OXYGEN ISOTOPE RATIOS OF REFRACTORY MINERAL PHASES FROM LARAMIDE-RELATED TERTIARY INTRUSIONS OF THE BLACK HILLS, SOUTH DAKOTA

Oxygen isotope analyses of primary magmatic refractory mineral phases, garnet and titanite, were completed from unmetamorphosed Tertiary intrusive bodies of the northern Black Hills. These igneous bodies were intruded shallowly as sills, laccoliths, and stocks after the Laramide Orogeny. Samples of Phono-tephrite, Phonolite, Trachy-andesite, Trachyte, and Rhyolite are all calc-alkaline in affinity. Most samples belong to the high-K calc-alkaline series and the remainder are of the shoshonitic series. Half the samples are characterized as within-plate granites while the others are volcanic-arc granite based on Y, Nb, and Rb content. Granite type does not correlate with rock type but rather with intrusive suite and location. Vanocker Canyon rhyolite has an average garnet d¹⁸O of 7.8±0.9‰. The Terry Peak summit rhyolite has titanite with a d¹⁸O of 6.6‰. Sugarloaf Mountain rhyolite, trachyte, and tinguaite have titanite of 5.3‰, 5.2‰, and 5.5‰ respectively. Whitewood Peak trachyte and phonolite have titanite of 6.9‰ and 6.1‰ respectively. The Mt. Roosevelt rhyolite unit has garnet that is 7.1‰. Unlike initial ⁸⁷Sr/⁸⁶Sr ratios from the region (Beintema, 1985), there is no clear correlation of rock type with d¹⁸O values. Phonolites have high d¹⁸O values despite low iSr ratios suggesting a mantle source (Beintema, 1985).

Titanite and garnet have been shown to be retentive of magmatic d¹⁸O values due to slow diffusion rates and high closure temperatures. For ease of comparison among all the intrusions, a fictive magmatic d¹⁸O(zircon) can be calculated from titanite and garnet values at T=800°C (King et al. 2001). The range of calculated d¹⁸O(zircon) is from 6.4‰ to 8.4‰ with an average of 7.3±0.7‰. Calculated d¹⁸O(WR) values based on SiO₂ content average 8.7±.9‰. The average calculated zircon d¹⁸O is higher than average mantle values (5.3‰) indicating a significant crustal component in the magma source for the Tertiary intrusive bodies of the Black Hills region. Most of the igneous bodies intrude lower-Paleozoic rock units in the Black Hills so there is not significant Paleozoic sedimentary rock at depth as a potential magma source. Calculated magmatic whole-rock d¹⁸O values are too high for typical unaltered Archean igneous rocks so the likely magma source would be Proterozoic crustal or Archean supracrustal rocks.