科研之友
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摘要
A number of ENE-WSW trending Paleoproterozoic dykes and plugs of mafic, ultramafic, alkaline and carbonatite rocks intrude Mahakoshal supracrustal belt (MSB), which is a part of the Central Indian Tectonic Zone (CITZ). Best exposures of these intrusions are found in the eastern parts of the MSB, particularly in and around Chitrangi area. Many of these intrusions have greenschist facies mineral composition and show sharp contact with supracrustal rocks. However, igneous textures, such as porphyritic/glomeroporphyritic, are still preserved in the form of partly pseudomorphed olivines, phlogopites and pyroxenes. Striking feature observed in some ultramafic samples is the presence of melanite garnet and rounded or elliptical carbonate ocelli. The petrographic characteristics suggest occurrence of carbonate-rich ultramafic lamprophyres; close to aillikite composition. Coarse-grained carbonatites show hypidiomorphic texture and mostly composed of calcite with appreciable amount of silicate minerals like clinopyroxene, phlogopite and olivine (often pseudomorphed by calcite, amphibole and chlorite). It is difficult to establish any direct genetic relationship between carbonatite and ultramafic lamprophyre samples on the basis of their chemistry; they were likely derived from distinct parental melts. High Mg# (up to similar to 78), and high Ni and Cr contents (up to similar to 4700 and similar to 1100, respectively) and low HREE concentration in few ultramafic lamprophyre samples apparently suggest their derivation from a near-primary mantle-derived melts originated at great depths. Geochemistry and presence of carbonate ocellae in ultramafic lamprophyre samples suggest genesis of these silicate rocks and associated carbonatites through liquid immiscibility, however possibility of their derivation through vein-plus-wall-rock melting model cannot be ignored. A multi-stage veined mantle melting model is suitable in the latter case. It is suggested that early stages of rifting in the Mahakoshal region due to lithospheric thinning caused by possible plume activity provided suitable conditions for the genesis of ultramafic lamprophyre (possibly aillikitic) and carbonatitic melts which ultimately crystallized as dykes and plugs.
- 出版日期2013-6
