Gondwana Research 23 (2013) 1567–1580

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Petrogenesis of Cretaceous mafic intrusive rocks, Fosdick Mountains, WestAntarctica: Melting of the sub-continental arc mantle along the Gondwana margin

S. Saito a,⁎, M. Brown a, F.J. Korhonen a,b, R.R. McFadden c, C.S. Siddoway d

a Laboratory for Crustal Petrology, Department of Geology, University of Maryland, College Park, MD 20742, USAb Geological Survey of Western Australia, East Perth 6004, Australiac Department of Geological Sciences, Salem State University, 352 Lafayette Street, Salem, MA 01970, USAd Department of Geology, Colorado College, Colorado Springs, CO 80703, USA

⁎ Corresponding author at: Food & Health Risk Projemanity and Nature, 457-4 Motoyama, Kamigamo, KitTel.: +81 75 707 2265; fax: +81 75 707 2506.

E-mail address: saitotetsu@chikyu.ac.jp (S. Saito).

1342-937X/$ – see front matter © 2012 International Ahttp://dx.doi.org/10.1016/j.gr.2012.08.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 November 2011Received in revised form 2 August 2012Accepted 2 August 2012Available online 14 August 2012

Handling Editor: A.S. Collins

Keywords:Fosdick MountainsGeochemistryMafic magmatismSr and Nd isotopesWest Antarctica

A diorite pluton and widely distributed mafic dykes occur in the Fosdick migmatite–granite complex, whichis interpreted to represent middle-to-lower crustal rocks of the paleo-Pacific active continental margin ofGondwana. The mafic dykes exhibit a variety of relationships with host rocks in the field ranging fromundeformed dykes with sharp contacts with host gneisses to dismembered dykes with commingled texturesand numerous back-veins of leucosome intruded from host migmatitic gneisses suggestive of significant in-teraction with crustal rocks. New U–Pb ages for magmatic zircon in these rocks yield Cretaceous crystalliza-tion ages ranging from ca 113 Ma to ca 98 Ma for the mafic dykes and ca 100 Ma for the diorite pluton. Thesemafic intrusive rocks, which contain abundant hydrous minerals, are medium- to high-K-series calc-alkalinerocks with basic–intermediate compositions (47–59 wt.% SiO2 for the mafic dykes and 52–56 wt.% SiO2 forthe diorite pluton). They have trace element patterns characterized by LILE enrichments and negative Nbanomalies indicating an origin from a hydrous mantle source metasomatized by slab-derived components.The samples without evidence of interaction with crustal rocks (11 of 14 samples), which are likely to betterreflect the mantle source composition, have positive εSr(100 Ma) values (+8.1 to +14.5) and negative toslightly positive εNd(100 Ma) values (−1.6 to +2.5) consistent with derivation from an enriched mantlesource. These eleven samples may be divided into two groups either characterized by higher LILE/HFSE ratios,less radiogenic εSr(100 Ma) values and more radiogenic εNd(100 Ma) values, or characterized by relatively lowerLILE/HFSE ratios, more radiogenic εSr(100 Ma) values and less radiogenic εNd(100 Ma) values suggesting differ-ences in the mantle source. The results of this study are consistent with the melting of a variablymetasomatized sub-arc mantle source during a transition from a wrench to a transtensional tectonic setting,but are inconsistent with a mantle plume origin.

© 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

The Fosdick migmatite–granite complex in western Marie Byrd Land,West Antarctica, has been interpreted as the former middle-to-lowercrust of the paleo-Pacific active continental margin of Gondwana that ex-perienced rapid exhumation as a result of intra-continental crustal exten-sion in the Cretaceous (Richard et al., 1994; Siddoway et al., 2004;Siddoway, 2008; Korhonen et al., 2010a,b; McFadden et al., 2010a,b). Re-cent petrological studies on the rocks of the FosdickMountains have shednew light on crustal processes at depth in an active continental arc, in-cluding the mechanism of intra-crustal differentiation through crustalanatexis and melt loss to shallower crustal levels during polyphase

ct, Research Institute for Hu-a-ku, Kyoto 603-8047, Japan.

ssociation for Gondwana Research.

high-temperaturemetamorphism (Korhonen et al., 2010a,b, 2012). How-ever, the nature of mantle activity beneath the region has not been fullyresolved despite numerous geochronological and kinematic studies onthe mafic dykes in the Fosdick Mountains (Richard et al., 1994; Smith,1997; Siddoway et al., 2005; McFadden et al., 2010a, b).

An important issue regarding the relationship between tectonicsandmagmatism inMarie Byrd Land during the Cretaceous is that of un-derstanding the transition from subduction related convergence to con-tinental rifting, which led to the formation of the West Antarctic Riftsystem (Siddoway, 2008). The transition is reflected in changes to thetype of magmatism, for example from 'I-type' (124–108 Ma) to'A-type' granites (ca 102–95 Ma) along the Ruppert–Hobbs coast(Weaver et al., 1994), and in the widespread emplacement of maficdykes throughout Marie Byrd Land (ca 112–95 Ma; Weaver et al.,1994; Storey et al., 1999; Siddoway et al., 2005). Mafic dykes are com-monly emplaced during crustal extension and provide valuable informa-tion on the orientation of the regional stress field as well as aboutprocesses operating in the mantle leading to generation of the magmas.

Published by Elsevier B.V. All rights reserved.

1568 S. Saito et al. / Gondwana Research 23 (2013) 1567–1580

The transition from subduction related plate convergence tointra-continental rifting has been argued to be due to various mecha-nisms, including ridge–trench interaction and transfer of Zealandia tothe Pacific plate (Luyendyk, 1995; Mukasa and Dalziel, 2000), possiblyin association with the presence of a mantle plume (Weaver et al.,1994; Storey et al., 1999), the collision of the Hikurangi plateau withthe trench (Davy and Wood, 1994; Mortimer et al., 2006; Davy et al.,2008), or horizontal stresses arising from buoyancy of the mantlewedge (Rey and Müller, 2010). The mantle plume hypothesis in partic-ular has been questioned by several groups. Arguments put forth

Fig. 1. (a) Geographical map of Marie Byrd Land. (b) Reconstruction of the East Gondwana mfor mafic rocks in this study.Panel a: after Pankhurst et al., 1998; panel b: modified after Tulloch et al., 2006; panel c: afteal. (2010b).

against the mantle plume hypothesis include the low magma produc-tion rates during the Cretaceous in the SW Pacific in general relativeto the voluminous magma production associated with plume-relatedlarge flood basalt provinces (Finn et al., 2005), the unrealistically shorttime interval in Marie Byrd Land (5–10 Ma) for the switch fromsubduction-related to plume-related magmatism (Dalziel et al., 2000;Mukasa and Dalziel, 2000) and the absence of regional uplift in MarieByrd Land at the time of breakup (LeMasurier and Landis, 1996).

A small diorite pluton and mafic dykes of Cretaceous age occur inthe Fosdick Mountains of western Marie Byrd Land (Richard et al.,

argin at ca 95 Ma. (c) Geological map of the Fosdick Mountains showing sampling sites

r McFadden et al., 2007, 2010b. * Signifies SHRIMP U–Pb titanite ages fromMcFadden et

Fig. 2. Field occurrence of the mafic dykes in the Fosdick Mountains. (a) Dismembered,commingled, and folded dykes with numerous back veins of leucosome intruded fromhost migmatitic gneisses (M5-ME145). Dismembered dyke blocks are subangular tosubrounded. (b) Planer dyke showing sharp contact with host gneisses (C5-Mj70A).This figure is modified from a similar version published in McFadden et al. (2010b) andis used in accordance with the publication rights policies of AGU journals.

1569S. Saito et al. / Gondwana Research 23 (2013) 1567–1580

1994; McFadden et al., 2010b). Recent kinematic analysis of the em-placement of granite sheets and contemporaneous mafic dykes inthe Fosdick Mountains shows that the mafic dykes were emplacedduring a transition from wrench to transtensional tectonics(McFadden et al., 2010a, b), which is consistent with a regional tec-tonic model of transcurrent tectonics for the Cretaceous Gondwanamargin (Siddoway et al., 2005). The mafic rocks provide an opportu-nity to investigate whether their geochemical features are consistentwith the plume hypothesis or whether an alternative petrogeneticmodel is required, which will contribute to a better understandingof the controversial tectonic setting of Cretaceous magmatism inthis area.

In this paper, we present new zircon U–Pb ages, whole rock majorand trace element geochemistry, and Sr–Nd isotope compositions formafic dykes and the diorite from the Fosdick Mountains to: (1) betterconstrain the range of crystallization ages for the mafic intrusiverocks, (2) determine the geochemical characteristics of these rocks,and (3) discuss the likely petrogenesis and type of mantle source im-plied by the new data. The results presented here is not consistentwith the plume hypothesis, and suggest that the mafic intrusive rocksin the FosdickMountainswere derived by themelting of sub-arcmantlethat was previously metasomatized by slab-derived components be-neath the proto-Pacific active continental margin of Gondwana.

2. Geological background

Marie Byrd Land is the largest tectonic element within West Ant-arctica (Fig. 1a) and one of a group of terranes that previously com-prised the Palaeozoic–Mesozoic active continental margin of EastGondwana (Fig. 1b; Siddoway, 2008). During prolonged subductionof the paleo-Pacific plate, a Neoproterozoic–Cambrian marine quartz-ose turbidite sequence, known as the Swanson Formation (Bradshawet al., 1983; Adams, 1986; Adams et al., 2005), accumulated in an ac-cretionary complex along the active convergent margin of East Gond-wana. In Marie Byrd Land a subduction-related Devonian–Carboniferouscalc-alkaline ‘I-type’ plutonic suite, termed the Ford Granodiorite Suite(Weaver et al., 1991), was emplaced into the Swanson Formation.

The Cretaceous geology of the Ruppert–Hobbs coast of central MarieByrd Land (Fig. 1a) is dominated by three distinct groups of igneousrocks that record the transition from subduction related convergenceto continental rifting (Weaver et al., 1994). The oldest group is a suiteof calc-alkalic and metaluminous ‘I-type’ hornblende-bearing granit-oids (124–108 Ma), which are found as isolated small plutons, that rep-resent part of the Jurassic–Cretaceous magmatic arc that formed alongthe Gondwana margin (Fig. 1b). Slightly younger is a suite of alkalicand metaluminous to peralkaline ‘A-type’ granitoids (102–95 Ma)found as small plutons and hypabyssal intrusions that form about 75%of the exposed rocks in central Marie Byrd Land. Mafic dikes and sills(110–95 Ma) intrude the ‘I-type’ granitoids, sometimes forming up to60% of the exposure.

The Fosdick Mountains are located in the west of Marie Byrd Land(Fig. 1a) inboard of the former Jurassic–Cretaceous magmatic arc(Fig. 1b); they are composed mainly of migmatitic paragneisses andorthogneisses, and granites at a variety of scales,which together comprisethe Fosdickmigmatite–granite complex (Korhonen et al., 2010a,b, 2012).The complex forms an elongate domal structure (80×15 km) of Creta-ceous age, bounded by a south-dipping, dextral-oblique detachmentzone on the south and by an inferred steep, dextral strike-slip zone onthe north (Fig. 1c; Siddoway et al., 2004; McFadden et al., 2007, 2010a,b). The protoliths for the para- and orthogneisses within the complexare interpreted to be the Swanson Formation and the Ford Granodioritesuite, respectively (e.g. Siddoway et al., 2004; Korhonen et al., 2010a,b).Recent geochronological studies on themetamorphic rocks of the Fosdickcomplex have revealed a polyphase history of high-temperature meta-morphism and partial melting temporally related to Devonian–Carbonif-erous subduction related plate convergence and a Cretaceous transition

from subduction related plate convergence to intra-continental rifting(Siddoway et al., 2004; Siddoway, 2008; Korhonen et al., 2010a,b,2012). In the Fosdick complex, Cretaceous migmatites with steep fabrics,which host concordant granite sheets (ca. 117–114 Ma), are overprintedby subhorizontal foliation, which is associated with shallow-dippinggranite sheets (109–102 Ma). The change records the transition fromwrench to transtension, which may have occurred in as little as 5 Ma(McFadden et al., 2010a, b). The Fosdick migmatite–granite complex re-cords crystallization of Cretaceous anatexic melts in the interval ca 118–96 Ma (Korhonen et al., 2010a; McFadden et al., 2010b) and a rapidcooling history in the interval ca 101–94 Ma (Richard et al., 1994;Siddoway et al., 2004).

In the Fosdick Mountains, mafic rocks are represented by a smalldiorite pluton at the western end of the complex (Fig. 1c) and thewidespread occurrence of dykes throughout the complex. The dykestrend WNW–ESE, with steep dips to both NNE and SSW (Siddowayet al., 2005). Sensitive high-resolution ion microprobe (SHRIMP) U–Pb titanite and 40Ar/39Ar hornblende ages of 100–96 Ma have beenreported for some mafic dykes (Richard et al., 1994; McFadden etal., 2010b). The mafic dykes were intruded over a period of time asevidenced by field relationships that range from the occurrence ofdykes that are dismembered by back-veins of leucosome intrudedfrom the host migmatitic gneisses (Fig. 2a) to the occurrence ofundeformed continuous dykes with sharp contacts with the hostmigmatitic gneisses (Fig. 2b). Based on the field observations of thecontact relationships, the mafic dykes are divided into two groups:group-1 dykes have numerous back-veins of leucosome intruded

1570 S. Saito et al. / Gondwana Research 23 (2013) 1567–1580

from the host migmatitic gneisses, suggesting significant interactionwith the host rocks, whereas group-2 dykes do not have back-veinsof leucosome, suggesting that there was no significant interactionwith the host rocks. All of the mafic intrusive rocks of this study con-tain hydrous minerals (mainly abundant hornblende and biotite).Petrographic features and field relations of the samples are summa-rized in Table 1. Unfortunately, the contact relationships betweenthe diorite pluton and the country rocks are concealed by glacial iceand could not be examined in the field.

Table 1Summary of field and petrographic remarks of the mafic intrusive rock samples from the FMineral abbreviations after Whitney and Evans (2010). Amphibole classification after Leak

Sample no. Textural remarks Main mineralconstituentsa

Accessoryminerals

DioriteK6-I57 Medium-grained

holocrystalline-granular tex.Pl(An=42–50),Bt(XMg=65–68),Hbl(XMg=73–75)

Qz, Ap, Zrn, Opq

K6-I58a Medium-grainedholocrystalline-granular tex.

Pl(An=32–49),Bt(XMg=63–66),pargasticHbl(XMg=58–63),Hbl(XMg=69–71),Di(XMg=75–79)

Qz, Ap, Zrn, Opq

Mafic dykeC5-I3a Medium-grained

holocrystalline-granular tex.Pl(An=31–43),Bt(XMg=63–65),Hbl(XMg=64–68)

Qz, Ap, Zrn, Opq, Sp

C5-Is5 Medium-grainedholocrystalline-granular tex.

Pl(An=30–58),Hbl(XMg=65–68)

Qz, Ap, Zrn, Opq, Sp

C5-Mj70A Medium-grainedholocrystalline-granular tex.

Pl(An=30–60),Bt(XMg=54–57),tschermakiticHbl-Hbl(XMg=57–62)

Qz, Ap, Zrn, Spn

C5-Mj70B Fine-grainedholocrystalline-granular tex.

Pl(An=51–77),Bt(XMg=54–59),tschermakiticHbl-Hbl(XMg=55–57)

Qz, Ap, Zrn, Opq, Sp

M5-Bb247 Medium-grainedholocrystalline-granular tex.with large poikilitic Pl(~5 mm in length)

Pl(An=37–56),Bt(XMg=65–67),Hbl(XMg=60–61)

Qz, Ap, Zrn, Opq

M5-EG63 Fine-grainedholocrystalline-granular tex.

Pl(An=32–61),Bt(XMg=53–56),Hbl(XMg=53–56)

Qz, Kfs(Or=89-98)

M5-I7 Medium-grainedholocrystalline-granular tex.

Pl(An=36–45),Bt(XMg=58–63),Hbl(XMg=62–64)

Qz, Ap, Zrn, Opq, Sp

M5-I118 Fine-grained mosaictex. with orientationof Pl, Bt and Hbl

Pl(An=33–53),Bt(XMg=52–54),tschermakiticHbl-Hbl(XMg=51–53)

Qz, Ap, Zrn, Opq, Al

M5-I132 Fine-grainedholocrystalline-granular tex.

Pl(An=27–46),Bt(XMg=57–59),Hbl(XMg=54–56)

Qz, Ap, Zrn, Opq, Al

M5-ME145 Fine-grainedholocrystalline-granular tex.

Pl(An=35–54),Bt(XMg=49–52),Hbl(XMg=51–58)

Qz, Ap, Zrn, Opq

M5-R28 Fine-grainedholocrystalline-seriate tex.

Pl(An=50–59),Bt(XMg=54–57),Hbl(XMg=53–56)

Qz, Ap, Zrn, Opq

M5-R159 Fine-grained mosaic tex. Pl(An=39–45),Bt(XMg=56–58),Hbl(XMg=55–58)

Qz, Ap, Zrn, Opq, Sp

XMg=100 Mg/(Mg+Fe). An=100Ca/(Ca+Na+K). Or=100 K/(Ca+Na+K).a The mineral compositions were acquired by SEM-EDS (LINK QX-2000) at the Graduate

(cf. Saito et al., 2007).b LA-ICP-MS U–Pb zircon age (this study).c SHRIMP U–Pb titanite age (McFadden et al., 2010b).

3. Analytical procedures

Zircon grains for U–Pb geochronologywere separated from five rep-resentative mafic rock samples (one diorite and four mafic dykes). TheU–Pb dating was undertaken using laser ablation inductively coupledplasma mass spectrometry (LA-ICP-MS) at the Department of Geology,University ofMaryland, with a UP-213 laser ablation system in conjunc-tion with a ThermoFinnigan Element 2 single-collector ICP-MS. Fixed30 μm diameter spots were used for analysis with a laser frequency of

osdick Mountains.e (1978).

Dominant hostrock type

Field evidence ofinteraction withhost rocks

Mafic dykegroup

U–Pbage (Ma)

– – – 100.3b

– – –

n Orthogneiss Not obvious Group-2 99.7c

n Orthogneiss Not obvious Group-2

Orthogneiss Not obvious Group-2 96.7c

n Orthogneiss Not obvious Group-2

Orthogneiss Not obvious Group-2 110.8b

, Ap, Zrn, Opq, Spn Paragneiss Back-veins ofleucosomes of hostmigmatite

Group-1 98.4b

n Paragneiss Not obvious Group-2

n Orthogneiss Not obvious Group-2

n Orthogneiss Not obvious Group-2 108.9b

Paragneiss Dismembered dykeswith back-veins ofleucosomes from hostmigmatite

Group-1

Orthogneiss Not obvious Group-2

n, Aln Paragneiss Not obvious Group-2 113.3b

School of Environment and Information Sciences, Yokohama National University, Japan

1571S. Saito et al. / Gondwana Research 23 (2013) 1567–1580

10 Hz. U–Pb mass-fractionation effects were corrected using zirconstandard 91500 (Wiedenbeck et al., 1995) and the same spot size onboth standard and unknown samples. Data reduction, isotope ratiosand apparent age calculations were carried out with an Excel macro(Chang et al., 2006). For each analysis the time resolved signals and U/Pb ratios were carefully checked in order to detect perturbations.Concordia ages were calculated using ISOPLOT ver. 2 (Ludwig, 1999).The Temora zircon standard (416.8 Ma, Black et al., 2003) yielded amean Concordia age of 419.6±9.0 Ma (2σ) (n=11). The analytical re-sults are given in Table 2. The sampling sites are indicated in Fig. 1c, to-gether with the mean Concordia ages in parentheses.

Major oxides and trace element concentrations, and Sr and Nd iso-tope compositions were determined from two diorite samples andtwelve mafic dyke samples from the Fosdick Mountains. Samplingsites are indicated in Fig. 1c. All samples were collected in situ exceptfor sample K6-I57 that was collected from a loose block of diorite atthe northern end of Mt Iphigene. Major oxides and trace elements

Table 2LA-ICP-MS U–Pb zircon data of mafic intrusive rocks in the Fosdick Mountains.

Radiogenic atomic rations Apparent age (M

Spot 207Pb/235U

2 σerror

206Pb/238U

2 σerror

207Pb/206Pb

2 σerror

rhoa 207Pb/235U age

2 σerror

K6-I57 (diorite)Jl25e02 0.1107 0.009 0.0166 0.0008 0.0482 0.0036 0.466 107 9Jl25e04 0.1121 0.010 0.0156 0.0008 0.0519 0.0042 0.438 108 9Jl25e06 0.1109 0.012 0.0158 0.0009 0.0509 0.0053 0.340 107 11Jl25e08 0.1049 0.009 0.0151 0.0007 0.0505 0.0041 0.433 101 9Jl25e10 0.1078 0.013 0.0149 0.0009 0.0525 0.0064 0.183 104 12Jl25e12 0.0972 0.009 0.0148 0.0007 0.0476 0.0045 0.306 94 9Jl25e14 0.1144 0.011 0.0162 0.0009 0.0511 0.0045 0.417 110 10Jl25e16 0.3265 0.031 0.0424 0.0020 0.0558 0.0052 0.281 287 24

M5-EG63 (group-1 mafic dyke)Jl25e74 0.1112 0.015 0.0150 0.0010 0.0537 0.0075 0.169 107 14Jl25e76 0.1090 0.012 0.0151 0.0009 0.0524 0.0052 0.377 105 11Jl25e78 0.1053 0.014 0.0154 0.0010 0.0497 0.0070 0.120 102 13Jl25e80 0.1077 0.015 0.0142 0.0011 0.0552 0.0073 0.313 104 13Jl25e82 0.1216 0.015 0.0143 0.0010 0.0617 0.0075 0.315 117 14Jl25e84 0.1043 0.012 0.0157 0.0009 0.0481 0.0054 0.276 101 11Jl25e86 0.1053 0.010 0.0159 0.0008 0.0479 0.0044 0.340 102 9Jl25e88 0.1039 0.010 0.0150 0.0008 0.0503 0.0042 0.448 100 9

M5-Bb247 (group-2 mafic dyke)Jl25e26 0.1138 0.019 0.0174 0.0013 0.0473 0.0084 0.072 109 17Jl25e28 0.1133 0.020 0.0169 0.0013 0.0486 0.0093 0.012 109 18Jl25e30 0.1244 0.026 0.0190 0.0018 0.0474 0.0107 0.037 119 23Jl25e32 0.1154 0.014 0.0168 0.0011 0.0499 0.0057 0.327 111 12Jl25e34 0.1192 0.014 0.0166 0.0010 0.0520 0.0059 0.276 114 12Jl25e36 0.1582 0.021 0.0172 0.0011 0.0666 0.0094 0.121 149 19Jl25e38 0.1228 0.015 0.0174 0.0011 0.0512 0.0061 0.265 118 13Jl25e40 0.1339 0.025 0.0176 0.0017 0.0553 0.0104 0.217 128 22

M5-I132 (group-2 mafic dyke)Jl25e50 0.1232 0.012 0.0159 0.0008 0.0560 0.0050 0.330 118 10Jl25e52 0.1126 0.009 0.0172 0.0008 0.0475 0.0032 0.516 108 8Jl25e54 0.1063 0.015 0.0165 0.0010 0.0468 0.0068 0.095 103 13Jl25e56 0.1255 0.012 0.0180 0.0010 0.0504 0.0046 0.375 120 11Jl25e58 0.1229 0.012 0.0175 0.0009 0.0509 0.0046 0.372 118 11Jl25e60 0.1048 0.010 0.0165 0.0009 0.0461 0.0039 0.435 101 9Jl25e62 0.1174 0.020 0.0162 0.0014 0.0524 0.0093 0.137 113 18Jl25e64 0.1107 0.013 0.0157 0.0011 0.0510 0.0056 0.405 107 12

M5-R159 (group-2 mafic dyke)Jl25e98 0.1320 0.017 0.0191 0.0012 0.0501 0.0068 0.185 126 16Jl25e100 0.1274 0.020 0.0187 0.0013 0.0493 0.0085 0.023 122 18Jl25e102 0.1438 0.022 0.0167 0.0014 0.0623 0.0092 0.296 136 19Jl25e104 0.1205 0.042 0.0160 0.0027 0.0546 0.0203 0.082 116 37Jl25e106 0.1302 0.019 0.0174 0.0014 0.0543 0.0079 0.268 124 17Jl25e108 0.1453 0.043 0.0177 0.0025 0.0596 0.0193 0.054 138 38Jl25e110 0.1129 0.010 0.0164 0.0008 0.0499 0.0039 0.472 109 9Jl25e112 0.1095 0.025 0.0154 0.0018 0.0517 0.0121 0.175 106 22

a Ratio between the relative standard deviation between 207/235 and 206/235 ratios.

(except for Nd and Sm) were analyzed by X-ray fluorescence spec-trometry at Franklin & Marshall College following their standard pro-cedures (Boyd and Mertzmann, 1987). FeO contents were analyzedby Fe2+ titration and Fe2O3 contents were calculated by difference.

Sr and Nd isotope compositions andNd and Smconcentrationswereacquired at the Department of Geology, University of Maryland. Rockpowders (0.02–0.04 g) were dissolved for three days in a mixture ofconcentrated HF-HNO3 in Parr Teflon bombs at 180 °C. All sampleswere spiked with a 149Sm–150Nd mixed solution prior to digestion. Sr,Nd and Sm fractions were separated in two stages. On the first column,filled with BioRad AG 50W-X8 (200–400 mesh) resin, Sr and total REEfractions were collected by eluting with 2.5 N HCl and 6.0 N HCl, re-spectively. The second-stage purification of the Sr fraction wasperformed using Eichrom Sr-SPEC resin. Nd and Sm fractions were sep-arated from the total REE fraction by eluting with 0.34 N HCl and 0.7 NHCl, respectively, using the second column filledwith Eichrom Ln-resin.Sr fractionswere loaded on Re filaments with a Ta activator and isotope

a)

206Pb/238U age

2 σerror

207Pb/206Pb age

2 σerror

Concordiaage

2 σerror

MSWD Probability

106 5 111 172 106 5 0.0034 0.95100 5 283 180 101 5 3.60 0.059101 6 720 231 101 6 1.19 0.2896 5 219 182 97 5 1.60 0.295 5 309 266 96 5 2.10 0.1495 5 80 217 95 5 0.015 0.9

104 5 316 194 104 5 1.80 0.17268 12 444 202 270 12 2.60 0.11

96 6 358 299 98 6 2.40 0.1297 6 301 220 97 6 2.90 0.08898 6 180 314 99 6 0.24 0.6391 7 419 283 92 7 4.20 0.04192 6 662 250 discordant

101 6 102 255 101 6 0.0002 0.99102 5 95 211 102 5 0.003 0.9696 5 209 186 96 5 1.30 0.25

111 8 573 403 111 8 0.048 0.83108 8 586 423 108 8 0.007 0.93121 12 609 433 121 11 0.04 0.85107 7 398 256 108 7 0.37 0.54106 6 510 258 107 6 1.80 0.19110 7 824 283 discordant111 7 394 265 112 7 0.97 0.32112 11 1328 413 114 10 1.80 0.17

102 5 453 194 discordant110 5 72 157 110 5 0.21 0.64105 7 0 414 105 6 0.13 0.71115 6 215 206 116 6 0.84 0.36112 6 236 201 112 6 1.30 0.25105 5 3 196 105 5 1.06 0.3104 9 304 382 105 8 0.88 0.35101 7 241 242 101 7 1.15 0.28

122 8 199 300 123 8 0.23 0.63120 8 164 380 120 8 0.049 0.82107 9 684 301 107 9 0.11 0.74102 17 395 741 104 16 0.43 0.51111 9 382 311 113 8 2.40 0.12113 16 588 634 116 15 1.40 0.23105 5 189 175 108 5 0.50 0.4898 11 270 498 99 11 0.37 0.54

Table 3Whole rock compositions for mafic intrusive rock samples from the Fosdick Mountains.

Rock type Diorite Mafic dyke

Sample no. K6-I57 K6-I58a C5-I3agroup-2

C5-Is5group-2

C5-Mj70Agroup-2

C5-Mj70Bgroup-2

M5-Bb247group-2

M5-EG63group-1

M5-I7group-2

M5-I118group-2

M5-I132group-2

M5-ME145group-1

M5-R28group-2

M5-R159group-2

Major element (wt.%)SiO2 55.06 52.34 56.83 51.25 49.22 52.69 53.68 57.40 54.77 49.61 54.58 51.89 47.03 52.53TiO2 0.64 1.06 1.77 1.07 1.40 1.27 2.26 1.07 1.70 1.96 1.33 1.25 2.34 1.61Al2O3 11.32 10.10 14.51 12.73 16.20 16.42 16.54 15.41 15.48 17.91 18.59 15.35 16.33 17.15Fe2O3 1.73 2.26 1.84 2.18 2.93 2.84 4.76 2.15 1.96 3.65 3.39 1.25 4.14 2.23FeO 5.92 6.64 5.16 6.61 6.98 6.26 4.17 5.06 5.44 5.97 4.33 8.14 8.20 6.36MnO 0.16 0.15 0.14 0.15 0.17 0.16 0.11 0.11 0.11 0.13 0.12 0.17 0.19 0.14MgO 11.11 11.43 6.90 9.84 6.93 5.82 3.94 4.54 6.94 4.80 3.93 6.78 6.39 5.14CaO 9.38 11.70 6.31 9.99 9.65 8.35 6.55 6.49 6.91 7.46 6.38 8.67 9.72 7.51Na2O 1.55 1.54 2.77 1.98 2.34 1.83 2.68 2.82 2.40 3.54 3.45 2.37 1.99 3.16K2O 1.31 1.35 2.30 1.05 1.77 1.97 2.29 3.00 2.37 2.19 2.05 1.27 1.58 1.91P2O5 0.25 0.24 0.15 0.36 0.59 0.55 1.36 0.36 0.21 0.84 0.48 0.22 0.67 0.62LOI 2.06 1.31 1.38 2.39 1.95 2.17 1.19 1.33 1.72 1.43 1.32 2.53 1.57 1.43Total 100.49 100.12 100.06 99.60 100.13 100.33 99.53 99.74 100.01 99.49 99.95 99.89 100.15 99.79Mg# 0.73 0.70 0.64 0.67 0.56 0.54 0.45 0.54 0.63 0.48 0.49 0.57 0.49 0.52

Trace element (ppm)Rb 89.1 55.8 115.7 44.2 79.1 102.9 74.3 144.9 147.1 106.9 91.2 56.6 100 107.2Sr 303 488 719 581 841 831 3040 525 871 1554 1128 412 799 1218Y 33.2 19.6 25.1 21.9 29.4 26 24.6 27.8 16 19.6 17.3 27.1 29.5 17.6Zr 91 78 56 79 147 142 240 218 42 219 198 111 203 168V 180 263 268 239 268 236 166 165 252 222 171 217 253 222Ni 139 4 62 112 37 30 20 31 61 32 20 87 59 32Cr 1120 632 87 820 130 78 11 156 75 49 26 208 56 90Nb 11.8 7.3 16.8 6.4 10.5 10.2 15.8 12 5.9 10.9 9 6.1 19.9 7.8Ga 16.2 11.4 19.8 13.1 17.6 16.3 25.6 18.4 19.6 24.7 24.9 17.3 20.4 23.5Cu 20 49 16 94 66 41 14 31 23 15 55 43 134Zn 123 90 110 107 128 113 170 95 103 159 142 110 136 138Co 44 48 33 41 34 29 24 23 32 32 22 38 44 27Ba 162 323 562 183 445 547 1285 634 460 819 827 242 292 643La 13 12 14 15 18 21 23 25 12 22 18 18 23Ce 27 28 50 28 49 47 129 66 38 56 76U 0.6 b0.5 4.5 0.5 b0.5 1.3 1.9 0.7 5 1.2 1.4 0.9 b0.5 1.2Th 1.5 4.8 5.4 3 2.8 3.7 4 9.5 5.3 4 2 6.9 2.6 4.2Sc 36 42 30 35 31 27 15 23 30 16 16 29 26 21Pb 6 6 2 2 2 0.5 7 b1 3 4 3 2 2Sm 6.06 4.98 7.51 5.83 8.03 7.00 12.66 7.31 5.53 9.01 6.32 4.70 7.10 7.15Nd 23.95 21.03 34.95 25.16 36.74 32.66 67.77 35.67 24.01 50.16 27.12 20.92 32.38 38.0487Rb/86Sr 0.85 0.33 0.47 0.22 0.27 0.36 0.07 0.80 0.49 0.20 0.23 0.40 0.36 0.2587Sr/86Sr 0.707431 0.705851 0.705944 0.705714 0.705537 0.705670 0.705111 0.707086 0.705814 0.7054030.705287 0.706483 0.705890 0.70562587Sr/86Sr(100Ma) 0.706222 0.705380 0.705282 0.705401 0.705150 0.705161 0.705011 0.705952 0.705120 0.705120 0.704954 0.705918 0.705375 0.705263εSr(100Ma) +26.11 +14.17 +12.77 +14.46 +10.89 +11.06 +8.92 +22.28 +10.47 +10.47 +8.12 +21.80 +14.10 +12.50

147Sm/144Nd 0.1531 0.1432 0.1300 0.1402 0.1321 0.1296 0.1129 0.1239 0.1392 0.1086 0.1409 0.1357 0.1325 0.1136143Nd/144Nd 0.512471 0.512573 0.512546 0.512681 0.512573 0.512583 0.512541 0.512435 0.512575 0.512513 0.512521 0.512582 0.512724 0.512536143Nd/144Nd(100Ma) 0.512370 0.512479 0.512461 0.512590 0.512487 0.512499 0.512467 0.512354 0.512483 0.512442 0.512428 0.512493 0.512637 0.512462εNd(100Ma) −2.71 −0.59 −0.94 +1.56 −0.44 −0.21 −0.82 −3.03 −0.51 −1.31 −1.58 −0.33 +2.49 −0.93U–Pb age(Ma)

100.3a 99.7b 96.7b 110.8a 98.4a 108.9a 113.3a

aLA‐ICP‐MS U–Pb zircon age (this study). bSHRIMP U–Pb titanite age (McFadden et al., 2010b).Mg#=MgO/[MgO+FeO (total)] using molar proportions.

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ratios were measured in dynamic collection mode on a VG Sector 54mass spectrometer. 86Sr/88Sr ratios were normalized to 0.1194 formass fractionation correction.Multiple analyses of theNBS987 standardduring this study averaged 87Sr/88Sr of 0.710251±0.000007 (1σ, n=8). Nd fractions were introduced into a Nu Plasma Multi-CollectorICP-MS using an APEX desolvating nebulizer. Nd isotope rations weremeasured in static collection mode. Mass fractionation was corrected

Fig. 3. Cathodoluminescence images of zircon grains from the mafic rocks of the Fosdick Mdetermined by LA-ICP-MS.

by normalizing to a 146Nd/144Nd of 0.7219. Repeated analyses of theAMES Nd standard during this study gave a 0.512187±0.000014 (1σ,n=13). All 143Nd/144Nd ratiosmeasured in the sampleswere correctedfor instrumental bias to an AMES standard 143Nd/144Nd value of0.512138 (in-house TIMS measurements). Using this method, average143Nd/144Nd for BHVO-2 standard (0.512983, Weis et al., 2005) is0.512975±0.000009 (1σ, n=8). Nd and Sm concentrations were

ountains. Circles indicate the position of the analysis. Ages shown are 206Pb/238U ages

Fig. 4. Tera–Wasserburg Concordia diagrams for LA-ICP-MS zircon U–Pb data. Data-point error ellipses are at the 2σ confidence level, which includes U decay constant uncertainty.Shaded ellipses are used for the mean Concordia age calculation.

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determined by isotope dilution analyses with the Nu Plasma Multi-Collector ICP-MS. The measured major and trace element compositionsand Sr and Nd isotopic compositions are listed in Table 3.

4. Analytical results

4.1. Zircon U–Pb geochronology

Cathodoluminescence (CL) images of zircon grains indicating theanalysis position and calculated 206Pb/238U ages are shown in Fig. 3.Most of analyzed zircon grains exhibit concentric oscillatory zoning.Tera–Wasserburg Concordia diagrams are shown in Fig. 4; a majorityof the data are concordant. These zircons yield Cretaceous mean con-cordant ages for the mafic rocks in the Fosdick Mountains rangingfrom ca 113 Ma to ca 98 Ma for the four mafic dykes (Fig. 4b–e)and ca 100 Ma for the diorite (Fig. 4a). One data point from the coreof a zircon from the diorite sample shows a significantly older age(ca 268 Ma of 206Pb/238U age, Fig. 3, Table 2) indicating an inheri-tance from older crustal material. In combination with previously

reported SHRIMP U–Pb titanite ages of ca 100 Ma and ca 97 Ma(McFadden et al., 2010b, Fig. 1c), these data suggest that the crystal-lization ages of mafic magmas in the Fosdick migmatite–granite com-plex range from ca 113 Ma to ca 97 Ma, which is comparable to therange of ca 112 to ca 95 Ma for mafic dykes along the Ruppert–Hobbs coast to the northwest of the Fosdick Mountains (Fig. 1b;Weaver et al., 1994; Storey et al., 1999).

4.2. Major and trace element compositions

Major oxides used in plots and reported in the text arerecalculated on a volatile-free basis. The mafic dykes and the dioriteshave basic to intermediate compositions (47–59 wt.% SiO2 for themafic dykes and 52–56 wt.% SiO2 for the diorites, Fig. 5a,b,e), and be-long to the subalkaline, medium- to high-K, and calc-alkaline magmatypes based on total alkali vs. SiO2, K2O vs. SiO2, and FeO–(Na2O+K2-

O)–MgO ternary diagrams, respectively (Fig. 5a–c). Except for twosamples, the mafic dykes and the diorites plot in the back-arc fieldon a FeO(total)/MgO vs. TiO2 diagram (Fig. 5d) used to classify

Fig. 5. Discrimination diagrams for the mafic rocks of the Fosdick Mountains. (a) Total alkali vs. SiO2 plot. (b) K2O vs. SiO2 plot. (c) FeO(total)−(Na2O+K2O)−MgO ternary diagram.(d) FeO(total)/MgO vs. TiO2 plot. (e) Mg number [Mg#, molecular MgO/(MgO+FeOtotal)] vs. SiO2 plot.Panel a: the nomenclature fields are from Le Maitre et al. (1989); panel b: the nomenclature fields are from Le Maitre et al. (1989); panel c: discrimination curve is from Kuno(1968); panel d: the discrimination line is from Kay et al. (1984). The field of Andes back-arc calc-alkaline volcanic rocks (Kay et al., 1994) is shown for comparison; panel e: thefields of primitive arc magmas and boninites are from Leat et al. (2002).

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continental margin arc rocks (‘arc front’) from ‘back-arc’ rocks in theAndes (Kay et al., 1984; Allen, 2000). The Mg number [Mg#, molecu-lar MgO/(MgO+FeOtotal)] for the mafic dykes and diorites rangesfrom 0.45 to 0.67 and from 0.70 to 0.73, respectively (Table 3,Fig. 5e). The diorites and three mafic dyke samples have relativelyhigh Mg numbers (>0.63), which are comparable to primitive arcmagmas and boninites (Leat et al., 2002; Fig. 5e).

In N-type mid ocean ridge basalt (N-MORB) normalized trace ele-ment plots (Fig. 6), the diorites and the mafic dykes both show distinc-tive negative anomalies in Nb coupled with enrichment in large ionlithophile elements (LILEs) relative to light rare earth elements (LREEs;e.g., Ba/La=12–92) and high field strength elements (HFSEs; e.g. Ba/Nb=14–92). These patternswith enrichment in the LILEs anddistinctivenegative anomalies in Nb are different from those of Cenozoic alkalinemafic volcanic rocks fromMarie Byrd Land, which have been interpretedto be plume related by Hole and LeMasurier (1994).

Storey et al. (1999) grouped the Cretaceous mafic dykes from theRuppert–Hobbs coast in the Marie Byrd Land (Fig. 1a) into threegroups: high-Ti, low-Ti and transitional types. The mafic rocks inthe Fosdick migmatite–granite complex have broadly comparabletrace element patterns (enrichment in LILEs with distinctive negativeanomalies in Nb) to those of the low-Ti type from the Ruppert–Hobbscoast (Fig. 6).

4.3. Sr and Nd isotopic compositions

Fig. 7 shows whole-rock Sr and Nd isotope ratios for the maficrocks from the Fosdick Mountains age-corrected to 100 Ma andexpressed as ε values, together with those for low-Ti type maficdykes of the Ruppert–Hobbs coast (also age-corrected to 100 Ma,Storey et al., 1999) and Cenozoic mafic volcanic rocks of Marie ByrdLand (age-corrected to 11.7–2.3 Ma; Hole and LeMasurier, 1994;

Fig. 6. N-type mid-ocean ridge basalt (N-MORB)-normalized trace element patterns forthe diorite samples and the group-1 dyke samples (a), and the group-2 dyke samples(b) in comparison with the Cenozoic mafic volcanic rocks from Marie Byrd Land (shad-ed area, Hole and LeMasurier, 1994) and the Cretaceous low-Ti type mafic dykes fromthe Ruppert–Hobbs coast (dotted area, Storey et al., 1999).The N‐MORB values are from Sun and McDonough (1989).

Fig. 7. εNd(t) vs. εSr(t) for the mafic rocks in the Fosdick Mountains in comparison withthe Cenozoic mafic volcanic rocks of Marie Byrd Land (Hole and LeMasurier, 1994; Hartet al., 1997; Panter et al., 2000) and the Cretaceous low-Ti type mafic dykes from theRuppert–Hobbs coast (Storey et al., 1999). The εNd(t) and εSr(t) values of the maficrocks in the Fosdick Mountains and the Ruppert–Hobbs coast mafic dykes are calculat-ed at 100 Ma. Those of the Marie Byrd Land Cenozoic mafic volcanic rocks are calculat-ed to the age reported in Hole and LeMasurier (1994), Hart et al. (1997) and Panter etal. (2000) (11.7–2.3 Ma). Broken curves are bulk mixing curves between twoend-member samples in the group-2 dykes (M5-I132, the least radiogenic εSr(100 Ma)

sample, and M5-R28, the least radiogenic εNd(100 Ma) sample) and the average valuesof orthogneisses and paragneisses in the Fosdick Mountains (Korhonen et al., 2010a).The shaded area represents the compositional field of bulk crustal assimilation withgroup-2 type mafic dyke magmas. The group-1 mafic dykes and diorite sampleK6-I57 plot within the shaded area. DM, HIMU, EM I and EM II are mantleend-member compositions at the present day (Faure, 2001; Faure and Mensing, 2005).

Fig. 8. εSr(100 Ma) vs. 1000/Sr for the mafic rocks in the Fosdick Mountains. Broken andsolid lines are bulk mixing lines and assimilation and fractional crystallization (AFC)lines (DePaolo, 1981), respectively. AFC models assume a bulk distribution coefficient(DSr) of 2.5 and a rate of assimilation to fractional crystallization (r) of 0.2.The field of the Cretaceous low‐Ti type mafic dykes from the Ruppert–Hobbs coast isfrom Storey et al. (1999). The average compositions of the paragneisses andorthogneisses are taken from Korhonen et al. (2010a).

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Hart et al., 1997; Panter et al., 2000). The mafic rocks in the FosdickMountains are characterized by relatively enriched Sr–Nd isotope com-positions as compared to the Marie Byrd Land Cenozoic mafic volcanicrocks and the low-Ti type Cretaceous mafic dykes of the Ruppert–Hobbs coast (Fig. 7). The group-1 dykes are characterized by radio-genic εSr(100 Ma) (+21.8–+22.3) with negative εNd(100 Ma) (−3.0to −0.3). In contrast, the group-2 dykes show relatively less radio-genic εSr(100 Ma) (+8.1–+14.5) and a wide range of εNd(100 Ma)

(−1.5–+2.5). The diorites have εSr(100 Ma) and εNd(100 Ma) compo-sitions of +14.6–+26.1 and −2.7 to −0.6, respectively.

5. Discussion

5.1. Crustal assimilation or source enrichment?

The mafic rocks in the Fosdick Mountains are characterized byenriched geochemical features such as high LILE concentrations andpositive εSr(100 Ma) and negative to slightly positive εNd(100 Ma) compo-sitions (Figs. 6 and 7). These features are suggestive of either (1) sourcemantle enrichment through geodynamic processes such as subduction-related metasomatism, or (2) crustal assimilation (i.e. mantle-derivedmagmas that assimilated country rocks during magma ascent). Field ob-servations including back-veins of leucosome from host migmatiticgneisses (Table 1, Fig. 2a) suggest that the group-1 mafic dyke magmashave interacted with country rocks during magma ascent. The more ra-diogenic εSr(100 Ma) values for these dykes compared to those of thegroup-2 mafic dykes are consistent with crustal assimilation (Fig. 7).Sample K6-I57 from the diorite pluton yielded one zircon with an olderinherited age (206Pb/238U age of ca 268 Ma, Table 2) and this together

with the more radiogenic εSr(100 Ma) value for this sample (Fig. 7) sug-gests assimilation of older crustal materials.

Since the trace element patterns of group-2 dykes (LILE enrich-ment and negative Nb anomalies) are broadly similar to those of thelow-Ti type Cretaceous mafic dykes of the Ruppert–Hobbs coast(Fig. 6), assimilation of para- and orthogneisses in the Fosdick Moun-tains by Ruppert–Hobbs type magma might explain their enriched Srand Nd isotope compositions of the group-2 dykes. However, theεSr(100 Ma) vs. 1000/Sr diagram (Fig. 8) demonstrates that the compo-sitions of group-2 dykes cannot be explained by either bulk

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assimilation of the Fosdick migmatitic gneisses into the Ruppert–Hobbs Coast type magmas or by a process of assimilation and frac-tional crystallization (AFC; e.g. DePaolo, 1981).

In Fig. 8, the group-2 dykes and one diorite sample (K6-I58a) plotalong the AFC lines from the sample with highest Sr content(M5-Bb247) at low assimilation/crystallization ratios (r=0.2). There-fore, an AFC process might explain the variation of Sr isotopes in thesesamples. However, this requires an extremely high degree of fractionalcrystallization (20%–60%) to explain the compositions (Fig. 8), which isinconsistent with their mafic nature. Furthermore, the group-2 dykesshow positive trends in the εNd(t) vs. εSr(t) diagram (Fig. 7), which can-not be explained by crustal assimilation. Therefore, we interpret thevariation in isotope compositions in the group-2 dykes and diorite sam-ple K6-I58a to reflect variation of the source mantle rather than a man-ifestation of crustal assimilation during magma ascent.

In summary, although the group-1 mafic dyke samples and one di-orite sample (K6-I57) experienced assimilation of crustal materials,the group-2 dyke samples and another diorite sample (K6-I58a) donot show evidence for significant crustal assimilation. Therefore, theenriched geochemical features of the group-2 mafic dykes and theK6-I58a diorite in the Fosdick Mountains indicate their derivationfrom an enriched mantle source. The trace element patterns withLILE enrichments and negative Nb anomalies are diagnostic of rocksformed in subduction-related settings (e.g. Pearce and Parkinson,1993) indicating that the mafic magmas likely formed by melting of

Fig. 9. Comparison of the mafic intrusive rocks of the Fosdick Mountains with mafic dykes frdykes (Scarrow et al., 1998). (c, d) Comparison with data of eastern Antarctic Peninsula dykely comparable trace elements and Sr–Nd isotope compositions to those of calc-alkaline dykesignificant crustal assimilation (group-1 dykes and the K6-I57 diorite) are not plotted in th

enriched sub-continental arc mantle metasomatized by slab-derivedcomponents. In addition, the similar isotopic compositions for the di-orite (K6-I58a) and the group-2 dykes together with their comparablezircon U–Pb ages suggest that the diorite represents a plutonic phasefrom the same magma source as the dykes.

5.2. Tectonic setting of mantle melting beneath the Fosdick Mountains

A Cretaceous mantle plume with a HIMU component beneathMarie Byrd Land has been proposed to explain the rifting of terranesfrom Antarctica that now occur in New Zealand (Weaver et al., 1994;Storey et al., 1999), although this hypothesis has been questioned byseveral groups, including LeMasurier and Landis (1996), Dalziel et al.(2000), Mukasa and Dalziel (2000) and Finn et al. (2005). Fig. 7 dem-onstrates, however, that the mantle source of the Cretaceous maficrocks in the Fosdick Mountains was quite different from HIMU typemantle, precluding the possibility of a mantle plume source with aHIMU component beneath this region during the Cretaceous.

Fig. 9 shows a comparison of normalized trace element patternsand Sr and Nd isotope compositions for the Fosdick mafic rockswith those for non plume related mafic dykes reported from the Ant-arctic Peninsula (Scarrow et al., 1998; Leat et al., 2002; Vaughan et al.,2012). This comparison suggests that the compositions of the Fosdickmafic rocks share similar geochemical characteristics with non plumerelated calc-alkaline dykes in the Antarctic Peninsula.

om the Antarctic Peninsula. (a, b) Comparison with data of western Antarctic Peninsulas (Leat et al., 2002; Vaughan et al., 2012). The Fosdick mafic intrusive rocks have broad-s from the Antarctic Peninsula. Symbols are as in Fig. 5. The samples with evidences ofis figure.

Fig. 10. (a) εNd (100 Ma) vs. Ba/Nb, (b) εSr(100 Ma) vs. Ba/Nb, (c) La/Sm vs. Ba/Nb, and (d) U–Pb age (this study; McFadden et al., 2010b) vs. Ba/Nb for the mafic intrusive rocks in theFosdick Mountains. Symbols are as in Fig. 5. Samples that show evidence of significant crustal assimilation (group-1 dykes and the K6-I57 diorite) are not plotted in this figure. Twomantle sources are identified for the mafic rocks; one source characterized by a higher Ba/Nb ratio, less radiogenic εSr(100 Ma) value and more radiogenic εNd(100 Ma) value, and an-other source characterized by lower Ba/Nb ratio, more radiogenic εSr(100 Ma) value and less radiogenic εNd(100 Ma) value. The samples with higher Ba/Nb ratio have older ages thanthe samples with lower Ba/Nb samples.

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Contrasting tectonic regimes have been proposed for the emplace-ment of the calc-alkaline dykes of the Antarctic Peninsula, includingboth a transtensional regime (Scarrow et al., 1997, 1998) and a com-pressional regime associated with collision of an oceanic terrane (Leatet al., 2002; Vaughan et al., 2012). Although the Fosdick mafic rocksshow geochemical similarities to calc-alkaline dykes from both tec-tonic settings in the Antarctic Peninsula (Fig. 9), it has been demon-strated that the Fosdick mafic dykes were emplaced during atransition from wrench to transtensional tectonics (McFadden et al.,2010a, b). Therefore, we interpret the Cretaceous melting event be-neath the Fosdick Mountains to have been induced by decompressionof metasomatized sub-continental arc mantle related to this transi-tion in tectonic regime that ultimately led to intra-continental rifting,possibly attributable to mantle wedge dynamics (e.g. Rey and Muller,2010), rather than to the collision of an oceanic terrain such as theHikurangi plateau.

5.3. Characteristics of the mantle sources and melting of enrichedsub-continental arc mantle

Based on the Sr–Nd isotope compositions shown in Fig. 7, it is likelythat therewere a number ofmantle source compositions beneathMarieByrd Land being tapped during the Cretaceous, including at least twoend-member mantle source compositions beneath the Fosdick Moun-tains. Beneath the Fosdick Mountains, one source has positiveεNd(100 Ma) (~+2.5) and positive εSr(100 Ma) (~+14) and anothersource has negative εNd(100 Ma) (~−1.3) and positive εSr(100 Ma)

(~+8) (Fig. 7). The geochemical features of these two mantle sourcescan be discerned from plots of εSr(100 Ma) and εNd(100 Ma) vs. Ba/Nbratio (Fig. 10a, b). The trends observed in these figures are interpreted

to represent mixing between melts derived from two chemically dis-tinct mantle sources; the one source characterized by higher Ba/Nbratio (i.e. higher LILE/HFSE ratios), less radiogenic εSr(100 Ma) valuesandmore radiogenic εNd(100 Ma) values, and the other source character-ized by lower Ba/Nb ratio (i.e. lower LILE/HFSE ratios), more radiogenicεSr(100 Ma) values and less radiogenic εNd(100 Ma) values (Fig. 10a, b).

The difference in LILE/HFSE ratios of the mafic intrusive rocks mightbe attributed to variable degrees ofmelting from a single sourcemantle.However, the La/Sm ratio, which would reflect degree of melting, doesnot show any correlation with Ba/Nb ratio (Fig. 10c). In addition, themagmas derived from a single source with variable degrees of meltingshould have similar Sr–Nd isotope compositions, which is not the casefor the Fosdick mafic rocks. Therefore, we interpret that the trends ob-served in Fig. 10a,b to reflect mixing between melts derived from twochemically distinct mantle sources.

High LILE/HFSE ratios together with radiogenic εNd(t) values arecommonly interpreted as the signature of addition of slab-derivedfluids to the sub-arc mantle wedge in a convergent margin (e.g.Kepezhinskas et al., 1997). Therefore, we interpreted the maficmagmas with higher LILE/HFSE ratios and radiogenic εNd(100 Ma)

values to have been derived from enriched mantle metasomatizedby LILE-enriched slab-derived fluids during prolonged subduction ofthe paleo-Pacific plate. On the other hand, the mafic magmas charac-terized by relatively lower LILE/HFSE ratios might be derived from ei-ther a mantle metasomatized by a slab-derived component with ahigh melt/fluid ratio, because an increase in the melt/fluid ratio ofthe slab-derived component causes a decrease in the LILE/HFSE ratios,or from a mantle metasomatized by fluids liberated by dehydration ofa residual slab that was depleted in fluid-mobile elements (e.g.Hochstaedter et al., 2001). Thus, spatial variation in the type of

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metasomatism due to the variable nature of the slab-derived compo-nents fluxing into the sub-continental arc mantle during prolongedsubduction is inferred to cause the variable LILE/HFSE ratios and Sr–Nd isotope compositions in the mantle source for the Fosdick maficrocks.

In addition, based on the available age data, the dykes with higherBa/Nb ratio have older ages than the dykes with lower Ba/Nb ratio(Fig. 10d). These data suggest that Cretaceous mantle melting initiatedin a region with higher LILE/HFSE ratios then shifted to a region withlower LILE/HFSE ratios. The mantle source region metasomatized byLILE-enriched slab-derived fluids is inferred to have had a relativelylower solidus temperature to enable it tomelt first under initial decom-pression during the transition fromawrench to a transtensional tecton-ic setting.

6. Summary

(1) Mafic dykes and a diorite pluton in the Fosdick Mountainmigmatite–granite complex are composed of medium- tohigh-K-series calc-alkaline rocks with basic–intermediatecomposition. They have Cretaceous crystallization ages rangingfrom ca 113 to 97 Ma.

(2) The mafic rocks contain abundant hydrous minerals and havetrace element patterns characterized by LILE enrichment and neg-ative Nb anomalies indicating their origin from hydrous mantlemetasomatized by subducted slab-derived components. PositiveεSr(100 Ma) values and negative to slightly positive εNd(100 Ma)

values suggest that the magmas were derived from an enrichedmantle source rather than from a plume-related HIMU source.

(3) The trace element and Sr-Nd isotope compositions suggest twoend-member mantle source compositions for the mafic rocks.One source was characterized by higher LILE/HFSE ratios, less ra-diogenic εSr(100 Ma) values and more radiogenic εNd(100 Ma)

values, whereas the other source was characterized by lowerLILE/HFSE ratios, more radiogenic εSr(100 Ma) values and less ra-diogenic εNd(100 Ma) values.

(4) The melting event beneath the Fosdick Mountains was likely in-duced by decompression of sub-continental arc mantle duringthe transition from a wrench to a transtensional tectonic setting.

Acknowledgments

We thank Alan S. Collins for editorial handling. We also thank twoanonymous reviewers for their thoughtful and constructive reviews.We acknowledge support from the University of Maryland, and theNational Science Foundation awards NSF-OPP 0338279 (Siddoway)and NSF-ANT 0734505 (Brown), and a Post-Doctoral Research Fel-lowship to Korhonen (NSF-OPP 0631324). The work is part of a col-laboration with C. Teyssier at the University of Minnesota. We thankM. Roberts and F. McCarthy who contributed to the field logisticsand ensured field safety, Jim Haffey and Kenn Borek Air, and RaytheonPolar Services personnel for transportation and logistical support. Weare grateful to I. S. Puchtel, R. Ash, W. F. McDonough, R. J. Walker, A. T.Mansur, P. M. Piccoli, B. L. Reno, A. J. Kaufman, R. L., Rudnick, J. M. D.Day and T. Yokoyama for their advice and help with the analyticalpart of this work at the University of Maryland. We are also gratefulto J. M. D. Day for the helpful review of an earlier version of this man-uscript. We thank M. Arima for help with mineral analysis at the Yo-kohama National University.

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