Palaeomagnetics and igneous activity of the area adjoining the south-westem margin of the Cuddapah basin, India

Abstract

Fifty-six orientated samples were collected from 13 sites on five dolerite dykes (between lat. 14"23'N, long. 77'43'E and lat. 14"08'N, long. 77"49'E), which adjoin the south-western margin of the Cuddapah basin in Anantapur district, Andhra Pradesh. After af demagnetization, two dykes (five sites) striking ENE possess similar magnetic directions, ( l ) D = 57", 1 = -6 9 " ( K = 5 2 , 0 1~~= 7 " ) and (2) D = 7 1 " , 1 = -7 2 " (K=26O,a9,=5"). Again dykes (3) (three sites), and (4) (two sites) have similar strike (NE) and magnetic directions, D = 64". I = -7" ( K = 142, ag5 = 8") and D = 53", I = -8" ( K = 142, ag5 = 6") and dyke (5) (two sites) striking NW shows D = 320", I = -34" (K = 68, ag5 = 13"). Remanent directions estimated from total field magnetic anomaly data agree well with these results. Synthesis of these data with 10 other published palaeomagnetic studies of Precambrian dolerite dykes on the Indian peninsula, suggest that these three systems of dykes adjoining the Cuddapah basin had been emplaced prior to the basin formation perhaps representing the initial thermal event responsible for the basin development and also that there have been at least three separate periods of dyke emplacement on this shield. The radiometric data, however, are very sparse and these periods cannot be dated with confidence.

India's changing place in global Proterozoic reconstructions: A review of geochronologic constraints and paleomagnetic poles from the Dharwar, Bundelkhand and Marwar cratons

“…Our sites yielded intermediate to shallow inclination that are similar to those obtained by previous workers from the area (Kumar and Bhalla, 1983; Table 3). Dyke I595 that yielded a concordant U-Pb age of 1025.6 ± 3.8 Ma also yielded a mean declination = 76.9 • and inclination = −50.6 • (k = 19.23, ␣95 = 12 • ; N = 9; Fig.…”

“…The larger dykes can be seen as persistent linear ridges, surrounding the Paleo-Mesoproterozoic intracratonic Cuddapah basin (Karunakaran, 1971;Halls, 1982;Drury, 1984;Murthy et al, 1987;Poornachandra Rao, 2005). Most of these dykes abruptly terminate at the boundary of the Cuddapah basin and are thus thought to be older than the basin (Kumar and Bhalla, 1983;Balakrishna et al, 1979;Bhattacharji and Singh, 1981). A close scrutiny of the field, petrographic, geochemical and radiometric data on these dykes suggest three major phases of dyke emplacement (all poorly dated) between Paleo-Neoproterozoic (1900-1700, 1500-1300 and 1200-1000 Ma) around the Cuddapah basin (Chalapathi .…”

“…The Dharwar craton experienced widespread mafic magmatism during the Proterozoic along with the intrusion of Meso-Neoproterozoic diamondiferous kimberlites and lamproites (Rao and Pupper, 1996;Murthy and Dayal, 2001;Chalapathi Rao et al, 2004). In addition to the kimberlites and lamproites, there are other mafic intrusives including metadolerites/metanorites, tholeiitic and alkali-olivine basaltic dykes forming dense E-W and NNW to NW trending swarms crosscutting the greenstone and gneissic basement of the Dharwar craton (Murthy et al, 1987;Kumar and Bhalla, 1983;Radhakrishna and Joseph, 1996;Chalapathi Rao et al, 2005). The E-W trending mafic dykes are well developed in the eastern portion of the craton around the Cuddapah basin.…”

“…The majority of these dyke swarms are dolerite and gabbro along with a few intrusions of peridotite, syenite and granophyres. The dykes are fine to coarse-grained with subophitic to ophitic or granular textures and tholeiitic to alkaline in compositions (Balakrishna et al, 1979;Kumar and Bhalla, 1983;Halls, 1982;Murthy, 1987;Murthy et al, 1987;Chalapathi Rao et al, 2005). The Anantapur gneissic basement is also intruded by a diamondiferous kimberlitic and lamproitic cluster at ∼1100 Ma (Chalapathi Rao et al, 2004).…”

“…6). Kumar and Bhalla (1983) also conducted a paleomagnetic study on ENE-WSW and NE-SW trending doleritic dykes in the region.…”