General geology and tectonics of the region
The metasediments exposed in and around the Big Thompson region of Colorado represent mature sediments deposited in a fore-arc (Condie and Martell 1983) or back-arc setting (Reed 1987). Detrital zircon ages suggest a maximum age of 1758+26 Ma for deposition of the Big Thompson sequence (Selverstone 2000). Figure 1 shows a regional geological map. Figure 2 shows a detailed geological map and the locations where samples were taken and FIAs measured. The geological history of Colorado has been well studied because of its spectacular mountains and mineral wealth (e.g. Karlstrom 1997). The basement is composed mainly of Paleoproterozoic rocks (2.5-1.6Ga, Sim and Peterman 1986) with some rocks having a Mesoproterozoic affinity (1.6-0.9 Ga, Sim and Peterman 1986; Reed 1987).
The rocks mainly consist of quartz-feldspar gneiss, biotite gneiss, amphibolite and migmatite (Reed 1987). They were generally deformed under amphibolite facies conditions (Reed 1987). Igneous intrusions at ~1.7, ~1.4 and 1.1 Ga reshaped the Paleoproterozoic rock distribution (Tweto 1987). The composition of the oldest (~1.7 Ga) and most dominant of these intrusions is intermediate with a calc-alkaline affinity. It is generally believed that these igneous bodies were emplaced synchronous with regional deformation during the Colorado orogeny, but a few formed post-tectonically (Reed 1987) at ~1.4 Ga. These intrusives are undeformed except locally on their margins, and were referred as A-type or “ anorogenic” plutons, mainly because of their composition (Anderson and Cullers 1999). Recently some workers have questioned their anorogenic classification and have argued that they were formed during deformation associated with the Berthoud orogeny (e.g. Nyman 1994).
Figure 2. Detailed geological map of study area and the sample locations
Detailed geological map of study area and the sample locations (after William 1970).
The two major deformation/metamorphic events previously recognized reached temperatures consistent with sillimanite-zone metamorphism (Sims and Stein 2003) although metamorphic conditions were very heterogeneous during these episodes. During the Colorado orogeny, several areas recorded a transition in metamorphic grade from the chlorite zone to the onset of migmatization (Selverstone 1997). Thermobarometric analyses by Selverstone (1997) and Shaw (1999) suggested that there were two intervals of heating during the Proterozoic Colorado and Berthoud orogenies. These were separated by an interval of cooling and decompression (Sims and Stein 2003). In the Big Thomson Canyon area (Fig. 3), thermobarometric analysis of metapelites have indicated that medium to high pressure metamorphism (7-10 kbars) was succeeded by the formation of garnet and staurolite porphyroblast down to the pressures of 4-6 kbars (Selverstone 1997).
Figure 3. The general geology of Big Thompson Canyon
The general geology of Big Thompson Canyon region of the northern Frontal Range and the metamorphic isograds (different thick lined patterns), demarcating the first appearance of metamorphic index minerals, staurolite (St), andalusite (A) and sillimanite (S). Map modified after Selverstone (1997).
According to Selverstone (1997), the whole region near the Big Thomson Canyon was reheated around 1.4-Ga. Geobarometric calculations on late garnet have shown that these rocks lay at ~ 10km depth when this reheating occurred, similar to that at the end of the Colorado orogeny (Selverstone 1997). The portion of their geological map reproduced in Fig. 3 shows a succession of isograds from staurolite through andalusite, sillimanite (and kyanite further west from this area) from SE to NW across the area. Monazite grains were dated within the foliations defining each FIA set (Sanislav and Shah 2010) with FIA sets 1, 2 and 3 forming at 1760.5±9.7, 1719.7±6.4 and 1674±11Ma, respectively (Table 1) and FIA set 4 forming around 1415±16 Ma (Table 1; Shah 2010).
Table 1. Summary of ages derived from the monazite grains preserved within porphyroblasts (Shah, 2010)
| Porphyroblast | Matrix | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Sample | Textural setting | Age and error | Total no of Spots | No of Monazites | Sample | Textural setting | Age and error | Total no of Spots | No of Monazites | |
| C117B | Grt M1 | 1756±22 | 17 | 2 | C75 | Bt M1 | 1664±38 | 7 | 1 | |
| FIA 1 | C75 | St M2 | 1765±23 | 16 | 1 | C75 | Bt M2 | 1762±35 | 7 | 1 |
| C84 | Grt M3 | 1762±21 | 24 | 1 | C43 | Mu M3 | 1724±37 | 7 | 1 | |
| C77 | Crd M4 | 1760±18 | 24 | 1 | C108 | Mta M4 | 1675±24 | 10 | 1 | |
| C51B | Crd M5 | 1762±32 | 12 | 1 | C77 | Bt M5 | 1677±19 | 17 | 1 | |
| C51B | Mt M6 | 1685±29 | 9 | 1 | ||||||
| C43 | St M6 | 1724±19 | 14 | 1 | C84 | Mta M7 | 1729±23 | 26 | 1 | |
| C65A | St M7, 8, 9 | 1717.6±9.5 | 53 | 3 | C83 | Mu M8 | 1723±34 | 7 | 1 | |
| FIA 2 | C108 | St M10, 11 | 1721±14 | 37 | 2 | C110 | Mu M9 | 1438±30 | 7 | 1 |
| C77 | Crd M12 | 1726±18 | 22 | 1 | C65 | Bt M10 | 1665±23 | 10 | 1 | |
| C75 | St M13 | 1712±25 | 10 | 1 | C65 | Bt M11 | 1742±29 | 8 | 1 | |
| C65 | Mta M11 | 1668±48 | 6 | 1 | ||||||
| FIA 3 | C83 | St M14 | 1681±27 | 10 | 1 | |||||
| C51A | Grt M15 | 1666±26 | 10 | 1 | Mta = Matrix | |||||
| C77 | And M16 | 1678±17 | 20 | 1 | ||||||
| C84 | St M17 | 1683±36 | 6 | 1 | ||||||
| C65 | St M18 | 1665±24 | 10 | 1 | ||||||
| FIA 4 | C51 B | Crd M19 | 1414±23 | 13 | 1 | |||||
| C77 | Crd M20 | 1410±26 | 10 | 1 | ||||||
| C110 | And M21 | 1432±39 | 5 | 1 | ||||||
Figure Table 2. Samples collected in and around Big Thompson Canyon region of northern Front Range
Samples collected in and around Big Thompson Canyon region of northern Front Range, Colorado (shown in Fig. 3), the geological formations from which they were taken, their latitude and longitude co-ordinates and the FIA trends measured in them. XQS= Quartzofeldspathic mica schist XKS= Knotted mica schist XBS= Porphyroblastic biotite schist.