COMPOSITION OF BIOTITE PHENOCRYSTS IN ORDOVICIAN TEPHRAS CASTS DOUBT ON THE PROPOSED TRANS-ATLANTIC CORRELATION OF THE MILLBRIG K-BENTONITE(UNITED-STATES) AND THE KINNEKULLE K-BENTONITE (SWEDEN)

Biotite phenocryst compositions in three thick, widespread Ordovician K-bentonites, the Deicke and Millbrig from Big Ridge, Alabama, and the Kinnekulle from Mossen, Vastergotland, Sweden, fall into three distin ct groups, and so the proposed intercontinental correlation of the Mil lbrig and the Kinnekulle is suspect. Because the biotites are nearly p ristine compositionally, electron microprobe analyses provide a precis e geochemical fingerprint of each bed. Millbrig and Kinnekulle biotite s contain more FeO and MnO and less MgO and TiO2 than do Deicke bioti tes, Millbrig biotites contain more MgO and less TiO2 than Kinnekulle biotites, and Kinnekulle biotites contain appreciably more Al2O3 than either Deicke or Millbrig biotites. Each of these tephras was unmistak ably the product of a gigantic explosive volcanic eruption, but the di fferences in phenocryst chemistry point to derivation from three compo sitionally different magma batches.