Abstract This study discusses the petrological and geochemical features of two monogenetic Miocene volcanoes, Mount Early and Sheridan Bluff, which are the above-ice expressions of Earth's southernmost volcanic field located at c. 87° S on the East Antarctic Craton. Their geochemistry is compared to basalts from the West Antarctic Rift System to test affiliation and resolve mantle sources and cause of melting beneath East Antarctica. Basaltic lavas and dykes are olivine-phyric and comprise alkaline (hawaiite and mugearite) and subalkaline (tholeiite) types. Trace element abundances and ratios (e.g. La/Yb, Nb/Y, Zr/Y) of alkaline compositions resemble basalts from the West Antarctic rift and ocean islands (OIB), while tholeiites are relatively depleted and approach the concentrations levels of enriched mid-ocean ridge basalt (E-MORB). The magmas evolved by fractional crystallization with contamination by crust; however, neither process can adequately explain the contemporaneous eruption of hawaiite and tholeiite at Sheridan Bluff. Our preferred scenario is that primary magmas of each type were produced by different degrees of partial melting from a compositionally similar mantle source. The nearly simultaneous generation of lower degrees of melting to produce alkaline types and higher degrees of melting forming tholeiite was most likely to have been facilitated by the detachment and dehydration of metasomatized mantle lithosphere.

Alkaline magmatism along the Cameroon Line has been active for at least 67 m.y. and is currently defined by an almost SW-NE geological lineament (mean value: N30°E). Available petrological, geochemical, and structural data obtained over the last 20 yr lead us to reappraise its mechanism of emplacement. Known as the second most important geological curiosity in Africa, after the East African Rift system, it displays a continental part and an oceanic part, a unique feature in Africa and even in the world. The continental part contains both plutonic and volcanic massifs, and the oceanic part consists only of volcanic massifs. Plutonic rocks as a whole define a complete series of gabbro-diorite-monzonite-syenite-granite type, whereas volcanic rocks display abundant basic (basalt-hawaiite) and felsic (trachyte-phonolite-rhyolite) lavas with very few intermediate ones (mugearite-benmoreite). The formerly entire alkaline nature of these rocks is here ruled out by the discovery of volcanoes with geochemically transitional affinities in some areas of the continental sector. On the other hand, new K-Ar and 40 Ar/ 39 Ar dates confirm the absence of any age migration associated with the SW-NE linear trend. This lack of steady time-space migration and the SW-NE trend have also been observed in the magmatic provinces of Nigeria and Benue Trough, which share similar geochemical features with the Cameroon Line, and along the NE-SW major igneous lineaments in South Africa. The mechanism of such episodic emplacement of alkaline magmatism can be better explained in terms of complex interactions between hotspots and lithospheric fractures during African plate motion.

Late Cenozoic volcanic rocks on the island of Sardinia are mildly alkaline-transitional lavas, dominantly hawaiites, mugearites, and transitional basalts with minor phonolites and trachytes, which form ∼80% of the entire sample population. Tholeiitic basaltic andesites form the remaining 20% of the analyzed rocks. The oldest lavas, the ca. 6.6–4.4 Ma radiogenic Pb volcanic group, are in southern Sardinia; they have geochemical characteristics very similar to most Circum-Mediterranean Anorogenic Cenozoic Igneous Province rocks. After a gap of ∼0.5 m.y., volcanism occurred in central and northern Sardinia, from ca. 3.9 to ca. 0.1 Ma. These products, the unradiogenic Pb volcanic group, are geochemically very different. Their geochemical characteristics (relatively high SiO 2 , low CaO, and CaO/Al 2 O 3 , relatively high Ni, relatively low high field strength elements, low heavy rare earth elements, high Ba/Nb and La/Nb, slightly high 87 Sr/ 86 Sr, and unradiogenic 143 Nd/ 144 Nd and 206 Pb/ 204 Pb ratios) are considered to be derived from an orthopyroxene-rich lithospheric mantle source. The origin of this enrichment in orthopyroxene is a consequence of SiO 2 -rich melt derived from delaminated and detached ancient lower continental crust reacting with mantle peridotite. The presence of two distinct groups of rocks (unradiogenic Pb volcanics and radiogenic Pb volcanics) in a very close geographic position is related to the existence of a lithospheric discontinuity running roughly E-W in southern Sardinia.

This study focuses on two issues that are still a matter of debate in subduction zones, particularly in western México: (1) the close association within the same volcanic complex of typical amphibole-free andesites to rhyolites and amphibole-bearing andesites to rhyolites, characteristic of the hydrated front of the Mexican arc; and (2) the occurrence of bimodal magmatism without evidence for interaction between mafic and intermediate to silicic magmas, which are in addition characterized by different petrogenetic affinities. Our case study is the San Pedro–Cerro Grande volcanic complex, a Quaternary silicic to intermediate dome complex located in western Mexico. Volcanic activity has been divided into two periods. In the middle Pleistocene, andesitic to dacitic magmas were emplaced along WNW-trending faults in the southern portion of the complex. The Las Cuevas pyroclastic sequence (older than ca. 500 ka) was emplaced during this episode, most likely from a local source. This first period of activity ended before ca. 280 ka with the emplacement of the Cuastecomate Plinian deposit, which is related to the formation of the San Pedro caldera, an ∼4-km-wide subcircular depression that is today partially buried by younger volcanic products. During the second period of activity (ca. 280–30 ka), rhyolitic and dacitic domes were mostly emplaced along the caldera rim and inside the caldera. In addition, hawaiites and mugearites built the Amado Nervo shield volcano on the caldera rim. Intermediate- to high-silica lava and pyroclastic rocks are subalkaline, whereas the Amado Nervo mafic lavas are transitional toward the alkaline series (Na-alkaline). No genetic relationships have been found between subalkaline and transitional Na-alkaline rocks, which are thought to represent different batches of magma from different mantle sources. Petrographic, geochemical, and isotopic variations observed in the transitional Na-alkaline Amado Nervo lavas point to a parental magma from a mantle melt that underwent limited olivine separation during its ascent to the surface. Among subalkaline rocks, two groups showing contrasting petrographical and geochemical features are recognized based on the presence of amphibole. Amphibole-bearing intermediate to silicic rocks are characterized by lower Ce and other incompatible trace element contents and lower 87 Sr/ 86 Sr (0.70382–0.70401) compared to amphibole-free rocks (0.70411–0.70424). On the basis of petrological characteristics, the two groups of magmas are interpreted to have evolved in two different magmatic reservoirs under different pressures and water contents in the mid-upper crust. Both groups of magmas were differentiated by open-system processes. We propose that assimilation and equilibrium crystallization (AEC) processes account for the amphibole-bearing rocks. Hotter and less evolved magmas interacted to a higher degree with the crust than the more evolved and colder magmas. This produced the observed higher 87 Sr/ 86 Sr in the less differentiated rocks of the amphibole-bearing group. On the other hand, amphibole-free rocks have chemical and isotopic characteristics that can be modeled by assimilation and fractional crystallization (AFC) processes. All data suggest that the two groups of subalkaline rocks have been generated by a common parental hydrous magma, but evolved in two different reservoirs. Amphibole-bearing magmas underwent amphibole fractionation in a mid-upper crustal reservoir and show assimilation of two types of basement: one akin to Oaxaquia and another akin to the Guerrero terrane. Amphibole-free magma only shows assimilation of an Oaxaquia-type basement.