Petrology of the Proterozoic Potato River Layered Intrusion, Northern Wisconsin, USA

The Potato River intrusion is a Keweenawan (1100 Ma) mafic plutonemplaced in Keweenawan volcanics and earlier Proterozoic metasedimentaryrocks along the southeastern flank of the Lake Superior syncline.It comprises the following lithostratigraphic zones: a thinto absent Border zone of altered olivine gabbro; a Lower zoneof olivine gabbro; a Picritic zone of picrite and troctolite;a Middle zone of olivine gabbro and leucogabbro; an Upper zoneof quartz leucogabbro and ferrogabbro; and a Roof zone of granophyricand granitic rocks. Fractional crystallization is evident fromcompositional changes in the rocks and cumulus minerals withstratigraphic height. Elements concentrated in the cumulus mineralsolivine and plagioclase (Mg, Fe²⁺, Al, Ca, Ni, Co, Cr, Sr) decreasewith height; elements concentrated in the trapped liquid (Na,K, La, Y, Zr, Nb, Rb, Ba) increase with height; and other elements(Ti, Fe³⁺, P, Ga, V, Sc, Cu, Zn) show complicated behavior relatedto the appearance of additional cumulus phases such as clinopyroxene,Fe-Ti oxides, and apatite. Lower zone rocks contain some sulfide,probably from sulfur derived from the country rock, and theUpper zone has sulfides probably precipitated from an immisciblesulfide liquid. The sulfide-bearing rocks have similaritiesto those of other intrusions, such as Bushveld, Stillwater,and Skaergaard. The picritic and troctolitic rocks of the Picritic zone indicatethat the intrusion was open to additional injections of maficmagma. Roof zone granophyric rocks are residual liquids intrudedalong the upper margin of the intrusion during regional tilting,but Roof zone granitic rocks are probably melted country rock.An attempt is made to estimate by reverse stratigraphic summationthe compositional path of the magma that solidified above thePicritic zone. The first compositions are highly aluminous,which suggests that the upper part of the intrusion has beenenriched in plagioclase by convection-aided crystal sorting.A complementary unit of mafic rocks is not exposed, but it couldbe present down dip. Some of the later compositions are similarto typical Keweenawan high-Al tholeiites. The magma did notundergo extreme iron enrichment, probably because of oxygenfugacity buffering.     

Geothermometry in the Kiglapait Aureole: Part II. Evaluation of Exchange Thermometry in a Well-Constrained Thermal Setting

Previous determination of a well-constrained thermal profilerepresenting peak conditions of metamorphism for the contactaureole of the Kiglapait Intrusion, Labrador, provides the basisfor critical comparative geothermometry of cation exchange thermometersapplied to mafic and ultramafic granulites. Knowledge of theshape of the profile and constraints on the temperature of theintruding magma allow calculation of cooling rates which rangefrom 150?C/m.y. at the contact to 30?C/m.y. at 2500 m from thecontact. Substitution of these rates in diffusion equationspermits discrimination between geothermometers which are eitheradequately or poorly calibrated, as well as thermometers whichare probably inappropriate for use in metamorphic terranes.Our results suggest that the two-pyroxene thermometer (Lindsley& Andersen, 1983) is accurate from 950 to 750?C. Below 750?C,both limbs of the miscibility gap require at least minor recalibration,or the projection scheme of Lindsley & Andersen (1983) requiresadjustment. Results from the cpx-ilm thermometer are erraticand imply that the solution models for pyroxene and ilmenite,as presently formulated in this thermometer, are inadequate.Results from the opx-ilm, ol-ilm, and opx-ol thermometers areinconsistent. Theoretical calculations at both high and lowtemperatures show that these three thermometers cannot giveinternally consistent results. Results from Fe-Ti oxide thermometrysuggest that these minerals are easily re-equilibrated due tocation interdiffusion between grains. However some grain pairsappear to retain compositions appropriate to peak thermal conditions.