Chemical, Isotopic, and Temporal Variations during Crustal Differentiation: Insights from the Dariv Igneous Complex, Western Mongolia

Claire Bucholz, Ph.D., 2016
Oliver Jagoutz, Advisor

Fractional crystallization of mantle-derived basaltic melts is a critical process in producing a compositionally stratified arc crust characterized by a silicic upper crust and a mafic lower crust. This thesis explores outstanding questions associated with fractional crystallization through detailed field, petrological, and geochemical studies of the Dariv Igneous Complex (DIC) in Western Mongolia. The DIC records the crystallization of a high-K primitive arc melt at shallow crustal levels, preserving both biotite-bearing ultramafic and mafic cumulates, as well as liquid-like evolved plutonics, such as (quartz-)monzonites. Chapter 2 establishes the petrogenetic groundwork to understand the conditions under which it formed.  Chapter 3 presents a quantitatively modeled liquid line of descent (LLD) for the complex based on whole rock geochemical analyses, which is able to explain the trends observed in monzonitic plutonic series. The oxygen isotope trajectory of fractionally crystallizing melts is rigorously constrained through modeling and mineral analyses in Chapter 4. Finally, Chapter 5 explores the timescales associated with fractional crystallization through high precision U-Pb geochronology of zircon from the DIC. Together, these studies advance our understanding of the compositional, isotopic, and temporal variations associated with the formation of the continental crust.