Chemical, isotopic, and temporal variations during crustal differentiation : insights from the Dariv Igneous Complex, Western Mongolia
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https://hdl.handle.net/1912/7597Location
Dariv Igneous Complex, Western MongoliaDOI
10.1575/1912/7597Abstract
Fractional crystallization of mantle-derived basaltic melts is a critical process in producing a
compositionally stratified continental 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 in Western Mongolia. The Dariv
Igneous Complex 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 presents comprehensive field and petrographic descriptions of the
complex and establishes the petrogenetic groundwork to understand the conditions under which it formed.
Results of this study indicate that the observed lithologies formed through the fractional crystallization of
a high-K hydrous basalt, typical of alkali-rich basalts found in subduction zone settings, at 0.2-0.5 GPa
and elevated oxygen fugacities. 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 the monzonitic plutonic series observed in continental arcs. The oxygen isotope trajectory of
fractionally crystallizing melts is rigorously constrained through modeling and mineral analyses in
Chapter 4. This study indicates that large (1 to 1.8‰) increases in δ18O as a melt evolves from basaltic to
granitic in composition due to the fractionation of low δ18O minerals. As such, the majority of δ18O values
of upper crustal silicic plutonics can be explained through fractional crystallization of primitive arc
basalts alone without needing to invoke assimilation of high δ18O crustal material. Finally, Chapter 5
explores the timescales associated with fractional crystallization through high precision U-Pb
geochronology of zircon from the Dariv Igneous Complex. Evolution from a basaltic melt to a silica-rich
monzonitic melt in the Dariv Igneous Complex occurred in <300 ka. If rates of fractional crystallization
are primarily a function of cooling, this study provides an end-member constraint for fractional
crystallization of a basaltic melt at relatively cool, shallow crustal levels. Together, these studies advance
our understanding of the compositional, isotopic, and temporal variations associated with the formation of
the continental crust.
Description
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2016
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Suggested Citation
Thesis: Bucholz, Claire E., "Chemical, isotopic, and temporal variations during crustal differentiation : insights from the Dariv Igneous Complex, Western Mongolia", 2016-02, DOI:10.1575/1912/7597, https://hdl.handle.net/1912/7597Related items
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