Arturo Gómez-Tuena

Source contamination versus assimilation: an example from the Trans-Mexican Volcanic Arc

Abstract Volcanic samples representing a wide range of lithologies and compositions were collected from the Miocene to Quaternary age Michoacan-Guanajuato volcanic field (MGVF) in the Trans-Mexican volcanic belt (TMVB). The samples were analyzed for major and trace elements, and 87Sr/86Sr and 187Os/188Os in an effort differentiate the importance of source contamination and assimilation in continental arc magmatism. Re concentrations in the MGVF samples range from 0.03 to 0.13 ppb and Os concentrations range from 0.05 to 0.001 ppb. The 87Sr/86Sr of the samples vary little, ranging from 0.7037 to 0.7047, despite a wide range in major element composition. However, the 187Os/188Os vary greatly, from 0.135 to 0.410. Decreasing Os concentration and increasing 187Os/188Os show a clear relation with indicators of fractionation such as MgO or Ni. A plot of 187Os/188Os versus Ba/Nb for all samples from the MGVF show two distinct trends: (1) a wide variation in Ba/Nb (50–200) associated with minor variations in 187Os/188Os (∼0.135–0.145), and (2) increasing 187Os/188Os (0.145–0.40) associated with restricted Ba/Nb (35–70). These trends are best explained through a dynamic multi-component process. Fluids are released from the subducting slab, resulting in melting of the overlying asthenospheric wedge. The pristine fluids have high Ba and low Re and Os concentrations. The resulting melts have variable Ba/Nb, but unradiogenic 187Os/188Os. Superimposed upon these melts are both assimilation and fractional crystallization processes, which affect both the Ba/Nb and 187Os/188Os systems as they ascend into the lower crust.

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.

Polyphase, High-Temperature Eclogite-Facies Metamorphism in the Chuacús Complex, Central Guatemala: Petrology, Geochronology, and Tectonic Implications

This paper describes the first discovery of eclogite-facies rocks in the Paleozoic Chuacús basement complex of north-central Guatemala. In this area, the complex comprises a thick, polydeformed sequence of high-Al metapelite, amphibolite, and quartzofeldspathic banded gneisses and schists characterized by garnet, phengite, and kyanite. Detailed petrographic, electronprobe microanalyses, and a late Carboniferous U-Pb zircon apparent age indicate that this deeply rooted orogenic terrane may be related to the Alleghenian suturing between Gondwana and Laurentia. Eclogite-facies metamorphism is established by assemblages with omphacite-garnet-rutile ± phengite ± zoisite in mafic rocks, which are consistent with garnet-kyanite-zoisite-rutile-quartz-phengite ± staurolite ± chloritoid assemblages in pelitic rocks, and amphibole-calcite/dolomite/aragonite?- rutile-quartz-zoisite ± clinochlore ± diopside in marbles. Moreover, various textural and mineralogical features (such as radial cracks in garnet and kyanite around quartz inclusions; palisade-like coronas of a silica mineral around quartz in some carbonates; lamellar inclusions of a titaniferous phase in garnet, zoisite, and phengite; and plagioclase or white mica in some omphacite; as well as the relatively high Na2O content of garnet [up to 0.12 wt%]), suggest relict ultrahigh-pressure metamorphism (UHPM). These conditions predated high-temperature-high-pressure hydration and decompression melting that occurred between 18 and 23 kbar and 700-770°C. This decompressional melting event of eclogitic rocks is dated as late Carboniferous by U-Pb on discordant zircons from a leucocratic neosome, and may be associated with the initial closure of Pangea. K-Ar ages of ~70-75 Ma on micas and amphibole, stable at 14 kbar and 597°C, are interpreted to record the Cretaceous obduction of Caribbean ophiolites and arc assemblages onto the Chuacúús complex and the southern edge of the Maya block, along the paleo-Motagua fault zone.

Refining the age of magmatism in the Altos Cuchumatanes, western Guatemala, by LA–ICPMS, and tectonic implications

The Altos Cuchumatanes Range is made up of a core of igneous and metamorphic rocks, surrounded by lower Palaeozoic and Mesozoic sedimentary strata. These units constitute the westernmost exposure of basement rocks in Guatemala and represent some of the most important crustal units in the Maya Block. New laser ablation–inductively coupled plasma mass spectrometry U-Pb zircon geochronology allows better definition of their igneous ages, inheritance and petrologic evolution. The Altos Cuchumatanes magmatism occurred during the Middle Ordovician (461 Ma) and lower Pennsylvanian (312–317 Ma), replicating similar age trends present in southern Mexico (Acatlán Complex) and the Maya Block, from Chiapas to central Guatemala (Rabinal-Salamá area) and Belize (Maya Mountains). The U-Pb inheritance from cores of the studied zircons makes it possible to decipher the pre-magmatic history of the area. During the Late Ordovician to Permo-Carboniferous, the Altos Cuchumatanes and Maya Block were located adjacent to northeastern Mexico, near the Mixteco terrane, where Ordovician megacrystic granites intruded a passive-margin sedimentary sequence. The Ordovician granites present at the southern limit of the Maya Block, in the Altos Cuchumatanes, in central Guatemala and in Belize, are the result of partial crustal melting during the initial opening of the Rheic Ocean, when both Maya and Mixteco terranes would have lain close to NW Gondwana until the closure of that ocean. The crystallization of the early Pennsylvanian granites seems to be the result of an E-dipping subduction zone that accommodated convergence between Laurentia and Gondwana.

A genetic link between silicic slab components and calc-alkaline arc volcanism in central Mexico

Abstract A fundamental question in the formation of orogenic andesites is whether their high melt SiO2 reflects the recycling of silicic melts from the subducted slab or the processing of basaltic mantle melts in the overlying crust. The latter model is widely favoured, because most arc magmas lack the ‘garnet’ signature of partial slab melts. Here we present new trace element data from Holocene high-Mg# >64–72 calc-alkaline basalts to andesites (50–62 wt% SiO2) from the central Mexican Volcanic Belt that crystallize high-Ni olivines with the high 3He/4He=7–8 of the upper mantle. These magmas have been proposed to be partial melts from ‘reaction pyroxenites’, which formed by hybridization of mantle peridotite (c. 82–85%) and heavy rare earth element-depleted silicic slab melt (>15–18%). Forward and inverse models suggest that the absence of a garnet signature in these melts reflects the efficient buffering of the heavy rare earth elements (Ho to Lu) in the subarc mantle. In contrast, all elements more incompatible than Ho – excepting TiO2 – are more or less strongly controlled by the silicic slab flux that also directly contributes to the silicic arc magma formation. Our study emphasizes the strong link between slab recycling and the genesis of orogenic andesites. Supplementary material: Methods, additional data and modelling parameters are available at http://www.geolsoc.org.uk/SUP18686

An introduction to orogenic andesites and crustal growth

Abstract This chapter provides an overview of the current state of research on orogenic andesites. While their importance as proxies to the evolution of the continental crust has long been recognized, andesite genesis has remained highly controversial with a broader consensus yet to be reached. The controversy is fuelled by the question of whether orogenic andesites are primary melts of slab and mantle materials, or instead derivative products of basaltic mantle melts that differentiate in the overlying crust. These hypotheses are addressed in three sections of the book devoted to slab–mantle processes, the complexities of melt differentiation at crustal levels, and models pertaining to arc crustal growth. We believe that cross-fertilization and discussion among seemingly opposite and irreconcilable hypotheses will smooth the pathway towards a holistic communal model of andesite petrogenesis.

Orogenic andesites and crustal growth

Orogenic andesites have long intrigued scientists because of their remarkable compositional similarities to the continental crust. The significance of orogenic andesites as proxies to continental crust formation has been recognized for over 30 years, but no consensus model of andesite genesis exists. Much of the controversy revolves around whether orogenic andesites are primary melts of slab and mantle materials, or instead evolve from basaltic mantle melts at shallower crustal levels. In three sections, this book provides an overview of andesite genesis at convergent margins that focuses on the slab–mantle interaction, crustal processing and andesite evolution through the life of volcanic arcs. Without favouring a particular view, the books aims to engender cross-fertilization and discussion that will smooth the pathway towards a holistic communal model of andesite petrogenesis and its role within the broader geochemical cycles of the Earth.

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.