Introduction
The structural and metamorphic evolution of low-pressure, high-temperature Proterozoic rocks in the Colorado frontal ranges, which contain an abundance of granites, is similar to such orogenic belts around the world including the Lachlan Fold Belt (Collins and Vernon 1992), the Pyrenees (Mezger and Passchier 2003) and western Maine (De Yoreo 1989). Mechanisms that have been proposed for the origin of such settings include compression of previously extended continental crust (Oliver 1991), mantle delamination (Loosveld and Etheridge 1990), heat advection via intrusions due to high mantle heat flow (Rubenach 1992; Sandiford 1995; Rubenach and Baker 1998), radiogenic heating due to enrichment of heat-forming elements in the upper crust and the burial of heat-producing stratigraphic sequences (Mildren and Sandiford 1995; Sandiford et al. 1998; McLaren 1999).
The relationship between metamorphism and granite emplacement has always been somewhat of an enigma. The question of which comes first has always been a point of debate and this, to some extent still remains unresolved, although many would favour that the heat generated by orogenesis eventually promotes melting and granite emplacement (Brown 1994a; Brown and Solar 1999). The common relationship observed, that the metamorphic grade increases towards igneous bodies, does not resolve this question because any heat source causing metamorphism could eventually generate a pluton rather than vice versa.
Until recently, generally only one isograd per mineral phase could be distinguished because routine quantitative separation of the timing of growth of multiple different phases of the one mineral from sample to sample across a region was not possible. With the advent of a technique for measuring foliation inflection/intersection axes in porphyroblasts (FIAs), it was realized that different periods of growth of a single porphyroblastic phase could be identified, correlated across a region (e.g. Bell et al. 1998) and dated (e.g. Bell and Welch 2002; Ali 2010; Sanislav 2010; Sanislav and Shah 2010). Consequently, one can potentially distinguish and thus map the distribution of isograds for different periods of growth of a single mineral phase .
Figure 1. Regional map of the Colorado Frontal Range
Regional map of the Colorado Frontal Range showing the Precambrian rocks and the location of the study area (box shows area of Fig. 3). BCSZ =Buckhorn Creek shear zone, CB = Cheyenne belt, ISRSZ = Idaho Springs-Ralston shear zone, MMSZ = Moose Mountain shear zone, SGSZ = Skin Gulch shear zone (modified after Cavosie and Selverstone, 2003).
The area described herein forms part of a Proterozoic orogenic belt in the southwestern United States and provides a classic example of a large high-T-low-P terrane and the problems associated with the tectonic interpretation of such regimes (Williams and Karlstrom 1996). The only indication of deformation events that occur before the peak of metamorphism, and how they affected the thermal structure of an orogenic belt, comes from combined quantitative micro and macro structural studies. Microstructural studies are also important in distinguishing between events at the peak of metamorphism and those significantly post-dating it (Thompson and Ridley 1987). This research has used the combined approach of integrating geochronological, metamorphic and structural studies (using FIAs).
The technique used requires the measurement of the orientation of foliation inflection intersection axes preserved by inclusion trails within porphyroblasts, which are called FIAs. The measurement of these structures within garnet, staurolite andalusite and cordierite porphyroblasts from the Colorado foothills of the Rocky Mountains has revealed a succession of FIAs within these rocks. This has enabled examination of whether or not the distribution of staurolite, cordierite and andalusite isograds changed with time. That is, whether they shifted with time as each FIA set developed. This paper reveals the role of granitic emplacement versus isograd migration with time, and their significance to the overall interpretation of PTtd paths.