TECTONICS OF THE CENTRAL BLUE RIDGE PROVINCE: INSIGHTS FROM MONAZITE DATING AND P-T EVOLUTION

The Winding Stair Gap in the Central Blue Ridge province exposes granulite facies schists, gneisses, granofelses and migmatites characterized by the mineral assemblages: garnet – biotite – sillimanite – plagioclase – quartz, garnet– hornblende – biotite – plagioclase – quartz ± orthopyroxene ± clinopyroxene, and orthopyroxene– biotite – quartz. Multiple textural populations of biotite, kyanite and sillimanite in pelitic schists support a polymetamorphic history characterized by an early clockwise P-T path in which dehydration melting of muscovite took place in the stability field of kyanite. Continued heating led to dehydration melting of biotite until peak conditions of c. 850 ± 30°C, 9 ± 1 kbar were reached. After equilibrating at peak temperatures, the rocks underwent a stage of near isobaric cooling during which hydrous melt ± K-feldspar were replaced by muscovite, and garnet by sillimanite + biotite + plagioclase.

Most monazite crystals from a pelitic schist display patchy zoning for Th, Y and U, with some matrix crystals having as many as 5 compositional zones. A few monazite inclusions in garnet, as well as Y – rich cores of some monazite matrix crystals, yield the oldest dates of c. 500 Ma, whereas a few homogeneous matrix monazites growing in the main foliation plane yield dates of 330 - 370 Ma. Culling and analysis of individual spot dates for eight monazite grains yields three age populations of 509 ± 14, 438 ± 5 Ma, and 360 ± 5 Ma. These data suggest that peak-temperature metamorphism and partial melting in the central Blue Ridge occurred during the Salinic or Taconic Orogenies, possibly explained by subduction of the Cartoogechaye terrane beneath Laurentia. Following near isobaric cooling, a second weaker thermal pulse, possibly related to intrusion of nearby igneous bodies, resulted in growth of monazite c. 360 Ma, coinciding with the Neoacadian/ Famennian events. Partial exhumation of the Cartoogechaye terrane then followed by vertical extrusion as the Dahlonega Gold Belt was thrust underneath it. Closing of the Rheic Ocean during the Alleghanian orogeny caused the overthrusting of these sheets onto Laurentia.