Oligocene crustal anatexis in the Nyalam region along the Himalayan orogenic belt
The Himalayan Cenozoic leucogranites record important information on the geochemical and tectonic physical effects of crustal partial melting and granite emplacement.Compared with the Eocene granites formed under thickened crustal conditions and Miocene granites formed during extensional process,the Oligocene granites are relatively distributed in a limited area,and there is still controversy about their sources and formation mechanisms.The Nyalam region is located in the central part of the Himalayan orogenic belt,where the 32.1Ma tourmaline-biotite-bearing granite,the 29.8Ma biotite-bearing granite,and the 26.6Ma tourmaline-biotite-bearing pegmatitic granite intruded into the upper part of the High Himalayan Crystalline Sequence.The three groups of Oligocene granites have the following characteristics:(1)high SiO2,Al2O3,K2O and Na2O contents,with A/CNK＞1.0;(2)negative anomalies of Ba,Nb,Ta,Sr and Ti;(3)slightly enriched in light rare earths,depleted in heavy rare earths,highly variable Eu anomalies and weak negative Nd anomalies;(4)uniform initial Sr and Nd isotope ratio,with 87Sr/86Sr(t)=0.7463～0.7471 and εNd(t)=-15.0～-14.6.Furthermore,the tourmaline-biotite-bearing granites and pegmatitic granites have higher Sr and Ba and lower Rb/Sr ratios than the other one,and their Rb/Sr ratios are not correlated with Ba and Sr contents,which suggest that they are products of B-rich fluid-present partial melting of metamorphic sedimentary rocks.While the biotite-bearing granites have lower Sr and Ba contents and higher Rb/Sr ratios,and Rb/Sr ratio negatively correlated with Ba and Sr contents,which suggest that they are products of fluid-absent melting of metamorphic sedimentary rocks.Combined with the published research results in the Himalayan orogenic belt,it is concluded that the source of the Oligocene granites is transformed from lower crustal basic material to middle crustal metamorphic sedimentary rocks,demonstrating the process of upward migration of the source,in addition,the Oligocene granites undergo an important period of magmatic hydrothermal metasomatism.The Oligocene partial melting recorded the response of the deep crust during the transition stage from thickening to extensional collapse in the Himalayan orogenic belt and promoted the exhumation of the High Himalayan Crystalline Sequence.
NyalamOligocene graniteFluid-present partial meltingExhumation of the High Himalayan Crystalline SequenceHimalayan orogenic belt
NETL
NSTL
万方数据
维普
