Geochemistry of North Himalayan Leucogranites: Regional Comparison, Petrogenesis and Tectonic Implications
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摘要: 北喜马拉雅出露一系列片麻岩穹窿, 这些穹窿被形成于27.5~10 Ma的淡色花岗岩侵入.淡色花岗岩的岩石类型为二云母花岗岩, 它们的主量元素组成为SiO2=70.97%~74.54%、K2O+ Na2O=6.27%~8.09%、K2O/Na2O=0.91~1.36及A/CNK=1.10~1.33.然而, 它们在微量元素组成上呈现出较大的变化: Rb=(41~322)×10-6、Sr=(26~139)×10-6、Ba=(135~594)×10-6、(La/Yb)N=0.97~17.31、Eu/Eu*=0.29~0.72.北喜马拉雅淡色花岗岩的主量元素和微量元素组成特征类似于高喜马拉雅中新世的二云母花岗岩, 而在Ti、Mg、Ca、Ba含量和Rb/Sr比值上明显不同于高喜马拉雅中新世的电气石-白云母花岗岩.北喜马拉雅淡色花岗岩(87Sr/86Sr)t=0.734 4~0.850 3(t=10 Ma), εNd(10 Ma)=-12.5~-19.3, 与高喜马拉雅淡色花岗岩无明显差异.在岩石成因上, 北喜马拉雅和高喜马拉雅中新世淡色花岗岩均起因于构造减压作用, 由此导致白云母发生脱水反应诱发高喜马拉雅结晶岩系的深熔.但北喜马拉雅淡色花岗岩形成的地质背景明显不同于高喜马拉雅淡色花岗岩, 前者具有较长的时间跨度, 开始形成于喜马拉雅渐新世的地壳增厚期, 之后形成于中新世穹窿片麻岩的折返时期, 而高喜马拉雅淡色花岗岩与中新世高喜马拉雅结晶岩系的构造挤出作用有关.因此, 北喜马拉雅和高喜马拉雅淡色花岗岩的形成反映了不同的构造过程.Abstract: The North Himalayan antiform is exposed a series of gneiss domes, intruded by the North Himalayan leucogranites(NHL)with magma emplacement ages from 27.5 to 10 Ma. The NHL are dominated by two-mica granites. They have SiO2=70.97%-74.54%, K2O+ Na2O=6.27%-8.09%, K2O/Na2O=0.91-1.36 and A/CNK=1.10-1.33. However, they display wide variations of trace element composition with Rb=(41-322)×10-6, Sr=(26-139)×10-6, Ba= (135-594)×10-6, (La/Yb)N=0.97-17.31, Eu/Eu* =0.29-0.72. Each body reveals distinct inter-granite trace element characteristics suggesting that each body results from distinctive conditions. The granites from the NHL resemble the two-mica granites from the High Himalayan leucogranites(HHL), but are distinct from tourmaline-muscovite granites from the HHL in their Ti, Mg, Ca, Ba contents and Rb/Sr ratio. The NHL have(87Sr/86Sr)t=0.734 4-0.850 3(for t=10 Ma) and εNd(10 Ma)=-12.5 to-19.3, which are indistinguishable from the HHL. Both the NHL and the HHL were derived from the anatexis of the High Himalayan Crystalline Series(HHCS)under the condition of fluid-absent melting, induced by muscovite breakdown due to decompression. The magma emplacement ages and the geological setting of the NHL are quite distinct from those of the HHL. Whilst the HHL resulted from southwards extrusion of the tectonic wedge of the HHCS during the Miocene, the NHL appear to have been generated over a much longer timespan, involving early melting during crustal thickening and subsequent melting during the exhumation of the gneiss domes. Thus the NHL and the HHL have different tectonic implications.
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Key words:
- leucogranite /
- geochemistry /
- petrogenesis /
- tectonic implication /
- North Himalaya /
- High Himalaya
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图 1 东喜马拉雅地质简图
+.北喜马拉雅淡色花岗岩; ×.高喜马拉雅淡色花岗岩; ITS.印度河-雅鲁藏布江缝合带; STDS.藏南拆离系; MBT.主边界逆冲断层; MCT.中央逆冲断层; GKT.吉隆-康马逆冲断层; TSS.特提斯沉积岩系; HHCS.高喜马拉雅结晶岩系.图内插图显示研究区在喜马拉雅构造带中的位置
Fig. 1. Simplified geological map in eastern part of Himalaya. Inset shows location of study area
图 2 北喜马拉雅和高喜马拉雅淡色花岗岩稀土元素组成模式
a— d.北喜马拉雅淡色花岗岩; e, f为高喜马拉雅淡色花岗岩
Fig. 2. Chondrite-normalized REE patterns for North Himalayan leucogranites and Dingge and Yadon leucogranites from HHL. Normalizing values from Taylor and McLennan(1995)
图 3 北喜马拉雅和高喜马拉雅淡色花岗岩主量元素Harker图
1.高喜马拉雅二云母花岗岩; 2.高喜马拉雅电气石-白云母花岗岩; 3.北喜马拉雅淡色花岗岩.高喜马拉雅淡色花岗岩资料据Castelli and Lombardo(1988)、Inger and Harris(1993)、Guillot and Le Fort(1995)、Ayres and Harris(1997)、Searle et al.(1997)、Visona and Lombardo(2002)及本文表 1
Fig. 3. Major element Haker diagram for North Himalayan and High Himalayan leucogranites
图 4 北喜马拉雅和高喜马拉雅淡色花岗岩微量元素Harker图
符号同图 3.高喜马拉雅淡色花岗岩微量元素资料据Castelli and Lombardo(1988)、Inger and Harris(1993)、Guillot and Le Fort (1995)、Ayres and Harris(1997)、Searle et al.(1997)、Visona and Lombardo(2002)及本文表 1
Fig. 4. Trace element Haker diagram for North Himalayan and High Himalayan leucogranites
图 5 喜马拉雅淡色花岗岩εNd(t)vs.(87Sr/86Sr)t图
HHL.高喜马拉雅淡色花岗岩; NHDN.北喜马拉雅穹窿片麻岩; HHCS.高喜马拉雅结晶岩系.HHL和HHCS的Sr-Nd同位素资料据Vidal et al.(1984)、Deniel et al.(1986)、Deniel et al.(1987); Stern et al.(1989)、Inger and Harris(1993)、Massey(1994)、Ahmad et al.(2000)、Miller et al.(2001)和表 2
Fig. 5. εNd(10 Ma)vs.(87Sr/86Sr)(t=10 Ma) diagram for North Himalayan leucogranite
图 6 北喜马拉雅和高喜马拉雅淡色花岗岩Rb/Sr vs.Ba图
右上方矢量图为据文献Inger and Harris(1993)确定的部分熔融反应.Mu(VA).缺乏蒸汽相的白云母熔融反应; Bi(VA).缺乏蒸汽相的黑云母熔融反应; Mu(VP).饱和蒸汽相的白云母熔融反应.高喜马拉雅淡色花岗岩的资料来源同图 4
Fig. 6. Rb/Sr vs. Ba diagram for North Himalayan and High Himalayan leucogranites
图 7 北喜马拉雅淡色花岗岩副矿物相温度估算图
TMon为根据独居石溶解资料(Rapp and Watson, 1986; Montel, 1993)计算的岩浆温度; TZr为根据锆石溶解资料(Harrison and Watson, 1983; Watson and Harrison, 1983)计算的岩浆温度
Fig. 7. Accessory phase thermometry for NHL
表 1 北喜马拉雅淡色花岗岩及部分高喜马拉雅淡色花岗岩主量元素和微量元素组成
Table 1. Major and trace element data for North Himalayan and some High Himalyan leucogranites
表 2 喜马拉雅淡色花岗岩和北喜马拉雅穹窿片麻岩Sr-Nd同位素组成
Table 2. Sr and Nd isotopic data for Himalayan leucogranite and North Himalayan dome gneiss
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