GEOCHEMISTRY AND TECTONIC SETTING OF THE « OPHITES » FROM THE EXTERNAL ZONES OF THE BETIC CORDILLERAS ( S . SPAIN )

Mesozoic basic magmatism in the External Zones of the Betic Cordilleras (S. Spain) is represented by small tectonic bodies (ophites) in Triassic formations, and submarine flows with abundant pillow-Iavas interbedded with Jurassic sediments. Both basic igneous manifestations suffered very lowto low-grade metamorphism, more intense in the case of the ophites. Two types of ophites are distinguished on the basis of their primary mineralogy. In the first type, orthopyroxene is present in the less differentiated products. Clinopyroxene and Ca-plagioclase are the main primary phases and quartz appears in the more evolved rocks. In the second type, olivine is present in the less differentiated products. Ti-rich augite and Ca-plagioclase are also important primary minerals, but quartz is absent. Whole-rock chemistry (major and trace elements, including REE) also allows us to discriminate between these two groups. The first group has higher SiOz and lower TiOz, PzOs and lower NazO/KzO ratios than the second. Normative quartz is almost invariably present in this first group, whereas normative nepheline (lower than 5%) is characteristic of the second group. Both groups are Sr, K, Ba, Rb, Th, Nb and Ce enriched with respect to normal MORB, but the first group has higher K, Rb, Ba, Th and lower Nb, Ce contents than the alkaline group. NblY and TiN ratios are also different and show a tholeiitic affinity for the first group and transitional to alkaline for the second. Chondrite-normalised REE patterns in both groups are similar and characterized by LREE enrichment with respect to HREE. LREE/HREE ratios are, however, slightly higher in the transitional to alkaline group. Various discriminant tectonic diagrams indicate a continental intraplate setting for both ophite groups. This magmatism is related to the first extensional period of the Betic Cordilleras, during the Triassic-Jurassic. Geochemical differences between the two groups could evidence different degrees of crustal contamination, and/or different mantle sources. Trace element ratios [(La/Ce)n> 1, La/Nb < 1.5] for both magmatism are indicative of an enriched mantle source. Differences in ThIYb, Zr/Nb, ZrIY and Ba/Zr ratios underline the greater influence of a lithospheric component in the case of the Triassic magmatism. Key wOl"ds: Ophites: Continental intraplate magmatism; Mesozoie; External Zones; Betie Cordilleras.


Introduction
Basic rocks of Mesozoic age are present as intercalations in Mesozoic sedimentary formations from the External Zones of the Betic Cordilleras, Southern Spain.Based on the geology of the outcrops and the geochemical data, these rocks can be divided into two types: a) subvolcanic rocks emplaced into the Triassic sedimentary units (Trias Keuper), mostly preserved as small tectonic blocks, and classically named as ophites in the Spanish geological literature 1; and b) basaltic volcanic rocks (pillow-lavas, sills and locally hyaloclastites) intercalated with the Jurassic sedimentary sequence.This magmatic province, known as the Jurassic Volcanic Province (Comas et al., 1986;Puga et al., 1989), is associated with a deep ENE-WSW fault system.
Based on major and trace elements, Puga and Ruiz-Cruz (1980), Comas et al. (1986), Puga and Dfaz de Federico (1988) and Puga et al. (1989) inferred a tholeiitic affinity for the ophites and an alkaline-sodic affinity for the basaltic rocks of the Jurassic Volcanic Province.Only a few petrological J In this paper the term «ophite» is applied to the small basic igneous bodies present in the Trias Keuper of the External Zones of the Betie Cordilleras, without consideration of their age, outcrop style or geochemical affinity.and geochemical studies have been carried out on the ophites, due to the relatively small size of the outcrops and the alteration of some of them.Recently, Morata et al. (1991), Morata (1993) and Morata and Puga (1993) have studied in detail the mineralogy and geochemistry of the ophites, suggesting the tholeiitic affinity of some of these basic rocks, and the more alkalic character of others.
The aim of this paper is to present new geochemical data (major and trace elements, including REE) to demonstrate this duality in the geochemical affinities of the ophites from the External Zones of the Betic Cordilleras.A geodynamic framework, based on the mineralogy and the geochemistry, together with radiometric ages, is proposed for this magmatism.

Geological setting and petrography
The Betic Cordilleras, along with the Rif in Northern Morocco, are the westernmost extension of the Mediterranean Alpine chains.The Betic Cordilleras are located in southern and southwestern Spain, and are divided into several major domains: the External and Internal Zones, the Flysch units, and the Neogene basins (fig. 1).The External Zones are located on the southern margin of the Iberian plate.Azema et al., 1979) with the location of the studied outcrops (black dots) and analysed rocks (number of analysed sample in table 1).
They received sediments during the Mesozoic and part of the Cenozoic and were subsequently deformed and detached from the South-Iberian passive margin.Their evolution began during the Triassic, with an extensional episode that continued until the Early-Mid Cretaceous boundary.Significant volumes of volcanic rocks were erupted during this period (see Garcfa-Hernandez et al., 1980;Sanz de Galdeano, 1993 and references therein).This volcanic activity is represented in two different domains with different outcrop styles.The first domain corresponds to small (several hundred metres), basic igneous bodies (ophites), mainly intercalated as tectonic blocks within Triassic evaporitic sediments (Trias Keuper, fig. 1),along a length of ca.500 km.The second domain corresponds to submarine volcanic rocks (pillow-Iavas) and sills intercalated with the Jurassic sedimentary rocks.These rocks crop out over an area 200 km long and 5 to 10 km wide, in the central part of the External Zones.Volcanic flows and sills are tenths up to hundred meters thick in some localities.

Petrography and age of the ophites
Three different petrographic facies can be distinguished in the ophites: chilled margins, central facies and pegmatoidal differentiated facies.The primary mineralogy is very homogeneous, and only slight variations in the compositions have been found.Clinopyroxene and plagioclase phenocrysts are the main primary phases; however, two groups can be distinguished among the ophites.In the first group, orthopyroxene (W04.5En7S.SFs20 to WOSEn76Fs19) is present in the less differentiated rocks.Calcic plagioclase (AnSO-4S) and calcic clinopyroxene (Wo36EnS2Fs12 to W037En39Fs24) are the major components.Pigeonite (WoSEn6SFs24 to WoIOEnS3Fs37) appears as a minor phase associated with augite.Quartz is present in the more evolved rocks as single crystals or intergrown with sodic plagioclase.Amphibole (hornblende and Fe-hornblende), biotite, minor apatite and Fe-Ti ores are accessory minerals.The second group is characterized by the presence of olivine (Fo n _ M ) in the more   .80 13.70 13.60 13.30 11.90 19.00 13.80 31.80 29.10 27.60 29.10 25.90 40.50 30.10 16.20 14.40 18.90 17.10 15.90 25.10All these basic igneous rocks have been affected by very low-to low-grade metamorphism, giving secondary assemblages in the prehnite-actinoiite, prehnite-pumpellyite and pumpellyite-actinolite facies (Puga et al., 1983;Morata, 1993;Morata et aI., 1992Morata et aI., , 1994)).The metamorphic minerals partly replace the primary phases, although the primary igneous textures are always preserved.This replacement can be complete, except for ciinopyroxene, which is preserved as a relic mineral.Clinopyroxenes are completely replaced by metamorphic assemblages only in some metabasites, in which metamorphic Na-pyroxene and metamorphic Na-amphibole are present (Morten and Puga, 1983;Morata et al., 1994).Radiometric age determinations (KJAr method) Betic Cordilleras can be distinguished from mineraand field relations (Morata, 1993;Portugal-Ferreira logy, radiometric ages and field relations as: et al., 1995) have shown that the orthopyroxene (1) Triassic ophites, and (2) post-Triassic ophites.bearing ophites are Late Triassic in age (182 ± 9 to 187 ± 4), while the olivine bearing ophites are post-Geochemistry Triassic, and probably Jurassic (137 ± 4).Therefore the two groups of ophites present in the Triassic The new chemical data set presented in this paper sedimentary rocks of the External Zones of the (table 1) was obtained in the X-Ray Assay Labora-    (Morata et al., 1996), confirm such an interaction  ---'_-'---'''"_-'---''"_"'--.....
tes (1 to 1.5% and 0.1 to 0.2%), and these elements exhibit a negative correlation with [mg].Nevertheless Ti0 2 has a contrasted behaviour in the pegmatoidal fades: in the first group, Ti0 2 clearly increases with differentiation, whereas in the second group Ti0 2 content decreases for [mg] values less than OA-0.5.This decrease can be explained as a consequence of the previous crystallization of high-Ti clinopyroxenes in these ophites.High P20S, contents in the pegmatoidal fades are due to an increase of the proportion of modal apatite.MnO exhibits small variations in both groups (within the range 0.1-0.3),and shows a slight negative correlation with [mgj.The differentiated rocks have high FeO IOI which, in both groups, exhibit a negative correlation with [mgj.Trace element variations with respect to [mg] values are presented in figure 4. Scatter in Ba, Rb and Sr contents may be a consequence of their high mobility during secondary alteration and low-grade metamorphic processes (Smith and Smith, 1976;  Dickin and Jones, 1983; Dostal and Strong, 1983;  Merriman et al., 1986, among others), with more dispersed values in rocks with higher L.O.I. contents.Zr, Nb, Y, Th, Hf and REE are more immobile during secondary processes (Ludden et al., 1982;Merriman et al., 1986), and show a good negative correlation with magmatic differentiation.However, Zr and Y show more scattered patterns in the post-  Triassic group than in the Triassic group.Transition elements (Cu, V, Sc, Ni, Cr) show rather less scattering, except for Co.In the post-Triassic rocks, V exhibits differentiation trend similar to that of Ti0 2 , with concentrations increasing when differentiation decreases (up to [mg] values close to 0.5) and decreasing after this value.Cu contents increase with decreasing [mg] values in the Triassic group.This «incompatible» behaviour of Cu was considered as characteristic of continental tholeiite suites (Dupuy and Dostal, 1984).In the post-Triassic rocks, Cu contents decrease with magmatic differentiation, and has therefore a compatible behaviour.REE abundances increase with decreasing [mg] (fig.4).No variation between LREE and HREE abundances (expressed as (Ce/Yb)n) and [mg] value is observed in the Triassic ophites.In the post-Triassic rocks, a slight increase of the LREE is observed in the most differentiated rocks.A slightly positive trend is observed between (Eu/Eu*)n and [mg] in both groups, in accordance with the crystallization of less calcic plagioclase in the more differentiated rocks.
Variations in trace element abundances are also shown on multi-element diagram (fig.5), normalized to N-type MORB (Pearce, 1982).Mean values and the total variation in the Triassic and post-Triassic ophites are shown for the three petrographic facies defined earlier.Both groups are enriched in Sr, K, Ba, Rb, Th, Nb and Ce, with higher contents in K, Rb, Ba, Th and lower values in Nb and Ce in the Triassic than in the post-Triassic rocks.In both groups, elements from P to Sc have concentrations similar to N-MORB, even if their abundances are slightly higher in the post-Triassic rocks.Slight differences exist in the patterns of the chilled margin and central facies in both types of ophites.Larger variations can be observed when comparing the central facies and pegmatoids: incompatible elements increase and Cr decreases.The opposite behaviour is observed in the picritic dolerite.Enrichment in Ba, Rb, Th, Sr and LREE and a decrease in Nb with respect to the N-MORB values as observed in the Triassic group is characteristic of continental tholeiites.
REE diagrams of the mean values in both ophite groups are shown in figure 6.Only slight differences are observed: the post-Triassic group has MREE and (La(Yb)n values higher than those of the Triassic group.In general, the ophites are enriched in LREE with mean (LalYb)n values ranging from 3.7 to 4.2 for the Triassic group and 4.4 to 4.8 for the post-Triassic, and (LalSm)n ratio ranging between  2.0 to 2.3 in the first group and 1.9 to 2.3 for the second.All samples plotted in the chondrite-normalized REE plot have approximately parallel patterns, and their (LREE/HREE)n ratios can be considered as typical of continental tholeiites and transitional basalts (Siders and Elliot, 1985, among others).

Geodynamic setting
sensitive to continental contamination processes.Thus, basaltic magmas contaminated with materials belonging to the upper continental crust would be richer in Th, and with high ThlTa ratios.The higher Tr contents of the Triassic ophites can be related to a thicker crust during the first extensional stages.This point will be discussed in more detail in the end of this paper.
A number of diagrams based on the geochemical composition are classically used to identify the tectonic setting of ancient basalts.A useful discriminant diagram for this purpose is the Th:Hf/3:Ta (fig.7) of Wood (1980).On this diagram, the Triassic ophites plot mainly in the field of the calc-alkaline destructive margin basalts (field D2), with some points falling in the field of within-plate basalts (field C).The post-Triassic ophites are scattered into the fields of the enriched MORB, tholeiitic within plate basalts and alkaline within-plate basalts (fields Band C, respectively).It is interesting to note that the Triassic ophites follow the crustal contamination trend proposed by Wood (1980).According to this author, the Th:Hf:Ta ratios are

Geochemical affinity of the ophites
The chemical affinity of these basic rocks can be established from the chemistry of their igneous clinopyroxenes as well as from their whole-rock geochemistry.In the Triassic ophites, low Ti and Ca contents of the clinopyroxene, together with the presence of pigeonite and orthopyroxene, are typical of tholeiitic basalts.On the other hand, in the post-Triassic ophites, the higher Ti and Ca contents of the clinopyroxenes are indicative of a more alkaline affinity (Morata and Puga, 1993).
Certain trace element ratios are useful to constrain the geochemical affinity of these basic rocks.All the Triassic ophites have a TiN ratio lower than  (Wood, 1980).
A =N-MORB; B =E-MORB and tholeiitic within-plate basalts; C = alkaline within-plate basalts; Dl = island-arc tholeiites; D2 = calc-alkaline basalts.Arrow shows the crustal contamination trend if the contaminant belongs to the upper continental crust (Wood, 1980).Same symbols as in figure 2. As Ta concentrations are lower than the detection limit.Ta contents were calculated using a Nbffa =16 ratio (c.! Wood et al., 1979).50, whereas in the post-Triassic ophites this ratio is close to or higher than 50.These values argue with the limit proposed by Shervais (1982) to discriminate between tholeiitic and alkaline basalts.On the other hand, the Nb/Y ratio (Winchester andFloyd, 1976, 1977) is considered as a good parameter to discriminate tholeiitic and alkaline basaltic rocks.Pearce (1982) concluded that rocks with Nb/Y ratios higher than 1.0 have an alkaline affinity, whereas those with Nb/Y ratios lower than 0.5 have a tholeiitic affinity; ratios between 0.5 and 1 characterize transitional affinity.Nb/Y ratio ranges between 0.2 to 1.2 (mean = 0.4) for the Triassic group, whereas for the post-Triassic rocks, its extreme values are between 0.2 and 3.7 (mean =0.6).These Nb/Y ratios are independent of the degree of evolution, but could be affected by secondary processes such as low-grade metamorphism.Higher contents of Ti, P, Zr and Hf (fig.5) and LREE (fig.6) in the post-Triassic ophites (fig.5) are also in agreement with their more alkaline affinity.
In summary, according to the clinopyroxene chemistry and whole-rock geochemical signatures, the Triassic ophites, in which orthopyroxene is present, have a tholeiitic affinity, whereas the post-Triassic ophites, which contain olivine, have a transitional to alkaline affinity.

Discussion and Conclusions
During the Triassic and Jurassic, basic magmatic activity intruded in the Triassic evaporitic formations of the External Zones of the Betic Cordilleras.Geochemical, radiometric ages and field data enable us to conclude that this magmatism began in the Late Triassic with a tholeiitic signature, and was followed by transitional-alkaline subvolcanic bodies.
Low Cr, Ni, and [mgJ values in both magmatic groups indicate the absence of true primitive liquids.Moreover [mgJ values close to 0.5-0.7 are typical of continental basalts which have experienced fractional crystallization (Cox, 1980;Dupuy and Dostal, 1984;Bellieni et aI., 1984).Variations in major and trace elements are consistent with lowpressure crystallization of clinopyroxene and plagioclase.The influence of orthopyroxene and olivine fractionation is reflected in the Cr and Ni contents, respectively.A narrow range in Si0 2 is typical of continental basalt magmatism, despite the large volumes of magma involved in this process (Bertrand, 1991).Some important differences have been identified between the two rock types and concern mainly TiO b P 2 0 S ' Th, Nb, Zr, Cu.The use of various diagrams for the geodynamic discrimination of paleobasalts (e.g.fig.7) have shown that both of ophites have a geochemical signature characteristic of continental intraplate magmatism.High Th and low Nb contents in the Triassic magmatism (fig.5) could be a consequence of crustal contamination of mantlederived melts during their ascent throught the crust, prior to their final emplacement into the Triassic sediments.Using major and trace element data, Puga et al. (1989) have shown chemical patterns indicative of granitoid contamination during the ophite ascent through the continental crust.Moreover, the presence of metapelitic xenoliths and xenocrysts in some post-Triassic ophites (Morata and Puga, 1992) indicates, that crustal contamination must have played a crucial role in their petrogenetic evolution.
The different chemical signatures of the Triassic and post-Triassic magmatism could be a consequence of different tectonic regimes during the emplacement of the magmas.According to the estimated age of the ophites, at the beginning of the extensional event, the continental crust would have been thicker.Under these conditions, continental contamination could have operated, resulting in relatively low Nb and high Th contents.Crustal contamination can also explain the production of SiOrrich melts, in which orthopyroxene and pigeonite crystallize.In the case of post-Triassic magmatism, occu-  Pearce, 1982, 1983).SHO =shoshonitic series; CA = calc-alkaline series; TH = tholeiitic series.U.C. =average composition of the upper continental crust (after Taylor and McLeannan, 1985).Primitive mantle (P.M.), N-MORB, E-MORB and om values from Sun and McDonough (1989).Tholeiitic dolerites from Northern Morocco (Bertrand, 1991) and Pyrenean ophites (Beziat et al., 1991) are respectively plotted as (+) and (x) for comparison.Ta calculated as in figure 7.
rring in a well established extensional regime evolving towards oceanic conditions, the basalts are less contaminated, allowing the eruption of more primitive olivine basalts.On the ThlYb versus Ta/Yb diagram (fig.8) Triassic rocks plot outside the mantle array, probably due to Th enrichment through crustal contamination, whereas the post-Triassic magmatism plots along the mantle array, with an evolution in accordance with fractional crystallization of within-plate basalts.In this last magmatism, chemical evidences of crustal contamination are restricted to some areas in which xenoliths and xenocrysts are present.In fact, in the MORB normalized diagrams (fig.5), the enrichment in Th (Ba, Rb, Sr) and LREE and decrease in Nb in the Triassic ophites can be interpreted in terms of crustal contamination  and Dostal, 1984;Dostal et al., 1986;Thompson et al., 1984), or as a consequence of their origin from an enriched subcontinental mantle source (Hawkesworth et al., 1983;Menzies et al, 1983, among others).
Thus, another possible genetic factor controlling the chemical differences recorded by these basalts is a different mantle source.Both magmatisms (Triassic and post-Triassic) have (La/Ce)n > I indicating an origin from an enriched mantle source (Marcelot et al., 1989).But higher ThlNb and lower Zr/Sm ratios in the Triassic magmatism could evidence some differences in their original mantle source.In this sense, the Zr/Nb vs Zr/Y plot (fig.9) also reflects important differences.Both magmatisms plot between the extreme OIB and E-MORB poles, having the post-Triassic magmatism more similitude with the OIB end-member.On the other hand (La/Nb) < 1.5 for both magmatism is indicative of an asthenospheric source (Thompson and Morrison, 1988), but higher La/Nb ratios in the Triassic magmatism could indicate a lithospheric (mantellic?, crustal?) component.Also, higher Ba/Zr values in the Triassic magmatism (in spite of the Ba secondary mobility) could indicate, according to Fitton et at. (1995), a greater lithospheric component.
In conclusion, multiple factors (magma source, crustal contamination, primary differentiation and secondary alteration) must contribute to the final bulk geochemical signature and it is difficult to elucidate the role of each one.This Mesozoic magmatism is related to the geodynamic evolution of the South-Iberian passive margin in connection with the opening of the North Atlantic.Its chemical characteristics suggest a progressive increase of the extension of the continental crust from the Triassic to the Jurassic, characterized by crustal thinning and rela- Th/Vb ted magmatism.This magmatism, generated from an enriched mantellic source, evolved from tholeiitic to transitional-alkaline, with a decreasing influence of continental crustal contamination and an increasing influence of an asthenospheric mantle component.According to this model, the tholeiitic signature of the Triassic magmatism could be a consequence of their higher lithospheric contamination during their petrogenetic evolution.
Fig. I.-Geological map of the Betic Cordilleras (modified fromAzema et al., 1979) with the location of the studied outcrops (black dots) and analysed rocks (number of analysed sample in table1).
Fig. 3.-Major element variations (oxides calculated on an anhydrous basis) versus [mg] values.[mg] calculation is indicated in table 1. Same symbols as in figure 2.

Fig. 5 .
Fig. 5.-Pearce (1982) multi-element diagram of basalts from the Betic Cordillera normalised to normal MORB.For each facies, mean values and ranges (shadow areas) are plotted.Fig. 6.-Chondrite-normalised REE patterns (values for chondrite according to Nakamura, 1974) for the mean and range values of the different petrographic facies.Mean (La/Yb)n values for each petrographic facies are indicated.

Fig. 8 .
Fig. 8.-TalYb vs ThlYb diagram in which mid-ocean ridge and within-plate volcanic rocks plot along the diagonal band.C indicates continental contamination trend, and W mantellic enrichment.Th/Yb and TaIYb increase due to fractional crystallisation according to f (afterPearce, 1982, 1983).SHO =shoshonitic series; CA = calc-alkaline series; TH = tholeiitic series.U.C. =average composition of the upper continental crust (after

Table I (
Puga et al. (1989)alyses of the ophites from the External Zones of the Betic Cordilleras (major element oxides in weight per cent calculated on an anhydrous basis, and trace elements in ppm).a)Triassictholeiiticophites.b)Post-Triassictransitional-alkalineophites.[mg]valuesarecalculated as Mg/(Mg+Fe 2 +) in atomic proportions, with Fe 2 +/Fe 3 +=0.15.b:chilled margins; c =central fades; p =pegmatoidal differentiated fades; p.d. =picritic dolerite.Q, By,Ne, and 01 are normative quartz, hypersthene, nepheline and olivine, respectively.Fe, Mg, Ca, Ba, Rb, Nb, Sr and Zr; ICP a relatively good concordance.Previous chemical for Mn, P, Y, Ni, Co, Sc, Pb, Cu, Zn and Li; NA for analyses inPuga et al. (1989)have been reinterpre-Th, Cr, Ta, Hf, and U; AA for Na and K; DCP for V ted based on the two groups here defined.With the Almost all the Triassic ophites are hypersthene aim to check the two analytical laboratories, double normative, and some of them are oversaturated in and ICP-MS for REE, and in the Laboratoire de Petrologie Magmatique, Universite d' Aix-Marseille III (France) (analyses 66 to 79), using AA for Mn, Whole rock chemistry Na, K, Rb, Nb, Sr, Y, Cr, V, Ni, Pb and Cu, and ICP for Si, Ti, AI, Fe, Mg, Ca, P, Ba, and REE.

Table 1 (
cont.)-Chemical analyses of the ophites from the silica (normative quartz; table 1).This is consistent External Zones of the Betic Cordilleras (major element