Zircon U-Pb Geochronology of Two Basement Cores (Kentucky, USA): Implications for Late Mesoproterozoic Sedimentation and Tectonics in the Eastern Midcontinent

Basement cores from two wells drilled west and east of the Grenville front consist of feldspathic litharenite and granitic orthogneiss, respectively. Detrital zircon U-Pb ages for the litharenite define a broad dominant U-Pb age mode at ca. 1115 Ma. The dominant mode matches that for the type locality of the Middle Run Formation in the Ohio subsurface and is interpreted to consist of detrital zircons sourced from East Continent Rift volcanic sources (ca. 1100 Ma) and Grenville Shawinigan granites/gneisses (1120–1180 Ma). The youngest detrital zircon ages (ca. 1020 Ma) require a maximum depositional age that is at least 70 My younger than the time of Midcontinent and East Continent rifting and magmatism. We propose that the litharenite is correlative with the Middle Run Formation in Ohio and was deposited in an evolving late Grenville rift/foreland basin adjacent to the exhuming Grenville orogen. Zircon U-Pb secondary-ion mass spectrometry ages from orthogneiss define a discordant array with intercepts of ca. 1500 and 1000 Ma. The oldest concordant dates (ca. 1450 Ma, from oscillatory-zoned cores) are interpreted as the crystallization age of the igneous protolith of the orthogneiss. Metamorphic zircon rims define a weighted mean U-Pb age of 1018 ± 19 Ma (2σ) Ma, interpreted to represent the time of high-grade metamorphism during the late Ottawan phase of the Grenville orogeny. This age pattern matches that of exposed basement in the Central Gneiss Belt of the Grenville Province (Ontario) and similar basement orthogneisses in Ohio and Kentucky that are interpreted to be of Eastern Granite-Rhyolite Province affinity. All age data are consistent with a provenance model of an actively exhuming Grenville orogen at ca. 1000 Ma producing sediment that is mixing with recycled East Continent Rift sediments

Calculation of CO 2 activities using scapolite equilibria: constraints on the presence and composition of a fluid phase during high grade metamorphism

Thermodynamic and phase equilibrium data for scapolite have been used to calculate CO 2 activities ( a CO 2 ) and to evaluate the presence or absence of a fluid phase in high-grade scapolite bearing meta-anorthosite, granulites, calc-silicates, and mafix xenoliths. The assemblage scapolite-plagioclase-garnet±quartz may be used to calculate or limit a CO 2 by the reaction Meionite+Quartz = Grossular+Anorthite+CO 2 . Granulites from four high-grade terranes (Grenville Province, Canada; Sargut Belt, India; Furua Complex, Tanzania; Bergen Arcs, Norway) yield a CO 2 =0.4-1, with most >0.7. For scapolite-bearing granulites from the Furua Complex, in which a CO 2 ≥0.9, calculated H 2 O activities ( a H 2 O) based on phlogopite dehydration equilibria are uniformly low (0.1–0.2). The a CO 2 calculated for meta-anorthosite from the Grenville Province, Ontario, ranges from 0.2 to 0.8. For Grenville meta-anorthosite also containing epidote, the a H 2 O calculated from clinozoisite dehydration ranges from 0.2 to 0.6. Calc-silicates from the Grenville, Sargur, and Furua terranes mostly yield a CO 2 1). The calculated fluid activities are consistent with metamorphism (1) in the presence of a mixed CO 2 −H 2 O fluid phase in which CO 2 is the dominant fluid species but other C−O−H−S species are minor, (2) in the absence of a bulk fluid phase (“fluid-absent metamorphism”), or (3) in the presence of a fluid-bearing melt phase. The results for many granulites and Grenville meta-anorthosite are consistent with the presence of a CO 2 -rich, mixed CO 2 −H 2 O fluid phase. In contrast the relatively restricted and low values of a CO 2 for calc-silicates require an H 2 O-rich fluid or absence of a fluid phase during metamorphism. The range of values for xenoliths are most consistent with absence of a fluid phase. The primary implication of these results is that a CO 2 -rich fluid accounts for the reduced a H2 O in scapolite-bearing granulites. However, scapolite may be stable with a wide range of fluid compositions or in the absence of a fluid phase, and the presence of scapolite is not a priori evidence of a CO 2 -rich fluid phase. In addition, close association of scapolite-free mafic granulites with scapolite-bearing granulites having identical mineral compositions in the Furua Complex, and the absence of scapolite from most granulite terranes implies that a CO 2 -rich fluid phase is not pervasive on an outcrop scale or common to all granulite terranes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47295/1/410_2004_Article_BF00307340.pd