New Paleomagnetic Results from Late Cretaceous Volcanics and Dykes, SouthWest India

Abstract:

Late Cretaceous magmatism is widely distributed along the south west coast of India, mainly in the form of dyke intrusions. Most prominent dykes were emplaced around 65 Ma and are distributed in central Kerala, north Kerala and Goa. The strike trend of these dykes is NW-SE in central Kerala, whereas two orthogonal directions with NW-SE and NE-SW trends are found in north Kerala. In Goa region, they are mostly perpendicular to the coast. 25 sites (232 samples) have been collected in Kerala and Goa areas. A subordinate magmatism, around 90 Ma in age, is also traced in Agali area and St Mary Archipelago off the Malpe coast. Nine dykes (79 samples) with NW-SE trend were collected, and 9 flows (63 samples) from 8 different islands of the St Mary Archipelago. Finally, following some field evidences, the Ezhimala igneous complex, south of St Mary Islands, that occurs as a linear ridge with a NNW-SSE trend and consists mainly of granite, granophyre and gabbro, cut by dolerites, was also sampled (9 sites, 58 samples) despite its inferred Precambrian age (Rb-Sr determinations at 678 Ma). Finally, in order to constrain the duration of the Deccan volcanism, some of the younger and of the older flows (55 sites, 309 samples) were also collected.

Most of the doleritic dykes are fine to medium grained with typical mineral assemblages of plagioclase, augite, olivine, and Fe-Ti oxides. All the mafic dykes of Kerala plot within the sub-alkalic field but three groups can be distinguished chemically. Fe-Ti oxides are found either as early inclusions within pyroxene or as interstitial and may constitute the late crystallization phases. Selected specimens from each site were subjected to low and high temperature susceptibility measurements to define the magnetic carriers and the thermal stability of the samples. They have indicated titanomagnetite as the main carrier of magnetization. Palaeomagnetic measurements were carried out by detailed step-wise alternating field and thermal demagnetizations. After removal of sometime significant secondary components of magnetization, characteristic remanent magnetizations could be defined for most samples. The directions include both normal and reverse polarities. Some Ar/Ar age determinations have also been carried out. These Late Cretaceous results will be discussed in a geodynamic context.