ABSTRACT Ferroan granite is a characteristic rock type of the Laurentian margin. It is commonly associated with anorthosite and related rocks. Ferroan granites are strongly enriched in iron, are alkalic to alkali-calcic, and are generally metaluminous. These geochemical characteristics reflect their tholeiitic parental magma source and relatively reducing and anhydrous conditions of crystallization. Their compositions distinguish them from arc magmas, which are magnesian and calcic to calc-alkalic. Ferroan granite magmas are hot, which promotes partial melting of their crustal wall rocks. Assimilation of these silica-rich and peraluminous melts drives the resulting magmas to higher silica and aluminum saturation values. Where Proterozoic ferroan granites intrude Archean crust, their mantle component is readily identified isotopically, but this is more difficult where they intrude relatively juvenile crust. Ferroan granite forms in tectonic environments that allow partial melts of tholeiitic mantle to pond and differentiate at or near the base of the crust. Phanerozoic examples occur in plume settings, such as the Snake River Plain and Yellowstone, or under certain conditions involving slab rollback, such as those that formed the Cenozoic topaz rhyolites of the western United States or ferroan rhyolites of the Sierra Madre Occidental. It is possible that the long-lived supercontinent Nuna-Rodinia, of which Laurentia was a part, formed an insulating lid that raised underlying mantle temperatures and created a unique environment that enabled emplacement of large volumes of mafic melt at the base of the crust. Ascent of felsic differentiates accompanied by variable crustal assimilation may have created large volumes of Proterozoic ferroan granite and related rocks.

ABSTRACT The Oaxacan Complex is the largest outcrop of Grenville-age rocks in Mexico, constituting the main crustal fragment in the backbone of Oaxaquia. It is mainly composed of scarce metasediments, intruded by arc, alkalic, and tholeiitic magmas (ca. 1.3 to ca. 1.01 Ga) and later affected (ca. 0.99 Ga) by granulite-facies metamorphism. A detailed study, combining U-Pb geochronology by laser ablation–inductively coupled plasma–mass spectrometry, with in situ Hf isotopes in zircon grains, allowed comparison of the age and isotopic patterns of the Oaxacan Complex granulite rocks with those from other similar outcrops in Mexico (Huiznopala and Novillo Gneisses, Guichicovi Complex) and with the neighboring orogens such as the Grenville Province of the eastern United States and Canada, the Sveconorwegian orogen of SW Baltica, and some of the localities in which Mesoproterozoic rocks border the Amazonian craton of South America (Colombia, Peru, Brazil). Detrital zircon ages show that most metasedimentary rocks are younger than 1.4 Ga (only three samples contained zircon grains between 1.6 and 1.4 Ga), whereas U-Pb dating of igneous rocks (1245–1161 Ma) confirmed previous findings. Hf isotopes of dated zircon grains show that few crystals have negative ε Hf(t) values, indicating a recycling component from an older crust, but most of them are moderately primitive, with ε Hf(t) values of up to +12, and linear arrays parallel to the 176 Lu/ 177 Hf average crustal evolution model. Those Hf values are indicative of partial assimilation of an older crustal component, with Hf model ages of ca. 1.65–1.50 Ga. Comparison of these data helps to constrain possible Mesoproterozoic conjugate margins of Oaxaquia and propose a paleogeographic model in which Oaxaquia acted as the leading edge of Amazonia, together with the Colombian terranes, and received sedimentary input from different sources such as the southern Sveconorwegian orogen, the U.S.–Canada eastern Grenville Province, and some of the Mesoproterozoic belts bordering the Amazon craton.

Abstract The Jurassic igneous rocks of Dronning Maud Land represent Karoo flood basalt magmatism in Antarctica. Fifty years of research has documented systematic differences between magmas associated with the Karoo rift-zone (Vestfjella and Ahlmannryggen) and the rift-shoulder (Sembberget, Kirwanveggen) settings. The 189–182 Ma rift-zone tholeiites were chemically diverse and mainly formed compound-braided flow fields which record several magnetic polarity reversals. In contrast, the c. 181 Ma rift-shoulder tholeiites were chemically uniform and formed thick tabular sheet lavas within a single normal polarity period. The volcanic architecture records a long initial phase of slow eruptions from shield volcanoes in the initial rift and a brief phase of voluminous fissure eruptions flooding the rift shoulder. All of the major magma types in the rift-zone and rift-shoulder settings belong to a Nb-depleted category of Karoo flood basalts and were mainly derived from depleted convective upper mantle by magmatic differentiation. Pyroxenite-rich mantle components may have been significant sources for the most enriched magma types. Geochemical fingerprints of recycled crustal material imply that the Nb-depleted Karoo tholeiites may have been derived from mildly subduction-modified parts of the same overall upper-mantle reservoir which has been associated with the Ferrar tholeiites.

Abstract The Antarctic Peninsula is distinguished by late Neogene volcanic activity related to a series of northerly younging ridge crest–trench collisions and the progressive opening of ‘slab windows’ in the subjacent mantle. The outcrops were amongst the last to be discovered in the region, with many occurrences not visited until the 1970s and 1980s. The volcanism consists of several monogenetic volcanic fields and small isolated centres. It is sodic alkaline to tholeiitic in composition, and ranges in age between 7.7 Ma and present. No eruptions have been observed (with the possible, but dubious, exception of Seal Nunataks in 1893) but very young isotopic ages for some outcrops suggest that future eruptions are a possibility. The eruptions were overwhelmingly glaciovolcanic and the outcrops have been a major source of information on glaciovolcano construction. They have also been highly influential in advancing our understanding of the configuration of the Plio-Pleistocene Antarctic Peninsula Ice Sheet. However, our knowledge is hindered by a paucity of modern, precise isotopic ages. In particular, there is no obvious relationship between the age of ridge crest–trench collisions and the timing of slab-window volcanism, a puzzle that may only be resolved by new dating.