Magnetic Susceptibility and Biotite Composition of Granitoids of Amritpur Region, Kumaun Lesser Himalaya: Implication on Granite Series Evaluation and Nature of Felsic Magma

Felsic magmatism in Amripur and adjoining localities of Kumaun Lesser Himalaya is represented by Palaeoproterozoic (ca 1890 Ma) Bt-Ms granitoids and quartz feldspar porphyries, referred to as Amritpur granitoids (AG) and Amripur porphyry (AP) respectively Magnetic susceptibility( MS) values (range of average ϰ=0062 to 0 522x10³ SI) measured for various types of AG correspond to ilmenite-series (reduced type) granites, which are consistent with their peralumnous nature. The MS of AP ranges from 0 399 to 0 912x10³ SI with an average MS of 0 528x10³ SI, which are typical to ilmenite series granite, although AP represents both metaluminous and peraluminous types Compositions of biotite from Medium-Grained, leucocratic equigranular variety of AG, referred to as Amritpur leucogranite (ALG), are primary in nature, and can be classified as Fe²⁺ (ferri)biotite enriched in siderophyllite component FeO^t/MgO of biotites from AtG vanes from 2 93 to 4 94 which typically crystallized into crustally derived, peraluminous (S-type) felsic magma, with probable 3Fe ⇋ 2AI substitution in biotites during ALG melt evolution but 3Mg⇋2AI substitution cannot be inferred Bulk Fe₂O₃,/FeO ratio of most AG corresponds to llmenite series (Fe₂O₃/FeO<O 5) consistent with the observed MS values whereas AP samples equally represent both llmemate and magnetite (Fe₂O₃/FeO > 0 5) senes. The results strongly favour that the AG magma evolved under reducing environment.