LITHOSTRATIGRAPHY, PETROCHEMISTRY AND U-PB (ZIRCON) AGE CONSTRAINTS OF VOLCANIC ROCKS OF THE BONAVISTA PENINSULA: IMPLICATIONS FOR THE INTERPRETATION OF THE MUSGRAVETOWN GROUP

Volcanic rocks of the central to northwestern Avalon Terrane in Newfoundland have traditionally been assigned to the Bull Arm Formation of the terminal Neoproterozoic Musgravetown Group. Recent U-Pb (zircon) geochronological and lithogeochemical investigations have facilitated identification of a time-progressive change in the chemical affinity of these rocks that necessitates designation of some of the volcanic rocks to lithostratigraphic units below and above the Bull Arm Formation. On Bonavista Peninsula, the basal Musgravetown Group (Cannings Cove Formation) is constrained to ca. 600 Ma, the age of a tuff interbedded with volcaniclastic boulder-conglomerate and calc-alkaline basalt. Bull Arm Formation magmatism heralds the onset of extensional magmatism, and includes continental tholeiites, weakly alkaline felsic volcanic rocks and tuffaceous and epiclastic rocks of the ca. 592 Ma Plate Cove volcanic belt. Undated alkaline basalts on the northeastern (Dam Pond area) and southwestern (British Harbour area) Bonavista Peninsula occur above the Plate Cove volcanic belt, below the Crown Hill Formation (uppermost Musgravetown Group), and at different stratigraphic levels with respect to the ca. 580 Ma glacial diamictite marker horizon known on Bonavista Peninsula as the Trinity facies (Gaskiers equivalent). The Dam Pond basalts are intercalated with mainly fine-grained, siliciclastic deposits (Big Head Formation) in a structural dome flanked to the north, west and south by Trinity facies diamictite. The more evolved British Harbour basalts occur above the Trinity facies, within an upward-coarsening sandstone sequence (Rocky Harbour Formation) overlain by red pebble conglomerate and sandstone of the Crown Hill Formation. Deposition of the largely siliciclastic Musgravetown Group is therefore interpreted to be mainly related to prolonged and episodic extension, consistent with either a back arc setting or intra-arc extension and rifting. These new chronological, lithological and petrological data collectively suggest a necessary correction to the extent and definition of the Musgravetown Group.