Abstract
The Munro Pluton is located in the extra-Andean region of the province of Chubut, Argentina, in northern Patagonia, northeast of the Sierra de Tecka and northwest of the Sierra de Languineo. It is a sub-volcanic pluton with a SHRIMP age of 60 Ma (Paleocene?). Studies of fission tracks on apatites of the studied region suggest that the exhumation of the region took place during the Paleogene, indicating that the lapse since its emplacement until its exhumation, took less than 35 Ma, and that the pluton has been exposed to weathering at least since the Miocene until present times. This pluton has ellipsoidal shape and a surface of 25 km2 and it is intruded by two dyke systems. The Munro Pluton develops a landscape whose weathering front exposes fresh rocks, regolith zones and boulders immersed in regolith. Granitic landforms have been recognized at different scales. Among the bigger landforms of the granitic landscape, the following have been identified: domes (bornhardts), nubbins, koppies and smaller landforms such as boulders, flared slopes, gnammas, rills/gutters/gullies, tafoni and pseudo-bedding. Structural and textural observations allowed the inference that many of the identified landforms are generated in the sub-soil, being followed by the regolith mobilization thus exposing the paleo-weathering front. Other landforms have a tectonic component associated for their development, such as pseudo-bedding, as well as the endogenous deformations, related to the emplacement type of the Munro pluton. In general, most of the landforms are convergent as they evolve along different pathways.
Highlights
O Plúton Munro está localizado na região extra-andina da província de Chubut, Argentina, no norte da Patagônia, a nordeste da Sierra de Tecka e a noroeste da Sierra de Languineo
Along their elevation towards the Earth surface, these rocks are exposed to lithostatic and tectonic forces that contribute to the final morphology developed on the granitic rock outcrops, highlighting the importance of the systems of rocky discontinuities and the active weathering processes
Differential weathering is the main process in the evolution of granitic landscapes, alteration that is produced by atmospheric agents, the hydrosphere and the biosphere
Summary
The igneous rocks that compose the basement of the region are schists and phyllites of Precambrian-Early Paleozoic age (the Cushamen Formation) and they are intruded by Early Paleozoic acid plutonic rocks, covered in erosional unconformity by the sedimentary rocks of the Tepuel Group (Carboniferous to Permian). These rocks are covered with angular unconformity by Jurassic sedimentary rocks, and both rock groups are the country rocks of a set of gabbro intrusive rocks of Early Cretaceous age, which reach very large dimensions (POMA, 1986). During the Early Jurassic, intense volcanism of andesitic-basaltic composition 49 (the Lonco Trapial Formation) took place These volcanic rocks are followed by a sequence of lacustrine sedimentary rocks with interbedded basaltic rocks of the Cañadón Asfalto Formation. Extensive aggradation deposits and psephitic ridges of the Laguna Agnia have been assigned to the Pleistocene, and more modern basaltic flows, saline, alluvial and colluvial sediments complete the sequence into the Holocene (MÁRQUEZ; NIETO, 2005)
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