Graduate Thesis Or Dissertation
 

Geochemical evolution of the Roseburg formation basaltic rocks, southern Oregon coast range

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  • The Paleocene-lower Eocene volcanic rocks of the southern Oregon Coast Range Roseburg Formation are the oldest remnant eruptive sequence of an oceanic islandseamount province which is presently exposed along a north-south lineament that stretches from Vancouver, B.C. into southwestern Oregon. This seamount terrane is thought to have formed on the Farallon plate and, been subsequently accreted into its present position along the Oregon and Washington coast during subduction of this oceanic plate beneath the North American Plate during the early Tertiary The Roseburg volcanic sequence is composed of a basal submarine tholeiitic section which grades upward into a subaerially erupted sequence of highly undersaturated alkalic basalts. Fields studies show that the morphological evolution of eruptive styles within the volcanic pile (i.e. sequences of pillowed and massive flows, pyroclastic beccias and hyaloclastite deposits, subareal flows, and erosional surfaces) closely follow successive stages of growth observed at present day oceanic islands such as Hawaii. The regional distribution of basalt types reveal a progressive increase in differentiation from exposures of tholeiitic basalts in the south to exposures of alkalic basalts in the north, and demonstrate a coherencey in the geochemical evolution between basalts which cropout on the east and west side of an intervening Eocene basin. The Roseburg basaltic sequence displays the greatest amount of chemical diversity yet reported for the early Tertiary volcanic centers which compise the Oregon-Washington Coast Range province. The tholeiitic basalts have trace element compositions which are typical of E-type MORB and tholeiites from hotspot generated oceanic islands. The alkalic suite is highly enriched in incompatible element constituents typically found in the late stage eruptive cycles of oceanic island volcanism. The generation of these two basaltic suites are related to a common mantle source composition which has undergone variable degrees of partial at different depths. Differentiation of the tholeiitic basalts results from uniform degrees of partial melting, whereas compositional variations in the alkalic basalts can be explained only by variable degrees of partial melting at deeper depths in the mantle. Fractional crystalization does not seem to be an important control on the differentiation of these two basaltic suites. In contrast, compositional variations of basalts from other volcanic centers in the Oregon-Washington Coast Range province indicate a more dominate control of differentiation by fractional crystalization. The trace element variations in the province is related to varying influences of mid-ocean ridge spreading and hotspot volcanism. The trace element data reported here does not support models for the generation of these seamount volcanoes by marginal basin spreading possibly caused by the highly oblique subduction of the Farallon plate during this time. The regional geochemical variations exhibited by basalts from different volcanic centers in the Oregon-Washington Coast Range province is shown to be geographically controlled rather than time dependent. Generally, the available data shows a decrease in the highly incompatible trace element constituents from the southern end of the Coast Range (Roseburg basalts) to the northern volcanic centers (Metchosin and Crescent Fm. basalts). This geochemical trend is thought to be caused by a declining influence of hotspot-type volcanism on the magma compositions of basalts erupted in the northern regions of the Coast Range and/or the increasing influence of spreading ridge magmatic activity in that area.
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