GEOCHEMISTRY OF THE WRANGELLIA FLOOD-BASALT PROVINCE - IMPLICATIONS FOR THE ROLE OF CONTINENTAL AND OCEANIC LITHOSPHERE IN FLOOD-BASALT GENESIS

The Wrangellia terrane of North America contains a large volume of Mid dle to Late Triassic oceanic flood basalts which were emplaced on top of a preexisting island are. Nd-, Sr-, and Pb-isotopic compositions re flect derivation from a plume source with epsilon(Nd)(T) similar to +6 to +7, Sr-87/Si-86 similar to 0.7034, and Pb-206/Pb-204(i) similar to 19.0. Major and trace element compositions suggest the Wrangellia flo od basalts (WFB) formed through relatively small degrees of partial me lting at greater depths than estimated for other oceanic plateaux such as Ontong Fava. It appears that the WFB did not form in a rifting env ironment, and that preexisting are lithosphere limited the ascent and decompression melting of the source plume. Rocks from the preexisting are are strongly depleted in high field strength elements (HFSEs) rela tive to large ion lithophile elements (LILEs), but the WFB are not. As similation of arc lithospheric mantle or crust was therefore generally minor However, some contamination by are components is evident, parti cularly in basalts erupted in the early stages of volcanism. Minor iso topic shifts, to lower epsilon(Nd)(T) and Pb-206/Pb-204(i) and higher Sr-87/Sr-86(i), are accompanied by shifts in trace element ratios towa rds more arclike signatures, e.g. low Nb/Th and Nb/La. Are contaminati on is greatest in the most evolved basalts, indicating that assimilati on was coupled with fractional crystallization. A comparison of the WF B with other continental and oceanic flood basalts reveals that contin ental flood basalts generally form through smaller degrees of melting than oceanic flood basalts and that the contribution of material from the crust and lithospheric mantle is significantly greater.