Elemental, Hf–Nd Isotopic and Geochronological Constraints on an Island Arc Sequence Associated with the Cretaceous Caribbean Plateau: Bonaire, Dutch Antilles

Lithos 74 (2004) 91–116 www.elsevier.com/locate/lithos

Elemental, Hf–Nd isotopic and geochronological constraints on an island arc sequence associated with the Cretaceous Caribbean plateau: Bonaire, Dutch Antilles

P.M.E. Thompsona,b,*, P.D. Kemptona, R.V. Whiteb, A.D. Saundersb, A.C. Kerrc, J. Tarneyc, M.S. Pringled

a NERC Isotope Geosciences Laboratory, Keyworth, NG12 5GG, UK b Geology Department, University of Leicester, University Rd., Leicester, LE1 7RH, UK c Cardiff University, PO Box 914, Cardiff, CF10 3YE, UK d Scottish Universities Environment Research Centre, East Kilbride, Glasgow, G75 OQF, UK Received 3 July 2003; accepted 26 January 2004

Abstract

On the Caribbean margins, Upper Cretaceous oceanic plateaux fragments are juxtaposed with island arc fragments of a similar age; until this study, the relationship between them was unknown. This work represents the first detailed study of one such island arc sequence, the Bonaire Washikemba Formation (BWF). These rocks display typical arc-like trace element and Hf–Nd isotopic characteristics (negative Nb and Ta anomalies, qHf of +12 to +14, qNd of +6.5 to +8). They show no indication for the involvement of oceanic plateau material in their source. This is confirmed by binary mixing hyperbolae, which indicate that the Hf–Nd isotopic composition can be modelled by mixing of <1% of Pacific pelagic sediment with average Pacific MORB. 87Sr/86Sr data for apatite mineral separates show significantly lower isotopic ratios than the whole rock, signifying that whole-rock Sr isotope ratios have been affected by alteration processes. New 40Ar– 39Ar ages indicate that the Formation is at least 96F4 Ma, which is older than the main phase of the Caribbean plateau (88–91 Ma). Together, the data presented in this study suggest that the Bonaire Washikemba Formation is part of an intra-oceanic arc unrelated to the Carribean plateau, but the Carribean plateau is probably indirectly responsible for the transport and ultimate preservation of this arc sequence. D 2004 Elsevier B.V. All rights reserved.

Keywords: Bonaire; Caribbean Plateau; Geochemistry; Nd–Hf isotopes; Island arc; Cretaceous

1. Introduction estimated area of 12Â105 km2 (Fig. 1). Whilst seismic studies show that the plateau underlies much of the The Cretaceous Carribean Plateau is one of the Caribbean Sea [Driscoll and Diebold, 1999], Carib- largest Pacific-derived oceanic plateaux, covering an bean plateau fragments also outcrop on land, includ-

* Corresponding author. Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK. Tel.: +44-116-252- 5355; fax: +44-116-252-3918. E-mail address: [email protected] (P.M.E. Thompson).

0024-4937/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2004.01.004 92 P.M.E. Thompson et al. / Lithos 74 (2004) 91–116 ing Colombia, Gorgona and Venezuela in the south, Along with oceanic plateaux fragments, island arc- and Hispaniola and Jamaica in the north. Detailed related sequences and tonalitic batholiths are found geochronological studies from across the province juxtaposed on the complex accretionary regions asso- show that a major phase of plateau magmatism ciated with the tectonic margins of the Caribbean occurred from about 88 to 91 Ma (Sen et al., 1988; plate. Intriguingly, the arcs, plateaux and related rocks Sinton and Duncan 1997; Kerr et al., 1997; Alvarado are all of a similar Upper Cretaceous age. Whilst et al., 1997; Lapierre et al., 1999; Walker et al., 1999; similar island arc sequences are found all around the Hauff et al. 2000a), with a secondary smaller pulse margins of the Caribbean, their origins remain enig- around 76 Ma (Kerr et al., 1997; Re´villon et al., matic, particularly given their close temporal and 2000). spatial association with the plateau.

Fig. 1. Tectonic map of the Caribbean, showing different oceanic terranes. After Mann et al. (1990), White et al. (1999) and Kerr et al. (1996). P.M.E. Thompson et al. / Lithos 74 (2004) 91–116 93

This study concentrates on Bonaire, one of the million years after plateau formation. The inherent ABC islands (Aruba, Bonaire and Curacßao), which is buoyancy of the plateau clogged this subduction zone located on the Southern Caribbean margin where (Duncan and Hargreaves, 1984; Pindell and Barrett, plateau, arc and batholith sequences are all now 1990; Pindell et al., 1998) andculminatedina juxtaposed. We also present new data for the batholith polarity reversal (Pindell and Barrett, 1990; Lebron exposed on Aruba. and Perfit, 1993; 1994), with proto-Caribbean crust The ABC islands are separated from each other by being subducted beneath Pacific crust for the first numerous strike-slip faults [Case, 1972; Ave´ Lalle- time (Lebron and Perfit, 1993; White et al., 1999). ment, 1996], so their tectonic relationships are un- Eastward-directed subduction then resumed on the clear. On Aruba, the most westerly island, an 89–91 trailing edge of the plateau, effectively back-stepping Ma oceanic plateau sequence, the Aruba Lava For- the Central American subduction zone (Pindell and mation is exposed. This has been identified as part of Barrett, 1990; Pindell et al., 1998). Thus, the Carib- the Caribbean plateau [Klaver, 1987; White et al., bean plateau, newly positioned on the overriding 1999], and is intruded by an 82–85 Ma tonalitic plate, moved into the widening gap between the batholith termed the Aruba batholith [White, 1999; two Americas, resulting in the plate configuration White et al., 1999]. The island of Curacßao consists of observed today (Mann et al., 1990). The strike-slip a thick 89–91 Ma succession of basaltic pillow lavas, motion of the plateau against the rigid continents was picrites and mafic intrusive rocks [Klaver, 1987; Kerr sufficient to cause uplift and accretion of the outer et al., 1996] identified as part of the Caribbean plateau portions of the plateau to North and South America [Kerr et al., 1996; Re´villon et al., 1999]. It has been (e.g. Kerr et al., 1997). Thus, oceanic plateau sec- postulated that it may represent a lower level of the tions are now exposed on land in the Caribbean plateau stratigraphy than the Aruba Lava Formation region, for example, on the islands of Curacßao and [Klaver, 1987]. On Bonaire, to the east, a Cretaceous Aruba (Kerr et al., 1996; White et al., 1999). Fur- volcanic sequence is exposed, known as the Wash- thermore, some arc fragments also experienced uplift ikemba Formation [Klaver, 1976]. Until this study, its and subsequent accretion onto neighbouring conti- tectonic affinity was not well understood [cf. Giunta et nents; these are found juxtaposed with oceanic pla- al., 2002]. In this paper, we characterise for the first teaux rocks throughout the Caribbean region today. time the BWF using geochemical, geochronological This study aims to determine how they relate to the and Hf–Nd isotope data in order to evaluate its origin Caribbean plateau itself, in both a temporal and and establish its association, if any, with the Caribbe- tectonic sense, through a detailed investigation of an plateau. one such arc sequence exposed on Bonaire in the ABC islands.

2. Generalised tectonic history of the Caribbean plateau 3. Analytical techniques

It is generally agreed that the Caribbean plateau is During two field seasons, we collected 325 igneous allochthonous relative to the Americas (e.g. Duncan and volcaniclastic rock samples, and a representative and Hargreaves, 1984; Meschede, 1998; Mann et al., subset was selected for X-ray fluorescence (XRF) 1990). Most workers also agree that it was extruded Spectrometry, inductively coupled plasma optical onto Pacific crust and may represent the initial out- emission spectrometry (ICP-OES), inductively cou- bursts of the Gala´pagos plume (Duncan and Har- pled plasma mass spectrometry (ICP-MS), Ar–Ar greaves, 1984; Richards and Griffiths, 1989; Hauff et geochronology and Sr–Nd–Hf–Pb isotopes. Samples al., 1997; Thompson et al., 2003). As Pacific crust were prepared for geochemical analysis at the Uni- was consumed through eastward subduction along versity of Leicester. Weathered surfaces were removed the Central American arc, the young buoyant plateau using a hand splitter and the remainder split into 3- was passively transported eastwards until it arrived at cm3 chips. These chips were placed in a flypress and the Central American subduction zone only a few reduced to small gravel-sized fragments. For major 94 P.M.E. Thompson et al. / Lithos 74 (2004) 91–116

Table 1 Representative major and trace element data for the BWF Northern Complex Sample BON BON BON BON BON BON BON BON BON BON BON BON BON BON BON BON no 94-08 96-35 94-17 00-33 00-44 94-04 96-12 99-87 99-104 99-182 94-14 99-144 99-171 99-173 99-176 99-180 Lithology v v b b b d d d d d r r r r r r Alteration 3 3 3 5 3 3–4 3 2 2 2 2 2 3–4 3 4 2–3 Majors

SiO2 50.72 67.67 51.73 46.22 52.27 49.70 68.80 49.13 58.68 50.57 72.52 70.74 68.44 77.31 70.51 72.81 TiO2 0.30 0.33 0.89 1.10 1.16 0.68 0.70 0.73 1.41 0.86 0.76 0.75 0.86 0.67 0.81 0.53 Al2O3 8.66 6.68 14.40 16.08 12.66 18.70 13.38 17.67 13.45 17.65 13.32 12.92 14.41 10.87 12.27 12.83 Fe2O3 3.90 3.42 13.08 11.81 9.95 10.46 5.15 9.34 9.50 10.27 4.06 3.81 3.63 2.40 5.28 2.98 MnO 0.18 0.19 0.21 0.12 0.21 0.16 0.25 0.16 0.12 0.17 0.05 0.07 0.06 0.08 0.09 0.09 MgO 1.55 1.49 3.70 7.13 4.55 4.28 2.04 4.49 2.56 4.58 1.18 0.88 0.86 0.22 1.51 0.83 CaO 17.07 7.60 7.01 7.11 6.82 8.52 0.74 8.11 3.26 8.51 0.73 0.41 0.39 0.44 0.30 0.78

Na2O 1.68 2.01 4.03 2.07 5.17 4.17 5.36 4.84 6.07 4.27 6.85 5.57 6.32 5.60 5.96 6.15 K2O 1.12 1.84 1.09 0.37 0.04 0.97 1.22 0.63 0.22 0.99 0.89 2.15 2.93 1.12 1.25 1.72 P2O5 0.12 0.09 0.15 0.19 0.18 0.17 0.18 0.15 0.27 0.16 0.22 0.13 0.22 0.18 0.17 0.12 SO3 n.a. n.a. n.a. n.a. n.a. n.a. n.a. 0.03 0.04 0.02 n.a. 0.04 0.04 0.02 0.03 0.02 LOI 14.87 7.51 3.57 6.91 6.92 2.61 2.35 3.47 2.71 2.89 1.03 1.43 1.43 0.55 1.51 0.71 Total 100.17 98.81 99.85 99.11 99.93 100.43 100.17 98.75 98.27 100.95 101.62 98.90 99.59 99.45 99.67 99.58

Traces Pb 6.27 6.2 2.6 0.5 n.d. 2.45 n.d. 1.09 2.24 2.52 27.1 2.99 2.1 4.22 4.2 2.65 Rb 38.46 20.8 12.4 2.2 n.d. 16.05 5.1 10.70 2.31 19.07 5.7 12.38 17.4 5.33 12.3 6.00 Ba 802.10 298.0 461.4 184.5 33.7 275.64 249.0 300.21 80.53 165.57 225.5 644.43 627.5 364.66 658.8 445.01 Th 2.25 2.0 n.d. 3.3 3.5 0.48 1.1 0.62 1.37 0.59 1.0 0.82 3.1 0.38 4.6 0.56 U 1.36 n.d. n.d. n.d. 0.5 0.12 n.d. 0.18 0.39 0.17 1.7 0.49 0.2 0.55 0.9 0.55 Nb 2.1 2.1 1.5 2.0 2.0 1.4 1.5 1.5 3.4 1.4 2.4 2.6 2.7 1.8 2.3 2.7 Sr 995.9 127.9 407.9 170.7 55.9 390.8 239.6 399.0 220.9 353.0 59.5 103.2 37.5 50.0 43.3 55.1 Zr 95.8 38.4 59.9 59.5 73.3 51.8 46.8 54.3 141.7 59.3 98.5 110.1 123.7 94.6 116.7 149.5 Sc 21.5 18.4 33.0 48.1 36.0 25.1 33.9 26.4 27.3 25.4 20.5 18.0 17.2 13.9 17.6 10.6 Co 11.6 11.6 43.8 47.2 35.9 36.1 45.0 34.5 32.4 36.7 8.7 8.4 8.2 3.9 13.6 6.3 V 76.3 88.1 357.1 380.1 364.5 228.7 285.4 218.8 234.4 256.8 13.9 12.4 15.1 18.2 27.6 21.4 Cr 8.8 34.4 67.0 18.7 96.0 15.6 7.4 23.2 n.d. 35.5 5.4 15.7 22.6 17.6 9.7 25.0 Cu 36.4 67.0 207.1 n.a. n.a. 154.4 42.4 51.0 10.9 106.4 3.9 n.d. n.d. n.d. 5.9 n.d. Ga 10.5 8.0 19.1 n.a. n.a. 21.4 16.5 18.3 16.9 18.6 13.6 14.3 14.8 11.3 14.8 14.1 Ni 4.6 20.6 13.0 23.9 13.5 24.7 11.3 24.6 6.2 23.3 n.d. n.d. n.d. n.d. 2.7 n.d. Zn 44.1 127.4 83.2 98.9 78.7 49.3 57.9 55.5 43.3 61.5 79.1 71.6 161.4 59.9 68.0 79.5 Hf 4.93 n.a. n.a. n.a. n.a. 1.22 n.a. 1.76 2.58 1.64 n.a. 3.50 n.a. 2.97 n.a. 4.36 Ta 0.25 n.a.* n.a.* n.a. n.a. 0.18 n.a. 0.42 0.27 0.22 n.a.* 0.16 n.a.* 0.12 n.a.* 0.20

REE La 12.23 7.0 4.4 4.56 8.66 5.27 1.35 5.50 10.98 5.23 9.4 4.89 9.7 2.28 9.2 3.45 Ce 22.74 n.d. 17.4 11.88 21.32 12.45 3.54 12.12 26.41 12.23 14.3 16.06 6.6 11.54 8.0 12.30 Pr 3.48 n.a. n.a. 1.69 2.95 1.93 0.56 1.80 4.15 1.88 n.a. 1.87 n.a. 0.96 n.a. 1.53 Nd 15.23 12.2 11 9.10 14.35 9.32 3.66 8.71 19.71 9.02 15.7 9.12 17.0 4.74 14.7 7.63 Sm 3.99 n.a. n.a. 3.12 4.06 2.58 1.31 2.48 5.45 2.55 n.a. 2.72 n.a. 1.62 n.a. 2.27 Eu 1.12 n.a. n.a. 1.21 1.42 1.02 0.85 0.99 1.69 0.98 n.a. 0.97 n.a. 0.67 n.a. 0.93 Gd 4.87 n.a. n.a. 4.25 4.60 2.94 2.67 2.83 6.18 2.99 n.a. 3.21 n.a. 2.11 n.a. 2.82 Tb 0.85 n.a. n.a. n.a. n.a. 0.52 n.a. 0.52 1.11 0.53 n.a. 0.60 n.a. 0.43 n.a. 0.54 Dy 5.53 n.a. n.a. 4.82 5.04 3.12 3.89 3.19 6.80 3.16 n.a. 3.96 n.a. 2.82 n.a. 3.51 Ho 1.22 n.a. n.a. n.a. n.a. 0.62 n.a. 0.66 1.39 0.66 n.a. 0.84 n.a. 0.63 n.a. 0.79 Er 3.58 n.a. n.a. 3.12 2.98 1.74 2.33 1.93 3.99 1.90 n.a. 2.48 n.a. 1.93 n.a. 2.28 Tm 0.57 n.a. n.a. n.a. n.a. 0.26 n.a. 0.27 0.56 0.27 n.a. 0.39 n.a. 0.31 n.a. 0.36 P.M.E. Thompson et al. / Lithos 74 (2004) 91–116 95

Southern Complex BON BON BON BON BON BON BON BON BON BON BON BON BON BON BON BON BON BON BON 00-09 99-04 99-08 99-27 99-52 00-01 00-34 96-26 96-27 99-42 99-50 00-19 00-23 00-29 00-52 96-22 99-35 00-50 00-67 v bb b b b b rdrdrdrdrdrdrdrdrr rr 4 3 3–4 3 3 2–3 2 3–4 3 3–4 2–3 2–3 2 2 2 3 3 3 3

71.98 49.01 48.44 50.59 48.77 51.41 49.79 72.34 72.37 77.97 70.03 68.71 58.95 70.16 69.31 79.04 78.54 69.33 71.37 0.60 0.89 0.85 0.99 1.07 1.32 1.19 0.53 0.54 0.57 0.60 0.60 1.38 0.59 0.58 0.37 0.64 0.74 0.76 13.59 18.00 17.83 16.21 13.99 15.55 14.96 14.01 14.31 11.59 14.21 14.82 16.62 14.67 15.02 9.75 10.61 13.52 13.76 3.10 10.51 10.52 11.99 12.79 11.47 13.14 3.87 3.78 2.35 4.26 4.30 7.38 4.10 4.06 3.29 1.99 4.82 5.05 0.10 0.24 0.24 0.25 0.21 0.15 0.16 0.08 0.05 0.06 0.09 0.09 0.16 0.07 0.08 0.03 0.07 0.07 0.08 1.07 6.66 8.18 7.14 3.88 4.73 4.66 0.89 0.89 0.17 0.75 0.78 3.00 0.91 0.87 0.85 0.23 1.10 1.01 0.70 3.87 2.29 1.03 10.03 6.53 6.91 0.63 0.41 0.25 0.76 1.52 2.49 0.84 1.40 0.33 0.81 0.40 0.38 6.42 3.97 5.01 6.07 3.50 5.59 5.37 7.75 7.17 6.98 7.76 6.90 7.43 7.88 7.46 4.84 6.28 5.41 6.67 1.55 1.06 1.05 0.32 0.59 0.55 0.39 0.14 0.93 0.16 0.73 1.42 0.97 0.60 1.42 0.71 0.11 2.55 1.16 0.17 0.11 0.10 0.15 0.15 0.17 0.11 0.12 0.13 0.10 0.15 0.13 0.40 0.13 0.13 0.04 0.32 0.23 0.19 n.a. 0.03 0.03 0.03 0.03 n.a. n.a. n.a. n.a. 0.03 0.03 n.a. n.a. n.a. n.a. n.a. 0.03 n.a. n.a. 1.13 5.28 6.08 4.87 4.90 2.60 2.71 0.97 1.07 0.51 1.23 1.02 2.55 1.08 1.27 1.58 0.52 1.52 1.24 100.41 99.64 100.63 99.64 99.91 100.08 99.39 101.34 101.64 100.74 100.62 100.29 101.33 101.03 101.60 100.82 100.13 99.67 101.66

4.44 1.5 n.d. 4.02 1.79 2.54 2.71 n.d. 0.84 3.68 n.d. n.d. n.d. n.d. 1.70 6.02 3.99 1.4 3.80 11.28 8.6 9.6 3.77 10.00 3.76 2.63 0.0 6.07 1.82 5.5 12.5 7.2 3.0 12.98 5.75 0.94 22.9 9.19 506.85 721.8 606.8 247.71 374.26 235.80 109.85 25.9 155.62 38.10 150.6 379.9 284.0 110.1 324.36 189.95 54.80 601.3 265.59 1.26 2.5 3.4 1.03 0.66 0.89 0.52 4.2 2.17 1.25 3.0 3.8 2.8 4.2 2.44 1.32 0.84 3.1 1.41 0.46 0.9 1.0 1.17 0.25 0.33 0.21 n.d. 0.72 0.47 0.3 0.1 0.8 0.7 0.84 1.20 1.23 0.6 0.57 2.6 1.1 0.9 2.4 1.7 3.3 1.0 8.1 8.3 6.4 8.3 8.4 6.7 8.0 8.0 2.4 2.0 2.2 2.5 121.03 202.4 147.9 124.7 532.1 115.66 70.32 57.3 98.74 40.60 108.1 188.6 151.5 72.5 93.68 65.97 78.2 62.4 73.58 98.3 50.4 42.2 62.9 49.8 97.6 56.9 277.2 282.5 219.7 293.9 295.5 210.2 285.9 285.5 106.5 88.2 108.6 109.1 12.3 45.1 46.7 39.4 40.5 n.a. n.a. 10.1 12.3 9.0 12.7 13.2 20.8 12.0 11.1 11.3 16.1 18.6 22.2 6.6 40.4 41.1 44.7 48.0 44.4 50.7 7.3 7.6 3.9 8.7 8.9 21.1 7.9 7.4 8.3 3.2 9.8 10.9 22.1 311.0 296.7 318.5 384.2 10.3 17.2 21.4 15.7 9.8 88.4 9.9 9.4 95.0 17.9 11.9 8.9 14.5 46.6 40.2 27.7 159.9 72.4 n.d. 8.8 7.4 5.2 19.8 21.5 87.1 2.2 19.6 25.7 8.6 2.9 n.d. 3.8 132.2 117.2 156.9 161.3 95.1 270.1 n.d. n.d. n.d. n.d. n.d. 4.7 n.a. n.a. 45.2 n.d. n.a. n.a. 14.4 18.9 16.9 19.7 17.4 17.2 18.2 18.2 17.0 12.7 19.3 19.0 19.4 n.a. n.a. 8.7 10.9 n.a. n.a. n.d. 24.8 34.4 24.1 18.9 0.3 n.d. n.d. n.d. 1.2 n.d. n.d. 1.2 n.d. n.d. 8.9 n.d. n.d. n.d. 74.9 66.7 59.8 77.5 89.5 71.3 85.9 47.7 60.4 37.9 45.9 59.2 86.2 47.0 54.3 68.7 72.6 89.6 71.7 3.93 n.a. n.a. 1.98 1.74 3.44 2.08 n.a. 9.25 5.27 n.a. n.a. n.a. n.a. 9.83 3.26 2.91 n.a. 4.59 0.18 n.a. n.a. 0.13 0.19 0.30 0.37 n.a. 0.39 0.42 n.a. n.a. n.a. n.a. 0.37 0.18 0.13 n.a.* 0.17

8.67 4.91 3.83 12.50 5.27 5.96 3.70 15.8 10.95 12.89 15.53 18.81 10.49 10.74 17.59 7.54 7.05 7.1 8.60 19.40 9.80 8.78 22.39 12.28 14.70 9.08 27.5 22.45 32.05 35.80 46.11 25.36 27.15 41.80 17.27 17.65 2.8 21.68 3.21 1.23 1.35 3.36 1.98 2.44 1.56 n.a. 4.34 4.51 4.23 5.68 3.50 3.60 6.57 2.39 2.51 n.a. 3.72 15.21 6.60 6.04 14.64 9.64 11.46 7.62 26.9 20.08 20.12 19.68 25.17 16.67 16.07 29.45 10.79 12.14 13.4 18.49 4.29 2.16 2.20 3.72 3.02 3.55 2.41 n.a. 5.52 5.19 5.84 7.46 5.20 4.77 7.75 2.93 3.71 n.a. 5.74 1.77 0.89 0.70 1.35 1.26 1.31 1.01 n.a. 2.13 1.57 2.05 2.40 1.84 1.57 2.50 0.78 1.19 n.a. 1.82 4.97 2.85 2.21 4.09 3.61 4.19 3.12 n.a. 6.12 5.77 6.84 7.77 6.02 5.22 8.70 3.31 4.37 n.a. 6.96 0.93 n.a. n.a. 0.70 0.67 0.79 0.61 n.a. 1.11 1.06 n.a. n.a. n.a. n.a. 1.59 0.62 0.83 n.a. 1.30 5.89 3.33 2.74 4.07 4.19 4.91 3.82 n.a. 6.65 6.46 7.24 7.79 5.72 5.50 9.74 4.08 5.42 n.a. 8.06 1.30 n.a. n.a. 0.78 0.89 1.02 0.80 n.a. 1.42 1.30 n.a. n.a. n.a. n.a. 2.05 0.86 1.23 n.a. 1.70 3.84 1.88 1.58 2.22 2.51 2.97 2.30 n.a. 4.21 3.93 3.27 4.67 2.69 3.18 5.80 2.55 3.67 n.a. 5.06 0.59 n.a. n.a. 0.32 0.38 0.42 0.33 n.a. 0.68 0.56 n.a. n.a. n.a. n.a. 0.93 0.41 0.57 n.a. 0.77 (continued on next page) 96 P.M.E. Thompson et al. / Lithos 74 (2004) 91–116

Table 1 (continued) Northern Complex Sample BON BON BON BON BON BON BON BON BON BON BON BON BON BON BON BON no 94-08 96-35 94-17 00-33 00-44 94-04 96-12 99-87 99-104 99-182 94-14 99-144 99-171 99-173 99-176 99-180 Lithology v v b b b d d d d d r r r r r r Alteration 3 3 3 5 3 3–4 3 2 2 2 2 2 3–4 3 4 2–3 REE Yb 3.75 n.a. n.a. 2.63 2.55 1.60 2.29 1.81 3.45 1.70 n.a. 2.58 n.a. 2.05 n.a. 2.40 Lu 0.61 n.a. n.a. 0.39 0.43 0.25 0.39 0.28 0.53 0.27 n.a. 0.42 n.a. 0.31 n.a. 0.37 Y 65.82 30.0 26.7 19.6 25.8 18.5 22.1 19.3 42.1 19.4 43.5 22.7 44.8 18.2 43.6 19.6 Alteration is graded from 0–5, with 0 being unaltered and 5 being completely altered. Lithological abbreviations: r—rhodacite, rd—rhyodacite dome, b—basalt and d—dolerite. n.a.: not analysed, n.d.: not detected. *Only XRF data available.

and trace element analysis, 50 cm3 of this material multi-dynamic mode; Nd was run in static mode. The was placed in an Agate Teman swingmill and ground effects of fractionation during runs were eliminated by to produce a fine powder. A tungsten carbide swing- normalising 87Sr/86Sr to a 86Sr/88Sr value of 0.1194 mill was used for samples selected for isotopic anal- and 143Nd/144Nd to a 146Nd/144Nd value of 0.7219. ysis. For ICP-OES analysis, 0.300 g of rock powder Sample values for 87Sr/86Sr and 143Nd/144Nd are was dissolved, using open vessel digestion techni- reported relative to an accepted value of NBS 987 ques, prior to column separation and analysis with a of 0.71024 and 0.51186, respectively, for La Jolla Philips PV 8060 instrument using the technique of international standards. Minimum uncertainty is de- Walsh et al. (1981). For ICP-MS analysis, 0.05 g was rived from external precision of standard measure- dissolved using microwave digestion prior to analysis ments, which over the course of analysis average 14 at Cardiff University Earth Sciences Department using ppm (1j)for87Sr/86Srand43ppm(1j)for a Perkin Elmer Elan 5000. Rh was used as the internal 143Nd/144Nd. Pb isotopes were analysed on the VG standard for Rb, Sr, Y, Nb, Cs and Ba and Re for the P54 MC-ICP-MS, since this instrument allows us to rare earth elements (REEs). Representative data are correct for mass fractionation during the run using the presented in Table 1. Tl-doping method. We have used a 205Tl/203Tl value The trace element and petrographic data indicate of 2.388, which was determined empirically by cross the rocks have experienced not only secondary alter- calibration with NBS 981. All Pb isotope ratios have ation but also low-temperature metamorphism. There- been corrected relative to the NBS 981 composition of fore, samples were not acid leached prior to isotopic Todt et al. (1996). Based on repeated runs of NBS analysis. Acid leaching tends to be non-specific in its 981, the reproducibility of whole-rock Pb isotope attack and can remove glass or primary minerals, such measurements is better than F 0.01% (2j). Blanks as apatite, which preserve the magmatic isotopic were < 1 ng for Sr, < 200 pg for Nd and 500 pg for signature (Thompson and Kempton, in prep.).Sr, Pb. For the apatite analysis, Nd, Sm and Sr blanks Pb, Nd and Hf isotope ratios were determined at the were less than 60, 10 and 570 pg, respectively. NERC Isotope Geosciences Laboratory, Keyworth Hf isotopes were run on a VG Plasma P54 MC- (NIGL). The data are presented in Table 2. Procedures ICP-MS at NIGL and within-run standard error used in the analysis of Sr, Pb, Nd and Hf isotopes are for Hf isotope measurements is normally less than given in Royse et al. (1998), Kempton et al. (2001) 22 ppm (2j). Minimum uncertainties are derived and Kempton and McGill (2002). Sr and Nd were run from external precision of standard measurements, as the metal species on single Ta filaments and double which average 44 ppm (2j). Replicate analysis of Re–Ta filaments, respectively, using a Finnigan MAT our internal rock standard, pk-G-D12, over the 262 multicollector mass spectrometer, Sr was run in course of analysis yielded 0.283050 F 12 (2j, P.M.E. Thompson et al. / Lithos 74 (2004) 91–116 97

Southern complex BON BON BON BON BON BON BON BON BON BON BON BON BON BON BON BON BON BON BON 00-09 99-04 99-08 99-27 99-52 00-01 00-34 96-26 96-27 99-42 99-50 00-19 00-23 00-29 00-52 96-22 99-35 00-50 00-67 vbbbbbbrdrdrdrdrdrdrdrdrr r r 433–4332–323–433–42–32–32223333

3.96 1.74 1.48 1.97 2.35 2.72 2.23 n.a. 4.48 3.66 3.65 4.37 1.89 2.56 5.86 2.69 3.83 n.a. 4.95 0.60 0.26 0.23 0.29 0.36 0.43 0.33 n.a. 0.75 0.56 0.49 0.64 0.22 0.34 0.91 0.42 0.60 n.a. 0.78 31.8 18.3 15.8 21.6 25.3 26.5 20.3 57.4 34.46 38.0 65.0 65.8 50.1 56.6 54.0 23.8 37.0 n.a. 44.87

n = 45), which is indistinguishable from our previ- 4. Results ously reported value determined by PIMMS (0.283049 F18, 2j, n = 27; Kempton et al., 2000). 4.1. Field relations on Bonaire The data are corrected for mass fractionation during the run by normalization to 179Hf/177Hf of 0.7325 Two main igneous complexes (termed the Wash- and are reported relative to an accepted value of JMC ikemba Formation; Klaver, 1976) are exposed on the 475 of 0.282160, as recommended by Nowell et al. island of Bonaire. These are separated by Tertiary to (1998). In all cases, within-run precision is less than Recent limestones (Klaver, 1976; Thompson, 2002; external reproducibility. Fig. 2) and are termed the Northern and the Southern Eight rhyodacite samples were selected for Ar–Ar Complex according to their geographic location. isotopic analysis, on the basis of the freshness of their The Bonaire Washikemba Formation (BWF) is feldspar phenocrysts. They were crushed using a essentially bimodal in composition, comprising cleaned flypress, sieved, and the relevant size fractions basalts, dolerites and rhyodacites, along with felsic leached in warm 3 N HCl for 30 min in an ultrasonic volcaniclastic lithologies. The Northern Complex con- bath, prior to being rinsed in distilled water and dried sists of a 5-km-thick sequence of intrusive dolerites prior to hand picking. Feldspars were separated using a and rhyodacitic lava flows, with intercalations of binocular microscope and fine tweezers. The selected volcaniclastic lithologies, cherts and cherty lime- mineral grains, 50 per sample, were wrapped in copper stones. Volcaniclastic rocks are volumetrically most foil packets and placed in quartz vials, separated by abundant, followed by rhyodacite and dolerite. The monitor samples from the USGS standard Taylor Creek middle of the sequence also includes shallow-level, Rhyolite (TCR). The packets were placed in a nuclear columnar-jointed, rhyodacitic sills. Pillow basalts and reactor for 24 h in the cadmium-lined RODEO facility cherty limestones are more abundant towards the at the EC 45 MW rector at Petten, Holland. The isotopic upper parts of the sequence. composition of Ar was determined at SUERC, East The Southern Complex, in contrast, is comprised Kilbride, using a MAP 215 rare gas mass spectrometer of a poorly exposed succession of basaltic pillowed connected to a CO2 laser. The results are presented in flows with interstitial chert, interbedded with fine- Table 3. The irradiation flux factor ( J) was monitored grained volcaniclastic sediments, all intruded by a using the TCR 85G003 at 27.92 Ma. All errors are series of endogenous rhyodacite domes. Pillow basalts reported as one standard deviation of analytical preci- and volcaniclastic lithologies predominate, with sub- sion unless otherwise stated. A more detailed descrip- ordinate rhyodacite. In addition, coarse-grained brec- tion of the 40Ar/39Ar analytical procedure can be found cias are present, which locally grade upwards into in Singer et al. (1999) and details of the irradiation crystal-rich, coarse-grained sandstones and fine- conditions can be found in Pringle (1993). grained turbiditic units. Table 2 98 Hf–Nd–Sr–Pb isotope data for the Bonaire Washikemba Formation and the Aruba batholith Sample # Area Rock 143 Nd/ Sm Nd 143 Nd/ qNd 176 Hf/ Lu Hf Initial Hf qHf Apatite 87Sr/86Sr 1j error 143 Nd/ 1j ppm type 144 Nd 144 Nd (t = 95) 177 Hf (t = 95) sample # measured 144 Nd error measured initial measured measured Bonaire BON 96-22 SC v 0.512968 2.93 10.79 0.512866 6.83 0.283127 0.42 3.26 0.283094 13.11 BON 96-29 0.70335 0.000035 0.513026 6 BON 96-27 SC rd 0.513027 5.53 20.08 0.512923 7.95 0.283116 0.75 7.79 0.283091 13.52 Nd ppm 1060 BON 96-29 SC rd 0.513016 3.30 12.27 0.512915 7.79 0.283118 0.44 7.01 0.283102 13.86 Sm ppm 289 BON 99-27 SC b 0.512972 3.72 14.64 0.512876 7.03 0.283122 0.29 1.98 0.283084 13.39 Sr ppm 564 BON 99-35 SC r 0.513004 3.71 12.14 0.512889 7.28 0.283171 0.60 2.91 0.283117 14.50 Bon 99-180 0.703 0.0055 0.513039 5 BON 99-87 NC d 0.513021 2.48 8.71 0.512914 7.77 0.283138 0.28 1.76 0.283097 13.74 Nd ppm 950 BON 99-104 NC d 0.513049 5.45 19.71 0.512945 8.37 0.283155 0.53 2.58 0.283101 13.94 Sm ppm 271 BON 99-144 NC r 0.513011 2.72 9.12 0.512899 7.47 0.283159 0.42 3.50 0.283128 14.83 Sr ppm 406 ...Topo ta./Lto 4(04 91–116 (2004) 74 Lithos / al. et Thompson P.M.E. BON 99-180 NC r 0.513015 2.27 7.63 0.512903 7.56 0.283136 0.37 4.36 0.283114 14.32 BON 00-01 SC b 0.513013 3.55 11.46 0.512896 7.42 0.283154 0.43 3.44 0.283121 14.15 BON 99-182 NC d 0.513034 2.55 9.02 0.512928 8.03 0.283135 0.27 1.64 0.283092 13.59 BON 99-173 NC r 0.513027 1.62 4.74 0.512898 7.46 0.283145 0.31 2.97 0.283117 14.43 BON 00-52 SC b 0.513045 7.75 29.45 0.512946 8.39 0.283124 0.91 9.83 0.283100 13.82 BON 00-34 SC b 0.513060 2.41 7.62 0.512941 8.29 0.283164 0.33 2.88 0.283134 14.84 BON 00-67 SC r 0.513008 5.74 18.49 0.512891 7.32 0.283148 0.78 4.58 0.283104 13.83

Aruba ARU 96-131 t 0.512960 2.35 9.98 0.512878 6.85 0.283144 0.19 2.65 0.283126 14.53 ARU 96-42 g 0.512998 1.33 4.70 0.512900 7.28 0.283165 0.13 0.61 0.283113 14.07 ARU 96-53 di 0.513005 1.39 4.85 0.512905 7.38 0.283124 0.17 0.91 0.283079 12.86 ARU 96-154 lg 0.513036 0.69 1.83 0.512905 7.38 0.283127 0.09 0.61 0.283092 13.30

Sample # Area Rock 87 Sr/86 Sr Rb Sr 87Sr/86Sr UThPb206 Pb/ 207 Pb/ 208 Pb/ 206 Pb/ 207 Pb/ 208 Pb/ type measured initial 204 Pb 204 Pb 204 Pb 204 Pb 204 Pb 204 Pb (t = 95) measured measured measured initial initial initial BON 96-22 SC v 0.706407 5.75 66.31 0.706068 1.20 1.32 6.02 19.015 15.618 38.669 18.825 15.618 38.601 BON 96-27 SC rd 0.704542 6.07 99.00 0.704303 0.72 2.17 0.80 19.899 15.623 39.560 19.021 15.619 38.691 BON 96-29 SC rd 0.704359 0.15 48.90 0.704347 0.63 0.63 0.79 19.714 15.627 39.294 18.943 15.624 39.039 BON 99-27 SC b 0.706504 3.77 125.39 0.706387 1.17 1.03 4.02 19.085 15.609 38.710 18.808 15.608 38.630 BON 99-35 SC r 0.705600 0.94 79.16 0.705554 1.23 0.84 3.99 19.116 15.614 38.619 18.823 15.612 38.553 BON 99-87 NC d 0.704083 10.70 399.35 0.703978 0.18 0.62 1.09 19.019 15.592 38.735 18.862 15.591 38.557 BON 99-104 NC d 0.704161 2.31 221.12 0.704120 0.39 1.37 2.24 19.241 15.612 38.817 19.074 15.611 38.625 BON 99-144 NC r 0.704988 12.38 103.52 0.704521 0.49 0.82 2.99 18.982 15.610 38.699 18.826 15.609 38.613 BON 99-180 NC r 0.704987 6.00 54.43 0.704556 0.55 0.56 2.65 18.779 15.573 38.391 18.583 15.572 38.326 BON 00-01 SC b 0.704840 3.76 116.00 0.704713 0.33 0.89 2.54 18.977 15.597 38.678 18.854 15.597 38.568 Within run errors are less than external reproducibility, except where reported for apatite. Ages are corrected to 95 Ma for the Washikemba Formation and 88 Ma for the Aruba batholith. Note that all samples for Hf isotope analysis were prepared in Savillex bombs rather than high-pressure vessels and thus accessory phases, such as zircon will not be dissolved. The likely zircon-bearing sample in this suite is the Aruba batholith tonalite, and thus we recommend caution in interpreting its Hf isotope characteristics. SC: Southern Complex, NC: Northern Complex. Lithological abbreviations: t—tonalite, g—gabbro, lg—layered gabbro, di—diorite; others as for Table 1. Numbers in italics denote data taken from White et al. (1999). P.M.E. Thompson et al. / Lithos 74 (2004) 91–116 99

Table 3 A summary of single crystal incremental heating Ar–Ar data Step 40Ar/ 37Ar/ 36Ar/ 40Ar 39Ar 37Ar 36Ar K/Ca 39Ar Apparent F 1 (a) 39Ar 39Ar 39Ar S.D. S.D. S.D. S.D. (%) (%) age (Ma) (c) S.D. (b) (b) (b) (%) (%) (%) (%) (Ma) BON 94-09, 53.5 F 1 Ma, 3.6 64.84 0.6477 0.2142 0.09 0.23 0.42 0.37 0.76 51.4 32.69 F 5.99 Northern MSWD = 74.91, 3.7 5.362 0.875 0.0123 0.17 0.27 0.26 1.89 0.56 9.9 36.87 F 1.35 Complex, 5 of 5 increments 3.9 6.568 0.9772 0.017 0.21 0.28 0.3 3.62 0.5 6.6 33.24 F 3.5 Experiment used 4.2 9.404 1.3838 0.0219 0.07 0.33 0.1 0.65 0.35 25 61.69 F 0.87 no: pl1l00602 10 16.457 3.92 0.0456 0.1 0.93 0.17 1.25 0.13 7.2 66.94 F 3.34 BON 94-09, 75.5 F Ma, 3.8 34.33 2.026 0.1038 0.05 0.49 0.14 0.5 0.24 14.2 77.45 F 3.33 Northern MSWD = 75.14, 4.1 10.75 1.8007 0.0257 0.1 0.43 0.07 1.01 0.27 15.6 66.93 F 1.5 Complex, 2 of 6 increments 4.3 12.468 1.6809 0.0316 0.06 0.4 0.1 0.77 0.29 12.6 66.17 F 1.43 Experiment used 4.5 17.784 2.671 0.0489 0.04 0.62 0.08 0.26 0.18 29.3 71.98 F 0.92 no: pl1l0062 10 21.44 7.553 0.0596 0.08 1.75 0.1 0.5 0.07 10.3 89.85 F 1.84 BON 94-09, 79.1 F 2 Ma, 3.5 130.74 0.979 0.4185 0.04 0.38 0.94 0.64 0.5 2.5 142 F 18.3 Northern MSWD = 0.59, 3.6 15.802 3.052 0.0367 0.21 0.76 0.16 3.35 0.16 2.7 104.83 F 6.75 Complex, 2 of 9 increments 3.7 57.62 2.893 0.1822 0.05 0.69 0.15 0.41 0.17 11.2 81.49 F 5.1 Experiment used 3.8 16.76 4.234 0.0462 0.09 1 0.16 1.62 0.12 5 69.94 F 4.27 no: pl1l0072 3.9 8.39 3.764 0.017 0.15 0.89 0.27 3.89 0.13 5.5 74.22 F 3.54 4.1 7.69 3.25 0.0162 0.05 0.8 0.07 2.29 0.15 9.1 64.13 F 2.06 4.3 19.469 3.045 0.0546 0.06 0.71 0.06 0.21 0.16 32.1 72.45 F 1.03 4.5 26.01 6.179 0.0746 0.03 1.43 0.09 0.5 0.08 9.5 79.74 F 2.29 4.7 26.08 8.446 0.0779 0.06 1.96 0.1 0.99 0.06 4.7 75.87 F 4.49 10 21.6 11.29 0.0604 0.06 2.62 0.07 0.46 0.04 17.6 94.32 F 1.75 BON99-180, 119.6 F 26 Ma, 3.7 27.7 2.951 0.0896 0.2 0.83 0.4 3.31 0.17 34 30.33 F 17.4 Northern MSWD = 4.24, 3.8 45.97 1.5802 0.1478 0.14 0.55 0.85 1.43 0.31 36.5 49.82 F 13.2 Complex, 1 of 3 increments 10 155.04 3.565 0.5055 0.07 0.92 0.23 0.67 0.14 29.4 119.62 F 25.7 Experiment used no: pl1l0065 Bon 99-180, 86.2 F 6 Ma, 3.6 8.244 0.041 0.0134 0.51 0.28 11.5 13.47 12 37 86.73 F 10.1 Southern MSWD = 1.08, 4.2 3.999 0.0931 0.0006 1.71 0.42 8.18 496.29 5.26 20.2 77.51 F 16.5 Complex, 3 of 3 increments 10 13.246 0.2022 0.0296 0.25 0.27 1.88 4.86 2.42 42.8 91.47 F 8.04 Experiment used no: pl1l0070 BON 00-19, 78.7 F 1 Ma, 3.6 8.933 0.0277 0.0172 0.09 0.2 1.06 0.9 17.7 56.5 77.69 F 0.99 Southern MSWD = 1.97, 3.9 6.951 0.0134 0.0089 0.4 0.13 30.25 13.04 36.6 6 87.23 F 6.47 Complex, 3 of 3 increments 10 6.41 0.034 0.0083 0.11 0.08 1.21 2.76 14.4 37.4 80.12 F 1.3 Experiment used no: pl1l0066 BON 00-19, 83.4 F 3 Ma, 3.6 24.3 0.0403 0.0671 0.08 0.24 6.03 1.61 12.2 41.8 90.15 F 6.24 Southern MSWD = 1.10, 3.8 6.84 0.087 0.0127 1.02 0.37 10.77 19.92 5.63 10.2 63.08 F 14.3 Complex, 4 of 4 increments 4.2 13.377 0.1542 0.0315 0.21 0.52 1.72 2.25 3.18 33 82.43 F 4.33 Experiment used 10 28.97 0.3683 0.0845 0.17 0.38 1.5 1.86 1.33 15 81.53 F 9.27 no: pl1l0068 1j errors are used throughout. (a) Steps labeled as laser power (W). (b) Corrected for 37Ar and 39Ar decay, half-lives 35.1 days and 259 years, respectively. (c) Ages calculated relative to 85G003 TCR Sanidine at 28.02 Ma with ke = 0.581E À 10/year and kb = 4.692E À 10/year.

4.2. Petrography and mineralogy tropical weathering (Thompson, 2002). The ground- mass of most rocks is at least partially altered Field and petrographic studies confirm that alter- ( f 50%) to clays. Many primary phases, such as ation has been extensive for much of the BWF, as the olivine and some clinopyroxene, have experienced a rocks have undergone prehnite-pumpellyite facies similar fate and are replaced by chlorite, pumpellyite, burial metamorphism, seawater alteration and surface prehnite or Fe-oxyhydroxide. In this section, we 100 P.M.E. Thompson et al. / Lithos 74 (2004) 91–116

Fig. 2. Geological map of Bonaire, Dutch Antilles. After Klaver (1976) and this study. describe the nature of this alteration, in addition to tabular and turbid, and alteration products, such as inferred primary characteristics, in order to place the sericite and clays, are common along fractures and geochemical data into context. within the grains. In many cases, plagioclase displays The rhyodacites typically contain up to 15% K- strong oscillatory zoning, fractures and resorbed mar- feldspar and plagioclase phenocrysts, along with var- gins, indicating much of it may be xenocrystic in iable amounts of quartz. The K-feldspar is commonly origin. Quartz phenocrysts are typically irregular in P.M.E. Thompson et al. / Lithos 74 (2004) 91–116 101 shape, and some show signs of resorption and undu- 4.3. Major and trace elements lose extinction indicative of straining. The rhyodacite matrix is typically fine-grained to microcrystalline and As discussed above, alteration and metamorphism consists of plagioclase, K-feldspar, quartz and mag- are a problem for the rocks of the BWF, such that netite, with interstitial patches of chlorite/pumpellyite. some major element abundances, along with the more A trachytic texture is common, and in some cases the mobile trace elements (e.g. Pb, U, Th and the large ion matrix has partially or completely devitrified, produc- lithophile elements or LILEs) are no longer represen- ing a granophyric texture. Rhyodacite domes can be tative of the original rock composition (Fig. 3A,B,C). distinguished petrographically from the flows and sills Therefore, in this study, we focus on the more by the seriate texture of the feldspar in the former. In immobile trace elements, such as the high field general, rhyodacites tend to be 50–60% altered. strength elements (HFSE) and the REEs, which have Basalts are typified by elongate plagioclase laths not been visibly affected by the alteration processes surrounding patches of altered clinopyroxene, now experienced by the BWF (Fig. 3D,E,F). This resis- partially replaced by chlorite and clay minerals. Pum- tance of these elements to alteration is further con- pellyite appears in olivine psuedomorphs, although firmed by the similar concentrations of REE and the high degree of alteration of most basalts (>50%) HFSE displayed by two different basalts from the makes estimating the original olivine abundance spec- same outcrop, despite having different visible degrees ulative at best. In the pillow basalts, plagioclase of alteration (BON 99-04 and BON 99-08; Table 1). typically displays either a skeletal branching form or A chondrite-normalised multi-element plot for the is subvariolitic, indicative of quenching. BWF is shown in Fig. 4. The BWF clearly displays The dolerites are characterised by an interlocking typical island arc characteristics, with negative Nb and framework of plagioclase and clinopyroxene, with Ti anomalies. All lithologies, except for the domes subordinate amounts of orthopyroxene and magnetite. from the Southern Complex, show roughly parallel Partially to completely pseudomorphed olivine is trends, with the felsic rocks displaying greater enrich- visible in some of the less altered sections, where it ments of most incompatible trace elements. This is comprises up to 10% of the rock. The dolerites are consistent with a cogenetic origin for these rocks. In typically very altered (>50%), and plagioclase laths contrast, the Southern Complex rhyodacite domes are usually seriticised and turbid, whilst clinopyrox- have elevated concentrations of most incompatible ene is partially to completely altered to pumpellyite trace elements, except P, Ti, Yb and Lu. In addition, and cryptocrystalline clays. Chlorite and Fe-oxyhydr- they have significantly larger negative P anomalies, oxides are also present as secondary phases, probably suggesting fractionation of apatite. Given the field replacing orthopyroxene and olivine, respectively. relationships (the domes appear synchronous with the Volcaniclastic lithologies vary from fine-grained bulk of the BWF), it is apparent that both geochemical volcaniclastic sandstones to a coarse-grained breccia. groups are contemporaneous, but derived from com- Petrographically, the coarse-grained breccia consists positionally distinct magma sources or different mag- of a variety of angular to rounded lithic clasts (e.g. ma chamber processes/plumbing systems. fine-grained igneous rocks, scoria, mudstones and some relic perlite), varying from 1 mm to upwards 4.4. Sr, Nd, Pb and Hf isotopes of 1 m in size, depending on distance from source. These are enclosed by a heterogeneous mix of angular Sr isotope ratios are plotted against qNd and fragments of quartz and feldspar (some showing compared to other Caribbean plateau localities in complex zoning), all in a matrix of brownish recrystal- Fig. 4. A pronounced spread to radiogenic Sr lized ash. Randomly orientated tube pumice, up to 10 ( f 0.7065) for a limited range in qNd is immediately mm in size, is volumetrically abundant (up to a apparent, not only for the new data from Bonaire, but maximum of 60%) in some specimens. The fine- also for other Caribbean plateau lithologies (Kerr et grained volcaniclastic sandstones predominantly con- al., 1996; Re´villon et al., 2002). Given the degree of sist of microcrystalline ash, with rare, poorly pre- alteration apparent in the Bonaire rocks, and the served, radiolarian fossils. preferential mobility of Sr compared with Nd, elevat- 102 P.M.E. Thompson et al. / Lithos 74 (2004) 91–116 P.M.E. Thompson et al. / Lithos 74 (2004) 91–116 103

Fig. 4. Plot of initial 87Sr/86Sr vs. qNd for samples from the Bonaire Washikemba Formation, with other fields shown for reference. All data are age corrected to assume age of emplacement (95 Ma for the Washikemba Formation). Data for the Aruba batholith from White (1999); Caribbean plateau data from White (1999) and Hauff et al. (2000b). Gala´pagos data from White et al. (1993); Northern Lesser Antilles data from White and Dupre´ (1986); EPR data from Mahoney et al. (1994); and Central American arc data from Feigenson and Carr (1986). ed 87Sr/86Sr ratios are probably due to seawater this primary phase, yet left behind some of the alteration. However, in a study of basalts and picrites secondary (and metamorphic) phases identified petro- from Curacßao, Kerr et al. (1996) found that high graphically, thus preserving the altered 87Sr/86Sr ra- 87Sr/86Sr ratios of whole-rock samples were not re- tios. Therefore, we conclude that the whole-rock duced by repeated leaching; they therefore argued that 87Sr/86Sr has been significantly affected by secondary this could be a primary feature due to assimilation of alteration, and we will exclude it from further inter- altered, high 87Sr/86Sr basalt. In contrast, Re´villon et pretation and discussion. al. (2002) found that unaltered clinopyroxenes from Pb isotope data (age corrected to 95 Ma) are Gorgona have much lower values of 87Sr/86Sr than the presented in Table 2 and Fig. 5. The BWF has higher whole rock, and therefore concluded that the displace- 207Pb/204Pb for a given 206Pb/204Pb than the Carib- ment to high 87Sr/86Sr was the result of subsolidus bean plateau field. Whilst the data overlap the Lesser alteration rather than a primary characteristic. This Antilles field in 207Pb/204Pb, some samples are dis- hypothesis was tested for the Bonaire rocks by mea- placed to lower 206Pb/204Pb. The observed range in suring the 87Sr/86Sr of primary igneous apatite, sepa- 206Pb/204Pb is surprising, particularly given that the rated from the whole rock. As Table 2 illustrates, BWF appears to be, for the most part, cogenetic on apatite separates for two different samples show the basis of trace element data (Section 4.3), while considerably lower Sr isotopic ratios (up to 0.0025 the wide range of rock types from the Northern lower) than the whole rock. Note that standard leach- Lesser Antilles show more restricted isotopic com- ing procedures (e.g. warm HCl) would have removed positions. Some Bonaire samples with virtually iden-

Fig. 3. Plots of trace elements vs. Zr. An indication of the degree of mobility of an element is represented by its correlation with Zr: an immobile element unmodified by alteration will show a good correlation, assuming that all rocks belong to the same suite and are related by crystal fractionation. All values are quoted in parts per million (ppm). All data obtained by ICP-MS where available, otherwise XRF, except for Zr (XRF). NC and SC are abbreviations for Northern Complex and Southern Complex, respectively. Note that A, C and E show considerably more scatter than B, D, F, G and H, and the latter elements are considered to be immobile. 104 P.M.E. Thompson et al. / Lithos 74 (2004) 91–116

207 204 206 204 Fig. 5. Plot of ( Pb/ Pb)i vs. ( Pb/ Pb)i for samples from the Bonaire Washikemba Formation with other fields shown for reference. Data for the Aruba batholith from White (1999); Caribbean plateau data from White (1999) and Hauff et al. (2000b). Gala´pagos data from White et al. (1993); pelagic sediment data from Schmincke et al. (1998); EPR data from Mahoney et al. (1994); and Lesser Antilles data from White and Dupre´ (1986) and Davidson (1987).2j errors are less than symbol size. tical major and trace elements have significantly domes, which are largely indistinguishable in terms different Pb isotope ratios (both measured and age of their trace element characteristics, have age cor- corrected). For example, the two SC rhyodacite rected 206Pb/204Pb values of 19.021 and 18.943,

Fig. 6. Plot of qHf vs. qNd for the Bonaire Washikemba Formation and the Aruba batholith with other fields shown for reference, all ages corrected to the assumed age of emplacement (95 Ma for the Washikemba Formation). Note that there is some uncertainty concerning the qHf of the tonalite sample: this rock is likely to contain zircon, which would not be dissolved using normal dissolution procedures and therefore may appear to have artificially high qHf. Caribbean plateau (defined by DSDP Sites and Aruba Lava Formation) and Gala´pagos data from Thompson et al. (2003), and Curacßao data from Geldmacher et al. (2003). Lesser Antilles data from White and Patchett (1984); Iceland data from Kempton et al. (2000); and Pacific MORB data from Nowell et al. (1999), Chauvel and Blichert-Toft (2001) and Kempton et al. (2002). P.M.E. Thompson et al. / Lithos 74 (2004) 91–116 105

Fig. 7. Plots of age spectra for the Bonaire Washikemba Formation. Age spectra shown represent the best analyses. 106 P.M.E. Thompson et al. / Lithos 74 (2004) 91–116 respectively (Table 2). Given that the Sr isotopic not extend to the low ratios observed for most of these system has been affected by alteration, it seems recent arc rocks. likely that the Pb isotopic system has been similarly There are no obvious systematic variations among affected by secondary alteration and/or metamor- the different lithological groups within the BWF. The phism processes; hence, its usefulness in determining rhyodacite domes, despite their unique trace element the magmatic origin of the Bonaire Washikemba patterns, are not distinguishable in terms of their Hf– Formation is limited. Mobility of Pb (and U) are Nd isotopes, and plot within the main cluster of the well known in altered oceanic crust (Kelly et al., BWF. 2001; Kempton et al., 2002). Nonetheless, the high 207Pb/204Pb for a given 206Pb/204Pb in Fig. 5 sug- 4.5. 40Ar– 39Ar geochronology gests a role for pelagic sediment, even if we cannot rigorously quantify this from the age-corrected data. The reliability of each 40Ar– 39Ar date is assessed In contrast, the nearly uniform qNd values, as using the criteria of Pringle (1993). Age spectra that compared with 87Sr/86Sr (Table 2, Fig. 4), implies have passed (or only narrowly failed) the reliability the Nd isotope systematics of the BWF have not been criteria are presented in Fig. 7 and Table 3; spectra significantly affected by secondary alteration process- that failed the reliability criteria by a greater margin es, which is consistent with the known behaviour of are included in Table 3 in italics. As with other Nd (e.g. Brewer and Aitken, 1989; Kempton et al., geochemical data, it is apparent that secondary 2002). Similarly, Hf isotopes are considered to be alteration has affected 40Ar– 39Ar systematics. As a insensitive to at least moderate degrees of alteration result, a number of cleaning and degassing steps and metamorphism (e.g. Pearce et al., 1999; Kempton were required for each sample. This involved the et al., 2002), due to the low mobility of Hf in aqueous removal of loosely bound 39Ar from likely alteration fluids. Thus, coupled Hf–Nd isotope systematics are sites, via lower temperature laser heating steps. The an ideal tool for interpreting rocks where the Sr and Pb weighted mean plateau ages are therefore based on isotopic systems may have been affected by alteration. only 50–70% of the 39Ar, and the 40Ar– 39Ar ages Nd and Hf isotope data for the BWF and the Aruba presented must accordingly be treated as minimum batholith are shown in Fig. 6, with the Caribbean ages. The ages range from 76 F 7 to 120 F 26 Ma. plateau (including the Aruba Lava Formation) and The most reliable age obtained is 96 F 4 Ma for a modern-day plume and arc data from Gala´pagos and rhyodacite sill from the Northern Complex (BON the Lesser Antilles plotted for comparison. Bonaire 94-09; Fig. 7a). This age is based on the final two rocks collectively form a sub-horizontal array that heating steps, which yielded 71% of the 39Ar, and plots above the field of recent basalts from Gala´pagos represents the oldest step heating age obtained for and the Caribbean plateau. Rocks from the Aruba the BWF. batholith partially overlap with Bonaire, but extend to slightly higher qHf for a given qNd. Komatiites and depleted basalts from Gorgona have both higher qHf 5. Discussion and qNd relative to the rest of the Caribbean plateau rocks (including the Gorgona enriched basalts). This The BWF is a volcanic sequence that appears to be has been interpreted by Thompson et al. (2003) as both temporally and spatially associated with the evidence for a distinct depleted component within the Caribbean plateau. But is it an island arc sequence, Caribbean plume. and what is its relationship with the Caribbean plateau Both the BWF and the Aruba batholith are clearly and the Aruba batholith? The Aruba batholith is distinct from the Caribbean plateau array, although thought to have formed from subduction of normal they show significant overlap with the enriched oceanic crust beneath the buoyant Caribbean plateau basalts from Gorgona. They also show some similar- (White et al., 1999; White, 1999). Given the geo- ities to the rocks from Curacßao (Geldmacher et al., graphical proximity of the BWF to Aruba, it is 2003). The BWF overlaps with the radiogenic (high reasonable to suppose that the BWF may have a qHf–qNd) end of the Lesser Antilles field, but does similar connection to the oceanic plateau. This hy- P.M.E. Thompson et al. / Lithos 74 (2004) 91–116 107 pothesis is considered below, with the aim of placing 2. enrichments of LILEs relative to LREE, Th and the island of Bonaire into a wider tectonic context and particularly HFSE, such as Nb (Tatsumi et al., evaluating its role in the Mesozoic evolution of the 1986; Hawkesworth et al., 1993). Caribbean region. The first of these criteria is difficult to apply to 5.1. Does the BWF represent an island arc ancient or altered rocks. In the lithologies of the succession? Washikemba Formation, primary hydrous mineral phases have either been replaced by, or are not easily Although an arc affinity for Bonaire has been distinguishable from, secondary hydrous phases due assumed based on a limited amount of major and to low-temperature metamorphism. Similarly, the trace element data (Klaver, 1976; Larue et al., 1991; LILEs have been extensively mobilised by alteration but see Giunta et al., 2002), this has not been proven. and/or metamorphism (Fig. 3), limiting their use. Given its temporal and spatial connections, it is also However, HFSE contents are relatively immune to possible that the BWF is plume-derived and forms these processes, and Fig. 8 shows that the BWF is part of the Caribbean plateau. In order to place the typified by negative Nb and Ti anomalies, features BWF into tectonic context, it is first necessary to typical of subduction-related rocks. Furthermore, the confirm its island arc origin. Subduction zone signa- BWF suite has similar immobile trace element pat- tures are characterised by: terns (Fig. 8) to an average basalt from Grenada. Therefore, geochemical evidence is consistent with 1. high volatile contents, as evidenced by the the interpretation of the BWF as an island arc presence of hydrous mineral phases, such as sequence. Given its geographic position, and the hornblende and kaersutite (Gill, 1981); and similar geochemical characteristics, the Lesser Antil-

Fig. 8. Chondrite-normalised (McDonough and Sun, 1995) multi-element plot for averaged analyses from the Washikemba Formation, Bonaire, with an average Grenada basalt plotted for comparison (Thirlwall and Graham, 1984). Caribbean plateau and Aruba batholith field from White (1999). 108 P.M.E. Thompson et al. / Lithos 74 (2004) 91–116 les island arc is considered a likely analogue for the position of the Aruba batholith (White, 1999; White BWF. et al., 1999). Having established an island arc origin for the BWF, we can use ratios such as Zr/Y and Nb/Y to infer mantle source compositions. Fitton et al. (1997) 6. Relationship between the BWF and the showed that Zr/Y and Nb/Y ratios only display small Caribbean plateau variation with low-pressure crystal fractionation and partial melting processes. They used this plot to In this section, we evaluate three possible models distinguish between entrained MORB-sourced de- for the origin of the arc-related sequences on Bonaire: pleted mantle and the Iceland plume (Fitton et al., 1997). The diagram has subsequently been success- (A) the arc is intrinsically related to the plateau fully used to identify a plume source for rocks from through remelting of plateau crust; other localities (e.g. Baksi, 2001). In this diagram (B) the arc is the product of subduction of oceanic (Fig. 9), the two parallel lines denote the bounds in crust beneath the plateau; which all plume-derived basalts from Iceland plot (C) the arc is unrelated to the plateau, but has been (termed the Iceland array); N-MORB plots below accreted to its margins due the unsubductable this array. The dotted field encompasses arc rocks nature of plateau crust. from Tonga-Fiji and Vanuatu arcs (Pearce and Kemp- ton, unpublished data), which overlaps the MORB 6.1. Model A: subduction beneath plateau field. The Caribbean plateau plots within the Iceland array, as expected for plume-derived oceanic plateau The presence of arc sequences spatially associated rocks, whereas the BWF plots entirely within the with the margin of the Caribbean plateau, many of MORB field. This strongly suggests the BWF was which have ages indistinguishable from those of the sourced from a non-plume mantle, in contrast to the plateau, suggests that the two have a related origin.

Fig. 9. Plot of Nb/Y vs. Zr/Y for the Bonaire Washikemba Formation, with the Aruba batholith, Caribbean plateau (represented by data from the Aruba Lava Formation and Curacßao) and Tonga-Fiji-Vanuatu fields shown for comparison. Note that the Aruba batholith suite includes felsic and plutonic rocks: the diagram was only intended for basaltic compositions (Fitton et al., 1997) and thus caution must be placed on interpreting its Nb/Y and Zr/Y systematics. Data from White (1999), White et al. (1999) and Kerr et al. (1997). Field for modern-day Tonga-Fiji-Vanuatu island arcs from Pearce and Kempton (unpubl. data). P.M.E. Thompson et al. / Lithos 74 (2004) 91–116 109

The hypothesis considered is that the Washikemba 6.2. Model B: remelting of the plateau Formation has resulted from subduction of proto- Caribbean oceanic crust beneath the Caribbean plateau A second model for the origin of at least the felsic and hence represents the extrusive equivalent of the rocks of the BWF is that they represent partial melting Aruba batholith, as suggested by the abundance of of the thin, hot, basaltic crust of the Caribbean evolved rocks formed by typical fractional crystallisa- plateau. This could occur by coalescence of low tion or assimilation fractional crystallisation processes. temperature felsic segregation veins, as has been This model can be refuted using Hf–Nd isotope observed on Iceland (Marsh et al., 1991; Jońasson et systematics. Because of the relative immobility of al., 1992; Jońasson, 1994). The resulting volcanic hafnium in aqueous fluids, modification of the mantle products on Iceland are significantly depleted in source by incorporation of subduction-related fluids HFSE and HREE, i.e. they have a geochemical or pelagic sediment would principally affect the qNd signature that superficially resembles that of island rather than the qHf, driving the resulting arc magma arcs (Jońasson et al., 1992). This would provide a composition towards the left on Fig. 10. This diagram convenient way of explaining the juxtaposition of arc illustrates that mixing between either of the likely and plateau fragments around the Caribbean, without Caribbean plateau end members C2 and G2 (defined necessitating the existence of a subduction zone. by Thompson et al., 2003) and Pacific pelagic sedi- This model is disregarded on the basis of geo- ment is unlikely to reproduce the Hf–Nd character- chemical arguments similar to those used to refute istics of the BWF. Many Pacific MORB compositions Model A. The new Ar–Ar data presented in this study would provide obvious end members, however, as reveal that the BWF predates plateau formation by at illustrated by the symbol M on Fig. 10. least 1 million years, although probably significantly Thus, it is unlikely that the BWF was formed by more. Furthermore, and importantly, this model fails melting of a plume-composition mantle wedge during to explain the occurrence of basaltic rocks with arc subduction beneath the Caribbean plateau. Instead, it signatures (Fig. 8). Finally, the BWF has Nb/Y, Zr/Y was probably derived from subduction-modified nor- and Hf–Nd isotopic characteristics that are distinct mal Pacific MORB. from the Caribbean plateau (Figs. 7 and 10). If it were

Fig. 10. Plot showing mixing in Hf–Nd isotope space between Pacific pelagic sediment and both Caribbean plateau (C2) and Gorgona (G2) end members (from Thompson et al., 2003) along with an arbitrary MORB end member (M). Numbers on mixing line refer to percentage of sediment. Gorgona source compositions have been calculated assuming 15% melting to produce an average Gorgona depleted basalt. Pacific pelagic sediment data are from Pearce et al. (1999). 110 P.M.E. Thompson et al. / Lithos 74 (2004) 91–116 derived from remelting of plateau lithologies, one batholith were generated by local remelting of Ca- would predict that it would have identical Hf–Nd ribbean plateau crust during an event of gabbroic isotopic characteristics to the plateau. Therefore, the plutonism. The gabbros were attributed to melting of hypothesis that the BWF formed from remelting of the the anomalous (plume-derived?) mantle wedge mod- Caribbean plateau is rejected. ified by addition of subduction-derived fluids. This hypothesis is supported by the observation that the 6.3. Model C: accretion of plume-unrelated arc silicic members of the batholith have isotope and key sequence trace element ratios (e.g. La/Nb) that are more similar to those of the plateau lava sequences than to arc Having rejected Models A and B, we are left with rocks. The Aruba batholith, therefore, is interpreted the conclusion that the BWF represents part of an as a consequence of subduction of normal oceanic island arc that is geochemically/petrologically uncon- crust beneath the oceanic plateau (White, 1999; nected to the Caribbean plateau, despite the fact that White et al., 1999). Age constraints dictate that this it is now juxtaposed with oceanic plateau fragments must have occurred immediately after subduction on the margins of the Caribbean. However, this polarity reversal, since the Aruba batholith is only juxtaposition of oceanic plateau and arc rocks is marginally younger than the main phase of plateau not just a coincidence. Most authors acknowledge volcanism (82–85 Ma as opposed to 88–91 Ma; that the Caribbean plateau has moved from the White et al., 1999; Sinton and Duncan, 1997; Walker Pacific realm into the space created by the diverging et al., 1999). Americas. The buoyant nature of the Caribbean Our new data reveal that, in spite of its clear plateau may have assisted the accretion of fragments association with the Caribbean Plateau, the Aruba in its path to its leading edge and moved them, batholith has similar EHf–ENd characteristics to the effectively eastwards (e.g. Burke et al., 1978; Dun- Bonaire Washikemba Formation, and higher EHf for a can and Hargreaves, 1984; Pindell and Barrett, 1990; given ENd than the main Caribbean plateau array Pindell et al., 1998; Kerr et al., 1999). Therefore, (Figs. 7 and 11). This is perhaps not surprising for given that the BWF is at least 96 F 4 Ma and is the gabbroic rocks, which have strong subduction deemed to be older than at least the main phase of signatures (White et al., 1999), as incorporation of Caribbean plateau magmatism, the most reasonable subduction-related fluids or pelagic sediment into a interpretation is that the BWF was part of a pre- Caribbean plateau mantle source could lead to a existing arc sequence that became accreted onto the reduction in ENd while leaving EHf relatively unaf- margins of the Caribbean, along with portions of the fected (Pearce et al., 2002). Thus, the batholith mantle dissected oceanic plateau. source could either have a composition that plots to the right of the batholith in EHf–ENd space (i.e. EHf c 13–14, ENd>9) if subduction fluids were in- 7. Implications of the EHf–ENd systematics of the volved (Pearce et al., 1999), or a composition that plots Aruba batholith above and to the right of the batholith if sediment The Aruba batholith, located only 100 km to the melting was a factor (Woodhead et al., 2001). How- west of Bonaire, has similar EHf–ENd characteristics ever, we note that the main Caribbean plateau array to the Bonaire Washikemba Formation (Fig. 6) and does not extend to the high EHf compositions required bears geochemical evidence for an association with by the latter scenario, making this less likely. subduction (White, 1999; White et al., 1999). Gab- More difficult to explain is the observation that broic and dioritic members are present within the the tonalitic Aruba batholith sample also has high predominantly tonalitic batholith, which are all ob- EHf for a given ENd (Fig. 6), more similar to the served intruding Caribbean plateau lava sequences of Bonaire Washikemba Formation than to the Caribbe- the Aruba Lava Formation. Based on Pb–Nd–Sr an plateau basalts from which it was inferred to have isotopes and incompatible trace element ratios, to- been derived (White et al., 1999). There are at least gether with field evidence and Ar–Ar age data, three possibilities for explaining the EHf–ENd sys- White (1999) proposed that the tonalites of the Aruba tematics of the Aruba batholith: (1) they indicate P.M.E. Thompson et al. / Lithos 74 (2004) 91–116 111

Fig. 11. Cartoon model for the origin and evolution of the Bonaire Washikemba Formation, illustrating the position of the Aruba batholith (A), Bonaire (B) and Curacßao (C). Symbols enclosed by stars and squares denote active and inactive arc sequences, respectively. General tectonic reconstructions after Pindell and Barrett (1990) and White et al. (1999). derivation from a similar setting and similar (MORB- and trace element signatures; or (3) the Caribbean like) mantle source to that of the BWF, with any plateau is more heterogeneous than has been previ- component of intra-plateau remelting to produce ously recognised, and the Aruba batholith is ulti- tonalites being insufficiently large to be detected in mately derived from a plume-derived mantle source the isotopic signature; (2) the Hf isotope systematics that is isotopically distinct from the main Caribbean have become decoupled from the Pb–Sr–Nd isotope plateau array. 112 P.M.E. Thompson et al. / Lithos 74 (2004) 91–116

The first hypothesis contradicts the available field ces on Gorgona and Curacßao. This may imply some and trace element evidence. In particular, the mafic preservational bias during the accretion process. For members of the batholith show no similarities with example, in the Gala´pagos archipelago it is apparent Bonaire rocks in terms of their Nb–Y–Zr character- that volcanoes from the MORB-dominated, geochem- istics, plotting within the Caribbean plateau field on ically depleted sector, e.g. Genovesa (Harpp and Fig. 9. Moreover, the Aruba batholith is younger than, White, 2001; Thompson et al., 2003), have noticeably and intrudes, the Caribbean plateau sequence, whereas lower elevations than those with more plume-like the BWF is older than the Caribbean plateau; thus, the enriched compositions, such as Fernandina. If this tectonic setting of the two sequences cannot be the archipelago were to impact the South American con- same. tinent today as a result of subduction processes, it is The second hypothesis is difficult to discredit with likely that volcanoes with more enriched composi- the limited data available, but we can currently tions would be preferentially accreted and preserved, envisage no geological process that would increase by virtue of their greater topography and consequent the EHf without affecting the ENd. Instead, we con- resistance to subduction. Whether the compositional sider the third hypothesis the most probable, i.e. that aspects are a fortuitous co-incidence, or whether the Caribbean plateau is more heterogeneous than volcanoes with more enriched compositions can erupt previously recognised. We discuss this in greater a greater volume of lava, is unclear. Nevertheless, it is detail in the following section. important to note that accreted Caribbean plateau sequences present a biased view of the relative distri- 7.1. Compositional heterogeneity of the Caribbean bution of different compositional groups for the 90 plateau Ma Caribbean plateau. Our analysis of the Nd–Hf systematics of the ABC Hf–Nd isotope data from the Caribbean basin islands also force us to conclude that Hf–Nd isotopes (DSDP Leg 15), along with accreted sequences on can only be used as a discriminant between plateau Aruba (Aruba Lava Formation) and Gorgona (Thomp- and arc-derived rocks in this region when viewed in son et al., 2003) suggest that most Caribbean plateau the context of other data (e.g. 207Pb/204Pb and Nb– rocks fall on the ‘‘main’’ Caribbean array between C1 Zr–Y). Subduction processes drive compositions to and C2 (Fig. 10). Depleted basalts and komatiites lower ENd (and possibly lower EHf) on the EHf–ENd from Gorgona deviate from this trend, defining a third diagram (Fig. 6), where they can potentially overlap component, G2. Thompson et al. (2003) argued that with Caribbean plateau lithologies, such as Curacßao. there were two possible explanations for this. Either Thus, we advise caution in the use of Hf–Nd isotope the plume that gave rise to the Caribbean plateau systematics to distinguish Caribbean plateau from contained more than one depleted component, or Caribbean arc rocks, particularly when the Pb and Gorgona was derived from a completely different Sr isotope systems have been compromised through Pacific plume system (e.g. Sala y Gomez). As out- alteration. lined in Section 7, the new data for the Aruba batholith (Figs. 7 and 11) suggest that the former is the more likely explanation. Moreover, new data for 8. Model for the origin and evolution of the BWF Caribbean plateau rocks from Curacßao (Geldmacher et al., 2003) also have higher EHf for a given ENd. Both As the BWF represents an arc sequence older than the Aruba batholith and Curacß ao data most likely the Caribbean plateau, it is envisaged that it originated indicate mixing between the enriched Caribbean pla- in the Pacific realm as part of the long-lived, oceanic, teau end member (C1) and the depleted Gorgona end intra-American arc at >95 Ma (Fig. 11). The Carib- member G2. bean plateau was extruded onto Farallon (Pacific) It is interesting that the ‘‘in situ’’ plateau samples crust shortly afterwards (somewhere to the west of from the Caribbean basin (together with the Aruba the BWF), and began to approach the subduction zone Lava Formation) plot on a different trend from that of where proto-Caribbean crust was overriding and con- the Aruba batholith and the accreted plateau sequen- suming Farallon crust. By 85 Ma, the buoyant and P.M.E. Thompson et al. / Lithos 74 (2004) 91–116 113 thick oceanic plateau arrived at the subduction zone 3. Hf–Nd isotope systematics are robust for moderate and subsequently clogged it. This resulted in a new degrees of alteration and thus are extremely suited subduction zone (with opposite polarity), which de- to the rocks of the BWF. In terms of Hf–Nd veloped to the east of the now-relict arc, as proto- isotopes, the BWF overlaps with the Lesser Caribbean crust started to subduct beneath the juxta- Antilles field and plots on a mixing line between posed arc and plateau sequences. The Aruba batholith pelagic sediment and Pacific MORB, but not the was intruded at this stage into plateau crust. As the Caribbean plateau. plateau continued to move passively between the two 4. New 40Ar– 39Ar ages obtained for the BWF yield a Americas, parts of the plateau and arc collided with minimum age of 96 F 4 Ma, which is older than the northern margin of the South American continent, the bulk of the Caribbean plateau (88–91 Ma). causing portions of the plateau to become dissected by 5. Trace element, Hf–Nd isotopic and geochronolog- strike-slip faults, and eventually accreted in slices ical data indicate that the BWF represents part of onto the Southern Caribbean margin. This accounts an arc sequence essentially unrelated to the plateau, for the present-day juxtaposition of plateau and arc which was probably accreted onto the plateau’s rocks in the Dutch Antilles. leading edge as it moved from the Pacific to the Thus, there were at least two stages of Mesozoic Caribbean realm. Arc and plateau fragments arc development in the Caribbean region: an earlier became juxtaposed on the continental margins as phase, which the Washikemba Formation represents, the Caribbean plate collided with North and South and a later phase of subduction beneath the margins of America. the Caribbean plateau itself, represented by the Aruba 6. The Aruba batholith has a Hf–Nd isotope batholith. Due to the close spatial relationship be- composition, which has higher EHf for a given tween the leading edge of the plateau and the arc ENd compared to the main Caribbean plateau array. sequence, as predicted by the model presented above, Once the effects of subduction are accounted for, one might also expect to see subduction occurring this implies that the source of the Aruba batholith, beneath the earlier stage arc, and a second stage arc the Caribbean plateau, is more heterogeneous then being built on relict arc basement. This scenario is previously recognised. found in the Solomon Isles, where the attempted subduction and eventual obduction of the Ontong Java Plateau has resulted in a subduction polarity Acknowledgements reversal, and is represented today by a complex collage of crustal units, including plateau, island arc, Sidonie Re´villon and an anonymous reviewer are arc developed on arc basement, and arc intercalated thanked for their constructive reviews of this with plateau (Petterson et al., 1997; 1999). Thus, the manuscript, and Mike Norry is thanked for stimulat- Solomon Isles may represent a modern-day analogue ing discussions. This research was supported by of the Southern Caribbean margin. Natural Environment Research Council (NERC) studentships GT4/98/ES0135 to PMET and GT4/95/ 157/E to RVW, and a block grant from NERC to 9. Conclusions NIGL. This paper represents NIGL publication number 597. RVW is currently supported by a Royal 1. New trace element data confirm that the Bonaire Society Dorothy Hodgkin Fellowship. Washikemba Formation represents a typical intra- oceanic arc sequence. 2. Post-magmatic alteration processes have resulted in References a spread to radiogenic 87Sr/86Sr in rocks of the BWF. This is evidenced by the substantially lower Alvarado, G.E., Denyer, P., Sinton, C.W., 1997. The 89 Ma Tortu- ratios obtained for primary apatite mineral sepa- gal komatiitic suite, Costa Rica: implications for a common rates compared with those of the whole rock. Pb geological origin of the Caribbean and eastern Pacific region isotopes are similarly affected. from a mantle plume. Geology 25, 439–442. 114 P.M.E. Thompson et al. / Lithos 74 (2004) 91–116

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