NEW DATA ON METAMORPHIC CHLORITE AS A PETROGENETIC INDICATOR MINERAL, WITH SPECIAL REGARD TO GREENSCHIST-FACIES ROCKS

Abstract

For rock-forming minerals, the extent and the behavior of solid solution are understandable only if considered in ttre context of a rigorous petrological framework, as has been shown repeatedly in the met?morphic perology literature. Herein, we present a study of the composition of trioctahedral chlorites, with special focus on nen-limigillg minsl4l 455e6!lages in rocks of low metamorphic grade. The goal has been to discern and understand any systematic changes of chlorite composition due to changing T' P' and bulk-rock composition. The three main compositional variations shown by the metamorphic chlorite in this study include the ratio Fe/Mg, the extent ofTschermak substitution, and deviation from trioctahedral toward dioctahedral chlorite. Our database includes 2619 chlorite compositions, of which 450 are selected from literature covering the temperature range subgreenschist - amphibolite facies, and,2169 are newly determined compositions from greenschist-facies rocks. All samples used are classified according to metemorphic grade, pressure, and brtk-rock composition, thereby establishing groups and subgroups of analytical data. These data are plotted on diagrams aimed at enabling one to discern more specifically how bulk-rock composition, temperatue and pressure affect the three main compositional variations of chlorite from typical meramorphic rocks. Each of ttrese parameters controls chlorite composition to some extent, but the control by the bulk-rock chemistry is clearly dominant. Comm661y it largely obscures the systematic compositional changes caused by temperatue and pressure. Thus, despite numerous attempts, it is evident that chlorite composition by itself is non-viable for geothermobarometric purposes in the case of the non-lirniting assemblages tlpical of the greenschist facies. Attempts to use chlorite from such assemblages for geothermobarometry should be restricted solely to approaches involving cation exchange with some coexisting phase(s).