Mineralogy of New Caledonian metamorphic rocks |
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| Authors: | Philippa M Black |
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| Institution: | 1. Department of Geology, University of Auckland, Auckland, New Zealand
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| Abstract: | Variations in chemistry and related physical properties of sheet silicates in the Ouégoa district with metamorphic grade are investigated. Weakly metamorphosed rocks prior to the crystallization of lawsonite contain phengite (d 006=3.317–3.323 Å), chlorite and occasionally paragonite while interstratified basaltic sills contain chlorite, minor phengite and stilpnomelane. Pyrophyllite crystallizes before lawsonite in some metamorphosed acid tuffs and is also stable in the lawsonite zone. Paragonite, phengite and chlorite appear to be stable through the sequence from weakly metamorphosed rocks into high-grade “eclogitic” schists and gneisses. Optical, chemical and some X-ray diffraction data is given for representative sheet silicates. Electron probe analyses of 55 phengites, 21 paragonites, 57 chlorites, 12 vermiculites, 2 stilpnomelanes, and 2 chloritoids are presented in graphical form. All K-micas analysed are consistently phengitic (3.29–3.55 Siiv ions per formula unit) and show limited solid solution with paragonite (4 to 13% Pa). The K∶Na ratio of the phengite is strongly dependant on the assemblage in which it occurs; the amount of phengite component and its Fe∶Mg ratio depends on bulk-rock composition. Phengites from acid volcanics have the highest Fe∶Mg ratio, highest phengite component and β refractive indices. Phengites from basic volcanics and metasediments of the epidote zone have the lowest Fe∶Mg ratio. Phengites from lawsonite-zone metasediments have intermediate Fe∶Mg ratios. The phengites show a small decrease in phengite component with increasing metamorphic grade. d 006 for phengites varied from 3.302 to 3.323 Å but at least in the lawsonite and epidote zones appears to reflect composition and had little systematic variation with metamorphic grade; phengites from very low-grade rocks showed the longest values of d 006. Paragonite shows almost no phengite-type substitution and only limited solid solution (4–12%) with muscovite. All paragonites (6) and most phengites (20) which have been examined are 2M1 polymorphs; one Fe2+-phengite appears to be a 1M polymorph. The chemistry of chlorites closely reflects parent-rock chemistry. Chlorites from metasediments have distinctly higher Fe/(Fe+Mg) ratios than chlorites from basic igneous rocks; chlorites from the lawsonite and lawsonite-epidote transitional zone metasediments have the highest Fe/(Fe+Mg) ratios. In metabasalts Fe/(Fe+Mg) ratios appear to reflect individual variations in bulk-rock chemistry and show no direct correlation with metamorphic grade. There is little difference in Al/(Si+Al) ratio between chlorites from sediments and basic igneous rocks although in both lithologies the chlorites from the epidote zone appear to be slightly more aluminous. Fe-rich chlorites of the lawsonite zone metasediments have been altered by a process involving leaching of Fe and Mg and introduction of alkalies to a brown pleochroic Fe-vermiculite. Chemical and physical data for this vermiculite are given. The decrease in Fe/(Fe+Mg) ratio in chlorites and phengites on passing from the lawsonite to the epidote zone can be correlated with the crystallization of Fe-rich epidote and almandine in the epidote zone. Elemental partitioning between coexisting minerals has shown Ti to be partitioned into phengite, while Fe and Mn are strongly partitioned into chlorite. When either stilpnomelane or chloritoid coexists with phengite or chlorite, Fe and Mn are slightly enriched in the stilpnomelane or chloritoid relative to the chlorite. |
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