Magnetic susceptibility and biotite composition of granitoids of Amritpur and adjoining regions, Kumaun Lesser Himalaya

Abstract

Felsic magmatism in Amritpur and adjoining localities of Kumaun Lesser Himalaya is represented by Palaeoproterozoic (~1890 Ma) Bt-Ms granitoids and quartz feldspar porphyries, referred to as Amritpur granitoids (AG) and Amritpur porphyry (AP) respectively. The AG are porphyritic and equigranular as well, and extend from Suraijula (west) to Barhoan (east) in the Nainital district of Kumaun Lesser Himalaya. The Main Boundary Thrust (MBT) forms the southern boundary of AG, and quartzite-metabasalts association is exposed in the north. The equigranular variety of AG, referred to as Amritpur leucogranite (ALG) dominates over porphyritic ones, and is medium- to coarse-grained, hypidiomorphic, mainly composed of K-feldspar (perthite), quartz and plagioclase (sericite). Muscovite is present associated with biotite but varies in proportion ranging from rare to noticeable amount. The magnetic susceptibility (MS) measurements of AP and AG were carried out in the field on smooth rock surfaces using hand-held SM-20 magnetic susceptibility meter. The obtained MS value (SI unit) was further corrected according to the degree of the rock-surface unevenness. Primary biotites from ALG were analyzed by wavelength dispersive electron-probe microanalysis. Ferrous and ferric iron from total iron (FeO) of biotite was estimated following charge-balance procedures. The MS values of AP vary from 0.399 to 0.912×10 -3 SI unit with an average of 0.528×10-3 SI (N=17), which typically represent ilmenite series (χ ≤3.0×10 -3 SI) granite (Ishihara 1977). Medium- grained ALG measures relatively lesser MS values (χ= 0.003- 0.148×10 -3 SI) with an average of 0.062×10 -3 SI (N=55) compared to coarse-grained ALG which vary from 0.025 to 0.195×10 -3 SI with an average MS of 0.117×10-3 SI (N=18), both being related to ilmenite series granite. Coarse-grained gneissic AG, exposed at places, record the MS values ranging from 0.295 to 0.527×10 -3 SI with an average MS value of 0.379×10 -3 SI (N=26). Porphyritic AG, an older lithounit, are xenolith-bearing and xenolith-free, and their MS values range from 0.368 to 0.629×10-3 SI and from 0.142 to 0.247×10 -3 SI with average MS of 0.522×10 -3 SI (N=10) and 0.190×10 -3 SI (N=10) respectively. The observed MS variations of porphyritic AG appear related with xenolith incorporation, which might have slightly oxidized the porphyritic AG. The MS values of AP and AG including the ALG typically correspond to ilmenite series granites, and the noted MS variations may be related with their differing contents of ferromagnesian minerals, ilmenite and textural variations. The AG including the ALG are typically peraluminous (S-type) consistent with their nature similar to ilmenite series granites. About 60% of whole-rock data (N=13) of AP (Gupta et al. 1994; Nautiyal and Rawat 1990) are metaluminous (I-type) despite of their exclusively ilmenite series nature. In Japan nearly all the ilmenite series are I-type, and it has been observed that magnetite-/ilmenite-series classification is not exactly equivalent to the I-/S-type classification in terms of alumina saturation index (ASI). The ALG biotites have ΣX variation between 1.02 and 1.25 atoms in their structural formulae, which are dominantly contributed by potassium. The observed ΣX values more than one in ALG biotites may be due to the replacement of K by the elements Ba, Rb and Cs. Biotites of ALG plot into the field of primary biotite crystallized in felsic magma but scatter at and above the Ni-NiO buffer. However, presence of ilmenite and substantially low MS values (0.003-0.157×10 -3 SI) of ALG suggest the prevalence of elevated reducing conditions during the evolution of ALG magma. The ALG melt was further reduced at emplacement level as evident by the presence of graphite pods and patches hosted in the ALG near the contact with country- rocks. The biotites of ALG are ferri-biotites showing enrichment in siderophyllite component, which is commonly available in crustally-derived felsic melt. The ALG biotites in terms of MgO- FeO t