NEGATIVE CE ANOMALIES IN ARCHEAN AMPHIBOLITES AND LARAMIDE GRANITOIDS, SOUTHWESTERN MONTANA, USA

Ce is apparently fractionated from other REE only under the oxidizing conditions of near-surface environments. Therefore, Ce anomalies in fresh igneous rocks indicate that source regions of such rocks included some material that had previously existed near the surface. Negative Ce anomalies in two epizonal Laramide granitoids in Montana are outside analytical error. The Ce anomalies were probably inherited from their supracrustal Archean basement rocks. Similar rocks now close to the granitoids have a range of Ce anomalies. Biotite-schists are relatively enriched in Ce, whereas orthoamphibolites show no fractionation of Ce or have negative Ce anomalies. The magnitude of the Ce anomalies correlates well with initial Pb isotopic compositions within one intrusion, indicating higher U/Pb, Ph/Th in rocks with more depleted Ce. The Ce anomalies correlate very poorly with initial Sr and Nd isotopic composition of the rocks. These anomalies may be the result of alteration of these rocks in a marine environment in which Ce was preferentially leached from the rocks and U and, to a lesser extent, Th were added to the altered rocks. Source regions of the granitoids were plausibly dominated by rocks similar to the amphibolites. Ce anomalies provide a powerful tracer of previously supracrustal material in source regions of igneous rocks. © 1990.