Comparative Study of Low-Grade Metamorphism in the California Coast Ranges and the Outer Metamorphic Belt of Japan
Published:January 01, 1970
W. G. Ernst, Y. Seki, H. Onuki, M. C. Gilbert, 1970. "Comparative Study of Low-Grade Metamorphism in the California Coast Ranges and the Outer Metamorphic Belt of Japan", Comparative Study of Low-Grade Metamorphism in the California Coast Ranges and the Outer Metamorphic Belt of Japan, J. D. H. Donnay, Wkrner Nowacki, the collaboration of Gabrielle Donnay
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Comparative field and laboratory investigations have been undertaken of representative portions of the Franciscan and Sanbagawa blueschist terranes of the California Coast Ranges and central Shikoku, respectively. Four colored maps at various scales have resulted from this work: the Goat Mountain area at 1:10,000; the San Luis-Pacheco Pass area at about 1:18,300; and the Shirataki and Oboke Districts, both at 1:50,000. In all, modes were measured for nearly 700 thin sections of rocks from the two metamorphic belts; bulk densities for more than half of the corresponding samples were obtained. Also, selected optical and X-ray properties for some minerals from these rocks have been measured. All these data are tabulated elsewhere (Seki and others, 1969), but are summarized graphically in the present report. Chemical data listed in the present paper include 93 conventional gravimetric rock analyses, 121 “wet” chemical analyses of minerals, and 107 electron microprobe analyses of minerals.
Rocks representative of three distinct metamorphic events and P-T environments are tectonically juxtaposed in the Goat Mountain area. These include (1) feebly metamorphosed pumpellyite-bearing greenstones and albite + calcite ± lawsonite metagraywackes, (2) glaucophane-crossite-bearing blueschists and jadeitic pyroxene + aragonite metaclastics, and (3) relict albite + garnet amphibolites and rare albite and/or jadeitic pyroxene-bearing metakeratophyres (?). Although chaotically deformed, rocks of group (1) appear to be more or less in place, whereas those of groups (2) and (3) evidently have been tectonically transported to their present location along faults.
In situ metagraywackes of the extended Pacheco Pass area exhibit the following progressive metamorphic sequence of critical bit the following progressive metamorphic sequence of critical assemblages, proceeding from west to east: (1) albite + quartz + lawsonite ± calcite; (2) albite + quartz + lawsonite ± aragonite; and (3) jadeitic pyroxene + quartz + lawsonite ± aragonite. Nearly all mafic metavolcanics carry lawsonite and either glaucophane-crossite or omphacite. Relatively open antiforms and synforms, and steeply dipping faults have been recognized in the San Luis Flat-Pacheco Pass area. The relationship between metamorphism and deformation is unclear; no evidence was found to support either multiple recrystallization of in situ rocks, or of a causal relationship between metamorphism and faulting.
Investigation of compositions of co-existing minerals from blue-schist tectonic blocks associated with California Coast Range serpentinites suggests that such inclusions formed under crustal conditions, evidently at temperatures lower than those attending glaucophane schist metamorphism in Shikoku. Tectonic blocks of amphibolite, some of which are also spatially related to serpentinite, evidently were recrystallized under physical conditions of metamorphism more intense than those attending production of the blueschist inclusions.
In both the Shirataki and Oboke Districts, virtually flat-lying, feebly recrystallized rocks on the south are succeeded by a more highly deformed, intensely recrystallized section toward the north; this northern portion of the terrane, which consists of schists carrying porphyroblastic albite, seems to have moved upward and southward relative to the southern part of the belt, which contains fine-grained non-spotted metamorphic rocks. In the Shirataki District, the high-grade metamorphic rocks constitute an overturned syncline and anticline, with axial planes and foliations dipping steeply to the north. The progressive metamorphic parageneses in albite-bearing metaclastics involve the sequential entrance of epidote, garnet, rutile and biotite at higher grades; corresponding mineral sequences in meta-volcanics reflect the conversion of the critical assemblage albite + epidote + actinolite ± crossite to blue-green hornblende, garnet, and lesser amounts of albite under more intense metamorphic conditions. Zoned and partly replaced crystals, and the rare preservation of low-grade phases in helical swirls within porphyroblasts, indicate that the higher grade metamorphic rocks passed through an initial low-grade state contemporaneous with tectonism. Metamorphism and deformation appear to have been roughly coeval in both Shirataki and Oboke Districts.
Eclogitic schlieren in a deformed peridotite, and in situ pods of eclogite in the surrounding amphibolite have been studied at Higashi-akaishi-yama, in the most intensely metamorphosed part of the Sanbagawa terrane in central Shikoku. Garnets are rich in pyrope, and clinopyroxenes are diopsidic in the eclogitic bands of the peridotite. These minerals are less pyropic and more jadeite-rich, respectively, in the amphibolitic lenses of eclogitic material. Hornblende is confined to subsilicic schlieren in peridotite, but occurs in both low and high silica varieties of in situ eclogitic amphibolite. The ferrous iron-magnesium fractionations between co-existing garnet + clinopyroxene and garnet + hornblende are slightly more pronounced in the eclogitic pods in amphibolite compared to the bands in the ultramafic mass. Evidently the peridotite was emplaced in the terrane from a higher temperature, higher pressure environment, probably as a hot but solid body.
Albite is a major phase in mafic, pelitic and psammitic schists of the Shirataki and Oboke Districts. It becomes slightly more calcic and distinctly more porphyroblastic with increasing metamorphic grade; the albite of Shikoku metavolcanic rocks is enriched in iron and calcium compared to analogues found in metasediments. Plagioclase is less common both in the Stoneyford-Goat Mountain area and at Pacheco Pass, but where present is virtually pure sodic albite in all rock types.
Calcic amphibole is a major phase in mafic schists of central Shikoku. Although actinolitic in weakly metamorphosed rocks, it gradually becomes more sodic and aluminous at higher grades. Calcic amphibole is virtually absent from in situ Pacheco Pass rocks; however, blue-green hornblende, similar in composition to the higher grade metamorphic amphibole in central Shikoku, occurs in relict amphibolites at Goat Mountain.
Sodic amphiboles are abundant in both Sanbagawa and Franciscan terranes. In Shikoku, crossites occur in mafic schists, whereas siliceous schists typically carry a member of the riebeckite-magnesio-riebeckite series. Na-amphiboles are absent from metaclastics. Contrasting with the Japanese rocks of similar bulk composition, metavolcanics from the Diablo Range contain glaucophane-ferro-glaucophane, or glaucophane-crossite, and some metagraywackes carry minor glaucophane; however, the sodic amphibole of Franciscan meta-cherts is chemically more nearly comparable to the analogous phase found in siliceous schists of central Shikoku. Many low-grade mafic schists from Shikoku contain both actinolite and crossite; with increasing grade the blue amphibole is replaced by successively more sodic and aluminous Ca-amphibole. Except for some of the South Fork Mountain schists, presumably stable, two-amphibole assemblages are rare in the Diablo Range and its northern extension. Zoned, composite grains commonly exhibit an actinolite or hornblende core armored or replaced by sodic amphibole.
Five compositionally distinct varieties of clinopyroxenes from the California Coast Ranges have been recognized: (1) diopsidic pyroxene associated with garnet in amphibolites; (2) omphacite in tectonic blocks of eclogite and in situ metabasaltic rocks; (3) jadeitic pyroxene in quartzose metagraywackes; (4) jadeite occurring in inclusions in serpentinites, but not associated with quartz; and (5) acmitic pyroxene in metacherts. Somewhat similar phases are present in the schists of central Shikoku; however, clinopyroxene does not occur in metaclastic rocks, and except for analogues in siliceous schists, clinopyroxenes are systematically impoverished in Na and AlVI relative to those from the Franciscan.
White micas are common minerals in nearly all rock types of the investigated areas. A few samples from the Stoneyford-Goat Mountain and Pacheco Pass areas represent the 1M polytype, but most of the Diablo Range samples, and all those studied from Shikoku, are 2M1 polytypes. They are uniformly phengitic. In the Shirataki District, progressive metamorphism involves an increment of sodium and aluminum, decrease in iron, magnesium, and silicon of the micas, which accordingly more closely approach the composition of K-rich muscovite-paragonite solid solutions at high grade.
Chlorite is abundant both in clastic metasediments and in metavolcanic rocks of the California Coast Ranges, and in the Shirataki and Oboke areas. The Sanbagawa chlorites studied are more aluminous and ferric iron-rich and less silicic than analogous phases from the Franciscan. Within the Shirataki District, both A1IV and A1VI of the chlorite increase with progressive metamorphism.
Except for spessartine-bearing metacherts, garnet is very rare from in situ Franciscan metamorphics; it is a common mineral in some tectonic blocks of blueschist associated with serpentinite, and in amphibolite masses. Where present in mafic schists thought to be in place, garnets are more manganiferous and slightly less Mg-rich compared to those from exotic blocks. Garnet is an essential phase in many of the more thoroughly recrystallized Shikoku schists, but in the lowest grade metamorphics it is confined to occurrences of spessartine in metacherts. Under conditions of progressive metamorphism, garnets of the Shirataki-Besshi District become successively depleted in Mn, and enriched in magnesium; those from eclogitic schlieren in peridotite represent intermediate solid solutions among grossular, almandine, and pyrope.
Aragonite, nearly stoichiometric CaCO3, is abundant in blue-schists and related rocks of the Franciscan, but has not been identified with certainty from the Sanbagawa belt. In both terranes, veins and stringers of calcite are abundant and, at least in the California Coast Ranges, this phase, in part, replaces an earlier orthorhombic polymorph. The frequency of carbonate-bearing rocks is higher in the Shirataki District than for the other areas studied. In this district, the proportion of calcite-bearing schists is inversely proportional to metamorphic grade. In all areas investigated, a greater percentage of mafic schists carry calcium carbonate than do metasediments.
Among the calcium-aluminum silicates, lawsonite is confined to the most feebly recrystallized Oboke greenstones from central Shikoku, but is a widespread phase in metagraywackes and metabasalts from the Diablo Range and its northern extension. Sparse analytical data suggest stoichiometry, with only minor substitution of ferric iron for aluminum. Of the areas studied, pumpellyite is abundant only in feebly metamorphosed volcanics of the Oboke District and in the Stoneyford-Goat Mountain area. Except for the South Fork Mountain region, epidote is uncommon in in situ Franciscan mafic metavolcanics; ignoring rare relict detrital grains, it is absent from metaclastic rocks. In contrast, an epidote mineral is an essential phase in the metasediments and metaigneous rocks of Shikoku; piemontite occurs in siliceous schists and as cores of epidote crystals in some metaclastic rocks. In the Shirataki District, the epidote becomes more aluminous at higher metamorphic grade.
Metamorphic rocks from Goat Mountain and Pacheco Pass have slightly higher specific gravities than compositional equivalents from central Shikoku. This is due to the fact that the investigated Californian blueschists and allied metamorphics contain dense phases such as lawsonite, sodic pyroxene, glaucophane-crossite and aragonite; in contrast, Sanbagawa schists contain in abundance the more open framework silicate, albite, as well as the less dense CaCO3 polymorph, calcite. Within the Shirataki-Oboke area, progressive metamorphism has resulted in a gradual increase in rock bulk density at higher grades, reflecting diminution in the proportions of volatile-rich minerals.
With regard to element fractionation between co-existing phases, iron is concentrated in sodic amphibole relative to calcic amphibole and white mica, and in garnet relative to chlorite. Titanium is enriched in white mica with respect to chlorite and sodic amphibole, and in garnet relative to chlorite; manganese is concentrated in Ca-amphibole relative to Na-amphibole, in garnet relative to chlorite, and in both chlorite and sodic amphibole relative to white mica. Except for iron partitioning in garnets, where distribution constants for mineral pairs from the investigated areas differ systematically, the Sanbagawa fractionations more closely approach unity than do those from the California Coast Ranges. These relations suggest that Shikoku schists recrystallized at slightly higher temperatures, and probably at somewhat lower lithostatic pressures than the Franciscan metamorphics.
Among mafic volcanic rocks of the Franciscan, two mutually gradational types of paragenetic sequence are distinguished. The relatively low-pressure series ranges from greenstones and schistose equivalents through higher grade amphibolites, whereas the relatively higher pressure series ranges from omphacitic “greenstone” and blueschist to eclogite. Occurrences of eclogites and amphibolites are confined to tectonic blocks in this terrane. In place metaclastics from the California Coast Ranges display four critical mineral compatibilities. In order of increasing pressure they are: albite + quartz + pumpellyite ± calcite; albite + quartz + lawsonite ± calcite; albite + quartz + lawsonite ± aragonite; and jadeitic pyroxene + quartz + lawsonite ± aragonite. Metabasaltic rocks from the Outer
Metamorphic Belt of Japan show a progressive gradation from green-stones through interlayered greenschists and blueschists to amphibolites and eclogitic amphibolites. Similar to the Franciscan occurrences, fragments of true eclogite are restricted to tectonic inclusions in ultramafic solid-state intrusions. Within the regionally metamorphosed terrane, albite + quartz-bearing Sanbagawa metaclastics have developed the characteristic index minerals epidote, garnet, and biotite, at successively higher grades.
Systematic differences in compositions of minerals, in rock bulk densities, and in element fractionations between co-existing phases characterize the two terranes, and suggest that the investigated Franciscan rocks recrystallized at relatively lower temperatures or higher pressures than the Sanbagawa schists, or both. Comparison of the contrasts in phase compatibilities from nearly isochemical rocks of the two belts with experimentally determined phase and isotopic equilibria indicate the following physical conditions of metamorphism: 150 to 300° C, and from 5 to more than 8 kb lithostatic pressure for generation of the in situ metamorphics of the Diablo Range and its northern extension; 200 to at least 400° C, and from less than 4 to nearly 7 kb lithostatic pressure for recrystallization of the in place schists of central and eastern Shikoku.
From stratigraphic and fossil evidence, deposition of Franciscan group rocks began at least as early as the Late Jurassic and apparently continued into mid- or Late Cretaceous time locally; in contrast, Sanbagawa rocks have depositional ages ranging from Permian to at least as young as mid-Jurassic. Accumulation evidently was not penecontemporaneous along the extent of either belt. The time of metamorphism in the California Coast Ranges must have closely followed the initial deposition, as demonstrated by apparent radio-metric ages of schists: local values are between about 100 and 150 m.y. Stratigraphic evidence in the Outer Metamorphic Belt of Japan indicates that metamorphism involved Jurassic rocks and took place prior to Early Cretaceous time; however, for rocks from central Shikoku, the few currently available values for apparent K-Ar ages range from approximately 80 to 100 m.y., evidently dating the cessation of argon leakage.
Based on structural, stratigraphic, radiometric, petrologic-geochemical and geophysical lines of evidence, the following tentative conclusions have been drawn. (1) The Franciscan coarse, first-cycle clastic sediments were derived chiefly from a Sierran + Klamath plutonic source. Along with subordinate mafic volcanics and siliceous units, they were deposited in Late Jurassic to at least mid-Cretaceous time at the margin of the continent on oceanic crust in one or a series of rapidly foundering troughs. Tectonic thickening of the ensimatic prism was produced due to overriding of the basin-trench by the continental block; this also contributed to an enhanced depression of the geothermal gradient. Prior to the attainment of isostatic and thermal balance, relatively high-pressure, low-temperature meta-morphism, obliteration of the trench and emplacement of hydrated fragments of mantle material closely followed the accumulation of clastic debris, minor volcanics and chert. The ages of these events range over the interval 100 to 150 m.y. ago; locally they cover a much shorter time span, but differ from place to place along the continental margin. (2) Sanbagawa volcanogenic sediments probably were derived from continental Asia and the pre-existing Japanese basement complex, as well as from a local volcanic island arc. They were deposited as extensive aprons on thin continental crust near the margin of the Pacific basin, principally during late Paleozoic but also including early Mesozoic time. Accompanying somewhat later metamorphism, which evidently terminated near the end of the Jurassic, the continentalward side of the belt appears to have been thrust up and over the portion lying nearer the oceanic margin. This orogeny resulted in an initial depression of the geothermal gradient, and in the generation of blueschist-greenschist assemblages in the tectonically loaded rocks. The relatively high-pressure, low-temperature phase compatibilities were partly replaced by those more characteristic of Barrovian-type regional metamorphism as a more normal thermal regime was subsequently re-established.
The occurrence of these and similar low-grade metamorphic belts peripheral to the Pacific basin reflects a profound, chiefly late Mesozoic, tectonic event, which apparently involved relatively rapid convergence of continental and oceanic crust.