Magnetic susceptibility mapping of felsic magmatic lithounits in the central part of Bundelkhand Massif,central India |
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| Authors: | Santosh Kumar S Raju Manjari Pathak Abhishek Pandey |
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| Institution: | 1.Department of Geology,Kumaun University,Nainital,India;2.Geological Survey of India,Dimapur,India;3.MIDC,Rolta Academy,Mumbai,India |
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| 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. |
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