Association of hydrothermal plagioclase alteration with micropores in a granite: Petrographic indicators to evaluate the extent of hydrothermal alteration

Abstract

This study presents the use of petrographic plagioclase alteration indicators as a new method for quantitatively evaluating the extent of plagioclase alteration within granites, using the Toki granite, central Japan, as an example. The new petrological indicator enables us to discuss the similarities/differences in the extent of alteration within a rock body and between rock bodies. Alteration indicators and areal fractions of microvoids in the plagioclase grains were obtained through the analysis of backscattered electron images. The volume of the micropores in the altered plagioclase was estimated using the areal fraction of microvoids in the grains. The plagioclase alteration indicators were obtained as the ratio between the alteration product domain and the original plagioclase domain. We found positive correlations between the plagioclase alteration and biotite chloritization indicators presented in Yuguchi et al. (2021), indicating that each alteration indicator can be used independently as a representative value for the sample. The positive correlations between the areal fraction of microvoids in the altered plagioclase and the alteration indicator in the samples and petrographic observations indicated the following: 1) the altered plagioclase contains the incipient micropores and the alteration micropores, 2) the incipient micropores, which were caused by the dissolution of plagioclase during the incipient stage of plagioclase alteration, acted as a pathway of hydrothermal fluid within the plagioclase, resulting in alteration progress, and 3) the hydrothermal alteration resulted in the production of new alteration micropores. In the Toki granite, the progress of plagioclase alteration is essentially dominated by the progress of biotite chloritization. The progress of biotite chloritization essentially influenced the progress of plagioclase alteration.