PRELIMINARY REASSESSMENT OF THE TECTONOTHERMAL EVOLUTION OF THE SMITH RIVER ALLOCTHON, VA: IMPLICATIONS FOR TERRANE AFFINITY

The Smith River allochthon (SRA) is a dominantly metaclastic terrane in the Piedmont zone of southwest Virginia that is faulted against native Laurentian rocks to the west. The crustal affinity of the SRA has been controversial; some workers view the SRA as correlative with adjacent Laurentian rocks, while others interpret the allochthon as exotic with respect to Laurentia. The crux of this controversy is the interpretation of the timing and nature of early and main phase tectonothermal events recorded in these rocks. We are directing new structural, metamorphic, and geochronologic studies towards resolving the tectonothermal evolution and crustal affinity of the SRA. A synthesis of previous work and our preliminary studies indicates that the earliest tectonothermal event in the SRA is represented by sericitic and chloritic pseudomorphs of staurolite, biotite, andalusite, sillimanite, and garnet poikiloblasts that reflect a low P, high T thermal event; this assemblage appears to be related to an early, highly transposed foliation. The main phase event is characterized by recumbent folds with an axial planar foliation defined by micas and chlorite and a medium P, medium T assemblage of weakly to randomly oriented staurolite+chloritoid+kyanite+chlorite+sericite+garnet. The SRA was intruded by a bimodal suite of plutons at c. 440-430 Ma; this magmatic pulse resulted in widespread static metamorphism. Field studies are underway in order to constrain the relationship of plutonism to the main phase event. Electron microprobe (EMPA) and laser ablation-inductively coupled plasma mass spectrometry (LAMS) U-Pb monazite geochronologic studies are being conducted in order to constrain the timing of the tectonothermal events; to date, the two methods yield conflicting results. EMPA monazite ages agree with previous U-Pb TIMS staurolite ages that are interepreted to reflect a c. 535 Ma age for the early event and a c. 485 Ma age for the main event. LAMS monazite ages suggest the early phase occurred at c. 485 Ma and that the main phase is coincident with the Silurian magmatic event. LAMS detrital zircon geochronologic studies are also underway as an independent constraint on the cratonic provenance of the SRA. Preliminary results from the SRA yield a unimodal population ca. 1 Ga, similar to that found in adjacent Laurentian rocks.