| 1 | Tourmaline from the Archean G.R.Halli gold deposit,Chitradurga greenstone belt,Dharwar craton(India):Implications for the gold metallogeny显示文摘Tourmaline occurs as a minor but important mineral in the alteration zone of the Archean orogenic gold deposit of Guddadarangavanahalli(G.R.Halli) in the Chitradurga greenstone belt of the western Dharwar craton,southern India.It occurs in the distal alteration halo of the G.R.Halli gold deposit as(a) clusters of very fine grained aggregates which form a minor constituent in the matrix of the altered metabasalt(AMB tourmaline) and(b) in quartz-carbonate veins(vein tourmaline).The vein tourmaline,based upon the association of specific carbonate minerals,is further grouped as(i) albite-tourmaline-ankerite-quartz veins(vein-1 tourmaline) and(ii) albite-tourmaline-calcite-quartz veins(vein-2 tourmaline).Both the AMB tourmaline and the vein tourmalines(vein-1 and vein-2) belong to the alkali group and are classified under schorl-dravite series.Tourmalines occurring in the veins are zoned while the AMB tourmalines are unzoned.Mineral chemistry and discrimination diagrams reveal that cores and rims of the vein tourmalines are distinctly different.Core composition of the vein tourmalines is similar to the composition of the AMB tourmaline.The formation of the AMB tourmaline and cores of the vein tourmalines are proposed to be related to the regional D1 deformational event associated with the emplacement of the adjoining ca.2.61 Ga Chitradurga granite whilst rims of the vein tourmalines vis-à-vis gold mineralization is spatially linked to the juvenile magmatic accretion(2.56-2.50 Ga) east of the studied area in the western part of the eastern Dharwar craton. | Susmita Gupta M.Jayananda Fareeduddin | 2014 | Geoscience Frontiers2014，5，6： | 5 |
| 2 | Geochemical constraints on komatiite volcanism from Sargur Group Nagamangala greenstone belt,western Dharwar craton,southern India:Implications for Mesoarchean mantle evolution and continental growth显示文摘We present field,petrographic,major and trace element data for komatiites and komatiite basalts from Sargur Group Nagamangala greenstone belt,western Dharwar craton.Field evidences such as crude pillow structure indicate their eruption in a marine environment whilst spinifex texture reveals their komatiite nature.Petrographic data suggest that the primary mineralogy has been completely altered during post-magmatic processes associated with metamorphism corresponding to greenschist to lower amphibolite facies conditions.The studied komatiites contain serpentine,talc,tremolite,actinolite and chlorite whilst tremolite,actinolite with minor plagioclase in komatiitic basalts.Based on the published Sm-Nd whole rock isochron ages of adjoining Banasandra komatiites(northern extension of Nagamangala belt) and further northwest in Nuggihalli belt and Kalyadi belt we speculate ca.3.2-3.15 Ga for komatiite eruption in Nagamangala belt.Trace element characteristics particularly HFSE and REE patterns suggest that most of the primary geochemical characteristics are preserved with minor influence of post-magmatic alteration and/or contamination.About 1/3 of studied komatiites show Aldepletion whilst remaining komatiites and komatiite basalts are Al-undepleted.Several samples despite high MgO,(Gd/Yb)_N ratios show low CaO/Al_2O_3 ratios.Such anomalous values could be related to removal of CaO from komatiites during fluid-driven hydrothermal alteration,thus lowering CaO/Al_2O_3 ratios.The elemental characteristics of Al-depleted komatiites such as higher(Gd/Yb)_N(>1.0),CaO/Al_2O_3 (>1.0),Al_2O_3/TiO_2(<18) together with lower HREE,Y,Zr and Hf indicate their derivation from deeper upper mantle with minor garnet(majorite?) involvement in residue whereas lower(Gd/Yb)_N(<1.0), CaO/Al_2O_3(<0.9),higher Al_2O_3/TiO_2(>18) together with higher HREE.Y,Zr suggest their derivation from shallower upper mantle without garnet involvement in residue.The observed chemical characteristics (CaO/Al_2O_3,Al_2O_3/TiO_2,MgO,Ni,Cr,Nb,Zr,Y,Hf,and REE) indicate derivation of the komatiite and komatiite basalt magmas from heterogeneous mantle(depleted to primitive mantle) at different depths in hot spot environments possibly with a rising plume.The low content of incompatible elements in studied komatiites suggest existence of depleted mantle during ca.3.2 Ga which in turn imply an earlier episode of mantle differentiation,greenstone volcanism and continental growth probably during ca. 3.6-3.3 Ga which is substantiated by Nd and Pb isotope data of gneisses and komatiites in western Dharwar craton(WDC). | Tushipokla M.Jayananda | 2013 | Geoscience Frontiers2013，4，3： | 0 |
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