TY  - GEN
ID  - heidok23679
KW  - geochemistry
KW  -  lamprophyres
KW  -  subduction
KW  -  mantle metasomatism
KW  -  xenoliths
TI  - Geochemistry and petrology of lamprophyres from the Hellenides and the European Variscides
Y1  - 2017///
AV  - public
UR  - https://archiv.ub.uni-heidelberg.de/volltextserver/23679/
N2  - Mantle-derived potassic to ultrapotassic magmatism is a typical feature of collisional orogens and is also occasionally present above oceanic subduction zones. Lamprophyres are part of this magmatism and are of particular interest, as they carry information about the chemical and mineralogical conditions of the uppermost mantle. They offer insights into the process of element cycling during subduction and collision, when a strong physical and chemical interactions between the geochemical reservoirs, specifically between the continental crust, the oceanic crust as well as the upper mantle, take place. This thesis presents examples from active subduction settings (the Greek Hellenides) as well as fossil subduction and collision zones (the European Variscides). Both studies focus on detailed whole-rock geochemical and isotopic data sets. Additional information about the lithospheric evolution are obtained from high-pressure xenoliths hosted in these lavas. Xenoliths from Variscan lamprophyres provide a snapshot of the lower orogenic crust during the Variscan collisional process and record the dynamics of regional extension. 
The island of Kos at the eastern end of the South Aegean Volcanic Arc hosts numerous late Neogene amphibole and mica lamprophyres. The Sr and Nd isotope ratios of these mantle-derived rocks correlate negatively in narrow ranges, extending from the undepleted end of the mantle array towards values typical for continental crust. The crust-like signature is linked to the addition of subducted SE Mediterranean sediments, which dominate the Pb isotopic composition of all lamprophyres. The mantle endmember of the Sr?Nd isotope mixing array is strongly enriched in incompatible trace elements and shows anorogenic affinities. Whole-rock geochemistry indicates that the lamprophyres originated from the depleted lithospheric mantle above the Hellenic subduction zone that was modified by (i) K-rich silicic partial melts or supercritical fluids from subducted metasediments and (ii) melts from the asthenosphere below the subducting slab. The first metasomatic component formed phlogopite-orthopyroxene-rich veins, whereas the second led to the formation of amphibole-bearing clinopyroxenites. Subsequent melting preferentially affected these enriched domains. Variable dilution by contributions from ambient peridotite and mixing between both components caused the large chemical variation observed in the lamprophyres. The asthenosphere-derived component represents incipient melt with a carbonatite-like trace element signature (e.g. superchondritic Nb/Ta and Zr/Hf; low Ti/Eu). This metasomatic agent was probably introduced into the mantle wedge along ruptures in the slab of the retreating Hellenic subduction zone. Lamprophyres dominated by the sediment signature have isotopic and chemical similarities to basaltic rocks erupted along the South Aegean Volcanic Arc. The much stronger enrichment in incompatible trace elements of the lamprophyres is related to only minor dilution by melts from the ambient mantle. Partial melting in the lithospheric mantle is attributed to extensional tectonics, probably during a stage of rapid slab rollback, with a limited availability of fluid as a fluxing agent. Lamprophyre emplacement occurred along sinistral ?en-échelon? structures in a crustal-scale shear zone, which separates Anatolia from the faster extending Aegean back-arc basin. 
In the Variscides, K-rich magmatic rocks form intrusions throughout the internal zones of the orogen (durbachites, melanocratic quartz-monzonites/-syenites, lamprophyres, lamproites, trachybasalts, basaltic trachyandesites, shoshonites). Based on a detailed whole-rock geochemical dataset of post-collisional dykes from SW Germany and E France, a generic petrogenetic model for this chemically and isotopically heterogeneous mantle-derived magmatism is presented. The lamprophyres have crust-like trace element patterns and elevated initial 87Sr/86Sr and 207Pb/204Pb as well as low 143Nd/144Nd ratios. High mantle-compatible element concentrations reflect significant contributions from mantle peridotite to the melts. This hybrid nature requires at least two source components: continental material and mantle peridotite. Continental subduction during Variscan collisional processes resulted in the physical and chemical interaction between the crust and the upper mantle. Systematic sampling of dyke rocks across tectonic zones of contrasting development reveals two groups of mantle-derived potassic to ultrapotassic rocks with distinct trace element patterns and isotopic compositions. Deeply subducted crustal lithologies were affected by high-degree partial melting at mantle depth due to the breakdown of hydrous phases like phengite. Whether the trace element signature is transferred unmodified into these melts or not, largely depends on the behaviour of accessory phases (e.g. zircon, allanite or monazite) that sequester significant amounts of incompatible trace elements. For instance, high Th/La?Sm/La and low Th/U ratios of some lamprophyres are related to residual allanite during generation of the metasomatic crustal melts. The melts migrate from the slab into the overlying lithospheric mantle, react with peridotite and solidify to phlogopite?pyroxenites. Reaction between siliceous melt and peridotitic wallrock leads to the crystallisation of orthopyroxene ± garnet at the expense of olivine, resulting in a depletion in Al2O3 and garnet-compatible trace elements in the coexisting melt. Progressive wall-rock interaction causes enrichment in incompatible trace elements and eventually produces peralkaline melt compositions. Post-collisional extension preferentially triggers melting within the metasomatic domains. Mobilisation of the diverse olivine-poor hybridisation products generates geochemically and mineralogically heterogeneous melt-compositions, ranging from medium-K basalts to peralkaline lamproites. 
Crustal xenoliths entrained in post-collisional Variscan lamprophyres from the crystalline Odenwald (Mid-German Crystalline Zone, MGCZ) include felsic granulites and retrogressed eclogites. Classical thermobarometry, Zr-in-rutile thermometry and equilibrium phase diagrams (pseudosections) reveal temperatures of 700?800°C and pressures of 1.7?1.8 GPa. Both lithologies record isothermal decompression and provide evidence for the presence of partial melt. The felsic granulite is partially molten at peak conditions. Instead, the solidus of the eclogite is crossed during isothermal decompression, causing fracturing of mineral grains and crystallisation of a plagioclase-bearing assemblage along grain boundaries and cracks crossing garnet, indicating pressures of 1.2±0.2 GPa. Both lithologies got entrained into the mantle-derived magma, causing a high-temperature overprint at temperatures in excess of 1100°C that resulted in the development of diverse fine-grained microstructures. As no deformation or cooling related to a tectonic exhumation process occurred, rare mineralogical and textural features are preserved. Compositional sector zonation in garnet is caused by rapid crystallisation, related to reaction overstepping. Sudden nucleation and crystallisation possibly takes place due to shearing and fluid-inflow during incipient decompression. During continuing extension, the wet solidus is crossed. Small amounts of partial melt cause a drastic reduction in rock strength due to brittle failure. A consequence of rheological weakening may be a localisation of deformation, resulting in crust-scale shear zones. Such shear zones may accommodate orogenic extension, provide pathways and trigger the ascend of the mantle-derived lamprophyres. For this reason, the lower crustal xenoliths are rare documents
of the Variscan collision process, resulting in crustal thickening to at least 60 km, and the subsequent regional extension. A Lower Carboniferous age of metamorphism is supported by SIMS U-Pb zircon dating, which gave ages of 332±7 (2?) Ma (felsic granulite) and 335±6 Ma (eclogite). An age of 440±10 Ma obtained for the oldest zircon population in the eclogite is interpreted as the age of the magmatic protolith. Abundant discordant zircons with oscillatory zonation in the felsic ganulite have an upper concordia intercept at 2110±25 Ma, giving the age of crystallisation of the granitic protolith. While Silurian magmatism is well established within the MGCZ, Paleoproterozoic basement was previously unknown and is generally very rare in the
European Variscides.
A1  - Soder, Christian
ER  -