Ofelia Pérez-Arvizu

Andesite petrogenesis by slab-derived plume pollution of a continental rift

Abstract The western Mexican subduction zone is characterized by steep subduction of the Rivera plate, and by the existence of a continental rift at the rear arc under which the slab rests at >300 km deep. Mafic magmatism at the volcanic front is potassic lamprophyric, interpreted to be influenced by deep and hot slab melts or supercritical fluids. In contrast, mafic rocks at the rear arc are intraplate-like basalts that derive from low extents of melting of a dryer mantle source. Although a transition from a volcanic arc front to an extensional rear arc is apparent, calc-alkaline andesitic stratovolcanoes with trace element characteristics that suggest a key role of residual amphibole have been constructed at the rear arc during the past ∼200 ka. Crystal fractionation of basalts and partial melting of crustal amphibolites are not viable mechanisms for andesites, whereas melting of slab amphibolites beneath the rear arc is also problematic because the oceanic plate rests too deep. We thus suggest that andesites are partial melts of rising diapirs made by mixtures of hydrous mantle, sediments, and possibly eroded crustal blocks, which detach buoyantly from the downgoing slab as discrete plumes that ‘pollute’ the upwelling regime of a continental rift.

The origin of intraplate magmatism in the western Trans-Mexican Volcanic Belt

Alkaline basalts with geochemical features similar to those of intraplate ocean islands have been emplaced along the main trace of the Tepic-Zacoalco rift (TZR), a unique tectonic structure of the western Trans-Mexican Volcanic Belt in which extension is superimposed to a convergent margin. New geochemical and petrologic data on mafic volcanic rocks along the rift indicate the existence of a highly heterogeneous pre-subduction mantle wedge that has been slightly overprinted by slab-derived chemical agents. Most mafic volcanic rocks display geochemical and isotopic compositions that are indistinguishable from those of the Pacific islands Socorro and Isabel, and confirm the existence of an ancient, recycled, high-μ component (HIMU; μ = 238 U/ 204 Pb) in their mantle source. Olivines separated from samples carrying the HIMU signature have NiO and CaO contents similar to olivines from mid-ocean ridge basalt (MORB), indicating that the source of enrichment must be entirely hosted in peridotite. In contrast, more evolved rocks within the TZR have stronger subduction signatures and water contents, and display a distinctive isotopic array that points to slab-derived contributions. Olivines from these rocks are slightly less forsteritic but also extend to higher NiO and lower CaO contents than those from more mafic magmas, suggesting provenance from a secondary pyroxenite source. The overall geochemical evidence thus indicates that the pre-subduction background mantle wedge in western Mexico must be identical, and just as diverse, as that below the Pacific basin. Extension-driven mantle upwelling in a continental setting can only melt a dry peridotitic mantle to its lowest extents, and therefore preferentially sample its most enriched and easily fusible components. Yet the addition of even a small amount of slab-derived silica promotes a secondary petrologic transformation to pyroxene-rich lithologies that upon melting create magmas with compositions that are more akin to a volcanic arc setting.

Petrogenesis and thermobarometry of the ∼50 Ma rapakivi granite-syenite Acapulco intrusive: Implications for post-Laramide magmatism in southern Mexico

The Acapulco intrusion is a composite pluton that belongs to the coastal batholithic belt of southern Mexico, intruding the Xolapa metamorphic complex and cropping out in the neighboring area of Acapulco city. The Acapulco intrusion has been considered as an anomaly based on its age, which contrasts with the surrounding plutons and the general age trend from the coastal batholithic belt and corresponds to an Eocene–Oligocene age. It ranges in composition from granite (sensu stricto) to syenite and diorite. The most distinctive characteristic of the Acapulco intrusion is the rapakivi texture developed in the granites, which are characterized by biotite, amphibole, allanite, and fl uorite as distinctive minerals, plus titanite, zircon, and apatite as accessory phases. Geochemically, the Acapulco intrusion varies from metaluminous to peraluminous, and displays the distinctive signatures of arc-related magmas. The studied rocks show strong negative Sr, Ba, and Eu anomalies, coupled with incompatible element enrichments and high Ga/Al ratios, which are typical characteristics of A-type granites that underwent strong plagioclase fractionation from a formerly metaluminous magma. Initial isotopic ratios ( 87 Sr/ 86 Sr from 0.7035 to 0.7100, and eNd from +5.50 to +1.78) indicate a range from depleted mantle compositions to compositions consistent with crustal contamination by continental crust, particularly from the surrounding Xolapa Complex. U-Pb geochronology in zircons by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) established crystallization ages of 49.40 ± 0.40 Ma, 50.20 ± 1.0 Ma, 50.42 ± 0.39 Ma, and 50.56 ± 0.39 Ma for different lithologies of the Acapulco intrusion. These geochronological data, together with previous published works, confi rm that post-Laramide plutonism between 50 and 60 Ma is widespread in the southern continental margin of Mexico as a major magmatic event. Finally, new thermobarometric determinations established emplacement conditions of ~700 °C at 8–10 km depth (2.08–2.8 kbar), indicating an exhumation rate of ~0.21 km/m.y. between 50 and 20 Ma, which is slower than the previous estimated rate of 0.44 km/m.y. These results call for a review on models suggesting fast and/or slow exhumation of the southern Mexico coastal batholitic belt.