Late Paleozoic-early Mesozoic magmatism in the Cordillera de Carabaya, Puno, southeastern Peru: Geochronology and petrochemistry

The Inner Arc domain, the easternmost magmatic manifestation of the post-Paleozoic Central Andean orogeny in southeastern Peru and western Bolivia, comprises a remarkably diverse assemblage of plutonic and volcanic rocks, many of which would be more characteristic of ensialic rifts or collisional mountain belts than of Andean-type convergent plate boundaries. Marked petrologic contrasts with the more homogeneous Main Arc domain, which underlies the westerly provinces of the orogen, have been maintained since the initiation of Andean orogeny in the Late Triassic. Constraints on the chronology and petrogenesis of the early stages in the protracted evolution of the Inner Arc and its Permian antecedents are provided herein by, respectively, KAr and RbSr geochronologic data and major and minor element analyses of representative pre-Cretaceous igneous rocks of the Cordillera de Carabaya, southeastern Peru. Our studies confirm the following sequence of magmatic events, which temporally overlapped with the initial stages of Andean orogeny: 1.i) Intrusion of the gabbroic-to-granitic San Gabán (Corani) complex, a calc-alkaline, but crustally contaminated, suite that cores an extensive area of high-grade, low-pressure metamorphism in lower Paleozoic strata. The complex has been assigned to the mid-Paleozoic, but its age remains poorly defined. The foliated, markedly peraluminous, two-mica granites of the smaller Limacpampa pluton may also have been emplaced during the Paleozoic, but a Triassic age is favored on the basis of our RbSr data.2.ii) Eruption of alkali basaltic lavas of the Lower Permian Mitu Group along the northeastern margin of a longitudinal ensialic rift that developed in response to extensional tectonism in the interval between the pre-Andean (“late Hercynian”) and Andean orogenies.3.iii) Emplacement of large granitoid plutons (Coasa, Limbani, and Aricoma centers), with I-Caledonian affinities, along the northeastern boundary of the Mitu rift during the Late Triassic (ca. 225 Ma). The metaluminous to weakly peraluminous monozogranites and granodiorites comprising the greater part of the Carabaya Batholith (new term) were closely associated with mafic dikes of alkaline composition, similar in many respects to the preceding alkali basalts.4.iv) Development of the Allinccápac Group or peralkaline complex (new term), an assemblage of Middle (and Lower(?) Jurassic lavas, pyroclastics, and plutons exhibiting alkaline to peralkaline affinities.Whereas each of the above suites may be assigned to either a predominant mantle or crustal source, it is evident from the chemical and isotopic data that varied mantle and crustal environments have been involved. Thus, the distinctive chemistries of the coeval granitoid intrusions — as expressed, for example, in the trace element contents of whole-rocks and biotites, the oxidation states of both rocks and biotites, and the initial strontium isotope ratios — demonstrate the contributions of several distinct protoliths. The close spatial and temporal association of mantle and crustal suites during the Permian-to-Jurassic interval strongly implies a cause-and-effect relationship. In particular, the role of basaltic injection in generating large volumes of peraluminous granitoid magmas is amply supported.