ABSTRACT Miocene–Pliocene volcanism around Lake Tahoe, California/Nevada, USA, part of the Southern Ancestral Cascades arc, ceased at around 3 Ma as the southern edge of the subducting Juan de Fuca plate migrated north of the region. Post–3 Ma, arc volcanism continued north of Lake Tahoe, but modern subduction and arc volcanism now occur only north of the Lassen volcanic center. Miocene–Pliocene Tahoe arc lavas appear to include an older mantle source component that is not common in Quaternary Lassen arc rocks. The goal of this work was to investigate how magma sources and/or volcanic processes transitioned in the northern Sierra Nevada between Lake Tahoe and Lassen. The Sierra Nevada between Lake Tahoe and Lassen, or the North Sierra segment of the Ancestral Cascades, includes eroded remnants of Ancestral Cascades volcanic rocks, including lava flow complexes, intrusions, and landslide/debris-flow deposits. Lava samples from the North Sierra segment include calc-alkaline basalts to dacites, with rare rhyolites. All North Sierra segment lavas exhibit normalized incompatible-element patterns with negative Nb, Ta, and Ti anomalies and positive Pb, Sr, and Ba anomalies. The North Sierra segment is geochemically split into two parts: a northern group including lavas from the Susanville area, and a southern group consisting of arc rocks from the Portola, Sierraville, Henness Pass, and Sagehen areas. With the exception of the Sagehen area, the North Sierra segment shows little variation in radiogenic isotope ratios with SiO 2 , indicating that assimilation of crustal rocks was outweighed by liquid-crystal crystallization during magma evolution. Trace-element and isotopic ratios in mafic rocks of the northern group are more typical of Lassen area Quaternary volcanic rocks, whereas those of southern group mafic rocks are more typical of Miocene–Pliocene arc lavas of the Lake Tahoe area. The isotopic distinction between Lassen-like and Tahoe-like arc lavas is likely controlled by basement age and lithology, where Lassen-like magmas were derived largely by mantle wedge melting and Tahoe-like magmas were primarily partial melts of metasomatized Sierran lithospheric mantle. The Susanville area represents the “transition zone” between these two geochemically distinct primary magma sources.

Abstract Aspects of the evolution of the Pan-African–Cadomian arc have been recognized in several European massifs. The Ossa–Morena Complex (SW Iberian Massif) is one of the best-preserved sections of this palaeo-Gondwana margin. In this domain, recent studies suggest that arc magmatism followed a cyclical pattern during the Upper Ediacaran and Lower Cambrian. However, its initial and more mature stages remain unclear. Upper Ediacaran magmatism ( c. 602 Ma) appears to be uninterrupted and driven by slab–mantle wedge–upper plate interactions. The early Paleozoic was a period of significant change along the Gondwana margin. In the Ossa–Morena Complex, the beginning of the Cambrian ( c. 541 Ma) is marked by a strong unconformity over the Ediacaran basement, which is linked to destabilization of the arc. However, subduction-related magmatism continued with increasing mantle input, driving the geochemistry to more alkaline compositions. This paper summarizes the geochemical and isotopic evolution of the peri-Gondwana arc preserved in SW Iberia during this period. These results highlight shifts in geochemistry related to a higher slab angle during each magmatic episode, suggesting a tectonic switch toward an extensional regime in this section of the Gondwana margin.

Abstract Mid- to late-Miocene continental arc volcanism on the North Island of New Zealand is found in the Coromandel Peninsula, the Kiwitahi volcanic chain and the Taranaki Basin (Kora Volcano) offshore the western margin of the North Island. Coeval oceanic arc volcanism is also found along the offshore Colville Ridge/Kermadec Ridge north of New Zealand. This Pb–Sr–Nd–Hf isotopic study aims to evaluate mantle sources and potential crustal contaminants along these sections of the Miocene arc system. The Colville/Kermadec Ridge and Kora lavas have the lowest Sr (0.7029–0.7045) and highest Nd (0.51305–0.51292) ratios; the Coromandel and Kiwitahi lavas overlap (Sr = 0.704–0.706; Nd = 0.51268–0.51296). The Colville/Kermadec Ridge, Kora, and Coromandel/Kiwitahi rocks form three distinct arrays on Pb–Pb plots, all above the Northern Hemisphere Reference Line, but none trend towards local Mesozoic basement greywackes. Isotopic and trace element ratio variations suggest that subducted sediments are a component in Coromandel/Kiwitahi mafic lava sources. The younger, southern Kiwitahi lavas have a more depleted mantle source than that for the older, northern Kiwitahi chain. Evolved lavas commonly have interacted with Waipapa basement rocks. Kora rocks have compositions similar to those of back-arc lavas and have been emplaced as sills in a rift environment above a distinct subduction-modified mantle.