Understanding orogenic processes is crucial for evaluating tectonic models of mountain belts such as the Himalaya. The channel flow model predicts that the Himalayan metamorphic core is exhumed by the buoyant flow of melt-bearing rocks in the mid-crust. Such melts, generated in the kyanite stability field during prograde metamorphism, may herald the transition from burial to exhumation tectonics. This thesis investigates the evidence for this prediction by examining the petrogenesis of kyanite migmatites, through detailed petrography, high-resolution mineral-scale geochemistry (including the first LA-ICP-MS trace element mapping of kyanite) and geochronology.
In Eastern Bhutan, melting of metapelites from the lowermost parts of the Greater Himalayan Sequence (GHS) generated ‘in-source’ kyanite migmatites by low-volume, fluid-present muscovite melting. Much of the kyanite in the leucosomes is xenocrystic, having been entrained from the metapelitic source rocks. Kyanite growth continued in the melt, producing both distinctive new rims observed in cathodoluminescence images, and fresh, Ge-enriched crystals. Back-reaction of kyanite with melt produced muscovite rims, also with elevated Ge concentrations. Peritectic kyanite growth is limited but is prevalent in structurally higher migmatites. Variations in kyanite Cr/V composition reflect disequilibrium melt production and changing melt compositions.
Zircon U-Pb geochronology suggests melt formed episodically between ~34 and 12 Ma in response to periodic water availability. Pulses of zircon growth at ~21 Ma and ~14 Ma may relate to movement on the Main Central Thrust, which would facilitate fluid percolation into the overlying rocks, triggering melting. This is later than predicted by the classic channel flow model but consistent with recent composite models that propose segmentation of the GHS into tectonic slices. As the lowermost GHS is the final slice to be exhumed, prograde Miocene melting in the kyanite field could still have driven the change from burial to exhumation in this crustal section.
Abstract Aluminosilicates (kyanite, sillimanite and andalusite) are useful pressure–temperature (P–T) indicators that can form in a range of rock types through different mineral reactions, including those that involve partial melting. However, the presence of xenocrystic or inherited grains may lead to spurious P–T interpretations. The morphologies, microtextural positions, cathodoluminescence responses and trace element compositions of migmatite-hosted kyanite from Eastern Bhutan were investigated to determine whether sub-solidus kyanite could be distinguished from kyanite that crystallised directly from partial melt, or from kyanite that grew peritectically during muscovite dehydration reactions. Morphology and cathodoluminescence response were found to be the most reliable petrogenetic indicators. Trace element abundances generally support petrographic evidence, but protolith bulk composition exerts a strong control over absolute element abundance in kyanite. Sample-normalised concentrations show distinctive differences between petrogenetic types, particularly for Mg, Ti, V, Cr, Mn, Fe and Ge. LA-ICP-MS element maps, particularly combined to show Cr/V, provide additional information about changing geochemical environments during kyanite growth. Most kyanite in the studied migmatitic leucosomes is of sub-solidus origin, with less widespread evidence for peritectic crystallisation. Where present, grain rims commonly crystallised directly from the melt; however, entire grains crystallised exclusively from melt are rare. The presence of kyanite in leucosomes does not, therefore, necessarily constrain the P–T conditions of melting, and the mechanism of growth should be determined before using kyanite as a P–T indicator. This finding has significant implications for the interpretation of kyanite-bearing migmatites as representing early stages of melting during Himalayan evolution.
Recent improvements in analytical capabilities allow us to reveal details of magmatic processes at an increasingly finer spatial and temporal scale. In situ analyses of the isotopic and trace element composition of accessory minerals at the sub-grain scale have proven to be effective tools for solving a wide range of geological problems. This study presents new data on accessory minerals including monazite & zircon, examined by in situ LA-ICP-MS and Laser Ablation Split Stream (LASS) techniques, analyzing multiple isotopic systems (U-Pb + Sm-Nd, and U-Pb + Lu-Hf in monazite and zircon, respectively) in order to track geochemical changes over time through a magmatic system. The late Cretaceous granitoids of the Old Woman Mountains in the Mojave Desert, California, provide an excellent opportunity to apply these analytical techniques. The peraluminous granites of the Sweetwater Wash, Painted Rock, and North Piute plutons represent different depths of the magmatic system, and are well understood in terms of field relations and whole-rock geochemistry. A preliminary study on the Sweetwater Wash monazites (Fisher et al., in preparation) has revealed significant inter-grain isotopic heterogeneity in the composition of the source region (~1700 Ma); however, the U-Pb ages show an isotopic resetting during emplacement at ~75 Ma. This decoupling of U-Pb and Sm-Nd isotopic systems is suggested by Fisher et al. to be due to recrystallisation and/or dissolution-reprecipitation of monazite. If grain boundary diffusion of Pb overrides the more kinetically limited volume diffusion, then the U-Pb systematics will be reset while Sm and Nd remain immobile in the monazite structure as essential structural components of the lattice. This new data will allow the further investigation of these preliminary results, providing new insights into the observed isotopic disequilibrium, with the LASS technique accurately linking the multiple isotopic systems. This will provide important insights into monazite isotope systematics, which will have implications for the geochronology community. Systematic sampling through transects of each pluton will also allow the geochemical homogeneity of each pluton to be assessed. Additionally, this study is the first application of the LASS technique to a magmatic system, and thus will provide further insight into the petrogenesis of the Old Woman and North Piute Mountains, and continental arc granites in general.
Aluminosilicates (kyanite, sillimanite and andalusite) are useful pressure-temperature (P-T)indicators that can form in a range of rock types through different mineral reactions, including thosethat involve partial melting. Their involvement in melting reactions means that the presence ofaluminosilicates in migmatite mineral assemblages can help to (broadly) constrain the P-T conditionsof melt formation, which then has implications for evaluating models of orogenic tectonics.Xenocrystic grains could lead to spurious tectonic interpretations, so being able to distinguishbetween different petrogenetic sources is important. Petrological and geochemical investigation ofmigmatite-hosted kyanite from Eastern Bhutan shows that kyanite petrogenesis may be constrainedby combining information from morphology, cathodoluminescence response, microtextural positionand geochemical zoning patterns. Mg, Ti, Ca, Fe, Cr and Ge concentrations provide diagnostic cluesthat distinguish sub-solidus kyanite from kyanite that crystallised directly from melt, or grewperitectically during muscovite dehydration reactions. The abundance of these elements in kyanite isalso strongly controlled by protolith composition, with considerable inter-sample variation observedin this sample set. LA-ICP-MS maps, especially of Cr/V, provide additional information aboutchanging geochemical environments during kyanite growth. These data and observations show thatmost kyanite is of xenocrystic origin in the analysed samples, and therefore that its presence does notnecessarily constrain the P-T conditions of the melt reaction(s). This finding has significantimplications for the interpretation of kyanite-bearing migmatites as representing early stages ofmelting during Himalayan evolution.
Abstract Mapping the age and trace element and Sm-Nd isotope compositions of monazite grains from a peraluminous Cretaceous granite using laser ablation–split stream analysis reveals a wide range in Nd isotope and rare earth element (REE) compositions within and between single grains. These data corroborate isotopic variability indicated by Hf isotope analysis of zircon in the same granite sample. The REE variations indicate that monazite grew during fractional crystallization. Hf and Nd isotopes indicate that the granitic magma was generated from at least two distinct Proterozoic sources of approximately the same age: one component that had highly radiogenic initial 176Hf/177Hf and 143Nd/144Nd and a second component that was notably less radiogenic. This study highlights the utility of in situ REE and Sm-Nd isotope data in monazite in magmatic systems. Further, it refines the zircon-based constraints on magmatic processes because of sensitivity of light REEs to fractional crystallization, lower probability of complications owing to inheritance, and smaller analytical volumes required.
The Old Woman-Piute Range Batholith (OWPB) in the Mojave Desert of south-eastern California is a suite of metaluminous and peraluminous Cretaceous granites that intrudes Proterozoic basement. The peraluminous Sweetwater Wash, Painted Rock and North Piute plutons were sampled to investigate geochemical heterogeneity. Zircon and monazite crystals were analysed for U–Pb & Lu–Hf and U–Pb & Sm–Nd isotopes, respectively, using the high-spatial resolution and the recently developed Laser Ablation Split Stream (LASS) approach. Inherited cores are widespread in zircon, limited in monazite, and yield U-Pb ages that range from 1800-1400 Ma, consistent with regional Proterozoic crustal building events. Zircon and monazite rims give a range of crystallisation ages between 70–75 Ma. The OWPB shows a large range in eHfi and eNdi in both inherited and magmatic populations, a characteristic that is derived primarily from the Proterozoic crustal source, but also influenced by the partial dissolution and preservation of inherited grains.
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