THE ROCKS DON’T LIE, BUT PLUTONS SPEAK A LANGUAGE THAT IS EASY TO MISINTERPRET

“The rocks don’t lie” is a common axiom in geology and speaks to the primacy of field evidence in studies of the Earth. This axiom is true—whatever happened, happened—but plutonic rocks have been, and continue to be, misinterpreted because they are speaking a language that we barely understand. Since the recognition that plutonic rocks were once magmas, observations made in plutonic terranes have been interpreted in terms of processes that we can observe at the surface, such as fluid flow, debris flows, particle settling, and granular flow; these interpretations are typically colored by vivid published images of red magma blobs and red seismic anomalies under volcanoes. However, many of these self-evident "truths" are revealed to be incorrect when simple and robust physical and chemical tests are applied.

Long-held but generally incorrect field interpretations that persist in the study of plutons include: that plutons are somehow born as km-scale magma bodies; that K-feldspar megacrysts grew early in an abundance of liquid; that intersecting modal layers form by erosional truncation; that crystals settle to the bottoms of magma chambers, pile up, and avalanche; that crystals are free to move independently of one another; that pluton floors are unseen, providing a convenient sewer system into which problems can be flushed; and so on. The rocks may seem to be saying these things, but they have been misunderstood, because most of these processes can be ruled out, or at least seriously questioned, by objective tests, application of results from phase equilibria, and impartial field observation.

Understanding what the rocks are saying requires recognition that their textures reflect long cooling histories spent largely under amphibolite and greenschist facies metamorphic conditions. Typically dismissed as “alteration”, the ubiquitous resetting of mineral compositions to lower-T (metamorphic) conditions is a fundamental process that also dramatically resets plutonic textures. Most plutonic rocks were contact-metamorphosed by heat and fluids from later increments. They have a lot to tell us, but we need to better learn their language.