Laura Mori

The origin of a primitive trondhjemite from the Trans-Mexican Volcanic Belt and its implications for the construction of a modern continental arc

A remarkable suite of Miocene high-silica trondhjemites discovered in the central Trans-Mexican Volcanic Belt indicates that slab melts can ascend through the mantle and crust while suffering only minor compositional modifications. Despite carrying an assortment of deep crustal xenoliths, the trondhjemites preserve the most depleted isotopic compositions ever measured in the Mexican arc, with values that are nearly identical to those of the Pacific mid-oceanic-ridge basalts. These rocks also have high Sr/Y ratios, and extremely fractionated heavy rare earth element patterns at relatively high Mg number (Mg#), features that are all consistent with melts from the subducted oceanic crust that had only limited interaction with mantle peridotite during ascent. Nonetheless, modeling results indicate that these unusual geochemical features can be modified by more extensive mantle assimilation, resulting in compositions that could match those of more typical intermediate rocks from Mexico. The data thus indicate that the slab melt component uniquely recorded by the Miocene trondhjemites represents a likely constituent for most volcanic sequences of the Mexican arc, and suggest that a modern andesitic continental crust can be constructed directly from mantle-modified slab melts without a basaltic precursor.

Sandstone Provenance of the Arperos Basin (Sierra de Guanajuato, Central Mexico): Late Jurassic-Early Cretaceous Back-Arc Spreading as the Foundation of the Guerrero Terrane

Three paleogeographic scenarios have been proposed for the Mesozoic volcano-sedimentary successions that compose the Guerrero terrane, western Mexico. In the type 1 scenario, the Guerrero terrane is an exotic Pacific arc accreted to nuclear Mexico by the consumption of a pre-Cretaceous oceanic basin, named Arperos Basin. The type 2 scenario considers the Guerrero terrane as a fringing multiarc system, accreted by the closure of pre-Cretaceous oceanic basin substrates at multiple subduction zones with varying polarities. In the type 3 scenario, the Guerrero terrane is interpreted as a North American west-facing para-autochthonous arc, which was drifted in the paleo-Pacific domain by the opening of the Cretaceous back-arc oceanic Arperos Basin. To test these reconstructions, we present here a combined study that includes geologic mapping, stratigraphy, U-Pb geochronology, and sandstone provenance data from the Arperos Basin in the Sierra de Guanajuato, central Mexico. Our data document that the Arperos Basin developed in a back-arc setting and evolved from continental to open oceanic conditions from the Late Jurassic to the Early Cretaceous. Sandstone provenance analysis shows an asymmetric distribution of the infill sources for the Arperos Basin: continent-recycled sedimentary rocks were deposited along its northeastern side, whereas magmatic arc-recycled clastic rocks developed at its southwestern side. Such asymmetric distribution closely fits with sedimentological models proposed for present-day continent-influenced back-arc basins. On the basis of this evidence, we favor a type 3 scenario for the Guerrero terrane, which is then considered to represent a detached slice of the Mexican leading edge that drifted in the paleo-Pacific domain during back-arc extension and subsequently accreted back to the Mexican craton.

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.

Thermomechanical maturation of the continental crust and its effects on the late Eocene–early Oligocene volcanic record of the Sierra Madre del Sur Province, southern Mexico

We interpret the voluminous late Eocene–early Oligocene volcanic successions of the north-central Sierra Madre del Sur as the eruptive manifestation of a progressive thermomechanical maturation of the crust, driven by sustained igneous activity that affected the region since the early Eocene. Widespread Eocene magmatism and injection of mantle-derived melts into the crust beneath the Michoacán-Puebla area promoted the development of a hot zone extending to upper crustal levels, and the formation of a mature intracrustal magmatic system. Within this context, the intermediate siliceous compositions of the Tilzapotla, Muñeca, and Goleta explosive centres were generated through fractional crystallization, crustal contamination, and anatexis. In particular, decreasing bulk-rock Sr and Eu concentrations and Nd isotopes with increasing silica in the Tilzapotla and Muñeca suites document an evolution through low-pressure fractional crystallization of plagioclase-dominated assemblages, simultaneous with the assimilation of middle–upper crustal materials. In contrast, marked Eu, Sr, and Ba depletions coupled with high and variable Rb/Nd at constant 143Nd/144Nd in the Goleta rhyolites suggest their derivation from partial melting of biotite-bearing quartz-feldspathic lithologies. Ascent of the thermal anomaly induced by magma emplacement and accumulation at shallow depths shifted the brittle–ductile crustal transition close to the surface, and produced an ignimbrite flare-up through caldera-forming eruptions. A different petrogenetic–volcanologic scenario developed in north-western Oaxaca, where less profuse early–middle Eocene igneous activity and an ancient lower crustal basement made up of refractory granulitic lithologies inhibited the expansion of the hot zone to shallow levels, and constrained magmatic evolution at depth. Here, composite and monogenetic volcanoes with intermediate compositions were produced through high-pressure fractional crystallization and crustal contamination. Specifically, increasing La/Yb and Sm/Yb with increasing silica in the Oaxaca suite, and negative correlations of Nd isotopes with SiO2 at low Rb/Nd, suggest garnet fractionation from parental basalts, coupled with the assimilation of Rb-depleted lower crustal materials.

Geochronology and magmatic evolution of the Huautla volcanic field: last stages of the extinct Sierra Madre del Sur igneous province of southern Mexico

The Huautla volcanic field (HVF), in the Sierra Madre del Sur (SMS), is part of an extensive record of Palaeogene magmatism reflecting subduction of the Farallon plate along the western edge of North America. Igneous activity resulting from Farallon subduction is also exposed to the north, in the Sierra Madre Occidental (SMO) and Mesa Central (MC) provinces. We present the results of a stratigraphic and K–Ar, Ar–Ar, and U–Pb geochronological study of the Huautla volcanic successions, in order to refine our knowledge on the petrologic and temporal evolution of the northern SMS and gain insights on magmatic–tectonic contrasts between the SMS and the SMO–MC provinces. The HVF is made up of lava flows and pyroclastic successions that overlie marine Cretaceous sequences and post-orogenic continental deposits of Palaeogene age. In the study area, the main Oligocene succession is pre-dated by the 36.7 million years its caldera west of the Sierra de Huautla. The HVF succession ranges in age from ∼33.6 to 28.1 Ma and comprises a lower group of andesitic–dacitic lava flows, an intermediate sequence of ignimbrites and dacitic lavas, and an upper group of andesitic units. The silicic succession comprises a crystal-poor ignimbrite unit (i.e. the Maravillas ignimbrite; 31.4 ± 0.6, 32.0 ± 0.4 Ma; ∼260 km3), overlain by a thick succession of dacitic lavas (i.e. the Agua Fría dacite; 30.5 ± 1.9, 31.0 ± 1.1 Ma). Integration of the new stratigraphic and geochronological data with prior information from other explosive centres of the north-central SMS allows us to constrain the temporal evolution of a silicic flare-up episode, indicating that it occurred between 37–32 Ma; it consisted of three major ignimbrite pulses at ∼36.5, ∼34.5, and ∼33–32 Ma and probably resulted from a progressive, mantle flux-driven thermomechanical maturation of the continental crust, as suggested in the HVF by the transition from andesitic to voluminous siliceous volcanism. The information now available for the north-central sector of the SMS also allows recognition of differences between the temporal and spatial evolution of magmatism in this region, and of that documented in the southern SMO and MC provinces, suggesting that such contrasts are probably related to local differences in configuration of the subduction system. At ∼28 Ma, the MC and southern SMO provinces experienced a trenchward migration of volcanism, associated with slab rollback; on the other hand, the broad, more stable distribution of Oligocene magmatism in the central and north oceanic plate was subducting at a low angle.

Petrology of high-grade crustal xenoliths in the Chalcatzingo Miocene subvolcanic field, southern Mexico: buried basement of the Guerrero-Morelos platform and tectonostratigraphic implications

The Miocene Chalcatzingo trondhjemitic volcanic field, sited along the southern margin of the Trans-Mexican Volcanic Belt, is a newly discovered locality with deep-seated crustal xenoliths that provide fundamental petrologic information on the nature of the unexposed metamorphic basement. The volcanic field lies along the eastern edge of the Cretaceous Guerrero-Morelos platform, which juxtaposes the Guerrero and Mixteco terranes of southern Mexico. Xenoliths consist of high temperature to ultra-high temperature metapelites as well as mafic and quartzofeldspathic gneisses, all of which show evidence of multiple granulite to amphibolite facies metamorphism and ductile deformation. A detailed petrologic study of representative xenoliths indicates a metamorphic evolution that apparently followed a clockwise pressure–temperature path leading from biotite-sillimanite1/kyanite(?)-quartz assemblages (M1) to the assemblage plagioclase-garnet-sillimanite2-rutile/ilmenite (M2) with a peak at ∼9–11 kbar and >870°C. These conditions were followed by rapid uplift to <6 kbar and >800°C, which produced the decompression assemblage spinel-cordierite-sillimanite3-corundum ± orthopyroxene ± quartz (M3) before shallow emplacement of the xenolith-bearing trondhjemitic magma. Three possible sources for the xenoliths are considered: (1) early Mesozoic metasediments buried in the middle crust; (2) Precambrian lower crust; and (3) subducted Cenozoic sediments trapped in the mantle wedge. Based on the deep-seated, polymetamorphic nature of the xenoliths, the Nd depleted mantle model age of an orthogneissic xenolith, and on regional tectonostratigraphic considerations, we suggest that the xenolith source was Proterozoic continental crust. Although old zircon inheritance in the host trondhjemite is minimal, it may be explained by a lack of interaction of the magma with the traversed lithosphere. Studies of Palaeogene shallow intrusions exposed 140 km west of Chalcatzingo in the Guerrero terrane (Pepechuca plug) and 80 km southeast of that place in the Mixteco terrane (Puente Negro dikes) reveal the presence of similar very high-grade aluminous xenoliths. However, these magmas were probably generated by partial melting of Triassic–Jurassic metasediments of the Guerrero terrane underplated by basaltic magmas in Jurassic–earliest Cretaceous times or from Precambrian crust assimilated by underplated mafic magmas of Oligocene age, respectively.