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11.
Whereas the glacial geomorphology of lower and central Wright Valley is reasonably well understood, the upper valley region, including the North and South Forks and the Dais, has received little attention. Our studies suggest that a wet-based glacier overrode the area and deposited what has been mapped elsewhere as the Peleus till. While we did not observe this silt-rich till in the Labyrinth, it occurs on the Dais and in protected areas of the North and South Forks. Most importantly, our findings imply that the Wright Upper Glacier, an outlet glacier of the East Antarctic Ice Sheet, deposited lateral moraines and lateral-moraine segments along the north and south walls of both the North and South Forks following deposition of the Peleus till. We argue that this drift is correlative with Taylor IVa drift in central Taylor Valley and Alpine III drift in central Wright Valley, based on landscape features, surface-boulder weathering, soil development, and stratigraphy. The presence of meltwater channels, outwash, kame moraines, esker-like features, and kames suggests that ice-marginal or supraglacial water may have accompanied this glaciation. 相似文献
12.
Phosphates are present on the surface of the Mio-Pliocene unconformity in the Otway, Port Phillip and Gippsland basins of south-east Australia. The phosphates occur as lenticular lag deposits and include reworked phosphatic intraclasts, vertebrate bone and teeth. In situ phosphatized burrows are also found in sediments of Late Miocene and Early Pliocene age. The phosphatic intraclasts on the unconformity are interpreted as reworked phosphatized burrows derived from latest Miocene sediments (6 to 5 Ma). The phosphatization of these intraclasts is temporally related to the unconformity. The timing of phosphogenesis coincides with a period of transgression across the south-east Australian margin following Late Miocene uplift. This transgression is responsible for initial marine erosion of the underlying Miocene sequence, creation of a period of very slow sedimentation that was favourable to phosphate formation and subsequent deposition of the latest Miocene through to Pliocene sediments. The continental weathering of the uplifted highlands adjacent to the sedimentary basins, global phosphorus enrichment in the Late Miocene oceans and localized upwelling may all have contributed to phosphatization in south-eastern Australia. 相似文献
13.
Petrogenesis and Geodynamic Implications of Late Cenozoic Basalts in North Queensland, Australia: Trace-element and Sr-Nd-Pb Isotope Evidence 总被引：3，自引：0，他引：3
ZHANG MING; STEPHENSON P. J.; O'REILLY SUZANNE Y.; McCULLOCH MALCOLM T.; NORMAN MARC 《Journal of Petrology》2001,42(4):685-719
Radiogenic isotopic (Sr–Nd–Pb) and trace-elementcompositions of late Cenozoic basalts from two discrete geographicalregions in North Queensland, Australia, can be used to identifycontributions from geochemically distinctive mantle source components.The North Queensland basalts have positive 相似文献
14.
40Ar/39Ar Geochronology of the Rajmahal Basalts, India, and their Relationship to the Kerguelen Plateau 总被引：3，自引：0，他引：3
KENT RAY W.; PRINGLE MALCOLM S.; MULLER R. DIETMAR; SAUNDERS ANDREW D.; GHOSE NARESH C. 《Journal of Petrology》2002,43(7):1141-1153
During the mid-Cretaceous, extensive magmatism occurred in theIndian Ocean to form volcanic portions of the southern and centralKerguelen Plateau, Elan Bank and Broken Ridge. Basalt was eruptedalso along the rifted margin of eastern India (Rajmahal). Weinvestigated the ages of these Indian basalts using 40Ar/39Arincremental-heating experiments on whole rocks. Our resultsare consistent with the hypothesis that the lava pile of 相似文献
15.
NIELSEN FRANK M.; CAMPBELL IAN H.; MCCULLOCH MALCOLM; WILSON J. RICHARD 《Journal of Petrology》1996,37(1):171-193
The 230 km2 Proterozoic Bjerkreim—Sokndal layered instrusionhas a monzonoritic bulk composition and comprises a 6 km thick,broadly leuconoritic Layered Series (LS) overlain by unlayeredmangerite and quartz mangerite. In the Bjerkreim lobe the LScomprises six megacyclic units (MCU 0, IA, IB, II, 11 and IV)in a syncline. This lobe is surrounded by migmatitic gneisses;the roof to the instrusion is missing. The mg-number opx, An% and whole-rock initial 87Sr/86Sr isotope ratios (Sr0) displaybroadly parallel trends through MCUs II and III, and into MCUIV, with decreasing mg-number (75–58) and An % (50–40)accompanying increasing Sr0 (0.7050–0.7085). This correlationbreaks down in the upper part of MCU IV and mangerite; the quartzmangerite has Sr0 values of 0.7085. Abrupt reversals occur acrossthe MCU boundaries. The LS crystallized on the floor of a periodically replenishedmagma chamber that was continually assimilating country rockgneisses. Strong compositional zoning of the magma developedas a result of repeated replenishments with relatively dense,primitive ferrobasaltic magma (Sr0 = 0.7049) along the floorand the development of extensive buoyant roof melts. Assimilationtook place on a massive scale (up to 50%) in the upper partsof the chamber. KEY WORDS: assimilation; fractional crystallization; hybrid magma; layered intrusion; Sr-isolopes
*Corresponding author. 相似文献
16.
The Stability of Igneous Anhydrite: Experimental Results and Implications for Sulfur Behavior in the 1982 El Chichon Trachyandesite and Other Evolved Magmas 总被引：11，自引：1，他引：11
Hydrothermal experiments on natural samples of trachyandesiteand dacite bulk composition show that anhydrite (CaSO4) mayoccur as a stable phenocryst phase at oxygen fugacities greaterthan or equal to 1.0 to 1.5 log fO2 units above the Ni–NiOequilibrium. The dissolved sulfur concentration in anhydritesaturated melts from MnO–Mn3O4 buffered experiments decreaseswith decreasing temperature, from approximately 2300 p.p.m.Sat 1025?C to 250 p.p.m.S at 850?C (all at 2 kb Pfluid = Ptotal).In FeS-saturated melts equilibrated at the Ni–NiO bufferand 2 kb pressure, the concentration of dissolved sulfur alsodecreases with decreasing temperature, varying from approximately400 p.p.m. S at 1025?C to less than 100 p.p.m. S at 850?C. AtNNO or lowerfO2s, decreasing melt FeO content due to crystalfractionation may explain the observed decrease in sulfur solubilitywith decreasing temperature. Sulfur solubility values equivalent to the approximately 0.6wt. per cent S present in fresh bulk pumice samples from the1982 eruptions of El Chichon volcano are not readily achievedunder any reasonable combinations of pressure, temperature,and oxidation state. Dissolved sulfur contents approaching 0.6wt. per cent might occur if the source regions of melts parentalto the El Chichon trachyandesite were at an fO2 several logunits above the Ni–NiO equilibrium. Because such elevatedoxidation states are far greater than the generally acceptedvalues for mantle-derived partial melts we believe the highsulfur content of the El Chichon pumices is not a primary feature;it reflects reaction with sulfur enriched material at some unknowndepth beneath the volcano. Published sulfur isotopic and petrologicdata suggest that hydrothermally altered rocks similar to thepyrite- and anydritebearing lithic fragments found in the 1982pumices could have provided a source of sulfur for crystallizationof magmatic anhydrite. The anhydrite was an important sourceof sulfur for evolution of a sulfur-rich vapor phase duringeruption of the magma. Although many calc-alkaline dacites and rhyolites appear toattain oxidation states high enough to stabilize anhydrite,the characteristically low bulk sulfur contents of these rockswill limit anhydrite abundances to less than approximately 0.1wt. per cent, assuming sufficient sulfur is present to achievesaturation. Such small amounts of a water soluble mineral couldbe easily removed by subaerial weathering processes, dissolvedduring vapor exsolution from a magma, or simply overlooked duringroutine petrographic examination. 相似文献
17.
Experimentally Determined Conditions in the Fish Canyon Tuff, Colorado, Magma Chamber 总被引：8，自引：7，他引：8
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, fO2, fH2O, 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 fO2 was -12.4 (intermediatebetween the Ni-NiO and MnO-Mn3O4 oxygen buffers) in the magmachamber. This fO2.102 is supported by the experimentally determinedvariations in hornblende and melt Mg-numbers as functions offO2 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. XH2O 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 differentXH2O and Ptotals. 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, XH2O=0.5,and log fo2=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
18.
MALCOLM W. WALLACE 《Sedimentology》1990,37(1):105-122
A thick, areally extensive subsurface sequence of Upper Devonian carbonates occurs on the Barbwire Terrace in the Canning Basin of Western Australia. It is a platform sequence in which most of the shallow water lithologies have been thoroughly dolomitized. Slightly deeper water marls have remained as limestones. The major, regional dolomite type in the sequence is not restricted to peritidal lithologies and forms large thicknesses of dolomite (up to 600 m) with no primary calcite. A small volume of evaporitic, supratidal dolomite is present at one location. This dolomite is derived from highly saline fluids developed in an arid supratidal environment. Replacement dolomite of the regional dolomite type has a xenotopic form, with undulose extinction, and irregular crystal boundaries. In addition, saddle dolomite cements appear to have precipitated contemporaneously with the major phase of replacement dolomite. This suggests the regional dolomite type was precipitated at slightly elevated temperatures. Dolomitized stylolites and cements appear to indicate that dolomitization occurred after cementation and pressure solution. Geochemically, the synsedimentary supratidal and regional dolomite types are quite distinctive. Supratidal dolomites have δ18O values which are significantly higher (δ18O=?2 to +1‰ (PDB)) than the regional dolomite type (δ18O=?9 to ?2‰ (PDB)). Assuming the lowest δ18O values for the sabkha dolomite represent replacement in marine waters, the oxygen isotopic composition for Upper Devonian Canning Basin marine dolomite would be around δ18O=?2‰ (PDB). The petrographic and geochemical characteristics of the regional dolomite type support a burial diagenetic origin. However, sources of magnesium in current burial dolomitization models appear insufficient to account for the large volume of dolomite on the Barbwire Terrace. Therefore, it is suggested that dolomitization may have taken place in a near-surface environment with a major recrystallization event superimposed during burial diagenesis. 相似文献
19.
Petrogenesis of Mafic to Felsic Plutonic Rock Associations: the Calc-alkaline Querigut Complex, French Pyrenees 总被引：1，自引：0，他引：1
ROBERTS MALCOLM P.; PIN CHRISTIAN; CLEMENS JOHN D.; PAQUETTE JEAN-LOUIS 《Journal of Petrology》2000,41(6):809-844
The Quérigut mafic–felsic rock association comprisestwo main magma series. The first is felsic comprising a granodiorite–tonalite,a monzogranite and a biotite granite. The second is intermediateto ultramafic, forming small diorite and gabbro intrusions associatedwith hornblendites and olivine hornblendites. A U–Pb zirconage of 307 ± 2 Ma was obtained from the granodiorite–tonalites.Contact metamorphic minerals in the thermal aureole providea maximum emplacement pressure of between 260 and 270 MPa. Petrographiccharacteristics of the mafic and ultramafic rocks suggest crystallizationat <300 MPa, demonstrating that mantle-derived magmas ascendedto shallow levels in the Pyrenean crust during Variscan times.The ultramafic rocks are the most isotopically primitive components,with textural and geochemical features of cumulates from hydrousbasaltic magmas. None of the mafic to ultramafic rocks havedepleted mantle isotope signatures, indicating crustal contaminationor derivation from enriched mantle. Origins for the dioritesinclude accumulation from granodiorite–tonalite magma,derivatives from mafic magmas, or hybrids. The granitic rockswere formed from broadly Proterozoic meta-igneous crustal protoliths.The isotopic signatures, mineralogy and geochemistry of thegranodiorite–tonalites and monzogranites suggest crystallizationfrom different magmas with similar time-integrated Rb/Sr andSm/Nd isotope ratios, or that the granodiorite–tonalitesare cumulates from a granodioritic to monzogranitic parent.The biotite granite differs from the other felsic rocks, representinga separate magma batch. Ages for Quérigut and other Pyreneangranitoids show that post-collisional wrenching in this partof the Variscides was under way by 310 Ma. KEY WORDS: Variscan orogeny; Pyrenees; Quérigut complex; epizonal magmatism; post-thickening; mafic–felsic association 相似文献
20.