Figure 16 6 c feomgo vs sio2 diagram distinguishing

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Unformatted text preview: vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series. Figure 16-6. c. FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series. 6 sub-series if combine tholeiite and C-A (some are rare) May choose 3 most common: May most 3 3 3 Low-K tholeiitic Med-K C-A Hi-K mixed Figure 16-5. Combined K2O - FeO*/MgO diagram in which the Low-K to High-K series are combined with the tholeiitic vs. calcalkaline types, resulting in six andesite series, after Gill (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag. The points represent the analyses in the appendix of Gill (1981). Tholeiitic vs. Calc-alkaline differentiation Tholeiitic Figure 16-6. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Tholeiitic vs. Calc-alkaline differentiation Tholeiitic Tholeiitic silica in the Skaergård Intrusion No change C-A shows continually C-A increasing SiO2 and lacks increasing dramatic Fe enrichment dramatic Calc-alkaline differentiation Calc-alkaline 3 Early crystallization of an Fe-Ti oxide phase Early Fe-Ti Probably related to the high water content of calcProbably alkaline magmas in arcs, dissolves → high fO2 alkaline 3 High water pressure also depresses the plagioclase High liquidus and → more An-rich As hydrous magma rises, ∆ P → plagioclase liquidus As moves to higher T → crystallization of considerable Anrich-SiO2-poor plagioclase 3 3 The crystallization of anorthitic plagioclase and lowThe anorthitic silica, high-Fe hornblende is an alternative mechanism for silica, hornblende the observed calc-alkaline differentiation trend the Figure 16-8. K2O-SiO2 diagram of nearly 700 analyses for Quaternary island arc volcanics from the SundaBanda arc. From Wheller et al. (1987) J. Volcan. Geotherm. Res., 32, 137-160. Other Trends Other q Spatial 3 3 q “K-h”: low-K tholeiite near trench → C-A → low-K alkaline as depth to seismic zone increases alkaline Some along-arc as well Some v Antilles → more alkaline N → S Antilles v Aleutians is segmented with C-A prevalent Aleutians in segments and tholeiite prevalent at ends in Temporal 3 Early tholeiitic → later C-A and often latest Early alkaline is common alkaline q REEs 3 3 Trace Elements Trace Slope within series is Slope similar, but height varies with FX due to removal of Ol, Plag, and Pyx Ol, (+) slope...
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