Jane Selverstone

Quantitative P-T paths from zoned minerals: Theory and tectonic applications

An analytical approach to the analysis of zoning profiles in minerals is presented that simultaneously accounts for all of the possible continuous reactions that may be operative in a given assemblage. The method involves deriving a system of simultaneous linear differential equations consisting of a Gibbs-Duhem equation for each phase, a set of linearly independent stoichiometric relations among the chemical potentials of phase components in the assemblage, and a set of equations describing the total differential of the slope of the tangent plane to the Gibbs free energy surface of solid solution phases. The variables are the differentials of T, P, chemical potentials of all phase components, and independent compositional terms of solid solution phases. The required input data are entropies, volumes, the compositions of coexisting phases at a reference P and T, and an expression for the curvature of the Gibbs functions for solid solution phases. Results derived are slopes of isopleths (dP/dT, dX/dT or dX/dP) which can be used to contour P-T diagrams with mineral composition.To interpret mineral zoning, T and P can be expressed as functions of n independent composition parameters, where n is the variance of the mineral assemblage. The total differentials of P and T are differential equations that can be solved by finite difference techniques using the derivatives obtained from the analytical formulation of phase equilibria.Results calculated from Zone I and Zone IV garnets of Tracy et al. (1976) indicate that Zone I garnets grew while T increased (ΔT≈+72° C) and P decreased sharply (ΔP≈−3 kb). Zone IV garnets zoned in response to decreasing T (ΔT≈−17° C) and P (ΔP≈−1 kb). A P-T path calculated for a zoned garnet from the Greinerschiefer series, western Tauern Window, Austria, also indicates growth during decompression (Δ∼−3kb) and heating (ΔT∼+15° C). A P-T path calculated for the Wissahickon schist (Crawford and Mark 1982) indicates growth during cooling and compression (ΔT∼−25δ C, ΔP∼+2.2 kb). The calculated P-T paths differ according to structural environment and can be used to relate mineral growth to tectonic processes.

Trace-element-rich brines in eclogitic veins: implications for fluid composition and transport during subduction

Primary and pseudosecondary fluid inclusions occur in oscillatory-and sector-zoned omphacite in eclogitic veins from the Monviso ophiolitic complex in the Western Alps. The inclusions contain aqueous brines and daughter crystals of halite, sylvite, calcite, dolomite, albite, anhydrite and/or gypsum, barite, baddeleyite, rutile, sphene, Fe oxides, pyrite and monazite. This daughter mineral suite indicates high solubilites of Na, K, Ca, Mg, Fe, Zr, Ti, P, Ba, Ce, La, Th, and S species and provides direct evidence for transport of high-fieldstrenght, large-ion-lithophile, and light-rare-earth elements as dissolved species during subduction. Fluid-inclusion heterogeneities preserved within and between adjacent grains in the veins, however, suggest that the scale of fluid equilibration was small. A crack-seal geometry in some of the veins implies that fluid release in pulses rather than steady flow controlled mineral deposition and growth in the veins. From these observations, we develop a model of fluid release and entrapment in which pulses of fluid are associated in time with increments of shear and tensile failure; the rate of fluid release and the reduction in porosity both depend on the rate of plastic flow. Vein fluids may initially be derived from decreptitation of early fluid inclusions in the host eclogites, Small-scale fluid heterogeneities implied by the fluid inclusions in the veins are best interpreted in terms of limited fluid flow, and hence limited metasomatism. We conclude that element recycling into the mantle wedge during subduction will depend at least as strongly on fluid transport mechanisms as on element solubilities in the fluid phase. At Monviso, despite evidence for high trace element solubilities in saline brines, the elements were not removed from the downgoing slab prior to teaching depths of ∼40 km.

Fluid variability in 2 GPa eclogites as an indicator of fluid behavior during subduction

Fluid activity ratios calculated between millimeter- to centimeter-scale layers in banded mafic eclogites from the Tauern Window, Austria, indicate that variations in aH2O existed between layers during equilibration at P approximately equal to 2GPa and T approximately equal to 625°C, whereas aCO2 was nearly constant between the same layers. Model calculations in the system H2O−CO2−NaCl show that these results are consistent with the existence of different saturated saline brines, carbonic fluids, or immiscible pairs of both in different layers. The data cannot be explained by the exisience of water-rich fluids in all layers. The model fluid compositions agree with fluid inclusion compositions from eclogite-stage veins and segregations that contain (1) saline brines (up to 39 equivalent wt. % NaCl) with up to six silicate, oxide, and carbonate daughter phases, and (2) carbonic fluids. The formation of crystalline segregations from fluid-filled pockets or hydrofractures indicates high fluid pressures at 2 GPa; the record of fluid variability in the banded eclogite host rocks, however, implies that fluid transport was limited to local flow along individual layers and that there was no large-scale mixing of fluids during devolatilization at depths of 60–70 km. The lack of evidence for fluid mixing may, in part, reflect variations in wetting behavior of fluids of different composition; nonwetting fluids (water-rich or carbonic) would be confined to intergranular pore spaces and would be essentially immobile, whereas wetting fluids (saline brines) could migrate more easily along an interconnected fluid network. The heterogeneous distribution of chemically distinct fluids may influence chemical transport processes during subduction by affecting mineral-fluid element partitioning and by altering the migration properties of the fluid phase(s) in the downgoing slab.

P-T paths from garnet zoning: A new technique for deciphering tectonic processes in crystalline terranes

A procedure to calculate quantitative rock pressure-temperature ( P-T ) paths based on chemical zoning profiles in garnet, combined with mineral chemistry from the other minerals in the assemblage, has been applied to samples from diverse tectonic settings with the result that the calculated P-T path is shown to be a sensitive monitor of tectonic processes. Terranes where metamorphic recrystallization paths are controlled by uplift and erosion show heating during decompression in the “prograde” P-T path and cooling during decompression in the “retrograde” path. In nappe terranes where hot rocks are emplaced over cool rocks, the upper plate shows cooling during decompression, whereas the lower plate shows heating during compression. In nappe terranes where cool rocks are emplaced over hot, the lower plate shows cooling during compression. Calculation of P-T paths from mineral zoning should provide a powerful new tool for deciphering tectonic processes in crystalline terranes.

Correlation by Rb-Sr geochronology of garnet growth histories from different structural levels within the Tauern Window, Eastern Alps

In order to evaluate rates of tectonometamorphic processes, growth rates of garnets from metamorphic rocks of the Tauern Window, Eastern Alps were measured using Rb-Sr isotopes. The garnet growth rates were determined from Rb-Sr isotopic zonation of single garnet crystals and the Rb-Sr isotopic compositions of their associated rock matrices. Garnets were analyzed from the Upper Schieferhülle (USH) and Lower Schieferhülle, (LSH) within the Tauern Window. Two garnets from the USH grew at rates of 0.67−0.13+0.19mm/million years and 0.88−0.19+0.34mm/million years, respectively, indicating an average growth duration of 5.4±1.7 million years. The duration of growth coupled with the amount of rotation recorded by inclusion trails in the USH garnets yields an average shear-strain rate during garnet growth of 2.7−0.7+1.2×10-14 s-1. Garnet growth in the sample from the USH occurred between 35.4±0.6 and 30±0.8 Ma. The garnet from the LSH grew at a rate of 0.23±0.015 mm/million years between 62±1.5 Ma and 30.2±1.5 Ma. Contemporaneous cessation of garnet growth in both units at ∼30 Ma is in accord with previous dating of the thermal peak of metamorphism in the Tauern Window. Correlation with previously published pressure-temperature paths for garnets from the USH and LSH yields approximate rates of burial, exhumation and heating during garnet growth. Assuming that theseP — T paths are applicable to the garnets in this study, the contemporaneous exhumation rates recorded by garnet in the USH and LSH were approximately 4−2+3mm/year and 2±1 mm/year, respectively.

Trace-element zoning in a metamorphic garnet

Trace-element zoning has been measured in an amphibolite garnet from the Tauern window, Austria, by using an ion microprobe. Humps in the zoning profiles of Na, Sc, V, Y, and the heavy rare-earth elements mark a period of open-system behavior. These humps correspond to a part of the major-element zoning profile that is interpreted as a P-T reversal. The source of the mass excesses of these elements remains ambiguous: they were derived either on a thin-section scale by the breakdown of trace-element-enriched refractory minerals or externally from unusual trace-element-enriched fluids. P-T paths determined from garnet zoning may require modification if open-system behavior is important during garnet growth.

Metamorphic consequences of thrust emplacement, Fall Mountain, New Hampshire

Metamorphic pressure-temperature (P-T) paths from the upper and lower plates of the Fall Mountain nappe, southwest New Hampshire, reveal different thermal histories in the two structural levels. Upper-plate rocks experienced early low-P, high-T (contact?) metamorphism to peak P-T conditions of 700-750 °C, 3-4.5 kbar. These P-T conditions were followed by loading to approximately 5-6 kbar, and then by nearly isobaric cooling to approximately 500 °C, 4.5 kbar. Upper-plate rocks then experienced minor heating with unloading and finally cooling. The lower-plate rocks also experienced early low-P, high-T (contact?) metamorphism but only to P-T conditions of 480-510 °C, 2-3.5 kbar. Nappe emplacement resulted in nearly isothermal loading to 500 °C, 5-6 kbar, which was followed by either heating or cooling, depending on the proximity of the sample to the upper plate. The early pressures recorded in both the upper- and lower-plate rocks suggest a depth of approximately 10 km for origination of the nappe; the post-nappe pressures (5-6 kbar) imply a total tectonic thickness of approximately 20 km or a doubling of the overburden thickness. Diffusion calculations suggest that the emplacement of the nappe took less than approximately 10 m.y., and U/Pb chronology on zircon suggests that it occurred between 400 and 410 Ma. 4O Ar/ 39 Ar cooling ages on micas indicate that the terrane was cooled to approximately 300 °C by 340 Ma.

Structural expression of a rolling hinge in the footwall of the Brenner Line normal fault, eastern Alps

The kinematic and temporal sequence of structures observed to overprint mylonites along the Brenner Line low-angle normal fault may record passage of the footwall through two rolling hinges, at the top and bottom of a ramp in the shear zone. The structures comprise west down brittle and brittle-ductile structures and east down brittle structures. PT conditions of formation (250° to >400°C and 2–23 km depth), obtained from analysis of oriented fluid inclusion planes, indicate that west down structures were formed at greater depths and temperatures, and therefore earlier, than the east down structures. These data suggest that the brittle structures formed under conditions that permit crystal-plastic deformation at long-term geologic strain rates and therefore probably reflect transient rapid strain rates and/or high fluid pressure. Structures inferred to have formed at a lower hinge are consistent with viscous flow models of rolling-hinge deformation and support the concept of a crustal asthenosphere. Such high temperatures at shallow crustal depth also suggest significant upward advection of heat by extensional unroofing of warm rocks, which may have reduced the flexural rigidity of the footwall and thus affected mechanical behavior at the upper rolling hinge. Exposed mylonitic foliation within a few hundred meters of the Brenner line and on top of the east–west trending anticlines in the footwall dips ∼15° west. Our data favor a ramp dip of ∼25° but permit a dip as great as 45°. Fluid inclusion data suggest that structures related to the hinge at the base of the ramp formed at depths of 12–25 km. If the average dip of the Brenner shear zone to those depths was 20°, intermediate between the favored ramp dip and the dip of exposed foliation, then the horizontal component of slip could be as high as 33–63 km. The two discrete sets of structures with opposite shear senses, formed in the temporal sequence indicated by PT data, are consistent with subvertical simple shear models of rolling-hinge strain. This kinematic pattern is not predicted by the flexural-failure model for rolling hinges. However, the predominance of normal slip at the upper hinge, which extends rather than shortens the mylonitic foliation, fails to match the subvertical simple shear model, which predicts shortening of the foliation there. One possible solution is that superposition of regional extension upon hinge-related stresses modified the rolling-hinge kinematics. Such a modified subvertical shear model can account for the observed small foliation-parallel extensional strains if the foliation was bent <5°–10° passing through the upper hinge. If more bending than that occurred, the data suggest rolling-hinge kinematics in which deformation is achieved by uniform-sense simple shear across the shear zone as in the subvertical simple shear model but in which material lines parallel to the shear-zone foliation and the detachment fault undergo very small length changes, presumably indicating that footwall rocks retained significant resistance to shear and underwent minimal permanent strain. The mechanics that would generate such a rolling hinge are uncertain but may incorporate aspects of both subvertical simple shear and flexural failure. An important kinematic consequence of such a rolling hinge is that all of the net slip across a normal fault, not only its horizontal component, is converted into horizontal extension. This implies a significantly larger magnitude of crustal extension across dipping normal faults whose footwalls passed through a rolling hinge than for those that did not develop along with a hinge.

Sm-Nd dating of multiple garnet growth events in an arc-continent collision zone, northwestern U.S. Cordillera

Integrated petrologic and Sm−Nd isotopic studies in garnet amphibolites along the Salmon River suture zone, western Idaho, delineate two periods of amphibolite grade metamorphism separated by at least 16 million years. In one amphibolite,P−T studies indicate a single stage of metamorphism with final equilibration at ∼600°C and 8–9 kbar. The Sm−Nd isotopic compositions of plagioclase, apatite, hornblende, and garnet define a precise, 8-point isochron of 128±3 Ma (MSWD=1.2) interpreted as mineral growth at the metamorphic peak. A40Ar/39Ar age for this hornblende indicates cooling through ∼525°C at 119±2 Ma. In a nearby amphibolite, garnets with a two-stage growth history consist of inclusion-rich cores surrounded by discontinuous, inclusion-free overgrowths. Temporal constraints for core and overgrowth development were derived from Sm−Nd garnet — whole rock pairs in which the garnet fractions consist of varying proportions of inclusion-free to inclusion-bearing fragments. Three garnet fractions with apparent “ages” of 144, 141, and 136 Ma are thought to represent mixtures between late Jurassic (pre-144 Ma) inherited radiogenic components preserved within garnet cores and early Cretaceous (∼128 Ma) garnet overgrowths. These observations confirm the resilience of garnet to diffusive exchange of trace elements during polymetamorphism at amphibolite facies conditions. Our geochronologic results show that metamorphism of arc-derived rocks in western Idaho was episodic and significantly older than in arc rocks along the eastern margin of the Wrangellian Superterrane in British Columbia and Alaska. The pre-144 Ma event may be an expression of the late Jurassic amalgamation of marginal oceanic arc-related terranes (e.g., Olds Ferry, Baker, Wallowa) during the initial phases of their collision with North American rocks. Peak metamorphism at ∼128 Ma reflects tectonic burial along the leading edge of the Wallowa arc terrane during its final penetration and suturing to cratonic North America.

Xenolithic evidence for Proterozoic crustal evolution beneath the Colorado Plateau

Tertiary diatremes of the Navajo volcanic field brought a wide variety of Proterozoic xenoliths to the surface of the Colorado Plateau. Examination of crustal xenoliths from the Navajo volcanic field diatremes permits reconstruction of Proterozoic pressure-temperature ( P-T ) histories beneath the Colorado Plateau. Diatremes from the northwest part of the Navajo volcanic field carry the greatest variety of xenoliths, including metasedimentary rocks, amphibolites, felsic gneisses, mafic granulites, and crustally derived eclogites; these rock types show variable degrees of hydrous alteration and evidence for complex reaction histories. In contrast, diatremes from the southeast part of the Navajo volcanic field contain primarily mafic and felsic granulites that show fewer reaction textures and less alteration; metasedimentary and eclogitic xenoliths are absent in these diatremes. The P-T paths from the northwest xenoliths are counterclockwise and reach temperatures as high as 850 °C and pressures equal to or greater than 10 kbar. Later hydrous alteration occurred around 500 °C and 8–12 kbar. Xenoliths from the southeast diatremes preserve little evidence of their P-T evolution, but paths involve heating and/or decompression. The differing rock types, P-T paths, and alteration histories of the northwest and southeast populations suggest that Proterozoic tectonism juxtaposed two distinct crustal blocks beneath the Colorado Plateau. Previous workers have postulated that the boundary between the Proterozoic Yavapai and Mazatzal provinces occurs in the region; our data support the existence of a northeast-trending boundary beneath the Four Corners area. If the eclogitic metamorphism and hydrous alteration are Proterozoic in age, their restricted occurrence beneath the northwest part of the plateau suggests that subduction was northwest dipping during Proterozoic accretion of the Mazatzal province onto North America.