The Petrology of the Lower Series Volcanics, ODP Site 642

Abstract

Between 1086.6 and 1229.4 m below seafloor at Site 642 on the Outer Wring Plateau, a series of intermediate volcanic extrusive flow units and volcaniclastic sediments was sampled. A mixed sequence of dacitic subaerial flows, andesitic basalts, intermediate volcaniclastics, subordinate mid-ocean ridge basalt, (MORB) lithologies, and intrusives was recovered, in sharp contrast to the more uniform tholeiitic T-type MORB units of the overlying upper series. This lower series of volcanics is composed of three chemically distinct groups, (B, A2, Al), rather than the two previously identified. Flows of the dacitic group (B) have trace-element and initial Sr isotope signatures which indicate that their source magma derived from the partial melting of a component of continental material in a magma chamber at a relatively high level in the crust. The relative proportions of crustal components in this complex melt are not known precisely. The most basic group (A2) probably represents a mixture of this material with MORB-type tholeiitic melt. A third group (Al), of which there was only one representative flow recovered, is chemically intermediate between the two groups above, and may suggest a repetition of, or a transition phase in, the mixing processes.

Archaean tectonics: a review, with illustrations from the Slave craton

“…Groves et al 1978). Some partial analogues of these settings do occur in postArchaean time: for instance, the c. 1.1 Ga basalt • rhyolite and sediment-filled Keweenawan rift of North America (Green 1977;Behrendt et al 1988;Ojakangas et al 2001), the complex volcanic rifted V6ring margin off the coast of Norway (Eldholm et al 1989;Parson et al 1989;Reemst & Cloetingh 2000), and perhaps the collapsed marginal basin of the Cretaceous Rocas Verdes Complex, Chile (e.g. Tarney et al 1976).…”

“…Wendt et al 1997;Turner & Hawkesworth 1998), the occurrence of calc-alkaline dacites with Nb depletions in Iceland's central volcanoes (Jdnasson et al 1992), and the occurrence of dacites and andesites in extensional environments (e.g. Parson et al 1989). Hence, given the known complexity of modern processes, the non-uniqueness of many geochemical signatures, and the incomplete preservation of ancient environments, are we not overinterpreting the Archaean record in terms of simple end-members?…”

Sill and lava geochemistry of the mid‐Norway and NE Greenland conjugate margins

“…Lavas collected in the 642E core on the Vøring Plateau (Figure a) consist of an Upper Series (US) of tholeiitic E‐MORB‐type basalts, and a very heterogeneous Lower Series (LS) that comprises rhyolitic ignimbrites, tholeiitic basalts, basaltic andesites, and dacites [ Viereck et al ., 1988, ; Parson et al ., ]. Based on trace elements and Sr‐Nd isotope data, Viereck et al .…”

“…[1988, ], Parson et al . [], and Meyer et al . [] concluded that the 642E US lavas are mostly uncontaminated and have essentially preserved the trace element and isotopic signature imprinted by a mantle source somewhat more radiogenic than the depleted MORB mantle (DMM).…”

“…[], is probably due to a less rigorous selection of unaltered rocks. The positions of the different series in Figure are in perfect agreement with the conclusions that the 642E LS and NE Greenland US are contaminated [ Parson et al ., ; Thirlwall et al ., ; Meyer et al ., ]. The Utgard sills fall at the transition between “uncontaminated” and “contaminated” 642E and NE Greenland series.…”