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Mesoproterozoic and Paleoproterozoic subcontinental lithospheric domains beneath southern Patagonia: Isotopic evidence for its connection to and

Andrea Mundl1, Theodoros Ntaflos1, Lukas Ackerman2,3, Michael Bizimis4, Ernesto A. Bjerg5, and Christoph A. Hauzenberger6 1Department of Lithospheric Research, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria 2Institute of v.v.i., Academy of Sciences of the Czech Republic, Rozvojová 269, 165 00 Prague 6 – Lysolaje, Czech Republic 3Czech Geological Survey, Geologická, 152 00 Prague 5, Czech Republic 4Department of and Sciences, University of South Carolina, 701 Sumter Street, Columbia, South Carolina 29208, USA 5INGEOSUR (CONICET-UNS) and Departamento de Geología, Universidad Nacional del Sur, San Juan 670, B8000ICN Bahía Blanca, 6Institute for Earth Sciences, University of Graz, Heinrichstrasse 26, 8010 Graz, Austria

ABSTRACT 72° 68° DC New isotopic studies on mantle xenoliths from Santa Cruz Province, southern Patagonia, DUB Argentina, reveal that at least three discrete subcontinental lithospheric mantle (SCLM) 46° TL POKA domains—the , Tres Lagos, and Pali Aike—form the southernmost part of SA . Re-Os systematics yield early Paleoproterozoic (up to 2.5 Ga) SCLM forma- ELRU

tion ages (rhenium depletion ages, TRD) for Pali Aike spinel peridotites, while samples from the Deseado Massif and Tres Lagos indicate a younger SCLM origin with Neoproterozoic to DESEADO Mesoproterozoic (0.9–1.3 Ga) and Mesoproterozoic to late Paleoproterozoic (1.3–1.9 Ga) T RD MASSIF ages, respectively. Hf-Sr-Nd isotopic compositions indicate metasomatic overprinting of the majority of the samples, which, however, has not affected the Os isotopic system. Based on similar formation ages, the geological evolution of the Deseado Massif is most likely connected

to the evolution of the Namaqua-Natal belt of . TRD ages from SCLM domains underneath Tres Lagos and Pali Aike indicate a common origin with crustal sections from Shackleton Range, Antarctica, positioning the southern tip of South America closer to west 50°

Antarctica in the reconstructed Rodinia supercontinent than previously assumed. a

INTRODUCTION tive to mantle metasomatism, providing reliable The origin of the Patagonian continental ter- information on the timing of melt extraction rane is highly debated. Whether Patagonia is a events leading to the formation of the SCLM Patagoni uniform part of the southernmost South Ameri- and its overlying crust (Walker et al., 1989; Carl- 0 100 Km PAVF can continental plate or an assemblage of frag- son, 2005). We have used the Re-Os isotopic ments with different mantle roots is a matter of system on carefully selected mantle xenoliths Figure 1. Simplified geological map of controversy (Stern et al., 1999; Pankhurst et al., to determine the timing of the stabilization of southern Patagonia. Light gray area is Santa 2006; Ramos, 2008; Schilling et al., 2008; Wang SCLM domains beneath southern Patagonia. In Cruz Province. Dark gray fields are Tertiary/ et al., 2008). As a result of the rarity of southern addition, Hf-Nd-Sr isotopic data obtained from Quaternary flood . Green outlined field is Pali Aike volcanic field (PAVF). DC— Patagonian crustal basement outcrops, which leached clinopyroxene (cpx) separates of those Don Camilo; DUB—Gobernador Gregores; could provide precise evidence on the timing of samples analyzed for Os isotopic composition TL—Tres Lagos; POKA—Potrok Aike; SA— crustal formation, Patagonia’s geological evolu- are presented to provide better insight into the Salsa; ELRU—El Ruido. tion has remained enigmatic. origin and complex history of South Patagonia. The formation of ancient continental is related to the formation of their underlying GEOLOGICAL SETTING al., 1989). Since then, subsidence and sedimenta- subcontinental lithospheric mantle (SCLM) Patagonia, in southern South America, is geo- tion have dominated the area, forming the Magel- roots, as is formed by the graphically defined as the south of Río lan Basin (Wang et al., 2008). Phanerozoic rhe-

extraction of melt of a mantle section that has Colorado and east of the Andean Cordillera. nium depletion ages (TRD) obtained by Wang et experienced variable degrees of partial melt- The only basement outcrop in southern Patago- al. (2008) have led them to interpret the basement ing. This residual depleted lithospheric mantle nia is the Deseado Massif which covers 60,000 beneath Pali Aike (Fig. 1) to be a young accre- coevally forms the root of the overlying conti- km2 (Fig. 1). Pankhurst et al. (2003) presented tionary that was either part of the nental crust (e.g., Griffin et al., 1999a, and ref- Mesoproterozoic (1.3–1.6 Ga) Sm-Nd mantle Deseado Massif or a separate part that accreted to erences therein; Pearson, 1999). This relation- separation ages from Deseado Massif , the Deseado Massif during the Paleozoic. ship permits us to use the determined timing of amphibolites, and schists. The basement south lithospheric mantle formation as a proxy for the of the Deseado Massif is covered by sedimen- SAMPLES AND RESULTS timing of the genesis of continental terranes. tary sequences >7 km thick, reflecting several Hence, mantle xenoliths brought to the surface episodes of basin formation of Triassic to Creta- Sample Description by can provide detailed information on ceous age. The Late breakup of Gond- From a large number (>200) of mantle xeno- SCLM sections. The Re-Os isotopic system is, wana resulted in voluminous rhyolitic volcanism liths collected at six different sites through- compared to other isotopic systems, less sensi- in southern Patagonia (Bruhn et al., 1978; Kay et out Santa Cruz Province, 11 anhydrous spinel

GEOLOGY, January 2015; v. 43; no. 1; p. 39–42; Data Repository item 2015029 | doi:10.1130/G36344.1 | Published online 21 November 2014 ©GEOLOGY 2014 Geological | Volume Society 43 | ofNumber America. 1 | www.gsapubs.org Open Access: This paper is published under the terms of the CC-BY license. 39

Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/43/1/39/3714264/39.pdf by guest on 02 October 2021 lherzolites and 13 anhydrous spinel harzburgi- ite, as well as sulfide alteration products (Figs. ( DR3). Hence, calculated TMA ages can be tes were chosen and studied in detail (see the DR3g–DR3l). considered reliable. Hf isotopic ratios from the GSA Data Repository1 for selection criteria). remaining South Patagonian peridotites yield

Host rocks are Pliocene and Quaternary alkali Re-Os Concentrations and Os-Hf-Nd-Sr unrealistic future or impossibly old TMA ages basalts, basanites, and pyroclastics (D’Orazio et Isotopic Systematics and suggest a resetting of the Lu-Hf system, al., 2000) that erupted in the back-arc region of Bulk-rock osmium concentrations vary from also evidenced by enriched cpx trace element the Andean zone (Stern et al., 1990). 0.5 to 3.3 ppb while rhenium contents are gen- concentrations (Table DR3). Sample localities are Don Camilo (DC) and erally low, between 0.002 ppb and 0.105 ppb. Sr and Nd isotopic compositions in the Gobernador Gregores (DUB) within the Deseado Initial 187Os/188Os ratios, calculated back to time Deseado Massif mantle xenoliths yield vari- Massif; Tres Lagos (TL); and Potrok Aike of eruption (4 Ma), are subchondritic, ranging able 87Sr/86Sr ratios ranging from 0.70278 to (POKA), El Ruido (ELRU), and Salsa (SA) from from 0.1114 to 0.1266, except in one sample 0.70588 and 143Nd/144Nd ratios between 0.51223

within the Pali Aike volcanic field (Fig. 1). from the Deseado Massif (DC08) exhibiting and 0.51328 (present-day eNd of -7.9 to +11.2). Bulk-rock major element analyses show that a suprachondritic Os isotopic ratio of 0.1327 Samples from Tres Lagos show Sr and Nd iso- samples from Pali Aike (POKA, SA, ELRU) (Fig. 2). Osmium isotopic ratios do not correlate topic ratios between 0.70225 and 0.70320 and 187 188 and Tres Lagos have mean Al2O3 contents and with either Re/ Os or bulk-rock Al2O3 (Figs. 0.51283 and 0.51321 (eNd +3.32 to +11.2),

Mg# [Mg/(Mg + Fetot)] of 1.46 wt% and 0.91, 2A and 2B). In samples from the Deseado Mas- respectively. Sr and Nd isotopic compositions 187 188 respectively, and are generally more depleted sif (DUB and DC), Os/ Os ratios lead to TRD in Pali Aike cpx range from 0.70282 to 0.70769

than mantle xenoliths found in the Deseado ages between 0.5 and 1.3 Ga. Except for sample and 0.51209 to 0.51318 (eNd -10.7 to +10.6),

Massif with mean Al2O3 contents and Mg# of TL142 (0.6 Ga), samples from Tres Lagos yield respectively, and correlate well with each other

2.08 wt% and 0.90, respectively. Bulk-rock older TRD ages ranging from 1.3 to 1.9 Ga. The as well as with Hf isotopes (Figs. 3A–3C). Sr

rare earth element (REE) compositions reveal largest variation in calculated TRD ages can be and Nd isotopic results combined with cpx trace elevated light REE (LREE) in the majority of found in the Pali Aike sample suite, with Neo- element analyses reveal a cryptic metasomatic

Pali Aike samples (ELRU and SA). All POKA to early Paleoproterozoic TRD ages (0.7–2.5 Ga). overprint of the majority of our studied samples. samples exhibit a hump in middle REE (MREE) Hf isotopic analyses from Deseado Mas-

suggesting melt percolation processes. Except sif cpx separates yield eHf ranging from +5.8 DISCUSSION for sample TL143, Tres Lagos samples have to +41.5. One sample (DC18) yields a model

slightly elevated LREE compositions. Deseado age (TMA) of 1.2 Ga, consistent with Mesopro- SCLM Formation Ages

Massif samples exhibit roughly flat REE pat- terozoic TRD ages determined from Os isotopic A scatter in Os isotopic ratios and the absence

terns with a minor increase in LREE in DC and analyses for Deseado Massif samples. eHf values of a correlation thereof with melt depletion indi-

a minor MREE and stronger LREE enrichment from Tres Lagos samples lie between +15.2 and ces (e.g., bulk rock Al2O3) within a sample suite

in DUB samples (Table DR2 and Fig. DR2 in +52.0. Sample TL142 yields a calculated TMA is often interpreted to indicate metasomatic the Data Repository). age of 1.8 Ga. Samples from Pali Aike show a overprint (i.e., Pearson et al., 1995; Brandon et

broad range in eHf values from -17.1 to +23.0. al., 1996; Handler et al., 2005). However, our

Sulfide Petrography Sample ELRU3, with eHf of +23.0, yields a TMA samples do not show significant variations in The modal abundances of sulfides in the stud- age of 2.7 Ga, slightly older than the oldest Pali their highly siderophile element abundances

ied mantle xenoliths are generally low and do Aike TRD ages (2.4–2.5 Ga). Laser ablation anal- (i.e., enrichment in the palladium group of the not correlate with fertility indices (e.g., bulk yses of cpx reveal no metasomatic addition of platinum-group elements, Ir-Ru fractionation;

rock Al2O3). Sulfides occur either interstitially or Hf or Lu to samples DC18, TL142, or ELRU3 Fig. DR1) as is typical for many metasomatized as spherical inclusions and trails preferentially in and orthopyroxene (Table DR4). Pri- mary enclosed sulfides with diameters ranging 0.134 PM from 30 to 150 µm are multiphase grains formed DC AB 0.132 DUB TL mainly of monosulfide solid solution, pentland- POKA Deseado Deseado 0.130 TRD ite, and chalcopyrite, with rare Pt- and/or Ir-rich SA Massif Massif [Ga] 0.128 ELRU microphases (Figs. DR3a–DR3f). Interstitial

(EA) 0.126 0.5 sulfides are commonly irregularly shaped and s 0.124 can be found in melt pockets, around spongy 18 8 1.0 cpx, and as up to 10 µm elongated or round sul- 0.122 Os /O

18 7 0.120 fide droplets in intergranular propagating fresh Tres 1.5 host glass consisting mainly of Ni mono- 0.118 Lagos Pali Aike Tres Lagos sulfide solid solution, pyrrhotite, or pentland- 0.116 2.0 0.114 Pali Aike 1GSA Data Repository item 2015029, detailed infor- 0.112 2.5 mation on sample localities; analytical methods; isoto- 0.110 pic results and HSE abundances (Table DR1 and Figure 0.1 0.2 0.3 0.4 1234 Al O DR1); bulk rock major and trace element compositions 1871Re/O88 s 23 (Table DR2) and bulk rock primitive mantle normal- [wt.%] 187 188 187 188 ized REE patterns (Figure DR2); clinopyroxene major Figure 2. Bulk-rock Os/ Os versus Re/ Os (A) and bulk-rock Al2O3 [wt%] (B), respec- and trace element concentrations (Table DR3); sulfide tively, of 24 spinel peridotites with reference to primitive mantle (PM) values from Meisel occurrences (Table DR4); backscattered electron im- et al. (1996) (187Os/188Os = 0.1296 and 187Re/188Os = 0.4353) and McDonough and Sun (1995)

ages and photomicrographs of primary and secondary (Al2O3 = 4.45 wt%). Rhenium depletion ages (TRD) on right are mantle formation ages calcu- sulfides (Figures DR3 and DR4), is available online lated using PM values from Meisel et al. (1996) at time of eruption (4 Ma). Gray dashed line at www.geosociety​ .org​ /pubs​ /ft2015.htm,​ or on request is 187Os/188Os chondrite value (0.127; Walker and Morgan, 1989). Fields for subcontinental from [email protected] or Documents Secretary, lithospheric mantle domains: Deseado Massif (DC and DUB), Tres Lagos (TL), and Pali Aike GSA, P.O. Box 9140, Boulder, CO 80301, USA. (POKA, SA, ELRU). Symbols and abbreviations are as in Figure 1.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/43/1/39/3714264/39.pdf by guest on 02 October 2021 Figure 3. Isotopic ratios of clinopyrox- 0.5135 Jokat, 2006). U-Pb zircon data from exposed DMMb BSE ene separates. A: 143Nd/144Nd versus DMMa DC basement rocks of the Natal belt and Mesopro- 87Sr/86Sr, showing a well-defined nega- DUB TL terozoic ages from Dronning Maud Land in East tive correlation. B: e versus e . C: e Hf Nd Hf POKA Antarctica suggest a continental crust formation versus 87Sr/86Sr of the three subconti- SA nental lithospheric mantle domains. PREMA ELRU between 1.2 and 1.0 Ga (McCourt et al., 2006; BSE—bulk silicate earth. Fields in A for 0.5130 Jacobs et al., 1996). These ages are similar to

depleted mantle (DMMa and DMMb), d our determined TRD ages of up to 1.3 Ga for the prevalent mantle (PREMA), high m HIMU

144 Deseado Massif mantle xenoliths, as well as to (HIMU), and enriched mantle I and II Mesoproterozoic ages (1.1–1.0 Ga) obtained for (EM-I and EM-II) are from Zindler and Nd /N Hart (1986). Mid-oceanic-ridge basalt 143 basement rocks from Cape Meredith Complex, (MORB) and ocean island basalt (OIB) 0.5125 –Malvinas (Thomas et al., 2000, fields in B and C are from Nowell et al. and references therein). We suggest that the Pata- (1998, and references therein). Sym- gonian SCLM fragment represented by Deseado bols and other abbreviations are as in EM-II Massif mantle xenoliths was accreted together Figure 1. EM-I A with other microcontinents during the forma- 60 B C tion of Rodinia and stayed contiguous to South Africa and until the breakup of Gondwana. Tres Tres 40 Lagos Lagos Tres Lagos and Pali Aike

TRD ages of the Tres Lagos samples are dis- Deseado Deseado tinctly older than Deseado Massif ages, indicating Massif 20 Massif a different origin. Ntaflos et al. (2007) proposed εHf MORB that the SCLM beneath Tres Lagos represents OIB OIB an isolated old Proterozoic lithospheric mantle MORB fragment, based on the depleted nature and low 0 pressure-temperature conditions of the mantle xenoliths. Our data support this model and fur- Pali Aike Pali Aike ther reveal distinct isotopic and trace element -20 compositional differences between Tres Lagos -20-10 010 0.702 0.7030.704 0.7050.706 0.7070.708 and other Patagonian mantle xenoliths. εNd 87Sr/86Sr Previous studies suggest a Mesoproterozoic to early Paleozoic (Stern et al., 1999; Schilling et al., 2008) or even younger (Wang et al., 2008) peridotites (e.g., Lorand et al., 2013, and refer- likely controlled by primary sulfides and yields formation of the SCLM beneath Pali Aike.

ences therein). minimum but reasonable TRD ages. However, our Re-Os data clearly show that the

Two populations of sulfides have been petro- Calculated TRD ages (Fig. 4) from DUB and SCLM beneath Pali Aike was formed at least as

graphically identified in our South Patagonian DC group at ca. 1 Ga, with the oldest TRD age long ago as early Paleoproterozoic times.

samples (Table DR4). While the Os isotopic com- at 1.3 Ga. The Tres Lagos samples yield TRD Crustal rocks of similar age close to the posi- position in the majority of the samples is most ages of up to 1.9 Ga. Samples from Pali Aike tion of southernmost Patagonia in Gondwana

likely controlled by primary sulfide inclusions exhibit the broadest range in TRD ages with three occur only at Shackleton Range, a composite ter- in silicates (Alard et al., 2000), some samples samples displaying a peak at ca. 2.4 Ga. Even rane located at the northwestern edge of the East also exhibit an abundance of secondary sulfides, though samples from within one sample suite

which may lead to mixed whole-rock Os isoto- yield variable TRD ages, Figure 4 clearly shows 6 n=24 Deseado Massif pic compositions. Clinopyroxene and bulk-rock that the oldest samples come from Pali Aike Tres Lagos primitive mantle–normalized REE patterns sug- with an age of up to 2.5 Ga, followed by the Tres 5 Pali Aike gest that percolating melts have affected the bulk Lagos sample suite with the oldest determined compositions of all POKA samples (Tables DR2 T ages of 1.9 Ga, and the “youngest” sample RD y 4 and DR3; Fig. DR2). This may have led to an suite from Santa Cruz Province, the Deseado addition of secondary sulfides prior to host basalt Massif, with T ages of up to 1.3 Ga. RD 3

entrainment, and hence, resulted in a refertiliza- Frequenc

tion of the samples leading to TRD ages younger SCLM Domains than their actual melt depletion ages. Second- 2 ary sulfides in most other samples are found in Deseado Massif fresh intergranular propagating glass and have The determined SCLM formation ages for 1 likely been added to the xenoliths by host basalt DC and DUB mantle xenoliths match that of 0 infiltration. It is important to point out that the the Mesoproterozoic basement interpreted by 012 3 T samples with the oldest calculated T ages from Pankhurst et al. (2003), as well as Meso- to Paleo- RD RD [Ga] each studied sample suite also exhibit the high- proterozoic TRD ages from Cerro Redondo and est primary to secondary sulfide ratio, where sec- Lote 17 (Schilling et al., 2008). Rodinia Figure 4. Histogram of calculated rhenium depletion ages (TRD), comprising all 24 Pa- ondary sulfides can only rarely be found within and Gondwana reconstructions have placed the tagonian spinel peridotites, using primi- a thin section (Table DR4). Thus, we infer that Deseado Massif close to South Africa and West tive mantle values from Meisel et al. (1996) the Os isotopic composition in those samples is Antarctica (e.g., Dalziel et al., 2000; König and (187Os/188Os = 0.1296).

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ACKNOWLEDGMENTS Lorand, J.-P., Luguet, A., and Alard, O., 2013, Walker, R.J., and Morgan, J.W., 1989, Rhenium- This work was financed by Austrian Science Fund Platinum-group element systematics and pet- grant P 23557-N19 (Principal Investigator: Th. Nta- osmium isotope systematics of carbonaceous rogenetic processing of the continental upper chondrites: Science, v. 243, p. 519–522, doi:​ flos). We gratefully acknowledge W. Wegner (Uni- mantle: A review: Lithos, v. 164–167, p. 2–21, versity of Vienna) for Sr and Nd isotope analyses; V. 10.1126​/science​.243.4890.519. doi:​10.1016​/j​.lithos​.2012.08.017. Walker, R.J., Carlson, R.W., and Shirey, S.B., 1989, Erban (Czech Geological Survey) for Os isotope anal- McCourt, S., Armstrong, R.A., Grantham, G.H., and yses; and J. Ďurišová (Czech Academy of Sciences) Os, Sr, Nd, and Pb isotope systematics of south- Thomas, R.J., 2006, Geology and evolution of ern African peridotite xenoliths: Implications for inductively coupled plasma–mass spectrometry the Natal belt, South Africa: Journal of African analyses of highly siderophile elements. We thank for the chemical evolution of the subcontinen- Earth Sciences, v. 46, p. 71–92, doi:10.1016/j​ tal mantle: Geochimica et Cosmochimica Acta, W.L. Griffin, J. Bédard, and two anonymous review- .jafrearsci.2006.01.013. ers for their constructive comments to improve this v. 53, p. 1583–1595, doi:10.1016/0016-7037​ (89)​ ​ McDonough, W.F., and Sun, S., 1995, The composition 90240-8. manuscript. The editorial handling and suggestions by of the Earth: Chemical Geology, v. 120, p. 223– J.B. Murphy are greatly appreciated. Wang, J., Hattori, K.H., Li, J., and Stern, C.R., 2008, 253, doi:10.1016/0009-2541(94)00140-4. 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