The Pipestone Pond Complex, central Newfoundland: complex magmatism in an eastern Dunnage Zone ophiolite

Abstract

Ophiolitic rocks are preserved in both the Notre Dame and Exploits subzones in the Dunnage Zone of the Newfoundland Appalachians. Ophiolites in the Exploits Subzone are generally less well preserved and exposed than their Notre Dame Subzone counterparts and, consequently, have received less attention in the literature.The Pipestone Pond Complex is an Exploits Subzone ophiolitic sequence, which outcrops on the western side of a structural window through the Exploits Subzone into the underlying Gander Zone. It includes a basal harzburgite, which passes upwards into a cumulate pyroxenite and gabbro sequence, and thence into isotropic gabbro intruded by pegmatitic gabbro, diabase, and plagiogranite. There is no sheeted dyke unit. Pillow lava occurs at the top of the sequence but is not observed to be in stratigraphic contact with the intrusive rocks. The ophiolitic rocks are structurally disrupted and no single cross section traverses the complete ophiolitic stratigraphy.Although the stratigraphic sequence of the Pipestone Pond Complex is relatively straightforward, whole-rock geochemical and Nd/Sm isotopic data provide evidence for a complex magmatic history. The intrusive rocks have εNd(t) ranging from −1.1 to + 4 and geochemical signatures indicating derivation from depleted and refractory mantle sources that were clearly influenced by subduction. Within the intrusive rocks, there are no simple petrogenetic relationships among the gabbros and dykes and trondhjemites. The extrusive rocks, in contrast, have εNd(t) of + 7.3 and geochemical signatures similar to those of normal mid-ocean-ridge basalts. They represent magmatism derived from depleted oceanic mantle, not affected by the subducted slab.The tectonic interpretation of the Pipestone Pond Complex is hampered by a lack of definitive evidence for the relative age of the arc-related plutonic rocks and the non-arc-related extrusive rocks. Two possible interpretations are (i) the initiation of subduction under oceanic crust and (ii) arc rifting. The Pipestone Pond Complex is geologically and geochemically analogous to the Boil Mountain Complex in Maine, and together they may form part of an extensive ophiolitic terrane that was emplaced upon the Gondwanan continental margin prior to its collision with Laurentia.