Low-pressure granulite metamorphism in the Ivanpah and Southern McCullough Mountains: A case study on partial melting drivers

Abstract

Granulite metamorphism and associated partial melting are fundamental components of the continental crust’s evolution, contributing to its chemical and structural stratification. Granulite-grade metamorphic rocks, often prevalent in the middle to lower crust, commonly preserve evidence of partial melting, melt migration, and melt extraction, thus offering insights into crustal differentiation and rheology. Situated in the Mojave crustal province along the California-Nevada border, the Ivanpah and Southern McCullough Ranges (IV-MR) possess well-exposed Palaeoproterozoic pelitic migmatites and mafic granulites. Understanding the heat sources responsible for driving metamorphism and partial melting in this area can provide more insight into the geological history of the Southwestern United States and consequently the evolution of North America as well as add to the larger understanding of crustal evolution. This study combines petrographic and field observations, phase-equilibrium modelling, U/Th–Pb geochronology, and utilisation of trace element geochemistry to determine the mechanisms of heating responsible for these migmatites.

Phase equilibrium modelling of garnet–sillimanite–cordierite–biotite migmatites constrain metamorphic conditions of ≥ 800 °C at ~4–5 kbar. Melt reintegration calculations for several pelitic migmatites suggest a near isobaric P–T path. Monazite and zircon geochronology coupled with monazite trace element analyses indicate a protracted high-grade metamorphism spanning c. 1750–1650 Ma with simultaneous mafic magmatism. These rocks were potentially formed in an extensional setting. A likely tectonic setting capable of sustaining a long-lived (≥ 100 Ma) high-temperature, low-pressure metamorphic event and allowing for mafic intrusion emplacements would be that of a thinned lithosphere, such as that of a backarc basin. In the case of the migmatites of IV-MR, partial melting was driven by a combination of elevated mantle heat flow from a thin mantle lithosphere and the emplacement of mafic intrusions.