GEOCHEMISTRY AND TECTONICS OF CENOZOIC MAGMATISM IN THE CARPATHIAN-PANNONIAN REGION

Carpathian - Pannonian Region (CPR) geodynamic system made of two microplates: Alcapa and Tisia (Tisza)-Dacia is a result of the Cretaceous to Neogene collision of Africa with Europe that modified the mantle lithosphere with various subduction components. Early Miocene to Quaternary magmatic rocks of highly diverse compositions were generated in response to complex post-collisional tectonic processes, as back-arc extension, in response to interplay of compression and extension (e.g. subduction with roll-back, collision, slab break-off, delamination, strike-slip tectonics, core complex type extension and block rotations breakup). At the climax of competition between the different tectonic processes melting was triggered at various levels in the lithosphere and asthenosphere, as mirrored in variations of the associated magmatism. No volcanic activity directly related to pre-collisional subduction is recorded.

Major, trace element and isotopic data of lavas and mantle xenoliths attests that subduction components already preserved in the lithospheric mantle after the Cretaceous-Miocene subduction were reactivated by various processes. Changes in the composition of the magmas through time are linked to the evolution of the Alcapa (1) and Tisia-Dacia (2) and their boundary relations:

1a. In the main Pannonian Basin, magmatism occurred in a back-arc setting producing calc-alkaline felsic volcanic rocks at 21-18 Ma, felsic - intermediate at 18-8 Ma and ended with Na-alkalic basaltic rocks at10-0.1 Ma. Rock geochemistry suggests a change through time in source from a crustal to a mixed crustal/mantle ending with a mantle one. Extrusion tectonics, block rotation and continental collision triggered partial melting by delamination and/or asthenosphere upwelling. Na-alkalic magmatism suggests a north-east-directed mantle flow which triggered magma generation at the base of the lithosphere along NW-SE strike-slip faults; counterclockwise rotation and push of the Adriatic microplate was responsible. Continuous volcanic activity in central Slovakia, as calc-alkaline (16.5-11 Ma), transitional (11-8 Ma) and as Na-alkalic basalts (8-0.13 Ma) supports a mantle plume scenario;

1b. Westernmost Styrian basin contains felsic and intermediate calc-alkaline, K-alkalic and ultrapotassic volcanic rocks generated at 17.5-14 Ma. They were related to extension and extrusion tectonics and core-complex generation at 21.9-13.4 Ma that produced strong mantle perturbations. Na-alkali basalts at 4-1.8 Ma suggest Adria push to cause a north-east directed mantle flow and melt generation.

1c. 15-9 Ma felsic and normal calc-alkaline volcanism in the north-easternmost Transcarpathian basin, is situated at a triple junction between Alcapa, Tisia and the European foreland. It resulted via counter-clock extensional rotation of easternmost Alcapa, causing core-complex exhumation. Geochemical studies indicate a heterogeneous lithospheric mantle as main source followed by fractionation-assimilation in crustal magma chambers. Melting was triggered by rotational extension and asthenosphere uprise;

2a. Calc-alkaline magmatism at 12-8 Ma in the northern part of the Tisia-Dacia microplate follows an important transcurrent fault and is entirely intrusive, ranging from basalts to rhyolites. Each body evolved independently with fractionation, crustal assimilation and/or magma mixing processes; decompression melting of the local heterogeneous mantle lithosphere is suggested. Sinistral transtensional stress regimes at 12-10 Ma controlled the generation and emplacement of the intrusive bodies that further were strongly uplifted and exposed by erosion.

2b. Normal and adakite-like calc-alkaline magmas were erupted in the Apuseni Mountains at 15-9 Ma. Lithosphere breakup during extreme westernmost Tisia rotations (~60 degree) was responsible for extension with core-complex formation. This led to decompression melting of an enriched heterogeneous lithospheric source. Volcanism ended with small volume Na-alkalic basalts (2.5 Ma), K-alkalic (1.6 Ma) and ultrapotassic (1.3 Ma) magmas. Inversion tectonics along the South Transylvanian fault triggered melt generation via decompression melting of diverse lithospheric and asthenospheric sources;

2c. Calc-alkaline Călimani-Gurghiu-North Harghita volcanic chain occurred at 10-3.9 Ma along the easternmost margin of Tisia-Dacia with a southward diminishing age and volume. Magma generation was associated with progressive break-off of the slab and asthenosphere uprise. Fractionation and crustal assimilation were typical;

2d. At ca. 3 Ma, magma changed in South Harghita to adakite-like calc-alkaline until recent times (< 0.03 Ma) interrupted at 1.6-1.2 Ma by simultaneous generation of Na and K alkalic varieties in nearby areas, indicative of various sources and melting mechanism. Two main geodynamic events were responsible: (a) slab-pull and steepening, with opening of a tear-window in the Vrancea lithospheric block hanging into the asthenospheric mantle (forming adakite-like magmas) and (b) inversion tectonics along reactivated fault systems allowed decompression melting of various asthenospheric and lithospheric sources;