Upper Cretaceous to Holocene Magmatism and Evidence for Transient Miocene Shallowing of the Andean Subduction Zone under the Northern

KEYWORDS: Andes, volcanism, tectonics, Neuquén basin, shallow subduction, geochemistry, Neogene ABSTRACT Evidence for a Miocene period of transient shallow subduction under the Neuquén

Upper Cretaceous to Holocene Magmatism and Evidence for Transient Miocene Shallowing

of the Andean Subduction Zone under the Northern Neuquén Basin

Suzanne Mahlburg Kay and W Matthew Burns*

INSTOC and Department of Earth and Atmospheric Sciences, Snee Hall, Cornell University,

Ithaca, NY 14853 USA, smk16@cornell.edu

* Now at U.S Geological Survey, Reston, Virginia, 20192, USA; wburns@usgs.gov

Peter Copeland Dept Geosciences, University of Houston, Houston, Texas, 77204, USA, copeland@uh.edu

Oscar MancillaREPSOL-YPF, Buenos Aires, Argentina omancillad@repsolypf.com

KEYWORDS: Andes, volcanism, tectonics, Neuquén basin, shallow subduction, geochemistry,

Neogene

ABSTRACT

Evidence for a Miocene period of transient shallow subduction under the Neuquén basin

in the Andean backarc, and an intermittent Upper Cretaceous to Holocene frontal arc with a relatively stable magma source and arc to trench geometry comes from new 40Ar/39Ar, major and trace element, and Sr, Pb and Nd isotopic data on magmatic rocks in a transect at ~36° to 38°S

Older frontal arc magmas include Early Paleogene volcanic rocks erupted after a strong Upper Cretaceous contractional deformation and mid-Eocene lavas erupted from arc centers displaced slightly to the east Following a gap of some 15 million years, ~ 26 to 20 Ma mafic to acidic arc-like magmas erupted in the extensional Cura Mallín intra-arc basin and alkali olivine basalts with intraplate signatures erupted across the backarc A major change followed as ~ 20-15 Ma basaltic andesite/dacitic magmas with weak arc signatures and 11.7 Ma Cerro Negro andesites with stronger arc signatures erupted in the near to mid backarc They were followed by 7.2 to 4.8 Ma high-K basaltic to dacitic hornblende-bearing magmas with arc-like high field strength element depletion that erupted in the Sierra de Chachahuén some 500 km east of the trench The chemistry of these Miocene rocks along with the regional deformation pattern support a transientperiod of shallow subduction that began at ~ 20 Ma and climaxed near 5 Ma The subsequent widespread eruption of Pliocene to Pleistocene alkaline magmas with an intraplate chemistry in the Payenia Large Igneous Province signals a thickening mantle wedge above a steepening subduction zone A pattern of decreasingly arc-like Pliocene to Holocene backarc lavas in the Tromen region culminates with the eruption of a 0.175±0.025 Ma mafic andesite The

northwest-trending Cortaderas lineament which generally marks the southern limit of Neogene backarc magmatism is considered to mark the southern boundary of the shallow subduction zone

INTRODUCTION

The history of the continental lithosphere, mantle wedge and subducting plate of the south Central Andes, between 36.5°S and 38°S, is reflected in the temporal and spatial distribution andchemistry of the Upper Cretaceous to Holocene arc and backarc magmatic rocks of the Neuquén

basin Together with the structural and geophysical characteristics of the region, the distribution and geochemical features of these magmatic rocks can be used to formulate a model for the magmatic and deformational history and the evolution of the subducting slab in a west to east transect between 36° and 37.5°S through the Neuquén basin

GEOLOGIC AND TECTONIC SETTING

The east-west transect through the Neuquén basin between 36.5° and 38°S latitude lies justsouth of the transitional Southern Volcanic Zone (SVZ) arc where the Holocene volcanic arc front of the SVZ is displaced to the west (Fig 1) At this latitude, the currently subducting Nazca plate corresponds to Chrons 8 to 13 (~ 33 to 25 Ma; Cande and Kent, 1992) The

Holocene centers of the transitional SVZ dominantly erupt andesitic lavas, whereas SVZ centers

to the south dominantly erupt high-Al basalts (see review by Stern, 2004) To the east, the backarc can be divided into two regions by the northwest-trending Cortaderas lineament (Fig 2),which broadly intersects the southern end of the transitional SVZ segment North of the

Cortaderas lineament, Miocene to Holocene backarc magmatic rocks are widespread in a retroarcregion where Mesozoic rifting was less important and Neogene contractional deformation more important than to the south (see Ramos and Kay, this volume) Particularly notable in the

backarc are the largely Pleistocene to Holocene backarc Tromen and Payún Matrú volcanic centers and the extensive mafic flows that constitute the Payunia and Auca Mahuída volcanic fields (Figs 1 and 2) To the south of the Cortaderas lineament, Miocene to Holocene backarc magmatic rocks are essentially absent

The ages and locations of the Upper Cretaceous to Holocene magmatic rocks discussed in this paper are summarized in Table 1 They largely overlie or intrude the Mesozoic to early Paleogene sedimentary strata of the Neuquén basin The history of the Neuquén basin can be divided into three general stages (e.g., Vergani et al., 1995): (1) a Triassic to Early Jurassic pre-rift and rift stage, (2) an Upper Jurassic to Cretaceous subsidence stage, and (3) a Paleocene to Holocene modification stage The first stage was largely shaped by the extension and fault-controlled subsidence that preceded and accompanied the initial breakup of the Pangea

supercontinent The widespread Triassic Choiyoi rhyolitic volcanic rocks (e.g., Kay et al., 1989) exposed in the Cordillera del Viento (Fig 2) and underlying much of the Neuquén basin erupted

at this time The active rifting of this stage generally terminated as Middle Jurassic Andean tectonism and magmatism began to the west During the second stage, the discrete rifts and intervening basement blocks of the first stage generally merged into a broad post-rift basin that was filled by Middle Jurassic to Paleogene sedimentary strata In the last stage, the Neuquén basin strata were modified by Tertiary to Holocene extensional and contractional deformation and affected by periodic magmatic events across the basin The magmatic rocks of this third stage are the principal topic of this paper

DISTRIBUTION, AGE AND CHEMISTRY OF NEUQUÉN BASIN MAGMATIC

ROCKS

The distribution and ages of the Upper Cretaceous to Holocene Neuquén basin magmatic rocks described below are shown on maps in Figures 1 to 4 and summarized in Table 1 Twelve new 40Ar/39Ar ages are listed in Table 2 Age spectra are presented in Appendix 1 and analytical

techniques are the same as in Jordan et al (2001) New major and trace element data for 90 samples and isotopic data for 12 samples are listed in Tables 3 to 7 and plotted along with another 300 unpublished and published data in the fields in Figures 5 to 9 Sources of published data are in the figure captions Analytical techniques are as described in Kay et al (this volume, Chapter 10) Sample locations are listed in Appendix 2 The range of magmatic rocks types in the Neuquén basin region is illustrated in the SiO2 versus total alkali (Na2O+K2O) concentration diagrams in Figure 5

Major element, trace element and isotopic data are used below to characterize the Neuquénbasin magmatic rocks Characteristics of basaltic to mafic andesitic samples, particularly those with low FeO/MgO ratios (~ 0.7 to 1.0) and high Cr and Ni (>200-300 ppm) concentrations, are useful in interpreting mantle and subcrustal processes, whereas characteristics of silicic andesite

to rhyolites are useful in interpreting crustal processes Ratios and concentrations of

incompatible elements (concentrated in melts) provide insights into mantle magma sources and tectonic settings Slab-related processes are reflected in ratios of Ti group elements (Ta, Nb) to REE and alkali (K, Rb, Cs)/ alkaline earth (Ba, Sr) elements Ratios of La/Ta, Ba/La, and Ba/Ta ratios in Neuquén basin samples are compared with those of SVZ frontal arc magmas (La/Ta ~

40 – 95, Ba/La > 20, and Ba/Ta > 500) and mid-ocean ridge (MORB) and intraoceanic intraplate (OIB) magmas (La/Ta < 12; Ba/La < 15) from Hickey et al (1986) in Figure 6 Indicators of mantle source conditions include the high field strength (HFS) elements plotted in Figure 7 High Ta/Hf ratios reflect an enriched intraplate mantle source whereas low Ta/Hf ratios indicate depleted MORB or arc mantle sources High Th/Hf ratios are typical of calc-alkaline arc

sources Relative concentrations of incompatible elements also serve as guides to percentages ofpartial melting in mantle and crustal source regions

Ratios and concentrations of compatible elements (concentrated in minerals) reflect

residual minerals that are either left in the magma source after melting or removed by

fractionation processes The residual mineral assemblage reflects the pressure, temperature, and fluid conditions under which the magma last equilibrated Trace elements are useful in

determining residual mineral assemblages as: a) olivine, orthopyroxene, and micas have little affinity for REEs, but take Ni and Cr, b) feldspar takes Eu2+ and Sr, c) clinopyroxene and to a greater extent amphibole take middle and heavy REEs and Sc, d) garnet takes heavy REEs, and e) accessory titanite and apatite take middle REEs and zircon takes heavy REEs, Hf, Th, and U Increasing pressure can produce a change from pyroxene to amphibole to garnet in the residual mineralogy that can be detected by increasing La/Yb and Sm/Yb ratios La/Yb ratios provide a guide to the overall steepness of the REE pattern (Fig 8a) whereas La/Sm and Sm/Yb ratios (Fig 8b) provide a guide to light and heavy REE behavior Nd, Sr and Pb isotopic ratios (Figs 9 and 10) contain independent source region information as they reflect parent/daughter ratios in closed systems and contaminant addition in open systems Magmas with higher 87Sr/86Sr,

206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb and lower 143Nd/144Nd ratios are said to be relatively isotopically “enriched”

Upper Cretaceous to Eocene Magmatic Rocks.

The Upper Cretaceous to Paleogene magmatic history of the Neuquén basin is not wellknown Volcanic and plutonic rocks of this age generally occur along a north-south trending beltthat runs through northwestern Neuquén (Fig 2a) Unlike younger magmatic rocks, they occuronly in the western Neuquén basin Radiometric ages support dividing them into: (1) UpperCretaceous, (2) Paleocene, and (3) latest Paleocene to Eocene groups All intrude or overlie

deformed Mesozoic strata, but are not themselves significantly deformed (Llambías et al., 1978;Llambías and Rapela, 1989; Franchini et al., 2003).

Upper Cretaceous to Paleocene magmatism is summarized by Franchini et al (2003).Upper Cretaceous activity is confirmed by K/Ar ages of 74.2±1.4 Ma for a biotite from anandesite dike in the Campana Mahuída region (Sillitoe, 1977), a whole rock K/Ar age of 71.5±5

Ma for an amphibole-bearing andesite sill cutting the Pelán Unit in the Cerro Nevazón region onthe east side of the Cordillera del Viento (Llambías et al., 1978; Linares and González, 1990),whole rock K/Ar ages from 69±4 to 65±3 Ma for tuffs and veins between Andacollo andHuinganco (Vilas and Valencio, 1978), and a whole rock age of 67±3.2 Ma for a tonalite stocknear El Maitenes in the southern Cordillera del Viento (Domínguez et al., 1984) Franchini et al.(2003) also refer to an unpublished age of 64.7±3.2 Ma for a pluton in the Cordillera del Viento

A new 40Ar/39Ar biotite age of 69.09±0.13 Ma from a granodiorite pluton near Varvarcó in thewestern Cordillera del Viento in Table 2 is interpreted as a cooling age Paleocene magmatism isconfirmed by hornblende K/Ar ages of 59.1 ±2.9 Ma and 56.5±1.7 Ma from a gabbro and adiorite in the Cerro Nevazón region and of 60.7±1.9 Ma from a diorite in the Campana Mahuídaregion (Franchini et al., 2003) Franchini et al (2003) further show that all of these magmaticrocks have chemical signatures similar to those of Holocene SVZ arc volcanic rocks (see Figs 5

to 8) The chemical analyses for the Varvarcó granodiorite in Table 3 shows a similar REEpattern (La/Yb = 6.6) and arc-like La/Ta (28) and Ba/La (31) ratios

Latest Paleocene and Eocene events produced most of the magmatic rocks mapped as theSerie Andesitic and Molle Formations by Groeber (1946) and reassigned to a volcanic CayantaFormation by Rapela and Llambías (1985) and a plutonic Collipilli Formation by Llambías and

Cayanta Formation is dominantly composed of the extensive breccias flows and necks on thewest side of the Cordillera del Viento (Figs 2a and 3) Their age is confirmed by 40Ar/39Arhornblende ages of 56.0±0.6 Ma and 50.3±0.6 Ma on two flows (Jordan et al., 2001) and wholerock K/Ar ages of 54.2 ±2.7 Ma on an aplitic stock and 46.1 ±2.3 Ma on an andesitic dike(Rovere, 1998) The plutonic Collipilli Formation included most of the hornblende-bearingplutonic rocks on the east side of the Cordillera del Indio and was assigned an Eocene age based

on K/Ar dates of 49.9±3.2 Ma for the Las Mellizas laccolith in the Collipilli region, 48.4±2.4 Ma

at Cerro del Diablo, and 44.7±2.2 Ma at Cerro Caicayén (Llambías and Rapela, 989) Cobboldand Rossello (2003) report a whole rock 40Ar/39Ar age of 39.7±0.2 Ma on a sill at Cerro Mayal.Just north of the Rio Barrancas in Mendoza, Linares and González (1990) report a K/Ar age of50±5 Ma for hornblende-bearing volcanic rocks in the Cerro Bayo de la Esperanza complex Toavoid confusion with new ages that show that some of the magmatic rocks included in theCollipilli Formation in the Collipilli region are Cretaceous in age (Zamora Valcarce et al., thisvolume); the magmatic rocks east of the Cordillera del Viento are here informally called theCaicayén group

New chemical analyses of Cayanta Formation, Cerro Bayo de la Esperanza region, andCerro Mayal region samples listed in Table 3 supplement those from Paleogene samples inRapela and Llambías (1985), Llambías and Rapela (1989) and Franchini et al (2003) As seen inFigures 5 to 8, all of these samples have arc-like features indicated by relative HFSE depletion(La/Ta > 28; Ta/Hf < 0.15) and fluid mobile element enrichment (Ba/La > 20) In detail, thereare differences among them

The Cayanta Formation samples west of the Cordillera del Viento are generally similar toHolocene SVZ arc samples In both cases, basaltic to mafic andesitic samples have high Al and

low Ti contents, arc-like La/Ta (40-64), Ba/La (21-27), and Ta/Hf (0.07 to 0.12) ratios, andrelatively flat REE patterns (La/Yb = 4-7, La/Sm = 3.3-4.1, Sm/Yb =1.5-1.9) The initial

87Sr/86Sr and 143Nd/144Nd ratios of a Cayanta basaltic andesite (Table 7) are near those of the SVZlavas (Fig 9) The Cayanta Formation flows do differ from the SVZ lavas in typically havingamphibole phenocrysts

In contrast, Cerro Bayo de La Esperanza and Cerro Caicayén region samples are more likeearly Paleocene samples in that they have higher alkali contents, higher La/Ta, Ba/Ta, La/Yb,La/Sm, and Sm/Yb ratios, and lower Ta/Hf ratios than Cayanta samples (Figs 5 to 8) MaficBayo de la Esperanza region (48-57% SiO2) are particularly notable for their high Na2O (4.4-6.0%), Sr (604-1402 ppm) and Ba (to 1835 ppm) contents and La/Ta (64-72), La/Sm (up to 6.7)and Sm/Yb (2.9-4.5) ratios Cerro Caicayén quartz diorites (59-62% SiO2) also have high La/Ta*(55-100 where Ta* = Nb/16), Ba/Ta* (most > 1400), La/Yb (> 9) and La/Sm (up to 14) ratios,but differ in having lower Sm/Yb ratios (<2.3) (Franchini et al., 2003)

Cerro Mayal region samples have alkali contents and La/Ta (29-50), Th/Hf (1.3-1.6),La/Yb (5-9), La/Sm (2.6-4.3), and Sm/Yb (1.6-2.2) ratios more similar to the Cayanta samples,but differ in having higher Ta/Hf ratios Chemical differences between Cerro Caicayén andCerro Mayal samples could be temporal given the 39.7±0.2 Ma age on the Cerro Mayal sill(Cobbold and Rossello, 2003)

Miocene Magmatic Rocks West of the Cordillera del Viento.

Latest Oligocene to Miocene arc and backarc magmatism in the Neuquén basin began after

a regional magmatic hiatus that lasted from ~ 39 Ma to ~ 26 Ma The oldest magmatic rocks ofthis age in the arc region are in the Cura Mallín Formation that crops out between the Holocene

SVZ arc front and the Cordillera del Viento (Fig 2a and 3) Most of these magmatic rocks are insequences of reworked silicic pyroclastic deposits that locally reach thicknesses of ~ 3 km(Suarez and Emparan, 1985; Burns et al., this volume) These sequences include rare basalticandesitic lavas and are locally cut by mafic dikes and granitoid intrusives Jordan et al (2001)reported hornblende 40Ar/39Ar ages of 24.6 ±1.8 Ma for a basaltic dike in the lower part of theCura Mallín Formation and of 22.8±0.7 Ma for an ash flow tuff near the top Burns et al (thisvolume) report a zircon fission track age of 26.3±1.5 Ma for a granitoid cutting the pyroclasticsequence.

The top of the Cura Mallín Formation is generally taken at a change from predominantlysedimentary rocks to volcanic tuffs, agglomerates and lava flows of Miocene age The volcanicrocks and related dikes, sills and granitoid intrusives in Chile are mapped in the Trapa TrapaFormation (Niemeyer and Munoz, 1983) Their ages are constrained by K/Ar ages that rangefrom 19.7±1.4 Ma to ~ 12 Ma (Niemeyer and Muñoz, 1983; Muñoz and Niemeyer, 1984; Suárezand Emparan, 1995) Temporally equivalent volcanic rocks in Argentina are assigned to theTrapa Trapa, Cajón Negra, Quebrada Honda, and Pichi Neuquén Formations (see Burns et al.,this volume; Figs 2a and 3) Those in the Trapa Trapa Formation generally occur near theChilean border Their ages are constrained by whole rock K/Ar ages of 18.5±0.2 Ma on abasaltic flow, of 12.6±0.2 Ma on an andesite dike, and of 12.3±0.2, 12.1±0.6, and 10.8±0.1 Ma

on granodiorite and aplitic intrusives (Rovere, 1998)

Volcanic rocks assigned to the Cajón Negro Formation, Quebrada Honda Formation, andPichi Neuquén Volcanic Complex (Pesce, 1981) crop out north of 37°S and east of the CuraMallín and Trapa Trapa Formation (Fig 3) They occur north of the westward projection of theCortaderas lineament (Fig 2a) The Cajón Negro Formation dominantly consists of mafic

andesitic to dacitic flows and agglomerates in the west To the east are pyroclastic units thatinclude surge deposits, ash falls, tuffs characterized by large pumice fragments, and columnarjointed ignimbrite flows in various stages of welding Jordan et al (2001) report a hornblende

40Ar/39Ar age of 16.2±0.2 Ma from one of the well-preserved silicic tuffs The composition andpreservation of the dated tuff raise the question if the hornblende could be a xenocryst Other

40Ar/39Ar ages from the Cajón Negro Formation are a biotite age of 11.7±0.3 Ma and ahornblende age of 10.8±1.6 Ma (Burns et al., this volume) The overlying Quebrada HondaFormation dominantly consists of massive gray olivine and pyroxene-bearing columnar jointedandesitic flows The Pichi Neuquén Complex includes andesitic to dacitic lavas, dikes, necks andstocks Jordan et al (2001) report a hornblende 40Ar/39Ar age of 9±2 Ma for a basal flow.Similarities in style and preservation with volcanic units to the south suggest that part of thePichi Neuquén complex could be Pliocene in age as argued by Pesce (1981)

Chemical analyses of Cura Mallín, Trapa Trapa, Cajón Negro, Quebrada Honda, and PichiNeuquén volcanic rocks and related plutons are listed in Tables 4 and 5 and plotted in Figures 5

to 8 Overall, these samples are similar to the Eocene Cayanta samples Among their chemicalcharacteristics are arc-like La/Ta (most 35-55), Ba/La (15-35, most 17-24), and Ta/Hf (most 0.9-0.11) ratios The Cura Mallín basaltic samples are distinctive in having higher Ta/Hf ratios (0.14-0.18, Table 4)

Miocene Magmatic Rocks East of the Cordillera del Viento

Miocene magmatic rocks are also abundant east of the Cordillera del Viento (Table 1; Fig.2a) Between 36.5°S and 38°S, the volcanic rocks in the backarc include early Miocene alkalibasaltic flows extending from the mid to the far backarc, subsequent early Miocene volcanic

centers in the mid-backarc, Miocene volcanic rocks near the east side of the Cordillera delViento, and the latest Miocene Chachahuén volcanic complex in the far backarc.

The oldest of these magmatic rocks are alkali olivine basalts erupted from mono andpolygenetic centers in the Sierra de Huantraico (e.g., Ramos and Barbieri, 1988) and Sierras deChachahuén/Matancilla (e.g., González Díaz, 1979) regions Kay and Copeland (this volume)showed that these basalts, which were mapped by previous workers in the Palaoco Formation,have ages from ~ 24 to 20 Ma and are contemporaneous with the Cura Mallín Formation in thearc Chemical and isotopic data for these basalts from Kay and Copeland (this volume) areplotted in Figures 5 to 9 As discussed by these authors, their chemical characteristics are those

of intraplate basalts generated by partial melting of an isotopically enriched garnet-bearingmantle (La/Yb ~ 13-30, Sm/Yb >3.5; єNd = +3.6 to +4.2; 87Sr/88Sr =0.7037 to 0.7040) virtuallydevoid of subducted components (La/Ta < 14; Ba/La <16, Ta/Hf >0.45) The least arc-likesignatures are found in the Sierra de Chachahuén/Matancilla region magmas that erupted farthestfrom the arc (La/Ta <12; Fig 6) No evidence for contractional deformational structures has beenreported for this period Outcrop scale extensional faults are present in the basalts in the Sierra deChachahuén (Ragona, personal communication, 1999; Kay, 2001b)

The >20 Ma alkali basalts are succeeded by ca.19-18 and 16-15 basaltic to hornblendebearing mafic andesitic to trachydacitic magmas erupted from volcanic complexes (circles inFig 2a) in the Sierras de Huantraico/Negra and in southern Mendoza (Nullo et al., 2002;Cobbold and Rossello, 2003; Kay and Copeland, this volume) As shown in Figures 5 to 9, thechemistry of these younger magmas differs in that they have weak arc-like signatures (La/Ta =15-26; Ba/La = 15-32; Ta/Hf = 0.2-0.45), flatter REE patterns (Sm/Yb < 3, La/Yb = 5-25) andless enriched isotopic signatures (єNd = +3.9 to +4.7; 87Sr/88Sr = 0.7033 to 0.7037; 206Pb/204Pb =

18.51 to 18.55) Kay and Copeland (this volume) used these characteristics to argue that theseyounger magmas were generated in a mantle containing subducted components They furtherargued that these magmas erupted in a contractional setting Evidence for a contractional regime

in the Sierra de Huantraico comes from seismic lines that show growth strata associated withfolded sequences (Viñes 1990; see Cobbold and Rossello, 2003) that contain early Miocenevolcanic rocks

The next magmas that erupted east of the Cordillera del Viento in Neuquén are theandesites at Cerro Negro (Fig 2a) A hornblende at that locality yielded the new 40Ar/39Ar age of11.7±0.2 Ma in Table 2 Chemical analyses in Table 3 show that these flows are medium-Kandesites (60% SiO2) with arc-like La/Ta (37-41), Ba/La (34 to 35), and Ta/Hf (0.10) ratios andrelatively flat REE patterns (La/Yb = 4.4 to 6.4, La/Sm = 3 to 4; Sm/Yb = 1.4-1.6) As seen inFigures 5 to 8, their chemistry is similar to that of Eocene Cerro Mayal lavas to the south andcontemporaneous Trapa Trapa andesites erupted in the arc to the west The initial 87Sr/86Sr ratioand єNd of a Cerro Negro andesite is also in the same general range as those of Eocene toHolocene arc lavas to the west (Fig 9)

The eruptions of the Miocene Trapa Trapa arc volcanic rocks and the Cerro Negro backarclavas overlap in time with those of the Farellones Formation/Teniente Complex in the arc to thenorth (e.g., Kay et al., 2005) and the Huincán Formation in the backarc in Mendoza (Fig 2b;Nullo et al., 2002) 40Ar/39Ar ages reported by Baldauf (1997) for the Huincán Formationbetween 34°S and 35°S are 13.57±0.9 Ma and 13.94±0.08 Ma for their Huincán I group samplesand 10.42±0.05 Ma and five between 7.49 and 5.3 Ma for their Huincán II group samples.Analyses in Baldauf (1997) and Nullo et al (2002) show that the chemical character of Huincánandesites changes with time As shown in Figures 5 to 8, Huincán II andesites have more arc-like

La/Ta (41-61), Ba/La (35-46), and Ta/Hf (0.09-0.13) ratios and steeper REE patterns (La/Yb =13-17; La/Sm = 4.9-6.5; Sm/Yb = 2.1-2.3) than Huincán I andesite (La/Ta = 34-37; Ba/La = 19-

24, Ta/Hf = 0.13-0.17; La/Yb = 9.5-14.5; La/Sm = 4.3-6.4; Sm/Yb = 1.8-3.1) All have steeperREE patterns (higher La/Yb; Fig 8), than the Cerro Negro andesites to the south

The easternmost Miocene eruptions in the Neuquén basin occurred at the Chachahuénvolcanic complex in the Sierra de Chachahuén (Fig 2b); some 500 km east of the modern trench(Fig 1) As described by Kay et al (this volume, Chapter 10), the Chachahuén complex consists

of the ~7.3 to 6.9 Ma Vizcachas group orthopyroxene-bearing dacites and rhyolites and the ~ 6.8

to 4.9 Ma high-K Chachahuén group basalts, hornblende-bearing andesites, and dacites All ofthese magmas erupted from a nested caldera complex located at the intersection of a NW-SE and

a NE-SW trending fault system These faults are among those inverted in the late Miocenecontractional deformation that uplifted the Sierra de Chachahuén (Pérez and Condat, 1996; Kay

et al., this volume, Chapter 10) Chemical and isotopic analyses of the Chachahuén complexrocks presented by Kay et al (this volume, Chapter 10) are summarized in Figures 5 to 9 Thesemagmatic rocks can be distinguished from those erupted elsewhere in the Neuquén basin by acombination of high alkali contents (Fig 5; particularly K), variable La/Ta (10-55; Fig 6), highTa/Hf ratios (>1.8, Fig 7), variable La/Yb, La/Sm and Sm/Yb ratios (Fig 8), low 87Sr/86Sr ratios

at a given εNd (Fig 9), and high 206Pb/204Pb ratios (Fig 10) Kay et al (this volume, Chapter 10)attribute the intraplate-like character of the older Vizcachas lavas (La/Ta <22; Ta/Hf = 0.4-0.7)

to contamination of mantle-generated magmas by intraplate-like crustal melts and the arc-likecharacter of the younger Chachahuén lavas (La/Ta = 32-55; Ta/Hf <0.3) to melting in the mantlewedge above a subducting slab The high La/Yb >18, La/Sm ~ 6 - 11, Sm/Yb >3 of theVizcachas lavas are attributed to removal of accessory REE-bearing phases (e.g., titanite) whose

importance was diminished in the Chachahuén lavas (La/Yb = 7-15; La/Sm ~2.5-7; Sm/Yb 3.2) The Chachahuén volcanic complex is considered contemporaneous to the petrologicallysimilar Plateado/Nevado center to the north (Fig 2b; Bermudez, 1991; Bermudez et al., 1993).

~1.8-Pliocene to Holocene Magmatic Rocks West of the Cordillera del Viento

Pliocene to Pleistocene volcanic rocks are widespread between the Holocene SVZ arc andthe western side of the Cordillera del Viento (Fig 2b and 3) Early Pliocene volcanic rocks thatform the base and surrounding region of the Pleistocene to Holocene arc are assigned to the Cola

de Zola Formation whose age range is ca 5.6 to 3 Ma (Vergara and Muñoz, 1982) Pliocenebasaltic to dacitic rocks to the east in Argentina between 37°S and 37.5°S are assigned to theCentinela Formation whose K/Ar ages range from 3.2±0.2 to 2.6±0.1 Ma (Rovere, 1998).Further north, a similar age range seems reasonable for the younger part of the Pichi NeuquénComplex The Pliocene Centinela Formation flows near the western side of the Cordillera delViento are followed by the Pleistocene olivine basalt flows of the Guañacos Formation whose K/

Ar ages range from 1.4±0.2 to1.2±0.1 Ma (Rovere, 1998) The Cerro Colorado olivine basaltflow mapped on the west side of the Cordillera del Viento to the north by Pesce (1981) is likely

to be of this age The characteristics of other similar late Miocene to Quaternary volcanic rocksnorthwest, west and southwest of the Cordillera del Viento are summarized by Miranda et al.(this volume), Lara and Folguera (this volume), and Folguera et al (this volume, Chapter 12).Holocene SVZ centers are discussed by Hildreth and Moorbath (1988), Tormey et al (1991),Dungan et al (2001) and references therein SVZ rocks from the Copahue volcano near 38°S areaddressed by Varencamp et al (this volume)

The chemistry of representative samples from the Pichi Neuquén, Centinela and GuañacosFormations are presented in Table 5 and plotted in Figures 5 to 8 These analyses along withthose in Rovere (1998) show that these Pliocene to Pleistocene centers have arc-likecharacteristics similar to those in the Eocene Cayanta Formation, Miocene Cura Mallín andTrapa Trapa Formations, and younger SVZ centers (Planchon, Cerro Azul, San Pedro, Antuco,Llaima, Villarrica and Puyehue; see Tormey et al., 1991) A new analyses of an Antuco basalt(51% SiO2) in Table 5 with La/Ta = 47, Th/Hf = 0.679, Ta/Hf = 0.09, Ba/La =23, La/Yb = 5.0,and Sm/Yb = 2.7 is representative of the SVZ samples

Pliocene to Holocene Magmatic Rocks East of the Cordillera del Viento

Pliocene to Holocene arc magmas are contemporaneous with voluminous backarc magmaserupted east of the Cordillera del Viento (Table 1; Fig 2b) Between 36°S and 38°S, theseeruptions produced the small mafic Pliocene flows west of the Auca Mahuída and Payún Matrúfields, the Pliocene to Holocene mafic to silicic volcanic rocks of the Tromen region (Fig 4), andthe voluminous latest Pliocene to Pleistocene alkaline basalt flows in the Auca Mahuída, PayúnMatrú, and Llancanelo fields

Early Pliocene backarc alkaline flows The oldest post-Miocene alkaline volcanic rocks

in the transect (Fig 2b) include the flows west of the Payún Matrú field (González Díaz, 1979)and the Parva Negra and Horqueta Norte flows west of the Auca Mahuída field (Ramos andBarbieri, 1988) González Díaz (1979) argued for a Pliocene age for the flows west of the PayúnMatrú field on the basis of geomorphology and two whole rock K/Ar ages of 8±4 Ma and 4±1

Ma Ramos and Barbieri (1998) reported a K/Ar whole rock age of 4.5±0.5Ma for the ParvaNegra flow Chemical analyses for the Parva Negra and Horqueta Norte flows are presented in

Table 6 and plotted in Figures 5 to 8 These flows are olivine alkaline basalts (46-48% SiO2)with intraplate-like low La/Ta (14-16) and Ta/Hf (0.36-0.43) ratios and Ba/Ta (192 to 570) andBa/La ratios (14 to 37) that show a variable arc to intraplate signature.

The near to mid backarc – Tromen Region The distribution of post-Miocene volcanic units in

the Tromen region is shown on the maps in Figures 2b and 4 The more detailed map in Figure 4

is based on the Chos Malal and Buta Ranquil geologic maps of Zollner and Amos (1973) and Holmberg (1976) and uses the volcanic stratigraphy of Groeber (1946) New 40Ar/39Ar ages in Table 2 and locations of samples with analyses in Table 6 are shown on Figure 4 The magmatic sequences on Figure 4 are divided into three groups in the discussion below: (1) an older

andesitic and rhyolitic group that includes the Coyocho (Basalto II) and Tilhué (Andesite III) Formations, (2) an intermediate age basaltic to mafic andesitic group that includes the Chapúa (Basalto III), Maipo (Basalto IV), and El Puente (Basalto V) Formations and is here designated the Chos Malal group, and (3) a younger, mostly andesitic Tromen Formation group that

includes Basaltos VI and VIII

The oldest group incorporates the eroded Coyocho Formation mafic flows west of Cerro Waile and Cerro Tilhué, east of Cerro Michico and in the Cerro Bayo region They are mapped

as being older than the Tilhué Formation which includes the hydrothermally altered rhyolite complex in the Cerro Bayo region, rhyolites on the south side of Cerro Tromen, the well

preserved biotite-bearing rhyolitic (75-76% SiO2) ignimbrite-dome complex and pyroclastic flows at Cerro Tilhué, and the tuffs west of Cerro Tromen (Tilhué Fm on map) The Cerro Bayo,Cerro Tromen and Cerro Tilhué complexes could be aligned along a northeast to southwest

trending fault zone Biotite in a Cerro Bayo rhyolite yielded the new biotite 40Ar/39Ar age of 4.04±0.4 Ma in Table 2

Samples analyzed from the Coyocho mafic flows have basaltic andesitic to andesitic compositions and are characterized by arc-like Ba/La ratios (up to 35), La/Ta (24-34), and Ta/Hf (0.17-0.22) ratios, relatively flat REE patterns (La/Yb = 10-16) and moderate Eu negative anomalies (0.62-0.90) Samples from Cerro Tilhué and Cerro Bayo and a pumice clast from a tuff northeast of the Chapúa School are rhyolites with 75-76% SiO2, 4.3-4.9% Na20, and 4.0-4.7% K2O Their trace elements show relatively flat REE patterns (La/Yb = 11-16), large

negative Eu anomalies (Eu/Eu* is 0.2 in Cerro Tilhué to 0.6 in Cerro Bayo), arc-like Ba/La 29) ratios and intraplate La/Ta (12-17) and Ta/Hf (0.37-0.44) ratios The pumice is

(18-compositionally most like the Cerro Tilhué rhyolite 87Sr/86Sr ratios (0.7041-0.7042), єNd (+2.1 to+2.7) and Pb isotopic ratios in the Cerro Tilhué and Cerro Bayo rhyolites overlap those of

younger Tromen region and SVZ magmas (Figs 9 and 10)

The intermediate age Chos Malal flows are concentrated in the depression between Cerro Tromen and the Cordillera del Viento that Zapata et al (1999) designated the Chos Malal trough.Published K/Ar ages for these flows include duplicate ages of 2.3±0.5 and 2.1±0.5 Ma for a flownorth of the Chapúa School and an age of 1±0.2 Ma for a flow near Tricao Malal (Valencio et al.,1970) New 40Ar/39Ar ages in Table 2 are 1.44±0.08 Ma for the groundmass of a flow near the Chapúa School and 1.04±0.06 Ma for the groundmass of a silicic andesite from Cerro Waile (Fig4)

As shown in Table 6, most Chos Malal flows have basaltic to basaltic andesite

compositions (50-55% SiO2) Some are characterized by large plagioclase phenocrysts A sample from Cerro Waile is a silicic andesite Overall, they are characterized by relatively flat

REE patterns (La/Yb 7-16) with minimal HREE depletion (Sm/Yb < 3), moderate to small negative Eu anomalies (0.7-0.9), and arc-like La/Ta (27-34), Ba/La (17-24), and Ta/Hf ratios (0.15 to 0.24) ratios As shown in Figure 9 and Table 7, Chos Malal mafic flows are slightly lessisotopically enriched (87Sr/86Sr = 0.7038 to 0.7040; єNd near +3.2) than the Cerro Waile andesite (87Sr/86Sr = 0.7044; єNd = +2).

The last group includes the youngest flows from Cerro Tromen and the surrounding region.New analyses in Table 6 and plotted in Figures 5 to 8 along with those in Llambías et al (1982)and Stern et al (1990) show that these flows are primarily andesitic in composition Duplicate

40Ar/39Ar groundmass determinations presented in Table 2 yield an average age of 0.175±0.025

Ma for the young “escorial” flow Chemically, Cerro Tromen andesites in Table 6 have ~ 60%SiO2, 2.5-3% K2O, 1% TiO2, and FeO/MgO ratios ~ 2 Compared to the Cerro Waile ChosMalal trough andesite, they have higher K2O contents, generally less arc-like La/Ta (17-23), Ba/

Ta (327-550) and Ta/Hf ratios (0.19-0.2), and higher La/Sm and Sm/Yb ratios Andesitic blocks

in the Agua Carmonina Formation deposits east of Cerro Tromen (Fig 3) are chemically similar

A basalt (7% MgO, 206 ppm Cr) from south of Cerro Michico (Fig 3) has distinctly like La/Ta (20), Ba/La (19), and Ta/Hf (0.20) ratios and a flat REE pattern (La/Yb = 6.0, Sm/Yb

intraplate-= 2) The 87Sr/86Sr ratio (0.7039), єNd (+3.4), and Pb isotopic ratios of the “escorial flow” (Table

7, Figs 9 and 10) overlap those of SVZ flows to the west

Auca Mahuída and southern Payún Matrú fields Further east are the extensive late

Pliocene to Pleistocene Auca Mahuída, Payún Matrú and Llancanelo alkaline volcanic fields(Fig 2b) These fields are composed of basaltic to hawaiitic shield volcanoes, monogenetic topolygenetic cones, and differentiated mugearite to trachyandesite flows, domes and pyroclasticrocks that are concentrated in the high standing regions The general characteristics of these

fields are summarized by Bermúdez et al (1993) and Muñoz et al (1989) Only the southernPayún Matrú and Auca Mahuída fields are discussed here

The general distribution of centers in the southern Payún Matrú region is shown on maps

by Holmberg (1964), González Díaz (1972; 1979), and Pérez and Condat (1996) The regionincludes extensive flows in and north of the Sierra de Chachahuén region as well as the conesand flows near the Río Colorado (Fig 4) Dated flows have largely yielded Pleistocene ages.Among these are whole rock K/Ar ages of 1.1±0.5 Ma and 1.0±0.4 Ma for Sierra de Chachahuénflows west of Cerro Ureta (37°2’ 68°56’) and north of Cerro Ratón (37°2.11’ 68°51’) in Pérezand Condat (1996) Pérez and Condat (1996) report a K/Ar age of 2.26±0.07 Ma for a flow fromCerro Tanque in the northern Sierra de Chachahuén A new 40Ar/39Ar groundmass age of1.48±0.14 Ma for a flow from Cerro Méndez north of the Río Colorado is in Table 2

The distribution of centers in the Auca Mahuída field is shown on maps by Holmberg(1964), Ardolino et al (1996), and Delpino and Bermúdez (personal communication) 40Ar/39Arages in Rossello et al (2002) that range from 1.7±0.2 Ma to 0.88±0.03 Ma and new 40Ar/39Arages in Table 2 show that these rocks erupted in the last 2 Ma in accord with the generalPleistocene age assignments in Holmberg (1964) and Uliana (1979) The groundmass 40Ar/39Ar

in Table 2 referred to the mapping units of Ardolino et al (1996) are: 1.78±0.1 Ma for a Pampa

de las Yeguas basalt, 1.55±0.07 Ma for a Cerro Las Liebres basalt, 1.39±0.14 Ma for a CerroGrande basalt, and 0.99 ±0.04 Ma for a Cerro Auca Mahuída mugearite The 40Ar/39Ar ages are

in accord with the relative volcanic sequence proposed by Ardolino et al (1996), but not their lateMiocene to Pleistocene age assignments

The chemistry of southern Payún Matrú and Auca Mahuída field samples analyzed by Kay(2001b) are plotted on Figures 5 to 9 As in other analyses from this region (e.g Delpino and

Bermúdez, 1985; Bermúdez and Delpino, 1989; Muñoz and Stern, 1988, 1989; Muñoz et al,1989; Stern et al., 1990; Bermúdez et al., 1993; Saal et al., 1993, Saal and Frey, 1994; Saal,1994), the southern Payún Matrú and Auca Mahuída volcanic rocks are olivine-bearing alkalibasalts, hawaiites, benmorites, trachyandesites and trachytes (Fig.5) characterized by intraplateLa/Ta (11-16) and Ta/Hf (0.38-0.52) ratios and transitional arc-like Ba/La ratios (most 16 to 25).La/Yb ratios range from 8 to 13 in the basalts and from 12 to 18 in more differentiated silicicsamples The higher ratios reflect light REE enrichment in the more silicic flows; all Sm/Ybratios are < 3.5 Overall the samples show little isotopic variation with 87Sr/86Sr ratios rangingfrom 0.70373 to 0.70388, єNd from +2.3 to +3.9, and 206Pb/204Pb from 18.40 to 18.52 (Figs 9 and10).

DISCUSSION: TECTONIC AND MAGMATIC SYNTHESIS

The distribution and chemistry of the magmatic rocks of the Neuquén basin provideinsights on the ages of contractional and extensional deformation across the arc and backarc, theevolution of the underlying crust and mantle, and the geometry of the subducting oceanic plate.The evolution of the Neuquén basin transect inferred from these data is summarized in cartoonlithospheric cross sections in Figure 11 The cartoons are drawn to scale where possible Severalfactors are important in constructing and interpreting these sections One is changes ingeochemical signals that reflect changes in the mantle and crustal source regions of the magmaticrocks and their implications for the geometry of the underlying continental lithosphere andsubducting oceanic plate Another is constraints from the timing and style of deformation, which

are discussed below in terms of the magmatic rocks, and relative to other criteria as summarized

in papers by Ramos and Kay (this volume) and Mosquera and Ramos (this volume)

Latest Cretaceous to Paleocene Magmatic and Tectonic History of the Neuquén Basin

The characteristics of the Upper Cretaceous to Eocene magmatic rocks in the Neuquénbasin are consistent with their eruption over a relatively steeply dipping subducting plate like thatshown in Figure 11 Such subduction geometry is predicated on Upper Cretaceous to Eocenemagmatic rocks between 36°S and 38°S being confined to the western part of the Neuquén basin(Fig 2a), the frontal arc-type chemical signatures of the magmatic rocks, and the concentration

of concurrent deformation near the frontal arc The distribution of the magmatic rocks is inaccord with the arc front having been east of the Miocene to Holocene arc front The chemistry

of the magmatic rocks is generally similar to that of the Miocene to Holocene arc magmas asshown by similar high La/Ta, Ba/Ta, and Ba/La ratios and low Ta/Hf ratios (Figs 6 and 7).Generally, similar La/Yb, La/Sm, and Sm/Yb ratios (Fig 8) fit with all of these magmas havingequilibrated with a similar residual mineral assemblage at a similar temperature and pressure.This commonality suggests that to a first order the subarc geometry and magma source regionwas not very different from that today A lack of backarc magmatism and deformation isconsistent with no influence from a subducting slab in the backarc

On the next level, the magmatic and tectonic evolution during this period is not wellunderstood Available ages indicate long gaps in magmatism, but the distribution and age pattern

of the magmatic rocks is only known to a first order Another factor is that convergencedirections between the South American and oceanic plate were oblique for much of this time

Convergence models based on oceanic plates are unconstrained before 69 Ma, show extremeobliquity between 69 and 49 Ma (Pardo Casas and Molnar, 1987), and show lower, but still highdegrees of obliquity (> 35° in Somoza, 1998) from 49 to 26 Ma The convergence data permit amore normal convergence regime in the Upper Cretaceous before 69 Ma The distribution ofmagmatic rocks is consistent with the arc front having been west of or on the western margin ofthe Cordillera del Viento from ca 70 to 50 Ma, and then shifting to the eastern side at the time ofmajor change in convergence obliquity at ca 49 Ma The magmatic record and regionalstratigraphic indicate that the principal period of pre-Miocene contractional deformation in theNeuquén basin was in the Upper Cretaceous Evidence for a major Eocene contractionaldeformation suggested by Cobbold et al (1999) is less clear The timing of major periods ofdeformation is discussed from a magmatic and tectonic viewpoint below.

Latest Upper Cretaceous (Campanian-Maastrichtian).

Support for Upper Cretaceous deformation of Lower Cretaceous and older strata in the

western Neuquén basin and Upper Cretaceous uplift of the Cordillera del Viento comes frommagmatic and stratigraphic evidence Stratigraphic evidence is based on Upper Cretaceous toPaleogene Neuquén and Malargüe Group strata being deposited in a foreland basin east of theCordillera del Viento (Kozlowski et al., 1987; Barrio, 1990) Magmatic evidence comes fromdeformed Neuquén basin strata being cut or overlain by Upper Cretaceous to Eocene magmaticrocks as old as ~ 74 Ma (see Franchini et al, 2003) A lack of discordance between UpperCretaceous Neuquén Group and latest Upper Cretaceous/earliest Paleocene Malargüe Groupstrata to the east shows that deformation was restricted to the general region of the arc in thewestern Neuquén basin (Kozlowski et al., 1987)

Support for deformation in the latest Upper Cretaceous comes from the biotite 40Ar/39Ar cooling age of 69.09 ± 0.13 Ma from the Varvarcó leucogranodiorite in the western Cordillera del Viento (Table 2) As unrealistically high geothermal gradients are needed to keep biotite from cooling below 300°C at depths of less than ~ 3 km (see Kurtz et al., 1997), the present outcrop must have been at 3 km depth or less at 69 Ma The relatively course-grain size (up to ½mm) and the amphibole-bearing mineral assemblage of the Varvarcó pluton are consistent with

an initial emplacement depth of at least 5-6 km The case for an Upper Cretaceous emplacement age for the pluton comes from ages of similar magmatic rocks in the Neuquén basin (see

Franchini et al., 2003 and above) and from the Lo Valle volcanic rocks to the north in Chile (40Ar/39Ar ages of 70 - 69 Ma; Gana and Wall, 1997) Given an Upper Cretaceous emplacement age for the pluton, 2 to 3 km of Upper Cretaceous uplift is needed in the Cordillera del Viento region Contractional deformational at this time fits with a more normal plate convergence regime and evidence for synchronous deformation along much of the Andean margin (see

Cobbold and Rossello, 2003)

Evidence that the Varvarcó pluton was near the surface by 56.50.6 Ma age comes from ages of the Cayanta volcanic rocks (Jordan et al., 2001) that lie unconformably on the western margin of the Cordillera del Viento A 5 to 6 km uplift of the pluton between the Upper

Cretaceous and Paleocene is consistent with the ~ 7000 meters of pre-Eocene uplift called upon

by Kozlowski et al (1996) to explain the discordance between the Carboniferous Andacollo Group and the Eocene lavas Further evidence for Upper Cretaceous deformation comes from fission track ages presented by Burns (2002)

Paleocene to Eocene

Ages of magmatic rocks in the Neuquén basin do not provide evidence for the major Eocene contractional deformational event postulated by Cobbold et al (1999) This event is postulated on: a) model ages and orientations of bitumen veins that cut deformed Cretaceous strata on the east side of the Cordillera del Viento, and b) ages of magmatic rocks in the Sierra deHuantraico (Cobbold and Rossello, 2003) The age for bitumen vein emplacement comes from thermal models that project that organically rich Mesozoic sedimentary source rocks reached thermal maturation in the Eocene Contemporaneous deformation is predicated on high fluid pressures related to hydrocarbon generation triggering motion on regional detachments A northwest-southeast orientation for both the veins and major regional contractional structures canthen be due to tensile failure perpendicular to the least compressive regional stress This

orientation is argued to be the one expected in the right-lateral Eocene transpressional regime predicted from plate convergence vectors (Pardo-Casas and Molnar, 1987) The lack of

contemporaneous foreland basin strata is explained by a transpressional regime Problems for this model are that deformed Neuquén basin strata are cut by Upper Cretaceous magmatic rocks and that latest Paleocene-earliest Eocene Cayanta lavas unconformably overlie uplifted

Cordillera del Viento rocks The argument for Eocene deformation in the Sierra de Huantraico is complicated by the possibility that the Eocene magmatic rocks used to delimit the deformation age are actually early Miocene in age (Kay and Copeland, this volume)

While support for major deformation in the Eocene is unclear, evidence for changes in the magmatic and tectonic regime in the Neuquén basin as the convergence obliquity changed at ca

49 Ma come from the eastward displacement of the arc front This offset is marked by the cessation of Cayanta volcanism west of the Cordillera del Viento and the initiation of Caicayén group magmatism to the east The combination of high La/Ta and Sm/Yb ratios in the ca 50-45

Ma magmatic rocks from Caicayén and Bayo de Esperanza (Figs 6 and 8) are reminiscent of regionally high ratios in magmatic rocks associated with eastward offsets of the Miocene arc front west of the northern SVZ (Kay et al., 2005) These Miocene offsets are associated with periods of compressional deformation and it is reasonable to expect that some Eocene

deformation accompanied the eastward shift of the Eocene arc in the Neuquén basin Kay et al (2005) argue that high La/Ta and Sm/Yb ratios at the time of arc migration reflect adjustments inthe slab geometry and peaks in forearc subduction erosion (see Fig 11)

A Model for Shallowing and Steepening of the Nazca plate to Explain the Miocene to Holocene Magmatic and Deformational Characteristics of the Neuquén Basin

Many aspects of the Miocene to Holocene magmatic and structural evolution of the

Neuquén basin north of the Cortaderas lineament can be explained by shallowing followed by steepening of the subducting Nazca plate as argued by Kay (2001a, 2001b, and 2002)

Lithospheric scale cross sections illustrating the model in a transect near 37°S are shown in Figure 11b and discussed below They draw heavily from the model for shallowing of the subduction zone below the Pampean flatslab between 28°S and 33°S (e.g., Kay et al., 1991, 1999; Kay and Abbruzzi, 1996)

Early Miocene (Extensional Regime over a Steep Subduction Zone)

The lithospheric cross section at ca 24 to 20 Ma shows an active volcanic arc andwidespread backarc eruptions in an extensional tectonic regime over a relatively steeplysubducting slab The period began as the oceanic Farallón plate broke up and the near normal

subduction regime between South America and the Nazca plate that persists today emerged (seePardo Casas and Molnar, 1987; Somoza, 1998) Unlike some Oligocene volcanic rocks studied

by Munoz et al (2000) in the arc region near to the south, all of the magmas erupted in the arc Cura Mallín basin show a frontal arc character as indicated by high La/Ta, Ba/Ta, and Ba/Laratios (Fig 6) Higher Ta/Hf ratios relative to older and younger arc magmas in the region (Fig.7) indicate that the mantle wedge had a more intraplate-like character than before or after (Burns,2002) The largely bimodal basaltic/basaltic andesite and rhyolitic compositional range of theCura Mallín magmas (Fig 5) and their relatively flat REE patterns (low La/Yb and Sm/Ybratios; Fig 8) are in accord with their eruption through a thin crust in an extensional setting.Further discussion of the setting of the Cura Mallín intra-arc basin can be found in Jordan et al.(2001) and Burns et al (this volume)

intra-In the backarc, a steep subduction zone is consistent with the lack of a subduction-relatedgeochemical signature in the alkali olivine basalts that erupted as far as 550 km east of themodern trench Their intraplate/OIB-like chemical signatures (see Kay and Copeland, thisvolume) are shown by low La/Ta, Ba/Ta and Ba/La (Fig 6) and high Ta/Hf ratios that reach anextreme in the lavas erupted the furthest east of the arc (Fig 7) A non-contractional orextensional backarc regime fits with the widespread eruption of alkali olivine basalts The lack

of reported structural evidence for major extension in the backarc appears to suggest that thestress regime was only mildly extensional at best

Late Early Miocene (Change to a Contractional Regime and Initial Shallowing)

The lithospheric section at 19 to 16 Ma shows volcanism from the arc into the mid-backarcover a shallower subduction zone than that before 20 Ma Evidence for a change to a non-

extensional stress regime in the arc comes from the end of sedimentation in the Cura Mallín intra-arc basin (Burns et al., this volume) A change from a bimodal volcanic assemblage in the Cura Mallín Formation to a more restricted mafic andesitic composition range in the Trapa TrapaFormation (Figs 5 and 6) is in accord with a longer residence time for magmas in the crust A frontal arc setting for the Trapa Trapa lavas is consistent with high La/Ta, Ba/Ta and Ba/La ratios (Fig 6) Evidence for a change in the mantle source region comes from a return to typical arc Ta/Hf ratios (Fig 7) Similarities in La/Yb, La/Sm, and Sm/Yb ratios between Cura Mallín and Trapa Trapa mafic magmas show that no major pressure change occurred in residual mineralassemblages in equilibrium with the erupted magmas.

Evidence for a shallower subduction zone under the backarc comes from changes in the character of backarc magmatism This change is marked by the cessation of widespread alkali olivine basalt eruptions, and their replacement by mafic andesitic to trachydacitic magmas erupting from volcano/caldera complexes in the mid backarc Kay and Copeland (this volume) pointed to the higher La/Ta, Ba/Ta, and Ba/La (Fig 6) and lower Ta/Hf (Fig 7) ratios in these mid-backarc magmas to argue for introduction of a subducted component into the mantle source below the Sierra de Huantraico by 19 Ma Evidence that the magmas were more hydrous by 19

Ma comes from the presence of large clinopyroxene phenocrysts in the basalts and hornblende inthe mafic andesites The presence of water is in accord with subduction related fluids entering the mantle source The easiest way to explain a more hydrated mantle richer in arc-like

subducted components is for a shallower slab to have extended under the Sierra de Huantraico by

20 Ma The presence of clinopyroxene and amphibole phenocrysts require a period of magma evolution in the mid to lower crust as is expected if magma rise is slowed in a contractional stress regime Other evidence for a contractional stress regime is discussed above

Middle to Late Miocene (Continued Contraction and Shallowing).

The lithospheric section at 14 to 10 Ma shows a shallower subduction zone with volcanic centers in the arc and near backarc and contractional deformation in the backarc The most important changes are in the backarc Evidence for little magmatic change in the arc comes frommiddle to late Miocene (Trapa Trapa, Cajón Negro, and Quebrada Honda) magmas having the same general petrologic character, arc-related La/Ta, Ba/Ta, Ba/La and Ta/Th ratios (Figs 6 and 7), and REE ratios (Fig 8) as early Miocene Trapa Trapa magmas

Three general stages can be recognized in the magmatic history of the backarc The first ischaracterized by a virtual magmatic lull from 16 to 14 Ma, the second by andesitic eruptions from ~14 to 10 Ma, and the third by another lull from ~ 10.7 to 7 Ma The backarc magmas of this period have chemical signatures that are consistent with subducted components influencing amantle wedge above a subducting slab Among the lavas erupted are the ~12 Ma Cerro Negro hornblende andesites whose high La/Ta (> 30) and Ba/Ta (>1000) and low Ta/Hf ratios are markedly more arc-like than those of the early Miocene backarc lavas (Figs 6 and 7) Similarly, the Miocene Huincán magmas in southern Mendoza (Fig 2a) show clear trace element evidence for a subducted component (Baldauf, 1997)

Support for backarc contractional deformation in the late Miocene comes from chemical signatures in magmatic rocks that are interpreted to reflect relative crustal thickening The argument is based on REE patterns whose shapes are influenced by pressure sensitive mafic mineral assemblages that equilibrate with mantle-derived magmas in the crust (Hildreth and Moorbath, 1988; Kay et al., 1987 and 1991) The best depth-indicators are Sm/Yb and La/Yb ratios as they can reflect amphibole that is stable at intermediate pressure and garnet that is stable

at higher pressure Using this reasoning, similar Sm/Yb and La/Yb ratios in ~ 12 Ma Cerro Negro andesites and Eocene Cayanta andesites are consistent with little crustal thickening under the western Neuquén basin between 56 and 12 Ma.

The picture changes in the late Miocene as shown by La/Yb (Fig 7a) and Sm/Yb (Fig 7b)ratios in Miocene to Pliocene Neuquén basin magmatic rocks Comparisons show that these ratios are: a) lower in ~14 Ma Huincán I andesites than in ~ 10-6 Ma Huincán II andesites, b) lower in ~ 12 Ma Cerro Negro andesites than in < 4 Ma Tromen andesites, and c) lower in Cerro Negro andesites than in Huincán andesites Based on the logic above, Baldauf (1997) and Nullo

et al (2002) argued that higher ratios in Huincán II than Huincán I andesites reflected a crustal thickness increase in Mendoza in response to crustal shortening between ca.14 and 10 Ma In the same way, lower ratios in ~ 12 Ma Cerro Negro andesites than in < 4 Ma Tromen andesites can

be interpreted as reflecting crustal thickening in response to crustal shortening between 12 Ma and ~ 4 Ma in Neuquén Generally higher ratios in Mendoza than Neuquén andesites are

consistent with greater crustal thicknesses in Mendoza where crustal shortening estimates are higher (Zapata et al., 1999) Overall, the data support an episode of backarc crustal thickening between 12 and 10 Ma under the Neuquén basin

Latest Miocene (Maximum Development of Shallow Subduction Zone).

The lithospheric section at ca 8 to 5 Ma is drawn as the shallow subduction zone reached its maximum development under the Neuquén basin A key feature in this interpretation is the Chachahuén volcanic complex that erupted in the block faulted Sierra de Chachahuén, some 500

km east of the modern trench To the west, the status of ~8 to 5 Ma magmatic activity is poorly known Candidates for arc magmas at this time are volcanic rocks in the Pichi Neuquén

Complex on the west side of the Cordillera del Viento Analyses of Pichi Neuquén basaltic to basaltic andesites (Table 5, late Miocene in Figs 5 to 8) show that they have generally higher La/

Ta (> 56) and Ba/Ta (>1300) and lower Ta/Hf (< 0.9) ratios than the other Miocene arc magmas (compare with Trapa Trapa) as would be expected if they erupted above a dehydrating, shallowlydipping slab The position of the Pichi Neuquén Complex north of the westward projection of theCortaderas Lineament is consistent with a location west of a shallowly dipping slab Similarities

in REE ratios with other Miocene arc samples (Fig 8) indicate minimal pressure changes in residual mineral assemblages as expected if crustal thicknesses changed little in the arc region during the Miocene (Fig 8)

The best evidence for a late Miocene shallow subduction zone under the Neuquén basin comes from the K-rich 7.3 to 4.8 Ma Chachahuén volcanic complex in the far backarc (Kay et al., this volume, Chapter 10) The high La/Ta and Ba/Ta and low Ta/Hf ratios in the 6.8 to 4.8

Ma Chachahuén sequences indicate the presence of an arc-like component in their mantle source.This component is most obvious in the late Chachahuén mafic lavas whose La/Ta and Ba/Ta ratios approach those of frontal arc lavas (Fig 6) Evidence for a hydrated, oxidized arc-like magma source also comes from the presence of clinopyroxene, amphibole, Fe-Ti oxides and titanite phenocrysts Intraplate-like low La/Ta and high Ta/Hf ratios in ~ 7.3 – 6.9 Ma Vizcachas dacites and rhyolites can be explained by the strong influence of a older crustal component on the first erupted silicic magmas (see Kay et al., this volume, Chapter 10) Overall, the easiest explanation for the arc-like features in the Chachahuén magmas is to introduce a subducted component into the mantle source over a shallowly dipping slab This subducted component must be transient as it is absent in both early Miocene and Plio-Quaternary alkali basalts erupted

in the Sierra de Chachahuén (Fig 6) The Cerro Nevado/Plateado centers (Bermúdez, 1991) to the north in Mendoza are considered to have originated in the same way.

Further support for a late Miocene shallow subduction zone under the Neuquén basin comes from similarities in eruption style, setting, and chemistry between the Chachahuén

magmas and those in the late Miocene Pocho field that is located ~ 700 km east of the modern trench over the Pampean flatslab (Kay and Gordillo, 1994) The shape of the subducting plate inthe lithospheric section in Figure 11 is predicated on the Chachahuén magmas erupting ~ 180 -

200 km above the slab as did the Pocho magmas Mantle melting at this depth in both cases is argued to be related to breakdown of phlogopite and other hydrous phases in the slab (e.g., Poli and Schmidt, 1995)

A shallow subduction zone also provides a rationale for a late Miocene episode of

contractional deformation that inverts older extensional structures leading to uplift of structural blocks across the northern Neuquén basin Deformation would be facilitated by hydrous

weakening of the underlying lithosphere (Kay and Abbruzzi, 1996) as argued for the central Andes on geophysical grounds by James and Sacks (1999) Evidence for deformation of the Miocene Trapa Trapa arc sequences and some uplift of the Cordillera del Viento respectively comes from structural evidence and fission track cooling ages discussed by Jordan et al (2001), Burns (2002) and Burns et al., 2000) This deformation must be over by the middle Pliocene when the Centinela arc volcanic sequences erupt (Rovere, 1998) west of the Cordillera del Viento Further east, the Tromen Massif is a high standing basement block (e.g., Ramos, 1978; Llambías et al., 1982) bounded by relatively steep faults (Zapata et al., 1999) The discussion above is consistent with uplift of the Tromen Massif between ~ 12 Ma and 4 Ma The most likelytimes are at ca 12 to 10 Ma and during the period of maximum shallow subduction ca 8 to 5

Ma These periods are synchronous with those identified by Baldauf (1997) in Mendoza The late Miocene uplift of the Sierra de Chachahuén on inverted normal faults is discussed by Kay et

al (this volume, Chapter 10) The principal times of Miocene deformation correspond with those of magmatism as expected if hydration is important in triggering both

Pliocene to Holocene (Steepening of the Slab, Extension, and Mantle Melting).

The last two lithospheric sections in Figure 11 are for the Pliocene and the Pleistocene toHolocene Dramatic changes across the Neuquén basin during this period are interpreted toreflect steepening of the subducting slab north of the Cortaderas lineament Important changesinclude mafic volcanism stretching from the arc to the backarc with voluminous eruptions in thefar backarc, the end of contractional deformation, and the onset of mild extension

Reasons for tectonic changes in the arc region west and south of the Cordillera del Vientoare discussed by Folguera et al (this volume, Chapter 12), Melnick et al (this volume), and Laraand Folguera (this volume) and references therein Evidence for extension in that area comesfrom the formation of the Lancopué Trough in a transtensional arc setting (Ramos, 1978;Folguera and Ramos, 2000) A broad NW-SE-trending magmatic belt that developed south of37°S (e.g., Muñoz and Stern, 1988, 1989) in the Late Pliocene narrowed westward into the NE-SW-trending SVZ by the Pleistocene (Lara and Folguera, this volume) Through these events,the chemistry of Pliocene to Holocene arc magmas west of the Cordillera del Viento retained anarc character with REE patterns similar to older arc magmas in the transect (Figs 5 to 8)

In contrast, the chemistry of magmatic rocks erupted in the backarc changed dramatically

as mafic andesitic to rhyolitic magmas erupted in the Tromen region, and voluminous alkalinemagmas erupted in the Auca Mahuída and Payunia volcanic fields In Figure 11, large eruptions