Abstract

--- - The Arrayan Formation comprises a thick, well stratified, strongly folded turbiditic succession, exposed along the Chilean coast, between 31 degrees 30 ' and 32 degrees S. It is interpreted as the frontally accreted portion of the Choapa Accretionary Complex formed in the south western Gondwana margin in Carboniferus and Permian times. The Array & aacute;n deposits are unconformably overlain by the marine neritic and richly fossiliferous late Early to early Late Permian Huentelauquen Formation. The contact with the basally accreted metamorphic units of the Choapa Accretionary Complex is tectonic. A previous and a recently dated sample from the Arrayan Formation (PS-8, this paper) yielded mid-Early Carboniferous (Visean) maximum depositional ages of 340 +/- 5 Ma and 342 +/- 4 Ma respectively. This age (i) contradicts the so far accepted Late Devonian to Early Carboniferous age assigned to the Arrayan Formation, based on its fossiliferous content, (ii) indicates that the age of the psylophytal remains found in this formation are post-Visean, (iii) confirms that the Arrayan Formation is younger than the mid-Early Carboniferous and older than the late Early to early Late Permian Huentelauqu & eacute;n Formation, and (iv) locates its stratigraphic position between the late Early Carboniferous and Early Permian. Previous dates from the compositionally similar metaturbiditic Agua Dulce Formation, which is exposed next to the Arrayan Formation and grades into a m & eacute;lange, yielded a similar age, which indicates that deposition of both units was coeval. The maximum depositional ages of the Array & aacute;n and the Agua Dulce formations are older than the maximum depositional ages obtained from the metamorphic units in the Choapa Accretionary Complex, which confirms that frontal accretion began earlier than basal accretion, and that the latter reached the Permian and was coeval with deposition of the Huentelauqu & eacute;n Formation. The stratigraphic position and the zircon age distribution pattern suggest that the El Toco, Sierra El Tigre and the Array & aacute;n formations are correlative. The Las T & oacute;rtolas Formation is somewhat younger than these formations. The age distribution pattern of the zircon grains in sample PS-8 shows two well-defined age peaks at similar to 340 (Visean) and similar to 480 Ma (Ordovician) and a barren interval between them. The barren interval coincides partly with the Middle Ordovician to Late Carboniferous passive stage proposed for this region of Gondwana. The Ordovician peak is present in all analyzed samples, between 22 degrees and 39 degrees 20 ' S, whereas the Visean peak and the barren interval are less developed in samples north of 27 degrees S. This similarity indicates the existence of roughly similar sources of sediments along this large section of the western Gondwana margin and, possibly, a rather similar paleogeographic context and depositional environment in the trench. A peak younger than 300 Ma in the Llano de Chocolate Beds indicates a younger depositional age and a different paleogeographic setting in the active continental margin, possibly in the forearc basin, like the Huentelauqu & eacute;n Formation. Broken-formation or melange facies occur in all complexes, except in the El Toco Formation, and affect the frontally accreted portion of the subduction wedge. The much higher metamorphic grade shown in the Chanaral m & eacute;lange compared to the much lower grade present in the Agua Dulce m & eacute;lange suggests formation at different levels along a mega-splay or thrust rooted in the subduction channel. - The overall mid-Mississippian age of the frontally accreted deposits analyzed in this study indicates that subduction was active already in the Early Carboniferous. The age, the tectonic setting and the geographic location of these deposits is totally different from the mostly older, marine deposits exposed to the east, in the Frontal Cordillera, which accumulated during a passive stage, in the Middle/Late Ordovician to late Early Carboniferous.