Trace Element Studies of the Origin of Igneous Rocks
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Downloaded from http://jgs.lyellcollection.org/ by guest on September 29, 2021 Jl geol. Soc. Lond. Vol. 135, 1978, pp. 591-595, 1 table. Printed in Northern Ireland. Conference Report Trace element studies of the origin of igneous rocks W. J. Rea TRACE ELEMENT STUDIES are increasingly im- subject of a paper by Harrison & Wood. portant in igneous petrogenetic investigations in as Mathematical modelling, using geochemical data, much as they place constraints on possible modes of now allows the quantitative assessment of petrogenetic origin of igneous rocks and also give evidence of the processes such as closed and open system fractiona- nature and composition of the mantle/crust systems tion, progressive and single stage partial melting, within which magmas are generated and modified. crystal accumulation and magma mixing (Pearce). The The conference saw trace element studies applied to models, however require valid mineral-liquid partition a wide range of igneous rock compositions, occurring coefficients for the trace elements concerned and there within a wide range of environments. Four papers is sometimes a substantial range in the available litera- were concerned with areas in which basalt lavas pre- ture values. The problem arises mainly because parti- dominate, i.e. Mull (Beckinsale et al.), the islands tion coefficients vary with physical conditions such as associated with the axial trough of the Red Sea (Hen- temperature (Harrison & Wood) and are also heavily derson & Parry), the East Scotia Sea (Saunders & dependent on melt structure which in turn reflects Tarney) where basalts are generated within an intra- chemical parameters such as asio~ and Primo. In this oceanic marginal basin and the South Shetland Islands sense the experimental determination of partition and Antarctic Peninsula (Weaver et al.) where differ- coefficients (Harrison & Wood) is particularly wel- ent periods of magma genesis have been related to come as is any increase in the variety of natural rock subduction and extension. Other papers dealt with systems for which natural partition coefficients have metabasalts from ophiolite complexes (Pearce), K-rich been determined. volcanics (Parker), diorites and granodiorites (Brown et al.), granites and charnockites (Petersen) and the Report of a joint meeting of the Mineralogical Society and the ultramafic to granitic rocks of the Fongen-Hyllingen Volcanic Studies Group, held during the Third Meeting of the Gabbro Complex (Esbensen). Geological Societies of the British Isles at Swansea University An important recent development in geochemistry on 26 September 1977. The meeting was organised by Mr has been to extend the range of material which can be Ewen Home and Dr W. J. Rea. usefully analysed. For example, important petrogene- Wn_LtmM JOHN REA, Department of Geology, University tic information is now being obtained from altered and College of Wales, Llandinam Building, Aberystwyth, Dyfed metamorphosed igneous rocks using ratios based on SY23 3DB. trace elements such as Ti, Zr, Y, Nb, P, Ce, Sm, Yb, abundances of which appear to be unchanged by sec- Geocheml~ and petrogenesis ot the early TertiaO lava pile ondary processes (Pearce). Volcanic rock geochemis- of the Isle ot Mull, Scotland R.D. Beckinsale, R. J. Pank- try, which until recently was virtually synonymous with hurst, R. R. Skelhorn & J. N. Walsh lava geochemistry, has now expanded its scope to Major and trace element analyses and strontium isotope include pyroclasties, which are volumetrically ex- ratios are presented for twenty-four samples of lavas and tremely important at many volcanoes and which can plugs from the early Tertiary lava pile in Mull. The samples yield important petrogenetic information (Parker). were selected on the basis of petrographic freshness from a Developments in analytical techniques also have an large collection from outside the hydrothermally altered important influence on the direction of geochemical 'zone of pneumatolysis' which occupies the central region of and petrogenetic research. The rare earth elements the volcanic complex. Most of the analyses yield normative (REE), difficult to separate and analyse by classical hypersthene and we argue that these are essentially unaltered methods, can now be analysed, e.g. by neutron activa- magmatic compositions and do not result from alteration. The analytical data indicate that the samples may be divided tion analysis, and appear to give important petrogene- into three groups on the basis of major element chemistry, tic information, e.g. on plagioclase fractionation or initial 87Sr/86Sr ratios and correlations between lithophile ele- garnet melting and fractionation. Eight of the papers ment contents. Group I comprises an alkaline series (basalt- included REE data, Pearce presented mineral-liquid hawaiite-mugearite) with extremely low initial 87Sr/S6Sr partition coefficients for three rare earth elements and ratios (~<0.7030) and generally low lithophile element con- an experimental study of the partition of samarium tents. Apart from their alkalinity and high Sr and Zr contents between garnet and hydrous melt at 30 kb was the these samples have affinities with abyssal tholeiites. Group II Downloaded from http://jgs.lyellcollection.org/ by guest on September 29, 2021 592 W.J. Rea contains hypersthene-normative basalts with more tholeiitic Trace element evidence for tim origin of tim diorite, and characteristics but (as in the case of the Skye Main Lava ~rite, of tim Choaet lmnlmtda, Gmmmey, Clmnml Series) the more evolved rocks are trachytes. This group is M. Brown, G. M. Power & C. Topley characterized by more normal levels of lithophile element concentrations and relatively high initial 87Sr/86Sr ratios of Within an area of 1 sq km even-grained diorite is transi- about 0.7055. Group III is less clearly defined and contains tional into acicular diorite and intruded by granodiorite and basalts that are generally sparsely olivine-phyric and in most all these appear to be older than an inhomogeneous diorite chemical respects fall between Group I and Group II-- which ranges from mesodiorite to leuco quartz diorite. Each including initial 87Sr/86Sr ratios (0.7033 to 0.7043). They rock-type has distinctive petrography and major element may represent mixtures of Group I and Group II type chemistry. Trace element distributions suggest that the series sources or magmas. Groups I and II appear to be similar, as a whole may not be explained in terms of crystal fractiona- respectively, to the relatively sodic iron-rich and the rela- tion. Of particular interest are trace element variations within tively potassic iron-poor silica enrichment trends disting- the inhomogeneous diorite and the acicular diorite as indi- uished in the Skye Main Lava Series. In the Group I magma cators of the probable origins of these rocks. series the behaviour of Y and Sr relative to other incompati- ble elements can only be explained by differential partial Trace element distributions in potassium-rich pumice and melting of a deep garnet-lherzolite mantle source. Fractional lava samples from Vuisini Volcano, central Italy R. J. crystallization has undoubtedly occurred at some stage during Parker the ascent of these magmas from the mantle, as indicated by the behaviour of Ni and Cr, but has not been a major factor A considerable amount of work has been carried out on in the production of evolved magma compositions. The the K-rich lavas from central Italy but the associated ignim- Group II magmas appear to have originated from a source brites and pyroclastics from this region have received little more enriched in lithophile elements, and a relatively shallow attention. Sparks (1975) has made detailed studies on the (<50 km) plagioclase-lherzolite mantle source is suggested tephrochronology and geology of Vulsini Volcano and six for these magmas because they have Sr/Ba ratios between major ignimbrite eruptions were identified and their eruption one and two orders of magnitude lower than those charac- sequence determined. Several smaller ignimbrites and pyro- teristic of Group I. Rb-Sr systematics suggest that the vertical clastic falls were also identified. Pumice clasts were subse- heterogeneity of the mantle which was largely responsible for quently collected (this work) from these ignimbrites and the chemical differences between these three groups may pyroclastics. A number of lava samples were also collected. have existed for a very long time prior to Tertiary The samples were examined for alteration effects such as magmatism. hydration/palagonitization of the pumice glass and samples showing these effects were discarded. Leucite is present in Petrogenetic studies ot metabasalts using immobile trace most of the lavas and in many of the pumice samples. In the element mtkm J.A. Pearce latter samples this mineral shows all degrees of alteration to analcime. The concentration of analcime in the pumice sam- The table below gives mineral-liquid partition coefficients for ples was determined by quantitative X-ray diffraction elements thought to be immobile during most weathering and (Parker, in press). Pure mineral separations of leucite and metamorphic processes. These coefficients, which are strictly analcime were also chemically analysed. These data were only valid for melts of tholeiitic basalt composition, will vary then used to correct the whole pumice major and trace according to melt composition, temperature and pressure. element geochemistry for this alteration. They can, however, be used with