Petrology and geochemistry of the mafic dyke rocks from precambrian almora crystallines of Kumaun Lesser Himalaya

Abstract

Mafic dykes of Almora region intrude the Precambrian crystalline rocks of Kumaun Lesser Himalaya. Mafic dykes exhibit fine grained margin and medium to coarse grained core, melanocratic, low to highly ferromagnetic (MS=0.85-38.58×10 -3 SI) in nature commonly showing subophitic to ophitic textures with ol-pl-cpx-hbl-bt-mt-ap-sp assemblage, and modally correspond to leucogabbro and olivinegabbro (sensu stricto). Olivine (Fo 61 -Fo 33 ), clinopyroxene (Wo 46 -En 42 -Fs 22 to Wo 40 -En 36 -Fs 15 ) and plagioclase (An 58 -An 12 ) have crystallized in the temperature range of ca1400-980 o C at pressure <2 kbar in an olivine tholeiitic basalt parent. Low acmite (Na pfu =0.033-0.025), (Mg # =0.64-0.82), Ti-Al contents of clinopyroxenes and their evolution along enstatite-ferrosilite join (i.e. MgFe substitution) strongly suggest tholeiitic nature of mafic dyke melt with changing activities of alumina and silica. Clinopyroxene compositions of mafic dykes differ markedly as compared to those observed for adjoining Bhimtal volcanics but closely resemble to that crystallized in tholeiitic melts of Deccan province. Observed Cr vs Mg # variation, enriched LILE (Sr, Ba)-LREE and positive Eu-anomaly of the studied mafic dykes are indicative of fractional crystallization of olivine-clinopyroxene -plagioclase from a crustally-contaminated tholeiitic basalt magma derived from enriched mantle source. The mafic dykes of Almora are geochemically identical to mafic dykes of Nainital, but are unrelated to Precambrian mafic volcanic flow and dykes of NW Himalaya and dykes of Salma and Rajmahal regions.