nETPOmma, 2010, m0M 18, № 5, c. 567-576
THE ORIGIN OF PSEUDOLEUCITE IN TINGUAITE, GHORI, INDIA:
A RE-EVALUATION
© 2010 S. G. Viladkar
Emeritus, Geology Department, St. Xavier's College, Mumbai Carbonatite Research Centre, Amba Dongar Kadipani, District Vadodara, India-390117; E-mail: sviladkar@gmail.com Received July 28, 2008, in final form February 7, 2010
Abstract—Pseudoleucite occurs as large megacrysts (giant phenocrysts) in tinguaite at Ghori which is located in the Panwad-Kawant carbonatite-alkalic complex, India. In thin sections the pseudoleucite crystals show mainly an oriented intergrowth of nepheline and orthoclase with additional minerals phases such as white mica, analcime, sodalite and cancrinite. From mineralogy and geochemistry it is inferred that the scheme involving subsolidus breakdown of leucite and subsequent reaction with Na-rich fluids can satisfactorily explain origin of pseudoleucite crystals in tinguaites.
INTRODUCTION
Pseudoleucite is the name attributed to an intergrowth assemblage of K-feldspar + nepheline after primary leucite. Many publications based on experimental results explain the conditions for the formation of such intergrown in "pseudoleucite" (Bowen and Ellestad, 1937; Fudali, 1963; Seki and Kennedy, 1964; Davidson, 1970; Taylor and MacKenzie, 1975; Roux and MacKenzie, 1978; Git-tins et al., 1980; Korobeinikov et al., 1998). The main problem in explaining this intergrowth is that the natural leucite does not contain excess amount ofsodium to form this type of intergrowth on decomposition.
The only occurrence ofpseudoleucite in India was first reported from a tinguaite dykes at Ghori (Fig. 1), Chhota Udaipur, Gujarat by Sukheswala and Sethna (1967). These dykes belong to the Panwad-Kawant carbonatite-alkaline dyke complex (Sukheswada and Avasia, 1971) which is part of the Chhota Udaipur carbonatite-alkaline province (Viladkar and Avasia, 1992, 1994). Sukheswala and Sethna used petrographic and chemical data to characterize the pseudoleucite and attributed its origin to the sub-solidus breakdown of leucite during cooling. The morphology of pseudoleucite crystals reveal that majority of crystals do not show a sharp contact with the ground-mass. There are also hair thin fractures within pseudoleu-cite crystals (Fig. 2). Within pseudoleucite crystal, nepheline sometimes has been sometimes replaced by analcime, sodalite, white mica.
Our petrographic, mineralogical and geochemical data suggest that only a subsolidus breakdown ofleucite cannot answer all questions related to the origin of the pseudoleucite in tinguaite rock. It is therefore suggested that the scheme involving the subsolidus breakdown of original leucite and a subsequent reaction with Na- and Cl-rich fluids can satisfactorily explain the origin and composition of pseudoleucite crystals in tinguaites.
GEOLOGICAL SETTING
The Panwad-Kawant dyke complex, comprising of alkaline igneous rocks and carbonatites lies to the north of the Amba Dongar carbonatite diatreme. A large number of dykes and plugs, which include analcime bearing mafic phonolite, felsic phonolite, syenite porphyry and lampro-phyre, are emplaced in two structurally weak zones. One trends east-west parallel to the Heran River fault zone, and the other runs northwest-southeast parallel to the Kara River fault zone (Fig. 1). The pseudoleucite tinguaite dyke exposed near Ghori village belongs to the Heran River fault zone.
A major dyke oftinguaite about 9 m long and 3 m wide is exposed in the river bed. During our subsequent visits, we realized that there could be a few more dykes, now partly exposed between Murangana and Phenai Mata. The major part of these dykes seems to be still hidden by the soil cover.
PETROGRAPHY
Sukheswala and Sethna (1967) provided a detailed petrographic description of the pseudoleucite tinguaite. The most striking feature is the large size ofthe pseudoleucite phenocrysts: they vary from 5 to 8 cm in diameter. They are embedded in a fine-grained tinguaitic ground-mass. In thin sections the pseudoleucite crystals show an oriented intergrowth of nepheline and K-feldspar and the oriented growth of nepheline and K-feldspar is well seen in BSE image (Fig. 3). However, the mineralogy of original pseudoleucite has been modified due to formation of new phases such as white mica, cancrinite, analcime, and sodalite which usually replace original nepheline. The matrix of tinguaite is composed of small crystals of pseudoleucite, nepheline, K-feldspar and mafics. The most abundant mafic phenocrysts in the groundmass tin-
Fig. 1. Geological map of the Panwad-Kawant carbonatite-alkalic complex; GPD stands for Ghori pseudoleucite-tinguaite dykes.
guaite are mauve-coloured zoned clinopyroxene surrounded by a rim of aegirine. Fine needles of aegirine are dispersed throughout the groundmass. The matrix also contains sparse flakes of strongly pleochroic biotite; eu-herdal crystals of zoned melanite which, range from reddish brown to almost opaque. Euhedral titanite and per-ovskite are minor phases. Some of the samples contain glomeroporphyritic aggregates of titanite, aegirine and apatite. Apatite occurs as irregular blebs and also as slender euherdal prisms or hexagonal crystals. Stray grains of analcime have also been noticed in the groundmass.
ANALYTICAL TECHNIQUES
Minerals in tinguaite and pseudoleucite were analysed using CAMECA SX100, electron microprobe operating at an accelerating voltage of 15 kV 10 nA beam current and beam defocused to 5micron diameter; at the Miner-alogical-Geochemical, Institute, Freiburg University, and Joel Superprobe JXA-8200 microprobe with accelerating voltage of 15 kV and beam current 10 nA, at the Max-Planck Institute for Chemistry (MPI), Mainz. Major and trace-element analyses of separated crystals of pseudoleucite and the matrix tinguaite were carried out using a Philip PW 1450/20 X-ray fluorescence spectrom-
eter at Mineralogical-Geochemical Institute, Freiburg University. Concentrations of the rare-earth elements (REE) in the pseudoleucite and matrix tinguaites were determined by the neutron activation method at the Bhabha Atomic Research Centre, Trombay, Mumbai.
COMPOSITION OF MINERALS
Clinopyroxene. The clinopyroxene is titaniferous di-opside (Table 1). All Al and some Ti enter the tetrahedral site to compensate for insufficient Si. The presence of Al(IV) of the diopside reflects the low activity of Si in the alkaline magma (Kushiro, 1960). The inferred absence of Al(VI) in all pyroxene samples is consistent with their crystallization at low pressure (Le Bas, 1962). The diopside crystals are weakly zoned and rimmed by green aegirine which, however, has not been analyses.
Nepheline. Nepheline from both nepheline-ortho-clase intergrowth that replaced primary leucite and the groundmass have been analysed. Matrix nepheline is almost identical with composition close to (Ne75Ks18Qtz7) while the ones in pseudoleucite show slight change in their composition from (Ne66Ks29Qtz5) to (Ne74KS24Qtz2). Where intergrown with feldspar, the nepheline is exten-
sively altered to analcime, white mica, cancrinite and so-dalite (Table 2).
K-feldspar. Composition of K-feldspar within pseudoleucite (Or97Ab3An0 to Or90.Ab9.5An0) and in the matrix of tinguaite (Or95Ab5An0 to Or89Ab11An0) do not show large variation (Table 3, Fig. 4). No sign of grid-twinned microcline has been found K-feldspar. However, the only variation noticed is in the BaO content of these two types. High BaO content (up to 2.39%) is found only in the sanidine that is associated with orthoclase in pseudoleucite crystal while the feldspar in the matrix of tinguaite shows low concentration of BaO (maximum up to 0.67%). Pivec et al. (2004) report similar high BaO (BaO 1.61%) feldspar in the core of pseudoleucite phe-nocrysts and also in the core of K-feldspar crystals in the matrix ofphonolite ofBohemian massif, Czech Republic. No such zonation is found in feldspars of the study area.
Analcime and sodalite. These minerals are found as replacement product of nepheline in pseudoleucite crystal (Fig. 5); and their compositions are given in Table 2. Analcime, however, is also noticed in the matrix of tinguaite. The two analyses ofanalcime shown in Table 2 are rather poorer in Na2O while one of them is relatively rich in Al2O3 as compared to the analyses listed by Deer et al. (2004). One analysis also shows low total (though we used 10 nA defocused beam current and a beam diameter of 5 micron). Both contain only 0.10% K and negligible Ca. Two analyses of sodalite presented in Table 2 are not perfectly stoichimitric compared to the typical sodalite analyses (Deer et al., 2004). One shows large deficiency in Na2O while the other is slightly Si-rich with high total. In addition, white mica, and cancrinite also occur in small amounts.
PSEUDOLEUCITE COMPOSITION
The results of analyses of the bulk composition of separated pseudoleucite crystals are given in Table 4. The pseudoleucite from Ghori resembles that from the Bear-paw Mountain, Montana (Zies and Chayes, 1960), whereas it differs from the pseudoleucite of Tzu Chin Shan, China (Ygi, 1954); the latter contains a higher Ca level and a lower Na. Our pseudoleucite also shows large variation in K (10.82 to 13.09%). We also, carried out alkali determinations on only three more pseudoleucite crystals, which showed variation in both K (10.70 to 13.50%) and Na (4 to 5.5%). The variation in alkali contents and H2O content (Table 4) in different pseudoleucite crystals could be attributed to the secondary alteration.
The accessory minerals, melanite biotite, titanite and perovskite have also been analysed (Table 5). Biotite compositions do not show much variation. They contain moderate amount of TiO2 and MnO and their Mg/Fe ratio are almost constant. Melanite is zoned with Si, Al and Ca decreasing and Ti, Fe increasing from core to rim. All
Fig. 2. (a) and (b) Pseudoleucite Giant phenocrysts in matrix
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