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Abstract: The Sacawee block is a narrow belt of Paleo- to Mesoarchean crust that extends for ~70 km across the northern Granite Mountains. It is composed of the ~3.3 Ga [...]

Abstract: The Archean rocks in western Wyoming, including the Teton Range, the northern Wind River Range, and the western Owl Creek Mountains, preserve a record of a 2.68-2.67 Ga orogenic [...]

Abstract: Neoarchean supracrustal sequences in the south-central Wyoming Province are exposed as relatively small belts in Laramide uplifts. Some sequences are composed of materials derived mainly from pre-existing Wyoming province [...]

Abstract: The 2.95-2.82 Ga quartzofeldspathic gneisses and granitoids in the Bighorn, western Owl Creek, and north-eastern Wind River uplifts in the central Wyoming Province include low-K tonalite-trondhjemite-granodiorite (TTG) and high-K [...]

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.

The Sybille intrusion ([approximately equal to]100 [km.sup.2]) is one of three large monzonitic intrusions in the 1.43 Ga Laramie anorthosite complex of southeastern Wyoming. The petrographic, geochemical, isotopic, and geophysical characteristics of Sybille monzonitic rocks are consistent with an origin by extensive crystallization of liquids residual to nearby anorthositic cumulates (ferrodiorites) and contamination by Archcan wall rocks. The exposed part of the intrusion is composed mainly of coarse-grained monzosyenites with abundant alkali feldspar phenocrysts. The monzosyenites preserve mineralogical evidence for high crystallization temperatures ([greater than]1000 [degrees] C), mid-crustal eraplacement pressures ([approximately equal to]3 kbar), relatively reduced crystallization conditions (2 log units below the fayalite + magnetite + quartz [FMQ] oxygen buffer), and they crystallized in the presence of a C[O.sub.2]-rich fluid phase (Fuhrman et al., 1988; Frost and Touret, 1989). The eastern monzosyenites, those adjacent to contemporaneous anorthosite, are distinguished by an anhydrous mineral assemblage ([Fo.sub.16]-[Fo.sub.8] olivine, high-Ca pyroxene) lacking modal quartz, silica contents of [less than]60 wt%, and relatively large Eu anomalies (Eu/[Eu.sup.*] = 2.1 to 2.7). In contrast, the western monzosyenites, in proximity to Archean gneisses, are distinguished by the presence of modal quartz (3% to 28%), primary hornblende, Fe-enriched fayalitic olivine ([Fo.sub.7]-[Fo.sub.4]), silica contents of [greater than]60 wt%, and smaller Eu anomalies (Eu/[Eu.sup.*] = 1.2 to 1.3). Abundant xenoliths of Archean wall rocks and anorthosite from the adjacent intrusions in all monzosyenites attest to a stoping eraplacement mechanism near the roof of the chamber. We propose that the monzosyenites represent a relatively thin, 0.5-1.0-km-thick, roof to a magma chamber dominated by dense fer-rodioritic cumulates at depth. Extensive, open-system fractionation of a ferrodioritic parent magma, residual after crystallization of anorthosite, produced Fe-enriched monzodioritic and/or monzonitic magma in the upper part of the chamber and complementary Fe- and Ti-rich cumulates in the lower levels. We have corroborated the production of monzonitic liquids from crystallization of ferrodiorite through a series of reconnaissance equilibrium-crystallization experiments. The presence of dense ferrodioritic cumulates at depth is consistent with the prominent positive gravity anomaly associated with the Sybille intrusion (Hodge et al., 1973). In the upper parts of the chamber, the fractionated monzodioritic and/or monzonitic magmas eventually became saturated in alkali feldspar. Owing to density contrasts, the alkali feldspar phenocrysts floated to the roof of the chamber, thus producing the exposed porphyritic monzosyenites. In addition, the roof of the chamber was the site of significant melting of Archcan gneiss and, locally, metapelite. The Sr and Nd isotopic compositions of the monzosyenites, with Sr isotopic ratios becoming increasingly radiogenic from east ([I.sub.Sr] = 0.7059 and initial [[Epsilon].sub.Nd] = -2.5) to west ([I.sub.Sr] = 0.7092 and initial [[Epsilon].sub.Nd] = -2.6), are consistent with a 5% to 15% addition of Archean orthogneiss to a ferrodioritic parent magma that had isotopic characteristics similar to adjacent anorthositic rocks. The stratigraphic and compositional similarity of the Sybille monzosyenites to mangerites in the Bjerkreim-Sokndal intrusion of the Rogaland anorthosite complex, southern Norway, indicates that similar open-system magmatic processes are capable of having produced high-temperature, K-rich monzonitic rocks in other Proterozoic anorthosite complexes.

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.

The calibrated pyroxene QUIlF equilibria is used in determining the temperature, pressure and silica activity of various volcanic and plutonic rocks. It is assumed that the assemblage is in equilibrium, mineral composition is retained and accurately known, other components' effects on the pyroxene equilibria is solved and that the thermodynamic model is valid before the equilibria can be applied to the various rock samples.

The use of the QUILF equilibrium in determining the composition of Fe-Ti oxides combined with iron-rich olivine and quartz reduces the uncertainty produced in the magnetite-ilmenite thermometer. The equilibrium is also used in calibrating the Ca-free system. Five-phase assemblages are suitable for determining the parameters of crystallization in Fe-Ti oxides, pyroxenes, olivines and quartz.

In the system CaO-MgO-FeO-[Fe.sub.2][O.sub.3]-Ti[O.sub.2]-Si[O.sub.2], equilibria among titanite, pyroxene(s), olivine, spinel phase, ilmenite, and quartz constrain [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], T, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], [a.sub.Fe], and the compositional variables, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], [micro]Ca[(Mg,Fe)].sub.-1] and consequently [micro]Mg,[Fe.sub.-1] among the silicate and oxide phases. These equilibria can provide the framework for a better understanding of relevant titanite-bearing natural assemblages. In addition, such equilibria can constrain the variables that operate during crystallization of igneous and metamorphic rocks. Titanite-bearing assemblages with pyroxene(s), Fe-Mg-Ti ilmenite and spinel, olivine, and quartz are rare in metamorphic rocks but are common in igneous rocks where they are more commonly reported in plutonic than in volcanic rocks. Probably because, at high temperatures, titanite cannot coexist with the relative common in volcanic rocks sub-assemblage augite + orthopyroxene + olivine + spinel phase + ilmenite. Moreover, decreasing pressure and temperature conditions appear to favor titanite. Thus, it is not surprising that titanite is commonly observed in slowly cooled rocks, albeit, most typically in association with amphibole. We argue that the titanite + amphibole association is favored by high [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and, because [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] typically increases during crystallization of a pluton, titanite + amphibole (and consequently titanite) is more common in plutonic than in volcanic rocks. We have used a modified version of the program QUILF that includes our thermodynamic data for titanite and appropriate titanite-bearing assemblages to estimate the equilibrium crystallization conditions of samples from: (1) a tonalite from the Myoken-Zan granitic complex in Japan; (2) the Sandwich Horizon in the Skaergaard intrusion in Greenland; (3) rhyodacites, dacites, and andesites from the Sajama, Porquesa, and Parinacota volcanoes in the Andes; (4) the Fish Canyon Tuff in Colorado; and (5) a quartz-rich, calc-silicate gneiss adjacent to the Horse Creek Anorthosite Complex in Wyoming. The calculations suggest that: (1) these titanite-bearing assemblages crystallized at values of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] that ranged from slightly below FMQ to more than 3 [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] units above FMQ at 500-800 [degrees] C; and (2) the stability of titanite is controlled by T, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], and the compositions of the coexisting oxides and silicates.

The Archean rocks of the Wind River Range in western Wyoming record a Late Archean history of plutonism that extends for more than 250 m.y. The range is dominated by granitic plutons, including the 2.8 Ga Native Lake gneiss, the 2.67 Ga Bridger batholith, the 2.63 Ga Louis Lake batholith, and late 2.54 Ga granites. These plutons provide a means of distinguishing the complex metamorphism and deformation that affected the range in the Late Archean. Five deformation events are recorded. [D.sub.1] is a penetrative deformation that occurred during the earliest granulite-facies metamorphism; [D.sub.2] is a folding event, probably in amphibolite facies, that deforms porphyritic dikes that cut the [D.sub.1] fabrics. Both [D.sub.1] and [D.sub.2] predate the intrusion of the ca. 2.8 Ga Native Lake gneiss. [D.sub.3] is a folding event, accompanied by upper amphibolite to granulite metamorphism, that deformed the Medina Mountain sequence, a sequence of rocks that was either deposited or thrust upon the Native Lake gneiss. [D.sub.4] is a fabric-forming event associated with the Mount Helen structural belt (MHSB). It is represented by mylonites in the MHSB, a penetrative fabric in the Bridger batholith, and folding of the [D.sub.3] structures in the Medina Mountain sequence. We consider [D.sub.3] and [D.sub.4] to be coeval with the emplacement of the Bridger batholith, and hence to date at ca. 2.67 Ga. The latest structures ([D.sub.5]) are fabrics associated with the folding and thrusting of the 2.65 Ga South Pass sequence. We recognize at least four metamorphic events. [M.sub.1] is associated with the [D.sub.1] fabrics and occurred at high T ([is greater than] 750 [degrees] C) and high P (~7-8 kilobars). [M.sub.2] (650-750 [degrees] C and 4-5.5 kilobars) is associated with the intrusion of the Bridger batholith and formation of the [D.sub.3] and [D.sub.4] structures. The [D.sub.5] structures of the South Pass sequence record [M.sub.3], which is low P (~2-3 kilobars) and low T (~500 [degrees] C). The final metamorphism, [M.sub.4], is a contact metamorphism around the Louis Lake batholith. In the south against the South Pass sequence, the metamorphism occurred at ~3 kilobars and at temperatures [is less than] 700 [degrees] C. In contrast, in the north where the Louis Lake batholith is charnockitic, the metamorphism occurred at 6 kilobars and 800 [degrees] C. This pressure gradient is probably a reflection of tilting of the Wind River block during the Laramide orogeny. The composition of the plutons and the structural and metamorphic history of the Wind River Range indicate that during the Late Archean this area occupied the active margin of the Wyoming province. This tectonic environment is similar to the long-lived Phanerozoic margins of North America. The Wind River Range represents the best-documented active margin of Archean age. Keywords: active margin, Archean, metamorphic petrology, structure, tectonics, Wyoming province.