Alleghanian-aged deformation and grain size reduction of quartzofeldspathic and mafic rocks of the Late Precambrian Ponaganset Gneiss proceeded at amphibolite grade conditions ( 600°C, 4-6 kb), and were accompanied by considerable mobility of Si, Na, K, Ca, Fe and Mg. Fluids present during and after ductile deformation facilitated hydration reactions within a narrow ( > 10 cm), foliation-parallel mafic layer composed largely of clinopyroxene and albitic plagioclase (An₃). The final product of these reactions was a K-feldspar/plagioclase (An₉)-rich amphibolite. Mass balance relations show that the amphibolite-forming reactions required an influx of Fe²⁺ and Mg²⁺ cations, and expulsion of Na⁺ into the surrounding gneisses. The source for Fe²⁺ and Mg²⁺ was probably the adjacent, actively-deforming quartzofeldspathic mylonite which, with reference to the protolith gneisses less than 30 cm from the gneiss-amphibolite contact, is reduced by 60% of its original concentration of these elements. Where no amphibolite layer is present, quartzofeldspathic mylonites display a similar depletion in Fe and Mg, but also carry textural and geochemical evidence for fluid flow on a millimeter to centimeter scale (e.g. foliation-parallel quartz veins walled by monomineralic layers of plagioclase or K-feldspar, and pronounced cm scale gradients in whole rock K, Na and Si concentrations).
Despite the evidence in favor of fluid flow, reaction progress calculations in the amphibolite layer suggest that minimum volumetric fluid-rock ratios never exceeded 0.05 liters H₂O/liter of rock. The local presence of strained clinopyroxene microporphyroclasts and laminae of clinopyroxene-albite mylonite in the amphibolite layer suggest that hydration reactions did not go to completion. The presence of biotite in the amphibolite layer elsewhere at the outcrop suggests that fluid-rock ratios were not uniform during, or shortly after, deformation. Post deformational metamorphic re- actions in the amphibolite resulted in the formation of actinolitic rims on amphibole grains which continued to grow at the expense of adjacent, strained clinopyroxene microporphyroclasts. Textural relationships indicate that the actinolitic rims grew via diffusive processes in the presence of an intergranular fluid film.
The ability of the zircon U-Pb system to yield useful geochronologic information on the timing of deformational events is a complex function of the degree of metamictization of the zircons, the chemistry and mineralogy of the enclosing rock, the P-T conditions of metamorphism and the presence and composition of metamorphic fluids. The possible effects of dynamic metamorphism on the isotopic ages obtained from zircons range from no apparent Pb loss to complete resetting of the isotopic "clock". The two examples presented here represent the two extremes: the mid-Paleozoic, greenschist-grade Rockfish Valley Fault Zone, and the PennsylvanianPermian amphibolite-grade Hope Valley Shear Zone. In the mylonites of the Rockfish Valley Fault Zone, zircons underwent brittle failure and were comminuted in the ductilely deforming matrix. Comparative U-Pb isotopic studies of zircons from the mylonites and from the Grenville-aged charnockitic protolith demonstrate that no appreciable Pb loss occurred as the result of intense mylonitization, despite the presence of aqueous fluids during deformation which affected the hydration of the mylonitic mineral assemblages.
A detailed U-Pb isotopic study of a single outcrop of the late Precambrian Ponaganset gneiss was also undertaken. Amphibolite-grade mylonitization associated with movement along the Hope Valley Shear Zone caused some grain size reduction of the zircons in the mylonite zones. However, the primary cause of isotopic disturbance in these zircons is the extensive growth of U-rich metamorphic zircon during, or shortly after, metamorphism. Corrosion of pre-existing zircon also occurred during metamorphism/deformation, but it is difficult to quantify its effects on U-Pb systematics. The lower intercept ages of 289± 24 m.y. and 265± 62 m.y. from the pink gneiss and gray gneiss, respectively, both fall within the range of other published age estimates (e.g. Zartman et al. 1988) for Alleghanian deformation and metamorphism in southeastern New England. / Ph. D.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/106531 |
| Date | January 1990 |
| Creators | Wayne, David Matthew |
| Contributors | Geology |
| Publisher | Virginia Polytechnic Institute and State University |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation, Text |
| Format | xvii, 324 leaves, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 23070293 |
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