Geochronology, geochemistry and petrogenesis of rhyodacite lavas in eastern Jamaica: A new adakite subgroup analogous to early Archaean continental crust?

Chemical Geology 276 (2010) 344–359

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Geochronology, geochemistry and petrogenesis of rhyodacite lavas in eastern Jamaica: A new adakite subgroup analogous to early Archaean continental crust? Alan R. Hastie a,⁎, Andrew C. Kerr b, Iain McDonald b, Simon F. Mitchell c, Julian A. Pearce b, Ian L. Millar d, Dan Barfod e, Darren F. Mark e a

School of Geography, Geology and the Environment, Kingston University, Penrhyn Road, Kingston upon Thames, Surrey, KT1 2EE, UK School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff, CF10 3YE, UK Department of Geography and Geology, University of the West Indies, Mona, Kingston 7, Jamaica d NERC Isotope Geoscience Laboratories, Keyworth, Nottingham, NG12 5GG, UK e NERC Argon Isotope Facility, SUERC, Rankine Avenue, East Kilbride, G75 0QF, UK b c

a r t i c l e

i n f o

Article history: Received 11 January 2010 Received in revised form 2 July 2010 Accepted 5 July 2010 Editor: R.L. Rudnick Keywords: Adakite Trondhjemite–tonalite–granodiorite/dacite (TTG/D) Archaean Oceanic plateau Jamaica 40 Ar/39Ar

a b s t r a c t Rhyodacite lavas (Newcastle Volcanic Formation) from the Wagwater Basin in eastern Jamaica dated at 52.74 ± 0.34 Ma (2σ) have adakitic-like major element compositions, low Y and heavy rare Earth element (REE) concentrations and negative Nb and Ta anomalies on a normal mid-ocean ridge basalt normalised multi-element diagram. They also have lower Sr (b 400 ppm), MgO (≤ 2.0 wt.%), Ni (mostly ≤ 30 ppm) and Cr (mostly ≤ 40 ppm) concentrations compared to other modern adakites and middle-late Archaean (3.5– 2.5 Ga) trondhjemite, tonalite and granodiorite/dacites (TTG/Ds). εNd(i) and εHf(i) values indicate that the Newcastle adakite-like lavas cannot be formed by assimilation and fractional crystallisation processes involving any other igneous rock in the area and so the composition of the lavas is largely the result of the residual mineralogy in the source region. Low Sr and Al2O3 contents indicate a fluid/vapour-absent source with residual plagioclase and REE systematics point to residual amphibole and garnet in the source region. Similarly, high silica values and constant Zr and P2O5 concentrations suggest residual quartz and accessory zircon and apatite. The plagioclase and garnet residue implies that the Newcastle magmas were derived from partially melting a metabasic protolith at 1.0–1.6 GPa, which would intersect the amphibole dehydration partial melt solidus at ~ 850–900 °C. Radiogenic isotopes along with the low MgO, Ni and Cr concentrations in the Newcastle lavas demonstrate that the garnet amphibolite source region cannot be part of (1) the lower Jamaican arc crust, (2) delaminated lower crust or (3) subducted Proto-Caribbean “normal” oceanic crust that may, or may not, have detached. This data, in addition to partial melting models involving a theoretical garnet-amphibolite source region for the Newcastle lavas, shows that the adakite-like rocks are derived from metamorphosed Caribbean oceanic plateau crust that underthrust Jamaica in the early Tertiary. The underplated oceanic plateau crust partially melted by either (1) influx of basaltic magma during lithospheric extension in the early Tertiary or (2) direct partial melting of the underthrusting (subducting) plateau crust. The Newcastle magmas ascended and erupted without coming into contact with a mantle wedge thus forming the low MgO, Ni and Cr contents. Most Cenozoic adakites have compositions similar to the middle-late Archaean TTG/D suite of igneous rocks. In contrast, early (N 3.5 Ga) Archaean TTG/D crustal rocks have lower Sr, MgO, Ni and Cr concentrations and prior to this study had no modern adakite analogue. However, the Newcastle adakites have similar compositions to the, early Archaean TTG/D. The discovery of these rocks has important implications for our understanding of the formation of the Earth's earliest continental crust and so it is proposed that the Newcastle lavas be classified as a unique subgroup of adakites: Jamaican-type adakite. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Adakites are Cenozoic intermediate to felsic volcanic rocks that have similar compositions to middle-late Archaean Na-rich, high-

⁎ Corresponding author. E-mail address: [email protected] (A.R. Hastie). 0009-2541/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2010.07.002

aluminium trondhjemites, tonalites and granodiorites/dacites (TTG/ Ds) (Defant et al., 1992; Drummond et al., 1996; Martin 1999). ‘Type’ adakites are defined in Defant et al. (1992) as having SiO2 N56%, Al2O3 N15%, MgO b6%, low Y and heavy rare earth element (HREE) (Y and Yb b18 and 1.9 ppm respectively) and high large ion lithophile element (LILE) contents with Sr N400 ppm. Recently, Martin et al. (2005) studied adakites classified using the criteria of Defant et al. (1992) and further separated them into high-SiO2 (N60 wt.%) and

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low-SiO2 (SiO2 b60 wt.%) groups. In contrast, the term “adakite” has also been used in the literature to describe intermediate to felsic lavas with variable major and trace element compositions not consistent with the original geochemical definition (e.g. Wang et al., 2005). As a consequence, this study uses the term “adakite” based on the moderately restrictive geochemical criteria of Defant et al. (1992) and Martin et al. (2005). Some workers (e.g., Castillo et al., 2002, 2007) argue that fractional crystallisation of a basaltic magma can produce adakite and TTG/D suites. However, the most widely accepted model for generating adakitic and TTG/D magmas involves the partial melting of a mafic protolith which has been transformed into either amphibolite, garnet amphibolite or eclogite (e.g., Martin, 1986; Beard and Lofgren, 1989; Drummond and Defant, 1990; Beard and Lofgren, 1991; Rapp et al., 1991; Atherton and Petford, 1993; Sen and Dunn, 1994; Rapp and Watson, 1995; Drummond et al., 1996; Martin, 1999; Smithies, 2000; Rapp et al., 2003; Smithies et al., 2003; Martin et al., 2005; Macpherson et al., 2006; Rushmer and Jackson, 2008; Moyen, 2009). Nonetheless, the tectonic setting of the metamorphosed mafic protolith of adakitic magmas is also controversial (e.g., Rushmer and Jackson, 2008). Adakites are regarded by many to be derived from partially melting young (b25 Ma), hot and buoyant oceanic crust of normal (6–7 km) thickness, which has been subducted at a convergent margin (e.g., Kay, 1978; Drummond and Defant, 1990; Defant et al., 1992; Sajona et al., 1993; Yogodzinski et al., 1995; Drummond et al., 1996; Kepezhinskas et al., 1996; Martin, 1999; Rapp et al., 1999; Gutscher et al., 2000a,b; Yogodzinski et al., 2001; Aguillón-Robles et al., 2001; Smithies et al., 2003; Martin et al., 2005). On the other hand, it is clear that some adakitic lavas are not formed by the subduction of young and hot oceanic crust, while others are not obviously associated with a subduction zone (e.g., Xu et al., 2002). This has led to suggestions that adakitic magmas can also be formed by re-melting mafic lower crustal material which may, or may not, have been delaminated (e.g., Beard and Lofgren, 1989; Atherton and Petford, 1993; Xu et al., 2002; Chung et al., 2003; Garrison and Davidson, 2003; Wang et al., 2005; Macpherson et al., 2006). Recently, however, the classification of these so called “continental adakites” as “adakites” has been questioned on a geochemical basis in Moyen (2009). Experimental work has demonstrated that both the subduction of young oceanic crust and re-melting of mafic lower crust would be able to produce the major and trace element composition of adakites and high-Al TTG/Ds by dehydration (fluid-absent/vapour-absent) and hydrous partial melting of a basaltic source at P–T conditions of N1.0 GPa and N650 °C to leave an amphibolite, garnet amphibolite, eclogite or granulite residue (e.g., Beard and Lofgren, 1989, 1991; Rapp et al., 1991; Rushmer, 1991; Sen and Dunn, 1994; Wolf and Wyllie, 1994; Rapp and Watson, 1995; Winther, 1996; Rapp et al., 1999; Prouteau et al., 2001; Rushmer and Jackson, 2008). It is therefore difficult to geochemically distinguish between the melting of young oceanic crust and re-melting of mafic lower crust. Nevertheless, if adakites are generated from a subducting slab, the ascending melts should interact to variable degrees with the overlying mantle wedge. Conversely, this would not take place if the melts are derived from a lower crustal source. Rapp et al. (1999) and Prouteau et al. (2001) have shown that, when a “pristine” slab-derived magma passes through the mantle wedge, the magma assimilates mantle peridotite. The resultant hybridised melt usually has higher concentrations of MgO (N2.0 wt.%), Ni (N40 ppm) and Cr (mostly N50) than the original slab-derived magma because of the breakdown of the original peridotite mineral assemblage (e.g., Rapp et al., 1999; Prouteau et al., 2001; Smithies et al., 2009). Consequently, it is possible to distinguish a slab melting vs. lower crustal origin for adakites. Martin et al. (2005) have argued that ~ 90% of the Archaean continental crust is represented by the TTG/D suite of igneous rocks.

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The majority of middle-late Archaean (3.5–2.5 Ga) TTG/Ds belong to a high-Al TTG/D group which are compositionally similar to Cenozoic adakites (e.g., Defant et al., 1992; Drummond et al., 1996; Winther, 1996). Therefore, studying modern-day adakites, in addition to analysis of TTG/D suites (e.g., Moyen and Stevens, 2006), has proven useful in assessing the petrogenesis of middle-late Archaean TTG/Ds suggesting that they are formed by partial melting of a subducting slab (e.g., Martin, 1999; Smithies, 2000; Smithies et al., 2003; Martin et al., 2005; Smithies et al., 2009). In contrast, the petrogenesis of the earliest Archaean TTG/Ds (N3.5 Ga) remains controversial, particularly in relation to the origin of the early Archaean continental crust. Early Archaean TTG/Ds differ from the younger TTG/Ds and most modern adakites because: (1) they lack a “mantle wedge component” in their genesis i.e., they have lower MgO, Ni and Cr concentrations (Drummond et al., 1996; Smithies, 2000; Smithies et al., 2003; Martin et al., 2005) and (2) they have lower Sr abundances (e.g., Martin and Moyen, 2002). It is clear that studying recent adakites with analogous compositions to the early Archaean TTG/Ds would be of key importance in helping to determine how the early continental crust was formed. However, as noted by Martin et al. (2005) prior to the present study, and Hastie et al. (2010), no Phanerozoic adakites had been discovered with compositions comparable to those of early Archaean TTG/Ds. This study interprets the geochemistry of Cenozoic adakites in eastern Jamaica which were recently described in Hastie et al. (2010). Unlike the previous study we examine the full petrogenesis of these Tertiary adakite-like rhyodacitic rocks, including new radiogenic isotope and Ar/Ar geochronological data. In particular we describe the likely composition and tectonic setting of their source region. We also show that these Cenozoic lavas could represent modern analogues the earliest Archaean continental crust and discuss the tectonic implications of our findings for the evolution of the Caribbean region and the origin and development of the continental crust in the Archaean. 2. Geological background 2.1. The evolution of the Caribbean plate The Caribbean plate is juxtapositioned between the North and South American continents, with active subduction zones on the eastern and western margins (Fig. 1). The northern and southern boundaries of the plate are complex areas of strike slip motion and rifting, with sinistral strike slip motion along the northern boundary and dextral motion along the southern boundary (Fig. 1). Most of the Caribbean plate consists of a Cretaceous oceanic plateau (~6 × 105 km2), which formed in the Pacific realm at ~90 Ma (e.g., Kerr et al., 2003) by erupting onto the Farallon plate [possibly from the Galapagos hotspot (Duncan and Hargraves, 1984; Thompson et al., 2003)]. It subsequently moved to the northeast to collide at ~80 Ma with a large intra-oceanic arc [the Great Arc of the Caribbean, Burke (1988)], which was located at the western end of the proto-Caribbean seaway between North and South America (e.g., Duncan and Hargraves, 1984; Burke, 1988; Kerr et al. 2003; Hastie, 2009). However, unlike the Farallon oceanic crust the Caribbean oceanic plateau would have been too thick, hot and buoyant to completely subduct (e.g., Burke, 1988; Saunders et al., 1996). According to most models the southern portion of the plateau collided with South America in the late Cretaceous to form significant accreted sequences in Colombia and Ecuador (e.g., Kerr et al., 2002). When the northern portion subsequently collided with the Great Arc of the Caribbean, it initiated subduction polarity reversal and subduction back-step whereby the proto-Caribbean crust began subducting in a westerly direction beneath the oceanic plateau (e.g., Duncan and Hargraves, 1984; Burke, 1988; Kerr et al. 2003). Throughout the Tertiary, the plateau and the Great Arc were tectonically emplaced between the westward moving North and South American continents to

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Fig. 1. (a) Map of the Caribbean region, (b) location of volcano-sedimentary Cretaceous inliers in Jamaica and (c) geological map of the Wagwater Basin (modified from Jackson and Smith, 1978; Hastie et al., 2010), which shows the location of the Newcastle and Halberstadt Volcanic Formations in eastern Jamaica.

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form the Caribbean plate (Duncan and Hargraves, 1984; Burke, 1988; Sinton et al., 1998; Kerr et al., 2003; Hastie and Kerr, 2010).

2.2. Jamaican geology — the Tertiary Wagwater Basin Two thirds of Jamaica is covered with Tertiary limestone, which overlies volcanic and sedimentary rocks that presently form Cretaceous inliers (Fig. 1) (Draper, 1986). The island can be divided into three major structural blocks. The main block, which represents the central portion of the island, is known as the Clarendon Block (Fig. 1). To the east lies the smaller Blue Mountain Block that is separated from the Clarendon Block by the Wagwater Basin (Fig. 1) (Jackson and Smith, 1978). The Wagwater Basin has an area of ~ 950 km2, runs in a NW–SE direction and is a faulted extensional basin bounded by the Wagwater Fault to the west and the Silver Hill – Yallahs – Plantain Garden fault in the east (Fig. 1) (Jackson and Smith, 1979; Jackson et al., 1989). Horsfield (1974) proposed that these faults became active in the early Tertiary and that, during this period, down-throw on the Wagwater Fault was to the east and down-throw on the Yallahs–Plantain Garden Fault was to the south and west. On either side of these faults, the Blue Mountains and the Clarendon Blocks were uplifted, so providing a sedimentary source to fill the newlyformed basin (Fig. 1). Approximately 3700 m of folded and faulted late Paleocene to early Eocene sedimentary and volcanic rocks are exposed in the basin and make up the Wagwater Group (Jackson and Smith, 1978). This Group is separated into lower, middle and upper subgroups (Jackson and Smith, 1979). The lower and upper subgroups consist of sedimentary rocks, whereas the middle subgroup contains both sedimentary and volcanic rocks. The Wagwater Group is overlain by

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the mid-Eocene Yellow and White limestone groups to the north and south respectively (Jackson and Smith, 1979). The middle subgroup contains the Halberstadt and Newcastle Volcanic Formations that only crop out in the northern and central portions of the Wagwater Basin (Fig. 1). The Halberstadt volcanics are composed of extrusive basalts and the Newcastle volcanics consist of adakitic rhyodacites (Jackson and Smith, 1978, 1979; Jackson et al., 1989; Hastie et al., 2010). 3. Field locations and petrography The majority of the Newcastle samples (AHWG11–34) were collected along road cuttings from Irish Town to Freetown and from Happy Gate to Woodford in the central Wagwater Basin (Fig. 2). Samples AHWG01–05 were collected in a network of quarries located to the south of Bito. The samples collected in the quarry came from small blasted sections of benches. The Newcastle lavas are renowned for their autobrecciation and high levels of jointing, and this, coupled with the damage caused by the blasting, made identifying original flows impossible. Exposure along the road sections is quite extensive and, although many of the outcrops are substantially weathered and covered with abundant vegetation, there is enough exposure to locate and collect relatively fresh samples (AHWG11–34) (Fig. 2). Jackson and Smith (1979) considered the igneous rocks in the Newcastle Volcanic Formation to be lava flows and not intrusives. In the course of this study, the outcrops were found to be massive and not to show any flow textures. However, we continue to use the interpretation of Jackson and Smith (1979) and regard the Newcastle rocks as lava flows. It is worthwhile noting that there are very few adakite intrusions described in the literature and that most occurrences are extrusive (cf., Martin, 1999). The rocks are porphyritic and have a light or dark grey/blue colour with a greenish tint because of the presence of secondary

Fig. 2. Location of the Newcastle samples collected in this study. GPS coordinates for each sample can be found in Tables 1 and S2. Diagram modified from Hastie et al. (2010).

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chlorite. In thin section, the rocks are predominantly composed of plagioclase, amphibole and quartz phenocrysts in a groundmass of plagioclase, amphibole, quartz, and oxide minerals, similar to other modern adakites (e.g., Defant et al., 1992; Martin, 1999). The primary mineralogy of these samples has been partially replaced with clay minerals, sericite and chlorite. In some of the samples the chlorite replacement can be extensive e.g., AHWG21. 4.

40

Ar/39Ar geochronology

Based on their stratigraphical position (Section 2.2) we can infer that the Newcastle lavas have an approximate age of ~ 50 Ma. In order to refine this age the lavas were dated using 40Ar/39Ar geochronology. 4.1. Analytical technique Following removal of altered zones and crushing to 250– 500 μm, groundmass from AHWG03 was prepared using magnetic separation and cleaned thoroughly in dilute HNO3. Subsequently the groundmass was screened by hand to remove contamination (e.g., phenocrysts and xenocrysts). Two splits (8A and 8B) were obtained. The samples were loaded into Cu packets, placed into quartz vials and then positioned in an Al can for irradiation. Adjacent to samples we placed Al packets of Taylor Creek Rhyolite sanidine (TCR—2 s; 28.34 ± 0.16 Ma, 1σ; Renne et al., 1998) to permit characterization of the irradiation flux to the samples. The

samples were irradiated in the Petten HFR reactor for 10 h. Samples were step-heated for 5 min in a fully automated all-metal, double vacuum, resistively-heated furnace over a temperature range from 500 to 1750 °C. Extracted gases were cleaned for 10 min using 3 SAES GP50 getters (two operated at 450 °C and one at room temperature). Data were collected using a fully automated ARGUS multi-collector mass spectrometer at the NERC Argon Isotope Facility, SUERC. The ARGUS employs five high-gain, lownoise Faraday detectors with 1011 (40Ar) and 1012 (39–36Ar) ohm resistors for simultaneous collection of all five isotopes of Ar (Mark et al., 2009). The ARGUS has a measured sensitivity of 7 × 10− 14 mol/V at 200 μA trap current. Furnace blanks were stable at less than 1.49 × 10− 15 mol 40Ar and 8.84 × 10− 18 mol 36Ar. Isotope data are corrected for blanks, radioactive decay, mass discrimination and interfering reactions. 40Ar/39Ar ages also include a 0.5% error assigned to the J-parameter. Raw data, correction factors and the J-parameters are found in Table S1 (Supplementary Material). The criteria for fitting of plateaus are that they must include at least 60% of 39Ar in three or more contiguous steps. There is no resolvable slope on plateau and any outliers or trends at upper or lower steps. The probability of fit of plateau to the data is N0.05. 4.2. Results 40

Ar/39Ar age spectra and inverse isochron ages are within 2σ error of each other for both 8A and 8B (Fig. 3). The inverse isochrons that

Fig. 3. (a) Inverse isochron and plateau diagrams for groundmass split 8A from AHWG03. (b) Inverse isochron and plateau diagrams for groundmass split 8B from AHWG03. Full analytical procedures can be found in Section 4.1. For the inverse isochron diagrams the data-point error ellipses are 2σ and the dashed circles represent the points that make up the plateau age. For the plateau diagrams the box heights are 2σ.

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include all steps are within error of the atmospheric 40Ar/36Ar (Nier, 1950, Fig. 3). We quote ages and uncertainties from the 40Ar/39Ar age spectra for both splits as the plateau steps in the inverse isochron plots all bunch close to the x-axis and there is geological scatter in the remaining points as indicated by inverse isochron MSWD's well in excess of the norm (Fig. 3). ARGUS dates 8A as 52.98 ± 0.52 Ma (2σ, MSWD = 0.47, Fig. 3a) and 8B as 52.56 ± 0.44 Ma (2σ, MSWD = 1.09, Fig. 3b). These rocks have not been exposed to high temperatures following cooling of the lava flow and as such given the consistency between the two plateau ages, we accept that the 40Ar/39Ar data is recording the true crystallisation age of the Newcastle lava. Therefore we can calculate an average for the two 40Ar/39Ar ages (two splits of the same material): 52.74 ± 0.34 Ma (0.64%, 2σ, MSWD = 1.5). This age will be reported throughout the rest of the manuscript.

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5. Geochemical results 5.1. Analytical techniques A full major and trace element dataset is presented in Hastie et al. (2010) and Table S2 in the Supplementary Material; however, a representative set can be found in Table 1. Nd and Hf isotope compositions were analysed at the NERC Isotope Geoscience Laboratories, Nottingham, UK. For Hf isotope analysis, samples were fused with Li-metaborate flux, and dissolved in 3 M HCl. Hf was separated using a single column procedure using LN-Spec resin, following Münker et al. (2001), and run on a Nu-Plasma multicollector ICP-MS. Hf blanks are b100 pg. Correction for Lu and Yb was carried out using reverse-mass-bias correction of empirically

Table 1 Representative major and trace element compositions of the Newcastle lavas. JB-1a certified values in the Table are taken from Govindaraju (1994). The JB-1a international standard data are based on 10 analyses carried out within the Jamaica analytical run. Relative standard deviation = r.s.d. Detection limits are based on blank analyses. A complete analytical description can be found in the supplementary Table S2 and McDonald and Viljoen (2006). AHWG03

Majors Bito Quarry (wt.%) N17°57.075

AHWG12

AHWG19

AHWG21

AHWG25

AHWG26

AHWG32

AHWG33

Irish Town to North of Hopewell Redlight

South of Newcastle

North of Newcastle

N18°04.566

N18°03.737

N18°03.927

N18°03.030

N18°03.981

N18°04.321

W076°42.926 W076°44.659 W076°44.786

N18°03.587

Average JB-1a certified JB-1a value for values this analysis

r.s.d.

Detection limits

W076°39.306 W076°43.480 W076°43.352 W076°42.528 W076°42.670 SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total Traces V Cr Co Ni Ga Rb Sr Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U

70.95 0.34 15.54 2.86 0.03 1.39 1.01 4.24 0.58 0.10 1.78 98.82 (ppm) 23 20 8.5 14 14.1 2.84 136 8.4 131.0 7.31 581 17.07 20.49 2.33 8.44 1.51 0.61 1.50 0.21 1.29 0.23 0.74 0.11 0.70 0.11 2.93 0.62 3.06 0.88

69.63 0.30 15.21 3.27 0.05 1.42 1.46 5.13 0.67 0.08 2.12 99.34

69.38 0.36 15.28 3.32 0.03 2.08 0.34 6.56 0.50 0.11 2.11 100.05

70.58 0.34 15.75 3.11 0.05 1.78 0.34 6.25 0.18 0.11 1.68 100.17

70.14 0.33 15.29 3.43 0.05 1.67 0.34 6.44 0.16 0.10 1.52 99.46

70.48 0.34 15.31 3.57 0.05 1.22 0.85 5.81 0.75 0.10 1.64 100.11

71.81 0.33 15.83 3.22 0.03 1.37 0.31 5.02 0.67 0.10 1.52 100.21

70.45 0.33 15.58 2.08 0.02 0.93 0.41 7.55 0.21 0.10 1.66 99.33

52.16 1.30 14.51 9.10 0.15 7.75 9.23 2.74 1.42 0.26

52.80 1.28 14.71 8.95 0.15 7.94 9.52 2.58 1.37 0.26

0.86 2.56 1.45 1.65 3.61 1.24 2.07 6.88 6.96 2.84

0.0119 0.0002 0.0055 0.0044 0.0194 0.0004 0.0029 0.0029 0.0169 0.0044

36 31 7.7 13 15.2 9.56 160 8.2 138.6 4.92 378 11.28 15.70 1.74 5.95 1.22 0.46 1.27 0.20 1.24 0.24 0.68 0.11 0.72 0.13 3.23 0.41 2.68 0.39

39 52 8.8 30 14.8 6.00 97 9.7 133.5 6.93 128 15.25 23.78 2.67 9.39 1.75 0.49 1.66 0.24 1.44 0.27 0.77 0.12 0.78 0.15 3.22 0.56 3.04 0.98

37 20 7.8 N1 15.3 2.13 76 8.1 134.0 6.09 64 9.61 16.73 2.02 7.49 1.54 0.38 1.45 0.22 1.32 0.24 0.68 0.11 0.70 0.13 3.21 0.50 2.75 0.90

38 39 6.8 4 15.8 1.94 107 8.0 133.9 6.39 43 14.68 22.35 2.47 8.48 1.60 0.57 1.46 0.21 1.22 0.23 0.65 0.10 0.66 0.13 3.21 0.51 2.82 0.98

32 15 7.9 16 15.2 10.86 131 6.7 137.3 5.32 275 13.25 22.79 2.35 8.12 1.47 0.48 1.36 0.19 1.16 0.21 0.62 0.10 0.63 0.10 3.15 0.45 2.99 0.85

37 104 7.6 62 16.5 3.74 141 6.3 134.3 3.57 448 17.55 26.81 2.90 9.71 1.68 0.54 1.45 0.20 1.08 0.19 0.53 0.08 0.56 0.10 3.13 0.32 3.03 0.94

32 20 4.3 N1 17.0 2.22 138 5.4 142.2 3.30 74 13.77 23.56 2.70 9.20 1.64 0.44 1.35 0.18 0.96 0.17 0.50 0.08 0.53 0.11 3.28 0.27 2.91 0.88

206 415 39.5 140 18.0 14 443 24.0 146.0 27 497 38.1 66.1 7.3 25.5 5.07 1.47 4.54 0.69 4.19 0.72 2.18 0.31 2.1 0.32 3.48 1.6 8.8 1.6

196 423 38.1 139 17.9 14 466 24.0 137.5 28 502 37.6 66.4 7.2 25.8 5.12 1.49 4.60 0.68 4.06 0.74 2.13 0.31 2.07 0.31 3.36 1.6 8.8 1.6

2.25 4.26 3.31 7.23 1.89 91.24 9.32 1.58 4.86 5.46 2.18 3.65 1.88 2.14 1.52 3.53 1.44 3.31 3.30 1.67 2.65 2.59 2.65 2.38 6.44 5.12 2.41 3.28 9.15

0.07 0.21 0.03 0.34 0.022 0.031 0.29 0.02 0.05 0.09 0.41 0.011 0.006 0.003 0.006 0.005 0.002 0.028 0.009 0.003 0.001 0.003 0.001 0.003 0.004 0.002 0.001 0.002 0.004

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predetermined 176Yb/173Yb (0.7950) and 176Lu/175Lu (0.02653). Replicate analyses of the JMC475 standard across the period of analysis gave 176Hf/177Hf = 0.282174 ± 0.000010 (2σ, n = 37); reported data are normalised to a preferred value of 0.282160 (Nowell and Parrish, 2001). Replicate analyses of BCR-1 gave a mean value of 0.282872 ± 0.000009 (2σ, n = 4), comparable to previously reported values 0.282879 ± 0.000008 (Blichert-Toft, 2001). Replicate analyses of the in-house standard PK-g-D12 gave 0.283050 ± 0.000005 (2σ, n = 5), comparable to previously reported values of 0.283049 ± 0.000018 (2σ, n = 27; Kempton et al., 2002) and 0.283046 ± 0.000016 (2σ, n = 9; Nowell et al., 1998). Determinations of Nd isotopes followed the procedures of Kempton (1995) and Royse et al. (1998). Samples were leached in 6 M HCl prior to analysis. 143Nd/144Nd ratios are normalised to 146Nd/144Nd = 0.7219. Nd was run as the metal species using double Re–Ta filaments on a Finnigan Triton mass spectrometer. Replicate analysis of the in-house J&M standard gave a value of 0.511184 ± 0.000022, 2σ, n = 24); Nd isotope data are reported relative to a value of 0.511123 for this standard, equivalent to a value of 0.511864 for La Jolla. 5.2. Alteration Studies on altered Cretaceous igneous rocks in the Caribbean have demonstrated that the majority of the large ion lithophile elements (LILE) have been variably mobilised by a range of weathering, hydrothermal and metamorphic processes (e.g., Révillon et al. 2002; Thompson et al. 2003; Escuder Viruete et al. 2007; Hastie et al. 2007, 2008; Hastie, 2009). Fortunately, the Newcastle lavas are younger and are petrographically less altered than the Cretaceous Jamaican igneous rocks i.e., less of their original mineralogy has been altered to secondary minerals. Assessing the mobility of incompatible elements in an altered suite of lavas often relies on studying the intra-formation differentiation patterns on Harker variation diagrams (e.g., Hastie et al., 2007, 2009). Unfortunately, this cannot be employed in the Newcastle lavas because the vast majority of the elements that are incompatible in mafic compositions become more compatible in acidic melts (e.g., Pearce, 1982). On the other hand, the loss on ignition values for the Newcastle lavas range from 0.98 to 4.79 with ~80% of the samples having approximately ≤2.0 wt.% (Tables 1 and S2). These low volatile concentrations provide evidence that the lavas were probably not substantially modified by the post-eruptive low temperature alteration processes similar to other modern adakite studies (e.g., Willbold et al., 2009). This is important, because some mobile elements are useful for interpreting the petrogenesis of adakitic lavas (e.g., Sr). 5.3. Major elements All the Newcastle lavas have a high, but restricted, range of SiO2 (64.6–72.2 wt.%; Table S2). They commonly have high Al2O3 (14.5– 16.0 wt.%) and very low MgO abundances of ≤2.0 wt.%. Interestingly, relative to average island arc and continental arc andesites–dacites– rhyolites (ADR) (e.g., Table 1, Drummond et al., 1996), the Newcastle lavas have high Na2O values (3.8–7.6 wt.%) with low K2O and CaO contents (0.1–1.1 and 0.3–4.0 wt.% respectively). These major element values are compositionally similar to modern adakites and liquids derived from amphibolite and eclogite partial melting experiments (e.g., Winther, 1996). 5.4. Trace elements The Newcastle lavas have adakite-like (La/Yb)cn ratios, low Y and HREE (Yb) concentrations (5.4–12.8 and 0.5–0.9 ppm respectively) and negative Nb and Ta anomalies on a normal mid-ocean ridge basalt (N-MORB) normalised multi-element diagram (Figs. 4, 5 and Tables 1

and S2). Negative Ti and P anomalies, concave middle and heavy REE patterns and N-MORB normalised (Gd/Yb)n–mn ratios N1 are also evident in Fig. 4. However, the Newcastle lavas also have several geochemical features that are distinct from modern-adakitic lavas (e.g., Rapp et al., 1999) and middle-late Archaean TTG/Ds (e.g., Smithies, 2000): (a) They lack positive Sr anomalies (apart from AHWG34) and (b) they have lower Sr/Y ratios (Fig. 5) (e.g., Fig. 13, Martin, 1999; Table 1, Martin et al., 2005). The Newcastle adakite-like geochemistry is similar to the earliest Archaean TTG/D suites, which have lower Sr (b400 ppm) and Sr/Y (~30) (Martin and Moyen, 2002; Table 1, Martin et al., 2005). Importantly for this study, the Newcastle lavas also have low MgO, Ni (mostly ≤30 ppm) and Cr (mostly ≤ 40 ppm) concentrations, which are also comparable to early Archaean TTG/Ds (e.g., Smithies et al., 2009), which have similar SiO2 ranges to the Newcastle lavas (Condie, 2005) (Fig. 6).

5.5. Nd–Hf radiogenic isotopes Based on the average 40Ar/39Ar plateau ages (Fig. 3), Nd and Hf radiogenic isotope data for samples AHWG12, 19, 21, 32 and 33 have been age-corrected to 52.74 Ma. The lavas have εNd(i) = + 6.06 − 7.03 and εHf(i) = +12.05 − 12.50 (Fig. 7a and Table 2). Whereas modern adakites have isotopic compositions similar to those of N-MORB (Martin, 1999), the Newcastle adakite-like lavas have εNd(i) and εHf(i) which are distinct from the Atlantic and Pacific N-MORB (Nowell et al., 1998; Kempton et al., 2000) and have isotopic compositions which are more similar to Cretaceous Caribbean oceanic plateau lavas (Fig. 7a) (Hastie et al., 2008).

6. Discussion Cenozoic adakites are thought to be derived from high pressure (N1 GPa), ~ 20% partial melting of amphibolite, garnet amphibolite or eclogite source regions (e.g., Drummond and Defant, 1990; Rapp and Watson, 1995; Smithies 2000; Willbold et al., 2009). In the following sections, the composition of the Newcastle lavas will be used to determine the possible mineralogy of the Newcastle source region.

6.1. Assimilation and fractional crystallisation (AFC) The only other igneous rocks erupted on Jamaica ~ 50 Ma are the Halberstadt basalts, however, the different εNd(i) and εHf(i) values preclude the Halberstadt magmas fractionating to form the Newcastle magmas (Fig. 7b). Additionally, the Newcastle lavas do not plot on mixing lines between the Halberstadt lavas and average Jamaican island arc lavas from the Benbow and Sunning Hill Inliers (Fig. 7b). Thus, assimilation and fractional crystallisation of a Halberstadt basaltic parental magma with the aforementioned lavas cannot generate the distinctive major and trace element concentrations of the Newcastle adakite-like rocks. Furthermore, lavas from the Above Rocks Inlier and some of the Newcastle lavas have comparable εNd(i) and εHf(i) ratios and, thus, the Halberstadt magmas cannot evolve into the Newcastle magmas with AFC processes involving the Above Rocks lavas because it would require 100% assimilation. The Newcastle lavas are also not derived from the Above Rocks magmas by solely fractional crystallisation processes because the Above Rocks lavas have compositionally distinct major and trace elements and are already evolved with silica values up to ~ 68 wt.% (Hastie, 2007). Consequently, it is unlikely that the intermediate/acidic Newcastle lavas fractionated from a basic parental magma. Thus, it is probable that the lavas formed by partial melting of a metabasic protolith.

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Fig. 4. (a) N-MORB-normalised multi-element modified from Hastie et al. (2010) and (b) chondrite-normalised REE diagrams. Normalising values are from Sun and McDonough (1989) and McDonough and Sun (1995) respectively. Grey field represents late Archaean and Cenozoic adakite data from Defant et al. (1991, 1992), Polat and Kerrich (2000), Beate et al. (2001), Aguillón-Robles et al. (2001), Castillo et al. (2007) and Pallares et al. (2007).

6.2. Low Al2O3 and Sr concentrations: plagioclase and garnet? It has been shown that high abundances of Al2O3 (≥19 wt.%) in an amphibolite-derived liquid are the result of high H2O (watersaturated) and/or high anorthite contents in the mafic protolith source (e.g., Fig. 13 in Beard and Lofgren, 1991; Fig. 9 on Wolf and Wyllie, 1994). However, all the Newcastle lavas have lower Al2O3 contents (14.5–15.9 wt.%) compared to the liquids formed during H2O-saturated amphibolite partial melting experiments. The lavas also have low CaO contents and high Na2O abundances ruling-out a large albite component in the source. Hence, the Newcastle magmas were probably formed under fluid-absent/vapour-absent conditions (with the only H2O derived from the breakdown of hydrous minerals). However, Winther (1996) also demonstrates that lower Al2O3

contents in amphibolite-derived melts are caused by residual garnet in the source region. Adakites with low Sr contents (b400 ppm) have been interpreted as having residual plagioclase in the source region (e.g., Smithies et al., 2009). Rapp and Watson (1995), Martin (1999), Martin and Moyen (2002), Martin et al. (2005) and Clemens et al. (2006) show that plagioclase stability is strongly reliant on depth (normally stable up to ~ 1.6 GPa), and thus, so is the Sr concentration of a mafic protolith-derived acidic melt. Martin and Moyen (2002) and Martin et al. (2005) describe the work of Zamora (2000) and note that residual plagioclase buffers the Sr content of a liquid derived from a basaltic protolith, to concentrations well below the Sr abundances of “average” adakites (≪ 400 ppm). Therefore, the high Sr contents in modern adakites

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Fig. 5. (a) Sr/Y–Y modified from Hastie et al. (2010) and (b) (La/Yb)cn–(Yb)cn diagrams showing that the Newcastle lavas have low ratios relative to modern-day adakites. Adakite, TTG/D and island arc fields are taken from Martin (1999) and Martin et al. (2005). These aforementioned studies also demonstrate that middle-late Archaean TTG/D suites predominantly plot within the adakite field although they can display higher Sr/Y and (La/Yb)cn ratios. Symbols as in Fig. 4.

indicates that plagioclase is not a common residual phase (Martin, 1999; Clemens et al., 2006) and that most adakite magmas are derived from depths N1.6 GPa. As a consequence, we infer that the low Sr (b400 ppm) and Al2O3 contents in the Newcastle lavas are the result of residual plagioclase and possibly garnet in a relatively shallow (≤1.6 GPa), fluid/vapour-

Fig. 7. (a) εNd(i) – εHf(i) plot showing the Newcastle and Halberstadt lavas and Jamaican island arc lavas from the Benbow, Above Rocks and Sunning Hill inliers. Diagram modified from Hastie et al. (2008) and Hastie and Kerr (2010). Data for NMORB, Iceland, Ontong Java and Jamaican island arc lavas are taken from Nowell et al. (1998); Kempton et al. (2000), Tejada et al. (2004), Hastie (2007) and Hastie et al. (2009). (b) εNd(i) – εHf(i) diagram showing mixing lines between the Halberstadt lavas and the average composition of island arc volcanic successions from (1) the Above Rocks lavas, (2) a Sunning Hill lava and (3) the IAT and CA Benbow Inlier lavas. Ticks on the mixing curves represent successive 20% increments. Diagram modified from Hastie et al. (2010).

absent source region, which negates the possibility of the Newcastle lavas being derived from an eclogite source region. 6.3. Low middle and heavy REE concentrations and positive Zr and Hf anomalies: amphibole and garnet?

Fig. 6. MgO–SiO2 diagram showing that the Newcastle lavas plot in the early Archaean field. Diagram modified from Martin et al. (2005) and Hastie et al. (2010).

An amphibolite source is supported by concave middle and heavy (M)/(H)REE chondrite-normalised patterns for the Newcastle lavas (Fig. 4), as M/HREE (Sm to Lu) and Y are compatible in amphibole at a range of temperatures and pressures when in equilibrium with a TTG/ D liquid, with the MREE (Gd to Er) being more compatible (e.g., Defant et al., 1991; Sen and Dunn, 1994; Klein et al., 1997; Martin, 1999; Garrison and Davidson, 2003). The effect of residual amphibole in the source region also explains several other features of the Newcastle lavas: (a) positive Zr and Hf anomalies (Fig. 6) can be explained by the smaller amphibole/acid melt partition coefficients of Zr and Hf compared to the MREE (e.g., Klein et al., 1997) and (b) negative Ti anomalies (Fig. 6) can also be explained by the greater amphibole/melt partition coefficient for Ti compared to the MREE (e.g., Klein et al., 1997). Significantly, however, (Gd/Yb)n–mn ratios are also N1 (average = 1.8). This cannot be generated solely by residual source amphibole because the partition coefficient for Gd in amphibole is

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Table 2 Nd–Hf isotope data for the Newcastle lavas. The chondrite values used to calculate the epsilon Hf and Nd values are 0.28273 and 0.51256 respectively. Initial and epsilon values calculated using the average 40Ar/39Ar plateau age of 52.74 Ma. 176

Newcastle lavas AHWG12 AHWG19 AHWG21 AHWG32 AHWG33 Halberstadt lavas AHHB03 AHHB07

Lu/177Hf

176

Hf/177Hf measured

176

Hf/177Hf initial

εHf(i)

147

Sm/144Nd

143

Nd/144Nd measured

143

Nd/144Nd initial

εNd(i)

0.0056 0.0064 0.0058 0.0046 0.0047

0.28309 0.28309 0.28309 0.28310 0.28309

0.28308 0.28308 0.28309 0.28309 0.28309

12.25 12.05 12.36 12.50 12.28

0.124 0.113 0.125 0.104 0.108

0.51294 0.51292 0.51293 0.51297 0.51296

0.51289 0.51288 0.51289 0.51293 0.51292

6.32 6.06 6.18 7.03 6.80

0.0134 0.0116

0.28307 0.28308

0.28306 0.28307

11.37 11.56

0.168 0.152

0.51298 0.51297

0.51292 0.51292

6.80 6.77

higher than for Yb in equilibrium with an acidic melt (Klein et al., 1997). Garnet is a common phase in amphibolites and, as a residual mineral, would be able to produce (Gd/Yb)n–mn ratios N1 (e.g., Martin, 1999; Klein et al., 2000; Martin et al., 2005). 6.4. Negative Nb–Ta anomalies: rutile or amphibole? Negative Nb and Ta anomalies in volcanic rocks are commonly attributed to subduction zone processes or contamination of a mantle-derived magma by continental crust (e.g., Foley et al., 2002). In modern subduction zones, dehydration reactions in down going slabs release aqueous fluids into the overlying mantle wedge, lowering the mantle solidus and triggering partial melting to form continental and island arc magmas (e.g., Pearce and Peate, 1995; Elliott, 2003; Tatsumi, 2003). The slab-derived aqueous fluids enrich the mantle wedge in fluid-mobile LILE and LREE while the fluidimmobile HFSE and HREE remain in the slab to a large extent within accessory minerals such as rutile [high concentrations of Nb and Ta (Ayers and Watson, 1993)] (Tatsumi et al., 1986; Keppler, 1996; Foley et al., 2000; Audétat and Keppler, 2005; Gaetani et al., 2008). A siliceous melt from a subducting plate (an adakite) can also leave residual rutile in the subducting slab and so generate negative Nb and Ta anomalies in resultant magmas similar to “normal” arc magmas (e.g., Ayers and Watson, 1993; Gaetani et al., 2008). However, amphibole, in equilibrium with an acidic melt, has lower partition coefficients for Th and La than for Nb and Ta (Klein et al., 1997). Thus, a rutile-free amphibolite source region may also be able to generate melts which have negative Nb and Ta anomalies on an NMORB normalised plot. Accordingly, the negative Nb and Ta anomalies in Fig. 4 may be explained by either (1) residual rutile (e.g., Xiong et al., 2005) or (2) residual amphibole (e.g., Martin, 1999; Martin et al., 2005) in the Newcastle source region. As discussed in Sections 6.1 and, 6.3, and modelled later in the paper, the negative Ti anomalies in the Newcastle lavas are the result of residual amphibole in the source region and not fractional crystallisation of amphibole or a Fe–Ti phase. The low MgO, Cr and Ni and high SiO 2 concentrations also indicate that the TiO 2 concentrations of the Newcastle magmas would not have been increased in response to interaction with peridotite material (e.g., Rapp et al., 1999). Consequently, the TiO2 concentration of the Newcastle lavas (0.29–0.37 wt.%) may represent maximum values for the Newcastle magmas prior to fractional crystallisation (Section 6.1). Rutile saturation percentages from Ryerson and Watson (1987) and Ayers and Watson (1993) indicate that, if rutile was present in the residual source region for the Newcastle melts, the TiO2 contents of the magmas would be N1 wt.% because rutile would partially melt and buffer the magmas at the rutile saturation value of 1–3 wt.%. However, subsequent saturation modelling by Green and Adam (2002), Xiong et al. (2005), Hayden and Watson (2007) and Gaetani et al. (2008) demonstrates that rutile saturation concentrations vary considerably with different temperatures, pressures and starting compositions (including H2O content) (Fig. 4; Hayden and Watson, 2007).

Nevertheless, the Newcastle lavas are rhyodacites with similar compositions to other adakites and Archaean TTG/Ds and have probably been derived from a pressure range of ~ 1.0–1.6 GPa (to stabilise plagioclase and garnet in the residue) and would thus intersect the amphibole fluid/vapour-absent solidus at ~ 850–900 °C (e.g., Peacock et al., 1994; Martin, 1999). If this is the case then rutile should saturate in the Newcastle lavas at ~ 0.85 wt.% TiO2 (Fig. 4; Hayden and Watson, 2007). However, the Newcastle lavas contain 0.29–0.37 wt.% TiO2 and so it is unlikely that rutile was a residual phase when the Newcastle magmas segregated and ascended. This is confirmed by melting experiments on mafic rocks (Rapp and Watson, 1995; Xiong et al., 2005) which show that, in contrast to the Newcastle source region, rutile is stable in high-pressure metamorphic rocks over ~1.6 GPa from ~ 900 to 1075 °C. Continental crust contamination can also be ruled out as a mechanism for forming the negative Nb–Ta anomalies (e.g., Foley et al., 2000 and references therein) because of the absence of any continental rocks beneath Jamaica, which is largely composed of Cretaceous–Tertiary carbonate and Cretaceous island arc igneous rocks overlying an assumed late Jurassic–Cretaceous altered oceanic basement (Robinson et al., 1972; Draper, 1986). Contamination of Halberstadt mafic magmas by an island arc protolith and subsequent fractionation has already been ruled out as a mechanism for forming the Newcastle volcanics and, thus, cannot be responsible for forming the negative Nb and Ta anomalies (Fig. 7b). The only remaining likely method of forming the negative Nb and Ta anomalies involves residual amphibole in the Newcastle source region (e.g., Martin, 1999; Martin et al., 2005). 6.5. High SiO2 concentrations: quartz? Garnet amphibolite source regions commonly contain quartz (e.g., Beard and Lofgren, 1991; Rushmer, 1991; Sen and Dunn, 1994; Winther, 1996) and it has been suggested that quartz can buffer melts generated by up to 15% partial melting to give high silica concentrations (Beard and Lofgren, 1991; Sen and Dunn, 1994). Thus, the high, and restricted, SiO2 abundances in the Newcastle lavas may be derived from residual quartz. 6.6. Constant Zr and P2O5 concentrations: accessory zircon and apatite? Tables 1 and S2 show that Zr and P2O5 concentrations are constant from low silica to high silica values suggesting that they are being buffered in the melt by residual phases. Watson and Harrison (1984) indicate that Zr concentrations in crustal magmas are ~ 100 ± 50 ppm (similar to the Newcastle lavas: 83–156 ppm) and that these abundances are enough to saturate zircon. Therefore, zircon was a residual phase in the Newcastle source region and buffered the Zr concentration of the Newcastle lavas. Similarly, Watson and Harrison (1984) show that P2O5 is saturated in high silica granites at a concentration of 0.02–0.15 wt.%. Therefore, the constant P2O5

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contents of the Newcastle lavas are derived from residual apatite in the source region. 6.7. Summary Major and trace element systematics strongly propose that the Newcastle lavas have been derived from a garnet amphibolite source region and that as the Newcastle magmas segregated and ascended they left behind a residue composed of amphibole, plagioclase, quartz and garnet with accessory zircon and apatite. 7. Characterising the source of the Newcastle magmas The Jamaican adakite-like lavas are derived from partially melting a mafic protolith that has been transformed into a garnet amphibolite. Hence, the Newcastle magmas could be generated from (a) partial melting of delaminated lower crust, (b) partial melting of subducted Proto-Caribbean “normal” oceanic crust that may, or may not, have detached, (c) partial melting of lower Jamaican crust, or (d) partial melting of underthrust/underplated Caribbean oceanic plateau crust. Although adakitic magmas do undergo fractional crystallisation, adakites that have interacted with overlying peridotite mantle still retain high MgO, Cr and Ni contents (e.g., Rapp et al., 1999; Xu et al., 2002; Smithies et al., 2009). Therefore, if the Newcastle magmas are derived from subducted or delaminated crust then they would be expected to have higher MgO, Cr and Ni contents (Fig. 6). The Newcastle lavas have different radiogenic isotope ratios relative to N-MORB and the arc lavas from the surrounding Cretaceous inliers (Fig. 1 and Section 6.1). These arc lavas, together with Tertiary limestones, make up the Jamaican crust (Robinson et al., 1972; Draper, 1986). As such, because radiogenic isotope values are not modified by partial melting or fractional crystallisation, it is unlikely that the Newcastle magmas are derived from an N-MORB source or the lower Jamaican crust. Consequently, models (a), (b) and (c) are improbable and the Newcastle magmas can only be realistically formed from the partial melting of a Caribbean oceanic plateau protolith. In order to form the garnet-bearing metabasic Newcastle source region in the late Palaeocene–early Eocene, either (a) the Jamaican arc crust had to have been ≥ 30 km thick and/or (b) similarly thick mafic crust must have been juxtaposed close to, or thrust under, Jamaica. The crustal thickness of eastern Jamaica at present is ~ 30 km (Wiggins-Grandison, 2004), thus, equating to the lowermost depth needed for garnet formation; nevertheless, it is uncertain to whether the crust was this thick in the early Tertiary. However, during the Tertiary the Caribbean oceanic plateau was being tectonically emplaced between the American continents (Section 2.1) (e.g., Hoernle et al., 2002; Kerr et al., 2003; Geldmacher et al., 2003; Thompson et al. 2003; Hastie and Kerr, 2010). Presently, the Caribbean–South American plate boundary is one of the most oblique convergent margins in the world (Ave Lallemant, 1997) and from the Miocene to the present-day the thickened oceanic plateau crust of the Caribbean plate has been underthrusting South America (Van der Hilst and Mann, 1994). If, as seems likely, a similar scenario existed on the northern Caribbean boundary at ~ 50 Ma, it is possible that the 8–20 km thick Caribbean oceanic plateau (e.g., Edgar et al., 1971; Mauffret and Leroy, 1997) could have underthrust the Jamaican crust. This would potentially form a crustal thickness well in excess of 30 km which would form garnet amphibolites in the underthrust Caribbean oceanic plateau crust (Hastie et al., 2010). 8. Composition of the Newcastle source region Tectonic blocks of greenschist, blueschist and amphibolites are found in the Plantain–Garden fault zone in Jamaica (Fig. 1) (e.g., Kemp, 1971; Draper, 1986). The amphibolites are known as the

Westphalia Schists and are located close to the oceanic plateau rocks of the Bath–Dunrobin Formation (Kemp, 1971; Draper, 1986; Hastie et al., 2008). However, the Westphalia Schists are predominantly composed of meta-sediments (Draper, 1986) and do not represent a mafic igneous protolith for the Newcastle magmas. Experimental determination of the compositions of melts and the mineralogy and compositions of resultant restites during partial melting of amphibolite is particularly complex and is dependent on the temperature and pressure of the system as well as the composition of the amphibolite starting material (e.g., Beard and Lofgren, 1991; Rapp et al., 1991; Rushmer, 1991; Sen and Dunn, 1994; Wolf and Wyllie, 1994; Winther, 1996). This variability means that modelling the partial melting of a theoretical amphibolite composition for the Newcastle magmas is extremely difficult. The Newcastle magmas are derived from a garnet–amphibolite source region at ~ 1.0–1.6 GPa, which means that the rock would intersect the amphibole dehydration melting solidus at ~ 850–900 °C (Peacock et al., 1994). Pearce et al. (1992) developed a parameterisation of the dehydration partial melting of a theoretical amphibolite source region using amphibolite phase proportions from Wyllie (1977) to derive shallow boninitic melts. Wolf and Wyllie (1994) present an updated phase proportion diagram which shows the mineral proportions present in a garnet amphibolite at a range of temperatures and 1.0 GPa. Their study shows that at ~900 °C and 1 GPa the estimated mineral assemblage of a garnet–amphibolite is 55% amphibole, 25% plagioclase, 8% garnet and 12% pyroxene (nearly all cpx). This mineralogy is very similar to that proposed in Section 6 for the Newcastle source region. However, the Newcastle lava compositions also imply the presence of residual quartz and accessory zircon and apatite. Unfortunately, no published experimental study has presented such a mineral assemblage for a potential amphibolite source region. Nevertheless, the mineralogy of a metamorphosed mafic rock depends on its major element composition, with the major element composition of an oceanic plateau basalt being similar to N-MORB. Sen and Dunn (1994) have shown that when an N-MORB-like metabasalt is subjected to the pressures and temperatures required to generate a potential Newcastle metabasic protolith, 2.3% quartz is present and pyroxene does not form until higher temperatures are attained. Therefore, we propose replacing 2.3% of the pyroxene in the mineral proportion of Wolf and Wyllie (1994) with quartz and 0.1% with accessory zircon and apatite respectively to produce a theoretical metabasic protolith from which the Newcastle magmas are derived. In order to model partial melting of the theoretical protolith, partition coefficients for 850–900 °C were taken from mineral studies in equilibrium with TTG/D liquids (Klein et al., 1997, 2000; Bédard, 2006; Willbold et al., 2009) and used along with experimental melting modes from Sen and Dunn (1994) and Wolf and Wyllie (1994), which are derived from the changing phase proportions during dehydration partial melting. As an example, non-modal batch melting calculations using an NMORB (Sun and McDonough, 1989) starting composition cannot produce the enriched incompatible trace element signature of the Newcastle lavas (Fig. 8a). Furthermore, if the E-MORB composition from Sun and McDonough (1989) is used, the modelled melts are too enriched in incompatible elements. However, if the average composition of the Bath–Dunrobin oceanic plateau basalts (e.g., Hastie et al., 2008) is used as the starting composition the geochemical patterns of the Newcastle lavas including the negative Nb–Ta, fractionated Sm– Nd, positive Zr and Hf, concave MREE and HREE, and fractionated (Gd/ Yb)n–mn ratios can be closely replicated with 15–30% partial melting (Fig. 8b). Martin (1999) has shown that most adakites do not have positive Zr and Hf anomalies on multi-element diagrams. The fact that the Newcastle lavas exhibit this feature is indicative of the presence of abundant amphibole and plagioclase in the source region which have

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Fig. 9. Gd/Yb – Nb/Sm trace element diagram showing quantitative partial melt models of possible source regions for the Newcastle lavas. Numbers on the curves represent percentage of partial melting.

Fig. 8. N-MORB normalised multi-element diagrams showing the results of non-modal batch partial melting of a modified theoretical Newcastle source region from Wolf and Wyllie (1994) with an initial mineralogy of 55% amphibole, 25% plagioclase, 9.5% cpx, 8% garnet, 2.3% quartz, 0.1% zircon and 0.1% apatite. Normalising values from Sun and McDonough (1989). Grey field represents the Newcastle data and each model displays data for 1, 5, 10, 15, 20 and 30% non-modal batch partial melting. Partition coefficients are taken from mineral studies in equilibrium with TTG/D liquids at 850–900 °C: Klein et al. (1997, 2000), Bédard (2006) and Willbold et al. (2009). The melting mode of the garnet amphibolite is taken from melt reactions in Sen and Dunn (1994) and Wolf and Wyllie (1994) and includes quartz (0.08) + plagioclase (0.20) + amphibolite (0.72) = cpx (− 0.4) + garnet (− 0.1) + melt (− 0.5). (a) models an N-MORB starting composition and (b) models the composition of partial melts with a Caribbean oceanic plateau starting composition.

higher partition coefficients for Sm and Eu than for Zr and Hf (e.g., Klein et al., 1997) An eclogitic source composed of largely clinopyroxene and garnet would not produce such a positive Zr and Hf anomaly. Residual amphibole is also responsible for forming the negative Ti anomalies in the Newcastle volcanics [DTi for amphibole in equilibrium with a TTG/D melt is 2.32; Klein et al. (1997)]. In addition, Fig. 9 is a Gd/Yb–Nb/Sm trace element ratio plot that utilises elements that are affected by varying amounts of garnet and amphibole in the residue (e.g. Klein et al., 1997, 2000). Quantitatively modelled partial melting curves are shown for a garnet-free amphibolite and eclogite source region based on the mineral and melt modes of Sen and Dunn (1994). It can be seen that the two melting curves do not plot with the Newcastle data (Fig. 9). It is worth noting that the eclogite source from Sen and Dunn (1994) is composed of 86% cpx and 14% garnet, which is quite a conservative modal volume of garnet. If more garnet rich eclogites are plotted the eclogite partial melting curves have even higher Gd/Yb ratios. Conversely, the modified theoretical source from Wolf and Wyllie (1994) produces a partial melt trend that passes through the Newcastle data. In spite of this, the Newcastle data is slightly scattered with a poor trend running obliquely to the modified Wolf and Wyllie (1994) partial melt trend. This oblique trend is the result of (1) variable modal volumes of the major and accessory minerals and/

or (2) source compositional heterogeneity. Interestingly, if the modal abundances of amphibole and garnet are varied slightly, most of the data can be explained. Partial melting curves for sources containing 52–59% amphibole and 4–11% garnet can explain most of the data (Fig. 9). Nevertheless, in order for us to more accurately quantify the model we would have to determine the mineral and melt modes of all the possible minerals in our source region (including accessory minerals). Unfortunately, to the authors' knowledge, this information is not available in the literature. This emphasises the fact that modelling a source region for the Newcastle lavas is extremely difficult. However, the theoretical source region from Wolf and Wyllie (1994) appears to be an excellent approximation of the composition of the Newcastle source region. In conclusion, 15–30% partial melting of the modified theoretical metabasic protolith from Wolf and Wyllie (1994) demonstrates that the Newcastle lavas can be derived from Caribbean oceanic plateau crust, which is metamorphosed to garnet amphibolite. 9. Petrogenetic and tectonic context of the Newcastle adakite-like rocks The Newcastle lavas have very similar compositions to the early Archaean TTG/Ds and thus represent Phanerozoic analogues of these rocks (Table 1, Smithies, 2000; Table 1, Martin et al., 2005; Hastie et al., 2010). However, our understanding of the tectonic regime in the early Archaean is controversial (e.g., Kröner and Layer, 1992; Rudnick, 1995). Some propose that plate tectonic processes were absent (e.g., Hamilton, 1998; van Thienen et al., 2004) while others contend that the convergent margin affinity of TTG/D suites and greenstone belts are evidence of subduction zone processes (e.g., Kröner and Layer, 1992; Kusky and Polat, 1999; Clemens et al., 2006; Kerrich and Polat, 2006). The lack of a “mantle wedge component” in the early TTG/D suites has led to suggestions, that if subduction did develop in the early Archaean, it was unlike present-day subduction zone processes (e.g., Smithies, 2000; Martin et al., 2005). Smithies et al. (2003, 2009) propose that where a modern subducting plate is descending into the mantle at a low angle, or flat, a substantial mantle wedge is still present, and if any adakite magma ascends through the peridotite it is likely that it will assimilate mantle material and generate high MgO (N2.0 wt.%), Ni (N40 ppm) and Cr (mostly N50) contents. Therefore, in order to form the early Archaean TTG/Ds and the Newcastle adakite-like rocks from a

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propose “Archaean flat subduction”, whereby hotter, thicker and more buoyant Archaean oceanic crust underthrust another plate to exclude a mantle wedge. The underthrust oceanic crust subsequently partially melts to form TTG/D magmas that lack a “mantle wedge component” (e.g., Winther, 1996). Recently, Hastie et al. (2010) propose that a similar model may generate the Newcastle lavas. Most of the Caribbean plate consists of an oceanic plateau that erupted onto the Farallon Plate at ~ 89–93 Ma and was transported northeastwards to collide with the southern margin of the Great Arc of the Caribbean by ~ 85 Ma (Fig. 10a) (Kerr et al., 2003; Hastie and Kerr, 2010). Unlike the Farallon crust, the plateau crust would have been too thick, hot and buoyant to subduct, thus initiating subduction polarity reversal along the Great Arc (Fig. 10b). In the late Campanian to lower early Maastrichtian the Jamaican part of the Great Arc of the Caribbean collided and accreted as a microplate to the Yucatan/Maya Block and from the late Maastrichtian to Paleocene northerly to northeasterly dipping subduction zone developed beneath the Jamaican microplate (Fig. 10c). Consequently, in the late Paleocene to early Eocene, the northern margin of the Caribbean oceanic plateau collided with the Jamaican microplate and underthrust Jamaica to form the garnet amphibolite source region of the Newcastle lavas in a similar way to the Archaean flat subduction model of Smithies et al. (2003). Once the Newcastle source region formed it potentially partially melted via two differing processes. In the first model proposed by Hastie et al. (2010), the underthrusting or subducting? oceanic plateau crust directly partially melts to form the Newcastle magmas (e.g., Peacock et al., 1994) and the Halberstadt lavas may be derived from a thin, slab-melt hybridised mantle wedge. However, in this study we propose a second model to explain the generation of the Newcastle lavas. The oceanic plateau crust may have underplated the Jamaican crust, such that the lower Jamaican crust is composed of oceanic plateau material, and subsequent extension of the Jamaican lithosphere, which produced the Wagwater Basin (Horsfield, 1974), produced decompression melting in the underlying asthenosphere (e.g., McKenzie and Bickle, 1988). The ascending mantle-derived mafic magmas (the Halberstadt lavas?) intruded the underplated oceanic plateau material resulting in partial melting to generate the Newcastle magmas (e.g., Petford and Gallagher, 2001). 10. A Phanerozoic analogue for early Archaean TTG/D suites: a unique adakite subgroup?

Fig. 10. (a–c) Illustrations showing the evolution of the Caribbean plate from the late Cretaceous to the beginning of the Tertiary. COP, Caribbean oceanic plateau. Diagram modified from Hastie et al. (2010).

subducting slab and still maintain low MgO, Ni and Cr abundances there must be little, or no, mantle wedge overlying the subducting slab (e.g., Martin and Moyen, 2002). Additionally, the presence of residual plagioclase and garnet shows that the source region of the Newcastle adakites partially melted between 30 and 50 km (e.g., Peacock et al., 1994; Sen and Dunn, 1994; Rapp and Watson, 1995; Martin and Moyen, 2002; Smithies et al., 2003). In order to explain the geochemistry of the early Archaean TTG/D suite, Smithies et al. (2003)

Both TTG/Ds and adakites have been classified into compositionally distinct subgroups: e.g., Martin et al. (2005) divide adakites into high-SiO2 (N60 wt.%) and low-SiO2 (SiO2 b60 wt.%) groups and Smithies (2000) and Smithies et al. (2003) divide Archaean TTG/Ds into the middle-late Archaean (3.5–b3.0 Ga) and early Archaean (N3.5 Ga) subgroups. The high-SiO2 adakites are considered to represent “type” adakites that were originally defined by Defant et al. (1992) and these are analogous to middle-late Archaean TTG/D suites (Smithies 2000; Martin et al., 2005). The low-SiO2 adakites (high-Mg andesites) are analogous to late Archaean sanukitoids and are thought to have formed by partial melting of the mantle wedge after it has been modified by slab melts (Martin et al., 2005). The modern analogues of early Archaean TTG/Ds reported in this study represent a distinct compositional end-member of the “adakite group” (e.g., low Sr, MgO, Ni and Cr concentrations) and so we propose that the Newcastle lavas should be categorised into a separate adakite subgroup known as Jamaican-type adakites. 11. Conclusions The Newcastle lavas have adakitic-like major element compositions, (La/Yb)cn ratios, low Y and HREE concentrations and negative

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Nb and Ta anomalies. However, they have lower Sr concentrations (b400 ppm) and lower Sr/Y ratios compared to modern-day adakites and middle-late Archaean (2.5–3.5 GPa) TTG/D suites. The geochemistry of the Newcastle lavas indicates that they are derived from partially melting a plagioclase-bearing garnet amphibolite source region at 1.0–1.6 GPa and N900 °C. Radiogenic isotopes and partial melt models indicate that the garnet amphibolite source region represents metamorphosed Caribbean oceanic plateau crust that underthrust Jamaica in the early Tertiary. Low MgO, Ni and Cr concentrations in the Newcastle adakite-like lavas are similar to early Archaean TTG/D suites (N3.5 Ga) and demonstrates that when the lavas segregated and ascended from their source region they did not interact with mantle wedge peridotite. Consequently, the Jamaican lavas are modern analogues for the earliest Archaean continental crust and are classified as a unique subgroup of adakites: Jamaican-type adakite. Acknowledgements Alan Hastie acknowledges NERC PhD Studentship NER/S/A/2003/ 11215. Thanks to Eveline DeVos for her analytical expertise on the ICP-OES and ICP-MS at Cardiff University, U.K. The authors are grateful to B. Leake for helpful comments and to Trevor Jackson, Arnott Jones, Geoffrey Edwards and Vencott Adams for help with fieldwork and logistics in Jamaica. Javier Escuder-Viruete and an anonymous reviewer are thanked for very constructive reviews that improved the manuscript. References Aguillón-Robles, A., Calmus, T., Benoit, M., Bellon, H., Maury, R.C., Cotton, J., Bourgois, J., Michard, F., 2001. Late Miocene adakites and Nb-enriched basalts from Vizcaino Peninsula, Mexico: indicators of East Pacific Rise subduction below southern Baja California? Geology 29, 531–534. Atherton, M.P., Petford, N., 1993. Generation of sodium-rich magmas from newly underplated basaltic crust. Nature 362, 144–146. Audétat, A., Keppler, H., 2005. Solubility of rutile in subduction zone fluids, as determined by experiments in the hydrothermal diamond anvil cell. Earth and Planetary Science Letters 232, 393–402. Ave Lallemant, H.G., 1997. Transpression, displacement partitioning, and exhumation in the eastern Caribbean/South American plate boundary zone. Tectonics 16, 272–289. Ayers, J.C., Watson, E.B., 1993. Rutile solubility and mobility in supercritical aqueous fluids. Contributions of Mineralogy and Petrology 114, 321–330. Beard, J.S., Lofgren, G.E., 1989. Effect of water on the composition of partial melts of greenstone and amphibolite. Science 244, 195–197. Beard, J.S., Lofgren, G.E., 1991. Dehydration melting and water-saturated melting of basaltic and andesitic greenstones and amphibolites at 1, 3, and 6·9 kb. Journal of Petrology 32, 365–401. Beate, B., Monzier, M., Spikings, R., Cotton, J., Silva, J., Bourdon, E., Eissen, J.-P., 2001. Mio-Pliocene adakite generation related to flat subduction in southern Ecuador: the Quimsacocha volcanic center. Earth and Planetary Science Letters 192, 561–570. Bédard, J.H., 2006. A catalytic delamination-driven model for coupled genesis of Archaean crust and sub-continental lithospheric mantle. Geochimica et Cosmochimica Acta 70, 1188–1214. Blichert-Toft, J., 2001. On the Lu–Hf isotope geochemistry of silicate rocks. Geostandards Newsletter — the Journal of Geostandards and Geoanalysis 25, 41–56. Burke, K., 1988. Tectonic evolution of the Caribbean. Annual Review of Earth and Planetary Science 16, 201–230. Castillo, P.R., Solidum, R.U., Punongbayan, R.S., 2002. Origin of high field strength element enrichment in the Sulu Arc, southern Philippines, revisited. Geology 30, 707–710. Castillo, P.R., Rigby, S.J., Solidum, R.U., 2007. Origin of high field strength element enrichment in volcanic arcs: geochemical evidence from the Sulu Arc, southern Philippines. Lithos 97, 271–288. Chung, S.-L., Liu, D., Ji, J., Chu, M.-F., Lee, H.-Y., Wen, D.-J., Lo, C.-H., Lee, T.-Y., Qian, Q., Zhang, Q., 2003. Adakites from continental collision zones: melting of thickened lower crust beneath southern Tibet. Geology 31, 1021–1024. Clemens, J.D., Yearron, L.M., Stevens, G., 2006. Barberton (South Africa) TTG magmas: geochemical and experimental constraints on source-rock petrology, pressure of formation and tectonic setting. Precambrian Research 151, 53–78. Condie, K., 2005. TTGs and adakites: are they both slab melts? Lithos 80, 33–44. Defant, M.J., Richerson, P.M., de Boer, J.Z., Stewart, R.H., Maury, R.C., Bellon, H., Drummond, M.S., Feigenson, M.D., Jackson, T.E., 1991. Dacite genesis via slab melting and differentiation: petrogenesis of La Yeguada Volcanic Complex, Panama. Journal of Petrology 32, 1101–1142.

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