UNRAVELING LOW-TEMPERATURE COOLING IN THE BLUE RIDGE, VA AND NC, THROUGH 40AR/39AR ANALYSIS AND GRAIN-SCALE CHARACTERIZATION OF K-FELDSPAR

Preexisting bedrock structures in tectonized terranes provide a template that future deformation uses to imprint its own story. Here we examine the (mostly) post-Alleghanian history of the Eastern (EBR) and Western Blue Ridge (WBR) in NC and VA using ⁴⁰Ar/³⁹Ar K-feldspar data to assess differences in the low-temperature cooling history in a cross-strike transect from Mt. Rogers to the Brevard fault, crossing the Fries fault— a major tectonic boundary.

Grenville-age rocks from the WBR include metavolcanic and metaigneous rocks with varying degrees of greenschist facies overprinting. In contrast, metasedimentary and metavolcanic rocks from the EBR contain primary metamorphic mineral assemblages consistent with amphibolite facies metamorphism. From the WBR and EBR, samples containing K-feldspar were examined using cathodoluminescence (CL) imaging, Raman spectroscopy, and X-ray diffraction to assess the intra-grain and inter-sample variability. CL images of K-feldspars from the WBR show pervasive patch perthite textures and “dead” CL regions associated with high abundances of fluid-filled pores. Correlative Raman analysis of the various CL regions reveals Al/Si ordering in association with porous regions are close to maximum microcline, and moderate to high CL intensity regions are less ordered. In contrast, CL imaging from the EBR K-feldspars reveal remarkably homogenous textures and a high degree of Al/Si ordering based on Raman analysis. High-end K-feldspar ⁴⁰Ar/³⁹Ar cooling ages from the WBR show a large age range of ~380–500 Ma, whereas the low-end ⁴⁰Ar/³⁹Ar K-feldspar cooling ages are less variable, ranging from ~250–310 Ma. In contrast, both the high- and low-end ⁴⁰Ar/³⁹Ar cooling ages from the EBR show little scatter, ranging between ~280–325 Ma and ~160–210 Ma, respectively. The scatter observed in the high-end ages from the WBR likely reflect partial greenschist facies, fluid-assisted recrystallization of K-feldspar during Paleozoic tectonic events. While the timing of thermal convergence of the EBR and the WBR cannot be fully explained by this dataset, it is clear that the EBR and WBR came to their present structural configuration after the end of the Alleghanian orogeny, suggesting reactivation of the Fries fault or perhaps larger scale regional tilting of the Blue Ridge Province.