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

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 Mancilla REPSOL-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 transient period of shallow subduction that began at ~ 20 Ma and climaxed near 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 and chemistry 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 just south 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 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 retroarc region 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 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 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 prerift 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 faultcontrolled 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 to and summarized in Table Twelve new 40Ar/39Ar ages are listed in Table Age spectra are presented in Appendix 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 to and plotted along with another 300 unpublished and published data in the fields in Figures to 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 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 Major element, trace element and isotopic data are used below to characterize the Neuquén basin 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 Indicators of mantle source conditions include the high field strength (HFS) elements plotted in Figure 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 of partial 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 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, 206 Pb/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 well known Volcanic and plutonic rocks of this age generally occur along a north-south trending belt that runs through northwestern Neuquén (Fig 2a) Unlike younger magmatic rocks, they occur only in the western Neuquén basin Radiometric ages support dividing them into: (1) Upper Cretaceous, (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 an andesite 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 on the 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 and Huinganco (Vilas and Valencio, 1978), and a whole rock age of 67±3.2 Ma for a tonalite stock near 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 the western Cordillera del Viento in Table is interpreted as a cooling age Paleocene magmatism is confirmed by hornblende K/Ar ages of 59.1 ±2.9 Ma and 56.5±1.7 Ma from a gabbro and a diorite in the Cerro Nevazón region and of 60.7±1.9 Ma from a diorite in the Campana Mahuída region (Franchini et al., 2003) Franchini et al (2003) further show that all of these magmatic rocks have chemical signatures similar to those of Holocene SVZ arc volcanic rocks (see Figs to 8) The chemical analyses for the Varvarcó granodiorite in Table shows a similar REE pattern (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 the Serie Andesitic and Molle Formations by Groeber (1946) and reassigned to a volcanic Cayanta Formation by Rapela and Llambías (1985) and a plutonic Collipilli Formation by Llambías and Rapela (1989) A distinctive feature of these rocks is the presence of amphibole phenocrysts The Cayanta Formation is dominantly composed of the extensive breccias flows and necks on the west side of the Cordillera del Viento (Figs 2a and 3) Their age is confirmed by 40 Ar/39Ar hornblende ages of 56.0±0.6 Ma and 50.3±0.6 Ma on two flows (Jordan et al., 2001) and whole rock 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-bearing plutonic 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) Cobbold and 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 of 50±5 Ma for hornblende-bearing volcanic rocks in the Cerro Bayo de la Esperanza complex To avoid confusion with new ages that show that some of the magmatic rocks included in the Collipilli Formation in the Collipilli region are Cretaceous in age (Zamora Valcarce et al., this volume); the magmatic rocks east of the Cordillera del Viento are here informally called the Caicayén group New chemical analyses of Cayanta Formation, Cerro Bayo de la Esperanza region, and Cerro Mayal region samples listed in Table supplement those from Paleogene samples in Rapela and Llambías (1985), Llambías and Rapela (1989) and Franchini et al (2003) As seen in Figures 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, there are differences among them The Cayanta Formation samples west of the Cordillera del Viento are generally similar to Holocene 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, and relatively flat REE patterns (La/Yb = 4-7, La/Sm = 3.3-4.1, Sm/Yb =1.5-1.9) The initial 87 Sr/86Sr and 143Nd/144Nd ratios of a Cayanta basaltic andesite (Table 7) are near those of the SVZ lavas (Fig 9) The Cayanta Formation flows differ from the SVZ lavas in typically having amphibole phenocrysts In contrast, Cerro Bayo de La Esperanza and Cerro Caicayén region samples are more like early 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 to 8) Mafic Bayo de la Esperanza region (48-57% SiO 2) are particularly notable for their high Na 2O (4.46.0%), 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% SiO 2) 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 (