Continuous metamorphic gradient documented by graphitization and K-Ar age, southeast Otago, New Zealand
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Yujiro Nishimura
,
Douglas S. Coombs
Abstract
The Chrystalls Beach-Brighton coastal section, southwest of Dunedin, New Zealand, exposes Otago Schist, including the weakly metamorphosed Triassic accretionary melange of the Chrystalls Beach Complex and higher-grade rocks of a largely felsic, arc-derived provenance. Melange zones contain pelagic sediments, some of which are manganiferous, and metabasites. The grade of metamorphism increases progressively from southwest to northeast. Four mineral zones are recognized largely on the basis of mineral assemblages in psammitic and semipelitic rocks: pumpellyite-chlorite, pumpellyite-actinolite, epidote-actinolite, and biotite. d002 data for carbonaceous material, b0 values for phengite, and metamorphic phengite K-Ar ages are reported for 32 pelitic and semipelitic samples. Remarkably tight correlations are revealed between d002 for progressively graphitized carbonaceous material, K-Ar ages that range from 184.6 to 138.7 Ma, and linear distance along the section. Both the d002 values and the K-Ar ages decrease progressively toward the northeast through the four mineral zones and with progressive textural changes. All rocks cropping out in the section have undergone a coherent episode of progressive metamorphism associated with terrane collision and have not been disturbed by any later major displacements. A convex trend for phengite b0 values plotted against advancing graphitization suggests P/T conditions during metamorphism as in the high-pres sure intermediate facies series. The age of the metamorphic peak is inferred to be 175-155 Ma (Middle to Late Jurassic), with cooling to the mica closure temperature at 155-135 Ma (Late Jurassic to Early Cretaceous). Unloading, denudation, and cooling were accompanied by localized hydrothermal events. The oldest covering strata were deposited at ca. 100 Ma and denudation was completed by ca. 75-80 Ma. Middle Triassic radiolarian nodules in melange zones are dated as preceding the metamorphic peak by about 65 Ma. Associated turbidites contain suspected late Middle to early Late Triassic tube fossils
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- Patterns of mineral occurrence in metamorphic rocks
- A double magmatic heat pump at the core-mantle boundary
- High-temperature contact metamorphism of calc-silicate xenoliths in the Kiglapait Intrusion, Labrador
- Multiple pyroxene and amphibole assemblages in the amphibolite facies: Bulk compositional controls
- Margarite-corundum phyllites from the Appalachian orogen of South Carolina: Mineralogy and metamorphic history
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- Kyanite eclogite thermobarometry and evidence for thrusting of UHP over HP metamorphic rocks, Nordøyane, Western Gneiss Region, Norway
- Monazite geochronology of UHP and HP metamorphism, deformation, and exhumation, Nordøyane, Western Gneiss Region, Norway
- Solubilities of noble metals in Fe-containing silicate melts as derived from experiments in Fe-free systems
- Water speciation in silicate glasses and melts: Langmuir limited site model
- The ab initio study of the stability of low temperature Al/Si ordered albite, NaAlSi3O8
- The heat capacity of MgCr2O4, FeCr2O4, and Cr2O3 at low temperatures and derived thermodynamic properties
- Thermodynamics of the R3̅ to R3̅c phase transition in the ilmenite-hematite solid solution
- An ab initio study of hydrogarnets
- Short-range order in synthetic aluminous tremolites: An infrared and triple-quantum MAS NMR study
- In-situ Raman spectroscopy of quartz: A pressure sensor for hydrothermal diamond-anvil cell experiments at elevated temperatures
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- Reactivity of surface sites on fractured arsenopyrite (FeAsS) toward oxygen
- Surface area and porosity of primary silicate minerals
- Charge contrast imaging of geological materials in the environmental scanning electron microscope
- Chemical fine structure of Franciscan jadeitic pyroxene from Ward Creek, Cazadero area, California
- Crystal structure of CaMg2Al6O12, a new Al-rich high pressure form
- The crystal structure of “Tetragonal Almandine-Pyrope Phase” (TAPP): A reexamination
- The crystal structure of tetranatrolite from Mont Saint-Hilaire, Québec, and its chemical and structural relationship to paranatrolite and gonnardite
- Elyite, Pb4Cu(SO4)O2(OH)4·H2O: Crystal structure and new data
- Dukeite, Bi3+24Cr86+O57(OH)6(H2O)3, a new mineral from Brejaúba, Minas Gerais, Brazil: Description and crystal structure
- Pseudosinhalite is a structural isotype of chondrodite
- Structure type and bulk modulus of Fe3S, a new iron-sulfur compound
- Single crystal elasticity of lawsonite
- In situ measurement of viscosity of liquids in the Fe-FeS system at high pressures and temperatures
