Metamorphism of mafic dikes from the central White-Inyo Range, eastern California

Thin mafic dikes, possibly correlative with the Independence dike swarm of SE California, transect uppermost Proterozoic–Cambrian metasedimentary strata in the White-Inyo Range. Textures and bulk-rock chemistry indicate that the protoliths were diabases and microdiorites, accompanied by Ca + Mg + Fe +Ni + Cr-rich hornblende (± minor augite) cumulates. Analytical data suggest crystal settling and fractionation at shallow depths. Most of the dikes lie in the mapped aureoles of – and were metamorphosed by – voluminous Late Jurassic granitoid plutons; however, a few metadikes cut these plutons and must have been recrystallized during the emplacement of Cretaceous granitic stocks. The mafic metadikes thus include members of two or more temporally distinct suites, pre-Late Jurassic, and latest Jurassic–Cretaceous. Neoblastic mineral assemblages and element partitioning within these nonfoliated mafic metadikes reflect lower-to-upper greenschist facies overprints; metamorphic parageneses, coincident with those developed in the metasedimentary wallrocks, are defined by the production of chlorite, biotite, white mica, epidote, and actinolite, and by albitization of the igneous plagioclase. Based on analytical and mineralogic data obtained in this study, the following conclusions regarding subsolidus recrystallization of the mafic metadikes are advanced: (1) Newly grown minerals and phase assemblages are systematic in their areal distributions. (2) Metamorphic grade increases chiefly toward the north and east, toward the Late Jurassic granitoids. (3) Element fractionation among coexisting neoblastic phases is regular, and compatible with a close approach to chemical equilibrium. (4) Assemblages 3–5 km from the granitic intrusive contacts reflect lowermost greenschist facies physical conditions. (5) Investigated mafic dikes exhibit mineral parageneses isofacial with the regional/contact metamorphic assemblages previously documented for the enclosing pre-Mesozoic clastic country rocks. Clearly, mafic dikes of several ages of injection and recrystallization are present in the central White-Inyo Range, making correlation with the Independence dike swarm problematic. In any case, the dikes record localized contact metamorphism that took place sporadically over portions of an approximately 100 million year interval. Received: 13 March 1996 / Accepted: 24 December 1996     

New P–T constraints on the Tamayen glaucophane‐bearing rocks,eastern Taiwan: Perple_X modelling results and geodynamic implications

New pseudosection modelling was applied to better constrain the P–T conditions and evolution of glaucophane‐bearing rocks in the Tamayen block of the Yuli belt, recognized as the world's youngest known blueschist complex. Based on the predominant clinoamphibole, textural relationships, and mineral compositions, these glaucophane‐bearing high‐P rocks can be divided into four types. We focused on the three containing garnet. The chief phase assemblages are (in decreasing mode): amphibole + quartz + epidote + garnet + chlorite + rutile/titanite (Type‐I), phengite + amphibole + quartz + garnet + chlorite + epidote + titanite + biotite + magnetite (Type‐II), and amphibole + quartz + albite + epidote + garnet + rutile + hematite + titanite (Type‐III). Amphibole exhibits compositional zoning from core to rim as follows: glaucophane → pargasitic amphibole → actinolite (Type‐I), barroisite → Mg‐katophorite/taramite → Fe‐glaucophane (Type‐II), glaucophane → winchite (Type‐III). Using petrographic data, mineral compositions and Perple_X modelling (pseudosections and superimposed isopleths), peak P–T conditions were determined as 13 ± 1 kbar and 550 ± 40 °C for Type‐I, 10.5 ± 0.5 kbar and 560 ± 30 °C for Type‐II (thermal peak) and 11 ± 1 kbar and 530 ± 30 °C for Type‐III. The calculations yield higher pressures and temperatures than previously thought; the difference is ~1–6 kbar and 50–200 °C. The three rock types record similar P–T retrograde paths with clockwise trajectories; all rocks followed trajectories with substantial pressure decrease under near‐isothermal conditions (Type‐I and Type‐III), with the probable exception of Type‐II where decompression followed colder geotherms. The P–T paths suggest a tectonic environment in which the rocks were exhumed from maximum depths of ~45 km within a subduction channel along a relative cold geothermal gradient of ~11–14 °C km^?1.     

An introduction to ultrahigh-pressure metamorphism

Abstract Ultrahigh-pressure (UHP) metamorphism refers to mineralogical and structural readjustment of supracrustal protoliths and associated mafic-ultramafic rocks at mantle pressures greater than ∼ 25 kbar (80-90 km). Typical products include metapelite, quartzite, marble, granulite, eclogite, paragneiss and orthogneiss; minor mafic and ultramafic rocks occur as eclogitic-ultramafic layers or blocks of various dimensions within the supracrustal rocks. For appropriate bulk compositions, metamorphism at great depths produces coesite, microdiamond and other characteristic UHP minerals with unusual compositions. Thus far, at least seven coesite-bearing eclogitic terranes and three diamond-bearing UHP regions have been documented. All lie within major continental collision belts in Eurasia, have similar supracrustal protoliths and metamorphic assemblages, occur in long, discontinuous belts that may extend several hundred kilometers or more, and typically are associated with contemporaneous high-P blueschist belts. This paper defines the P-T regimes of UHP metamorphism and describes mineralogical, petrological and tectonic characteristics for a few representative UHP terranes including the western gneiss region of Norway, the Dora Maira massif of the western Alps, the Dabie Mountains and the Su-Lu region of east-central China, and the Kokchetav massif of the former USSR. Prograde P-T paths for coesite-bearing eclogites require abnormally low geothermal gradients (approximately 7°C/km) that can be accomplished only by subduction of cold, oceanic crust-capped lithosphere ± pelagic sediments or an old, cold continent. The preservation of coesite inclusions in garnet, zircon, omphacite, kyanite and epidote, and microdiamond inclusions in garnet and zircon during exhumation of an UHP terrane requires either an extraordinarily fast rate of denudation (up to 10 cm/year) or continuous refrigeration in an extensional regime (retreating subduction zone).     

Bifurcation of volcanic plumes in a crosswind

Bent-over buoyant jets distorted by a crosscurrent develop a vortex pair structure and can bifurcate to produce two distinct lobes which diverge from one another downwind. The region downwind of the source between the lobes has relatively low proportions of discharged fluid. Factors invoked by previous workers to cause or enhance bifurcation include buoyancy, release of latent heat at the plume edge by evaporating water droplets, geometry and orientation of the source, and the encounter with a density interface on the rising path of the plume. We suggest that the pressure distribution around the vortex pair of a rising plume may initially trigger bifurcation. We also report new experimental observations confirming that bifurcation becomes stronger for stronger bent-over plumes, identifying that bifurcation can also occur for straight-edged plumes but gradually disappears for stronger plumes which form a gravity current at their final level and spread for a significant distance against the current. Observations from satellites and the ground are reviewed and confirm that volcanic plumes can show bifurcation and a large range of bifurcation angles. Many of the bifurcating plumes spread out at the tropopause level and suggest the tropopause may act on the plumes as a density interface enhancing bifurcation. Even for quite moderate bifurcation angles, the two plume lobes become rapidly separated downwind by distances of tens of kilometers. Such bifurcating plumes drifting apart can only result in bilobate tephra fall deposits. The tephra fall deposit from the 16 km elevation, SE spreading, bifurcating volcanic plume erupted on 15 May 1981 from Mt Pagan was sampled by previous workers and clearly displayed bilobate characteristics. Examples of bilobate tephra fall deposits are reviewed and their origin briefly discussed. Bilobate deposits are common and may result from many causes. Plume bifurcation should be considered one of the possible mechanisms which can account for come examples of bilobate tephra fall deposits.     

Regional policy in Costa Rica: The problems of implementation

In most Third World countries regional planning and regional policy is formally well-developed, but in practice is not implemented. In spite of increasing regional disparities, narrow sectoral and centrally decided programmes and projects dominate over territorial and decentralized ones. This article examines theoretically and empirically why, in the case of Costa Rica, the implementation problem is so difficult and which strategic approaches would best suit an effective regional policy.     

Future ocean uptake of CO2: interaction between ocean circulation and biology

We discuss the potential variations of the biological pump that can be expected from a change in the oceanic circulation in the ongoing global warming. The biogeochemical model is based on the assumption of a perfect stoichiometric composition (Redfield ratios) of organic material. Upwelling nutrients are transformed into organic particles that sink to the deep ocean according to observed profiles. The physical circulation model is driven by the warming pattern as derived from scenario computations of a fully coupled ocean-atmosphere model. The amplitude of the warming is determined from the varying concentration of atmospheric CO₂. The model predicts a pronounced weakening of the thermohaline overturning. This is connected with a reduction of the transient uptake capacity of the ocean. It yields also a more effective removal of organic material from the surface which partly compensates the physical effects of solubility. Both effects are rather marginal for the evolution of atmospheric pCO₂. Running climate models and carbon cycle models separately seems to be justified. Received: 9 August 1995 / Accepted: 22 April 1996     

Statistische Bearbeitung einer einjährigen Grobaerosol-Messreihe

Zusammenfassung Die Ergebnisse von Untersuchungen über das Grobaerosol (Partikelgröße über 0.5 ) in einer Großstadt (Hamburg), wurden mit Hilfe modernster statistischer Verfahren eingehend bearbeitet. Eine ursprünglich durchgeführte grobe Bearbeitung, wie sie heute noch von den meisten Meteorologen vorgenommen wird, ergab praktisch nur eine Abhängigkeit von den Jahreszeiten (Heizperiode) und von den Inversionslagen.Mit Hilfe moderner statistischer Methoden war es u. a. möglich, statistisch gesicherte Abhängigkeiten der Partikelzahlen und Partikelgrößen von der Windrichtung, der Partikelzahlen von der Windstärke und der Partikelzahlen von den Luftkörpern, aufzudecken. Auch ein statistisch gesicherter Zusammenhang zwischen der Sichtweite und der Partikelzahl des Grobaerosols konnte festgestellt werden.Diese Untersuchungen zeigen deutlich die Vorteile und die Notwendigkeit der Bearbeitung von Meßreihen mit den neuen Verfahren der mathematischen Statistik.     

Blueschist metamorphism and P-T regimes in active subduction zones

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.