PETROGENESIS OF CARBONATITE AND ASSOCIATED ALKALINE ROCKS, PURULIA, W.B., INDIA

Abstract

The carbonatite and other alkaline rocks are exposed at Sushina, Chirugora-Purdaha, Kutni- Dandodih-Gamardih, Mednitanr and Beldih from east to west along the Northern Shear Zone (NSZ). One of the study areas, Beldih (86°1l'E to 86°23'E, 23°00'N to 23°07'N) is situated 38 Km south of the Purulia town, where the carbonatite-alkali-pyroxenite association occur within the host rock chlorite-phyllite and chlorite-schist. The other study area Sushina (22°57'N, 86°37E) lies about 40 Km west of Beldih, where the varieties of nepheline-syenite is exposed along a lenticularshaped NW-SE running hill. Carbonatite around Beldih is essentially composed of calcite and minor apatite with amphibole, biotite and magnetite as common accessories. It is characterized by the presence of two varieties of amphibole: richterite and magnesiokatophorite, which are low-Al, sodic-calcic in composition. The amphiboles are characteristic of middle to late stage development of carbonatite. The co-existence of these two types of amphibole indicates rapid ascent of the carbonatite magma along NSZ. The carbonatites are found to be calico-carbonatite and enriched in Sr (8500-11200 ppm), Ba (797- 1784 ppm) and SREE (1567-2114). The Nb and U concentrations are variable and primitive mantle normalized plot reveals that the Purulia carbonatite is relatively poorer in Rb (0.30 times), Zr (0.40 times) and Hf (0.80 times) while enriched in Sr (500 times), La (600 times), Ce (500 times) and Sm (150 times) and Nb (13 times for two samples and about 100 times for other two). However, Nb shows prominent negative anomaly when it is compared to average carbonatite and the pattern is characteristics of Precambrian carbonatites. Based on low HFSE content a carbothermal fluid derivation could be postulated. The Grenvillian metamorphic event resulted post-magmatic changes in the carbonatite as reflected from their bulk rock (Rb-Sr) isotopic studies. This shows that the measured 87Sr/86Sr ratio of the bulk rock and apatite (0.70340) is well within the range of mantle values. The present study rather support genesis of Purulia carbonatite from a parent primary carbonatitic magma than genesis from a carbothermal fluid. Moreover the 87Sr/86Sr ratio suggests that the existence of long-lived heterogeneity in the mantle during the time of formation of these rocks which is well in accord with the world-wide mantle heterogeneity during 2800 to 1000 Ma period. Pb-Pb model age suggest that the Purulia carbonatite is at least > 1.37 Ga of age. The alkali-pyroxenite, associated with the carbonatite had undergone extensive alkalimetasomatism during carbonatite intrusion. Such alkali-metasomatism is exemplified by two major changes: alteration of the primary mineral assemblages and formation of a vein-filling assemblage. The alteration process of the primary magmatic assemblage resulted transformation of pyroxene to amphibole (taramite/katophorite) and biotite respectively. The vein-filling assemblage is dominated by the calcite and apatite. Along with the apatite and calcite, albite is also found within the vein filling assemblage indicating dominance of sodic metasomatism. On the other had introduction of K resulted transformation of pyroxene and amphibole to biotite. The alteration process was accompanied by the introduction of incompatible element, including REE, in the system. The role of fluoride complexing was dominant as evident from the higher concentration of Y, REE in the apatite of the alkali-pyroxenite. The overall pressure is estimated to be around 2-3 kbar or less with temperature ranging around 400-450°C. They02-condition was low and suppressed by presence of CO2and alkalis. Two varieties of under-saturated syenite (banded and massive syenite) are found in the Sushina area. Based on the intensity of banding, the banded syenite can further be subdivided into two sub categories: poorly and strongly banded syenite gneiss. Both the varieties of banded syenite are essentially composed of albite, orthoclase, nepheline and aegirine. The strongly banded syenite gneiss characterized by the presence eudialyte and complex sodium-zirconium silicates. The massive syenite is characterized by the complete absence of mafic constituents except few late stage Mn-rich biotites. Pure end member compositions of the feldspars along with the nepheline compositions converging towards the Morozewicz-Buerger field of plutonic/metamorphicmetasomatic assemblage suggest that the banded variety of syenite represent a recrystallized mineral assemblage and can be best termed as nepheline gneisses. Textural and mineralogical study reveals that these rocks of miaskitic character were originally emplaced under plutonic condition and pervasive subsolidus or deuteric alteration related to autometasomatism changed the original miaskitic composition towards the agpaitic one. The metamorphic effect is also evident by the preferential partitioning of Al in the octahedral coordination in pyroxene of the banded variety of syenite. The metasomatic effect and subsequent change in the alkalinity (pH) is best exemplified by the apatite group of minerals. The maximum alkalinity stage is represented by the Sr-LREE enriched apatite of the banded syenite. Maximum alkalinity stage resulted formation of eudialyte and complex sodium-zirconium (Na-Zr) bearing silicate, partial replacement of nepheline by sodalite, analcime. However the effect of deuteric alteration in the massive syenite is somewhat less which can be attributed by the presence of Sr-Na-LREE apatite which indicates a lower/decreasing alkalinity stage of the deuteric fluid. The dissolution of zircon at the maximum alkalinity stage is evident by the absence of zircon in the strongly banded syenite gneiss. However, Pb mobility is observed in one of the studied sample of banded syenite reflecting dissolution and re-precipitation of the zircon which yields an age of 877±52 Ma. Zircons from the poorly banded syenite gneiss give an age range of 1.3-0.96 Ga indicating a complex poly-phase metamorphic activity of Grenvillian metamorphic event under amphibolite to greenschist facies condition. The 1.51 Ga zircon age of the massive syenite is somewhat similar to the intrusive age of the near by Bengal anorthosite, indicating at least > 1.51 Gaage of the Sushina nepheline syenites. in There is no conclusive evidence to support consanguineous relation between carbonatitepyroxenite of Beldih and nepheline syenite of Sushina. The present study proposes mantle derivation of these rocks under similar geodynamic conditions controlled by NSZ.