UC Santa Barbara

The process of subduction has significant influence on the geochemical evolution of the crust and mantle, and subduction-related magmatism may have been an important mechanism for the growth of continental crust over time. Extensive exposure of the mid-crustal levels of an ancient, exhumed continental arc in the Transantarctic Mountains provides an exceptional opportunity to study the metamorphic and magmatic processes associated with an archetypal subduction zone. The belt of multiply deformed metamorphic rocks and granitoid batholiths records convergence and subduction of paleo-Pacific oceanic lithosphere beneath East Antarctica during the Neoproterozoic–Paleozoic Ross orogeny. These rocks are especially well exposed in the southern Victoria Land (sVL) segment of the Transantarctic Mountains––the largest ice-free area of Antarctica. The metamorphic and igneous rocks in sVL provide insights into the early stages of convergent-margin tectonism, the compositional diversity of subduction-related magmatism, and the relative roles of crustal growth and recycling in continental arcs.

In Chapter 1 of this dissertation, garnet Lu-Hf and monazite U-Pb geochronology—combined with petrography, mineral chemistry, and thermobarometry—reveal a Barrovian-style metamorphic history that predated the dominant phase of magmatism in sVL. The geochronology data from this study provide one of the oldest records of tectonism along the Ross orogen. The results are consistent with a tectonic model that involves shortening across the margin of East Antarctica prior to the major phase of subduction-related magmatism.

Chapter 2 explores the age and magma sources of a large subduction-related igneous complex in the Dry Valleys area. Zircon U-Pb geochronology demonstrates that the period of magmatism in the Dry Valleys was relatively short-lived compared to other segments of the Ross orogen. Whole-rock geochemistry and Hf isotopes in zircon reveal the assimilation of ancient crust during the differentiation of juvenile magmas that were likely derived from an enriched sub-continental lithospheric mantle source. A compilation of Nd and Sr isotope data from granitoids from along the Ross orogen suggest that enriched lithospheric mantle may have been a common juvenile magma source along the arc.

In Chapter 3, a comprehensive geochemical, geochronologic, and isotopic investigation of the magmatism in sVL explores the conspicuous occurrence of alkaline silicate rocks and carbonatites—most commonly associated with intraplate and continental-rift magmatism—within the continental arc. The alkaline magmatism was partially contemporaneous with the emplacement of large sub-alkaline igneous complexes in adjacent segments of the arc. The isotopic and trace element composition of the alkaline and subalkaline rocks suggests derivation from geochemically enriched sources—potentially metasomatized sub-continental lithospheric mantle. Despite enriched isotopic and trace element compositions that broadly resemble recycled continental crust, binary mixing and assimilation-fractional crystallization models indicate that crustal growth may have been dominant over crustal reworking in the sVL magmatism.