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The Wisconsin magmatic terranes constitute a major easttrending belt of mainly subduction-related Early Proterozoic volcanic and plutonic rocks and associated volcanogenic sedimentary rocks in northern Wisconsin. Magmatic rocks within the terranes have been subdivided, on the basis of lithology and structure. into two subterranes: the Pembine-Wausau terrane on the north and the Marshfield terrane on the south. Rocks in · the Monico area are part of an island arc suite of the PembineWausau terrane that developed and was tectonically deformed during convergence and docking with Archean rocks of a continental margin to the north. In most areas the magmatic terranes are covered by a thick blanket of Pleistocene deposits. As a result. volcanic stratigraphy and structure within the volcanic belt are poorly known. However. outcrops are relatively abundant locally in the Monico area. which is one of the few places where stratigraphic and structural relations can be studied in the field. Rocks exposed in the Monico area consist of banded gneisses. greenschist-grade metavolcanic rocks with well-preserved primary features. synvolcanic mafic and felsic intrusions. posttectonic granitic rocks. and a Middle Proterozoic diabase dike that is probably related to volcanic rocks of the Keweenawan Supergroup. The northern part of the map area consists of banded gneisses of uncertain age. The gneisses are overlain to the south by a bimodal suite of mafic pillow lavas and pillow breccias intercalated with dacitic to rhyolitic flows, breccias, tuffs. and welded tuffs. Numerous pillows consistently indicate that the volcanic sequence faces stratigraphically. south. Mafic and felsic volcanic units are interbedded on both a small and large scale with the felsic component generally increasing southward. The volcanic sequence is tectonically repeated by major shear zones that are approximately parallel to bedding. A large subvolcanic pluton in the northern part of the map area was emplaced approximately along the contact between the older gneisses and the overlying volcanic sequence. The pluton is a quartz (and feldspar) porphyry with a gray aphanitic matrix. Tuffaceous layers and subtle layering suggest the porphyry is. at least in part. extrusive in eastern exposures. but abundant xenoliths in the western part suggest that in this area it is intrusive. Chemically, the quartz porphyry is almost identical to volcanic rocks associated with the Pelican River massive sulfide deposit that occurs within the map area. A post-tectonic pink to red granite, dated at 1. 739 Ma. is exposed in the southern part of the map area. The granite body U.S. Geological Survey and University of Wisconsin-Oshkosh, Oshkosh, WI 54901, emeritus 2U.S. Geological Survey and Western Illinois University, Macomb, IL 61455, emeritus is bulbous in shape and cuts the general volcanic stratigraphy. An unmetamorphosed Middle Proterozoic (Keweenawan) dike with an associated significant low magnetic anomaly transects the entire map area in a N. 70° E. direction. The dike appears to be part of a regional swarm emplaced near the beginning of the Midcontinent Rift. · Structurally the area is characterized by polyphase deformation resulting from the series of events related to docking of the island arc with a continental margin of mostly Archean rocks to the north. Among the oldest structural fabrics recognized is a north-trending gneissic foliation (SA1) in the northwestern part of the map area. North-trending foliation was not observed elsewhere in the map area, nor has it been reported elsewhere in the Wisconsin magmatic terranes. A widespread east-trending foliation (S1) is well developed in the volcanic rocks to the south of the gneissic rocks. S1 is, in turn, cut by a northeast-trending foliation (S2) consisting of discrete, isolated shear zones. The S2 fabric is cut by well-defined, east-trending, north-side-up shear zones (S3) that are responsible for the repetition of the volcanic succession. A conspicuous, N.88° E.trending foliation, along the southern margin of the gneiss. is tentatively interpreted to be an s3 structure that overprints the north-trending structure in the gneiss. 0 1, 0 2, and 03 structures are truncated by the red granite in the southern part of the map area, and are crosscut by the N. 70° E.-trending undeformed diabase dike.

Geology and Tectonic Significance of Early Proterozoic Rocks in the Monico Area, Northern Wisconsin