The Magmatic Evolution of the Cordillera Oriental, Southeastern Peru
Magmatism within the Central Andean Inner Arc of the Cordillera Oriental of southeastern Perú is characterized both by its episodicity (ca. 350, 270–250, 240–230, 180, 90–70 and 26–4 Ma) and its variable chemical/petrological signature. In both respects, marked contrasts are evident with the quasi-continuous and petrochemically unified subduction-related magmatism of Triassic-Quaternary age in the Cordillera Occidental. The geochemical and petrographic features of the volcanic and plutonic rocks reveal highly variable crustal and mantle contributions within, and between, episodes of magmatism. For example, the 270–250 Ma Mitu Group volcanics (Sri = 0.704), and the 180 Ma Macusani syenite complex represent mantle-derived melts which experienced negligible crustal modification. In contrast, the 26–20 Ma (K-Ar, biotite) peraluminous (A/CNK > 1.2) cordierite-sillimanite-bearing granitoids (Sri = 0.718), and 17–4 Ma (K-Ar, biotite) peraluminous (A/CNK > 1.2) andalusite-sillimanitemuscovite ± cordierite-bearing rhyolitic ash-flow tuffs (Sri = 0.722) of the Macusani area represents melts of crustal derivation. The remaining suites, however, are chemically and petrographically more ambiguous, and in some cases variable proportions of mantle and crustal components were probably involved. The Permo-Triassic Coasa granitoid batholith, for example, comprises suites of both mantle and crustal derivation which may be distinguished on the basis of their mineralogy and whole-rock and mineral chemistry.
The relationship between the periods of Inner Arc magmatism and regional Andean tectonics (e.g. subduction, deformation) remains obscure. However, magmatism was associated with compressional, extensional and ‘neutral’ tectonic regimes. We attribute the continued juxtapositioning of multifarious igneous rocks in this relatively small area (8800 km2) to the existence of a zone of structural weakness, probably of Precambrian origin. The structure was variably reactivated during the Phanerozoic in response to regional stress systems and acted as a zone along which magmas of varying origin were channelled, and in some instances (i.e. the crustal-derived suites) may have been partially responsible for generating some of these magmas.
KeywordsCrustal Contamination Tectonic Regime Granitoid Rock Igneous Suite Magmatic Episode
Unable to display preview. Download preview PDF.