A COMPARISON OF THE STRUCTURAL SEQUENCES BETWEEN BLOWING ROCK GNEISS AND LATE PROTEROZOIC MAFIC INTRUSIONS IN THE GRANDFATHER MOUNTAIN WINDOW

In this report, we focus on the Blowing Rock Gneiss(coarse-grained biotite augen gneiss), and its structural relationships with metadiabase dikes, presumably part of the Linville metadiabase. The Blowing Rock Gneiss was exposed to at least two pervasive metamorphic events. The original higher-grade metamorphism (amphibolite to granulite facies) and pervasive deformation were associated with the Grenville Orogeny. This original metamorphism was later overprinted by retrograde metamorphism. Ductile shear zones and diffuse regions of saussuritization in the augen gneiss, and chloritized faults may represent either advanced alteration in localized high strain partitions from this overprinting event,.The metadiabase dikes are concordant to slightly discordant with the primary foliation in Blowing Rock Gneiss. These dikes are greenstones that contain strong cleavage parallel to dike margins and a fairly widespread crenulation cleavage deforming the primary cleavage. We consider these dikes to be part of the Late Proterozoic rifting sequence superimposed on Grenville tectonites.

Recent road cuts on Highway 321 near Blowing Rock, NC exposes the contacts between these rocks. The primary foliation in augen gneiss is a strong composite schistosity defined by mica and quartz, and by stretched, rotated, and flattened feldspar porphyroclasts. In other cases, the contacts appear to be gradational and display increased amounts of shear within the augen gneiss marked by extreme flattening of feldspar porphyroclasts . In some of the dike contacts there are localized shear zones resulting in.very fine-grained phyllonite from alteration of the metadiabase along these shear zones. Our preliminary structural mapping suggests that all of the metadiabase dikes sustained the same deformation that produced the flattening foliation in these country rocks.

Some dike contacts show original intrusive interfingering relationships with the augen gneiss that predate the cleavage within the dikes. We observe, however, that some of these apparent interfingering boundaries are actually small amplitude tight folds. Those boundaries structures are commensurate with tightly folded lamellar veins within some dikes. At the mesoscopic scale cleavage within the dikes is axial planar to intra-dike folds.