Duncan Keller
Rice University
 

Topic: Reading the record of Earth’s changing magmatic styles from massif-type anorthosites

Academic host: Spencer Zeigler

Abstract: How have Earth processes changed through time? This question motivates much work in geoscience. The rock record of Earth’s Proterozoic eon showcases one of the most conspicuous and mysterious examples of an igneous process absent from modern Earth environments: massif-type anorthosites. These batholith-sized mafic intrusions emplaced into the crust are nearly entirely composed of plagioclase feldspar cumulates separated from their parent magma and were produced worldwide during the Proterozoic. Despite progress in the last 50 years that has constrained some of the physical mechanisms by which these intrusions formed, major open questions persist about which tectonic processes generated the magmas that formed the anorthosites and why these rocks are restricted to Earth’s “middle age.”

I will present recent work in which, alongside collaborators, I tested hypotheses of the formation and origins of parent magmas for two large, well-studied Proterozoic massifs: the Marcy in the Adirondacks of New York state and the Morin in southern Quebec. We combined Nd, Sr, O, and B isotope analyses with rock bulk chemistry and petrogenetic modelling to evaluate magma source age, inputs from mantle and crustal reservoirs, and magma chamber processes. Our findings indicate a major role for melting of subducted oceanic crust alongside mantle melting to produce the mafic magmas parental to the anorthosite massifs. Considering geochronological, tectonic, and geodynamic factors, this surprising result seems to point to intense slab melting during the Proterozoic. I will present these findings in the context of Earth’s physical evolution and discuss our hypothesis for the restriction of massif-type anorthosites to the middle of Earth’s history that can be tested for examples worldwide.