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Spidergrams
Prior knowledge: plate tectonic environments; names of rare earth elements; meaning of chondrite.
Spidergrams are histogram like plots of the abundances of a set of elements in an analyzed sample relative to their abundance in some standard. One of the earliest spidergrams (Jahn, B-M, Chi-Yu, S., and Rama Murthy, V., 1974. Trace element geochemistry of Archean volcanic rocks. Geochim. Cosmochim Acta, 38, 611-627) plots the relative abundance of the alkali elements Sr, K, Rb, Ba in various volcanic rocks in comparison to their abundance in oceanic floor rocks (MORB). Unfortunately, the alkali elements, and also U and Th, are mobile and susceptible to alteration. Spidergrams therefore now include the less mobile Rare Earths, Nb (Ta), Zr, Ti, P, and Y, ordered in a manner that reflects their relative compatibility in mantle derived melts, as well as their relative mobility in hydrous phases (spider1 ). (Elements of the Rare Earth group possess a number of physical and chemical properties that make them especially useful in geochemical studies of igneous and sedimentary rocks, ocean water/rock ratios, and the weathering of continental rocks. All the REE are refractory with oxide condensation temperatures similar to Sr, U and Th. They were not fractionated during the formation of the Earth, and REE patterns are similar to those of chondrites and the solar photosphere.) What is important is the variation in the pattern of the relative abundances rather than the abundance values themselves (spider2 ).
The MORB normalized patterns of Within Plate basalts (spiderwp)typically show element enrichment from right (Yb) to left (Nb-Sr), whereas immature arc basalts ( spiderlkt) exhibit MORB type patterns but with marked relative alkali enrichment. Mature arcs ( spiderca) display more jagged patterns with typical depletions in Nb-Ta and Zr-Hf and enrichments in P. Basaltic rocks which have undergonecrustal contamination and continental derived sediments (spidercrustcontam) also tend to show these characteristics, and they commonly show up in continental flood basalts (spiderthomp).
The element abundance standard used for normalization is commonly that of chondrite ( spiderchond) or primitive Earth, rather than ocean floor basalt (spiderpearce). The element abundances used in the chondrite bulk earth standard are about 1.5 times those of chondrite "C1", therby taking into account the volatile loss in the formation of the Earth from material of "C1" composition. The REE abundances used in the primitive Earth standard (present mantle plus crust) is further enriched 1.5 times relative to their abundance in the chondritic bulk earth standard.
References
Pearce, J.A., 1975. Basalt geochemistry used to
investigate past tectonic environments on Cyprus. Tectonophysics,25, 41-67
Pearce, J.A., 1982. Trace element characteristics
of lavas from destructive plate boundaries: Thorpe, R.S., ed., Andesites,
John Wiley and Sons, 525-548. Comment- QE 462.A5
Pearce, J.A., 1984. Role of the Sub-continental
Lithosphere in Magma Genesis at Active Continental Margins p. 230-249
in Hawkesworth, C.J. and Norry, M.J. (Eds), Continental Basalts and Mantle
Xenoliths, Shiva Geology Series
Pearce, J.A., Alabaster, T., Shelton, A.W., 1981.
The Oman ophiolite as a Cretaceous arc-basin complex: evidence and
implications. Phil. Trans, R. Soc. Lond. A 300, 299-317.
Pearce, J.A. Harris, N.B.W., and Tindle, A.G.,
1984. Trace element discrimination diagrams for the tectonic interpretation
of granitic rocks: Journal of Petrology, 25, 956-983.
Pearce, J.A. Lippard, S.J., and Roberts, S.,
1984. Characteristics and tectonic significance of supra-subduction zone
(SSZ) ophiolites: In Marginal basin geology; volcanic and
associated sedimentary tectonic processes in modern and ancient
marginal basins. Edited by B.P. Kokelaar and M.F. Howells. Geological Society
of London, Special publication 16, 77-94.
Gale, G. H. and Pearce, J. A , 1982. Geochemical
patterns in Norwegian greenstones. Can. Jour. Earth Sci., 19, 3, 385-397
Thirlwall, M.F. & Bluck, B.J. , 1984. Sr-Nd
isotope & geochemical evidence that the Ballantrae "ophiolite", SW
Scotland, is polygenetic: p. 215-230 in Gass, I.G., Lippard,
S.J., & Shelton, A.W., eds., Ophiolites & Oceanic Lithosphere,
Spec. Pub Geol. Soc. 13.
Thirlwall, M.F., Graham, A.M , 1984. Evolution
of high-Ca, High-Sr C-series basalts from Grenada, Lesser Antilles:the
effects of intra-crustal contamination Jour. Geological Society:
140, p. 427-446.
Thirlwall, M.F. , 1986. Lead isotope evidence
for the nature of the mantle beneath Caledonian Scotland: EPSL, 80,, 55-70.
Thirlwall, M.F. , 1989. Movement on proposed
terrane boundaries in northern Britain: constraints from Ordovician-Devonian
igneous rocks: JGS, 146, 3, 373-376. Comment- correlation using Sr
and Pb; Sr decrease to the SE; rocks south of the SUF have high Pb207/Pb204;
"indeed, all igneous rocks just south of the SUF have incompatible
element concentrations difficult to explain on the subduction model
of Thirlwall (1981), and are rather close to the supposed subduction trace
at the Iapetus suture. Most importantly, igneous rocks
just south of the the SUF show consistently higher initial 207Pb/204Pb
at constant 206Pb/204Pb than Midland Valley ORS lavas, implying
a source component not seen N of the SUF
FIGURES
Structural Provinces of North America.
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