Paleomagnetic studies of volcanic rocks in Ireland, Newfoundland and Labrador.

Abstract

Twenty-five samples from four bands of Ordovician ignimbrite exposed near Killary Harbour, Eire, were studied in detail, yielding strong evidence for magnetic stability: (1) At two collection sites, the natural remanent magnetization was significantly misaligned with the present field. At all three sites its direction was substantially stable against (2) a.c. demagnetizing fields up to 795 oe (peak), and (3) heating to 400-550°C; both treatments tended to reduce directional scatter. (4) A positive fold test suggests that the stable magnetization is primary. Assuming a "reversed" axial dipole field, a geographic north pole in the Pacific Ocean (9°N, 146°W; dp = 12°, dm = 22°) was calculated from the resultant of the four band mean directions relative to bedding planes, after a.c. treatment of all samples to 540 oe. Allowing for possible incomplete averaging of the secular variation, this result is compatible with the few published Ordovician poles relative to Europe. -- An oven for stepwise demagnetization, used in the present study, was calibrated. Various tests showed that field-nulling errors would be within 50-100γ during runs. Systematic directional errors from this cause can be greatly reduced by random specimen grouping. -- Basalts from three Lower or Middle Paleozoic single flows in southern Labrador (Henley Harbour, Table Head), and at Cloud Mountain in northern Newfoundland, were thermally tested for stability. Like the Irish ignimbrites, these rocks tend to be highly oxidized, with hematite partly replacing the primary titanomagnetite. The three lavas showed comparable thermomagnetic behaviour, with blocking temperatures mostly in a 100-200°C range below 450°C. Treatment to 350-450°C destroyed some secondary components, yielding new mean directions that were similar at the three sites, though with poor vector grouping. Being spot readings of the ancient field, these directions agree broadly with published directions from Lower Paleozoic rocks.