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Book Chapter

Magmatic and Metallogenic Evolution of the Central Andes

By
Ulrich Petersen
Ulrich Petersen
H. C Dudley Professor of Economic Geology, Emeritus, Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138-2902
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Published:
January 01, 1999

Abstract

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.

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Contents

Special Publications of the Society of Economic Geologists

Geology and Ore Deposits of the Central Andes

Brian J. Skinner
Brian J. Skinner
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Society of Economic Geologists
Volume
7
ISBN electronic:
9781629490311
Publication date:
January 01, 1999

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