Mineral chemistry and geochemistry as proxy for petrogenetic evaluation of charnockites: evidences from Marandahalli, Salem Block of Southern India

The present work highlights the petrogenesis of charnockites from the Marandahalli, Dharmapuri District, Tamil Nadu that are conformably emplaced into an older basement of the Precambrian Peninsular Gneissic Complex (PGC). These charnockites form a part of the southern granulitic terrain in the Salem Block (SB) of Tamil Nadu that is marked by diverse lithology that ranges from ultramafic rocks, syenites, quartz-syenites, carbonatites to granites. The charnockites are medium to coarse grained, in-equigranular, displaying porphyroblastic and granophyric textures, and are composed of felsic minerals like plagioclase feldspars (~ 49–66 vol.%), K-feldspar (~ 1–5 vol.%) and quartz. The mafic minerals are dominated by pyroxene (~ 2–7 vol.%) and biotite (~ 1–12 vol.%), and are marked by the absence of garnet. The accessory minerals include magnetite and apatite. The chemistry of these rocks show a distinctive calc-alkaline signatures marked by high SiO 2 (~ 65–68 wt.%), Na 2 O > K 2 O, with a marginally high CaO (~ 3–5 wt.%), MgO (~ 2.5-4 wt.%) and FeO (~ 1.5-5 wt.%), that are typical of charnockite rocks and they are surmised to have originated from the amphibolite source material and of magnesian nature. The LREE-rich and HREE-depleted patterns indication low degree of partial melting. V, Rb and Ni trace element of the Marandahalli suggest the role of partial melting in the genesis of these rocks. In summary, from the studies carried out it is envisaged that the Marandahalli charnockites melts were derived from amphibolitic sources that are garnet-free.