Journal of the Virtual Explorer

Isograds can potentially be obliterated by further heating (e.g. Braddock and Cole 1979). Shaw (1999) and Selverstone (1997; Fig. 3) claimed that the staurolite isograd resulted from two generations of growth. They had isotopic age evidence for 2 periods of tectonism separated by ~300 million years but did not attribute these two periods of growth to periods of tectonism that far apart. Rather, they suggested that andalusite growth took place at the end of the phase of tectonism that produced the staurolite (~1.7Ga) and then reoccurred some 300 million years after staurolite growth (~1.4Ga). The ability to quantitatively measure FIAs and distinguish many periods of growth of the same porphyroblastic phase has changed this. The recognition of a succession of 4 FIAs preserved within the staurolite porphyroblasts in this region plus at least 2 phases of staurolite growth within each FIA set (see also Sanislav and Bell, 2011), significantly contrasts with most concepts on how reactions take place during deformation and metamorphism.

Quantitative measurement of inclusion trails within porphyroblasts reveals that the reactions from which porphyroblasts such as staurolite grow do not simply start and go to completion. Rather they take place over and over again, once the PT and bulk composition are suitable and are a function of some other factor that previously has not been considered as significant. Bell and Hayward (1991), Spiess and Bell (1996) and Bell and Bruce (2007) have shown that the initiation of crenulation hinges controls where porphyroblast nucleation and growth takes place during regional metamorphism and that growth ceases as soon as a differentiated crenulation cleavage begins to develop. They argue that this results from strain softening and the cessation of microfracture and thus access of the components need for the reaction to take place to the porphyroblast crystal faces. Consequently, when another deformation initiates such that crenulations can begin to form again (which requires shortening near orthogonal to the previous event) the same porphyroblastic phase can regrow (Sanislav and Bell, 2011). Such regrowth of staurolite took place in this region during 4 different events, the first and last of which were spaced 250 million years apart!

A transition from andalusite to cordierite during FIAs 3 and 4 is suggested by a concomitant increase in the latter phase and accords with the overall migration of the staurolite, andalusite and cordierite isograds during the development of FIAs 1 through 4 from W to E across Figs. 18 and 19. This succession suggests a negative slope for the growth of cordierite on a PT pseudosection during metamorphism accompanied by deformation. The younger cordierite, andalusite and sillimanite isograds that lie nearly orthogonal to those defined by the FIA succession (compare Figs. 19a-b and 19c-d) appear to have switched andalusite for cordierite. This suggests a positive slope for the growth of cordierite on a PT pseudosection due to contact metamorphism. They appear to have resulted from a heat source to the SW instead of the WNW.

The isograds mapped by Selverstone (1997) shown in Fig. 3 resemble the youngest isograds determined through this research. This is to be expected because they were unable to distinguish the different phases of growth of staurolite, andalusite and cordierite that the FIA approach allowed.

Rocks are subjected to heterogeneities at all scales (e.g. Bell 1981; Williams 1994; Bell et al. 2004). In a given outcrop some aspects of the deformation history can have affected some portions while others will be affected by other different events. Microstructural (Bell and Hayward 1991; Spiess and Bell 1996; Bell and Bruce 2007) and FIA data (Bell et al. 1998) strongly suggest that porphyroblast nucleation and growth is a function of the partitioning of deformation at the scale of a porphyroblast. This provides a simple explanation for why reactions take place at different times from sample to sample in the same or adjacent outcrops (e.g. Sanislav and Bell 2011) and contrasts with the assumption that mineral growth is only a function of pressure, temperature and bulk composition (e.g. Thompson 1957).

Different distributions on a histogram from FIA to FIA for garnet, staurolite, andalusite and cordierite (Fig. 23) reveal changes in the partitioning of deformation within a region. Garnet dominates FIA set 1 and staurolite FIA set 4 (Fig. 23a). The total distribution in each FIA set reveals that there are more samples which preserve FIA set 3 trails as compared to those which contain FIA set 1, 2 or 4 (Fig. 23b). The number of FIAs measured decreases from garnet to staurolite, cordierite and andalusite (Fig. 23c). Differences from region to region preserve changes in the effects of deformation partitioning at a large scale (e.g. Bell et al. 2004; Abu Sharib and Bell, 2011). The disbursement of FIAs 1, 2, 3 and 4 in garnet and staurolite porphyroblasts across the area strongly suggests that the P, T and bulk composition were suitable for growth of these mineral phases from FIA 1 onwards. Yet staurolite porphyroblasts containing FIA 1 have only been found in seven samples (Fig. 14a). This is more likely to be the result of a change in the pattern of deformation partitioning during the development of this FIA set rather than a change in the P and T (e.g. Bell et al. 2004; Sanislav and Bell, 2011).

Figure 23. Histograms showing the distribution of FIAs

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Histograms showing the distribution of FIAs. (a) The distribution frequency of FIA sets in four major porphyroblastic mineral phases (garnet, staurolite, andalusite and cordierite). Garnet dominate in FIA set 1 and staurolite in FIA set 4. In (b), the total distribution in each FIA set is plotted. There are more samples which preserve FIA set 3 trails as compared to those which contain FIA set 1, 2 or 4. In (c), the total frequency distribution of FIAs in each mineral species is shown. Garnet contains the maximum FIAs followed by staurolite, cordierite and andalusite.

The only plutonic rocks known to have intruded during the peak of metamorphism in the Colorado orogeny are the pegmatite swarms close to the Big Thompson River (Fig. 2). However, there is no broad first-order spatial relationship between pre-metamorphic granites and medium grade rocks. The isograd patterns show no relationship to pegmatite contacts. Granites exposed elsewhere in the region are enriched in radiogenic heat-producing elements. They may have contributed to the thermal budget but were not the primary cause of the low-pressure metamorphism during the Colorado and Berthoud orogenies as they lie too far away (e.g. Foster and Rubenach 2006). Granite emplacement has been a controversial topic for many years. Some believe that extensional tectonics results in ascent and emplacement (Scaillet 1995; Brown and Dallmeyer 1996) while others argue that it occurs during contractional orogenesis (Brown 1994a; Solar et al. 1998; Brown and Solar 1999).

The eastward migration of the staurolite isograds with time and their near orthogonal orientation to S0,1 suggests that the flux of heat through rock might be controlled by the orientation of the foliation parallel to compositional layering. Maximum heat flux may occur along a well developed compositional layering that can be reactivated or reused during deformation, rather than a newly developing oblique foliation through enhanced diffusion (e.g., Bell and Cuff 1989). Metamorphism should be directly related to diffusion rates and the grade will increase closer to the source of heat. The garnet staurolite, cordierite and andalusite mineral succession revealed by the FIAs, the appearance of fibrolitic sillimanite during younger deformation and the absence of kyanite suggest a low pressure, high temperature metamorphic sequence.

Heat flux, like deformation, will be heterogeneous and partitioned through the crust if foliation development is important to diffusion. The eastward shift in the distribution of the staurolite isograds suggests a temperature increase with time even though the overall migration is minimal. The first three of the FIAs developed over period of ~100 Ma during the Colorado orogeny. Five fold FIA successions elswehere have been dated as lasting 100 (Bell and Welch 2002) to 200 (Cihan 2006) million years. If such time lengths are involved, why was the eastward migration of index minerals so limited. One possibility is that the isograds have steep dips rather than gentle ones. The garnet isograd in the Homestake mine is sub-vertical through the mine centre and formed that way (CC Bell and TH Bell unpublished data). Steep dips of the isograds in this region would readily explain the minimal migration over a potentially a long time period.