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Due to limited exposure and the challenges of field access, the magmatic record of the Ross Orogeny along the Transantarctic Mountains, Antarctica remains ill-constrained. This results in a lack of clarity for Gondwanan tectonics and geodynamics along the paleo-Pacific margin. To better understand spatiotemporal trends in the timing, duration, and source(s) of magmatism, whole-rock geochemistry, U-Pb ages, and Lu-Hf isotopes were measured on zircon in 132 granitoids and 40 gneisses from northern Victoria Land (nVL), the Miller Range (MR), and the Ohio Range (OHR) mountains, spanning ~4,000 km of the Antarctic segment of the paleo-Pacific margin of Gondwana. U-Pb data reveal magmatism in the northern Transantarctic Mountains at nVL initiated by at least ca. 552 Ma, lasting until ca. 467 Ma, magmatism in the central Transantarctic Mountains (cTAM) at the MR spans ca. 573 Ma to ca. 487 Ma, and, the OHR in the southern Transantarctic Mountains (sTAM) records magmatism from ca. 540 Ma to ca. 423 Ma. These age ranges suggest that the duration of magmatism was near-synchronous along the entire margin, but that magmatism first initiated in the cTAM through sVL (ca. 595-590 Ma), before migrating northward and southward into nVL (ca. 552 Ma) and sTAM (i.e., OHR, ca. 540 Ma), respectively. Termination of magmatism followed a similar trend, ending in the cTAM through sVL ca. 485 Ma, then in nVL ca. 467 Ma, and finally in OHR ca. 423 Ma. Thus, our data do not support the long-held suggestion of arc magmatism decreasing in age northward along the margin.Zircon Hf isotope data indicate different magma sources contributed to magmatism as it migrated along the margin. Granitoids from nVL contain minimal mantle input, and were instead dominantly sourced from melting of (meta-)sedimentary source(s). Granitoids from the Ohio Range contain some juvenile input from a relatively young mantle source and contain almost no inherited zircon xenocrysts. Magmatic and metamorphic rocks from the

Temperatures of the lower crust and Moho are critical boundary conditions for models of heat transfer and production in the lithosphere, as well as models of the bulk composition of the continental crust, yet they remain uncertain. This dissertation focuses on xenoliths—fragments of in situ deep crust transported rapidly to the surface by volcanism—and accessory phase U-Pb and trace-element petrochronology in order to provide direct insights into the long-term thermal history of lower continental crust. Chapter I constitutes a study of xenoliths and basement outcrops from northern Tanzania aimed at unraveling crustal vs. mantle heat contributions to variable surface heat flow across the tectonically-stable craton and adjacent rifting crust. Lower-crustal xenoliths erupted on the craton edge contain rutile with U-Pb dates as old as 1000 Ma, whereas xenoliths from the rift have rutile and apatite with near-zero Ma U-Pb dates but significantly older titanite dates (up to 560 Ma). These and other data suggest that Moho temperatures beneath the craton margin are cooler than below the rifting crust, such that regional differences in surface heat flow reflect variable mantle heat flow. Chapter II focuses on kimberlite-borne xenoliths from the central Siberian craton. Rutile and apatite in garnet-granulite xenoliths preserve U-Pb dates between 1.8 Ga and 360 Ma (the timing of kimberlite eruption). Such spreads in U-Pb dates have been interpreted previously to reflect partial Pb loss during slow cooling through the Pb closure temperatures of rutile/apatite, but could also result from brief heating pulses in the lower crust prior to eruption. Laser ablation depth profiling reveals that U-Pb age and elemental gradients in rutile are coupled, contrary to diffusive decoupling expected for thermally-mediated volume diffusion over billions of years. These data are instead best explained by multiple transient heating episodes during distinct thermotectonic events, including immedia

A bimodal suite of lamprophyric and felsic porphyry dikes, forming part of the Neoproterozoic-Ordovician Ross orogeny, occurs throughout the Dry Valleys of Southern Victoria Land, Antarctica. Lamprophyres are mafic dikes enriched in incompatible elements and hydrous minerals and, in orogenic settings, are frequently associated with post-subduction extension, marking the end of subduction-related magmatism. Although the geologic units comprising the Ross orogen have been extensively mapped and studied, only sparse geochemical and geochronological data for these dikes exist; the Ross orogen provides an opportunity to gain insight into orogenic processes, particularly those underlying the end of subduction. A primary goal of this project was to use field relationships and U-Pb zircon geochronology to clarify cross-cutting relationships between lamprophyres, porphyry dikes, and other intrusive units in order to document the timing of the youngest magmatism and end of the Ross orogeny in Southern Victoria Land. Another was to use whole rock major and trace-element geochemistry to determine the petrogenetic relationship between lamprophyre and porphyry dikes and finally improve our understanding of the geochemistry of lamprophyre dikes. Geochronological data from 19 samples in tandem with field observations indicate that lamprophyres are cross-cut by, and older than, porphyry dikes, and that emplacement of these dikes occurred in quick succession following the cessation of voluminous calc-alkaline magmatism. Dates are consistent with similar dikes from Northern Victoria Land, suggesting similar emplacement ages for dikes along much of the Ross orogen margin. Whole-rock geochemical data from 213 samples indicate that the most primitive lamprophyres are enriched in Mg, Cr, and Ni as well as incompatible elements such as LILE and LREE, indicating that they drive from an enriched mantle source. The presence of negative HFSE anomalies and Pb enrichment suggests a subcontinenta

Investigating the secular geochemical and geodynamic evolution of accretionary orogens with zircon petrochronology: A case study from West AntarcticabyDemian Alan Nelson Subduction-related processes in accretionary orogens modulate Earth’s geochemistry. Uncovering the secular geodynamic evolution of accretionary orogens, therefore, is the key to Earth’s geochemical history. A record of the secular geodynamic evolution of accretionary orogens is preserved in the tempo, geochemistry, and distribution of subduction-related magmatism. Zircon is ubiquitous in subduction-related magmas and provides a valuable time integrated geochemical proxy for the secular geodynamic evolution of accretionary orogens via U-Pb geochronology, Hf isotopes, and trace-elements (i.e., zircon petrochronology). The paleo-Pacific margin of Gondwana, including South America, eastern Australia, and West Antarctica, provides an ideal opportunity to investigate the secular geodynamic evolution of accretionary orogens with zircon petrochronology because it contains the most long-lived and best-preserved record of accretionary orogenesis on Earth. This dissertation represents the first comprehensive zircon petrochronologic study of Phanerozoic subduction and accretionary orogenesis in West Antarctica and combines the results with existing data from adjacent regions in eastern Australia and South America to refine our understanding of the secular evolution of accretionary orogens. Chapter 1 investigates volcaniclastic sedimentary rocks in the central Transantarctic Mountains and documents episodic isotopically depleted magmatism along the Antarctic margin of Gondwana. Comparisons of these data from the central Transantarctic Mountains demonstrate a shared tectonic history between Antarctica, Zealandia, and Australia that contrasts with that of South America. Chapter 2 applies zircon petrochronology to the plutonic rocks in eastern Marie Byrd Land and Thurston Island. These new data combined wit

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 di

Monazite [(LREE)PO4] and xenotime [(HREE,Y)PO4] occur in ore-grade concentrations within the Pinto Gneiss in the Music Valley region (MVR) of southern California. However, both the age and petrogenesis of this potentially economically significant rare earth element (REE) deposit remain uncertain. A combined petrologic and geochronologic study yields insight into the textural and temporal relationships between REE mineralization and the host rock. Ore-forming monazite and xenotime are typically restricted to biotite folia within the host Pinto Gneiss with highest modal abundances occurring within a few meters of contacts between the host gneiss and a diorite/amphibolite intrusive unit, the latter of which is cross-cut by pegmatite veins that appear to have been generated by partial melting of the Pinto Gneiss. Ore-forming monazite and xenotime preserve complex internal elemental zoning. Textures inferred to be the result of dissolution re-precipitation reactions overprint relict oscillatory zoning with xenotime and uranothorite [(U,Th)SiO4] forming from the breakdown of monazite and monazite and uranothorite forming from the breakdown of xenotime, suggesting late-stage fluid mediated alteration of the ore minerals. Localized breakdown of monazite, anorthite and biotite to apatite and allanite provides further evidence for post-mineralization metasomatism of the ore bodies. In-situ monazite and xenotime U-Pb geochronology constrains REE mineralization to &sim1.71 Ga, consistent with zircon dates obtained from the Pinto Gneiss. Zircon from the diorite/amphibolite are &sim1.4 Ga, inferred to represent the emplacement age of this unit, whereas zircon in the pegmatite veins yield ages of &sim165 Ma. Zircon from a porphyritic hornblende diorite pluton exposed in the northern part of the MVR also yield an age of 165 Ma, suggesting a genetic link between emplacement of this intrusive suite and melting of the Pinto Gneiss to form the pegmatite veins. Based on the

Accessory phases, such as monazite, xenotime, and zircon, record a wealth of information regarding the timing, duration, and sources of crustal melting. Combined U-Th/Pb and REE analysis of these petrochronometers by laser ablation split stream inductively coupled plasma mass spectrometry (LASS-ICPMS) reveals complex spatiotemporal relationships on a range of scales, from distinct chemical domains within a single crystal, to cross-cutting dikes within heterogeneous plutons composed of multiple melt batches. The anatectic core of the Himalaya exposes mid-crustal rocks well suited for investigations of the time-scales involved in melt processes, such as generation, segregation, amalgamation, mobilization, and emplacement. Three examples from different settings within the Himalayan orogen, including 1) the Leo Pargil leucogranite injection complex exposed in a gneiss dome in the hinterland, 2) the Manaslu pluton at the interface between the anatectic core and overlying metasediments, and 3) Everest region and Mahabharat granites from the anatectic core to the crystalline thrust sheet of the foreland, illustrate the value of monazite for deciphering crystallization in source rocks and/or earlier melt batches in addition to determining granite emplacement age.