Petrogenesis of Glaucophane Schists

Some glaucophane schists are chemically indistinguishable fromgreenschists and epidote amphibolites. Provided all three rocktypes represent equilibrium assemblages, they must have formedunder differing physical conditions. The mineralogy of suchglaucophane schists taken in conjunction with experimental evidencesuggests that these rocks formed at low temperatures and atrelatively elevated pressures. The relatively high-pressure,low-temperature phases lawsonite, jadeitic pyroxene, and metamorphicaragonite are diagnostic of physical conditions attending thismetamorphism. Differential stress may aid in the attainmentof the appropriate mean pressure necessary for the productionof these phases. Graphic analysis and approximated thermodynamic calculationsindicate that relatively elevated pressures, or relatively lowtemperatures, or both, promote the formation of glaucophanein rocks of a wide range of bulk compositions while restrictingthe compositional range of albite-bearing rocks. It is concludedthat the coexistence of glaucophane with carbonate, calcium-aluminumsilicate or paragonite results from such physical conditions,and it is on the basis of these associations or, equally well,the presence of lawsonite, jadeitic pyroxene, or metamorphicaragonite that the blueschist facies should be defined. High pressures are not required for the production of glaucophaneitself. It is stable under physical conditions present in thegreenschist and epidote amphibolite facies in rocks deficientin CaO and rich in Na₂O and MgO relative to A1₂O₃. Such bulkcompositions might result from exchange of material betweenserpentinite and albite-bearing country rocks, and could accountfor glaucophane aureoles around, and inclusions of glaucophanerock within, some serpentinites.     

Chemically Distinct Mafic Dike/Sill Sequences of Contrasting Age Ranges, Sawyers Bar Area, Central Klamath Mountains, Northern California

Mafic hypabyssal rocks in the western Triassic and Paleozoicbelt provide important clues to the nature of accretion andarc evolution along this sector of the North American margin.In the east-central part of the belt, near Sawyers Bar, somediabases have been metamorphosed before and accompanying emplacementof the mid-Jurassic English Peak and Russian Peak granitoidswithin the North Fork/Salmon River + Stuart Fork amalgamatedterrane. Certain other dikes/sills, chiefly mafic microdiorites,cut the calc-alkaline plutons but are themselves deutericallyaltered; at least two of these mafic microdiorites near theEnglish Peak body possess hornfelsic textures. Thus, althoughmost mafic microdioritic hypabyssals seem to have been injectedafter granitoid emplacement, a few must have preceded plutonicintrusion. Macroscopic appearances, phase assemblages, mineralcompositions, and textures of the mafic microdioritic and metadiabasicdikes/sills are sufficiently alike to preclude the ready fieldand petrographic distinction of the different magma series.Bulk-rock chemistries fall into two groups, however, with slightlymore porphyritic, altered, synplutonic mafic microdiorite samplesbeing distinctly richer in Si, K, P, Rb, Sr, Zr, and light rareearth elements (LREE) relative to the Mg + Cr + Ni-rich, preplutonicmetadiabases. Analyzed mafic microdiorites have bulk-rock chemicaland isotopic compositions similar to the more ferromagnesianportions of the mid-Jurassic English Peak and Russian Peak plutoniccomplexes, whereas the metadiabases are comparable with theearly Mesozoic Salmon River metabasalts. Although the two groupsof dikes/sills probably overlap in age of emplacement, the maficmicrodiorite group is predominantly younger and uniform in oxygenisotopic composition (bulk-rock ¹⁸O 11•37, 11•4 and11•46) compared with the older, more intensely metamorphosed,and variably metasomatized Salmon River metadiabases (bulk-rock5¹⁸ 9•4, 11•0, and 15•3). Both types of maficdike/sill locally intrude the more easterly Stuart Fork terrane.Therefore, suturing and regional metamorphism of the outboardNorth Fork/Salmon River oceanic-island arc and inboard StuartFork subduction complex must have occurred during terminal stagesof injection of the pregranitoid diabases into the North Fork(oceanic-island basalts)/Salmon River (island-arc tholeiites)arc + Stuart Fork terrane, but before invasion of the amalgamatedterrane assembly by the calc-alkaline plutons and most compositionallyrelated synplutonic mafic microdiorite dikes/sills. Becauseof their lateral continuation both north and south of the SawyersBar area, the North Fork/Salmon River igneous suite documentsthe construction of an oceanic arc of considerable lateral extentin the central Klamaths before terrane accretion. Suturing wasimmediately followed by the mid-Jurassic intrusion of calc-alkalineplutons + syngranitoid mafic microdioritic hypabyssals.     

Petrology of the Great Abitibi Dyke, Superior Province, Canada

The Great Abitibi Dyke (GAD) which can be traced northeast,for >> 700 km, across the Abitibi Belt in the southeasternSuperior Province of the Canadian Shield, is composed of olivinegabbro to monzodiorite, weakly saturated to undersaturated insilica. All rocks of the GAD can be derived by mainly plagioclase andolivine fractionation from a parental magma corresponding incomposition to chilled margin samples. Two units can be distinguished,a marginal unit (Unit 1) representing 0–50% crystallizationand a central unit (Unit 2), found over about half of the dykelength, representing 50–70% crystallization. Modelling,using Pearce elemental ratio analysis, quantifies the fractionationhistory and allows mass–balance calculations over thepresent exposure level of the dyke. The approximate balancebetween the amounts of cumulate and fractionated rocks suggeststhat Unit 1 differentiated essentially in situ as a closed system.In contrast, Unit 2 rocks were formed by loss of substantialplagioclase and olivine from the parental magma. This fractionatemust have either been lost to depth or left behind in an externalchamber. Unit 2 rocks show depletion in plagioclase and enrichmentin mafic minerals along strike towards the southwest (deeperexposure level?), a trend explained by density stratification.Regional variation in Unit 1 chemistry is interpreted in termsof lateral magma injection towards the northeast from the locusof Keweenawan rift volcanism. Feldspar, olivine, and augitecompositions are linearly correlated with equilibrium temperatureand extent of magma evolution. The parent magma had a trace element chemistry correspondingto a ‘within–plate’ setting and was probablyderived from an incompatible–element enriched mantle similarto the source for ocean island basalts (OIBs).     

Cosmic-ray produced,radiogenic, and solar noble gases in lunar meteorites Queen Alexandra Range 94269 and 94281

Abstract— We measured the noble gas isotopic abundances in lunar meteorite QUE 94269 and in bulk-, glass-, and crystal-phases of lunar meteorite QUE 94281. Our results confirm that QUE 94269 originated from the same meteorite fall as QUE 93069: both specimens yield the same signature of solar-particle irradiation and also the cosmogenic noble gases are in agreement within their uncertainities. Queen Alexandra Range 93069/94269 was exposed to cosmic rays in the lunar regolith for ～1000 Ma, and it trapped 3.5 × 10^?4 cm³STP/g solar ³⁶Ar, the other solar noble gases being present in proportions typical for the solar-particle irradiation. The bulk material of QUE 94281 contains about three times less cosmogenic and trapped noble gases than QUE 93069/94269 and the lunar regolith residence time corresponds to 400 ± 60 Ma. We show that in lunar meteorites the trapped solar ²⁰Ne/²²Ne ratio is correlated with the trapped ratio ⁴⁰Ar/³⁶Ar, that is, trapped ²⁰Ne/²²Ne may also serve as an antiquity indicator. The upper limits of the breccia compaction ages, as derived from the trapped ratio ⁴⁰Ar/³⁶Ar for QUE 93069/94269 and QUE 94281 are ～400 Ma and 800 Ma, respectively. We found very different regolith histories for the glass phase and the crystals separated from QUE 94281. The glass phase contains much less cosmogenic and solar noble gases than the crystals, in contrast to the glasses of lunar meteorite EET 87521, that were enriched in noble gases relative to the crystalline material. The QUE 94281 phases yield a ⁴⁰K-⁴⁰Ar gas retention age of 3770 Ma, which is in the range of that for lunar mare rocks.     

Petrochemical Study of Lherzolitic Rocks from the Western Alps

An integrated geologic and petrochemical comparative study offive peridotite complexes in the western Alps has been undertaken.Investigated bodies are exposed at Alpe Arami in southern Switzerland,and at Finero, Balmuccia, Baldissero and Lanzo in northwesternItaly. The Alpe Arami mass has been tectonically emplaced withinthe Lepontine gneiss terrane of the in part subducted, morenortherly European lithospheric plate, whereas the other fourultramafic masses represent original portions of the non-subductedSouth Alpine plate. Eighty samples were examined petrographically. Most are lherzolites,but a few are clinopyroxene-bearing harzburgites, olivine websteritesor clinopyroxenite. Alpe Arami peridotites locally contain primarygarnet (± minor spinel); in contrast, the South Alpinelherzolites are spinel bearing, and in addition, the Lanzo massifcontains widespread plagioclase. All masses display strain effectssuch as bent lamellae in pyroxenes, gradational or sectoraloptical extinction, and minor recrystallization. Twenty-fourbulk XRF analyses demonstrate that the investigated rocks fairlyclosely match pyrolite composition, but are slightly impoverishedin alumina. The normative olivines of all analyzed specimenshave Fa contents ranging between 8 and 10 mole per cent. Electronmicroprobe analyses for 26 olivines, 27 orthopyroxenes, 23 calcicpyroxenes, three garnets, 18 spinels, three plagioclases, 13calcic amphiboles, two chlorites and two phlogopites are presented.Phases in a particular sample are remarkably homogeneous, anobservation consistent with an inferred close approach to chemicalequilibrium. Use of various two-pyroxene geothermometers allows the followingassignments of grand average apparent temperatures for the fiveperidotite complexes: Alpe Arami, 966±78°C; Finero,893 ± 94°C; Balmuccia, 973 ± 50°C; Baldissero,1002 ± 37°C; and Lanzo, 1069 ± 85°C. Pressureestimates, determined using the lherzolitic petrogenetic grid,Al₂O₃^cpx and Al₂O₃^opx isopleths, involve large uncertainties.The Alpe Arami ultramafic body evidently crystallized at a pressureof 40 ± 10 kilobars, the nominal value depending on themethod of computation. In contrast, the South Alpine spinellherzolites seem to have last equilibrated at pressures of approximately5–20 kb. The deep upper-mantle source region of the AlpeArami body apparently was subjected to a subcontinental-typegeothermal gradient, consistent with solid-state rise of thismass and Late Alpine tectonic insertion in the old Lepontinesialic crust of the European lithospheric plate. The mechanismand driving force of this process remain obscure. In contrast,the investigated spinel peridotite complexes of the South Alpinelithospheric plate seem to have been derived from shallow, uppermostmantle sections characterized by oceanic geothermal gradients.For this reason it is conjectured that these massifs were derivedfrom the northern margin of the southern plate where continentalcrust evidently was moderately thin or absent. Their upliftand overthrusting involved P-T paths which in most cases alloweddecompression partial recrystallization and incipient fusion,the latter indicated by the presence of transecting mafic dikesand segregations associated with the peridotites. The Finerobody appears to have reached its present position at the baseof the South Alpine continental crust in pre-Mesozoic time,whereas the Lanzo complex was tectonically involved in the EarlyAlpine orogeny.     

Metamorphic P–T conditions and thermal structure of Chinese Continental Scientific Drilling main hole eclogites: Fe–Mg partitioning thermometer vs. Zr-in-rutile thermometer

Core rocks recovered from the main hole (5158 m deep) of the Chinese Continental Scientific Drilling (CCSD‐MH) project, southern Sulu UHP terrane, east‐central China, consist of eclogites, various gneisses and minor metaperidotite cumulates; this lithological section underwent subduction‐zone UHP metamorphism. Coesite‐bearing eclogites are mainly present between the depths of 100–2000 m, but below 2000 m, mafic eclogites are rare. Selected elements (Zr, Nb, Cr, Fe, Si, Mg, Al & Ti) in rutile from 39 eclogite cores from 100 to 2774 m, and major elements of minerals from representative eclogites were analysed by electron microprobe. Zirconium and Nb concentrations of rutile cluster ～100–400 and 200–700 ppm respectively. However, Zr and Nb contents in rutile from strongly retrograded eclogites show larger variations than those of fresh or less retrograded eclogites, implying that somehow fluid infiltration affected rutile chemistry during retrograde metamorphism. Zr contents in rutile inclusions in garnet and omphacite are slightly lower than those of the matrix rutile, suggesting that the rutile inclusions formed before or close to the peak temperature. The P–T conditions of the CCSD‐MH eclogites were estimated by both Fe–Mg exchange and Zr‐in‐rutile thermometers, as well as by the Grt–Cpx–Phn–Ky geothermobarometer. The maximum temperature range of 700–811 °C calculated at 40 kbar using the Zr‐in‐rutile thermometer is comparable with temperature estimates by the Fe–Mg exchange thermometer. The temperature estimates of eclogites in a ～3000 m thick section define a continuous gradient, and do not show a distinct temperature gap, suggesting that the rocks from 100 to 3000 m depth might belong to a single, large‐scale UHP slab. These data combined with P–T calculations for CCSD‐MH peridotites yield a low geotherm (～5 °C km^?1) for the Triassic subduction zone between the Sino‐Korean and Yangtze cratons; it lies ～30–35 mW m^?2 conductive model geotherm.     

Dendroclimatic studies on conifers from central Europe and Great Britain

Schweingruber, Fritz H., BrÄker, Otto U. & SchÄr, Ernst 1979 1201: Dendroclimatic studies on conifers from central Europe and Great Britain. Boreas, Vol. 8, pp. 427–452. Oslo. ISSN 0300–9483. The use of X-ray densitometry for measuring tree rings makes it possible to consider the relations between climate and tree rings in a new light. Investigations so far have shown that it is the maximum density of the annual rings that provides the most important and best climatological information. Comparisons between density sequences are possible when the tree-ring samples come from ecologically similar sites. Site characteristics are expressed by six different tree-ring parameters. Relations between the maximum densities of various species from different regions are revealed by calculating the percentage of agreement. The growth rate of cell walls in latewood of all coniferous species from cool humid regions is limited mainly by summer temperatures. In Scotland, where the climate is cool-oceanic and relatively stable, the cell walls grow from June to October. In the subalpine zone of the Alps and the Rocky Mountains near the timber line growth takes place mainly in August and September. Maximum density in trees of temperate sites is also primarily influenced by the summer temperature. Minimum density shows the relationship to the xerothermic site characteristics. On dry sites cambial activity and cell wall growth in latewood are mainly limited by precipitation during the growth period. Samples from the subalpine zones (1500–2200 m above sea level) between latitudes 45d? and 50d? North, i.e. those from the Western Carpathians, the Alps, and the Northern Appalachian Mountains, seem to form a unity. Samples from the oceanic region (20–600 m above sea level) between 52d? and 58d?N latitudes show a different and incoherent picture. Only the present low level density during 1950–1975 is evident at all sites investigated for the last 200 years. By cross-dating the annual tree-ring sequences of living trees with samples of timber taken from ancient houses a 700-year-long chronology has been developed for the Northern Prealps in Switzerland. This chronology shows the approximate annual and decennial temperature changes; the sensitivity curve shows the different climatic variability in temperature.     

Diabase-Granophyre Relations in the Endion Sill, Duluth, Minnesota

The gently dipping 1,500-ft thick Endion sill intrudes Keweenawanflows at Duluth, Minnesota. In a general way, from the bottomto the top of the body, there is a gradual transition from diabasethrough intermediate rock (granodiorite) to granophyre (adamellite).These latter two rock types together constitute about 40 percent of the exposed mass. Fractional crystallization of basalticmagma produced a great thickness of basic and intermediate rocktypes which accumulated predominantly in the lower portionsof the sill, and probably in the end-stage production of anaqueous, salic, alkalic liquid. It is proposed that, due toinitial inclination of the sill, part of this salic fractionmigrated up dip, accumulated and reacted with portions of thediabase and intermediate rock, and completed crystallizationat the presently exposed level. Alternatively, much or all ofthe Endion sill granophyre may represent a separate intrusionunrelated to differentiation of the earlier diabase. In eithercase the sill is composite. Bulk compositions of cryptoperthitesindicate both granophyre and intermediate rock crystallizedat magmatic temperatures. Compositional uniformity of the clinopyroxene and lack of ironenrichment in felsic portions of the Endion sill may be theresult of accumulation of H₂O and the presumed maintenance ofnearly constant partial oxygen pressure during crystallization.This mechanism would furthermore account for inferred late magmaticsolid solution between alkali feldspar and KFe³?Si₃O₈, withsubsequent subsolidus exsolution of haematite.     

Diabase--Granophyre Relations in the Endion Sill, Duluth, Minnesota

The gently dipping l,5OO-ft thick Endion sill intrudes Keweenawanflows at Duluth, Minnesota. In a general way, from the bottomto the top of the body, there is a gradual transition from diabasethrough intermediate rock (granodiorite) to granophyre (adamellite).These latter two rock types together constitute about 40 percent of the exposed mass. Fractional crystallization of basalticmagma produced a great thickness of basic and intermediate rocktypes which accumulated predominantly in the lower portionsof the sill, and probably in the end-stage production of anaqueous, salic, alkalic liquid. It is proposed that, due toinitial inclination of the sill, part of this salic fractionmigrated up dip, accumulated and reacted with portions of thediabase and intermediate rock, and completed crystallizationat the presently exposed level. Alternatively, much or all ofthe Endion sill granophyre may represent a separate intrusionunrelated to differentiation of the earlier diabase. In eithercase the sill is composite. Bulk compositions of cryptoperthitesindicate both granophyre and intermediate rock crystallizedat magmatic temperatures. Compositional uniformity of the clinopyroxene and lack of ironenrichment in felsic portions of the Endion sill may be theresult of accumulation of H₂O and the presumed maintenance ofnearly constant partial oxygen pressure during crystallization.This mechanism would furthermore account for inferred late magmaticsolid solution between alkali feldspar and KFe³+Si₃0₈, withsubsequent subsolidus exsolution of haematite.     

Corundum-bearing residues produced through the evaporation of natural and synthetic hibonite

Abstract— We have produced corundum-bearing residues through the evaporation of natural and synthetic hibonite samples. The sequence of major element losses as well as volatility related trace element fractionations in these residues are similar to those previously observed in residues from the evaporation of chondritic starting material, which suggests that the processes by which these fractionations occur may be largely independent of the starting material used. However, the mineralogy of the residues does depend on the composition of the starting material and, to some extent, on the conditions under which evaporation took place. Similarly, the degree of isotopic mass fractionation observed in the residues is composition-dependent. This observation means that it may be possible to use isotopic data for several elements to constrain the compositions of precursor materials of Ca-Al-rich inclusions, which have an evaporation origin. Although corundum-bearing inclusions are known, their origins are complex and variable, and the scarcity of such inclusions indicates that melting of hibonite, with or without concomitant evaporation, must have been a rare process in the solar nebula. By evaporating mixtures of synthetic oxides of the rare earth elements, we have reproduced the patterns of Group III inclusions and some of the characteristics of ultrarefractory patterns. However, the extreme conditions required to do so indicate that refractory inclusions with these patterns probably have a condensation rather than evaporation origin.