Shear Zones, Veins, and Metasomatism as Evidence of Latest Tectono-Magmatic Activity in the South Dakota Black Hills

Three coincident features: complex, cross-cutting veins, late-to post-kinematic porphyroblasts (metasomatic) and macro-scale shear zones, characterize broad regions of the Precambrian Black Hills (BH). The potential relationship among the three has received little notice until recently. The best reconstructed temporal setting places these features at the culmination of a tectono-petrological model based on extensive structural analyses (completed via mentoring of undergraduate students; Allard and Portis, 2013). All previous attempts in deciphering the chronological relationship among magmatism, metamorphism, and tectonic-structural elements in the BH, have been over-specified, considering gaps in field and age data.

The best supported model currently involves two distinct events related to Trans-Hudsonian suturing. Upright isoclinal folds with subhorizontal hinge lines accompany regional bt or bt-grt grade metamorphism at 1750 Ma.These are overprinted by sub-vertically plunging folds (F₃) and semi-ductile shear zones associated with left-lateral transpression. Late static metasomatism confined within these younger fold and shear structures is texturally identical to that described in the Harney Peak aureole (Nabelek, et al, 2006). In this context, veins, shearing, and late porphyroblasts are most compatible with the last folding, thrusting, and magmatic influences. There is a very strong implication from continued field studies, that the three features are linked via the invasion of metasomatizing fluids from the Harney Peak Granite. Some of the most intriguing evidence is exposed where the granite is not at the surface but strongly implicated in the subsurface. The co-occurrence of tourmaline, K-bearing alkali feldspars (antiperthite), and muscovite are associated with the latest veins where they cut metabasaltic flows and sills up to 35km north of granite exposures. The regional isograds that are constructed for the Black Hills display maximum metamorphic intensity adjacent to granite and associated pegmatites. The dominant impression from porphyroblast formation, is that the BH is both Buchan-like in lower pressure conditions and quite volatile-rich in creating late- to post-kinematic metasomatic phases, even well beyond direct association with leucogranite intrusion.