Antonio García-Casco

Cooling and exhumation of the Western Betic Cordilleras, 40Ar/39Ar thermochronological constraints on a collapsed terrane

New 40Ar/39Ar data on amphiboles, muscovites, biotites and potassium-feldspars from different tectono-metamorphic units of the Western Alpujarrides (Betic Cordilleras, southern Spain) help to constrain the P-T-t evolution of this Alpine collisional belt. During an initial stage of plate convergence between Africa and Eurasia, the Alpujarride metamorphic rocks evolved along increasing pressure-temperature paths, locally reaching eclogitic conditions, but the timing of peak metamorphism is only constrained to be earlier than 25 Ma. In the interval of 25–22 Ma, the Alpujarride rocks underwent strong adiabatic decompression related to the collapse of the previously thickened crust. We propose that the main phase of synmetamorphic ductile deformation and thinning of the metamorphic pile was related to this extensional event. The last step is marked by fast cooling of the hot Alpujarride rocks below 600°C, resulting in a striking convergence of our 40Ar39Ar determinations in the range 19–20 Ma. Cooling rates in the range 100–350°C/m.y. are indicated for this period, associated with exhumation rates of less than 3 km/m.y. We suggest that fast cooling took place primarily as a result of thermal relaxation of the abnormally steep geotherm resulting from extensional tectonics which has the effect of juxtaposing thinned rock bodies with contrasting temperatures along shear zones and faults. The main implication of this data set is that the Western Alpujarrides present a structural and metamorphic development which should be regarded as characteristic of “collapsed terranes”.

CSpace: an integrated workplace for the graphical and algebraic analysis of phase assemblages on 32-bit wintel platforms

Abstract CSpace is a program for the graphical and algebraic analysis of composition relations within chemical systems. The program is particularly suited to the needs of petrologists, but could also prove useful for mineralogists, geochemists and other environmental scientists. A few examples of what can be accomplished with CSpace are the mapping of compositions into some desired set of system/phase components, the estimation of reaction/mixing coefficients and assessment of phase-rule compatibility relations within or between complex mineral assemblages. The program also allows dynamic inspection of compositional relations by means of barycentric plots. CSpace provides an integrated workplace for data management, manipulation and plotting. Data management is done through a built-in spreadsheet-like editor, which also acts as a data repository for the graphical and algebraic procedures. Algebraic capabilities are provided by a mapping engine and a matrix analysis tool, both of which are based on singular-value decomposition. The mapping engine uses a general approach to linear mapping, capable of handling determined, underdetermined and overdetermined problems. The matrix analysis tool is implemented as a task “wizard” that guides the user through a number of steps to perform matrix approximation (finding nearest rank-deficient models of an input composition matrix), and inspection of null-reaction space relationships (i.e. of implicit linear relations among the elements of the composition matrix). Graphical capabilities are provided by a graph engine that directly links with the contents of the data editor. The graph engine can generate sophisticated 2-D ternary (triangular) and 3D quaternary (tetrahedral) barycentric plots and includes features such as interactive re-sizing and rotation, on-the-fly coordinate scaling and support for automated drawing of tie lines.

A 21 ± 2 Ma age for the termination of the ductile alpine deformation in the internal zone of the betic cordilleras, South Spain

Abstract Rb-Sr dating of WR-muscovite pairs from two mica schists and two gneisses from the Velez-Malaga-Torrox area, 40 km E of Malaga, gives tie lines indicating ages of 23.4 ± 2.7, 19.3 ± 2.2, 19.5 ± 0.7 and 22.4 ± 0.7 Ma, respectively. Geological evaluation suggests that these analytical ages indicate an age of 21 ± 2 (2σ) Ma for the metamorphic culmination connected with the latest phase of ductile deformation in the area. This Early Miocene (Aquitanian) age compares well with published radiometric ages for major orogenic processes in the westernmost Mediterranean and it is suggested that a significant part of the Alpine orogeny in the region took place in the restricted period of 19–23 Ma ago. Uplift rates in the order of 3–5 km/Ma are tentatively suggested.

HP-LT rocks exhumed during intra-oceanic subduction: the example of the Escambray massif (Cuba).

High-pressure (HP) metabasites from the Sancti Spiritus dome (Escambray massif, Central Cuba) have been studied in order to better understand the origin and evolution of the Northern Caribbean boundary plate during the Cretaceous, in a global subduction context. Geochemical and petrological studies of these eclogites reveal two groups with contrasting origins and pre-subduction metamorphic histories. Eclogites collected from exotic blocks within serpentinite (mélange zone) originated from a N-MORB type protolith, do not record pre-eclogitic metamorphic history. Conversely eclogites intercalated in Jurassic metasedimentary rocks (non-mélange zone) have a calc-alkaline arc-like origin and yield evidence for a pre-subduction metamorphic event in the amphibolite facies. However, all the studied Escambray eclogites underwent the same eclogitic peak (around 600 C at 16 kbar), and followed a cold thermal gradient during their exhumation (estimated at around 13.5 C km), which can suggest that this exhumation was coeval with subduction. Concordant geochronological data (Rb ⁄ Sr and Ar ⁄Ar) support that the main exhumation of HP ⁄LT rocks from the Sancti Spiritus dome occurred at 70 Ma by top to SW thrusting. The retrograde trajectory of these rocks suggests that the north-east subduction of the Farallon plate continued after 70 Ma. The set-off to the exhumation can be correlated with the beginning of the collision between the Bahamas platform and the Cretaceous island arc that induced a change of the subduction kinematics. The contrasting origin and ante-subduction history of the analysed samples imply that the Escambray massif consists of different geological units that evolved in different environments before their amalgamation during exhumation to form the present unit III of the massif.

Tectonic blocks in serpentinite mélange (eastern Cuba) reveal large-scale convective flow of the subduction channel

Detailed petrological study of mid-oceanic ridge basalt−derived high-pressure amphibolite blocks from a fragment of the Caribbean subduction channel (La Corea serpentinite-matrix melange, eastern Cuba) has revealed contrasted zoning patterns of garnet porphyroblasts, including well-defi ned complex oscillatory prograderetrograde concentric zoning in one sample. Calculated pressuretemperature (P-T) conditions for this sample using mineral inclusion assemblages and isochemical P-T projections reveal large P-T recurrences best explained by large-scale convective movement of the tectonic block in a serpentinitic subduction channel. The P-T conditions attending garnet growth followed an overall counterclockwise path as a consequence of continued refrigeration of the subduction channel during ongoing underfl ow after its onset ca. 120 Ma. These fi ndings constitute the fi rst report of large-scale convective circulation of deeply subducted material in the subduction channel, and are consistent with the thermomechanical behavior of the channel predicted by numerical models.

Paleogene Foredeep Basin Deposits of North-Central Cuba: A Record of Arc-Continent Collision between the Caribbean and North American Plates

Paleogene deposits of north-central Cuba have been identified as a deformed foredeep basin, whose stratigraphy recorded very well the collision of the Bahamas—Proto-Caribbean realm (North American plate) with the Caribbean plate, a process that occurred since latest Cretaceous to early Late Eocene time. The debris incorporated in the foredeep basin has two provenance regions and four tectonostratigraphic sources, including: (1) the Caribbean Plate (1a = allochthonous Cretaceous arcs, 1b = serpentinite mélanges and ophiolites); (2) the North American plate (2a = Pre-Paleogene sedimentary rocks derived from the substrate of the foredeep basin, 2b = Cretaceous Bahamian carbonate platform rocks). Evaluation of the age, size, and volume of the debris demonstrate the formation of a forebulge within the Bahamas platform in response to the collision between the Caribbean and North American plates, and the northeastward migration of the axis of maximum subsidence of the foredeep basin since the Paleocene. By the early Late Eocene, structural NE-SW shortening ended in central Cuba, with uplift and deep erosion, followed by a quick transgression before the end of the Eocene. The resulting Upper Eocene sediments unconformably cover the deformed foredeep deposits and underlying rocks, finishing the formation of the North Cuba—Bahamas fold-and-thrust belt. Palinspastic reconstructions suggest that this belt accommodated nearly 1000 kilometers of shortening, during underthrusting of the Proto-Caribbean crust below the Caribbean Plate.

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.

Thermal structure of the crust in the Gibraltar Arc: Influence on active tectonics in the western Mediterranean

We have modeled thermal structure of the crust in the western Mediterranean on the basis of inversion of heat flow and elevation in the context of Airys isostatic equilibrium. Modeling results reveal dramatic variations in crustal temperatures within the Gibraltar Arc region. The steep gradients in crustal thickness, together with the regional heat flow pattern lie at the origin of temperature anomalies. Temperatures at the base of the crust range between 700°C in the eastern Betics and the connection between the Rif and Tell belts in North Africa, where a high heat flow anomaly occurs. High-temperature zones define a hotter region (>650°C) running SW–NE across the central part of the Alboran Sea. These results agree with other geophysical evidence (e.g., low deep-crust Vp and Pn values) suggesting the occurrence of high temperatures in the deepest crust. According to the estimated silicic composition of the deep crust in the area, temperatures in some regions are appropriate for partial melting under muscovite dehydration conditions (>700°C). We have estimated that average partial melting ranges from ∼12% (maximum XL) in the western Tell region to ∼6% in the eastern Betics. This process modifies physical and mechanical properties of the deep crust enhancing crust-mantle decoupling and deep crustal flow with concomitant surface uplift. These mechanisms explain why high topography and active E–W extension occur transecting the overall orogenic trend of the Gibraltar Arc.

Timing and Evolution of Cretaceous Island Arc Magmatism in Central Cuba: Implications for the History of Arc Systems in the Northwestern Caribbean

SHRIMP and conventional zircon dating place temporal constraints on the evolution of the Cretaceous Volcanic Arc system in central Cuba. The arc has a consistent stratigraphy across strike, with the oldest and deepest rocks in the south (in tectonic contact with the ∼5–10-km-wide Mabujina Amphibolite Complex [MAC]) and younger rocks in the north. The MAC is thought to represent the deepest exposed section of the Cretaceous Volcanic Arc and its oceanic basement in Cuba. We undertook a single zircon geochronological study of five gneisses and two amphibolites from the MAC and seven rocks from the Manicaragua Batholith, which intrudes both the MAC and the Cretaceous Volcanic Arc. A SHRIMP zircon age of Ma for a trondhjemitic orthogneiss (MAC) from the Jicaya River dates the oldest phase of granitoid magmatism in this area and the entire Caribbean (Antillean) region. A tonalitic gneiss collected near the previous sample yielded an age of Ma, and a further tonalitic gneiss had an age of Ma, with one inherited zircon at Ma. Two trondhjemitic orthogneisses from the central part of the MAC yielded ages of and Ma, whereas two amphibolites from the eastern part of the complex provided similar ages of ca. 93 Ma and zircon inheritance at 315, 471, 903, and 1059 Ma. Two weakly foliated Manicaragua granitoids from the eastern part of the massif provided ages of and Ma, whereas five unfoliated granitoid samples from the central and eastern part of the massif yielded ages of , , , , and Ma. Our age data support the view that the Mabujina Protholiths are exotic and formed somewhere NNW along strike of the nonmetamorphosed Cuban arc since pre–Middle Hauterivian time (before ∼133 Ma). The MAC became part of the Cuban Volcanic Arc during the Turonian (ca. 90–93 Ma), when it was intruded by plutonic rocks of the Manicaragua Batholith (Turonian-Campanian; ca. 89–83 Ma). The geology and geochronology of central Cuba do not support the idea of a polarity reversal event at any stage of the Cretaceous Arc–building process. Because most of our dated samples come from the narrow Mabujina Belt, the polarity reversal model would imply that the axis of a newly developing arc (with opposite polarity) would spatially coincide with the older arc, which appears unlikely. Inherited Precambrian and Palaeozoic zircons in the MAC granitic rocks (similar to inherited zircon populations in the Guerrero terrane from central-western Mexico) suggest a Neocomian proximal setting close to a cratonic area (probably SW Mexico/Maya Block) for the protolith of the MAC relative to the synchronous Primitive Island Arc of central Cuba.

Hydrothermal origin and age of jadeitites from Sierra del Convento Mélange (Eastern Cuba)

Jadeitite (jadeite jade) from Sierra del Convento (eastern Cuba) occurs in a subduction-related serpentinite-matrix melange associated with a variety of high-pressure tectonic blocks including garnet-amphibolites and related anatectic trondhjemites. The eastern Cuban jadeitite is massive and characterized by rare quartz inclusions and omphacite exsolution in jadeite crystals, as well as replacement or infilling by omphacite. Minor minerals include epidote, biotite, albite, phengite, titanite, rutile, zircon, and apatite. Oscillatory zoning in jadeite crystals and zircon ages suggest hydrothermal crystallization in veins formed in serpentinized peridotite, probably of the mantle wedge. Al-Na-Mg-Ca-bearing fluids of variable composition but high pH (capable of mobilizing Zr and Hf at SiO2-subsaturated conditions) deposited jadeitite in veins during episodic opening of the fractures at depth in the subduction environment. Late-stage crystallizations include omphacite, albite and epidote from fluids nearly saturated in SiO2. The compositional gap of two coexisting pyroxenes indicates a temperature of jadeite formation higher than 500 °C. Zircon 206Pb/238U ages of 107.4 ± 0.5 Ma and 107.8 ± 1.1 Ma attest formation during the earliest stages of subduction in the region. These ages and the high temperature of formation of jadeitite suggest a genetic link between the jadeite-forming fluids and fluids derived from associated anatectic trondhjemites crystallized at depth (15 kbar).