Trace Element Discrimination Diagrams for the Tectonic Interpretation of Granitic Rocks

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作者：

PEARCE JULIAN A.，HARRIS NIGEL B. W.，TINDLE ANDREW G.

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摘要：

Granites may be subdivided according to their intrusive settings into four main groups—ocean ridge granites (ORG), volcanic arc granites (VAG), within plate granites (WPG) and collision granites (COLG)—and the granites within each group may be further subdivided according to their precise settings and petrological characteristics. Using a data bank containing over 600 high quality trace element analyses of granites from known settings, it can be demonstrated using ORG-normalized geochemical patterns and element-SiO2 plots that most of these granite groups exhibit distinctive trace element characteristics. Discrimination of ORG, VAG, WPG and syn-COLG is most effective in Rb–Y–Nb and Rb–Yb–Ta space, particularly on projections of Y–Nb, Yb–Ta, Rb–(Y + Nb) and Rb–(Yb + Ta). Discrimination boundaries, though drawn empirically, can be shown by geochemical modelling to have a theoretical basis in the different petrogenetic histories of the various granite groups. Post-collision granites present the main problem of tectonic classification, since their characteristics depend on the thickness and composition of the lithosphere involved in the collision event and on the precise timing and location of magmatism. Provided they are coupled with a consideration of geological constraints, however, studies of trace element compositions in granites can clearly help in the elucidation of post-Archaean tectonic settings.

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关键词：

Bibliography, Experimental/ Earth composition Earth crust geochemistry oceanic crust rocks tectonics/ trace element discrimination diagrams, Y-Nb projection Earth composition geochemistry lithosphere composition lithosphere thickness Earth crust discrimination boundaries postArchaean tectonic settings oceanic crust syncollision granites postcollision granites plagiogranites magmatism timing tectonic interpretation granitic rocks intrusive settings ocean ridge granites volcanic arc granites within plate granites collision granites petrological characteristics ORG-normalized geochemical patterns element-SiO 2 plots Rb-Y-Nb Rb-Yb-Ta space Yb-Ta Rb-(Y+Nb) Rb-(Yb+Ta) petrogenetic histories granite groups/ A9135G Earth crust and upper mantle A9135L Geochemistry A9145D Plate tectonics A9150E Seafloor processes A9165 Geophysical aspects of geology and mineralogy