Variations in style of volcanism and massive sulfide deposition in Early to Middle Ordovician island-arc sequences of the Newfoundland Central Mobile Belt

Abstract

The Newfoundland Central mobile belt records the Early Paleozoic birth, development, and destruction of the Iapetus ocean. Following opening of the ocean in the Cambrian, a compressional plate margin developed in the Early Ordovician and an island arc was formed above an east-dipping subduction zone. Volcanism was active at this plate margin from late Tremadocian to early Caradocian, at which time subduction ceased and the products of the volcanism were blanketed by argillite and flysch.The pre-Caradocian island-arc volcanic sequences of the Central mobile belt are host to numerous volcanogenic massive sulfide occurrences. Both the volcanic rocks and the associated massive sulfide occurrences show distinct contrasts across the volcanic belt. In the northern part of the belt (Notre Dame Bay area), submarine, dominantly mafic, volcanic rocks host pyritic Cu-Zn deposits associated with local felsic domes. These sequences pass southward into the Victoria River area, where volcanic rocks are characterized by laterally extensive felsic pyroclastic sheets which host polymetallic Zn-Pb-Cu deposits. In southern Newfoundland (the Hermitage flexure area), partially emergent, preponderantly felsic volcanic rocks host Pb-Zn-Ag massive sulfide mineralization formed during the waning stages of volcanism.Lead isotope ratios in massive sulfide deposits in the Hermitage flexure area are consistently more radiogenic than in deposits hosted by more mafic volcanic sequences to the north. This, coupled with geologic evidence, suggests that these sequences formed under the influence of sialic crustal material. It is inferred that the Early-Middle Ordovician island-arc volcanism in the Newfoundland Central mobile belt occurred in both oceanic and continental margin settings. The compositions and geologic environments of the associated massive sulfide deposits reflect these variations in tectonostratigraphic settings.Time and space relationships within the volcanic arc during the interval of active volcanism are not entirely clear. All Early to Middle Ordovician volcanic rocks in central Newfoundland may have formed from contemporaneous volcanic activity. If this is the case, simultaneous volcanism must have occurred both in oceanic environments and at or near the continental margin. Alternatively, the volcanism may have been slightly diachronous with an early episode of ensimatic mafic volcanism followed by migration of the volcanic centers toward the continental margin.The massive sulfide deposits of the pre-Caradocian island arc exhibit lead isotope ratios which are distinctly more radiogenic than those in the Buchans, Roberts Arm, and post-Caradocian Cutwell Groups. This appears to support geologically based inferences that the latter sequences are not part of the main phase of island-arc activity in central Newfoundland.