have high ASI (Alumina Saturation Index, Al2O3/(CaO+Na2O+K2O)=1.01–1.46 molecular ratios), with 0.6–5.6wt.% of normative corundum, and are characterized by moderately negative Eu anomalies (Eu/Eu*=0.38–0.98) and strong depletion in Ba, Nb and Sr elements. Our data suggest that these intrusions were emplaced from 419 to 393Ma, consistent with a period of intensive magmatic activities and high temperature metamorphism in the Chinese Altai. While in situ zircon Hf isotopic analyses for these granites give predominantly positive εHf(t) values (+0.8 to +12.8), a few inherited zircons yield negative εHf(t) values from −12.5 to −1.53. The U–Pb age and Hf isotopic data of these inherited zircons are similar to that of the widespread metasediments. In addition, the peraluminous granitic rocks have near-zero or negative εNd(t) values (−3.3 to −0.5) and relatively high initial87Sr/86Sr ratios (0.7079–0.7266), distinct from those of the I-type granites in the study region, but similar to the Early Paleozoic Habahe sediments. These isotopic compositions suggest that the newly accreted metasediments of Habahe Group may be the major source rock of the peraluminous granites. The geochemical compositions indicate that their precursor magmas were derived from a relatively shallow crustal level (P≤5kbar) and zircon saturation temperatures suggest that these granitic intrusions were emplaced at 672–861°C. The peraluminous granitic magmas were generated by dehydration melting of newly accreted materials, which were possibly brought to at least middle crustal depth by subduction-related processes in an active margin, and were subsequently molten by strikingly high ambient temperature probably caused by upwelling of the hot asthenosphere associated with ridge subduction in the Paleozoic.