Alkaline rocks of Samchampi-Samteran, District Karbi-Anglong, Assam, India |
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| Authors: | S Nag S K Sengupta R K Gaur A Absar |
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| Institution: | (1) Geological Survey of India, 226 024 Lucknow, India;(2) Geological Survey of India, 781 021 Guwahati, India |
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| Abstract: | The Samchampi-Samteran alkaline igneous complex (SAC) is a near circular, plug-like body approximately 12 km2 area and is emplaced into the Precambrian gneissic terrain of the Karbi Anglong district of Assam. The host rocks, which
are exposed in immediate vicinity of the intrusion, comprise granite gneiss, migmatite, granodiorite, amphibolite, pegmatite
and quartz veins.
The SAC is composed of a wide variety of lithologies identified as syenitic fenite, magnetite ± perovskite ± apatite rock,
alkali pyroxenite, ijolite-melteigite, carbonatite, nepheline syenite with leucocratic and mesocratic variants, phonolite,
volcanic tuff, phosphatic rock and chert breccia.
The magnetite ± perovskite ± apatite rock was generated as a cumulus phase owing to the partitioning of Ti, Fe at a shallow
level magma chamber (not evolved DI = O1). The highly alkaline hydrous fluid activity indicated by the presence of strongly
alkalic minerals in carbonatites and associated alkaline rocks suggests that the composition of original melt was more alkalic
than those now found and represent a silica undersaturated ultramafic rock of carbonated olivine-poor nephelinite which splits
with falling temperature into two immiscible fractions—one ultimately crystallises as alkali pyroxenite/ijolite and the other
as carbonatite. The spatial distribution of varied lithotypes of SAC and their genetic relationships suggests that the silicate
and carbonate melts, produced through liquid immiscibility, during ascent generated into an array of lithotypes and also reaction
with the country rocks by alkali emanations produced fenitic aureoles (nephelinisation process). Isotopic studies (δ18O and δ13C) on carbonatites of Samchampi have indicated that the δ13C of the source magma is related to contamination from recycled carbon. |
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| Keywords: | Alkali pyroxenite ijolite-melteigite carbonatite phonolite aegirine augite aegirine sodi-calcic amphibole pyrochlore monazite phlogopite scapolite major element chemistry δ 18O and δ 13C isotopic studies liquid immiscibility fractional crystallisation |
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