Secondary Ca-Al silicates as low-grade alteration products of granitoid biotite |
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| Authors: | A J Tulloch |
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| Institution: | (1) Geology Department, University of Otago, New Zealand |
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| Abstract: | Secondary Ca-Al silicates, including andradite-grossular garnet, epidote, pumpellyite, and prehnite are shown to be extremely widespread as low-grade alteration products of granitoid biotite. All may occur within metadomains on the scale of a single biotite pseudomorph. They all generally have a lensoid habit parallel to the host biotite cleavage, which may, or may not be deformed. A cleavage or parting inherited from the host biotite cleavage is usually present.Prehnite, pumpellyite and epidote are unusually Fe-rich: 100 Fe/Fe + Al for prehnite and pumpellyite reaching 19.1 and 49.1 respectively and epidote Ps36-Ps48. Garnet compositions are relatively uniform averaging approximately andradite56 grossular35 hydrogrossular7.Correlation of prehnite Fe3+ with host biotite Fe3+ and oxidation state support the textural evidence that prehnite in this paragenesis often replaces the host biotite. Sericitization of plagioclase and commonly a complete lack of hornblende indicate that plagioclase (An50-An15) is the chief source of Ca. Chlorite and muscovite (whose compositions are both directly related to the host biotite composition), aluminian sphene (Al2O3 reaching 7.72%), and K-feldspar (Or99+) are complimentary to the biotite breakdown reaction.Host rock composition to some extent controls the development of the Ca-Al silicates which do not occur in granitoids whose whole-rock CaO is less than 1%. An aqueous pore fluid with locally varying activities of ions enabled concentration of Ca in micro-metadomains, allowing development of Ca-Al silicates in relatively low-Ca granitoids. Individual phases generally appear to have developed independently due to fluctuation in chemical environment in micro-space and/or T, P fluctuation with time. Textures suggest that grandite may have locally replaced epidote in response to increasing temperature.In granitoids of the Victoria Range, South Island, New Zealand the alteration has probably occurred with P<2kb and T< 300 °–350 °C. Here alteration can be ascribed to strictly deuteric activity in only one of the two groups of granitoids so affected. |
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