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Magmatism in North-Delhi Fold Belt, NW India: Evidence for Pre-Rodinia Tectonics
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Jacobs, J., Hansen, B.T., Henjes-Kunst, E, Thomas, R.J., Bauer, W., Weber, K., Armstrong, R.A. and Cornell, D.H. (1999) New age constraints on the Proterozoic/lower Palaeozoic evolution of Hemefrontfjella, East Antarctica, and its bearing on Rodinia/Gondwana correlations. Terra Antarctica, v. 6, pp. 377-389. Ohta, Y. (1993) Nature environment map, Gjelsvikfjella and Western Muhlig-Hofmannfjella, Dronning Maud Land, Antarctica, 1: 100000. Sheet 1 and 2. Norsk Polarinstitutt Temakart 24. Ohta, Y., Torudbakken, B.O. and Shiraishi, K. (1990) Geology of Gjelsvikfjella and western Muhlig-Hofmannfjella, Dronning Maud Land, East Antarctica. Polar Res., v. 8, pp. 99-126.
Ravich, M.G. and Soloviev, D.S. (1966) Geologiya i petrologiya central 'noj chasti gor zemli Korolevy Mod [Geology and petrology of the central part of the mountains of central Dronning Maud] . Trudy Naucno-Isseldovatel'skogo Instituta Geologii Arktiki, Nedra, Leningrad, v. 141, 290p. Rex, D.C. (1972) K-Ar age determinations o n volcanic and associated rocks from the Antarctic peninsula and Dronning Maud Land. In: Adie, R.J. (Ed.), Antarctic geology and Geophysics. Universitetsforlaget, Oslo, pp. 133-136. Shackleton, R.M. (1996) The final collision zone between East and West Gondwana: where is it? J. African Earth Sci., v. 23, pp. 271-287.
Gondwana Research, K 4, No. 2, pp. 149-150. 0 2001 International Association for Gondwana Research, Japan. ISSN: 1342-937X
GR
Gondwana Research
Magmatism in North-Delhi Fold Belt, NW India: Evidence for Pre-Rodinia Tectonics S. Biju-Sekharl, M. Yoshidal, M. Santoshl and M.K. randit* Department of Geosciences, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585,Japan Department of Geology, University of Rajasthan, Jaipur - 302 004, India
The Precambrian of the Aravalli Mountain Region, Rajasthan, NW India encompasses Paleoproterozoic Aravalli Fold Belt and the Meso- to Neoproterozoic Delhi Fold Belt, cumulatively spanning the major part of the Proterozoic (2000 million years), The rocks of the Delhi Fold Belt are further exposed in two domains (Sinha Roy et al., 1998), the North Delhi Fold Belt (NDFB) and South Delhi Fold Belt (SDFB). 'Itvo distinct periods of magmatism have been reported from the Delhi Fold Belt; the Mesoproterozoic activity is confined to the NDFB and a younger and more extensive Neoproterozoic (commonly referred to as Erinpura Granites) to the SDFB (Chaudhary et al., 1984). The granites of the Alwar basin in central NDFB are intrusive into the Delhi metasediments. Five such plutons are exposed in Jaipur and Alwar districts, namely, Ajitgarh (AJ), Barodia (BA), Bairat (BT), Harsora (HA) and Dadikar (DR). Individual exposures are limited to few kilometers and their relationship with the host metasediments is often obscured by alluvial sand cover. The DR, BT and HA show well developed foliation, BA is crudely foliated while AJ is massive and undeformed.
Among the Alwar basin granitoids, the AJ pluton exhibits compositional heterogeneity (Pandit et al., 1996), while the others are mostly homogeneous. Late pegmatite veins are also common in most of the plutons. Mineralogically, these plutons carry perthitic alkali feldspar as the dominant constituent. Plagioclase and quartz occur in subordinate amounts. The major maf$ minerals are hornblende and biotite. The major accessories include zircon, sphene, apatite and iron oxides. In the AJ and BA plutons sphene is ubiquitous. Zircon grains show euhedral shape and zoning. The BA, however has predominant sodic plagioclase with quartz and minor K-feldspar that indicates a granodioritic composition. Geochemical characters of these plutons indicate that they range from metaluminous to peraluminous and alkaline. All the plutons show alkaline affinities and plot in the within-plate field in geochemical discrimination diagrams. Previous geochronological work (by Rb-Sr whole rock isochron method) on the AJ and BT granites reported ages around 1450 Ma and 1650 Ma respectively (Chaudhary et al., 1984; Crawford, 1970). Rb-Sr and Sm-Nd whole
150
rock isochron age of the BT granite carried out under this study yielded ages of 1701 and 1921 Ma respectively. This is further supported by electron microprobe chemical dating of zircons in the AJ and BA plutons, where ages spanning between 1750-1950 Ma were obtained. The geochronological data on the NDFB granites thus signify a major magmatic event in the time span between 1700-1950 Ma. More precise geochronological data are awaited to constrain the timing of magmatism, and to evaluate whether the time span represents one protracted event or different events. These ages are also very close to post-Aravalli magmatic events reported from the Aravalli Fold Belt of southern Rajasthan. New age data presented here, thus necessiate a re-examination of the Meso-Proterozoic post-Aravalli and NDFB magmatic events on account of their proposed coevality. The geochemical signature of these plutons, particularly their within-plate affinity, suggests a possibility of a major intracratonic rift. In the regional scenario, extensive Neoproterozoic magmatism ranging from peralkaline granites to rhyolites with ages spanning between 750-1000 Ma have been reported from the SDFB (Sinha-Roy et al., 1998 and references therein). However, it is interesting to note that this younger event has not been detected from isotopic signatures in any of the NDFB plutons presently studied. Apart from the sporadic 700-850 Ma Rb-Sr reset mineral ages reported from one granite body near Khetri, more than 100 km west of the present study area in NDFB (Gopalan et al., 1979), there are at present no data indicating any major tectonothermal activity during Neoproterozoic in the NDFB. This would indicate that the NDFB was largely unaffected by the Neoproterozoic tectonothermal event. However, since the contact between SDFB and NDFB is covered by extensive alluvium, it has not been possible to bring out the tectonic relations between these two terrains.
At present the data are insufficient to draw correlations for the Paleoproterozoic/Mesoproterozoic event in NDFB reported in this study with similar events in other crustal fragments of the globe. Although the existence of the supercontinent Rodinia at ca. 1.0 Ga and Gondwana at ca. 0.5 Ga are generally accepted by now (Unrug, 1997), the configurations of pre-Rodinia supercontinents are still vague (Rogers, 1996). The possibility of existence of a major supercontinent during the Paleoproterozoic/ Mesoproterozoic has recently emerged (Rogers et al., in prep.), but its configuration has to be brought out through detailed future studies. The NDFB granitoids are likely to provide critical information on the tectonics of this supercontinental assembly.
References Chaudhary, A.K., Gopalan, K. and Sastry, C.A. (1984) Present status of the geochronology of the Precambrian rocks of Rajasthan. Tectonophys., v. 105, pp. 131-140. Crawford, A.R. (1970) The Precambrian geochronology of Rajasthan and Bundelkhand, northern India. Can. J. Earth Sci., v. 7, pp. 91-110. Gopalan, K., Trivedi, J.R., Balasubramanyam, M.N., Ray, S.K. and Sastri, C.A. (1979) Rb-Sr chronology of the Khetri Copper Belt, Rajasthan. J. Geol. SOC.India, v. 20, pp. 450-456. Pandit, M.K., Khatatneh, M.K. and Saxena, R. (1996) Trondhjemite of the Alwar basin, Rajasthan: implications of late Proterozoic rifting in North Delhi Fold Belt. Curr. Sci., v. 71, pp. 636-641. Rogers, J.J.W. (1996) A history of continents in the past three billion years. J. Geol., v. 104, pp. 91-107. Rogers, J.J.W., Santosh, M. and Yoshida, M. Mesoproterozoic Supercontinent. Gond. Res., spl. issue (in prep). Sinha-Roy, M., Malhotra, G. and Mohanty, M. (1998) Geology of Rajasthan. Geol. SOC.India, 278p. Unrug, R. (1997) Rodinia to Gondwana: the geodynamic map of Gondwana supercontinent reassembly. GSA Today, v. 7, pp. 1-6.
Gondwana Research, I/:4, No. 2,2001
Jacobs, J., Hansen, B.T., Henjes-Kunst, E, Thomas, R.J., Bauer, W., Weber, K., Armstrong, R.A. and Cornell, D.H. (1999) New age constraints on the Proterozoic/lower Palaeozoic evolution of Hemefrontfjella, East Antarctica, and its bearing on Rodinia/Gondwana correlations. Terra Antarctica, v. 6, pp. 377-389. Ohta, Y. (1993) Nature environment map, Gjelsvikfjella and Western Muhlig-Hofmannfjella, Dronning Maud Land, Antarctica, 1: 100000. Sheet 1 and 2. Norsk Polarinstitutt Temakart 24. Ohta, Y., Torudbakken, B.O. and Shiraishi, K. (1990) Geology of Gjelsvikfjella and western Muhlig-Hofmannfjella, Dronning Maud Land, East Antarctica. Polar Res., v. 8, pp. 99-126.
Ravich, M.G. and Soloviev, D.S. (1966) Geologiya i petrologiya central 'noj chasti gor zemli Korolevy Mod [Geology and petrology of the central part of the mountains of central Dronning Maud] . Trudy Naucno-Isseldovatel'skogo Instituta Geologii Arktiki, Nedra, Leningrad, v. 141, 290p. Rex, D.C. (1972) K-Ar age determinations o n volcanic and associated rocks from the Antarctic peninsula and Dronning Maud Land. In: Adie, R.J. (Ed.), Antarctic geology and Geophysics. Universitetsforlaget, Oslo, pp. 133-136. Shackleton, R.M. (1996) The final collision zone between East and West Gondwana: where is it? J. African Earth Sci., v. 23, pp. 271-287.
Gondwana Research, K 4, No. 2, pp. 149-150. 0 2001 International Association for Gondwana Research, Japan. ISSN: 1342-937X
GR
Gondwana Research
Magmatism in North-Delhi Fold Belt, NW India: Evidence for Pre-Rodinia Tectonics S. Biju-Sekharl, M. Yoshidal, M. Santoshl and M.K. randit* Department of Geosciences, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585,Japan Department of Geology, University of Rajasthan, Jaipur - 302 004, India
The Precambrian of the Aravalli Mountain Region, Rajasthan, NW India encompasses Paleoproterozoic Aravalli Fold Belt and the Meso- to Neoproterozoic Delhi Fold Belt, cumulatively spanning the major part of the Proterozoic (2000 million years), The rocks of the Delhi Fold Belt are further exposed in two domains (Sinha Roy et al., 1998), the North Delhi Fold Belt (NDFB) and South Delhi Fold Belt (SDFB). 'Itvo distinct periods of magmatism have been reported from the Delhi Fold Belt; the Mesoproterozoic activity is confined to the NDFB and a younger and more extensive Neoproterozoic (commonly referred to as Erinpura Granites) to the SDFB (Chaudhary et al., 1984). The granites of the Alwar basin in central NDFB are intrusive into the Delhi metasediments. Five such plutons are exposed in Jaipur and Alwar districts, namely, Ajitgarh (AJ), Barodia (BA), Bairat (BT), Harsora (HA) and Dadikar (DR). Individual exposures are limited to few kilometers and their relationship with the host metasediments is often obscured by alluvial sand cover. The DR, BT and HA show well developed foliation, BA is crudely foliated while AJ is massive and undeformed.
Among the Alwar basin granitoids, the AJ pluton exhibits compositional heterogeneity (Pandit et al., 1996), while the others are mostly homogeneous. Late pegmatite veins are also common in most of the plutons. Mineralogically, these plutons carry perthitic alkali feldspar as the dominant constituent. Plagioclase and quartz occur in subordinate amounts. The major maf$ minerals are hornblende and biotite. The major accessories include zircon, sphene, apatite and iron oxides. In the AJ and BA plutons sphene is ubiquitous. Zircon grains show euhedral shape and zoning. The BA, however has predominant sodic plagioclase with quartz and minor K-feldspar that indicates a granodioritic composition. Geochemical characters of these plutons indicate that they range from metaluminous to peraluminous and alkaline. All the plutons show alkaline affinities and plot in the within-plate field in geochemical discrimination diagrams. Previous geochronological work (by Rb-Sr whole rock isochron method) on the AJ and BT granites reported ages around 1450 Ma and 1650 Ma respectively (Chaudhary et al., 1984; Crawford, 1970). Rb-Sr and Sm-Nd whole
150
rock isochron age of the BT granite carried out under this study yielded ages of 1701 and 1921 Ma respectively. This is further supported by electron microprobe chemical dating of zircons in the AJ and BA plutons, where ages spanning between 1750-1950 Ma were obtained. The geochronological data on the NDFB granites thus signify a major magmatic event in the time span between 1700-1950 Ma. More precise geochronological data are awaited to constrain the timing of magmatism, and to evaluate whether the time span represents one protracted event or different events. These ages are also very close to post-Aravalli magmatic events reported from the Aravalli Fold Belt of southern Rajasthan. New age data presented here, thus necessiate a re-examination of the Meso-Proterozoic post-Aravalli and NDFB magmatic events on account of their proposed coevality. The geochemical signature of these plutons, particularly their within-plate affinity, suggests a possibility of a major intracratonic rift. In the regional scenario, extensive Neoproterozoic magmatism ranging from peralkaline granites to rhyolites with ages spanning between 750-1000 Ma have been reported from the SDFB (Sinha-Roy et al., 1998 and references therein). However, it is interesting to note that this younger event has not been detected from isotopic signatures in any of the NDFB plutons presently studied. Apart from the sporadic 700-850 Ma Rb-Sr reset mineral ages reported from one granite body near Khetri, more than 100 km west of the present study area in NDFB (Gopalan et al., 1979), there are at present no data indicating any major tectonothermal activity during Neoproterozoic in the NDFB. This would indicate that the NDFB was largely unaffected by the Neoproterozoic tectonothermal event. However, since the contact between SDFB and NDFB is covered by extensive alluvium, it has not been possible to bring out the tectonic relations between these two terrains.
At present the data are insufficient to draw correlations for the Paleoproterozoic/Mesoproterozoic event in NDFB reported in this study with similar events in other crustal fragments of the globe. Although the existence of the supercontinent Rodinia at ca. 1.0 Ga and Gondwana at ca. 0.5 Ga are generally accepted by now (Unrug, 1997), the configurations of pre-Rodinia supercontinents are still vague (Rogers, 1996). The possibility of existence of a major supercontinent during the Paleoproterozoic/ Mesoproterozoic has recently emerged (Rogers et al., in prep.), but its configuration has to be brought out through detailed future studies. The NDFB granitoids are likely to provide critical information on the tectonics of this supercontinental assembly.
References Chaudhary, A.K., Gopalan, K. and Sastry, C.A. (1984) Present status of the geochronology of the Precambrian rocks of Rajasthan. Tectonophys., v. 105, pp. 131-140. Crawford, A.R. (1970) The Precambrian geochronology of Rajasthan and Bundelkhand, northern India. Can. J. Earth Sci., v. 7, pp. 91-110. Gopalan, K., Trivedi, J.R., Balasubramanyam, M.N., Ray, S.K. and Sastri, C.A. (1979) Rb-Sr chronology of the Khetri Copper Belt, Rajasthan. J. Geol. SOC.India, v. 20, pp. 450-456. Pandit, M.K., Khatatneh, M.K. and Saxena, R. (1996) Trondhjemite of the Alwar basin, Rajasthan: implications of late Proterozoic rifting in North Delhi Fold Belt. Curr. Sci., v. 71, pp. 636-641. Rogers, J.J.W. (1996) A history of continents in the past three billion years. J. Geol., v. 104, pp. 91-107. Rogers, J.J.W., Santosh, M. and Yoshida, M. Mesoproterozoic Supercontinent. Gond. Res., spl. issue (in prep). Sinha-Roy, M., Malhotra, G. and Mohanty, M. (1998) Geology of Rajasthan. Geol. SOC.India, 278p. Unrug, R. (1997) Rodinia to Gondwana: the geodynamic map of Gondwana supercontinent reassembly. GSA Today, v. 7, pp. 1-6.
Gondwana Research, I/:4, No. 2,2001