NEWLY RECOGNIZED PALEOPROTEROZOIC MAFIC MAGMATISM, CENTRAL PENOKEAN OROGEN (WISCONSIN)

Precambrian mafic dikes exist throughout the southern Lake Superior region; their location and trend are determined from field mapping and aeromagnetic surveys. The dike population is dominated by the NW-trending 2.1 Ga Kenora-Kabetogama swarm in Minnesota, and the 1.1 Ga Keweenawan swarms surrounding Lake Superior. In central Wisconsin, however, a number of mafic dikes intrude Penokean basement and are metamorphosed to lower amphibolite facies, indicating they predate midcontinent rifting. Further, their minimum age is constrained by geon 16 Ar-Ar cooling ages. We recently dated three metamorphosed diabase dikes from bedrock exposures at Biron Dam and Conants Rapids: ion microprobe analyses on zircon resulted in Pb-Pb ages of 1815 ± 5 Ma, 1817 ± 4 Ma, and 1815 ± 7 Ma (all with MSWD: ~1.0). Metamorphic conditions in central Wisconsin during (and after) the Penokean orogeny never attained the greater-than-amphibolite facies temperatures required to produce neogrowth zircon and the basement arc rocks are 1840 Ma or older, so the dates are interpreted to be crystallization ages. These dike ages are remarkable, since they indicate intrusion during a proposed period of tectonic quiescence within the orogen. Conventional interpretation suggests Proterozoic mafic volcanism and related intrusions in the Lake Superior region represent the incipient or failed rifting phases of continental break-up. Clearly, our new ages document a period of mafic magmatism that must be interpreted differently. For instance, the 1815 Ma mafic magmatism may represent a relaxation of Penokean compression and a change to intra-arc extension specifically related to subduction flip. Switching between tectonic extension and compression has recently been inferred to explain episodic, short-lived orogenic contraction of an otherwise continuously extending upper plate formed by ongoing slab retreat. A 1740 Ma Th-Pb monazite age recently obtained by us on a metapelite from central Wisconsin (Hamburg schist), and a preliminary 1722 Ma U-Pb sphene age on a metamorphosed dike reported by Van Wyck (1995) may indicate a geon 17 metamorphic episode related to renewed contraction.