Very fast exhumation of high-pressure metamorphic rocks with excess 40Ar and inherited 87Sr, Betic Cordilleras, southern Spain (original) (raw)
Related papers
Repeated thermal resetting of phengites in the Mulhacen Complex (Betic Zone, southeastern Spain) shown by 40Ar/39Ar stepheating and single grain laser probe dating. , 1992
This study reports the results of the first 40Ar/39Ar combined induction furnace and laser probe dating of phengites from the Mulhacen HP/LT metamorphic complex in the Betic Cordilleras, southern Spain. Laser step heating and spot fusion analyses on different halves of a split single grain were made with a continuous laser probe. Spot fusion analysis resulted in ages of about 30–31 Ma in the core and ages as low as 25–26 Ma in the rim. Laser step heating on the other half of the grain gave a spectrum with apparent ages increasing from about 25 Ma to 29.5 Ma. The age spectrum and the decreasing ages towards the rim of the grain may imply that resetting essentially occurred by volume diffusion of radiogenic 40Ar due to late stage reheating resulting from extensional tectonics. Ages around 30 Ma in the core of the grain are interpreted as minimum estimates of the cooling age of the main tectono-metamorphic phase D2. Induction furnace step heating on phengite separates from mica schists and one gneiss resulted in two types of age spectra. Type I spectra show monotonously rising apparent ages from 14.5 ± 1.9 Ma to 20.7 ± 0.2 Ma, and in a second sample from 16.9 ± 0.7 to 29.7 ± 0.2 Ma. Type II spectra are characterized by plateaus of 14.4 ± 0.1 Ma (the gneiss sample), 17.3 ± 0.1 Ma and 17.6 ± 0.1 Ma. Type II spectra show low temperature apparent ages significantly below the plateau age, implying resetting subsequent to initial cooling. Modelling of the age spectra demonstrated that the plateau ages are possibly the result of strong resetting (75–85% of argon loss) of an older isotope system. Total fusion of a number of phengite single grains from marbles taken close to type II mica schists yielded ages of 15.4 ± 1.2 Ma and 17.0 ± 0.7 Ma. The observed repeated resetting is coeval with major volcanic activity in basins adjacent to the metamorphic ranges, pointing to a resetting by advective fluid transport related to volcanism.
Journal of Petrology, 2020
Linking mineral growth and time is required to unravel the evolution of metamorphic rocks. However, dating early metamorphic stages is a challenge, due to subsequent retrograde overprinting. A perfectly fresh eclogite and a former eclogite retrogressed under amphibolite facies from the southern French Massif Central (Lévézou massif, Variscan belt) were investigated with a large panel of geochronometers (U-Pb on zircon, rutile and apatite, Lu-Hf and Sm-Nd on garnet) in a petrological context tightly constrained by petrographic observation, trace element analyses and phase equilibrium modelling. Both samples recorded similar HP conditions at 18-23 kbar and 680-800°C, while the retrogressed eclogite later equilibrated at 8-9.5 kbar and ca. 600°C. In the retrogressed sample, most of the zircon grains are characterized by negative Eu anomalies and HREE enrichment, and yield an Ordovician U-Pb date of 472.3 ± 1.7 Ma, interpreted as the emplacement age of the mafic protolith. In agreement with other data available for the Variscan belt, and based on zircon trace elements record and whole rock geochemistry, this age is considered to represent the magmatism associated with the extreme thinning of the continental margins during the Ordovician. In the same sample, few zircon rims show a weaker HREE enrichment and yield a date of 378 ± 5.7 Ma, interpreted as a prograde pre-eclogitic age. Lu-Hf garnet dating from both samples yields identical dates of 357 ± 13 Ma and 358.0 ± 1.5 Ma inferred to approximate the age of the high-pressure metamorphic peak. Fresh and retrogressed samples yield respectively 350.4 ± 7.7 Ma and 352 ± 20 Ma dates for Sm-Nd garnet dating, and 367.8 ± 9.1 Ma and 354.9 ± 9.5 Ma for U-Pb rutile dating. Apatite grains from the retrogressed sample give a mean age of 351.8 ± 2.8 Ma. The similarity between all recorded ages from distinct chronometers and radiometric methods (U-Pb – rutile, apatite; Lu-Hf – garnet; Sm-Nd – garnet) combined with P–T estimations from high-pressure metamorphic rocks equilibrated under different conditions testifies to very fast processes that occurred during the Variscan orogeny, highlighting a major decompression of 15-8.5 kbar in less than 7 Myr, and suggesting mean exhumation rates in excess of 6.3 mm/yr.
Ar timing of emplacement and exhumation and isotope-geochemical features
Ar= 39 Ar) are used to unravel the Late Alpine high-grade metamorphism, migmatisation and exhumation of Variscan granitoids within the core of the Central Rhodopean dome, Bulgaria. The age of the granitoid protolith is 300 AE 11 Ma, as determined by U-Pb analyses on single zircons selected from the core of the dome structure.
Earth and Planetary Science Letters
This study reports the results of the first 4°Ar/39mr combined induction furnace and laser probe dating of phengites from the Mulhacen HP/LT metamorphic complex in the Betic Cordilleras, southern Spain. Laser step heating and spot fusion analyses on different halves of a split single grain were made with a continuous laser probe. Spot fusion analysis resulted in ages of about 30-31 Ma in the core and ages as low as 25-26 Ma in the rim. Laser step heating on the other half of the grain gave a spectrum with apparent ages increasing from about 25 Ma to 29.5 Ma. The age spectrum and the decreasing ages towards the rim of the grain may imply that resetting essentially occurred by volume diffusion of radiogenic 4°Ar due to late stage reheating resulting from extensional tectonics. Ages around 30 Ma in the core of the grain are interpreted as minimum estimates of the cooling age of the main tectono-metamorphic phase D 2.
Bulletin De La Societe Geologique De France, 2023
We present new constraints on the age, nature, and tectonic setting of mafic eclogite protoliths from the Maures-Tanneron Massif, southern Variscan belt. Whole-rock major and trace element geochemistry was combined with zircon dating using 206 Pb/ 238 U by LA−ICP−MS to improve the understanding of this key-target of the European Southern Variscides. Geochemical data show that protoliths of the mafic eclogites are typical MORBs, while REE and HFSE patterns suggest an E-MORB affinity. However, the geochemical study shows several signs of crustal contamination that increases with the degree of retrogression. A comparison with Sardinian eclogites, which belong to the same Variscan microplate, namely, "MECS" (Maures-Estérel-Corsica-Sardinia), demonstrates that the eclogites are included in migmatites, which is the case for the studied samples, are the most contaminated. The Maures-Tanneron mafic eclogites represent the remnant of an oceanic basaltic crust. Zircon cores display homogeneous Th/U ratios (0.3-0.4), which are consistent with a magmatic origin, and define an age peak at 499.5 ± 2.9 Ma that is interpreted as the most likely emplacement age of the basaltic protolith. This age suggests that this protolith was part of an oceanic floor that was older than the Rheic Ocean and located to the north of the Gondwana active continental margin as predicted by recent unified full plate reconstruction models. Although the studied eclogites are retrogressed, the study of mineral inclusions trapped in garnets combined with thermodynamic modelling yields a P−T range of 17.2-18.5 kbar and 640-660°C, which is consistent with the standard oceanic subduction palaeo-geotherm. These new data suggest that eclogites recognized in the "MECS" Variscan microplate represent the closure of oceanic domains of different ages (Cambrian or Ordovician).
Mafic rocks containing eclogite relics are fairly widespread in the crystalline nappe Stack of lhe Swiss Central Alps. This study addresses the spatial distribution of eclogite relics in the Central Alps, their field relations. structural and petrological characteristics. and their PTt-history. Implications for the assembly of the nappe Stack are explored. The majority of eclogite-facies relics is confined lo a single super-unit of tectonic melange. interpreted as a tec¬ tonic accretion Channel (TAC). Numerous mafic high-pressure (HP) lenses have been discovered through systematic fieldwork in the TAC units of the Central Alps, an up-lo-date inventory of which is presented. Systematic documentation of select samples with HP imprint yields clockwise PT-paths. Prograde phase relations are seldom preserved. except in the chemical zoning of garnet porphyroblasts. However. when present. relic assemblages indicate HP-LT conditions indicative of a subduction setting. Maximum recorded pressures are substantially different from one loca¬ tion to the next (1.9 to 3.3 GPa). Depending on the degree of rehydration. reaction sequences are derived from observed relics. local replacement relations and assemblages. Quantitative constraints on the detailed PT-path are extracted by combining isochemical phase diagrams and 7Wß-thermobarometry with petrographie information. HP lenses from different locations display substantially different paths. both within and between different melange zones of the TAC. PT-conditions reflecting the late-Alpine Barrovian overprint of mafic HP lenses are in agreement with the coherent regional pattern derived from metasediments. i.e.. maximum temperatures (-600°C in the central Lepontine belt. 700-750°C in the southern parts) were reached at pressures between 0.75 and 0.55 GPa Four samples have been dated by Lu-Hf isotopic analysis of garnet. clinopyroxene. malrix phases and wholerock powders.The age span Covers a ränge from >70 to-.36 Ma.much larger than previously documented for Alpine HP rocks from the Central Alps. Petrological data of the samples and their Lu-Hf isotopic system indicate a protracted HP historv for at least some of the sub-units of the TAC. with garnet growth under eclogite-facies conditions starting before 70 Ma in some parts of the TAC. and continuing as late as 36 Ma in others. Iliese data have implications for the dynamics of melange formation within the TAC with internal fragmentation and mixing. and pronounced mobility of lhe tectonic zones, probably during the early. subduclional stages and again during the post-collisional extrusion along the plate boundary. After 32 Ma. when the Barrovian overprint reached its maximum temperature. the TAC appears to have been exhumed as part of the then-coherenl crystalline nappe Stack.
International Geology Review, 2007
A well-preserved phengite-amphibole eclogite (Br2F) from the UHP Brossasco-Isasca Unit (BIU) of the Dora-Maira Massif was studied in detail. The eclogite consists of the peak assemblage omphacite, garnet, phengite, rutile, and quartz. A porphyroblastic blue-green amphibole statically overgrows the eclogitic foliation defined by the preferred orientation of phengite flakes, and by the alignments of abundant accessory rutile grains. Both omphacite and phengite are partially replaced by fine-grained symplectites, consisting of clinopyroxene + albite and biotite + oligoclase, respectively. The metamorphic evolution of eclogite Br2F was reconstructed combining microstructural observations, conventional thermobarometry, and pseudosection analysis. A first pseudosection was calculated in the NKCFMASH system in the pressure range 5-45 kbar to model the peak and early retrogressive conditions, whereas a second pseudosection, calculated in the NCFMASH system, was used to model the albite + clinopyroxene symplectite after omphacite. Peak metamorphic conditions of P = 37.7 kbar and T = 732°C were estimated. The decompressional P-T path is associated with significant cooling from about 730°C at 38 kbar to 630°C at 14 kbar. These data, obtained combining pseudosection analysis with conventional thermobarometric methods, are in agreement with the P-T paths estimated from other lithologies by Hermann (2003), Castelli et al. (2004), and Groppo et al. (2006), and confirm that the BIU equilibrated within the diamond stability field.
Mineralogy and Petrology, 2006
In the Eastern Alps, different tectonic units contain eclogites of Carboniferous, Cretaceous (eo-Alpine), and Cenozoic age: (i) In the structurally lower Penninic unit, which is exposed in the Tauern window, eclogites formed at peak P-T conditions of 2.4 GPa and 630 C as a result of subduction of the former European continental margin at the base of an imbricate stack of Mesozoic oceanic rocks below the Austroalpine domain. Amphibole and phengite 40 Ar-39 Ar and Rb-Sr multimineral isochron ages suggest an Eocene-Early Oligocene (<45-31 Ma) age for the stage of deepest subduction and rapid exhumation=cooling. (ii) Variscan MORB-type eclogites are present in the central Ö tztal basement (2.7 GPa=730 C; mean Sm-Nd age: 347 AE 9 Ma), the eastern Silvretta (ca. 350 Ma), and the Ulten high-grade crystalline (336 AE 4 Ma). These domains pertain to the N Apulian (Austroalpine) crust. (iii) In the southern Austroalpine units, eclogites and high-P metapelites are known from the Texel complex (1.3 GPa=520-600 C), the Schober area (1.8 GPa= 690 C), and the Saualpe-Koralpe-Pohorje domain (2.2-2.7 GPa= 750 C). Their tectonic position and near-identical Cretaceous ages suggest a common subduction-exhumation history. This ''eo-Alpine high-pressure belt'' (EHB) resulted from burial of the strongly thinned, distal passive Neotethyan margin of the Meliata back-arc basin and, further west, pre-Alpine Austroalpine crust along an intracontinental subduction zone during the Late Mesozoic convergence of Apulia and Europe. Peak metamorphism and initial decompression=exhumation is dated by the Sm-Nd, Lu-Hf, U-Pb and Rb-Sr systems as close to 90 Ma, with mean exhumation rates in the range of 5-10 km=Ma for the time between 90 and 85 Ma B.P.
Journal of Metamorphic Geology, 2006
Eclogite boudins occur within an orthogneiss sheet enclosed in a Barrovian metapelite-dominated volcano-sedimentary sequence within the Velke´Vrbno unit, NE Bohemian Massif. A metamorphic and lithological break defines the base of the eclogite-bearing orthogneiss nappe, with a structurally lower sequence without eclogite exposed in a tectonic window. The typical assemblage of the structurally upper metapelites is garnet-staurolite-kyanite-biotite-plagioclase-muscovite-quartz-ilmenite ± rutile ± silli-manite and prograde-zoned garnet includes chloritoid-chlorite-paragonite-margarite, staurolite-chlorite-paragonite-margarite and kyanite-chlorite-rutile. In pseudosection modelling in the system Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O (NCKFMASH) using THERMOCALC, the prograde path crosses the discontinuous reaction chloritoid + margarite ¼ chlorite + garnet + staurolite + paragonite (with muscovite + quartz + H 2 O) at 9.5 kbar and 570°C and the metamorphic peak is reached at 11 kbar and 640°C. Decompression through about 7 kbar is indicated by sillimanite and biotite growing at the expense of garnet. In the tectonic window, the structurally lower metapelites (garnet-staurolite-biotite-muscovite-quartz ± plagioclase ± sillimanite ± kyanite) and amphibolites (garnet-amphibole-plagioclase ± epidote) indicate a metamorphic peak of 10 kbar at 620°C and 11 kbar and 610-660°C, respectively, that is consistent with the other metapelites. The eclogites are composed of garnet, omphacite relicts (jadeite ¼ 33%) within plagioclase-clinopyroxene symplectites, epidote and late amphibole-plagioclase domains. Garnet commonly includes rutile-quartz-epidote ± clinopyroxene (jadeite ¼ 43%) ± magnetite ± amphibole and its growth zoning is compatible in the pseudosection with burial under H 2 O-undersaturated conditions to 18 kbar and 680°C. Plagioclase + amphibole replaces garnet within foliated boudin margins and results in the assemblage epidote-amphibole-plagioclase indicating that decompression occurred under decreasing temperature into garnet-free epidote-amphibolite facies conditions. The prograde path of eclogites and metapelites up to the metamorphic peak cannot be shared, being along different geothermal gradients, of about 11 and 17°C km )1 , respectively, to metamorphic pressure peaks that are 6-7 kbar apart. The eclogiteorthogneiss sheet docked with metapelites at about 11 kbar and 650°C, and from this depth the exhumation of the pile is shared.