Early Oligocene intrusions in the central Coast Range of Oregon : petrography, geochemistry, geochronology and implications for the Tertiary magmatic evolution of the Cascadia forearc

Graduation date: 2007The Early Oligocene Oregon Coast Range Intrusions (OCRI) consist of gabbroic rocks and lesser alkalic intrusive bodies that were emplaced in marine sedimentary units and volcanic sequences within a Tertiary Cascadia forearc basin. The alkalic intrusions include nepheline syenite, camptonite, and alkaline basalt. The gabbros occur as dikes and differentiated sills. Presently, erosional remnants of the intrusions underlie much of the high topography along the axis of the central Oregon Coast Range. The intrusive suite is most likely associated with Tertiary oblique rifting of the North America continental margin. Dextral shear and extension along the continental margin may have been a consequence of northeast-directed oblique Farallon plate convergence. However, while both the gabbroic and alkaline OCRI appear to be related to tectonic extension, major and trace element geochemistry reveal separate parental magma sources between the two suites. The OCRI are part of a long-lived (42-30 Ma) magmatic forearc province that includes the Yachats Basalt and Cascade Head Basalt. The timing of Cascadia forearc magmatism correlates with a significant decrease in convergence rates between the Farallon and North American plates from 42 to 28 Ma. The OCRI are also contemporaneous with magmatism that occurred over a broad area of the Pacific Northwest (e.g. Western Cascades, John Day Formation).\ud New radiometric age data acquired from the [superscript 40]Ar-[superscript 39]Ar incremental heating experiments indicate that OCRI magmatism occurred between ~36 to 32 Ma; a longer\ud interval than previously thought. The alkaline OCRI are closely associated with the late Eocene OIB-like Yachats Basalt and Cascade Head Basalt forearc volcanic centers in both space and time. In addition, trace element geochemical data suggest similar parental magma sources for the alkaline OCRI and the forearc volcanic centers.\ud The geochemistry of the OCRI camptonites and alkaline basalts is typical of continental rift zones and oceanic intraplate environments. Major element geochemical data indicate that differentiation by crystal fractionation was the primary process involved in the magmatic evolution of the alkaline OCRI. The OCRI camptonites are volatile-rich equivalents of alkaline basaltic or basanitic parental magmas. Both trace element and major element geochemical data imply that the nepheline syenites and the camptonites were derived from similar alkaline parental magma sources. However, the nepheline syenites are not simple derivatives of the camptonite magmas. Instead, they appear to be alkaline variations of trachyandesites and rhyolites found in the forearc volcanic centers. The alkaline OCRI parental magmas were generated from small degrees of partial melting of an enriched OIB-like mantle source. There are two possible mantle sources for the alkaline OCRI: 1) An enriched lithospheric Siletzia mantle source that was previously metasomatized by deep-sourced alkaline melts or 2) upwelling asthenospheric mantle from beneath the subducting slab which would require a slab window. If the alkaline OCRI were derived from the lithospheric Siletzia mantle, then the mantle wedge would have been significantly thicker and possibly hotter during the Tertiary. Presently, the crustal rocks of Siletzia rest directly on the subducting Juan de Fuca plate.\ud The tholeiitic OCRI gabbros are a separate, more widespread, and voluminous pulse of forearc magmatism that was contemporaneous with alkaline OCRI and Cascade Head Basalt magmatism. The [superscript 40]Ar-[superscript 39]Ar geochronology and previous K-Ar age data reveal that the gabbroic OCRI magmatism was short-lived. The Marys Peak sill and nearby dikes were emplaced between approximately 32 and 33 Ma. The major element geochemistry of the gabbros indicates that their magmatic evolution was driven by crystal fractionation processes at crustal levels. In contrast with the alkaline OCRI, the gabbros are characterized by tholeiitic compositions with lower degrees of incompatible element enrichment. Their trace element chemistry also displays an arc-like signature which\ud indicates the involvement of a subduction-modified mantle source component in their petrogenesis.\ud The formation of the OCRI gabbroic magmas involved upwelling and decompressional melting within the mantle wedge. Oligocene forearc extension may have become more widespread for a short duration (~2 m.y.) which aided the ascension of relatively dense, iron-rich gabbroic OCRI magmas. During this period, the mantle wedge beneath the forearc experienced larger degrees of decompressional partial melting allowing for the generation of voluminous tholeiitic magmas. Since the gabbros are in close proximity to the Western Cascades arc, their petrogenesis was also linked to the Cascades subduction system, as indicated by their trace element geochemistry.\ud Overall, the OCRI represent the final episode of widespread forearc magmatism and record significant changes in tectonic interactions between the Farallon and North America plates