ARE PLUTONS IGNEOUS OR METAMORPHIC ROCKS? YES. THEY ARE MELTAMORPHIC (Invited Presentation)

Mineral textures in intrusive igneous rocks are used to define the most basic intrusive rock map unit - the pluton. The textures are interpreted by most to reflect the magmatic history of the pluton: flow during intrusion, settling or floating during fractionation, sorting during channelized flow, crystallization in a melt-dominated chamber, and myriad other inferred processes. However, these interpretations overlook the impact on textures of most of the thermal history of plutonic rocks. We present multiple lines of evidence in support of the interpretation that the textures of plutons – fundamental features used to define them – reflect the complex thermal histories below the nominal solidus and, except for the most quickly cooled intrusions, reveal little about intrusion and the melt-dominated history of a magma.

Plutons grow slowly by incremental addition of magma. Consequently, early increments (like all other wallrocks) experience complex histories characterized by temperature cycling and volatile flux as later increments expel the hydrous phase. Geo- and thermochronologic data show that, whereas an increment added to a growing pluton may spend a few 10’s of ka above its solidus temperature (and consequently an even shorter duration as a melt-dominated body), it may spend 100’s to 1000’s of ka at subsolidus temperatures in amphibolite to greenschist facies conditions. Classical interpretations of the textures of plutons generally assume that they record only the melt-dominated stage, ignoring the rest of the thermal history. However, many otherwise enigmatic observations in plutonic rocks (low equilibration T in quartz and K-feldspar, megacrystic and porphyritic textures, modal layering, amphibole crystals altered to greenschist facies mineral assemblages, and chloritization of biotite) all suggest that the rocks reflect significant modification at subsolidus temperatures. Ignoring the obvious impact of subsolidus reequilibration (“meltamorphism”) hampers progress in understanding plutons, pluton emplacement and plutonic-volcanic rock connections.