Rafael Luís Torres-Roldán

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.

Fractionated melting of metapelite and further crystal-melt equilibria—The example of the Blanca unit migmatite complex, north of Estepona (Southern Spain)

Abstract The Blanca Unit is a tectonic element of the Western Betic Zone (Betic Cordilleras of southern Spain) whose rock-sequence—essentially built of metamorphic derivatives from pelites. semipelites and carbonatic rocks—is characterized by the prevalence of high-, to very high-grade metamorphic assemblages. This paper presents a first attempt at interpreting some of these rocks in terms of melting, and its implications regarding both origin of metamorphism and geological evolution of the region. The investigated migmatite complex consists essentially of two contrasting types of granitoid mobilizates: Type I mobilizates bear indications of a comparatively higher mobility, have a wide range of compositions (granitic to tonalitic) and lack of cogenetic enclaves; Type II mobilizates have a limited compositional range (mainly granitic) and contain abundant restitic material, particularly in the form of small aluminous (cordierite-garnet-sillimanite) enclaves. Field and compositional relationships suggest that Type I mobilizates represent early fractionated water-saturated melts, derived from rock-systems initially having the highest Ab/An ratios. Type II mobilizates may represent a residual fraction that underwent further melting at higher temperatures, but under water-undersaturated conditions. Phase relations among solidus minerals in Type II mobilizates and their contained aluminous enclaves indicate that melting may have started at pressures higher than 5 kb, but also that this event was followed by nearly adiabatic decompression before final cooling and crystallization. As a consequence of this decrease of total pressure, extensive reaction between suspended crystals and melt took place, accompanied by further melting due to an increase in water activity towards saturation. These processes are analyzed for consequences regarding the diversity and generation of granitic rocks through crustal anatexis. Several possible modes of origin of metamorphism in the Blanca Unit are reviewed, and it is concluded that it was most probably caused by heat transfer from the Sierra Bermeja peridotite intrusion, during initial “hot” emplacement of the latter into the crust. Furthermore, a comparison of the estimated P − T evolution of the Blanca Unit rocks and that of adjacent metamorphic series is possible, which suggests that the late decompression of the complex was related to the occurrence of syn-metamorphic crustal thinning around the intruded ultramafic body.