Mihir K. Bose

Sedimentation pattern and tectonic evolution of the Proterozoic Singhbhum Basin in the eastern Indian shield

Abstract The Proterozoic Singhbhum Basin (PSB) located on the flank of an Archaean platform (ca. 3400 Ma) is a distinct lithotectonic unit of the Singhbhum crustal province of the eastern Indian shield. The present contribution traces the evolution of this basin filled by volcanics-dominated submarine supracrustals and sedimentary formations. The Proterozoic Singhbhum Basin has been identified to be a marginal basin evolved in a back-arc setting. An axial volcanic belt manifesting MOR-like basaltic eruptions divided the basin into two domains with parallel but distinct sedimentation patterns. An asymmetry in sedimentation pattern and source polarity are documented in litho-as-semblages and REE distribution patterns of cover rocks in the basinal domain flanking the southern platform. Prevailing tectonics strongly controlled the mechanism of sedimentation which, coupled with the volcanic history of the basin, indicates its riftogenic character. Progressive lithospheric stretching successively produced picritic volcanics and MOR-type basalts along the axial zone of the basin, being related to a stretching factor of ⩾ 2. Continental margin rifting ultimately resulted in an oceanic transition as supported also by available geophysical data. The evolution of the Proterozoic Singhbhum Basin in its broad perspective was controlled by subduction-related processes.

Evaluation of the tectonic setting of precambrian dalma volcanic belt, eastern India, using major and trace element characters

Geochemical characterisation of Dalma basalt, the upper member of a volcanic belt occupying the spine of a Precambrian basin, has been used to elucidate the eruptive environment. Dalma basalt in general is high magnesian in character as well as depleted in incompatible elements. The major and trace element distributions are strongly indicative of ocean-ridge magmatism with a feeble impression of arc volcanism. Such a transitional tectonic environment, corroborated by the overall geologic setting of the volcanic belt, is strongly indicative of a marginal basin situation during Proterozoic time in this part of the Indian shield.

Petrology and geochemistry of the igneous complex of Mount Girnar, Gujarat, India

The Girnar massif of Kathiwar plateau, Western India, occurs within the Deccan volcanic province and manifests a wide range of fractional crystallisation of the parent basic magma. Mineralogical variation in the suite comprising: gabbro-diorite-lamprophyric rock-nepheline syenite, has been studied in detail. The alkalinity of the suite is pronounced after crystallisation of gabbro and increases with fractionation of the magma. The history of fractional crystallisation has been influenced by growing water content of the magma followed by a depletion of water in the final residual liquid. Silicic porphyries developed in close spatial association with the Girnar massif, are established to have no genetic relation with the alkalic suite. Chemical variation for both major and minor elements in the suite is discussed.

Petrological observations on alkali syenites of Kunavaram, A. P., India

The alkali syenite assemblage of Kunavaram, Khammam district, Andhra Pradesh in south-eastern India, comprises perthite syenite and nepheline syenite. The massive perthite syenite (locally grading to alaskite) is fringed on either side by foliated nepheline syenite, the latter developing composite gneisses along the contact zones with the country rocks. The syenites are essentially hypersolvus although the alkali feldspar, the dominant constituent of the rocks, shows varying degrees of unmixing and Al/Si order. The mafic constituents (e.g. sodic pyroxene, amphibole and biotite) are appreciably rich in ferrous iron. Mineralogical and chemical data suggest that the primary crystallization characteristics of the syenites were not totally obliterated during the post-magmatic history of the pluton.

Petrology of the nepheline syenite of Mount Girnar, India

Hypersolvus nepheline syenite and haplosyenite, in close spatial and temporal association with lamprophyric rock, occur as small intrusions in the gabbrodiorite pluton of Mount Girnar, Gujrat, Western India. The syenite crystallized in a relatively dry condition from a residual liquid following crystallization of lamprophyric rock with profuse hydrous mafic phases. Distribution of some of the more common trace elements with respect to the major elements in the syenites has been interpreted.

K-Rb relations in the alkaline suites of the Eastern Ghats precambrian belt, India

K-Rb relations in the major alkaline suites from the Eastern Ghats Precambrian belt of India have been studied. All the suites show progressive enrichment of Rb with differentiation, but the Sivamalai suite with a dominance of feldspathoidal members has extremely low Rb concentrations. The K/Rb ratios, except those for Kunavaram, lie above the MT for average igneous rocks. Possible cause for fluctuation in K/Rb is discussed. The high K/Rb ratio is a possible inherent character of the magmas corresponding to their derivation from deeper parts of the upper mantle.

On garnet coronites from Koraput, Orissa

In the gabbro-anorthosite at Koraput, India, coronites are commonly developed. The garnet coronite found in the northwestern part occurs as a dark band with bluish-gray feldspar laths, pink garnet, and darker mafic minerals (pyroxene, amphibole, and biotite). The mineralogy is described and the petrogenesis discussed. The Koraput coronas are not late magmatic; they probably mark a particular stage in the metamorphic evolution of the gabbros.

Inter-element relations in some alkaline suites of the Eastern Ghats Precambrian belt

Element interrelations, with particular emphasis on alkaline earth metals, have been studied quantitatively for three alkaline suites of the Eastern Ghats Precambrian belt. Geochemical characterisation brings the Koraput and the Kunavaram suites closer, relative to the Elchuru suite. K-Ba and K-Rb correlations vary during the fractionation process, being strongly positive for the early members and almost noncorrelatable for the late fractions. The covariant relation between Ba and Sr is not well developed in any of the suites. Significant positive correlation between Rb and the degree of differentiation has been observed for the Koraput and the Kunavaram suites but not for the Elchuru suite. Liappears to be fractionated with the early mafic phases and is negatively correlated with Na. Zr shows a significant positive correlation with differentiation in the Elchuru but not in the Koraput suite although Ti/Zr falls remarkably with advancing differentiation for both the suites. P and Ti are mutually positively correlated in all the three suites and both tend to manifest significant negative correlation with progressive fractionation. K-(P + Ti + Sr) seems to be a good indicator of the fractionation process in the suites investigated.