Structure and petrography of the basement complexes east of the Hot Springs Window, Blue Ridge Province, western North Carolina

Author

Roger Clay Brewer

Date of Award

8-1986

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Nicholas B. Woodward

Committee Members

Harry Y. McSween Jr., Theodore C. Labotka

Abstract

Recent tectonic models of the Blue Ridge Province of western North Carolina correlate the contact between the Amphibolitic Basement Complex ("layered gneiss and migmatite" of Hadley and Nelson, 1971) and the structurally underlying Cranberry Granitic Basement Complex with the hypothesized Ordovician-Devonian Hayesville-Fries Fault. This study involves the detailed petrographic and structural study of these two basement complexes and the nature of the contact separating them. The study area is located in the area just east of the Hot Springs Window, central western North Carolina.

The Cranberry Basement is composed chiefly of biotite granitic gneisses and semischists intruded by small scattered bodies of weakly foliated granodiorite to quartz diorite. Other lithologies less typical of the complex include biotite hornblende granitic gneiss and calc-silicate rock. As is characteristic elsewhere in the Blue Ridge, the western edge of the granitic basement is marked by the presence of a pink, epidote-rich granite called the Max Patch Granite. Published radiometric ages from exposures of the Cranberry Basement to the north of the study area range in age from 1.8by to 870my.

Roughly three-fourths of the Amphibolitic Basement Complex is composed of locally migmatitic biotite hornblende granitic gneiss intruded by and complexly intermixed with amphibolite. Other intermixed lithologies typical of the complex include weakly foliated granitic rock, talc-rich ultramafic bodies (high in Cr and Ni), metapyroxenite, garnet chlorite schist, feldspar augen semischist, metabasalt porphyry, diopsidic marble, and calc-silicate rock. No stratigraphic order could be developed for the complex. Field observations in conjunction with geochemical analyses suggest that the bulk of the amphibolites represent tholeiitic, continental basalts. A single published Rb/Sr whole rock date on a granitic gneiss from just east of the study area yielded an age of 1.2by.

Based on; 1) the truncation of amphibolite units and deformation zones elsewhere along the contact and 2) the dominant restriction of amphibolites and ultramafic rock to the Amphibolitic Basement Complex, the contact between the Amphiboltic Basement Complex and the structurally underlying Cranberry Granitic Basement Complex is interpreted to be a fault. In the study area, granitic and gabbroic rocks correlated with the Crossnore and Bakersville plutonic suites intrude both Cranberry and Amphibolitic Basement complexes. The Crossnore-like granite (Indian Grave Metagranite) appears to crosscut the contact between the two complexes. The Late Proterozoic (734-700my) age of the two plutonic suites establishes a minimum age for the fault contact separating the two complexes.

As in areas to the north, the Cranberry Granitic Basement Complex is interpreted to represent a Grenville-age granitic plutonic complex once attached to the eastern edge of the North American Craton. The Amphibolitic Basement Complex is interpreted to represent an intensely disrupted, melange-like complex composed chiefly of transitional crustal material. The complex is interpreted to have developed during Middle to Late Proterozoic incipient rifting of granitic crust, accompanied by the intrusion of tholeiitic basaltic magmas. Closure of the rift complex later during the Proterozoic resulted in the intense disruption of the transitional crust and the incorporation of blocks of lower crustal/upper mantle ultramafic rock and sedimentary carbonate into the complex.

Contrary to previous interpretations, the contact between the Amphibolitic Basement Complex and the structurally underlying Cranberry Granitic Basement Complex cannot be correlated with the hypothesized Ordovician-Devonian Hayesville-Fries Fault. If a single, continuous "Hayesville-Fries" fault can be drawn in the Blue Ridge of western North Carolina, it must lie no further west than the upper contact of the Amphibolitic Basement Complex.

Recommended Citation

Brewer, Roger Clay, "Structure and petrography of the basement complexes east of the Hot Springs Window, Blue Ridge Province, western North Carolina. " Master's Thesis, University of Tennessee, 1986.
https://trace.tennessee.edu/utk_gradthes/13656