SR-ND ISOTOPE COMPOSITIONS OF NORTHERN BLACK HILLS PALEOCENE-EOCENE IGNEOUS ROCKS: RELATIONSHIP TO CENTRAL MONTANA ALKALIC PROVINCE

Strontium and neodymium isotopic compositions of Black Hills Laramide-age igneous rocks overlap published values from Missouri Breaks, Haystack Butte, Porcupine Dome, and a malignite sample from the Crazy Mountains in the central Montana alkalic province. Black Hills compositions of initial 87Sr/86Sr are 0.704591-0.706670, and compositions of eNdi are 1.04000 to -6.94173; these values straddle that of bulk earth. Isotopic values for a basement xenolith from a diatreme at Missouri Buttes, Wyoming, are (87Sr/86Sr)i=0.716841 and eNdi=-34.8472 with a Nd model age of 2.9 Ga. Mixing calculations suggest little, if any, crustal assimilation of the xenolith nor of other Archean rocks from the Black Hills or Laramie Range; assimilation of Proterozoic schist or Harney Peak granite from the Black Hills is also unlikely.

The Black Hills, Porcupine Dome, Haystack Butte, and Missouri River Breaks are spatially aligned, trending west-northwest, and have similar ages and isotope compositions. Black Hills isotopic samples are latite, trachyte, phonolite, rhyolite, and pyroxenite; carbonatites are also present. Montana samples with similar isotopic composition are kimberlite, mica peridotite, monticellite peridotite, alnoite, and carbonatite (Scambos, 1991), suggesting the possibility of a genetic relationship among these very different rocks of subalkalic to extremely alkalic compositions.

Ages, composition, and isotopic ratios of northern Black Hills igneous rocks follow a rough geographic trend from east to west. Ages decrease from east to west along the N70-80W belt; in addition, compositions exhibit generally increasing alkali content, decreasing silica content, increasing eNdi, and decreasing 87Sr/86Sr. High initial 87Sr/86Sr (0.706382) and lowest eNdi values (-6.941728) are at the eastern end (latite sample; 58 Ma). Intermediate compositions are found in the central portion (ages 55-54 Ma), and lowest initial 87Sr/86Sr and highest eNdi values are found in the youngest samples (49.6 to 46 Ma) at the western end of the belt. These trends may reflect changing lithospheric geometries and thicknesses across an Archean - Proterozoic boundary inferred to exist beneath the western flank of the Black Hills.