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KERR ANDREW C.; TARNEY JOHN; KEMPTON PAMELA D.; PRINGLE MALCOLM; NIVIA ALVARO 《Journal of Petrology》2004,45(9):1877-1906
The fault-bounded Bolívar Ultramafic Complex (BUC) onthe eastern fringes of the Western Cordillera of Colombia wastectonically accreted onto the western coast of South Americain the late Cretaceous–early Tertiary, along with pillowbasalts of the Caribbean–Colombian Oceanic Plateau (CCOP).The complex consists of a lower sequence of ultramafic cumulates,successively overlain by layered and isotropic gabbroic rocks.The gabbros grade into, and are intruded by, mafic pegmatitesthat consist of large magnesiohornblende and plagioclase crystals.These pegmatites yield a weighted mean 40Ar–39Ar step-heatingage of 90·5 ± 0·9 Ma and thus coincidewith the timing of peak CCOP volcanism. The chemistry of theBUC is not consistent with a subduction-related origin. However,the similarity in Sr–Nd–Pb–Hf isotopes betweenthe CCOP and the BUC, in conjunction with their indistinguishableages, suggests that the BUC is an integral part of the plume-derivedCCOP. The parental magmas of the Bolívar complex wereprobably hydrous picrites that underwent 20–30% crystallization.The residual magmas from this fractionation contained 相似文献
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Experiments in the system KAlSi04-NaAlSi04-SiOj-Fe-0-H indicatethat approximately 20 mole per cent of sodium annite [NaFe3AlSiO10(OH)2]can enter into solid solution with potassium annite. This substitutionreduces the biotite stability field. With excess alkali feldsparand no free quartz, the following reactions limit the biotitestability: Feldspar(s)+biotite feldspar(s)+magnetite+vapor. (1) Feldspar+biotite feldspar+fayalite+leucite+vapor. (2) Feldspar+biotite feldspar+fayalite+liquid+vapor. (3) Biotite feldspar+fayalite+nepheline+vapor. (4) On the QFM buffer (PTotal = 2 Kb), reaction 1 occurs at 685°C in the sodium free system and at 625 °C when twofeldspars are present; on NNO, it occurs at 630 and 590 °C.On the G-CH buffer, reaction 2 is stable at high temperatures(795–830 °Q and potassic compositions (), reaction3 at intermediate conditions (T = 740–95 °C; ) andreaction 4 at lower temperatures (710–40 °C) and sodiccompositions (). In the presence of excess quartz, reaction1 remains stable below the QFM buffer, but the reaction Feldspars)+biotite+quartz feldspars)+fayalite+vapor (5) is stable at higher hydrogenfugacities. On the G-CH buffer, reaction 5 occurs at 610 °Cin the sodium free system and at 595 °C when two alkalifeldspars are present. On the MW buffer, the temperatures are585 and 515 °C (M.I.). The experimental data presented suggest that biotite will notbe stable in the presence of granitic liquids at total pressuresbelow 4 kb, providing the fluorine and titanium content of thebiotite is low. They also suggest that gradients in the a£1S,l0 in a rock could produce variations in the biotite Fe/Fe+Mgratio and in extreme cases could result in the complete breakdown(decreased ) of biotite or its crystallization (increased )where previously there was none. 相似文献
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MALCOLM WHITWORTH WILLIAM MURPHY DAVID GLLES DAVID PETLEY 《The Geographical journal》2000,166(2):139-155
The Broadway area of the Cotswolds has been extensively investigated in an attempt to delineate areas of slope instability, including'relict'landslides, which may have been initiated under periglacial climatic conditions. It is, therefore, useful both in terms of the geomorphology and of the understanding of the evolution of the area, to investigate the rates and timing of any reactivation and subsequent movements of landslides in this area. The remains of ridge and furrow cultivation can be seen extensively throughout the Cotswolds. This particular agricultural practice, which documentary evidence suggests dates back to Anglo-Saxon times (approximately 10th century AD), has been disturbed in many places by slope movements. A detailed study of Parish Records and other local his-torical sources has revealed that ridge and furrow cultivation ceased on Parliamentary Enclosure, which in this area, occurred in 1771. Therefore, it is possible to identify patterms of slope movement between these dates. Landslides have been identified by ground mapping and aerial photography, and can be divided into three categories. Firstly, active landslides which are those which have shown evidence of movement during the study period. Secondly, suspended landslides, which show evidence of movement, but have not been observed to move during the study period. In this context, this has been taken to mean landslides, which have shown evidence of movement since 1771. Thirdly, relict landslides which have shown no evidence of movement since 1771 and thus were probably active under a different climatic regime. By incorporating historical data with the geomorphological survey it has been possible to identify areas of potentially difficult ground for engineering geomorphological purposes. 相似文献
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MALCOLM B. HART 《Geology Today》1992,8(4):137-141
Micropalaeontology and biostratigraphy have been, and still are, a vital part of the Channel Tunnel Project. The use of foraminiferal assemblages and the determination of an accurate biozonation have given the tunnel builders a comprehensive view of the geological succession below the English Channel between Dover and Calais. This work, which was begun in 1958, has continued intermittently to the present day. 相似文献
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The experimental work on biotites has primarily involved compositionsalong the annite-phlogopite join, but most natural biotitescontain significantly larger amounts of aluminum. At the sametime, the aluminum content of natural biotites varies considerably.The available evidence indicates that these variations in thealuminum content of biotite depend on the conditions of formationand the whole rock chemistry. Experiments on the phase relations of aluminous iron biotitesin the silica deficient system KAlSiO4-KAlSi3O8-Al2O3-Fe-O-H(pfluid = 2 kb) indicate that compositions up to Ann75 can besynthesized on the join annite [K2Fe6Al2Si6O20(OH)4]-aluminumbiotite [K2Al6Al2Al6O20(OH)4]. The aluminous biotites are stableto higher temperatures than annite. An isobaric divariant equilibrium,Bioss-Mtss-Sa-Lc-V, extends to higher oxygen fugacities fromthe Ann-Mt-Sa-Lc-V curve of Eugster & Wones (1962). Compositioncontours on this surface indicate that both the magnetite andbiotite become more aluminous with increasing temperature and/oroxygen fugacity. The Bioss-Mtss-Sa-Lc-V reaction surface isterminated by equilibria involving the additional phases muscovite,corundum, and hercynite respectively as the conditions becomemore reducing. At 2 kb fluid pressure; aluminum-rich iron biotiteis stable to 555 °C on the HM buffer, 763 °C on theMt-Hc-Cor buffer, 820 °C on NNO, and about 860 °C onQFM. The data obtained can be applied to a number of biotitesyenites and appears to explain why iron-rich aluminum biotitesoccur in these rocks. 相似文献
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In a system open to both potassium and water, the conditionsfor two different chloritebiotite equilibrium reactions maybe depicted on an isobaric temperature–(K)/(H) diagram.At moderate values of the (K)/(H) ratio in the fluid phasepervading the system, muscovite is involved in the equilibriumreaction: chloritequartzKOH= biotitemuscoviteH2O. At high values of (K)/(H) K-feldspar is involved in the equilibriumreaction: chlorite quartzKOH = biotiteK-feldsparH2O. When the same system is closed to potassium, assemblages formingat lower values of (K/(H) resemble those developed on progressivemetamorphism, whereas higher (K)/ (H assemblages in the closedsystem resemble certain retrogressive mineral assemblages. Theassemblage muscovitebiotite is characteristic of higher temperaturesand lowyer values of µK/µS than the assemblageK-feldsparchlorite in carbonate-free rocks where both potassiumand water were perfectly mobile components and SiO2 was presentin excess. 相似文献
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Magmatic Conditions and Magma Ascent as Indicated by Hornblende Phase Equilibria and Reactions in the 1995-2002 Soufriere Hills Magma 总被引：3，自引：0，他引：3
A study of amphiboles and associated minerals in samples ofSoufrière Hills andesite erupted from 1995 to 2002 showssignificant compositional variations within hornblende phenocrysts,a separate set of small pargasitic crystals in the groundmass,and two types of reaction rims on the phenocrysts. The compositionof the amphiboles and coexisting phases defines the thermalhistory of the erupting magma. As many as seven zones (<200µm wide) in the hornblende phenocrysts begin with a sharpincrease in Mg and Si, and then change gradually to a more Fe-and Al-rich hornblende, a transition that is consistent witha temperature rise. Analyses of the hornblende phenocrysts andassociated Fe–Ti oxides verify previous conclusions thatthe pre-eruption magma was at 130 MPa and 830 ± 10°C,but was variably heated before eruption. The heating occurredwithin 相似文献
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