SUBALKALIC AND ALKALIC RHYOLITES IN THE BLACK HILLS ALKALIC SUITE, SOUTH DAKOTA AND WYOMING: DIFFERENCES IN TIMING, SOURCES, AND INTRUSIVE MECHANISMS

In the Black Hills alkalic suite, subalkalic rhyolites differ from alkali rhyolites in age, texture and mineralogy, geochemistry, and intrusive style. Alkalic rocks range from lamprophyre and tephrite to phonolite, alkali rhyolite and comendite, whereas subalkalic rocks include only trachyte and subalkalic rhyolite. Earliest magmatism (~58 Ma) was subalkalic, and was followed by at least two alkalic pulses (~55-54 and ~49-46 Ma; 40Ar/39Ar; Duke et al., 2002). Published K/Ar ages of subalkalic rhyolite at 56 Ma and 51-52 Ma indicate that subalkalic magmatism may have continued sporadically, but consistently preceded alkalic activity.

Subalkalic rhyolite differs from alkali rhyolite in terms of mean total Th concentration (subalkalic rhyolites (SR): 2.6 ppm; alkali rhyolite and comendite (AR): 57 ppm); mean total U (SR: 4.6 ppm; AR: 11.8 ppm); mean Th/U (SR: 2.9; AR: 5.3); mean total REE (SR: 105 ppm; AR: 334 ppm); mean La/Yb (SR: 37; AR: 49). Subalkalic rhyolite has lowest Na2O, MgO, FeO(tot), P2O5, Sr, Zr, Ti, Nb, and Y contents of the entire suite. Published initial Sr isotopic ratios are higher for SR than AR, and previous Pb isotopes denote multiple sources for SR, including lower crust. Mean La/Nb of subalkalic rhyolite is 1.4, similar to that of mafic lower crustal xenoliths.

Multi-element diagrams of all alkalic rocks reveal patterns parallel to those of local carbonatite, but at lower concentrations. Alkalic magmas, including alkali rhyolite, were differentiated from a primitive ultramafic alkalic parent magma. In contrast, variable trace-element patterns of subalkalic rocks, with lower concentrations and flatter patterns than their alkalic counterparts, suggests significant assimilation of various types of crustal material.

Subalkalic rhyolitic activity included an explosive component: several diatremes and numerous breccia pipes are located throughout the province. At certain centers, evidence exists of initial explosive rhyolitic activity followed by intrusion of phonolite or other alkalic magma along the same vertical route. The substantial, but variable, crustal component in subalkalic rhyolite attests to lengthy residence at various levels in the crust; however, it is not known if interaction with alkalic magma, with its high volatile content, triggered rhyolite eruptions.