In situ hydroclastic fragmentation of subaqueous ponded lavas; New Senator caldera, Abitibi greenstone belt, Quebec, Canada

Abstract

Subaqueous ponded lavas have been recognized within the structure of the New Senator caldera in the Blake River Group of the Abitibi greenstone belt, Quebec, Canada. Sub-vertical to near-vertical dips of extrusive volcanic facies within the southern sector of the caldera permit the identification of the internal architecture of an Archean subaqueous volcanic complex. Detailed facies analyses of the dominantly mafic sequences have shown two localities with atypical primary volcanic structures and facies inconsistent with those observed at modern subaqueous mafic seamounts. Combined with the use of facies-specific geochemical analyses, a coherent model for the two localities accounting for all observed volcanic structures and features has been developed.Mapping completed at a scale of 1:100 at Localities 1 and 2 reveals a series of sub-parallel to parallel hyaloclastite-rich horizons separating aphanitic to medium grained ponded units. Several primary volcanic structures are observed within the hyaloclastite horizons and are identified as pillowed forms, cigar forms and v-shaped structures. Ponded units range in thickness from 2.5 to 5m, with the exception of one unit which is approximately 30m thick. In central portions, this thick unit is medium-grained and has a sub-ophitic texture. Units at Locality 1 strike approximately NW, while those at Locality 2 strike ENE. Several families of mafic dykes cross-cut Locality 1 and trend E-W and NNE. Both volcaniclastic and effusive facies have similar compositions, affinities and trace and rare earth element signatures. All facies have a tholeiitic affinity, range in composition from basalt to andesite and have MORB-type trace and rare earth element signatures. Locality 1 appears slightly more evolved than Locality 2, with later dykes and a late sill being the most evolved facies.Volcanic facies at Localities 1 and 2 are consistent with subaqueous ponded lavas. Bounding synvolcanic structures and volcanic facies validate this interpretation and indicate these ponded lavas are hosted within a large synvolcanic depression at the summit of a mafic shield complex. Primary volcanic structures within hyaloclastite horizons formed via in situ fragmentation when water entered the system as ponded lavas began to cool. Water entered the system through synvolcanic fractures and encountered semi-molten pockets of lava, leading to hydroclastic fragmentation of the lavas. V-shaped and cigar structures formed during more energetic events and changed laterally into pillowed forms as energy dissipated from the system.