New geochronological and isotope data for the Las Chacras – Potrerillos and Renca batholiths: A contribution to the Middle-Upper Devonian magmatism in the pre-Andean foreland (Sierras Pampeanas, Argentina), SW Gondwana. (August 2019) - Dahlquist, Juan A.
Abstract: The Las Chacras-Potrerillos together with the Renca batholith (Sierras de San Luis) are the largest Devonian igneous bodies in the Sierras Pampeanas after the Achala batholith (Sierra de Córdoba), all emplaced in the Devonian foreland region. The Achala batholith has U–Pb zircon ages (using SHRIMP or LA-ICP-MS methodology, ranging from 366 to 379 Ma), while comparable ages for the Las Chacras-Potrerillos batholith remain absent (a single U–Pb conventional zircon age is available for this batholith), and one weighted mean age (U–Pb zircon SHRIMP data) is available for Renca batholith. U–Pb zircon LA-MC-ICP-MS data sets from a monzogranite sample of the biotite-amphibole porphyritic granite unit (CHA-101), the largest granitic unit of the Las Chacras-Potrerillos batholith, yielded three ages: 377 ± 3, 384 ± 2, and 393 ± 3 Ma. These ages (outside error limits), suggest the existence of a long-lived magmatic plumbing system, of around 15 Ma, with intrusion of younger batches into older crystal mushes. This interpretation leads us to assume a long period of heating in the source through the earliest Middle - early Upper Devonian ranging from 377 to 393 Ma. Based on previous studies of ascent and emplacement of granitic magmas, we propose that the majority of the zircon ages measured could be inherited from a deeper level of the magmatic system (i.e., antecrysts) and only some could be assigned to in-situ crystallization during the emplacement in middle-upper crust. Additionally, U–Pb zircon SHRIMP data sets from a monzogranite sample of the biotite-amphibole porphyritic granite unit (REN-103), the largest granitic unit of the Renca batholith, yielded two ages: 353 ± 4 Ma and 393 ± 3 Ma. The older age is equal to that obtained for the CHA-101 sample and confirms that the magmatism started in the earliest Middle Devonian. The younger age is comparable to previous ages determined in micas crystallized in intragranitic pegmatites as well as cooling ages from biotite hosted in granites of the Las Chacras-Potrerillos batholith, strongly suggesting a long-lasting cooling history of ca. 24 or 40 Ma (according to the crystallization age assumed, 377 or 393 Ma) or a reheating effect at around 350 Ma as was suggested in previous works. Recent Carboniferous crystallization ages determined in granites of the Sierras Pampeanas produced similar values (i.e., close to 350 Ma), and suggest a longer time of magmatic activity embracing the Middle-Late Devonian – Early Carboniferous (∼40 Ma), in the Devonian foreland (present-day Eastern Sierras Pampeanas). Nd and Sr isotopes data from CHA-101 and REN-103 samples (εNdt and 87 Sr/ 86 Srt ranging from −3.0 to −1.1 and from 0.706861 to 0.707275, respectively), are consistent with previous data reported for the studied granitic units and suggest a continental source with probable participation of subcontinental lithospheric mantle. TDM values (1.2 and 1.4 Ga) support the idea of an older Gondwana lithosphere source for the studied monzogranites. Highlights: Three ages outside error limits suggest a long-lived magmatic plumbing system for granites. Reported and previous ages support continuous Devonian – Carboniferous magmatism. TDM values support the idea of an older Gondwana lithosphere source.