field guide

ARICA > PARINACOTA

FIELD GUIDE

SOTA7

Continental arc magmatism on thick, old and hot crust: Geology, volcanology and petrology along a w-e transect from Arica to Volcan Taapaca and Parinacota Sept. 9-15, 2018

Georg August Universität Göttingen, Germany

Universidad Católica del Norte Antofagasta, Chile

Oregon State University Corvallis, USA

International Association of Volcanology and Chemistry of the Earth's Interior

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FIELD GUIDE CONTINENTAL ARC MAGMATISM ON THICK, OLD AND HOT CRUST: GEOLOGY, VOLCANOLOGY AND PETROLOGY ALONG A W-E TRANSECT FROM ARICA TO VOLCAN TAAPACA AND PARINACOTA

Sept. 9-15, 2018

G. Wörner Georg August Universität Göttingen, 37077 Göttingen, Germany [email protected]





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Contents

page

Overview

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Plate tectonic context

4

Types and distribution of volcanoes in the Central Andes

6

Stratigraphy and geological units in the field trip area

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Description of stops and material for discussion

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Day 1 (Sept. 9): Drive from San Pedro de Atacama to Arica (c. 700 km)

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Day 2 (Sept. 10): Stop 1 Azapa valley stratigraphy Stop 2 Top Poconchile: Overview Lluta collapse Stop 3 Base Poconchile: Oxaya ignimbrite and Azapa formation Stop 4 Cardones Ignimbrite, welded facies

12 13 14 15

Origin, volume and composition of large-volume ignimbrites

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Stop 5 Stop 6 Stop 7 Stop 8

Pucara Copaquilla: structural evolution of the western slope of the Central Andes Lauca ignimbrite Mirador „Socoroma“ Mirador “Putre“, Taapaca dome complex; deposits and stratigraphy

Taapaca and Parinacota: two contrasting magma regimes Taapaca petrography and geochemistry Taapaca sanidine megacrysts Parinacota stratigraphy, petrography and geochemistry P-T conditions of crystallisation at Taapaca and Parinacota P-T-t : Putting time on magmatic regimes

19 20 23 24

27 28 32 38 41 43

Transcrustal magma systems in the Central Andes

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Day 3 (Sept 11): Taapaca, Huaylas sediments, Lauca Ignimbrite on the slope of the Western Cordillera Stop 9 Taapaca block-and-ash flows / debris avalanche deposits Stop 10 Quebrada Allane: Huaylas Formation and Lauca ignimbrite Stop 11 Lauca ignimbrite: facies and flow units

48 48 48 49 49

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Stop 12 Highly altered core of upper Miocene glaciated stratovolcano

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Day 4 (Sept 12): Parinacota stratigraphy Stop 13 Taapaca block-and-ash flows Stop 14 „Cota Cotani“, Parinacota sector collapse, Old Cone – Young Cone Stop 15 Lago Chungará, Parinacota stratigraphy, and views Stop 16 Parinacota village Stop 17 Distal Parinacota debris avalanche deposit, secondary mud flows Stop 18 Mirador “Pampa Chuca”: Parinacota / Pomerape at sunset

51 51 51 52 55 56 56

Day 5 (Sept. 13): Parinacota Petrology Stop 19 Full day: Parinacota stratigraphy, volcanology, petrography (involves 4-WD shuttle / walking, total ca. 10 km at 4600 m)

57 57

Altiplano Salars and Lakes and the metamorphic basement

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Day 6 (Sept. 14): Belen basement Stop 20 Salar de Surire, Borax and flamingos Stop 21 Hotspring (optional) Stop 22 Lauca Basin sediments and Lauca Ignimbrite Stop 23 Overview Portezuelo Chapiquiña Stop 24 Chapiquña diorite (Miocene intrusion into Lupica Formation) Stop 25 various stops along the road: Belen Metamorphic Complex (1.8 Ga): basement gneisses, amphibolites, serpentinites Stop 26 Folded ignimbrites of Oxaya Formation (19 to 22 Ma)

58 58 58 59 59 60 60 61

Return to the Jurassic Coasta Cordillera

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Day 7 (Sept. 15): Putre to Arica, Lauca descending Stop 27 Overview near Putre Stop 28 = Stop 6 Mirador „Socoroma“: Valley-filling Lauca ignimbrite Stop 29 Lauca Ignimbrite filling Lluta valley at Molinos Stop 30 Lauca Ignimbrite at Panamericana near Pacific coast Stop 31 Jurassic rocks of the Coastal Cordillera: submarine sheet-flows, pillows and breccias, sunset, evening welfare-dinner at restaurant “Maracuyá”

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Acknowledgements

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References

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Tour map

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Overview Active continental margins are shaped by subduction-related magmatism, and the Central Andes of South America are a prime example. The “modern” Andean orogeny with its present elevations and landscape characteristics has evolved over the past 25 My via magmas ascending from the mantle and interacting with increasingly thickened continental crust. This process is reflected in the volumes and compositional variations of the magmas that erupt at the surface and their relation to the tectonic history and in the evolution of trans-crustal magma systems that feed the iconic andesite stratovolcanoes on the Central Andes. In order to understand this magmatism, this field trip covers magmatism from crustal to crystal scale, covering the following topics: 1) Landscape and structural evolution of the Western Margin of the Central Andes at 18°S between the coast at Arica and Volcán Parinacota on the Altiplano. 2) Large-volume, plateau-forming ignimbrite volcanism of the Oxaya formation (23 to 19 Ma) 3) Facies and deposition of the post-tectonic Lauca ignimbrite (2.7 Ma) descending the western Andean escarpment from >4500 m to the ocean. 4) Distinct magmatic regimes feeding stratovolcanoes: slow and deep, fast and shallow -

Taapaca Volcano: 1.5 My of dacite magmatism

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block-and-ash flow deposits

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sector collapse and debris flows

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sanidine megacrysts

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Parinacota Volcano: 160 kyr from basalt to rhyolite

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“Old Cone” rhyolite domes and amphibole andesites

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sector collapse and debris avalanche deposits (Cota Cotani and Lago Chungará)

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“Young Cone” high-flux cpx andesites and Ajata flank eruptions

5) Lauca Basin lacustrine sediments: 7 Ma years of climate history of the western Altiplano

Note: Minor parts of the introductory text were taken in modified form from a recent article by Wörner et al (2018) in ELEMENTS, August 2018. The descriptions of Stops necessarily will treat different topics in a sequence that are determined by logistics rather than logic. The first time a particular topic is touched at any given stop, the guide provides detailed information that will be picked up again at later stops.

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Plate tectonic context Subduction the Central Andes (Fig. 1) has been an active since Jurassic times; however, significant shortening of the crust, crustal thickening, and formation of the Altiplano–Puna Plateau began only at about 35 Ma (Late Eocene), with accelerated shortening during the last 10 My (Fig. 2). Consequently, mantle-derived magmas must now traverse the thickest crust (>70 km; Beck and Zandt, 2002) of any subduction zone on Earth. Because of the increasingly arid climate on the western margin of the Central Andes, volcanic edifices and ignimbrite deposits are extremely well-preserved, and their composition and distribution can be studied back in time. Because the chemical and isotopic composition of magmas are strongly affected by interaction with crustal material during ascent, and because the thickness of the crust has changed through time, the Central Andes are an excellent natural laboratory to study the interaction between crustal evolution and magma genesis.

Fig. 1: (a) Plate tectonic map of south America, (b) Nazca-plate convergence rates through time, (c): tectonic shortening along the Central and southern Andes (from Horton, 2018). The active plate boundary of western America is known for its abundance earthquakes (see recent compilation of Ruiz and Maradagia, 2018, Fig. 2). A “giant” >8 M mega-thrust earthquake that ruptured the deeper part of the plate interface over several hundreds of km has not occurred in northern Chile since 1877. This has resulted in considerable concern that such an event – and associated large tsunamis - should be expected in the near future. 4

Fig. 2: (a) Compilation of earthquake data 1900 to 2017 for northern Chile Epicenters (> M 4.5) from the NEIC catalog. Color code relates to hypocenter depth. Vertical lines represent estimated extends of rupture. The black stars: epicenters of major intra-plate intermediate depth events. (b) Hypocenters of main earthquakes since 1900 in the area of the field trip in Northern Chile. From Ruiz and Maradagi (2018). Thickening of the Central Andean crust to 70 km was episodic and mostly associated with tectonic shortening during increased westward plate movement of South America since about 40 Ma (Fig. 1b, c). Underthrusting of the Brazilian shield from the E, lower crustal flow of a thermally weakened crust and, possibly, forearc erosion and underplating are the main causes of thickening since 35 Ma (Isaacks, 1988; Schildgen and Hoke, 2018). Delamination may have contributed, but is not the main cause of crustal thickening (de Silva and Kay, 2018). Magmatic addition has previously been often cited as a major process but is clearly insufficient to explain the present thick crust: First, it would involve extremely high magmatic fluxes (on the order of a flood basalt province) and second, is inconsistent with evidence of extensive crustal shortening.

Types and distribution of volcanoes in the Central Andes The spatial distribution of volcanic edifices (Miocene to Recent) and ignimbrites (27 Ma to recent) in the Central Andes is shown in Figure 3.

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Fig. 3: Distribution of stratovolcanoes (Miocene to Holocene) and monogenetic volcanic centers (Pliocene to Holocene) in the Central Andes. Active volcanoes in red; the volcanoes of Parinacota (P), Taapaca (T) and Aucanquilcha (A) are marked in large blue letters. Large-volume ignimbrites are color-coded according to age (age column bottom right); caldera structures are outlined in yellow. The location of the mid-crustal Altiplano–Puna Magmatic Body (red dashes) is based on geophysical data (Zandt et al. 2003). Ignimbrites from the Southern Peruvian Volcanic Complex are mostly between 20 Ma and 5 Ma.

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There are four main modes of volcanism (de Silva and Francis, 1991): (1) Modern (2,000 m in height and many have summit elevations well over 6,000 m. Ojos del Salado in northern Chile is the world’s highest active volcano at 6,887 m. These large clustered volcanoes are the products of intra-crustal magmatic systems that have typical life times from between a few 100 ka to several My (e.g. Hora et al. 2007; Walker et al. 2013).(2) Fields of small, monogenetic volcanoes and related individual lava flows are rare and concentrated in a few regions, e.g. the Andagua valley, at Negrillar, as well as in the back-arc region. (3) The Central Andes boast one of the largest ignimbrite provinces on Earth (de Silva and Kay, 2018). Monotonous or crystal-rich dacites to rhyolites of Miocene age contain individual flows of thousands of cubic kilometers. These “ignimbrite flare-ups” relate to increased mantle input and to a zone of anomalously low seismic velocities in the middle crust of the southern Central Andes (Ward et al. 2014). These large-volume ignimbrites contrast with local, often valley-filling smaller ignimbrite deposits. (4) Isolated dacitic domes that can reach more than 500 m in elevation are either related to advanced differentiation and formation of small volumes of dacites and rhyolites in magma systems below individual stratovolcanoes (e.g. Parinacota) or represent the surface expression of large volumes of evolved melts associated with caldera systems. Basalts are exceedingly rare in stratovolcanoes and monogenetic fields: out of more than 1,500 analyzed samples of lavas, the most mafic magma in the Central Andes during Holocene times, and the only true basalt lavas, were erupted in the Andagua/Huambo monogenetic field (Mg# = 65.3; SiO2 = 51.8 wt.%) [Mg# = MgO/(FeOt + MgO) x 100, molar] with a few occurrences of shoshonites in the Peruvian back-arc (Mg# = 69.6; SiO2 = 51.6 wt.%) (Mamani et al. 2010). A simple SiO2 wt.% histogram (Fig. 4) is instructive to summarize the major element characteristics of magmas erupted in the Central Andes since Miocene times. Compositions more mafic than andesite are rare because such primitive magmas are too dense and will stagnate, cool, and crystallize during ascent to the surface. This highlights the effective crustal density filter in processing mantle-derived magmas through magmatic differentiation and crustal assimilation in this thick-crust continental arc. Subsequent assimilation and compositional differentiation leads to magmas of more evolved compositions. Appropriately, andesites with a range from 55–68 wt.% SiO2 and that are formed by differentiation, assimilation, and mixing in (trans-) crustal magma systems represent the most abundant magma types. Magmas having 68–72 wt.% SiO2 rarely erupt in the Central Andes, but where they do they typically form crystal-rich domes (or “tortas”) indicative of high magma viscosities. These domes may represent the crystal mushes from which more silicic and voluminous ignimbrites are derived by melt extraction. However, the maximum of the SiO2 distribution between 64–67 wt.% SiO2 for (older) intrusive rocks falls close to this minimum in compositions for erupted lavas. Further differentiation and mixing with crustal melts produce large volumes of silicic magmas that can feed large-volume ignimbrite eruptions (Fig. 4). 7

Fig. 4: SiO2 histogram for Central Andean magmas: intrusive rocks, lavas and ignimbrites. SiO2 values are distinctly bimodal as controlled by density, viscosity, and the eutectic composition. Intrusive rocks have a wide range and their maximum is between the modes of the lavas and ignimbrites (From Wörner et al. 2018).

There are three main questions with respect to Andean magmatism. First, how do magmas form beneath the Central Andes? Second, how do magmatic trace-element and isotopic compositions reflect changing conditions of magma evolution during the past 35 My of Andean orogeny and increasingly thickened continental crust? Third, what is the role of the Andean crust in explaining the variation in isotopic ratios in lavas that are spatially distinct?

Stratigraphy and geological units in the field trip area Links between tectonic evolution and magmatism in the e field trip area is conveniently documented by a typical stratigraphic sequence and radiometric ages of deposits on the western slope of the Central Andes near Arica (Wörner et al. 2002; Garcia et al., 2017 and references therein). This sequence is exposed along the road from Arica to Chungará (Stops 1-8) and can be found along the western margin of the Central Andes for over 1500 km. Here, we observe four general geological units that are related to the evolution of the modern Central Andes: First, molasse-type sediments (Azapa Fmt, Stops 1, 2, 28) were deposited during a magmatic lull (~35–22 Ma) that was caused by a period of flat-slab subduction. As a consequence of the flat slab, increased plate coupling and plate shortening resulted in uplift, erosion, and sedimentation. Second, deposition of plateau-forming ignimbrites (Stops 1-6), which represent large volumes of mixed mantle- and crust-derived silicic magmas containing 70 wt.% and 78 wt.% SiO2. Third, these ignimbrites are locally overlain by flat-lying, phenocryst-poor andesite shield lavas that indicate hotter and dryer magmas. Fourth, the development of andesitic and dacitic stratovolcanoes of the Central Andes with typical compositions of 55–68 wt.% SiO2 (Fig. 4). True rhyolites (>69 wt.% SiO2) are exceedingly rare in stratovolcanoes, yet it is such rocks that dominate the compositional spectrum of the ignimbrites.

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Fig. 6: Compiled stratigraphic sections (from Garcia et al., 2017)

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Description of stops and material for discussion This guide provides information on geological stops with GPS-coordinates, some with GOOGLE-Earth images with view directions and brief descriptions. Additional “informal” stops, which are mere scenic “geo-stops” are not listed here.

Day 1 (Sept. 9): Drive from San Pedro de Atacama to Arica (c. 700 km). Start (8:00) via Calama – Panamericana with several scenic stops along the way, views of Miocene lake sediments (Arcas fan near Quillagua), Quebrada Camarones. Arrival in Arica late after sunset at Hotel Savona.

Fig. 7: Space Shuttle image of the Central Andes with route to Arica indicated.

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Stop 1: Top of Oxaya- and Diablo Formation looking SE across the Azapa Valley 18°30’28’’S 70°08’15’’W, 784 m Driving up from the base Valley Azapa, view across the valley on Azapa Fmt sediments overlain by ignimbrites of the Oxaya Fmt and conglomerates of the Diablo Fmt. Note that welding degrees change strongly within these thick ignimbrite flows: The base (few m) is unwelded, overlain by a vitrophyre grading into a welded facies which makes up 50-70% of thickness. The top is again unwelded (pink to white). This zonation reflects the interplay of temperature, cooling rate and load after deposition. The unwelded top of Cardones ignimbrite has been eroded (here) between c. 21.9 and 19.4 Ma. These ignimbrites can be correlated across several major valleys and to the top of the Altiplano where they underlie younger sediments and volcanic edifices (e.g. at Parinacota). The sequence is overlain by andesitic conglomerates of the Diablo Formation (exposed at the stop).

Fig. 8: Stratigraphy of Azapa-, Oxaya-, and Diablo Formations exposed on the southern flank of the middle reach of Azapa Valley. Ages from Wörner et al (2000)(1) and van Zalinge et al (2016)(2)

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Stop 2 : Top of Lluta Valley above village of Poconchile 18°28’21’’S 70°04’42’’W, 1006m A section identical to that at Stop 1 is seen below and across the Lluta Valley to the NE. The flat lying Azapa, Oxaya and Diablo deposits are, however, “disturbed” by a gigantic landslide that has displaced these units.

Fig. 9: View to the N from above village of Poconchile across the Lluta Valley. Most of the northern flank of the valley across is made up of a giant gravitational collapse (Lluta Collapse). The Lluta collapse is sourced at 3500 m on the steep western limb of the Oxaya anticline. The thickness reaches more than 800 m at 25 km run-out and >300 km2 in area. The age of the collapse cannot be constrained precisely. (1) The collapse must be younger than the formation of the anticline (< 10.5 Ma, see below) and (2) significantly older than the Lauca ignimbrite (2,7 Ma) which covers its eroded surface. (3) The Lluta valley makes a conspicuous and unusual bend through the center part of the collapse (seen here ) and must be younger as well. (4) Valley incision at the western flank of the Central Andes starts around 10 Ma (Thouret et al, 2007). (5) The collapse is – in a few places - directly covered by undisturbed diatomite beds, indicating abundance of water, possibly from aquifers cut by the collapse. (6) Sediments on the Altiplano (e.g. in the Lauca and Allane basins, see later stops; Kött et al., 1995; Gaupp et al, 1999)) indicate a wet period around 7 Ma. The basal shear-plane at the source is at the contact between older intrusives and sediments of the Azapa Formation. Increased porepressure in the aquifer at this base may have aided the collapse. Thus, it is likely that the collapse occurred at less arid conditions around 7 Ma. Exposed at the viewpoint are conglomerates of the Diablo Formation (age here: 11-16 Ma). 13

Stop 3 : Base of Oxaya Formation in the Lluta Valley near village of Poconchile. 18°26′56″ S, 70°03′58″ W : 555 m The road cuts through the entire Oxaya Formation. Exposed at the level of the road is the “Molinos” (aca “Willi”) ignimbrite (22.72±0.12 Ma Sanidine Ar-Ar age, Wörner et al., 2000).

Fig. 10: View of Poconchile section through Oxaya ignimbrites (23-20 Ma. Top at 1006 m, base at 550 m). At the lower valley slope, sandstones, mudstones, and gravel of the Azapa Formation are poorly exposed. These formed by erosion after the initial phase of uplift of the Andes 25 to 20 million years (Ma) ago. "Cardones-" and “Oxaya-“ ignimbrites show typical zonation in degrees of welding, unwelded, white at the base, quickly grading into pink and a black vitrophyre (obsidian) about 2 ma above base. The main volume is comprised of welded (brown) ignimbrite with fiamme in a mostly devitrified matrix, changing again to pink and white unwelded at the upper third of the thickness. The top ignimbrite ("Oxaya-Ignimbrite") is eroded and overlain the Diablo conglomerates.

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Fig. 11: The scar of the Lluta Collapse at Pampa Plazuela is a giant amphitheater. To the north and northeast, across Lluta Valley, the surface of the sediment + ignimbrite “wedge” is undisturbed. Volcan Taapaca appears at the horizon towards the northeast.

Driving further into the Cardones valley, older rocks (granites of Cretaceous age: >65 Ma) are covered by the ignimbrites. The “Lauca-ignimbrite” erupted 2.7 Ma ago as a hot ash cloud North of Lago Chungará and left thick deposits in the valleys and plains. The Lauca ignimbrite is exposed in patches along the road overlying the Lluta collapse and filling the Cardones Valley. GPS: 18°20′15.5″ S, 69°01′35.2″ W; Elevation: 1300 m. The road crosses the Ausipar thrust fault (Garcia et al, 2018) that thrusts and uplifts older intrusive rocks towards the W. Thick sections on both sides of the Cardones Valley are comprised of Oxaya ignimbrites. Intercalated sediments that were seen in stop 2 are reduced to mere few m between each flow and the underlying Azapa Formation was not deposited here. The ignimbrites are directly overlying Cretaceous and Eocene intrusive rocks. These uplifted older intrusive rocks are part of the source region for the Azapa sediments below further to the W.

Stop 4 : Cardones ignimbrite welded facies, 18°28′13″ S 69°49′54″ W : 2015 m Exposure of welded facies of Cardones ignimbrite dated at 21.92±0.17 (U-Pb zircon age, van Zalinge et al. (2016). Fiamme contain abundant phenocrysts of quartz, sanidine and biotite. This is a good spot for sampling an Oxaya ignimbrite.

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Discussion: Origin, volume and composition of large-volume ignimbrites Ignimbrites of the Oxaya Formation are rhyolitic in composition and phenocryst-rich (qz, san, plg, biot, ap, zr). HREE element patterns are relatively flat (in contrast to REE in Lauca ignimbrite, see later), indicating processing at relatively shallow level. Sub-populations of phenocryst types and abundances suggest mixing of different rhyolitic magma batches (van Zalinge et al., 2018). Sr- and O-isotope data on minerals (qz, san) indicate that the source of the ignimbrites is dominated by juvenile components from the mantel wedge with only 20% of assimilation of an Belen-type metamorphic basement of Proterozoic age (Freymuth et al., 2015; day 6). Alternatively, the source could be made entirely of crustal rocks with relatively unradiogenic Sr and mantle-like O-isotopes (e.g. re-melts from Cretaceous intrusive rocks). In that case, however, the heat source fro melting would be unexplained. Therefore, Freymuth et al. (2015) preferred a model with 80% of magmas derived from differentiation of mantlederived magmas and 20% of crustal melts. This scenario is supported by negative εHf in zircon from Oxaya (and other northern Chilean) ignimbrites; Keeman, Wörner, Turner, unpublished data).

Fig. 12: After the passage of the Juan Fernandez Ridge, the slab steepens again and causes a sequence of ignimbrite flare-ups (Fig. 14). The large volume (< 3000 km) of ignimbrites of the Oxaya Formation and the wide distribution of other ignimbrites in southern Peru of the same age (see Brandmeier et al., 2016) then indicates a major flux of mantle derived magma into the Andean crust at around 23 to 20 Ma, i.e. just after the magmatic lull and flat slab subduction. Such ignimbrite “flare-us” are likely 16

caused by rapid steeping of the slab, influx of hot asthenosphere into the mantle wedge and extensive mantle melting at this time are all linked processes that follow the flat-slab episode that was caused by the passage of the Juan Fernandez aseismic ridge (Fig. 12; Kay and Coira 2009, Wörner et al, 2000, 2002; de Silva and Kay, 2018). Older ignimbrites (>9 Ma) in the northern part of the Central Volcanic Zone (CVZ) have flatter REE patterns compared to younger ignimbrites (such as the Lauca ignimbrite, 2.7 Ma) and ignimbrites to the S (Fig. 13, 14, from Brandmeier et al. 2016).

Fig. 13: A statistical analysis of ignimbrites in the central Andes indicates a significant change in REE pattern around 9 Ma, coincident with the second phase of increased shortening and thickening of the crust (see Fig. 14), from Brandmeier et al. (2016).

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Fig. 14: Magmatic fluxes (km3 magma per Ma and 1 km of arc length) estimated from mapped ignimbrites and the crust-mantle mass balance based on Sr-O isotope modeling (Freymuth et al., 2015). These minimum estimates indicate that ignimbrite flare-ups are caused by massive input of mafic magma from the mantle wedge after slab steepening in the wake of the passage of the Juan Fernandez Ridge. Crustal shortening from Oncken et al. (2006.)

Freymuth et al (2015) and Brandmeier et al (2016) provide a compilation of compositional, age and volume data for known and mapped ignimbrite deposits from the Central Andes which can be accessed in our Andean Ignimbrite Database (AIDA) : http://www.uni-geochem.gwdg.de/en/andes-database This website presents data in a clickable map where data for individual mapped ignimbrite sheets a documented. Note, that no data compilation can ever be complete and this compilation is based on data published until 2014. Fig. 15: Landing page of AIDA. Zoom into any mapped ignimbrite sheet, click and obtain age, volume and reference data. Individual points on map represent analyzed samples where compositional data can be accessed. Compilation as of 2014 data available.

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Stop 5 : Mirador Pucara de Copaquilla , 18°23′29″ S, 69°38′33.2″ W, 2900 m The Western Cordillera in the east formed by strong uplift along a series of thrust faults. Badlands below indicate unconsolidated sediments of the Huaylas Formation, which is equivalent to the Diablo Formation on the lower slopes of the western Andes. These sediments rest on eroded 20-23 Ma old Oxaya ignimbrites by which they are also overthrust from the E. (a) (b)

Fig. 16: Mirador Pucara: (a) view to the Western Cordillera (WC). Proterozoic metamorphic basement (MB, rocks of the Oxaya Formation and volcaniclastic rocks of the Lupica Formation (OVR) are thrusted over Miocene Huaylas sediments (SED). (b) view to the S. Cerro Marquez volcano (ca. 8 Ma, Wörner et al, 2000) at the horizon. To the W, the steep Chapiquina – Belen escarpment displaces the Oxaya Formation from c. 3000 m to > 5000 m at the horizon. Oxaya ignimbrites are tilted to the east, against the general slope of the Andes as part of the eastern limb of the N-S trending Oxaya anticline. This tilting resulted in a sediment trap now filled by debris from the Cordillera: flat lying gray sediments fill the depression above the tilted block in front of you. The sediments have a maximum age of 10.5 Ma as indicated by a date on an intercalated ignimbrite near Tignamar to the S of here (Wörner et al., 2000a). The dark band onto of the Oxaya ignimbrite is a lava flow that runs” up slope” and thus must be older than the tilting. This flow has been dated at 11.5 Ma (Garcia et al, 2004). Tilting thus occurred at about 11 Ma. 2.7 Ma ago. The Huaylas sediments are horizontally covered by the Lauca ignimbrite, which is also exposed at the road across from the parking at Copaquilla. Rio Azapa has cut back into the titled “Oxaya” block, the sediments and the Lauca ignimbrite. This process is still going on. The rotation of the Oxaya Block against the general western slope of the Andean margin about c. 7 Ma resulted in the over-steepening of the western limb and extension along the crest of the Oxaya anticline and this - likely in conjunction with a large earthquake – has caused the gravitational movement of the Lluta collapse.

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Fig. 17: Satellite image of the tilted Oxaya Block and the Lluta collapse

Stop 6 : Lauca ignimbrite across from the Inca fortress, 18°23'28''S 69°38'40'', 3120 m The Lauca ignimbrite emplaced here has descended down from the Altiplano, across the Western Cordillera to the W at 4400 m down to 3000 m at this location. From here it continued along the same path as the road we have just taken: it entered that Cardones valley flowing down and over the Lluta collapse into the Lluta valley and on to the Pacific coast (Stops 29 and 30 on the last day, Fig. 18. The Lauca ignimbrite has been dated at several locations by Ar-Ar on sanidine at 2.72±0.01 Ma (Wörner et al, 2000a). Two flow units are identified but the depositional facies is highly variable do to the extremely different topography and flows paths from source to the ocean. In some outcrops, a well-sorted but slightly laminated basal pumice layer is observed, which is NOT a pre-ignimbrite air fall deposit but rather pumice fragments that were concentrated by elutriation of fines at the front of the highly mobile ignimbrite flow. The lower flow unit is finegrained, whereas the upper flow unit shows pumice enrichments with clasts up to 20 cm. Crystals contents change from aphyric at the base to strongly porphyritic qz-rich pumice at the top (with sanidine, plagioclase, biotite and minor magnetite next to quartz). Chemical composition is rhyolitic with relatively little changes from base (77.41% SiO2, 0,06 % MgO) to top (76.65% SiO2). However, individual pumices show mingling with more mafic magmas and these hybrid pumices have SiO2 down to 67.74% SiO2 and 2.63 % MgO. REE patterns (Fig. 19) are very consistent with increasing negative Eu-anomaly, strong LREE enrichment and flat heavy REE. A slight depletion around Dy-Er may indicate the role of amphibole and/or titanite. Freymuth et al. (2015) show Sr- and O-isotope data for separated minerals from ignimbrites throughout the CVZ including Lauca and Oxaya ignimbrites.

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Fig. 18: Mapped and inferred distribution of the Lauca ignimbrite in northern-most Chile, southern Peru and western Bolivia. Fig. 19: REE patterns (chondritenormalized) for Lauca ignimbrite pumices from different localities indicate diff-errent degrees of fractionation at rather shallow pressure (data from Brandmeier and Wörner, 2016, and unpublished data).

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Fig. 20: O-isotope melt compositions (calculated from mineral data) for stratovolcanoes and ignimbrites from the Central Andes. Note the general overlap between andesite and ignimbrite data for stratovolcanos and Oxaya ignimbrites at 18°S an in the northern CVZ 5 cm), strongly porphyritic, qz-rich pumices. This layer is equivalent to the upper flow unit near Quebrada Allane (Stop 11). This indicates that the hotter, coarsergrained flow unit was more mobile and ran put a longer distance than the finer-grained, more coherent lower flow unit. Depending on the topography over which the Luaca ignimbrite has travelled, both flow units may be present (typically on flat ground) whereas only the upper flow unit is found behind topographic barriers that the lower unit could not surpass. The Lake sediments of the Lauca Formation are age equivalent to gravel deposits of the Huaylas Formation below the Lauca ignimbrite at the Copaquilla viewpoint (Stop 5) and also at Quebrada Allane (Stop 10). Ages are also similar to the Diablo Formation that are exposed on the lower flanks of the western slope of the Andes (Stop 1) Stop 23: Overview Portezuelo Chapiquiña, 18°10'30''S 69° 29'54''W, 4439 m The pass at 4439 is one of the gates through which the Lauca Ignimbrite descended to the valley and basins below. Exposed here are strongly folded, sometime vertically oriented sediments of the Lupica Formation with equally folded members of the Oxaya ignimbrites. Lupica sediments are strongly hydrothermally altered by intrusions which gave ages between 12 and 19 Ma (Garcia et al., 2018). Oxaya ignimbrites with clear facies indicators of proximal deposition overly the Lupica Formation. This indicates that alteration of Lupica Formation, dated at 17 to 9 Ma (Garcia et al., 2018) and the magmatic activity of the Oxaya Formation are closely linked in space and time. Thus it is likely that the exposed and hydrothermally strongly overprinted rocks of the Western Cordillera between here, Belen to the S and Putre to the N may be related to a 22 to 19 Ma old "Oxaya Caldera" complex. Discussion: The age of the Lupica Formation This strong hydrothermal overprint on sediments of the Lupica Formation, which is closely superimposed on tectonically with Oxaya ignimbrites and intruded by magmas of similar age has always puzzled and hampered stratigraphic correlation with other Formation in the area. Garcia et al. (2004) propose that the Lupica Formation is age equivalent to the Azapa Formation. Both underlie the Oxaya Formation. However, the lithologies are very different: Lupica rocks are made of lavas and volcaniclastic breccias, are strongly folded and alteration ages always give ages similar to or younger than ignimbrites of the Oxaya Formation. By contrast, Azapa sediments are not deformed, never hydrothermally altered and their clasts include rocks from the Belen metamorphic basement, Jurassic rocks of the Livilcar Formation, Cretaceous and lower Tertiary intrusive rocks as well as altered andesitic clasts similar to rocks of the Lupica Formation (Decou et al., 2013 Wotzlaw et al. 2011) This, then, indicates that the Azapa formation represents the clastic debris from the uplifting Western Cordillera that already includes rocks from the Lupica Formation. The base of the Lupica Formation is not exposed and tectonically reworked by overthrusting onto the Belen metamorphic complex and 59

limited extends of Permian marine rocks overlying it (Quichoco Beds; Garcia et al., 2004) which contains lower Permian brachiopods (Wörner et al, 2000b) The view to the W overlooks the tilted Oxaya Block with its topography and incised valleys (Cardones valley) filled with Lauca ignimbrite. The tilting caused the formation of the Huaylas basin which is filled with sediments from the uplifting western Cordillera which is comprised of Oxaya Ignimbrites, Lupica Formation volcanic and volcaniclastic sediments and the oldest rocks exposed in Chile, the Belen metamorphic Complex. We observe that the Lupica Formation represents a belt of volcanic edifices that produced abundant andesite lavas and breccias indicative of water-aided transport and fragmentation. The volcanics and volcaniclastic rocks are intercalated by fine-grained sediments which indicate locally extensive stagnant water bodies. Thus, the facies and the lithology of Lupica rocks are very distinct from the conglomerates and sandstones of the Azapa. The Lupica Formation is therefore likely older than the Azapa Formation and represents a narrow belt of lower Tertiary volcanic centers into which magmas intruded in Miocene times. Hydrothermal alteration a lower greenshist facies conditions of these andesites makes it impossible to reliably date Lupica rocks. Therefore, close tectonic and hydrothermal association with Miocene intrusives and Oxaya ignimbrites and the thermal overprint of the Oxaya Caldera complex is the reason why no older radiometric ages can be extracted from rocks of the Lupica formation. Stop 24 Chapiqiuña diorite ,18°22'21''S 69°33'21''W, 3315 m This microdiorite intrusion into strongly hydrothermally overprinted rocks of the Lupica Formation gave an U/Pb zircon age of 16.0±0.6 Ma (Garcia et al, 2004, 2018) and is a good example for the arguments given above. Stop 25 Belen Metamorphic Complex 18°25'57''S 69°30'25''W, 3761 m We will see gneisses and amphibolites of the Belen Metamorphic Complex which gave U-Pb zircon upper concordia intercepts of 1,877+139 −131 Ma and 1,745±27 Ma (Wörner et al., 2000b).

Fig. 61: Metamorphic evolution of the Belen Metamorphic Complex (Wörner et al., 2000b.) 60

Terrane accretion is associated with younger metamorphic overprints between 366 and 456 Ma (Wörner et al. 2000b; Loewy et al. 2004). Intrusives associated with convergence and collision of the terrane blocks are associated locally with serpentinites. Amphibolites have arc-tholeiitic geochemical character. Metamorphic grades during the Ordovician metamorphic overprint reached high temperatures in granulites and amphibolites (500 to 700°C, and 3 to 7 kbar, Wörner et al., 2000b, Fig. 61). The Belen basement rocks are unusual in having rather unradiogenic Pb isotope compositions (Wörner et al., 1992) which identifies the Arequipa basements block as a litho-tectonic unit in the Central Andes (Mamani et al., 2008; Loewy et al. 2004). Stop 26: at Belen, 18°27'03'S 69° 31''30'W', 3473 m Folded ignimbrites of the Oxaya Formation have been dated here (19 Ma).

Fig. 62: folded Oxaya ignimbrites near Belen (19 Ma). Lava flows from Cerro Marquez at the horizon have been dated at 9.3±0.3 Ma (Wörner et al., 2000a). Folding is related to a west-vergent thrust system that has displaced older rocks of the Belen Metamorphic Complex over Lupica sediments. Oxaya ignimbrites are folded and overthrusted together with younger (18 to 11 Ma) andesitic volcanic edifices (e.g. Copoquilla, Stop 5) onto sediments of the Huyalas Formation (10.5 to 2.7 Ma) (Garcia et al., 2004; Garcia and Hérail, 2005).

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Return to the Jurassic Coastal Cordillera Day 7 (Sept. 15): Putre to Arica, Lauca descending Stop 27: Overview near Putre We will start this day with a "morning" view from a view point near Putre to re-iterate what we have seen in the past few days. This includes.. Stop 28: = Stop 7 Mirador „Socoroma, 18°16′21.8″S 69°34′48.9″W, 3490 m The Valley-filling Lauca ignimbrite is today much better appreciated with the morning light. Stop 29: Lauca Ignimbrite filling Lluta valley at Molinos, 18°22'38''S 69°56'16'', 1060 m We have driven the road back from the Western Cordillera to the base of the Lluta valley near the village of Molinos. Just 18 m above the present valley floor, we find remnants of eroded Lauca ignimbrite overlying ancient Lluta gravels beds. Apparently, erosion here has incised the Lluta valley by 18 m within 2.72 Ma. Fig. 63: Lauca ignimbrite exposed 18 m above present level of Lluta valley. Stop 30: Lauca Ignimbrite near Pacific coast: 18°22'56''S 70°18'01''W, 83 m We follow the road back to the Pacific coast through the Lluta valley at Arica. Slumped blocks of unwelded Oxaya ignimbrites on the valley flanks are easily confused with remnants of Lauca Ignimbrite deposits in the valley. Lauca ignimbrites have been verified at several locations, including a small hummock at 18°24'47''S 70'11'48'' at 241 m elevation. Here, the ignimbrite is at the same level of the present valley. We will stop at a road cut on the Panamericana between Arica and the border post to Peru. Here, the distal facies of the Lauca ignimbrite consists of numerous small flow units, each representing a depositional wave after traveling between 235 km and 160 km (depending on the flow path) down from the Altiplano through the Western Cordillera and into the Lluta valley. Stop 31: Coastal Cordillera: 18°32'15'' 70°19 35''S at sea level Jurassic rocks of the Coastal Cordillera are exposed with submarine sheet-flows, pillows and breccias, sunset, evening welfare-dinner at restaurant “Maracuyá”

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Acknowledgements I am thankful to Magdalene Blum-Oeste, John Hora and Smruti Sourav letting me include their unpublished results into this field guide. Hopefully their interesting result will be published soon.

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Mamani M, Wörner G, Sempere T (2010) Geochemical variations in igneous rocks of the Central Andean orocline (13°S to 18°S): tracing crustal thickening and magma generation through time and space. Geological Society of America Bulletin 122: 162-182 McMillan N, Davidson J, Wörner G, Harmon RS, Lopez-Escobar L, Moorbath S (1993) Influence of crustal thickening on arc magmatism: Nevados de Payachata Region, northern Chile. Geology, 21: 467-470. DOI: 10.1130/0091-7613(1993) Mutch, E.J.F., Blundy, J.D., Tattitch, B.C., Cooper, F.J., and Brooker, R.A., 2016, An experimental study of amphibole stability in low-pressure granitic magmas and a revised Al-in-hornblende geobarometer: Contributions to Mineralogy and Petrology, v. 171. Oncken O Hindle D, Kley J, Elger K, Victor P, Schemman K (2006) Deformation of the Central Andean Upper Plate System – facts, fiction, and constraints for plateau models. In: Oncken O and 7 coeditors (eds) The Andes. Frontiers in Earth Sciences. Springer, Berlin pp 3-27. Putirka, K.D., 2008, Thermometers and barometers for volcanic systems, Minerals, Inclusions and Volcanic Processes, Volume 69: Reviews in Mineralogy & Geochemistry, p. 61-120. Ridolfi, F. & Renzulli, A., 2012, Calcic amphiboles in calc-alkaline and alkaline magmas: thermobarometric and chemometric empirical equations valid up to 1,130°C and 2.2 GPa. Contrib Mineral Petrol (2012) 163: 877. doi:10.1007/s00410-011-0704-6 Ruiz S, Madariaga R (2018) Historical and recent large megathrust earthquakes in Chile. Tectonophysics 733:37-56. https://doi.org/10.1016/j.tecto.2018.01.015 Ruprecht P, Wörner G (2007) Variable regimes in magma systems documented in plagioclase zoning patterns: El Misti stratovolcano and Andagua monogenetic cones (S. Peru). J Volcanol Geoth Res 165: 142-162 Sourav S, Schmidt BC, Wörner G (2018) Non-isothermal diffusive analysis: experimental validation and application to sanidine megacrysts from Taapaca volcanic complex (Northern Chile). GeoBonn abstracts, Meeting of the Deutsche Mineralogische Gesellschaft Sept. 2-6, 2018 in Bonn.. Székely B, Koma S, Karátson D, Dorninger P, Wörner G, Brandmeier M, Nothegger C (2014) Automated recognition of quasi-planar ignimbrite sheets as paleosurfaces via robust segmentation of digital elevation model: an example from the Central Andes. Earth Surface Processes and Landforms 39:1386-1399 DOI: 10.1002/esp.3606. Thouret JC, Wörner G, Singer B, Gunnell Y, Zhang X, Souriot T (2007) Geochronologic and stratigraphic constraints on canyon incision and Miocene uplift of the Central Andes in Peru. Earth Planet Sci Lett 263:151-166. van Zalinge ME, Sparks RSJ, Blundy JS (2018) Petrogenesis of the large-volume Cardones ignimbrite, Chile; Development and destabilization of a complex magma-mush system. J Petrol 58: 1975-2006.doi: 10.1093/petrology/egx079 van Zalinge ME, Sparks RSJ, Cooper FJ, Condon DJ (2016) Early Miocene large-volume ignimbrites of the Oxaya Formation, Central Andes. Journal of the Geological Society 173: 716–733. doi:10.1144/jgs2015-123 Walker BA Jr and 5 coauthors (2013) Crystal reaming during the assembly, maturation, and waning of an eleven- million-year crustal magma cycle: thermobarometry of the Aucanquilcha Volcanic Cluster. Contributions to Mineralogy and Petrology 165: 663-682 Ward KM, Zandt G, Beck SL, Christensen DH, McFarlin H (2014) Seismic imaging of the magmatic underpinnings beneath the Altiplano-Puna volcanic complex from the joint inversion of

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surface wave dispersion and receiver functions. Earth and Planetary Science Letters 404: 43-53 Wörner G, Harmon RS, Davidson J, Moorbath S, Turner D, McMillan N,Nye C,Lopez-Escobar L, Moreno H (1988) The Nevados de Payachata volcanic region (18°S/69°W, N.Chile): I. Geological, geochemical and isotopic observations. Bull Volc 50: 287-303. Wörner G, Moorbath S, Harmon, RS (1992) Andean Cenozoic volcanics reflect basement isotopic domains. Geology 20: 1103-1106. Wörner G, Moorbath S, Horn S, Entenmann J, Harmon RS, Davidson JD, Lopez-Escobar L (1994) Large and fine scale geochemical variations along the Andean Arc of Northern Chile (17.5 22˚S). In : Tectonics of the Southern Central Andes. Structure and evolution of an active continental margin. Reutter KJ, Scheuber E, Wigger PJ (eds), Springer, Berlin : 77-91. Wörner G, Hammerschmidt K, Henjes-Kunst F, Lezaun J, Wilke H (2000a) Geochronology (40Ar39 Ar, K-Ar and He-exposure ages) of Cenozoic magmatic rocks from Northern Chile (1822°S): implications for magmatism and tectonic evolution of the central Andes. Revista Geológica de Chile 27: 205-240 Wörner G, Lezaun J, Beck A, Heber V, Lucassen F, Zinngrebe E, Rößling R, Wilke HG (2000b) Precambrian and Early Paleozoic evolution of the Andean basement at Belen (N. Chile) and C. Uyarani (W. Bolivian Altiplano). J South Am Earth Sci 13 (8) : 717-737 Wörner G, Ulig D, Kohler I, Seyfried H (2002) Evolution of the West Andean Escarpent at 18°S (N. Chile) during the last 25 Ma: uplift, erosion and collapse through time. Tectonophysics 345: 183-198 Wörner G, Wegner W, Kiebala A, Singer B, Heumann A, Kronz A, Hora J (2004) Evolution of Taapaca Volcano, N. Chile, evidence from major and trace elements, Sr-, Nd-, Pb-, and Useries isotopes, age dating and chemical zoning in sanidine megacrysts. IAVCEI General Assembly 2004, Chile/Pucon (on CD) DOI: 10.13140/RG.2.1.4524.2965 Wörner G, Mamani M, Blum-Oeste M (2018) Magmatism in the Central Andes. Elements 14-4: 237-244 Wotzlaw JF, Decou A, von Eynatten H, Wörner G, Frei D (2011) Jurassic to Paleogene tectonomagmatic evolution of northern Chile and adjacent Bolivia from detrital zircon U-Pb geochronology and heavy mineral provenance. Terra Nova 23: 399-406, doi: 10.1111/j.1365-3121.2011.01025.x Yáñez GA, Ranero CR, Huene R, Díaz J (2001) Magnetic anomaly interpretation across the southern central Andes (32–34 S): The role of the Juan Fernández Ridge in the late Tertiary evolution of the margin: Journal of Geophysical Research, Solid Earth 106: 63256345. Zandt G, Leidig M, Chmielowski J, Baumont D, Yuan X (2003) Seismic detection and characterization of the Altiplano-Puna magma body, Central Andes. Pure and Applied Geophysics 160: 789-807 Zellmer GF, Clavero JE (2006) Using trace element correlation patterns to decipher a sanidine crystal growth chronology: An example from Taapaca volcano, Central Andes. J Volcanol Geotherm Res 156: 291-301.

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