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Genesis of Analcime and Nepheline-Potassium Feldspar-Kalsilite Intergrowths: a Review

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Genesis of Analcime and Nepheline-Potassium Feldspar-Kalsilite Intergrowths: a Review See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/255708575 Genesis of analcime and nepheline-potassium feldspar-kalsilite intergrowths: a review Article · January 2009 CITATIONS READS 6 83 7 authors, including: Piero Comin-Chiaramonti Ettore Ruberti Università degli Studi di Trieste ENEA 124 PUBLICATIONS 1,942 CITATIONS 62 PUBLICATIONS 786 CITATIONS SEE PROFILE SEE PROFILE Angelo De Min John Gittins Università degli Studi di Trieste University of Toronto 85 PUBLICATIONS 1,410 CITATIONS 66 PUBLICATIONS 1,028 CITATIONS SEE PROFILE SEE PROFILE All content following this page was uploaded by Piero Comin-Chiaramonti on 05 January 2016. The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the original document and are linked to publications on ResearchGate, letting you access and read them immediately. Special Issue «Acta Vulcanologica» · Vol. 21 (1-2), 2009: 81-90 GENESIS OF ANALCIME AND NEPHELINE-POTASSIUM FELDSPAR-KALSILITE INTERGROWTHS: A REVIEW Pietro Comin-Chiaramonti1,* · A. Cundari2 · E. Ruberti3 · A. De Min1 J. Gittins4 · C. B. Gomes3 · L. Gwalani5,6 1. Dipartimento di Scienze della Terra, Università di Trieste, Via Weiss 8, I 34127 Trieste, Italy 2. Geotrak International Pty Ltd., P. O. Box 4120, Melbourne University, Victoria aus 3052, Australia 3. Instituto de Geociências, University of Sao Paulo, Cidade Universitária, Rua do Lago 562, br 05508-900 São Paulo, Brazil 4. Department of Geology, University of Toronto, Toronto, cdn M5S 3B1, Ontario, Canada 5. C/o D. I. Groves, cet, segs, The University of Western Australia, Crawley, Perth aus WA6009, Australia 6. Present address: C/o Ken Rogers, Level-22, Mineral Securities Limited, Allendale Square, 77 St Georges Terrace, Perth aus WA6000, Australia Abstract The controversial origin of potassic rock suites (lava flows and dykes) containing analcime and pseudoleucite, which may occur as gi- ant trapezohedra {211}, has remained for several decades a hot topic in petrological research. This controversy still persists and as a contribution to the genesis of these minerals, we here examine a few interesting occurrences of analcime and pseudoleucite exhibit- ing complex intergrowths of alkali-feldspar-analcime-leucite or nepheline-alkali-feldspars in potassic rocks of Iran (North-Eastern Azerbaijan volcanics), Brazil (Poços de Caldas and Banhadão) and Paraguay. Our study supports the view that the phenomenon of ion exchange involving Na+ and H2O, and K+ diffusion in presence of Na-rich water results in the conversion of leucite to analcime. Under hydrous conditions alteration or interaction takes place between pre-existing Na-rich phases, such as, nepheline, feldspar and glass. Also, at lower temperatures below the solidus, and at high CO2 activity, sodium-rich leucite breaks down to a complex inter- growths of orthoclase and nepheline resulting in the development of pseudoleucite, which may retain the original trapezohedral form of leucite. We describe several typologies corresponding to the processes leading to the formation of analcime and complex inter- growths in pseudoleucites occurring in rare potassic suites found in widely separated localities. The study of various topologies cor- roborates the conclusions drawn from experimental petrology, which indicates that the replacement of leucite by analcime is a solid- state reaction involving cation exchange by volume diffusion. Keywords: xxxxxx, xxxxxx, xxxxxx, xxxxxx Foreword through similar processes (Cundari 1979, Comin-Chia- ramonti et alii 1979, Cundari and Comin-Chiaramonti he paper is dedicated to Fabrizio Innocenti, Pro- 1996, Prelevic´ et alii 2004, Moradian 2008). Particularly, fessor of Petrology at the Pisa University. Fabrizio, T Karlsson and Clayton (1991), while discussing the prob- passed away at 29 January 2009, for more of 40 years lem of origin of analcime phenocrysts, suggested that was friend of Piero Comin-Chiaramonti and worked this mineral either resulted from subsolidus oxygen iso- together on the petrogenesis and geodynamics of the topic exchange or it was formed from preexisting volcanic rocks from Iranian country. leucite (cf. also Putnis et alii 2007). Although there are many papers dealing with the Introduction problem of pseudoleucite genesis (e.g., Knight 1906; SiO2-undersaturated alkaline magmatic rocks of potas- Bowen and Ellestad 1937; Larsen and Buie 1938; Fudali sic to ultrapotassic affinity, particularly leucite norma- 1963; Seki and Kennedy 1964; Hamilton and McKenzie tive rock-types (e.g., Group i, ii and iii of Foley 1992), 1965; Davidson 1970; Taylor and MacKenzie 1975; often show euhedral (trapezohedral) megacrysts of Gittins et alii 1980, and the enclosed references), we here analcime or pseudoleucite (sensu Gittins 1980) usually examine some interesting occurrences of analcime and associated with nepheline. The former two minerals pseudoleucites where phenocryst-like analcime ex- tend to cluster along the analcime-leucite tie line of Pet- hibits complex intergrowths of alkali-feldspar-anal- rogeny’s Residua System (Fudali 1963, Comin-Chiara- cime-leucite/plagioclase or nepheline-alkali-feldspar monti and Gomes 1996). It is well known that nepheline intergrowths, which may have been derived from an may alter to analcime during subsolidus interactions original leucite-bearing igneous rock through sub- with deuteric and/or hydrothermal fluids (Deer et alii solidus processes. 1992, 2004; Wilkinson and Hensel 1994) and that leucite Preliminary description of rare analcime and readily changes to compositions approaching analcime pseudoleucite occurrences from the Upper Cretaceous * Corresponding Author: P. Comin-Chiaramonti: [email protected] 81 P. Comin-Chiaramonti et alii Fig. 1. Typology I. (cf. Table 1): A, macrocrysts of analcime in analcimite; B: phenocryst-like analcime in lava flow. Crossed Nicols; the square represents the Fig. I-C: analcime (Anl) with K-rich rims (Table 1); S, sanidine. Typology II. (Table 1): Phenocryst-like analcime in a glassy groundmass (Gl). to Miocene volcanism of Iranian Azerbaijan (Alberti and Comin-Chiaramonti 1979, Comin-Chiaramonti Analcime and complex Intergrowths 1979, Comin-Chiaramonti et alii 1979) and from the in potassic Suites Lower and Upper Cretaceous magmatic complexes in The origin of potassic rock suites containing analcime Southern Brazil and Eastern Paraguay, have been de- is a controversial subject. Multiple stages of igneous ac- scribed in earlier work (Hussak 1890; Valença and Edgar tivity and low-grade metamorphism (not considered 1974; Comin-Chiaramonti and Gomes 1996, 2005; Sgar- herein) have been responsible for the formation of anal- bi et alii 2000; Ulbrich et alii 2005; Ruberti et alii 2010, cime-bearing potassic rocks occurring as lava flows as and included references). From a few other localities in- well as dykes. Analcime and the complex intergrowths dividual trapezohedral phenocrysts of pseudoleucite in the alkaline rock suites of Iran, Brazil and Eastern attaining very large dimension, up to 15 cm long, have Paraguay may occur in various typologies summarized been reported from Montana, usa (Larsen and Buie in Appendix and briefly described below. 1938) and the Panwad-Kawant sector of the Chhota 1. Conspicuous phenocryst-like icositetrahedra, Udaipur alkaline subprovince in the Deccan Traps of which are occasionally enveloped by potassium-rich West-Central India (Sukeheswala and Sethna 1967, anisotropic rims (Fig. 1, Pls. i-A-B-C). Gwalani et alii 1993). 2. Euhedral to subhedral or subrounded analcime Representative whole rock chemical analyses are in- phenocrysts occurring in a cryptocrystalline to glassy cluded in the Appendix. groundmass (Fig. 1, Pl. ii). 82 Genesis of Analcime and Nepheline-Potassium Feldspar-Kalsilite Intergrowths: a review Fig. 2. Typology III. (cf. Table 2): A, B macrocrystals of analcime (Anl) in phonolitic lava HK-1, Iran, including leucite (Lc) and nepheline (Neph); Note Ca-K feldspar (Ca-K) and glass at the contact with plagioclase (Pl) and Anl (B). Parallel polars. Typology IV. Phenocryst-like analcime in a microcristalline groundmass; Crossed polars. The numbers correspond to the analyses in Table 3. Note the ghost of plagioclase (12) enclosed in Anl in Fig. IV-B. 3. Wedge-shaped analcime interfaced predominantly that later changed into a more sodic chemistry with with large plagioclase crystals, which may be associat- falling temperature (Gittins 1980, and enclosed refer- ed with metasomatic Ca-K feldspar (Fig. 2: Pls. iii-A ences). Subsequent cooling caused exsolution of and iii-B). nepheline and alkali feldspar where the leucite struc- 4. Subhedral to subrounded analcime interfaced with ture is destroyed, but retaining the leucite crystal mor- plagioclase laths and comparatively fine-grained phology. groundmass. In Figure 2 (Pls. ivA and ivB), plagioclase Notably, leucite (pseudoleucite) and analcime tend to is enclosed in analcime and is partially to completely cluster along the analcime-leucite tie-line of the Pet- resorbed. rogeny’s Residual System and fall in the primary leucite 5. Complex intergrowths comprising analcime, alka- field, coexisting with analcime rich compositions li-feldspar, leucite, plagioclase, glass and kalsilite. (Comin-Chiaramonti et alii 1992). 6. Pseudoleucite occurring as large polygonal crystals or as phenocryst and crystals with irregular outline Processes of Analcime Formation commonly exhibit coarse-grained intergrowths, which
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