Volcanism in the Central Volcanic Zone (CVZ) of the Andes is a prime example of active continental arc magmatism. Young (e.g., <6 Ma) volcanic rocks in the CVZ are dominantly andesitic to dacitic in composition with geochemical and petrographic features indicative of extensive crustal contamination during magma differentiation (e.g., [1] -[3] ). Previous studies of CVZ volcanic rocks have greatly contributed to our understanding of the regional setting of the magmas [4] [5] ; and references therein in addition to ignimbrite forming eruptions [6] -[9] ; among others and volcanism along the modern arc-front [3] [10] . However, comprehensive studies of back-arc composite volcanoes are scarce, due in part, to their rarity and remote locations. As a result, studies of these systems are typically reconnaissance in nature and associated with regional studies [4] [5] [11] -[13] .

The lack of comprehensive studies on individual back-arc composite volcanoes in the CVZ prompted a detailed field, petrologic and geochemical study of the back-arc center Cerro Uturuncu (6010 m; 22˚15'S, Figure 1). The objectives of this study are to discuss the volcanic evolution of an individual back-arc composite volcano and broadly assess petrogenetic processes affecting magma composition in the back-arc of the CVZ. This study of Uturuncu results from a reconnaissance petrologic and geophysical investigation by Sparks et al. [16] . These authors concluded Uturuncu exhibits stages of volcanism separated by periods of quiescence common at other intermediate composite centers in the CVZ. Currently, the system is in a period of dormancy as the most recent eruption occurred at ~270 ka. Relatively long periods of quiescence are common at composite volcanoes as they been observed at other intermediate composition systems in the CVZ and elsewhere such as at Soufrière Hills Volcano, Montserrat [3] [17] [18] . In this paper, we build on the reconnaissance investigation of Sparks et al. [16] by furthering discussion of the volcanic history, field relationships, and petrography of volcanic rocks at Uturuncu. We also present a broad geochemical model to account for the restricted compositional diversity of magmas erupted at the volcano. More detailed discussion of the petrology and petrogenesis of Uturuncu magmas will ensue in a forth-coming paper.

The Andes are considered the classic example of a modern Cordilleran-type orogen formed by long-term subduction of oceanic lithosphere beneath continental lithosphere. The central Andes represents the type locality for this process owing to the great width of the orogen, immense crustal thickness (up to 70 - 80 km) [19] and high elevations (~4 - 6 km above sea level) [19] that occur over a vast area (Figure 1). The region composes one of the youngest and largest active silicic volcanic provinces with recent caldera formation. The CVZ contains ~20 calderas that have erupted ignimbrites (large ash flow sheets) less than 10 Ma and over 1100 late Cenozoic volcanic edifices [6] [7] [20] .

The southern central Andes between 21˚S - 24˚S is divided into three NW to SE trending geologic provinces based on structural similarities [19] . From east to west, these are the Eastern Cordillera (Cordillera Oriental), the Altiplano, and the Western Cordillera (Cordillera Occidental). The Eastern Cordillera comprises a classic thin- skinned fold and thrust belt. The Altiplano is a broad plateau where undeformed late Miocene and younger ignimbrites overlie variably deformed mid-Miocene and older volcanic and sedimentary rocks. The Western Cordillera consists of a westward dipping monocline and the modern volcanic arc-front [19] [21] . Depths to the subducting slab at these latitudes range from 130 km at the arc-front to ~250 km along the eastern boundary of the Altiplano [22] [23] .

Late Cenozoic to modern volcanic rocks in the CVZ is divided into three groups based on similarities in composition and eruptive style [21] [24] . First, between 21˚S - 24˚S, large-volume, regionally extensive ignimbrites erupted from large caldera complexes on the Altiplano and Western Cordillera [6] [7] [25] . These rocks are calc-alkaline, homogeneous dacites to rhyolites. Second, basaltic andesitic to dacitic lava flows erupted from 23 Ma to the present. At 21˚S - 22˚S, the largest volumes of these magmas erupted from composite volcanoes forming the peaks of the Western Cordillera. Baker and Francis [12] estimated the Western Cordillera contains ~3000 km³ of these lavas between 21˚S and 22˚S latitude. Lavas associated with composite volcanoes extend for ~200 km eastward onto the Altiplano, although volumes decrease sharply to <800 km³. Uturuncu is associated with this group. Third, volumetrically minor alkali basalts erupted from small monogenetic centers to the east of the arc-front on the Altiplano north of 21˚S [13] [26] .

New whole rock samples of Uturuncu volcanic rocks were analyzed for major and trace element compositions at Washington State University, Pullman. Major and trace element compositions for 121 samples were acquired by X-ray fluorescence spectrometry (XRF) on a ThermoARL Advant’XP+ automated sequential wavelength spectrometer. Methods and errors for XRF analyses are described in Johnson et al. [27] . Additional trace element compositions, including the rare earth elements, for 45 samples, were acquired by inductively coupled plasma mass spectrometry (ICP-MS) on an Agilent 7700 ICP-MS. Methods and errors for trace element analyses are presented in detail in Jarvis [28] . Whole rock Nd and Sr isotopic ratios, for 30 samples, were acquired by thermal ionization mass spectrometry (TIMS) on a VG Sector 54, analyzed by five Faraday collectors in dynamic mode at New Mexico State University, Las Cruces. Pb isotopes were analyzed on a Thermo Finnigan Neptune multi-collector ICP-MS equipped with nine Faraday collectors and an ion counter at the University of California at Santa Barbara. Michelfelder et al. [29] describe in detail the methods for radiogenic isotopic analyses. Geologic mapping was performed using commercially purchased ASTER satellite imagery and field mapping.

In this section, we summarize the volcanic and petrographic features of the lava flows and domes at Cerro Uturuncu. Cerro Uturuncu is classified as a composite volcano [20] resulting from volcanism over ~620,000 years [16] . Construction of the edifice of Cerro Uturuncu took place the back-arc of the CVZ approximately 125 km behind the modern arc front in the Altiplano region (Figure 1). The volcano is built upon a series of 5 Ma to 1 Ma ignimbrites erupted from large calderas complexes in the area [8] [9] [30] [31] . The eruptive history of Uturuncu is dominantly effusive with no evidence of explosive activity. Eruptive activity at Uturuncu predates the last glacial episode (~11,000 ky [26] ) in the area as evident by glacial valleys incising lava flows and the reformed southern and northern flanks mantled in places by moraines (Figure 2). Glacial incision has allowed for access to the potentially oldest lava flows erupted from Uturuncu.

Active fumaroles producing hydrothermally altered volcanic rocks are present in the saddle between the two peaks of Uturuncu. Hydrothermal alteration, mining of altered rocks for sulfur and erosion of the stratigraphically youngest volcanic rocks has removed the interpreted location of the central vent. The presence of fumaroles suggests current activity beneath Cerro Uturuncu. Pritchard and Simmons [32] identified a 70 km diameter regional uplift centered on Uturuncu. This uplift, suggested to be the result of diapiric rise of new magma into a shallow chamber beneath the center [33] , could lead to new activity at Uturuncu in the future.

Relatively few radiometric ages are available for Uturuncu lavas and domes. The available ages suggest two stages of volcanism between 890 - 271 ka [16] and a period of quiescence from 540 - 470 ka. The stages described by Sparks et al. [16] are compositionally and petrographically similar, though erupted volumes of volcanic rocks decrease with younger age.

In this section, we describe a simple working model to explain the limited compositional diversity observed in Uturuncu volcanic rocks. The intent of the proposed model is not a rigorous description of the petrogenesis of the magmas nor is the variables and compositions selected for assimilation-fractional crystallization and mag-

mamixing models intended to quantify the magmatic system beneath Uturuncu. The intent is to constrain the processes affecting compositional diversity of the rocks and serve as a point of reference in future studies of Uturuncu volcanic rocks.

The lavas and domes have a large range of ⁸⁷Sr/⁸⁶Sr ratios for a narrow range of ¹⁴³Nd/¹⁴⁴Nd ratios (Figure 9(B)). Assimilation of crustal rocks with high ⁸⁷Sr/⁸⁶Sr ratios but similar ¹⁴³Nd/¹⁴⁴Nd ratios explains the observed trend. Figure 10 illustrates six scenarios using different composition contaminants and different partition coefficients dependent on the crustal composition. The lower crustal contaminant is a garnet-sillimanite-gneiss xenolith described by McLeod et al. [40] from the southern CVZ. We selected this composition because of petrographic similarities to a sillimanite-gneiss xenolith described by Sparks et al. [16] from Uturuncu lavas. The upper crustal contaminant used in the models is Paleozoic granite from Sierra de Moreno described by Lucassen et al. [41] . These granites are petrographically similar to the xenolith observed in this study. We contaminate two primitive basalt compositions with lower and upper crustal contaminants: one from Davidson and de Silva [13] for basalts from the central Altiplano, and a second from Hernando et al. [42] for pre-caldera basalts from the Payún Matrú volcanic field in western Argentina’s Southern Volcanic Zone. Table 3 presents concentrations and isotopic ratios of the contaminants and basalts.

Model curves labeled AFC (D = 1.7) and AFC (D = 1.5) were constructed to simulate the effects of differentiation on two potential parental magmas compositions contaminated by a highly evolved crustal contaminant (Figure 10). Bulk distribution coefficients of 1.5 and 1.7 are high for andesites, but the decrease of Sr content with increasing SiO₂ content and in increase of Rb/Sr ratios with increasing ⁸⁷Sr/⁸⁶Sr ratios suggest Sr was compatible during differentiation. The curve AFC (D = 0.1) was constructed to simulate the effects of differentiation in the lower crust compared to curves for the upper crust. A bulk distribution coefficient of 0.1 is considered appropriate for this composition based on regional studies that have shown an increase in Sr/Y ratio with a decrease in ⁸⁷Sr/⁸⁶Sr ratios reflects incompatibility of Sr in garnet-stable/plagioclase-unstable-hybridized crust [29] [44] [45] .

⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios for Uturuncu lavas and domes create an array consistent with assimilation of old radiogenic basement rocks with relatively non-radiogenic Nd and radiogenic Sr. High Rb/Sr ratios suggest these basement rocks were also felsic in composition as opposed to noritic as could be interpreted from the presence of noritic composition xenoliths described by Sparks et al. [16] . High silica, quartz-feldspathic xeno-

liths, observed in this study, and similar to xenoliths interpreted by Sparks et al. [16] to be remnants of a crustal hot zone could represent this contaminant.

Cerro Uturuncu has historically been inactive and glacial erosion suggests that the volcano is in a period of dormancy. However, new geophysical data has shown a concentric uplift centered on Uturuncu for the past 20 years with the potential to initiate future eruptions [32] [33] . According to the spatial distribution of previously erupted lava flows, it is difficult to predict the likeliness of where a lava flow will occur during a future potential eruption. In addition, the injection of new magma beneath the central vent could cause instability of the highly altered hydrothermal rocks composing the summit potentially leading to partial collapse of the vent and debris avalanches towards the north. This event has precedence in other CVZ composite cones [10] [21] [26] , but has not been described in volcanic history of Cerro Uturuncu. These events at other intermediate centers have also been associated with pyroclastic flows and volcanic blasts [46] - [48] . The only village within the possibly affected area is Quetena Chico approximately 30 km to the northeast potentially affecting the community with ash fall in the event of one of these more explosive eruptions.

In this study we combine field, petrographic and geochemical data to describe the volcanic evolution of Cerro Uturuncu, at 22˚15'S, in the CVZ. The volcano has erupted uniform, crystal-rich orthopyroxene, biotite dacite and andesite lavas for ~620,000 years. Hydrothermal activity and glaciation resulted in the removal of the main vent suggesting a long period of dormancy since the last eruption. Field relationships between the lava flows and domes suggest eruptions are effusive with an average repose interval between 6000 and 8000 years. Sparks et al. [16] identified 50 lava flows; we have identified an additional 45 lava flows and 10 domes with the total number of erupted units to 105. Erupted volumes ranged between 0.1 km³ and ~10 km³ per eruption with a total volume erupted of ~89 km³. Construction of the edifice took place in one stage in the volcanic history.

Typical phenocryst assemblage is plagioclase > OPX > biotite >> Fe-Ti oxides and quartz. Micro-inclusions and glomerocrysts contain trace olivine. Phenocrysts of plagioclase and OPX show multiple textures consistent with thermal disequilibrium. Lava flows and domes were erupted with a limited compositional range (61 wt% - 67 wt% SiO₂) of magma for ~620,000 years. Magmatic inclusions and xenoliths observed in nearly all lava flows and domes record the magmatic system beneath Uturuncu. Repeated injections of contaminated parental magma produce the compositional diversity observed in these magmatic inclusions. Xenocrysts/antecrysts of olivine and quartz in both the magmatic inclusion and the lava flows and domes, and the presence of quartz-rich xenoliths suggests crustal contamination is an important process in magma genesis.

Lavas and domes at Uturuncu show abundant evidence for thermal and chemical disequilibrium through the presence of sieved and zoned phenocryst phases and magmatic inclusions. Magma mixing controlled the limited compositional range observed in Uturuncu volcanic rocks. It is possible to explain the geochemical trends of the andesite and dacite volcanic rocks by differentiation in old, Sr radiogenic, Nd non-radiogenic felsic basement rock. The variation in isotopic composition suggests large amounts of assimilation fractional crystallization during differentiation.

The authors wish to thank Frank Ramos for use of and assistance with the thermal ionization mass spectrometer at New Mexico State University; the staff at the Geoanalyitical lab at Washington State University and all members of the PLUTONS Research group for insightful discussions. Jamie Kern for assistance mapping and collecting samples in the field, and the residents of Quetena Chico, Bolivia and Lipiko Tours for logistical support and hospitality. We thank the anonymous reviews for detailed comments that substantially improved this manuscript. Support for this work came from the U.S. National Science Foundation EAR-0901148 to TCF and grants to GSM from the Department of Earth Sciences at Montana State University.