NEW INSIGHTS INTO THE NEOPROTEROZOIC GEOLOGY OF THE CENTRAL AVALON PENINSULA (PARTS OF NTS MAP AREAS 1N/6, 1N/7 and 1N/3), EASTERN NEWFOUNDLAND

Abstract

New mapping and geochronological studies of the central Avalon Peninsula reveal a geological record of protracted and episodic volcanicity, having varied composition, lithofacies, and depositional environments, spanning a period of 160 Ma in the late Neoproterozoic. Deposition of these disparate volcanic successions was punctuated by both marine and terrestrial sedimentation, and by the emplacement, uplift and erosion of intrusive suites of differing age and composition. The new data clearly demonstrate, for the first time, that volcanic rocks previously included within the Harbour Main Group cannot be categorized as a single lithostratigraphic unit of group status. These Neoproterozoic rocks are described below in a framework of six lithostratigraphically and chrono-stratigraphically discrete units that can be mapped on a regional scale. A seventh volcanic unit occurs east of the Topsail Fault and is in part coeval with the youngest (ca. 585 Ma) subaerial volcanism west of this structure. Several contrasting ages, settings and styles of volcanic-hosted mineralization are preserved; these include volcanic-redbed copper, VMS-style zinc–lead–copper, and high- and low-sulphidation styles of precious metals. The new data also document a plutonic history that is complex, protracted and metallogenically important, especially visa-vis porphyry-style copper–gold mineralization. Several chronologically disparate, lithologically distinctive plutons have been mapped within the Holyrood Intrusive Suite. Additional subdivision and reassignment of rocks historically grouped within this suite likely will be required, as further geochemical and geochronological information becomes available. Three separate ages of Neoproterozoic marine sedimentation have been identified. The oldest siliciclastic rocks lie unconformably on pre-640 Ma volcanic successions in the central Holyrood Horst, and contain ca. 620 Ma tuff beds. The earlier of two periods of post-585-Ma marine sedimentation records the initial submergence of the epithermally altered rocks along the east side of the Holyrood Horst, and was accompanied by submarine mafic volcanicity, the emplacement of intermediate hypabyssal intrusions, and the formation of VMS-style mineralization. Younger marine siliciclastic sequences, such as those found east of the Topsail Fault, typically lack intercalated volcanic rocks, and lie in stratigraphic continuity with the 565 Ma, Ediacaran-bearing strata of the upper Conception Group.