Campanian magmatism the Fuegian Andes: New SHRIMP age of Jeujepen pluton. Argentina.

CERREDO, M. E.; TASSONE A.; RAPALINI, A.; HERVÉ, F.; FANNIN, X.

Neuquén

Congreso; XVIII. Congreso Geológico Argentino; 2011

Asociación Geológica Argentina

The Jeujepen pluton (JP) is a small (<10 km2), epizonal, intrusive body located at the southeastern edge of the Lago Fagnano (Tierra del Fuego) discordantly emplaced in the Lower Cretaceous mudstones of the Beauvoir Formation. Magnetic and gravimetric modelling of the area has revealed the presence of a geometrically complex body partly exposed at Cerro Jeujepen which, at the shallower depths, can be viewed as a composite plug, with roots located at subsurface depths of about 8 Km (Cerredo et al. 2000; Tassone et al., 2005). The shape and position of the JP suggest that its emplacement was localized in a releasing bend of the Magallanes–Fagnano fault system (MFFS), which forms the boundary between the Scotia and South American plates (Tassone et al., 2005). The JP comprises rocks within the monzogabbro to syenite span bearing small cumulates rich in ferromagnesian minerals. JP shares with other Fuegian intrusions a dominant shoshonitic nature (Cerredo et al., 2005, 2010; Peroni et al., 2009; González Guillot et al., 2009) and displays a ferroan-calc-alkalic to alkalic trend. The magmatic series of amphibole ± clinopyroxene ± biotite-gabbro to syenite is metaluminous (A/CNK: 0.6-0.97,) and typically depicts high Ba (250/1600 ppm), Sr (350-2000 ppm), and moderately to strongly fractionated REE patterns (LaN/YbN 10-17). Monzogabbros, monzonites, syenites and the ultrabasic enclaves show comparable patterns on multielement chondrite-normalized plots with a general increase in LIL elements and troughs at the HFSE Ti and Nb, Ta, as typical for subduction-related magmas (Cerredo et al., 2005). Recent comprehensive study based on EMP, single-crystal XRD, SREF, FTIR, microthermometric fluid-phase equilibria and thermobarometry carried out on samples bearing amphibole megacrysts of the dominant monzonite facies revealed that water-rich nature of monzonite melts (very high, up to 17 wt%), the presence of CO2 species in the fluid phase. The crystallization of amphibole megacrysts occurred at mantle conditions, at the upper experimental limit of amphibole stability, during the magma ascent path which could be bracketed between ~ 25 Kbar/~ 1000 ºC and the emplacement conditions between 1-3 Kbar (Ridolfi et al., 2010). Reported WR K/Ar data yielded a Late Cretaceous (Cenomanian-Turonian) age of 93 ± 4 Ma for JP (Acevedo et al. 2000). The purpose of this contribution is to communicate new U-Pb SHRIMP zircon data for a sample of the monzonite facies of JP and to evaluate its relationship and correlation with other intrusions and units of the Fuegian Andes. The analyzed sample contains a relatively uniform population of zircons. A weighted mean of 8 of the 13 analyses gives 72.01±0.75 Ma (MSWD =1.4) which is interpreted as the time of igneous zircon crystallization in the monzonite facies. Two grains gave Early Permian (282.4 ± 3.5 Ma) and Latest Silurian (418.7 ± 4.8 Ma) ages. No Silurian inheritance ages have been reported for the Southern Patagonian Batholith (Hervé et al., 2007), whereas Permian inheritance ages are very rare (only two intrusions, out of 70, show peaks within the 255-295 Ma range). The non-metamorphic character of Permian zircon in JP (Th/U: 0.61) rules out the correlation of the inherited zircon grain with the Tierra del Fuego Igneous and Metamorphic Complex-TFIMC- (Hervé et al., 2010a) characterized by Permian metamorphism and Cambrian plutonism. Scarce Silurian peaks have been reported for the cover sequences of the Cordillera Darwin Metamorphic Complex (CDMC), but Permian inherited ages are relatively common in the Tobífera Formation of western Beagle Channel (Barbeau et al., 2009a; Hervé et al., 2010b). Therefore, incorporation of inherited zircons from underlying Tobífera/Lemaire Formation seems possible. The here presented Campanian age for the intrusion of JP is contemporaneous with the recently reported SHRIMP age of ca. 74 Ma for the Ushuaia Pluton (Barbeau et al., 2009b). Both intrusions are exposed along major strike-slip structures of the Fuegian Andes: the MFFS and the Beagle Channel Fault System. Contemporaneous sedimentation is represented by the Late Campanian-? Early Maastrichtian Bahia Thetis Formation (Oivero et al., 2009) with a microfaunal content indicative of poorly oxygenated waters of fairly limited depth, likely in silled basins (Malumián and Jannou, 2010). Within the Fuegian Andes, strike-slip tectonics has been generally considered as restricted to the Late Cenozoic transcurrent faulting (eg. Klepeis and Austin, 1997). However, it has been long recognized the transpressional nature of the closure and inversion of the Rocas Verdes Basin during the Late Cretaceous (Cunningham 1993, 1995). A component of left-lateral wrenching was identified for the Cretaceous of the Fuegian Cordillera and foot-hills (Nelson et al., 1980; Cunningham 1995). The apparent synchronism of epizonal magmatic intrusions and sedimentation in restricted basins could be signaling the upper crustal expressions of Latest Cretaceous strike slip tectonics in the southernmost Andes.