New Mexico Geological Society Annual Spring Meeting — Abstracts

Recent work in the Proterozoic of southwest North America suggests that the ca. 1.4 Ga metamorphic and deformational events are more important and more widespread than previously thought (Nyman et al., 1994; Kirby et al., 1995; Karlstrom et al., 1996). These ca. 1.4 Ga events involved regional heating at low to moderate pressure (3-5 kbar), large-scale, widespread granitoid intrusions and movement on some shear zones and deformation localized around some 1.4 plutons. The Big Thompson Canyon region of the Yavapai Province in the northeastern Colorado Front Range was previously thought to have experienced one low-pressure, hightemperature metamorphism ca. 1.7 Ga during the collision of this region with the Archean Wyoming Province. Recent workers (Selverstone et al., 1995; Shaw et al., 1995) in this area have found abundant evidence for a more complex, polymetamorphic history which includes an early high pressure, high temperature metamorphism ca. 1.7 Ga and a later low pressure, high temperature metamorphism ca. 1.4 Ga.

Metamorphosed Proterozoic supracrustal rocks are exposed approximately 100 km south of the suture with the Archean Wyoming Province in the Front Range of Colorado. This region, centered on the Big Thompson Canyon, consists of subvertical, poly deformed metaturbidite sequences that still preserve relict graded bedding. The metasedimentary rocks range in grade from biotite zone to the onset of partial melting and are concordantly intruded by ~1.7 Ga trondhjemites parallel to foliation. The high-grade rocks of this region are intruded by discordant bodies of ~1.4 Ga granite. This is the only area in the Front Range of Colorado where a complete progression from low to high grades is preserved and it therefore presents the opportunity to determine the P-T-t-deformation history of Proterozoic tectonism uncomplicated at lower grades by the effects of diffusional reequilibration. The 1.7 Gainetamorphism reached garnet-staurolite grade over a large part of the area following isoclinal folding and development of axial planar foliation and prior to the development of steep, asymmetric crenulation cleavages. The initial prograde metamorphic assemblages were retrogressed to chlorite + white mica ± cordierite pseudomorphs by post-deformational fluid-rock interaction. New euhedral garnet and staurolite growth in some of these pseudomorphs indicates a subsequent, short-lived heating event. Ar-Ar age spectra from hornblende, muscovite and biotite indicate that this rcheating event reached temperatures of about 500 to 550° C ca. 1.4 Ga followed by rapid cooling (Shaw et al., 1995). The isograd pattern of this region is interpreted as a combination of 1.7 and 1.4 Ga metamorphism rather than just 1.7 Ga tectonism, as previously thought. The isograds and locations where Ar age spectra indicate 1.4 Ga resetting are not spatially related to 1.4 Ga plutons and therefore may represent a widespread mid-crustal heating event. PT data indicate that both the 1.7 and 1.4 Ga metamorphic events occurred at pressures of 2 to 4 kbar, however some samples preserve earlier pressures of 8 to 10 kbar that probably pre-date the emplacement of the 1.7 Ga intrusions. These data are interpreted to indicate that the early metamorphism involved deep burial of the sedimentary sequences during the collision with the Wyoming craton ca. 1.8 Ga. The rocks were then transported to shallower depths and intruded by 1.7 Ga trondhjemites, metamorphosed to gt-st grade and further deformed. The region experienced pervasive post-deformational hydration and metamorphism to T>500 °C ca. 1.4 Ga. Field relations and geochronologic data are incompatible with a model of early, ca. 1.7 Ga metamorphism followed by slow cooling, and indicate that the 1.4 Ga metamorphic event is more significant in this area than previously thought. Many observations made in the Front Range are similar to those made in the Proterozoic rocks of New Mexico and Arizona. This relationship indicates that similar processes occurred throughout the southwest U.S. ca. 1.4 Ga and that the heating event of that age is a widespread, regional metamorphism.