Articles written in Journal of Earth System Science
Volume 119 Issue 5 October 2010 pp 675-699
The Samchampi –Samteran alkaline complex occurs as a plug-like pluton within the Precambrian granite gneisses of Mikir Hills,Assam,northeastern India and it is genetically related to Sylhet Traps.The intrusive complex is marked by dominant development of syenite within which ijolite – melteigite suite of rocks is emplaced with an arcuate outcrop pattern.Inliers of alkali pyroxenite and alkali gabbro occur within this ijolite –melteigite suite of rocks.The pluton is also traversed by younger intrusives of nepheline syenite and carbonatite.Development of sporadic,lumpy magnetite ore bodies is also recorded within the pluton.Petrographic details of the constituent lithomembers of the pluton have been presented following standard nomenclatorial rules.Overall pyroxene compositions range from diopside to aegirine augite while alkali feldspars are typically orthoclase and plagioclase in syenite corresponds to oligoclase species.Phase chemistry of nepheline is suggestive of Na-rich alkaline character of the complex.Biotite compositions are typically restricted to a uniform compositional range and they belong to ‘biotite ’ﬁeld in the relevant classiﬁcation scheme.Garnets (developed in syenite and melteigite)typically tend to be Ti-rich andradite,which on a closer scan can be further designated as melanites.Opaque minerals mostly correspond to magnetite.Use of Lindsley ’s pyroxene thermometric method suggests an equilibration temperature from ∼450°$–$600°C for melteigite/alkali gabbro and ∼400° C for syenite.Critical assessment of other thermometric methods reveals a temperature of equilibration of ∼700°$–$1350°C for ijolite –melteigite suite of rocks in contrast to a relatively lower equilibration temperature of ∼600° C for syenite. Geobarometric data based on pyroxene chemistry yield an equilibration pressure of 5.32 –7.72 kb for ijolite,melteigite,alkali pyroxenite,alkali gabbro and nepheline syenite.The dominant syenite member of the intrusive plug records a much higher (∼11 kb)equilibration pressure indicating a deeper level of intrusion.Major oxide variations of constituent lithomembers with respect to differentiation index (D.I.)corroborate a normal magmatic differentiation.A prominent role of liquid immiscibility is envisaged from ﬁeld geological,petrographic and petrochemical evidences. Tectonic discrimination diagrams involving clinopyroxene chemistry strongly suggest within plate alkaline affinity for the parental magma which is in conformity with the regional plume tectonics.
Volume 120 Issue 3 June 2011 pp 459-473
The Mylliem granitoids of the Meghalaya Plateau, northeastern India, represent one of the disharmonic Neoproterozoic igneous plutons, which are intrusive into low-grade Shillong Group of metasediments. Field studies indicate that the Mylliem granitoids cover an area of about 40 km2 and is characterized by development of variable attitude of primary foliations mostly marked along the margin of the pluton. Xenoliths of both Shillong Group of metasediments and mafic rocks have been found to occur within Mylliem granitoids. Structural study of the primary foliation is suggestive of funnel-shaped intrusion of Mylliem granitoids with no appreciable evidence of shearing. Petrographically, Mylliem granitoids are characterized by pink to white phenocrysts of prismatic microcline/perthite and lath-shaped plagioclase (An20$–$An29). Groundmass material is characterized by quartz, microcline, plagioclase, muscovite and biotite. Sphene and apatite occur as accessory minerals. Petrographically Mylliem granitoids have been discriminated as granite and granodiorite according to IUGS system of classification.
Critical evaluation of geochemical data and variation trends of major oxides/trace elements suggests a significant role of fractional crystallization in the evolution of Mylliem pluton. Th/U ratios (3.22–6.77) indicate a relatively higher abundance of Th over U. Chondrite-normalized REE diagram characteristically shows an enriched LREE pattern and prominent negative Eu anomaly (Eu/Eu* = 0.16–0.42) indicating the significant role of plagioclase fractionation from the parent magma. An overall strong REE fractionation pattern has been envisaged for Mylliem granitoids. The strong REE fractionation of the Mylliem granitoids is depicted by (Ce/Yb)$_N$ values, which show a range of 1.39 to 1.65. The aluminium saturation index (ASI) (ranging from 1.0 to 1.3), A/CNK ratios (ranging from 1.4 to 2.11) and A/NK ratios (ranging from 1.75 to 2.43) provide evidences for the peraluminous, S-type nature of the Mylliem granitoids. The peraluminous, S-type character is further supported by geochemical parameters such as Fe* and MALI (modified alkali lime index). Normative corundum < 1.0 wt.% is suggestive of the S-type nature of Mylliem granitoids. This is indicative of parent melt-extraction from metasedimentary source rocks by partial melting. Distinct geochemical parameters suggest a post-orogenic tectonic environment for the Mylliem granitoids. The peraluminous, calc-alkalic to alkali-calcic, post-orogenic Mylliem granitoids are geochemically correlatable with the post-orogenic Caledonian granitoids of Ireland and Britain.
Volume 121 Issue 1 February 2012 pp 91-108
Several basaltic lava flows have been identified in the study area in and around Linga, in the Eastern Deccan Volcanic Province (EDVP) on the basis of distinctly developed structural zones defined by primary volcanic structures such as columnar joints and vesicles. These basaltic lava flows are spatially distributed in four different sectors, viz., (i) Bargona–Gadarwara (BG) sector (ii) Shikarpur–Linga (SL) sector (iii) Arjunvari–Survir Hill (AS) sector and (iv) Kukrachiman–Morand Hill (KM) sector. A threetier classification scheme has been adopted for the characterization and classification of individual lava flows. Each lava flow consists of a Lower Colonnade Zone (LCZ) overlain by the Entablature Zone (EZ) and Upper Colonnade Zone (UCZ). The LCZ and UCZ grade into a distinct/indistinct Lower Vesicular Zone (LVZ) and Upper Vesicular Zone (UVZ), respectively. The LCZ and UCZ of the flows are characterized by columnar joints while the EZ is marked by multi-directional hackly jointing. The geometry of different joint patterns corresponds to different styles of cooling during solidification of lava flows. Detailed petrographic studies of the investigated lava flows reveal inequigranular phenocrystal basalts characterized by development of phenocrystal phases including plagioclase, clinopyroxene and olivine, whereas groundmass composition is marked by tiny plagioclase, clinopyroxene, opaque mineral and glass. Electron microprobe analyses indicate that the olivine has a wide range ∼Fo22 to Fo66 revealing a wide spectrum of compositional variation. Pyroxene compositions are distinctly designated as Quad pyroxenes. Phenocrystal pyroxenes are mostly diopsidic, while the groundmass pyroxenes mainly correspond to augite with a minor pigeonite component. Pyroxene phenocrysts are characterized by a prominent Tienrichment. Phenocrystal plagioclase grains are calcic (An52.7$–$An72.9), whereas groundmass plagioclase are relatively sodic (An39.2$–$An61.6). Groundmass opaque minerals are characteristically found to be Ti–magnetite/ilmenite/pyrophanite. Pyroxene thermometry reveals a temperature span of 850° to 1280°C for the studied lavas while olivine–clinopyroxene thermometry yields a temperature range from 1040°$–$1160°C. The variation of temperature for the lava flows is ascribed to their normal cooling history after eruption.