high-Sr BEM
BEM 47 wt% SiO 2 shoshonitic
2. V OLCANOLOGICAL BACKGROUND G EOLOGICAL SETTING GEOLOGICAL SETTING GEOLOGICAL SETTING
2.1.
Taapaca Volcanic Complex (TVC, 18°S, 69°W), also known as Nevados de Putre is located in an active volcanic chain developed in the central Andes, on the western margin of the South American continent. The Andes represent the largest recent magmatic continental arc in the world, and result from subduction of the Nazca and Antarctic oceanic plates beneath the South American continental lithosphere. The Quaternary Andean volcanic activity occurs in four segments, the Northern, Central, Southern and Austral volcanic zones, which are separated by volcanic gaps. The TVC is a part of the Andean Central Volcanic Zone (CVZ) extending between latitudes 14° and 28°S (Figure 1).
Plateau, the largest non-collisional plateau on the Earth (Isaacks, 1988). The Western Cordillera comprises the Quaternary volcanic chain aligned NW-SE to the border between Chile and Bolivia, characterized by high topography with numerous active volcanoes exceeding elevation of 6000 m asl. Taapaca lies about 30 km to the west from the main volcanic chain forming the central part of the CVZ, the Altiplano-segment (15-23°S)( 7 ). At 23°S, the volcanic chain changes to the N-S alignment including volcanic centers of the Puna region (23-28°S).
Volcanism in the CVZ occurred since about 300 Ma (e.g. Scheuber et al., 1994). The CVZ is underlain by subducting slab, descending at a relatively steep angle of ~25° (Cahill and Isaacks, 1992) at a current convergence rate of 75-80 mm/a (Samoza, 1998). The volcanic front of the CVZ lies approximately 120-150 km above the subducted slab and remarkably thick continental crust exceeding 70 km below the Altiplano region (James, 1971; Zandt et al., 1994; Allmendinger et al.
1997; Scheuber & Giese, 1999; Yuan et al., 2002; Tassara et al., 2006).
The crust below the Altiplano consists of Palaeoproterozoic (2.0-1.8 Ga) Arequipa terrane represented by metamorphic and igneous rocks exposed in southern Peru. Proterozoic amphibolites and gneisses of metamorphic Belen Complex as well granulites and charnockites of Cerro Uyarani, exposed in northernmost Chile, on the western Altiplano and along the Chilean Precordillera have been reported by Wörner et al. (2000) and are assumed to underlie significant portions of the CVZ in northern Chile.
O
RIGIN OF THECVZ Q
UATERNARY MAGMAS2.2.
The predominantly mafic composition and old age of the Arequipa crustal domain is reflected by the specific isotopic and trace element composition of the Quaternary magmas erupted in the Altiplano region, characterized by low 206Pb/204Pb and 143Nd/144Nd, elevated 87Sr/86Sr and high Sr/Y (e.g. Mamani et al., 2010 and references therein). The adakite-like signature (Sr/Y>40) found in the Central Andean magmas since Miocene time is commonly assumed to result from an involvement of garnet and/or amphibole in partial melting of thickened lower crust, high pressure fractional crystallization of mantle-derived magmas, and assimilation of crustal rocks into mantle-derived magmas (Mamani et al., 2010 and references therein). It is broadly accepted that these processes can occur simultaneously and set up the “baseline” chemical and isotopic features of erupted magmas, in the MASH (melting, assimilation, storage, and homogenization) zones at the mantle-crust transition (Hildreth & Moorbath, 1988; Davidson et al., 1990, 1991).
Magmas erupted at Quaternary volcanic centers near 18°S yield two kind of parental mafic magmas characterized by different baseline geochemical features. These baseline magmas contrast in Ti, Cr, Sr- and Ba-contents, and HREE patterns, which suggest interactions at different MASH-levels in the upper crust (Ginibre & Wörner, 2007; Hora et al., 2009; Mamani et al., 2008). They emphasize the role of a more mafic composition of the Arequipa domain in generation of the adakite-like signature in the CVZ, which does not occur in the Puna region that is also characterized
7 23° is the geographic boundary between Altiplano and Puna Plateau presented by e.g. Stern et al. (2004). Based on paleogeographic features, Allmendinger et al. (1997) set the boundary across the volcanic arc at ~20°S.
by relatively thick crust (>65 km). Minor variations in isotopic composition within a wide range of SiO2-contents in magmas erupted at 18°S (Davidson et al., 1990) imply generation of intermediate and silicic magmas in deep-seated baseline MASH zones, as suggested by Ginibre & Wörner (2007) for Parinacota volcano (CVZ: 18°S), and Goss et al. (2010) for Incapillo Caldera and Dome Complex (CVZ: 28°S). These data are in agreement with the deep hot zones differentiation model of Annen et al. (2006).
In contrast to the broadly accepted MASH-based explanation of the geochemical properties of the Neogene CVZ lavas, a multivariate statistical modeling using Polytopic Vector Analysis (PVA), presented in Chapter 2 of this work, has detected three distinct magmatic components involved in the petrogenesis of the recent Central Andean lavas. These (near) primary magma compositions represent shoshonitic, high-Al calc-alkaline and high-K calc-alkaline magmatic series. Two-stage magma mixing between these three end-member magmas with minor contribution of assimilation and fractional crystallization may be responsible for the entire compositional variability of the Quaternary CVZ lavas.
Figure 1. Schematic map of the Andean Central Volcanic Zone (CVZ) modified from Stern (2004), illustrating location of Taapaca volcano and other volcanic centers in the CVZ. Inset shows the position of the CVZ in relation to the Northern (NVZ), Southern (SVZ), and Austral (AVZ) Volcanic Zones. Nazca Plate convergence
E
RUPTIVE HISTORY OFT
AAPACAV
OLCANICC
OMPLEX2.3.
The eruptive history of the TVC, previously described by Kohlbach & Lohnert (1999), have been refined and presented in detail by Clavero et al. (2004), who recognized four evolutionary stages based on geochronological and morphological criteria, including migration of the main vent system.
A geological map modified from Clavero et al. (2004) is presented in Figure 3.
The volcanic edifice of Taapaca is constructed atop three main uppermost basement units: Upper Oligocene - Lower Miocene Lupica Formation, Miocene andesitic volcanic deposits, and Upper Pliocene rhyolitic Lauca Ignimbrite. The Taapaca edifice consists of elongated dome clusters with three exposed summit domes of which the highest reaches 5850 m asl. The main edifice volume is estimated to be 35 km³; the eruptive products of Taapaca cover an area of 250 km² (Clavero et al., 2004).
The initial eruptive stage I formed a shield-like stratocone consisting of moderately porphyritic two-pyroxene andesitic lava flows containing small amounts of sanidine and hornblende. Stage I is estimated to be older than 1.5 Ma; the oldest 40Ar/39Ar age from earliest dacitic stage II samples from Taapaca is 1.46 ±0.07 Ma (Clavero et al., 2004). This stage formed the main volume of TVC between 1.5 and 0.5 Ma. It consists of viscous dacitic lava flows, which formed a stratovolcano with steeply dipping flanks. A major collapse event is documented by a voluminous debris avalanche/lahar, dated at 1.27 ±0.04 Ma by Wörner et al. (2000). The dacites of stage II contain sanidine megacrysts, plagioclase, amphibole and biotite phenocrysts, rare quartz and titanite, and very rare, small anhedral clinopyroxene. Stage III consists of small volume lava domes and block-and-ash flow deposits concentrated mainly in the central part of the dome complex erupted during a short period between 0.5 and 0.47 Ma. The eruptive products differ from stage II only by higher amounts of sanidine megacrysts and mafic enclaves. Partial collapse of the southern part of ancestral stage II edifice and stage III domes marks the start of the youngest and morphologically most complex stage IV. The dacites generated during Late-Pleistocene – Holocene eruptive activity of TVC are petrographically undistinguishable from those of unit III. Clavero et al. (2004) observed higher amounts of mafic enclaves comprising up to 6 vol% of the juvenile material and increasing sizes and abundance of sanidine megacrysts. Stage IV form the main edifice of TVC, characterized by extrusion of voluminous domes and associated block-and-ash flows, blasts, tephra fallout, pyroclastic flows, debris avalanche, and lahars. The pyroclastic flows, surges, and tephra fallout are associated with dome growth-collapse explosions.
Numerous debris avalanche deposits are evidence for frequent edifice collapse events at TVC.
Clavero et al. (2004) recognize two types of debris avalanche at Taapaca. First, debris avalanches that are a consequence of extensive hydrothermal alteration causing edifice weakening, and second, by intrusion of a cryptodome causing deformation and instability of the edifice. The latter collapse triggering mechanism is evidenced by blast deposits integrated in the debris avalanche, which result from a rapid decompression after abrupt mass unloading. Despite the catastrophic mass unload events observed at TVC, a change in the composition and mineralogy of erupting products after
edifice collapses, as documented for Parinacota volcano (Wörner et al, 1988; Hora et al., 2007;
Chapter 4 of this work) has not been observed.
G
EOCHEMICAL CHARACTERISTICS OFT
AAPACA LAVAS2.4.
2.4.1. T
AAPACA HYBRID DACITESAccording to the classification of Le Maitre et al. (1989), Taapaca rocks plot in the transition zone between andesite-dacite and trachyandesite-trachyte fields (Figure 2a). The prefix “trachy”- denotes genetic link to the alkaline intra-plate volcanism, which cannot be used for classification here, otherwise as suggested by Higgins (2011) due to the high alkali contents in Taapaca rocks.
Using the division of Irvine & Baragar (1971), Taapaca rocks are subalkaline and classify as high-K calc-alkaline andesite and dacite, due to the subdivision of Rickwood (1989), (Figure 2b).
Previous studies of the Taapaca whole rock chemistry (Kohlbach, 1999; Clavero et al., 2004;
Higgins, 2011) report a limited range in silica content through all eruptive stages I-IV (60-68 SiO2
wt%) characterized by a strong (linear) correlation of the most major and trace elements. Moreover, the dacites show a compositional dispersion, which is greater than an analytical error in each data point. Considerable compositional scatter of Taapaca dacites, reported also in Chapter 2 (section 4.2.1, Figure 2, 4, 5) does not follow any spatial or temporal trends. The variable modal percentage of sanidine megacrysts may be considered as one reason for data scatter. However, an influence of sanidine megacrysts on the whole rock composition is small due to their small amount (<5 vol%) and similar composition to the host dacites (Higgins, 2011). Nonetheless, two main compositional trends can be extracted from the scatter in the Taapaca dacites, forming a “main-group” dacites and
“subtrend” dacite, previously recognized by Kohlbach (1999) as well.
These two trends are also known from other Andean volcanic centers. Two distinct calc-alkaline trends of different Na2O, K2O, TiO2, P2O5, LILE and HREE have been defined by Davidson et al.
(1990) for Nevados de Payachata twin volcanoes, Parinacota and Pomerape (Figure 2c). The younger trend (<1 Ma), called PP-trend shows an enrichment in incompatible elements in contrast to an older, Neogene (>1 Ma) trend, called N-trend, from the same Nevados de Payachata region. Both trends converge at ~68-70 wt% SiO2. The main group of Taapaca dacite overlaps the PP-trend; the subgroup overlaps the N-trend (Figure 2c).
An involvement of whole rock compositions of the mafic enclaves into a geochemical examination of Taapaca magmas reveals a wide range of different compositions of the mafic input magmas. The compositions of the Taapaca mafic enclaves span contrasting both, major and trace element characteristics, e.g. low- and high- TiO2 contents (1.0-1.9 wt%), low- and high-LILE contents (647-1439 ppm Sr), low- and high-LILE/HFSE and -LREE/HREE ratios (e.g.
27<Sr/Y<161, 2.0<Sm/Yb<11.7, respectively). Such distinct geochemical signatures imply different petrogenetic processes generating the parental magmas forming the mafic enclaves.
Thus, the compositional variability of the mafic recharge magma is responsible for the compositional scatter observed in the Taapaca dacites. The minor compositional differences
most apparent for Sr contents illustrated in Figure 2c.
2.4.2. C
OMPARISON TO OTHERA
NDEAN DACITIC COMPLEXESFigure 2 presents Taapaca rocks with selected, morphologically and compositionally similar Andean volcanic centers. A narrow compositional gap between ~60 to ~61 wt% SiO2 between the dacites and more mafic compositions found at this volcanoes can be observed in all presented examples. Due to the results obtained from multivariate statistical Polytopic Vector Analysis (PVA, Chapter 2), the geochemical characteristics of Taapaca rocks, result from two-stage magma mixing between three magmatic end-member magmas, representing shoshonitic (BEM) basaltic magmas, low-Mg high-Al calc-alkaline (AEM) basaltic andesite magma, and high-K calc-alkaline (RDEM) rhyodacite magma. It is strongly suggested, that the selected dacitic centers, showing similar petrological characteristics to Taapaca samples, may originate via the same differentiation processes, and furthermore, contain a silicic end-member generated in the thick crust, as argued for RDEM in Chapter 2.
A crucial geochemical signature leading to the above mentioned conclusion is the high Sr/Y ratio found in Taapaca (27-161), Haunyaputina (50-94) Irruputuncu (32-57), as well as Cayambe (32-79) and Longaví (14-81). Sr/Y ratios >30 are connected to the “adakitic” signature, as mentioned in section 1, and connected to the magma interactions with an exceptionally thick crust in the CVZ, with garnet as a residual phase. Although the two latter volcanoes Cayambe and Longaví are located in the NVZ and SVZ, respectively, characterized by a significantly lower thickness of the underlying crust (e.g. Tassara, & Yanez, 2003; Tassara, 2005), they also show high Sr/Y ratios. In contrast to the mainly high-K calc-alkaline CVZ dacites, they plot in medium-K calc-alkaline field. They do not contain high-K - high-Sr/Y shoshonitic component, as observed for the CVZ magmas. The origin of the adakitic signature in these highly porphyritic dacites must result from high Sr/Y ratios in the RDEM-type silicic magmas, due to presented issue in section 6.6.1, Figure 15 in Chapter 2.
Figure 2. Next page: a) Total Alkali vs. Silica (TAS) diagram according to Le Maitre et al. (1989) with alkaline/subalkaline division of Irvine & Baragar (1971), showing Taapaca and selected Quaternary Andean dacitic volcanic centers. b) K2O vs. Silica subdivision of Rickwood (1989) presenting the subalkaline lavas from a). c) Sr vs. Silica plot presents Taapaca rocks in comparison to two main compositional trends occurring in the CVZ, defined by Davidson et al. (1990). Main Taapaca dacite group overlaps with the highly in incompatible elements enriched PP-trend; the Taapaca subgroup dacites overlaps with the Neogene trend.
Abbr.: BEM-basaltic shoshonitic, AEM-high-Al basaltic andesitic calc-alkaline, RDEM-high-K calc alkaline PVA end-members.
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