FROM OROGENIC TO ANOROGENIC SETTINGS - EVOLUTION OF GRANITOID SUITES AFTER A MAJOR OROGENESIS

Many orogenic belts display associations of calc-alkaline and alkaline igneous centres which are closely related in space and time. The sequence: calc-alkaline batholith-->uplift and unroofing-->alkaline plutonic-volcanic complexes, lasts less than 100 Ma and documents a very fast switch from orogenic to anorogenic geodynamic conditions. The magmatic suites have a mantle origin with decreasing crustal contribution, but the sources and the types of differentiation processes differ. Alkaline granites can be subdivided in two groups: (1) Post-orogenic Ba and Sr-rich red granites having Mg-rich mafic minerals and high content of Mn whether they are peralkaline or peraluminous. Crustal contribution is indicated by Sr isotopic signatures. Water-rich fluids are responsible for the subsolvus crystallization of alkali feldspars and for the high oxygen fugacity. Magmatic centres were emplaced less than 10 Ma after the late-orogenic formations. (2) Early anorogenic Ba and Sr-poor greenish to whitish hypersolvus granites having Fe-rich mafic minerals, low contents of Mn, and virtually no Mg F-rich aqueous fluids are probably less abundant and promote subsolidus hydrothermal alteration with only subordinate late-stage oxidation. Crustal contamination can be ruled out for the non-mineralized complexes, but can be important in the hydrothermal mineralized areas. The magmatic centres are considerably younger than the first group. During the first 100 Ma following the end of a major orogeny, a new mantle source replaces the old complex system of mixed oceanic-continental crust-mantle sources and produces alkaline melts which are subsequently less and less contaminated by crustal host rocks. Relations between hydrothermal events, mineralizations, and crustal isotope signatures suggest that crustal contribution relates to percolating fluids and not to anatexis of lower and/or middle crust.