The Central Indian Basin (CIB, 10°-19°S) encompasses morphotectonic features such as seamounts, abyssal hills, faults, fracture zones and lineations and basic to silicic volcanics that were derived from diff...The Central Indian Basin (CIB, 10°-19°S) encompasses morphotectonic features such as seamounts, abyssal hills, faults, fracture zones and lineations and basic to silicic volcanics that were derived from different sources. Instances of incipient hydrothermal event (s) occurred as evident from: (1) spilites that probably formed due to low-temperature hydrothermal action on the precursory basalts, (2) slabs and fragments of zeolitites which are the consequent products of palagonitization of the basaltic glasses, (3) chemical variations in the ferromanganese crusts, and (4) abundance of volcanogenic-hydrothermal material (vhm) that includes ochrous sediments and magnetite spherules. The vhm suggests its origin by interaction of Fe-rich hydrothermal solutions with surrounding sediments. The CIB spherules occur in sediments of~10 ka and ~425 to 650 ka age and are orders of magnitude younger than the 50 to 60 Ma age of the basin. Incidentally, to the north of the basin at ODP Site 717-719(1 °S/81 °E) hydrothermal precipitates of inorganic calcite and deposition of sulphide minerals (~7.5 to 9 Ma and 0.5 Ma) occur as a result of the on-going intraplate deformation. This paper reviews the above evidences that possibly indicate episodes of localized hydrothermal events in the CIB.展开更多
The Precambrian greywacke of Ribandar-Chimbel belonging to the Sanvordem Formation of the Goa Group, India, has been studied for petrography and analyzed for major trace elements. The greywacke is characterized by ang...The Precambrian greywacke of Ribandar-Chimbel belonging to the Sanvordem Formation of the Goa Group, India, has been studied for petrography and analyzed for major trace elements. The greywacke is characterized by angular to sub-round grains of quartz, feldspar, biotite, chlorite and clay minerals. The abundance of clay in the matrix seems to have influenced the Al2O3 content and the K20/Al2O3 ratio. The variation diagrams indicate a decreasing trend of TiO2, Al2O3, Fe2O3 and MgO; whereas Na2O and CaO exhibit a scatter which could be a result of the variable presence of feldspar within the sediments. The immobile elements, vanadium （25 to 144 ppm）, nickel （up to 107 ppm） and chromium （up to 184 ppm）, reflect abundance of clay minerals. The greywacke shows strongly fractionated REE patterns with LaN/YbN = 8 to 26 and with higher total REE abundances （up to 245 ppm）. The low REE enrichment and depletion in heavier REE with prominent negative Eu anomaly （Eu/Eu^＊= 0.54 to 0.79） suggest a derivation of the greywacke from an old upper continental crust composed chiefly of felsic components. Petrological evidence and geochemical data suggest that the deposition of the greywacke largely took place in a deep to shallow basin that progressively chang- ed from that of a continental island arc to an active continental setting.展开更多
Mid-Ocean Ridge Basalts （MORB） from the Northern Central Indian Ridge （NCIR） were recovered between latitudes 3° and 11° S and are olivine tholeiite with higher abundances of K and Rb. They are of typica...Mid-Ocean Ridge Basalts （MORB） from the Northern Central Indian Ridge （NCIR） were recovered between latitudes 3° and 11° S and are olivine tholeiite with higher abundances of K and Rb. They are of typical transitional MORB （T-MORB） variety and appear to have been generated from an enriched-mantle peridotite source. The primitive NCIR MORBs having Mg^# 〉 0.68 are the product of partial melting at an estimated pressure of - 1 GPa. It is inferred that the magma was subsequently modified at a pressure 〉 1 GPa by crystal fractionation and spinel was the first mineral to crystallize followed by separation of relatively Fe-rich olivine with subsequent decrease in pressure. During progressive fractionation at lower pressure （between 1-0.5 GPa）, the bulk composition of the magma became systematically depleted in MgO, and enriched in ∑FeO, TiO2, P2Os and Na20. There was, however, limited gradual depletion in Al2O3 and CaO and concomitant enrichment in K20. With the progressive fractionation these basalts became gradually enriched in V, Co, Y, Zr and to some extent in Sr, and depleted in Ni and Cro In addition, the T_JtEE of the magma also increased with fractionation, without any change in （La/Yb）n value.展开更多
文摘The Central Indian Basin (CIB, 10°-19°S) encompasses morphotectonic features such as seamounts, abyssal hills, faults, fracture zones and lineations and basic to silicic volcanics that were derived from different sources. Instances of incipient hydrothermal event (s) occurred as evident from: (1) spilites that probably formed due to low-temperature hydrothermal action on the precursory basalts, (2) slabs and fragments of zeolitites which are the consequent products of palagonitization of the basaltic glasses, (3) chemical variations in the ferromanganese crusts, and (4) abundance of volcanogenic-hydrothermal material (vhm) that includes ochrous sediments and magnetite spherules. The vhm suggests its origin by interaction of Fe-rich hydrothermal solutions with surrounding sediments. The CIB spherules occur in sediments of~10 ka and ~425 to 650 ka age and are orders of magnitude younger than the 50 to 60 Ma age of the basin. Incidentally, to the north of the basin at ODP Site 717-719(1 °S/81 °E) hydrothermal precipitates of inorganic calcite and deposition of sulphide minerals (~7.5 to 9 Ma and 0.5 Ma) occur as a result of the on-going intraplate deformation. This paper reviews the above evidences that possibly indicate episodes of localized hydrothermal events in the CIB.
文摘The Precambrian greywacke of Ribandar-Chimbel belonging to the Sanvordem Formation of the Goa Group, India, has been studied for petrography and analyzed for major trace elements. The greywacke is characterized by angular to sub-round grains of quartz, feldspar, biotite, chlorite and clay minerals. The abundance of clay in the matrix seems to have influenced the Al2O3 content and the K20/Al2O3 ratio. The variation diagrams indicate a decreasing trend of TiO2, Al2O3, Fe2O3 and MgO; whereas Na2O and CaO exhibit a scatter which could be a result of the variable presence of feldspar within the sediments. The immobile elements, vanadium （25 to 144 ppm）, nickel （up to 107 ppm） and chromium （up to 184 ppm）, reflect abundance of clay minerals. The greywacke shows strongly fractionated REE patterns with LaN/YbN = 8 to 26 and with higher total REE abundances （up to 245 ppm）. The low REE enrichment and depletion in heavier REE with prominent negative Eu anomaly （Eu/Eu^＊= 0.54 to 0.79） suggest a derivation of the greywacke from an old upper continental crust composed chiefly of felsic components. Petrological evidence and geochemical data suggest that the deposition of the greywacke largely took place in a deep to shallow basin that progressively chang- ed from that of a continental island arc to an active continental setting.
文摘Mid-Ocean Ridge Basalts （MORB） from the Northern Central Indian Ridge （NCIR） were recovered between latitudes 3° and 11° S and are olivine tholeiite with higher abundances of K and Rb. They are of typical transitional MORB （T-MORB） variety and appear to have been generated from an enriched-mantle peridotite source. The primitive NCIR MORBs having Mg^# 〉 0.68 are the product of partial melting at an estimated pressure of - 1 GPa. It is inferred that the magma was subsequently modified at a pressure 〉 1 GPa by crystal fractionation and spinel was the first mineral to crystallize followed by separation of relatively Fe-rich olivine with subsequent decrease in pressure. During progressive fractionation at lower pressure （between 1-0.5 GPa）, the bulk composition of the magma became systematically depleted in MgO, and enriched in ∑FeO, TiO2, P2Os and Na20. There was, however, limited gradual depletion in Al2O3 and CaO and concomitant enrichment in K20. With the progressive fractionation these basalts became gradually enriched in V, Co, Y, Zr and to some extent in Sr, and depleted in Ni and Cro In addition, the T_JtEE of the magma also increased with fractionation, without any change in （La/Yb）n value.
