Depth of emplacement and tilting of the Middle Proterozoic (1470 Ma) Wolf River batholith, Wisconsin: Ar-40/Ar-39 thermochronologic constraints

摘要

The 1470 Ma Wolf River batholith of northeastern Wisconsin is one of the oldest and largest intrusive sequences of the Middle Proterozoic magmatic suite that trends across the continent from Labrador to southern California. The presence of miarolitic cavities in portions of the batholith and in associated plutons (Wausau syenite complex) and equilibrium considerations suggest a shallow emplacement depth (i.e., <4 km). However, these data contrast with published Rb/Sr biotite ages, which suggest much of the batholith was uplifted through mid-crustal temperatures well after intrusion. To address this problem better, we have obtained four biotite Ar-40/Ar-39 dates from a transect across the southwestern portion of the batholith, one date from the similar to 1520 Ma Wausau syenite complex, and another from rocks metamorphosed during the Early Proterozoic Penokean orogeny. The two oldest dates are concordant (within error) with the crystallization age of the batholith, suggesting rapid cooling after intrusion and hence shallow emplacement depth. Cooling dates young to the south-southeast, however, and the youngest dates within the batholith (similar to 1400 Ma) are concordant with previous Rb/Sr biotite dates and with the Ar/Ar biotite date from the adjacent country rock. The data suggest slow cooling and southeast-side-up tilting of this portion of the exposed batholith from mid-crustal temperatures (similar to 300 degrees C). Postemplacement uplift and tilting of the batholith appear related to differential isostatic uplift driven by substantial original thickness variations of the intrusion. [References: 31]