Journal of Petrology Advance Access originally published online on February 25, 2005
Journal of Petrology 2005 46(6):1155-1201; doi:10.1093/petrology/egi013
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Post-Collisional Transition from Subduction- to Intraplate-type Magmatism in the Westernmost Mediterranean: Evidence for Continental-Edge Delamination of Subcontinental Lithosphere
1 GEOMAR RESEARCH CENTRE FOR MARINE GEOSCIENCES, DEPARTMENT OF VOLCANOLOGY AND PETROLOGY, WISCHHOFSTR. 13, 24148 KIEL, GERMANY
2 IFMGEOMAR LEIBNIZ INSTITUTE FOR MARINE SCIENCES, DYNAMICS OF THE OCEAN FLOOR, WISCHHOFSTR. 13, 24148 KIEL, GERMANY
3 INSTITUTE FOR GEOSCIENCES, UNIVERSITY OF KIEL, OLSHAUSENSTR. 40, 24118 KIEL, GERMANY
RECEIVED NOVEMBER 12, 2003; ACCEPTED JANUARY 14, 2005
| ABSTRACT |
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Post-collisional magmatism in the southern Iberian and northwestern African continental margins contains important clues for the understanding of a possible causal connection between movements in the Earth's upper mantle, the uplift of continental lithosphere and the origin of circum-Mediterranean igneous activity. Systematic geochemical and geochronological studies (major and trace element, SrNdPb-isotope analysis and laser 40Ar/39Ar-age dating) on igneous rocks provide constraints for understanding the post-collisional history of the southern Iberian and northwestern African continental margins. Two groups of magmatic rocks can be distinguished: (1) an Upper Miocene to Lower Pliocene (8·24·8 Ma), SiK-rich group including high-K (calc-alkaline) and shoshonitic series rocks; (2) an Upper Miocene to Pleistocene (6·30·65 Ma), Si-poor, Na-rich group including basanites and alkali basalts to hawaiites and tephrites. Mafic samples from the SiK-rich group generally show geochemical affinities with volcanic rocks from active subduction zones (e.g. IzuBonin and Aeolian island arcs), whereas mafic samples from the Si-poor, Na-rich group are geochemically similar to lavas found in intraplate volcanic settings derived from sub-lithospheric mantle sources (e.g. Canary Islands). The transition from Si-rich (subduction-related) to Si-poor (intraplate-type) magmatism between 6·3 Ma (first alkali basalt) and 4·8 Ma (latest shoshonite) can be observed both on a regional scale and in individual volcanic systems. SiK-rich and Si-poor igneous rocks from the continental margins of southern Iberia and northwestern Africa are, respectively, proposed to have been derived from metasomatized subcontinental lithosphere and sub-lithospheric mantle that was contaminated with plume material. A three-dimensional geodynamic model for the westernmost Mediterranean is presented in which subduction of oceanic lithosphere is inferred to have caused continental-edge delamination of subcontinental lithosphere associated with upwelling of plume-contaminated sub-lithospheric mantle and lithospheric uplift. This process may operate worldwide in areas where subduction-related and intraplate-type magmatism are spatially and temporally associated.
KEY WORDS: post-collisional magmatism; Mediterranean-style back-arc basins; subduction; delamination; uplift of marine gateways
| INTRODUCTION |
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Subduction-related and intraplate-type igneous rocks occur together worldwide (e.g. Mediterranean area, Colorado Plateau and Andes) (Kay & Mahlburg Kay, 1993
Combination of systematic geochemical and geochronological data allows reconstruction of the temporal and spatial transition from post-collisional subduction-related to intraplate-type magmatism in the western Mediterranean. The transition occurs regionally in the westernmost Mediterranean realm and locally in individual volcanic systems such as the Gourougou and Guilliz stratovolcanic complexes in northwestern Africa. The new geochemical and geochronological data for igneous rocks from the continental margins of southern Spain and northern Morocco are integrated with published data for rocks from the westernmost Mediterranean sea floor (Duggen et al., 2004
) and northwestern Algeria (Louni-Hacini et al., 1995
; Coulon et al., 2002
) providing the basis for a case study of this transition.
| REGIONAL GEOLOGY |
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Neogene post-collisional igneous rocks occur along the European and African margins of the western Mediterranean (Wilson & Bianchini, 1999
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The Alboran Basin area includes Middle to Late Miocene igneous rocks from the Alboran Sea floor (ridges and seamounts), the Alboran Island and coastal stratovolcanic complexes bordered by major strike-slip faults in southern Spain (the Cabo de Gata and Aguilas block) and northern Morocco (Ras Tarf and Trois Furches) (Duggen et al., 2004
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The Iberian and African continental areas surrounding the Alboran Basin are marked by igneous activity with a different geochemical composition. Upper Miocene to Pleistocene post-collisional magmatism along the southern Iberian and northwestern African continental margins is characterized by Si-rich, K-rich igneous rocks and Si-poor, Na-rich lavas (Fig. 1). Upper Miocene to Lower Pliocene SiK-rich igneous rocks include latites, shoshonites and ultrapotassic rocks such as lamproites (Fig. 2) (Bellon & Brousse, 1977
A large number of models have been proposed to explain the geodynamic evolution of the Alboran region in the westernmost Mediterranean since the Eocene and can be divided into three groups: (1) mantle diapirism (Weijermars, 1985
); (2) subduction of oceanic lithosphere involving slab roll-back and slab detachment (Royden, 1993
; Zeck, 1996
; Lonergan & White, 1997
; Hoernle et al., 1999
; Coulon et al., 2002
; Gutscher et al., 2002
; Duggen et al., 2003
, 2004
); (3) convective removal (detachment) and peeling-off (delamination) of subcontinental lithosphere (Platt & Vissers, 1989
; Docherty & Banda, 1995
; Houseman, 1996
; Seber et al., 1996a
; Comas et al., 1999
; Turner et al., 1999
; López-Ruiz et al., 2002
; Platt et al., 2003
). In this study, the temporal and spatial geochemical evolution of the magmatism in the westernmost Mediterranean is integrated with geophysical and geological data from the literature to distinguish between these models. We developed a three-dimensional model that describes the mantle geodynamic setting in the westernmost Mediterranean area near the MiocenePliocene boundary.
| ANALYTICAL METHODS |
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40Ar/39Ar isotope ratios were measured on K-bearing phases such as amphibole, biotite and feldspar phenocrysts and glass and matrix chips, at the GEOMAR Tephrochronology Laboratory. The freshest possible glass and matrix chips and phenocrysts were hand-picked from crushed and sieved splits and cleaned using an ultrasonic disintegrator. Plagioclase phenocrysts were additionally etched in hydrofluoric acid (15%). For several samples, fresh matrix chips were separated and analysed together with phenocrysts to examine the reliability of age data measured for matrix chips from aphyric igneous rocks. The samples were neutron-irradiated at the 5 MW reactor of the GKSS Reactor Centre in Geesthacht (Federal Republic of Germany), with crystals and glass and matrix chips in aluminium trays and irradiation cans wrapped in 0·7 mm cadmium foil. Age determinations by laser 40Ar/39Ar analysis were performed by fusing single crystals and glass and matrix chips in a single step or by laser-step heating analysis when necessary. Purified gas samples were then analysed using a MAP 216 series noble gas mass spectrometer. Raw mass spectrometer peaks were corrected for mass discriminations background, and blank values determined every fifth analysis. Neutron flux during irradiation was monitored using TCR sanidine (Taylor Creek Rhyolite, 27·92 Ma) (Dalrymple & Duffield, 1988
confidence level.
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For bulk-rock geochemical analyses, whole-rock sample chips (0·11 cm in diameter) were hand-picked using a binocular to minimize the effects of alteration; these were then cleaned ultrasonically with de-ionized water to extract salts, and then dried and pulverized in an agate mill. Bulk-rock analyses of major and selected trace elements (Ba, Co, Cr, Cu, Ga, Rb, Sr, V and Zr) were performed on fused tablets with a Philips X'unique PW 1480 X-ray fluorescence spectrometer (XRF) at the GEOMAR Research Centre. International reference standards JB-2, JB-3 (basalts), JA-2, JA-3 (andesites), JR-2, JR-3 (rhyolites), JG-2, GM (granites) and JF-1 (feldspar) were used to evaluate the precision and accuracy of the measurements. Na contents were normalized to the standard values to account for X-ray tube drift for this element. H2O and CO2 were determined using a Rosemont Infrared Photometer CSA 5003. The new data are presented in Table 2 and Electronic Appendix 2. Values of major elements given in the text and used for plotting and rock classification were calculated on a volatile-free basis following a correction of Ca contents assuming that CO2 comes from CaCO3.
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Additional trace elements (Li, Y, Nb, Mo, Sn, Sb, Cs, REE, Hf, Ta, W, Tl, Pb, Th and U) were determined from mixed acid (HFaqua regiaHClO4) pressure digests by inductively coupled plasma-mass spectrometry (ICP-MS) at the Institute for Geoscience, University of Kiel, using an upgraded (High-Performance Interface, torch box) VG Plasmaquad PQ1. The precision of analytical results as estimated from duplicate measurements was better than 3% RSD. Details of the analytical procedure have been described in the literature (Garbe-Schönberg, 1993
Sr, Nd and Pb isotopic analyses were carried out on whole-rock powders. To minimize the effects of seawater alteration, the powders were leached with hot distilled 6 M HCl for 1 h (except the highly evolved samples). After dissolution with Merck Ultrapur HF, HNO3 and HCl, Sr, Nd and Pb were extracted by conventional ion exchange chromatographic techniques. Isotope ratios of all samples were measured by thermal ionization mass spectrometry (TIMS) in static (Sr, Nd, Pb) and dynamic (Sr, Nd) mode on a Finnigan MAT262 system at GEOMAR. Measured 87Sr/86Sr and 143Nd/144Nd ratios were normalized within-run to 86Sr/88Sr = 0·1194 and 146Nd/144Nd = 0·7219. The Sr and Nd isotope ratios were further normalized to 87Sr/86Sr = 0·71025 (NBS 987 standard) and 143Nd/144Nd = 0·51185 (La Jolla standard) or 143Nd/144Nd = 0·51171 (internal Nd-Spex standard calibrated to La Jolla) to correct for mass spectrometer drift. Replicate analyses of standards NBS 987 (Sr), La Jolla and Nd-Spex (Nd) used for the evaluation of the external precision gave mean values of 87Sr/86Sr = 0·710242 ± 0·000017 (2 SD) (n = 39), 143Nd/144Nd = 0·511841 ± 0·0000010 (2 SD) (n = 13) and 143Nd/144Nd = 0·511706 ± 0·000008 (2 SD) (n = 12). Replicate analyses of the NBS 981 (Pb) standard yielded a mean value of 206Pb/204Pb = 16·894 ± 0·004 (2 SD), 207Pb/204Pb = 15·433 ± 0·005 (2 SD) and 208Pb/204Pb = 36·515 ± 0·017 (2 SD.) (n = 23). Pb-isotope data were corrected for 0·1238
mass discrimination per atomic mass unit (a.m.u.). Total analytical blanks were on average <0·100 ng and are thus negligible given the generally high Pb content of the samples.
| RESULTS |
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40Ar/39Ar geochronological data
Laser probe 40Ar/39Ar age data for 45 volcanic rocks from the Iberian and African continental margins in southeastern Spain and northern Morocco are reported in Table 1. Phenocryst ages are identical within error, or in very good agreement with each other, in all samples (e.g. feldspar and amphibole in sample CX110599 [GenBank] -3, biotite and amphibole in sample TA200400-1, biotite and feldspar in GZ280300-1). Matrix chip ages are also identical within error and in accordance with the phenocryst ages (e.g. matrix and feldspar in GG140699 [GenBank] -8, matrix and feldspar and biotite in GZ280300-3b, matrix and feldspar in GZ180699-4, matrix and biotite in OD190699-1a), showing that analyses of carefully prepared matrix chips can provide reliable age data. Two SiK-rich samples (lamproites) from southeastern Spain yielded phlogopite ages (FT120599-5, BQ170599 [GenBank] -9) with high MSWD values as a result of a large variation in single phenocryst ages. These phlogopite ages are significantly older than the matrix ages and are unlikely to reflect eruption ages. Three other SiK-rich (lamproite) samples (CT270300 [GenBank] -2c, ZN130599-1, VE290699-10A) provided phlogopite ages having low MSWD and initial 40Ar/36Ar ratios close to the atmospheric value and are thus considered to represent eruption ages. Below we systematically discuss the laser probe 40Ar/39Ar age data for the SiK-rich igneous rocks followed by the Si-poor lavas from four areas on the Iberian and African continental margins: southeastern Spain and the Gourougou, Guilliz and Oujda volcanic areas in northwestern Africa (Fig. 1).
The southern Iberian continental margin
Laser probe 40Ar/39Ar age data for 13 SiK-rich rocks (lamproites and their derivatives) from southeastern Spain range from 8·19 to 6·37 Ma (Table 1 and Figs 1 and 2) and are within the range of published K/Ar ages (8·66·2 Ma) (Nobel et al., 1981
; Bellon et al., 1983
). The 10·8 ± 1 Ma K/Ar age for a lamproite from the Vera basin (Bellon & Brousse, 1977
) was not confirmed by laser 40Ar/39Ar age dating. The 6·76 ± 0·04 Ma matrix 40Ar/39Ar age from La Celia near Jumilla is identical within error to a published Ar/Ar whole-rock age of 6·8 ± 0·4 Ma (Turner et al., 1999
). A systematic relationship between the new Ar/Ar ages and older K/Ar age data was not detected. As inferred from geological maps, the laser 40Ar/39Ar age data are consistent with stratigraphic evidence showing that lamproitic rocks are generally associated with Upper Miocene marine sediments (mostly marls and sandstones) in the Murcia area. Field observations in the Vera basin show that lamproitic lava flows, showing pillow structures, were emplaced and chilled in soft, wet Messinian marine muds (peperites). Two lamproitic peperites were dated at 6·44 Ma and 6·37 Ma and are consistent with the Messinian ages of their host marine sediments. A newly discovered lamproitic dyke (VE290699-10A) yielded a phlogopite age of 7·45 ± 0·08 Ma. It intrudes rhyodacitic breccias cut by the Palomares fault and the dyke itself has been tectonically deformed. Thus the new age data also provide constraints for the minimum age of the breccia and the tectonic history of the Palomares fault zone.
Si-poor, Na-rich volcanic rocks from the Tallante volcanic field (two hawaiites and one trachybasalt), which are spatially associated with the SiK-rich (lamproitic) rocks, yielded amphibole + biotite, glass and matrix mean apparent ages and step heating ages ranging from 2·93 to 2·29 Ma (Table 1 and Figs 1 and 2). For Tallante, the ages are consistent with field evidence showing that the volcanic units overlie Tortonian, Messinian and Pliocene sediments and are in good agreement with published K/Ar ages ranging from 2·8 to 2·7 Ma (Bellon et al., 1983
). The 10·5 Ma Ar/Ar phlogopite age for a Tallante lava reported by Turner et al. (1999)
is not consistent with these Pliocene ages. Phlogopite is not a common phenocryst phase in basaltic magmas, as it is stable only at high pressures and becomes unstable upon extrusion (Deer et al., 1996
). Ultramafic mantle xenoliths frequently associated with the alkali basalts from Tallante contain phlogopite (Dupuy et al., 1986
; Capedri et al., 1989
) and, therefore, the phlogopites dated by Turner et al. (1999)
may be xenocrysts from disaggregated mantle xenoliths. In conclusion, the transition from SiK-rich to Si-poor, Na-rich volcanic rocks occurred between 6·4 to 2·9 Ma at the southern Iberian continental margin.
The northwestern African margin
SiK-rich igneous rocks from the Gourougou volcanic field (basaltic trachyandesites, trachyandesites, andesite and one diorite) yielded matrix mean apparent and isochron ages on biotite and feldspar ranging from 7·58 to 4·8 Ma (Table 1 and Figs 1 and 2). These ages are largely consistent with the range of whole-rock and biotite-separate K/Ar age data (9·0 to 4·6 Ma) (Bellon & Brousse, 1977
; Hernandez & Bellon, 1985
; El Bakkali et al., 1998
) and 40Ar/39Ar age data (6·73 to 6·0 Ma) (Roger et al., 2000
) for volcanic tuffs interbedded with Messinian marine sediments, which were deposited on the slope of the Gourougou stratovolcanic complex. Si-poor, Na-rich alkali basalts and trachybasalts were erupted on the flanks of the Gourougou stratovolcano between 6·3 and 3·73 Ma (Table 1). Lava flows directly overlie Messinian marine sediments (e.g. Haidoun flow, dated at 3·73 Ma). Previously determined K/Ar ages generally tend to be younger (e.g. 2·5 Ma for Haidoun) (Bellon & Brousse, 1977
) than the laser 40Ar/39Ar age data. In the Gourougou volcanic system, the transition from SiK-rich to Si-poor, Na-rich igneous activity occurred between 6·3 and 4·8 Ma.
SiK-rich igneous rocks (trachyandesite to trachyte) from the Jebel Guilliz stratovolcano located in the centre of the Guilliz volcanic field yielded mean apparent ages for feldspar and biotite + feldspar ranging from 6·90 to 6·82 Ma (Table 1 and Figs 1 and 2). Mean apparent ages for matrix and feldspar + matrix from Si-poor, Na-rich alkali basalts in the Guilliz volcanic field range from 6·3 to 0·65 Ma (Table 1 and Figs 1 and 2). Most published K/Ar ages (8·0 to 4·9 Ma) (Hernandez & Bellon, 1985
) fall within this range but some K/Ar ages tend to be older. Major discrepancies between the new laser 40Ar/39Ar age data (0·65 Ma and 0·88 Ma) and the K/Ar (1·74 Ma and 2·5 Ma) (Hernandez & Bellon, 1985
) data from the literature exist for the Si-poor lavas from the Ain Zora and Jebel El Kehal lava flows in the westernmost Guilliz area. The new age data suggest that volcanism in the westernmost Guilliz volcanic field was active contemperanously with the Quaternary alkali basaltic volcanism of the nearby Middle Atlas mountains (1·8 to 0·5 Ma, K/Ar ages) (Bellon & Brousse, 1977
; Harmand & Cantagrel, 1984
). In the Guilliz area, the transition from SiK-rich to Si-poor, Na-rich igneous rocks occurred between 6·8 and 6·3 Ma.
The OujdaAlgeria area includes igneous rocks from northwestern Algeria, the Oujda volcanic field and the Plateau du Rekkam area situated south of the Oujda area. Based on K/Ar age data, SiK-rich magmatism (high-K calc-alkaline and shoshonitic series) in northwestern Algeria occurred at the MiddleUpper Miocene boundary and ceased in the Upper Miocene (11·7 to 7·5 Ma) (Louni-Hacini et al., 1995
; Coulon et al., 2002
). Si-poor, Na-rich rocks (basanites, alkali basalts and trachyandesites) from the Oujda volcanic field yielded matrix and biotite mean apparent ages ranging from 3·78 to 3·10 Ma (Table 1). These ages show that the eruption history of the Oujda volcanic field has a much narrower range than hitherto thought based on published whole-rock K/Ar age data (6·2 to 1·5 Ma) (Bellon & Letouzey, 1977
; Tisserant et al., 1985
; Andries & Bellon, 1989
). Two altered lava flows outcropping in the vicinity of the Oued Isly southwest of Oujda were K/Ar dated at 6·2 and 5·6 Ma (Andries & Bellon, 1989
). These flows overlie Tortonian blue marls (Marnes bleues) and so may correlate with Messinian marine sediments (Marnes jaunes) in the Oujda area (Jadid et al., 1999
). However, redating of the flows in this area with the laser 40Ar/39Ar technique (3·78 to 3·10 Ma) was not able to confirm the Late Miocene K/Ar ages (Table 1). Based on K/Ar age data, Si-poor volcanic rocks from northwestern Algeria show a much wider age range (10·07·2 Ma and 4·30·8 Ma) (Fig. 2) (Louni-Hacini et al., 1995
; Coulon et al., 2002
) than Si-poor volcanic rocks from the Oujda volcanic field (3·783·10 Ma) (this study). However, additional laser 40Ar/39Ar age dating is necessary to confirm the older K/Ar ages from northwestern Algeria, in particular the onset and termination of SiK-rich and Si-poor magmatism. Plateau du Rekkam volcanic rocks SE of the Oujda volcanic field probably erupted in the Plio-Quaternary (Saadi et al., 1985
). In the OujdaAlgeria area, the transition from SiK-rich to Si-poor, Na-rich magmatism apparently took place between 10·0 and 7·5 Ma.
In summary, the transition from SiK-rich to Si-poor volcanism occurred between 6·4 and 2·9 Ma in southern Spain, between 6·3 and 4·8 Ma in the Gourougou area, between 6·8 and 6·3 Ma in the Guilliz area, and possibly between 10·0 and 7·5 Ma in the OujdaAlgeria area.
Major and trace element data
Major and trace element data for 145 igneous rocks from the southern Iberian and northwestern African continental margins are presented in Table 2 and Electronic Appendix 2. The Electronic Appendix includes the full dataset and is available from the Journal of Petrology website at http://www.petrology.oupjournals.org, whereas Table 2 is a reduced version that includes major and trace element data only for samples that were selected for SrNdPb-isotope analysis.
The comprehensive dataset allows division of the post-collisional igneous rocks from the westernmost Mediterranean into a Si-rich and a Si-poor group (Fig. 3a). Based on the K2O content, the Si-rich samples can be further subdivided into a relatively K-poor group and a K-rich group (Fig. 3b). The Middle to Upper Miocene (12·1 to 6·1 Ma) K-poor group includes low-K (tholeiitic) and medium- to high-K (calc-alkaline) basalts through rhyolites outcropping in the Alboran Basin area (Hoernle et al., 1999
; Duggen et al., 2004
; Gill et al., 2004
). The occurrence of Upper Miocene to Lower Pliocene (8·2 Ma and 4·8 Ma) K-rich magmatism, including high-K to shoshonitic series basaltic andesites and basaltic trachyandesites to dacites and trachytes and lamproitic rocks is restricted to the southern Iberian and northwestern African continental margins. Upper Miocene to Pliocene (6·3 and 0·65 Ma) Si-poor volcanic rocks are spatially associated with the SiK-rich group rocks, and range from alkali basalts and basanites to hawaiites and tephrites and tend to have higher Na2O contents for a given K2O concentration than the K-rich group. Below we describe the major and trace element composition of the SiK-rich and Si-poor group rocks from the continental margins of southern Iberia and northwestern Africa.
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The southern Iberian continental margin
SiK-rich volcanic rocks from southeastern Spain (lamproites and their derivatives) show a large range in most major and trace element contents. MgO (1·512·9 wt %) correlates inversely with SiO2 and Al2O3 and positively with TiO2, Nb, Ta, Nd, Sm, Zr, Eu, Hf, Gd, Sn, Cr and Ni (with r2 for the regression line >0·7, except for Ni, which shows a curved correlation with MgO) (Fig. 4). Most of these systematic geochemical variations can be explained by fractional crystallization of common phenocryst phases in the Spanish lamproites such as olivine (containing Cr-spinel), phlogopite, apatite and sanidine (Venturelli et al., 1988
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Si-poor volcanic rocks from Tallante in southeastern Spain have MgO contents ranging from 3·6 to 8·5 wt %. MgO correlates negatively with Al2O3, Nb, Ta, Zr and Hf and positively with TiO2, Cr and Ni (r2 > 0·7) (Fig. 4ad), generally consistent with the fractionation of the observed phenocryst assemblages (e.g. olivine, clinopyroxene and magnetite). The Si-poor group samples from the Iberian continental margin generally exhibit contrasting multi-element patterns to the SiK-rich group showing relative enrichments of fluid-immobile Nb and Ta and relative depletion of fluid-mobile elements K, ± Pb, and have lower Rb, Ba, Th, U, K and Pb, but higher Sr and Ti concentrations than the SiK-rich volcanic rocks. The Si-poor group lavas have geochemical signatures similar to those of ocean-island basalts (OIB) and lavas from continental intraplate volcanic settings (e.g. Massif Central, St. Helena and Canary Islands in Fig. 5d). The Tallante lavas, however, show deviations from typical intraplate-type, multi-element patterns with positive ThU anomalies, highly variable K anomalies and Pb anomalies ranging from negative to positive.
The northwestern African continental margin
The SiK-rich igneous rocks from the northwestern African margin also show a large variation in major and trace element composition. MgO ranges from 0·1 to 6·6 wt % and correlates positively with CaO, TiO2, FeO, CaO/Al2O3, Co (Gourougou) and MnO, Co, V (Guilliz) and negatively with SiO2, K2O, total alkalis, Rb (Gourougou) and K2O and Li (Guilliz) (r2 > 0·7) (Fig. 4eh). In multi-element diagrams, SiK-rich group rocks from Gourougou at the northwestern African margin show spiked patterns as a result of enrichment of fluid-mobile elements Rb, U and K relative to less fluid-mobile or fluid-immobile elements (e.g. REE, Nb and Ta) (Fig. 6a). However, the behaviour of Pb is more complex, showing both positive and negative anomalies, and the troughs in Nb and Ta of some samples are only moderately pronounced. The more evolved SiK-rich samples from Guilliz have smooth patterns and show neither positive nor negative anomalies for the fluid-mobile elements Rb, Th, U, K and Pb and fluid-immobile elements Nb and Ta. The only negative anomalies are for Ba, Sr and Ti, and there is a slight positive anomaly for Zr (Fig. 6c). The SiK-rich sample from the OujdaAlgeria area (Ahfir sample; open circle, Fig. 6e) shows enrichment of fluid-mobile Rb, Ba, Th and U relative to fluid-immobile Nb and Ta but lacks peaks for fluid-mobile elements K and Pb. The multi-element pattern of the Ahfir sample is largely subparallel to that of both the mafic and evolved SiK-rich samples from northwestern Algeria (Fig. 6e) (Louni-Hacini et al., 1995
; Coulon et al., 2002
). Volcanic rocks having distinct troughs in Nb and Ta but only slight positive peaks for fluid-mobile elements can be found in the active Aeolian subduction zone (Fig. 5c).
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For the Si-poor, Na-rich lavas, MgO (1·812·6 wt %) correlates positively with Cr, Ni, Ga (Gourougou), Cr, Ni (Guilliz) and CaO, CaO/Al2O3, Cr, Ni (Oujda area) and inversely with Al2O3, K2O, Th (Gourougou), Al2O3 (Guilliz) and Al2O3, MnO, Cs, Zr, Hf, Eu, Yb, Lu and Y (Oujda) (r2
0·7, except for Cr and Ni, which show curved correlations with MgO) (Fig. 4eh). The Oujda samples also show a strong decrease in TiO2, FeO, Co and V as MgO decreases below 3 wt %. These systematic correlations between major and trace elements point to the fractional crystallization of the observed phenocryst assemblages (olivine, clinopyroxene and magnetite). In multi-element diagrams (Fig. 6b, d, f), the Si-poor lavas from northwestern Africa generally show contrasting patterns to the SiK-rich group as a result of a relative depletion in fluid-mobile elements Rb, Th, U, K and Pb, compared with less fluid-mobile elements such as the REE, Nb and Ta. These patterns are similar to those of lavas from oceanic intraplate-type volcanic settings (e.g. St. Helena and Canary Islands in Fig. 5d) (Chaffey et al., 1989It should be emphasized that several samples from both the SiK-rich group and the Si-poor group show deviations from typical subduction-related or intraplate-type geochemical fingerprints and have hybrid compositions between the two end-member geochemical signatures (Figs 5 and 6). Some of the SiK-rich samples lack relative enrichment and even show slight relative depletion in some fluid-mobile elements (in particular Th, U, K and Pb for Guilliz and Pb for Gourougou lavas). These samples also display large ranges in fluid-mobile to fluid-immobile element ratios, which have values between those found in OIB and subduction zone lavas. The SiK-rich volcanic rocks from Gourougou, for example, have U/Nb = 0·090·33 and K/La = 7321143, whereas similar rocks from Guilliz have U/Nb = 0·030·07 and K/La = 461582. Some mafic Si-poor samples exhibit less pronounced depletion or even slight enrichment in fluid-mobile elements (e.g. Th, U, K and Pb for Guilliz and Gourougou lavas and Th, U, K and Pb for Tallante lavas). Selective addition of strongly fluid-mobile elements relative to much less fluid-mobile elements, which have almost identical partition coefficients (such as Pb relative to Ce) cannot be significantly controlled by differences in the degree of partial melting or by fractional crystallization processes in mafic magmas but could reflect binary mixing of magmas from different sources, derivation from a subduction-modified mantle source or crustal assimilation.
SrNdPb-isotope data
SrNdPb-isotope data for 52 igneous rocks from the southern Iberian and northwestern African continental margins are reported in Table 3. Figures 7, 8, 10 and 11 display age-corrected SrNdPb-isotope ratios.
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The southern Iberian continental margin
SiK-rich, lamproitic rocks have high initial 87Sr/86Sr ratios (0·71610·7230) and low initial 143Nd/144Nd ratios (0·512150·51223) (Fig. 7a, c, d). The Sr-isotope ratios reported here slightly extend the range of published data (0·71660·7207) (Nelson, 1992
7/4 (14·316·5) and
8/4 (61·476·8) (Fig. 7b) [
7/4 and
8/4 notations denote the vertical deviation from the Northern Hemisphere Reference Line (NHRL), in the 206Pb/204Pb vs 207Pb/204Pb and 208Pb/204Pb diagrams, respectively] (Hart, 1984
Si-poor, Na-rich lavas from southeastern Iberia have relatively low initial 87Sr/86Sr ratios (0·70370·7046) and relatively high initial 143Nd/144Nd ratios (0·512830·51298) (Fig. 7a, c, d). Despite higher initial 206Pb/204Pb (18·8818·98) compared with the SiK-rich, lamproitic rocks, the Si-poor lavas from Tallante have similar 207Pb/204Pb and 208Pb/204Pb ratios (15·6515·67 and 38·939·0) to the SiK-rich rocks from the same area and therefore plot well above the NHRL, also having relatively high
7/4 and
8/4 (10·213·3 and 38·654·7; Fig. 7b).
The northwestern African continental margin
SiK-rich igneous rocks from the African continental margin show a wide range of initial 87Sr/86Sr ratios (0·70400·7100) and initial 143Nd/144Nd ratios (0·512360·51284), similar to the range reported for the OujdaAlgeria area (87Sr/86Srinitial = 0·70430·7096 and 143Nd/144Ndinitial = 0·512260·51254) (Coulon et al., 2002
) (Fig. 7a, c, d). On Pb-isotope diagrams, they show a limited range for initial 206Pb/204Pb (18·719·0) and 207Pb/204Pb (15·6315·70) ratios and plot above the NHRL (
7/4 = 7·112·9,
8/4 = 30·756·9; Fig 7b).
Most Si-poor, Na-rich lavas from the northwestern African continental margin have lower initial 87Sr/86Sr (0·70290·7045) and higher 143Nd/144Nd (0·512740·51303) than the spatially associated SiK-rich rocks and overlap with the range reported for Si-poor rocks from northern Algeria (87Sr/86Srinitial = 0·70320·7060 and 143Nd/144Ndinitial = 0·512550·51292) (Coulon et al., 2002
) (Fig. 7a, c, d). The African Si-poor rocks exhibit a large range in initial Pb-isotope ratios (206Pb/204Pb = 18·8520·75, 207Pb/204Pb = 15·6515·67, 208Pb/204Pb = 38·939·0), although the range within the individual volcanic areas is much more limited, with the exception of lavas from the Plateau du Rekkam volcanic field in the OujdaAlgeria area (Fig. 7b). Si-poor samples from Gourougou and Guilliz plot well above the NHRL (except one sample) (
7/4 = 0·19·2,
8/4 = 2·243·8) and the data fields overlap those of the spatially associated SiK-rich rocks but extend to more radiogenic Pb isotopic composition. Most Si-poor volcanic rocks from the OujdaAlgeria area plot below the NHRL (
7/4 = 7·75·8,
8/4 = 42·134·8) and fall within the field for Ahaggar volcanic rocks (southern Algeria) (Allègre et al., 1981
) and two samples have 206Pb/204Pb ratios as high as St. Helena OIB (Chaffey et al., 1989
). The variation in 206Pb/204Pb ratio for Si-poor lavas from the northwestern African margin almost covers the field for the modelled composition of young HIMU plotting below the NHRL, reflecting the composition of MORB with a 238U/204Pb of 10 and a 0·51·5 Gyr recycling time (Thirlwall, 1997
) (Fig. 7b). In contrast to the young HIMU field, most Si-poor lavas plot closer to or lie above the NHRL; Si-poor Oujda lavas, however, overlap with the low-velocity component (LVC) (Hoernle et al., 1995
).
| DISCUSSION |
|---|
|
|
|---|
Temporal evolution of volcanism at the Iberian and African margins
Dating of the westernmost Mediterranean igneous rocks suggests that the transition from SiK-rich to Si-poor, Na-rich compositions occurred between 6·4 and 2·9 Ma in Spain along the southern Iberian continental margin, between 6·8 and 4·8 Ma in Morocco and possibly between 10·0 and 7·5 Ma in Algeria within the northwestern continental African margin (Fig. 8ah) (Louni-Hacini et al., 1995
The transition also occurred in individual volcanic systems such as the Gourougou and Guilliz stratovolcanic complexes over shorter timescales (Figs 1, 2 and 8ah). At the Gourougou stratovolcanic complex, SiK-rich igneous activity occurred from the Late Miocene to Early Pliocene (7·58 to 4·8 Ma) partially overlapping the Latest Miocene to Early Pliocene Si-poor volcanism (6·3 to 3·73 Ma), which erupted from localized vents on the flanks of the stratovolcano. In the Guilliz volcanic field, a short interval of c. 0·4 Myr separates Late Miocene Si-rich explosive and intrusive volcanism (6·90 to 6·69 Ma) from the eruption of lava flows from scoria cones with Si-poor compositions in the Latest Miocene to Pleistocene (6·3 to 0·65 Ma). The SiK-rich Gourougou igneous rocks show a systematic change in their geochemical characteristics with decreasing age over a time span of about 3 Myr (7·58 to 4·8 Ma). This is observed in their major and trace element and Nd isotope ratios (e.g. K2O/Na2O, K2O/Al2O3, Ba/Sr, Th/Nb, K/Nb, K/La, Pb/Ce, 87Sr/86Sr,
7/4Pb decrease and Nb/La, Nb/Zr and 143Nd/144Nd increase) (Fig. 8ac, f, h). A similar geochemical evolution is observed in the Si-poor Gourougou lavas, which show a systematic decrease of incompatible element ratios (e.g. Ba/Sr, K/La, K/Nb, Pb/Ce) with decreasing age (Fig. 8a, f). The systematic temporal variations of both isotope and incompatible element ratios with very similar partition coefficients cannot be explained by differences in the degree of partial melting or by fractional crystallization, but instead must reflect crustal assimilation or binary mixing between different components from a heterogeneous mantle source.
Mantle source identification
As illustrated in the SrNdPb-isotope diagrams (Fig. 7ad), a number of distinct mantle and/or source components appear to play a role in the petrogenesis of the SiK-rich and Si-poor groups: (1) lithospheric components such as the continental crust and subduction-modified lithospheric mantle (e.g. Enriched Mantle 2, EM2) having high 87Sr/86Sr,
7/4 and
8/4, moderate 206Pb/204Pb ratios and generally low 143Nd/144Nd ratios, and (2) sub-lithospheric components such as young and old HIMU mantle with low 87Sr/86Sr, high 143Nd/144Nd and moderate 206Pb/204Pb ratios combined with negative
7/4 (e.g. Canary Islands and Ahaggar) or high 206Pb/204Pb ratios combined with positive
7/4 (e.g. St. Helena) (Thirlwall, 1997
). Below we explore the role of the continental crust, the subcontinental lithospheric mantle, sub-lithospheric mantle reservoirs and the possible depths of melting in the origin of SiK-rich and Si-poor group magmas from the southeastern Iberian and northwestern African continental margins.
The SiK-rich group from the southern Iberian margin (8·24·8 Ma)
The SiK-rich volcanic rocks (lamproites) from the southeastern Iberian continental margin form a compositional continuum with cordieritegarnet-bearing lavas from southern Spain, which are thought to result from crustal anatexis of a metapelitic protolith (Fig. 4ad) (Zeck, 1970
; Munksgaard, 1984
; Cesare et al., 1997
; Cesare & Maineri, 1999
). The geochemical similarity of the Spanish low-MgO lamproites (some of which contain cordierite and garnet, e.g. the lamproitic rocks from Mazarron based on petrographic observations) and the cordieritegarnet-bearing lavas from Cerro Hoyazo suggests that the cordieritegarnet-bearing igneous rocks themselves may be highly crustally contaminated derivatives of mafic lamproitic melts rather than pure crustal anatexites. This is consistent with the results of geochemical modelling by Benito et al. (1999)
, who proposed a mantle-derived origin for the parental melts of the Cerro Hoyazo lavas. However, as inferred from the previously mentioned compositional continuum between high-MgO lamproites, low-MgO lamproites and cordieritegarnet-bearing lavas, the SiK-rich, lamproitic lavas with MgO below 68 wt % appear to have apparently assimilated significant amounts of continental crust. Although the high-MgO lamproites (MgO >8 wt %) seem to be unaffected by crustal contamination, they have isotopic compositions very similar to crustal materials such as local continental crust and marine sediments (Figs 5a and 7ad). High-MgO lamproites also show extreme enrichments of most incompatible elements (e.g. K2O = 3·610·2 wt %) and have high Cr (7401230 ppm) and Ni (100340 ppm) contents (Fig. 5a and Table 2). These more mafic magmas appear, therefore, to represent primitive mantle melts, despite their crustal trace element and isotopic geochemical signatures. This suggests that they are derived from enriched subcontinental mantle lithosphere modified by fluids or melts probably released from subducted marine sediments, resulting in extremely high 87Sr/86Sr and low 143Nd/144Nd ratios as well as a large positive deviation from the NHRL (high positive
7/4 and
8/4) (Venturelli et al., 1984
, 1988
; Nelson et al., 1986
; Benito et al., 1999
; Turner et al., 1999
).
Based on results from experimental studies, the primary Spanish lamproite melts are thought to originate from depths of about 3050 km within the uppermost portion of the subcontinental mantle lithosphere (Foley, 1993
), i.e. within the stability field of spinel lherzolite. Derivation of the southern Iberian primary SiK-rich melts from a spinel lherzolite mantle source, even if it contains K-rich amphibole, is not, however, consistent with their high Dy/Yb ratios (Fig. 9a). High Dy/Yb ratios require residual garnet in the source of the magmas, which preferentially incorporates the HREE. Very low degrees of partial melting of a garnetphlogopite lherzolite with K contents ranging from 3000 to 9000 ppm could explain the variation of K/Yb and Dy/Yb (Fig. 9a). According to Foley, however, petrogenetic models for lamproites based on partial melting of phlogopite- (and amphibole-) bearing lherzolitic or harzburgitic mantle sources belie a more complex melting history including phlogopiteamphibole-rich pyroxenitic veins and veinmeltwall-rock interaction (Foley, 1992
, 1993
). Based on geochemical modelling, 130% partial melts from a vein having the composition Orthopyroxene0·05Clinopyroxene0·20Garnet0·05Phlogopite0·40Amphibole0·25Apatite0·03Rutile0·02 [as estimated from the diagrams of Foley (1992)
] ought to have relatively low Dy/Yb and high K/Yb ratios. The partial melting curve for such vein material plots far outside the diagram in Fig. 9a, as the modelled melts have extremely high K/Yb (130 000140 000) combined with relatively low Dy/Yb ratios (1·301·35) (i.e. to the right of the legend box in Fig. 9a). The position of the modelled curve, however, is relatively robust to moderate changes in the orthopyroxene, clinopyroxene, phlogopite, amphibole and rutile contents in the vein but very sensitive to minor changes in the amount of garnet and apatite. For a wide range of 010% garnet and apatite contents, however, K/Yb and Dy/Yb ratios stay above 100 000 and below 2·5, respectively. Therefore vein melting alone seems to be inadequate to explain the high Dy/Yb ratios observed for the lamproites (2·83·8). Foley (1992)
proposed an attractive model of interaction of vein melts with a garnet peridotite wall rock. Significant interaction of lamproitic vein melts with garnet lherzolite or melts therefrom could increase the Dy/Yb and lower the K/Yb ratios to the range observed in the lamproites. High Dy/Yb, however, indicates interaction of lamproite melts with garnet lherzolite and thus an origin from within the garnet peridotite stability field at depths
7080 km.
|
The Si-poor group from the southern Iberian margin (2·932·29 Ma)
Si-poor lavas from the Tallante volcanic field at the southeastern Iberian continental margin show correlations between SrNdPb-isotope ratios and major and trace element concentrations and inter-element ratios. These correlations provide important information about the possible role of crustal and mantle components in their petrogenesis. 87Sr/86Sr correlates positively with SiO2, Th, U, Pb, (Th, U)/Nb, Nb/La and Pb/Nd, and negatively with CaO, 143Nd/144Nd and 206Pb/204Pb (see examples in Figs 10 and 11). 143Nd/144Nd shows inverse correlations with U, Th, Pb, (U, Th)/Nb and Pb/Nb. 206Pb/204Pb displays inverse correlations with CaO, U/Nb and Pb/Nd (with r2 > 0·7). Whereas the most mafic samples are likely to reflect the composition of their mantle source, the more evolved samples are likely to provide information about the composition of assimilants during fractional crystallization (AFC). On this basis, it can be inferred that the contaminant has higher 87Sr/86Sr, (Th, U)/Nb and Pb/Nd but lower 143Nd/144Nd and 206Pb/204Pb than the mafic Si-poor Tallante magmas and their mantle sources.
|
|
Such geochemical variations may result from interaction of primitive Si-poor Tallante magmas with the continental lithosphere, e.g. the continental crust or metasomatized subcontinental mantle. Volcanic rocks from the Tallante volcanic field frequently contain lower crustal xenoliths and mantle xenoliths (Vielzeuf, 1983
7/4 and
8/4 well above the NHRL (Fig. 7b); (3) the hybrid geochemical characteristics between intraplate-type and subduction-related geochemical signatures in multi-element diagrams (e.g. positive ThU anomalies, variable Nb, Ta, K and Pb anomalies) (Fig. 5b). However, the geochemical similarity to lavas from intraplate volcanic areas such as the Canary Islands in terms of major and most trace elements and SrNd-isotope ratios points to derivation of the primary Si-poor melts from sub-lithospheric mantle sources and has already been pointed out in the literature (Wilson & Bianchini, 1999
The FeO and SiO2 contents of mafic partial melts in intraplate volcanic settings can vary as a function of melting depths (Hirose & Kushiro, 1993
). As illustrated in Fig. 12a, the FeO and SiO2 contents of the more mafic Tallante lavas combined with experimental data for partial melting of dry peridotite at high pressures (Hirose & Kushiro, 1993
) point to a derivation from a peridotitic source (Mg-number between 85 and 89) at depths between c. 40 and 80 km, i.e. presumably primarily within the spinel stability field. It should be emphasized that neither SiO2 nor FeO in the mafic Si-poor, Na-rich Tallante lavas (MgO >6 wt %) correlates with MgO. Therefore the variations in SiO2 and FeO observed for the mafic Tallante lavas are unlikely to result from fractional crystallization of magnesian phenocrysts and thus more probably reflect mantle processes such as partial melting and/or mixing with high-SiK, low-Fe lamproitic melts. However, the moderately high Dy/Yb ratios at around three in the most mafic Si-poor Tallante lavas (>6 wt % MgO) require the presence of residual garnet in the source of the Tallante melts.
|
In the K/Yb vs Dy/Yb diagram (Fig. 9a), the Si-poor samples from Tallante plot close to partial melting curves modelled for three different scenarios: (1) partial melting of a garnet lherzolite source; (2) a spinelgarnet lherzolite source; (3) a garnet-facies amphibole lherzolite source with low K contents (250 ppm, i.e. a Na-rich rather than a K-rich amphibole). Garnet in peridotite is stable at depths >7080 km, which is the approximate depth for the spinelgarnet transition zone in the upper mantle. Amphibole is unstable at depths exceeding 70100 km (Green, 1973
In conclusion, the Si-poor Tallante lavas from the southeastern Iberian continental margin probably represent low-degree partial melts of garnet-bearing sub-lithospheric mantle material such as garnet lherzolite or lherzolite from the garnetspinel transition zone at depths
7080 km. Variable interaction of these Si-poor melts with SiK-rich components or melts from the metasomatized subcontinental mantle lithosphere is interpreted to have partially modified the intraplate geochemical fingerprint of the sub-lithospheric melts towards subduction-related signatures. How sub-lithospheric mantle material may undergo partial melting at depths between c. 70 and 100 km to form the Si-poor Tallante lavas in an area in which the normal sub-continental lithosphere thickness is c. 150 km will be addressed subsequently in the discussion of the geodynamic evolution of this region.
The SiK-rich group from the northwestern African margin (7·584·82 Ma)
SiK-rich igneous rocks from Gourougou and Guilliz show linear correlations between radiogenic isotope ratios and major and trace elements. 87Sr/86Sr correlates positively with La and Nd (Gourougou) and negatively with Na2O (Guilliz), 143Nd/144Nd increases with increasing Sr (Gourougou and Guilliz) and CaO (Guilliz) and 206Pb/204Pb correlates positively with Na2O (Guilliz) (with r2 > 0·7). Hyperbolic correlations exist between Sr-isotope ratios and incompatible trace element ratios such as U/Nb and Nb/La (Fig. 11a). Such correlations, in which several samples achieve elevated 87Sr/86Sr up to 0·710, indicate that AFC processes affected the geochemical composition of some SiK-rich magmas in the Gourougou and Guilliz stratovolcanic complexes. As illustrated in the SiO2 and 1/Sr vs 87Sr/86Sr diagrams (Figs 10a and 11c), three Gourougou igneous rocks, one Guilliz sample and the most SiK-rich igneous rocks from the OujdaAlgeria area have elevated 87Sr/86Sr coupled with high SiO2 and 1/Sr (Coulon et al., 2002
). However, several SiK-rich samples from Gourougou and Guilliz (and one SiK-rich sample from the OujdaAlgeria area) have low Sr-isotope ratios despite a significant variation in SiO2 and Sr contents, which argues against significant crustal contamination. The Sr and Nd isotopic composition of the SiK-rich igneous rocks from Gourougou and Guilliz with low Sr-isotope ratios overlaps the field for the Si-poor lavas from the same area despite different Pb isotopic compositions. This is not consistent with a co-genetic derivation of the SiK-rich lavas from the Si-poor ones by AFC (Fig. 11) and, therefore, we infer that the primary magmas of the SiK-rich and Si-poor volcanic rocks at the northwestern African continental margin were derived from different mantle sources. Major and trace element and SrNdPb radiogenic isotope ratios of the SiK-rich igneous rocks, therefore, suggest derivation from a K-enriched, metasomatized mantle source with a subduction-related geochemical signature, possibly inherited from an older subduction zone. Because of its vicinity to the Alboran Basin (Fig. 1), the source of the Gourougou magmas may also have been affected by hydrous fluids and melts from the Miocene Alboran subduction zone (Hoernle et al., 1999
; Duggen et al., 2004
; Gill et al., 2004
).
The K/Yb vs Dy/Yb diagram in Fig. 9b provides constraints on the depths of melting for the SiK-rich Gourougou magmas. Two of the more mafic Gourougou samples with MgO > 4 wt % and low 87Sr/86Sr are plotted. The Guilliz SiK-rich trachyandesites are not included in Fig. 9b because of uncertainties arising from their evolved nature such as fractionation of REE through fractional crystallization of apatite, which is evident from a negative correlation of SiO2 and P2O5 (r2 = 0·9) for SiK-rich samples with >57 wt % silica. The more mafic SiK-rich Gourougou lavas (SiO2 <55·5 wt %) (and one mafic SiK-rich sample from Algeria) have Dy/Yb ratios slightly below two and elevated K/Yb ratios. Elevated Dy/Yb ratios require residual garnet in the source of the melts but they are too low to result from partial melting of garnet lherzolite alone (Fig. 9b). Different scenarios can be considered: (1) mixing between 110% partial melt of a garnet-facies phlogopite lherzolite with 0·510% partial melt of a spinel lherzolite or 115% melt from a spinel-facies amphibole lherzolite; (2) mixing of 115% garnet-facies amphibole lherzolite melts with spinel lherzolite melts or spinel-facies amphibole lherzolite melts; or (3) mixing of garnet lherzolite melts or spinelgarnet transition zone melts with spinel-facies amphibole lherzolite melts. Each scenario requires residual garnet in the source region and the presence of K-bearing phases. A plausible explanation would be a melting column with the lower end located in the upper garnet-stability field and the upper end situated in the spinel lherzolite field. In conclusion, SiK-rich melts from the northwestern African continental margin were probably derived from a K-enriched, metasomatized mantle source from depths between 50 and 100 km, i.e within the mechanical boundary layer of the subcontinental lithosphere. K enrichment may have been associated either with fluids or hydrous melts introduced from the adjacent Miocene subduction zone in the Alboran Basin (e.g. Gourougou) or with an earlier subduction event (e.g. Guilliz) or both.
The Si-poor group from the northwestern African margin (6·250·65 Ma)
Several samples from the Si-poor group from the northwestern African continental margin plot significantly below the NHRL and therefore have negative
7/4 values up to 7·7 (e.g. igneous rocks from Plateau du Rekkam, the Oujda volcanic field except for one sample, and one sample from Guilliz) (Fig. 7b). On the African plate, basaltic lavas plotting on or below the NHRL can be found in the adjacent intraplate volcanic areas of the Canary Island Archipelago (eastern North Atlantic) and Ahaggar (southern Algeria); these have geochemical characteristics suggesting derivation from sub-lithospheric mantle sources (Allègre et al., 1981
; Hoernle & Schmincke, 1993a
, 1993b
). It is very unlikely that lavas with negative
7/4 values have been significantly affected by continental crustal contamination, as the continental crust plots well above the NHRL and has a high Pb content (Taylor & McLennan, 1985
; Thirlwall, 1997
). Therefore, such lavas provide direct geochemical information about their mantle sources, in terms of radiogenic isotope ratios and ratios of elements having very similar partition coefficients during partial melting and fractional crystallization in basaltic melts (e.g. the pairs UNb, KLa and PbNd). It can, thus, be inferred that the source of the Si-poor melts with negative
7/4 had low 87Sr/86Sr <0·7039, high 143Nd/144Nd >0·5127, 206Pb/204Pb ranging from 19·3 to 20·8, and U/Nb = 0·01720·0230, K/La = 80·2411·1 and Pb/Nd = 0·03260·0751. In terms of SrNd-isotope and inter-element ratios this source is very similar to that of lavas from the Canary Islands derived from sub-lithospheric mantle sources but it tends to have higher 206Pb/204Pb ratios (e.g. Gran Canaria 87Sr/86Sr <0·7034, 143Nd/144Nd >0·5128, 206Pb/204Pb = 19·520·0, U/Nb = 0·01460·0229, K/La = 98·8418·7, Pb/Nd = 0·03690·0557) (Hoernle et al., 1991
; Thirlwall, 1997
). Interestingly, both Oujda and Plateau du Rekkam volcanic rocks with negative
7/4 yield regression lines subparallel to the Ahaggar array in the uranogenic Pb-isotope diagram (Fig. 7b). If interpreted as PbPb isochrons, these regression lines correspond to c. 840 Ma and 975 Ma ages, respectively. Similar PbPb isochron ages were found for the neighbouring Madeira hot spot system in the eastern North Atlantic and were interpreted to reflect short-term recycling of oceanic lithosphere in the Earth's mantle (Geldmacher & Hoernle, 2000
; Geldmacher et al., 2000
). Many Oujda lavas, however, also plot close to the low-velocity component (LVC) (Fig. 7bd), which is observed in intraplate-type lavas from Central and Western Europe, the Central Mediterranean and the Eastern North Atlantic (Hoernle et al., 1995
). This suggests that most Si-poor igneous rocks from the OujdaAlgeria area were derived from a sub-lithospheric mantle source with plume-like characteristics.
The Si-poor igneous rocks from the Gourougou and Guilliz volcanic fields extend from the NHRL on Pb isotope diagrams to the field for SiK-rich rocks with high
7/4 values (e.g. Fig. 7b). For Si-poor Gourougou lavas
7/4 correlates negatively with Nb/Zr (r2 = 0·92) and Nb/Yb (r2 = 0·85), whereas for the Si-poor Guilliz samples 87Sr/86Sr correlates negatively with Eu anomaly (Eu/Eu* ranging from 1·070·95, r2 = 0·92) (Fig. 10d). These correlations provide information about the high
7/4 end-member or contaminant of the sublithospheric melts, which has 87Sr/86Sr >0·7045, 143Nd/144Nd <0·5127,
7/4 >8·6,
8/4 >44, which can also be inferred from the SrNdPb isotope diagrams (Fig. 7ad). Components with such a geochemical fingerprint can be found in the continental crust or metasomatized subcontinental mantle lithosphere or melts therefrom, for example, the SiK-rich melts in the Gourougou and Guilliz volcanic centres (Figs 7, 10 and 11). As no clear correlations exist between MgO or SiO2 with Nb/Zr, Nb/Yb, Eu anomaly or SrNdPb-isotope ratios, it seems unlikely that fractional crystallization of magnesian phenocryst phases was associated with an increase of 87Sr/86Sr,
7/4 and
8/4 and a decrease of 143Nd/144Nd, Nb/Zr and Nb/Yb. These observations argue against significant crustal contamination of the Si-poor Gourougou and Guilliz magmas by AFC requiring significant fractional crystallization. Instead, the geochemical systematics point to an interaction of the Si-poor melts with the metasomatized subcontinental mantle lithosphere or melts therefrom. Unfortunately, AFC cannot conclusively be excluded for some of the samples having elevated
7/4 and
8/4. However, the aforementioned geochemical correlations and the composition of the more mafic samples (MgO >68 wt %) provide information about the composition of the mantle source of the primary Si-poor magmas. It can be inferred that the source of the Si-poor Gourougou lavas had 87Sr/86Sr <0·7033, 143Nd/144Nd <0·5130,
7/4 <4·3 and
8/4 <9·0, and that of the Guilliz melts had 87Sr/86Sr <0·7029, 143Nd/144Nd <0·5130, negative
7/4 and
8/4 < 2·2. This isotopic composition along with the intraplate-type multi-element patterns (Figs 6b, d, f, and 7ad) is consistent with derivation of the primary Si-poor Gourougou and Guilliz lavas from a sub-lithospheric mantle source. As can be inferred from the uranogenic Pb-isotope diagram (Fig. 7b), this source was probably contaminated by plume material having a young-HIMU geochemical fingerprint similar to that of the Canary Islands (Hoernle & Tilton, 1991
; Hoernle et al., 1991
; Thirlwall, 1997
).
The major and trace element compositions of the mafic Si-poor igneous rocks from the northwestern African continental margin can provide information about the melting regime of their mantle source. As indicated by the SiO2 vs FeO diagram in Fig. 12b, Si-poor melts from the Plateau du Rekkam volcanic field could have been generated by partial melting of a peridotite with decreased fertility at depths exceeding 100 km, i.e. outside the amphibole stability field. Many Oujda samples plot at the end of the experimental field for the melting of a peridotite with Mg-number of
89, consistent with derivation from depths exceeding 100 km. Several Oujda samples, however, lie close to the experimental field for partial melts of a more fertile peridotite with Mg-number = 85, pointing to derivation at depths exceeding 7080 km depth from a more fertile source. In contrast, most of the Si-poor lavas from Algeria have SiO2 and FeO contents pointing to magma generation at shallower depths (c. 40100 km) than the Plateau du Rekkam and Oujda lavas. These estimates are consistent with geochemical modelling results in the K/Yb vs Dy/Yb diagram in Fig. 9b. Lavas from the Plateau du Rekkam area have very high Dy/Yb ratios (>3·5) combined with low K/Yb ratios and therefore plot very close to a garnet lherzolite partial melting curve (45% partial melting). Very high Dy/Yb and relatively low K/Yb argue for the absence of spinel, phlogopite and K-bearing amphibole in the source of Plateau du Rekkam lavas. The situation is slightly different for the remaining samples from the OujdaAlgeria area. They have high Dy/Yb but show a large variation in K/Yb ratios. The OujdaAlgeria lavas therefore plot close to the garnet lherzolite, the garnet-facies amphibole lherzolite and the spinelgarnet transition zone model partial melting curves. Several Oujda samples with elevated K/Yb, shown in Fig. 9b, plot close to the high-pressure tip of the experimental field for partial melting of a peridotite with Mg-number = 89 indicating derivation from >100 km, i.e. outside the stability fields for amphibole and spinel. Those Oujda lavas plotting close to the Mg-number = 85 field could have been derived from 70100 km depths where spinel and amphibole may have been stable. However, the mafic samples from Oujda do not plot along one of the partial melting curves. Instead, they define an array with a positive correlation of K/Yb with Dy/Yb, which suggests binary mixing of two melt end-members (Fig. 9b). Therefore, a more plausible model to explain the composition of the Si-poor lavas from the Oujda volcanic field is mixing of low-degree (<1%) partial melts from a garnet lherzolite source with moderate-degree (510%) partial melts from the spinelgarnet transition zone.
The Si-poor Gourougou and Guilliz magmas were apparently generated from a more fertile peridotite source than the Si-poor lavas from the OujdaAlgeria area. As neither SiO2 nor FeO correlates with MgO for the mafic Si-poor Gourougou and Guilliz samples, elevated SiO2 and lowered FeO are unlikely to result from fractional crystallization. Therefore, SiO2 and FeO contents indicate shallower depths of melting on the order of 4080 km. The Dy/Yb ratios of mafic Gourougou and Guilliz lavas are relatively high (>2·0) and, therefore, require the involvement of melts from the garnet stability field. One mafic Guilliz sample (GZ160699-5, MgO 11·2 wt %) has higher Dy/Yb ratios than the other samples in Fig. 9b and also has the lowest SiO2 (46·4 wt %) and the highest FeO content (10·7 wt %) of the mafic samples shown in Fig. 12b, which is consistent with a derivation from the spinelgarnet transition zone at 7080 km depth. The other Guilliz samples plot below the spinelgarnet transition zone partial melting curve in Fig. 9b. Their higher SiO2 and lower FeO contents are not consistent with a significant melt component from a garnet lherzolite source (
80 km) and point to involvement of a spinel lherzolite source (Fig. 12b). Therefore we propose derivation by mixing of low- to moderate-degree partial melts from the spinelgarnet transition zone with low- to moderate-degree partial melts from a spinel lherzolite (Fig. 9b). Interaction of Si-poor Guilliz melts with SiK-rich material in the metasomatized subcontinental mantle lithosphere may also be involved, and could result in a decrease of Dy/Yb, FeO and increase of K/Yb and SiO2. Such a process is more clearly indicated for the mafic Si-poor melts from Gourougou as they form an array with their mafic SiK-rich associates in Fig. 9b. Relatively low FeO contents and elevated SiO2 and K/Yb in the mafic Si-poor Gourougou melts may result from interaction with SiK-rich melts in the subcontinental lithosphere (Figs 9b and 12); lavas with the lowest K/Yb would be derived from the deepest sources in this scenario. Taking into account both K/Yb and Dy/Yb ratios and SiO2 and FeO contents, we infer that the Si-poor Gourougou melts were derived by low to moderate degrees of partial melting in the garnetspinel transition zone at c. 7080 km depth, and mixed with partial melts (SiK-rich) from the (amphibole-bearing) spinel lherzolite field in the subcontinental lithosphere.
Summary of the inferred mantle sources and the partial melting regime
Based on major and trace element and SrNdPb-isotope ratios, the mantle sources of Upper Miocene to Lower Pliocene SiK-rich igneous rocks from the Iberian and African continental margins appear to have been modified by Miocene or more ancient subduction events. The mafic SiK-rich igneous rocks require garnet and K-bearing phases such as phlogopite and/or amphibole in their mantle source, which are different for the Iberian and African SiK-rich rocks. The Spanish lamproites are inferred to have been generated by interaction of melts from phlogopiteamphibolepyroxenegarnet veins with garnet peridotite wall-rock (Foley, 1993
) and were generated deeper than 70 km based on the geochemical modelling results in this study. For the SiK-rich rocks from the northwestern African margin, partial melting probably involved a melting column ranging from uppermost garnet lherzolite to well within the spinel lherzolite stability field, i.e. from depths between 50 and 100 km. K-bearing phases such as phlogopite or amphibole could have been stable within the full range of the melting column. The mantle sources of the SiK-rich igneous rocks from the Iberian and African continental margins, however, were probably located in the mechanical boundary layer of the subcontinental lithosphere.
Based on intraplate-like geochemical signatures, the Late Miocene to Pleistocene mafic Si-poor igneous rocks were probably derived from sub-lithospheric mantle sources contaminated with mantle plume material such as young HIMU mantle (Thirlwall, 1997
) or the low-velocity component (LVC; Hoernle et al., 1995
). Mafic Si-poor melts with negative
7/4 (e.g. lavas from the OujdaAlgeria area and Guilliz) have probably not interacted with the continental lithosphere. Si-poor melts with positive
7/4 (e.g. lavas from southeastern Spain, Gourougou and Guilliz) show interaction with components in, or melts from, the metasomatized subcontinental lithospheric mantle. Depths of partial melting range from >100 km (garnet lherzolite stability-field) to about 40 km (spinel lherzolite stability-field). Partial melting and generation of Si-poor melts in several volcanic areas obviously involved melting columns ranging from garnet- to spinel-facies depths.
In summary, the depths of partial melting of the younger Si-poor melts (c. 40 to >100 km) overlap with the depths inferred for the spatially associated but generally older SiK-rich igneous rocks (c. 50100 km) despite fundamental differences in their major and trace element and SrNdPb-isotope compositions. A discussion of the westernmost Mediterranean geodynamic evolution may help to solve the puzzle of why two fundamentally different mantle sources existed at the same depths but at different times.
| GEODYNAMIC EVOLUTION |
|---|
|
|
|---|
The geochemical transition from subduction-related to intraplate-type geochemical signatures in mantle-derived rocks strongly points to major changes in mantle geometry beneath the westernmost Mediterranean close to the MiocenePliocene boundary (El Bakkali et al., 1998
The Alboran Basin
Several models have been proposed for the formation of the Alboran Basin that invoke (1) subduction of oceanic lithosphere (Royden, 1993
; Lonergan & White, 1997
; Hoernle et al., 1999
; Gutscher et al., 2002
; Duggen et al., 2003
, 2004
; Gill et al., 2004
); (2) detachment of near-vertical subducted oceanic lithosphere (Blanco & Spakman, 1993
; Zeck, 1996
, 1999
; Hoernle et al., 1999
); (3) detachment or delamination of subcontinental mantle lithosphere (Docherty & Banda, 1995
; Comas et al., 1999
; Turner et al., 1999
; Calvert et al., 2000
; López-Ruiz et al., 2002
; Platt et al., 2003
). Subduction of oceanic lithosphere and detachment or delamination of subcontinental lithosphere should produce volcanism with characteristic geochemical compositions. Therefore, we can use the geochemistry of the Alboran Basin volcanism to distinguish between these fundamentally different tectonic processes for the formation of the Alboran Mediterranean-style back-arc basin (Horvath & Berckhemer, 1982
).
Volcanic rocks from the Alboran Basin (Fig. 1) belong to the low-K (tholeiitic) and medium- to high-K (calc-alkaline) series with major and trace element and OSrNdPb-isotopic compositions very similar to those of volcanic front and rear-arc lavas found in active subduction zones (e.g. IzuBonin) (Hoernle et al., 1999
; Duggen et al., 2004
; Gill et al., 2004
). Their geochemical characteristics provide evidence for magma generation involving high degrees of partial melting of depleted (harzburgitic) upper mantle material triggered by hydrous fluids and melts released from subducted oceanic lithosphere in the Middle to Late Miocene (c. 126 Ma) (Hoernle et al., 1999
; Duggen et al., 2004
). This is consistent with the boninitic affinity of mafic lavas from Alboran Island and the Alboran sea floor (Hoernle et al., 1999
; Gill et al., 2004
). The geochemical characteristics of the Alboran Basin lavas such as mantle-like
18O as low as 5·3
(Duggen et al., 2003
, 2004
; Duggen et al. unpublished data) undoubtedly rule out an origin exclusively by anatexis of continental crust as proposed in the literature (Zeck et al., 1998
, 1999
).
Detachment or delamination models advanced for the formation of the Alboran Basin are difficult to envisage for several reasons. Detachment of a near-vertical slab beneath the centre of the Alboran Basin in the Early Miocene following northward subduction of Tethys oceanic lithosphere under Iberia (Zeck, 1996
) requires subduction prior to detachment and should involve typical subduction-zone volcanic activity before the slab detached. There is, however, no evidence for large-volume Late OligoceneEarly Miocene continental subduction-zone volcanism on the southern Iberian margin. Additionally, slab detachment should cause a rapid cessation of tholeiitic and calc-alkaline arc volcanism (Hoernle et al., 1999
). As volcanism with a clear subduction-related geochemical fingerprint was active between 12·1 and 6·1 Ma in the Alboran Basin (Hoernle et al., 1999
; Duggen et al., 2004
), slab detachment in the Early Miocene seems to be very unlikely.
Models involving detachment or delamination of subcontinental lithosphere beneath the Alboran Basin at the OligoceneMiocene boundary are unable to explain a number of key geological, geochemical and geophysical observations in the westernmost Mediterranean. These are: (1) the oceanic subduction zone geochemical characteristics of Middle to Late Miocene Alboran Sea lavas (Hoernle et al., 1999
; Duggen et al., 2004
; Gill et al., 2004
); (2) the transition from thinned continental to Neogene oceanic crust in the easternmost Alboran Basin (Dewey et al., 1989
; Comas et al., 1999
) (Fig. 1); (3) directed rather than radial Miocene crustal nappe emplacement onto the southern Iberian and northwestern African continental margins associated with vertical axis-rotations of crustal nappes anti-clockwise in the Betics and clockwise in the Rif mountains (Lonergan & White, 1997
); (4) an east-dipping positive seismic anomaly beneath the Alboran Basin that shows westward continuity with the Atlantic oceanic lithosphere (Gutscher et al., 2002
; Gutscher et al., 2003
); (5) a concentration of seismicity along a northsouth-directed line at 60120 km depth (as is commonly observed within subducting slabs where the slab curvature rapidly increases) beneath the westernmost Alboran Basin (Gutscher et al., 2002
); (6) geophysical evidence for an active accretionary wedge due west of Gibraltar (Gutscher et al., 2002
). These important observations can convincingly be explained with westward slab roll-back and steepening of subducted, east-dipping oceanic Tethys lithosphere since the Oligocene (Lonergan & White, 1997
; Wilson & Bianchini, 1999
; Gutscher et al., 2002
; Duggen et al., 2004
). A slab roll-back model for the Alboran Basin fits well into the widely accepted view that the western and central Mediterranean geodynamic evolution was dominated by slab roll-back of old subducting Tethys oceanic lithosphere (Dewey et al., 1989
; Lonergan & White, 1997
; Carminati et al., 1998
; Wilson & Bianchini, 1999
; Jolivet & Faccenna, 2000
; Wortel & Spakman, 2000
; Gutscher et al., 2002
; Duggen et al., 2004
). However, below we argue that the situation for the continental margins of southern Iberia and northwestern Africa is different.
The southern Iberian and northwestern African continental margins
During the Late Miocene to Pleistocene, igneous activity along the southern Iberian and the Moroccan part of the northwestern African continental margin was marked by the occurrence of SiK-rich igneous rocks (8·24·8 Ma) with subduction-like geochemical signatures and Si-poor lavas (6·30·65 Ma) with intraplate-type signatures (Figs 1 and 2). Available K/Ar age data indicate that the onset of both SiK-rich and Si-poor igneous activity was earlier in Algeria than in Spain and Morocco (Louni-Hacini et al., 1995
; Coulon et al., 2002
) (Fig. 2). As inferred from the major and trace element data, geochemical modelling and interpretation of experimental partial melting of spinel and garnet peridotites presented above (e.g. Figs 9 and 12), the parental SiK-rich and Si-poor magmas are considered to be derived by partial melting of geochemically different mantle sources that existed largely at similar depths in a given areametasomatized, subcontinental lithosphere and plume-contaminated sub-lithospheric mantle. New Ar/Ar age data suggest that the transition from SiK-rich to Si-poor magmatism at the continental margins of Iberia and Africa (Fig. 8) reflects the progressive replacement of subcontinental lithosphere by sub-lithospheric mantle.
We propose that delamination of subcontinental lithosphere occurred beneath the continental margins of southern Iberia and northwestern Africa. It could be argued that slab detachment was responsible for the transition from SiK-rich, subduction-related to Si-poor, intraplate-type igneous activity. Any geodynamic model, however, has to explain why the transition from SiK-rich to Si-poor magmatism occurred both along the southern Iberian and northwestern African continental margins. If slab detachment occurred in the Alboran region, then double slab detachment, following north- and south-directed subduction beneath the Iberian and African margins, is required. There is no evidence in the available geophysical data that indicates such a process. Finally, as mentioned above, slab detachment is not consistent with recent results from seismic tomography and the observed temporal and spatial geochemical variation of the westernmost Mediterranean volcanism.
Detachment of subcontinental lithosphere has been proposed before based on geochemical and geochronological data for igneous rocks from southern Spain, by Turner et al. (1999)
. Those workers extended their detachment model to the entire Alboran region including the Alboran Basin area. As discussed above, this model is not supported by geophysical evidence or by the geochemical composition of Miocene volcanic rocks from the Alboran Basin area. Instead, we propose that the removal of the sub-continental lithosphere beneath the continental margins of southern Iberia and northwestern Africa was progressive (delamination) rather than convective (detachment).
Both delamination and detachment of subcontinental lithosphere could be associated with the generation of SiK-rich, subduction-related igneous rocks and Si-poor, intraplate-type lavas. Delamination of subcontinental lithosphere in conjunction with subduction of oceanic lithosphere has been proposed for the petrogenesis of the Neogene to Quaternary K-rich magmatism of central Italy (Serri et al., 1993
) and the association of SiK-rich and Si-poor magmatism of the Andean Puna Altiplano Plateau (Kay & Mahlburg Kay, 1993
). Detachment of subcontinental lithosphere following continental thickening is thought to be responsible for the generation of K-rich melts in the Tibetan Plateau (Turner et al., 1996
) and the calc-alkaline to K-rich igneous rocks of Eastern Anatolia (Turkey) (Pearce et al., 1990
). Based on mantle mineral stabilities, mantle solidi and geothermal gradients, Pearce et al. (1990)
showed that continental thickening can also bring metasomatized subcontinental lithosphere to greater depths, causing partial melting.
Delamination of subcontinental mantle lithosphere beneath the continental margins of southern Iberia and northwestern Africa is also consistent with geophysical data such as the location of earthquake hypocentres and seismic-wave velocities pointing to the presence of a thin wedge-shaped layer (c. 2040 km) of (possibly hotter) asthenospheric material between the Moho and a slab of lithospheric mantle down-bending beneath southern Spain and northern Morocco near Gibraltar in the western part of the Alboran Basin (Seber et al., 1996b)
. This process can explain why Si-poor, intraplate-type magmas were formed at largely similar depths in the same areas where SiK-rich magmatism had ceased, as the upwelling sub-lithospheric mantle that replaced the delaminating subcontinental lithosphere at a given depth was probably the source for the primary Si-poor magmas. Such sub-lithospheric mantle could either be depleted convecting asthenosphere capable of producing MORB melts at sufficiently high degrees of melting or enriched mantle plume material. The major and trace element and SrNdPb-isotope composition of most of the Si-poor lavas is consistent with derivation from a HIMU-type mantle source rather than depleted MORB-source mantle (Fig. 7bd). The HIMU-type mantle could be plume material or could reflect concentrations of pyroxenitic and eclogitic material within the upper mantle. A few Si-poor samples, however, have 87Sr/86Sr, 143Nd/144Nd and 206Pb/204Pb similar to enriched MORB that might indicate a marginal involvement of a depleted MORB mantle (DMM) component (Fig. 7c and d), which, however, does not seem to be supported by the Pb isotope data (Fig. 7b). An overlap with enriched MORB composition in the 206Pb/204Pb vs 87Sr/86Sr and 143Nd/144Nd diagrams is more likely to result from interaction of sub-lithospheric melts with the subcontinental mantle or melts derived therefrom (indicated by arrow 2 in Fig. 7c and d). Involvement of a MORB source therefore appears to be unnecessary.
Low magma volumes and the absence of an age progression in the western Mediterranean Si-poor rocks (Fig. 1) and lack of evidence from seismic tomography for low-velocity cylindrical structures beneath the Alboran region (Seber et al., 1996b
; Calvert et al., 2000
; Gutscher et al., 2002
) makes derivation of the Si-poor lavas directly from mantle plumes such as found below the Canary Islands unlikely. The similarity in geochemistry of the western Mediterranean Si-poor group lavas with intraplate-type volcanic rocks from Central Europe, the eastern North Atlantic and NW Africa provides evidence for derivation from a common source beneath these regions (Allègre et al., 1981
; Hoernle et al., 1991
, 1995
; Wilson & Downes, 1991
; Hoernle & Schmincke, 1993a
, 1993b
; Granet et al., 1995
; Thirlwall, 1997
; Geldmacher & Hoernle, 2000
). Global seismic tomography data suggest that a large low-velocity structure extends from >2000 km depth to the base of the upper mantle (c. 660 km) and spreads out in the upper mantle beneath Europe and the Mediterranean (Hoernle et al., 1995
; Goes et al., 1999
) thereby probably contaminating the convecting asthenosphere with plume material. Subduction of oceanic lithosphere may trigger sub-lithospheric mantle material to well up around slab graveyards localized at the base of the upper mantle or slab structures present in the upper mantle (Goes et al., 1999
). We propose that further upwelling of such plume-contaminated, sub-lithospheric mantle material may be triggered by delaminating or detaching subcontinental mantle lithosphere. Melting of upwelling plume-contaminated asthenosphere replacing delaminated subcontinental lithosphere can explain the occurrence of Si-poor lavas on the continental margins of southern Iberia and northwestern Africa.
Reconciling the Miocene to Pliocene geodynamic evolution of the Alboran Basin with that of the continental margins of southern Iberia and northwestern Africa
In Figure 13, we present a three-dimensional geodynamic model that illustrates the connection between the changes in mantle geometry beneath the westernmost Mediterranean area and the transition from subduction-related to intraplate-type magmatism occurring at the MiocenePliocene boundary. As the Alboran Block was forced to move to the SW in the Miocene it overrode subducting east-dipping Tethys oceanic lithosphere (Gutscher et al., 2002
; Duggen et al., 2004
). Hydrous fluids and melts were released from the subducted oceanic lithosphere into the mantle wedge beneath the Alboran Basin, which consisted of asthenospheric mantle and attenuated continental lithosphere. These hydrous fluidsmelts caused high degrees of partial melting in the mantle wedge, resulting in the eruption of low-K (tholeiitic) and medium-K (calc-alkaline) series lavas in Alboran Basin area. These Middle to Late Miocene lavas have typical subduction-related geochemical signatures similar to those from modern subduction zones (e.g. IzuBonin arc) (Hoernle et al., 1999
; Duggen et al., 2004
; Gill et al., 2004
). Roll-back and steepening of the subducted Tethys oceanic lithosphere could have had several effects, as follows.
- It could cause peeling off (delamination) of bands of subcontinental mantle lithosphere beneath the southern Iberian and northwestern African continental margins (continental-edge delamination). We propose that the lithospheric bands were attached to the cold and dense subducting oceanic lithosphere through mechanical coupling, which would be an effective mechanism for the removal of even buoyant subcontinental lithosphere.
- It could cause asthenosphere contaminated with plume material to well up from depth around the margins of the delaminating subcontinental lithosphere.
|
Perturbation of the metasomatically enriched subcontinental lithosphere resulted in the generation of SiK-rich, subduction-type melts in the Late Miocene to Early Pliocene. Decompressional partial melting of the upwelling asthenosphere contaminated with plume material led to the formation of Si-poor, intraplate-type alkali basalts in the Late Miocene to Pleistocene. Interaction of Si-poor melts with the metasomatized subcontinental mantle or SiK-rich melts derived from it could have generated magmas with hybrid geochemical characteristics between subduction- and intraplate-type. Westward slab roll-back and steepening coupled with delamination of subcontinental lithospheric mantle is consistent with the earlier onset of the subduction- to intraplate-type magmatic transition in Algeria (10·0 to 7·5 Ma) (Louni-Hacini et al., 1995
| CONCLUSIONS |
|---|
|
|
|---|
The transition from subduction-related, SiK-rich to intraplate-type, Si-poor magmatism in the westernmost Mediterranean at the MiocenePliocene boundary can be observed both on a regional scale in the entire Alboran region and on a local scale in individual volcanic centres. Whereas the Alboran Basin area is marked by the eruption of low-K (tholeiitic) series and medium- to high-K calc-alkaline series volcanic rocks related to Middle to Late Miocene subduction (Duggen et al., 2004
The mafic SiK-rich group magmas at the Iberian and African continental margins resulted from low degrees of partial melting of subcontinental mantle lithosphere metasomatically enriched by fluids or melts from earlier subduction processes. Mafic intraplate-type, Si-poor magmas were generated by low-degree decompressional melting of upwelling asthenospheric mantle contaminated with enriched plume material. The hybrid geochemical characteristics of the mafic magmas generated close to the MiocenePliocene boundary reflect interaction between subduction-type, SiK-rich magmas or their source regions and intraplate-type, Si-poor magmas of sub-lithospheric origin.
At the MiocenePliocene boundary, the primary source of the magmatism shifted from subduction-modified subcontinental lithosphere to plume-contaminated sub-lithospheric sources not previously affected by subduction processes. The evolution of the Gourougou stratovolcanic complex in Morocco exhibits the progressively decreasing influence of the subduction-type component and increase of the intraplate-type component through time.
The transition from subduction-related to intraplate-type magmatism points to major changes in the geometry of the westernmost Mediterranean mantle. Roll-back and steepening of an east-dipping, subducting slab of old Tethys oceanic lithosphere in the Miocene triggered (1) the peeling off (delamination) of bands of the subcontinental lithosphere at the southern Iberian and northwestern African continental margins and (2) the upwelling of plume-contaminated sub-lithospheric material beneath southern Iberia and northwestern Africa, replacing the delaminating subcontinental lithospheric mantle. Continental-edge delamination associated with slab roll-back and steepening provides a plausible explanation for the complex spatial and temporal geochemical evolution of magmatism in the westernmost Mediterranean since the Oligocene, and is consistent with the Late Miocene uplift and closure of marine gateways connecting the Mediterranean Sea to the Atlantic Ocean, causing the desiccation of the Mediterranean and the Messinian Salinity Crisis.
Wider implications are that subcontinental lithosphere can effectively be delaminated when attached to dense subducting oceanic lithosphere. This process may be responsible for continental uplift and the generation of post-collisional magmatism in the Mediterranean area and elsewhere in the world.
| SUPPLEMENTARY DATA |
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Supplementary data for this paper are available on Journal of Petrology online.
| ACKNOWLEDGEMENTS |
|---|
We express our gratitude to K. Abdelmalki, M. Bouabdellah, Y. Bouabdellah, M. Chaieb, M. Jadid, A. Milhi and A. Moukadiri (from the Universities of Oujda and Fez and the Centre Régional Géologique d'Oujda) for their valuable help with our fieldwork in Morocco. We thank T. Arpe, D. Garbe-Schönberg, F. Hauff, L. Hoffmann, S. Krolikowska, S. Laukat, S. Plagmann, D. Rau, J. Sticklus, S. Vetter, K. Wolff and E. Zuleger for their help with the analytical work in Kiel. Martin Menzies and Matthew Thirlwall are gratefully acknowledged for comments on the manuscript prior to submission. We are grateful to José-Maria Cebriá, Szabolcs Harangi, Julian Pearce and Marjorie Wilson for their thorough reviews and valuable comments on the manuscript. The project was supported by the Deutsche Forschungsgemeinschaft (German Research Council) (HO1833/5).
* Corresponding author. Present address: Geological Institute, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen K, Denmark. E-mail: sduggen{at}geol.ku.dk
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, 0·007, 0·0015;
, 0·022, 0·042;
, 0·33, 0·28;
, 1·06, 4·01;
, 0·01, 0·01;
, 0·029, 0·03;
, 0·78, 0·59, respectively (these values were selected from the GERM website, at 







