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CHAPTER 5

Central Atlantic Magmatic Province (CAMP): The Palisade Connection

Abstract

The 200 Ma Palisade Sills, exposed along the Hudson River in northeastern

North America are an expression of the Central Atlantic Magmatic Province (CAMP)

magmatism. On the basis of similar ages of eruption, Palisade Sill tholeiites have

been correlated to other CAMP exposures in four different continents. We provide an

isotopic tracer study of the Palisade Sill basalts and relate them to low-Ti (<2 wt %)

CAMP related tholeiites from North and South America, Europe, and West Africa.

We report Nd-Sr-Pb isotopic and multiple trace element data of nineteen basalts and

gabbros, three chilled margin basalts, and four sandstones spanning the entire length

and thickness of the Palisade Sill.

The Palisade Sill basalts of this study yield the typical composition of low-Ti

CAMP tholeiites with small LREE enrichments (LaN/SmN = 1.7 to 2.3), radiogenic Sr

and negative εNd(I) values (87Sr/87Sr(I) = 0.70668 to 0.71037; εNd(I) = -0.64 to -3.8), and

Pb-isotopic ratios (e.g. 206Pb/204Pb = 18.11 to 18.69) above the NHRL and subparallel

to it. The combined geochemical data of the Palisade Sill basalts and their correlation

with other low-Ti CAMP related lavas imply a slightly enriched mantle source with

likely contamination by the continental crust and with no contribution from an EM-I

or depleted MORB like component.

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Although both plume and lithospheric mantle sources have been previously

proposed for the low-Ti CAMP lavas, we use the geochemical data of our study to

propose the source of the Palisade Sill basalts as well as other low-Ti CAMP

tholeiites to be derived from ~15% melting of a slightly depleted spinel peridotite

which experienced up to ~20% contamination by the continental crust prior to or

during the emplacement of these lavas. We suggest the geodynamic emplacement

processes of the CAMP to be similar to the present day East African Rift System

which is sourced from the African superplume via multiple feeder stems that span a

large aerial extent.

5.1. Introduction

The extensively studied 200Ma Central Atlantic Magmatic Province (CAMP,

Fig. 5.1a) is considered to be the biggest Large Igneous Province (LIP) on this planet

covering up to 7 X 106 km2 (Marzoli et al., 1999; Olsen, 1999; Hames et al., 2000).

This igneous province has been linked to the early Mesozoic initial opening of the

Central Atlantic Ocean (Dalrymple et al., 1975; Bertrand et al., 1982; Alibert, 1985;

Dupuy et al., 1988; Bertrand, 1991; Sebai et al., 1991; Deckart et al., 1997; Hames et

al., 2000; McHone, 2000; Cebria et al., 2003; DeMin et al., 2003). The opening of

the Central Atlantic Ocean fragmented the CAMP into several segments that occur on

four different tectonic plates today. This magmatic event has been compared to

formation of flood basalt provinces such as the Siberian and Deccan Traps, in that

each may be genetically linked to a global faunal extinction (e.g. Olsen, 1999). For

215

CAMP this is the event recorded at the Triassic-Jurassic boundary (Marzoli et al.,

1999). Although many authors have done extensive work on CAMP-related tectonics

and magmatic processes, there is still no consensus on its origin, and many aspects of

the CAMP remain controversial (e.g. Deckart et al., 1997; Olsen, 1999; Hames et al.,

2000; McHone, 2000; Puffer, 2002).

The CAMP related LIP is different from others in that it constitutes almost

entirely of dykes and Sills with scarce volcanic outflows (Cebria et al., 2003). In the

pre-Atlantic Ocean reconstruction, this dike swarm defines an overall radiating

pattern extending nearly 300 km from its focal point (May, 1971; Ernst et al., 1995).

Although the distribution of dikes on a local scale shows more complex patterns (e.g.

Bertrand, 1991; Hames et al., 2000), on a large scale it represents the best example on

this planet of a complete radiating dike swarm system (Greenough and Hodych, 1990;

Ernst et al., 1995; Dalziel et al., 2000; Beutel, 2009), comparable only with some of

the radial dike swarms of Venus (Ernst et al., 1995).

Extensive Ar-Ar dating and limited U-Pb dating have shown this even to be of

short duration with magmatism in all regions occurring within a few million years at

200 Ma (e.g. Dunning and Hodych, 1990; Sebai et al., 1991; Hodych and Dunning,

1992; Deckart et al., 1997; Olsen, 1997; Marzoli et al., 1999; Olsen, 1999; Hames et

al., 2000). Although non-plume models have been considered for the CAMP event

(McHone, 2000), a mantle plume appears to be necessary to explain the radiating dike

pattern and the generation of such a huge area of basaltic magmatism within only a

few million years (e.g. Wilson, 1997; Ernst and Buchan, 2002).

216

Figure 5.1a. General distribution of Early Jurassic tholeiitic dikes, lavas and sills in a

pre-drift reconstruction at 200Ma (May, 1981; Schermerhorn et al., 1978; Belleni et

al., 1990; McHone, 2000). The square represents the Palisade Sill region.

217

The study of regionally extensive dike and sill systems represents one of the

fundamental tools in the analyses of LIPs to clarify their origin (e.g. plume vs. non-

plume) and geodynamic setting. In this context the detailed geochemical study of the

Palisade Sill reported here may shed light on some of the unsolved questions linking

LIPs and regional dike swarms, such as (1) the association of radiating dikes with

either sublithospheric plume impingement (Ernst et al., 1995) or mantle insulation

beneath highly refractory cratons (Yale and Carpenter, 1998) and (2) vertical

emplacement versus far reaching (>3000 km) and nearly instantaneous (in less than a

few million years) lateral migration of magma from its source (Greenough and

Hodych, 1990; Ernst et al., 1995; Elliot et al., 1999).

It is generally agreed that the Palisade Sill basalts are an expression of the

CAMP magmatism that related to the earliest stages of the opening of the Central

Atlantic (Ernst et al., 1995; Oyarzun et al., 1997; Wilson, 1997). In terms of trace

elements and multiple element isotopic systematics, this poorly studied area of

CAMP, presently located in northeastern America is similar to the voluminous low-Ti

tholeiites associated with CAMP and distinctly different from the smaller volumes of

high-Ti magmatism related to the CAMP.

In this study we report the detailed trace element concentrations and Nd-Sr-

Pb-isotopic ratios of nineteen basalts and gabbros, three chilled margin basalts, and

four sandstones from the entire length of the Palisade Sill (Fig. 5.1b). These

geochemical data are essential to understand the relationship between mantle

218

geodynamic processes involved in the generation of the CAMP tholeiites prior to the

formation of the of the Atlantic Ocean crust.

5.2. Geological History of the Palisade Sills

The initial fragmentation of the Pangaea supercontinent was accompanied by

extensive tholeiitic magmatism now represented by Sills, dikes and minor lava flows

in four continents along both sides of the Central Atlantic Ocean, on the eastern

margin of north America (between Nova Scotia and Florida), Western Europe (Spain

and France), West Africa (Morocco to Ivory Coast), and northern south America

(French Guyana, Surinam and Brazil) (e.g. Cebria et al., 2003).

The Palisades Sill is one of several diabase intrusions located in the system of

Eastern North American rift basins, formed by significant crustal extension during the

break-up of Pangea during the late Triassic. This early Jurassic Palisade Sill that

intruded into the Triassic continental sedimentary rocks of the Newark basin has been

cites as the classic example of a vertically differentiated Sill (e.g. Lewis, 1908;

Walker, 1940; 1956; Carmichael et al., 1974; Shirley, 1987; Husch, 1992; Gorring

and Naslund, 1995). The Palisade Sill and other sills in the Newark basin are used as

markers to define the Triassic-Jurassic boundary (Dunning and Hodych, 1990; Kent

et al., 1991). The intrusion is a ~200Ma (Erickson and Kulp, 1961; Dallmeyer, 1975;

Dunning and Hodych, 1990), 300m thick diabase Sill intruded into sandstones and

arkoses of the Newark basin (Walker, 1969b). The outcrop extends for 80 km in a

north-south direction from central Staten Island in New York, through a significant

219

Figure 5.1b. Distribution of early Jurassic igneous rocks throughout the northern Newark

Basin (Puffer, et al., 2009). The location of the Palisade sill basalts discussed in this study

relative to eastern North America is shown in the inset.

220

portion of New Jersey, along the western bank of the Hudson River (Fig. 5.1b)

(Naslund, 1998). Here it turns westward and is discordant with the local strata and

hence referred to as a dike locally (Walker, 1969b). It may connect under cover with

the Rocky Hill and Lambertville Sills to the south for a total strike length of 150 kms

(Husch, 1992). Field relations and petrography have been described by a number of

authors (Lewis, 1908; Walker, 1940; Walker, 1969a; Shirley, 1987; Husch, 1992;

Steiner et al., 1992).

Several authors have done major element analyses of the various layers of the

Palisade Sill to understand compaction and differentiation processes experienced by

this Sill after emplacement (e.g. Shirley, 1987; Husch, 1992; Puffer, 2002). However

very limited trace element and isotopic geochemistry related work has been done in

this region (Pegram, 1990).

5.3. Analytical Results

In this section we present the geochemical results of the Palisades Sill samples

of this study that include 17 basalts and gabbros, 4 sandstones, and 3 chilled margins.

These data comprise multiple trace element concentrations including the rare earths,

and the isotopic compositions of Nd, Sr, and Pb. The data are presented in tables 5.1-

5.2 and figures 5.2-5.7 and are compared with similar data obtained from literature on

volcanic rocks related to CAMP activity from North America, South America,

Europe, Africa, and Canada. Analytical methods are described in Appendix-2.

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Table 5.1. Trace element concentration of the basalts and sandstones from the

Palisade Sill analyzed for this study.

PS-01 PS-02 PS-03 PS-05 PS-07 PS-08 PS-10A PS-10B

Basalt Basalt Basalt Basalt (Dyke)

Basalt (Dyke) Gabbro Basalt

(Massive) Basalt

(Weathered) Rb 28.4 19.8 23.1 24.7 19.0 26.8 19.0 20.8 Ba 89 90 169 88 180 207 159 164 Sr 147 154 171 144 165 168 159 123 Pb 5.27 5.38 14.1 16.4 20.1 16.8 6.24 15.0 La 11.4 9.03 9.38 8.55 13.1 9.96 10.5 7.29 Ce 24.5 19.4 20.2 19.2 24.9 22.2 21.8 15.7 Pr 3.07 2.58 2.69 2.60 3.60 2.99 2.88 2.05 Nd 12.3 10.9 11.2 11.1 15.1 12.8 12.2 8.57 Eu 0.98 0.96 0.99 0.91 1.15 1.14 0.99 0.77 Sm 3.14 2.92 2.93 3.00 3.95 3.46 3.13 2.24 Gd 3.39 3.09 3.14 3.16 4.18 3.55 3.35 2.34 Tb 0.57 0.54 0.55 0.56 0.74 0.65 0.57 0.41 Dy 3.63 3.34 3.52 3.55 4.55 4.11 3.50 2.66 Ho 0.77 0.71 0.73 0.76 0.97 0.88 0.75 0.57 Er 2.10 1.93 1.94 2.01 2.56 2.38 2.02 1.52 Tm 0.32 0.29 0.30 0.30 0.38 0.36 0.30 0.24 Yb 1.99 1.84 1.92 1.91 2.41 2.30 1.97 1.51 Lu 0.28 0.27 0.28 0.28 0.36 0.34 0.29 0.22 Y 21.3 19.85 19.9 20.5 27.7 23.9 21.2 15.8 Th 1.85 1.79 2.10 1.74 1.97 2.26 2.23 1.68 U 0.64 0.43 0.51 0.43 0.61 0.52 0.51 0.45 Zr 85 89 99 74 95 103 95 84 Hf 2.27 2.36 2.74 2.02 2.65 2.75 2.47 2.11 Nb 9.75 11.3 15.1 10.2 12.2 14.0 9.19 7.45 Ta 0.77 0.72 0.95 0.66 0.96 0.89 0.58 0.91 Sc 36.3 33.2 31.5 32.1 39.6 32.8 33.2 22.7 V 311 313 321 325 351 303 332 252

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Table 5.1 continued

PS-11 PS-12 PS-13 PS-14 PS-17 PS-20 PS-21 PS-23 PS-24 Gabbro Basalt Basalt Basalt Gabbro Basalt Basalt Basalt Basalt

Rb 33.5 17.2 76 25.0 31.5 22.0 13.3 23.5 11.1 Ba 213 92 392 190 248 216 88 219 72 Sr 208 157 244 171 236 207 205 182 150 Pb 9.47 31.7 133 8.88 51 14.1 34.8 35.8 6.20 La 10.2 7.11 21.6 10.3 16.0 12.7 10.3 12.0 7.76 Ce 21.7 17.1 46.1 22.8 32.8 27.6 24.1 25.9 17.4 Pr 2.84 2.01 5.95 3.04 4.20 3.63 3.04 3.39 2.40 Nd 11.9 8.42 24.4 12.9 17.0 15.4 12.6 13.8 10.2 Eu 1.14 0.73 1.72 1.08 1.41 1.31 1.13 1.32 0.93 Sm 3.08 2.12 6.17 3.49 4.30 4.08 3.44 3.80 2.95 Gd 3.27 2.18 6.56 3.71 4.55 4.43 3.63 4.20 3.20 Tb 0.57 0.36 1.10 0.65 0.73 0.77 0.62 0.73 0.56 Dy 3.55 2.25 6.59 4.04 4.36 4.71 3.89 4.56 3.54 Ho 0.76 0.46 1.42 0.85 0.90 1.00 0.81 0.98 0.75 Er 1.99 1.24 3.76 2.26 2.48 2.66 2.17 2.64 2.01 Tm 0.31 0.18 0.58 0.35 0.36 0.40 0.33 0.41 0.31 Yb 1.97 1.11 3.66 2.20 2.29 2.59 2.09 2.63 1.96 Lu 0.28 0.16 0.53 0.33 0.32 0.38 0.30 0.38 0.28 Y 20.4 10.1 40.2 22.8 26.4 27.1 21.2 26.7 19.0 Th 2.08 1.34 5.51 2.14 3.48 2.67 2.07 2.13 1.44 U 0.50 0.52 1.31 0.51 1.06 0.61 0.48 0.54 0.34 Zr 94 106 209 101 164 126 83.1 105 76.2 Hf 2.57 2.85 5.08 2.81 3.99 3.26 2.25 2.79 2.19 Nb 8.71 10.6 15.8 9.76 12.7 11.2 8.07 9.71 6.19 Ta 0.54 0.65 1.07 0.63 0.81 0.68 0.51 0.58 0.38 Sc 32.9 10.8 22.4 33.9 20.9 32.4 25.0 39.4 34.8 V 338 278 274 345 249 373 247 368 316

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Table 5.1 continued

PS-24 PS-04 PS-15 PS-22 PS-06 PS-09 PS-16 PS-19

Basalt Chilled Margin

Chilled Margin

Chilled Margin

Silicified Sandstone

Sand- stone

Sand- stone

Sand- stone

Rb 11.1 187 123 25.1 93 76 69 39 Ba 72 558 301 261 885 652 940 262 Sr 150 134 123 205 289 86 331 94 Pb 6.20 39.8 20.1 14.2 18.5 25.5 75 21.3 La 7.76 31.0 18.4 11.7 33.6 55.6 16.9 6.33 Ce 17.4 82 33.3 25.2 70 218 34.1 18.4 Pr 2.40 8.04 3.96 3.26 8.12 13.7 4.80 3.17 Nd 10.2 30.5 13.9 13.7 29.2 49.6 18.2 16.1 Eu 0.93 1.39 0.94 1.24 1.36 1.25 1.07 0.82 Sm 2.95 6.86 2.70 14.3 6.11 9.92 4.14 6.19 Gd 3.20 6.79 2.56 3.94 5.63 8.61 3.99 6.00 Tb 0.56 0.99 0.34 0.67 0.80 1.01 0.63 1.22 Dy 3.54 5.44 1.93 4.11 4.50 4.42 3.75 7.80 Ho 0.75 1.02 0.40 0.90 0.91 0.71 0.78 1.62 Er 2.01 2.51 1.09 2.38 2.42 1.82 2.11 4.30 Tm 0.31 0.35 0.18 0.36 0.36 0.24 0.33 0.67 Yb 1.96 2.10 1.11 2.31 2.27 1.64 2.12 4.08 Lu 0.28 0.29 0.17 0.33 0.33 0.25 0.31 0.54 Y 19.0 35.0 11.5 24.0 25.4 18.5 23.2 44.3 Th 1.44 16.8 6.23 2.34 13.8 25.2 6.61 8.82 U 0.34 2.75 1.59 0.56 2.66 2.37 2.45 1.13 Zr 76.2 77 22.2 118 127 105 168 68 Hf 2.19 1.92 0.72 3.00 4.11 3.45 4.91 2.31 Nb 6.19 31.2 11.3 10.2 5.74 13.2 6.43 8.33 Ta 0.38 2.15 0.79 0.61 0.43 0.88 0.40 0.62 Sc 34.8 16.3 9.24 31.0 2.98 8.50 2.68 6.18 V 316 153 151 374 24.0 25.4 22.5 25.0

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5.3.1. Trace Element Geochemistry

Trace element data for all the Palisades Sill basalts and gabbros and related

sandstones and chilled margins of this study are presented in Table 5.1. The

Chondrite normalized (Evensen et al., 1978) rare earth element (REE) patterns for the

basalts and gabbros are shown in figure 5.2a, while those of the sandstones and

chilled margin basalts are shown in figure 5.2b. The basalts and gabbros show

uniform patterns with slight light rare earth element (LREE) enrichment (LaN/SmN =

1.7 to 2.3) and a relatively gentler slope for the heavy rare earth elements (HREE)

(GdN/YbN = 1.3 to 1.6). In contrast to the basalts and gabbros, sandstones and chilled

margins show a much wider range of chondrite normalized REE patterns, especially

LREEs (LaN/SmN = 1.1 to 23). These sandstones and chilled margin basalts show

LREE enrichment, ~20-100 times that of chondrite, HREE ~10-40 times that of

chondrite (GdN/YbN = 1.2 to 4.3), and strong negative Eu anomalies (Fig. 5.2b).

Twenty-two compatible and incompatible trace element concentration patterns

for the Palisades Sill lavas and associated sandstones and chilled margin basalts are

shown normalized to primitive mantle (Sun and McDonough, 1989) in figures 5.3a

and b, respectively. Primitive mantle normalized basalts and gabbros show low Ba,

high U, Pb, Zr-Hf concentrations, mildly negative Nb-Ta anomalies, and gently

sloping HREEs. In general, basalts and gabbros of the Palisades Sill are similar to and

~30 times more enriched than primitive mantle. Sandstones and chilled margin

basalts (Fig. 5.3b) have enrichments and depletions in the same elements as the

Palisade sill basalts and gabbros, but the magnitude of these enrichments and

225

Figure 5.2. Chondrite normalized REE patterns of basalts from the Palisades sill

samples of this study: (a) Basalts and Gabbros and (b) sandstones (open circles) and

chilled margins (gray filled circles). Palisade Sill samples of this study have been

compared to high and low-Ti CAMP basalts from Europe (Alibert, 1985; Demant et

al., 1996), Brazil (DeMin et al., 2003), average Guinea, and average Guyana (Deckart

et al., 1997).

226

Figure 5.3. Multiple trace element concentrations normalized to primitive mantle for

the Palisades sill samples of this study: (a) Basalts and Gabbros and (b) sandstones

and chilled margins. Symbols as in figure 5.2

227

depletions are much larger for the sandstones and chilled margin basalts. In addition,

they also have negative Sr and positive Th peaks. The Nb-Ta anomaly is much

stronger in the sandstones than in the chilled margin basalts.

The La/Ba (0.02-0.08) and La/Nb (0.8-8.3) ratios of the Palisades Sill data as

well as associated sandstones and chilled margins are plotted in figure 5.4a and

compared to various mantle reservoirs (Saunders et al., 1992) as well as CAMP data

from France (Jourdan et al., 2003), Guinea, and Guyana (Deckart et al., 1997). Notice

the close correspondence of the Palisades basalts and gabbros with OIBs, and low-Ti

CAMP related tholeiites from Guinea and France.

Various trace element concentrations and ratios have been plotted in figure 4.

The plot of Nb (ppm) versus Nb/Ta (Fig. 5.4b) compares the basalts and gabbros of

this study with sediments (Govindaraju, 1994; Vroon et al., 1995), ocean island

basalts (OIB) (Clague and Frey, 1982; Frey and Clague, 1983; Palacz and Saunders,

1986; Weaver et al., 1987; Chauvel and Hofmann, 1992), average mid-ocean ridge

basalt (MORB) and primitive mantle (PM) (Sun and McDonough, 1989). Also shown

is calculated variation of Nb with Nb/Ta in melt and residue at variable degrees of

batch melting of a peridotitic source with primitive mantle Nb (0.713 ppm) and Ta

(0.041 ppm) concentrations. Peridotite melting models are based on starting

composition of olivine (60%), orthopyroxene (25%), clinopyroxene, (10%), and

garnet (5%). ‘D’ values are from Green et al. (1989).

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Figure 5.4a. Comparison of the Palisade sill basalts and associated sandstones and

chilled margins with high and low-TI CAMP basalts from Guyana, Guinea (Deckart

et al., 1997), Brazil (DeMin et al., 2003), and France (Jourdan et al., 2003) in the

diagram La/Ba versus La/Nb (Saunders et al., 1992; Nomade et al., 2002). DM:

depleted mantle; CC: continental crust; PM: primitive mantle; OIB: ocean island

basalts.

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Figure 5.4b. Plot of Nb/Ta versus Nb (ppm) of the Palisade sill basalts and gabbros

of this study compared with sediments (Govindaraju, 1994, Vroon et al 1995), ocean

island basalts (OIB) (Clague and Frey, 1982; Frey and Clague, 1983; Palacz and

Saunders, 1986; Weaver et al., 1987; Chauvel et al., 1992), average mid-ocean ridge

basalt (MORB) and primitive mantle (PM) (Sun and McDonough, 1989). Also shown

is calculated variation of Nb and Nb/Ta in melt and residue at variable degrees of

melting of a peridotitic source (see text for details).

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5.3.2. Nd-Sr-Pb Geochemistry

Rb-Sr and Sm-Nd isotope systematics data for all the rocks of this study are

reported in Table 5.2. The initial εNd and 87Sr/86Sr(I) ratios of all the Palisade Sill

basalts of this study are plotted in figure 5.5. Initial εNd and 87Sr/86Sr have been

calculated at 200 Ma which is the 40Ar-39Ar age for these intrusive rocks (e.g. Marzoli

et al., 1999) (Fig. 5.5). The initial εNd values for the Palisade Sill basalts and gabbros

range from -0.64 to -3.8 whereas the sandstones and chilled margin basalts have a

much more negative εNd(I) range of -6.3 to -12.3. The initial 87Sr/86Sr values of basalts

from the Palisade Sill range from 0.70668 to 0.71037, with the sandstones and chilled

margins displaying the most radiogenic 87Sr/86Sr(I) from 0.70736 to 0.72267. In figure

5.5 all basalts, sandstones, and chilled margins of this study have been compared to

the Central Atlantic MORB (Janney and Castillo, 1996) as well as CAMP related

intrusives from Liberia (Dupuy et al., 1988), eastern North America (Peagram, 1990;

(Pegram, 1990; Heatherington and Mueller, 1999), Spain (Alibert, 1985; Cebria et al.,

2003), Guinea and Guyana (Deckart et al., 1997), Brazil (DeMin et al., 2003), France

(Jourdan et al., 2003), and Nova Scotia (Pe-Piper and Reynolds, 2000).

In the Nd-Sr isotopic correlation (Fig. 5.5) the Palisade Sill basalts show

affinity with the low-Ti CAMP tholeiites and lie in the enriched quadrant with

negative εNd and radiogenic 87Sr/86Sr in contrast to the high-Ti CAMP tholeiites

which have a smaller isotopic range and lie in the quadrant of mantle derived rocks. A

mixing is constructed with the Central Atlantic Plume (CAP) (Cebria et al., 2003) as

231

the uncontaminated plume end member and sandstone from the Palisade Sill region as

the contaminant end member.

Initial 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb of the Palisade Sill basalts and

gabbros at 200 Ma have ranges of 18.11-18.69, 15.57-15.69, and 37.74-39.82 and the

sandstones and chilled margin have ranges of 18.45-18.67, 15.62-15.77, and 38.06-

39.78 respectively as reported in Table 5.3. Pb-Pb isotopic ratios of these rock are

plotted in figure 5.6 along with Central Atlantic MORB (Janney and Castillo, 1996)

as well as CAMP related intrusives from eastern North America (Pegram, 1990;

Heatherington and Mueller, 1999), Spain (Alibert, 1985; Cebria et al., 2003), Guinea

and Guyana (Deckart et al., 1997), France (Jourdan et al., 2003), Nova Scotia (Pe-

Piper and Reynolds, 2000), and mafic and felsic gneisses from Honeybrook Upland,

Pennsylvania (Sinha et al., 1996). Various continental crustal and mantle reservoirs as

well as the Northern Hemisphere Reference Line (NHRL) are also plotted in the Pb-

Pb plots for reference (Zartman and Doe, 1981; Hart and Zindler, 1989). Notice the

correspondence of the Palisade Sill basalts and gabbros with the other low-Ti CAMP

related tholeiites in figure 5.6b.

Initial εNd and 87Sr/86Sr vs. 206Pb/204Pb(I) at 200 Ma for all the rocks of this

study are shown in figure 5.7a and b respectively and compared with relevant CAMP

related basalts from eastern North America (Pegram, 1990; Heatherington and

Mueller, 1999), Spain (Alibert, 1985; Cebria et al., 2003), Guinea and Guyana

(Deckart et al., 1997), France (Jourdan et al., 2003), Nova Scotia (Pe-Piper and

Reynolds, 2000), Central Atlantic MORB (Janney and Castillo, 1996), and various

232

Table 5.2. Initial Nd-Sr-Pb-isotopic data of the Palisade Sill samples of this study

corrected to 200 Ma

147Sm/

144Nd 143Nd/

144Nd(0) 143Nd/

144Nd(I) εNd(I)

87Rb/ 86Sr

87Sr/86Sr(0) 87Sr/86Sr(I)

PS-01 Basalts 0.16 0.512492 0.512291 -1.8 0.51 0.708061 0.706614 PS-02 0.16 0.512453 0.512242 -2.7 0.34 0.706832 0.705857 PS-03 0.16 0.512509 0.512299 -1.6 0.38 0.707276 0.706186 PS-05 0.16 0.512547 0.512331 -1.0 0.47 0.707573 0.706226 PS-07 0.16 0.512388 0.512177 -4.0 0.33 0.707229 0.706299 PS-08 0.17 0.512533 0.512316 -1.3 0.44 0.707887 0.706639

PS-10A 0.15 0.512472 0.512276 -2.0 0.31 0.707238 0.706356

PS-10B 0.15 0.512522 0.512322 -1.2 0.43 0.706683 0.705454

PS-11 0.15 0.512500 0.512305 -1.5 0.44 0.708068 0.706825 PS-12 0.14 0.512488 0.512303 -1.5 0.29 0.706820 0.706002 PS-13 0.14 0.512506 0.512322 -1.12 0.83 0.710371 0.708008 PS-14 0.15 0.512489 0.512296 -1.7 0.39 0.707397 0.706301 PS-17 0.14 0.512467 0.512286 -1.7 0.36 0.706937 0.705926 PS-20 0.15 0.512509 0.512311 -1.4 0.29 0.706914 0.706096 PS-21 0.15 0.512474 0.512275 -2.1 0.18 0.706960 0.706454 PS-23 0.16 0.512451 0.512242 -2.7 0.36 0.707408 0.706381 PS-24 0.16 0.512562 0.512348 -0.6 0.21 0.706817 0.706219 PS-04 Chilled

Margins 0.14 0.512044 0.511861 -10.1 2.77 0.721529 0.713646

PS-15 0.11 0.512108 0.511968 -8.1 2.27 0.722667 0.716224 PS-22 0.60 0.512537 0.511754 -12.3 0.34 0.707355 0.706378 PS-06 Sand-

stones 0.13 0.512104 0.511933 -8.7 0.91 0.714436 0.711853

PS-09 0.12 0.512105 0.511953 -8.4 2.36 0.721286 0.714571 PS-16 0.13 0.512138 0.511966 -8.1 0.55 0.714879 0.713314 PS-19 0.21 0.512338 0.512061 -6.3 1.08 0.717792 0.714725

233

Table 5.2 continued.

206Pb/ 204Pb(0)

207Pb/ 204Pb(0)

208Pb/ 204Pb(0)

238U/ 204Pb

235U/ 204Pb

232Th/ 204Pb

206Pb/ 204Pb(I)

207Pb/ 204Pb(I)

208Pb/ 204Pb(I)

PS-01 18.56 15.62 38.50 9.23 0.07 27.99 18.27 15.61 37.96 PS-02 18.55 15.70 40.20 5.77 0.04 25.03 18.37 15.69 39.72 PS-03 18.33 15.57 38.14 2.66 0.02 11.63 18.25 15.57 37.92 PS-05 18.33 15.64 38.26 2.13 0.02 8.88 18.26 15.64 38.09 PS-07 18.50 15.63 38.31 2.29 0.02 7.67 18.42 15.62 38.16 PS-08 18.39 15.65 39.15 2.63 0.02 12.04 18.31 15.64 38.92 PS-10A 18.52 15.63 40.05 5.49 0.04 24.06 18.35 15.62 39.59 PS-10B 18.42 15.65 39.99 2.12 0.02 8.29 18.35 15.65 39.83 PS-11 18.48 15.67 38.52 3.61 0.03 15.68 18.37 15.66 38.22 PS-12 18.24 15.62 38.14 1.25 0.01 3.36 18.20 15.62 38.08 PS-13 18.25 15.63 38.13 0.70 0.01 2.90 18.23 15.62 38.08 PS-14 18.43 15.69 38.53 4.53 0.03 19.36 18.29 15.68 38.16 PS-17 18.25 15.63 38.17 1.49 0.01 4.89 18.21 15.62 38.07 PS-20 18.22 15.58 38.03 3.30 0.02 14.85 18.11 15.57 37.74 PS-21 18.22 15.59 38.04 0.95 0.01 4.29 18.19 15.58 37.96 PS-23 18.73 15.64 38.37 1.22 0.01 5.10 18.69 15.64 38.27 PS-24 18.44 15.65 38.45 4.84 0.04 21.84 18.28 15.64 38.03 PS-04 18.76 15.65 39.00 5.54 0.04 34.99 18.59 15.64 38.33 PS-15 18.86 15.64 38.60 5.96 0.04 23.38 18.67 15.63 38.15 PS-22 18.54 15.62 38.44 2.71 0.02 11.83 18.45 15.62 38.22 PS-06 18.84 15.72 20.26 7.98 0.06 43.28 18.58 15.71 39.43 PS-09 18.85 15.78 40.18 8.19 0.06 87.68 18.59 15.77 38.50 PS-16 18.59 15.66 39.90 2.42 0.02 6.50 18.51 15.65 39.78 PS-19 18.61 15.65 38.72 4.19 0.03 34.37 18.48 15.64 38.06

234

mantle reservoirs (Zartman and Doe, 1981; Hart and Zindler, 1989). Here again, the

Palisade Sill basalts show affinity towards other CAMP related low-Ti rocks.

5.4. Discussion of Palisade Sill Geochemistry compared to existing high and low-

Ti CAMP from literature

The participation of a mantle plume as the main cause of the CAMP is a

matter of intense debate and there are many arguments that can be put forward to

either support (e.g. Ernst et al., 1995; Wilson, 1997) or reject (e.g. Bertrand, 1991;

McHone, 2000) the participation of a Central Atlantic Plume (CAP) in the

petrogenesis of the primary magmas. In this section we discuss the geochemical

results presented in section 5.3 and attempt to identify either a plume or a

subcontinental mantle lithospheric source for the Palisade Sill basalts as well as other

low-Ti CAMP tholeiites.

5.4.1. Discussion of Trace Element Geochemistry

The Chondrite normalized REE patterns of the basalts and the gabbros from

the Palisade Sill display only minor variability along the entire Sill (Fig. 5.2a). There

is slight LREE enrichment in these rocks which is typical of both high and low-Ti

CAMP tholeiites (e.g. Alibert, 1985; Dupuy et al., 1988; Bertrand, 1991; DeMin et

al., 2003). In particular, the REE patterns of the Palisade Sill basalts strongly

resemble the low-Ti tholeiites from Europe (Jourdan et al., 2003) and Brazil (DeMin

et al., 2003). The Palisade Sill basalts as well as other high and low-Ti CAMP

235

tholeiites are distinctly different from Central Atlantic MORB (Janney and Castillo,

1996) in their REE patterns and are likely derived from an enriched mantle. In

contrast to the restricted REE patterns of the Palisade basalts, the sandstones and

chilled margins have a much wider range of REEs (Fig. 5.2b) with a strong negative

Eu anomaly that is characteristic of continental crust. The sandstones of this study are

the likely contaminants of the Palisade Sill basalts. The chilled margins that formed

by the sudden cooling of the intrusive lava with the surrounding rock have REE

patterns similar to the sandstones (Fig. 5.2b) and likely experience nearly 100%

contamination by the sandstones.

On the multi-element primitive mantle normalized diagram the Palisade

basalts are flat with the exception of low Ba and very high Pb (Fig. 5.3a). There are

very small negative Nb-Ta and positive Zr-Ha anomalies in a few samples. Tholeiites

from French Guyana (Nomade et al., 2002) and Spain (Cebria et al., 2003) also show

similar patterns. The nearly flat HREE pattern implies an absence of garnet in the

source; the depth of melting for these rocks may have been in the field of stability of

spinel peridotite. These rocks were possibly derived from a mantle ~20-50 times

more enriched than the primitive mantle. The sandstones and chilled margins have

low Ba, Sr and Eu, negative Nb-Ta anomalies, and high Pb when normalized to the

primitive mantle (Fig. 5.3b). Notice that both Palisade Sill basalts as well as

sandstones show the enrichments and depletions in the same element, but the

magnitude of enrichment and depletion is higher in the Sandstones compared to the

Palisade basalts. This indicates that the Palisade Sill lavas to be contaminated by

236

these sandstones prior to or during emplacement. The chilled margins experienced

very large degrees of contamination by the sandstones and hence mimic their trace

element patterns (Figs. 5.2, 5.3).

Most continental flood basalts (CFBs) are characterized by both high and low-

Ti basalts (Saunders et al., 1992; Hawkesworth et al., 1999). The Palisade Sill basalts

appear to be derived from a source region similar to other low-Ti CAMP (e.g.

Pegram, 1990; Deckart et al., 1997; Jourdan et al., 2003) as well as other continental

flood basalts such as the Parana and (Hawkesworth et al., 1999; Nomade et al., 2002)

and in figure 5.4a. These basalts have similar La/Ba ratios and slightly higher La/Nb

ratios compared to ocean island basalts. They are distinctly different from the high-Ti

CAMP tholeiites from Guyana (Deckart et al., 1997) which lie entirely within the

ocean island basalt field and have relatively higher La/Ba ratios.

The systematic differences between the high and low-Ti CAMP tholeiites may

indicate that these two types of tholeiites have different mantle sources (e.g. Cebria et

al., 2003). However, it is also possible that both high and low-Ti tholeiites are derived

from an enriched mantle and the low-Ti tholeiites have been contaminated by the

continental lithosphere whereas the high-Ti tholeiites lie close to the primitive mantle

and may or may not have asthenospheric contamination. Another possibility is that

the high and low-Ti lavas of the CAMP provinces may have been derived by different

depths and degrees of melting of the same mantle.

Studies of Nb-Ta variation in MORBs, komatiites, depleted mantle xenoliths,

and chondritic meteorites (Jochum et al., 1986; Jochum et al., 1989) have suggested

237

that the Nb/Ta of the Earth’s mantle has a chondritic value of 17.5 and that there is no

significant fractionation of these elements, at least at large degrees of partial melting.

However, experimental studies (Green and Pearson, 1987; Green et al., 1989; Green,

1995) of the partitioning of these elements between various melt compositions and

potential residual phases in the mantle suggest that in certain circumstances this pair

of geochemically similar elements may be fractionated. These effects have been

modeled assuming primitive Nb and Ta concentrations, and the calculated likely

maximum variation of Nb concentration with Nb/Ta in the melt and the residue at

various degrees of batch meting in figure 5.4b.

At very low degrees of melting (<1%) Nb/Ta values may be similar to

leucites, but calculated Nb concentrations are much higher, and Zr/Nb values are

much lower than for leucites (Stolz et al., 1996). However, if a slightly depleted

mantle source is used as the starting composition, the calculated Nb/Ta values for the

melt are much lower (~14) due to the significant reduction of Nb/Ta in the residue

after removal of a very small melt fractions. Hence from figure 5.4b we can infer that

the low-Ti Palisade Sill basalts were derived from ~15% melting of a slightly

depleted peridotite. The high-Ti tholeiites may have been products of the initial small

melt fractions that left behind a slightly depleted peridotite source for the low-Ti

CAMP tholeiites.

The Palisade Sill basalts and most of the CAMP investigated throughout the

four circum-Atlantic continents are typically tholeiitic low-Ti continental flood

basalts, which differ fundamentally from MORB by higher concentrations of LREE

238

(Figs. 5.3-5.4) (e.g. Bertrand et al., 1982; Alibert, 1985; Dupuy et al., 1988; Bertrand,

1991; Puffer, 2002; Cebria et al., 2003; DeMin et al., 2003; Jourdan et al., 2003). On

the basis of our trace element data discussed above, the low-Ti Palisade Sill basalts

appear to have been derived by melting of a spinel lherzolite and are likely

contaminated by the continental crust represented by the sandstones and chilled

margin basalts of this study.

5.4.2. Discussion of Nd-Sr-Pb Geochemistry

Despite their apparent similarity in the normalized trace element diagrams, the

high and low-Ti CAMP magmatic rocks exhibit two distinct ranges of Sr-Nd

compositions. The enriched Nd-Sr composition of the Palisade basalts suggests

contamination by a more radiogenic component. Several studies have explained the

Nd-Sr isotopic composition of the low-Ti CAMP rocks by fractional crystallization of

an enriched lithospheric mantle coupled with assimilation of lower crustal granulites

(e.g. Cebria et al., 2003). One of the key questions concerning the Palisade Sill and in

general the CAMP giant dike swarms is to determine if the dikes are a result of lateral

injection of magmas radiating from a CAP or if they originate from other mantle

sources. Dike emplacement models confirm that magmas can travel laterally long

distance (>3000km) from their focal point (Elliot et al., 1999). Although the larger

number of CAMP related magmas with radiogenic Sr and negative εNd may suggest

the mantle lithosphere to be the main source of these rocks, the intermediate

composition of these low-Ti tholeiites between CAP and crust-like compositions

239

points towards continentally contaminated plume derived melts for these dikes and

sills.

The presence of a plume related mantle source component in the CAMP is

supported by data published for the oldest (160-120Ma) Atlantic Oceanic crust

(Janney and Castillo, 1996), which suggest the involvement of a plume type mantle

component during the early stages of the Central Atlantic opening. Similarly, the

primitive olivine dolerites and lamprophyre dikes from Nova Scotia, Canada, also

suggest the presence of plume sources in CAMP (Pe-Piper and Reynolds, 2000).

Furthermore, as we have shown in our trace element data the Palisade Sill basalts as

well as other low-Ti CAMP data support the participation of an OIB type mantle

source (Figs. 5.2, 5.3).

We use the age-corrected (200Ma) isotopic composition of the Central

Atlantic Plume (CAP) (Cebria et al., 2003) as the starting composition of the Palisade

Sill basalts. As observed from Fig. 5.5 the assumed CAP derived tholeiites

assimilated ~ 10-20% continental crust, as represented by the sandstone, to produce

the Palisade Sill basalts. Also note the absence of a Central Atlantic MORB like

contaminant in both the high and low-Ti tholeiites of the CAMP as well as the

Palisade Sill basalts of this study. Since the CAMP consists of intrusive rocks,

assimilation of mid-continental crust during emplacement and ponding of the magmas

in the crust is expected.

207Pb/204Pb(I) and 208Pb/204Pb(I) for the CAMP magmatic rocks correlate

positively with 206Pb/204Pb(I) defining a tight linear array subparallel to the NHRL but

240

displaced towards higher 207Pb/204Pb and 208Pb/204Pb (Fig. 5.6). Taken as a whole, all

the low-Ti CAMP data collectively define a single positive correlation displaced

vertically above the field of MORB and resembles OIB from localities such as

Gough, Reunion, and French Polynesia (Pegram, 1990, and references therein). Such

207Pb/204Pb traits are characteristics of the majority of continental flood basalts as

well.

The Palisade Sill basalts overlap almost entirely with other low-Ti tholeiites

associated with the CAMP (Fig. 5.6). Collectively the Palisade Sill basalts and low-Ti

CAMP tholeiites trend towards EM-II which is considered to be middle or upper

continental crust (Fig. 5.6a). There is no correspondence of these low-Ti lavas with

either the average lower crust or with EM-I which represents sublithopheric mantle

compositions. There is also no correspondence of these data with Grenville age

granulites from Honeybrooks Creek, Pennsylvania (Sinha et al., 1996), which is

representative of the lower crust-mantle lithosphere. Hence it is likely that the low-Ti

CAMP basalts were derived from a mantle plume and assimilated continental crust

during emplacement. The overlap of the high and low-Ti CAMP related lavas with

the Central Atlantic MORB in their Pb-Pb correlation (Fig. 5.6) is not unexpected as

the oldest Atlantic oceanic crust has experienced widespread plume contamination

(Janney and Castillo, 1996). The absence of a depleted asthenosphere for the CAMP

rocks and specifically for the Palisade Sill basalts is confirmed by their normalized

trace element patterns (Fig. 5.2, 5.3) as discussed previously.

241

Figure 5.5. Initial εNd vs. 87Sr/86Sr at 200 Ma for the Palisade sill basalts and gabbros

compared to global CAMP data and the Central Atlantic MORB (Janney and Castilo,

2001). All CAMP data from literature are corrected to 200Ma. References for CAMP

data from literature are as follows: Liberia (Dupey et al., 1988); eastern North

America (Peagram, 1990; Heatherington and Mueller, 1991); Spain (Alibert, 1985;

Cebria et al., 2003); Guinea and Guyana (Deckart et al., 2005); Brazil (DeMin et al.,

2003); France (Jourdan et al., 2003); Nova Scotia (Pe-Piper and Reynolds, 2000).

242

Figure 5.6. Initial 208Pb/204Pb(I) vs. 206Pb/204Pb(I) and (b) 207Pb/204Pb(I) vs. 206Pb/204Pb(I)

plots of the Palisade sill basalts and gabbros compared to global CAMP data and

Central Atlantic MORB. Grenville granulites are from Sinha et al., (1996). Also

shown are the domains of the BSE – Bulk silicate Earth; NHRL – Northern

Hemispheric Reference Line; and DM – Depleted mantle (Hart and Zindler, 1989);

Upper crust and lower continental crust (Zartman and Doe, 1981). All other data

sources and symbols are as in figure 5.5.

243

The relatively small variation in Nd-Sr-Pb isotopic compositions of the

Palisade basalts suggests that the amount of crustal material assimilated was not very

significant (Fig. 6.7). The trend of the Palisade basalts towards the sandstones, chilled

margin basalts and EM-II is clear in the variation of Nd-Pb (Fig. 6.7a). The absence

of an EM-I or a Central Atlantic MORB like component in the Palisade lavas as well

as the low-Ti CAMP basalts in Nb-Pb-Sr variations (Fig. 6.7) re-emphasize our

inference that these rocks are likely derived from a plume source that was

contaminated by the continental crust, rather than my the melting of a heterogeneous

mantle lithosphere due to plume impingement. The overall similarity of CAMP

tholeiites has been used to support the involvement of a common mantle source

(Marzoli et al., 1999). The inferences made for these dikes and Sills, when considered

individually, imply the contamination of the original magma by the local crustal

component (e.g. Bertrand et al., 1982; Dunn et al., 1998).

Geochemical characteristics common to all low-Ti CAMP related tholeiites

including the Palisade Sill basalts are slight LREE enrichment (Fig. 5.2, 5.3),

enriched Nd-Sr compositions, and positive correlation in the Pb-Pb plot (Fig. 5.6)

subparallel to the NHRL. These trace element and Nd-Sr-Pb isotopic signatures

cannot unequivocally discriminate between a heterogeneous subcontinental

lithosphere and a plume source contaminated by the continental crust. However, the

absence of any EM-I component in any of the low-Ti CAMP along with presence of

an EM-II like contaminant in indicative of the latter. Also, major element chemistry

of the Palisade Sill rocks indicate 50-55 weight % SiO2 in these rocks

244

Figure 5.7. (a) Initial εNd vs. 206Pb/204Pb(I) and (b) 87Sr/86Sr(I) vs. 206Pb/204Pb(I) for the

Palisade sill basalts and gabbros compared to global CAMP data and Central Atlantic

MORB. Data sources and symbols are as in figures 5.5 and 5.6.

245

(e.g. Shirley, 1987; Husch, 1992; Puffer, 2002) indicating contamination and

assimilation of a silica rich continental crust rather than derivation from a silica poor

mantle lithosphere.

5.5. Geodynamic implications of our geochemical data for a plume source for the

Palisade Sill basalts and other high and low-Ti CAMP lavas

The relative enrichment of these elements in CAMP has been long debated

and explained by different mechanisms: (1) derivation from an enriched

subcontinental lithospheric mantle source, with no or limited crustal contamination

during magma ascent (e.g. Bertrand et al., 1982; Alibert, 1985; Dupuy et al., 1988;

Pegram, 1990; Bertrand, 1991; Heatherington and Mueller, 1999; DeMin et al.,

2003); (2) derivation from an asthenospheric MORB-like source with a more

significant crustal contamination (Dupuy and Dostal, 1984; Dostal and Durning,

1998); and (3) derivation from an incipient plume head (White and McKenzie, 1989;

Hill, 1991; Oyarzun et al., 1997; Wilson, 1997; Courtillot et al., 1999; Ernst and

Buchan, 2002; Morgan, 19893). With respect to (3) several authors discuss the

geochemical composition of the possibly involved magmas (Oliviera et al., 1990;

Janney and Castillo, 1996; Cebria et al., 2003).

Our geochemical data show no evidence of a depleted asthenospheric MORB

source for the CAMP derived dikes and sills. Derivation from an enriched

subcontinental lithosphere requires lithospheric thinning to drive decompressional

melting (White and McKenzie, 1989), which is unlikely to produce effusive, rapid

246

volcanism. In plume models, mantle material originates at or near the core-mantle

boundary and rises buoyantly through the mantle (e.g. Hill, 1991), spreading laterally

as the plume head encounters the thin eroded lithosphere and producing voluminous

melt. Is it possible that all the CAMP (6000 km) is a result of a single process:

melting of a heterogeneous lithospheric mantle initiated by plume impingement? A

more simple answer is to attribute the CAMP to a hotspot system. Geochronological

data show that most parts of the investigated CAMP were active in the early Jurassic

(~200Ma) (Deckart et al., 1997; Marzoli et al., 1999; Hames et al., 2000). This time

coincidence of the CAMP age argues for one plume for all the CAMP magmatism.

The brief and extremely widespread tholeiitic magmatism associated with the CAMP

implies that an anomalously hot mantle extended over a very wide area and melted

extensively. In general, our data and previous geochemical and geochronologial data

on CAMP are consistent with models that suggest that an upwelling plume head

separated from the plume tail (Leitch et al., 1998) and that the plume material spread

over a very large area by ambient mantle flux (Wilson, 1997).

The CAMP can be compared to the East African Rift System (EARS) which is

a classic example of ongoing continental rifting and provides an excellent framework

to investigate magmatism in an extensional setting. The EARS rift system extends

over 4000 km from the Red Sea in the north to Mozambique in the south representing

a ∼150 km wide zone of NW–SE trending extension (Chakrabarti et al., 2009). The

seismically and volcanically active EARS is the youngest mantle plume province

worldwide, with one or more upwellings impinging on thick cratonic lithosphere

247

since ~45 Ma (e.g. WoldeGabriel et al., 1990; Furman et al., 2006) caused by an

anomalously hot asthenosphere (Ebinger and Furman, 2003; Furman et al., 2004).

Discrete rifting episodes have recently been observed in the Afar triple junction in

Ethiopia. In this region batches of molten mantle rocks have risen into cracks and

fractures to form long, thin vertical sheets of new crust in the form of dikes, often

feeding surface eruptions of basalts. These dikes serve to transport melt percolating

upward from mantle source zones and also had accumulated in magma chambers or

thin horizontal magma sheets within the crust. The dikes along with faults constitute

plate boundary separation within the crust in this region (Barberi and Varet, 1977;

Hayward and Ebinger, 1996). Tomography and seismic images (Nyblade and

Robinson, 1994; Weeraratne et al., 2003), numerical models (Beutel et al., 2010), and

geochemical data (Marty et al., 1993; Furman et al., 2006; Pik et al., 2006;

Chakrabarti et al., 2009) collectively support a single plume with multiple stems

originating in this African super-plume.

The CAMP was derived from a compositionally heterogeneous super-plume

similar to the present day EARS, with multiple stems acting as feeders over a large

aerial extent.

5.6. Conclusions

In this study we have correlated the Palisade Sill basalts and gabbros to other

low-Ti CAMP related magmatism from eastern North America (Pegram, 1990;

Heatherington and Mueller, 1999), South America (DeMin et al., 2003), Europe

248

(Alibert, 1985; Cebria et al., 2003; Jourdan et al., 2003), West Africa (Deckart et al.,

1997), and Canada (Pe-Piper and Reynolds, 2000) by their geochemical and Nd-Sr-

Pb isotopic signatures. When compared to the small volume high-Ti CAMP related

magmatism, the low-Ti CAMP lavas have a more radiogenic Sr and less radiogenic

Nd component as well as higher values of 207PPb and 208Pb. Based on the

geochemical data presented in this study the Palisade Sill basalts were derived from a

slightly enriched OIB-like mantle source (Fig. 5.2, 5.3, 5.4a). Further, these rocks

were derived from ~15% (Fig. 5.4b) melting of a slightly depleted spinel peridotite

(Fig. 5.4c). Since other low-Ti CAMP lavas have similar geochemistry as well as

eruption ages as the Palisade Sill basalts of this study, it is safe to assume the same

source for these tholeiites across the four continents where they are emplaced.

Although the collective trace element and Nd-Sr-Pb isotopic signatures of

low-Ti CAMP magmatism cannot unequivocally discriminate between a

heterogeneous subcontinental lithosphere and a plume source contaminated by the

continental lithosphere, we argue for the latter based on the short time of eruption of

all these lavas as well as the absence of an EM-I like component in the low-Ti lavas.

We suggest that the CAMP was derived from a compositionally

heterogeneous super-plume similar to the present day East African Rift System, with

multiple stems acting as feeders over a large aerial extent. Compositional

heterogeneity may have been caused due to variability in lithosphere-asthenosphere

boundary, or due to different degrees and depths of melting as has been suggested for

the heterogeneous basalts derived from the EARS (Chakrabarti et al., 2009).

249

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