High-T metamorphism, crustal melts, and a large igneous province: The Paleozoic to Cenozoic margin of the Central Andes, N Chile and NW Argentina

BECCHIO, R.; LUCASSEN, F.,; FRANZ, G.

Cenozoic geology of the Central Andes of Argentina

Instituto del Cenozoico, Universidad Nacional de Salta

Lugar: Salta; Año: 2010;

The present orogen of the Central Andes provides the opportunity to compare exhumed crust of two different types, from compressional and extensional tectonic settings. Thick crust with high mid-crustal temperatures  in a compressional tectonic regime occurs twice at the Phanerozoic active continental margin of the Andes, in the exhumed Early Palaeozoic orogen of the Central Andes and in the active Cenozoic orogen and plateau. The Early Palaeozoic orogen is characterized by mid-crustal temperatures above the wet granite solidus (~650°C). Widespread felsic migmatite and rare granulite formed at pressures ca 0.5 to 0.7 GPa, locally 1.0 GPa and represent the deepest exhumed sections of the overthickened Palaeozoic crust. High pressure – low temperature rocks are absent. Granitoid magmatism, which mainly recycles the (meta) sedimentary crust, is abundant whereas juvenile additions to the crust are rare. Sm-Nd model ages of Palaeozoic metamorphic, granitoid, and sedimentary rocks have their peak between 1.7 and 2.0 Ga. The ages of high temperature metamorphism and granitoid magmatism range between ~ 530-420 Ma and indicate a long-standing near stationary high temperature regime. In the Cenozoic orogen, the large spatial distribution of partially molten mid-crust at ca 20 km depth beneath the Central Andean high plateau is indicated by geophysical anomalies. The partially molten crust has its surface expression in the widespread occurrence of large volume, mainly crustal-derived ignimbrite. The latter resembles largely the isotope signature of the Palaeozoic crust. We propose that the present situation at the western continental margin of South America is the analogue of fossil, now exhumed large-scale orogenic events, active over a period of ≈ 100 Ma, which lead to extremely extended areas of migmatized upper amphibolite facies (transitional to granulite facies) terranes. The explanation of massive, longstanding heat transfer into the mid-crust remains a major issue, because the crust is predominantly felsic and voluminous mafic intrusions are precluded. In the Jurassic to Lower Cretaceous, a different type of hot crust formed in an extensional  magmatic arc at the Andean margin (~18°S to 27°S). It is characterized by abundant juvenile, prevailing mafic intrusions and a thick sequence of mafic volcanic rocks. Large sections of the crust comprise this compositionally homogeneous material and form a prominent geophysical anomaly indicating a mafic crust. The deepest exhumed part of this section indicates granulite facies temperature of ~800°C at pressures below the garnet-in reaction (< 0.7 GPa) in recrystallized igneous rocks of the same arc. The heat transfer in this prevailing extensional setting is advection by voluminous intrusions of mafic magmas over a time period of ≈ 60 Ma.