TRIASSIC THOLEIITIC DOLERITES (<<OPHITES») OF THE EL GRADO DIAPIR (PYRENEES, HUESCA, SPAIN): EMPLACEMENT AND COMPOSITION

Mesozoic dolerites in the south Pyrenean sector of El Grado (Huesca, Spain) preserve emplacement structures (fluidity structures at the top and load structures at the base) developed during their intrusion into unconsolidated marly-evaporitic Triassic sediments (Keuper facies). By analogy with other dolerites in the south Pyrenean domain, their emplacement age is equivalent to the uppermost Keuper facies terms, but prior to the final Triassic-early Liassic carbonated sediments. Radiometric ages (l87-197±7 Ma) show that the emplacement occurred during the lower Liassic. The petralogical differentiation fram the chilled margin facies to the central facies, and also to the late pegmatitoids, is consistent with that obtained fram major elements, trace elements and REE. Their tholeiitic affinity, as defined by their geochemical composition, is equivalent to that of similar racks in the Pyrenean domain. However, the rocks analyzed here, which are located at the external sector of this domain, display a greater petralogical and geochemical differentiation as compared to similar rocks in the central sectors of the Pyrenean domain.


Introduction
The Triassic tholeiitic dolerites (<<ophites») of the El Grado diapir (Huesca, Spain) preserve the main features of a subvolcanic emplacement mechanism, and constitute a useful reference in the study of this type of magmatism, which is wideiy represented in the Pyrenean domain (fig.lA).These dolerites, which outcrop as two sills, show a series of ideal characteristics for their study: 1) the present exceptionally well preserved fluidity structures, 2) they induced a very low grade contact metamorphism on their hosting sediments and 3) the three well represented rock-types (chilled margin, central facies and pegmatitoid 1 differentiate) display mineralogical and geochemical compositions typical of this magmatism and its crystallization conditions.In this sense, the El Grado ophites are even more interesting than the Estopiñán and Laredo outcrops (Pyreneean tholeiitic peri-atlantic domain) which were used as references for this type of magmatism in the Pyrenees (Lago &Pocovi, 1982 and1984).Finally, our paper offers new data on the mineral composition evolution of dolerites, specially in their late stages (amphiboles and biotites), as well as more comprehensive geochemical data regarding the dolerites of the south Pyrenean domain (major, trace elements and REE).

Geological setting, age and emplacement conditions
The dolerite outcrop of the El Grado diapir (figs.lB and l C), was emplaced within marly-evaporitic sediments of the Triassic Keuper facies.This diapir belongs to a lineation of diapirs connecting the «Sierras Marginales Aragonesas» and the «Sierras Marginales Catalanes» (Martínez & Pocovi, 1988).Two different kinds of processes were involved in this diapirism: 1) piercing of the Mesozoic and Tertiary cover in anticlinal cores and thrust fronts, and 2) diapir development by halokinesis of Late Triassic evaporitic sediments.The former process belongs to the Pyrenean tectogenesis, characterized by folding and sliding of cover materials towards the south, to form a complex thrusting sequence during the Late Eocene-Oligocene (Martínez, 1982;Cámara & Klimowitz, 1985;Martínez & Pocovi, 1988).The later represents the rise of plastic and less dense materials from zones where they had concentrated during the first pro-  which underwent very low grade contact metamorphism (Lago, 1980).The plasticity of these hosting sediments greatly favoured the magmatic transport and the development of fluidity structures at the top of the sill, and load structures at its base.These processes are the result of the weight of the magmatic mass acting over the hosting materials, under confinement conditions.Fluidity structures and fragments of the dolerite chilled margin are observed at the top of the sill, under the hosting marly-c1ayey sediments (fig.2C  and 2E).These fluidity structures show displacement of the magma in different (even opposite) directions within the hosting plastic sediment stratification plane (fig.2A).Typical examples of fluidity structures are constituted by «pillows» (pillow-sill type, without radial fractures, fig.2B), flow undulations and «ropy» forms (figs.2C, 2D, 2E and 2F).Their surfaces are helicoidal (figs.2C and 2D), and they show multidirectional patterns of magma displacement.Similar structures have been described by Kokelaar (1982 and1986), Kano (1989) and Morata (1993).These «ropy» structures, similar to those observed in subaerial flows can be explained by the conjunction of three factors: a) the low viscosity of the basaltic magma and also h) the low viscosity of the hosting plastic sediments, still unconsolidated and water-saturated.These two conditions Iead to a low viscosity contrast; finalIy e) the injection of the magma as a wedge, expanding laterally between relatively more competent sedimentary Ievels.
In a few cases, isolated or coalescent degassing vesicles appear (usualIy filled with epidote).Load and Fig. 6.-PlagiocIase cIassification in the Ab-Or-An system.A progressive variation of the composition is observed from the chilled margin to the central facies and pegmatitoid...  et al., 1988).b) Cr vs mg* plot.mg* stands for the Mg/(Mg+Fe 2 +) ratio (cations per formula unit), c) Ti \'.1' Ca+Na plot (Leterrier et al., 1982).
Pyroxenes in the marginal and central facies plot into the subalkaline (S) field and only the more evolved compositions of the pegmatitoid falI into the alkaline field (A).-    - el)   ãccomodation structures of the chilled margin to the hosting plastic sediments are common at the base of the sill.These elements, as well as the weak contact metamorphism of the hosting Keuper sediments, and the absence of aerial structures, indicate an emplacement coetaneous or slightly subsequent to the Late Triassic sedimentation.This kind of emplacement into unconsolidated materials (fig.3A and 3B) excludes the effect of the lithostatic pressure caused by the weight of a significant sedimentary column (Lago & Pocovi, 1982).Thus, emplacement happened before the sedimentation of the Rhaetian and Liassic calcareous series.The range of radiometric ages (187-197±7 Ma) obtained in other Pyrenean dolerites (Montigny el al., 1982, Walgenwitz, 1976 & Beziat,  1983), compared with the new Mesozoic age scale (Gradstein el al., 1994), shows that the dolerite was probably emplaced during the Liassic (between the Hettangian and Toarcian).This implies that the upper-Established Iithotypes most section of the Keuper facies and the lowermost calcareous series are Early liassic in this south Pyre-The established lithotypes (chilled margin, cennean domain, unless the available radiometric ages tral facies and pegmatitoid differentiate) are simi-(which show a broad dispersion of values) require lar to those described in other Pyrenean tholeiitic revision and confirmation using other methods.
dolerites (Lago, 1980;Azambre et al., 1987).A  altered, but they are abundant in the pegmatitoid differentiate (2 %).They are ususally ilmenite or magne-Mineral composition was obtained with CAME-tite (table 6).These ilmenites crystallized later; they CA SX-50 microprobes (Barcelona, Toulouse and have microscopic rutile inclusions and their edges are Oviedo universities) under standard conditions leucoxene or pseudo-brookite.These microtextural (15Kv, lOnA, 1 mm beam diameter and 10 seconds criteria indicate that ilmenite may have undergone integration time).Except for the olivine (altered to late destabilization processes, thus precluding the use clinochlore: table 1), the centre-to-edge profiles of geothermometers.Sulfides are scarce « 1 %), and reflect the complete evolution of mineral phases.
-- abundant in the pegmatitoid.Prehnite appears occasionally in these rocks.

Geochemical composition
Table 9 shows 8 new analyses of unaltered rocks (average L.O." in 1.18 %) analyzed by FRX and ICP-MS at the XRAL Laboratory (Toronto, Canada), together with the average of 6 other south Pyrenean «ophites» (Beziat el al., 1991).Their subalkaline composition might be deduced from the %Si0 2 vs Nb/Y diagram (fig.9a; Winchester & Floyd, 1973).The mg* (Mg/Mg+Fe) range of 0.61-0.53,together with the Ni (86-43 ppm) and Cr ranges (240-41 ppm) suggest differentiated liquids with chilled margin fractionation in relation to the central facies.This is even more clear in the pegmatitoid.Fractionation (AFM in fig.9b) is also shown by the relations between the compatible (e.g.Cr; fig.9c) and incompatible (e.g.Ce, Th; fig.9d) elements.These fractionation trends are mainly controlled by the pyroxene modal proportion and evolution (Cr and Co).The rare olivine (with a high partition coefficient for Ni) has almost no significance in our rocks, in contrast to other south Pyrenean ophites with higher proportions of this mineral (Azambre el al., 1987;Beziat el al., 1991).Plagioclase fractionation has been confirmed by the increase in Eu, with higher values for the pegmatitoid.These patterns indicate a slightly higher differentiation in our rocks (from the southern Pyrenees) in relation to other ophites located c10ser to the central Pyrenees (Beziat el al., 1991).Ti (0.98 % in the chilled margins) presents higher values in the pegmatitoid (1.24The multielemental pattern normalized to chon-(table 10).Additionally, the alkalinity index (A.1.= drite (Sun & McDonough, 1989) shows (fig.10) Na20 + K 2 0/(SiO r 43) X 0.17; Midd1emost, 1975) high Ba and Th contents, a strong negative anoma1y values, with an 1.99 average a1so indicate this thoin P, low Ti values, and slightly decreasing values leiitic affinity, when plotted against A1 2 0 3 of La-Nd and Zr-y.The REE pattern (fig.11), nor-(fig.12A).The obtained values coincide well with ma1ized to MORB-N (Hoffmann, 1988) shows a those from other Pyrenean continental tholeiitesslight enrichment in LREE and a depletion in -«ophites»- (Alibert, 1985;Beziat et al., 1991).HREE, together with a weak positive Eu anomaly.However, the mineral associations of our rocks Both diagrams evidence a weak fractionation of the show, a higher degree of differentiation than other chilled margin and central facies, a process which is samples from the internal Pyrenees, where occasioslightly stronger in the pegmatitoid.On the other nal olivine cumulates or slight enrichments in this hand, the scarce data from other south Pyrenean mineral might be observed (Azambre et al., 1987).«ophites» (fig. 11;values from Beziat et al., 1991) This pattem of higher differentiation can be dedufit the observed pattern.ced in the Cr vs mg* (fig.12B) diagram, where the A tholeiitic affinity can be deduced by the avera-values of Azambre et al. (1987) and Beziat et al. ge ratios (tab1e 10) of Nb/Y, Th/La, Th/Hf, La/Lu, (1991) have a1so been plotted.This behaviour sug-La/Yb and K/Rb, together with low Ti0 2 and P 2 0 S gests an evolution of these tholeiitic liquids from values, which are consistent with the ratios of the centre (first stage) towards the external zones of Zr/Ti0 2 , Ti/Zr, Ti/V, Nb/Zr and Y/Zr relations the Pyrenees (late stages).Conclusions fluidity structures and a very low grade contact metamorphism at the top of the sills indicate that the dole-The El Grado tholeiitic dolerites were emplaced as rites where emplaced after the deposition of these subvolcanic sills beneath a poorly compacted sheet of sediments and before the Liassic carbonates, concormarly-evaporitic Keuper sediments.Development of dant with the available radiometric ages.The dolerites   1988).Only the pegmatitoid facies displays a significant fractionation.

Fig. 3 .
Fig. 3.-(a) Schematic dolerite emplacement model with location of structures shown in Fig. 2. (b) idealized outcrop section,showing the relative location of ropy structures and lava toes.
Fig.9.-a) Rock classification in the %Si0 2 vs Nb/Y system (Winchester & Floyd, 1973).b) AFM diagram, showing the compositions of the studied rocks and, also, composition of the less evolved «ophites» analysed by Beziat el al. (1991) and Azambre el al. (1987).Note the increase of the F/M ratio, typical of tholeiitic series.c) Cr vs Ce plot and d) Ce vs Th plot show nearly linear differentiation trends, typical of a fractional crystallization process.

Table 3
tiate consists of a centimetric vein of irregular geometry, cross-cutting the other lithotypes.It has ilmenite, potassic feldspar and quartz filling the a doleritic texture, with major augite and plagio-intercrystalline spaces (sometimes with microclase development, and with amphibole, biotite, graphic textures; fig.4).

Table 5 .
-Biotite compositions in central facies (22be observed from the chilled margin to the central