Petrology and Geochemistry of the Back-Arc Lithospheric Mantle Beneath Eastern Payunia (La Pampa, Argentina): Evidence from Agua Poca Peridotite Xenoliths
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Geochemical Journal, Vol. 47, pp. 219 to 234, 2013 Petrology and geochemistry of the back-arc lithospheric mantle beneath eastern Payunia (La Pampa, Argentina): Evidence from Agua Poca peridotite xenoliths GUSTAVO W. BERTOTTO,1* MAURIZIO MAZZUCCHELLI,2 ALBERTO ZANETTI3 and RICCARDO VANNUCCI3,4 1INCITAP. UNLPam-CONICET. Uruguay 151, Santa Rosa (6300), La Pampa, Argentina 2Dipartimento di Scienze Chimiche e Geologiche, Università di Modena e Reggio Emilia, P.le S. Eufemia 19, I-41121 Modena, Italy 3Istituto di Geoscienze e Georisorse - CNR, U.O.S. di Pavia, Via Ferrata 1, I-27100 Pavia, Italy 4Dipartimento di Scienze della Terra, Università di Pavia, Via Ferrata 1, I-27100 Pavia, Italy (Received February 5, 2012; Accepted March 13, 2013) This paper presents the results of new petrochemical studies carried out on mantle xenoliths hosted in Pleistocene basaltic rocks from the Agua Poca volcano in central-western Argentina. Mantle xenoliths studied are shown to be mainly anhydrous spinel lherzolites with minor amounts of harzburgite and banded pyroxenite, showing highly variable equilib- rium temperatures ranging from 820°C to 1030°C at 1.0 to 2.0 GPa. This constitutes evidence that the mantle xenoliths are representative of a large portion of the lithospheric mantle column and that the geothermal gradient is not very elevated as reported in some other Patagonian provinces. Geochemical characteristics of clinopyroxene in the mantle xenoliths allow classification into two groups; Groups 1 and 2. Group 1 contains most of the lherzolites and has light-REE depletion, with slightly positive anomalies of Eu in some samples and extreme Nb and Ta depletion. Group 2 consists of two harzburgitic samples, has flat REE patterns with lower Sm to Lu concentrations, with enriched Sr and negative HFSE anomalies. Based on mineral and residua compositions estimated assuming equilibrium with clinopyroxenes, Group 1 can be considered to be refractory residua after up to 7%, non-modal, near-fractional melting of a spinel-facies Primitive Mantle. Group 2 can be considered to be after ca. 13% of partial melting. It is inferred that partial melting events in the lithospheric mantle beneath the Agua Poca occurred in different ages since the Proterozoic, but compared with Group 1, the metasomatic overprint is dominant in Group 2 mantle xenoliths. The calculated melt compositions from Group 2 are interpreted to be transient liquid compositions developed during melt-peridotite interaction, and are different from the host alkaline basalts. The HFSE-depleted composition estimated for the rising melt suggests the presence of a slab-derived component, al- though the possibility cannot be disregarded (on the basis of present data) that such a geochemical feature is due to segregation of HFSE-bearing minerals during the interaction with the peridotite. Thus, we attribute the metasomatic agent to a basaltic melt and to a minor amount of slab-derived fluids. Keywords: mantle xenoliths, fractional melting, mantle metasomatism, depleted mantle reservoir, Argentina et al., 2005; Wang et al., 2007; Schilling et al., 2008; INTRODUCTION Dantas et al., 2009; Scambelluri et al., 2009) since the Tectonically, southern Argentina, (commonly known pioneering works of Villar (1975), Niemeyer (1978), as Patagonia), is part of the back-arc region of the East Gelós and Hayase (1979), Muñoz (1981), and Labudía et Andean Cordillera, in which Pliocene–Quaternary alka- al. (1984). Among these localities, only two contain line basalt-forming volcanic centers are sporadically dis- garnet-bearing peridotites; Pali Aike located in southern tributed between 37°S and 53°S. In recent times, more Patagonia (Skewes and Stern, 1979; Douglas et al., 1987; than twenty localities of basalt hosted mantle xenoliths Stern et al., 1989; Kempton et al., 1999) and Prahuaniyeu, have been reported in the Patagonian region (e.g., Bjerg located in northern Patagonia (Ntaflos et al., 2001; Bjerg et al., 1994, 1999, 2005, 2009; Varela et al., 1997; Gorring et al., 2009; Gervasoni et al., 2009). Bjerg et al. (2005) and Kay, 2000; Laurora et al., 2001; Ntaflos et al., 2001, suggest that the Patagonian subcontinental lithospheric 2007; Kilian and Stern, 2002; Acevedo and Quartino, mantle experienced only minor melt extractions in the 2004; Rivalenti et al., 2004a, 2004b, 2007a; Conceição garnet peridotite field, while melt extraction was more extensive in the spinel lherzolite field. Furthermore, it was considered that the lithospheric mantle had been *Corresponding author (e-mail: [email protected]) moderately to strongly deformed and recrystallized, fre- Copyright © 2013 by The Geochemical Society of Japan. quently in the presence of migrating melts (Rivalenti et 219 al., 2004a; Bjerg et al., 2005). Calculation of equilibrium temperatures and pressures indicates an elevated geothermal gradient similar to that of southeast Australia, and an oceanic lithosphere, which led Ntaflos et al. (2001, 2003) and Bjerg et al. (2005) to propose the presence of rising mantle plume(s) in the back-arc region during the Neogene time. The mantle xenoliths sampled in Patagonia exhibit geochemical and petrological variations as a function of their distance from the Chilean Trench. Cryptic and mo- dal metasomatism of variable intensity affected the lithospheric mantle in this region (Rivalenti et al., 2004a; Bjerg et al., 2005). Metasomatic agents recorded in sam- pled mantle xenoliths vary with distance to the trench and are linked to the presence of subduction components, such as in the localities of Cerro de Los Chenques (Rivalenti et al., 2007a) and Cerro del Fraile (Kilian and Stern, 2002; Wang et al., 2007). As shown by the recrystallization and growth of newly formed minerals, such as amphibole and phlogopite, the modal metasomatism becomes increas- ingly noticeable as the distance increases from the trench, and the strongest influence is indicated in Gobernador Gregores xenoliths (Laurora et al., 2001; Aliani et al., 2004, 2009; Rivalenti et al., 2004b). These processes, observed in the distal region, have been attributed to both carbonatitic fluids (Ntaflos et al., 1999; Gorring and Kay, 2000) and silicic fluids derived from the subducted plate (Laurora et al., 2001; Rivalenti et al., 2004b), whereas Fig. 1. Location of Agua Poca within the basaltic units of the metasomatism by tholeiitic and alkaline melts has been western La Pampa province. proposed to have affected a lherzolite–websterite xenolith suite in Cerro de Los Chenques and the Cerro Clark vol- canoes (Dantas et al., 2009). Metasomatism by silicate melts has been indicated for the mantle xenoliths in the mantle xenoliths performed by Jalowitzki et al. (2010), Pali Aike volcanic field (Kempton et al., 1999; Stern et led to the conclusion that the variability of whole rock al., 1999). composition for the Agua Poca xenoliths might have been In southern Argentina, most of the mantle xenolith caused by varying degrees of partial melting processes in localities are distributed south of 40°S parallel; and only the lithospheric mantle, having either a Depleted Mantle three localities with spinel peridotites have been recog- MORB (DMM) or a Primitive Mantle (hereafter PM) with nized north of the Colorado River (38°S) (Bertotto, 2000, metasomatic enrichment, and involving the interaction of 2002a, 2002b; Jalowitzki et al., 2010). In order to under- the lithospheric mantle with up to 3% of aqueous fluids stand the compositional variation of the whole lithospheric derived from a subducted slab. However, whole rock com- mantle and its petrologic evolution, more detailed stud- position does not usually provide full constraints in the ies of the mantle xenolith are needed in the northern re- case of multi-stage petrologic evolution, particularly when gions. Thus, we focus on the Agua Poca mantle xenolith a later metasomatic event caused petrochemical zoning site, located on the back-arc area of Payunia, La Pampa within the samples. Moreover, the whole rock composi- province, Central-Western Argentina (Fig. 1). tion of mantle xenoliths can be affected by an interaction Bertotto (2000) first reported the mantle xenoliths at with melts and fluids related to the volcanic event that the Agua Poca volcano as having protogranular and carried them to the surface (e.g., Dantas et al., 2009; protogranular to porphyroclastic textures. The four man- Kempton et al., 1999; Schilling et al., 2008; Stern et al., tle xenoliths collected were classified into two types, 1999). Consequently, such events can hinder the spinel lherzolite and spinel harzburgite, with textures and unraveling of the petrochemical processes on the pristine whole rock compositions indicative of an upper mantle subcontinental lithospheric mantle. To overcome this origin with a minimum provenance depth of 40 km. Sub- problem, a mineral micro-analysis investigation is needed, sequently, a detailed geochemical study of the Agua Poca in addition to the detailed petrographic inspection of the 220 G. W. Bertotto et al. textural relationships, to achieve a comprehensive frame- work in which to reconstruct the petrologic evolution of the samples. Here, we present the results of mineral microanalyses for major and trace element compositions obtained from the Agua Poca mantle xenoliths, in order to unravel the petrochemical processes in the underlying pristine lithospheric mantle, and to suggest a petrological model using a combination of results obtained in this study and the whole rock compositions reported in previous