Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Santosh Kumar ¹, Brajesh Singh ¹, Charu C. Josh ¹, Abhishek Pandey ¹

Affiliations
1 Department of Geology, Kumaun University, Nainital - 263 002, Uttranchal, IN

Abstract

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.

Keywords

Palaeoproterozolic Granitoids, llmenite Series, Biotlite Composition, Amritpur, Kumaun Lesser Himalaya.

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Santosh Kumar ¹, S. Raju ², Manjari Pathak ¹, Abhishek Pandey ³

Affiliations
1 Department of Geology, Kumaun University, Nainital - 263 002, IN
2 Geological Survey of India, Dimapur - 797 112, IN
3 Rolta Academy, MIDC, Marol, Andheri East, Mumbai - 400 093, IN

Abstract

Late Archaean to Palaeoproterozoic felsic magmatic lithounits exposed in the central part of the Bundelkhand massif have been mapped and their redox series (magnetite vs ilmenite series) evaluated based on magnetic susceptibility (MS) data. The central part of Bundelkhand massif comprises of multiple felsic magmatic pulses (∼2600-2200 Ma), commonly represented by coarse grained granite (CGG-grey granite, CPG-pink granite), medium grained pink granite (MPG), fine grained pink granite (FPG) , grey and pink rhyolites and granite porphyry (GP). However, the pink colour of these felsic rocks is the result of hydrothermal fluid-flushing leading to potassic alteration of grey granites. MS values of CGG vary from 0.058 to 14.75×10^-3 SI with an average of 6.35×10^-3 SI, which mostly represent oxidized type, magnetite series (73%) granites involving infracrustal (igneous) source materials. CPG (av. MS=3.95×10^-3 SI) is indeed a pink variety of CGG, the original oxidizing nature of which must have been similar to the bulk of CGG, but has been moderately to strongly reduced because of distinctly more porphyritic nature together with partial assimilation of metapelitic (supracrustal) materials, surmicaceous enclaves, carbonaceous material included in the source materials, and to some extent, induced by hydrothermal and later deformational processes. MPG (av. MS= 1.15×10^-3 SI) as lensoidal stock-like bodies intrudes the CPG and represent both magnetite series (18%) and ilmenite series (82%) granites, which are probably formed by heterogeneous (mixed) source rocks. GP (av. MS=6.26×10^-3 SI) occur as dykes (mostly trending NE-SW) intrudes the MPG, CPG and migmatites and bears the nature similar to oxidized type, magnetite series granite. FPG (av. MS= 0.666×10^-3 SI) trending NE-SW occur as lensoid bodies including a large outcrop, is intrusive into both CPG and MPG, and is moderately to very strongly reduced type, ilmenite series granites, which may be derived by the melting of metapelitic crustal sources. FPG hosting microgranular (mafic magmatic) enclaves commonly exhibit high MS values (7.31-10.22×10^-3 SI), which appear induced by the mixing and mingling of interacting felsic and mafic magmas prevailed in an open system. Grey (av. MS=10.30×10^-3 SI) and pink (av. MS=6.72×10^-3 SI) rhyolites represent oxidized type, magnetite series granites, which may have been derived from infracrustal (magmatic) protoliths. Granite series evaluation of felsic magmatic rocks of central part of Bundelkhand massif strongly suggests their varied redox conditions (differential oxygen fugacity) mostly intrinsic to magma source regions and partially modified by hydrothermal and tectonic processes acting upon them.

Keywords

Magnetic Susceptibility, Magnetite Series, Ilmenite Series, Granitoids, Bundelkhand Massif, Central India.

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