Robert T. Gregory

Tectonics of the Arabian margin associated with the formation and exhumation of high-pressure rocks, Sultanate of Oman

Continental crustal rocks, now structurally beneath the allochthonous Samail ophiolite, underwent blueschist to eclogite facies metamorphism prior to the emplacement of the Oman ophiolite onto the Arabian margin. The recognition of a major low angle fault within this polydeformed and polymeta-morphosed sequence of metacarbonates, metabasites, quartzites and quartz mica schists greatly simplifies the interpretation of the structure and metamorphic zonation within the Saih Hatat window, NE Oman. Carpholite-bearing upper plate rocks consist of pre-Permian to Jurassic rocks that have been folded into large recumbent nappe structures which exhibit a marked increase in deformation intensity toward the boundary with the lower plate. The lower plate rocks have higher peak pressure and temperature assemblages; they are exposed in two windows separated by Jabal Abu Daud. Although more intensely deformed, the lower plate rocks are recognizable as metamorphosed continental platform sediments. Fold axes are parallel to the regional NNE-SSW lineation. Sense of shear indicators yield a transport direction of south over north in the lower plate, opposite to the sense of motion inferred for the emplacement of the ophiolite. Lower plate eclogite-facies metabasalts are only preserved in kilometer-scale megaboudins found in the easternmost window at As Sifah. These metamorphic assemblages along with their remnant east-west fabrics define the existence of a short-lived, Arabian platform-directed, nascent subduction zone. The stretching lineation elsewhere within the upper and lower plates, the Hatat Schist (the basement), and the metamorphic sole of the ophiolite is consistently NNE-SSW, suggesting that the exhumation of high-pressure metamorphic rocks of Saih Hatat is related to the ophiolite obduction. Exhumation of the high pressure rocks was accompanied by intense deformation involving regional-scale fold nappes in a convergent margin setting. The geotherm remained suppressed for a period (>30 Myr) greater than the thermal relaxation time of the crust. The geometric and thermal constraints from Oman may be applicable to the general problem of the formation and preservation of high-pressure, low-temperature rocks.

Exhumation of high-pressure rocks in northeastern Oman

The exhumation of high- P rocks in the northeast Saih Hatat window, Oman Mountains, formed regional nappes associated with greenschist-facies metamorphism. Exhumation resulted in the juxtaposition of two plates with different structural style, stratigraphy, and metamorphic history. A major crustal discontinuity separates the two plates, with lower-metamorphic-grade rocks in the hanging wall. Lower-plate units were exhumed during strong top-to-the-northeast shearing, resulting in the formation of regional closures that evolved by folding and transposition of the high-pressure fabrics while the lithostatic pressure was decreasing. Upper-plate units were later exhumed with the juxtaposition of the two plates, truncation of lower-plate closures, and the formation of upper-plate nappes that are attenuated and sheathlike toward the upper and lower plate boundary. In some high- P terranes, apparent increases in metamorphic-grade down structural section and low-grade C′-type shear bands are often attributed to extensional exhumation. In Oman, however, these exhumation features formed during crustal-scale contraction at a convergent margin. The low-geothermal-gradient regimes afforded by convergent margins are critical for the generation and preservation of high- P –low- T rocks.

The importance of diffusion, advection, and host-rock lithology on vein formation: A stable isotope study from the Paleozoic Ouachita orogenic belt, Arkansas and Oklahoma

More than 600 stable isotope analyses from veins and their metasedimentary host rocks from the Ouachita orogenic belt of Arkansas and Oklahoma provide an opportunity to study fluid-rock interaction processes associated with vein formation during deformation and low-grade regional metamorphism. The δ18O values of vein quartz vary from 16.0 to 26.4‰, whereas coexisting host rocks have a greater range from 12.9 to 27.4‰. The oxygen isotopic compositions of quartz vein versus those of the coexisting host rocks follow an array described by δ18Ovein quartz ≈ δ18Owhole rock + e, where e ≈ 8–0.3(δ18Owhole rock). This relationship emphasizes the dependence of δ18O values of vein quartz on host-rock oxygen isotopic composition. The e term empirically monitors the difference between the quartz-water fractionation factor and the compositional dependence of the bulk-rock–water fractionation factor. Vein-quartz–host-rock Δ18O fractionations are ∼0‰ in chert, novaculite, quartzite, and siliceous shale and typically between 1 and 4‰ in sandstones and shales. In quartzite and sandstone units that are bounded by shales and associated with significant quartz-crystal deposits, vein-quartz–host-rock fractionations are often unusually large, near 7‰. Quartz-calcite oxygen isotope geothermometry indicates that veins from the Ouachita Mountains formed over a temperature interval of 100 °C, consistent with fluid-inclusion temperatures previously obtained from quartz crystals. Individual quartz veins are homogeneous, with <0.4‰ variation, for all vein orientations at all scales, even though vein formation occurred over a temperature interval in which quartz-water fractionation varies by 5‰. This homogeneity highlights the insensitivity of vein-quartz δ18O values to temperature when veins form under rock-buffered conditions. The similarity between vein and host-rock δ18O values in quartz-rich lithologies, and between vein and host-rock δ13C values in calcite-bearing rocks, indicates that diffusion was an important mass-transport mechanism. The variability in δ18O values between calcite-bearing veins and host rocks and large vein-quartz–whole-rock fractionations in some sandstones and quartzites indicates that advection also played a major role in mass transport associated with vein formation. This inference leads to the interpretation that veins from the Ouachita Mountains formed by a combined diffusion-advection process, whereby 18O and 13C from the host rock was transported into the veins with the assistance of a rock-buffered fluid on outcrop scales of 10–100 m.

CATCHMENT-SCALE COUPLING BETWEEN PYRITE OXIDATION AND CALCITE WEATHERING

Abstract Integrated time series of major element concentrations and flow rates from a small watershed (White Rock Creek, Dallas, TX) are used to examine the general hydrogeochemical controls on calcite weathering and the specific role of trace pyrite oxidation in base cation export. White Rock Creek is a perennial gaining stream occupying a catchment underlain by expanding clay soils, and an uppermost fractured bedrock weathering zone (which acts as a surface aquifer) below which there is low permeability Austin Chalk. Seasonal variations in cation export are controlled predominantly by temperature effects on carbonate solubility, dilution by variable amounts of rainfall, and possibly organic activity. Creek water PCO2 is 7 to 10 times atmospheric PCO2 throughout the year, and does not correlate with surface temperature. Non-pyrite sulfate inputs are well-constrained; moreover, integrated runoff and solute export from the basin are similar to global mean values. Pyrite, although present only as a trace component, accounts for a disproportionate amount of bedrock weathering (∼40%) when oxidized to produce sulfuric acid. Irreversible dissolution of pyrite is insensitive to seasonal changes in temperature, runoff, and organic activity and probably reflects the slow movement of a weathering front at depth through the aquifer.

Low-(18)O Silicic Magmas: Why Are They So Rare?

LOW-180 silicic magmas are reported from only a small number of localities (e.g., Yellowstone and Iceland), yet petrologic evidence points to upper crustal assimilation coupled with fractional crystallization (AFC) during magma genesis for nearly all silicic magmas. The rarity of 10W-l `O magmas in intracontinental caldera settings is remarkable given the evidence of intense 10W-l*O meteoric hydrothermal alteration in the subvolcanic remnants of larger caldera systems. In the Platoro caldera complex, regional ignimbrites (150-1000 km3) have plagioclase 6180 values of 6.8 + 0.1%., whereas the Middle Tuff, a small-volume (est. 50-100 km3) post-caldera collapse pyroclastic sequence, has plagioclase 8]80 values between 5.5 and 6.8%o. On average, the plagioclase phenocrysts from the Middle Tuff are depleted by only 0.3%0 relative to those in the regional tuffs. At Yellowstone, small-volume post-caldera collapse intracaldera rhyolites are up to 5.5%o depleted relative to the regional ignimbrites. Two important differences between the Middle Tuff and the Yellowstone 10W-180 rhyolites elucidate the problem. Middle Tuff magmas reached water saturation and erupted explosively, whereas most of the 10W-l 80 Yellowstone rhyolites erupted effusively as domes or flows, and are nearly devoid of hydrous phenocrysts. Comparing the two eruptive types indicates that assimilation of 10W-180 material, combined with fractional crystallization, drives silicic melts to water oversaturation. Water saturated magmas either erupt explosively or quench as subsurface porphyrins bejiire the magmatic 180 can be dramatically lowered. Partial melting of low- 180 subvolcanic rocks by near-anhydrous magmas at Yellowstone produced small- volume, 10W-180 magmas directly, thereby circumventing the water saturation barrier encountered through normal AFC processes.

Geological and geochronological constraints on the exhumation of a high-pressure metamorphic terrane, Oman

Abstract New 40Ar-39Ar age data from high-pressure former continental shelf rocks, now structurally beneath the Samail ophiolite, support a two-stage exhumation process for these rocks. High-P rocks occur as stacked fold-nappes within a less deformed upper plate and a strong to intensely deformed lower plate separated by a major, ductile, crustal discontinuity. The highest-grade lower plate rocks yield 40Ar-39Ar ages that range from 82 to 131 Ma. The majority of these ages are inferred to represent cooling by the progressive emplacement of colder units during convergent margin tectonism, before emplacement to higher structural levels. In the lower plate, 82–79 Ma 40Ar-39Ar ages are associated with lower-grade assemblages defining both transposition fabrics and C’-type shear bands which overprint the high-P assemblages during a NE-directed shearing event. Partial exhumation of these lower plate sequences resulted in the formation of regional closures by folding and transposition of earlier high-pressure fabrics. The lower plate closures are truncated by the structural break separating the two plates. Emplacement of the lower plate units to shallower crustal levels may have occurred before, or synchronously with, peak metamorphism of the upper plate units. The lower plate 40Ar-39Ar cooling(?) ages are older than mica crystallization ages of 76–70 Ma from the upper plate. These upper plate micas grew in axial surface fabrics associated with regional-scale fold-nappes which formed during the exhumation of the upper plate units and their transport over the higher-grade lower plate units. This nappe-forming event was possibly synchronous with structurally higher low-angle normal faulting and/or rapid erosion of the nappe pile before the deposition of Maastrichtian autochthonous units at c. 67–68 Ma.

Significance of Melt-Wall Rock Reaction: A Comparative Anatomy of Three Ophiolites

The ascent of basaltic melts through the upper mantle results in chemical disequilibrium between the melts and the pyroxene and plagioclase of the wall-rock peridotite. Phase and cryptic variations in ophiolitic peridotites demonstrate that the resulting reactions deplete the mantle in magmatophile components and enrich ascending melts in Ca, Na, Al, and incompatible trace elements while buffering their Mg/Mg + Fe ratios at primitive values (>0.6). A comparative anatomy of the Trinity, Oman, and Darb Zubaydah ophiolites illustrates both the significance of this process in shaping the composition of the shallow lithospheric mantle and how the integrated effects may reflect tectonic setting. A first-order correlation between crustal thickness and degree of mantle depletion exists, but multiple rifting events may remove part of the crustal record so that melt/rock ratios are difficult to quantify. Most impressive in ophiolitic peridotites is the abundance of melt crystallization and reaction products in zones of focused porous flow and(or) conduits indicating that melt segregation occurs at depths >20 km in extensional tectonic settings. Within these zones of melt transport, melt/rock reaction ratios will vary as the composition of the wall rocks evolves with melts becoming chemically insulated from the wall rocks when reaction zones of dunite develop. This may help explain why many MORBs retain trace-element signatures of a deep garnet-bearing source.

Effect of deformation on oxygen isotope exchange in the Heavitree Quartzite, Ruby Gap duplex, central Australia

Abstract δ18O values of the Heavitree Quartzite, central Australia, vary systematically as a function of deformation and recrystallization in a duplex composed of 5 superposed thrust sheets which formed under greenschist-grade conditions. Undeformed quartzite of the basal thrust sheet 1 and moderately strained and partially recrystallized quartzite of thrust sheet 2 range over 4%. in δ18O values and have an average value of ~12.8%., Entirely recrystallized quartzite of overlying thrust sheet 3 retains a similar average value of ~12.3%. but shows a narrow range of ~1.5%., suggesting isotopic homogenization. Recrystallized quartzites of sheet 5 have a lower δ18O value than the other thrust sheets. The homogenized δ18O values of sheet 3 relative to sheets 1 and 2 could not have resulted from mechanical mixing, isotopic exchange during dissolution-reprecipitation, or exchange by diffusion between the quartzite and a fluid in microfractures or along grain boundaries. The homogenized δ18O values of sheet 3 and lowered values of sheet 5 are interpreted to have been produced by isotopic exchange during recrystallization by grain-boundary migration. This is probably an important mechanism of isotopic exchange for rocks undergoing dynamic recrystallization.

Oxygen isotope disequilibrium between quartz and sanidine from the Bandelier Tuff, New Mexico, consistent with a short residence time of phenocrysts in rhyolitic magma

Abstract Oxygen isotope analyses are reported from co-existing quartz and feldspar from the Bandelier Tuff and Cerro Toledo high-silica rhyolitic pyroclastic deposits erupted from the Valles caldera, New Mexico. Quartz shows little variation outside analytical error, but δ 18 O in feldspar varies over >1‰. In most samples, 18 O/ 16 O fractionation between quartz and feldspar is significantly less than is predicted for equilibrium at temperatures appropriate for rhyolitic magma. In the Tshirege (upper) Member of the Bandelier Tuff, isotopic fractionation between mineral pairs is close to equilibrium in the later erupted ignimbrite, but non-equilibrium in the initial Plinian deposit. These relationships are interpreted in terms of a model where most phenocrysts are derived from a highly porphyritic carapace around the magma chamber that was disrupted by eruption, thus scattering crystals throughout the magma. Carapace quartz and feldspar are initially isotopically lighter than the bulk aphyric magma, due to chemical communication with low-δ 18 O country rock in the meteoric/hydrothermal system surrounding the chamber. We assume that quartz and feldspar were initially in isotopic equilibrium. Diffusive re-equilibration of crystals begins when the carapace disintegrates and the minerals are immersed in the bulk magma just prior to and during eruption. Feldspar is isotopically lighter than quartz at equilibrium, but responds more rapidly than quartz to an external change, due to a higher diffusion coefficient for oxygen. Hence, immersion in the isotopically heavier bulk magma causes feldspar and quartz δ 18 O values to initially converge over ∼10 2 years, and then diverge over 10 3 –10 4 years as first feldspar, and then quartz, re-equilibrate with the new magma. Higher δ 18 O variability of feldspar than quartz indicates that the shorter timescale applies to the Bandelier and Cerro Toledo rhyolites. Two important implications of this interpretation are (1) that the Bandelier magmas developed in an aphyric condition, and their porphyritic character is an artifact of eruption; and (2) that a protective, mechanically rigid cognate carapace around a silicic magma chamber may limit interaction with low-δ 18 O hydrothermally altered crust, thus hindering the development of significant volumes of low-δ 18 O silicic magma.

Age and Stratigraphic Relationships of Structurally Deepest level Rocks, Oman Mountains: U/Pb SHRIMP Evidence for Late Carboniferous Neotethys Rifting

Igneous zircons in felsic schist, or metatuff, infolded with mafic schist, calcschist, and quartz mica schist of the As Sifah lower plate window, NE Oman, have yielded a U/Pb SHRIMP crystallization age of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape