北喜马拉雅淡色花岗岩地球化学:区域对比、岩石成因及其构造意义

Geochemistry of North Himalayan Leucogranites: Regional Comparison, Petrogenesis and Tectonic Implications

北喜马拉雅出露一系列片麻岩穹窿,这些穹窿被形成于27.5～10Ma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=0.7344～0.8503(t=10Ma),εNd(10Ma)=-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。

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 = 0.7344-0.8503 (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.