Petrogenesis of early syn-tectonic monzonite-granodiorite complexes – Crustal reprocessing versus crustal growth

摘要: Abstract The 563.7 ± 6.1 Ma old, early-syntectonic Mon Repos complex is a predominantly metaluminous, magnesian, calc-alkalic granodioritic to granitic intrusion. Major and trace element variations imply that the rocks evolved through fractional crystallization processes involving amphibole, biotite, Fe-Ti oxides, zircon, apatite. Rocks less than granodiorite show evidence of hornblende accumulation. Initial Sr (87Sr/86Sr: 0.7090–0.7111) Nd (eNd: −5.3 −12.3) isotopic compositions granodiorites granites are highly heterogeneous vary with SiO2 contents indicating assimilation older crustal components occurred. 206Pb/204Pb (16.65–17.65) 207Pb/204Pb ratios (15.52–15.60) similar other mafic-intermediate complexes from Damara belt. One monzodiorite two quartz monzonites K2O-, LILE-, HFSE enriched have as granodiorites. These samples post-collisional mafic magmas elsewhere in world. Their e values (−3.8 −4.3), 87Sr/86Sr (0.7051–0.7073) trace-element characteristics their unexposed parental melts derived lithospheric mantle source was contaminated metasomatized by material during ancient subduction processes. data explained assuming “flat” model where melting involves continental lithosphere crust limited, if any, underlying asthenospheric mantle. During flat subduction, sliver consisting buoyant oceanic amalgamated base overriding plate. dehydrated (but did not melt) these fluids lowered solidus overlying crust. This scenario can explain occurrence rare K2O- LILE-enriched monzodiorites crustal-like observed this study well some more alkaline Because has lower temperatures peridotitic mantle, it very likely such may melt early subduction. Thus, K2O-enriched monzogranites restricted late stages evolution an orogen but already form at onset orogeny.