By Scott Zylstra and Jake Voorhees; Dr. Ron Harris, Mentor, Department of Geological Sciences
Utah’s rocks, though incredibly complete in later eons, hold very little information about what was occurring in that area before 700 million years ago. In Southern Utah, only two relatively small exposures of these ancient Precambrian exist, in the west of the Beaver Dam Mountains west of St. George and the west of the Mineral Mountains west of Beaver. Therefore, it is vitally important to study these exposures, as they constitute the only clues to what was happening in Utah 1.7 billion years ago.
Many field excursions have been made and are ongoing to both metamorphic complexes. A small team traveled to the Beaver Dam Complex in January 2015, returning for a longer stay in May of that year. A brief but informative trip was made to the Mineral Mountains in February 2015. On these excursions, crosscutting and numerous lithological and other field observations were made and samples collected. From these samples thin sections were made and more detailed petrologic and mineralogically analyses could be conducted. Electron microprobe and x-ray diffraction experiments have begun and are ongoing.
The Beaver Dam Metamorphic Complex consists in a large body or bodies of what has been interpreted to be orthogneiss, a metamorphosed granitic pluton surrounded by paragneiss, metamorphosed thinly banded sediments of possible turbiditic origin. The mineralogy of the orthogneisses and paragneisses is similar, with quartz, plagioclase, k-feldspar, and biotite being the primary minerals. Garnet is not always present, but is common and can be found in either variety of gneiss. Foliation in the gneisses is defined by biotite rich layers. In the paragneiss, biotite forms well-defined foliation planes that cause the rock to break along the planes. The orthogneiss has more disseminated biotite so it doesn’t break along foliation planes. Much of the orthogneiss is protomylonitic with grain size reduction and weak s-c fabric, but some of the orthogneiss is relatively undeformed, having less grain size reduction and weak to absent s-c fabric. The Mineral Mountains Metamorphic Complex also consists in banded gneiss similar in mineralogy to the Beaver Dam Metamorphic Complex, but with a conspicuous absence of garnet. Sibbett and Nielson (1980) have suggested that this gneiss has a sedimentary protolith, but current field observations have yet to determine their origin.
Schist in the Beaver Dam Mountains is composed of quartz, -kfeldspar, plagioclase, and biotite. Garnet is abundant and sillimanite is common. The Mineral Mountains also contains a schist of the same mineralogy but high in sillimanite and biotite. Due to the high percentage of mica in both schists, the protolith of the schist was most likely sedimentary.
Dikes of pegmatite and leucogranite are common throughout the Beaver Dam Mountains, and pegmatite dikes are common in the Mineral Mountains. They are composed of large crystals of quartz and k-feldspar, the leucogranite also containing plagioclase and abundant large euhedral garnets. Pegmatite in the northern part of the Beaver Dams has magnetite but lacks garnet. Amphibolite is common in the Beaver Dam Mountains and a pure white quartzite is present in the Mineral Mountains.
In the Beaver Dam Mountains, a somewhat consistent pattern has emerged with generally higher-grade rocks occurring closer to the orthogneiss body. Pegmatite dikes appear farthest from the ancient pluton, with leucogranites closer and amphibolite often occurring in contact with the pluton. Migmatites are common in both complexes, and the amphibolite seems to often contain a restitic composition, with variable amounts of quartz and k-feldspar. This presence of quartz and k-feldspar seems to at least be an indication that the amphibolite is not a metabasalt, but has a more felsic protlith. The amphibolite also often contains skarn mineralization, which may have originally been pockets of limestone or marl. Much of the amphibolite could represent melanosomes and the leucogranite and pegmatite leucosomes obtained from melt extraction.
Thin section analysis reveals much about the metamorphic history and path of these rocks. The presence of sillimanite, which is the hightemperature aluminosilicate polymorph, indicates temperatures greater than 500°C at peak metamorphism, which is confirmed by the presence of grain boundary migrationtype quartz recrystallization, which only occurs at temperatures above 500°C (Stipp et al. , 2002). The presence of subgrain rotation recrystallization (200-500° C, Stipp et al., 2002) and greenschist alteration by chlorite and epidote indicate later retrograde reactions. This back-and-forth metamorphism is manifest in many porphyroblastic garnets and k-feldspars which are seen retrograding to biotite and sillimanite, or else the reverse prograde reaction is taking place.
Mylonites are pervasive in both complexes, ranging in intensity from proto-mylonites with minimal shear to river-like ultramylonites consisting of rounded k-feldspar porphyroblasts in an extremely fine-grained matrix which looks glassy in hand sample. These are indications of variable strain intensities which have occurred during the history of the rocks.
Much has been done and much is yet to be done to decipher the meaning behind the 1.7 Ga (billion-year-old) Beaver Dam and Mineral Mountain Complexes, but so far they belie an intense convergent world with plutons and dikes intruding and melting thinly layered sediments, a world changed almost beyond recognition by burial, shearing and recrystallization, summoned again from the depths to tell their story.
Sibbett, B. S., and Nielson, D. L., Geology of the Central Mineral Mountains, Earth Science Laboratory: University of Utah Research Institute, 1980.
Stipp, M., Stunitz, H., Heilbronner, R., and Schmid, S. M. The eastern Tonale fault zone: a ‘n ‘natural laboratory’ for crystal plastic deformation of quartz over a temperare range from 250 to 700 degrees C, Department of Earth Sciences, Basel University, Bernoullistrasse 32, 4056 Basel, Switzerland, 2002.