Palaeomagnetic studies on the dykes of Mumbai region, West coast of deccan volcanic province: implications on age and Span of the deccan Eruptions

Through detailed AF and thermal demagnetizations of 35 samples (161 specimens) collected from 6 dykes located at Murud region, south of Mumbai, we report the first mean characteristic remanent magnetization direction as D=341°; I= -42°(μ95=5.7; N=6 dykes) for the dykes intruded into the West Coast zone of the Deccan Volcanic Province. The virtual geomagnetic pole (VGP) position (44°N; 83°W; A95=5.8°; N=6 dykes) calculated from this study is compared with those of the Narmada-Tapti zone dykes and with the Deccan Super Pole, proposed by Vandamme et al. (1991). Based on the concordance of these poles and recently reported 40Ar/39Ar and 87Rb/86Sr dates on the dykes of the studied region (Murud), it is proposed that the entire Deccan flows and the associated dykes were erupted in a short interval close to the Cretaceous-Tertiary Boundary at 65.5 Ma. Thermal demagnetization spectra along with isothermal remanent magnetizations and LF test experiments indicated that the SD type titano-magnetite was the major ChRM carrying magnetic mineral in the samples. Out of the 6 studied dykes, one dolerite dyke showing the “reverse magnetic polarity”, could have acted as a feeder dyke to the Poladpur Formation flows, which host the dykes in the studied area. The remaining four dolerite dykes and one lamprophyre dyke, yielding “normal polarity” directions, might have fed to the youngest formations of Wai Subgroup flows, viz., Panhala Formation and Desur Formation, which were eroded in the studied region. INTRODUCTION It is a widely accepted view that the Deccan flood basalts of Indian sub-continent, covering at present an area of 500,000 sq.km., is the result of outburst of immense magma material from the Reunion Hot spot source that impinged on the northerly drifting Indian lithosphere in the Late Cretaceous (Morgan, 1981). The Deccan Province is traversed by three major rift zones, namely the Narmada-Tapti-Son rift, the Cambay rift and the West Coast rift, which form a triple junction at the CambayBasin (Sheth and Chandrashekharam, 1997). Over the last 30 years the Deccan Traps have attained the attention of geochronologists, palaeomagnetists, geochemists and petrologists alike with its impressive horizontal lava piles, enormous size and volume along with its rapid eruptions at the K/T boundary coinciding with the much debated mass extinctions (Sen, 2001; Subbarao, 1999a and 1999b). Palaeomagnetic investigations on the Deccan Traps have been successful in strengthening the plate-tectonic theory as well as establishing the magnetostratigraphy of the flow sequence permitting constraints on the ages and span of eruption of the flows. From the compilation of the available good quality palaeomagnetic data on the flows, Vandamme et al. (1991) proposed a normal-reversenormal polarity sequence that correspond to the chrons 30 N-29 R-29 N; a major part of the eruptions is limited to the chron 29 R. From detailed geochronological studies on the 2.5 km thick lava pile in the Western Ghats, Duncan and Pyle (1988) have suggested the age to be 67.4±0.7 Ma as compared to 65.5±2.5 Ma reported by Vandamme et al. (1991). In both these studies, a short duration of less than 1 Ma long has been proposed for the Deccan Traps. Recent 40Ar/39 Ar dating of mineral separates and whole rocks from the Western Ghats lava pile, has reinforced these views by (Hofmann et al., 2000). In contrast, Venkatesan et al. (1993, 1994) suggested the flow duration to be not less than 3 Ma with eruption pre-dating the KTB by at least 1.0 Ma. Dhandapani and Subbarao (1992) had also favored a longer duration of more than 6 Ma for the Deccan flows by identifying normal polarity in the lowermost Deccan flow coinciding with the Cretaceous Long Normal Superchron. Most of the rock magnetic and palaeomagnetic studies till date have focused on measurements from different lava flows. In order to have a better understanding of the Deccan volcanic episode, in particular, the precise age, span and mechanism of eruption as well as the original extent of the Deccan Province, it is desirable to extend palaeomagnetic studies to the associated intrusive phases. There are only three published palaeomagnetic reports on the Deccan traps dykes -(i) Mandaleshwar dykes in the south of Narmada river (Subbarao et al., 1988), (ii) Dhadgaon and Nandurbar dykes respectively from the south of Narmada and Tapti rivers (Prasad et al., 1996) and (iii) Goa dykes, which exist around 70 km beyond the present day southern margin of the Deccan Province, along the west coast of India (Patil and Rao, 2002). Besides these three reports, Radhakrishna et al. (1994) have studied the central Kerala dykes, based on palaeomagnetic and 40Ar/39 Ar ages (69±1 Ma), as the feeders to the Deccan volcanic Patil, S.K. and Arora, B.R. 2003. Palaeomagnetic Studies on the Dykes of Mumbai Region, West Coast of Deccan Volcanic Province: Implications on Age and Span of the Deccan Eruptions. Journal of the Virtual Explorer 12, 107-116. Patil and Arora, 2003. Journal of the Virtual Explorer 12, 107-116 107 sequences and suggested that the Deccan eruptions were widespread and extended southward into Central Kerala. The most prominent region of Deccan Traps Province, the Western Ghat Section, where flows yielded good quality palaeomagnetic, isotopic dates and geochemical data sets, is lacking in detailed palaeomagnetic studies on the intrusive bodies. Only one report by Subbarao et al. (1992) identifying the dyke polarities is available to the best of authors” knowledge. However, at present palaeomagnetic and geochemical investigations along with geochronological studies are initiated on the dykes and recently Sahu et al. (2003) have presented 87 Rb/86 Sr ages and Sr isotopic composition of alkaline dykes near Mumbai. In this present article, we have reported new palaeomagnetic results from the dykes along the West Coast, south of Mumbai (Bombay), and compared the results with those of the previous studies for constraining the age and duration of the Deccan trap flows and also discussed their genetic relation with the flows. GEOLOGY OF THE STUDY AREA Mafic dykes are concentrated in clusters and swarms in the two tectonic belts paralleling the N-S trending West Coast and Sahyadri Mountains and E-W trending Narmada-Tapti-Satpura lineament zones in the Deccan volcanic province (Figure 1). According to Deshmukh and Sehgal (1988) dyke swarms cover areas of 32,500 km2 and 87,000 km2 in the Narmada-Tapti and West Coast belts respectively. These dykes are mainly dolerites of tholeiitic character and they occupy dilatory tensional fractures, which are formed due to tectonic movements in the two tectonic belts. The Murud region, the present study area (Figure 2), lies in the Konkan Coastal belt of the Western Ghat Section, and is predominantly covered by flows of Poladpur and Ambenalli Formations of Wai Subgroup of the Deccan Basalts (Subbarao and Hooper, 1988). Powar and Vadetwar (1995) identify these flows as simple and compound types that range in thickness from about 5m to 40m or more. The “Panvel Flexure” is the most prominent tectonic feature in this region (Auden, 1949). In this area tholeiitic to mildly alkaline flows of Deccan basalts are intruded by a number of basic (dolerite) dykes associated with plugs of gabbrodiorite. The dykes of this region are mostly oriented N-S, parallel to the “Panvel lineament”. According to Powar and Vadetwar (1995), over 32% of the dykes occur in the azimuth range N 0-9° and nearly 60% in the range N 030°. The dykes show sharp contacts with the basalts and have chilled margins. The dykes are vertical or steeply inclined up to 70°. Based on their field characteristics, Dessai and Viegas (1995) distinguished these dykes into four generations. Powar and Vadetwar (1995) observed close similarity in the mineralogy and chemistry of basalts and dolerite dykes of this region and suggested that both dykes and flows represent the Poladpur magma-type. They also opined that the dykes were emplaced immediately after the outpouring of basalts of Poladpur Formation, but are not the feeders to the flows. SAMPLING AND MEASUREMENTS 35 oriented block samples were collected from 5 dolerite dykes (4 dykes, RID 1 to 4, belong to south of Rat Island, 1 dyke (TBD-1) from north-west of Borlai (Figure2b) and 1 lamprophyre dyke (ELD-1) from Beacon Hill region (Figure 2-c). It has been taken care that minimum 5 samples from each dyke was collected. Rat Island dykes trend N10°W and their widths range from 1 to 2 m. Borlai dyke trends N50°W and has the width of around 2.5 m, whereas, Beacon Hill lamprophyre dyke trends NNESSW direction with a width of 3 m. From the collected 35 samples around 160 standard cylindrical specimens of size 2.2 cm height and 2.5 cm diameter were prepared in the laboratory. The Natural Remanent Magnetization (NRM) intensities of all the prepared specimens were measured with JR-5A Spinner magnetometer (M/S Agico, Czech Republic) having the sensitivity of 3 pT. NRM intensities of dolerite dyke specimens were in the range of 0.22 to 3.31 A/m, whereas, lamprophyre dyke specimens were in the range of 0.60 to 1.19 A/m. The magnetic susceptibilities were measured by MS-2B Bartington susceptibility meter. The magnetic susceptibilities of dolerite and lamprophyre dyke specimens were in the ranges of (1599 – 5932) x 10-5 SI and (11610 – 12220) x10-5 SI respectively. Koenigsberger (Qn) ratios were calculated and it was noticed that the dolerite specimens had relatively higher values (mean=0.68) than those of lamprophyre dykes specimens (mean=0.20). Both the alternating field (AF) and thermal demagnetization methods were used to isolate the characteristic remanent magnetization (ChRM) directions from the specimens. For the AF demagnetizations M/S. Molspin AF demagnetizer was used and for the thermal demagnetizations, MAVACS system manufactured by Geofyzica, Brono, was used. Initially 36 specimens representing 6 dykes