Magmatism of the British Tertiary Volcanic Province
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This review concentrates on those aspects of research into the magmatism of the British Tertiary Volcanic Province (BTVP) which have seen rapid accumulation of data and changes of views during the last few years. There seems to be general agreement at present amongst those who have made detailed geochemical studies of the problem that the pervasive hydrothermal metamorphism which affects most BTVP igneous rocks has not caused significant changes in the compositions (other than hydrogen and oxygen isotopes) of samples collected away from the immediate vicinities of the major passageways for hydrothermal fluids. Wide ranges of silica saturation characterize the basic members of both the major traditional BTVP magma types—Plateau and Non-Porphyritic Central (NPC). It is only the evolved members of these suites, the ne-normative Plateau-type hawaiites and the Q-normative NPC tholeiitic andesites, which show clearly the long-emphasized contrast between alkalic and tholeiitic compositions. Prior to their final uprise and eruption or emplacement, the Plateau-type magmas appear to have equilibrated near the base of the continental crust, whereas the NPC magmas evolved at upper-crustal depths. The NPC basalts (as traditionally defined) seem to be polygenetic in terms of mantle-derived magma types. Most of them are related to the MORB(Mid Ocean Ridge Basalt)-like Preshal Mhor basalts of Skye and Mull but some appear to be batches of Plateau-type magma which equilibrated in the upper crust. The Rio Grande Rift, southwestern U.S.A., provides a present-day analogue to the Palaeocene magmatic plumbing envisaged beneath the BTVP.
The mantle-derived basic magmas upwelling beneath the BTVP were affected by fractional crystallisation and sialic contamination to varying extents as they rose through the continental crust. The earlier magma batches mostly interacted only with granulite-facies Lewisian gneisses, from which they appear to have melted selectively the most-fusible minor acid members. In addition, there is little doubt that Pb equilibrated selectively, in a vapour or non-silicate liquid phase, between the magmas and Archean sial, whilst Sr- and Nd-isotope ratios in the BTVP igneous rocks may also be in part the products of selective magma-crust interactions. If those BTVP basic rocks which contain negligible components of sialic contaminant are considered, it is apparent that there are consistent differences in various incompatible-element ratios between the Plateau-type basalts of Skye and Mull, the two areas studied in most detail at present. All of the BTVP Plateau-type basalts are exceptionally depleted in the strongly-incompatible elements, relative to worldwide examples of basalts with similar major-element compositions. This feature is consistent with genesis from a volume of upper mantle which had previously lost a very small fraction of strongly-alkalic melt. The Permian lamprophyres of the region are tentatively identified as samples of this postulated pre-Tertiary alkalic magma. A case is also made for considering the Caledonian Iapetus Suture as the tectonic feature along which most of the BTVP igneous centres are distributed. Seen from this point of view, the origins of BTVP magmatism are to be found in upper-mantle convection during and after the subduction which closed Iapetus—subsequently re-activated by the regional tension and Palaeocene mantle convection associated with the opening of the North Atlantic. At the climax of BTVP magmatism, when the bulk of the dyke swarms and the plutons of the central complexes were emplaced, a final increment of the progressive partial fusion of the upper mantle beneath the region gave rise to the MORB-like Preshal Mhor-type magmas.
Although it is apparent from their Sr-, Nd- and Pb-isotope ratios that most of the BTVP acid magmas contained appreciable fractions from sialic sources, there are few instances where mantle-derived magma does not appear to have been the dominant ingredient in the hybrid liquids which evolved—by fractional crystallisation—to acid residua. Nevertheless, when the sequence of granite emplacement within individual BTVP intrusive centres is considered, it is apparent that the evolution of each acid magma batch was a complex and variable process, involving combinations of such mechanisms as fractional crystallisation, fusion of both sialic crust and earlier BTVP acid rocks, and mixing of magmas at various states of evolution.
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Tài liệu tham khảo
AOKI K., KUDO A. M. 1976. Major-element variations of late Cenozoic Basalts of New Mexico. New Mexico Geol. Soc. Spec. Publ. 5, 82–8.
ANDERSON F. W., DUNHAM K. C. 1966. The geology of northern Skye, Mem. Geol. Surv. G.B.
BAILEY E. B., CLOUGH C. T., WRIGHT W. B., RICHEY J. E., WILSON G. V. 1924. Tertiary and post-Tertiary geology of Mull, Loch Aline and Oban. Mem. Geol. Surv. G.B.
BALDRIDGE W. S. 1979. Petrology and petrogenesis of Plio-Pleistocene basaltic rocks from the Central Rio Grande Rift, New Mexico and their relation to rift structure. In , Riecker R. E. (ed.) Rio Grande Rift: tectonics and magmatism, 323–53, Am. Geophys. U.
BOWEN N. L. 1928. The evolution of the igneous rocks, Princeton University Press.
CLAGUE D. A., FREY F. A. 1981. Petrology and trace-element geochemistry of the Honolulu Volcanic Series, Oahu : implications for the oceanic mantle below Hawaii. J. Petrol, (in press).
CLARKE E., BECKINSALE R. D., SKELHORN R. R., THORPE R. S., WALSH J. N. 1980. A basic-to-acid fractional crystallisation sequence from Mull. Mineral. Soc. Bull. 47–8, 2.
DURANT G. P. 1978. The geochemistry of the Tertiary Islay and Jura dyke swarms. J. Geol. Soc 135, 463.
EATON G. P. 1979. A plate tectonic model for Late Cenozoic crustal spreading in the western United States. In , Riecker R. E. (ed.) Rio Grande Rift: tectonics and magmatism, 7–32, Am. Geophys. U.
EMELEUS C. H., DUNHAM A. C., THOMPSON R. N. 1971. Iron-rich pigeonites from acid rocks in the Tertiary Igneous Province of Scotland. Am. Mineral. 56, 940–51.
FLANAGAN F. J. 1978. Descriptions and analyses of eight new USGS rock standards. U.S. Geol. Surv. Prof. Paper 840, 1–192.
FORD C. E., MACDONALD R. 1978. Melting relations of a Midland Valley alkali basalt. NERC Pub. D11, 196–7.
GAMBLE J. A., OLD R. A., PRESTON J. 1976. Subsurface exploration in the Tertiary central complex of gabbro and granophyre at Slieve Gullion, Co. Armagh, Northern Ireland. Rep. Inst. Geol. Sci. No. 76/8.
GASS I. G., THORPE R. S. 1976. Igneous case study: the Tertiary igneous rocks of Skye, NW Scotland. In , Aprahamian F. (ed.), Science: a third level course. Earth science topics and methods, Open University Press.
HALSALL T. J. 1978. The emplacement of the Tertiary dykes of the Kildonan shore, south Arran. J. Geol. Soc. 135, 462.
HARKER A. 1904. The Tertiary igneous rocks of Skye. Mem. Geol. Surv. G. B.
HARRISON R. K., JEANS C. V., MERRIMAN R. J. 1979. Mesozoic igneous rocks, hydrothermal mineralisation and volcanogenic sediments in Britain and adjacent areas. Bull. Geol. Surv. G.B. 70, 57–69.
JOHNSON G. A. L. 1978. European plate movement during the Carboniferous. In , Tarling D. H. (ed.), Evolution of the Earth's crust, 343–60, Academic Press.
KENNEDY W. Q. 1930. The parent magma of the British Tertiary Province. Geol. Surv. G.B. Summ. Prog. II, 61–73.
KING P. 1977. The secondary minerals of the Tertiary lavas of northern and central Skye—zeolite zonation patterns their origin and formation. Univ. Aberdeen Ph.D. thesis (unpubl.).
Le BAS M. J. 1967. On the origin of the Tertiary granophyres of the Carlingford Complex, Ireland. Proc. R. Ir. Acad. 65B, 325–38.
LONG L. E., LAMBERT R. St. J. 1963. Rb-Sr ages from the Moine Series. In , Johnson M. R. W., Stewart F. H. (eds.), The British Caledonides, 217–47, Oliver and Boyd.
LYLE P. 1980. A petrological and geochemical study of the Tertiary basaltic rocks of Northeast Ireland. J. Earth Sci. R. Dubl. Soc. 2, 137–52.
MACDONALD R. 1980. Trace element evidence for mantle heterogeneity beneath the Scottish Midland Valley in the Carboniferous and Permian. Philos. Trans. R. Soc. Lond. A297, 245–57.
MASON B. 1979. Data of geochemistry. Chapter B. Cosmochemistry. Part 1. Meteorites. Prof. Pap. U.S. Geol. Surv. 440-B-1, 1–132.
MATTEY D. P. 1980. The petrology of high-calcium low-alkali tholeiite dykes from the Isle of Skye regional swarm. Univ. London Ph.D. thesis (unpubl.).
MEIGHAN I. G. 1979. The acid rocks of the British Tertiary Province. Bull. Geol. Surv. G.B. 70, 10–22.
MORRISON M. A. 1977. Interactions between water and Palaeocene basalt, Mull, NW Scotland. Abstr. IASPEI/IAVCEI meeting, Durham, 250.
MORRISON M. A. 1980. Igneous and metamorphic geochemistry of Mull lavas. Univ. London Ph.D. thesis (unpubl.).
MORRISON M. A., THOMPSON R. N., GIBSON I. L., MARRINER G. F. 1980. Lateral chemical heterogeneity in the Palaeocene upper mantle beneath the Scottish Hebrides. Philos. Trans. R. Soc. Lond. A297, 229–44.
MURATA K. J., RICHTER D. H. 1966. Chemistry of the lavas. 1959–60 eruption of Kilauea volcano, Hawaii. Prof. Paper U.S. Geol. Surv. 537-A, 1–26.
O'HARA M. J. 1980. Nonlinear nature of the unavoidable long-lived isotopic, trace and major element contamination of a developing magma chamber. Philos. Trans. R. Soc. Lond. A297, 215–27.
PANKHURST R. J., BECKINSALE R. D. 1979. Rare-earth evidence for the petrogenesis of Tertiary Mull volcanics. J. Geol. Soc. 136, 327.
RIDLEY W. I. 1973. The petrology of volcanic rocks from the Small Isles of Inverness-shire. Rep. Inst. Geol. Sci. No. 73/10.
RINEHART E. J., SANFORD A. R., WARD R. M. 1979. Geographic extent and shape of an extensive magma body at midcrustal depths in the Rio Grande Rift near Socorro, New Mexico. In , Riecker R. E. (ed.), Rio Grande Rift: tectonics and magmatism, 237–51, Am. Geophys. U.
SHERATON J. W., SKINNER A. C., TARNEY J. 1973. The geochemistry of the Scourian gneisses of the Assynt district. In , Park R. G., Tarney J. (Eds.), The Early Precambrian of Scotland and related rocks of Greenland, 13–30, Dept. of Geol., Univ. of Keele.
SUN S.-S. 1980. Lead isotopic study of young volcanic rocks from mid-ocean ridges, ocean islands and island arcs. Philos. Trans. R. Soc. Lond. A297, 409–45.
SUTHERLAND D. S. (ed.) 1981. Igneous rocks of the British Isles. Wiley.
TARNEY J., WOOD D. A., SAUNDERS A. D., CANN J. R., VARET J. 1980. Nature of mantle heterogeneity in the North Atlantic: evidence from deep sea drilling. Philos. Trans. R. Soc. Lond. A297, 179–202.
THOMPSON R. N. 1965. The geology and petrology of the Marsco area Skye. Univ. Oxford D.Phil thesis (unpubl.).
THOMPSON R. N. 1981a. The Tertiary Province: geochemistry and magma genesis. In , Sutherland D. S. (ed.), Igneous rocks of the British Isles, 461–77, Wiley.
THOMPSON R. N., MORRISON M. A., GIBSON I. L., DICKIN A. P. 1981. The Staffa Magma Type revisited . . . . and re-instated. J. Geol. Soc., in press.
VANN I. R. 1978. The siting of Tertiary vulcanicity. Geol. J. Spec. Issue 10, 393–414.
WAGER L. R., BROWN G. M. 1968. Layered igneous rocks, Oliver and Boyd.
WAGER L. R., VINCENT E. A., BROWN G. M., BELL J. D. 1965. Marscoite and related rocks of the Western Redhills Complex, Isle of Skye. Philos. Trans. R. Soc. Lond. A257, 273–307.
WALKER G. P. L. 1970. The distribution of amygdale minerals in Mull and Morvern (western Scotland). In , Murty T. V. V. G. R. K., Rao S. S. (eds.), Studies in earth sciences (West Commem. Vol.), Univ. Saugar.
WILLIAMS S., MURTHY V. R. 1979. Sources and genetic relationships of volcanic rocks from the northern Rio Grande Rift: Rb-Sr and Sm-Nd evidence. EOS Trans. Am. Geophys. Union 60, 407.
WOODHALL D., KNOX R. W. O'B. 1979. Mesozoic volcanism in the northern North Sea and adjacent areas. Bull. Geol. Surv. G.B. 70, 34–56.