MINERALOGY AND GEOCHEMISTRY OF DETRITAL MAGNETITES FROM THE BEACH SANDS OF COXS-BAZAR, BANGLADESH, AND THEIR PETROGENETIC IMPLICATIONS

Ti-poor magnetites from Cox's Bazar beach are seen to be remarkably ho mogeneous, chemically and optically, containing less than 2% ulvospine l. Mg, Ti, Ct, Al and V, though in insignificant amounts, are found to be responsible in contributing to the reduction in hyperfine field of the Mossbauer spectrum, which showed linewidths of 0.18 mm/s at B-sit e and 0.26 mm/s at A-site. Certain fraction of these magnetites manife sts authigenic growth, presumably in a locally reduced condition. Inte rgrowth of ilmenite with titanomagnetites possibly resulted from oxida tion of ulvospinel-magnetite and a regular partitioning of Cr, V, Al, Mg and Mn observed between coexisting titanomagnetite and ilmenite sug gests a chemical equilibrium. The temperature and oxygen fugacity of c oexisting titanomagnetite-ilmenite phases as calculated ranged from 62 5-850 degrees C and 10(-15) to 10(-21) respectively. These temperature s and oxygen fugacities equilibrated in a T-fO(2), field very near to FMQ (Fayalite-Magnetite-Quartz) buffer curve, lending support to its g enesis in the upper crust, more being of igneous sources rather than m etamorphic.