VARIABILITY IN BIOTITE COMPOSITION AMONG FOUR ORDOVICIAN TEPHRAS, AND COMPARISON WITH CENOZOIC VOLCANIC BIOTITES TO CONSTRAIN TECTONOMAGMATIC SETTING

To characterize the setting and nature of the explosive volcanism that generated four voluminous Ordovician K-bentonites (tephras)—the Deicke, Millbrig, Ragland (North America) and Kinnekulle (Europe) beds—600 probe analyses of biotite phenocrysts are compared with 800 analyses of biotite from petrologically well-characterized Cenozoic lavas and tephras comprising 450 original and 350 literature analyses. The Ordovician biotites separate into two groups, Mg and Ti biotites (Deicke and Ragland), and Fe and Mn biotites (Millbrig and Kinnekulle). Deicke biotite compositional averages (all wt%) are 11.76 MgO, 5.12 TiO₂, 18.43 FeO*, and 0.22 MnO, and these exhibit more compositional variability in Al₂O₃ – FeO – MgO space than do biotites from the other three beds. Ragland biotite averages are 10.39 MgO, 5.37 TiO₂, 18.65 FeO*, and 0.14 MnO, Millbrig biotite averages are 8.27 MgO, 3.68 TiO₂, 24.45 FeO*, and 0.37 MnO, and Kinnekulle biotites averages are 7.68 MgO, 4.59 TiO₂, 23.66 FeO*, and 0.32 MnO.

Cenozoic biotite that best matches the magnesian-titanian biotite of the Deicke and Ragland beds is in quartz phyric to quartz sparsely-phyric metaluminous to weakly peraluminous (based on Al/K+Na+Ca < or > 1), dacitic to trachytic calc-alkaline lavas. These are associated with subduction zone or hotspot volcanism and include lavas and tephras from volcanic arcs in Japan, Chile, Costa Rica, and Kamchatka, and the Yellowstone hot spot.

Ferroan biotite in the Kinnekulle and Millbrig K-bentonite is most like that in mildly peraluminous calc-alkaline Cenozoic lavas (not nearly as common as metaluminous calc-alkaline lavas) that are quartz-phyric rhyodacite to rhyolite and associated with volcanism on continental crust. The Toba system of Sumatra, the Bishop Tuff of California, the peraluminous lava of Ambon in the Banda Arc, the Whakamaru Ignimbrite of New Zealand, and the Cerro Panizos Ignimbrite of the Bolivian tin belt have generated biotite-bearing lavas that are all suitable analogs for the Millbrig and Kinnekulle biotites. The Toba and Ambon lavas, from evolved aluminous magmas with biotite much like those in the Millbrig and Kinnekulle, are a good match because their setting (island arc on a basement of continental crust) fits with Ordovician paleogeographic reconstructions of Laurentia and Baltoscandia.