A New Look at the Virgin Islands Batholith

The Virgin Islands batholith (VIB) comprises a large volume of silicic plutonic rocks on the northeastern corner of the Caribbean plate. Field observations suggest that the VIB is more compositionally and petrologically heterogeneous than previously appreciated. The VIB has been interpreted to represent a single Eocene magmatic episode, but likely includes at least three or more plutons of differing age and character. Here we describe new field observations and provide preliminary informal names for these bodies.

The Baths pluton, which crops out in southwestern Virgin Gorda, eastern Tortola, Fallen Jerusalem, Beef, Ginger, and Cooper islands, is a relatively homogenous enclave-rich hornblende granodiorite that locally has a hornblende foliation. It is cut by a north-striking fault zone near Copper Mine Point, responsible for hydrothermal alteration and mineralization. On western Virgin Gorda, near Little Dix Bay, this pluton contains a heterogeneous hornblende gabbro body with zones of modal layering and orbicules. The North Sound pluton of northeastern Virgin Gorda, east of the Copper Mine fault zone, is a fine-grained, heterogeneous, silicic body. The Peter Island pluton on Peter and Norman Islands is characterized by sheets of tonalite, diorite and metamorphic wall-rock screens that strike roughly north, and range in thickness from 1-4 meters. Tonalite sheets exhibit internal contacts with chilled margins and include granodiorite xenoliths resembling the Baths pluton, suggesting the Peter Island cuts the Baths pluton. The northwestern part of the VIB includes north-striking mafic dikes cutting granodiorite on Little Camanoe Island; granodiorite on Little Camanoe may represent extensions of the Baths pluton or be a separate pluton. Steep, locally slickensided, north-striking faults are common throughout the area; these and large granitic boulders define several key dive spots. New U-Pb zircon ages and geochemical data should clarify the tectonic significance and history of northeastern Caribbean magmatism.