by Michael DePangher (1982)
The Quinnesec Group in the Pembine 15' Quadrangle, Marinette County, Wisconsin, consists of lower Proterozoic, metamorphosed, mafic to felsic, flows, pyroclastics, and minor volcaniclastics. Five lithostratigraphic units of formation status are recognized: the Pembine Formation, the Golden Lake Formation, the Pemebonwon Complex, the McAllister Pond Formation, and the Miscauno Island Formation. They are intruded by igneous rocks ranging in composition from diorite to granodiorite. These rock units form the easternmost exposure of the Penokean foldbelt greenstones which host a number of volcanogenic massive sulfide deposits.
Analysis of major elements and selected trace elements was done for 23 volcanic rocks and 5 intrusive rocks. Alteration of original major element compositions due to secondary processes is assessed using a graphical technique and shown to be significant. Regional silicification, decalcification, hydration, and carbonation are shown to have occurred. Fluxes of alkali and ferromagnesian components are variable and shown to be related to initial bulk composition and primary and secondary mineralogies.
Twenty-one volcanic rock analyses are plotted on a series of variation diagrams to compare interpretations of tectonic environment of deposition based on uncorrected and corrected rock chemistry. Uncorrected data is shown to be unreliable for this purpose while corrected data gives a reliable story consistent with geologic, petrographic, and "immobile" trace element evidence. Rocks of the Quinnesec Group in the study area are interpreted to have formed in a lower Proterozoic, volcanic island arc with a north-dipping subduction zone.