SILURIAN AND LATE ORDOVICIAN K-BENTONITES AS A RECORD OF LATE CALEDONIAN VOLCANISM IN THE BRITISH-ISLES

Silurian and late Ordovician K-bentonites of the British Isles provide a record of prolonged volcanism during the convergence of terranes as sociated with closure of the Iapetus and Tornquist Oceans. In the Sout hern Uplands-Longford-Down and Midland Valley terranes, they range fro m late Caradoc to Telychian, with further early Homerian occurrences. South of the Iapetus suture, in Eastern Avalonia, the range is Hirnant ian to early Ludlow in northern England and early Telychian to earlies t Ludfordian in the Welsh Borderland and English Midlands. In both cas es, the distributions indicate that volcanism was more long-lived and probably more extensive than is depicted in current plate tectonic rec onstructions. Average intervals between K-bentonites are estimated, ba sed on Harland er al. (1990), as: c. 65 000 years at Dob's Linn (late Aeronian to early Telychian); c. 39 000 years in the Cautley area (Tel ychian); c. 51 000 years in a borehole at Walsall (late Llandovery to Sheinwoodian). Trace element geochemistry suggests mostly subalkaline dacitic to rhyolitic magmas in which LILE-enrichment accompanies varia ble enrichment in crustally derived elements (Ta, Nb). The geochemistr y suggests comparison with continental are volcanism of 'within-plate, attenuated lithosphere' character. Ta-Nb enrichment and an absence of Eu anomalies from REE profiles are consistently present north of the Iapetus suture, but trace element patterns are less consistent south o f the suture where negative Eu anomalies are generally present. Discri minant function analysis successfully distinguishes Llandovery, Wenloc k and Ludlow K-bentonites from south of the Iapetus suture, and Llando very K-bentonites from north and south of the suture. Those from north of the Iapetus suture probably originated in volcanism along the sout hern margin of Laurentia before final closure of the Iapetus Ocean. Th ose from south of the suture may have been derived from volcanism asso ciated with late destruction of Iapetus and Tornquist oceanic crust, a lthough an alternative involving volcanism at the southern margin of E astern Avalonia or Baltica may accord better with the distribution of K-bentonites and the geographical trend of the fragmentary outcrops of Silurian volcanic rocks from southern Ireland to Belgium.