Tertiary-Quaternary subduction processes and related magmatism in the Alpine-Mediterranean region

During Tertiary to Quaternary times, convergence between Eurasia and Africa resulted in a variety of collisional orogens and different styles of subduction in the Alpine-Mediterranean region. Characteristic features of this area include arcuate orogenic belts and extensional basins, both of which can be explained by roll-back of subducted slabs and retreating subduction zones. After cessation of active subduction, slab detachment and post-collisional gravitational collapse of the overthickened lithosphere took place. This complex tectonic history was accompanied by the generation of a wide variety of magmas. Most of these magmas (e.g. low-K tholeiitic, calc-alkaline, shoshonitic and ultrapotassic types) have trace element and isotopic fingerprints that are commonly interpreted to reflect enrichment of their source regions by subduction-related fluids. Thus, they can be considered as ‘subduction-related’ magmas irrespective of their geodynamic relationships. Intraplate alkali basalts are also found in the region generally postdated the ‘subduction-related’ volcanism. These mantle-derived magmas have not been, or only slightly, influenced by subduction-related enrichment. This paper summarises the geodynamic setting of the Tertiary-Quaternary “subduction-related” magmatism in the different segments of the Alpine-Mediterranean region (Betic-Alboran-Rif province, Central Mediterranean, the Alps, Carpathian-Pannonian region, Dinarides and Hellenides, Aegean and Western Anatolia), and discusses the main characteristics and compositional variation of the magmatic rocks. Radiogenic and stable isotope data indicate the importance of continental crustal material in the genesis of these magmas. Interaction with crustal material probably occurred both in the upper mantle during subduction (‘source contamination’) and in the continental crust during ascent of mantle-derived magmas (either by mixing with crustal melts or by crustal contamination). The 87Sr/86Sr and 206Pb/204Pb isotope ratios indicate that an enriched mantle component, akin to the source of intraplate alkali mafic magmas along the Alpine foreland, played a key role in the petrogenesis of the ‘subduction-related’ magmas of the Alpine-Mediterranean region. This enriched mantle component could be related to mantle plumes or to long-term pollution (deflection of the central Atlantic plume and recycling of crustal material during subduction) of the shallow mantle beneath Europe since the late Mesozoic. In the first case, subduction processes could have had an influence in generating asthenospheric flow by deflecting nearby mantle plumes due to slab roll-back or slab break-off. In the second case, the variation in the chemical composition of the volcanic rocks in the Mediterranean region can be explained by “statistical sampling” of the strongly inhomogeneous mantle followed by variable degrees of crustal contamination

Tectonic significance of changes in post-subduction Pliocene-Quaternary magmatism in the south east part of the Carpathian-Pannonian Region

The south-eastern part of the Carpathian–Pannonian region records the cessation of convergence between the European platform/Moesia and the Tisza–Dacia microplate. Plio-Quaternary magmatic activity in this area, in close proximity to the ‘Vrancea zone’, shows a shift from normal calc-alkaline to much more diverse compositions (adakite-like calc-alkaline, K-alkalic, mafic Na-alkalic and ultrapotassic), suggesting a significant change in geodynamic processes at approximately 3 Ma. We review the tectonic setting, timing, petrology and geochemistry of the post-collisional volcanism to constrain the role of orogenic building processes such as subduction or collision on melt production and migration. The calc-alkaline volcanism (5.3–3.9 Ma) marks the end of normal subduction-related magmatism along the post-collisional Călimani–Gurghiu–Harghita volcanic chain in front of the European convergent plate margin. At ca. 3 Ma in South Harghita magma compositions changed to adakite-like calc-alkaline and continued until recent times (< 0.03 Ma) interrupted at 1.6–1.2 Ma by generation of Na and K-alkalic magmas, signifying changes in the source and melting mechanism. We attribute the changes in magma composition in front of the Moesian platform to two main geodynamic events: (1) slab-pull and steepening with opening of a tear window (adakite-like calc-alkaline magmas) and (2) renewed contraction associated with deep mantle processes such as slab steepening during post-collisional times (Na and K-alkalic magmas). Contemporaneous post-collisional volcanism at the eastern edge of the Pannonian Basin at 2.6–1.3 Ma was dominated by Na-alkalic and ultrapotassic magmas, suggesting a close relationship with thermal asthenospheric doming and strain partitioning related to the Adriatic indentation. Similar timing, magma chamber processes and volume for K-alkalic (shoshonitic) magmas in the South Apuseni Mountains (1.6 Ma) and South Harghita area at a distance of ca. 200 km imply a regional connection with the inversion tectonics

Simultaneous cathodoluminescence hyperspectral imaging and X-ray microanalysis

A facility has been developed to acquire hyperspectral cathodoluminescence (CL) images simultaneously with X-ray composition data. Based around an electron microprobe, the system uses a built-in Cassegrain microscope to efficiently couple emitted light directly into the entrance slit of an optical spectrograph. A cooled array detector allows the parallel acquisition of CL spectra, which are then built up into a multidimensional data-cube containing the full set of spectrally- and spatially-resolved information for later analysis. This setup has the advantage of allowing wavelength-dispersive X-ray (WDX) data to be recorded concurrently, providing a powerful technique for the direct comparison of luminescent and compositional properties of materials. The combination of beam and sample scanning thus allows the correlation of composition and luminescence inhomogeneities on length scales ranging from a few cm to sub-micron

Post-collisional Tertiary–Quaternary mafic alkalic magmatism in the Carpathian–Pannonian region: a review

Mafic alkalic volcanism was widespread in the Carpathian–Pannonian region (CPR) between 11 and 0.2 Ma. It followed the Miocene continental collision of the Alcapa and Tisia blocks with the European plate, as subduction-related calc-alkaline magmatism was waning. Several groups of mafic alkalic rocks from different regions within the CPR have been distinguished on the basis of ages and/or trace-element compositions. Their trace element and Sr–Nd–Pb isotope systematics are consistent with derivation from complex mantle-source regions, which included both depleted asthenosphere and metasomatized lithosphere. The mixing of DMM-HIMU-EMII mantle components within asthenosphere-derived magmas indicates variable contamination of the shallow asthenosphere and/or thermal boundary layer of the lithosphere by a HIMU-like component prior to and following the introduction of subduction components. Various mantle sources have been identified: Lower lithospheric mantle modified by several ancient asthenospheric enrichments (source A); Young asthenospheric plumes with OIB-like trace element signatures that are either isotopically enriched (source B) or variably depleted (source C); Old upper asthenosphere heterogeneously contaminated by DM-HIMU-EMII-EMI components and slightly influenced by Miocene subduction-related enrichment (source D); Old upper asthenosphere heterogeneously contaminated by DM-HIMU-EMII components and significantly influenced by Miocene subduction-related enrichment (source E). Melt generation was initiated either by: (i) finger-like young asthenospheric plumes rising to and heating up the base of the lithosphere (below the Alcapa block), or (ii) decompressional melting of old asthenosphere upwelling to replace any lower lithosphere or heating and melting former subducted slabs (the Tisia block)

Time-space evolution and volcanological features of the Late Miocene-Quaternary Calimani-Gurghiu-Harghita Volcanic Range, East Carpathians, Romania. A Review.

The Carpathian-Pannonian Region (CPR) hosts one of the major Cainozoic volcanic provinces of Europe extending in space over 6 eastern European countries.The lithospheric evolution of this large area governed by large-scale asthenospheric processes is recorded by products of volcanic activity occurred during a time interval of more than 21 million years. According to their surface occurrence areas, ages and composition the Neogene volcanics of CPR were systematized in three main groups: 1) mostly explosive products of felsic magmas generated at the beginning of volcanism in the whole CPR and in their particular occurrence areas (21-12 Ma) developed in the actual intra-Carpathian Pannonian Basin, 2) mostly intermediate calc-alkaline rocks emplaced in both the intra-Carpathian areas and along the arcuate Carpathian fold-and-thrust belt, and 3) Na- and K- alkaline and ultra-alkaline products clustered in a number of monogenetic volcanic fields across the whole intra-Carpathian realm developed in the final stages of volcanic activity of the CPR as a whole and of their particular occurrence areas. The ca. 160 km long Călimani-Gurghiu-Harghita volcanic range (CGH) developed as part of the intermediate calc-alkaline volcanism closely related in space with the fold-and-thrust belt of the Carpathians, representing the south-eastern segment of the CPR. Although its map view and general petrochemical and volcanological characteristics are quite similar with those of other segments of the orogene belt- tied calc-alkaline volcanic segments, at a closer look CGH displays a number of unique features. The time-space evolution of CGH is particular not only in that it is the youngest (10.5 to < 0.05 Ma) dominantly calc-alkaline segment in CPR but also it shows a transient character. Unlike other segments along which volcanism occurred simultaneously forming true subduction-related 400 to 800 km long volcanic fronts which were stable in time for millions of year, in CGH volcanic activity migrated continuously along the range from NW to SE. So, during any given 1 Ma time interval active volcanism was restricted to very limited areas and to just a few active volcanic centers. The along-range shift of volcanic foci was concurrent with progressively lower volumes of magma erupted and decreasing magma output rates. As a result, gradually lower-volume and less complex volcanic edifices were built up. Moreover, at the range-ending and youngest South Harghita sub-segment, magma compositions gradually changed from normal calc-alkaline to high-K calc-alkaline and shoshonitic, and adakitic features emerged at the end of volcanic activity, after a time gap of 0.5 Ma. This marks a major geodynamic event in the development of the East Carpathians themselves. During the transient volcanism of CGH, edifices of varying volume and complexity were built up forming a row of tightly- packed adjoining stratovolcanoes/composite volcanoes whose peripheral volcaniclastic aprons complexly juxtaposed, overlapped and merged with each other. The largest ones (Călimani caldera, and Fâncel-Lăpuşna) developed until caldera stage. Some of them (Rusca-Tihu in the Călimani Mts., Vârghiş in the North Harghita Mts.) became unstable during their growth and collapsed, generating widespread large-volume debris avalanche deposits. Edifice instability was solved by volcano-basement interaction processes, such as volcano spreading, at some large-volume volcanoes (in particular those in the Gurghiu Mts.). Volcano typology changed at the smaller-volume constructs toward the southeastern terminus of the range in the South Harghita Mts. from typical large stratovolcanoes to smaller composite volcanoes, dome clusters and isolated domes and simpler internal structures. As a whole, CGH displays an extremely particular evolutionary pattern strongly suggesting a transient character and decreasing to extinguishing volcanic activity along its length from NW to SE

Origin of the Laleaua Albă dacite (Baia Sprie volcanic area and Au-Pb-Zn ore district, Romania): evidence from study of melt inclusions

Crystal inclusions (plagioclase, biotite, magnetite) and melt inclusions were studied in minerals of the Laleaua Albă dacite (Baia Sprie, Romania). Electron microprobe analysis of 29 melt inclusions in the plagioclase, K-feldspar, and quartz confirm that crystallization of these minerals took place from typical silicic melts enriched in potassium relative to sodium (K2O/Na2O = 1.5). The sum of the petrogenic components is 92–99 wt%. This points to a possible change in water content from 8 to 1 wt% during crystallization of phenocrysts. According to ion microprobe analysis of 11 melt inclusions, the minimum water content is 0.5 wt%, and the maximum water content is 6.1 wt%. The presence of high-density water fluid segregation in one of the melt inclusions suggests that the primary water content in the melt could reach 8.4 wt%. Ion microprobe data revealed a high concentration of Cu (up to 1260 ppm) as well as higher U content (from 5.0 to 14.3 ppm; average 11.5 ppm) in some melt inclusions as compared to the average U contents in silicic melts (2.7 ppm in island-arc settings and 7.9 ppm in continental rift settings). Chondrite-normalized trace-element patterns in melt inclusions suggest a complex genesis of the studied magmatic melts. Contents of some elements (for instance Sr and Ba) are close to those in island-arc melts, while others (for instance Th, U, and Eu) resemble those in melts of continental settings

Origin of basaltic magmas of Perşani volcanic field, Romania: A combined whole 6 rock and mineral scale investigation

The Perşani volcanic field is a low-volume flux monogenetic volcanic field in the Carpathian–Pannonian region, 24 eastern-central Europe. Volcanic activity occurred intermittently from1200 ka to 600 ka, forming lava flow fields, 25 scoria cones andmaars. Selected basalts fromthe initial and younger active phaseswere investigated for major and 26 trace element contents and mineral compositions. Bulk compositions are close to those of the primitive magmas; 27 only 5–12% olivine and minor spinel fractionation occurred at 1300–1350 °C, followed by clinopyroxenes at about 28 1250 °C and 0.8–1.2 GPa. Melt generation occurred in the depth range from 85–90 km to 60 km. The estimated 29 mantle potential temperature, 1350–1420 °C, is the lowest in the Pannonian Basin. It suggests that no thermal 30 anomaly exists in the uppermantle beneath the Perşani area and that themaficmagmas were formed by decom- 31 pressionmelting under relatively thin continental lithosphere. Themantle source of themagmas could be slightly 32 heterogeneous, but is dominantly variously depleted MORB-source peridotite, as suggested by the olivine and 33 spinel composition. Based on the Cr-numbers of the spinels, two coherent compositional groups (0.38–0.45 and 34 0.23–0.32, respectively) can be distinguished that correspond to the older and younger volcanic products. This in- 35 dicates a change in themantle source region during the volcanic activity as also inferred from the bulk rockmajor 36 and trace element data. The younger basaltic magmas were generated by lower degree of melting, from a deeper 37 and compositionally slightly different mantle source compared to the older ones. The mantle source character of 38 the Perşanimagmas is akin to that ofmany other alkaline basalt volcanic fields in theMediterranean close to oro- 39 genic areas. The magma ascent rate is estimated based on compositional traverses across olivine xenocrysts using 40 variations of Ca content. Two heating events are recognized; the first one lasted about 1.3 years implying heating 41 of the lower lithosphere by the uprisingmagma,whereas the second one lasted only 4–5 days,whichcorresponds 42 to the time of magma ascent through the continental crust. The alkaline mafic volcanismin the Perşani volcanic 43 field could have occurred as a response to the formation of a narrow rupture in the lower lithosphere, possibly 44 as a far-field effect of the dripping of dense continental lithospheric material beneath the Vrancea zone. Upper 45 crustal extensional stress-field with reactivation of normal faults at the eastern margin of the Transylvanian 46 basin could enhance the rapid ascent of the mafic magmas