地学前缘 ›› 2016, Vol. 23 ›› Issue (1): 264-276.DOI: 10.13745/j.esf.2016.01.024

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藏南冲巴淡色花岗岩的地球化学特征、成因机制及其构造动力学意义

王晓先，张进江，闫淑玉   

  1. 1. 中国地震局 地壳应力研究所 地壳动力学重点实验室, 北京 100085
    2. 北京大学 地球与空间科学学院 造山带与地壳演化教育部重点实验室, 北京 100871
    3. 中国石油勘探开发研究院, 北京 100083
    4. 中国地质科学院 地质研究所 大陆构造与动力学国家重点实验室, 北京 100037
  • 收稿日期:2015-01-04 修回日期:2015-04-13 出版日期:2016-01-15 发布日期:2016-01-15
  • 作者简介:王晓先(1986—)，男，博士，助理研究员，构造地质学专业，主要从事青藏高原地质研究工作。E-mail：xiaoxianwang@pku.edu.cn
  • 基金资助:

    中国地震局地壳应力研究所中央级公益性科研院所基本科研业务专项(ZDJ2014 - 09)；国家自然科学基金项目(41402175，41172176，41121062)

Geochemical characteristics of the Chongba leucogranites, southern Tibet: Formation mechanism and tectonic implications.

  • Received:2015-01-04 Revised:2015-04-13 Online:2016-01-15 Published:2016-01-15

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摘要：

冲巴淡色花岗岩位于大喜马拉雅淡色花岗岩带的东部，岩石地球化学研究结果显示，其具有高的SiO2(73.87%~74.95%)、Al2O3(14.20%~14.74%)和K2O(4.44%~4.89%)，高的K2O/Na2O比值(1.19~1.42)和A/CNK比值(1.18~1.22)，富集Rb、Th和U，亏损Ba、Nb、Sr和Zr，具有强烈的负Eu异常(δEu=0.27~0.37)，属于高钾钙碱性过铝质S型花岗岩。冲巴淡色花岗岩具有较高的Rb/Sr比值(2.6~8.6)，低的CaO/Na2O比值(0.18~0.20)，指示源区可能为泥质岩区，(87Sr/86Sr)i和εNd(t)值分别为0.763 199~0.778 799和-16.7~-16.3，与大喜马拉雅结晶杂岩(GHC)中变泥质岩一致，表明其来自GHC变泥质岩的部分熔融。淡色花岗岩具有高的(87Sr/86Sr)i而低的Sr含量，且随着Ba含量增加，Rb/Sr比值降低。这些特征表明，冲巴淡色花岗岩是无水条件下变泥质岩发生白云母脱水熔融的产物，部分熔融可能与STDS伸展拆离导致的构造减压密切相关。

关键词: 藏南, 冲巴淡色花岗岩, 地球化学, SrNd同位素, 成因机制

Abstract:

The Chongba leucogranite pluton is situated in the eastern part of the Greater Himalayan leucogranite belt. Geochemical data show that these rocks are characterized by high SiO2(73.87%74.95%), Al2O3(14.20%14.74%), K2O(4.44%4.89%), K2O/Na2O(1.191.42), A/CNK values (1.181.22), and enrichment in Rb, Th, U, depletion in Ba, Nb, Sr, Zr, and strong negative Eu anomalies (δEu=0.270.37). These features suggest that they are high potassium calcalkaline and peraluminous Stype granites. The high Rb/Sr (2.68.6) and low CaO/Na2O (0.180.20) ratios imply that the source rocks were probably pelites. The relatively high (87Sr/86Sr)i(0.7631990.778799) and low εNd(t) (-16.7-16.3) compared with those of the metasedimentary rocks in the Greater Himalaya crystalline complex (GHC), indicating that the leucogranites were generated from partial melting of the GHC metasedimentary rocks. The features of high (87Sr/86Sr)i and low Sr content, and the Rb/Sr values decreasing with increased Ba content indicate that the Chongba leucogranites were derived from muscovite dehydration melting of the GHC metapelite under waterabsent condition, which is probably closely associated with the structural decompression caused by the extensional decollement of the south Tibet detachment system (STDS).

Key words: southern Tibet, Chongba leucogranites, geochemistry, SrNd isotopes, formation mechanism

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