Petrology and geochemistry of the back-arc lithospheric mantle beneath the Payenia volcanic province (La Pampa, Argentina)

Insights into the petrochemical composition and evolution of the lithospheric mantle beneath the Payenia volcanic province (PVP) (Mendoza and La Pampa provinces, centralwestern Argentina) are provided thanks to the characterization of mantle xenoliths hosted in Pleistocene basaltic rocks from the Agua Poca and Huanul volcanoes. The PVP is related to the mantle wedge of the Argentine back-arc area in correspondence to the Transitional Southern Volcanic Zone. Agua Poca is a pyroclastic cone formed 0.6 Ma and located in the eastern PVP at 37°01′S and 68°07′W, at about 530 km east of the Chilean trench. The Cerro Huanul is instead located in the southern part of PVP, at 37º17'S and 68º32'W, about 480 km east of the Chilean trench. It is a shield volcano crowned by a ring of lava 3.9 km in diameter and 20 m height. Inside this ring, there are various heights formed by pyroclastic cones and a lava flow. The latter contains ultramafic xenoliths mostly smaller than 5 cm. According to K-Ar dating, Huanul volcanic rocks are 0.84 ± 0.05 Ma (Bertotto et al. 2006). The studied mantle xenoliths from both the localities are mainly anhydrous spinel lherzolites, with very subordinate amounts of harzburgites, wehrlites and pyroxenites. Most of the peridotites show porphyroclastic texture, whereas pyroxenites are equigranular. These lithologies show highly variable equilibrium temperatures ranging from 780°C to 1080°C at 1.0 to 2.0 GPa, with the relevant exception of two peridotite samples from Huanul, reaching 1200°C of equilibrium T. This constitutes evidence that the studied ultramafic xenoliths are representative of a large portion of the lithospheric mantle column. Major element mineral composition of Agua Poca and Huanul spinel lherzolites is characterized by fertile character, as evidenced by low Fo in olivine, large Al content in pyroxenes and spinel, large Na in clinopyroxene. CI-normalised REE patterns of lherzolite clinopyroxenes are LREE depleted with flat M-HREE region at 10 xCI (see also Bertotto et al. 2013). They also show extreme depletion in highly incompatible elements such as Nb, Ta, U and Th. REE whole rock and clinopyroxene composition indicate that lherzolites can be considered to be refractory residua after 1% to 7%, non-modal, near-fractional melting of a spinel-facies Primitive Mantle. These estimates are consisting with those obtained by means of spinel composition. Isotope composition of clinopyroxene separates from Agua Poca samples is characterized by low radiogenic Sr and large radiogenic Nd. As a whole, all the petrochemical data converge in indicating that most of the mantle column of this sector of PVP consists of rocks belonging to Depleted Mantle reservoir. Spinel harzburgites show bulk and trace element composition characterized by LREEenrichment over HREE. These enrichments are the evidence of interaction of the mantle peridotite with LREE-enriched melts, which induced the development of transient chemical gradients, but that locally was also able to induce a complete metasomatic overprint of the trace element composition. The Sr and Nd isotopic composition of clinopyroxenes from Agua Poca harzburgites lies very close to the DM field, thus not evidencing any clear geochemical fingerprint imparted by contributions from slab-derived crustal components. A banded websterites from Agua Poca shows peculiar major and trace element composition. In particular, REE patterns are characterized by slight LREE depletion (La down to 0.8 xCI) and flat HREE, with the content of these latter down to 3 xCI. Isotope composition of the websterite clinopyroxene exhibits very low radiogenic Sr, similar to DM. Also the Nd isotope composition in the pyroxenite slab at the contact with the peridotite is very close to that of the Agua Poca lherzolites, but it becomes markedly more enriched in radiogenic Nd in the central part of pyroxenite. As a whole, the geochemical data suggests a derivation of the parent melt by partial melting of an ultra-depleted DM. In conclusion, the lithospheric mantle beneath the PVP is apparently dominated by a DM reservoir, which melted, according to Nd and Re-Os systematic (Schilling et al. 2008), in Proterozoic times. Both pyroxenites and metasomatised harzburgites so far investigated do not provide any clear evidence of the presence of slab-derived crustal components in the migrating melts. The petrochemical features of the Payenia lithospheric mantle are significantly different with respect to that documented for the Patagonian back-arc region, where the lithospheric mantle column is mainly constituted by harzburgite and/or strongly metasomatised by multiple events of melt migration, sometimes bearing evidence of slabderived components. Thus, the results of this study point to a significantly different geodynamic evolution for Payenia and Patagonian mantle domains.