Field Relations (field + relation)

Distribution by Scientific Domains


Selected Abstracts


High-Si phengite, mineral chemistry and P,T evolution of ultra-high-pressure eclogites and calc-silicates from the Dabie Shan, eastern China

GEOLOGICAL JOURNAL, Issue 3-4 2000
Robert Schmid
Abstract A suite of coesite,eclogites and associated calc-silicate rocks from the ultra-high-pressure (UHP) belt in the Dabie Shan (eastern China) was investigated petrologically. Field relations and the presence of UHP minerals such as coesite, omphacite and high-Si phengite in the eclogites and the enclosing calc-silicates testify to a common metamorphic evolution for these two lithologies. Except for one sample, all bear phengite with unusually high silica contents (Si up to 3.7 per formula unit). Phengite occupies various textural positions indicating that different metamorphic stages are reflected by these white micas, which correlate with distinct mineral zonation patterns. Using the latest thermobarometric calibrations for eclogite-facies rocks, maximum pressure,temperature (P,T) conditions of 40,48 kbar at <,750°C were estimated for the peak-metamorphic mineral assemblages. These P,T conditions were calculated for both eclogitic garnet porphyroblasts with diffusion-controlled zoning as well as garnet porphyroblasts with prograde growth zonation patterns. Most samples were affected by a strong retrograde overprint mainly under eclogite- and amphibolite-facies conditions. Thermobarometry using mineral sets from different textural positions reveals cooling and decompression of the UHP rocks down to <,20 kbar at <,600°C for the bulk of the samples. Decompression and heating indicated by a few samples is interpreted to result from mineral chemical disequilibrium or late thermal influence. These new data show that subduction of continental crust in the Dabie Shan was deeper than previously thought, and also that some cooling and decompression took place at upper-mantle depths. Copyright © 2000 John Wiley & Sons, Ltd. [source]


Late Cretaceous blueschist facies metamorphism in southern Thrace (Turkey) and its geodynamic implications

JOURNAL OF METAMORPHIC GEOLOGY, Issue 9 2008
G. TOPUZ
Abstract A blueschist facies tectonic sliver, 9 km long and 1 km wide, crops out within the Miocene clastic rocks bounded by the strands of the North Anatolian Fault zone in southern Thrace, NW Turkey. Two types of blueschist facies rock assemblages occur in the sliver: (i) A serpentinite body with numerous dykes of incipient blueschist facies metadiabase (ii) a well-foliated and thoroughly recrystallized rock assemblage consisting of blueschist, marble and metachert. Both are partially enveloped by an Upper Eocene wildflysch, which includes olistoliths of serpentinite,metadiabase, Upper Cretaceous and Palaeogene pelagic limestone, Upper Eocene reefal limestone, radiolarian chert, quartzite and minor greenschist. Field relations in combination with the bore core data suggest that the tectonic sliver forms a positive flower structure within the Miocene clastic rocks in a transpressional strike,slip setting, and represents an uplifted part of the pre-Eocene basement. The blueschists are represented by lawsonite,glaucophane-bearing assemblages equilibrated at 270,310 °C and ,0.8 GPa. The metadiabase dykes in the serpentinite, on the other hand, are represented by pumpellyite,glaucophane,lawsonite-assemblages that most probably equilibrated below 290 °C and at 0.75 GPa. One metadiabase olistolith in the Upper Eocene flysch sequence contains the mineral assemblage epidote + pumpellyite + glaucophane, recording P,T conditions of 290,350 °C and 0.65,0.78 GPa, indicative of slightly lower depths and different thermal setting. Timing of the blueschist facies metamorphism is constrained to c. 86 Ma (Coniacian/Santonian) by Rb,Sr phengite,whole rock and incremental 40Ar,39Ar phengite dating on blueschists. The activity of the strike,slip fault post-dates the blueschist facies metamorphism and exhumation, and is only responsible for the present outcrop pattern and post-Miocene exhumation (,2 km). The high- P/T metamorphic rocks of southern Thrace and the Biga Peninsula are located to the southeast of the Circum Rhodope Belt and indicate Late Cretaceous subduction and accretion under the northern continent, i.e. the Rhodope Massif, enveloped by the Circum Rhodope Belt. The Late Cretaceous is therefore a time of continued accretionary growth of this continental domain. [source]


Hydrothermal alteration of late- to post-tectonic Lyon Mountain Granitic Gneiss, Adirondack Mountains, New York: Origin of quartz,sillimanite segregations, quartz,albite lithologies, and associated Kiruna-type low-Ti Fe-oxide deposits

JOURNAL OF METAMORPHIC GEOLOGY, Issue 1 2002
J. Mclelland
Abstract Quartz,sillimanite segregations, quartz,albite lithologies (Ab95,98), and Kiruna-type low-Ti iron-oxide deposits are associated with late- to post-tectonic (c. 1055 Ma) leucogranites of Lyon Mountain Gneiss (LMG) in the Adirondack Mountains, New York State. Most recent interpretations of these controversial features, which are global in occurrence, favour hydrothermal origins in agreement with results presented here. Field relations document that quartz,sillimanite veins and nodules cut, and therefore post-date, emplacement of host LMG leucogranites. Veins occur in oriented fracture networks, and aligned trains of nodules are interpreted as disrupted early veins. Late dykes of leucogranite cut veins and nodules demonstrating formation prior to terminal magmatism. Veins and nodules consist of sillimanite surrounded by quartz that commonly embays wall-rock feldspar indicating leaching of Na and K from LMG feldspar by acidic hydrothermal fluids. Subsequent, and repeated, ductile flow disrupted earlier veins into nodular fragments but produced little grain shape fabric. Geochemical and petrographic studies of quartz,albite rock indicate that it formed through metasomatic replacement (albitization) of LMG microperthite by sodic hydrothermal fluids that resulted in diagnostic checkerboard albite. Low-Ti iron-oxide ores are commonly associated with the quartz,albite sub-unit, and it is proposed that hydrothermal fluids related to albitization transported Fe as well. The regional extent of sodic alteration suggests large quantities of surface-derived hydrothermal fluids. Fluid inclusion and oxygen isotope data are consistent with high temperature, regionally extensive fluids consisting primarily of evolved surface-derived brines enriched in Na and Cl. Quartz,sillimanite veins and nodules, which are significantly more localised phenomena and require acidic fluids, were most likely formed from local magmatic fluids in the crystallizing carapaces of LMG plutons. [source]


Transition from arc- to post-collision extensional setting revealed by K,Ar dating and petrology: an example from the granitoids of the Eastern Pontide Igneous Terrane, Arakl,-Trabzon, NE Turkey

GEOLOGICAL JOURNAL, Issue 4 2005
Sabah Yilmaz-
Abstract The Eastern Pontide Igneous Terrane (EPIT) includes several Cretaceous to Neogene intrusive rocks, ranging in composition from low-K tholeiitic gabbros through calc-alkaline and high-K calc-alkaline metaluminous granitoids or peraluminous leucogranites to alkaline syenites. Such high diversity in age and composition is also accompanied by a broad spectrum in terms of geodynamics,i.e. from arc through syn-collisional thickening to post-collisional extensional regimes. Shallow-seated porphyritic acidic to intermediate rocks are from oldest to youngest, on the basis of field relations, the Gündo,du altered microgranite, the Bo,al, K-feldspar-megacrystic monzogranite and the Uzuntarla porphyritic granodiorite. These rocks, exposed in the southern part of the Arakl, region, east of Trabzon, Turkey, were studied in terms of their mineralogy and petrography, whole-rock geochemistry and hornblende K,Ar dating. The mineralogical and geochemical data reveal an apparent diversity in incompatible-element enrichment and depletion, for the Bo,al, unit and Uzuntarla unit, respectively. The Bo,al, and Uzuntarla units yield hornblende K,Ar ages ranging from 75.7,±,1.55 to 61.4,±,1.47,Ma and from 42.4,±,0.87 to 41.2,±,0.89,Ma, respectively. The diversity in both mineralogy,geochemistry and hornblende K,Ar ages suggests that the Bo,al, and Uzuntarla units are parts of the Cretaceous arc and Eocene post-collision extensional-related igneous activity, respectively, in the EPIT of northern Turkey. Copyright © 2005 John Wiley & Sons, Ltd. [source]


An efficient method for solving the nonuniqueness problem in acoustic scattering

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 11 2006
A. Mohsen
Abstract The problem of acoustic wave scattering by closed objects via second kind integral equations, is considered. Based on, combined Helmholtz integral equation formulation (CHIEF) method, an efficient method for choosing and utilizing interior field relations is suggested for solving the non- uniqueness problem at the characteristic frequencies. The implementation of the algorithm fully utilizes previous computation and thus significantly reduces the CPU time compared to the usual least-squares treatment. The method is tested for acoustic wave scattering by both acoustically hard and soft spheres. Accurate results compared to the known exact solutions are obtained. Copyright © 2006 John Wiley & Sons, Ltd. [source]