Contrasting responses of Rb–Sr systematics to regional and contact metamorphism, Laramie Mountains, Wyoming, USA

Abstract

Archean supracrustal sequences of pelitic, quartzitic, calcareous and mafic compositions in the central Laramie Mountains, Wyoming, have been affected by two metamorphic events: a 1.78 Ga amphibolite‐grade regional metamorphism, and a 1.43 Ga contact metamorphism resulting from the intrusion of the Laramie Anorthosite Complex (LAC). Rb–Sr whole‐rock isotopic data from both outside and within the LAC contact aureole define a linear array that lies along a 1.78 Ga isochron. This date has been independently established as the time of amphibolite facies regional metamorphism associated with collision of the Archean Wyoming province and the Proterozoic Colorado province along the Cheyenne belt. The Rb–Sr isotopic data require that Sr was redistributed during regional metamorphism on a scale of at least tens of metres. Although within the 2 km‐wide aureole of LAC the pelitic rocks were thermally metamorphosed at temperatures greater than 800 °C, none of the whole‐rock Rb–Sr data from samples within the LAC aureole show evidence of resetting at 1.43 Ga. It is interpreted that the regional metamorphism involved fluid transport which facilitated Sr isotopic resetting, whereas the contact metamorphism occurred in a relatively dry environment in which isotopic mobility was restricted to centimetre‐scale or less. Rb–Sr data for biotite, feldspar and whole rock from a regional metamorphosed pelitic schist give an isochron age of 1450±40 Ma, which is interpreted as a cooling age resulting from crustal uplift. Rb–Sr data for biotite, quartz+feldspar and whole rock from a pelitic schist affected by contact metamorphism give an isochron age of 1420±43 Ma, the time of isotopic re‐equilibration in response either to crustal uplift or to both contact metamorphism and crustal uplift. This study demonstrates that although the response of isotopic systems to metamorphism is complex, isotopic data provide insight into metamorphic processes that is difficult to obtain by other means.

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

“…We interpret the increase in 87 Sr/ 86 Sr ratios in carbonate samples from Lake Uinta at~53 Ma as the result of a large-scale reorganization of the lake's drainage system with new and substantial inflows from Lake Gosiute and the catchments of the Greater Green River Basin that eventually extended into the Challis Volcanic Field at~49 Ma (KentCorson et al, 2006;Carroll et al, 2008 Sr ratios of samples from the Laney Member of the Green River Formation (~50 Ma) in the Greater Green River Basin (~285 km from our sampled section over the Uinta Uplift) give a mean of 0.71245 ± 0.00014 (n = 22, p b 0.05) (Rhodes et al, 2002). The more radiogenic Sr isotope composition of these samples reflects the radiogenic composition of Sr in adjacent, craton-cored Laramide structures (e.g., cores of the Wind River, Granite, Owl Creek, Laramie and Sierra Madre Uplifts were exposed in the Eocene Carroll et al, 2006), where whole rock 87 Sr/ 86 Sr ratios can be in excess of 1.0 (Divis, 1977;Peterman and Hildreth, 1978;Zielinski et al, 1981;Mueller et al, 1985;Frost et al, 1998;Patel et al, 1999). Although the Uinta Uplift bounding the Uinta Basin to the north is also basement-cored, the east-trending axis of the range is beyond the southwestern margin of the Wyoming craton, so that basement rocks at the core of the Uinta Uplift are a mixture of sediments deposited during the Neoproterozoic accretion of terranes (Ball and Farmer, 1998;Condie et al, 2001;Nelson et al, 2002;Foster et al, 2006 Sr ratios to also be high during the later stages of Lake Uinta's lifespan, when braided streams flowing south from the Uinta Uplift deposited fluvial units along the northern margin of the lake (Anderson and Picard, 1972;Davis et al, in press .…”

Long-term tectonothermal history of Laramide basement from zircon–He age-eU correlations

“…As discussed above, a few K-Ar ages from the southernmost Wind Rivers hint at cooling below ~300 °C at approximately 1500 Ma. The next nearest constraints are Rb/Sr data from the Laramie Range in southeastern Wyoming, which suggest an ~1.78 Ga metamorphic cooling event (Patel et al, 1999). Similarly, K-Ar hornblende and U-Pb apatite ages within the Laramie Peak shear zone and Cheyenne Belt are interpreted to record rapid uplift during Cheyenne belt deformation between 1.8-1.6 Ga (Chamberlain et al, 1993).…”