XENOLITHS AS EVIDENCE FOR EXPLOSIVE MAGMATISM IN THE MESOZOIC RING DIKES OF NEW HAMPSHIRE

Evidence for eruptive magmatism in the Mesozoic ring dike complexes of New Hampshire has been documented in the Ossipee Mountains (Carr, 1980), Belknap Mountains (Modell, 1936), and White Mountain batholith (Creasy, 1974). Though several ring dikes in New Hampshire have been described as calderas (e.g. Creasy & Eby, 1993), little has been written regarding the role of eruptive magmatism in the ring dike emplacement process. In a study of the Ascutney complex of eastern Vermont, Schneiderman (1987) concluded that xenoliths present in the exposed stock and ring dike represent lithologies that overlay the area at the time of emplacement, and therefore constitute volcanic breccia formed during the emplacement of the complex. Similar xenoliths exist in the Mesozoic ring dikes of New Hampshire, and based on their shapes and lithologies it is probable that they were formed in a manner similar to that of the Ascutney breccia blocks. Xenoliths observed in New Hampshire ring dikes include unaltered to highly-altered metasediments, trachyte, basalt, and assorted intrusive igneous lithologies. In the case of the basalt and intrusive igneous rock types, the xenolith source is easily identifiable as a lithological unit which lies adjacent to the ring dike, indicating wallrock fracture during the emplacement process. The trachyte xenoliths are evidence that ring fractures may have reached the level of a volcanic edifice (Schneiderman, 1989). Finally, the metasedimentary xenoliths are identifiable as Devonian-age Littleton lithologies that are believed to have been deposited over much of the region in pre-Acadian time, and therefore constituted the highest level roof rocks of most Mesozoic intrusions in New Hampshire (Schneiderman, 1987). The presence of these xenoliths in the New Hampshire ring dikes indicates an emplacement process involving explosive magmatism, volcanism, and fracture of roof and wallrocks above magma chambers.