The Alisitos arc segment is the southernmost and only part of the western Peninsular Ranges batholith accreted during the Cretaceous. Collision-related deformation is concentrated along the northern and eastern margins of the arc segment. While shortening within the Alisitos arc produced similar amounts of crustal thickening throughout the arc, suppression of parts of the lower crust of the Alisitos arc due to throw across the terrane-bounding faults varies substantially. Geobarometric change across the Main Mártir thrust suggests that ~15 km of additional crust was thrust onto the central Alisitos arc. Geochemical and geochronologic data from intrusive rocks of the Alisitos arc indicate arc magmatism was active before, during, and after collision. The data suggest that all Peninsular Ranges batholith intrusive rocks within the Alisitos arc were derived from a broadly similar primitive source, lacking interaction with evolved continental lithologies. Postcollisional intrusions from the central Alisitos arc adjacent to the Main Mártir thrust yield trace elemental signatures suggesting melt derivation at depths where garnet would be a stable residual phase. The spatial and temporal coincidence of these intrusions with the Main Mártir thrust suggests that the increased pressure of anatexis inferred for the depth of generation of these melts was generated by displacement on this fault. Further, close temporal and spatial characteristics, and similar geochemical characteristics between the central Alisitos arc intrusions and La Posta intrusions east of the Main Mártir thrust suggest that the Alisitos arc intrusions represent precursors to the much larger flare-up event. This observation supports models suggesting collision as a cause of magmatic flare-ups in arcs.

The ca. 570 Ma Catoctin volcanics, exposed in the Blue Ridge of northern Virginia, include metamorphosed rift-related basalts extruded during breakup of the supercontinent Rodinia. Field relationships, petrography, and geochemistry are used to decipher the stratigraphy for two areas of the volcanics, one at the base of the formation, and the other near its top. Geochemical characteristics of sequential flows can be explained by fractional crystallization of minerals commonly occurring in basalts. Intervening flows with slightly different geochemical features that cannot be explained by fractional crystallization from magma corresponding to an underlying flow, or by crustal contamination, most likely represent new pulses of magma. Positive Pb and negative Nb anomalies, coupled with ε Nd values of +1.5 to +4.5 and model ages that exceed crystallization ages by over 500 m.y., suggest that an enriched component was added to the mantle prior to melting. This component resulted from devolatilization of subducting sediments prior to assembly of Rodinia, likely during Grenville orogenic events. Stratigraphic control shows that lower lavas in the sequence contain more of the enriched component than upper flows.

In this review we summarize the major lithological and geochemical characteristics of the Mesoarchean (ca. 3075 Ma) Ivisaartoq greenstone belt, Nuuk region, southern West Greenland. In addition, the geological characteristics of the Ivisaartoq greenstone belt are compared with those of other Archean greenstone belts in the area. The Ivisaartoq greenstone belt is the largest Mesoarchean supracrustal lithotectonic assemblage in the Nuuk region. The belt contains well-preserved primary magmatic structures including pillow lavas, volcanic breccias, and cumulate (picrite) layers. It also includes variably deformed gabbroic to dioritic dikes, actinolite schists, serpentinites, siliciclastic sediments, and minor cherts. The Ivisaartoq rocks underwent at least two stages of postmagmatic metamorphic alteration, including seafloor hydrothermal alteration and syn- to post-tectonic calc-silicate metasomatism, between 3075 and 2961 Ma. The trace element systematics of the least altered rocks are consistent with a subduction zone geodynamic setting. On the basis of lithological similarities between the Ivisaartoq greenstone belt and Phanerozoic forearc/backarc ophiolites, and intra-oceanic island arcs, we suggest that the Ivisaartoq greenstone belt represents a relic of dismembered Mesoarchean suprasubduction zone oceanic crust. The Sm-Nd isotope system appears to have remained relatively undisturbed in picrites, tholeiitic pillow lavas, gabbros, and diorites. As a group, picrites have more depleted initial Nd isotopic signatures (ɛ Nd = +4.2 to +5.0) than gabbros, diorites, and tholeiitic basalts (ɛ Nd = +0.3 to +3.1), consistent with a strongly depleted mantle source. In some areas gabbros include up to 15 cm long white inclusions (xenoliths). These inclusions are composed primarily (>90%) of Ca-plagioclase and are interpreted as anorthositic cumulates of the lower oceanic crust brought to the surface by upwelling gabbroic magmas. Alternatively, the inclusions may represent the xenoliths from older (>3075 Ma) anorthositic crust onto which the Ivisaartoq magmas were emplaced as an autochthonous sequence. However, no geological evidence has been found for such older anorthositic crust in the region. The anorthositic cumulates have significantly higher initial ɛ Nd values (+4.8 to +6.0) than the surrounding gabbroic matrix (+2.3 to +2.8), suggesting two different mantle sources for these rocks.

The inland edge of the Jurassic magmatic belt passes through the eastern Mojave Desert, where it was emplaced in ancient continental crust. Three intrusive units exposed there—the Ship and Clipper Mountains plutons and a dike swarm in the Old Woman and Piute Mountains and Kilbeck Hills—are broadly similar to each other and to other intrusions of Jurassic age, but they differ from one another in detail and all show very clear evidence for interaction with the ancient crust. All three intrusive units are primarily metaluminous and range from mafic to moderately felsic in composition. The Ship Mountains pluton and dikes included both mafic and felsic magmas that mingled locally. The Clipper Mountains pluton comprises a compositional continuum from hornblende gabbro through granodiorite, at least partly a result of crystal accumulation processes. The ca. 160-Ma Clipper Mountains pluton was emplaced syntectonically with thrusting at a depth of approximately 15 km. The ca. 145-Ma dike swarm intruded at approximately 12 km, and the Ship Mountains pluton at <5 km. The Ship Mountains pluton, which is not well dated, initially overlay the dike swarm prior to Late Cretaceous and Tertiary extension and may have a similar age. The intrusions are all enriched in incompatible elements and have isotopic compositions that are more evolved than any plausible mantle source (high 87 Sr/ 86 Sr, low ε Nd , high 207 Pb/ 204 Pb and 208 Pb/ 204 Pb compared with 206 Pb/ 204 Pb). Ship Mountains and most dike samples are less evolved in Nd and Sr than the Mojave crust, but the Clipper Mountains Nd-Sr array is coincident with the less evolved portion of the field of ancient Mojave crust. Extremely strong U-Pb inheritance in Clipper zircons and moderate inheritance in dike zircons verifies the crustal component. We interpret Ship and dike rocks to be hybrids of ancient enriched mantle-derived mafic magmas and the ancient crust; the Clipper Mountains pluton could represent a restite-rich magma entirely derived from the Mojave crust, although a modest mantle contribution is likely.