Geological evolution and metallogenetic relationships of the Farallon Negro Volcanic Complex, northwest Argentina

The 700 km{dollar}sp2{dollar} region encompassing the Farallon Negro Volcanic Complex and associated outlying intrusive centres represents the locus of the greatest metal endowment currently recognized in Argentina. The early volcanic history of the region is characterized by the extrusion from discrete centres from 12.56 to 8.51 Ma of basalt, basaltic andesite and dacite flows of the Farallon Negro Group. These precursor volcanic events were subsequently covered by flows and breccias of the Main Farallon Negro Stratovolcano, centred on Alto de la Blenda, which erupted from ca. 8.51 to 7.49 Ma, but with the minor extrusion of pyroclastics persisting as late as 6.72 Ma. The stratocone-building event was accompanied by intrusive activity, which peaked between 6.8 and 8.6 Ma and continued until 5.2 Ma with the emplacement of dacite and rhyolite dykes representing the terminal stages of magmatic activity in the region. Porphyry Cu-Au and epithermal Au-Ag alteration-mineralization formed early in this terminal stage.; The geochemical continuum represented by the Farallon Negro high-K calc-alkaline and shoshonite suites strongly suggests a common source. Whereas shoshonitic units represent some of the oldest dated units at Farallon Negro, they are interbedded with high-K calc-alkaline units. The absence of features indicative of magma mixing in the shoshonitic suggests a lesser; involvement of deep-crusial MASH processes for the more potassic members. The calc-alkaline melts show evidence for mixing and may have experienced greater interaction with the lower crust through slower magma ascent and/or longer crustal residence time. They were erupted quasi-continuously throughout the igneous history at Farallon Negro.; The igneous rocks of the Farallon Negro region were emplaced in a pull-apart basin related to coalescing zones of transtension, nucleated at the intersection of N-S striking faults and the NE-trending dextral Tucuman Transfer Zone. Initiation of the localized extension occurred in a stress regime dominated by E-W compression related to the convergence of the Nazca and South American Plates. A reorientation of stress regimes at ca. 8.0 Ma, evident from the NW orientation of the Aguila dyke swarm, heralds the overriding of the subduction-related stresses by NW-SE directed stresses related to the uplift of the Puna.; The study-area delimits the eastern extension of a transect of Chile and Argentina centred at ca. 27{dollar}spcirc{dollar} S which hosts a definable, world-class, concentration of Cu-Au deposits. It constitutes the northern boundary of a distinct Cu-Au metallogenetic subprovince. Tears in the subducted slab, possibly nucleated on inhomogeneities in the oceanic crust related to the Easter Island volcanic chain, allowed asthenosphere to traverse the slab and rise into the overlying mantle wedge. This provided higher heat flow and possibly allowed greater devolatilization of the subducted slab, which is inferred to have resulted in the generation of melts which record the Middle Miocene are broadening at this latitude. (Abstract shortened by UMI.)...