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Emplacement age and isotope geochemistry of Sung Valley alkaline–carbonatite complex, Shillong Plateau, northeastern India: implications for primary carbonate melt and genesis of the associated silicate rocks
Authors:Rajesh K Srivastava  Larry M Heaman  Anup K Sinha  Sun Shihua
Institution:

aIgneous Petrology Laboratory, Department of Geology, Banaras Hindu University, Varanasi 221 005, India

bDepartment of Earth and Atmospheric Sciences, University of Alberta, Alberta, Canada T6G 2E3

cRCMRE, Chinese Academy of Sciences, Datum Road, Beijing 100101, PR China

Abstract:The early Cretaceous (Albian–Aptian) Sung Valley ultramafic–alkaline–carbonatite complex is one of several alkaline intrusions that occur in the Shillong Plateau, India. This complex comprises calcite carbonatite and closely associated ultramafic (serpentinized peridotite, pyroxenite and melilitolite) and alkaline rocks (ijolite and nepheline syenite). Field relationship and geochemical characteristics of these rocks do not support a genetic link between carbonatite and associated silicate rocks. There is geochemical evidence that pyroxenite, melilitolite and ijolite of the complex are genetically related. Stable (C and O) and radiogenic (Nd and Sr) isotope data clearly indicate a mantle origin for the carbonatite samples. The carbonatite var epsilonNd (+0.7 to +1.8) and var epsilonSr (+4.7 to +7.0) compositions overlap the field for Kerguelen ocean island basalts. One sample of ijolite has Nd and Sr isotopic compositions that also plot within the field for Kerguelen ocean island basalts, whereas the other silicate–carbonatite samples indicate involvement with an enriched component. These geochemical and isotopic data indicate that the rocks of the Sung Valley complex were derived from and interacted with an isotopically heterogeneous subcontinental mantle and is consistent with interaction of a mantle plume (e.g. Kerguelen plume) with lithosphere. A U–Pb perovskite age of 115.1±5.1 Ma obtained for a sample of Sung Valley ijolite also supports a temporal link to the Kerguelen plume. The observed geochemical characteristics of the carbonatite rocks indicate derivation by low-degree partial melting (not, vert, similar0.1%) of carbonated mantle peridotite. This melt, containing a substantial amount of alkali elements, interacted with peridotite to form metasomatic clinopyroxene and olivine. This process could progressively metasomatize lherzolite to form alkaline wehrlite.
Keywords:Carbonatite  Silicate rocks  Sung Valley  India  U–Pb perovskite Age  Sr and Nd isotopes  Primary carbonate melt  Kerguelen plume
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