Low-grade retrogression of a high-temperature metamorphic core complex: Naxos, Cyclades, Greece

Retrogressive deformation and metamorphism are often reported from the main low-angle shear zones and detachments of metamorphic core complexes, but their importance is not sufficiently emphasized for the footwall interior. In order to contribute to a better understanding of exhumation-related retrogression processes within and at the top of metamorphic core complexes, an integrated detailed microstructural, textural, 40Ar/39Ar geochronological, and thermobarometric study on the Naxos metamorphic core complex within the Aegean Sea is presented that provides a new perspective on low-grade retrogression during exhumation through shallow ductile levels. We found variable retrogressive deformation within the Naxos metamorphic core complex, which even pervasively affected significant portions of the migmatite-grade metamorphic core and remnant high-pressure areas of the metamorphic core complex, where retrogression led to pervasive formation of new fabrics within greenschist-facies metamorphic conditions during brittle-ductile transition. Within a continuum of retrogression, 40Ar/39Ar white mica dating allowed us to deduce three retrogressive ages at 16.52 ± 0.39 Ma (within the Naxos metamorphic core complex), 12.6 ± 0.28 Ma (Moutsounas detachment shear zone on the eastern boundary of the metamorphic core complex), and 10.43 ± 0.44 Ma to 8.40 ± 0.76 Ma (last ductile activity along the Naxos-Paros shear zone to the north of the metamorphic core complex). A further stage of retrogression at 12−11 Ma occurred along distinct low-angle normal faults within the middle Miocene Naxos Granite. Retrogressive microstructures, low-temperature calcite fabrics in marbles, and chloritization in metapelites (at temperatures of ∼350−130 °C) in the metamorphic core complex core resulted mainly from late-stage E-W shortening and folding. Late-stage flow of hydrous fluids resulted in resetting of fabrics and enhancement of ductile deformation. The middle−late Miocene retrogression events are also reflected by a similarly aged tectonic collapse basin in the hanging-wall unit above the detachment. The wide temporal range of retrogression within the Naxos metamorphic core complex coincides in age with retrogressive deformation within other metamorphic core complexes of the Aegean Sea. We interpret the long temporal range of retrogression to reflect outward, southwestward retreat of the subduction and sequential activation of major detachment zones

Geochemical and isotopic evidence for Carboniferous rifting : mafic dykes in the central Sanandaj-Sirjan zone (Dorud-Azna, West Iran)

In this paper, we present detailed field observations, chronological, geochemical and SrNd isotopic data and discuss the petrogenetic aspects of two types of mafic dykes, of alkaline to subalkaline nature. The alkaline mafic dykes exhibit a cumulate to foliated texture and strike NWSE, parallel to the main trend of the region. The 40Ar/39Ar amphibole age of 321.32 0.55 Ma from an alkaline mafic dyke is interpreted as an indication of Carboniferous cooling through ca. 550 C after intrusion of the dyke into the granitic Galeh-Doz orthogneiss and Amphibolite-Metagabbro units, the latter with Early Carboniferous amphibolite facies grade metamorphism and containing the Dare-Hedavand metagabbro with a similar Carboniferous age. The alkaline and subalkaline mafic dykes can be geochemically categorized into those with light REE-enriched patterns [(La/Yb)N = 8.329.28] and others with a rather flat REE pattern [(La/Yb)N = 1.16] and with a negative Nb anomaly. Together, the mafic dykes show oceanic island basalt to MORB geochemical signature, respectively. This is consistent, as well, with the (Tb/Yb)PM ratios. The alkaline mafic dykes were formed within an enriched mantle source at depths of 90 km, generating a suite of alkaline basalts. In comparison, the subalkaline mafic dykes were formed within more depleted mantle source at depths of 90 km. The subalkaline mafic dyke is characterized by 87Sr/86Sr ratio of 0.706 and positive Nd(t) value of + 0.77, whereas 87Sr/86Sr ratio of 0.708 and Nd(t) value of + 1.65 of the alkaline mafic dyke, consistent with the derivation from an enriched mantle source. There is no evidence that the mafic dykes were affected by significant crustal contamination during emplacement. Because of the similar age, the generation of magmas of alkaline mafic dykes and of the Dare-Hedavand metagabbro are assumed to reflect the same process of lithospheric or asthenospheric melting. Carboniferous back-arc rifting is the likely geodynamic setting of mafic dyke generation and emplacement. In contrast, the subalkaline mafic sill is likely related to the emplacement of the Jurassic Darijune gabbro.(VLID)223441

Electron microprobe analysis and Ar-Ar age dating of polyhalite from the evaporitic Permian Haselgebirge Formation

To test polyhalite age dating of the mineral polyhalite [K2Ca2Mg(SO4)4·2H2O], samples of the evaporitic Permian Haselgebirge Formation were collected in the Eastern Alps. Samples were taken from two salt bodies. The salt body of Altaussee (UTM 33T 405316 5278325) has a vertical thickness of >800 m. Samples were collected in the Altaussee mine (ALT). The salt body of Bad Dürrnberg-Berchtesgaden (UTM 33T 351091 5278007) is at least 1000 m thick. The salt body comprises two separate mines, where samples were collected, Bad Dürrnberg (DÜ) and Berchtesgaden (BGD) salt mines. The samples were collected during several field trips, and investigated at the University of Salzburg during the years 2007-2015. Electron microprobe analyses were conducted to determine possible chemical variations of polyhalite

Electron microprobe analysis of polyhalite from the evaporitic Permian Haselgebirge Formation

Electron microprobe analyses were conducted at the University of Salzburg, Department of Geography and Geology, in the year 2011. Measurements were performed on a JEOL electron microprobe (JXA-8600), equipped with a wave-length dispersive system. An acceleration voltage of 15 kV and a low sample current of 20 nA were applied to prevent decomposition of polyhalite under the electron beam. Additionally, the spot was defocused to a diameter of 15 µm and after measurement of sulfur, the sample was moved one beam diameter to start measurement of potassium and calcium. Sulfur, potassium and calcium were all measured with the same analyzing crystal. Synthetic and natural mineral standards were used to analyse the emitted wave lengths of the sample and to quantify their amount. Standard ZAF correction calculation revealed the composition in oxide weight percent. The calculation method after Love/Scott1 revealed the formula units of polyhalite (doi:10.1594/PANGAEA.942485)

Raw data and plots of 40Ar/39Ar age dating of polyhalite from the evaporitic Permian Haselgebirge Formation

The measurements were conducted at the University of Salzburg, Department of Geography and Geology, in the years 2007-2015. Polyhalite samples were manually reduced to small pieces with a hammer. They were washed with destilled water and dried with isopropanol to free them from dust and Cl-ions of halite. Chlorine produces Ar isotopes during irradiation, which may tamper the proportion of Ar isotopes from polyhalite. Grains of 200–250 µm size were selected under the microscope. A sufficient number of grains of each sample were packed into aluminium-foil and put into quartz vials. Details of the analytical 40Ar/39Ar technique is described in Leitner et al. (2014) and Cao et al. (2017). Irradiation was conducted for 16 hours in the Magyar Tudományos Akadémia (MTA) Központi Fizakai Kutato Intézet (KFKI) reactor (Debrecen, Hungary). Flux-monitors were placed between the samples for calculation of the J-values. The distance between adjacent flux-monitors was c. 5 mm. Corrections for interfering isotopes were the same as described earlier: Correction factors were calculated from 45 analyses of co-irradiated Ca-glass samples and 70 analyses of K-glass samples, and are: 36Ar/37Ar(Ca) = 0.000225, 37Ar/39Ar(Ca) = 0.000614, 38Ar/39Ar(K) = 0.0117, and 40Ar/39Ar(K) = 0.0266. Variation in the flux of neutrons were monitored with DRA1 sanidine standard for which a 40Ar/39Ar plateau age of 25.26 ± 0.05 Ma has been reported (van Hinsbergen et al. 2008). 40Ar/39Ar analyses were carried out at the Department of Geography and Geology at the University of Salzburg. The equipment used was the same as described earlier: 40Ar/39Ar analyses are carried out using a ultra high vacuum Ar-extraction line equipped with a combined MERCHANTEKTM UV/IR laser system, and a VG-ISOTECHTM VG-3600 noble gas mass spectrometer. Stepwise heating analyses of samples are performed using a defocused (~1.5 mm diameter) 25 W CO2-IR laser operating in Tem00 mode at wavelengths between 10.57 and 10.63 µm. The laser is controlled from a PC, and the position of the laser on the sample is monitored on the computer screen via a video camera in the optical axis of the laser beam through a double-vacuum window on the sample chamber. Gas clean-up is performed using one hot and one cold Zr-Al SAESTM getter. Gas admittance and pumping of the mass spectrometer and the Ar-extraction line are computer controlled using pneumatic valves. The VG-3600 is an 18 cm radius 60° extended geometry sector field mass analyzer instrument, equipped with a bright Nier-type source operated at 4.5 kV. Measurements are performed on an axial electron multiplier in static mode, peak-jumping and stability of the magnet is controlled by a Hall-probe. For each increment the intensities of 36Ar, 37Ar, 38Ar, 39Ar, and 40Ar are measured, the baseline readings on mass 35.5 are automatically subtracted. Intensities of the peaks are back-extrapolated over 16 measured intensities to the time of gas admittance either by a straight line or a curved fit, depending on intensity and type of pattern of the evolving gas. Inspection of intensities was applied with regard to background, system blanks, interfering isotopes and post-irradiation decay of 37Ar. Calculations of isotope ratios, errors, ages and plateau ages followed suggestions of McDougall and Harrison (1999), Scaillet (2000), Steiger and Jäger (1977) and Ludwig (2012)

Formula units of polyhalite from the evaporitic Permian Haselgebirge Formation

Electron microprobe analyses were conducted at the University of Salzburg, Department of Geography and Geology, in the year 2011. Measurements were performed on a JEOL electron microprobe (JXA-8600), equipped with a wave-length dispersive system. An acceleration voltage of 15 kV and a low sample current of 20 nA were applied to prevent decomposition of polyhalite under the electron beam. Additionally, the spot was defocused to a diameter of 15 µm and after measurement of sulfur, the sample was moved one beam diameter to start measurement of potassium and calcium. Sulfur, potassium and calcium were all measured with the same analyzing crystal. Synthetic and natural mineral standards were used to analyse the emitted wave lengths of the sample and to quantify their amount. Standard ZAF correction calculation revealed the composition in oxide weight percent (doi:10.1594/PANGAEA.942486). The calculation method after Love/Scott1 revealed the formula units of polyhalite

Polyphase exhumation in the western Qinling Mountains, China: Rapid Early Cretaceous cooling along a lithospheric-scale tear fault and pulsed Cenozoic uplift

The western sector of the Qinling–Dabie orogenic belt plays a key role in both Late Jurassic to Early Cretaceous “Yanshanian” intracontinental tectonics and Cenozoic lateral escape triggered by India–Asia collision. The Taibai granite in the northern Qinling Mountains is located at the westernmost tip of a Yanshanian granite belt. It consists of multiple intrusions, constrained by new Late Jurassic and Early Cretaceous U–Pb zircon ages (156 ± 3 Ma and 124 ± 1 Ma). Applying various geochronometers (40Ar/39Ar on hornblende, biotite and K-feldspar, apatite fission-track, apatite [U–Th–Sm]/He) along a vertical profile of the Taibai Mountain refines the cooling and exhumation history. The new age constraints record the prolonged pre-Cenozoic intracontinental deformation as well as the cooling history mostly related to India–Asia collision. We detected rapid cooling for the Taibai granite from ca. 800 to 100 °C during Early Cretaceous (ca. 123 to 100 Ma) followed by a period of slow cooling from ca. 100 Ma to ca. 25 Ma, and pulsed exhumation of the low-relief Cretaceous peneplain during Cenozoic times. We interpret the Early Cretaceous rapid cooling and exhumation as a result from activity along the southern sinistral lithospheric scale tear fault of the recently postulated intracontinental subduction of the Archean/Palaeoproterozoic North China Block beneath the Alashan Block. A Late Oligocene to Early Miocene cooling phase might be triggered either by the lateral motion during India–Asia collision and/or the Pacific subduction zone. Late Miocene intensified cooling is ascribed to uplift of the Tibetan Plateau

Geochemical and isotopic evidence for Carboniferous rifting: mafic dykes in the central Sanandaj-Sirjan zone (Dorud-Azna, West Iran)

In this paper, we present detailed field observations, chronological, geochemical and Sr–Nd isotopic data and discuss the petrogenetic aspects of two types of mafic dykes, of alkaline to subalkaline nature. The alkaline mafic dykes exhibit a cumulate to foliated texture and strike NW–SE, parallel to the main trend of the region. The 40Ar/39Ar amphibole age of 321.32 ± 0.55 Ma from an alkaline mafic dyke is interpreted as an indication of Carboniferous cooling through ca. 550 °C after intrusion of the dyke into the granitic Galeh-Doz orthogneiss and Amphibolite-Metagabbro units, the latter with Early Carboniferous amphibolite facies grade metamorphism and containing the Dare-Hedavand metagabbro with a similar Carboniferous age. The alkaline and subalkaline mafic dykes can be geochemically categorized into those with light REE-enriched patterns [(La/Yb)N = 8.32–9.28] and others with a rather flat REE pattern [(La/Yb)N = 1.16] and with a negative Nb anomaly. Together, the mafic dykes show oceanic island basalt to MORB geochemical signature, respectively. This is consistent, as well, with the (Tb/Yb)PM ratios. The alkaline mafic dykes were formed within an enriched mantle source at depths of ˃ 90 km, generating a suite of alkaline basalts. In comparison, the subalkaline mafic dykes were formed within more depleted mantle source at depths of ˂ 90 km. The subalkaline mafic dyke is characterized by 87Sr/86Sr ratio of 0.706 and positive ɛNd(t) value of + 0.77, whereas 87Sr/86Sr ratio of 0.708 and ɛNd(t) value of + 1.65 of the alkaline mafic dyke, consistent with the derivation from an enriched mantle source. There is no evidence that the mafic dykes were affected by significant crustal contamination during emplacement. Because of the similar age, the generation of magmas of alkaline mafic dykes and of the Dare-Hedavand metagabbro are assumed to reflect the same process of lithospheric or asthenospheric melting. Carboniferous back-arc rifting is the likely geodynamic setting of mafic dyke generation and emplacement. In contrast, the subalkaline mafic sill is likely related to the emplacement of the Jurassic Darijune gabbro

Postcollisional cooling history of the Eastern and Southern Alps and its linkage to Adria indentation

Indentation of rigid blocks into rheologically weak orogens is generally associated with spatiotemporally variable vertical and lateral block extrusion. The European Eastern and Southern Alps are a prime example of microplate indentation, where most of the deformation was accommodated north of the crustal indenter within the Tauern Window. However, outside of this window only the broad late-stage exhumation pattern of the indented units as well as of the indenter itself is known. In this study we refine the exhumational pattern with new (U–Th–Sm)/He and fission-track thermochronology data on apatite from the Karawanken Mountains adjacent to the eastern Periadriatic fault and from the central-eastern Southern Alps. Apatite (U–Th–Sm)/He ages from the Karawanken Mountains range between 12 and 5 Ma and indicate an episode of fault-related exhumation leading to the formation of a positive flower structure and an associated peripheral foreland basin. In the Southern Alps, apatite (U–Th–Sm)/He and fission-track data combined with previous data also indicate a pulse of mainly Late Miocene exhumation, which was maximized along thrust systems, with highly differential amounts of displacement along individual structures. Our data contribute to mounting evidence for widespread Late Miocene tectonic activity, which followed a phase of major exhumation during strain localization in the Tauern Window. We attribute this exhumational phase and more distributed deformation during Adriatic indentation to a major change in boundary conditions operating on the orogen, likely due to a shift from a decoupled to a coupled system, possibly enhanced by a shift in convergence direction.ISSN:1437-3254ISSN:1437-326