AR-40 AR-39 EVIDENCE FOR MIDDLE PROTEROZOIC (1300-1500 MA) SLOW COOLING OF THE SOUTHERN BLACK-HILLS, SOUTH-DAKOTA, MIDCONTINENT, NORTH-AMERICA - IMPLICATIONS FOR EARLY PROTEROZOIC P-T EVOLUTION AND POSTTECTONIC MAGMATISM/

Ar-40/Ar-39 total gas and plateau dates from muscovite and biotite in the southern Black Hills, South Dakota, provide evidence for a period of Middle Proterozoic slow cooling. Early Proterozoic (1600-1650 Ma) m ica dates were obtained from metasedimentary rocks located in a synfor mal structure between the Harney Peak and Bear Mountain domes and also south of Bear Mountain. Metamorphic rocks from the dome areas and und eformed samples of the similar to 1710 Ma Harney Peak Granite (HPG) yi eld Middle Proterozoic mica dates (similar to 1270-1500 Ma). Two sampl es collected between the synform and Bear Mountain dome yield intermed iate total gas mica dates of similar to 1550 Ma. We suggest two end-me mber interpretations to explain the map pattern of cooling ages: (1) s ubhorizontal slow cooling of an area which exhibits variation in mica Ar retention intervals or (2) mild folding of a Middle Proterozoic (si milar to 1500 Ma) similar to 300 degrees C isotherm. According to the second interpretation, the preservation of older dates between the dom es may reflect reactivation of a preexisting synformal structure (and downwarping of relatively cold rocks) during a period of approximately east-west contraction and slow uplift during the Middle Proterozoic. The mica data, together with hornblende data from the Black Hills publ ished elsewhere, indicate that the ambient country-rock temperature at the 3-4 kbar depth of emplacement of the HPG was between 350 degrees C and 500 degrees C, suggesting that the average upper crustal geother mal gradient was 25 degrees-40 degrees C/km prior to intrusion. The th ermochronologic data suggest HPG emplacement was followed by a similar to 200 m.y. period of stability and tectonic quiescence with little u plift. We propose that crust thickened during the Early Proterozoic wa s uplifted and erosionally(?) thinned prior to similar to 1710 Ma and that the HPG magma was emplaced into isostatically stable crust of rel atively normal thickness. We speculate that uplift and crustal thinnin g prior to HPG intrusion was the result of differential thinning of th e subcrustal lithosphere beneath the Black Hills. If so, this process would have also caused an increase in mantle heat flux across the Moho and triggered vapor-absent melting of biotite to produce the HPG magm a. This scenario for posttectonic granite generation is supported, in part, by the fact that in the whole of the Black Hills, the HPG is spa tially associated with the deepest exposed Early Proterozoic country r ock.