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Peridotite denuded by tectonic extension and exposed at the seafloor adjacent to slow-spreading centers hosts hydrothermal circulation of seawater. The reaction of seawater with peridotite causes serpentinization, which generates a high-pH, strongly reducing fluid rich in methane and hydrogen, and is accompanied by as much as 40% volume expansion. Complete serpentinization of peridotite requires tectonic activity to open fluid paths sealed by volume expansion. Diffuse venting of serpentinization fluids causes lithification of calcareous ooze on the seafloor to chalk-like limestone. This may be the main mechanism of deposition of ophicarbonates common in ophiolites. The degree of induration is a function of the fluid flux through the sediment column. Calcareous ooze infiltrates faults and fractures and can be deformed following lithification. Focused venting of serpentinization fluids may lead to deposition of large chimneys composed of calcite, aragonite, and brucite, such as those in the recently documented Lost City vent field (30[degrees]N, Mid-Atlantic Ridge). Geophysical implications of serpentinization include (1) creation of magnetic anomalies due to growth of magnetite in serpentinite and (2) lowered seismic velocity. Integrated studies of geologic and geophysical effects of serpentinizatiou may aid in a more complete understanding of the structure of oceanic lithosphere and the mechanisms that expose mantle peridotite at the seafloor. Keywords: carbonate, lithification, ridge, serpentinization, slow spreading.

In this paper, we show that deformation can be dated by combining mesoscopic and microscopic structural observations with an understanding of metamorphic mineral reactions and U-Pb ages of newly grown sphene (titanite). This approach can be used on a variety of rock types that have been deformed at a wide range of metamorphic conditions. In an example from the Proterozoic Laramie Peak shear zone of southeastern Wyoming, a single period of syntectonic sphene growth in sheared mafic dikes is documented both by a strong spatial relationship between deformation and metamorphism and by sphene microtextures. U-Pb analyses of sphene separates give overlapping concordant or nearly concordant ages with a weighted mean of the 207Pb/206Pb ages of 1763 [+ or -] 7 Ma. We interpret this direct age of deformation as evidence that the Laramie Peak shear zone records the cratonic response to the 1.78-1.74 Ga Cheyenne belt collisional event.

Two major mylonite zones are exposed in the southeastern Archean Wyoming province: the Cheyenne belt, which marks the boundary between the Wyoming and Colorado provinces, and the Laramie Peak shear zone, across which Archean rocks have been uplifted differentially at least 10 km. Between the two mylonite zones is a 60-100-km-wide belt of Archean migmatitic gneiss, which has been intruded by diabase dikes and peridotites and contains enclaves of high-grade metasedimentary rocks. We interpret this belt of high-grade rocks as a tectonically reactivated block of Archean crust, uplifted during the development of the Cheyenne belt collisional zone at ca. 1.8 Ga. This block does not exhibit a decreasing metamorphic gradient away from the mylonite zone, but is at uniformly high grade. Thus, rather than a thick-skinned thrust-ramp model for uplift, we invoke vertical uplift along a high-angle reverse fault without rotation. This style of uplift may be a tectonic response to transpression, underplating, or tectonic interfingering at depth during collisional orogeny, and it may be a more common feature of basement uplifts within foreland fold-and-thrust belts than has been previously recognized.

Presents information on reduced rapakivi-type granites, which are the most enriched and reduced of the `anorogenic' granite association. Features of granites; Composition of granites; Uses of the granites.