The calc-alkaline igneous rocks in the central Wasatch Mountains were emplaced between 36-30 Ma. They form a belt comprised of eleven stocks and the Keetley volcanic field aligned along the crustal suture between the Archean Wyoming province and accreted Paleoproterozoic terranes. Magmatism associated with this belt and its westward continuation into the Bingham mining district has been related to midCenozoic extension. These rocks consist of two types of stocks based on texture: a western type, which is coarse grained, and mostly equigranular, and an eastern type (including the Keetley volcanic rocks), which is fine grained and porphyritic. The compositional variation in the western stocks (Little Cottonwood, Alta, and Clayton Peak stocks) forms three distinct compositional groups. The compositional variation in the eastern stocks is similar to the compositional variation in the Alta stock. Major and trace element variations in these rocks resemble those of subduction-related magmas. However, the high KzO contents and low values are not consistent with this origin. These magmas formed from melting of mafic igneous rocks. We propose that magmas were generated by decompression melting due to gravitational collapse of the crust that had been thickened during Cretaceous to early Cenozoic deformation. Magmas rose to varying levels in the crust along a n east-west lineament. The igneous rocks of the central Wasatch Mountains have E~~(~~ similar to most of the Phanerozoic igneous rocks in the miogeocline (MG), but have significantly lower E ,~(~) .That anomaly has been explained as due to melting of a basement long depleted in Rb (Farmer andDePaolo, 1983, 1984). However, the Wasatch igneous belt rocks are high-potassium, calc-alkaline rocks and all have very similar incompatible trace element patterns, whereas only a few MG rocks are calc-alkaline or high potassium. Furthermore, in the MG rocks incompatible trace element patterns are variable. One possible explanation for the dilemma of long-time depletion of Rb in these high-potassium, calc-alkaline rocks is that the crust may have been recently charged with Rb and K during the Sevier-Laramide event (100-40 Ma) by dehydration of the subducting slab. This event was followed by melting during mid-Cenozoic collapse of the orogen (ca. 40-20 Ma). The source of the magmas was melting of mafic rocks in the lower crust. Some of these magmas ponded, formed magma chambers, and differentiated. Some involved little ponding and erupted directly on the surface in the form of the Keetley volcanic field. Continued melting and extension produced new magmas from a similar crustal source. These magmas were emplaced below a series of pull-apart structures associated with strike-slip displacement along an east-west suture. This suture may have been controlled by the ArcheanProterozoic boundary. Some magma bodies were emplaced quickly to the surface without significant fractionation. Others coalesced and fractionated over a protracted period of time. These magma bodies interacted with crus...

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Compositionally zoned ash flow sheets provide convincing evidence for chemically zoned magma bodies. Most workers have assumed that the high-silica portions of these magma bodies evolved largely by differentiation processes that occurred within the magma chamber. However, chemical heterogeneities within some ash flow sheets are not consistent with these differentiation processes. The chemical variation of pumice fragments in the large volume (>1200 km3), Rainier Mesa ash flow sheet ranges from 55 to 76.3% silica. These pumice fragments occur in three distinct chemical groups. A low-and high-silica group is separated by a compositional gap at about 72% silica, and within the high-silica group there are two distinct populations based on trace element variations. There is little overlap between populations. These three magma types have been resident in same magma chamber at the same time and cannot be produced by any differentiation process of a single magma body. They must reflect discrete magma batches generated in the source area. Furthermore, the lower silica portion (<72% SiOn) of the Rainier Mesa ash flow sheet is chemically distinct from the lower silica portion of the overlying Ammonia Tanks ash flow sheet, even though they erupted within 200,000 years of each other. These ash flow sheets from the SW Nevada volcanic field are associated in time and place with Basin and Range extension, and all models for extension involve detachment surfaces that extend to great depth. A model for the relationship of these compositional heterogeneities and the regional extension involves (1) the generation of magma batches by either continuous melting of the source at different temperatures, or by melting of different sources, (2) the use of faults (shears) as conduits for transport of magma, and (3) the use of a dilatant releasing step on a detachment as storage chamber for the magma. We first address the evidence for separately generated magma batches and then propose a model for their coalescence in a single chamber prior to eruption. [ 1981 ], Baker and McBirney [ 1985], and Trial and Spera [ 1990]). In almost all cases, the inferred trends toward the topCopyright 1995 by the American Geophysical Union. Paper number 95JB00515 0148-0227/95/95JB-00515505.00 of these magma bodies are increasing silica content, together with decreasing temperature and phenocryst content. Because of the occurrence of compositionally distinct pumice fragments within the upper portions of many large-volume ash flow sheets from the Southwest Nevada Volcanic Field, Vogel et al. [ 1987, 1989a], Flood et al. [ 1989a], Schuraytz et al. [1989], and Mills [1991] concluded that discrete compositional layers existed in their parental magma chamber. Regardless of the exact spatial configuration of these compositionally discrete magmas in a chamber, most workers generally accept the fact that large-scale zoning or layering was common. Most of these workers have assumed that an initial magma body evolved, to a large extent, by in situ differentiation from ...

A powerful north-east-trending fault cuts right across the north-west corner of Ireland and forms the most important member of a system of steep wrench-faults that dissect the Dalradian rocks of Donegal into parallel strips. The contrast between the rocks flanking this Leannan fault is very marked. On the north-western side the outcrops of the same succession of metasediments run parallel to the fault-line for over 60 miles; over this distance they show little change in lithology, structure, or metamorphism. On the other side the geology varies considerably along the fault, and, except in the north-east, the outcrops run at a high angle to the fault-line; as a result, several different rock-groups are brought against the fault. The metamorphism increases progressively to the south-west so that rocks of kyanite grade are finally brought against others of chlorite grade. The main movements on the fault are shown to have occurred between the deposition of red beds, presumably of Lower Old Red Sandstone age, and the deposition of Viséan marine sediments. The fault-zone generally consists of one main break accompanied by two others of lesser importance: together these fractures isolate narrow strips of metasediments in which inverted sequences are the rule. It is argued that the inversion of these strata predates the faulting, and that they were derived from the now eroded limb of a great recumbent fold. Along the main fault a total of 60 yards of intense deformation is usual, and black schist, platy siliceous mylonites, and quartzitic breccias are the result; the steep platiness is always set at an acute angle to the fault. Two separate movements are shown to have occurred; a downthrow to the south-east amounting to several thousand feet is combined with a left-handed displacement, the amount of which, though difficult to estimate, may be as much as 25 miles. The Leannan fault is directly in line with the Great Glen fault and the possibility that it is a continuation of this structure is discussed.

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