Int J Earth Sci (Geol Rundsch) DOI 10.1007/s00531-007-0253-4
ORIGINAL PAPER
Geochemical constraints on the origin of the late Jurassic proto-Caribbean oceanic crust in Hispaniola J. Escuder Viruete Æ A. Pe´rez-Estau´n Æ D. Weis
Received: 25 January 2007 / Accepted: 12 September 2007 Springer-Verlag 2007
Abstract The nature of the oceanic crust produced through rifting and oceanic spreading between North and South America during the Late Jurassic is a key element for the Caribbean plate tectonic model reconstruction. Located in the Cordillera Central of Hispaniola, the Loma La Monja volcano-plutonic assemblage (LMA) is composed of gabbros, dolerites, basalts, and oceanic sediments, as well as metamorphic equivalents, which represent a dismembered fragment of this proto-Caribbean oceanic crust. Petrologic and geochemical data show that the LMA have a relatively broad diversity in composition, which represent the crystallization products of a typical low-pressure tholeiitic fractionation of mid-ocean ridge basalts (MORB)-type parental magmas, ranging from Nto E-MORB. Three geochemical groups have been distinguished in the volcanic sequence: LREE-flat to slightly
J. E. Viruete Instituto Geolo´gico y Minero de Espan˜a, C. Rı´os Rosas 23, 28003 Madrid, Spain A. Pe´rez-Estau´n I.C.T. Jaume Almera-CSIC, Lluı´s Sole´ i Sabarı´s s/n, 08028 Barcelona, Spain
LREE-enriched basalts of groups II and III occur interlayered in the lower stratigraphic levels; and LREEdepleted basalts of group I in the upper levels. Mantle melt modeling suggests that group III magmas are consistent by mixing within a mantle melt column of lowdegree (\1%) melts of a deep garnet lherzolite source and high-degree ([15%) melts of a shallow spinel source, and groups II and I magmas are explained with moderate to high (14–18%) and very high ([20%) fractional melting degrees of a shallower spinel mantle source, respectively. Thus, upward in the volcanic sequence of the LMA, the magmas represent progressively more extensive melting of shallower sources, in a plume-influenced spreading ridge of the proto-Caribbean oceanic crust. Nb/Y versus Zr/Y systematics combined with recent plate tectonic model reconstructions reveal that Caribbean Colombian oceanic plateau fragments in Hispaniola formed through melting of heterogeneous mantle source regions related with distinct plumes during at least from Aptian–Albian ([96 Ma) to Late Campanian. Keywords Spreading ridge Mantle plume Mantle melting Oceanic crust Hispaniola Caribbean plate
Introduction D. Weis Pacific Centre for Isotopic and Geochemical Research, University of British Columbia, 6339 Stores Road, V6T 1Z4 Vancouver, BC, Canada J. E. Viruete (&) Instituto Geolo´gico y Minero de Espan˜a, Area de Geologı´a y Geofı´sica, C. La Calera 1, 28760 Tres Cantos, Madrid, Spain e-mail:
[email protected] URL: www.igme.es
The Caribbean region consists of a rim of CretaceousRecent arc terranes and associated back-arc basins molded about a core consisting of a continental fragment in the western Caribbean, or Chortı´s block, and a large igneous province beneath the central and eastern Caribbean (Mann 1999). In the Chortı´s block, a continental metamorphic basement of Paleozoic age is unconformably overlain by
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Int J Earth Sci (Geol Rundsch)
Middle Jurassic through Early Cretaceous rift-related clastic rocks and Aptian–Albian shallow marine limestones (Pindell et al. 2005). The large igneous province is part of the Cretaceous Caribbean-Colombian oceanic plateau (CCOP), which is now exposed in and around the Caribbean plate, the Pacific coast of Central America and western Colombia (Spadea et al. 1989; Donnelly et al. 1990; Kerr et al. 1997; Sinton et al. 1998; Mann 1999; Hoernle et al. 2004; Pindell et al. 2006). The CCOP consists of volcanic rocks erupted during three phases of broadly different age, which include (compiled by Kerr 2003): 124–112 Ma (Aptian), a 91–83 Ma (the most voluminous), and a 78–72 Ma. Using the more recent plate reconstruction models of Pindell et al. (2005, 2006), rift basins of Late Jurassic age in the Caribbean region form part of a band of rifts associated with the early opening of the central and northern Atlantic. The widening gap between North and South America was presumably occupied by oceanic crust generated at a proto-Caribbean spreading ridge. This Oxfordian early oceanic corridor between the Atlantic and Pacific widens through continued rifting and oceanic spreading into the Tithonian (Mann 1999). The ocean crust that underlies the central Gulf of Mexico and the outcroping of mid-ocean ridge basalts (MORB)-type pillow lavas in western Cuba support these reconstructions. In the Duarte Complex of central Hispaniola, an interpreted fragment of the CCOP, Lapierre et al. (1999) describe basalts associated with Late Jurassic ribbon cherts and dolerites, whose major, trace element and low Pb–Pb isotopic ratios are consistent with those of MORB. Following Lapierre et al. (1999), these rocks represent the remnants of the proto-Caribbean oceanic crust, formed around 150 Ma from an oceanic ridge possibly close to a hot spot. Therefore, these rocks offer an unique opportunity to study on land the nature of the Late Jurassic oceanic crust of the proto-Caribbean, which is a key element for the Caribbean plate tectonic reconstructions, and their relation with the plume-related magmas that later intruded and extruded over forming the deep levels of the CCOP. In this paper, we present new regional petrologic, geochemical and Sr–Nd isotopic data on the igneous rocks and metamorphic equivalents of the Loma La Monja volcanoplutonic assemblage (LMA), recently investigated throughout the Cordillera Central, during mapping of the Dominican Republic funded by the EU (SYSMIN Project). We argue that these oceanic rocks are genetically related and record the magmatic processes in a Late Jurassic plume-influenced spreading ridge. We also discuss the implications that provide about the origin of the protoCaribbean crust and the relations of this assemblage with other plume-derived units present in Hispaniola.
123
Geologic setting of the Central Cordillera of Hispaniola Located in the northern margin of the Caribbean plate, the tectonic collage of Hispaniola results from the WSW to SW-directed oblique-convergence of the continental margin of the North American plate with the Cretaceous Caribbean island-arc system, which began in Eocene to Early Miocene times and continues today (Donnelly et al. 1990; Draper et al. 1994; Mann 1999; Escuder Viruete et al. 2006). The arc-related rocks are regionally overlain by Upper Eocene to Holocene siliciclastic and carbonate sedimentary rocks that post-date island-arc activity, and record the oblique arc-continent collision in the north, as well as the active subduction in the southern Hispaniola margin (Dolan et al. 1998; Mann 1999). Central Hispaniola is a composite of oceanic derived units bound by the left-lateral strike-slip Hispaniola and San Juan-Restauracio´n fault zones (Fig. 1). In the study area of Cordillera Central, five lithostratigraphic and geochemical units have been mapped (Figs. 2, 3): the Loma Caribe serpentinized peridotite; the LMA; the El Aguacate Chert; the Duarte Complex; and the Tireo Group. All these units underwent variable deformation and low-grade metamorphism. The Loma Caribe serpentinized peridotite consists of serpentinized harzburgite, dunite, and lherzolite, with dykes and tectonic blocks of gabbro and dolerite (Fig. 4a). The El Aguacate Chert consists of about 150-m thick sequence of ribbon chert with locally interlayered limestone (Fig. 4d). Radiolarian microfauna of Pacific affinity provided an Oxfordian to Tithonian age (Montgomery et al. 1994). The Duarte Complex comprises a *3-km thick sequence of picrites and high-Mg basalts of [96 Ma age (probably Aptian), chemically related to plume-generated magmas (Draper et al. 1994; Lewis et al. 2002) and similar to the more enriched CCOP lavas (Lapierre et al. 1997, 1999; Escuder Viruete et al. 2007a). Duarte Complex-like mafic dykes cut the LMA and the El Aguacate Chert. The Tireo Group consists of [3 km thick sequence of arc-related volcanic, sub-volcanic, and sedimentary rocks of Cenomanian to Maastrichtian age (Lewis et al. 2002; Escuder Viruete et al. 2004, 2007b).
The Loma La Monja volcano-plutonic assemblage: field relationships and petrography The LMA consists of a \3-km thick sequence of isotropic gabbros, minor cumulate-layered olivine gabbros, dolerites, and massive basalts, that grade upward into pillow lavas, hyaloclastites, and are overlain by phyric basalts and deep-marine sediments below the El Aguacate Chert (Fig. 3). In the La Vega area, these rocks form several
Int J Earth Sci (Geol Rundsch) Fig. 1 Geological map of Central Hispaniola. SFZ Septentrional fault zone, HFZ Hispaniola fault zone, BGFZ Bonao-La Gua´cara fault zone, SJRFZ San Juan-Restauracio´n fault zone, EPGFZ EnriquilloPlantain Garden fault zone. Box show location of the Fig. 2
Fig. 2 a Geological map of the SW La Vega area, Cordillera Central. Text labels in boxes show the locations of La Monja volcano-plutonic assemblage samples, and stars the situations of amphibolites of the Rı´o Baiguaque shear zone and obtained ages for regional shearing by Escuder Viruete et al. (2007a)
WNW-trending fault bounded blocks, distributed south of the Hispaniola fault.
Gabbros Gabbros forming lenticular bodies about 0.5 km long and 100 m thick, except for a much larger one (1.5 km long and 350 m thick) located west Los Velazquitos. The gabbroic bodies are surrounded or cut by strike-slip shear
zones showing a marked decrease in grain size and transformed into S-L amphibolites. The lower contact to the serpentinized peridotites is tectonic and actually strongly overprinted by retrograde shear deformation. Internally, the bodies are generally characterized by the absence of magmatic layering and of systematic vertical changes (Fig. 4b), but show a relatively high diversity in composition, from primitive Mg-rich olivine gabbro to evolved Fe–Ti gabbro. The Mg-rich gabbro part of the bodies (\20%) is of coarse- to medium-grained, equigranular
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Int J Earth Sci (Geol Rundsch)
olivine, amphibole, Ti-magnetite, ilmenite, and apatite. In the evolved gabbros, amphibole is hastingsitic hornblende, Ti-rich pargasite or hornblende, and occurs as interstitial grains between plagioclase and clinopyroxene or may form rims around or patches within clinopyroxene. Most of the amphibole was formed as alteration products of clinopyroxene, being often the only Fe–Mg silicate in the rock. Plagioclase is often altered to a fine-grained mixture of albite, chlorite, epidote, prehnite, and pumpellyite. Gabbros are cut by dolerite and basaltic dykes without chilled margins indicating that emplacement of the dykes took place during cooling of the gabbros.
Dolerite dykes Sub-volcanic rocks consist of 0.5–1 km thick sequence of isotropic dolerites, that grade upward into extrusive basaltic rocks (Fig. 3). Dolerite and minor microgabbro occur as [250 m long and about 100 m thick lenticular bodies, overlying the Loma Caribe serpentinized peridotites throughout a fault contact. Also, individual dolerite dykes cutting across serpentinized peridotites. As gabbros, dolerites range in composition from Mg-rich dolerites to evolved Fe–Ti dolerites. They are massive and show finegrained intersertal to intergranular and sub-ophitic textures, with euhedral plagioclase (40–50%), interstitial clinopyroxene (30–40%), and round patches of chlorite which may represent former olivine (rarely fresh). In the Fe–Ti dolerites, interstitial brown hornblende and Ti-magnetite (5– 10%) occur as well as acicular grains of apatite. All dolerites have undergone prehnite–pumpellyite to low-T greenschist facies hydrothermal metamorphism, resulting in cloudy plagioclase and replacement of mafic minerals by chlorite ± actinolite. In addition, Fe3+-rich epidote, chlorite, albite, pumpellyte, quartz, and calcite locally fill a network of veins. The microgabbro dykes are mediumgrained with ophitic/sub-ophitic textures and composed mainly of plagioclase and clinopyroxene.
Fig. 3 Lithostratigraphic relationships between geological units in the Cordillera Central, Dominican Republic. In the extrusive sequence of the Loma La Monja volcano-plutonic assemblage, I–III are the geochemical groups defined in the text
texture, with subhedral olivine, euhedral plagioclase, and sub- to anhedral clinopyroxene as the main constituents, and accessory Cr-spinel (inclusions in Ol). Mg-rich gabbro grades from medium- to fine-grained clinopyroxene–plagioclase gabbro and Fe–Ti gabbro, with plagioclase and intergranular clinopyroxene, and variable amounts of
123
Volcanic rocks Volcanic rocks occur as massive basaltic flows, pillow lavas, breccias, hyaloclastites, and basaltic feeder dykes, showing chilled margins against the country rocks. Pillow lavas and hyaloclastites form a 50 to 150-m thick sequence (Fig. 4c), which is overlain by Fe–Ti basalts, tuffaceous sediments, siltstones, and shales below the El Aguacate Chert (Fig. 3). The basaltic flows are nonvesicular and contain microphenocrysts of plagioclase, clinopyroxene, and less frequently olivine, though they are generally aphyric. The textures are generally fine-grained
Int J Earth Sci (Geol Rundsch) Fig. 4 a Field-aspect of the Loma Caribe serpentinized peridotite with sheared doleritic dykes; b Massive gabbro affected by a network of epidote–calcite veins from the Loma La Monja volcanoplutonic assemblage; c Pillowlava basalts of the Loma La Monja volcano-plutonic assemblage; d Folded chertribbons of the El Aguacate Fm
intergranular and intersectal, but also include sub-ophitic/ ophitic types. Compositionally, they grade from plagioclase–clinopyroxene tholeiitic basalts with some high-Mg olivine basalts, to highly evolved tholeiitic Fe–Ti basalts. High-Mg basalts are characterized by abundance of euhedral skeletal olivine phenocrysts and accessory Crspinel in a mesostasis rich in Fe and Ti oxides. Fe–Ti basalts contain between 3 and 15% modal of Ti-magnetite, plus ilmenite, apatite, and often Fe–Cu sulphides. Pillows are generally \1 m in diameter and lack amygdales, but sometimes display radiating fractures. They have microphenocrysts of clinopyroxene and olivine in a groundmass of needle-like plagioclase, clinopyroxene, and probably interstitial devitrified glass. Pillows rims are aphyric and show variolitic to arborescent textures. A dark hyaloclastite breccia occurs as material interpillow. Low-grade alteration is widespread in all basalts, which contain albite pseudomorphs after plagioclase, chloritized clinopyroxene and olivine, and a recrystallized ground-mass of finegrained albite, chlorite, pumpellyite, epidote, sericite, and calcite.
Amphibolites Along sub-vertical shear zones a metamorphism was developed in the LMA rocks synchronous to the Late Cretaceous regional deformation (Escuder Viruete et al. 2006). The resulting amphibolites have a mineral assemblage in the foliation planes composed of actinolite + albite + epidote + chlorite or hornblende + oligoclase + epidote (+white mica ± sphene ± quartz), which indicate a syn-kinematic
metamorphism in the low-P greenschist to amphibolite facies conditions. Foliated amphibolites of the Duarte Complex yield 40 Ar–39Ar hornblende plateau ages of 93.9 ± 1.4 and 95.8 ± 1.9 Ma (Fig. 2) that demonstrate an older age of the protholiths, probably Aptian ([96 Ma).
Geochemistry Analytical procedures Samples were powdered in an agate mill, and analyzed for major oxides and trace elements by inductively coupled plasma-mass spectrometry (ICP-MS) analysis with a LiBO2 fusion. This analytical work was done at ACME Analytical Laboratories Ltd., in Vancouver and results for selected LMA samples reported in Table 1. For major elements oxides, the detection limits are \0.01% except for K2O (0.04%), SiO2 (0.02%), and Al2O3 (0.03%). The detection limits for trace elements are typically \0.1 ppm, except for Ba, Ce, La, Ga, and Zr (0.5 ppm); for some trace elements, they are as low as 0.05 ppm. Analytical accuracy and reproducibility are estimated from measurements of international rock standards SO17/CSB and duplicate analyses of samples. The accurancy of the standards is within ±10% of the working values, but generally better than ±4% for rare earth elements (REE). Duplicate analyses show reproducibility to be better than 2.5% except for the transition metals (e.g., Cr, Ni, Cu, and Zn) and elements with very low concentrations (e.g., Tl), which show deviations of up to 20%.
123
123
44.0
0.12
2.5
0.2
1.48
3.5
0.1
0.60
31.0
4.1
1.47
38.40
1.2
0.47
1.30
Nb
Ta
La
Ce
Pb
Pr
Sr
Nd
Sm
Zr
Hf
Eu
Gd
428
V
Th
59
Ni
Ba
46
Co
0.21
32
Cr
Rb
3.0
100.0
Total
44
0.04
Cr2O3
Mg#c
0.21
LOI
0.10
MnO
11.22
CaO
P2O5
5.56
MgO
1.54
14.42
Fe2O3
0.02
14.00
Al2O3
Na2O
1.31
TiO2
K2O
51.58
SiO2
2J100b
1.60
0.54
1.4
48.25
1.24
3.5
35.2
0.74
0.1
4.5
1.82
0.2
3.4
0.15
48.0
0.2
67
53
48
45
2.4
100.0
0.05
0.20
0.10
0.03
1.64
10.53
5.42
14.60
13.81
1.41
52.20
5J99
PIL
Sample wt.%b
Rocka
PIL
335130
2119300
321371
2127817
I
Y(UTM)
I
X(UTM)
Group
1.89
0.55
0.8
27.5
1.30
3.7
102.4
0.77
0.2
5
1.9
0.1
2
0.1
97.6
11.8
245.0
161.4
48.5
649.9
68
3.9
100.0
0.10
0.14
0.04
0.52
2.00
10.92
10.54
9.74
14.42
0.68
50.89
5je32
MGAB
2125088
328225
II
2.39
0.66
1.3
38.1
1.54
5.0
79
0.97
0.5
6.1
2.3
0.2
2.6
0.2
35.0
7.6
289.0
10.7
48.5
396.8
63
0.9
100.0
0.06
0.22
0.08
0.08
2.03
12.12
9.38
10.96
14.10
0.90
50.06
FC9103
AMPH
2143504
265475
II
2.92
0.79
1.4
48.1
2.20
5.4
116.3
1.19
0.1
8.8
2.7
0.2
4
0.2
62.4
2.5
330.0
117.0
46.3
335.2
59
1.2
100.0
0.05
0.17
0.11
0.13
2.15
11.77
8.43
11.53
14.10
1.04
50.51
JE9076
BAS
2151553
248664
II
3.43
0.89
1.8
53
2.40
6.9
116
1.29
0.6
8.2
3.4
0.2
3.4
0.2
54.6
0.7
355.0
16.5
44.2
143.7
51
1.4
100.0
0.02
0.20
0.07
0.13
2.28
10.62
6.95
13.33
13.59
1.16
51.65
5JE56
FBAS
2127989
316503
II
4.85
1.32
3.0
79
3.70
11.1
82
2.20
10.0
12.6
4.9
0.025
4
0.5
55.0
3.0
364.0
96.0
46.0
127.0
45
1.65
100.0
0.02
0.19
0.16
0.27
2.31
10.17
6.28
15.33
13.13
1.62
50.52
02J103
FBAS
2127789
322374
II
4.12
1.14
3.0
71
3.35
9.7
116
1.94
10.0
11.4
4.7
0.025
4
0.5
49.0
2.6
921.0
89.0
58.0
63.0
44
0.55
100.0
0.01
0.21
0.11
0.24
2.72
8.61
6.85
17.46
13.32
2.23
48.25
02J96
FBAS
2122375
320155
II
Table 1 Major and trace element data for the Loma La Monja volcano-plutonic assemblage
4.91
1.27
3.0
80
4.00
10.6
145
1.99
10.0
10.9
3.7
0.025
5
0.6
31.0
1.2
431.0
26.0
48.0
143.0
30
0.4
100.0
0.02
0.25
0.14
0.13
3.61
6.39
3.95
17.93
12.87
2.18
52.52
02J14
AMPH
2079900
361700
II
2.01
0.6
1.3
34.3
1.60
4.5
75.2
0.88
0.7
5.9
2.50
0.1
2.8
0.08
21.5
0.3
288.0
169.7
49.2
581.5
66
3.5
100.0
0.09
0.18
0.05
0.04
2.06
11.01
11.43
11.52
13.98
0.84
48.80
5je01
BAS
2119200
335100
III
2.52
0.66
1.2
39
1.80
5.5
145.8
0.97
0.5
7.2
2.8
0.2
3.1
0.09
69.0
3.4
292.0
88.3
44.2
369.4
62
2.7
100.0
0.06
0.16
0.08
0.34
4.12
8.46
9.09
10.92
13.92
0.99
51.86
5JE40a
DOL
2125262
323655
III
2.5
0.7
1.1
37.9
1.80
5.4
97.1
0.98
0.4
6.6
2.5
0.1
3
-
96.2
0.5
295.0
41.9
36.5
260.0
62
1.8
100.0
0.04
0.16
0.07
0.07
1.95
12.82
8.68
10.66
14.74
0.94
49.87
5je50
DOL
2132285
314473
III
3.72
1.07
1.9
66.6
2.80
8.8
113.3
1.71
0.4
11.8
4.9
0.3
5.4
0.4
83.4
6.2
402.0
50.9
45.1
123.1
56
2.6
100.0
0.02
0.20
0.10
0.61
3.63
8.01
8.60
13.50
14.00
1.44
49.89
5je38a
PIL
2125152
323689
III
3.56
0.84
1.9
58
2.40
8.2
112.7
1.56
0.4
10.3
4.3
0.3
4.5
0.2
38.2
3.3
379.0
55.3
44.3
205.2
56
2.5
100.0
0.03
0.16
0.10
0.34
3.62
9.06
7.87
12.11
14.84
1.29
50.58
5je38b
PIL
2125152
323689
III
4.11
1.07
2.3
69.1
3.10
10.6
220
1.87
0.6
12.8
5.1
0.4
5.7
0.6
50.6
6.7
466.0
32.8
46.5
54.7
45
2.6
100.0
0.01
0.20
0.11
0.45
4.14
8.73
6.13
14.73
13.09
1.60
50.80
5je41
FGAB
2125280
323630
III
4.27
1.09
1.9
67.1
3.00
9.3
108.7
1.75
0.2
12.3
5.1
0.3
5.5
0.4
17.0
473.0
27.9
47.0
61.6
43
1.9
100.0
0.01
0.21
0.11
0.04
2.39
10.13
5.94
15.57
13.49
1.58
50.52
5je42
DOL
2131259
317087
III
Int J Earth Sci (Geol Rundsch)
1.07
1.22
3.11
0.48
0.48
3.11
1.11
4.91
30.0
A representative sub-set of samples (Table 2) was also analyzed for Sr and Nd isotopic compositions at the Pacific Centre for Isotopic and Geochemical Research at the University of British Columbia. Rb, Sr, Sm, and Nd were re-analyzed with a Thermo Finnigan Element2, a double focussing (i.e., high resolution) ICP-MS. Separation of Sr and Nd was done following the method described in Weis and Frey (2002). Samples were repeatedly leached with HCl6N to remove effects of secondary alteration (Weis et al. 2006). Isotope ratios measurements were carried out on a Thermo Finnigan Triton-TI TIMS in static mode with relay matrix rotation on single Ta filament and double Re–Ta filament for Sr and Nd isotopic analyses, respectively. Sample Sr and Nd isotopic compositions were corrected for fractionation using 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219. During the course of analyses, La Jolla Nd standard gave an average value of 0.511851 ± 0.000008 (n = 3) and the SRM987 standard gave an average of 0.710243 ± 0.000025 (n = 7). 147Sm/144Nd ratio errors are *1.5%, or *0.006.
1.10
1.07
0.52
3.14
0.46
3.16
1.11
32.2
4.84
1.04
1.17
0.42
2.56
0.38
2.57
0.92
25.9
3.85
0.26
1.24
1.07
0.66
0.78
0.99 1.16
1.08
0.5
1.08
0.98
0.59
0.98
1.10
0.39
1.10
1.12
0.34
0.97
0.88
0.3
1.05
1.15
0.21
1.38
0.71
1.12
0.31
1.87
1.35
0.22
1.54
0.25
1.13
0.69
Er
Tm
Yb
Lu
(La/ Nd)N
(Sm/ Yb)N
Major oxides recalculated to an anhydrous basis
1.66 4.5 3.2 3.8 2.72 2.23 1.95
0.27 0.23 0.28
0.45 Ho
Mg# = 100 · mol MgO/mol (FeO + MgO); for Fe2O3/FeO = 0.2
0.25 0.66 0.47 0.59 0.43 0.32
0.77
2.06 1.80
0.66 0.47
1.36 1.66
0.55
11.5 Y
Chemical changes due to alteration and tectonometamorphism
c
1.63 4.31 2.64
0.91
26.4 20.9 17.4 14.2 14.5
1.94 Dy
Rock type abbreviations: PIL pillow lava basalt, MGAB microgabbro, AMPH amphibolite (metabasalt), BAS basalt, FBAS Fe–Ti basalt, DOL dolerite
0.98
1.14
0.28
2.03
0.27
1.85
0.67 0.58 1.45 1.13
3.33 3.92
1.32
31.0
0.93
6.04 3.98
0.65 0.48
3.57 3.07
0.43 0.32
2.13 2.38
0.34 0.27 Tb
Results
b
1.12
1.03
0.3
1.78
0.29
1.88
0.66
19.0 18.7 16.8 31.0 27.0
5.02
0.8
II II II II II II II I I Group
Table 1 continued
a
1.08
1.24
0.42
2.87
0.44
2.70
1.02
26.6
4.24 2.92 2.88
0.41
2.52
0.98
III
6.44
III
0.44
III
0.44
III
0.69
III
0.63
III
0.79
III
0.72
Int J Earth Sci (Geol Rundsch)
Loma La Monja volcano-plutonic assemblage rocks have been variably altered, deformed, and metamorphosed. Consequently, changes of the bulk-rock chemistry are expected as a consequence of selected mobility of relevant elements. It is known that some major (e.g., Si, Na, K, and Ca) and trace (e.g., Cs, Rb, Ba, and Sr) elements are easily mobilized by late and/or post-magmatic processes involving fluids and during metamorphism. However, the concentration of the high field-strength elements (HFSE) Y, Zr, Hf, Ti, Nb, and Ta, the REE, the transition elements V, Cr, Ni, and Sc, the large ion lithophile element (LILE) Th, and the Nd isotopic composition are generally unchanged under a wide range of metamorphic conditions, including seafloor alteration (Bienvenu et al. 1990). In the LMA samples, relatively immobile trace elements exhibit a good correlation of Zr with Th, Nb, La, Sm, Ti, and Yb (diagrams not shown), with small differences of interelement ratios reflecting primary features that are discussed below. Thus, the discussion on the petrogenesis and tectonic setting of the LMA igneous rocks is based mostly on the REE and HFSE geochemistry, as well as the Nd isotopes systematics, as we can assume that they were not significantly affected by seafloor alteration and tectonometamorphism at whole-rock scale.
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Int J Earth Sci (Geol Rundsch) Table 2 Sr–Nd isotope ratios for representative rock groups of the Loma La Monja volcano-plutonic assemblage 87
(87Sr/86Sr)i
Sm
31.2
0.703663 (7)
0.703603
1.47
33.1
0.703689 (9)
0.703638
1.3
7.6
79.2
0.704329 (7)
0.703694
5JE38b
3.3
112.7
0.704796 (8)
5JE40a
3.4
145.8
0.705670 (9)
5JE41
6.7
220.0
0.705958 (7)
Type
Sample
Rb
I
2JE100a
0.20
I
2JE100b
0.21
II
FC9103
III III III
Sr
Sr/86Sr
143
(143Nd/144Nd)i
(eNd)i
4.1
0.513010 (6)
0.512782
6.83
3.5
0.513002 (6)
0.512768
6.56
1.5
5.0
0.513056 (5)
0.512858
8.32
0.704603
2.4
8.2
0.512994 (6)
0.512809
7.35
0.705517
1.8
5.5
0.513000 (5)
0.512793
7.04
0.705758
3.1
10.6
0.512997 (7)
0.512812
7.41
Nd
Nd/144Nd
Rocks: 2JE100a and 2JE100b, pillow lavas basalts; FC9103, amphibolite; 5JE38b, pillow lava basalt; 5JE40a, dolerite; and 5JE41, fine-grained Fe–Ti gabbro. Calculated initial ratios (i) and eSr- and eNd-values calculated at t = 160 Ma. Number in brackets is the absolute 2r error in the last decimal places. eNd-values are relative to 143Nd/144Nd = 0.512638 and 147Sm/144Nd = 0.1966 for present day CHUR and lambda 147Sm = 6.54 · 10–12 per year
Major and trace elements The igneous rocks of the LMA have a restricted range in SiO2 content, ranging from 49.9 to 53.4 wt.% SiO2, with some slightly lower value in foliated amphibolites (Table 1). The more evolved rocks are Fe–Ti basalts with more than 14 wt.% FeOT, enriched in Ti and V, and with lower MgO (4.0–6.3 wt.%). On the basis of immobile trace elements classification schemes, as the Nb/Y versus Zr/TiO2 diagram (Winchester and Floyd 1977), the samples of the LMA cluster in the sub-alkaline basalt and andesite basalts fields. Selected compatible and incompatible major and trace elements are plotted against MgO, which show (not all in Fig. 5) an increase of SiO2, Fe2O3T, alkalis, TiO2, Zr, and Nb; and a decrease in Cr and Ni for decreasing MgO. Al2O3 and CaO increase slightly to reach a maximum at about 7–8 wt.% MgO, then decrease in the evolved basalts. These trends are tholeiitic and can be attributed to the low-pressure fractionation and/or accumulation of olivine plus Cr-spinel, plagioclase, and clinopyroxene (Tribuzio et al. 2000), which is compatible with the igneous mineralogy. However, the TiO2–MgO variation shows at least two distinct trends of relative low- and high-Ti samples, related not only to mineral fractionation/accumulation, but also to distinct, source related, geochemical features of the magmas (see below).
Chemical classification To describe this compositional diversity, igneous rocks of the LMA and metamorphic equivalents can be classified into three geochemical groups, on the basis of TiO2 content (Fig. 5), the primitive mantle-normalized extended-REE pattern (Fig. 6), and the incompatible trace-elements ratios (Fig. 7), although a probably continuum of compositions exists: group I, LREE-depleted basalts; group II, LREE-flat gabbros, dolerites, and basalts; and group III, relatively high-Ti, slightly LREE-enriched dolerites and basalts. In
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the volcanic sequence of the LMA, basalts of groups II and III occurs interlayered in the lower stratigraphic levels, and group I forms the basaltic flows and pillow lavas of the upper levels. The sampled group I basalts are pillow lavas, which include the more fractionated compositions of all samples (Mg# = 45–44; with Mg# = 100 mol MgO/[mol MgO + mol FeOT]). TiO2 is *1.2 wt.% (Fig. 5b), Zr and Nb are about 50 and 3 ppm at 6 wt.% MgO, respectively. Cr (45– 30 ppm) and Ni (75–55 ppm) are lower than in rocks of the II and III groups (Fig. 6e; Table 1). The REE patterns (Fig. 6) are similar to normal mid-ocean ridge basalt (NMORB; Sun and McDonough 1989; Perfit et al. 1994) having similar HREE abundances (3–5 · primitive mantle), slight LREE-depleted patterns ([La/Nd]N = 0.9–1.2; Table 1), positive Nb anomaly (Nb/Nb* = 2.1–2.2), and flat HREE ([Sm/Yb]N = 0.7–0.8). The group II samples are Mg-rich gabbros and microgabbros, dolerite dykes, and massive Fe–Ti basalts (FeOT [ 14 wt.%). These rocks range from unfractionated to relatively fractionated (Mg# = 68–41). The less or unfractionated rocks have relatively low TiO2 that ranges from 0.7 to 1.0 wt.%, but the more evolved Fe–Ti basalts have high-TiO2 contents (1.4–2.2 wt.%). Concurrently, they contain over 27–33 ppm Zr and 2 ppm Nb at 10 wt.% MgO. The more compatible trace elements show a wide range of values, 650–60 ppm for Cr and 185–15 ppm for Ni. The REE patterns are similar in the analyzed samples, and differ from those of group I basalts for the higher absolute HREE abundances (3–9 · primitive mantle) at same Mg#. These tholeiites show REE patterns that are LREE ([La/Nd]N = 0.8–1.1) and HREE flat ([Sm/Yb]N = 0.7–0.95), with a small positive Nb anomaly (Nb/Nb* = 0.9–2.5). The more evolved rocks have slightly positive Hf and Ti and negative Y anomalies. As group I samples, the incompatible element ratios (e.g., Zr/Nb [ 14 and La/Sm \ 1.5; Fig. 7) are characteristics of transitional to normal MORB (Sun and McDonough 1989; Mahoney et al. 1993).
Int J Earth Sci (Geol Rundsch) Fig. 5 Plots of selected major and trace elements against MgO for the diverse geochemical groups of the Loma La Monja volcano-plutonic assemblage. In b and c, MORB field is from Hawkins (1995); and in g the fields of CCOP samples from Sites 146, 150, 152, and 153 of DSDP Leg 15, and Colombian basalts (Kerr et al. 1997, 2002) are shown for comparison with the LMA. Major (a–d), metal (e), and trace element (g, h) modeled fractional crystallization trends using PELE program (Boudreau 1999) for two representative starting compositions: 5JE32 (group II) and 5JE01 (group III) are also shown. Ticks on the trends represent 10% crystallization intervals. Figure 5d includes the modeled sequence of crystallizing phase assemblages at wt.% MgO intervals for group III 5JE01 sample
The group III is represented by relatively high-Ti and LREE-enriched basaltic flows and pillow lavas, isotropic massive gabros and dolerite dykes. They grade from unfractionated to fractionated (Mg# = 66–43). Generally, they are high in TiO2 (0.9–1.6 wt.%) and Fe2O3T (13.5– 15.5 wt.%), and the more fractionated samples are therefore classified as Fe–Ti basalts. However, a sub-group of moderately evolved LREE-enriched metabasalts from Jicome´ area (Mg# = 57–54) have relatively low-Fe and Ti contents. They contain over 35 ppm Zr and 3 ppm Nb at 11.5 wt.% MgO. The more compatible trace elements show a wide range of values, 580–55 ppm for Cr and 170– 28 ppm for Ni, suggesting extensive fractional crystallization. In general, these rocks have higher TiO2 contents and LREE abundances than groups I and II for similar
Mg# (Figs. 5, 6). They have slightly LREE-enriched patterns ([La/Nd]N = 1.0–1.4), positive Nb anomaly (Nb/ Nb* = 1.2–2.3), and flat HREE ([Sm/Yb]N = 1.0–1.3). Some samples have slightly negative Hf, Eu, and Ti anomalies related to plagioclase and Fe–Ti oxide frationation. These features, as well as their incompatible element ratios (Zr/Nb \ 15 and La/Sm [ 1.5) are characteristics of enriched MORB (Sun and McDonough 1989; Mahoney et al. 2002).
Nd and Sr isotopes Initial (87Sr/86Sr)i versus (143Nd/144Nd)i variation of the LMA is restricted to high (eNd)i values between +6.5 and
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Int J Earth Sci (Geol Rundsch) Fig. 6 Primitive mantlenormalized extended-REE diagram patterns of the Loma La Monja volcano-plutonic assemblage (Table 1). Primitive mantle-normalizing values are from Sun and McDonough (1989). Three geochemical groups are distinguished: a group I, LREE-depleted pillow lava basalts; c, d group II, LREE-flat Mg-rich gabbros, microgabbro, and dolerite dykes, massive Fe–Ti basalts and amphibolites; and e, f group III, relatively high-Ti, LREEenriched basaltic flows and pillow lavas, isotropic massive gabros, and dolerite dykes. In b N-MORB, E-MORB, and OIB patterns of Sun and McDonough (1989), and average of East Pacific Ridge basaltic glasses (n = 262; Su and Langmuir 2003) are included for comparison
+8.3 (where i = 160 Ma; Fig. 8a; Table 2), in the range of enriched MORB (Su and Langmuir 2003). Despite acid leaching of the samples (87Sr/86Sr)i, ratios are highly variable (0.70360–0.70575) at near constant (eNd)i, similar to altered basalts in ridges and consistent with seawater hydrothermal alteration (e.g., Sinton et al. 1991). The effect of alteration is to shift the samples from a DMORB/E-MORB array to the right, and (87Sr/86Sr)i ratios therefore do no reflecting primary magmatic values. The (144Nd/143Nd)i ratios range between 0.51299 and 0.513056 and are relatively high and homogeneous, compatible with a source dominated by depleted mantle (DM). In the identified geochemical groups (eNd)i, values range between +6.56 and +6.83 in group I pillow lavas, is +8.32 in group II FC9103 amphibolite, and range from +7.0 to +7.4 in group III gabbros, dolerites, and basalts. A similar range of (eNd)i values between +6.8 and +9.3 was also described by Lapierre et al. (1999). However, slight LREE-enriched trace element compositions and 147Sm/144Nd ratios less than expected for DM
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(i.e., N-MORB-like source) indicate a more fertile component in the mantle source. On a 147Sm/144Nd versus (eNd)i diagram (Fig. 8b), hypothetical mantle sources involved in the petrogenesis of LMA rocks are: DM; OIB (ocean-island basalt source); FM (fertile mantle, interpreted as a 1% partial melt of OIB); and continental crust (CC or subducted continental material). Samples plot above the field for mixing of DM and CC, suggesting that the lowering of (eNd)i values is not due primary to the presence of recycled crustal material. The isotopic composition of the samples can be explained by mixing a DM component with a FM component in variable amounts, following a trend with positive slope on the diagram. An appropriate FM component would have slight to moderate LREE enrichment (i.e., low 147Sm/144Nd of 0.1–0.13), but positive eNd values (i.e., long-term history of LREEdepletion) about +5 to +7, which was obtained from 1% partial melting of OIB source. Groups I–III samples lie closer to the DM depleted and FM end-members, respectively (Fig. 8b), which is compatible with the
Int J Earth Sci (Geol Rundsch)
Fractional crystallization
Fig. 7 Plots of Zr/Nb and La/Sm trace elements ratios against MgO for the diverse geochemical groups of the Loma La Monja volcanoplutonic assemblage. Average values of N-MORB, E-MORB and Primitive Mantle are from Sun and McDonough (1989). Data of the East Pacific Ridge MORB (n = 242) and Cayman Trough N-MORB (Su and Langmuir 2003) are included for comparison. In b, the modeled fractional crystallization trends using PELE program (Boudreau 1999) for two representative starting compositions: 5JE32 (group II) and 5JE01 (group III) are also shown
geochemical interpretation that group III represent melts from a more enriched source.
Field relations and whole-rock chemistry in the LMA igneous rocks show that each individual flow or dyke occurrence represents a single batch of melt emplaced within oceanic lithosphere or extruded onto the seafloor. The group I pillow lavas displays a high degree of fractionation (Mg# = 45–44) and are enriched in HREE, Nb, Zr–Hf and Ti, and no Eu anomaly. The liquids producing such a signature are most likely residual, and result from segregation of olivine and pyroxene. In Fig. 9a, selected samples of the group II define a compositional evolution from primitive olivine-bearing Mg-gabbros and microgabbros, to moderately fractionated clinopyroxene and plagioclase-phyric tholeiitic basalts, to highly evolved tholeiitic Fe–Ti basalts and dolerites. This evolution is recorded by the continuous decrease of the Mg# and the increase in accessory minerals, which correlate with the increase of the bulk REE contents coupled with some changes in the modal abundance of plagioclase and clinopyroxene. Therefore, fractional or in situ crystallization (Langmuir 1989) was most probably the dominant process (see also modeling below). Group III samples show a crystallization history similar that of group II in that their total REE content does increase with the Mg# decrease (Fig. 9c), at constant Zr/Nb ratio (11.4–12.8, Fig. 7a). In both groups, the high-modal content of skeletal Ti-magnetite in Fe–Ti basalts and more incompatible element-rich whole-rock composition, are consequence of low-pressure crystallization sequence at high undercooling conditions, until strong enrichment of bulk Fe and Ti is achieved in residual liquids by segregation of olivine, plagioclase, and clinopyroxene (Natland 1991). The removal of plagioclase will produce the slight negative Eu anomalies seen in some evolved Fe–Ti basalts. The absence of related felsic igneous rocks in the LMA indicates that fractional crystallization did not produce more SiO2-rich ([53 wt.%) differentiates.
Discussion Fractional crystallization modeling Compositional diversity in LMA igneous rocks A compositional diversity is recorded within and between the different geochemical groups of the LMA. Upward in the stratigraphic sequence, the volcanic rocks of the LMA exhibit a change from slight LREE-enriched to LREE-depleted samples (Fig. 6). Likewise, the TiO2 contents, the incompatible element ratios and the radiogenic isotope ratios exhibit significant variations through the LMA. In the next sections, possible mechanisms to explain these variations are discussed which include fractional crystallization and partial melting of different source regions.
Simple modeling of closed-system, low-pressure fractional crystallization of selected near-primary liquid compositions using the PELE software (Boudreau 1999), successfully matches the mineralogy (Fig. 5f), trace-element ratios (Fig. 7), and extended REE-patterns (Fig. 9b, d) of rocks of the groups II and III. Critical in the modeling is the selection of parental magmas, because most MORB which erupted on the seafloor have been modified from their primary composition by fractional crystallization during ascent. Desmurs et al. (2002) estimated compositions of primary MORB melts using the compositions of olivine phenocrysts and relations between major oxides
123
Int J Earth Sci (Geol Rundsch)
Fig. 8 a Initial Sr–Nd isotopes ratios of the Loma La Monja volcanoplutonic assemblage samples. Initial ratios (i) and (eNd)i values were at t = 160 Ma. The fields for CCOP samples from the Cordillera Central (CC) MORB, Duarte Complex, Dumisseau Formation, DSDP Leg 15, Magellan Rise, and Gala´pagos, as well as samples from the Eastern Pacific Rise (EPR-MORB), are taken from Kerr et al. (1997, 2002), Lapierre et al. (1997, 1999), Sen et al. (1988), and Escuder Viruete et al. (2007a). PACPR = MORB samples from pre-arc complex of Puerto Rico (Jolly et al. 1998). BABB = Mesozoic back-arc basalts in Hispaniola (J. Escuder Viruete et al. unpublished
data). Depleted MORB mantle Sr–Nd isotopic compositions taken from Su and Langmuir (2003): DMM average for MORBs far from plumes; D-DMM is 2r depleted and E-DMM is 2r enriched over the average. b 147Sm/144Nd versus (eNd)i diagram for the same rocks showing the various end-member mantle components interpreted to be involved in its petrogenesis. Hypothetical mantle sources are: DM (depleted mantle), OIB (ocean-island basalt source), FM (fertile mantle, interpreted as a 1% partial melt of OIB), and CC (continental crust or subducted continental material). Discontinuous lines illustrate schematically mixing trends. See text for explanation
and Ni. These authors obtain for primitive liquids with MgO [ 8 wt.% compositions of olivine of Fo91–88, which are consistent with experimental data (Re´villon et al. 2002). In the LMA, the analyzed olivine compositions of Fo88–80 in gabbros, dolerites, and basalts with MgO [ 9– 10 wt.%, indicate that these rocks are close to primary liquids. In Fig. 9, the trace-element compositions of samples representative of the primary liquids are plotted and consist of a microgabbro dyke from the group II (5JE32;
MgO = 10.5 wt.%) and an aphyric massive basalt from the group III (5JE01; MgO = 11.4 wt.%). Group I rocks representative of primitive liquids were not sampled. The microgabbro have La/Sm = 1.4, shows no Nb anomaly (Nb/Nb* = 1.4) and have flat HREE. The lack of a positive Eu/Eu* anomaly indicates that this rock were not formed as cumulate, but instead crystallized from basaltic liquids that were emplaced and trapped. The olivine basalt is slightly enriched in LREE (La/Sm = 1.6) and other incompatible
Fig. 9 Comparison of the traceelement patterns of samples of the groups II (a) and III (c) of the Loma La Monja volcanoplutonic assemblage (left), with the results of modeled fractional crystallization of melts with 5JE32 (b) and 5JE01 (d) compositions with PELE software (Boudreau 1999). Values indicate the percentage of solid phases. Normalization is to Primitive Mantle from Sun and McDonough (1989). See text for discussion
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Int J Earth Sci (Geol Rundsch)
elements as Nb–Ta (Nb/Nb* = 2.3) compared to the HREE, and have a positive Hf anomaly. In both models, the first phase to crystallize is olivine (+Cr-spinel), followed in the fractionating assemblage by plagioclase, then clinopyroxene and finally Fe–Ti oxide (Ti-magnetite). For the 05JE1 composition, for example, ol + spl, ol + pl + spl, ol + pl + cpx, and pl + cpx + Fe– Ti oxide assemblages crystallize at [8.3, 8.3–7.7, 7.7–5.2, and \5.2 wt.% MgO intervals, respectively (Fig. 5f). The results for Zr and Nb (Fig. 5g, h) show that the two different ‘‘parental magmas’’ could fractionate to produce the incompatible trace element compositions of most of the groups II and III samples. In Fig. 9b, the extended-REE patterns of the most primitive Mg-rich melts predicted by the model for group II agree reasonably well with the ones observed. It shows that the olivine microgabbros formed after about 5–6% of fractional crystallization of olivine cumulates. Between 10 and 60% crystallization, clinopyroxene–plagioclase massive and pillowed basalts (JE9076 and FC9103 amphibolite), with \5% olivine are formed. Above *60% crystallization, the model predicts the occurrence of Timagnetite, which is characteristic of Fe–Ti basalts and dolerites (2J103, 2J103b, and 2J96). For group III, modeling shows that high-Mg olivine basalts (5JE01, FC9050) formed after 5–8% crystallization of olivine forming possibly at deep cumulates (Fig. 9d). Between 10 and 55% crystallization, clinopyroxene–plagioclase dolerites (5JE40a, 5JE50), massive and pillowed basalts (5JE38) with \5% olivine were formed (Fig. 5f). Above *55% crystallization, the model predicts the occurrence of a Fe– Ti oxide, which is characteristic of evolved Fe–Ti basalts and dolerites (5JE41, 5JE42), and accessory apatite appears after about 90% crystallization. This low-pressure tholeiitic crystallization sequence is consistent with the one found in the field. Furthermore, the model predicts no crystallization of orthopyroxene, which is also consistent with observations in LMA samples. In summary, bulk chemistry, mineral, and field relations in conjunction with modeling indicate that LMA igneous rocks were formed from a combination of different magmatic processes at low-pressure, ranging from predominantly fractional crystallization to solidification without fractionation. However, if the mineralogy and the trace elements of the gabbros are well simulated by the model, this is not the case for all major elements, especially for the most differentiated rock types. Indeed, the model predicts more fractionated compositions in terms of Mg# numbers than the ones observed in the most differentiated rocks. Also, a particularly greater enrichment in Ti and Nb is recorded in some samples of gabbros and dolerites (Fig. 5b, h) and not predicted by the modeling. Probably, this is due to the infiltration of late Ti-rich liquids into previously crystallized
rocks, leading to crystallization of ilmenite and to an increase in Ti and incompatible elements such as Nb and Ta. Thus, the genesis of some gabbros and dolerites could be explained by a process of fractional crystallization coupled with late percolation of differentiated liquids expelled from deeper-seated olivine gabbro cumulates (open-system crystallization; Scha¨rer et al. 2000). Alternatively, enrichment of Nb and low Zr/Nb ratios can be related to plumerelated sources (Kerr et al. 2002, Kerr 2003).
Variation of sources and degree of melting Fractional crystallization alone cannot explain the observed geochemical variations, particularly the ranges of Zr/Nb and La/Sm ratios and the Nd isotopes compositions, because these features are relatively insensitive to magmatic differentiation (Pearce and Peate 1995). In Fig. 7, the Zr/Nb (12–20) and La/Sm (1.1–1.4) values of groups I and II rocks are in the range of primitive mantle and enriched respect to average N-MORB (Sun and McDonough 1989), suggesting that both group samples were derived either from a similar ‘‘less depleted’’ MORB source. Particularly, groups I and II samples have Zr/Nb ratios lower than the Eastern Pacific ridge (18–60; Mahoney et al. 1993; Sinton et al. 1991; Su and Langmuir 2003) and the Cayman Through (24–40; Michael 1995) basaltic rocks and glasses. Samples from group III may have originated from a relatively more enriched mantle source, because they have slightly LREE-enriched patterns (La/Sm = 1.4–2.0), relatively high Nb contents and low Zr/Nb ratios (11–13), which define a horizontal array in Fig. 7b. As been described, the geochemical and isotopic characteristics of the LMA igneous rocks suggest derivation from an enriched MORB source. To address mantle sources and melting processes in these oceanic rocks, the ratios of incompatible trace elements unaffected by the addition or removal of olivine [i.e. (Sm/Yb)N and (La/Nd)N] are useful, because they reveal either the LREE depletion or enrichment and the presence of residual garnet (Fitton et al. 1997; Kerr et al. 1997, 2002). Moreover, these incompatible element ratios can provide qualitative information on the degree of partial melting. For this reason, selected compositions or each geochemical group were compared with the pooled fractional melting calculations for various possible mantle sources, which were modeled by Kerr et al. (2002) and range from depleted to enriched (primitive) mantle, and from garnet lherzolite through a 50:50 spinelgarnet lherzolite mixture to spinel lherzolite. To minimize the effects of fractional crystallization and crystal accumulation, a MgO value of 10 wt.% was chosen for parental magmas and the theoretical Zr10 content of each sample at this MgO value was calculated and plotted in Fig. 10.
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Int J Earth Sci (Geol Rundsch) Fig. 10 a, b Zr10 versus (La/ Nd)N, and c, d Zr10 versus (Sm/ Yb)N diagrams for the Loma La Monja volcano-plutonic assemblage ignous rocks, and models of fractional melting of various, possible mantle source regions (Kerr et al. 2002) used for petrogenetic interpretation. The mantle sources include depleted mantle (D), more enriched (P, primitive) mantle, garnet lherzolite (gt lhz), 50:50 or 75:25 spinel-garnet lherzolite mixtures, and spinel lherzolite (sp lhz). Numbered ticks on the melting curves indicate percentage of partial melting and Zr10 indicates the calculated Zr content for 10 wt.% MgO. See text for discussion
Group I basalts ([La/Nd]N = 0.9–1.16; [Sm/Yb])N = 0.7–0.8) are consistent with a DM source containing spinel, as shown by the lowest values in the (Sm/Yb)N ratio and the (eNd)i \ +7.0 values in the samples. The generation of group I magma can be modeled by a very high melting degree ([20%) of a shallower mantle source (Fig. 10b, d). The source of group II magmas ([La/Nd])N \ 0.8–1.1; [Sm/Yb]N = 0.95–1.1) was depleted to primitive spinel lherzolite mantle, that was slightly LREE-enriched with respect to MORB-source. This shallow source underwent a high melting degree of about 14–18% to form group II magmas. The source of group III magmas ([La/Nd])N \ 1.0–1.24; [Sm/Yb]N = 1.0–1.14) was primitive mantle consisting of spinel lherzolite. This source was enriched relative to N-MORB source, consistent with the LREEenriched patterns and lower (eNd)i \ +7.0 to 7.4 values (Fig. 8). However, the genesis of group III magmas requires a very high melting degree ([20–25%) of such a source. Therefore, the composition of group III magmas can be best explained by mixing of melts produced by low melting degree (\1%) of deep garnet lherzolite and higher melting degree ([15%) of a shallow spinel mantle source within a mantle melt column (Fig. 10b). In summary, the mantle source for group I was shallower and underwent a
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higher degree of melting than the sources of groups II and III, and group III probably involves a deeper and more enriched source. Therefore, upward in the volcanic sequence of the LMA, the magmas represent progressively more extensive melting at shallower depths, as those which can be achieved by decompression of peridotite in a plumeinfluenced spreading ridge. This moderate compositional heterogeneity has also been described at large and small scales in the East Pacific (Reynolds et al. 1992; Perfit et al. 1994; Wendt et al. 1999; Niu et al. 2002) and Southeast Indian Ridges (Mahoney et al. 2002), where similar trace element enriched basalts (E-MORB) are most commonly associated with hot spotinfluenced spreading centers (Langmuir et al. 1986; Perfit et al. 1994; Regelous et al. 1999). In this context, the geochemical characteristics of EMORB suggest that they represent melting of a heterogeneous DM reservoir which has plume-related or enriched domains (Niu et al. 2002). However, E-MORB also occur far from known hot spots in the Pacific and Atlantic ocean basins (Donnelly 2002) and alternative genetic models of melting of eclogite/pyroxenite veins or recycled crust as sources has been proposed. Quantitative modeling, however, shows that even an enriched source like eclogite veins
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requires low-F melting (\0.01; Donnelly 2002) to produce the observed fractionation among the incompatible elements. Low-degree melting of pyroxenite veins is a difficult model to support physically. The solidus of pyroxenite is lower than peridotite, and pyroxenite should melt more than peridotite. Also, low-F melts of eclogite or pyroxenite veins being enriched overall in all elements. Both melts have similar HREE steep patterns characteristic of garnet in the residue. In contrast, the trace element pattern of E-MORB is relatively flat. Recently, the evaluation of the trace-element patterns shows that E-MORB generation requires two melt stages (Donnelly et al. 2004). The first stage melting occurs at depth in subduction zones where the mantle wedge is enriched by the addition of lowdegrees melts of subducted crust, and the second stage of greater extents of melting occurs beneath ocean ridges, as we propose for the LMA. In the next section, a plume source for LMA samples is evaluated in terms of incompatible element ratios.
Mantle reservoirs and implications The deep DM source can be distinguished from the shallow N-MORB source in plume-related basalts using Zr/Y and Nb/Y ratios (Fitton et al. 1997). In a log–log Zr/Y versus Nb/Y diagram, the DNb line separating plume from nonplume basaltic sources seems to provide good discrimination (Fig. 11), because basalts plotting below the DNb line
are those deriving from either a shallow depleted source (DM), or from subduction zones, or from plume melts that have been contaminated by CC or/and sub-continental lithosphere. In the Nb/Y versus Zr/Y diagram, the LMA samples are characterized in terms of mantle components and compared to other plume-related igneous units of the CCOP in Hispaniola (Escuder Viruete et al. 2007a). A firstorder interpretation of the LMA groups is that all the gabbros, dolerites, and basalts plot above the DNb line in the mantle plume field, with most data falling near the PM composition. This is consistent with our previous inference that LMA magmas are derived from a heterogeneous plume source. In this figure, the heterogeneity in the lume source of LMA rocks is expressed by a vertical trend, or variable Nb/Y. Also, the different LMA geochemical groups define a common linear array sub-parallel to DNb line, which suggests derivation from a common PM-like peridotite source by variable extents of melting (Fitton et al. 1997; Revillon et al. 2000). In terms of Zr/Y–Nb/Y relationships, the LMA samples lack a significant contribution of the shallow depleted component (DM) from which N-MORB is derived and also indicated by a representative field for East Pacific Ridge MORB in Fig. 11 (Mahoney et al. 1993; Sinton et al. 1991; Su and Langmuir 2003). Some of group II and all group I samples with more high Zr/Y values have an enriched component (EN), which may be continental material, whether derived from CC or sub-crustal continental lithosphere (Weis et al. 2001).
Significance of Late Jurassic magmatism in Hispaniola
Fig. 11 Samples of the Loma La Monja volcano-plutonic assemblage plotted in a log–log diagram of Nb/Y versus Zr/Y after Fitton et al. (1997). Fields of fragments of the CCOP in Hispaniola: Dumisseau Formation (Sen et al. 1988), Duarte Complex (Escuder Viruete et al. 2007a), and Siete Cabezas Formation (Escuder Viruete et al. 2004), also plot within the tramlines defined by Icelandic mantle plume lavas. East Pacific Rise N-MORB lavas (Mahoney et al. 1993) plot below this field. Diagram shows the end-members of mantle components: UC upper continental crust, PM primitive mantle, DM shallow depleted mantle, EN enriched component, and REC recycled component. See text for further discussion
The geochemical similarity of the LMA samples and other plume-derived units present in Hispaniola open two main questions: (a) if all rocks were derived from the same, long lived, Gala´pagos-type mantle plume; and (b) if the plume had some role in the break-up of Pangea. Kerr et al. (1997) have shown that oceanic plateau basalts from the Caribbean-Colombian oceanic plateau plot between two ‘‘tramlines’’ defined in the log Nb/Y–log Zr/Y diagram by the plume-derived lavas from Iceland. For these authors, this similarity indicates that CCOP formed at, or near, an oceanic spreading ridge. In Fig. 11, LMA igneous rocks also plot between two Icelandic tramlines, which agree with that LMA rocks were formed in a plume-influenced spreading ridge. Also, the geochemical range of LMA rocks coincides with that of the East Pacific Rise MORB field if the Nb/Y ratio offset at a given Zr/Y the lower Icelandic tramline (i.e., Nb-enrichment by a superposed plume-component). In terms of incompatible element and Nd isotopic ratios (Fig. 8), the LMA igneous rocks are distinct from the overlying Duarte Complex lavas (Lapierre et al. 1997;
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Escuder Viruete et al. 2007a), particularly from the group III basalts, and the geochemical gap between them suggest two geochemical different plumes, separates by the Late Jurassic time interval of pelagic sedimentation of the El Aguacate Chert. Figure 11 also shows the elongated field of the Late Cretaceous (91–88 Ma) CCOP igneous rocks, including the basalts drilled during DSDP Leg 15 in the Caribbean seafloor (Sinton et al. 1998; Hauff et al. 2000; Kerr et al. 2002), Gala´pagos Islands basalts, and those from Dumisseau Fm in Haiti (Sen et al. 1988). Also, the Middle Campanian to Maastrichtian Siete Cabezas Fm from Central Hispaniola has been related to the CCOP (Sinton et al. 1998; Lewis et al. 2002). Although groups II and III samples of the LMA are similar to the more depleted CCOP lavas (Fig. 11), mostly of the CCOP field is more enriched, including the Duarte Complex, site 151, Dumisseau Fm, Siete Cabezas Fm, and Gala´pagos plume basalts. An interpretation of the data is that plume-derived units in Hispaniola were generated from heterogeneous source regions and diverse degrees of melting during at least from Albian ([96 Ma) to Late Campanian. Therefore, overlying Late Jurassic oceanic crust, several Cretaceous oceanic intraplate volcanic structures, such plateau and hotspot tracks are preserved in Hispaniola as suggested by Lapierre et al. (1997), Sinton et al. (1998), and Lewis et al. (2002). These results are not consistent with CCOP formation through a single mantle plume such as the Gala´pagos hotspot. Alternatively, we propose that CCOP fragments in Hispaniola results from several pulses of magmatism related with spatially distinct plumes and separated by protracted periods at a much lower rate, as has been proposed for other localities of the Caribbean plate (Hoernle et al. 2004; Kerr and Tarney 2005). Spatial clusters of large igneous provinces in time have been linked with supercontinent fragmentation (Storey 1995; Li et al. 2003). Specifically, at least five LIPs have been linked with the progressive breakup of Pangea. In eastern Pangea, Ingle et al. (2002) recently implicate the Kerguelen plume in the breakup of India, Australia, and Antarctica, at about 154–130 Ma ago. The breakup of western Pangea involved Middle Jurassic to Late Jurassic rifting and sea floor spreading in the Gulf of Mexico and the proto-Caribbean seaways (Fig. 12; Pindell et al. 2005). The anomaly M-21 (ca. 150 Ma) is the oldest known magnetic anomaly in the Venezuelan and Colombian Basins, which may have resulted from Tithonian seafloor spreading (Pindell et al. 2005). This Late Jurassic protoCaribbean oceanic crust could form the substrate of the future North Caribbean region and dismembered fragments are now preserved in Jamaica, Hispaniola (LMA), Puerto Rico, and La De´sirade (Fig. 12a). In the Early Cretaceous, diverse CCOP igneous units were constructed onto this proto-Caribbean ocean crust and located in a SW position
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Fig. 12 a Late Jurassic (Tithonian, anomaly M-21) and b Early Aptian plate reconstruction of the Gulf of Mexico and Caribbean area (mod. from Pindell et al. 2005, 2006). The Late Jurassic protoCaribbean oceanic crust could form the substrate of the future North Caribbean region and dismembered fragments are now preserved in Jamaica, Hispaniola (Loma La Monja volcano-plutonic assemblage), Puerto Rico, and La De´sirade. Note in a as a very oblique sinistral subduction of Kula–Farallon plate could not generate an arc until a later change in the plate kinematics. Cherts with Pacific-derived radiolaria (Montgomery et al. 1994) could be deposited in this area opened to the Pacific realm. In the Early Cretaceous (b), the new subduction zone related to the primitive NE-facing primitive Caribbean island arc could have nucleated along transcurrent faults or ridge segments. Note in b as remnants of the proto-Caribbean crust become part of the hanging wall above the new trench, escaping in Hispaniola to a severe deformation and high-P metamorphism. Later, diverse CCOP igneous units were constructed onto the ‘‘Pacific’’-Caribbean ocean crust and located in a SW position of the Aptian island arc. CCOP Caribbean-Colombian Oceanic Plateau, DC Duarte Complex, BR Beata Ridge, DF Dumisseau Fm, PR Puerto Rico
of the NE-facing primitive Caribbean island arc (Fig. 12b; Aptian). Therefore, it would be possible to relate the rocks of the LMA, which are overlain by the Oxfordian to Tithonian El Aguacate Chert, with this oceanic crust, and established a link between their plume-related geochemical signature with the break-up of western Pangea as an active plume-driven process (or rifting enhanced by a mantle plume). To further explore the tectonic setting of this early plume, we propose two posibilities: a pre- to sin-breakup plume; and a post-breakup oceanic plume. The first posibility does not explain the absence of continental flood basaltic volcanism in the stratigraphic record of the North
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American margin, as is typical of areas where plumes are present prior the onset of rifting (Kerr 2003). The second posibility better explains that initial magmatism related to the plume in the Caribbean area took place after continental rifting, which agree with geochemical and isotopical evidence of no contamination by CC in the LMA rocks.
Conclusions The LMA is composed of Late Jurassic gabbros, dolerites, basalts, and minor oceanic sediments, as well as metamorphic equivalents. Petrologic and geochemical data show that the LMA includes a relatively broad diversity in composition, which represent the crystallization products of a typical low-pressure tholeiitic fractionation of MORBtype parental magmas, ranging from N- to E-MORB. Mantle melt modeling and incompatible trace element systematics suggest that, upward in the volcanic sequence, the magmas represent progressively more extensive melting of shallower sources, in a plume-influenced spreading ridge of the proto-Caribbean oceanic crust. Nb/Y versus Zr/ Y systematics reveal that CCOP-related fragments in Hispaniola formed through melting of heterogeneous mantle source regions in relation to distinct plumes during at least from Aptian–Albian ([96 Ma) to Late Campanian. Acknowledgments The authors would like to thank John Lewis (George Washington University) and Gren Draper (Florida International University) for his introduction to the area, initial fieldwork, and continued discussions on the igneous rocks in the Dominican Republic. We are also grateful to many colleagues of the IGMEBRGM-Inypsa team for topic discussions and Piera Spadea () for help in petrological and geochemical interpretations. Elisa DietrichSainsaulieu is thanked for her help with the Sr–Nd isotopic analyses of the samples at PCIGR. This work from part of the MCYT projects BTE-2002-00326 and CGL2005-02162/BTE and also received aid from the cartographic project of the Dominican Republic funded by the SYSMIN Program of the European Union. Careful reviews by Andrew Kerr and Folkman Hauff substantially improved the manuscript.
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