DIVERSE, WIDESPREAD STATIC PORPHYROBLASTS: THE LAST GASP OF PROTEROZOIC METAMORPHISM IN THE BLACK HILLS, SOUTH DAKOTA?

The development of late, post-kinematic porphyroblasts is an undeniable, widespread feature of the Black Hills Precambrian exposure. Best known and studied of these static phases are garnet and staurolite. However, amphiboles, feldspars, hematite-magnetite, ilmenite, pyrite, biotite, muscovite, carbonates, and tourmaline also occur as late, euhedral grains overprinting fabrics in many areas of the Black Hills. A current and evolving hypothesis is that this latest metamorphism is associated with hydrothermal metasomatism and veining which accompanied the emplacement of the Harney Peak Granite (HPG) at ca. 1715 Ma. While this concept is well documented for areas of higher-grade metamorphism in the southern Black Hills nearer the exposed HPG, the coincidence of very similar late, post-kinematic evidence much farther to the north begs interpretation.

In terms of regional structure, the porphyroblasts of interest in the north-central Black Hills are most often spatially associated with D₃ structures overprinting planar fabrics in D₃ shear zones or F₃ fold hinge areas, and are commonly crystallized in crosscutting veins (quartz and/or carbonate or talc-chlorite). Veining with late porphyroblast development is associated with Paleoproterozoic metavolcanics, metagabbroic plutonic (sill) complexes, and pelitic to feldspathic metasediments. Stable isotope C and O data suggest that the fluids accompanying porphyroblast development are hydrothermal and neither completely igneous nor of in situ origin. Further age control is necessary to unequivocally establish a link to the HPG event.