Generation of magmatism under active continental margins: A thermodynamic study of subduction and translithospheric diapirs

A recent model of continental arc magmatism states that subducted sediments mix physically with the oceanic slab basalts in the subduction channel and produce diapirs that ascend through the mantle wedge, undergo melting and relaminate to the base of the continental crust. From here, melt or magma batches ascend through the crust, producing the Cordilleran batholiths and associated volcanism ranging in composition from 0 to 25 wt% maficity (FeOt + MgO) and 44–79 wt% SiO2, with a gap around 11.5 wt% maficity and 57–60 wt% SiO2. This model has been predicted by thermomechanical numerical studies and later supported by phase equilibrium experiments; however, thermodynamic modelling to verify whether the composition of the Cordilleran batholiths is reproduced by this model has not yet been carried out to complement the experimental approach. In this article, the evolution of the mélange along the subduction path and during the ascent of the diapirs is investigated, with focus on the conditions of generation of the diapirs, the composition and proportion of melt at the P-T conditions of relamination of the diapirs, the relationship between the basalt:sediment proportion (the composition) of the diapir to this melt composition and proportion and whether the composition of the melts match the Cordilleran batholiths. The key point to test is the process or processes that produce the compositional variability observed in these granitoids. Our findings indicate that (1) parental melts and/or magmas range in maficity from ∼1 to ∼11.5 wt%, with the compositional gap of the Cordilleran trend representing the most mafic composition possible of the parent, (2) the diapirs are restricted to having 50–60% of sediment component: diapirs with lower sediment contents may be too dense to detach from the slab and mélanges with higher sediment contents may be too less dense to be subducted to the depth of formation of diapirs, (3) melts at 1100 °C and 1.5 GPa from these diapirs have the bulk granodioritic composition of the batholiths, (4) restite unmixing occurs when magma batches segregate from the relaminated diapirs, (5) the lower the temperature of the region of the diapir from which the batch segregates, the higher the restite content, (6) the temperature, by controlling the restite content, is the ultimate factor controlling the compositional trends of Cordilleran batholiths, and (7) fractional crystallization or cotectic evolution is still needed to account for the full compositional range of these batholiths, from the parental 1–11.5 wt% to the observed 0–25 wt% in maficity.This investigation was funded by internal funds of the University of Los Andes, Colombia (FAPA – Fondo de Ayuda a Profesores Asistentes), FAPA number INV- 2019-63-1701

Metamorphic gradient modification in the Early Cretaceous Northern Andes subduction zone: A record from thermally overprinted high-pressure rocks

New field observations and petrological data from Early Cretaceous metamorphic rocks in the Central Cordillera of the Colombian Andes allowed the recognition of thermally overprinted high-pressure rocks derived from oceanic crust protoliths. The obtained metamorphic path suggests that the rocks evolved from blueschist to eclogite facies towards upper amphibolite to high-pressure granulite facies transitional conditions. Eclogite facies conditions, better recorded in mafic protoliths, are revealed by relic lawsonite and phengite, bleb- to worm-like diopside-albite symplectites, as well as garnet core composition. Upper amphibolite to high pressure granulite facies overprinting is supported by coarse-grained brown-colored Ti-rich amphibole, augite, and oligoclase recrystallization, as well as the record of partial melting leucosomes. Phase equilibria and pressure-temperature (P-T) path modeling suggest initial high-pressure metamorphic conditions M1 yielding 18.2–24.5 kbar and 465–580 °C, followed by upper amphibolite to high pressure granulite facies overprinting stage M2 yielding 6.5–14.2 kbar and 580–720 °C. Retrograde conditions M3 obtained through chlorite thermometry yield temperatures ranging around 286–400 °C at pressures below 6.5–11 kbar. The obtained clockwise P-T path, the garnet zonation pattern revealing a decrease in Xgrs/Xprp related to Mg# increment from core to rim, the presence of partial melting veins, as well as regional constraints, document the modification of the thermal structure of the active subduction zone in Northern Andes during the Early Cretaceous. Such increment of the metamorphic gradient within the subduction interface is associated with slab roll-back geodynamics where hot mantle inflow was triggered. This scenario is also argued by the reported trench-ward magmatic arc migration and multiple extensional basin formation during this period. The presented example constitutes the first report of Cretaceous roll-back-related metamorphism in the Caribbean and Andean realms, representing an additional piece of evidence for a margin-scale extensional event that modified the northwestern border of South America during the Early Cretaceous.The National University of Colombia is acknowledged for its financial support, providing execution of the Hermes 30362 project and a travel grant to D. S. Avellaneda-Jiménez for analytical data acquisition at the University of Arizona (Tucson, Arizona)

Subduction of young oceanic plates : a numerical study with application to aborted thermal‐chemical plumes

We investigated numerical models of initiation and subsequent evolution of subduction of young (10-30 Myr) oceanic lithosphere. Systematic numerical experiments were carried out by varying the age of the subducting plate (10, 12.5, 15, 17.5, 20, 25 and 30 Myr), the rate of induced convergence (2, 4 and 5 cm/yr) and the degree of hydration (0 and 2 wt% H(2)O) of the pre-existing weak oceanic fracture zone along which subduction is initiated. Despite the prescribed plate forcing, spontaneously retreating oceanic subduction with a pronounced magmatic arc and a backarc basin was obtained in a majority of the experiments. It was also found that the younger age of oceanic lithosphere results in more intense dehydration and partial melting of the slab during and after the induced subduction initiation due to the shallow dispositions of the isotherms. Partial melting of the subducted young crust may create thermal-chemical instabilities (cold plumes) that ascend along the slab-mantle interface until they either freeze at depth or detach from the slab and penetrate the upper plate lithosphere contributing to the nucleation and growth of a volcanic arc. Freezing of the plumes in the slab-mantle interface is favored by subduction of very young lithosphere (i.e., 10 Myr) at moderate rate (4 cm/yr) of convergence. Such aborted plumes may correspond to Cretaceous partially melted MORB-derived slab material and associated adakitic tonalitic-trondhjemitic rocks crystallized at ca. 50 km depth in the slab-mantle interface and exhumed in a subduction channel (serpentinite melanges) in eastern Cuba.We appreciated constructive comments by Erin Burkett and an anonymous reviewer and recommendations of Editor T. Becker. Blanco-Quintero would like to thank E. Sizova and Y. Mishin for provision of visualization routines, basic instructions and help during his stay in the ETH. We appreciate financial support from Spanish MCINN project CGL2009-12446 and support from Swiss National Science Foundation research grants 200020_129487, 200020_126832, 200021_113672/1, 200021_116381/1, 4D-Adamello, Swiss Federal Institute of Technology research grants ETH-06 09-2, ETH-08 07-2 and Crystal2Plate program to TVG. This is a contribution to IGCP-546 "Subduction zones of the Caribbean." Blanco-Quintero is supported by grant AP2005-5258 from the "Programa de Formacion del Profesorado Universitario" of the Spanish Ministry of Education

Introducing Geosciences in a blended Education Master degree

[EN] The aim of the project was to introduce the subject of geosciences to a group of teachers without previous knowledge of the subject. Since the students of module presented here work full time, the master was designed in a blended format. While all students had completed their university education, none had done any geoscience studies/courses before making the starting level basic for the whole course. Contents and activities were carefully selected to provide a good introduction to geosciences giving the students the basic concepts and letting them apply them in a problem based strategy. Students reported that besides the blended nature of the course collaborative learning was necessary and key to their learning process. They also reported the contents of the course to be clear, organized sufficient and interesting. We view the module as a success since all students reported enjoying the module and building novel knowledge in the field of geosciences. Additionally since the students are in turn teachers we were very happy to learn some of them will implement geosciences in their activities in the future.http://ocs.editorial.upv.es/index.php/HEAD/HEAD18Huguet, C.; Francisco Blanco-Quintero, I.; Henao Mejía, MC.; Moreno Vela, FJ.; Chimbí Sanchez, LA. (2018). Introducing Geosciences in a blended Education Master degree. Editorial Universitat Politècnica de València. 107-116. https://doi.org/10.4995/HEAD18.2018.7905OCS10711

High-pressure greenschist to blueschist facies transition in the Maimón Formation (Dominican Republic) suggests mid-Cretaceous subduction of the Early Cretaceous Caribbean Arc

The Maimón Formation (Cordillera Central, Dominican Republic) is formed of metamorphosed bi-modal mafic-felsic volcanic rocks and sedimentary horizons of Early Cretaceous age deposited in the forearc of the nascent Caribbean island arc. Two structural-metamorphic zones depict an inverted metamorphic gradient: the Ozama shear zone, which records intense mylonitic and phyllonitic deformation and ubiquitous metamorphic recrystallization, tectonically overlies the much less deformed and variably recrystallized rocks of the El Altar zone. The presence of ferri-winchite and high-Si phengite, first reported in this paper, in the peak metamorphic assemblage of rocks of the Ozama shear zone (actinolite + phengite + chlorite + epidote + quartz + albite ± ferri-winchite ± stilpnomelane) point to subduction-related metamorphism. Pseudosection calculations and intersection of isopleths indicate peak metamorphic conditions of ~ 8.2 kbar at 380 °C. These figures are consistent with metamorphism in the greenschist/blueschist facies transition, burial depths of ~ 25-29 km and a thermal gradient of ~ 13-16 °C/km. Our new data dispute previous models pointing to metamorphism of Maimón rocks under a steep thermal gradient related to burial under a hot peridotite slice. Instead, we contextualize the metamorphism of the Maimón Formation in a subduction scenario in which a coherent slice of the (warm) Early Cretaceous forearc was engulfed due to intra-arc complexities and regional-scale-driven tectonic processes operating in the late Early Cretaceous. Integration of our findings with previous studies on metamorphic complexes in Hispaniola suggests that a major tectonic event affecting the whole arc system took place at c. 120-110 Ma

Fluid flow in the subduction channel: Tremolite veins and associated blackwalls in antigoritite (Villa Clara serpentinite mélange, Cuba)

Exotic blocks of massive antigorite-serpentinite (antigoritite) document a deep-seated subduction channel in the Villa Clara serpentinite-matrix mélange, central Cuba. The petrological and geochemical characteristics of antigoritite allow distinguishing two types of rock: i) antigoritite and ii) dolomite-bearing antigoritite. Both types are intimately related in field exposures and represent deep peridotite infiltrated by H2O-CO2 fluid mixtures that triggered antigoritization and local carbonation. Fluid infiltration continued after antigoritization forming a vein network as a potential response to hydrofracturing that precipitated tremolitite in the veins and triggered fluid-antigoritite reaction forming blackwalls. The mineralogical and chemical zoning in the blackwalls (Atg + Chl + Tr adjacent to antigoritite and Chl + Tr adjacent to the tremolitite vein) attest for multi-step metasomatic processes during fluid-rock interaction characterized by advection of infiltrating fluid towards the blackwall and, possibly, by diffusion out of the blackwall towards the fluid-filled vein. Tentative thermodynamic modeling of the blackwall domain Atg + Chl + Tr points vein network formation at 400–500 °C and 5–10 kbar during exhumation in the subduction channel, suggesting the infiltration of deep-seated pressurized fluid that triggered hydrofracturing. The chemical compositions of antigoritites, veins and blackwalls indicate a LILE- and LREE-enriched fluid evolved from the subducting plate, while Srsingle bondNd isotope systematics are compatible with an external fluid composed of a mixture of fluids evolved from sediments and, probably to a lesser extent, altered oceanic crust.This research was funded by projects MICINN PID2019-105625RB-C21 (co-funded by Fondo Europeo de Desarrollo Regional, FEDER), Junta de Andalucía P20_00550, Catalonian project SGR 2014-1661 and the University of Granada. LD acknowledges PhD grant BES-2013-063205 of the Spanish Ministry of Economy and Competitiveness and scholarship of Fundació Universitària Agustí Pedro i Pons. Funding for open access charge: Universidad de Granada / CBUA

Mantle-to-crust metal transfer by nanomelts

The transfer of chalcophile metals across the continental lithosphere has been traditionally modeled based on their chemical equilibrium partitioning in sulfide liquids and silicate magmas. Here, we report a suite of Ni-Fe-Cu sulfide droplets across a trans-lithospheric magmatic network linking the subcontinental lithospheric mantle to the overlying continental crust. Petrographic characteristics and numerical calculations both support that the sulfide droplets were mechanically scavenged from the mantle source during partial melting and transported upwards by alkaline magmas rising through the continental lithosphere. Nanoscale investigation by high-resolution transmission electron microscopy (HR-TEM) documents the presence of galena (PbS) nanoinclusions within the sulfide droplets that are involved in the mantle-to-crust magma route. The galena nanoinclusions show a range of microstructural features that are inconsistent with a derivation of PbS by exsolution from the solid products of the Ni-Fe-Cu sulfide liquid. It is argued that galena nanoinclusions crystallized from a precursor Pb(-Cu)-rich nanomelt, which was originally immiscible within the sulfide liquid even at Pb concentrations largely below those required for attaining galena saturation. We suggest that evidence of immiscibility between metal-rich nanomelts and sulfide liquids during magma transport would disrupt the classical way by which metal flux and ore genesis are interpreted, hinting for mechanical transfer of nanophases as a key mechanism for sourcing the amounts of mantle-derived metals that can be concentrated in the crust.This research was supported by the BES-2017-079949 Ph.D. fellowship to E.S. The Spanish projects PID2019-111715GB-I00/AEI/10.13039/501100011033, NANOMET PID2022-138768OB-I00, and MECOBE ProyExcel_00705 provided funding for field emission gun–environmental scanning electron microscopy (FEG-ESEM), focused-ion beam (FIB) and high-resolution transmission electron microscopy (HR-TEM). M.F. and F.P. acknowledge financial support from the Australian Research Council through ARC Linkage Project LP190100785

Cadomian metabasites of the Eastern Pyrenees revisited

This study presents a new geochemical, petrological, and geochronological U–Pb dataset from Ediacaran metabasites of the Canigó and Cap de Creus massifs, Eastern Pyrenees. The rocks are composed of calcic amphibole + plagioclase + chlorite + epidote ± quartz plus titanite + apatite + ilmenite ± biotite ± rutile as accessory phases and show relict igneous textures. Peak pressure-temperature determinations share common conditions, ranging 452–482ºC and 5.2–7.7kbar. These intermediate P-T conditions suggest Barrovian-type metamorphism, most likely related to a collisional setting. The metabasites correspond to evolved basaltic rocks (Mg#<0.55) with moderate TiO2 content (up to 2.08wt.%) and relatively low Cr (43–416ppm). The rocks are moderately enriched in light rare earth elements (LREE) relative to heavy rare earth elements (HREE) (average (La/Lu)n of 2.7) and the N-MORB normalized multi-element patterns show negative slopes, with prominent negative Nb anomalies ((Nb/La)NMORB=0.33–0.78). These variations are akin to island arc tholeiites generated in back-arc basins and to other metabasites described in the Eastern Pyrenees with a putative Ediacaran age, and they differ from the Ordovician tholeiitic metabasites from the Canigó massif, which derived from a contaminated E-MORB source. The positive ƐNd(T) values (0.82–3.05) of the studied metabasites preclude a notable contribution from an older continental crust. U-Pb dating (LA-ICP-MS) of one chlorite-rich schist sample in contact with the metabasites from the Canigó massif yielded a main peak at ca. 632Ma. We argue that the Cadomian metabasites from the Pyrenees formed during back-arc extension in the continental margin of Gondwana and were later affected by (probably early Variscan) medium-P metamorphism before the HT-LP metamorphism classically described in the Pyrenees.This research was financially supported by the Spanish Projects CGL2017-87631-P and PGC2018-093903-B-C22, Ministerio de Ciencia, Innovación y Universidades/Agencia Estatal de Investigación/Fondo Europeo de Desarrollo Regional, Unión Europea. Additional funding was provided by a Margarita Salas grant to NPS by the Universitat de Barcelona with funds from the Ministerio de Universidades/NextGenerationEU/PRTR/Plan de recuperación, transformación y resiliencia

Metamorphosed volcanic ultramafic rocks associated with the Moa-Baracoa ophiolite complex in the Camarioca Sur sector

En el sector Camarioca Sur del municipio de Moa se han documentado bloques exóticos de rocas metamórficas que no guardan relación con las litologías comunes del complejo ofiolítico y sobre los cuales apenas se desarrolla corteza de meteorización. Esta investigación tuvo el propósito de identificar los rasgos estructurales, texturales y mineralógicos de las rocas componentes de estos bloques para establecer su protolito y ambiente de formación. Se encontró que los protolitos de las rocas metamórficas están constituidos de picro-basalto y basaltos, revelando por primera vez rocas vulcanógenas ultramáficas metarmorfizadas asociadas a rocas ultramáficas del complejo ofiolítico en el macizo Moa-Baracoa. Se fundamentó el carácter mantélico de las rocas vulcanógenas ultramáficas metamorfizadas, sustentado en la existencia y los contenidos de los elementos químicos: Ni, Cr, Cu, Co, V, Mg y Ti. Asimismo se corrobora que la génesis de estos basaltos presenta una afinidad con tipo MORB y de arcos de islas, respaldada por los valores de las relaciones entre Al2O3/TiO2 y CaO/TiO2 obtenidas en este estudio.In the Camarioca Sur sector of Moa municipality, exotic blocks of metamorphic rocks that are not related to the common lithology of the ophiolite complex and over which barely any crust develops have been documented. The aim of this study is identifying the textural, structural, and mineralogical features of the component rocks of these blocks to establish their protolith and formation environment. It was found that the protolith of the metamorphic rocks are made of picro-basalt and basalts, revealing for the first time ultramafic volcanic rocks metamorphosed associated with ultramafic rocks of the ophiolite complex in Moa-Baracoa massif. The mantle character of the metamorphosed ultramafic volcanic rocks is based on the existence and contents of the chemical elements: Ni, Cr, Cu, Co, V, Mg and Ti and it is corroborated that the genesis of these basalts has similarity with type MORB and island arcs, supported by the values of relationships between Al2O3/TiO2 and CaO/TiO2 obtained in this study

Cadomian metabasites of the Eastern Pyrenees revisited

This study presents a new geochemical, petrological, and geochronological U-Pb dataset from Ediacaran metabasites of the Canigó and Cap de Creus massifs, Eastern Pyrenees. The rocks are composed of calcic amphibole + plagioclase + chlorite + epidote ± quartz plus titanite + apatite + ilmenite ± biotite ± rutile as accessory phases and show relict igneous textures. Peak pressure-temperature determinations share common conditions, ranging 452-482ºC and 5.2-7.7kbar. These intermediate P-T conditions suggest Barrovian-type metamorphism, most likely related to a collisional setting. The metabasites correspond to evolved basaltic rocks (Mg#<0.55) with moderate TiO2 content (up to 2.08wt.%) and relatively low Cr (43-416ppm). The rocks are moderately enriched in light rare earth elements (LREE) relative to heavy rare earth elements (HREE) (average (La/Lu)n of 2.7) and the N-MORB normalized multi-element patterns show negative slopes, with prominent negative Nb anomalies ((Nb/La)NMORB=0.33-0.78). These variations are akin to island arc tholeiites generated in back-arc basins and to other metabasites described in the Eastern Pyrenees with a putative Ediacaran age, and they differ from the Ordovician tholeiitic metabasites from the Canigó massif, which derived from a contaminated E-MORB source. The positive ƐNd(T) values (0.82-3.05) of the studied metabasites preclude a notable contribution from an older continental crust. U-Pb dating (LA-ICP-MS) of one chlorite-rich schist sample in contact with the metabasites from the Canigó massif yielded a main peak at ca. 632Ma. We argue that the Cadomian metabasites from the Pyrenees formed during back-arc extension in the continental margin of Gondwana and were later affected by (probably early Variscan) medium-P metamorphism before the HT-LP metamorphism classically described in the Pyrenees