W. G. Ernst

Experimental phase-equilibrium study of Al- and Ti-contents of calcic amphibole in MORB; a semiquantitative thermobarometer

Abstract Calcic amphiboles were synthesized from a natural mid-ocean ridge basalt (MORB) in 39 experiments representing 24 sets of pressure-temperature (P-T) conditions ranging from 650-950 °C, 0.8-2.2 GPa, at ƒO₂ controlled by the fayalite-magnetite-quartz (FMQ) buffer, and Paqueous fluid = Ptotal. Experiments lasted up to 1630 h at low temperatures; in all cases, synthesized hornblendes were coarse-grained (5-7 × 10-15 μm) and chemically homogeneous. Over the investigated pressure range, Ca-amphibole coexisting with phases rich in Al and Ti gradually changes composition from sodic-calcic, Si-rich at low temperatures to calcic, Si-poor at high temperatures: it is barroisite at 650 °C, edenite at 700 °C, and pargasite at 800-950 °C. Electron microprobe data were combined with 41 comparable analyses from the literature for Ca-amphiboles synthesized from MORBs at intermediate ƒO₂ in order to erect a petrogenetic grid for the experimental range 0.0-2.2 GPa, 450-1050 °C. Isopleths for Al2O3 in Ca-amphibole exhibit markedly negative P-T slopes, indicating increasing Al2O3 contents with both P and T. In contrast, TiO2 isopleths are nearly independent of P, demonstrating that TiO2 in Ca-amphibole increases almost exclusively as a function of T. For natural metabasaltic assemblages that contain coexisting Al-rich and Tirich phases, and closely approached chemical equilibrium under crustal or uppermost mantle conditions, this semiquantitative petrogenetic grid allows the simultaneous assignment of attendant P and T employing Ca-amphibole Al2O3 and TiO2 contents. However, during slow cooling, natural Ca-amphiboles may exsolve TiO2 as rutile, titanite, and/or ilmenite, but in general do not redistribute Al2O3, so this thermobarometer must be applied with caution to inhomogeneous specimens.

Contrasting plate-tectonic styles of the Qinling-Dabie-Sulu and Franciscan metamorphic belts

The Dabie Mountains are part of the >2000-km-long Qinling-Dabie-Sulu suture zone juxtaposing the Sino-Korean and Yangtze cratons. An eastern extension apparently crosses Korea and lies along the Japan Sea side of Honshu as the Imjingang and Sangun terranes, respectively; a northeastern segment may be present in Sikhote-Alin, Russian Far East. This orogenic belt records late Paleozoic ocean-floor consumption and the Triassic collision of two Precambrian continental massifs in east-central China. Coesite and microdiamond inclusions in strong, refractory minerals of eclogite facies ultrahigh-pressure (UHP) metamorphic rocks in the Dabie-Sulu area attest to profound subduction of a leading salient of the old, cold Yangtze craton, now recovered through tectonic exhumation and erosion. Northward increase in intensity of subsolidus recrystallization of the suture complex is analogous to the internal progression in grade of high-pressure (HP) and UHP metamorphism documented in the Western Alps. In both regions, subduction of narrow prongs of continental material, UHP metamorphism, and return toward midcrustal levels of relatively lower density, buoyant microcontinental blocks resulted from delamination of these rocks from the descending, higher density, oceanic-crust-capped lithospheric plate. Such salients of continental crust represent an integral structural part of the downgoing slab, remain intact, and may be dragged to great depths before disengaging and rising differentially as coherent blocks. UHP parageneses include trace mineralogic relics requiring peak metamorphic conditions of 700–900 ° C and 28–35 kbar or more. In contrast, Pacific-type HP metamorphic belts, as represented by the Franciscan Complex of western California, recrystallized under physical conditions up to 200–500 ° C, 10 ± 3 kbar. In this setting, voluminous quartzo-feldspathic and graywacke debris was carried downward on oceanic-crust-capped lithosphere, choking the subduction zone with incompetent material. Sited between both plates, and strongly adhering to neither, this buoyant, largely sedimentary complex decoupled at 25–30 km depth, and ascended toward the surface. In both Alpine-type intracontinental collision and Pacific-type underflow, light sialic material displaced dense mantle; thus, the return to midcrustal levels was propelled dominantly by body forces.

Metamorphic zonations on presumably subducted lithospheric plates from Japan, California and the Alps

High-pressure blueschist-type mineral parageneses from the Sanbagawa belt of southwestern Japan, the Franciscan terrane of western California and the Sesia zone, Pennine and Helvetic realms of the central Alps may reflect metamorphic conditions attending lithospheric plate descent. The observed progressive metamorphic sequences seemingly have developed chiefly, but not exclusively within the confines of oceanic crust, and evidently mark the suture zones between pairs of convergent lithospheric plates. The downgoing slabs have developed relatively near-surface (1) zeolitized rocks and apparently at successively greater depths (2) pumpellyite-bearing rocks, (3) greenschists and/or blueschists, and (4) albite-amphibolites; eclogitic assemblages are characteristic of the highergrade environments. The sense of metamorphic progression (1)→(2)→(3)→(4) marks the direction of presumed lithospheric underflow.Profound pressure discontinuities revealed by mineral assemblage contrasts across the plate junctions indicate that the high-pressure terranes must have risen great distances subsequent to the blueschist-type recrystallization. This conclusion is supported in California and the Alps by the exposure of rocks interpreted as basal portions of the oceanic or continental crust+upper mantle in the overlying lithospheric slabs; such sections appear to have been dragged upwards adjacent to the plate junctions during the buoyant rise of the underlying and subducted blueschistic slabs subsequent to active plate convergence. The exposed widths of the high-pressure metamorphic belts roughly correlate with the depths of inferred crustal subduction now exhumed of 25–35 km or more.

Stability of hydrous phases in subducting oceanic crust

Experiments in the basalt-H2O system at 600–950°C and 0.8–3.0 GPa, demonstrate that breakdown of amphibole represents the final dehydration of subducting oceanic tholeiite at T≥650°C; the dehydration H2O occurs as a free fluid or in silicate melt co-existing with an anhydrous eclogite assemblage. In contrast, about 0.5 wt% of H2O is stored in lawsonite at 600°C, 3.0 GPa. Our results suggest that slab melting occurs at depths shallower than 60 km for subducting young oceanic crust; along a subduction zone with an average thermal gradient higher than 7°C/km, H2O stored in hydrated low-potassium, metabasaltic layers cannot be subducted to depths greater than 100 km, then released to generate arc magma.

Systematics of large-scale tectonics and age progressions in Alpine and Circum-Pacific blueschist belts☆

Abstract The early Alpine tectonics and relatively high-pressure metamorphic parageneses of the Helvetic, Pennine and Sesia—Lanzo realms are compared with analogous circumpacific terranes in western California, southwestern Japan, west-central Chile, and southern Alaska. Petrotectonic relationships appear to be compatible with a process involving syntectonic recrystallization and pervasive deformation in an imbricated subduction zone for each of these regions. Successive underthrusting of younger portions of the downgoing low-density material, followed by decoupling and rise towards the surface may account for the systematic decrease in both age and metamorphic intensity proceeding away from the old plate junction in these terranes. The environs of the present Aleutian trench serve as a modern analogue. Several other circumpacific blueschist belts (i.e., New Zealand, New Caledonia and eastern Papua) exhibit possible departures of timing, metamorphic polarity or direction of tectonic transport from that appropriate to the imbricated convergent plate-junction model presented in this paper, but the overall metamorphic sequences and tectonic settings are nevertheless strikingly similar. Many relatively high-pressure belts seem to be characterized by the occurrence of a major post-metamorphic transcurrent fault. This phenomenon suggests that blueschist belts, which must be exhumed rapidly in order to preserve the relatively high-pressure, low-temperature mineral assemblages, typically are obliterated through higher-temperature recrystallization unless a profound change from convergent to conservative plate motion occurs. Variation in convergence rate is yet another important factor, more rapid underflow promoting generation of blueschistic metamorphic parageneses, slower underflow favoring shallow decoupling and buoyant return towards the earths surface of the successive underplatings. Other things being equal, a steeper angle of underflow would provide lower temperatures at a given depth for the subducted slab, thus promoting glaucophane schist-type metamorphism.

Interpretative Synthesis of Metamorphism in the Alps

Compilation maps are presented showing major tectonic features, selected lithologies, and zones of progressive metamorphism in the eastern half of the western Alps (scale 1:400,000) and the eastern Alps (scale 1:800,000). Two principal complexes are distinguished on them: (1) the Caledonian and Hercynian metamorphosed terrane in the Southern Alps + Austroalpine sheets, overlain by deformed but largely unrecrystallized uppermost Paleozoic and younger platform-type sedimentary rocks; and (2), the tectonically lower Sesia-Lanzo + Lepontine-Pennine + Helvetic realms, a sequence of Hercynian and pre-Hercynian plutonic igneous + metamorphic rocks and a younger, chiefly Mesozoic cover sequence consisting of shelf, slope, and deep-sea sediments + ophiolites, incompletely to pervasively recrystallized during Alpine metamorphism. 1. The pre-Mesozoic metamorphism involved several cycles in both complexes, but it has not been possible to distinguish these on the maps. Judging from the mineral para-geneses, recrystallization events seem to have taken place under moderate to very high temperatures at low to moderately high pressures. 2. Three contrasting but intergradational, temporally overlapping episodes of Alpine metamorphism are recognized in the Sesia-Lanzo + Lepontine-Pennine + Helvetic terrane: (a) an early, high-pressure, low-temperature event syntectonic with nappe formation, which produced eclogites + albite amphibolites, glaucophane schists, and allied greenschists, with lower grade, more recently metamorphosed sections lying externally (that is, toward the European foreland) relative to the older, progressively higher grade, more internal, imbricated sections lying to the south and east; (b) a middle syntectonic to post-tectonic stage characterized by more “normal” physical conditions, resulting in the partial or complete conversion of the products of event (a) to greenschist (prasinite) and low-rank amphibolite facies metamorphic rocks; and (c), a late, and in most cases syntectonic to post-tectonic recrystallization involving moderately high temperatures and pressures, which locally obliterated the products of both (a) and (b). Of the recrystallization continuum, event (a) is best preserved in the Franco-Italian Alps, and in Switzerland in the cantons of Wallis and Graubunden, (b) is nearly ubiquitous in the Sesia-Lanzo + Pennine + Helvetic complex, and (c) is confined to the Lepontine gneiss area of the Italian and Swiss Alps, and to the central gneiss domes of the Tauern Fenster, central Austria. The timing of Alpine metamorphism evidently varied laterally along and across strike of the belt. For instance, in Austria, event (a) may have begun in Late Cretaceous (?) time, whereas it probably commenced during Paleocene-Eocene time in the western Alps. Moreover, “early” Alpine, low-grade zeolitization occurred in the external parts of the Helvetic realm probably during Oligocene time— nearly contemporaneously with the more internal “late” Alpine higher grade Lepontine recrystallization of event (c). The contact between (1) the Southern Alps + Austroalpine nappes on the one hand, and the structurally lower (2) Sesia-Lanzo + Lepontine-Pennine + Helvetic realms on the other, juxtaposes rocks of markedly contrasting petrologic and tectonic histories. This zone, here referred to as the Alpine Suture, is postulated to represent the crustal expression of a Late Mesozoic-early Tertiary convergent lithospheric plate junction. The early Alpine high-pressure paragenesis appears to reflect subduction and shuffling of the more northerly terrane beneath the stable lithospheric dab capped by the Southern Alps + Austroalpine sheet. If so, the observed blueschist-type metamorphic zoning probably was generated by progressively greater depths of underflow and a consequent depression of the isotherms. A variable rate or time at which the complex was exhumed locally could account for the later establishment of a more normal thermal regime, and thus, succeeding higher temperature mineral assemblages as displayed in the Lepontine gneiss area and the Tauern gneiss domes.

Blueschist metamorphism and P-T regimes in active subduction zones

Abstract Blueschist-type metamorphism involves the progressive development of some of the following minerals: at low grades, zeolites, pumpellyite, lawsonite + quartz, aragonite, jadeitic pyroxene + quartz; then at higher grades, zoisite-epidote, kyanite, omphacite, garnet and blue-green hornblende. Comparisons of natural assemblages with experimentally determined phase equilibria and oxygen isotopic analyses yield apparent physical conditions in the range 150–500° C at fluid (≈ lithostatic) pressures on the order of 3–8 + kbar. The unusually low metamorphic geothermal gradient indicated is on the order of 10–15°C/km. Blueschist belts seem to be confined to oceanic trench-type environments where they are hypothesized to represent subducted material which has buoyantly returned to the surface since recrystallization. Calculated downward deflections of the isotherms in the subducted lithospheric slabs yield similar or even lower geothermal gradients than deduced from the mineral parageneses. The disposition of relatively high-pressure isogradic surfaces in the upper portions of a model subduction zone demonstrates that the observed sequence of metamorphic facies reflects the direction of lithospheric plate descent. The approximate magnitude of inferred underflow can be gauged by the presence or absence of a contemporaneous high-temperature volcanic + plutonic + metamorphic complex in the non-subducted lithospheric plate. Greater amounts of underflow evidently are required to build up larger, relatively high-temperature, low-pressure terranes. Blueschist belts tend to be associated in time and space, indicating that since Late Paleozoic time, many convergent plate junctions have remained in approximately the same positions relative to the stable lithospheric slabs. The fact that, where preserved, older blueschistic belts, hence plate sutures, are located farther inland suggests that in some cases the subduction zones have stepped seaward with time, allowing, for the episodic return towards the surface of old trench melanges, hence growth of the non-subducted plates.

Mineral Parageneses in Franciscan Metamorphic Rocks, Panoche Pass, California

Approximately 33 square miles of Franciscan terrane near Panoche Pass in the Diablo Range of the central California Coast Ranges were mapped at an initial scale of 1:12,000. Serpentinite intrusions present in the area are not mantled by metasomatic glaucophane schist aureoles, but do appear to contain tectonic inclusions of chloritized eclogite, “glaucophane eclogite,” amphibolite, glaucophane schist, and greenstone. Petrographic and X-ray investigations of country rocks reveal systematic mineral parageneses in metabasalts, metacherts, and metagraywackes. Conversion of greenstones to blueschists involved reaction of albite + actinolite + chlorite to form crossite + lawsonite; greenstones contain calcite, mafic glaucophane schists calcite or aragonite. Least metamorphosed metacherts contain very minor albite + calcite; with progressive recrystallization these phases are replaced by acgirme, riebeckite, and aragonite. Metagraywacke paragenesis is from albite metagraywackes containing accessory lawsonite + calcite or aragonite, to jadeitic pyroxene + law-sonite metagraywackes carrying accessory glaucophane + aragonite. The “jadeitic pyroxene + lawsonite isograd” is recognized on the basis of metagraywacke assemblages. The aragonite isograd cannot be located precisely because of rapid postmetamorphic inversion to calcite. More extreme conditions of recrystallization appear to have been required to produce aragonite and the jadeitic pyroxene + lawsonite (+ quartz) compatibility than to convert greenstones to blueschists. Mineral parageneses and inferred phase relationships in the system CaCO 3 and in the pseudoternary system Ab-An-H 2 O suggest that metamorphism at Panoche Pass involved temperatures of about 200–300°C, lithostatic pressures approaching 7–8 kb, and high chemical potential of H 2 O. Presence of a separate aqueous phase during the metamorphism is indicated. Based on observed mineral assemblages, experimental phase equilibrium data, tectonic and spatial occurrences of blueschists, available geophysical evidence, fossil occurrences, and radiometric dates, the following tentative geologic model is proposed. Franciscan rocks principally of the Diablo Range are hypothesized to have been deposited in an oceanic trench bordering the continental margin in later Jurassic and possibly earliest Cretaceous time. Rapid deposition and subsidence could have accounted for thermal disequilibrium and consequent relatively high pressure, low temperature recrystallization in axial portions of the trench. Thickness of the Franciscan rocks in the Diablo Range is thought to have approached 30 km near the trough axis. Diastrophism nearly contemporaneous with deposition locally deformed the structural trough, resulted in periodic intrusion of serpentinized mantle material and in rapid uplift, and allowed preservation of the blueschist facies mineral assemblages. Elsewhere, portions of the trench apparently were not destroyed and continued to receive sediments well into Cretaceous time.

An Experimental Study of Tectonic Overpressure in Franciscan Rocks

Approximately 4 kb tectonic overpressure was required, according to one theory, for the formation of jadeite-aragonite-bearing rocks of the Franciscan. Strength of two common Franciscan rocks—massive graywacke, and thin-bedded shale and graywacke—was determined to see if this tectonic overpressure could have been generated under the conditions of metamorphism; that is, 4 kb confining pressure, 200 to 300° C, in the presence of aqueous pore fluid. The strength was found to depend markedly on both pore and total pressures. The required tectonic overpressure could only have been generated in massive graywacke, and only when pore pressure was less than hydrostatic. With even moderate amounts of interbedded shale, tectonic overpressure could not have exceeded 1 kb.

Metamorphism of Franciscan tectonostratigraphic assemblage, Pacheco Pass area, east-central Diablo Range, California Coast Ranges

The Pacheco Pass area, well known for the relatively high-pressure, low-temperature mineral assemblages developed in a well-ordered section of chiefly metaclastic strata, was restudied in order to elucidate the geologic, paragenetic, and textural relationships among the various lithologic units. The mapped 7½ minute quadrangle extends from 4 km west of the range crest eastward to the Ortigalita fault, tectonic contact of the Franciscan assemblage with coeval Great Valley strata. Semicontinuous metachert beds, underlain by scraps of Na-cpx- and/or Na-amphibole-bearing metabasaltic rock, mark the base of terrigenous tectonostratigraphic units, evidently decoupled from the downgoing, oceanic-crust-capped plate. Underplating along newly recognized bedding-plane faults has juxtaposed members of an apparently conformable Franciscan sedimentary sequence. Four depositional units, each consisting of monotonous metagraywacke and interlayered metashale, are stacked within the accretionary section: from top down, these are tectonostratigraphic members A, B, C, and D. Unit D differs from overlying members in possessing a greater abundance of bluestone/greenstone pods, minor serpentinite bodies, and rare metaconglomerate lenses; varying degrees of stratal disruption indicate that it has been arrested in the transformation to broken formation. The Ortigalita fault is nearly vertical, strikes north-northwest, and exhibits apparent dextral offset; it truncates the east-west-striking Gonzaga fault and other Franciscan structures. The topographic low occupied by San Luis Reservoir is a Cenozoic tectonic depression sited at a releasing bend on the Ortigalita fault. Newly acquired textural, chemical, mineralogic, and areal relations of albitic and jadeitic pyroxene-bearing Franciscan metagraywacke near Pacheco Pass demonstrate that (1) Na-cpx and Na-amphibole are metamorphic minerals, not clastic materials; (2) pumpellyite is a minor neoblastic phase in albite-bearing metagraywacke west of Pacheco Pass, and in the north-western corner of the quadrangle, but it does not occur in jadeitic pyroxene-bearing metaclastic rocks; (3) higher textural grade metagraywacke specimens generally contain higher modal proportions of jd; (4) abundance, textural grade, and chemistry of Na-cpx are not related to tectonostratigraphic units; and (5) because of sluggish reaction rates, albite persisted metastably into the higher pressure, jadeitic pyroxene + quartz P-T field. Production of Na-cpx resulted from a reaction of the sort ab + chl = jd + Iws + qtz ± gln ± H 2 O. Irregular, oscillatory zoning within single jadeitic pyroxene prisms and variable jd/ab modes for rocks of the same bulk composition reflect growth governed by diffusion rather than recrystallization within a P-T transition zone; high-pressure overstepping of the phase boundary explains variable modal proportions of ab and jd, as well as chemical heterogeneity of the jd. Elevated P attended metamorphism of Na-cpx-bearing rocks exposed in the Pacheco Pass area, as indicated by the wide-spread coexistence of quartz and NaAlSi 2 O 6 - rich pyroxene. Inferred physical conditions of prograde metamorphism were 150 ± 50 °C at 7-8 kbar or more. Franciscan rocks in the east-central Diablo Range underwent subduction-zone metamorphism accompanying paleo-Pacific lithospheric plate descent during mid-Cretaceous convergence. Underplating, contraction, and sequestering of the tectonostratigraphic assembly at depths of 25-30 km during underflow, followed by gradual rise, took place under conditions of refrigeration due to protracted descent of the oceanic slab. Paleogene diapiric uplift and extension followed, indicating more complete decoupling of the buoyant trench complex from the downgoing plate. Finally, passage of the East Pacific Rise triple junction in Neogene time initiated the present dextral-slip regime in the California Coast Ranges.