MELTAMORPHISM

Magmas accumulate into plutons slowly. Initial increments metamorphose unrelated wall rocks. However, as assembly continues, early portions of the pluton become wall rocks for later increments. Geo/thermochronologic data indicate that plutonic rocks may spend millions of years between the initiation of zircon crystallization (>700˚C) to Ar/Ar biotite closure (<400˚). Thus, it is inevitable that early intrusive phases will experience greenschist- to amphibolite-facies conditions for long periods of time. When increments are added prior to solidification of preceding increments, melt-assisted change should occur. We informally call this “meltamorphism” and suggest that it is widespread in plutons.

In the Tuolumne Intrusive Suite, the tonalite of Glen Aulin is the wall rock during assembly of the Half Dome Granodiorite. Within the Half Dome, the equigranular facies is wall rock for the porphyritic facies. Textural variations that define these map units are independent of composition. Rocks mapped as the equigranular Half Dome Granodiorite span the compositional range of the Tuolumne suite (~57-77% SiO₂). We suggest that the sequence of textures mapped as a sequence of plutons is the result of the thermal history of the suite, not textures characteristic of discrete, sequentially intruded magmas. This explains why the textural sequence mapped in the Sierran John Muir and Whitney Intrusive Suites is identical to that in the Tuolumne - they all result from similar thermal histories shared by slowly assembled granodiorites.

Meltamorphism likely accounts for many enigmatic observations in plutonic rocks including: K-feldspar megacrysts, low inferred temperatures of equilibration of K-feldspar, amphibole crystals altered to greenschist facies mineral assemblages, and chloritization of biotite. We also suggest the possibility that modal layering may result from extreme meltamorphism where fluids from late increments are focused, resulting in textures akin to gneissic banding.