Browsing by Subject "Metamorphism"
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Item Carbonatites(2009-07) Barker, Daniel S.Item An evaluation of quartz-inclusion barometry by laser Raman microspectrometry : a case study from the Llano Uplift of central Texas(2010-08) McDowell, Emily Allen 1985-; Carlson, William, 1952-A new barometric technique measuring stored stress in quartz inclusions via laser Raman microspectrometry was employed in an attempt to elucidate the extent of highpressure (HP) metamorphism in the Llano Uplift of central Texas. Rare lithologies within the Llano Uplift contain mineralogical evidence of HP metamorphism (pressures from 1.4 to 2.4 GPa at temperatures from 650 to 775°C), but much of the uplift is composed of felsic gneisses lacking any HP signature; these felsic gneisses may never have transformed to HP assemblages, or they may have been thoroughly overprinted by later low-pressure events. Barometry via laser Raman microspectrometry computes entrapment pressure for a quartz inclusion in garnet from measurement of the displacements of its Raman peak positions from those of a quartz standard at atmospheric pressure. Quartz inclusions in garnets that grew in felsic gneisses under HP conditions should retain HP signatures, despite later overprinting. Application of the Raman microspectrometry technique should therefore allow barometry of previously uncharacterizable rocks. For two localities in the Llano Uplift, entrapment pressures from Raman barometry (0.6-0.7 GPa and 0.2-0.3 GPa) were substantially lower than pressures expected based on conventional barometers (1.4 GPa and 1.6-2.4 GPa). This absence of any HP signatures in the Llano rocks contrasts with more successful applications of the Raman technique by previous workers in high P/T blueschist-facies rocks. A key difference in the Llano rocks is that they reached peak temperatures at which intracrystalline diffusion in garnet, driven by compositional gradients produced during growth, had noticeable effects: complete homogenization of growth zoning had occurred in the locality that produced the greatest discrepancies between Raman and conventional pressures, and modest relaxation of zoning occurred in the locality with the smaller discrepancies. The failure of the Raman technique to recover pressures consistent with conventional barometry in the Llano Uplift is therefore attributed to relaxation of stress on the quartz inclusions as the result of intracrystalline diffusion within the garnet. This conclusion suggests that use of the Raman barometric technique must be restricted to rocks whose time-temperature histories produce only very limited intracrystalline diffusion in garnet, typically those rocks whose peak metamorphic temperatures fall at or below upper amphibolite-facies conditions.Item Evidence from high-temporal-resolution strain rates for strain softening due to episodic fluid influx at Passo del Sole, Central Swiss Alps(2012-12) Stacy, Sarah Jean; Carlson, William, 1952-; Cloos, Mark; Ketcham, Richard ABerg (2007) determined hand-sample-scale high-temporal-resolution strain rates from rotated garnet for two samples of quartzose pelitic gneiss at Passo del Sole, Central Swiss Alps, documenting a correlation between dramatic increases in strain rate and compositionally anomalous garnet growth zones. Considering additional evidence that these anomalous zones resulted from externally derived ephemeral fluid flow, he concluded that increased strain rates at Passo del Sole are the result of strain softening caused by fluid influx. This study tests Berg's interpretation by calculating new hand-sample-scale high-temporal-resolution strain rates for two additional samples of the same gneiss: a control sample (Sample PDS 03-30) that shows no unusual zoning patterns, implying that it was unaffected by fluids; and another (Sample PDS 03-2) that features a prominent Ca spike, suggesting that it has been affected by fluid influx. Unique features of garnet from this locality--contemporaneity of chemical zones, near-simultaneous nucleation, size-proportional growth, and rock-wide chemical equilibrium--were exploited to calculate strain rates of unprecedentedly high temporal resolution. Thermodynamically modeled temperatures for several growth-zone boundaries in each garnet crystal were combined with a regional heating rate of 11.5 ± 3.5 °C/Myr (presumed constant) and measured deflections of inclusion trails in each zone to calculate strain rates for several discrete time increments during garnet growth. Sample PDS 03-2 displays a 2- to 16-fold increase in strain rate that correlates with growth of the high-Ca zone; strain rates are 0.4 x 10⁻¹⁴ s⁻¹ to 4.1 x 10⁻¹⁴ s⁻¹ for zones with normal Ca concentration and 9.1 x 10⁻¹⁴ s⁻¹ to 17.9 x 10⁻¹⁴ s⁻¹ for the high-Ca zone. Distinct amongst all analyzed samples from Passo del Sole, Sample PDS 03-30--which has not been affected by fluid influx--shows no fluctuations in strain rates, which remain low and similar (0.2 x 10⁻¹⁴ s⁻¹ to 2.6 x 10⁻¹⁴ s⁻¹) across all zones. Results from this study therefore further substantiate the correlation of high strain rates with compositionally anomalous zones, strengthening the interpretation that elevated strain rates at Passo del Sole result from strain softening caused by episodic, externally controlled flow of fluids through the system during synkinematic garnet growth.Item High-temperature carbonate replacement mineralization, metamorphism, deformation, and intrusion in the Bryant District, Beaverhead County, Montana(2003) McGuire, James B.; Kyle, J. RichardThe Bryant District, in the southwest Montana fold-and-thrust belt, contains many structurally and lithologically controlled high-temperature carbonate replacement Pb-Zn-Cu-Ag-Au deposits. Laramide crustal shortening prepared Cambrian and Devonian carbonate strata for fluid circulation through thrusting and folding. Thrust-controlled mineralization is present in the Lion Mountain mines, whereas fold-controlled chimney-style mineralization is present in the Cleve-Avon mines. Pb isotopic evidence indicates that base and precious metals in the district were not directly sourced from phases of the adjacent Late Cretaceous Pioneer Batholith. Ore Pb was probably scavenged from the Middle Proterozoic Belt Supergroup that underlies the district. A small pluton, satellitic to the Pioneer Batholith, underlies the Hecla Dome and may have set up a hydrothermal system which produced carbonate replacement as well as Mo skarn mineralization. Fluid inclusions in ore-associated quartz from the Bryant District are CO2-rich and have salinities from 3 to 8 wt% NaCl equivalent with homogenization temperatures from 260 to 330°C. Fluorine-deficient porphyry Mo deposits in Montana and Idaho, most notably Cannivan Gulch 5 km northwest of the Bryant District, have remarkably similar fluid inclusion characteristics, suggesting that the Bryant District carbonate replacement deposits may be the distal portion of a porphyry Mo system. The mineralization in the Bryant District appears to be related to a late satellitic intrusion of the Pioneer Batholith. Such an intrusion was hypothesized (Winchell, 1914; Karlstrom, 1948) and recently proven by exploratory drilling on the Hecla Dome. Field evidence suggests that Pb-Zn-Cu-Ag-Au mineralization took place after most Laramide shortening, intrusion of the bulk of the Pioneer Batholith, regional metamorphism, and contact metamorphism related to the emplacement of an intrusion below the Hecla DomeItem P-T-t paths and deformation of blueschist and associated graphite-schist blocks from the Franciscan mélange, San Simeon, California(2010-05) Ukar, Estibalitz, 1980-; Cloos, Mark; Ernst, Gary; Carlson, William D.; Mosher, Sharon; Garcia-Casco, AntonioThe Franciscan Complex forms the structurally complicated, locally chaotic basement of the Northern and Central California and southwestern Oregon Coast Ranges. It is an accretionary wedge formed during the Late- Jurassic-Tertiary subduction along the west coast of North America. In northern California, the Franciscan is subdivided into three belts, the Western, Central, and Coastal belts, which show a zonation in age, metamorphic “grade”, and structural style. Franciscan mélanges are present in the Central belt, as well as the Diablo Range --a tectonic window within the structurally overlying Great Valley Group--, and the Nacimiento Block, where the study area is located. One of the best exposures of Franciscan mélange, where contact relationships between blocks and matrix can be observed, crops out along 6 km of seacliffs near San Simeon. Boudinaged blocks of graywacke, greenstone, chert, and much rarer blueschist and graphite-schist are ix dispersed in the shale matrix. Block sizes range from 10 cm to 15 m. The discovery of interlayered blueschist and graphite-schists, and the presence of lawsonite in some graphite-schists demonstrates that these two lithologies were metamorphosed together. Graphite-schist blocks in the Franciscan have not been reported prior to this study. Two main mineral assemblages were recognized among the studied 34 mafic blueschist blocks: 1) Lws + Na-amp + Pmp + Phe + Ttn + Chl recrystallized at ~5 kbar and 200-250°C, and 2) Lws + Na-amp + Pmp + Phe + Ttn + Ep + Chl, which recrystallized under slightly higher temperatures but similar pressures (300-350°C, at 5 kbar). A pre-blueschist facies metamorphic event under greenschist facies conditions is recorded by calcic cores overprinted by Na-amp rims in about half of the blocks. Sodic amphibole rims with a higher Fe3+ content probably developed due to the breakdown of epidote during a decrease in T. These mafic blueschists followed a counterclockwise P-T path. Remnants of “actinolitic rinds”, which are reaction zones formed when the blocks were in contact with serpentine, were found associated with nine of the studied mafic blueschist blocks. Such rinds were thought to be unique to better-studied Franciscan high-T blocks. Graphite-schist blocks (30 studied) contain Qtz + Phen + Ab + Gr, and are of two types. One type has relict sedimentary textures with a weak foliation defined by graphite and pressure solution seams. The other type has a compositional layering with layers containing well recrystallized quartz. Nine of these blocks also contain lawsonite within the more graphitic layers. The geochemistry of the blueschist blocks indicates that they were derived from the MORB-like oceanic crust, and seamounts underplated during the initiation of subduction. The mainly mafic protolith contained a small volume of interlayered sediment, as indicated by the presence of associated metasedimentary graphite-schists. Blueschist facies conditions were attained at the bottom of the overriding plate during the initial states of Franciscan subduction (150-155 Ma). Mafic material continued to be underplated, and low-T dynamic blueschist-facies metamorphism continued to form until at least ~137 Ma. A model is proposed in which exhumation was facilitated by normal faulting near the surface driven by gravitational collapse driven by decrease in subduction plate dip around 80 Ma that is known to have caused the Laramide orogeny. During this time, blueschist and graphite-schist blocks were plucked from the bottom of the hanging wall, incorporated into the shaleand water-rich shear zone at the plate interface, and exhumed during the upward flow of mélange driven by the movement of the downgoing plate. Blocks were extended and boudinaged, and metasomatically altered in the cataclastic zones that developed along necks and margins of the blocks during upwelling and the dewatering that led to final compaction near the surface.Item Polyphase deformation and metamorphism of the Middle Proterozoic Coal Creek Serpentinite, Gillespie County, Texas(1989) Gillis, Gretchen; Mosher, Sharon, 1951-The Middle Proterozoic Coal Creek Serpentinite and adjacent schists and gneisses of the southeastern Llano Uplift, Gillespie County, Texas, have been multiply deformed under amphibolite facies metamorphism during a Grenville-age (1.0-1.3 Ga) orogeny. At least 5 phases of folding, as well as previously undocumented "chocolate tablet" boudinage, affected the southeastern Llano Uplift. The Coal Creek Serpentinite is one of few Grenville-age ultramafic rocks and is a critical part of tectonic models for Texas and North America during the Precambrian. The Coal Creek Serpentinite has been considered to be an altered remnant of an ophiolite. Detailed mapping of a traverse across the well-exposed Coal Creek Serpentinite body into adjacent Packsaddle Schist and Big Branch Gneiss shows that Coal Creek Serpentinite has been multiply deformed, indicating a complex post-emplacement (and perhaps emplacement) history. Packsaddle Schist shows 2 sets of isoclinal folds (FI and F2) producing a mushroom-type fold interference pattern that has been refolded by at least 2 sets of later folds. The predominant regional metamorphic layering (S2) is axial planar to F2 folds. Adjacent to the Coal Creek Serpentinite, extension of S2 in nearly orthogonal directions within a NNW-trending plane produced chocolate tablet boudinage recording high extensional strain, probably by foliation boudinage. Coal Creek Serpentinite adjacent to Packsaddle Schist shows a similar sequence of deformational features. Serpentinite texture varies from schistose to massive, with 2 orientations of cross-cutting mineralogical layering. Talc-rich Coal Creek Serpentinite at the margins of the body shows 2 crenulation cleavages that correspond in orientation to late-stage folds within Packsaddle Schist. Serpentinization of the Coal Creek ultramafic body occurred in at least 2 phases, an early phase of serpentinization best preserved in deformed reaction zones around the margins of the body and near amphibolite dikes within the Coal Creek body, and a post-tectonic pervasive serpentinization event probably associated with granite emplacement during the Precambrian. The polyphase deformation of the Coal Creek area is consistent with the complex deformational history documented elsewhere in the southeastern Llano Uplift and indicates that the serpentinite was emplaced early in the deformational history. If the Coal Creek Serpentinite is indeed a remnant from Grenville-age subduction, its subsequent deformational history requires collision of an island-arc or continental mass with the North American craton prior to 1.0 GaItem Structural evolution and metamorphism of mid-Proterozoic basement in the northwest Van Horn Mountains, Trans-Pecos, Texas(1987) Bristol, David Arthur, 1961-; Mosher, Sharon, 1951-Psammitic, pelitic and mafic schists in a NNW-trending horst in the NW Van Horn Mountains of west Texas show evidence of a multiphase, synmetamorphic, ductile Grenville-age deformation. Other basement exposures in the Van Horn area record a later phase of Grenville-age brittle deformation that has wholly or partly obscured the earlier-formed ductile features. Detailed mapping indicates that basement rocks exposed in the NW Van Horn Mountains underwent three phases of progressive isoclinal folding followed by two later, less intense, nearly coaxial folding episodes. The second phase of folding produced the dominant foliation and foliation intersection (S₁/S₂) lineation observed throughout the area. This deformation is inferred to have resulted from a large scale folding event, at or very near, peak metamorphic conditions, possibly associated with nappe formation. Garnet-biotite and garnet-hornblende Fe-Mg exchange thermometry indicate maximum temperatures of 640±50°C, corresponding to mid-amphibolite facies conditions, for this event. The intrusion of late syn- to post-orogenic pegmatite sills and dikes was widespread across the area. The associated pegmatitic fluids either facilitated or became the principal catalysts of a static recrystallization event. This event altered previously formed deformational fabrics and produced diagnostic strain-free textures in many minerals. The Grenville-age, polydeformational history recorded by the basement rocks of the NW Van Horn Mountains is similar to that noted in portions of the Llano Uplift of central Texas, about 650 km to the east. The strong similarity of deformational styles in these two areas suggests that large-scale folds with localized shear zones were produced in this region of the Grenville ProvinceItem The behavior of halogens (F, Cl, Br, I) in altered oceanic crust during prograde subduction zone metamorphism and devolatilization(2022-05-05) Beaudoin, Grace Margaret; Barnes, Jaime Danielle; Lassiter, John C; John, Timm; Penniston-Dorland, Sarah; Stockli, Daniel FHalogens (F, Cl, Br, I) are volatile elements enriched in Earth’s surface reservoirs. Following seafloor alteration and tectonic convergence, halogens are carried into subduction zones. Prograde metamorphism of altered ocean crust (AOC) results in hydrous mineral breakdown and the release of volatiles. Halogen devolatilization is not well constrained, particularly for Br and I. To explore the halogens fluxes and behaviors during subduction of AOC, this study investigates bulk rock concentrations from seafloor AOC drill cores (n=21) and paleo-subduction settings that expose exhumed metamorphic rocks (n=44) that record the chemical evolution of AOC during prograde metamorphism. Chapter 2 juxtaposes eclogitic and AOC samples to gauge broadscale changes to the halogen budget between the trench and the depths of ~80 km. Chapter 3 focuses on a suite of petrogenetically-related meta-ophiolites from the Western Alps that record progressive metamorphism and shed light on halogen loss across metamorphic grade, especially at low P-T conditions. To study how halogens partition during phase breakdown and fluid transport, Chapter 4 examines a ~1 m transect of eclogitic samples that preserve evidence of high pressure vein formation and fluid-rock interactions. Findings from this work reveal that halogens are not evenly distributed in AOC. Basalts are F-rich and gabbros are Cl- and Br-rich. During subduction, F is decoupled from the heavy halogens (Cl, Br, I) displaying compatibility and immobility. Chlorine and Br are closely coupled; they preferentially partition into fluid phases and are efficiently removed from the mafic slab. Iodine behaviors are more cryptic, with many high-P samples preserving protolith-level abundances. Halogen mobility in devolatilizing AOC is as follows: Cl ≈ Br > I ≫ F. New flux estimates find that ≤40% of initial F and I and ~70% of initial Cl and Br is removed from subducting AOC prior to depths of arc magma genesis, with much of this loss occurring early in the forearc, before the transition to blueschist assemblages. Fluorine is broadly distributed among phases. The heavy halogen inventory is not controlled by a dominant phase (i.e., amphibole). Rather, these elements are distributed at low concentrations among many phases or are predominantly hosted in non-lattice sites.Item Unrecognized complexities of metamorphism : crystallization kinetics, reaction affinity, and geochronology(2011-12) Kelly, Eric David; Ketcham, Richard Alan, 1965-; Carlson, William, 1952-; Gardner, James E.; Cloos, Mark; Pattison, David; Farver, John R.Unrecognized metamorphic complexities can produce erroneous interpretations when using equilibrium thermodynamics and isotope geochronology. Universally employed methods for determining pressure-temperature conditions during regional metamorphism are based on the assumption of chemical equilibrium, and geochronology in metamorphic rocks can suffer from cryptic redistribution of isotopes. In this research, the scales of disequilibrium in regionally metamorphosed rocks and the effects of garnet resorption on Lu-Hf garnet ages were examined through numerical simulations of these processes. Concerning scales of disequilibrium, thirteen porphyroblastic datasets, previously measured using X-ray computed tomography, were examined by numerically simulating diffusion-controlled nucleation and growth of garnet while tracking chemical potential gradients to determine reaction affinity Ar (-[Delta]rG). Maximum nucleation rates are 10⁻¹³̇⁶-10⁻⁹̇⁸ nuclei cm⁻³ s⁻¹, interfacial energies are 0.004-0.14 J m⁻² assuming shape factors of 0.1-1.0, and Al intergranular diffusion (QD = 140 kJ/mol⁻¹) is 10⁻¹⁴̇⁴-10⁻¹¹̇¹ m² s⁻¹ at 600 °C. Limitations in determining crystallization kinetics arise from difficulties in constraining rock-specific properties (e.g., porosity and Al solubility). Ar at the time and location of nucleation is 0.4-5.9 kJ/mol⁻¹ of 12-oxygen garnet ([Delta]T = 4.0-62.0 °C) for the earliest nuclei, and 5.3-29.0 kJ/mol⁻¹ ([Delta]T = 50-125 °C) for nucleation at maximum Ar. The results demonstrate potential for delayed nucleation and metastability that can generate spurious interpretations. The timing of metamorphic events is also critical for understanding geologic history. In the Makhavinekh Lake Pluton aureole, Labrador, garnet resorption caused redistribution of Lu and loss of Hf from consumed rims, creating spuriously young ages. Garnet-ilmenite Lu-Hf geochronology using bulk separates yields apparent ages that young toward the contact from 1876 ± 21 Ma (4025 m) to 1396 ± 8 Ma (450 m). Toward the contact, garnet crystals are progressively more resorbed. Numerical modeling was used to test retention of Lu and loss of Hf during resorption as the dominant control on age. More resorption and Lu retention produce younger apparent ages (false ages). Application of the model to the aureole yields model ages from 1850 Ma to 1374 Ma, younging toward the contact. Thus, Lu-Hf geochronology applied to resorbed garnets requires careful examination of Lu zoning.