Abstract Tholeiitic basalts (oceanic tholeiites) that form most of tne deeply submerged volcanic features in the oceans are characterized bv extremely low amounts of Ba, K, P, Pb, Sr, Th, U, and Zr as well as Fe 2 O 3 /FeO < 0.2 and Na/K > 10 in unaltered samples. Oceanic tholeiitles also have rare earth abundance-distribution patterns and ratios of K/Rb (1300) and Sr 87 /Sr 86 (0.702) similar to or overlapping those of calcium-rich (basaltic) achondritic meteorites. The close compositional similarities between the oceanic tholciites and calcium-rich achondrltes indicates the relatively primitive nature of the oceanic tholeiites. In.contrast, the alkali-rich basalts that cap submarine and island volcanoes arc relatively enrichcd in Ba, K, La, Nb, P, Pb, Pb 206 , Rb, Fe 2 O 3 , Sr, Sr 87 , Ti, Th, U, and Zr; i.e. in the same elements and isotopes that are concentrated ir. the sialic continental crusts by factors of 5 to 1000 more tljan the amounts readily inferred in the upper mantle. These analytical data coupled with the field relationships indicate that the,alkali-rich basalt are derivative rocks, fractionated from the oceanic tholeiites by processes of magmatic differentiation, and that the bceanif tholeiites are the principal or \ only primary magma generated in the upper irtantic under the oceans. Studies of the abundances and compositions of continental basalts show that essentially identical tholeiitic lavas, contaminated with Si, K, and the chemically coherent trace elements and radiogenic isotope from the sial, also have been the predominant or only magma generated in the mantle under the continents. The chemical properties of oceanic tholeiites suggesjt that the upper mantle probably contains less than(in parts per million): Ba, 10; K, 100; Pb, 0.4; Rb, 10; Th, 0.2; and U, 0.1. The Sr 87 /Sr 86 must be lesj than 0.7015; Th/U about 2; K/Rb about 1500-2000; and Fe 2 O 3 /FeO less than 0.1. The integration of field and petrochemical data with seismic, density, and shock-wave studies suggests that the oceanic tholeiites are cither complete melts of the upper mantle or are generated from a mix of- this tholeiite and a magnesium-rich peridotite or dunik in proportions up to perhaps 1:4. The Mhorovicic discontinuity under the oceans appears to mark the transition downward from a largely tholeiitic oceanic crust to either tholeiite reconstituted to blueschist or greenschist or to the ultramafic residue left after expulsion of oceanic tholeiite

Abstract The progressive metamorphism of amphibolite interlayers in the major para-gneiss of the northwest Adirondacks has been investigated throughout a belt 35 miles long. Maximum temperatures of metamorphism appear to have increased uniformly from about 525° C at one end of the belt to 625° C at the other. This gradient in temperature is calculated from the several geothermometers used to calibrate the metamorphism of the enclosing paragneiss (Engel and Engel, 1958). Depths of metamorphism, based upon geologic reconstructions, are at least 5 to 7 miles but may be considerably deeper. The chemical composition of the amphibolites changes, with increasing T and P of metamorphism, from that of typical saturated basalt to olivine basalt or pyroxene granulite. Mineralogical changes as a function of increasing temperature are marked and systematic. Average amphibolite metamorphosed at 525° C consists of (volume per cent) quartz 10, andesine 20, hornblende 68, and ilmenite and pyrite 2; at ~ 540° C green clinopyroxene appears; at ~ 575 ° C pink orthopyroxene appears; at 625° G the average amphibolite contains calcic andesine 39, hornblende 22, clinopyroxene 21, orthopyroxene 14, and ilmenite and pyrite 2. Principal changes i n chemical composition with increasing T are decreases in Si, K, H 2 O, F, CI, and Fe +++ . Amounts of Ca, Mg, and probably Alincrease. Along the same gradient the enveloping paragneiss also is distinctly depleted in Si, K, H 2 O, F, CI, and Fe +++ and enriched in total Fe, Ca, and Mg. Associated siliceous marble is partially decarbonated. Accordingly, during Adirondack orogenesis the metamorphism of rocks now exposed produced large volumes of “hydrothermal” fluids and, at highest temperatures, granite and possibly ore-forming elements. Threshold conditions for degranitization were about 550° C for paragneiss (metagraywacke ?) and about 600° C for basaltic amphibolite. This progressive metamorphism verging on, or resulting in, partialmelting of a gneiss-amphibolite complex is rarely exposed for study in other parts of the sial, principally because belts of regionally metamorphosed rocks reconstituted at T and P in excess of the amphibolite facies have been disrupted by faulting. Hence the rocks of the Colton type that commonly appear in other granulite terranes are tectonically disassociated from less highly reconstituted parental types. Very possibly many (most ?) rocks of the granulite facies are more mafic than their initial parent rocks. Metamorphic reconstitution o f metabasic rocks at some T and P higher than that deduced for the Adirondack metamorphism could, logically, lead to the formation of eclogites; the minimum T would be 700° C, the minimum depth some 15 miles. Eclogites may, therefore, form from metabasaltic rocks at the base of, or under, continental crusts. But they would seem to be most improbable products of reconstitution at the base of the much thinner oceanic crusts. The origin of the Emeryville-Colt on amphibolites remains conjectural. Most of them seem to have formed by simple recrystallization of basaltic sills or flows. This interpretation is not entirely convincing, however, lor the very thin-layered amphibolites and these, or all of the amphibolites, may have formed by metamorphic differentiation or by replacement of sheared beds of paragneiss.