| | VARIABILITY IN BIOTITE COMPOSITION AMONG FOUR ORDOVICIAN TEPHRAS, AND COMPARISON WITH CENOZOIC VOLCANIC BIOTITES TO ... |
 | | 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 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. |
| gsa.confex.com /gsa/2004AM/finalprogram/abstract_74126.htm (395 words) |