EXTENSIONAL TECTONICS IN COLLISIONAL OROGENS: A CASE STUDY OF EARLY SILURIAN EXTENSION IN THE CENTRAL APPALACHIANS ASSOCIATED WITH THE TACONIC OROGENY

In many orogens, an extensional event subsequent to a crustal thickening episode has been recognized through both structural and petrologic studies. We describe a similar temporal association in the central Appalachian orogen of Virginia. New ion probe zircon ages of igneous rocks along the axis of collision suggest a 441 Ma igneous event where plutons are identified as suturing tectonic blocks assembled during the collision process, thus marking the end of the compressional phase of the Taconic Orogeny. U/Pb zircon ages of mafic complexes (Rich Acres, Buckingham, Diana Mills and Green Springs) in the same region are well constrained between 434 and 431 Ma, and denote episodic emplacement of mantle derived magmas at mid-crustal levels. These mafic rocks show a range in Mg# (60 to 68), SiO2 from 46 to 59 wt.% and total alkalis of 2-6 wt.%, and can be classified as gabbro to diorite. HFSE data (Zr/Hf of 30 to 40; Zr/Nb of 10 to 20; Nb/Ta of 5 to 10) as well as Ba-La-Nb between the plutons are in a narrow range, suggesting a similar source, but different from MORB or OIB. The strontium isotopic signature (0.7047 to 0.7063) and enriched REE data suggest derivation from either an enriched (metasomatized) mantle or a product of crustal contamination. Although no extensional faults of Silurian age have been recognized, we suggest that the sequence of igneous events is most likely to be the result of extension following a mid-Ordovician collisional event. Two tectonic models are permissible (gravitational collapse or slab delamination); however, we propose extension was induced through plate delamination, as gravitational collapse is less likely because of the narrow width of the collision zone. Our data is consistent with a continent-arc collision subsequent to attempted subduction of the passive margin, leading to slab delamination and resultant uplift and extension. In such a tectonic environment hot asthenosphere is likely to intrude the lithospheric mantle of the overriding plate, leading to partial melting. These melts are transported to mid-crustal levels (presumably along structural discontinuities) and are recorded as the mafic plutons in the collision zone.