Plutonism from Antarctica to Alaska
The plutonic history of central Chile between 30° 00′ and 42°00′S developed through episodic intrusions that took place from the late Paleozoic to the late Cenozoic. Three main stages of plutonio development correlate with the major tectonic events of central Chile: pre-Andean (late Paleozoic), Transitional (mainly Triassic to Early Jurassic), and Andean (Middle Jurassic to late Tertiary).
The pre-Andean and Transitional granitoids north of 39°S are distributed in two en echelon belts in the Coast Range and in the High Andes Cordillera. The coastal plutonic belt is hosted in the low P/T metamorphics of the eastern half of the upper Paleozoic subduction complex, whereas the High Andes belt intrudes related volcanics. South of 39°S, only isolated patches of Paleozoic to Jurassic granitoids occur in the western slope of the Andes.
The Andean granitoids north of 39°S are distributed along three parallel north-south–trending belts of decreasing age from the Coast Range to the High Andes Cordillera. The Coast belt includes Jurassic and Cretaceous granitoids, the Central belt comprises lower Tertiary granitoids, and the High Andes belt is formed by upper Tertiary granitoids. South of 39°S, Miocene granitoids constitute the bulk of the High Andes, whereas Cretaceous granitoids occupy an eastern position, giving rise to a reverse plutonic zonation compared to that observed farther north.
Pre-Andean plutonism was dominated by tonalites and granodiorites, with abundant mafic inclusions and leucogranites almost free of inclusions. All the available pre-Andean 87Sr/86Sr initial ratios are higher than 0.705, the most frequent values being in the 0.706 to 0.707 range.
The Transitional plutonism gave rise to a bimodal association of leucogranites and quartz-diorite and gabbros. Mafic and felsic dike swarms spatially and temporally related with leucogranite plutons are recognized in the two belts. Most of the 87Sr/86 initial ratios obtained in the Transitional plutonic rocks are in the 0.704 to 0.707 range.
Compared to pre-Andean and Transitional plutonism, Andean plutonism is more primitive. Gabbros, diorites, and tonalites are the dominant lithologies. The 87Sr/86Sr initial ratios are lower, in the 0.7035 to 0.7045 range.
The available geochemical and isotopic data suggest differences between the Paleozoic–lower Mesozoic magmatic sources in which crustal material was important and the Meso-Cenozoic mantle-type source. Mixing between two-contrasting magmas has been suggested in the northern part of the pre-Andean Coast plutonic belt. In the case of the Transitional leucogranites and gabbros, crustal and mantle melting has been invoked for the origin of the respective magmas. Finally, most of the Andean granitoids seem to result from partial melting of a mantle or mantle-derived basaltic source.
Pre-Andean and Andean plutonic belts were important elements of magmatic arcs developed during plate convergence episodes. Transitional plutonism might have developed during a non-subduction stage dominated by an extensional tectonic regime, which favored crustal derivation of leucogranite and coeval mafic pluton and dike intrusions. The eastward-migrating Andean plutonism north of 39°S could be related to a combination of processes such as subduction-related tectonic erosion, changes in the dip of the subduction zone, and eastward propagation of mantle-crust decoupling. Westward-decreasing ages of Andean granitoids south of 39°S may reflect a westward migration of the trench caused by accretionary subduction.
- alkaline earth metals
- igneous rocks
- mineral composition
- plutonic rocks
- South America
- stable isotopes
- upper Paleozoic
- central Chile