- UW Department of Geology and Geophysics
Proterozoic rocks in the Park Range comprise an assemblage of highly deformed metavolcanic and metasedimentary supracrustal rocks intruded by numerous plutons. The oldest rocks in the region are a suite of bimodal metavolcanic and associated metasedimentary rocks on Farwell Mountain. These rocks are similar to the Green Mountain Formation to the north, and are geochemically similar to modern continental volcanic arcs related to subduction. The most widely exposed Proterozoic supracrustal unit in the Park Range consists of a highly metamorphosed and deformed, bimodal volcanic sequence with trace-element patterns similar to those of continental interior bimodal suites instead of subduction-related volcanism. The metavolcanic rocks are overlain by a suite of metasedimentary rocks. Protoliths for the basal metasedimentary rocks are graded sequences of feldspar-rich lithic sandstones and conglomerates interpreted as turbidites. These grade stratigraphically upward into a sequence of interbedded shales and finegrained quartz arenites, similar to those found on passive-margin shelves. The top of the sedimentary sequence consists of a medium- to coarse-grained, cross-bedded quartzite. The supracrustal rocks are intruded by plutonic rocks with compositions from gabbro to granite and ages of 1.78 to 1.735 Ga. The youngest Proterozoic unit in the area is the ∼1.4-Ga Mount Ethel pluton.
Upper amphibolite-facies regional metamorphism produced abundant sillimanite in aluminous rocks throughout the Park Range and overprints structural fabrics associated with the older intrusions. Garnet-biotite/GASP thermobarometry indicates temperatures for this event ranging from 550–700°C and pressures of 4 to 6 kbar for most of the region south and east of greenschist-facies rocks at Farwell Mountain. Evidence of an earlier period of higher pressure metamorphism is present near Lester Mountain where kyanite that contains small blebs of staurolite has partly broken down to sillimanite. Evidence for a late, lower-pressure rehydration event is also recorded at Lester Mountain.
The dominant fabric in the supracrustal rocks throughout the range consists of a steeply dipping northeast-southwest-striking foliation with a steeply plunging stretching lineation. This fabric is also recorded in the oldest of the felsic plutonic rocks, although the amount of strain in the dated plutonic rocks decreases with age. The intensely deformed Soda Creek-Fish Creek shear zone might be a suture between major terranes in the Park Range. There are also changes in metamorphic grade and ages of plutonic rocks that may be related to the shear zone. The main deformation along this shear zone predated the Mount Ethel pluton and was northside-down. However, some left-lateral displacements along late mylonite zones offset rocks attributed to the Mount Ethel magmatic system. A second candidate for a suture is the area between Farwell Mountain and Lester Mountain, where metamorphic grades and compositions of metavolcanic rocks change abruptly.