AGE CONSTRAINTS ON THE PALEOPROTEROZOIC PEMBINE OPHIOLITE-ISLAND ARC COMPLEX AND IMPLICATIONS FOR THE EVOLUTION OF THE PENOKEAN OROGEN

The Pembine ophiolite-island arc complex (POIAC) is located in northeastern Wisconsin and south of the Niagara fault zone, a Paleoproterozic suture zone. The POIAC consists of a lower suprasubduction zone ophiolite composed of primitive island-arc tholeiitic pillow basalt and diabase, boninitic pillowed flows and breccias, and peridotite-gabbro bodies locally intruded by sheeted tholeiitic and boninitic dikes. The ophiolite sequence is overlain to the south by low-K calc-alkaline andesite to rhyolite lava flows and volcaniclastic rocks with oceanic-arc compositional characteristics. The POIAC is intruded by syn-volcanic diorite-quartz diorite-tonalite bodies as well as syn- to post-tectonic diorite-tonalite-granite plutons.

Attempts to date the volcanic components of the POIAC have proven unsuccessful because of a lack of recoverable zircon, probably caused by the generally low zirconium content of these primitive arc rocks. However, we have dated the Twelve Foot Falls quartz diorite, which intrudes the upper part of the ophiolite sequence and overlying calc-alkaline volcanic rocks, at 1889 ± 6 Ma (ion microprobe U-Pb zircon age). This provides a minimum age for the POIAC and confirms that the ophiolitic sequence is significantly older than most Paleoproterozoic rocks in the Pembine-Wausau magmatic terrane (mostly 1875-1835 Ma). The POIAC also is at least 30 million years older than the Penokean collision between the Pembine-Wausau magmatic terrane with the southern margin of the Superior craton, which occurred at about 1860 Ma. Recent dating of the Chocolay Formation in upper Michigan suggests that these continental margin sediments are older than 2100 Ma (Vallini et. al., in review). The new age information on the Chocolay and our age constraint for the POIAC suggest 1) that a major Paleoproterozoic ocean basin evolved following rifting of Kenoraland at about 2200-2100 Ma, and 2) that subduction systems in this ocean led to the generation of new arc crust and repeated accretion events along a Pacific-type southern margin of the Superior craton.