Neogene Magmatism, Tectonism, and Mineral Deposits of the Central Ande (22° to 33° S Latitude)
Suzanne Mahlburg Kay, Constantino Mpodozis, Beatriz Coira, 1999. "Neogene Magmatism, Tectonism, and Mineral Deposits of the Central Ande (22° to 33° S Latitude)", Geology and Ore Deposits of the Central Andes, Brian J. Skinner
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The distribution and chemistry of late Oligocene to Recent central Andean magmatic rocks and mineral deposits between 22° 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 subduction-related 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 the 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 sub-Andean 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 of 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°–34° S), greater El Indio (ca. 29°–31 ° S), and Maricunga (26°–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 that were in equilibrium with erupted magmas dehydrated to yield high-pressure, garnet-bearing assemblages.
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Geophysical data relating the dynamic processes of plate motion and subduction to Andean orogenesis are interpreted in terms of a new model for magmatic and tectonic development of the central Andes. The model is based on changing subduction geometry—from normal to flat to normal—and the attendant magmatic and tectonic effects of slab dewatering, continental lithospheric hydration, and asthenospheric flow during closing and opening of the subduction zone mantle wedge. The model includes five stages:
1. Normal subduction extended into Eocene time.
2. A slab transition from normal to flat subduction occurred in late Eocene-early Oligocene time, coincident with extensive crustal deformation in the eastern Altiplano and Eastern Cordillera.
3. Flat subduction during much of Oligocene time was accompanied by a volcanic null throughout the central Andes, when water from the slab infiltrated and hydrated the overlying continental lithosphere, resulting in advective cooling and abnormally low heat flow values. Lithospheric hydration was concentrated not only in the usual fore-arc region but also within the inner arc, in the zone of resubduction where amphibole is presumed to break down and the slab dips steeply into the mantle.
4. The transition from flat to normal subduction in late Oligocene-earliest Miocene time brought about an influx of asthenospheric material from depth into the growing mantle wedge above the slab. Hot asthenospheric mantle in contact with hydrated lithosphere of the inner arc produced widespread melting of both mantle and crust beneath the eastern Altiplano-Eastern Cordillera and ushered in a period of ductile deformation associated with oroclinal formation. The magmatic activity and orogenic uplift that began in the inner arc broadened westward as hot asthenospheric material flowed into the mantle wedge above the sinking slab.
5. The westward broadening of volcanic activity culminated in a resumption of calc-alkaline volcanism all along the main volcanic arc by at least 20 to 15 Ma. The crust beneath the main arc, probably thickened by previous magmatic and deformational events, was further thickened and uplifted by the intrusion or underplating of massive volumes of mantle-derived magmas. Eruptive activity in the inner arc, much of it anatectic and correlated with periods of crustal deformation, gradually waned, with migration of minor magmatic centers eastward almost to the present day. The thermally thinned and weakened lithosphere of the Eastern Cordillera and sub-Andean belt formed a ductile block in which compressive stresses have been concentrated in Neogene time. The tectonic collapse of the inner