The application of a ThHfTa diagram to problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary Volcanic Province

The recently proposed [1] ThHfTa diagram is shown in the light of considerable additional data to be a sensitive indicator of the tectonic environment in which an unknown lava (basic of silicic) was erupted. With the presently available data it is now possible to distinguish calc-alkaline lavas from island arc tholeiites. Some of the fields of the different tectonic environments have been enlarged and the boundaries between the fields modified slightly, but the conclusions drawn by Wood et al. [1] are still supported by the data. It is not possible to discriminate between E-type MORB andtholeiitic within-plate basalts using this diagram, but when used in conjunction with the ZrTiY triangular diagram [23] (as originally suggested [1]) these magma types can be distinguished. The effects of different types of bulk lower and upper crustal contamination of a within-plate alkali basalt on the Th, Hf, Ta and radiogenic isotope concentrations of the residual liquids are calculated in detail. The ratios of ThHfTa are shown to be extremely sensitive to crustal contamination processes. Data published by Thompson and co-workers [2,5] for the Tertiary lavas of Skye, Scotland, are used to illustrate the complexity of crustal contamination and develop a petrogenic model for these lavas. These calculations suggest that the use of isotopic data alone to estimate the extent of crustal contamination a particular lava has suffered is fraught with errors and should be interpreted with care.

[1]  G. Hanson,et al.  Evolution of the mantle: Geochemical evidence from alkali basalt , 1975 .

[2]  R. Thompson,et al.  Major Element Chemical Variation in the Eocene Lavas of the Isle of Skye, Scotland , 1972 .

[3]  R. Thompson.  Primary basalts and magma genesis , 1974 .

[4]  D. Wood,et al.  A RE-APPRAISAL OF THE USE OF TRACE ELEMENTS TO CLASSIFY AND DISCRIMINATE BETWEEN MAGMA SERIES ERUPTED IN DIFFERENT TECTONIC SETTINGS , 1979 .

[5]  D. Wood.  Major and Trace Element Variations in the Tertiary Lavas of Eastern Iceland and their Significance with respect to the Iceland Geochemical Anomaly , 1978 .

[6]  H. Chapman,et al.  Lead isotope measurements from the oldest recognised Lewisian gneisses of north-west Scotland , 1977, Nature.

[7]  J. Tarney,et al.  Continental growth, island arc accretion and the nature of the lower crust—a reply to S. R. Taylor & S. M. McLennan , 1979, Journal of the Geological Society.

[8]  P. Cambon,et al.  Leg 55, emperor seamounts : trace elements in transitional tholeiites, alkali basalts, and hawaiites-mantle homogeneity or heterogeneity and magmatic processes , 1980 .

[9]  P. Hamilton,et al.  Neodymium and Strontium Isotope Evidence for Crustal Contamination of Continental Volcanics , 1978, Science.

[10]  H. Schmincke,et al.  Rare earth and other trace elements in historic azorean lavas , 1976 .

[11]  N. Gale,et al.  The significance of lead isotope studies in ancient, high-grade metamorphic basement complexes, as exemplified by the Lewisian rocks of Northwest Scotland , 1969 .

[12]  I. Gibson,et al.  The diagnostic geochemistry, relative abundance, and spatial distribution of high-calcium, low-alkali olivine tholeiite dykes in the Lower Tertiary regional swarm of the Isle of Skye, NW Scotland , 1977, Mineralogical Magazine.

[13]  J. Powell,et al.  Isotopic evidence for the age and origin of the “grey gneiss” complex of the southern Outer Hebrides, Scotland , 1975, Journal of the Geological Society.

[14]  H. Schock.  Distribution of rare-earth and other trace elements in magnetites , 1979 .

[15]  J. Tarney,et al.  Chemistry, thermal gradients and evolution of the lower continental crust , 1977, Journal of the Geological Society.

[16]  B. G. Jamieson,et al.  The Olivine-rich Lavas of Nuanetsi: a Study of Polybaric Magmatic Evolution , 1974 .

[17]  P. Hamilton,et al.  Basalt magma sources during the opening of the North Atlantic , 1979, Nature.

[18]  M. Prinz,et al.  Evolved lavas from the Snake River Plain: Craters of the Moon National Monument, Idaho , 1976 .

[19]  S. Moorbath,et al.  Lead isotope studies on igneous rocks from the isle of Skye, Northwest Scotland , 1968 .

[20]  S. McLennan,et al.  Discussion on 'Chemistry, thermal gradients and evolution of the lower continental crust' by J. Tarney & B. F. Windley , 1979, Journal of the Geological Society.

[21]  N. Rogers.  Granulite xenoliths from Lesotho kimberlites and the lower continental crust , 1977, Nature.

[22]  S. Drury.  The geochemistry of Precambrian granulite facies rocks from the Lewisian complex of Tiree, Inner Hebrides, Scotland , 1973 .

[23]  D. Wood,et al.  The petrology, geochemistry, and mineralogy of north atlantic basalts : a discussion based on ipod leg 49 , 1979 .

[24]  P. Hamilton,et al.  Sm—Nd systematics of Lewisian gneisses: implications for the origin of granulites , 1979, Nature.

[25]  J. Joron,et al.  Coefficient de partage de quelques éléments en trace entre plagioclase et verre dans les ignimbrites—Implications pétrogénétiques , 1977 .

[26]  M. Okrusch,et al.  Granulite-facies metabasite ejecta in the Laacher See area, Eifel, West Germany , 1979 .

[27]  Julian A. Pearce,et al.  Tectonic setting of basic volcanic rocks determined using trace element analyses , 1973 .

[28]  D. Wood,et al.  Elemental and Sr isotope variations in basic lavas from Iceland and the surrounding ocean floor , 1979 .

[29]  M. Lindstrom,et al.  Geochemistry and petrogenesis of a basalt-benmoreite-trachyte suite from the southern part of the Gregory Rift, Kenya , 1977 .

[30]  W. White,et al.  The petrology and geochemistry of the Azores Islands , 1979 .

[31]  S. Humphris,et al.  Influence of rock crystallisation history upon subsequent lanthanide mobility during hydrothermal alteration of basalts , 1978 .

[32]  W. Leeman,et al.  Petrology of McKinney Basalt, Snake River Plain, Idaho , 1976 .