Abstract

A strongly developed set of healed micro-cracks striking approximately northwest-southeast is present at 5 of 6 sample localities within the 1450 ± 20 Ma Wolf River batholith of central Wisconsin. Other minor sets are present but are less well defined. Fluid inclusions in healed microcracks show melting temperatures in 2 groups: 0 °C, indicating nonsaline water, and -3 to -30 °C, suggesting salinities above 4.5%. The in situ trapping temperatures of fluid inclusions, calculated from homogenization temperatures, show a consistent 250-400 °C for all the sets of healed microcracks. Rb-Sr dates of about 1440 Ma on biotites from the Wolf River batholith suggest that the 300 °C blocking temperature of biotite was reached very soon after intrusion and that no reheating event after primary cooling took place in the batholith. The fluid-inclusion data and Rb-Sr date interpreted together suggest that cracks were probably generated and healed during the initial cooling of the pluton, starting at near 400 °C. The preferred orientation of these microcracks implies that the maximum horizontal stress was northwest-southeast for this region about 1400-1450 Ma. A similar orientation of a major joint set in the area also might be correlated with the same event. Strongly developed N20°W fluid-inclusion planes in the Illinois deep borehole UPH-3, about 250 km to the southwest, with similar-age bedrock, may reflect a more distal effect of this same stress filed.

The northwest-southeast orientation for the maximum horizontal stress direction during the cooling of the Wolf River batholith is subparallel to the proposed orientation of the paleostress field prior to the emplacement of the Wolf River batholith during an unnamed event that deformed upper Proterozoic quartzites, and it is consistent with a long period of continental collision and accretion, which formed northeast-trending belts along the southern and southeastern margin of the continent. The orientation of the regional stresses may have remained fairly constant during much of the period from Penokean to the start of the Keweenawan, when the maximum regional horizontal stress changed from a northwesterly orientation to a northeasterly one. The data show that even in periods of relative tectonic quiescence, such as during intrusion of the Wolf River batholith, micro-cracks may provide a record of paleostress orientations.

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