THE PIPESTONE POND COMPLEX, CENTRAL NEWFOUNDLAND - COMPLEX MAGMATISM IN AN EASTERN DUNNAGE ZONE OPHIOLITE

Ophiolitic rocks are preserved in both the Notre Dame and Exploits sub zones 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 re ceived less attention in the literature. The Pipestone Pond Complex is an Exploits Subzone ophiolitic sequence, which outcrops on the wester n side of a structural window through the Exploits Subzone into the un derlying Gander Zone. It includes a basal harzburgite, which passes up wards into a cumulate pyroxenite and gabbro sequence, and thence into isotropic gabbro intruded by pegmatitic gabbro, diabase, and plagiogra nite. 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 t he intrusive rocks. The ophiolitic rocks are structurally disrupted an d no single cross section traverses the complete ophiolitic stratigrap hy. Although the stratigraphic sequence of the Pipestone Pond Complex is relatively straightforward, whole-rock geochemical and Nd/Sm isotop ic data provide evidence for a complex magmatic history. The intrusive rocks have epsilon(Nd(t)) ranging from - 1. 1 to +4 and geochemical s ignatures indicating derivation from depleted and refractory mantle so urces that were clearly influenced by subduction. Within the intrusive rocks, there are no simple petrogenetic relationships among the gabbr os and dykes and trondhjemites. The extrusive rocks, in contrast, have epsilon(Nd(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 tect onic interpretation of the Pipestone Pond Complex is hampered by a lac k of definitive evidence for the relative age of the arc-related pluto nic rocks and the non-arc-related extrusive rocks. Two possible interp retations are (i) the initiation of subduction under oceanic crust and (ii) arc rifting. The Pipestone Pond Complex is geologically and geoc hemically analogous to the Boil Mountain Complex in Maine, and togethe r they may form part of an extensive ophiolitic terrane that was empla ced upon the Gondwanan continental margin prior to its collision with Laurentia.