GEOCHEMISTRY AND EVOLUTION OF CARBONATITE-ALKALIC COMPLEX OF AMBADUNGAR, GUJARAT

Abstract

A carbonatite-alkalic complex is located in and around Ambadungar, 37 km south of Chhota Udepur (20°N, 74.5*E) in Baroda district of Gujarat state. Detailed field studies of the area revealed that the basement rocks of the Ambadungar complex consists of a sequence of sandstone and limestone with intercalated shales and conglomerates (Bagh beds. Upper Cretaceous). It is established that a series of basaltic lava flows of tholeiitic descent associated with Deccan volcanism preceded the emplacement of the carbonatite-alkalic complex. The Ambadungar area is intensively faulted and fractured. Two major fault patterns, trending E-W and N-S are observed. Those trending E-W appear to be older and are parallel to the main Narmada lineament. The most conspicuous structural feature of the area is updoming of the sedimen-* tary rock sequence during the emplacement of the carbonatite. It is observed that the intrusion took place at a steep angle producing a stress field, which gave rise to the formation of tension joints (with an inward dip of 45° or less) and shear joints (with an outward dip)0 This event was followed by extrusion of the carbonatite-breccia mainly along the tension fractures. They are exposed at Mogra# Bunjar, Nakhal, Rajawat, Saidiwasan, and Kostawat villages. (v) The last phase of volcanic cycle is associated with the intrusion of silica-undersaturated alkalic rocks along the shear joints. Many of these fractures associated with the joints are presumed to be extended to considerable depths as evidenced by the occurrence of fault breccia at depths of even 80m (recovered from drill core). Analyses of ten basaltic rocks partly covering the dome and associated in the neighbouring area show that they have a wide range of chemical composition (SiO varying from 45 to 54 wt.%# and all of them are basic). Both normative and modal compositions of the basalts show that they vary from tholeiitic andesite to tholeiitic basalts. When the normative compositions are plotted in the nepheline-diopsidehypersthene- quartz diagram (Yoder and Tilley), it is observed that they cluster around the diopside-hypersthene join lying between diopside-hypersthene-quartz and diopsideolivine- hypersthene ternary diagrams. A plot of the chemical compositions of the rocks in an alkali-silica diagram of Irvine and Baragar show that they are all tholeiitic. Eight representative samples of the alkalic rocks Na 0+K 0 Na have been analysed. The value of ., q —and mTjjj ratio Cm -J suggest that they are nepheline-syenites and are miascitic. This is further supported by their high CaO and MgO contents, and presence of CO and HO. It has been observed that the (vi) alkalic rocks are usually restricted very close to carbonatites along the shear and tension fractures. Mineralogical studies of the country rocks in the vicinity of the carbonatite are found to be metamorphosed to albite - epidote - hornfels facies. These rocks show incorporation of calcite, sodicpyroxene or veins of carbonatites. The fenitization is rather limited. Texurally the alkalic rocks are porphyritic with phenocrysts of K-feldspar, aegirine-augite, apatite, magnetite, calcite, fluorite and pl.gioclase. Nepheline and analcite with or without hauyne are also present. Groundmass constituents are mainly aegirine-augite and K-feldspar. The microprobe studies of pyroxenes show that they have the following structural formulae, ^0.03-0.32' C*0.72-0.73> ^0.02-0. 33< Fe0. 61-0.62' M„0 w. Cr0#00_o.01. *10.03» CSi2.03-2.04' 6- K-feldspar often displaysperthitic intergrowtn and plagioclase feIdspars.are ca-l, ciic (iA.il^bUltep30_40>). EPMA .«*» of tine celcites in alHalic rocKs sno„ *.* tney contain 11**1. «^3 and Pe«,3 in - solid solution One alkalic rock is founda *to» bbee ssiilliiccaa-saturated. When the ^ percent of aegirlne-angite. al*ali-feldspar^nd (nepheline ♦ analcite plowed in tne pyro*ene-feldsp JldspaWd ternary diagram it Unoted *nat tine co.posr.ron o£ *. alHalic roCc. plot in *e fields of pulas*ite. lusitanite, malionite anda ff^ovyaaiittee. A few have compositions BiBdlar to fcldspathic ijolite. (vii) If the normative compositions of the alkalic rocks are plotted in the A (alkali-feldspar) - P(plagioclase)- F(feldspathoids) diagram (streikensen), two trends of crystallization are noted. One trend is similar to that displayed by the feldspathoidal rocks from Birunga volcanic rocks, Zaire. In this case the crystallization trend follows from feldspathoid-tephrite through phonolitic feldspathoic - tephrite and latite to trachyte. The other trend is similar to the one displayed by volcanic rocks of the Vico volcanic complex, north of Roman Province, Italy, in this case the crystallization trend is through the fields of tephritic phonolite to alkali trachyte. It should be emphasized here that the feldspathoid-bearing rocks from Ambadungar are not tephritic as the plagioclase content of these rocks are very low or insignificant and anofcthite appearing in the norm is most likely tied with the CaTiAl206 and CaAl206 molecules. The above discussion is based on the plot of only three components (viz. alkalifeldspar, feldspathoid and plagioclase) to emphasize the following two facts, (i) the alkalic rock intrusions of Ambadungar represent different phases of a volcanic sequence and (ii) do not represent arbitrary bulk composition. If the bulk composition of the alkalic rocks are plotted in the nepheline-kalsilite-Si02 system, five compositions plot in the leucite field a'.ong a line, and

one has a composition lying near the nepheline-syenite minimum. Most of the compositions plot is the leucite field and one near the composition KAlSi20& appears to have the primitive composition. These compositions plot along a line depicting a distinct crystallization trend as demanded by the phase equilibria relation in the simplified nepheline-kalsilite - Si02 system. The one with composition plotting near nepheline-syenite minimum obviously has the most evolved composition. Based on 21 analyses ('3 4 major and minor elements) it is noted that the carbonatites can be classified into two broad groups: (i) silica-poor carbonatites, and (ii) silica-rich carbonatites. When the composition of the carbonatites is compared with that of average carbonatite composition of Gold, Pecora, Maravic and Morteani, Kresten and Burnfelt, and Harsen, it is noted that the Ambadungar carbonatites are more enriched in CaO (53.36-35.05 wt.%), Ba0(3.42-0.03 wt.%), SrO (0. 661-0. 15 wt.%), P^U. 52-0.02 wt.%), F(23.09-Tr. wt.%), S03(2.63-0.02 wt.%), La(1032-648 ppm), Ce (3387-491 ppm), Nd( 1543-155 ppm), Sc(364-'.>5 ppm), Gd(250-51 ppm), and Yb(100-38 ppm). Except for CaO content of the Ambadungar carbonatites, the concentrations of other major elements are similar to the average chemical composition of carbonatites reported by Gold, Pecora, Maravic and Morteani, Kresten and Burnfelt, and Harsen.

The carbonatites are porphyritic with phenocrysts of calcite, clinopyroxene, K-feldspar, magnetite, apatite, phlogopite fluorite, and nepheline. In the silica-rich variety instead of nepheline, quartz is present. Groundmass is generally carbonate-rich with the same phases in the matrix, EPMA data of carbonates show the following compositional range for the carbonate-bearing phases : (calcite)66_55 (magnesite)27_12 (dolomite)-. .Q. Based on EPMA data of magnetites present 22—18 in carbonatites, it is observed that the temperature of crystallization of magnetite varies between 625° and 660°C and -InfQ ranges between 17.5 and 19.7 bar, respectively. 2 The variation in the abundance of major and trace elements with respect to depth shows that the emplacement of carbonatitic rocks is associated with atleast three carbonatite flows. They occur at different levels of the hill between 531-520m, 520-470 m, and 470-410 m from the mean sea level. Beginning of each flow is marked by the enrichment in Si02, Ti02' ZFe 0 , V, Cr, Ni, Cu, Co, Zn, and Ga and deficiency in CaO, P205, A1203, K20, Na20, BaO, and S03. There may be more flows of carbonatite at greater depths. In Ambadungar, fenites are preserved in the outer margins parallel to the carbonatite dyke in the west, northwest, south, southwest and northeast in the contact zones with sandstones. Fenites are present in the northern fringe of the carbonatite dyke, which was exposed during the mining (x) operation of fluorite. Width of fenites vary from 20 to 100m and attain a maximum width (slightly more than 150m) in the north west contact of carbonatite and sandstones. Fenitization in this area is considered to be due to (l) reaction between the volatiles escaping from the carbonatite magma and the wall rock, and (2) diffusion of carbonatitic materials along the grain boundaries of the country rocks in the immediate vicinity. The fenitized basalts are found to re tain their original texture but often contain calcite and apatite occurring as veins or present in the grain boundaries as independent phases. Assuming a mantle comprising containing olivine(55%), orthopyroxene (38.5%), clinopyroxene ( 1%), garnet (5%)# and dolomite (0.5%), thermodynamic calculations have been made to construct a melting model. In such calculations; partition coefficients for different elements have been obtained from Frey et al.. It is noted that 0.003 to 5.5% melting of the mantle material is needed to explain the REE abundances as noted in the carbonatites from the Ambadungar. It is speculated that due to the accumulation of heat the break-down of accessory carbonate phases within the upper mantle took place releasing C02 . 0.2 to 0.7% melting of the mantle at a depth of 80 to 100 km should initiate a carbonate and alkali-rich liquid. Evolution of

such gases result in crack propagation. In case of the Ambadungar area presence of pre-existing fractures and zones of weakness in the lower crust before Cretaceous should further control its propagation. Emplacement of the carbonatite took place in several cycles, whereas that of the alkalic liquid took place at a later stage along the shear and tensional joints during the same volcanic episode.