Tab.1: Representative bulk-rock and trace element chemistry

High-pressure/low-temperature metabasites occupy a definite geological position within the structure of the Polar Urals and have a very important bearing on the understanding of the early history of the Ural Mountains. Recently obtained geological, petrographic, geochemical and isotope data allow some conclusions on this history. The metabasites of the Khord"yus and Dzela complexes contain relics of a Neoproterozoic (578 ±8 Ma) oceanic crust. This crust formed part of the base of the early Paleozoic (500 Ma) ensimatic island arc and experienced Ca-Al-Si±Na metasomatism and, probably, partial melting with the formation of boninite melts. However, so far no boninite volcanics have been found. The metabasites at the base of the island arc took part in the collision and as a consequence experienced glaucophane schist and greenschist facies metamorphism during the collision and obduction over the passive Baltic margin 350 ±11 Ma ago.

Seismic properties of volcanic rocks from DSDP Hole 62-465A

Hess Rise, a major structural feature in the northern Pacific, is one of several oceanic plateaus which apparently possess anomalous seismic structures (Sutton et al., 1971). Hence, Laboratory measurements of compressional- and shear-wave velocities in rocks from oceanic plateau regions are of considerable interest. Several questions come to mind: (1) Are compressional - wave velocities of volcanic rocks from oceanic plateaus similar to basalts of equivalent age from normal oceanic crust? (2) Do velocity-density relations for plateau rocks fit the well-established trends for Layer 2 basalts? (3) How do Poisson's ratios, determined from compressional- and shear-wave velocities, of oceanic plateau rocks compare with those of normal sea-floor basalts? To answer these questions, we have selected for velocity measurements five volcanic samples from different depths from Hole 465A, on southern Hess Rise. It is particularly significant that all the rocks are vesicular and have been highly altered, which significantly affects their elastic properties.