Comparative Lower-Middle Ordovician conodont oxygen isotope palaeothermometry of the Argentine Precordillera and Laurentian margins

摘要

Stratigraphic, palaeontologic, and palaeomagnetic data support a hypothesis that argues for the Argentine Precordillera rifting from the southwestern margin of Laurentia in low latitudes during the Cambrian, migrating across the Iapetus Ocean, colliding with the Gondwanan margin in the late Middle Ordovician, and receiving glaciogenic sediments in the Late Ordovician. An alternative model proposes that the Precordillera originated as a low-latitude segment of Gondwana, migrated southward through major transform faulting toward high latitudes in the late Middle Ordovician, to reach its present position in the Devonian. New conodont oxygen isotope compositions (delta O-18(phos)) have been determined by ion microprobe SHRIMP II using samples from both the Precordillera and Laurentia (Marathon area of Texas, Wilcox Pass in Alberta, and western Newfoundland). Significantly, the delta O-18 values of conodonts from all four widely separated areas show a consistent pattern of a cyclic but overall increasing trend in delta O-18 (ca. 16 to 18 parts per thousand) hence ocean cooling through the Early and Middle Ordovician. An apparent change occurs at the basal Late Ordovician, where delta O-18 values obtained from conodonts in the uppermost sample from the Precordillera are significantly higher (+1.5 parts per thousand) than those from Laurentia. Albeit from a single sample, this higher value implies significantly cooler conditions, as would be anticipated with a southerly (poleward) migration of the Precordillera (irrespective of either hypothesis). The virtual absence of conodont-bearing carbonates in most of the Precordilleran Upper Ordovician precluded analysis of younger samples. When combined with existing macrofaunal and palaeomagnetic data, the oxygen isotope data would tend to favour the model of a drift of the Precordillera from tropical to higher latitudes during the Ordovician; however, further studies are needed to determine unequivocally whether the Precordillera originated from southern Laurentia (Ouachita embayment). These new oxygen isotope values provide the best and regionally most consistent data through the Early-Middle Ordovician.