Spinifex-Textured Peridotitic Komatiite from Honnabetta Area, Nagamangala schist Belt, Karnataka

Metaultramafite exposure in the newly cut Hemavathi canal section in Honnnbetta area in the Nagamangala belt exhibits excellent spinifex texture with komaliitic chemistry implying their volcanic origin. Though, several such localities of spinifex textured peridotitic komaliitcs (STPKs) have already been recorded from Ghattihosahalli, Kummanghatta, Banasandra, Mayasandra and Karighatta areas, this new find of plate and random spinifex textured ultmmafic variant from Honnabetta is the first report from the Nagamangala belt. They show yellowish green colour with skeletal forms of olivine blades, which are totally altered to talc-tremolite-chlorite, anthophyllite, iddingsite and antigorite. Parallel sheafs and aggregates of bladed olivine pseudomorphs form plate spinifex with criss crossing random orientation pattern. The primary igneous mineralogy consists of altered relict olivine and clinopyroxene phenocrysts. A few samples exhibit micro-spinifex texture with randomly oriented, elongated platy, pseudomorphs of olivine separated by tremolite in the groundmass. Gradational change from fine to coarse-grained spinifex with long crystals (rare) of clinopyroxene in a fine-gained groundmass of tremolite and chloriteis also observed. Medium-grained spinifex rockshow crystals upto 5 to 2 mm thick and 10 to 50 mm long in sheafs with parallel grouping and abut against each other randomly at angles varying from 40 to 60° enclosing triangular to rectangular interstitial talc, chforitc, pale green finegrained chlorite and glassy material. Ilmenite-magnetite and chrornite are the important accessory opaque minerals, with ilrnenite at places showing oriented picotite inclusions. Normative plots indicate that the STPKs of Honnabetta are hurzburgitic peridotites.

Compared to the Ghattihosahalli-Kummanghatta and Banasandra STPKs, tlonnabella STPKs show characteristic plate and random spinifex texture. In texture they are similar to Karighatta STPKs and in composition they compare well with Mayasandra STPKs with their low CaO:Al₂O₃ ratios. lnspite of alteration (hydration), deformation and metamorphism, preservation of primary igneous structures and textures is almost perfect in Honnabetta. Such a zone with a possible thermal erosion model for the Komatiitic types imply some sulphide bodies at depth, as incidence of sulphide associated with gold minerrrlisation arc observed in Honnabetta area.