REGIONAL PATTERN OF 1700-1300 MA SLOW COOLING OF THE MIDCRUSTAL HARNEY PEAK GRANITE AND ITS EXTENSIVE METAMORPHIC AUREOLE, BLACK HILLS, SOUTH DAKOTA

The Precambrian crystalline core of the southern Black Hills, South Dakota, exposes an extensive, high-T low-P aureole of garnet- to 2^nd-sillimanite-zone schists centered on a voluminous plutonic core (the 1715 Ma Harney Peak granite). In many respects, present geological understanding of this Paleoproterozoic complex has been enhanced by Charles Guidotti and his colleagues, who published classic papers describing the field relations, mineralogy, petrology, and origins of analogous Paleozoic complexes in Maine. Here we report regional cooling-age patterns based upon 52 ⁴⁰Ar/³⁹Ar dates obtained for muscovite and biotite in schists and granite sampled from across the ~1000-km² area of the southern Black Hills metamorphic aureole and its plutonic center. These data (~30 are new) reveal radial patterns of mica cooling age becoming younger from outer aureole to granite core. Specifically, 22 muscovites appear to have cooled through ~400 °C (assumed T_C) at: 1650-1570 and 1550-1480 Ma in the lower- and upper-staurolite zones, respectively; 1450-1390 Ma in the sillimanite zone; and 1340-1320 Ma nearest to the 2^nd-sillimanite isograd. Likewise, 10 biotites cooled through ~300 °C at 1510-1460, 1400-1380, and 1280-1260 Ma in the staurolite, sillimanite, and 2^nd-sillimanite zones, respectively, whereas 20 other biotites contain excess Ar as largely inferred from their ⁴⁰Ar/³⁹Ar dates being apparently older than for coexisting muscovites. The regional trends in ⁴⁰Ar/³⁹Ar cooling age for muscovite and biotite are independently mapped as elliptical zones centered on the granite, which also mimic the mapped regional trends of both isograds and structures. The relatively Ar-retentive muscovites also exhibit a robust and convex age-distance trend, which indicates slower time-integrated cooling in the outer aureole (~1 °C/Myr between 1715-1480 Ma) than in the inner granite, as also predicted from thermal models. Judging from the biotite data, the granite core cooled through ~300 °C by 1280-1260 Ma, or ~450 Myr after intrusion into the midcrust. The nested, elliptical cooling-age zones defined above also account for regionally-variable published Rb/Sr ages of ~1690-1550 Ma, when Black Hills muscovite cooled through ~500 °C, and likewise for U/Pb cooling ages of nearby apatites that range from ~1700-1465 Ma. Finally, no compositional effects on mica cooling age are discerned within the data set.