Robert G. Coleman

Eclogites and Eclogites: Their Differences and Similarities

Eclogites are divisible into three groups based on mode of occurrence: Group A, inclusions in kimberlites, basalts, or layers in ultramafic rocks; Group B, bands or lenses within migmatite gneissic terrains; Group C, bands or lenses within alpine-type metamorphic rocks. The compositions range from olivine basalt for Group A to tholeiitic basalts for Group C. New analytical data on six eclogites from glaucophane schist terrains in California and New Caledonia now permit comparisons among the three eclogite types. The pyrope content of the garnets is distinctive for each group as follows: Group A, greater than 55 per cent py; Group B, 30–55 per cent py; Group C, less than 30 percent py. Pyroxenes coexisting with these garnets also reflect a compositional change related to their occurrence. The jadeite content progressively increases from Group A through Group B, whereas the diopside content decreases. A comparison of eclogites from different geologic occurrences but with similar bulk compositions demonstrates variation in Ca-Mg partition between coexisting garnet and pyroxene. The Ca/Mg ratio increases in garnet and decreases in pyroxene from Group A through Group B eclogites. This obvious difference in the Ca-Mg partition between coexisting garnet-pyroxene in eclogites of the same bulk composition indicates a broad range of pressure-temperature conditions obtained during crystallization. Experimental synthesis of eclogite-like material at high pressures and temperatures demonstrates that some eclogites may form in the earths mantle, but naturally occurring Group C eclogites have coexisting garnet-pyroxene with distinct Ca/Mg ratios when compared to Group A or B eclogites of similar bulk composition. This difference in the Ca/Mg ratio must reflect the pressure-temperature conditions characterizing the glaucophane schist facies. The formation of eclogites within different metamorphic facies is strong evidence of the divergent pressure-temperature conditions that allow basalts to recrystalhze into garnet-pryoxene rocks. In view of the rather compelling field evidence, it would seem advisable to discontinue the concept of an eclogite metamorphic facies.

O18/O16 Ratios of Coexisting Minerals in Glaucophane-Bearing Metamorphic Rocks

Oxygen isotope analyses have been obtained for coexisting minerals in several blue-schist-facies metamorphic rocks from California, Oregon, and New Caledonia. Detailed isotopic studies were made on a continuous exposure of schist in Ward Creek, California, previously described by Coleman and Lee (1962). The oxygen isotope fractionations among coexisting minerals in a variety of rock types, including metasediments and metabasalts, are systematic and larger than those measured in pelitic schists metamorphosed at the grade of biotite zone or higher. Therefore, these Ward Creek rocks (termed Type III) must have formed at lower temperatures than have such pelitic schists. Evidence for significant isotopic equilibration and homogenization is observed in the Ward Creek sequence. Six different metasediments and metavolcanics collected within 25 m of one another show almost identical mineral δ; -values: quartz (15.8 to 16.3), aragonite (13.1 to 13.3), glaucophane (9.8 to 10.0), muscovite (10.9 to 11.3), lawsonite (9.3 to 9.5), and garnet (8.0 to 8.4), given as per mil enrichment in O 18 relative to mean ocean water. These rocks seem to have reached equilibrium at about the same temperature in contact with abundant metamorphic pore fluids. Not all the Ward Creek rocks have completely equilibrated with the postulated metamorphic pore fluids. In particular, the metacherts seem to have been relatively impermeable to the aqueous fluids during metamorphism, as indicated by the large δ -values of quartz in such rocks (17 to 19.2) and by their appreciably higher Fe +3 /Fe +2 ratios. Gradients in O 18 /O 16 and Fe +3 /Fe +2 have been generally “smoothed out” in the rocks during metamorphism, but the process has gone to completion only locally. Cherts and limestones have apparently been lowered in O 18 content by 10 to 15 per mil, and the metabasalts are enriched by 3 to 4 per mil over their unmetamorphosed parent rocks. Using the calibrated quartz-muscovite and quartz—CaCO 3 geothermometers, the measured quartz-muscovite and quartz-aragonite fractionations indicate essentially “concordant” temperatures of formation for the Type III rocks of 270° to 315° C. Inasmuch as aragonite is part of the equilibrium assemblage, these rocks must therefore have been metamorphosed at pressures of at least 6.4 to 7.0 kb. Oxygen isotope fractionations for the mineral pairs aragonite-lawsonite, quartz-muscovite, and quartz-glaucophane progressively decrease from Type II (low-grade) through Type III to Type IV (high-grade tectonic blocks) metabasalts. The higher-grade blueschists from New Caledonia exhibit quartz-muscovite and quartz-glaucophane fractionations similar to the Type IV metabasalts at Ward Creek, indicating temperatures of formation of 400° to 550° C. Thus, glaucophane-bearing metamorphic rocks apparently form over a temperature range of 200° C to 550° C, encompassing the probable temperature range of the entire greenschist and epidote-amphibolite facies. This suggests that glaucophane schists should be separated into at least two metamorphic facies, a lower-grade, lawsonite-aragonite blueschist facies and a higher-grade, epidote-rutile blueschist facies, both representing higher pressures than are attained during ordinary low-rank and middle-rank regional metamorphism.

Distribution and Age of High-Grade Blueschists, Associated Eclogites, and Amphibolites from Oregon and California

Isolated blocks of high-grade blueschist and amphibolite facies metamorphic rocks occur within the Jurassic and Cretaceous eugeosynclinal deposits of the Coast Ranges of southwestern Oregon and California. The blocks range in size from individual rock masses commonly 5 to 1,000 ft in diameter to a few larger masses as much as 7 mi long and 2 mi wide. The high-grade blocks are predominantly basaltic in composition and include glaucophane schists, eclogites, and gneissic rocks of the amphibolite facies. Field relationships indicate that the blocks are closely associated with serpentine, that high-grade blueschist and amphibolite blocks, lower grade blueschists, volcanic rocks, and cherts occupy disturbed zones that may be related to thrusting, and that there is no exposed in situ provenance for the high-grade blueschists, eclogites, and amphibolites. Potassium-argon mineral ages of white mica and actinolite from the blueschists and of hornblende from the amphibolites indicate that these minerals crystallized approximately 150 m.y. ago, but the ages measured on glaucophane from the blueschist blocks are commonly younger. These data suggest that the high-grade blue-schist and amphibolite blocks represent fragments of a cryptic metamorphic terrane of pre-Tithonian age that have been tectonically mixed with younger rocks of the Franciscan Formation in California and Otter Point Formation in Oregon. The younger ages for glaucophane probably reflect metamorphic episodes in which lower grade in situ blueschist facies mineral assemblages were developed in the blocks after their emplacement within the Franciscan Formation. This pre-Tithonian cryptic metamorphic terrane probably developed as a result of interaction between oceanic and continental plates. The occurrence of tectonic blocks of this terrane within melange zones in Oregon and California may be related to later plate interaction.

Geochronology of the Arabian Shield, western Saudi Arabia: K-Ar results

An orogenic event, correlated with the Pan-African event in eastern Africa, affected the Arabian Peninsula between 510 and 610 m.y. ago and is well-recorded geochronologically. The event probably included two thermal pulses or maxima, the first occurring between 560 and 610 m.y. ago and the second between 510 and 540 m.y. ago. The earlier pulse, the more severe one, included the majority of the igneous activity and metamorphism. During the last part of the 510- to 610-m.y. period, left-lateral strike-slip faulting occurred along a set of northwest-trending en echelon fracture zones, whose composite displacement may be as large as 240 km. At least one and probably more orogenic events affected the Arabian Peninsula before the Pan-African event, but only minimum ages can be assigned to these, because thermal effects of the 510- to 610-m.y. event have reset K-Ar ages. Major diorite-granite batholiths, however, formed before 760 m.y. ago.

87Sr/86Sr ratios in some eugeosynclinal sedimentary rocks and their bearing on the origin of granitic magma in orogenic belts

Abstract Rb and Sr contents and 87 Sr/ 86 Sr values were determined for samples of eugeosynclinal sedimentary rocks, mostly graywackes, from Oregon and California. These data are compatible with the theory of anataxis of eugeosynclinal sedimentary rocks in orogenic belts to produce granitic magmas provided that the melting occurs within several hundreds of m.y. after sedimentation. The low ( 87 Sr/ 86 Sr) 0 values of the eugeosynclinal sedimentary rocks are related to the significant amounts of volcanogenic detritus present which probably were originally derived from the mantle.

Age of amphibolites associated with alpine peridotites in the Dinaride ophiolite zone, Yugoslavia

Abstract Amphibolites associated with alpine peridotites in the Central Ophiolite zone in Yugoslavia have K-Ar ages of 160–170 m.y. These amphibolites and associated peridotites underwent deep-seated metamorphism prior to tectonic emplacement into the sedimentary-volcanic assemblage of the Dinarides. The alpine peridotites and associated local rocks of the ophiolite suite are interpreted as Jurassic oceanic crust and upper mantle.

Red sea drillings.

Recent drilling in the Red Sea has shown that much of the basin is underlain by evaporites of a similar age to that of evaporites found in the Mediterranean Sea. These evaporites and their structural positions indicate that other brine areas are present—and, indeed, several others have been discovered.

Eclogites from Southwestern Oregon

Eclogite, high-grade blueschist, and amphibolite blocks occur within the Mesozoic Otter Point Formation of southwestern Oregon and are inferred to have been tectonically emplaced by eastward-directed overthrusting involving Colebrooke Schist and serpentinite. Eclogite from southwestern Oregon is very similar in bulk chemistry and mineralogy to the well-studied eclogite of California. Calculations of phase equilibria at load pressures of 7 and 10 kb and T = 400°C to 550°C suggest that many of the hydrates found in eclogite could have been stable at very low H2O fugacities. The lack of lawsonite and the presence of almandine-grossular garnet set a maximum limit on H2O fugacity for a given Ps-T. Chemically, Group C eclogite from Oregon and California characteristically is nepheline normative and is enriched in normative diopside relative to basaltic compositions. The present chemistry of this eclogite may be the result of metasomatism in an ultramafic environment with low a SiO2 and high a Ca, but outside the stability field of serpentine. The generally high jadeite content of clinopyroxene from Group C eclogite compared with Group A and Group B eclogite is largely a function of bulk rock chemistry. Crystallization under low a SiO2 conditions stabilizes jadeite in clinopyroxene at lower Ps for a given T.

THE MIOCENE TIHAMA ASIR OPHIOLITE AND ITS BEARING ON THE OPENING OF THE RED SEA

In early Miocene time (ca. 2. Ma ago), dike swarms, layered gabbros, granophyres, and basalts of the Tihama Asir ophiolite in S.W. Saudi Arabia were emplaced during the initial stages of Red Sea rifting and separation of the African and Arabian plates. As rifting began, dikes invaded Paleozoic and Mesozoic sedimentary and Precambrian metamorphic rocks of the Arabian plate along a northwesterly trend paralleling the axial trough of the Red Sea. With increased separation along the rift edge, new crust was accreted to the continental margin. This crust consisted of a 4 km wide zone of subparallel dikes having chilled margins against one another and lacking screens of older continental rock. This dike complex is analogous to the sheeted dike swarms described from the Cyprus, Oman, and Newfoundland ophiolites. Layered gabbros and granophyric intrusions within the dike swarm are products of magmatic differentiation developed during the initial stages of rifting. The differentiation trends indicate that the parent magma was tholeiitic and evolved by crystal fractionation in much the same manner as lavas at Thingmuli Volcano, Iceland. Late Miocene tilting of the sedimentary rocks and dike swarm toward the Red Sea axis and later erosion have exposed the continental-oceanic crust boundary along a narrow zone. The continental Baid Formation (19 Ma) rests unconformably on the Tihama Asir ophiolite.

Provenance of Eocene graywackes of the Flournoy Formation near Agness, Oregon — A geochemjcal approach

Systematic variations in major elements, U and Th contents, and Sr isotope ratios in graywackes of the Flournoy Formation near Agness, Oregon, are interpreted to indicate a multilithologic provenance. At the time of deposition, Rb-Sr systems in the graywackes defined a pseudo-isochron corresponding to an age of 110 m.y. Calculated 87 Sr/ 86 Sr ratios range from 0.7050 to 0.7074, and compositional variations relate to these values in simple mixing patterns. Rb, U, Th, and K 2 O show a positive correlation with increasing ( 87 Sr/ 86 Sr) O and decreasing MgO, CaO, and Sr. Average values of Rb/Sr and 87 Sr/ 86 Sr (at 50 m.y. ago) for Mesozoic graywackes—Otter Point, Dothan, Galice, and Colebrooke Formations–are coincident with the mixing line defined by the Rb-Sr systems of the Flournoy graywackes, suggesting that some combination of the Mesozoic graywackes comprised part of the felsic component of the source terrane. A mafic source is also required by chemical, isotopic, and mineralogic features of the Flournoy graywackes. This was probably equivalent to basalts in the Paleocene and Eocene Roseburg Formation beneath the Flournoy.