Jacques Kornprobst

Crustal splitting and the emplacement of Pyrenean lherzolites and granulites

Abstract Pyrenean lherzolites representing fragments of the upper mantle are embedded in Mesozoic sediments along the 500-km-long, 20-km-wide “North Pyrenean Zone”. These ultramafics and the nearby occurrence of granulites, the presence of undersaturated rocks intrusive in the Cretaceous, and the development of a narrow elongate zone of “Pyrenean metamorphism”, confer a fundamental orogenic and geodynamic significance to this zone. These phenomena and the emplacement of the lherzolites may be attributed to an important phase of crustal thinning followed by a succession of compressive stages. The crustal thinning involved in the North Pyrenean Zone is possibly related to a transcurrent movement mainly during Albian and Senonian times, indicating a transform zone [24]. The ascent and emplacement of lherzolites in Albian sediments may have occurred in response to alternating tensional and compressional stresses operating in transcurrent zones. The Pyrenean metamorphism in Mesozoic sediments may be related to convective (emplacement of lherzolites) as well as conductive (thermal anomaly in connection with the rifting) processes. This model is compared with existing transform domains. Finally, it is suggested that crustal thinning may permit the incorporation of lower crust-upper mantle associations in the upper levels of the crust during subsequent compressive stages.

Gabbro and related rock emplacement beneath rifting continental crust: UPb geochronological and geochemical constraints for the Galicia passive margin (Spain)

The thinned continental crust of the west Galicia margin is bound by a belt of serpentinized peridotites (‘peridotite ridge’) lying about 300 km off the coast in the North Atlantic ocean. From this ridge, a gabbro and a chlorite rock were studied in an attempt to substantiate rift-related subcontinental magmatism, occurring prior to sea-floor spreading. U-Pb dating of 13 different zircon fractions yields a precise age of 122.1 ± 0.3 Ma (2σ) for the emplacement of the chlorite rock protolith, from which more than 50% of Si and alkali-calc-alkali elements were lost during greenschist facies tectonometamorphism. Sr and Nd isotope signatures suggest that the gabbro and chlorite rock protoliths were derived from mantle sources that were moderately depleted in LILE, relative to a chondritic reservoir. No evidence for the presence of continental material in the magma source regions can be observed. From the new zircon age of 122.1 ± 0.3 Ma, and earlier determined39Ar40Ar age of 122.0 ± 0.6 Ma for amphibole from the same locality, it can be documented that magma formation, solidification and unroofing of the mantle rocks occurred during a short period of time of about 3.4 Ma, which means that the peridotite ridge detached from the continent and rose to the surface immediately after, or even coevally with mantle melting.

Proterozoic blueschist-bearing mélange in the Anti-Atlas Mountains, Morocco

Abstract Blueschists from the Bou Azzer inlier provide compelling evidence for Late Proterozoic subduction in the Anti-Atlas Mountains of Morocco. High-pressure/low-temperature metabasites containing blue amphibole minerals crossite and magnesioriebeckite record pressures in excess of 5 kbar. Together with regional relationships, the geologic setting of the blueschists constrains the polarity of Pan African subduction in this region, which occurred from ∼750 to 600 Ma. Blueschist facies rocks crop out in a heterogeneous assemblage of variably deformed and metamorphosed tectonic slices of ophiolitic fragments enclosed in a schistose serpentinite matrix. The melange belt containing the blueschist facies rocks is intruded by a number of diorite plutons, one of which has yielded a U/Pb radiometric age of 650 Ma. Together with Transaharan Belt to the southeast, the Anti-Atlas suture zone exposed within the Bou Azzer inlier contains among the oldest known blueschist-bearing, ophiolitic melanges in the world.

An Improved Experimental Calibration of the Olivine-Spinel Geothermometer

The calibration of the olivine-spinel geothermometer by Fabriès (1979) is commonly adopted by a number of petrologists. But the temperatures calculated in this way for ultramafic rocks are significantly lower than those obtained by the pyroxene geothermometers. These O1-Sp temperatures are also lower than those measured experimentally in the natural system (four-phase lherzolite). Different rates of cation diffusion cannot fully account for these differences. The temperature deviation is actually related to the inconsistencies between natural and experimental data which support the calibration. A re-evaluation of the calibration is proposed on the basis of a set of new experimental data.

The high-pressure assemblages at Milos, Greece: A contribution to the petrological study of the basement of the Cyclades Archipelago

Metamorphic rocks at Milos are known in small outcrops beneath the volcanic formations, as xenoliths in the Traphores volcanic breccia and as pebbles in the Paleochora Quaternary deposits. These rocks seem to belong to three different metamorphic units which probably have intricate relationships in the basement: — the “eclogites unit” shows garnetjadeite or garnet-omphacite primary associations, with apparently late crystallization of lawsonite phenoblasts an recrystallization features at relatively low temperatures; — the “glaucophane schist unit” exhibits Jadeite+quartz or glaucophane+lawsonite primary mineralogy; — the “greenschist facies unit” shows low-pressure main assemblages, most generally developed after high-pressure events. The two former units involve tholeiitic meta-igneous rocks, having been spilitized before metamorphism; they generally suffered more or less advanced recrystallization features in the low-pressure field of the greenschist facies. The third unit shows only meta-sedimentary rocks, with the sole exception of one single meta-doleritic pebble having calk-alkaline affinities.Petrological and mineralogical studies, based upon 15 bulk-rock compositions and 178 probe-analysed data points, lead to suspect at least 2, perhaps 3, different metamorphic events rather than one single metamorphic evolution, to account for the 3 U distinguished. From the observation of the mineralogical assemblages and their evolution, the former events (“stages 1 and 2”) could be related to rapid subduction of ocean-floor or back-arc basalts, whereas, during the latter event (“stage 3”), the rocks experienced crystallization conditions involving both decreasing pressures and increasing temperatures.

Interaction entre lithosphère et asthénosphère au cours de l'ouverture océanique : données isotopiques préliminaires sur la Marge passive de Galice (Atlantique-Nord)

Abstract The Galicia Margin ultramafic ridge has been cross-cut by diorites, pyroxenites and gabbros before the end of the rifting stage, and then by dolerites, after the continental breakup; it has been further overlaid by basaltic lava flows. The younger the rocks, the higher the initial ɛ Nd (2.2–8.8). This evolution would be the result of the contamination of liquids extracted from the asthenosphere, by the enriched (ɛ Ndt = 4.0) and partially melted previous continental lithosphere. Time-decreasing con- tamination is related to progressive litbospberic thinning from the end of rifting to the beginning of oceanic spreading.

CMAS 3D, a new program to visualize and project major elements compositions in the CMAS system

CMAS 3D, developed in MATLAB^(R), is a program to support visualization of major element chemical data in three dimensions. Such projections are used to discuss correlations, metamorphic reactions and the chemical evolution of rocks, melts or minerals. It can also project data into 2D plots. The CMAS 3D interface makes it easy to use, and does not require any knowledge of Matlab^(R) programming. CMAS 3D uses data compiled in a Microsoft Excel(TM) spreadsheet. Although useful for scientific research, the program is also a powerful tool for teaching.

Subsolidus phase relations in the system MgO-ZnO-SiO2 at high pressure

The subsolidus phase relations were determined experimentally in the system MgO-ZnO-SiO2 at 1.0 GPa and 1 200°C, by use of the high-pressure apparatus “piston-cylinder”. The results showed characteristics of the phases assemblage different from that in similar ternary systems. It is impossible to form complete Mg2SiO4-Zn2SiO4 olivine and Mg2Si2O6 Zn2Si2O6 pyroxene solid solution. This is controlled by the properties of Zn2+ with an outer layer of electron (Ar)3d10, different from others transition metal ions, like Fe2+, Ni2+, Co2+.

“Crustal splitting and the emplacement of Pyrenean lherzolites and granulites”—A reply to M.W. Fischer

Abstract The emplacement of Pyrenean lherzolites is a fascinating problem to tackle because of the unusual location of the lherzolites within sediments and the numerous constraints which are currently available on the Pyrenean belt (plates kinematics, petrology, structural geology, geochemistry, geophysics). Our model [1] represents an attempt to integrate the emplacement of lherzolites within the geodynamic evolution of the Pyrenees. However, on the basis of imprecise statements and observations, and neglecting or quoting incorrectly several previous works. Fischer [2] considers that we failed to address a number of problems relevant to the evolution of the North Pyrenean Fault Zone. Most of his comments find an answer in a careful reading of our article; nevertheless the following points must be clearly emphasized.