CHROME-SPINEL CONSTRAINTS ON PROVENANCE HISTORY OF CENOZOIC SEDIMENTS FROM ASSAM, NORTHEAST INDIA

The Digboi-Margherita area of Assam, NE India, near the eastern Himalayan syntaxis and Indo-Burman ranges, has experienced multiple convergence of the Indian plate with Eurasia, resulting in accumulation of several kilometers of sediment since the Eocene. Chemical analyses of chrome spinels from the Cenozoic sequences help decipher their source and its tectonic setting.

Chrome spinel, an accessory mineral in most mafic rocks, provides a key index of the source rocks. The lack of both high-Al and high-Mg spinels in the Digboi-Margherita sequence suggests strongly that the detrital spinels of the Assam basin did not originate in a mid-ocean-ridge source. The Assam chrome spinels are similar in composition (podiform chromites and harzburgites) to those of marginal basin and island-arc igneous suites, such as Alpine-type peridotites emplaced by obduction of marginal-sea lithosphere and back-arc crust onto continental crust.

Type I ophiolites are from MORB settings with low-Cr, high-Mg, high-Al, while Type III ophiolites are from arc-related settings with high-Cr, low-Mg, low-Al. Type II ophiolites are intermediate in composition. Assam basin spinels are compositionally similar mostly to those of type III ophiolites with some similarity to Type II ophiolites. The very location of the Assam foreland basin suggests that the spinels may have been derived from Himalayan arc material, Himalayan ophiolites, and/or Indo-Burmese ophiolites. Based on Ti content, continental flood basalts of Deccan Traps and the Rajmahal volcanics are not considered likely sources of the Assam spinels. Low-Al (Cr # 0.30 – 0.75) and low-Mg (Mg # 0.40 to 0.71) content and low-Ti (TiO2 wt. % <1.0%) suggest that the Assam basin chrome spinels most likely were derived from Alpine-type ophiolites exposed in the Indo-Burman ranges. These ophiolites are highly dismembered and form a belt, 2-15 km wide, resembling an ophiolitic mélange. They were obducted between the Jurassic and the Eocene, with deformation continuing until the late Oligocene when the Indo-Burman ranges attained their present configuration.