Eclogite subduction wedge intruded by arc-type magma: The earliest record of Variscan arc in the Bohemian Massif
Abstrakt
Previously unrecognized mafic and felsic plutonic rocks, formerly interpreted as an anatectic part of the amphibolite-eclogite Marianske Lazne Complex (MLC, Bohemian Massif) were examined together with a possible upper-crustal equivalent represented by the Cista pluton intruding continental crust farther east. These plutonic rocks were studied by whole-rock geochemistry (major and trace elements, Sr-Nd isotopes) and zircon U-Pb geochronology.
The MLC magmatic rocks range from pyroxene-amphibole gabbro to trondhjemite and oligoclasite. The Cista pluton consists of porphyritic granitoids.
The calc-alkaline nature of these rocks, relative enrichment of fluid-mobile elements (including large-ion lithophile elements), strong fractionation of light rare earth elements over heavy rare earth elements and depletion of high field strength elements are evidence for active margin magmatism. U-Pb zircon dating indicates that the magmatism was Mid-Devonian in both the MLC (ca. 385 Ma) and Cista pluton (ca. 390 up to ca. 365 Ma).
Both magmatic units show inherited cores probably linked to recycling of crustal material. Sr-Nd isotopic compositions indicate that the gabbro formed from a depleted MORB mantle source composition with a small contribution of a subduction and crustal component.
A three-stage geodynamic model is proposed: 1) ca. 390-380 Ma recycling of a high-pressure accretionary wedge and migration of arc melts into more distal parts of the upper continental plate, 2) ca. 380-370 Ma compression and thickening of the accretionary wedge-arc edifice, and 3) ca. 370-365 Ma development of a giant east dipping detachment that reworked the whole edifice in the west and also involved emplacement of the supracrustal Cista pluton along antithetic west-dipping shear zone in the east. The studied magmatic rocks record a Mid-Devonian magmatic arc related to the formation of a subduction wedge stacked beneath the upper plate that was followed by formation of a second and independent Carboniferous continental arc further east.