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Mónica López de Luchi, Instituto de Geocronologia y Geología Isotópica (Argentina)
Andre Steenken, Instituto de Geociências, Universidade de São Paulo (Brazil)
Siegfried Siegesmund, Geoscience Centre of the University of Göttingen (Germany)
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At the eastern part of the Sierra de San Luis (Sierras Pampeanas,) Devonian magmatism is represented by granodiorite-monzogranite batholiths that are associated with Qz-syenites, Qz-monzonites and minor Qz-diorites. Small stock or dykes of red monzogranites sharply intrude the batholiths. Minette dykes intruding the country rocks next to the contacts are preferentially emplaced in an E-W fracture system. These syntectonic batholiths are associated with major shear zones that switched with time (20Ma) from transpressional to transtensional (Siegesmund et al., 2003).
Batholiths intrude into the Cambrian Conlara and the Ordovician Pringles metamorphic complexes and crosscut Devonian (?) mylonitic shear zones. Two magmatic units are present: a high Mg-K association represented by both syenitic, monzonitic and monzodioritic microgranular enclaves and synplutonic dykes and a porphyroid monzodioritic to melagranodioritic facies with the more basic rocks included as enclaves in the synplutonic dykes and a high K calc-alkaline association that constitute the major part of the concentrically zoned batholiths with an external porphyroid granodiorite to monzogranite and a central equigranular monzogranite: The more basic representatives of this association are Qz-dioritic enclaves. The red granite small stocks are locally fluorite-bearing equigranular monzogranites.
Devonian magmatism may be related to gravitational collapse or back-arc extension of a distant subduction zone. Nb and Ti troughs in the monzonites together with the high LILE may be a geochemical indication of subduction-related sources. In present geographic coordinates, the voluminous Devonian granitoids are concentrated inboard the probable Gondwana margin where accretion of the Chilenia Terrane was assigned to the interval Silurian-Devonian. Granitoids were emplaced in a crust had cooled down to temperatures around biotite closure by their emplacement time. Delamination of the lithospheric mantle after crustal thickening is a possible mechanism to increase the heat input to the base of the crust, and may explain the reheating that is indicated by the regional c. 380 Ma biotite cooling ages. Upwelling mantle resulting from delamination of subducted lithosphere following an accretionary stage or post-accretion strike slip faulting that extended to mantle depths could control melting of a fertilized mantle source region. Magma mixing processes were favored in connection with the transtensional regimes that allowed considerable migration from the melting site up to the emplacement site (around 3.3-3.8 kbar).Isothermal decompression melting and melt segregation to mid-upper crustal levels may have been triggered by tectonic exhumation.
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