The Famatina belt, Central Andes, is part of an ancient accretionary margin built along Western Gondwana in the early Palaeozoic. U–Pb ion microprobe analysis of detrital zircons and Sm–Nd whole-rock analysis of two early Palaeozoic low-grade metasedimentary units record the early evolution of this region. Detrital zircons in the Negro Peinado and Achavil formations have ages ranging from Palaeoproterozoic to Cambrian, consistent with derivation from Gondwanan sources. T DM ages suggest that the sedimentary rocks were derived from a composite source area, which separated from the mantle during the Palaeoproterozoic ( c . 1.8–1.6 Ga). Constraints from the youngest detrital grains indicate accumulation in a Mid- to Late Cambrian foreland basin adjacent to the inboard Pampean orogenic tract. The dominance of Cambrian ages in the Negro Peinado Formation suggests derivation principally from the eastern Pampean belt whereas the dominance of late Neoproterozoic ages in the Achavil Formation suggests that input from the Pampean belt was overwhelmed by older sources. The paucity of Palaeoproterozoic ages argues against direct input from older areas such as the Río de la Plata craton. The predominance of Meso- and Neoproterozoic ages over older sources suggests that a Brasiliano-age magmatic arc developed on a Mesoproterozoic basement, probably a southern extension of the Arequipa–Antofalla massif.

[1] The Vinchina Foreland Basin, western Argentina, contains a ∼7 km thick nonmarine stratigraphy, chronologically constrained within the Mio-Pliocene (circa 19-3.4 Ma), and where distribution of Illite/Smectite interstratified phases has shown a progressive smectite-illitization progress (R0 → R1 → R3), is consistent with an incipient burial history. R0 represents randomly mixed-layered illite/smectite normally found at shallow depths, as this ordering is not stable at ∼120°C. In the Vinchina Basin, however, the R0 is still persistent at ∼7 km depth, and its appearance even in the deepest levels is consistent with previous interpretations of low burial temperatures based on thermochronologic studies of detrital apatites. The maximum paleotemperature estimation and basin depth imply geothermal gradient as low as ∼15°C/km, which allowed an estimate of heat flow values between 33 and 42 mW/m 2 , that would rise to between ∼40 and 51 mW/m 2 when the sedimentation rate (thermal blanketing) is taken into account. These values were only reported for cold basins and represent a paleothermal state of a refrigerated lithosphere. We suggest the central Andes were dominated since the Miocene by heat transfer derived mostly from crustal contributions with a minimum input from the asthenosphere. This refrigerated lithosphere is typical of segments affected by flat subduction. Preliminary thermal models based on previous geodynamic approaches support our conclusions.

We present mineralogic, isotopic and thermochronologic analyses on psammopelitic and tuffaceous levels from the Bermejo and Vinchina basins -both foreland depocentres of the Central Andes of Argentina -that define a low-temperature regime for the crust akin to a slab shallowing and flattening process. The contents of illite in illite/smectite interstratified (I/S) show a progressive illitization into the deeper parts of both basins. The distribution of I/S is compatible with theoretical simulations and predicted heat flow values of ca. 26 mW m À2 in the 8-3.4 Ma interval for the Vinchina Basin and ca. 42 mW m À2 since 9 Ma for the Bermejo Basin. The latter shows heat flow values that are comparable to those reported by magnetotelluric analysis (36-40 mW m À2 ) in agreement with previously published heat flow calculations along the modern Andean foreland. The Rb-Sr isochrones in psammopelites (<2 lm fractions) show ages between 125 and 165 Ma, whereas the K-Ar ages decrease as the grain size is smaller (136-224 Ma for 1-2 lm, 112-159 Ma for 0.2-1 lm, 76-116 Ma for <0.2 l and 39.3-42 Ma for <0.1 lm). These ages are significantly older than the sedimentation in the basins (ca. 16 Ma for the Vinchina Basin; U-Pb age), and can be explained by the presence of a significant amount of detrital components, mainly illite, even in the finer fractions. The preservation of detrital ages is consistent with the shallow diagenesis related to a low-temperature regime, proposed here for the basins. Younger K-Ar ages (21.3-12 Ma) were obtained for a basal tuffaceous level. Clay mineralogy and R0 ordering in the deepest part of the Vinchina Basin, together with the evolution model of I/S with depth, suggest that the burial temperatures would have not exceeded ca. 100°C in agreement with (U-Th)/He analyses performed on apatite extracted from two tuffaceous units. Thermal indicators from both studied basins confirm the existence of a low-temperature regime during flat subduction.