Untitled

Abstract

Present-day High Cascades arc magmatism was preceded by ~40 m.y. of nearly cospatial magmatism represented by the ancestral Cascades arc in Washington, Oregon, and northernmost California (United States). Time-space-composition relations for the ancestral Cascades arc have been synthesized from a recent compilation of more than 4000 geochemical analyses and associated age data. Neither the composition nor distribution of ancestral Cascades magmatism was uniform along the length of the ancestral arc through time. Initial (>40 to 36 Ma) ancestral Cascades magmatism (mostly basalt and basaltic andesite) was focused at the north end of the arc between the present-day locations of Mount Rainier and the Columbia River. From 35 to 18 Ma, initial basaltic andesite and andesite magmatism evolved to include dacite and rhyolite; magmatic activity became more voluminous and extended along most of the arc. Between 17 and 8 Ma, magmatism was focused along the part of the arc coincident with the northern two-thirds of Oregon and returned to more mafi c compositions. Subsequent ancestral Cascades magmatism was dominated by basaltic andesite to basalt prior to the post-4 Ma onset of High Cascades magmatism. Transitional tholeiitic to calc-alkaline compositions dominated early (before 40 to ca. 25 Ma) ancestral Cascades eruptive products, whereas the majority of the younger arc rocks have a calc-alkaline affi nity. Tholeiitic compositions characteristic of the oldest ancestral arc magmas suggest development associated with thin, immature crust and slab window processes, whereas the younger, calc-alkaline magmas suggest interaction with thicker, more evolved crust and more conventional subduction-related magmatic processes. Presumed changes in subducted slab dip through time also correlate with fundamental magma composition variation. The predominance of mafi c compositions during latest ancestral arc magmatism and throughout the history of modern High Cascades magmatism probably refl ects extensional tectonics that dominated during these periods of arc magmatism.Mineral deposits associated with ancestral Cascades arc rocks are uncommon; most are small and low grade relative to those found in other continental magmatic arcs. The small size, low grade, and dearth of deposits, especially in the southern two-thirds of the ancestral arc, probably refl ect many factors, the most important of which may be the prevalence of extensional tectonics within this arc domain during this magmatic episode. Progressive clockwise rotation of the forearc block west of the evolving Oregon part of the ancestral Cascades magmatism produced an extensional regime that did not foster signifi cant mineral deposit formation. In contrast, the Washington arc domain developed in a transpressional to mildly compressive regime that was more conducive to magmatic processes and hydrothermal fl uid channeling critical to deposit formation. Small, low-grade porphyry copper deposits in the northern third of the ancestral Cascades arc segment also may be a consequence ...

The earliest low and high δ18O caldera-forming eruptions of the Yellowstone plume: implications for the 30–40 Ma Oregon calderas and speculations on plume-triggered delaminations

“…rhyolites of the Tower Mountain and Wildcat Mountain calderas have Nb+Y abundances that overlap with those of the high and ancestral Cascade rhyolites (Figure 2B). In contrast, the correlated basalts have high field strength element (HFSE) abundances and Nb/Yb and Th/Yb ratios that are elevated relative to N-MORB (Figure 2A and Supplementary Material) and the modern (e.g., Mazama) and ancestral Cascades (Bacon, 1989;Bacon et al, 1997;du Bray and John, 2011). These relations are consistent with a deep, undepleted, sublithospheric mantle origin (Pearce and Peate, 1995).…”

“…Renewed Cascadia subduction and related arc magmatism built a north-south Cascade volcanic front, initiating in southern Washington and northern Oregon, with the first ancestral Cascade volcanoes and calderas appearing around 42 Ma (du Bray and John, 2011). East of the ancestral Eocene-Oligocene Cascade arc, voluminous 30-40 Ma ash-flow tuffs associated with large caldera forming eruptions were deposited as part of the Clarno and John Day formations.…”

Eocene initiation of the Cascadia subduction zone: A second example of plume-induced subduction initiation?

“…are known (Wells et al, 2014, their figure 2). The oldest lavas of the Ancestral Cascades arc are tholeiitic basalt (du Bray and John, 2011). Like the Caribbean PISI sequence , the record from clearly OIB-like (50-56 Ma) alkali basalt of Siletzia to Ancestral Cascades arc (<40 Ma) tholeiitic basalt shows no obvious breaks that could be ascribed to a collision or other major tectonic event.…”

“…2). A major tectonic reorganization occurred in Paleogene time, whereby the older, contorted convergent margin was abandoned and the new, straighter Cascadia convergent margin was established to the west of the Ancestral Cascades volcanic arc (Armstrong and Ward, 1991;Madsen et al, 2006;du Bray and John, 2011). This study addresses why the old convergent margin shut down and how the new one formed.…”

Facies architecture of a continental, below-wave-base volcaniclastic basin: The Ohanapecosh Formation, Ancestral Cascades arc (Washington, USA)

“…Despite the widespread extent of the Ohanapecosh Formation in the central Cascades (>400 km 2 ), and mapping of various sections, the depositional processes and paleo-environment remain debated, in part due to incomplete exposure. This voluminous volcaniclastic succession is basaltic to andesitic in composition, and records the northernmost eruptive activity of the Ancestral Cascades arc (Sherrod and Smith, 2000;du Bray et al, 2006;du Bray and John, 2011).…”