Geochemical signatures from the Atlantic coast of Tierra del Fuego and their provenance implications for Magallanes basin sediments

Chemical Geology 347 (2013) 69–81

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Geochemical signatures from the Atlantic coast of Tierra del Fuego and their provenance implications for Magallanes basin sediments Khandaker M. Zahid ⁎, David L. Barbeau Jr. Department of Earth and Ocean Sciences, University of South Carolina, Columbia, SC 29208, USA

a r t i c l e

i n f o

Article history: Received 3 April 2012 Received in revised form 1 March 2013 Accepted 10 March 2013 Available online 23 March 2013 Editor: L. Reisberg Keywords: Geochemistry Rare earth elements Nd isotopes Provenance South America

a b s t r a c t The Magallanes foreland basin, located along the northern periphery of the Scotia Arc and extending up to 51°S to its northern terminus, contains a near-complete sedimentary record of Fuegian Andes tectonics since the Early Cretaceous. Herein we report trace- and rare-earth-element (REE) geochemistry and Nd isotope ratios from Upper Cretaceous to lower Oligocene mudstones of the eastern Magallanes basin, Argentina. The REE patterns of all of these samples are typical of average post-Archean upper continental crust. The older Upper Cretaceous to middle Eocene samples contain an overall lower concentration of light REE (smaller LaN:SmN ratio) than do the younger, middle-upper Eocene and lower Oligocene samples, although a strict temporal change is not observed. These REE patterns suggest a Patagonian Batholith and mafic volcanic provenance for the older successions and an Eastern Andean metamorphic complex provenance for the younger sediments. The lower Th:Sc ratio of the older samples suggests a mafic origin, whereas the higher Th:Sc ratio of younger samples indicates a felsic origin. Nd isotope data show a broadly coeval shift in εNd values from less negative to more negative. These data, combined with previously reported geochronological and mineral composition provenance analysis, suggest an increased Eocene–Oligocene tectonic exhumation of the southern Andes, concurrent with independent evidence of the opening of the adjacent Drake Passage. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The southernmost Andes is a Meso-Cenozoic subduction-related mountain belt with associated strike-slip deformation, possible orocline development, and limited sedimentary recycling (Nelson, 1982; Wilson, 1991; Pankhurst et al., 2000; Augustsson and Bahlburg, 2008; Barbeau et al., 2009). Initial breakup of Gondwana in Patagonia marks the beginning of its tectonic history during Middle to Late Jurassic time, when an extensional phase of deformation resulted in widespread silicic volcanism and emplacement of associated felsic plutons in southern Patagonia and West Antarctica (Gust et al., 1985; Pankhurst et al., 2000; Calderón et al., 2007). Obduction of the basin floor (Fildani et al., 2003) onto the craton margin during Middle Cretaceous time is widely suggested to mark the transition from the extensional phase into contraction, with the resultant development of a retroarc fold-thrust belt along the east margin of the Cordillera (Klepeis, 1994a; Fildani and Hessler, 2005; Klepeis et al., 2010). Crustal thickening and shortening via folding and faulting have thus contributed to uplift the greater Andean Cordillera (Ramos, 1989; Kley and Monaldi, 1998; Fildani and Hessler, 2005; Fosdick et al., 2011)

⁎ Corresponding author at: Bureau of Economic Geology, The University of Texas at Austin, University Station, Box X, Austin, TX 78713-8924, USA. Tel.: +1 512 471 1506; fax: +1 512 471 0140. E-mail address: [email protected] (K.M. Zahid). 0009-2541/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chemgeo.2013.03.004

and subsequent development of a retro-arc foreland basin adjacent to the southern Andes. The region's possible relationship to the development of the Drake Passage and earliest Oligocene glaciation of Antarctica (Barker and Burrell, 1977; Scher and Martin, 2004; Eagles et al., 2006) makes the understanding of this region's kinematic history particularly important (Livermore et al., 2007). Ice-volume and ocean-temperature proxy data from benthic foraminifera δ 18O isotope ratios and water-mass tracer data from εNd preserved in fossil fish teeth indicate shifts in ocean geochemistry leading up to and including the Eocene– Oligocene transition (ca. 34 Ma; Barker and Thomas, 2004; Scher and Martin, 2004). A decrease in global δ 18O ratios from the middle Paleocene to the early Eocene was followed by a prolonged, gradual increase in these ratios until the end of the Eocene. This increased cooling trend concluded with an abrupt further increase in δ 18O ratios across the Eocene–Oligocene boundary (ca. 34 Ma) that is widely interpreted to record the onset of the Oi-1 Antarctic glaciation (Barker and Thomas, 2004). Formation of the Drake Passage, the deep-water gateway between South America and the Antarctic Peninsula, initiated the onset of a continental-scale revolving current known as the Antarctic Circumpolar Current (ACC). Initiation of this circumpolar current has been argued to be the cause of the thermal isolation of Antarctica from the remaining equatorial warm water (Barker and Thomas, 2004; Pfuhl and McCave, 2005; Lyle et al., 2007). Whether the development of the ACC made a significant contribution to Oi-1 glaciation (Livermore et al., 2007 and

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K.M. Zahid, D.L. Barbeau Jr. / Chemical Geology 347 (2013) 69–81 48oS

FUEGIAN ANDES TECTONIC ELEMENTS Magallanes Foreland Basin

Basement

Rocas Verdes Marginal Basin L. Cretaceous basin-fill ophiolite

Chon -Aike

4

Gondwana break-up Cordillera Darwin complex

Tertiary

1

Patagonian Andes

B

North Patagonian Batholith

2

Sample Location

Deseado Massif

1 EAMC

1

2 PBS

3 3

Ice Cap

3 JV

Batholith

4 PB-N 5 PB-S

Cretaceous-Tertiary 51oS o

o

50 W

A

30 W

B Scotia

3 5

Arc o

55 S

Drake Passage

Central Scotia Sea Scotia Plate

5 o

60 S

Antarctic Peninsula 0

KM

Malvinas Basin

South Patagonian Batholith

500

55oS

70oW

75oW

Fuegian Andes C

ATLANTIC OCEAN

C

PG1 PG08b

Cerros A RUTA3B

MC1AM, CCA2M, IDK1M CCD3M, CCDM3M CCB3M, CCC1M CL2A PN3 PD2A

54.5°S

55°S

SARMIENTO

0

10

20

30

40

50

55.5°S

km

69°W

67°W

65°W

K.M. Zahid, D.L. Barbeau Jr. / Chemical Geology 347 (2013) 69–81

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Table 1 Trace element concentration data (in ppm) from Magallanes basin mudstones. Sample

MC1AM

CCD3M

CCC1M

CCB3M

CCA2M

IDK1M

PG08b

CCDM3M

PN3

CL2A

PD2A

Age (Ma)

31

33.7

35

39

39

45

45

52

54.8

63

68

Latitude

−5429.60

−54 29.44

−54 29.51

−54 29.33

−54 28.80

−54 28.44

−54 21.18

−54 27.202

−54 38.44

−54 36.62

−54 39.08

Longitude

−6622.89

−66 23.01

−66 22.99

−66 22.97

−66 26.95

−66 28.35

−66 38.61

−66 29.149

−65 50.44

−65 55.96

−65 33.41

Sc V Cr Ni Y Zr Th U La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu Eu/Eu* Eu* Ce/Ce* Gd/Yb GdN/YbN LaN/SmN Zr/Y Th/U Th/Sc Y/Ni Cr/V

19.43 113.28 42.34 19.17 40.07 197.50 12.88 3.21 44.99 82.77 11.18 42.07 8.45 1.61 6.96 1.08 6.80 1.38 4.07 3.98 0.57 0.65 2.48 .90 1.75 1.46 3.25 4.93 4.02 0.66 2.09 0.37

13.22 93.63 35.60 26.28 18.67 132.43 11.07 2.06 26.08 63.78 6.54 25.06 5.01 0.92 3.98 0.61 3.53 0.68 2.14 2.23 0.31 0.64 1.44 1.19 1.79 1.49 3.17 7.09 5.37 0.84 0.71 0.38

15.67 127.39 47.33 22.68 30.55 147.33 10.70 2.45 36.91 89.33 9.21 35.57 7.41 1.53 6.33 0.99 6.03 1.12 3.19 2.82 0.39 0.69 2.21 1.17 2.24 1.87 3.04 4.82 4.38 0.68 1.35 0.37

18.63 157.92 51.52 24.86 27.37 168.42 9.82 2.76 32.49 69.50 7.55 29.11 5.76 1.22 4.89 0.79 4.76 0.98 2.92 2.97 0.43 0.71 1.72 1.06 1.65 1.38 3.44 6.15 3.56 0.53 1.10 0.33

15.95 120.22 43.59 20.04 20.18 125.23 7.57 1.76 21.78 50.42 5.41 20.42 4.37 1.01 3.78 0.61 3.68 0.72 2.25 2.10 0.31 0.77 1.31 1.13 1.79 1.50 3.04 6.21 4.30 0.47 1.01 0.36

17.15 94.16 37.74 21.56 11.97 94.56 5.91 1.65 15.17 37.35 3.27 12.46 2.50 0.60 2.14 0.36 2.11 0.44 1.34 1.41 0.21 0.80 0.75 1.26 1.52 1.27 3.71 7.90 3.58 0.34 0.56 0.40

15.60 106.97 41.99 72.25 24.85 107.91 8.00 4.68 26.86 54.85 5.86 21.87 4.45 0.96 4.20 0.64 3.94 0.78 2.30 2.21 0.32 0.69 1.40 1.05 1.89 1.58 3.68 4.34 1.71 0.51 0.34 0.39

22.35 136.60 46.58 24.46 23.25 126.54 9.61 2.07 29.01 57.98 7.20 27.44 5.57 1.25 4.71 0.74 4.53 0.89 2.60 2.68 0.38 0.76 1.65 0.98 1.76 1.47 3.18 5.44 4.64 0.43 0.95 0.34

19.92 160.06 55.16 24.01 23.39 159.49 9.11 3.08 27.59 61.87 7.10 27.07 5.76 1.65 4.75 0.74 4.26 0.87 2.52 2.61 0.36 0.98 1.69 1.08 1.82 1.52 2.92 6.82 2.96 0.46 0.97 0.34

26.61 109.20 50.80 25.53 27.90 94.19 12.25 3.15 35.17 57.15 8.85 33.92 6.71 1.50 5.80 0.91 5.23 1.07 3.08 2.82 0.41 0.74 2.02 0.79 2.06 1.72 3.20 3.38 3.88 0.46 1.09 0.47

18.94 133.10 57.49 21.16 18.48 124.51 8.11 2.01 23.37 44.62 5.45 19.85 4.02 0.88 3.56 0.53 3.26 0.64 2.00 1.85 0.28 0.72 1.22 0.97 1.92 1.60 3.55 6.74 4.04 0.43 0.87 0.43

Note: Eu/Eu* = EuN / (SmN × GdN)1/2. 2/3 1/3 Ce/Ce* = CeN / (LaN ×NdN ). where N is chondrite-normalized value from Anders and Ebihara, 1982.

references therein) has been the subject of much debate. Central to this discussion are various interpretations pertaining to the existence and/or configuration of the Drake Passage and the Scotia Sea between southern South America and the Antarctic Peninsula because the prior connection between these continents would have inhibited circumpolar circulation. A large Nd isotopic change in Southern Ocean seawater composition is evident ca. 41 Ma, when values in the Atlantic sector shifted toward less negative Pacific sector ratios, suggesting that Pacific seawater began entering the South Atlantic through the Drake Passage (Scher and Martin, 2004, 2006). Subsequent increases in Nd isotope ratios in the Atlantic sector, which occurred at 37 and 34 Ma, are interpreted to reflect deepening episodes in the Drake Passage (Scher and Martin, 2004). Paleocurrent and flow-speed analysis data from the Southern Ocean (Pfuhl and McCave, 2005; Lyle et al., 2007; Koenitz et al., 2008), however, are inconsistent with the timing of the gateway opening, which is identified by water-mass and sediment provenance data, suggesting a much later (post-Oi1) ACC development. Recent climate modeling (DeConto and Pollard, 2003) suggests nontectonic development of glaciation in Antarctica and argues for declining Cenozoic CO2 as the primary cause of massive ice-sheet formation. One approach to deciphering the unroofing history of an active orogenic arc is to study mineralogical and geochemical signatures of

the sediments that were deposited in the adjacent basin. Although the Scotia Arc has been dispersed and fragmented, the Magallanes foreland basin within the northern limb of this large subduction system (Fig. 1) contains an important and near-complete sedimentary record of the late Cretaceous to Miocene history of subduction, crustal shortening, and thickening, as well as the flexural subsidence that occurred in this important region. Sediment provenance studies based on single-grain detrital zircon and heavy minerals from the Magallanes basin have proven to be beneficial in identifying changes in source rocks as a result of tectonic uplifting in the southern Andes (Barbeau et al., 2009; Zahid and Barbeau, 2010). Detrital-zircon data (Barbeau et al., 2009) and heavy-mineral studies (Zahid and Barbeau, 2010) from the eastern Magallanes basin reveal a change in southern Andean tectonics at ~ 39 Ma, with a major shift in sediment provenance from a magmatic- and back-arc setting to metamorphic basement-complex rocks. On the basis of comparison with the tectonic history of the Patagonian Andes, these workers suggested that the rapid shift in sediment provenance may have been caused by the development of the Patagonian orocline, which could be genetically related to the opening of the Drake Passage. Whole-rock trace and rare earth element (REE) concentrations in mudstones have also been considered appropriate provenance indicators because of the short residence times of REE in solution and their

Fig. 1. Local and regional map of the study area. (A) Greater Scotia regions with 3000-meter contour line showing the position of South America and Antarctica, the Scotia Arc, and Drake Passage. (B) Simplified geologic map and different tectonostratigraphic elements of southern Andes showing Patagonia oroclinal bend and location of study area. (C) Detailed map of sample collection locations. Modified from Wilson (1991), Fildani and Hessler (2005), Livermore et al. (2007), Barbeau et al. (2009). EAMC: Eastern Andean Metamorphic Complex, PBS: Patagonian Batholith and Sarmiento, JV: Jurassic Volcanics, PB-N: Patagonian Batholith North, PN-S: Patagonian Batholith South.

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tendency to transfer unchanged from source rocks to clastic sedimentary detritus (McLennan and Taylor, 1983; Taylor and McLennan, 1985; McLennan, 1989; Hessler and Lowe, 2006). Chondrite-normalized REE data have been applied successfully in the Patagonian Andes to discriminate between felsic and mafic input (Fildani and Hessler, 2005). Nd isotopic composition of mudstones is another important tool because provenance studies based on the Sm–Nd isotopic system use age sensitivity of the source region in their analyses of deposited sediments (Gleason et al., 1995). Whole-rock Nd isotope data have been applied successfully to the study of the provenance of clastic sedimentary rocks (e.g., Michard et al., 1985; Frost and Winston, 1987; Gleason et al., 1995; Hemming et al., 1995; Garzione et al., 1997; Patchett et al., 1999; Robinson et al., 2001). Crustal provenances with different ages can be distinguished because the parameter εNd is more negative in older crust (e.g., Archean crust εNd = ~−30) and is less negative, or even positive, in younger crust (e.g., recent island arc volcanic rocks, εNd > 2; McLennan and Hemming, 1992; Gleason et al., 1995). By carefully analyzing REE and Nd isotope data, uncertainties due to mineralogical alteration of samples can be constrained (McDaniel and McLennan, 1991; McDaniel et al., 1994). In this study, we present trace-element, REE, and Nd isotope composition data from 14 eastern Magallanes basin mudstone samples collected from two transects in Argentine Tierra del Fuego having depositional ages between Late Cretaceous and late Oligocene. These data complement our previously published U–Pb detrital-zircon (Barbeau et al., 2009) and heavy mineral based provenance study (Zahid and Barbeau, 2010) for deciphering the tectonics of the southern Andean hinterland.

conditions of upper amphibolite facies inboard of the batholith and are known as the Cordillera Darwin Complex (Kohn et al., 1993; Cunningham, 1995). Although geochemical analyses are scarce for the Cordillera Darwin rocks, several major and trace elemental analyses from the EAMC and DYC are available. EAMC rocks have compositions typical of passive margin rocks, with an averages of 78% SiO2, 2.1% Na2O, and 1.7% K2O, whereas the DYC complex has an active continental margin affinity with 66% SiO2, 2.9% Na2O, and 3.2% K2O (Faundez et al., 2002). The REE patterns of Andean metamorphic complex rocks are generally enriched in light rare earth elements (LREEs), with an average sample/chondrite value exceeding 100 and a flat heavy rare earth element (HREE) pattern (Augustsson and Bahlburg, 2008). εNd analyses from this complex (both EAMC and DYC) show values ranging from −6.5 to −10.8, with an average value of −8.8 (Augustsson and Bahlburg, 2003, 2008). 2.2. Gondwana break-up of silicic volcanics and granitoids The early phase of Gondwana break-up during Late Jurassic to Early Cretaceous time erupted enormous amounts of basalt, rhyolite, and ignimbrite in the Chon-Aike Province of the southern Andes and in Antarctica (Pankhurst et al., 1998). Although the duration of basalt eruption was short (183–184 Ma), silicic volcanism continued for a period of 30 Ma in three different episodes (Pankhurst et al., 2000). 200

A

MC1AM CCD3M CCC1M CCB3M CCA2M IDK1M PG08B CCDM3M PN3 CL2A PD2A

2.1. Paleozoic–Mesozoic basement rocks The lowest exposed rocks of the southernmost Andes are composed of metasedimentary and metavolcanic rocks of late Paleozoic to early Mesozoic age (Hervé et al., 2003, 2010) that have been divided into the Eastern Andean metamorphic complex (EAMC; east of Patagonian batholith) and the Duque de York Complex (DYC) in the Patagonian Andes on the basis of their different mineralogical and geochemical contents (Faundez et al., 2002; Augustsson and Bahlburg, 2008). In the Fuegian Andes, these rocks reached peak metamorphic

150

100

50

0 La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Yb

Lu

REE 200

B Sample/Chondrite

The Fuegian Andes result from convergence along a MesoCenozoic subduction system that has been active since the Late Cretaceous and has transitioned into a sinistral transform boundary during the early Miocene (Klepeis et al., 1994b; Ghiglione and Ramos, 2005). The mountain belt is composed of a granitoid batholithic magmatic arc of partly exhumed, high-grade metasedimentary rocks and volcanic, volcaniclastic, and sedimentary cover rocks (Wilson, 1991; Pankhurst et al., 2000; Calderón et al., 2007; Hervé et al., 2007). Tectonic evolution of the Fuegian Andes involves an initial Middle-Late Jurassic extensional phase, with associated widespread silicic volcanism and normal faulting coinciding with the development of an ocean margin basin filled with volcaniclastic detritus (Wilson, 1991; Pankhurst et al., 2000; Fildani and Hessler, 2005; Willan and Hunter, 2005; Calderón et al., 2007). This extension was followed by a compressional phase, beginning in the mid-Cretaceous and involving increased plate convergence, closer proximity of the magmatic arc to the continent, closure of the marginal basin, and formation of a foreland basin (Dalziel and Palmer, 1979; Biddle et al., 1986; Klepeis et al., 1994a; Kraemer, 2003; Somoza, 2005). The Magallanes foreland basin, adjacent to this mountain belt, contains a 7-km-thick succession of Upper Cretaceous to Miocene siliciclastic sedimentary rocks that document the kinematic history of southern Andes evolution. We divided the tectonostratigraphic elements of Fuegian Andes into five units—described next—in terms of major and trace-element geochemical and Sm–Nd isotopic characteristics.

Sample/Chondrite

2. Regional tectonics

Samples: Up. Eocene - Low. Oligocene Overlap Maastrichtian - Mid. Eocene

150

Source rocks: E. Andean Meta. Complex Tobifera Mafic Volcanics Patagonian Batholith Sermiento Rocks

100

50

0

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Yb

Lu

REE Fig. 2. (A) Chondrite-normalized REE patterns of mudstones collected from eastern Magallanes basin. Data reported in Table 1. Chondrite values from Anders and Ebihara (1982). (B) Comparative diagram of analyzed samples with source rocks. Source rock data from Bruhn et al. (1978), Saunders et al. (1979), Stern and Stroup (1982), Faundez et al. (2002), Fildani and Hessler (2005), Hervé et al. (2007), Augustsson and Bahlburg (2008). Samples are separated into two different age groups (Maastrichtian to middle Eocene and upper Eocene to lower Oligocene) based on the previous provenance study of Barbeau et al. (2009) and Zahid and Barbeau (2010).

K.M. Zahid, D.L. Barbeau Jr. / Chemical Geology 347 (2013) 69–81

73

correlation to TiO2 concentration, indicating mid-oceanic ridge and marginal basin provenance (Saunders et al., 1979). εNd analyses of Patagonian basalts and Sarmiento rocks show juvenile crustal values (−0.35–7).

The Tobífera Formation, the southern South American component of the silicic volcanism and volcaniclastic rocks, shows a considerable variation in some components of its chemical composition, such as SiO2 75–78 %, Al2O3 11–13.4%, Na2O 0.7–4%, and K2O 0.8–4.6% (Bruhn et al., 1978). Trace-element studies reveal an average composition of Rb 117 ppm, Sr 126 ppm, Ba 872 ppm, Zr 159 ppm, and Nb 12 ppm. These silicic rocks have higher average Ba and Sr and lower average Rb, Zr, and Nb than do Patagonian batholithic rocks (Bruhn et al., 1978). These trace-element data indicate that the silicic volcanics of Patagonia originated from partial melting of calc-alkaline-rich sedimentary detritus (Bruhn et al., 1978). Pankhurst and Rapela (1995) determined εNd(0) values of Chon-Aike Province rocks, which provided a wide range of values from −1.1 to −10, with an average of −5.9.

2.4. Patagonian–Fuegian magmatic arc Westward migration of the Antarctic Peninsula and South America stopped silicic volcanism during the Early Cretaceous. This was followed by calc-alkaline magmatism throughout much of the west margin of South America that led to the formation of the Patagonian batholith (Tangeman et al., 1996; Millar et al., 2002). Formation of the Rocas Verdes basin detached the southernmost part of the Patagonian batholith into a separate body known as the Fuegian batholiths (Hervé et al., 2007). Geochemical analyses from the south Patagonian batholith show variations from gabbroic to granitic composition for the Upper Jurassic rocks and a lower alkali content for the Lower Cretaceous rocks. Major-element analysis shows an average of 62.7% SiO2, 2.7% Na2O, 1.4% K2O and a negative correlation between SiO2 and Ca, Fe, Mg, Al, and Mn (Hervé et al., 2007), both of which are typical of calc-alkaline series rocks. Trace-element analyses show a decrease in incompatible elements with increasing silica (Hawkesworth et al., 1979) and spatially variable Eu anomalies, depending on the age of the batholith and the amount of SiO2 present (Hervé et al., 2007). On the basis of a higher content of Rb, Th, U, Ta, Nb, Hf, and Y and a lower content of Ba, Sr, P, Zr, and Ti, the southern Patagonian batholith has been identified as a mature magmatic arc (Brown, 1982). Sm–Nd isotopic analyses were conducted on both the north and south Patagonian magmatic-arc rocks, which show markedly different εNd values for upper Mesozoic versus Cenozoic volcanic deposits. Whereas Upper Jurassic to Cretaceous rocks show relatively evolved εNd values

2.3. Rocas Verdes marginal basin Extension and westward movement of the South American plate, in conjunction with extensional silicic volcanism during Early Cretaceous time, formed the Rocas Verdes basin, which filled with fine-grained clastic and volcaniclastic sediment (Suárez and Pettigrew, 1976; Willan and Hunter, 2005; Klepeis et al., 2010). This basin overlies interpreted remnants of seafloor known as the Sarmiento-Tortuga ophiolite complex that were exposed sporadically south of 51° latitude inboard of the Patagonian batholith (Stern et al., 1992). The ophiolites are composed of gabbros, sheeted dikes, pillow basalts, and tonalites (Calderón et al., 2007). Trace-element compositions, such as Ti and Zr, confirm the origin of these ophiolites as ocean-floor basalts (De Wit and Stern, 1981). Major-element analyses of gabbros, dikes, and leucogranites from the Sarmiento complex reveal a fairly consistent amount of SiO2 (~50%) but a wide range of Fe/Mg with a positive

Table 2 Nd isotope analytical data from Magallanes basin mudstones. Sample MC1AM PG1 CCD3M CCC1M CCB3M CCA2M PG08b IDK1M CCDM3M PN3 RUTA3B CL2A PD2A CerrosA

±2σ (10−6)

εNd (T)

0.147833

2.62

−5.27

0.512335

0.148713

1.40

−5.70

14.57

0.512388

0.146655

2.40

−4.66

3.83

14.43

0.512384

0.159664

2.26

−4.79

39

3.27

13.30

0.512481

0.147954

2.10

−2.82

39

5.52

15.42

0.512470

0.215379

2.56

−3.38

45

2.63

10.22

0.512425

0.154619

2.52

−3.91

45

2.04

8.13

0.512461

0.150718

2.64

−3.19

52

5.95

19.47

0.512494

0.183801

2.14

−2.73

54.8

4.40

17.46

0.512587

0.151730

5.94

−0.68

56

3.39

15.79

0.512412

0.135336

1.40

−3.97

63

2.46

9.41

0.512351

0.157671

4.43

−5.29

68

2.51

9.82

0.512396

0.153850

2.88

−4.35

68

2.08

11.82

0.512331

0.110941

1.50

−5.25

Locations Lat/Long

Calculated age (Ma)

Sm (ppm)

Nd (ppm)

143

Nd

147

Sm

144

Nd

144

Nd

−5429.60 −6622.89 −54 21.11 −66 38.42 −54 29.44 −66 23.01 −54 29.51 −66 22.99 −54 29.33 −66 22.97 −54 28.80 −66 26.95 −54 21.18 −66 38.61 −54 28.44 −66 28.35 −54 27.20 −66 29.14 −54 38.44 −65 50.44 −54 23.12 −67 15.02 −54 36.62 −65 55.96 −54 39.08 −65 33.41 −54 21.18 −67 54.37

31

3.39

13.81

0.512358

33

4.77

20.22

33.7

3.55

35

λ = 6.54 × 10−12. 143 Nd/144NdCHUR = 0.512638. 147 SM/144NdCHUR, today = 0.1967. 147 SM/144NdDM, today = 0.217. 2σ = 1%.

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K.M. Zahid, D.L. Barbeau Jr. / Chemical Geology 347 (2013) 69–81

(2.5 to −8.5, with an average of − 4.25), post-Cretaceous εNd values are more juvenile (6.5 to − 0.5 with an average of 2.1) with lower to middle crust affiliations (Pankhurst et al., 1999; Hervé et al., 2007).

revealed that the dominant sediment-transport direction in the eastern Magallanes basin is generally perpendicular to the orogen (Biddle et al., 1986; Galeazzi, 1998; Martinioni et al., 1999; Olivero, 2002; Olivero and Malumián, 2008). These studies indicate that the mountain belts to the west and south-southwest of the basin were contributing voluminous sediments to the east part of the basin from where all of the samples were collected and analyzed (Romans et al., 2010).

2.5. Magallanes fold-thrust belt and foreland basin The Magallanes basin, located inboard of the southern Patagonian batholith and between the Fuegian Andes and the Deseado Massif (Fig. 1), is a foreland basin developed as a result of retro-arc foldthrust belt formation at the southern Andean arc-craton convergence. Initiation of the formation of this foreland basin marks the beginning of a state of change from an extensional to a contractional setting in the southern Andes. The Magallanes is a sizable basin that stretches along the southern part of South America, straddles across Chile and Argentina, and contains a nearly complete sedimentary record of hinterland exhumation and subsequent sediment flux from the Late Cretaceous through the Miocene of both Patagonian and Fuegian Andes (Biddle et al., 1986; Wilson, 1991; Olivero and Martinioni, 2001; Fildani and Hessler, 2005; Klepeis et al., 2010). Paleocurrent data, reflection seismology, and sediment petrography studies have

3. Methods Fourteen mudstone samples were collected from the Argentina sector of the Magallanes basin along the Atlantic coast of Tierra del Fuego for analysis (Fig. 1). Approximately 20 g of sample was used for initial crushing, 200-mg ball-mill powders of which were used for analysis. Each sample was ignited in a 900 °C muffle furnace for 16 h to remove carbon and volatiles. For acid bomb digestion, 7 N (~ 50% concentrated) nitric acid and 100% hydrofluoric acid were used at a proportion of 1:9 in high-pressure Teflon bomb vessels. To ensure complete dissolution of all heavy minerals, the bombs were maintained in stainless steel containers at 150 °C for 84 h. Once the

30 OLI.

E

RUPELIAN

33.7

35

L

PRIABONIAN

37.0 BARTONIAN

41.3

M

EOCENE

45

TERTIARY

Age (Ma)

40

LUTETIAN

49.0

50

E

MC1AM PG1 CCD3M CCC1M CCB3M CCA2M IDK1M PG08B CCDM3M PN3 RUTA3B CL2A PD2A Cerros A

YPRESIAN

54.8

55 THANETIAN

M

SELANDIAN

57.9

PALEOCENE

60

L

61.0

E

DANIAN

65 CRET.

65 MAASTRICHTIAN

70 -7

-6

-5

-4

-3

-2

-1

0

1

ε Nd(t) Patagonian Batholith North Cretaceous

Eocene

Patagonian Batholith South L. Jurassic to M. Cretaceous

M. Cretaceous to Neogene

Jurassic Volcanics Eastern Andean Metamor. C Patagonian Basalts & Sarmineto -12

-10

-8

-6

-4

ε Nd(0)

-2

0

2

4

6

Fig. 3. εNd(t) values from mudstone of Magallanes basin sediments plotted against their depositional age. Data given in Table 2; 0.2 epsilon units error bars shown for each analysis. Depositional ages from Malumián and Olivero (2006), Olivero and Malumián (1999), Olivero et al. (2003). At the bottom are ranges of εNd(0) values from possible source regions from previous studies of Hawkesworth et al (1979), Pankhurst and Rapela (1995), Pankhurst et al. (1999), Hervé et al. (2007), Augustsson and Bahlburg (2008). εNd(0) values for published data are plotted with their range (vertical bars connected to horizontal lines), mean (bold vertical bars), and standard deviation (gray horizontal bands). NPB: north Patagonian batholith; SPB: south Patagonian batholith; Jur. Volc.: Jurassic volcanics; EAMC: Eastern Andean metamorphic complex; PBS: Patagonian basalts and Sarmiento.

K.M. Zahid, D.L. Barbeau Jr. / Chemical Geology 347 (2013) 69–81

Whole-rock REE analyses of 11 samples from the Magallanes foreland basin reveal REE patterns that are generally characterized by an overall enrichment of light over HREEs, low to moderate negative Eu anomalies (Eu:Eu⁎), Ce anomalies (Ce:Ce⁎) in some samples, and a flat, heavy, REE pattern (see Table 1 notes for Eu⁎ and Ce⁎ definitions, Fig. 2A). Eu anomalies range from 0.63 to 0.97, with an average value of 0.74, which is slightly higher than the average upper continental crustal value of 0.65 (Taylor and McLennan, 1985; McLennan et al., 1990). Four samples exhibit Ce anomalies (CL2A and MC1AM show negative anomaly, whereas PD2A and CCC1M show positive anomaly), although they do not indicate any resetting of the Sm–Nd system, as indicated by their Nd isotope analysis (McDaniel et al., 1994). Many of the older Upper Cretaceous to Eocene samples have lower LaN:SmN than do most of the younger Oligocene samples, indicating a different concentration of LREE (Fig. 2B). However, no gradual temporal variations are evident in these samples for the REE. The εNd values from 14 samples deposited during Late Cretaceous to Oligocene time for this study are presented in Table 2, with overall values ranging from − 0.68 to − 5.70. The three Upper Cretaceous and Paleocene samples have εNd values of − 4.35 to −5.29, whereas the overlying seven lower to middle Eocene samples have much less negative εNd values, ranging from − 0.68 to − 3.97 (Fig. 3). Of the seven lower to middle Eocene samples, six occur within a small standard deviation of 0.53. Sample PN3 from the Punta Noguera Formation has the least negative (− 0.68) εNd value of the analyzed samples. The four upper Eocene and Oligocene samples, on the other hand, transition to more strongly negative values, ranging from − 4.66 to − 5.70. Previously published εNd values from the source area rocks for Magallanes basin sediments are plotted in Fig. 3 for the purpose of comparison with our data (see caption of Fig. 3 for references for these data). We have identified five areas that could potentially contribute sediments to the Magallanes basin. εNd values from the north Patagonian batholith range from − 7.43 to 2.54 (mean: − 3.20) for the Cretaceous unit and − 0.43 to 5.07 (mean: 1.94) for the Eocene unit. The south Patagonian batholith shows εNd values ranging from −8.49 to −2.07 (mean: −5.39) for Upper Jurassic to Middle Cretaceous rocks, whereas Middle Cretaceous to Neogene rock εNd values range from −1.56 to 6.53 (mean: 2.12). Among other possible source rocks, Jurassic volcanic rocks from the Chon Aike province have isotopic compositions of εNd = −10.07 to −1.17 (mean: −5.85), eastern Andean metamorphic complex rocks contain εNd values of between −10.75 and −6.59 (mean: −8.83),

12 MC1AM CCD3M CCC1M CCB3M CCA2M IDK1M PG08B CCDM3M PN3 CL2A PD2A

10

8

Zr/Y

4. Results

and those of Patagonian basalts and Sarmiento mafic rocks range from −0.35 to 7.06 (mean: 3.49). To exclude the presence of excess amounts of heavy minerals that could bias the REE patterns of our samples, we have plotted the concentration of Zr and Y for the detection of a high abundance of detrital zircon (Fig. 4A). Similar plotting was done with Gd and Yb for anomalous monazite (Fig. 4B) addition to our samples. A relatively flat pattern of heavy REE (Fig. 2A) and a lack of positive correlation in the Zr:Y versus Zr plot (Fig. 4A) indicate an absence of any unusual abundance of zircons in our samples (McLennan, 1989). Monazite has a Gd concentration of ~ 14,000 ppm (Lee and Bastron, 1967), and the concentration of excess monazite results in highly fractionated HREE, which is absent in post-Archean sediments. Sediments from our study area exhibit no unusual abundance of monazite either, as evidenced by its typical concentration of Gd (2–6 ppm) and a GdN:YbN ratio of 2.0 (McLennan, 1989). The ratio of Th:Sc can often be used to distinguish between mafic and felsic sources, with higher ratios of Th:Sc indicating more felsic composition, and vice-versa. Fig. 5 depicts plots of Th and Sc against one another and three other variables (age, εNd, and La concentration). The Th versus Sc plot in Fig. 5A shows an intermediate position of Magallanes foreland basin sediments between trailing-edge and active-margin setting values. Stratigraphically younger Oligocene sediments have a Th:Sc ratio closely resembling a Th:Sc = 1.0 (trailing edge), whereas older Upper Cretaceous to Eocene sediments are more

6

A

4

2

0 0

50

100

150

200

250

Zr (ppm) 100

B Shale

GdN

solutions were cooled to room temperature, repeated mixing with acids was performed using 5 mm of HNO3 (7 M) and HCl (6 M), followed by a complete drying using a hot plate and heating lamp. Trace elements were analyzed with a Thermo Scientific Element2® plasma mass spectrometer (ICP-MS) at the University of South Carolina. Nd isotope ratios were collected using a Thermo Scientific Neptune® multicollector plasma mass-spectrometer (MC-ICP-MS) at the University of California at Santa Cruz. Trace-element concentrations were determined by comparison with measured values of the U.S. Geological Survey rock standard BCR-2. Uncertainties were low (b2%) for REE measurements, given repeated measurement of USGS-basalt BHVO as unknown samples. Oxide interference corrections for Sm, Eu, and Gd were made based on previous runs of REE solutions. Laboratory blanks were negligible for Nd isotopic measurements ( 146Ndblank = 0.00070 ppm). Mass fractionation correction used 146Nd/ 144Nd = 0.7219 (Wasserburg et al., 1981). 147Sm was monitored for Sm correction. Data were normalized to 0.511858, using the mean of La Jolla analyses (0.511847) for the time period during which the samples were run (six analyses with standard deviation of 2.9E-6).

75

10

Sandstone

1 0.5

1

1.5

2

2.5

3

3.5

4

GdN/YbN Fig. 4. Plot of Zr versus Zr:Y and GdN:YbN versus GdN values of samples from study area. The two lines show typical values of Gd and Yb in sandstones and shales. The lack of preferred orientation in Zr vs. Zr:Y plot in A and GdN:YbN value of less than 15 in B indicates no excess presence of zircon or monazite heavy minerals. Sand and shale lines are from McLennan, 1989.

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100.00

20

Th/Sc=1.0 Th/Sc=0.1

Trailing edge & continental collision

A

B 30

MC1AM CCD3M CCC1M CCB3M CCA2M IDK1M PG08B CCDM3M PN3 CL2A PD2A

Active margin 1.00

0.10 10.0

Age (Ma)

Th (ppm)

10.00

50

60

70

100.0

Sc (ppm)

40

0

0.2

0.4

0.6

0.8

1

Th/Sc

La

15

C 10 MORB

D

Felsic Componenet Arc Andesite

εNd(t)

5

Upper Crust

Mafic Componenet

0 -5 Island Arc Bulk Crust

-10 Upper Crust

-15 -20 0.01

Oceanic Crust

Old Crust

0.1

1

10

Th

Sc

Th/Sc Fig. 5. (A) Sc versus Th plot of all samples. Two lines representing Th:Sc ratio of 1 and 10 indicate average values for trailing edge and continental collision and active margin sediment values, respectively. (B) Plot of Th:Sc against sample depositional age showing trend with age. C. Th:Sc ratio plotted against εNd(t) showing a mix in provenance between the upper crustal and arc andesite source. (D) Ternary plot of same sediment for La, Th, and Sc showing provenance similar to that of Th:Sc vs. εNd(t) plot, again indicating mix between these two aforementioned sources. Fields for different tectonic settings are from Taylor and McLennan, 1985.

associated with an active margin (Th:Sc = 0.1) setting. A plot of Th:Sc versus the age of samples shows a progressively higher Th:Sc ratio with a younger depositional age (Fig. 5B). A plot of Th:Sc with respect to εNd shows an intermediate composition for all samples occupying a position in between the upper crust and arc-andesitic field (Fig. 5C). Samples from the eastern Magallanes basin occupy an intermediate position between the upper crustal block and the Island Arc source in the La–Th–Sc ternary plot (Fig. 5D), indicating a possible mix of these two source materials (Taylor and McLennan, 1985; McLennan et al., 1990). A stratigraphic relationship also exists with these samples, as evidenced by the proximity of the older samples to island-arc composition and younger sediments to the upper crustal composition in this plot. Universal Eu depletion in the upper crust generally indicates intracrustal melting, in which the Eu is left in residual plagioclase in the lower crust. As a result, average upper crustal rocks have a net negative Eu anomaly in their REE composition. Fig. 6A shows the plot of Eu* versus Eu, in which Eu anomalies of all samples are close in an average upper crustal value of 0.65 (solid line), with one exception that falls closer to the 1.0 ratio line (broken line). The Eu:Eu* versus GdN:YbN plot in Fig. 6B indicates that many of the sampled units fall in an overlapping field of active tectonic and cratonic sources. Although four of the younger sampled units fall in the “cratonic” field, and all seven of the older samples fall in the “tectonic” field (Taylor and McLennan, 1995), we suggest that most samples derived from an active tectonic setting, given the overlapping position of these fields.

Plots of Th:U versus Th are used to determine mafic contribution in the source area, as well as the degree of weathering. A higher Th:U ratio generally indicates oxidation during sedimentary processes, and a low Th:U ratio reflects a source of depleted mantle materials (McLennan et al., 1993). Samples for our study show a trend in weathering (Fig. 7A) and no correlation to the average upper crustal value (Th:U = 3.5–4; McLennan et al., 1993). These samples show a broad range of Th:U from 1.7 to 5.4. Of the four samples that contain a Ce anomaly (both positive and negative), three also have a greater Th:U ratio, indicating a possible weathering effect. However, none of the samples has a Th:U ratio low enough to fall in the depleted upper mantle field proposed by McLennan et al. (1993). Fig. 7B shows Cr:V versus Y:Ni as an indicator of ultramafic components in the source region. Y:Ni is used to evaluate the importance of a mafic/ ultramafic source component and a Cr:V index is used to express the enrichment of chromium compared with that of other ferromagnesian elements. Samples from our study area have a moderately low and narrow range in Cr:V (~ 0.5) but a large range of Y:Ni (0.3–2.1), indicating the lack of excess chromium in the source-rock composition. Older samples have an overall Y:Ni which is lower than that of younger samples. 5. Interpretations Whole-rock REE analysis of 11 samples from the Magallanes foreland basin reveals REE patterns that are similar to those of the average

K.M. Zahid, D.L. Barbeau Jr. / Chemical Geology 347 (2013) 69–81

3 Eu/Eu*=0.65 Eu/Eu*=1

Average upper Crust

77

6

A

A 5 4

Eu*

MC1AM CCD3M CCC1M CCB3M CCA2M IDK1M PG08B CCDM3M PN3 CL2A PD2A

1

0

Th/U

2

0

1

2

Weathering trend

Typical Upper crustal rocks

3 Depleted Upper Mantle

2 1 0 0

4

6

8

10

14

3 3

B Ophiolite (Chromite)

1.4

B

MC1AM CCD3M CCC1M CCB3M CCA2M IDK1M PG08B CCDM3M PN3 CL2A PD2A

2

Cr/V

Post Archean active tectonic Post Archean cratonic GdN/YbN= 2.0 Eu/Eu*= 0.85

1.2

1

1

0.8 0

0

0.5

1

1.5

2

2.5

3

Y/Ni

0.6

0.4 0.5

12

Th

Eu (ppm)

Eu/Eu*

2

1

1.5

2

2.5

3

3.5

GdN/YbN Fig. 6. (A) Eu* versus Eu plot for Magallanes basin sediments showing closer affinity to typical average upper crustal source values of 0.65. (B) Eu:Eu* versus GdN:YbN plot showing all samples falling in individual and overlapping fields of both post-Archean active and cratonic fields. Fields for different tectonic settings are from McLennan, 1989.

post-Archean upper continental crust (McLennan, 1989; McLennan et al., 1993) throughout the Upper Cretaceous to middle Oligocene strata. Although there are no strict temporal trends and large overlap exists between the groups, the older Upper Cretaceous to Eocene samples have an overall lower LaN:SmN and a correspondingly lower concentration of LREE than the younger Oligocene samples (Fig. 2B). The concentrations of HREE also have overall lower values in older Upper Cretaceous to middle Eocene samples but indicate an overall flat pattern (low Gd:Yb ratio) in all samples typical of those of the upper continental crust (Taylor and McLennan, 1995). We compared the REE pattern of our Magallanes basin samples to those of published data from the source region (Bruhn et al., 1978; Saunders et al., 1979; Stern and Stroup, 1982; Faundez et al., 2002; Fildani and Hessler, 2005; Hervé et al., 2007; Augustsson and Bahlburg, 2008). Four end members (Eastern Andean metamorphic complex, Patagonian batholith, Sarmiento rocks, Tobífera volcanics) were identified as source regions, and comparisons were made on the basis of chondritenormalized concentrations of heavy and light REE. Although the trends are not completely clear, the data (LREE) suggest that Maastrichtian to middle Eocene mudstones from the Magallanes basin show an affinity to Tobífera mafic volcanics and Sarmiento rocks, whereas upper Eocene to lower Oligocene rocks show preferred association with the Patagonian batholith and Eastern Andean metamorphic complex (Fig. 2B). Both negative and positive Ce anomalies are present in REE patterns

Fig. 7. (A) Th versus Th:U plot showing weathering trend in samples as a result of oxidation and subsequent removal of U. A higher Th:U ratio indicates more weathering. (B) Cr:V versus Y:Ni plot showing Cr:V as an index for the enrichment of Cr in comparison with other ferromagnesian trace elements. Fields for different tectonic settings are from McLennan et al., 1993.

in some of the samples, regardless of their stratigraphic distribution (Table 1; Fig. 2). Ce anomalies present in Magallanes basin samples suggest a result of the weathering process, in which the oxidizing environment caused the Ce+4 to be removed from the system. In a diagram (Fig. 3) with εNd plots comparing values between the source region and our Magallanes basin samples, the older Upper Cretaceous to lower Paleocene mudrocks show an affinity to Jurassic volcanics and the Eastern Andean metamorphic complex (εNd = ~−4 − −6). The overlying upper Paleocene to middle Eocene samples show values similar to those of pre-Cretaceous Patagonian batholithic rocks (εNd = ~−1 − −4), with some degree of mixing with Jurassic volcanic rocks. The uppermost Eocene and Oligocene samples again shift back to a more negative value (εNd = ~−4.0− −5.75), suggesting more of the Jurassic volcanics, with probable input from Eastern Andean metamorphic rocks. Plots comparing the most incompatible element, Th, and the most compatible element, Sc, with other variables, such as εNd and La, show an intermediate composition of Magallanes basin samples between the upper crust and arc andesite (Fig. 5C) and oceanic crust (Fig. 5D). The samples also show an intermediate composition between an active margin and a continental collision setting in the Th versus Sc plot (Fig. 5A). All of these plots indicate a roughly homogenous mix of sediments from different tectonic regions. However, a change in sediment provenance occurs for the overlying youngest three samples (CCC1M, CCD3M, and MC1AM), starting at lower Oligocene time for all aforementioned plots, as well as for a plot of Th:Sc versus age (Fig. 5B). Because a lower Th:Sc ratio generally indicates a mafic provenance and a higher ratio is indicative of a felsic provenance, we suggest that a shift occurred in provenance in middle

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K.M. Zahid, D.L. Barbeau Jr. / Chemical Geology 347 (2013) 69–81

Table 3 Elemental concentration (n = 51) matrices used in principal component analysis. Eigenvalues:

F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11

Factor scores: Eigenvalue

Variability (%)

Cumulative %

Sample

F1

F2

F3

F4

F5

32.190 5.530 3.612 3.231 2.702 1.267 1.004 0.574 0.497 0.246 0.149

63.117 10.843 7.082 6.334 5.298 2.484 1.969 1.125 0.975 0.482 0.292

63.117 73.960 81.042 87.376 92.674 95.158 97.127 98.251 99.226 99.708 100.000

MC1AM CCD3M CCC1M ccb3m CCA2M IDK1M PG08b CCDM3M PN3 CL2A PD2A

10.619 −0.700 5.853 3.879 −2.480 −8.136 −1.425 1.254 2.126 3.308 −3.270

−2.453 −0.459 −0.627 −1.516 −0.677 1.892 4.143 −0.688 −0.309 3.635 1.539

−1.148 −2.616 −1.974 0.986 −1.369 −2.402 1.606 1.287 4.087 0.314 −0.048

−1.560 1.311 −0.016 1.076 1.263 1.178 −1.693 −0.506 3.178 −2.749 1.332

−0.319 −1.411 −0.431 −0.479 −0.369 1.278 −4.010 1.434 0.473 2.986 0.930

to late Eocene time from a mafic to nonmafic/felsic source (Barbeau et al., 2009; Zahid and Barbeau, 2010). To reduce the interference between significant and insignificant variables in our large data set (in terms of number of elements analyzed), we performed a principal component analysis (PCA) to obtain two-dimensional views of the global variability of our samples. PCA is a mathematical technique that reduces the dimensionality of a complex data set into a smaller number of dimensions that accounts for most of the data variance (Davis, 2002). The eigenvalues reflect the quality of the projection from the N-dimensional initial table (N = 51 which is the total number of elements analyzed) to a lower number of dimensions. In Table 3, we can see that the first eigenvalue is 32.19 and represents 63% of the total variability. Each eigenvalue corresponds to a factor (F), and each factor to one dimension. The first three factors represent 81% of the initial variability of the data. The eigenvalues and the corresponding factors are sorted by descending order of how much of the initial variability they represent (converted to %). The transformed data (Table 3) were plotted with the first three principal components showing the origin (Fig. 8). Variability in elemental concentration is what dominates the main factor F1 and no isotope ratio data were used for determining this factor. By plotting this figure with the three most important dimensions, we can see some minor correlation and trend between the samples in two-dimensions. For the first two factors (F1 and F2), we see that some of the older samples (PD2A, PG08b, and IDK1M) plot in a different

quadrant compared to younger samples like MC1AM, CCB3M, and CC1M. For the second two factors (F2 and F3), the four younger samples (MC1AM, CCD3M, CCC1M, and CCA2M) are also separated from the rest. This orthogonal position of the younger samples compared to the older ones represents a negative correlation and implies different chemical composition of the source area. We constructed a comparative provenance diagram (Fig. 9) by plotting previous estimates of sediment provenance based on detrital zircon ages (Barbeau et al., 2009) and distribution of heavy mineral occurrences (Zahid and Barbeau, 2010) along with geochemical data plot of Th:Sc ratios and εNd values from this study. The detritalzircon data record a prominent shift in sediment provenance in Magallanes basin sediments from the Patagonian–Fuegian magmatic arc to the Gondwana break-up-related silicic volcaniclastic and Cordillera Darwin metasedimentary rocks at ~ 39 Ma. Heavy-mineral study of Upper Cretaceous to lower Miocene sediments from the same region also indicates a rapid change in sediment input from the Patagonian magmatic arc and Rocas Verdes basin to the Cordillera Darwin metasedimentary provenance during middle to late Eocene time (Zahid and Barbeau, 2010). This fundamental shift in sediment provenance is consistent with our geochemical data in this study, in which a shift from a mafic to a felsic source in sediment is evident from the Th:Sc plot during middle to late Eocene time (Fig. 9A). Results from Nd isotope data also indicate a similar shift in εNd values, from a less negative (average: − 2.95), juvenile magmatic-arc-type

Observations (axes F2 and F3: 17.93 %)

Observations (axes F1 and F2: 73.96 %) 5

10

PN3

4 5

3

PG08b

IDK1M

F3 (7.08 %)

F2 (10.84 %)

CL2A PD2A CCD3M

0 CCA2M

PN3 CCC1M

CCDM3M

CCB3M

MC1AM

2

PG08b CCDM3M CCB3M

1

CL2A

0 PD2A

-1

-5

CCA2M

MC1AM

CCC1M

-2

CCD3M

-10 -15

-10

-5

0

F1 (63.12 %)

5

10

15

-3 -5

-4

-3

-2

-1

0

IDK1M

1

2

3

4

5

F2 (10.84 %)

Fig. 8. PCA plots with the first three important factors of the corresponding eigenvalues for 51 analyzed elements. The F1 vs F2 plot accounts for ~85% of the data variance of the analyzed samples. Total number of elements analyzed is 51 and total number of samples is 11. Eigenvalues and converted factor scores are given in Table 3.

K.M. Zahid, D.L. Barbeau Jr. / Chemical Geology 347 (2013) 69–81

GEOCHEMICAL ANALYSIS

Depositional Age (Ma)

C

B

20

20

30

30

D

20

JV

MetaSed

30

Evolved

Mixed

CDMC

40

50

HEAVY MINERALS

DETRITAL ZIRCON AGE

A

Felsic

79

40

40

Juvenile

50

50 Batholith

Mafic

60

0.2

0.4

0.6

Th/Sc

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1

60

60

70

0

Mafic

70

70

-7 -6 -5 -4 -3 -2 -1

0

ε Nd(t)

0

20

40

60

80

100

0

20 40 60 80 100

% of total zircon population % total h. mineral population

Fig. 9. Comparative provenance diagrams from geochemical, geochronological, and mineral composition analysis. (A) Th:Sc plot versus age from this study; (B) εNd(t) values plotted against age from this study; (C) U–Pb Detrital zircon ages from Barbeau et al. (2009); (D) Heavy mineral provenance data from Zahid and Barbeau (2010). For sample symbol legend in A and B, refer to Figs. 2 and 3 respectively. JV: Jurassic Volcanics; CDMC: Cordillera Darwin Metamorphic Complex; MetaSed: Metasedimentary.

source to a more negative (average: − 5.11), evolved basement-rocktype source at around the same time (McLennan and Hemming, 1992; Gleason et al., 1995). Scher and Martin (2004, 2006) analyzed fossil fish teeth for Nd isotopes from Southern Ocean drill-core samples and discovered a sudden influx of Pacific water into the Southern Atlantic Ocean ca. 41 Ma, which they interpreted as indicating an initial opening of the Drake Passage. Gombosi et al. (2009) conducted thermal modeling using fission-track and (U–Th–Sm)/He isotopic analysis of the Cordillera Darwin complex and adjacent rocks from the Fuegian Andes. Their data indicate rapid cooling during the middle and late Eocene (ca. 48–45 Ma), followed by a slow cooling to the recent. They interpreted this accelerated cooling event as a result of rapid exhumation by shortening and uplift of the Fuegian Andes crystalline hinterland rocks via a contractional mechanism. Although an alternative interpretation of basement uplift by an extensional mechanism has been proposed and supported (Dalziel and Brown, 1989; Ghiglione et al., 2008), recent structural and kinematic analyses of Paleogene tectonics of Fuegian Andes indicate a predominantly contractional mechanism for such exhumation (Nelson, 1982; Wilson, 1991; Klepeis, 1994a,b; Kraemer, 2003; Ghiglione and Ramos, 2005; Torres-Carbonell et al., 2008). Our geochemical data depicting a moderate shift in mudstone composition ca. 38 to 36 Ma are consistent with detrital-zircon and heavy-mineral provenance proxies, as well as thermal modeling of hinterland rocks and kinematic interpretation of deformational style, which indicate a rapid contractional exhumation during the middle to late Eocene of the Fuegian Andes hinterland. 6. Conclusions 1. Upper Cretaceous to middle Oligocene mudstone samples from the Magallanes basin have REE patterns typical of those of average post-Archean upper continental crust. Although there are no strict temporal trends, the older Upper Cretaceous to middle Eocene

samples contain overall lower LaN:SmN than do younger middle Eocene to lower Oligocene samples. The higher LaN:SmN ratios of the Eocene–Oligocene samples are comparable to those of the Eastern Andean metamorphic complex, which suggest a possible provenance for these rocks. No anomalously high abundance of heavy minerals seems to occur in these mudstones. Trace element ratios, such as Th:Sc, are important indicators of mafic versus felsic sources. Such Th:Sc values exhibit a lower value for older samples than for younger samples, suggesting a mafic origin for the Upper Cretaceous to middle Eocene mudstones. 2. When plotted on tectonic-setting discrimination diagrams, all samples show an intermediate position between active arc and upper crustal settings, indicating a homogenous mix of sediments from two possible major sources. In these plots, stratigraphically older (Late Cretaceous to Eocene) samples show a roughly trend toward the arc andesite and island arc setting, whereas younger samples (Oligocene) seem to have an affinity for the upper crustal setting. 3. Nd isotopic analyses from these mudstones have more negative εNd values (~− 4 − − 6) for Upper Cretaceous to lower Paleocene rocks but less negative values (~− 1 − − 4) for upper Paleocene to middle Eocene rocks. When compared with source-region Nd isotopic values, a provenance change occurs in the middle to upper Eocene strata. The younger upper Eocene to Oligocene samples suggest derivation from Jurassic volcanics and metasedimentary units, whereas underlying upper Paleocene to middle Eocene samples have values similar to those of Upper Cretaceous Patagonian batholithic rocks. 4. Geochemical analyses of Magallanes basin mudstones show a shift in sediment provenance around middle to late Eocene time, consistent with results from previous provenance studies based on detrital-zircon geochronology and sandstone heavy minerals. We interpret this shift as an indication of rapid exhumation of Fuegian Andes rocks.

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Acknowledgments This research was funded primarily by the National Science Foundation Office of Polar Programs Grant (ANT-0732995) awarded to DLB. Other sources of funding were the University of South Carolina Research and Productive Scholarship program awarded to DLB and a Geological Society of America Research Grant awarded to KMZ. Michael Bizimis and Howie Scher helped analyze the samples in their labs and provided further assistance in the post-analysis stage. We appreciate the two reviewers and the editor for substantially improving this manuscript by their thoughtful comments and suggestions.

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