The petrogenesis of the Eocene Challis Volcanic Group, Idaho, western U.S.A.

McKervey, John Antony (1998). The petrogenesis of the Eocene Challis Volcanic Group, Idaho, western U.S.A. PhD thesis The Open University.



The rocks of the Challis Volcanic Group, Idaho are investigated and constraints on their petrogenesis used to evaluate the tectonic control on the formation of early, extensionassociated magmatism in the western U. S. A. New and published 40Ar-39Ar analyses indicate that the rocks of the Challis Volcanic Group erupted between ~50 and 45 Ma at extrusion rates estimated at 0.01 to 0.03 km3 yr-1. The rocks have high-K calcalkaline/ shoshonitic compositions and incompatible trace element analyses show them to be LILE and LREE enriched ((La/Yb) ~7 to 20) but relatively depleted in Nb, Ta and Ti (e. g. Nb/La < 0.5 in the main). All rocks have high initial 87Sr/86Sr (0.70673 to 0.71135) and low 143Nd/144Nd (0.51151 to 0.51234) ratios in comparison to oceanic basalts. The rocks are interpreted to result from partial melting in both spinel and garnet facies of heterogeneousm, ajor elementd epleted,L REE enriched but Nb, Ta and Ti depletedp eridotite source regions in the lithospheric mantle. The origin of these LREE enriched source regions is most probably related to mantle metasomatism in a subduction zone tectonic setting, although the' age of these events are not constrained. The petrogenesis of the Challis Volcanic Group is broadly similar to early magmatism from areas of the Cordillera to the south (e. g. Colorado River Trough), but contrasts with areas to the north where crustal melting apparently dominates (e. g. northern Idaho: Omineca Belt). Thus it is suggested that the syn-compression thermal history of the Cordillera, immediately prior to extension and early magmatism, varies significantly between southern and northern Idaho. This variation correlates spatially with the northern limit of compression within the Laramide Foreland Province (~75 to 45/30 Ma). Compressional deformation within the Laramide Foreland Province may be coincident with a period of sub-horizontal subduction and therefore partial melting of the mantle lithosphere may be related to the removal of this subducted slab from beneath the lithosphere, although this remains poorly constrained. A comparison is made between the Challis Volcanic Group and Archaean sanukitoids, to suggest that the Tertiary rocks may provide a tectonomagmatic analogue for these particular late Archaean rocks. The implications of this comparison for late Archaean tectonics and crustal growth are discussed.

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