THE ORIGIN OF GARNET CORONAS IN GRANULITE XENOLITHS AND EVOLUTION OF THE LOWER CONTINENTAL CRUST

Continental crust forms via subduction zone magmatism, but how continents become Si-rich remains controversial. A process is needed to separate mafic from felsic components, such as crystal fractionation of arc basalts. If igneous cumulates formed thus lie within garnet (gt) stability, they may become dense enough to founder, driving remaining buoyant crust to more continental-like composition. However, whether such a process operated throughout Earth history is unknown. Here, we investigate origins of gt and gt reaction coronas in a suite of Paleoproterozoic lower crustal granulite xenoliths from the State Line (SL) kimberlite, CO. Xenoliths range from recrystallized gabbronorite to gt-bearing assemblages. A distinctive feature is abundance of gt and gt+clinopyroxene (cpx) coronas (>25%), which formed at expense of opx and plagioclase. Interestingly, some gt granulites resemble gt-cpx cumulates from Phanerozoic arcs (i.e. Sierra Nevada Batholith, CA), suggesting possible igneous cumulate origin; bulk geochemistry also supports arc-derived protoliths. To better constrain origin of gt, we apply thin section-based microanalytical techniques (WDS and EBSD large-area mapping) with pixel counting to reconstruct xenolith bulk compositions. Such methods circumvent exotic grain-boundary phases from the kimberlite that often plague deep crustal xenolith studies. We then calculate normative mineralogy of reconstructed compositions and compare them to natural arc rocks and experiments. Combining thermodynamic phase equilibria modeling of reasonable protoliths and “ground-truthing” to xenolith data, we test 3 hypotheses of corona origin: 1) primary igneous gt cumulate, 2) isobaric cooling and/or increasing pressure of initially gt-free cumulate, and 3) open-system melt-rock reaction/metasomatism. Each hypothesis has implications for origin of Paleoproterozoic arc lower crust and possible similarities to modern-day arc lower crustal processes: 1) parallels the ubiquitous gt-cpx “arclogites” in Phanerozoic arc xenolith suites. For 2), depending on protolith composition, an arc cumulate could cool into gt stability, but may not eclogitize and potentially remain neutrally buoyant on long timescales. Lastly, 3) supports increasing recognition of the role of melt-rock reaction in lower crust.