Concepción Lázaro

High pressure metamorphism of ophiolites in Cuba

High-pressure metamorphic complexes of ophiolitic material in Cuba trace the evolution of the northern margin of the Caribbean Plate during the Mesozoic. In the northern ophiolite belt of western and central Cuba, these complexes document cold (i.e., mature) subduction of oceanic lithosphere. Age data indicate subduction during pre-Aptian times followed by melange formation and uplift during the Aptian-Albian. The P-T evolution is clockwise with relatively hot geothermal gradient during exhumation (i.e., “Alpine-type”), suggesting that exhumation may have been triggered by unroofing processes ensuing arrest of subduction. It is hypothesized that tectonic processes related to termination of subduction led to formation of characteristic oscillatory zoning of garnet recorded in blocks separated by ca. 800 km along strike of the belt. In eastern Cuba, the complexes document hot subduction with peak conditions at ca. 750 oC, 15-18 kbar followed by near-isobaric cooling (i.e., counterclockwise P-T path). The contrasting petrologic evolution in the two regions indicates that the correlation of eastern and western-central Cuban melanges is doubtful. The age and tectonic context of formation of these hot-subduction complexes is uncertain, but available data are consistent with formation during the Aptian-Albian due to a) the birth of a new subduction zone and/or b) subduction of young oceanic lithosphere or a ridge. Furthermore, tectonic juxtaposition of high-pressure ophiolitic material and subducted platform metasediments in the Escambray complex (central Cuba) that were decompressed under relatively cold geothermal gradients (“Franciscan-type” P-T paths) indicates syn-subduction exhumation during the uppermost Cretaceous (ca. 70 Ma). The diversity of P-T paths, ages and tectonic settings of formation of the high-pressure complexes of ophiolitic material in Cuba document a protracted history of subduction at the northern margin of the Caribbean Plate during the Mesozoic.

Metamorphic evolution of subducted hot oceanic crust (La Corea Mélange, Cuba)

Thermobarometric estimates and predictions of theoretical and experimental isochemical P-T phase diagrams for epidote±garnet amphibolite blocks from the serpentinite mélange of La Corea (eastern Cuba) indicate partial melting of subducted oceanic lithosphere occurred at peak metamorphic conditions of ca. 700 °C and 14 to 15 kbar. These anomalously high geothermal conditions suggest onset of subduction of young oceanic lithosphere of the Proto-Caribbean. The amphibolites have basaltic composition and MORB affinity. Partial melting produced tonalitic-trondhjemitic-granitic melts that crystallized at depth associated with the amphibolites. Calculated retrograde conditions for the amphibolites (450 °C and 8-10 kbar) indicate counterclockwise P-T paths during exhumation in the subduction channel, in agreement with published predictions on thermo-mechanical modeling of onset of subduction of young lithosphere. These findings have important consequences for the plate tectonic configuration of the Caribbean realm since they corroborate the existence of fragments of early subducted young oceanic lithosphere in the eastern Cuba mélanges that indicate subduction of an oceanic ridge during mid-Cretaceous times.

Metamorphic evolution of Triassic rocks from the transition zone between the Maláguide and Alpujárride complexes (Betic Cordilleras, Spain).

A complete sequence of mineral assemblages ranging from late diagenesis to the epizone has been identified in Triassic rocks from “intermediate units” between the Alpujarride and the Malaguide complexes (Betic Cordillera, Spain). These units appear as a set of tectonic slices, the uppermost showing lithological characteristics similar to the Malaguide complex, which change as depth increases, towards lithologies typical of the Alpujarride complex. The study was carried out by X-ray diffraction, optical microscopy, electron microprobe and X-ray fluorescence. The mineralogical composition shows a clear vertical evolution: A dickite-bearing assemblage, characteristic of the upper slices is replaced by sudoite ± pyrophyllite-bearing assemblages, these by trioctahedral chlorite-rich assemblages, and finally by paragonite-bearing assemblages. Examination of bulk-rock chemistries indicates that most of the mineralogical changes are the result of the increasing metamorphic grade, with the exception of chloritoid; its growth does appear to be partly controlled by the rocks chemistry (it is Al 2 O 3 dependent). The evolution of the metamorphic assemblages occurred prior to emplacement of the slices by thrusting and reveals a progressive transition in metamorphic grade between both complexes, although some discontinuities in the KI values between consecutive slices are observed.

Late Jurassic terrane collision in the northwestern margin of Gondwana (Cajamarca Complex, eastern flank of the Central Cordillera, Colombia)

Medium-grade metabasites and metapelites from the Cajamarca Complex (Central Cordillera of Colombia) are in fault contact with the Jurassic Ibague batholith and show a penetrative foliation, locally mylonitic, suggesting intense dynamic–thermal metamorphism. The amphibolites are composed of calcic amphibole + epidote + plagioclase + quartz plus rutile + titanite + apatite + carbonate as accessory phases. Chlorite and albite appear as retrograde replacements. The metapelites are mainly composed of phengite + quartz + garnet + chlorite, plus epidote + albite + apatite + titanite + haematite as accessory phases. Bulk geochemistry of the amphibolites indicates basaltic protoliths with a mid-ocean ridge basalt (MORB) signature, although enrichment in the mobile large-ion lithophile elements compared to MORB suggests pre- and/or syn-metamorphic alteration by fluids. Peak pressure–temperature determinations for both types of rocks are similar, ranging 550–580°C and 8 kbar (approximately 26 km depth and an apparent geothermal gradient of 22°C/km). 40Ar-39Ar dating of amphibole from two amphibolite samples and one phengitic mica from a pelitic schist yielded plateau ages of 146.5 ± 1.1 Ma and 157.8 ± 0.6 Ma, and 157.5 ± 0.4 Ma, respectively. These Late Jurassic ages contrast with previously published (Permian)Triassic ages of metamorphism in the Cajamarca Complex. Taken together, our data indicate tectonic-driven burial of oceanic supracrustal sequences down to mid-crustal depths during Late Jurassic times and are best explained as the result of terrane collision-related metamorphism and deformation in a fore-arc/volcanic-arc environment of the active western margin of Gondwana rather than as a result of Jurassic thermal–metamorphic resetting of a (Permian)Triassic metamorphic sequence during intrusion of the Jurassic Ibague batholith. Our results represent the first report of Jurassic terrane collision tectonics involving supracrustal oceanic rocks in the northwestern margin of Gondwana in Colombia.

Did the Turonian–Coniacian plume pulse trigger subduction initiation in the Northern Caribbean? Constraints from 40Ar/39Ar dating of the Moa-Baracoa metamorphic sole (eastern Cuba)

The Güira de Jauco metamorphic sole, below the Moa-Baracoa ophiolite (eastern Cuba), contains strongly deformed amphibolites formed at peak metamorphic conditions of 650–660°C, approximately 8.6 kbar (~30 km depth). The geochemistry, based on immobile elements of the amphibolites, suggests oceanic lithosphere protholiths with a variable subduction component in a supra-subduction zone environment. The geochemical similarity and tectonic relations among the amphibolites and the basic rocks from the overlying ophiolite suggest a similar origin and protholith. New hornblende 40Ar/39Ar cooling ages of 77–81 Ma obtained for the amphibolites agree with this hypothesis, and indicate formation and cooling/exhumation of the sole in Late Cretaceous times. The cooling ages, geochemical evidence for a back-arc setting of formation of the mafic protoliths, and regional geology of the region allow proposal of the inception of a new SW-dipping subduction zone in the back-arc region of the northern Caribbean arc during the Late Cretaceous (ca. 90–85 Ma). Subduction inception was almost synchronous with the main plume pulse of the Caribbean–Colombian Oceanic Plateau (92–88 Ma) and occurred around 15 million years before arc-continent collision (75 Ma–Eocene) at the northern leading edge of the Caribbean plate. This chronological framework suggests a plate reorganization process in the region triggered by the Caribbean–Colombian mantle plume.

Palaeogeography and crustal evolution of the Ossa–Morena Zone, southwest Iberia, and the North Gondwana margin during the Cambro-Ordovician: a review of isotopic evidence

ABSTRACT Cambro-Ordovician palaeogeography and fragmentation of the North Gondwana margin is still not very well understood. Here we address this question using isotopic data to consider the crustal evolution and palaeogeographic position of the, North Gondwana, Iberian Massif Ossa–Morena Zone (OMZ). The OMZ preserves a complex tectonomagmatic history: late Neoproterozoic Cadomian orogenesis (ca. 650–550 Ma); Cambro-Ordovician rifting (ca. 540–450 Ma); and Variscan orogenesis (ca. 390–305 Ma). We place this evolution in the context of recent North Gondwana Cambro-Ordovician palaeogeographic reconstructions that suggest more easterly positions, adjacent to the Sahara Metacraton, for other Iberian Massif zones. To do this we compiled an extensive new database of published late Proterozoic–Palaeozoic Nd model ages and detrital and magmatic zircon age data for (i) the Iberian Massif and (ii) North Gondwana Anti-Atlas West African Craton, Tuareg Shield, and Sahara Metacraton. The Nd model ages of OMZ Cambro-Ordovician crustal-derived magmatism and Ediacaran-Ordovician sedimentary rocks range from ca. 1.9 to 1.6 Ga, with a mode ca. 1.7 Ga. They show the greatest affinity with the Tuareg Shield, with limited contribution of more juvenile material from the Anti-Atlas West African Craton. This association is supported by detrital zircons that have Archaean, Palaeoproterozic, and Neoproterozoic radiometric ages similar to the aforementioned Iberian Massif zones. However, an OMZ Mesoproterozoic gap, with no ca. 1.0 Ga cluster, is different from other zones but, once more, similar to the westerly Tuareg Shield distribution. This places the OMZ in a more easterly position than previously thought but still further west than other Iberian zones. It has been proposed that in the Cambro-Ordovician the North Gondwana margin rifted as the Rheic Ocean opened diachronously from west to east. Thus, the more extensive rift-related magmatism in the westerly OMZ than in other, more easterly, Iberian Massif zones fits our new proposed palaeogeographic reconstruction.

Petrogenesis and 40Ar/39Ar dating of proto-forearc crust in the Early Cretaceous Caribbean arc: The La Tinta mélange (eastern Cuba) and its easterly correlation in Hispaniola

ABSTRACT The La Tinta mélange is a small but singular ultramafic mélange sheet that crops out in eastern Cuba. It is composed of dolerite-derived amphibolite blocks embedded in a serpentinite matrix. The amphibolite blocks have mid-ocean ridge basalt (MORB)-like composition showing little if any imprint of subduction zone component, similar to most forearc and MOR basalts worldwide. Relict Cr-spinel and olivine mineral chemistry of the serpentinized ultramafic matrix suggest a forearc position for these rocks. These characteristics, together with a hornblende 40Ar/39Ar age of 123.2 ± 2.2 Ma from one of the amphibolite blocks, suggest that the protoliths of the amphibolite blocks correspond to forearc basalt (FAB)-related rocks that formed during the earlier stage of subduction initiation of the Early Cretaceous Caribbean arc. We propose that the La Tinta amphibolites correspond to fragments of sills and dikes of hypoabyssal rocks formed in the earlier stages of a subduction initiation scenario in the Pacific realm (ca. 136 Ma). The forearc dolerite-derived amphibolites formed by partial melting of upwelling fertile asthenosphere at the beginning of subduction of the Proto-Caribbean (Atlantic) slab, with no interaction with slab-derived fluids/melts. This magmatic episode probably correlates with Early Cretaceous basic rocks described in Hispaniola (Gaspar Hernandez serpentinized peridotite-tectonite). The dikes and sills cooled and metamorphosed due to hydration at low pressure (ca. 3.8 kbar) and medium to high temperature (up to 720ºC) and reached ca. 500ºC at ca. 123 Ma. At this cooling stage, serpentinite formed after hydration of the ultramafic upper mantle. This process might have been favoured by faulting during extension of the forearc, indicating an early stage of dike and sill fragmentation and serpentinite mélanges formation; however, full development of the mélange likely took place during tectonic emplacement (obduction) onto the thrust belt of eastern Cuba during the latest Cretaceous.

Trace-element geochemistry of transform-fault serpentinite in high-pressure subduction mélanges (eastern Cuba): implications for subduction initiation

ABSTRACT The Sierra del Convento and La Corea mélanges (eastern Cuba) are vestiges of a Cretaceous subduction channel in the Caribbean realm. Both mélanges contain blocks of oceanic crust and serpentinite subducted to high pressure within a serpentinite matrix. The bulk composition of serpentinite indicates spinel-harzburgite and -herzolite protoliths. The samples preserve fertile protolith signatures that suggest low melting degrees. High concentration of immobile elements Zr, Th, Nb, and REE contents (from ~0.1 to ~2 CI-chondrite) point to early melt–rock interaction processes before serpentinization took place. Major- and trace-element compositions suggest an oceanic fracture-zone–transform-fault setting. A mild negative Eu anomaly in most samples indicates low-temperature fluid–rock interaction as a likely consequence of seawater infiltration during oceanic serpentinization. A second, more important, serpentinization stage is related to enrichment in U, Pb, Cs, Ba, and Sr due to the infiltration of slab-derived fluids. The mineral assemblages are mainly formed by antigorite, lizardite, and chlorite, with local minor talc, tremolite, anthophyllite, dolomite, brucite, and relict orthopyroxene. The local presence of anthophyllite and the replacements of lizardite by antigorite indicate a metamorphic evolution from the cooling of peridotite/serpentinite at the oceanic context to mild heating and compression in a subduction setting. We propose that serpentinites formed at an oceanic transform-fault setting that was the locus of subduction initiation of the Proto-Caribbean basin below the Caribbean plate during early Cretaceous times. Onset of subduction at the fracture zone allowed the preservation of abyssal transform-fault serpentinites at the upper plate, whereas limited downward drag during mature subduction placed the rocks in the subduction channel where they tectonically mixed with the upward-migrating accreted block of the subducted Proto-Caribbean oceanic crust. Hence, we suggest that relatively fertile serpentinites of high-pressure mélanges were witness to the onset of subduction at an oceanic transform-fault setting.