GRANITES ARE UPPER GREENSCHIST FACIES ROCKS

Our studies of granitic rocks sensu lato (granite, granodiorite, tonalite) in the Sierra Nevada batholith of California demonstrate that their mineralogy, crystal chemistry, and texture reflect upper greenschist facies (~400 °C) conditions. These rocks and their archetypal textures are igneous by definition but they differ radically from volcanic rocks of comparable composition. Here we summarize some of these differences and implications for interpretation of plutonic rocks.

K-feldspar in calc-alkaline volcanic rocks is intermediate (e.g., Or₆₀) sanidine that shows simple concentric zoning in backscattered electron (BSE) and cathodoluminescence (CL) imaging. Sanidine is sparse or absent in dacites, consistent with phase equilibrium experiments that show late crystallization in dacitic/granodioritic melts. In granites, K-feldspar is K-rich (e.g., Or₈₈), and BSE and CL images are extremely complex and show multiple episodes of fracturing and brecciation along with significant microporosity. Plagioclase in granites is superficially similar to that in volcanic rocks but in CL shows complexity and porosity similar to K-feldspar. Quartz is superficially similar but again the plutonic variety shows complex internal fracturing, and O-isotope studies from plutons elsewhere demonstrate significant fluid exchange. Albite or myrmekite is nearly ubiquitous at contacts between plagioclase and K-feldspar in plutonic rocks but absent in volcanic rocks. Hornblende in plutonic rocks, although superficially fresh in hand specimen, is locally replaced up to 50% by an isochemical greenschist assemblage of epidote, K-feldspar, chlorite, magnetite, and other minerals. Biotite is locally replaced by chlorite. Magnetite in plutonic rocks has completely exsolved the Ti-component that is abundant in volcanic rocks.

These observations indicate pervasive re-equilibration at greenschist-facies conditions. Textural observations that conflict with well-known phase equilibria (e.g., K-feldspar megacrysts) require significant crystal ripening and textural rearrangement. This is unsurprising in view of results from geochronology and thermochronology that demonstrate growth and cooling over millions of years. Granites are autometamorphic igneous rocks without a prograde path.