Neogene Magmatism, tectonism and mineral deposits of the Central Andes (22º-33ºS latitude).

KAY, S.; MPODOZIS, C. AND B., COIRA

ECONOMIC GEOLOGY AND THE BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS

SOC ECONOMIC GEOLOGISTS, INC

Lugar: Nueva York; Año: 1999 vol. 7 p. 27 - 59

0361-0128

Abstract The distribution and chemistry of late Oligocene to Recent Central Andean magmatic rocks and mineral deposits between 22°S and 33°S latitude reflect changes in the dip of the subducting Nazca plate and the thickness of the overlying lithospheric mantle and crust. Correlations of major magmatic and tectonic events at ca. 18 to 16 Ma, ca. 10 Ma, ca. 7 to 5 Ma, and ca. 2 Ma with previously proposed Andean-wide pulses support external causes for major events and regional geometric control on local style. Evolving magmatic and tectonic patterns indicate that the slab has shallowed beneath the modern Chilean flat-slab region (28° to 33°S), steepened beneath the modern northern Puna plateau (ca. 25° and 22° S), and remained in a transitional state beneath the intervening modern southern Puna. Shallowing in the Chilean flat-slab region is indicated by eastward migration of subductionrelated magmatism and deformation, termination of main arc andesitic volcanism by ca. 10 Ma, and the virtual cessation of volcanic activity by ca. 5 Ma. Shallowing was accompanied by crustal thickening, lithospheric thinning and hydration, and substantial loss of the asthenospheric wedge. Steepening of the slab below the northern Puna is indicated by widespread deformation and basin formation associated with virtual volcanic quiescence in the late Oligocene to middle Miocene, followed by westward contraction of the middle Miocene to Recent volcanic arc. A westward-shifting focus of giant late Miocene to Pliocene ignimbritic eruptions reflects massive melting caused by introduction of a thickening asthenospheric wedge above a steepening subduction zone, and below a thinned hydrated lithosphere. A contemporaneous eastward shift in the major zone of thrusting to the Subandean Belt can be explained by compressional collapse of the hot, ductile crust beneath the plateau. Lithospheric thickening accompanied deformation above the steepening slab. A persistent intermediate dip of the slab beneath the intervening southern Puna is supported by the lack of a volcanic gap, and by a transitional magmatic and tectonic history compared to that to the north and south. Extreme crustal thickening over the intermediately dipping slab resulted in instabilities in eclogitic lower crust that led to Pliocene continental lithospheric foundering (delamination). Evidence for delamination comes from Pliocene to Recent eruptions of the Cerro Galán ignimbritic center, a concentration of primitive mafic lavas associated with normal and strike-slip faults, high average regional elevation, and seismic evidence for a thin underlying lithosphere and an abnormally hot subducting slab. Temporal variations in mantle-derived mafic magma chemistry indicate Neogene mantle enrichment by introduction of crustal material during the subduction process. Within this framework, major Central Andean Neogene Au and Cu deposits in the greater El Teniente (ca. 32° to 34°S), greater El Indio (ca. 29° to31°S), and Maricunga (26° to 28°) belts formed as crustal thicknesses reached 45 to 50 km over the shallowing and cooling subduction zone. The general southward younging of these deposits reflects a southward pattern of crustal thickening. Emplacement of the deposits took place in the waning stages of arc volcanism as the arc front migrated eastward or extinguished. Mineralization occurred as geochemically-inferred hydrous hornblende-based residual mineral assemblages in equilibrium with erupted magmas dehydrated to yield high pressure garnet-bearing assemblages.