The Early Carboniferous age of the Arrayán Formation in the Choapa Accretionary Complex: implications for its fossil floral content, tectonic setting and evolution of the southwestern Gondwana margin (north-central Chile)

Abstract

The Arrayán Formation comprises a thick, well stratified, strongly folded turbiditic succession, exposed along the Chilean coast, between 31°30’ and 32°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 Arrayán deposits are unconformably overlain by the marine neritic and richly fossiliferous late Early to early Late Permian Huentelauquén 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 Arrayán 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 Arrayán 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 Arrayán Formation is younger than the mid-Early Carboniferous and older than the late Early to early Late Permian Huentelauquén 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 Arrayán Formation and grades into a mélange, yielded a similar age, which indicates that deposition of both units was coeval. The maximum depositional ages of the Arrayán 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 Huentelauquén Formation. The stratigraphic position and the zircon distribution pattern suggest that the El Toco, Sierra El Tigre and the Arrayán formations are correlative. The Las Tórtolas 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 ∼340 (Visean) and ∼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° and 39°20’S, whereas the Visean peak and the barren interval are less developed in samples north of 27°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 Huentelauquén Formation. Broken-formation or mélange 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 Chañaral mélange compared to the much lower grade present in the Agua Dulce mélange 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.