| Abstrakt: |
Single-grain geochemistry of heavy minerals and its assemblage from beach sediments of Uttara Kannada coast, India, were studied to understand the provenance. The studied heavy minerals were identified under binocular microscope followed by electron probe micro analysis emphasising on spinel, rutile, tourmaline, ilmenite and zircon. Geochemistry of spinel from northern (Rabindranath Tagore) and central (Gokarna) zones suggests their island-arc volcanic origin, particularly peridotites implying the presence of ultramafic suite in the catchment area of Kali and Gangavali rivers. Tourmalines in Devbhag beach from the northern zone are schorl type confirming granitoids as their source rock, while from Rabindranath Tagore beach are dravite type suggesting their metamorphic origin. Conversely, central zone tourmalines are both schorl and dravite type exhibiting dual origin. Rutile geochemistry from northern zone depicts its derivation from metamorphic source. Heavy mineral assemblage of kyanite/sillimanite, rutile, zircon, garnet and staurolite in the southern zone indicates its origin from metamorphic sources. However, the absence of high-grade metamorphic rocks in catchment area of river Sharavati precludes the derivation of garnet and staurolite from this source. Therefore, the presence of such minerals in southern zone may have been brought from further south by northerly alongshore drift where high-grade metamorphic rocks are dominant. Minor heavies-hematite, V-hematite, magnetite, Ti-magnetite, V-magnetite, Ti–V-magnetite present in all three zones reflect their origin from iron ore bodies in the hinterland. From the geological map of Uttara Kannada district, an apparent correlation between hinterland lithology and occurrences of heavy minerals in beach sediments is observed which indicates their derivation from igneous and low-grade metamorphic suites, high-grade metamorphic minerals like garnet and staurolite suggest their derivation from outside. Despite the present work having been carried out in one particular area, the results presented and discussed in this article have wide applicability. The knowledge of sediment input from different sources (river, offshore or alongshore) and transport pathways which play a significant role in making up the composition of beach sands are useful for exploration and/or coastal management studies. |
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