ABSTRACT We conducted a detailed rock magnetic and mineralogical study of bole beds from the Deccan magmatic province, India. Magnetic susceptibility of 15 bole beds showed two contrasting patterns, with susceptibility values either increasing or decreasing up the profile. We then focused on two representatives red boles located in the Western Ghats, the RBB and RBAN profiles, to unravel the nature and origin of these contrasting magnetic susceptibility patterns. The presence of smectite argues against significant secondary thermal alterations. Major-elemental compositions obtained by X-ray fluorescence spectrometry of RBB and RBAN red boles are comparable to the parent basalt and show significant and typical depletion of mobile elements such as sodium and calcium compared to the parent basalt. The Ti/Al ratio of both the red boles and their overlying clay layers is close to the typical value of Deccan basalt (0.2), suggesting that the material of the red boles has been derived from weathering of the parent basalt. The chemical index of alteration varies from 40–50 in the parent basalt to 80–90 at the top of the bole beds, consistent with moderate to intense weathering of the bole beds. However, similar to other Deccan bole beds, indices of lateritization below 50 suggest that the state of lateritization has not been reached. Although the RBB and RBAN profiles share similar mineralogical signatures, their magnetic mineral assemblages are distinctly different. In the RBB profile, magnetic susceptibility decreases up-profile as a result of oxidation/dissolution of primary titanomagnetite inherited from the parent basalt, with subsequent formation of pedogenic hematite and superparamagnetic particles. In contrast, magnetic susceptibility in the RBAN profile, which contains magnetite, some hematite, and goethite, increases up-profile. The increase in the magnetic signal is mainly due to the increasing amounts of phyllosilicate and goethite, while the content of magnetite and hematite remains constant along the profile. We attribute the variation in the magnetic mineral assemblage to contrasting humid and dry environments during weathering, leading to the preferential formation of goethite or hematite, respectively. The combined mineralogical and rock magnetic data suggest the existence of a single weathering profile involving soil formation in the two studied red boles, with few or no contributions from an external source.

From the mantle plume model it would be expected that one to a few kilometers of regional, domal lithospheric uplift occurred 5–20 m.y. before the onset of flood basalt volcanism. This uplift resulted from heat conduction out of and dynamic support by the hot, buoyant, rising plume head. Field evidence for such uplift would comprise sedimentary sequences that reflect progressive basin shallowing before volcanism or (in the case of differential uplift along faults) widespread conglomerates derived from the basement rocks and underlying the first lavas. Local uplifts and subsidences cannot be used to invoke or rule out plume-caused uplift. Over large areas of the Late Cretaceous Deccan flood basalt province, the base of the lava pile is in the subsurface. Basalt-basement contacts are observed along the periphery of the province and in central India (the Satpura and Vindhya ranges), where substantial post-Deccan uplift is evident. Here, extensive horizontal Deccan basalt flows directly overlie extensive low-relief planation surfaces cut on various older rocks (Archean through Mesozoic) with different internal structures. Locally, thin, patchy Late Cretaceous clays and limestones (the Lameta Formation) separate the basalts and basement, but some Lameta sediments are known to have been derived from already erupted Deccan basalt flows in nearby areas. Thus, the eruption and flowage of the earliest Deccan basalt lava flows onto extensive flat planation surfaces developed on varied bedrock, and the nearly total absence of basement-derived conglomerates at the base of the lava pile throughout the province, are evidence against prevolcanic lithospheric uplift (both regional and local), and thereby the plume head model. There has been major (∼1 km) post-Deccan, Neogene uplift of the Indian peninsula and the Sahyadri (Western Ghats) Range, which runs along the entire western Indian rifted margin, well beyond the Deccan basalt cover. This uplift has raised the regional Late Cretaceous lateritized surface developed on the Deccan lava pile to a high elevation. This uplift cannot reflect Deccan-related magmatic underplating, but is partly denudational, is aided by a compressive stress regime throughout India since the India-Asia collision, and is possibly also related to active eastward flow of the sublithospheric mantle. The easterly drainage of the Indian peninsula, speculated to be dome-flank drainage caused by the plume head, predates the uplift. Field evidence from the Deccan and India is in conflict with a model of plume-caused regional uplift a few million years before the onset of volcanism.