摘要:

ABSTRACTPorphyroblast inclusion fabrics are consistent in style and geometry across three Proterozoic metamorphic field gradients, comprising two pluton‐related gradients in central Arizona and one regional gradient in northern New Mexico. Garnet crystals contain curved ‘sigmoidal’ inclusion trails. In low‐grade chlorite schists, these trails can be correlated directly with matrix crenulations of an older schistosity (S1). The garnet crystals preferentially grew in crenulation hinges, but some late crenulations nucleated on existing garnet porphyroblasts. At higher grade, biotite, staurolite and andalusite porphyroblasts occur in a homogeneous S2 foliation primarily defined by matrix biotite and ilmenite. Biotite porphyroblasts have straight to sigmoidal inclusion trails that also represent the weakly folded S1 schistosity. Staurolite and andalusite contain distinctive inclusion‐rich and inclusion‐poor domains that represent a relict S2 differentiated crenulation cleavage. Together, the inclusion relationships document the progressive development of the S2 fabric through six stages. Garnet and biotite porphyroblasts contain stage 2 or 3 crenulations; staurolite and andalusite generally contain stage 4 crenulations, and the matrix typically contains a homogeneous stage 6 cleavage.The similarity of inclusion relationships across spatially and temporally distinct metamorphic field gradients of widely differing scales suggests a fundamental link between metamorphism and deformation. Three end‐member relationships may be involved: (1) tectonic linkages, where similarP‐T‐time histories and similar bulk compositions combine to produce similar metamorphic and structural signatures; (2) deformation‐controlled linkages, where certain microstructures, particularly crenulation hinges, are favourable environments for the nucleation and/or growth of porphyroblasts; and (3) reaction‐controlled linkages, where metamorphic reactions, particularly dehydration reactions, are associated with an increase in the rate of fabric development. A general model is proposed in which (1) garnet and biotite porphyroblasts preferentially grow in stage 2 or 3 crenulation hinges, and (2) chlorite‐consuming metamorphic reactions lead to pulses in the rate of fabric evolution. The data suggest that fabric development and porphyroblast growth may have been quite rapid, of the order of several hundreds of thousands of years, in these rocks. These microstructures and processes may be characteristic of low‐pressure, firs

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00001.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe assemblages chlorite‐pumpellyite‐lawsonite‐albite‐quartz, chlorite‐lawsonite‐quartz‐epidote and chlorite‐epidote‐albite‐quartz occur in metabasaltic blocks and veins in a metamorphosed tectonic mélange in the structurally highest unit of the autochthonous and parautochthonous section underlying the Semail ophiolite in Saih Hatat, north‐east Oman. The pre‐Permian basement of this section contains mafic units characterized by the assemblage crossite‐epidote‐chlorite‐quartz‐albite /el actinolite. These assemblages indicate a down‐section increase in metamorphic grade from ‘lawsonite‐albite facies' conditions in the mélange to ‘epidote‐blueschist’ facies conditions in the basement.Application of empirically and experimentally based thermobarometers as well as petrogenetic grids calculated for a model basaltic system indicates that theP‐Tconditions of metamorphism ranged from 3 to 6 kbar and 250 to 300d̀ C for the mélange andP>6.8 kbar,T>310d̀ C for the basement units. Textural relations interpreted in the context of petrogenetic grids indicate that these units followed clockwiseP‐Tpaths of evolution. The estimatedP‐Tconditions and down‐section increase in metamorphic grade in central, western and northern Saih Hatat are consistent with the hypothesis relating metamorphism to the Late Cretaceous tectonic loading of the continental margin by an ophiolite slab<18 km in thickness. These results contrast with field and petrological observations documented for blueschists and eclogites exposed along the eastern coast of Saih Hatat which may have formed at an earlier st

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00002.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTJadeitites from Guatemala are found as weathered blocks in tectonized serpentinite in a 15‐km zone north of the Motagua Fault Zone. Rock types found with jadeitite include albitites, albite‐mica rocks, omphacite/taramitic amphibole‐bearing metabasites, chlorite‐actinolite schists, talc‐carbonate rocks and antigorite schists. In addition to the predominant jadeitic (Jd93_100) pyroxene, common phases in jadeitite include micas (paragonite and/or phengite ± rarer phlogopite), omphacite, albite, titanite /Pm zircon, apatite and graphite. Conditions of jadeitite formation are 100‐400d̀ C, 5‐11 kbar with 0.0>log10asio2≥= 0.7. Fluid inclusions, coarse textures, vein structures, and rhythmic zoning of pyroxene indicate an aqueuos fluid was involved. Jadeitites are either (1) metasomatic modifications of former felsic‐to‐pelitic inclusions that have undergone silica depletion plus efficient soda exchange and enrichment, or (2) solution precipitations derived from such a source. The close spatial relationship of faults and shear zones, serpentinites, and jadeitites suggests jadeitites form in a relatively high‐P/Tsetting with substantial flow of sodic fluid in a tectonized zone.Most Guatemalan jadeitites are extensively altered to analcime, albite, taramitic amphibole, (clino)zoisite ± nepheline and preiswerkite. This alteration reflects depressurization /Pm heating to below the jadeite + fluid = analcime reaction at highaNa. With progressive alteration, analcime and nepheline are replaced by albite; the increase in silica content may result from fluid flowing up a tectonized zone reaching saturation with an albite assemblage. Albitite phases, albite, actinolite, zoisite, /Pm chlorite, phengite, K‐feldspar and quartz, record conditions ofc. 3‐8 kbar atT<400d̀ C, indicating a clockwiseP‐Ttrajectory of the blocks.Barium aluminosilicates—banalsite, celsian, cymrite and hyalophane—are common minor late‐stage phases in jadeitites and albite‐rich rocks. Barian

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00003.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThis paper describes the progressive metamorphism and deformation of a series of metasediments, Le Conquet Schists and their higher grade equivalents, which occur as tectonically emplaced screens within a sequence of foliated gneisses, the Gneiss de Brest and Gneiss de Lesneven. The sequence exhibits a steep south to north increase in metamorphic grade from garnet‐staurolite schist to sillimanite gneiss and sillimanite‐K‐feldspar migmatite. The relationship of mineral growth to foliation development has been established for individual screens. At least five phases of deformation (D1‐D5) are preserved. Analysis of porphyroblast inclusion trails is used to demonstrate sequential mineral growth during the successive development of orthogonal foliations S1‐S4. Porphyroblasts continued to grow during the subsequent development of C‐S mylonite fabrics and extensional crenulation cleavages which are genetically related to a series of high‐strain zones (D5). Mineral assemblages, phase relations and mineral chemistry are consistent with porphyroblast growth being the result of continuous reactions. Microstructure‐porphyroblast relations are used to show that although mineral growth proceeded during continuous reactions, these only operated episodically. Phase relations, mineral chemistry andP‐Testimates are used to constrainP‐Ttrajectories and these are linked to the deformation histories within individual screens. A comparison between the resulting pressure‐temperature‐deformation paths is used to demonstrate that the metamorphic peak occurred progressively later and at successively lower pressures with increasing metamorphic grade. It is suggested that the early evolution of the belt is the result of crustal thickening by overthrusting. The subsequent history is one of progressive heating and unroofing of the higher grade rocks in a dextral strike‐slip

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00004.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe intracrystalline diffusion rate of oxygen in diopside was constrained based on natural isotopic variations from a granulite facies marble from Cascade Slide, Adirondacks (New York, USA). The oxygen isotope compositions of the diopsides, measured as a function of grain size, are nearly constant (20.9 ± 0.3‰ vs. SMOW) over the entire measured size range (0.3–3.2 mm diameter). The δ18O values of the cores of calcite grains are 23.0‰. Temperature estimates based on the Δ18O(calcite‐diopside) are 800d̀C, in agreement with the highest previous thermometric estimates for these rocks.The lack of isotopic variation in the diopsides as a function of grain size requires that the oxygen intracrystalline diffusion rate in diopside from the Adirondack samples was very slow. The maximum diffusion rates (D800d̀Cparallel to thec‐axis) were calculated with an infinite reservoir model (IRM) and a finite reservoir model (FRM) that incorporates mineral modal abundances and initial isotopic variations. For an assumed activation energy (Q) = 100 kJ/mol, the IRM diffusion rate estimate of 1.6 times 10‐20cm2/s is two orders of magnitude faster than from the FRM; atQ=500kJ/mol, theD800d̀Cestimate for both methods is c. 5.6 times 10‐20cm2/s. The present results require that a hydrothermal fluid significantly enhances the diffusion rate of oxygen in diopside if previous data are correct.The δ18O(SMOW) and δ13C(PDB) values of the calcite, measuredin situwith a CO2laser, are 22.9 ± 0.3, 0.1±0.3‰ in the grain cores, 22.1 ±0.3, 0.2 ±0.1‰ at the grain boundaries and 21.7 ±0.4, ‐0.6±0.1‰ abutting diopside grains. The δ18O and δ13δC values measured conventionally are: crystal cores, 22.96, ‐0.95‰; abutting diopside grains, 22.38, ‐0.93‰; bulk, 22.79, ‐0.95%. Use of the bulk δ18O(calcite) values for thermometry yields unreasonably high temperatures. The lower δ18O values at the calcite grain boundaries are not due to retrograde diffusional exchange with the diopside, they are thought to b

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00005.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTA petrogenetic grid is presented for the system KFMASH (K2O‐FeO‐MgO‐Al2O3‐SiO2‐H2O), including biotite, muscovite, K‐feldspar, chlorite, chloritoid, staurolite, cordierite, garnet, orthoamphibole, orthopyroxene, spinel, andalusite, sillimanite, kyanite, quartz and corundum with H2O in excess, which was calculated using the computer program THERMOCALC and the Powell and Holland internally consistent thermodynamic dataset. By removing the normal constraint of having quartz in excess, both quartz‐bearing and quartz‐absent equilibria are shown. Quartz‐absent equilibria are particularly relevant at high‐Tand low‐Pconditions, because of their common occurrence at these conditions. The calculated mineral assemblage and mineral compositional variations in terms of FeMg‐1 and (Fe, Mg)SiAl‐2exchange vectors are broadly compatible with observations on natural rocks, particularly when non‐KFMASH compon

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00006.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00007.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY