The magmatic evolution of the Cordillera Oriental, southeastern Peru.

Abstract

Magmatism within the Central Andes inner arc of the Cordillera Oriental of SE Peru is characterized both by its episodicity and its variable chemical/petrological signature. The exposed geological history of SE Peru was initiated in the early Palaeozoic when eugeosynclinal-type sedimentation was dominant and generated over 10 km of pelitic and psammitic strata by the late Devonian (approx 350 m.y.). Regional deformation and metamorphism occurred during the late Devonian-early Carboniferous interval followed by Permo-Carboniferous sedimentation, which ceased in the Mid-Permian during the 'late Hercynian' deformation episode. From the mid-Permian to the early Triassic, extensive alkaline and sub-alkaline volcanism associated with molasse-type sedimentation occurred along the entire Cordillera Oriental. Large (200-1200 km2) Permo-Triassic (240-230 m.y.) granitic intrusions were emplaced immediately following the Mitu volcanism and sedimentation in the area. An episode of peralkaline magmatism is represented by the Mid-Jurassic Macusani syenite complex. The Cretaceous (90-70 m.y.) is represented by three small (<0.5 km2) hypabyssal intrusions and several dolerite dykes located north of Crucero. The most recent episode of magmatism comprises two distinct suites: an older (26-20 m.y.) group of medium-grained biotite-cordierite-sillimanite-K-feldspar-megacrystic monzogranites, and a younger (17-14 m.y.) succession of biotite-andalusite-sillimanite-muscovite-cordierite-bearing rhyolitic tuffs. Although speculative, the area may have acted as a 'boundary layer' between the essentially stable craton to the E and the relatively unstable orogenic belt to the W.-J.M.H.