Geology and Petrology of the Wilder Lake Intrusion, Duluth Complex, Northeastern Minnesota

Abstract

Abstract The Wilder Lake Intrusion (WLI) is a well differentiated, northward-dipping, sheet-like mafic layered intrusion that is part of the 1.1 Ga Duluth Complex exposed in northeastern Minnesota. While several reconnaissance studies of the well exposed western quarter of the WLI have shown it to have several distinctive petrologic characteristics compared to other layered intrusions in the Duluth Complex, its relative inaccessibility within the Boundary Waters Canoes Area Wilderness precluded a detailed study of its field and petrologic attributes along its entire 10 km strike length. This changed in the Fall of 2011 when an intense forest fire (the Pagami Creek Burn) created easy access to excellent exposure of the WLI. Through field mapping, petrographic observations, and geochemical analysis, this study sought to document the igneous stratigraphy of the WLI along its entire strike-length with the goal of better understanding the petrogenesis of its unique petrologic attributes. These attributes, noted by others and confirmed here, include the up-section stratigraphic changes characterized by 1) a cumulus reversal from a olivine oxide gabbroic cumulate of Pl+Cpx+Ox+Ol to a troctolitic (Pl+Ol) cumulate; 2) a reversed cryptic variation of Fo in olivine and En’ in clinopyroxene; and 3) the cumulus arrival of Fe-Ti oxide before augite (Miller and Ripley, 1996). Detailed mapping conducted in 2012 and 2013 revealed that most cumulate units of the WLI can be followed along its entire strike length, but with some notable exceptions. Remapping in the western part of the WLI has confirmed that the 2 km-thick igneous stratigraphy exposed here starts with a basal unit of heterogeneous, intergranular olivine oxide gabbro that is in sharp contact with Anorthositic Series rocks. This marginal gabbro is overlain by a troctolitic unit of Pl+Ol cumulates, which can be subdivided into a heterogeneous subunit, a layered subunit and an anorthosite inclusion-rich subunit. The troctolite unit is overlain by a thin (20-100 m thick) oxide troctolite unit of Pl+Ol+ Ox cumulates that abruptly gives way to an olivine oxide gabbro unit of Pl+Cpx+Ox+Ol cumulates. This four-phase cumulate is abruptly overlain by another troctolitic cumulate unit demarking a cumulus reversal back to Pl+Ol cumulates. Mapping of the excellent exposures created by the Pagami Creek Burn reveal that the upper troctolite unit cross-cuts and locally scoured out the four-phase gabbro and part of the oxide troctolite unit. Thus it is interpreted as a recharge of more primitive magma into the upper part of the WLI chamber rather than a downward crystallizing roof zone unit as speculated by Miller (1986). Detailed mapping by overland traverses in the central and eastern extents of the WLI show it to thin from 700 m in the west to 500 m in the east. Moreover, several units pinch out in the eastern section of the intrusion. The oxide troctolite pinches out just east of center, but swells back to about 20 meters in stratigraphic thickness before pinching out again farther east. The lower gabbro also pinches out around the same place and is replaced by a taxitic unit that dominates at the eastern margin; a similar heterogeneous unit also can be found at the western margin. Petrographic study of 223 thin sections collected along three profiles across the WLI at its western, east-central and eastern extents helped to confirm and refine the stratigraphic variations in mineralogy and texture noted from field observations. In addition, olivine and pyroxene from many of the samples were analyzed by University of Minnesota Duluth’s SEM/EDS (Scanning Electron Microscope/Energy Dispersive X-Ray Spectroscopy) to document cryptic variation of mg# (=Mg/(Mg+Fe), cation %). This mineral chemical data was acquired to verify the reversed cryptic variation previously documented in the west and to determine if this variation persists along strike to the east. Reversed cryptic variation of upwardly increasing magnesium number (mg# =MgO/(MgO+FeO), mol %) in olivine (Fo) and pyroxene (En’) was confirmed in the west and shown to persist in the eastern profiles. However, the data also reveal that the mg# tends to decrease at a particular stratigraphic horizon from west to east. The reversed cryptic variation up-section is interpreted to reflect a trapped liquid shift within the oxide troctolite and olivine oxide gabbro units. Trapped liquid shift occurs where high mg# cumulus olivine re-equilibrates with low mg# intercumulus liquid. As evidenced by their strong foliation and the low abundance of augite in the oxide troctolite unit, these rocks are clearly adcumulates with very little postcumulus minerals (i.e., representing the trapped liquid component) and thus retain their high-mg#. The lateral decrease in mg# to the east, as well as the disappearance of the oxide troctolite unit, is thought to be caused by the thinning of the intrusion which in turn would result in the eastern portion of the intrusion to cool more rapidly than the west. This more rapid cooling would have trapped intercumulus liquid (and thus a stronger trapped liquid shift) and promoted oxide and pyroxene to crystallize more synchronously since their liquidus temperatures are not very different. Finally, whole rock geochemical analyses of the basal intergranular gabbro samples were evaluated to determine if they may be representative of a parental liquid composition. One piece of evidence that this is the case is that the WLI marginal gabbro composition is comparable to other tholeiitic magma compositions occurring in the North Shore Volcanic Group (NSVG). Another method used to evaluate the parental composition of the marginal gabbro is to apply its composition to a MELTS-based phase equilibrium program, Pele (Boudreau, 2006). This modelling indicates that the phases in equilibrium with the fractional crystallization of the marginal gabbro compositions can replicate the cumulate stratigraphy observed in the WLI with normal to reduced fO2 conditions (between QFM and 3 log units below QFM). These modeling results indicate that the cause of early oxide crystallization relative to augite was largely the result of an Fe-Ti enriched parental magma composition, and not elevated oxygen fugacity. References Joslin, G.D., and Miller, J.D., Jr., 2003, Stratiform Pd-Pt-Au mineralization in the Sonju Lake Intrusion, Minnesota. Geological Society of America Abstracts with Programs, v.34, no. 7, p. 101. Miller, J.D., Jr. 1999, Geochemical evaluation of platinum group element (PGE) mineralization in the Sonju Lake intrusion, Finland, Minnesota. Minnesota Geological Survey Information Circular 44, 32 p. Miller, J.D. Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.E., and Wahl, T.E., 2002, Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58, 207p. w/ CD-ROM Phinney, W.C., 1972. Northwestern part of Duluth Complex. In: Sims, P.K. & Morey, G.B. (eds.) Geology of Minnesota -A centennial volume. Minnesota Geological Survey, p. 335-345