PETROLOGIC INVESTIGATION OF EARLY CASCADE UNITS IN THE SOUTHERN WILLAMETTE VALLEY, OREGON, AND IMPLICATIONS FOR LONG-TERM GEOCHEMICAL EVOLUTION OF THE CASCADE ARC

Numerous studies have documented the chemical evolution of individual arc volcanoes over time, but there has been less work on the temporal evolution of entire arcs. The Cascade arc, with its ~43 Ma record of magmatism related to subduction of the Juan de Fuca plate, is well suited to such investigation. To identify long term trends we have examined >11,000 mafic and intermediate (< 60 wt. % SiO₂) analyses compiled from duBray & John (2011), McClaughry et al. (2010), and Pitcher & Kent (2019), supplemented by new data from four early units (42-30 Ma) in the Southern Willamette Valley, Oregon (SWV). In addition, we investigate whether the along-arc chemical segmentation recognized by Pitcher & Kent among Quaternary lavas was present during the earlier history of the arc within their 3 southern (47.75-41.25°N) segments: Washington (WS), Graben (GS), and South (SS).

Some contrasts between older and younger lavas are evident throughout the arc: Sr, Ba/Nb, La/Yb have increased over time, while V has decreased. Other changes are only discernable within individual segments, and differ from one to the next. Within the WS, La, Nb, Ta, Y, and K increase over time, while average Cr decreases and Rb appears unchanged. Within the GS, La increases over time, Nb, Ta, Cr, and K remain consistent, and Rb and Y are more abundant and variable in older rocks. Within the SS none of these elements show discernable trends over time with the exception of Nb, which decreases slightly. Our new SWV analyses greatly expand the early Cascade dataset for the GS-SS segments. Included are mafic and intermediate flows from the Fisher Formation (wt. % SiO₂ 49.3-62.1, Mg# 62-33) and the Basalt of Mt. Tom (wt. % SiO₂ 51.9-52.6, Mg# 61-57). Both units appear more heterogeneous than younger units, but have low Ba/Nb (5-61 and 23-33, respectively) and La/Yb (3.7-24 and 5.1-5.5) typical of early Cascade rocks.

The long-term rise in Ba/Nb largely reflects increasing Ba, and suggests progressive LILE enrichment of the mantle wedge; degree of melting appears relatively unchanged. Within the WS, rising incompatible element levels point to involvement of a more enriched mantle component. The absence of this enriched signature in older WS samples, and in the GS and SS, suggest it is a more recent and localized addition to the southern Cascade arc source region.