(Table 1) Isotopic composition of Sm and Nd in mantle restites of the Central Atlantic and in associated plutonic and volcanic rocks

Ultrabasic rock samples collected from two areas of the crustal zone of the Mid-Atlantic Ridge (MAR): (1) 13-17°N (near the intersection of the ridge axis with the 15°20'N prime fracture zone), and (2) 33°40'N prime (the western intersection of the MAR crest with the Heis fracture zone) were objects of this study. Samples of peridotite and of plutonic and volcanic rocks associated with it were used to measure their Sm/Nd, 143Nd/144Nd, and 147Sm/144Nd ratios, which allowed to test time and genetic relationships between evolution of mantle material under the ridge crest and products of its magmatic activity. Results of this work proved ubiquitous discrepancy between melting degree values of extremely depleted mantle peridotites in the MAR area between 14°N and 16°N, obtained using petrologic and geochemical methods. This discrepancy suggests large-scale interaction between mantle material and magmatic melts and fluids enriched in incompatible elements or fluids. The results obtained suggest that repeated melting of the mantle under the axial MAR zone is an universal characteristic of magmatism in low-velocity spreading centers. The results of this study also proved the crestal MAR zone in the Central Atlantic region show distinct indications of isotope-geochemical segmentation of the mantle. It is suggested that the geochemically anomalous MAR mantle peridotite in the zone of the MAR intersection with the 15°20'N prime fracture zone can be interpreted as fragments of mantle substrate, foreign for the Atlantic mantle north of the equator.

U-Pb (zircon) geochronologic analyses for rocks of the Transantarctic Mountains (Table 4.3-1)

The Byrd Glacier discontinuity us a major boundary crossing the Ross Orogen, with crystalline rocks to the north and primarily sedimentary rocks to the south. Most models for the tectonic development of the Ross Orogen in the central Transantarctic Mountains consits of two-dimensional transects across the belt, but do not adress the major longitudinal contrast at Byrd Glacier. This paper presents a tectonic model centering on the Byrd Glacier discontinuity. Rifting in the Neoproterozoic producede a crustal promontory in the craton margin to the north of Byrd Glacier. Oblique convergence of the terrane (Beardmore microcontinent) during the latest Neroproterozoic and Early Cambrian was accompanied by subduction along the craton margin of East Antarctica. New data presented herein in the support of this hypothesis are U-Pb dates of 545.7 ± 6.8 Ma and 531.0 ± 7.5 Ma on plutonic rocks from the Britannia Range, subduction stepped out, and Byrd Glacier. After docking of the terrane, subduction stepped out, and Byrd Group was deposited during the Atdabanian-Botomian across the inner margin of the terrane. Beginning in the upper Botomian, reactivation of the sutured boundaries of the terrane resulted in an outpouring of clastic sediment and folding and faulting of the Byrd Group.

(Table 3) Chemical composition of phosphatized rocks from the Kammu Seamount

Phosphatized biogenic limestones and phosphorites with initial Fe-Mn mineralization dredged from the summit surface of the Kammu Seamount (Milwaukee Seamounts, northwestern Pacific) are studied. The rocks are largely composed of nannofossils and planktonic foraminifers with an admixture of benthic foraminifers, bryozoans, and other organic remains, presumably including bacterial ones. The nannofosssil and foraminiferal assemblages indicate Quaternary age of sediments, and their phosphatization is consistent with the phosphatization age determined previously based on nonequilibrium uranium (within the limits of 1 My). The age of phosphatization and the Fe-Mn mineralization in the sediments from Pacific seamounts that young implies dependence of these ore-forming processes on oceanic environments favorable for ore accumulation rather than on their age.