An introduction to orogenic andesites and crustal growth

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

The Systematics of Olivine CaO + Cr-Spinel in High-Mg# Arc Volcanic Rocks: Evidence for in-Situ Mantle Wedge Depletion at the Arc Volcanic Front

We investigated the state of the arc background mantle (i.e. mantle wedge without slab component) by means of olivine CaO and its Cr-spinel inclusions in a series of high-Mg# volcanic rocks from the Quaternary Trans-Mexican Volcanic Belt. Olivine CaO was paired with the Cr# [molar Cr/(Cr + Al) *100] of Cr-spinel inclusions, and 337 olivine+Cr-spinel pairs were obtained from 33 calc-alkaline, high-K and OIB-type arc front volcanic rocks, and three monogenetic rear-arc basalts that lack subduction signatures. Olivine+Cr-spinels display coherent elemental and He–O isotopic systematics that contrast with the compositional diversity of the bulk rocks. All arc front olivines have low CaO (0.135 ± 0.029 wt %) relative to rear-arc olivines which have the higher CaO (0.248 ± 0.028 wt %) of olivines from mid-ocean ridge basalts. Olivine 3He/4He–δ18O isotope systematics confirm that the olivine+Cr-spinels are not, or negligibly, affected by crustal basement contamination, and thus preserve compositional characteristics of primary arc magmas. Variations in melt H2O contents in the arc front series and the decoupling of olivine CaO and Ni are inconsistent with controls on the olivine CaO by melt water and/or secondary mantle pyroxenites. Instead, we propose that low olivine CaO reflects the typical low melt CaO of high-Mg# arc magmas erupting through thick crust. We interpret the inverse correlation of olivine CaO and Cr-spinel Cr# over a broad range of Cr# (~10–70) as co-variations of CaO, Al and Cr of their (near) primary host melts, which derived from a mantle that has been variably depleted by slab-flux driven serial melt extraction. Our results obviate the need for advecting depleted residual mantle from rear- and back-arc region, but do not upset the larger underlying global variations of melt CaO high-Mg# arc magmas worldwide, despite leading to considerable regional variations of melt CaO at the arc front of the Trans-Mexican Volcanic Belt

Crustal recycling by subduction erosion in the central Mexican Volcanic Belt

Recycling of upper plate crust in subduction zones, or ‘subduction erosion’, is a major mechanism of crustal destruction at convergent margins. However, assessing the impact of eroded crust on arc magmas is difficult owing to the compositional similarity between the eroded crust, trench sediment and arc crustal basement that may all contribute to arc magma formation. Here we compare Sr–Nd–Pb–Hf and trace element data of crustal input material to Sr–Nd–Pb–Hf–He–O isotope chemistry of a well-characterized series of olivine-phyric, high-Mg# basalts to dacites in the central Mexican Volcanic Belt (MVB). Basaltic to andesitic magmas crystallize high-Ni olivines that have high mantle-like 3He/4He = 7–8 Ra and high crustal δ18Omelt = +6.3–8.5‰ implying their host magmas to be near-primary melts from a mantle infiltrated by slab-derived crustal components. Remarkably, their Hf–Nd isotope and Nd/Hf trace element systematics rule out the trench sediment as the recycled crust end member, and imply that the coastal and offshore granodiorites are the dominant recycled crust component. Sr–Nd–Pb–Hf isotope modeling shows that the granodiorites control the highly to moderately incompatible elements in the calc-alkaline arc magmas, together with lesser additions of Pb- and Sr-rich fluids from subducted mid-oceanic ridge basalt (MORB)-type altered oceanic crust (AOC). Nd–Hf mass balance suggests that the granodiorite exceeds the flux of the trench sediment by at least 9–10 times, corresponding to a flux of ⩾79–88 km3/km/Myr into the subduction zone. At an estimated thickness of 1500–1700 m, the granodiorite may buoyantly rise as bulk ‘slab diapirs’ into the mantle melt region and impose its trace element signature (e.g., Th/La, Nb/Ta) on the prevalent calc-alkaline arc magmas. Deep slab melting and local recycling of other slab components such as oceanic seamounts further diversify the MVB magmas by producing rare, strongly fractionated high-La magmas and a minor population of high-Nb magmas, respectively. Overall, the central MVB magmas inherit their striking geochemical diversity principally from the slab, thus emphasizing the importance of continental crust recycling in modern solid Earth relative to its new formation in modern subduction zones

A Biogeochemical Imprint of the Panama Basin in the North Andean Arc

Abstract Oceans and continents mingle at convergent margins. However, the effects of this interaction in the construction and evolution of the continental crust remain poorly understood. Here we use geochemical data from the Panama Basin and the Northern Volcanic Province of Colombia to reveal that the oceanological and biogeochemical processes of a subducted ocean basin are imprinted in the compositions of continental arc volcanoes. The Panama Basin is a biologically highly productive area of the Eastern Equatorial Pacific in which the strongly biogenic sedimentation is reclassified and preserved differently depending on tectonically controlled depositional environments. Due to a shallow lysocline, sediments deposited on newly formed spreading centers are carbonate‐rich, whereas those accumulated on older subsiding seafloor become gradually richer in terrigenous components, organic carbon and authigenic U. Volcanoes of the North Volcanic Province of Colombia erupt high‐Mg# andesites that are common in some arcs, but display unusually high U contents and a symmetrical or “parabolic‐shaped” along‐arc trace element and isotopic variations that appear unrelated to differentiation or the pre‐existent crustal architecture. Instead, the parabolic‐shaped elemental trends mirror the reconstructed compositional variations of sediments deposited across axis on the currently subducted Sandra and Buenaventura ocean ridges. We interpret that subduction of these ocean ridges delivered a compositionally variable sediment influx that influenced the compositions of arc magmas. These findings demonstrate a strong connectivity between oceans and continents, and further imply that arc volcanoes can be reliable records of the oceanological and biogeochemical conditions of long subducted ocean basins

Mesozoic geologic evolution of the Xolapa migmatitic complex north of Acapulco, southern Mexico: implications for paleogeographic reconstructions

The Xolapa Complex in the Acapulco-Tierra Colorada area, southern Mexico, is made up of orthogneisses and paragneisses, both affected by a variable degrees of migmatization. These rocks are intruded by several episodes of Jurassic-Oligocene, calc-alkaline granitic magmatism. Three phases of ductile deformation affected the gneisses and migmatites. D1 produced an amphibolite-facies metamorphic banding (M1) and a penetrative S1 foliation, axial planar to isoclinal, recumbent F1 folds with axes parallel to a NE to NW, gently plunging, L1 stretching lineation. D2 consists of a S2 foliation defined by hornblende, biotite and garnet, synchronous with M2 migmatization that generated leucosomes, which are generally parallel to S1. D3 is made up of asymmetric, chevron F3 folds that deform the composite S1/S2 foliation during greenschist to lower amphibolite metamorphism (M3). U-Pb SHRIMP (Sensitive High-Resolution Ion Micropobe) geochronology carried out on zircon separated from two orthogneisses yielded an Early Jurassic magmatic event (178.7 ± 1.1 Ma) and the age of migmatization (133.6 ± 0.9 Ma). Two episodes of Pb loss were also recognized, the first at 129.2 ± 0.4 Ma, and the second during the earliest Paleocene (61.4 ± 1.5 Ma); they are probably associated with two episodes of magmatism. The Early Jurassic magmatic arc may be correlated with a magmatic arc in the eastern Guerrero terrane. The Early Cretaceous migmatization is inferred to have resulted from shortening, possibly due to the accretion of an exotic block, such as the Chortís block along whose northern margin contemporaneous, high-pressure metamorphism has been recorded.El Complejo Xolapa es el terreno metamórfico más extenso en el sur de México. En el segmento que va de Acapulco a Tierra Colorada está constituido por ortogneises y paragneises, afectados por un grado variable de migmatización. El basamento del Complejo Xolapa está también afectado por diversos episodios de magmatismo granítico de afinidad geoquímica calcialcalina y edades que van del Jurásico al Oligoceno. Tres fases de deformación dúctil afectan los gneises y migmatitas. Un bandeamiento metamórfico se desarrolla durante D1, y está asociado con una foliación penetrativa S1, en facies de anfibolita, que actúa como plano axial de pliegues de recumbentes a isoclinales F1. Una lineación de estiramiento L1 está presente en toda el área de estudio, con buzamiento tanto al NE como NW. La orientación de las estructuras D1 controla el emplazamiento de los lentes de leucosoma D2, generados durante el proceso de migmatización. Afuera de los dominios leucosomáticos, la foliación S2 está caracterizada por la asociación hornblenda + biotita + granate. Pliegues asimétricos de tipo chevron relacionados a la fase F3 deforman la foliación compuesta S1/S2, en condiciones de facies de esquistos verdes hasta anfibolita. La geocronología de U-Pb por SHRIMP (microsonda iónica de alta resolución) realizada en zircones separados de dos muestras de ortogneises permitió reconocer un evento magmático jurásico temprano (178.7 ± 1.1 Ma), así como la edad de migmatización en esta porción del complejo de Xolapa calculada en 133.6 ± 0.9 Ma. Dos episodios de pérdida de Pb se pueden reconocer en algunos de los zircones de las muestras fechadas. El primero es posterior al pico de migmatización (129.2 ± 0.4 Ma), y el segundo ocurrió durante el Paleoceno temprano (61.4 ± 1.5 Ma), y probablemente ambos son un efecto térmico asociado a dos de los episodios magmáticos posteriores. Estos datos sugieren que un arco magmático del Jurásico temprano habría constituido el basamento del Complejo Xolapa; se sugiere que este mismo arco corresponda a la porción este del terreno Guerrero, en donde algunas edades magmáticas similares han sido reportadas. La migmatización ocurrió durante el Cretácico temprano como consecuencia del acortamiento producido por la accreción de un bloque exótico. El bloque de Chortís es un candidato posible para tal colisión, ya que registra un evento metamórfico de alta presión contemporáneo con el evento de migmatización en el Complejo Xolapa

El terreno Cuicateco: ¿cuenca oceánica con influencia de subducción del Cretácico Superior en el sur de México? Nuevos datos estructurales, geoquímicos y geocronológico

The southestern portion of the Cuicateco terrane, between Matías Romero and Juchitán, Oaxaca, is made up by a low metamorphic grade vulcanosedimentary sequence that shows at least three deformation events, D1: isoclinal-recumbent F1 folding of the S0 surfaces with northeastern verging and development of a S1 axial plane cleavage with southwestern dipping. Parallel to S1 surfaces can be observed the development of hornblende, epidote, chlorite, muscovite and quartz. Such structures are related to a post-Maastrichtian left transcurrent deformational event. D2: Paleocene(?) event characterized by thrust faulting with displacement direction to north-northeast, such as the Vista Hermosa mesostructure fault, and the development of the S2 cleavage without associated recrystallization. D3: Miocenic event, defi ned by normal and lateral faulting. Chemical composition of the amphibolitic rocks of the Cuicateco terrane resembles that of tholeiitic basalts (SiO2 45�50%), with a depletion in LREE and fl at HREE patterns. Trace elements show a pattern depleted in the highly incompatible elements with respect to the less incompatible elements, and negative anomalies of Nb and Th. A contribution of subduction-related magmas is also shown by the Ta/Yb vs. Th/Yb relationships. U-Pb dating by LA-MC-ICPMS on zircons belonging to the amphibolite yielded a Maastrichtian age (65.7 ± 1.2 Ma) that is interpreted as the crystallization age of the basaltic protolith. U-Pb analyses on detrital zircons of a vulcanoclastic phyllite suggest a maximum depositional Maastrichtian age (~78 Ma) with signifi cant peaks at 113 Ma, 542 Ma, 1198 Ma, 1662 Ma, 2791 and 3062 Ma. Zircons from a sandstone belonging to the Todos Santos Formation indicate a maximum depositional age of the Middle-Upper Triassic (228 Ma), with important peaks at 571 Ma, 1026 Ma, 1530 Ma, 1978 Ma and 2909 Ma. Petrological, structural and geochronological data suggest the development of a mafi c volcanism and sedimentation in an ocean basin and close relationships with a subduction process during the Late Cretaceous. This volcanic-sedimentary sequence was deformed by two events: a transpressive to compressive deformation, during the Latest Cretaceous � Paleocene, and a later, Miocenic normal-lateral faulting, which can be associated to the Polochic-Motagua faulting tectonic systemLa porción suroriental del terreno Cuicateco, entre Matías Romero y Juchitán, Oaxaca, está compuesta por una secuencia vulcanosedimentaria de bajo grado metamórfi co que presenta por lo menos tres eventos de deformación, D1: plegamiento recumbente a isoclinal F1 de la superfi cie de estratifi cación S0, con una vergencia general hacia el noreste y desarrollo de una esquistosidad de plano axial S1 con echado hacia el suroeste. Paralelo a las superfi cies S1 se observa el desarrollo de hornblenda, epidota, clorita, moscovita y cuarzo. Este deformación está relacionada a un evento transcurrente con cinemática sinestral de edad post-maastrichtiana. D2: evento paleocénico? caracterizado por cabalgaduras con dirección de desplazamiento al norte-noreste, como la falla Vista Hermosa, y el desarrollo de la superfi cie de esquistosidad S2 sin una aparente recristalización asociada. D3: evento miocénico defi nido por fallamiento de tipo normal y lateral. La composición química de las rocas anfi bolíticas del terreno Cuicateco es de basalto toleítico (SiO2 de 45 a 50%), con empobrecimientos relativos en tierras raras ligeras y patrones casi planos de tierras raras pesadas. Los elementos en traza muestran un patrón empobrecido en elementos más incompatibles con respecto a los menos incompatuble y anomalías negativas de Th y Nb en algunas muestras. Una componente de magmas relacionados con subducción es también evidente por su relación Ta/Yb vs. Th/Yb. Los fechamientos de U-Pb por LA-MC-ICPMS en zircones de una anfi bolita arrojaron una edad de 65.7 ± 1.2 Ma (Maastrichtiano) interpretada como la edad de cristalización del protolito basáltico. De igual manera, los zircones detríticos de una fi lita vulcanoclástica indican una edad máxima de depósito maastrichtiana (78 Ma) con picos signifi cativos en 113 Ma, 542 Ma, 1198 Ma, 1662 Ma, 2791 y 3062 Ma. Los zircones de una arenisca de la Formación Todos Santos arrojaron una edad máxima del depósito posterior al Triásico Medio - Superior (228 Ma) con picos importantes en 571 Ma, 1026 Ma, 1530 Ma, 1978 Ma y 2909 Ma. Los resultados petrológicos, estructurales y geocronométricos sugieren el desarrollo de un vulcanismo básico en un ambiente de cuenca oceánica con infl uencia de subducción durante el Cretácico Superior que sufrió los efectos de dos deformaciones, una transpresiva y otra compresiva, durante el Cretácico Superior¿Paleoceno, seguido por un fallamiento normal y lateral miocénico que se asocia a la dinámica del sistema de fallas Polochic-Motagua

Geochemical analysis on lavas from Nevado de Toluca, Central Mexican Volcanic Belt and from the Cocos plate

This study presents evidence that Quaternary frontal arc calc-alkaline lavas from Central Mexican Volcanic Belt (CMVB) contain contributions from partial melts of the subducting garnet-bearing eclogitic oceanic crust and sediment, based on chemical and Hf-Nd isotope data. The CMVB includes both calc-alkaline lavas with arc-type trace element patterns such as aqueous fluid mobile element enrichments and high field strength element depletions; and "high-Nb" alkaline lavas with trace element patterns similar to ocean island basalts. The two types of lavas are closely related geographically and temporally. Distinct from the high-Nb lavas, the calc-alkaline lavas show trends toward higher 176Hf/177Hf and 143Nd/144Nd ratios coupled with lower Lu/Hf. The high Hf-Nd isotope ratios fingerprint contributions of subducted basaltic ocean crust, while the correlation with low Lu/Hf indicates melting in the presence of residual garnet, which reflects conversion of the subducted oceanic crust to eclogite. Isotopic and chemical mass balance considerations indicate that the slab melts are ~ 80% basaltic oceanic crust and ~ 20% subducted sediment. The calc-alkaline lavas have higher SiO2 at a given Mg# compared to the high-Nb alkaline lavas, also reflecting melt contributions from the subducted slab. A survey of global arc lavas shows that calc-alkaline lavas with low Lu/Hf ratios, reflecting melting in the presence of residual garnet and preferential mobilization of Hf over Lu from the subducted slab, are generally associated with hot slab conditions. These include arcs where young (< 30 Ma old) ocean crust is subducted (e.g. Mexican Volcanic Belt, Cascades, Austral Andes, Luzon, Setouchi), where slab tearing occurred and hot asthenospheric mantle could upwell through the slab window (e.g., western Aleutians, Sunda, southern Scotia), and where oblique or slow subduction leads to higher slab temperatures (e.g. Lesser Antilles, western Aleutians). In some of these hot slab arcs, where low Lu/Hf ratios are coupled with high Nd-Hf isotope ratios, slab melt contributions are dominated by partial melts from the subducted oceanic basalt (e.g., Mexican Volcanic Belt, Aleutians and Cascades). In other hot slab arcs, low Lu/Hf ratios are coupled with low Nd-Hf isotope ratios, reflecting slab contributions dominated by sediment melts (e.g. Setouchi, Lesser Antilles, Luzon, Sunda, and southern Scotia). Arcs associated with colder subducted oceanic crust (e.g. Izu-Bonin-Marianas, Tonga-Kermadec, central and northern Scotia) erupt lavas with high Lu/Hf along with high Hf-Nd isotope ratios, similar to mid-ocean ridge basalts, thus they lack the signature of residual garnet as well as significant slab melt input