A MONOGENETIC ALKALI BASALT FIELD EAST OF THE ANDEAN ARC BETWEEN 34° AND 35° S: IMPLICATIONS FOR MANTLE COMPOSITION

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A MONOGENETIC ALKALI BASALT FIELD EAST OF THE ANDEAN ARC BETWEEN 34° AND 35° S: IMPLICATIONS FOR MANTLE COMPOSITION

ABSTRACT

This study analyzes a series of Quaternary basaltic lavas sampled ~50km east of the arc front of the Northern Southern Volcanic Zone (NSVZ) of the Andean arc, as well as two basaltic andesites from Casimiro parasitic cone in the Diamante Caldera, located on the arc front itself, for major element concentrations, trace element concentrations, and radiogenic isotope ratios of Sr, Nd, Pb, and Hf.  The basaltic field has previously been classified as a northern portion of the Payenia Volcanic Complex, the more southerly parts of which are interpreted to have resulted from adiabatic melting associated with changes in slab dip during the late Oligocene through Miocene.  The retro-arc basalt samples are alkaline basalts, enriched in fluid mobile elements such as Cs, Ba, Pb, Sr, and Li, with moderate relative depletions in Nb and Ta.  These characteristics are typical of rear-arc basalts from subduction zones.  We propose that these basalts should be classified as retro-arc basalts associated with the Quaternary volcanic arc rather than as an extension of the more extensive Payenia Volcanic field to the south.

It has been well documented that arc lavas in the NSVZ have distinctly higher 87Sr/86Sr and lower εNd values than arc magmas found south of 34.5°S.  This chemical distinction has been interpreted as indicating a significant crustal contribution to rising magmas within the NSVZ.  The mechanism for this crustal contribution, however, has been largely debated.  Two models have been proposed to explain the driving force for this increased crustal signature:  1) increased assimilation of crustal material with elevated 87Sr/86Sr isotope ratios and lower εNd values due to thicker, older crust in the NSVZ, or 2) the subducting Nazca plate is eroding crustal material with elevated 87Sr/86Sr isotope ratios and lower εNd values and subducting it beneath the arc, where it mixes with and alters the composition of the mantle wedge, i.e. subduction erosion.  If the elevated 87Sr/86Sr isotope ratios and lower εNd values observed in the arc front lavas from the NSVZ were due to crustal contamination of the mantle source from subduction erosion, it can reasonably be expected the isotopic evidence for crustal contamination would be observable in the retro-arc basalts of the NSVZ, located a few tens of kilometers west of the arc front.  Conversely, if the isotopic evidence for crustal contamination were due to assimilation of thicker, older crust beneath the arc, then one would not expect to see similar isotopic evidence for crustal contamination in the retro-arc basalts.  The retro-arc basalts from this study have 87Sr/86Sr isotope ratios of 0.7037 – 0.7043, which is statistically different than 87Sr/86Sr  isotope ratios found in arc lavas of the NSVZ (87Sr/86Sr  = 0.7046-0.759).  Similarly, εNd values retro-arc basalt samples from this study (εNd = 1.4 to 4.4) is statistically different than εNd values found in arc lavas of the NSVZ (εNd = -1.8 to -0.19). This evidence suggests that the mantle beneath the NSVZ does not contain the elevated 87Sr/86Sr isotope ratios and lower εNd values observed in arc lavas of the NSVZ. Therefore, subduction erosion cannot be the driving mechanism for crustal contribution to ascending magmas in the NSVZ.  Instead, assimilation of crustal material within the older, thicker crust beneath the NSVZ must be the driving force for elevated 87Sr/86Sr isotope ratios and lower εNd values observed in NSVZ arc lavas.

 

 

TABLE OF CONTENTS

List of Tables…………………………………………………………………………………………………………. v

List of Figures………………………………………………………………………………………………………. vi

Acknowledgements………………………………………………………………………………………………… ix

 

 

  • INTRODUCTION………………………………………………………….……….1
    • Classification of retro-arc basalts…….……………………………………….1
    • Continental crustal signature in the Northern Southern Volcanic Zone ……2

 

  • GEOLOGIC BACKGROUND….………………………………………………….6

 

  • METHODS……………………………………………………..………………….13
    • Sample Preperation…..………………………………………………….…..13
    • Major Element Concentrations……………………………………………….13
    • Trace Element Concentrations……………………………………………….14
    • Isotope Ratio Analysis………………………………………………………15

 

  • RESULTS ……………………………………………………………………18
    • Major Elements………………………………………………………………………………….18
    • Trace Elements…………………………………………………………………………………..18
    • Isotope Ratios……………………………………………………………….19

 

  • DISCUSSION……………………………………………………………………..29
    • Chemical Variability of retro-Arc Basalts…………………………………..29
    • Crustal Contamination and the State of the Mantle in the NSVZ…………..29
    • Crustal Contamination within the Retro-Arc Basalt Suite…………………..33
    • Pb Isotope Ratios……………………………………………………………35
    • Classification of Retro-arc Basalt Field……………………………………..36

 

  • CONCLUSION…………………………………………………………………….47

 

REFERENCES………………………………………………………………………….50

 

APPENDIX A:  Sample Descriptions………………………………………….53

 

APPENDIX B:  Rock Digestion Procedures for ICP-MS Trace Element

Analysis………………………………………………………………………….54

 

TABLE OF CONTENTS

 

 

APPENDIX C:  Dissolution Procedure at Lamont-Doherty Earth Observatory….60

 

APPENDIX D:  Ion Exchange Procedure used at Lamont-Doherty Earth

Observatory…………………………………………………………………………62

 

APPENDIX E:  Maipo Volcano Figures…………………………………………..65

 

APPENDIX F:  Data Tables for Maipo Volcano and Basement Rock……………67

1    INTRODUCTION

The Andes Mountains are the longest mountain chain in the world, extending over 7,500 km along the western margin of the South America plate as shown in figure 1(Stern 2004).  With over 200 potentially active Quaternary volcanoes (Stern 2004), the Andean subduction zone serves as the archetypal continental arc setting.  Due to the extensive range and number of volcanic edifices, as well as the relative inaccessibility of parts of the arc, some processes contributing to the current petrological, tectonic, and geochemical state of the crust-mantle system remain poorly understood.

The Northern Southern Volcanic Zone (NSVZ) of the Andean Volcanic Belt, located between 33° and 34.5°S, results from the subduction of the Nazca plate beneath the South American plate. This study focuses on a suite of Quaternary basalts from behind the volcanic front between 34.5° and 34.77° S, and two basaltic andesite samples from

Casimiro parasitic cone located on the wall of the Diamante Caldera at the same latitude.  The goal of this study is to illuminate the geochemical composition of the subarc mantle beneath the NSVZ.

                            1.1       Classification of retro-arc basalts

The isolated monogenetic basaltic lavas located ~50 km east of the Quaternary arc front between 34.5° and 34.77° S have previously been classified as the northern segment of the extensive Payenia Volcanic Province (Figure 2; Ramos and Folguera 2011).  Previous studies have focused on the more extensive central and southern segments of the Payenia (sometimes called Payunia) Volcanic Province, which includes the large shield volcano

Payun Matru at 36.42° S and 69.2°W (Kay and Copeland 2006).  Among studies of the monogenetic basalts of this northern area, most have focused on either dating of volcanic events (Stern et al. 1984; Folguera et al. 2009) or tectonic setting (Folguera et al. 2009).

                            1.2       Continental crustal signature in the Northern Southern Volcanic Zone

The NSVZ segment of the Andean arc is geochemically distinguished from the rest of the Southern Volcanic Zone (SVZ) by its elevated 87Sr/86Sr ratios, lower 143Nd/144Nd ratios, increased K20, Ba, and Ce concentrations, and higher Ce/Yb and Hf/Lu ratios (Hildreth and Moorbath 1988).  Hildreth and Moorbath (1988) attribute these features to a proposed model of Mixing, Assimilation, Storage, and Homogenization (MASH) in which rising magmas produced from fluid-induced partial melting of the mantle wedge stall at the base of the continental crust and assimilate lower crustal material.  An alternate hypothesis in which the subducting oceanic plate is eroding and subducting continental material has been proposed by other authors (Stern, 1991; Kay et al., 2005).  In this model, the subducted crustal material mixes with and changes the geochemical composition of the mantle wedge.  Subduction erosion is supported in the Central

Volcanic Zone (CVZ), north of the study area, by the disappearance of pre-Andean Paleozoic basement, along-strike disappearance of a Jurassic Andean plutonic belt, and the eastward shift of Andean plutonic belts in time (Stern 1991).  Kay et al. (2005) link erosion of crustal material by the subducting slab to episodes of crustal shortening and thickening and eastward shifts of the arc front in the NSVZ.  Since both processes may produce similar geochemical signatures within the arc, it has been difficult to resolve the dominant process in the NSVZ.

Since the retro-arc basalts from this study are located east of the arc front, where the crust is thinner, and are chemically more primitive than rocks found at the arc front, they present an opportunity to examine mantle composition and processes at these latitudes in samples that have undergone less interaction with crustal material than have the arc front lavas.  The Casimiro basaltic andesites are among the most primitive arc front lavas in the NSVZ.  They may represent a middle member between the retro-arc basalts and the more evolved rocks of the Andean arc front.

Ten samples from nine basaltic cinder cones, lava flows, and maar deposits, as well as two samples from Casimiro parasitic cone, were analyzed for major element concentrations, trace element concentrations, and radiogenic isotope ratios of Sr, Nd, Pb and Hf, in order to shed light on the composition of the mantle in this region.

 

 

Figure 1.  Figure from Stern 2004 showing schematic map of the four volcanic zones of the Andes Mountains.

 

 

Figure 2.  Image altered from Ramos and Folguera 2011 showing major volcanic fields for Payenia Volcanic Province.  Dashed black box represents the field area for this study. 2    GEOLOGIC BACKGROUND

The Andean Volcanic Belt extends discontinuously along the western margin of the South American plate from Colombia to the southern tip of Argentina.  This volcanic belt is a result of subduction of the Nazca plate (and in the south, a portion of the Antarctic plate) beneath the South American plate.  Subduction of these two plates continues today at a rate of 7-9 cm/year (Cembrano and Lara 2009) and an average dip of 30° (Ramos et al 1996).  The Andean arc is separated into four zones of active volcanism:  the Northern

Volcanic Zone (NVZ, 5°N-2°S), Central Volcanic Zone (CVZ, 14-27°S), Southern

Volcanic Zone (SVZ, 33-46°S), and Austral Volcanic Zone (AVZ, 49-55°S) (Figure 1).   Each of these zones may be subdivided into smaller volcanic arc segments based on differences in geological and/or tectonic setting (Stern 2004).

The SVZ can be sub-divided into several segments based on geologic offsets along the current arc; however, previous studies do not all agree upon naming or delineation of the segments.  In this work, the divisions of Stern (2004) are used: the Northern Southern Volcanic Zone (NSVZ, 33-34.5°S); the Transitional Southern Volcanic Zone (TSVZ,

34.5-37°S) the Central Southern Volcanic Zone (CSVZ, 37-41.5°S); and the Southern

Southern Volcanic Zone (SSVZ, 41.5-46°). The relatively thin crust (<30 km thick) in the

SSVZ and CSVZ thickens northward to >45 km thick in the NSVZ as shown in figure 5 (Zandt 2005).  Similarly, the subducting slab shallows northward to an average interpreted dip of 20° beneath the NSVZ (Stauder, 1973; Barazangui and Isacks, 1976).  This change in dip angle may be related to along-arc differences in the trench to arc gap, which may in turn be a primary reason for the observed offsets in the arc between segments (Stern 2004).   Wagner et al. (2005) observe that the slab goes from a flat-slab north of the NSVZ, to a normally subducting slab within 2° latitude, with an unresolved geometry in this transitional area.

The NSVZ encompasses three main volcanic complexes as well as numerous minor volcanic features including parasitic cones, cinder cones, maars, and flow basalts.  The three stratovolcano complexes, Tupungato-Tupungatito, Marmolejo-San Jose, and Diamante-Maipo, are comprised of dominantly andesites with basaltic andesites, dacites, and rhyolitic ignimbrites (Stern 2004).  Other minor volcanic edifices of primarily basaltic composition are present east of the main arc in the retro-arc foreland basin.

Payenia is a large volcanic province located behind the arc front in the foreland basin of the SVZ.   This province encompasses multiple volcanic fields containing more than 800 volcanic centers of primarily basaltic composition located from 33°30’°S to 38°S.  This province can be subdivided into three segments:  a northern segment containing isolated monogenetic basaltic fields (sampled in this study), a central segment containing three significant volcanic fields with extensive lava flows, and a southern segment containing two significant volcanic fields, but lacking extensive lava flows (Ramos and Folguera 2011).  The central and southern segments of Payenia have been the focus of multiple studies because they contain the largest volcanic fields, including Payun Matru and

Llancenelo (e.g., Ramos and Folguera 2011, Kay and Copeland 2006, Jordan et al. 1983, Ramos and Barbieri 1988).  Kay and Copeland (2006) interpreted much of the volcanic activity in this area to be the result of early Miocene extension followed by a contractional regime and slab shallowing.  Samples from early Miocene eruptions show little to no evidence of an arc-like trace element signature, while more recent (middle Miocene) lavas show an increasing, but still relatively small, arc-like signature (Kay and Copeland, 2006).

This study focuses on the northern segment of the Payenia volcanic province, which is primarily comprised of small monogenetic cones, maars, and flows of alkaline basaltic composition.  Previous studies have examined the tectonic state of the area (Sruoga and Cortes, 1998; Folguera et al. 2009) and dated several of the volcanic centers (Folguera et al. 2009); however, little published data exists relating major and trace element concentrations and isotope ratios to the state of the Andean arc.  Stern et al. (1990) analyzed a suite of basalts from ~34°S to ~52°S, two of which are located in the retro-arc region of the  NSVZ, for major and trace element concentrations, and Sr, Nd, Pb, and O isotopic compositions, but their study focused primarily on basalts in the intermediate arc and retro arc farther south than the current study area.  From a tectonic viewpoint, these volcanic centers reside in the foreland basin between the Andean Cordillera and the uplifted San Rafael Block located to the east.  They are typically found in NNW to NW linear chains that follow tectonic structures in the area associated with the thick-skinned Malargue foreland fold-and-thrust belt and early Miocene extension.  These structures are interpreted by Cortés (2000) as piedmont fault scarps and bedrock escarpments that correspond to reactivated normal faults associated with NW-trending transtensional lineaments.  In addition to determining the geochemical composition of the mantle in this region, a secondary goal of this study is to characterize these volcanic centers within the current and historic NSVZ tectonic setting.

 

 

Figure 3.  Area map showing sampled region within the Southern Volcanic Zone and relative to the Central Volcanic Zone (after Sruoga et al., 2005).  The Southern Volcanic Zone is often divided into four segments, the Northern Southern Volcanic Zone (NSVZ), Transitional Southern Volcanic Zone (TSVZ), Central Southern Volcanic Zone (CSVZ),

and Southern Southern Volcanic Zone (SSVZ) as shown.  Dashed lines represent depth to the top of the subducting Nazca plate.  

 

Figure 4. Tectonic map showing sample locations relative to the Cordillera Frontal to the west and the San Rafael Block to the east (after Folguera and Ramos, 2011).  Eruptive centers can be seen in line with previously mapped normal faults in the area.  

    

 

 

Figure 5.   Figure altered from Gilbert et al 2005 showing crustal thickness contours.  Red dashed box represents the field area for this study.  Black lines are depth to slab contours from Cahill and Isacks 1992.  

Table 1.  Sample location and descriptions.  Ages listed are from Ramos and Fulguera 2011.

 

Sample Location Description Latitude Longitude Age (Ma)
MD-109-13 Sepultura cinder cone 0.07 ±0.004
MD-109-14  flow NW of Sepultura flow basalt 34.27433 69.12033  
MD-109-15 Arroyo Honda cinder cone 34.50119 69.22733 0.49 ±0.03
MD-109-16 Maar NW of Arroyo Honda maar 34.47044 69.25258 0.434 ±0.3
MD-109-17 La Leña flow basalt 34.76633 69.42108  
MD-109-21 Las Bolas flow basalt 34.61500 69.02136 0.495 ±0.03
MD-109-22 Agua del Toro at dam flow basalt 34.59197 69.03506  
MF-212-15 Gaspar cinder cone 34.28352 69.04361 0.106 ±0.01
MF-212-18 el Pozo cinder cone 34.32798 69.12115 0.092 ±0.01
MF-212-19 el Pozo cinder cone 34.32798 69.12115 0.092 ±0.02
G0100112-1 Casimiro parasitic cone 34.21594 69.92058  
G0100112-2 Casimiro parasitic cone 34.21638 69.92742  

 

A MONOGENETIC ALKALI BASALT FIELD EAST OF THE ANDEAN ARC BETWEEN 34° AND 35° S: IMPLICATIONS FOR MANTLE COMPOSITION

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