JOURNAL OF PETROLOGY
VOLUME 38
NUMBER 1
PAGES 115–132
1997
Continental Lithospheric Contribution to Alkaline Magmatism: Isotopic (Nd, Sr, Pb) and Geochemical (REE) Evidence from Serra de Monchique and Mount Ormonde Complexes JEAN BERNARD-GRIFFITHS1∗, GE´RARD GRUAU1, GUY CORNEN2, BERNARD AZAMBRE3 AND JOEL MACE´1 GE´OSCIENCES RENNES, UPR 4661 CNRS, CAMPUS DE BEAULIEU, AVENUE DU GE´NE´RAL LECLERC, 35042 RENNES CEDEX,
1
FRANCE LABORATOIRE DE PE´TROLOGIE STRUCTURALE, UER DES SCIENCES DE LA NATURE, FACULTE´ DES SCIENCES DE
2
NANTES, 2 RUE DE LA HOUSSINIE`RE, 44072 NANTES CEDEX 03, FRANCE DE´PARTEMENT DE PE´TROLOGIE, UNIVERSITE´ P. ET M. CURIE, 4 PLACE JUSSIEU, 75252 PARIS CEDEX 05, FRANCE
3
RECEIVED NOVEMBER 1, 1995 REVISED TYPESCRIPT ACCEPTED JULY 30, 1996
Isotopic results (Sr, Nd, Pb), as well as concentrations of major and trace elements (REE) are reported for whole-rock samples and mineral separates from the onland alkaline complex of Serra de Monchique (South Portugal) and the offshore alkali basalt volcanic suite of Mount Ormonde (Gorringe Bank). These two genetically related alkaline complexes were emplaced at the east Atlantic continent–ocean boundary during the Upper Cretaceous, i.e. 66–72 m.y. ago. Taken together, Serra de Monchique and Mount Ormonde may be seen as one of the few examples of within-plate magmatism that straddles the continent–ocean boundary. Major and trace element compositions fail to reveal any significant differences between onland and offshore complexes. This is particularly true regarding less differentiated samples (mg-number [0·40) which show the same progressive and continuous enrichment of their trace element patterns, with no specific anomaly (e.g. negative Nb anomaly) being present in samples from the onland complex. Initial Pb and Sr isotopic compositions also do not allow any distinction to be made between Serra de Monchique and Mount Ormonde samples. Initial Pb isotope ratios are moderately high (19·1< 206Pb/204Pb < 19·8; 207 Pb/204Pb = 15·6) in both cases. Moreover, once the effects of Sr contamination by seawater are taken into account and the most
contaminated samples discarded using data from fresh clinopyroxene separates and results of leaching experiments, the initial Sr isotopic compositions of Mount Ormonde samples are found to be unradiogenic ( 87Sr/86Sr=0·7031±1) and identical to those obtained at Serra de Monchique ( 87Sr/86Sr=0·7032±1). In contrast, a systematic mean difference of 2 eNd units is observed between Serra de Monchique [eNd(T) = +4·8] and Mount Ormonde [eNd(T) = +6·6] whole-rock samples. Surprisingly, a variation is also observed at Mount Ormonde between the whole-rock samples and one of the two analysed clinopyroxene separates. Whereas Mount Ormonde whole-rock samples invariably yielded eNd(T)= +6·6 (mean value), a value of +0·5 is obtained for one clinopyroxene separate, whereas another gives +6·0. The above geochemical and isotopic results make it possible to assign respective roles to the asthenosphere, lithosphere and crust in the petrogenesis of Serra de Monchique and Mount Ormonde complexes. We propose that both complexes share a common mantle source whose isotopic characteristics are very similar to the source of oceanic island basalts. Continental mantle lithosphere, already characterized isotopically by studies of peridotite massifs within the Iberian peninsula, acts as a contaminant which is evident onland on the whole-rock scale,
∗Corresponding author. Telephone: 02 99 28 67 30. Fax: 02 99 28 67 72. e-mail:
[email protected]
Oxford University Press 1997
JOURNAL OF PETROLOGY
VOLUME 38
and also present offshore as discrete clinopyroxene xenocrysts. The continental crust appears to play no role in the petrogenesis of the Serra de Monchique alkaline rocks.
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shallow melting of mantle diapirs in areas of reduced crustal and lithospheric thickness, within-plate basic magmas are generally emplaced in zones of thicker crust and lithosphere. For this reason, the possibility exists that these magmas become contaminated during their ascent to the surface, in which case their measured isotopic compositions will not represent those of their source rocks deep in the mantle. Hence, two crucial questions may arise: (1) Do the four or more distinct isotopic endmembers identified in within-plate basic magmatic rocks really reflect the existence of a series of deep mantle domains, each possessing different chemical and isotopic compositions? (2) Is it possible that some of the identified end-members are in fact not representative of deep mantle domains, but rather reflect shallow heterogeneities located in the lithosphere or the crust? One way to shed light on these questions is to focus trace element and isotopic studies on within-plate magmatic lineaments that straddle the continent–ocean boundary.
alkaline complexes; continental lithosphere; isotope geochemistry; passive continental margin; within-plate volcanics KEY WORDS:
INTRODUCTION Isotopic data from within-plate basic magmatic rocks (i.e. oceanic island basalts, continental flood basalts, etc.) have provided part of the basis for the recognition of a heterogeneous mantle consisting of four or more isotopically distinct reservoirs (e.g. Zindler & Hart, 1986; Alle`gre, 1987; Macdougall, 1988; Peng et al., 1994). However, unlike mid-ocean ridge basalts which form by
Fig. 1. Sketch map showing location of Mount Ormonde and Serra de Monchique alkaline complexes [modified from Laughton et al. (1975)].
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BERNARD-GRIFFITHS et al.
COMPOSITION OF ALKALINE COMPLEXES
Table 1: Petrographic description of the whole-rock samples Samples Rock types
Texture
Alteration Primary minerals
Secondary minerals
intersertal
5%
Ne, Or, Ab, Cpx, Agt, Sdl, Phl/Bt, Ttn, Ap, Mag, Lav, Pcl
Zeo, Fl
Zeo
Serra de Monchique 11780
nepheline syenite
11781
alkali gabbro
granular
5%
Krs, Cpx, Pl, Or, Phl/Bt, Ttn, Ap, Mag
11782
monchiquite
porphyritic
5–10%
Spl, Ol, Cpx, Krs, Ap, Mag, Gl
Zeo, Carb
11783
tinguaite
porphyritic
5%
Cpx, Sa, Ne, Hyn/Nsn, Agt, Ttn, Ap
Zeo
11784
lamprophyre
porphyritic
5–10%
Spl, Ol, Cpx, Krs, Phl/Bt, Ne, Pl
Zeo, Smc
11785
lamprophyre
porphyritic
5–10%
Cpx, Krs, Mag, Ap, Gl
Zeo, Smc, Carb
11786
nepheline
intersertal
5%
Ne, Or, Ab, Cpx, Agt, Phl/Bt, Sdl, Ttn, Whl, Mag, Ap
syenite
Mount Ormonde 11286
limburgite
porphyritic
10%
Spl, Ol, Cpx
11287
nephelinite
porphyritic
20%
Cpx, Phl/Bt, Krs, Mag, Ilm, Ttn, Ap, Hyn/Nsn, Ne
Smc, Hydr, Carb Smc, Hydr, Carb
11288
tinguaite
porphyritic
20–30%
Cpx, Phl/Bt, Krs, Mag, Ilm, Ttn, Ap, Hyn/Nsn, Ne, Sa, Agt
Smc, Hydr
11289
phonolite
trachytic
20–30%
Cpx, Phl/Bt, Krs, Mag, Ilm, Ttn, Ap, Hyn/Nsn, Ne, Sa, Agt
Smc, Hydr
11290
phonolite
trachytic
