Abstract

MILLER, Hubert; LORK, Anette; TOSELLI, Alejandro J.  and  ACENOLAZA, Florencio G.. IGNEOUS ROCKS OF CACHI FORMATION: GEOCHEMISTRY AND GEOCHRONOLO-GY IN THE CALCHAQUI VALLEY, ARGENTINA. Ser. correl. geol. [online]. 2019, vol.35, n.1, pp.50-60. ISSN 1666-9479.

The basement of the Palermo, Cachi and Molinos ranges, in Salta province, are part of the Eastern Cordillera and is composed by a meta-sedimentary basement corresponding to the Puncoviscana Formation, with a maximum sedimentation age of 527 My, that is the culmination of Ediacaran-Terreneuvian period. This formation is affected by magmatic-metamorphic events that gives rise to the tonalitic-trondhjemitic and granodioritic-granitic intrusives of Cachi Formation and too the gneisses-migmatites, phyllites and spotted schists of La Paya Complex. The ages of the intrusives and associated metamorphites are between 488 and 453 My (Upper Cambrian - Upper Ordovician), which were determined by U-Pb zircons and monazites. Likewise, the ⁸⁷Sr/⁸⁶Sr ratios values varies from 0.70330 to 0.70394 are applied to the genetic interpretation of the intrusives, while the rocks of the Puncoviscana Formation register relations between 0.71275 and 0.71327. The igneous rocks are formed essentially by feldspars, quartz, biotite and pyroxenes. The wide variation of the plagioclase compositions, from labradorite to albite, evidences the rapid up rise and cooling of the plutons, as well as the survival of the pyroxenes. The tonalite-trondhjemite rocks are low in potassium and with ratios K₂O/Na₂O<1; while granodiorite-granite rocks are high in potas-sium, with K₂O/Na₂O>1 ratios. The distribution of chemical data projected in different diagrams, discards a common origin of the magmatic associations, which could not have been formed by fractional crystallization phenomena, but supports the hypothesis of different sedimentary protoliths with probably subordinate participation of the mantle material, such as Rio Blanco sub-alkaline basalts, with a somewhat older age. This magmatic association is unique in the Famatinian Cycle and would respond to a transtensive tectonics, with little or no convergence and related to an active continental margin during the development of a continental islandarc.

Keywords : Tonalites-trondhjemites and granodiorites-granites; Cachi Formation; La Paya Complex; Pampean/Famatinian Cycles.

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