LEAD ISOTOPIC EVIDENCE FOR MIXED SOURCES OF PROTEROZOIC GRANITES AND PEGMATITES, BLACK-HILLS, SOUTH-DAKOTA, USA

The lead isotopic compositions of K-feldspars separated from the ca. 1700 Ma Harney Peak Granite complex and spatially associated granitic pegmatites indicate that these rocks were derived from at least two sources. It has been reported previously that the core of the Harney Peak Granite complex is dominated by relatively lower deltaO-18 (avg. 11.5 parts per thousand) granites, whereas higher deltaO-18 (avg. 13.29 parts per thousand) granites occur around the periphery of the complex. The higher deltaO-18 granites and one simple pegmatite have low values of Pb-207/Pb-204 for their Pb-206/Pb-204. Thus, they likely were derived from a source with a short crustal residence time. This source may have been the pelitic schists into which the Harney Peak Granite complex and pegmatites were intruded. Feldspars from granites with lower deltaO-18 values have significantly higher Pb-207/Pb-204 for their Pb-206/Pb-204. The data define a linear array with a slope equivalent to an age of ca. 2.6 Ga with t2 defined to be 1.7 Ga. Such a slope could represent a mixing array or a secondary isochron for the source. These low deltaO-18 granites could have been derived from a source with a high U/Pb and with a crustal residence beginning before the Proterozoic. The source(s) of these granites may have been a sediment derived from late Archean continental crust. The highly evolved Tin Mountain pegmatite has lead isotopic systematics intermediate between those of the two granite groups, suggesting either a mixed source or contamination. Two late Archean granites, the Little Elk Granite and the Bear Mountain Granite, had precursors with high U/Pb and low Th/U histories. The Th/U history of the Bear Mountain Granite is too low for this rock to have been an important component of the source of the Proterozoic granites. However, crustal rocks with lead isotopic compositions similar to those of the Little Elk Granite were an important source of lead for some of the Proterozoic granitic rocks.