Two-mica andalusite-bearing granite with no primary muscovite: constraints on the origin of post-magmatic muscovite in two-mica granites

Abstract

The two-mica granite from Gęsiniec (Strzelin Granitic Massif, SW Poland) consists of quartz, K-feldspar, normally zoned plagioclase (30 ± 7 % An), subordinate biotite and muscovite and magmatic andalusite. Andalusite crystallised before the outer parts of plagioclase grains were formed. Biotite has constant Fe/(Fe + Mg) ratio of approximately 0.81. Five textural types of muscovite occur in the granite: (1) muscovite replacing andalusite, (2) embayed interstitial muscovite, (3) muscovite forming aggregates with biotite, (4) muscovite accompanying biotite and chlorite in microfissures and (5) fine muscovite forming fringes at the contact between larger muscovite plates and K-feldspar. They are commonly associated with albite. Crystallisation of muscovite started significantly below the granite solidus, mostly by the replacement of andalusite. Formation of muscovite continued during cooling of host rock. The growth of individual plates was initiated at different undercoolings and the plates whose crystallisation was frozen at different stages of growth occur. Those that were formed earlier are richer in titanium and iron relative to the later ones. As the rock contains no Ti and Fe saturating phases, the content of Ti and Mg in muscovite depends on their local availability. The homogeneous Fe/(Fe + Mg) ratio of biotite indicates that it was re-equilibrated at the post-magmatic stage.