Cassiterite of the Kuqu spodumene-bearing pegmatites in the eastern Himalaya,Tibet,and its implication
Cassiterite is one of the most important tin-bearing minerals in granitic pegmatite,which can be observed in many leucogranite-pegmatite systems in the Himalaya.The microstructures and chemical compositions of cassiterite from the lithium-rich pegmatites in this region have not been revealed so far and its significance on magma fractionation and evolution requires further study.In the field work,the spodumene-bearing pegmatite dykes in the Kuqu intrusion in the eastern Himalaya were identified and investigated,which are simply zoned and mainly composed of spodumene-quartz-albite zone or spodumene-blocky feldspar-quartz-(muscovite)zone with albite aplite.The cassiterite crystals from three spodumene-bearing pegmatite dykes were studied by cathodoluminescence(CL)and in-situ EMPA and LA-ICP-MS analyses,demonstrating that the mechanisms of the cassiterite substitution are dominant of 2(Nb,Ta)5++Fe2+→3Sn4+together with a homovalent substitution of(Ti4+,Zr4+,Hf4+,U4+)→Sn4+.In some of the cassiterite crystals in one pegmatite dyke,Sn and Nb were replaced by Fe.Black,grey,oscillatory and white domains are displayed on CL images of cassiterite,and the results show that the black domains have high Nb,Ta,Fe,Zr,U and Ga contents with a relatively high Hf content and a larger Ti content range,the grey zones display similar components to the oscillatory zones which are slightly rich in Nb,Ta and Fe and depleted in U,while the white domains are mainly poor in Nb,Ta,Fe,Zr,U,Ga and Hf.The CL characteristics of cassiterite are influenced by its compositions,i.e.,Zr,U,Ga and Hf are related with CL of cassiterite in some degree.The CL structures are homogeneous crystals,including black,grey and oscillatory grains,and heterogeneous patterns which are core-rim normal zoning,patch and core-rim reverse zoning.They are black core+oscillatory/grey border/patch,grey core+oscillatory/grey border/patch,oscillatory/black core+white convolute rim/patch,oscillatory/grey/black core+black border,etc.The sequence of the cassiterite CL structure is black core/crystal → grey or oscillatory domain/crystal →(white/black domain),and geochemically,the evolution trend is a gradual decrease of Nb,Ta,Fe,U,Ga,a slight loss of Zr,Hf,Y,Ti,an increasing of Ta/(Nb+Ta)and Zr/Hf values,and finally followed by an Fe enrichment.Comparisons on CL structures and chemical compositions of cassiterite reveal that the evolution degree of the pegmatite with sporadic small spodumene crystals is lower than that of the pegmatite hosting coarse spodumene grains,competence of coexisting minerals might produce Nb-rich and Ta-rich cassiterite in one pegmatite sample,and different spodumene-bearing pegmatite dykes are diversified in their compositions and evolution of magmas.Thus,CL structure and chemical composition of cassiterite could reflect the characteristics of magma chemistry and formation process in a spodumene-bearing pegmatite.
万方数据
维普
