Colorado’s Turbulent Past: Laramide Granitoid Plutons

We camped the first night in the Pike-San Isabel National Forest at the northern end of the Sangra de Cristo Mtns., after passing some large greenhouses filled with shrubs, either tomatoes or marijuana but probably the latter (legally grown in CO). The neighbors were all friendly as they drove by our camp site shown at location 2 in this map.

san isabel map with sample 2 location

We set up in a borrow quarry for road repair soil, which was surrounded by granitic rocks similar to those seen in the Wet Mountains, but looks can be deceiving. Here is a photo of the exposure at our camp site.

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I attached the 10x lens to my iPhone and took the following photo, which we will now examine.

san isabel granitoid hand sample

The K-feldspar (orthoclase) is the orange mineral; it comprises about (estimating from the photo) 30% of this sample. The Na-Feldspar (albite) is the white mineral, which is about 20% of the sample, and quartz (mostly gray and pinkish in this sample) is about 40% of the sample. I would estimate that amphibole (the dark mineral; however, some of this may be other minerals that could only be identified by thin section analysis with a microscope) is about 10% of the sample. I cheated to make them add up to 100% but the errors won’t matter when we compare it to the sample from the Wet Mts shown in the last post; that sample was approximately albite = 7%; orthoclase = 50%; quartz = 40%; and amphibole = 2%. Even taking account of my errors in estimating their composition, these rocks are very different; they are also very different in age. The Wet Mts sample is ~1.5 by old whereas the Sangre de Cristo Mts rock is less than 70 my old. We will discuss this in a later post. This young age comes from the USGS CO geologic map with data sources, which lists the rocks at site 2 as Laramide Granitoid.

We crossed through the intermontane valley and the town of Salida before following Poncha Creek to Marshall Pass, indicated by location 3 in the previous photo. This was a steep rise to an elevation of 10842 feet, with a great deal of blasting, cutting and filling of deep ravines in the surrounding peaks. This leg was interesting because it took us to the Continental Divide, which separates eastward-flowing rivers from those that flow westward. It also is not within a single mountain range but instead separates the Sawatch Mts to the north from lower hills to the south.

This area also includes some of the host/country rock that the granitic rocks were intruded into. This photo shows an exposure of the host rock for the granitic intrusions at the southern end of the Sawatch Range.

poncha pass exposure

This photo shows the orientation of the original bedding plane (horizontal surface of the sediments that became the rock before it was metamorphosed and intruded) of the silts and clays that were originally deposited during the Tertiary period (~65 – 2.5 mya). The tilting occurred during the Laramide Orogeny that created the Rocky Mts. The circle shows where the following photos were taken.

This first photo is at normal magnification; it shows striations on the bedding plane (indicated by black lines) that are probably from sliding during movement, overlaid on original bedding plane irregularities.

poncha creek fault surface

I have to admit, however, that there is always some uncertainty when only examining one exposure; many geologists have examined this region and I am implicitly including their conclusions in my discussion. When we examine the next photos, which are at 10x magnification, we can rely on our own judgement.

poncha creek vein

When we are examining igneous rocks that were crystallized from a relatively uniform temperature and composition, we can assume a lot, as I have been doing up until now. This photo shows some of the intruded rock within a few feet of the tilted rocks shown in the previous photo. I have circled regions of this small image (~1 cm in diameter) that would be of great interest to an economic geologist. I can only state with some confidence that this sample has a lot of orthoclase (reddish colored) and several different minerals that formed within small cavities when the intrusion occurred. Only microscopic study could positively identify the white mineral (albite?), the elongate crystals (orthoclase) and the very small dark minerals (pyrite?). I use question marks because these aren’t even guesses.

I took a closer look at one of these thick veins near the host rock with the 15x magnification lens to see if it would help.

poncha creek vein 15x

The quartz is easy to identify from its gray hue (in these samples) but the circled area could be pyrite, which is also known as “fool’s gold”…where there is smoke, there’s fire…

This photo at 15x magnification shows a bleb (tiny inclusion…this one < 1 cm in diameter) of light-colored minerals in the host rock, which appears orange in this photo, despite looking gray to the naked eye (see the outcrop photo above). The minerals in the bleb look like "salt and pepper", which has no specific meaning but it suggests that the host rock produced these inclusions when heated as the Tertiary granitic melt (liquid rock before crystallizing) invaded. The gray quartz is easily identified (no longer rosy looking due to heat and pressure) and the white could be albite? but the dark minerals would require microscopic examination to identify.

bleb poncha creek

I have to admit that I am writing this post several days after reaching Marshall Pass, where we discovered that I didn't coin that phrase. Someone felt it worth digging a mine to further investigate remineralization along the fault and intrusion zone following Poncha Creek, and they put their money where their mouth was to build this great road we used.

marshall pass mine

I have one last point to make about our ride to the Continental Divide; the flat meadow at the top suggested to me that this was a cirque, where a mountain glacier formed before pushing along the fault line that now delineates Poncha Creek. These photos show why I think this.

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I don’t think it was as thick as those that formed in Yellowstone and at Beartooth Pass, at the north end of Yellowstone Natl park.

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