Short-term tectonic phases associated with advancing and retreating orogens during the SW Gondwana break up, zircon U–Pb ages, and Lu-Hf isotopes of northern Patagonia Argentina

The Early Mesozoic intraplate extension along the SW Gondwana exhibits complex and polyphase activity, related to one of the major Phanerozoic continental breakup events. At present, mantle plume or subduction-related influence on this process is a matter of intense debate (e.g., Navarrete et al., 2024, among others), as well as the temporal constraints of the intervening tectonic pulses. In any case, isotopic analyses of the magmatic record are crucial for deciphering the nature of the orogeny and its evolutionary stages. South America, and in particular, Patagonia, constitutes a remarkable area for evaluating the magmatic sources, and the temporal constraints of this mantle-crust interaction for such continental fragmentation. This contribution intends to progress in recognising contractional and stretching episodes through a combination of stratigraphic, structural, and geochronological analyses of the magmatic successions, using isotopic data as a fundamental tool for providing mantle-crust insights and offer petrogenetic information to assess this geodynamic scenario.

The plate's interaction along the subduction zones and its evolution over significant periods, about tens of millions of years, define the advancing and retreating character and their changes along the orogenic evolution. The present contribution analyses six samples for U-Pb geochronology and seven for Lu-Hf isotopes in zircons. The study area includes the North Patagonian Andes, the sub cordilleran area, and the western Northpatagonian Massif, and covers 200 km along the crustal segment and 150 km across it. The analyzed samples include the Montes de Oca Fm, Piltriquiltrón Fm, and El Serrucho Granite. the Garamilla Fm, the Cañadón Chileno Complex, and Lipetrén Granite. The results combined with previous data, show the development of four contrasting short-term subduction phases for the 40 to 42° S, characterized by advancing and restraining orogenic features.

The geodynamic phase occurred during the Upper Rethian to Lower Sinemurian (205-196 Ma) shows a positive Lu-Hf signature (εHf; +1 to +3) indicating a metasomatised mantle source for the granite's magma origin. Lipetrén Granite (205 Ma) is located along the continental interior indicating an inland migration of the continental arc. The second phase marks the previous granitic rocks uplifting, linked to an intense sinistral strike-slip kinematics (195- 189 Ma; εHf -5 to +5; Cañadón Chileno and Loncón Complexes). This process is assigned to the beginning of the slab break-off regime developed during the Middle Sinemurian to Lower Pliensbachian.

During the Lower Pliensbachian (190-185 Ma) begun a tectonic switching to a restraining orogeny phase, evidenced by massive volcanic activity in the arc and back-arc zone. The continental arc migrates toward the trench (Montes de Oca Fm). It is characterized by a strong positive εHf–εNd trend, associated with the contemporary mantle ascent along the back-arc rift (Garamilla Fm εHf, depleted mantle signature, εHf +11.13 to +17.51;) and also in the retroarc zone (Marifil Fm). This phase shows a significant change characterised by a retraining orogen developed under significant stretching of the continental crust and the mantle ascent.

The last identified phase occurs during the Early to Middle Toarcian magmatic gap (184-178) and consists of the consolidation of an advancing orogen, and the subsequent pre-Andean basement uplift and deformation (Genge et al. 2021). This phase denotes the contractional stage acme and the related foreland basin development e.g., part of the Liassic Chubut Basin infill. The continental arc deposits migrate about 100 km inland and are characterized by strong negative εHf–εNd values (Piltriquiltrón Fm and La Angostura Granite; εHf:-11 to +0.5 and -17 to -5, respectively) indicating an advancing orogenic phase link to a significant crustal isotopic reservoir for these calc-alkaline magmas (Rapela et al. 2005).