Magmatic and Metallogenic Evolution of the Central Andes
The <10,000-yr-old volcanoes of the Central Volcanic zone (15°-27° S) form a 50-km-wide belt that widens locally to 100 to 150 km and has three outliers 100 to 200 km east of it. The locations of over 1,800 radiometrically dated igneous rocks and hydrothermal ore/alteration minerals between 6° S and 33° S are plotted for 25 time intervals (varying between 2 and 65 m.y.), from the Precambrian to the Holocene. For short time intervals, these locations define 25- to 75-km-wide belts that widen occasionally to 75 to 125 km and have local outliers of volcanic tuffs or ignimbrites. Nonmagmatic stretches, such as the current Northern (2°-15° S) and Southern (27°-34° S) Nonvolcanic zones, probably occurred at various times and locations in the past, but were distinctly subordinate in strike length and duration to the magmatic zones.
There are two roughly parallel belts that are 200 to 400 km apart (locally separated by only 125 km or up to 500 km). Over the chosen time intervals, both magmatic belts were often active. However, judging from presently active magmatic belts, they were probably seldom coeval over geologically very short time spans. The western belt corresponds to the conventionally envisaged magma generation by a subducting oceanic plate at 100- to 125-km depth. The eastern belt is akin to a back arc in an oceanic setting, except that it occurs in a continental plate. The apparent parallelism of both belts suggests that they were generated by linked mechanisms.
Pegmatites, granites, rhyolites, and rhyodacites occur in both belts, but as a group are more common in the eastern belt. Calc-alkaline igneous rock compositions also occur in both belts, but as a group predominate in the western belt. Although basaltic rocks occur in both belts, as a group the mafic igneous rocks appear to be largely restricted to the western belt. The two phonolites and the nepheline syenite dated are in the eastern belt.
In many areas, the location of the magmatic belt did not change significantly over a long period of time. The location of the magmatic belt gives the appearance of essentially continuous magmatism accompanied by occasional hydrothermal activity that resulted in the formation of ore deposits.
Significant changes in the activity and locations of magmatic belts can occur in about 5 million years. As magmatic belts shift eastward or westward, individual magmatic centers may be dragged along. Integrated over a long time, this process may give rise to transverse magmatic alignments or transbatholiths with associated hydrothermal ore deposits of different ages that appear to be controlled tectonically.
The relatively straight magmatic belts have local deflections. These deflections can be interpreted as smooth changes in the dip of the subducting plate or as faulting of either the oceanic or the continental plate.
Oceanic plate subduction below the central Andes has occurred since the Cambrian. Folding and overthrusting in the continental plate did not significantly disturb the geometry of the magmatic-hydrothermal belts.
Figures & Tables
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