Partial melting of metagreywacke: a calculated mineral equilibria study   总被引：2，自引：0，他引：2  

T. E. JOHNSON  R.W. WHITE  R. POWELL 《Journal of Metamorphic Geology》2008,26(8):837-853

Greywacke occurs in most regionally metamorphosed orogenic terranes, with depositional ages from Archean to recent. It is commonly the dominant siliciclastic rock type, many times more abundant than pelite. Using calculated pseudosections in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O system, the partial melting of metagreywacke is investigated using several natural protolith compositions that reflect the main observed compositional variations. At conditions appropriate for regional metamorphism at mid‐crustal depths (6–8 kbar), high‐T subsolidus assemblages are dominated by quartz, plagioclase and biotite with minor garnet, orthoamphibole, sillimanite, muscovite and/or K‐feldspar (±Fe–Ti oxides). Modelled solidus temperatures are dependent on bulk composition and vary from 640 to 690 °C. Assuming minimal melting at the H₂O‐saturated solidus, initial prograde anatexis at temperatures up to ～800 °C is characterized by very low melt productivity. Significant melt production in commonly occurring (intermediate) metagreywacke compositions is controlled by the breakdown of biotite and production of orthopyroxene (±K‐feldspar) across multivariant fields until biotite is exhausted at 850–900 °C. Assuming some melt is retained in the source, then at temperatures beyond that of biotite stability, melt production occurs via the consumption of plagioclase, quartz and any remaining K‐feldspar as the melt becomes progressively more Ca‐rich and H₂O‐undersaturated. Melt productivity with increasing temperature across the melting interval in metagreywacke is generally gradational when compared to metapelite, which is characterized by more step‐like melt production. Comparison of the calculated phase relations with experimental data shows good consistency once the latter are considered in terms of the variance of the equilibria involved. Calculations on the presumed protolith compositions of residual granulite facies metagreywacke from the Archean Ashuanipi subprovince (Quebec) show good agreement with observed phase relations. The degree of melt production and subsequent melt loss is consistent with the previously inferred petrogenesis based on geochemical mass balance. The results show that, for temperatures above 850 °C, metagreywacke is sufficiently fertile to produce large volumes of melt, the separation from source and ascent of which may result in large‐scale crustal differentiation if metagreywacke is abundant.  相似文献   

Thin section grain size analysis revisited     

M. R. JOHNSON 《Sedimentology》1994,41(5):985-999

Existing proposals for converting thin section data to their sieve equivalents are all flawed in various ways, while questions concerning the significance of the grain size of spherical grains measured on a volume frequency basis in thin section using φ units, and of non-spherical (ellipsoidal) grains using both millimetres and φ units, have not been -satisfactorily resolved in the literature. It can be shown mathematically that the mean thin section diameters of spherical grains, or axis lengths of a series of parallel sections through ellipsoidal grains, will underestimate the dimensions of the corresponding central section (i.e. one passing through the grain centre) by 0.2023 φ when measured on a volume frequency basis. In order to approximate the effect of measuring particle size on random cuts through ellipsoidal grains, the dimensions of a series of sections cut in 49 unique directions, symmetrically arranged and evenly spread with respect to the ellipsoidal axes, were calculated. This calculation was carried out for five different ellipsoids which between them covered the mean sphericity and thin section axial ratio values normally encountered among naturally occurring quartz grain populations. The data indicated that the mean true nominal diameters (D̄) of ellipsoidal quartz grains can be obtained in thin sections from the mean nominal sectional diameters (d̄′) and major axes (ā′) of the central sections (derived from the observed values by multiplying the millimetre means by 1·1318 and subtracting 0·2023 from the φ means) using the following equations: A rough estimate (to within c. 5%) of both the mean nominal diameters and the mean long axes of ellipsoidal quartz grains can be arrived at by applying a simple correction factor to the mean long axis lengths as measured in thin section using either millimetres or φ units.  相似文献   

THE EFFECT OF MINE TAILINGS ON LEVELS OF HEAVY METALS IN SEAWATER     

C. S. WONG  F. A. WHITNEY  W. K. JOHNSON  LU XIANKUN  WU JINPING  WU SHENGSAN  LI JINXIA 《海洋学报(英文版)》1986,5(3):401-415

Changes in levels of heavy metals in seawater resulting from the addition of mine tailings were studied using plastic enclosures. Nominal concentrations of 10 mg l-1 and 100 mg l-1 of mine tailings from a B. C. molybdenum mine were added, and the metal levels were followed together with sedimentation rates, particle size distribution and other oceanographic and biological time-series parameters over sixteen days. No substantial elevation in levels of zinc, cadmium and copper was observed in the tailing bags compared to that in a background control bag with no addition of tailings. Lead showed a substantial increase in seawater following tailing addition and its removal apparently was with detritus settling following a plankton bloom, and inorganic settling of fine tailing particles.  相似文献   

Manam Island, Papua New Guinea: Petrology and Geochemistry of a Low-TiO2 Basaltic Island-Arc Volcano     

JOHNSON  R. W.; JAQUES  A. L.; HICKEY  R. L.; MCKEE  C. O.; CHAPPELL  B. W. 《Journal of Petrology》1985,26(2):283-323

Manam volcano consists of relatively mafic and compositionallysimilar tholeiitic basalts and low-SiO₂ andesites that are characterizedby notably low (mainly 0?3–0?35 weight per cent) TiO₂contents. These rocks provide an ideal opportunity to investigateboth the extent of depletion in their peridotite magma-sourceregions (which are evidently similar in many respects to thehighly depleted sources of boninitic magmas), and the interplayof the high-level processes of magma mixing, crystal fractionation,and upper crustal contamination, in an island-are volcano. Manamrocks have pronounced enrichments in Rb, Ba, K, and Sr relativeto the light rare-earth elements and, especially, to the high-field-strengthelements (Sr/Ti values are exceptionally high). However, thereis no compelling evidence that these enrichments were causedby addition of a hydrous, slab-derived component to the peridotitesource region. Nd and Sr-isotope ratios plot within the oceanicmantle array; ²⁰⁷Pb/²⁰⁴Pb values are only slightly higher thanthose for oceanic rocks; and the absence of hydrous minerals,the early crystallization and modal preponderance of plagioclaseover pyroxene, high estimated quenching temperatures, and lowwater contents in the Manam rocks, are all evidence that themagmas crystallized under markedly water-undersaturated conditions.Unusually anorthite-rich plagioclase phenoerysts in the morediffrentiated rocks may correspond to crystallization underhigher water-vapour pressures, possibly caused by influxes ofgroundwater, or they may be accidental xenocrysts. Fractionationof olivine, clinopyroxene, and spinel (early chromite followedby magnetite) has dominated the evolution of the magma series.However, clear correlations between incompatible trace-elementratios, ⁸⁷Sr/⁸⁶Sr, and 100 Mg/(Mg ? Fe^2?) values are convincingevidence for an accompanying mixing process—either of(1) two basaltic magma types (one more fractionated and lowerin ⁸⁷Sr/⁸⁶Sr than the other), or (2) pristine magmas and contaminantfrom basaltic conduit and reservoir wall rocks. Wall-rock contaminationis the less likely process, and is the more difficult one toidentify, particularly if it accompanied magma mixing.  相似文献   

Experimentally Determined Conditions in the Fish Canyon Tuff, Colorado, Magma Chamber   总被引：8，自引：7，他引：8  

JOHNSON  MARIE C.; RUTHERFORD  MALCOLM J. 《Journal of Petrology》1989,30(3):711-737

The Fish Canyon Tuff, Colorado, forms one of the largest (3000km³ known silicic eruptions in Earth history. The tuff is ahomogeneous quartz latite consisting of 40% phenocrysts (plagioclase,sanidine, biotite, hornblende, quartz, magnetite, apatite, sphene,and ilmenite) in equilibrium with a highly evolved rhyoliticmelt now represented by the matrix glass. Melt inclusions trappedin hornblende and quartz phenocrysts are identical to the newlyanalyzed matrix glass composition indicating that hornblendeand quartz crystallized from a highly evolved magma that subsequentlyexperienced little change. This study presents experimentalphase equilibrium data which are used to deduce the conditions(P, T, f_O2, f_H2O, etc.) in the Fish Canyon magma chamber priorto eruption. These new data indicate that sanidine and quartzare not liquidus phases until 780?C temperatures are achieved,consistent with Fe-Ti oxide geothermometry which implies thatthe magmatic temperature prior to eruption was 760?30?C. NaturalFe-Ti oxide pairs also suggest that log f_O2 was -12.4 (intermediatebetween the Ni-NiO and MnO-Mn₃O₄ oxygen buffers) in the magmachamber. This f_O2.102 is supported by the experimentally determinedvariations in hornblende and melt Mg-numbers as functions off_O2 A new geobarometer based on the aluminum content of hornblendesin equilibrium with the magmatic assemblage hornblende, biotite,plagioclase, quartz, sanidine, sphene, ilmenite or magnetite,and melt is calibrated experimentally, and yields pressuresaccurate to ?0.5 kb. Total pressure in the Fish Canyon magmachamber is inferred to have been 2.4 kb (equivalent to a depthof 7.9 km) based on the Al-content of natural Fish Canyon hornblendesand this new calibration. This depth is much shallower thanhas been proposed previously for the Fish Canyon Tuff. Variationsin experimental glass (melt) composition indicate that the magmawas water-undersaturated prior to eruption. X_H2O in the fluidphase that may have coexisted with the Fish Canyon magma isestimated to have been 0.5 by comparing the An-content of naturalplagioclases to experimental plagioclases synthesized at differentX_H2O and P_totals. This ratio corresponds to about 5 wt.% waterin the melt at depth. The matrix glass chemistry is reproducedexperimentally under these conditions: 760?C, 2.4 kb, X_H2O=0.5,and log f_o2=NNO+2 log units. The fugacity of SO₂ (91 b) is calculatedfrom the coexistence of pyrrhotite and magnetite. Maximum CO₂fugacity (2520 b) is inferred assuming the magma was volatilesaturated at 2.4 kb.  相似文献   

Strategic Planning for the Forest Industry in North Canterbury, New Zealand     

S. E. JOHNSON  A. G. D. WHYTE 《New Zealand geographer》1992,48(1):11-20

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Sedimentation and water escape structures in some late Precambrian shallow marine sandstones from Finnmark, North Norway     

HOWARD D. JOHNSON 《Sedimentology》1977,24(3):389-411

Water escape structures are abundant in the Grønnes Formation, a tectonically undeformed, late Precambrian shallow marine sandstone deposit in North Norway. Trough cross-bedded sandstones of the current-dominated shallow marine environment were frequently liquefied, presumably due to recurring seismic shocks. Subsequent dewatering resulted in deformation of the cross-bedding and the formation of convolute lamination. A three-fold upward vertical sequence developed where liquefaction occurred below the sediment-water interface: convoluted bed → passively deformed bed → undeformed bed. The passively deformed bed resulted from differential subsidence of a relatively plastic bed above a liquefied bed. It is characterized by anticlinal ridges and sand volcanoes at the sites of vertical sediment extrusion, and synclinal troughs at the sites of lateral sediment movement. Liquefaction may have been induced by either tectonic (earthquake shocks) or non-tectonic (storm-induced microseisms) trigger mechanisms, or a combination of both. The restriction of such a high frequency of water escape structures to deposits immediately above a gentle regional unconformity lends support for a tectonic trigger mechanism.  相似文献   

Sandstone mounds and associated facies sequences in some Late Precambrian and Cambro-Ordovician inshore tidal flat/lagoonal deposits     

CHRISTOPHER T. BALDWIN  HOWARD D. JOHNSON 《Sedimentology》1977,24(6):801-818

Sandstone mounds occur in some shallow marine heterolithic deposits from the Late Precambrian Stangenes Formation (N. Norway) and the Cambro-Ordovician Crozon Formation (N. W. France) and Cabos Series (N.W. Spain). The sediments displaying the mounds accumulated in partially protected tidal flat/ lagoonal environments immediately before and during major transgressions. The mounds are erosional features typically occurring on the tops of sheet sandstones (ca. 50–500 mm thick) some of which may have a storm washover origin. Mound genesis related to periodic emergence and late stage run-off is supported by their intimate association with mudcracks and other very shallow water features (e.g. bidirectional current-formed structures, wave ripples, ladder and interference patterns, mudflakes, etc.). Variation in mound morphology suggests that post-depositional dissection began as elongate ridge-gully couplets with secondary erosion of the ridge flanks leading to the development of more characteristic hemispherical geometries. Emergence may have been a function of tidal fluctuations and/or subsidence of storm surge events. Facies sequences point to the repeated filling of these inshore environments by storm washovers superimposed on ambient tidal conditions which possibly resulted from the progressive decay of beach barriers during transgression.  相似文献   

Variable Impact of the Subducted Slab on Aleutian Island Arc Magma Sources: Evidence from Sr, Nd, Pb, and Hf Isotopes and Trace Element Abundances   总被引：5，自引：0，他引：5  

JICHA  B. R.; SINGER  B. S.; BROPHY  J. G.; FOURNELLE  J. H.; JOHNSON  C. M.; BEARD  B. L.; LAPEN  T. J.; MAHLEN  N. J. 《Journal of Petrology》2004,45(9):1845-1875

Major and trace element compositions and Sr, Nd, Pb, and Hfisotope ratios of Aleutian island arc lavas from Kanaga, Roundhead,Seguam, and Shishaldin volcanoes provide constraints on thecomposition and origin of the material transferred from thesubducted slab to the mantle wedge. ⁴⁰Ar/³⁹Ar dating indicatesthat the lavas erupted mainly during the last  相似文献   

The Mineralogy and Petrology of Mount Suswa, Kenya   总被引：1，自引：0，他引：1  

NASH  W. P.; CARMICHAEL  I. S. E.; JOHNSON  R. W. 《Journal of Petrology》1969,10(3):409-439

Mount Suswa, a Quaternary volcano in the Rift Valley of Kenya,is composed of sodalite-trachytes, sodalite-phonolites, andphonolites, the majority of which are mildly peralkaline. Thelavas are predominantly feldspathic with phenocrysts of alkalifeldspar and, less commonly, slightly sodic augite and fayaliticolivine. The groundmass typically contains alkali feldspar,augite, titanomagnetite, and may in addition contain sodalite,nepheline, alkali amphibole, aenigmatite, and glass. The lavasof four stratigraphically distinct episodes can be distinguishedon the basis of mineralogy and chemical composition. These lavasare the products of at least three parental magmas, none ofwhich appears to be a derivative of the other. Each ‘magmatype’ represents an independent episode of magma generation,emplacement, and eruption. The order of eruption in the finalepisode corresponds to increasing peralkalinity and undersaturationwith respect to silica, and indicates that these lavas weregenerated via the tapping of a differentiating magma, with thefirst lavas being the least differentiated. Utilizing coexistingfeldspar, residual glass, and bulk rock compositions, the derivationof peralkaline phonolitic residual liquids from a trachyticparent is shown to be a process controlled by feldspar fractionation.  相似文献