David A. Hewitt

Fluid interaction and element mobility in the development of ultramylonites

ABSTRACT Shear zones are often characterized by the presence of mylonitic fabrics. The textural development of such fabrics is enhanced by the presence of a fluid phase. In a single specimen of rock from the Brevard fault zone in North Carolina, we can demonstrate the development of ultramylonite domain by focused fluid flow. The ultramylonite interface retains a sharp textural and chemical discontinuity; this suggests that solute transport was dominantly parallel to the tectonic layering. Major chemical changes between the ultramylonite zone and the protolith include losses of Si(>2, Na 2 0, and K 2 0 and gain of CaO, FeO, and HjO in the high-strain domain. INTRODUCTION During the tectono-thermal evolution of mountain belts, zones of high strain are often recorded by the development of mylonites. Although mylonites and subcategories within mylonites (Sibson, 1977) can be generated by numerous well-documented physical processes, their chemi-cal and isotopic evolution is much more complex. Mechanical processes often associated with the development of high-strain zones include micro-fracturing, dilatancy, grain-boundary sliding, and dislocation glide. These properties are strongly dependent on the modal mineralogy (bulk compo-sition), heterogeneity of the rock, and the temperature and pressure con-ditions during deformation. Such variations in the mechanics of deforma-tion have led to the more common classification of shear zones as either ductile or brittle (Sibson, 1977; Ramsay, 1980). However, most shear zones are characterized by the availability of fluids that cause a drastic change in the style of deformation by altering the behavior and assem-blage of minerals (hydration reactions) and the rates of chemical and mechanical processes during progressive deformation. The presence of a fluid phase during deformation enhances solution transport, microfractur-ing, and recrystallization Suc.h chemically active zones generate mineral assemblages that record the metamorphic history of the mylonites, al-though it is not unusual to find minerals that persist through such an event. These relict minerals, if recognized, can identify equilibrium vs. nonequilibrium assemblages observed in shear zones. Clearly, the pres-ence of relict minerals would have a substantial impact on the trace-element and isotopic signature of the rock. To more carefully document the behavior of elements in a strain zone that has abundant fluid, we have chosen to study a transect across the Brevard fault zone, near Rosman, North Carolina. Within this tran-sect, one particular outcrop provided a single large specimen that over a distance of-25 cm showed the entire strain-gradient fabric from proto-mylonite to ultramylonite (see Sibson, 1977, for mylonite series) ob-served by us as discontinuous patches in the longer (15-km) transect. We report petrographic, textural, and chemical data from this block and pro-vide a physical model to explain these data.

Differential response of zircon U−Pb isotopic systematics to metamorphism across a lithologic boundary: an example from the Hope Valley Shear Zone, southeastern Massachusetts, USA

AbstractA detailed morphological, chemical and isotopic study of zircons from a single outcrop of two mineralogically and chemically distinct units of the late Precambrian Ponaganset gneiss was undertaken to investigate the effects of mylonitization and metamorphism on U-Pb isotopic systematics. Late Paleozoic, amphibolite-grade (approx. 600°C) mylonitization of the Ponaganset gneiss at this locality is associated with movement along the Hope Valley Shear Zone. The response of zircon to metamorphism in each gneiss unit is distinct: zircons in gray augen gneiss are uncorroded and not overgrown, whereas zircons from fluorite-bearing pink granitic gneiss are variably corroded and over 50% bear opaque overgrowths. The zircon overgrowths are chemically distinct from the primary cores, and contain high conentrations of Hf, U, HREE, and Th. Mylonite derived from the gray gneiss contains only a small population of Hf-U-rich metamorphic zircon, but zircons in the pink gneiss-derived mylonite are dominated by the Hf-U-rich metamorphic component. In terms of their U-Pb isotopic systematics, overgrowth-free zircons from both units are markedly discordant (gray, 10–20%, pink, 35%), but overgrown zircons from the pink gneiss are up to 70% discordant. Zircons from the mylonites yield younger Pb−Pb and U−Pb ages than those of the protolith gneisses, and isotopic data from each gneiss + mylonite pair define a linear array on concordia plots. Upper intercept ages of the gray gneiss (621+/−27 Ma) and the pink gneiss (635+/−50 Ma) indicate that the crystallization of both units was coeval, and the lower intercept ages (gray, 270+/−92 Ma; pink, 285+/−26 Ma) fall within the range of other published age estimates for Alleghanian metamorphism in southeastern New England (e.g., Zartman et al. 1988). New growth of zircon suggests that Zr was mobile during metamorphism. The presence of fluorite in the pink gneiss, and a discontinuity in log

Hydrogen fugacities in Shaw bomb experiments

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The hydrothermal stability of zircon: Preliminary experimental and isotopic studies

values obtained from biotite across the pink gneiss-gray gneiss contact indicates that dissolution and reprecipitation of zircon may be related to local variations in HF fugacity. Zircon dissolution/reprecipitation in the pink gneiss, and the lack of similar features in the contiguous gray gneiss, suggests that the degree of isotopic perturbation of zircon during metamorphism is related to bulk chemistry, fluid chemistry and/or the degree of fluid-rock interaction.

Experimental phase relations of the micas

We present a method to measure carbonate content of apatite that uses infrared spectroscopy, The method was calibrated against the amount of C02 released by reaction of apatite with phosphoric acid and may be used to study apatite with C02 content below I wt%. We have applied the method to apatite from carbonatite (>:::0.3-0.4 wt% C02) and metasomatic (>:::0.6-1.0 wt% C02) and alkaline silicate «0.3 wt% C02) rocks from the Jacupiranga alkaline complex. The results show that higher concentration of carbonate was found in apatite related to metasomatic processes and, in particular, to metasomatism related to the intrusion of carbonatite from Jacupiranga. Major and trace elements of the analyzed apatites do not show a simple substitution mechanism that would explain the presence of carbonate in the apatite structure. In a series of exploratory laboratory experiments in which apatite was synthesized under high temperature and pressure, no clear relationship was observed between the carbonate content of the synthetic apatite and the total C content in the coexisting fluid (molecular C02 or carbonate ions in solution). However, even though these experiments were not conclusive, they suggest that apatite coexisting with carbonate ions in solution has a C02 content above 0.3 wt%, whereas apatite coexisting with molecular C02 has a C02 content <0.3 wt%. These results indicate that the C02 content of apatite depends not only on total fluid C content, but also on the C speciation in the fluid.

Experimental evidence on the substitution of Ti in biotite

AbstractExperiments show that equilibration times across Shaw hydrogen diffusion membranes are variable. Using quartz-filled platinum membranes with fugacity differentials less than 20 bars, equilibration times at 750 ° C range from 1–3 days with new membranes to greater than 10 days for membranes with extensive use. New membranes at 650 ° C equilibrate in ∼7 days. At these temperatures the steady state hydrogen lag pressure between the bomb and membrane is less than one bar as long as relatively new membranes are in use.These slow hydrogen equilibrations necessitate that exacting procedures be followed when using the bombs for equilibrium determinations. The initial unequilibrated hydrogen fugacity in the bomb must be higher than the final equilibrated hydrogen fugacity for a reduced starting assemblage or must be lower than the final equilibrated hydrogen fugacity if the starting assemblage is oxidized. This criterion can be met by carefully controlling the initial hydrogen/argon ratio in the pressurizing fluid, and can be verified by monitoring changes in the membrane pressure.Determinations of the Ni-NiO equilibrium at 750 °C and 850 °C at 1 kbar have been performed in order to test the method. The corrected 1 bar values, log

Experimental observations on coexisting zoisite-clinozoisite

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