XENOLITHIC ORIGIN OF ORBICULES IN DEVONIAN GRANODIORITES AT EAST BETHEL, VT

Zones of granite-cored, biotite and muscovite-sheathed orbicules occur within and along the contacts of two-mica granodiorite dikes with schists of Waits River Formation at the well-known Bethel "white" quarries. The orbicules, as much as 7 cm in diameter, are spherical, oblate or discoidal depending upon the degree of local synintrusive flattening strain within the igneous rocks. At exposed contacts mobilized "restitic" schist forms protrusions into the granodiorite and shows all degrees of ductile pinching to form inclusions in the granodiorite. The size, shape and orientation of orbicules are maintained for as much as 2 m normal to the contact. A 10-cm thick dike of orbicular granite in the schist parallel to the contact contains smaller but similar orbicules that were generated in situ. These observations indicate that the orbicules formed largely by physical rather than igneous melt-crystal processes. Low ductility contrast between host and magma facilitated plucking of schist by viscous drag adjacent to lit par lit injected walls of dikes. Internal orbicular zones probably represent occult borders or screens between multiply injected dikes. Subsequent deformation of orbicules by flattening during crystallization of the granodiorite and also by ductile shear is recorded by patterns of crenulate folds of the micaceous sheathes of orbicules in what otherwise appears to be homogeneous "nondeformed" granite. The exceptional exposures at East Bethel rank among most illustrative examples of the formation of orbicules of this kind.