Origin and emplacement of igneous rocks in the central Wasatch Mountains, Utah

Abstract

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...

The effect of drainage reorganization on paleoaltimetry studies: An example from the Paleogene Laramide foreland

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Age and petrogenesis of volcanic and intrusive rocks in the Sulphur Spring Range, central Nevada: Comparisons with ore-associated Eocene magma systems in the Great Basin

“…Apparently several magma systems were episodically active beneath the Carlin trend during the Eocene and Oligocene (Grauch et al, 2003;Ressel and Henry, 2006). Concurrent magmatism occurred at similar latitudes in Utah, including that responsible for the Bingham, Clayton, and Alta stocks (Vogel et al, 2001;Deino and Keith, 1997).…”

Exhumation of the central Wasatch Mountains, Utah: 1. Patterns and timing of exhumation deduced from low‐temperature thermochronology data

“…Zircon U/Pb ages for the intrusions range from 36 Ma for the Clayton Peak stock to 30 Ma for the western Little Cottonwood stock [ Feher et al , 1996; Constenius , 1998]. The easternmost intrusions are porphyritic with microcrystalline to fine‐grained groundmass and include the Flagstaff, Ontario, Mayflower, and Pine Creek stocks, which range in age from ∼34 to ∼39 Ma [ Vogel et al , 1997; Constenius , 1998, and references therein]. The Keetley Volcanics crop out east of the easternmost intrusions dated in this study.…”

“…In Figure 2b, sample numbers are keyed to Tables 1 and 2. In Figure 2c, numbers in parentheses and brackets are intrusion age and depth ranges [ John , 1989; Feher et al , 1996; Vogel et al , 1997; Constenius , 1998]. LCC, Little Cottonwood Canyon; BCC, Big Cottonwood Canyon; LCS, Little Cottonwood Stock; AS, Alta Stock; CPS, Clayton Peak Stock; OFMS, Ontario, Flagstaff, and Mayflower stocks; PPS, Park Premier stock; SF, Superior Fault; SFF, Silver Fork Fault; AGT, Alta‐Grizzly Thrust; AFF, American Fork Fault; DCF, Deer Creek Fault; CNT, Charleston‐Nebo Thrust.…”