THE TRIASSIC ALKALINE DOLERITES OF THE VALACLOCHE-CAMARENA AREA ( SE-IBERIAN CHAIN , TERUEL ) : GEODYNAMIC IMPLICATIONS

The dolerite sills outcropping in the Valacloche-Camarena area (SE Iberian Chain), are the expression of an alkaline magmatism, emplaced in Keuper facies sedimentary rocks. Their pre-Hettangian age is deduced from the development of fluidality structures at the top of the sills together with the very low grade contact metamorphism of the host rocks. A differentiation trend, represented by two rock-types, with variable Ti-augite content, is confirmed by geochemical data (REE). The alkaline composition of this magmatism is close to that of the OIB type. Crust-derived enclaves (metapelites and granitoids) are common in these sills, suggesting that magma ascent took place through a fracture system, related to a distensive tectonic regime, that affected different levels of the crust. This magmatism was one of the expressions of the triassic rifting events that are well represented in the SE border of the Iberian Chain.


Introduction
. Preliminary petrological and geochemical data on this magmatism were obtained by Lago el al. (1996), showing its differences when com-pared with a coeval tholeiitic magmatism outcropping in the Valencia area (described in Lago et al., 1999).
This triassic magmatism is exposed in more than a hundred outcrops within seven big diapiric units in this part of the Iberian Chain: Villel, Valacloche-Camarena, Sarrión, Albentosa-Santa Cruz de Moya, San Agustin-Va 11 de Uxó, Torás and Altura (fig. lb).As exposed in Lago el al. (1996), the main features of this magmatism are: (l) Dolerite intrusions display a lenticular shape, with a thickened central mass, the intrusions in the Camarena area (figs.1b, 2 and 3b) being the most representative example.
(2) These sills are emplaced close to the top of the Keuper facies plastic rocks (Kl to K4, according to Ortí, 1990) or, less commonly, within older materials (e.g., Buntsandstein facies, in Pina de Montalgrao and to the E of Albentosa, fig. 1b).In aH cases, dolerite sills are emplaced below the dolomitic levels of the Imón Fm., which represents a pre-Hettangian age, according to Goy & Yébenes (1977).Neither explosive terms nor aeriallava flows outcrop.
(3) The alkaline affinity of this magmatism, expressed by high Nb, Y and P contents, agrees with its mineral composition (Ti-augite, K-rich plagioclase, apatite, titanite and Ti-magnetite).
The Valacloche-Camarena outcrop (figs.1b and 2) is the most interesting example of this magmatism, because it shows well preserved emplacement structures (fig.4) and a very low-grade contact metamorphism in the host rocks, allowing us to propose an emplacement age.On the other hand, crustal enclaves are common in this outcrop (fig.3b) whereas they are less abundant or totally absent in others.Taking into account aH this data, together with its age and composition, this magmatism can be related to the rifting events that affected the SE margin of the Iberian Chain during Triassic times [Salas & Casas (1993); Martínez et al. (1997)].
In this paper we deal with a petrological and geochemical characterization of the dolerites, an interpretation of their emplacement conditions, which allow for an age proposal and finaHy, we discuss the petrogenetic meaning of this magmatism within the geodynamical context of the Triassic rifting of the SE border the Iberian Chain.The detailed study (mineral and geochemical composition) of crustal enclaves included in the dolerites (figs. 2, 3b, 4A, 4D, 4E and 4F) is another aim of this paper; sorne previous studies on these enclaves were carried out by Sánchez Cela (1981 and1982) and Sánchez Cela et al. (1984, and 1987-1988).--..... P.,...
Emplacement condilions can be inferred from IwO main features.Firsl, lhe nuidalily structures exposed at Ihe upper conlact of Ihe sills in Valacloche (Iig.2) and sorne of Ihe examples to Ihe S of Camarena (ligs.2a and 3b).The masl common flui-dalilY struClures display ropy (lig.4C), bulbous or lobale (lig.48) developments whick are indicative of a magma movement into the loosely consolidated evaporitic and marly-c1ayey sediments of Ihe K4 unit.The lateral movement of this low-viscosity magma, acling as a wedge, was favoured by the small load-pressure of Ihe overlying sedimenls and also by the lower viscosity of Ihe unconsolidated sediment with respect to the inlruding magma.In second place, Ihe very low grade conlacl metamorphism, thal affecls only a thin sheel of the hosl rack suggesls a fasl cooling process.related 10 near surface intrusion of Ihe doleritic magma.
The intrusion thal oUlcrops lO the SOlllh of Camarcna (figs. 2 and 3b) shows concordanl sheet-like injeclions in lhe plastic sedimenls of the K4 unit that, at a map-scale, are configured as sills wilh variable thickness belween 3 and ISO m.In the Vala-c10che area (fig.2) Iwo sills were mapped and sludied.with <In almost constant thickness (15-25 m) and a wide lateral eXlension.Two rock-Iypcs, indicative of IwO differentiation slages, were idenlified in both intrusions: a Ti-augile rich, biotilc-poor dolerite.which is the main type in the Valacloche sills and a more differenliated biolite rich dolerile (with scarce Ti-augile cryslals) which, in contrast, is the best exposed rock-IYpe in Camarena, where il contains an anomalOllS accumulalion of cruslal enclaves (quartzilic, metapelites and rare granitoids; fig.3b, details in figs.4A.4D, and 4F), thal represent diffe• rcnl levels of the posl-variscan crus!.
Thus, Ihe emplacemenl model for Ihis magrnatisln musl consider IwO successive slages, very c10se in time.with and wilhoul crustal enclaves.The small volume or the sills -as shown in a geological map--suggesls small magmalic chambers.Imerprelalion of spatial relalionships belween the intrusions and Ihe hosl rocks and also amongst Ihe two dolerite types and cruslal enclaves, allow us lO propose that in a first stage, the unevolved liquid.wilh low water conlenl, would reach its emplacement level wilhin loosely consolidaled sediments, under an extensional teclonic regime thal favoured pervasive fracluring of cruslal rocks in the magma path.The second.more fraclionaled and more viscous liquid, with a higher water conlenl would have been eXlruded later.carrying mosl of Ihe enclaves ripped from different crustal levels.The reaction lextures observed in sorne of Ihe enclaves (granil oids and melapelites) wilh the hosl dolerile are explained by Iheir longer contacl with Ihe magma.80lh stagcs probably represenl the beginning of the Triassic-Jurassic rifting in lhe SE of the lberian Chain, characterizcd by an incipient fracturation in the uppcnnost pan of crust.
In orden lO reconslruct the general geomelry of the Valacloche-Camarena inlrusion three interpretat ive cross-sections have been made (fig.S).The main intrusion lO Ihe S of Camarena (2-2') is cha-raClerized by its lenticular shape and concordant sheel-like injeclions in Keuper facies rocks.To Ihe NW and to the S of the main outcrop (1 and 3) Ihe inlrusions display sill-like seclions.The current strllclllre of Ihe aren is the result of Alpine deforrnation slages lO which several inverse faulls and rolds affecling Muschelkalk facies rocks and also.Ihe verticalizalion of the Jurassic series in Ihe fool wall of the maio thrust (figs. 2 and 3) should be relaled.
(2) Quartzilic: Ihese are Ihe mosl common enclaves (90 %); Ihey are helcromelric in size (dm) and angular in shape (figs.4E and 4F).Their cantael with Ihe dalerile is generally sharp.with no evidence of reaction ar resorption by the magma.Some of the enclaves show clinopyroxene rims (2.5 mm thickncss on average).Clinopyroxene cryslals in Ihese rims (figs.4E and 4F; composition in figs. 7a.b and e, and analysis 9 lO 12 in lable 1) grew radially on the surface of the enclave, which acted as a crystallization nucleus.
(4) High grade melapeliles: Ihe most comman type cansists of quanz (30-35 %).alkali feldspar (30-35 %) and altercd plagioclase as the main camponcnts.Sillimanite (5-10 %) is frequent both as the fibrous variely (fibrolite) and idioblastic prisms, SUITounded occasionally by potasSiUlll feldspar relics.Anhedral spinel (hercinite: IIIg* = 0.31-0.32;table 7), rutile and anhedral amphibole (Mg-homblende and actinolite; table 5) are accessory pllases.Some enclaves contain associalions of small (0.1 mm) anhedral gamel crystals Of in olher samples.Ihe mineral ClSSOciation is composcd of spincl (Iablc 7) included in cordierile (Iable 9) togethef with prism:uic sillimanite (Iable 8).biolite (analysis 7 lO 10 in table 2), Ti-magnelite (Iable 10) and pOlassium feldspar.Mosl of these enclaves show sharp boundaries and reactional textures have been observed only in a few samples.The equilibrium conditions suggested for these crustal enclaves according to their mineral association without orthopyroxene), must be over 900 0 C for the 6-9 Kb (Spear, 1993) which may correspond to a 15 km depth.These crustal enclaves associated to the evolved liquid suggests, as previously exposed, that the ascent of the less evolved magma was favoured by pervasive fracturing of the middle-upper crust.A significant amount of magma remained in these discontinuities, reacting with sorne of the rock fragments (as expressed by the pyroxene rims wrapping quartzitic enclaves) and later the second, more evolved, magma batch was able to carry the enclaves together with their surrounding magma.Nevertheless, the absence of important reaction textures in enclaves points to a small-sized reservoir and a relatively fast ascent of the magma.

Petrology of the dolerites
The studied dolerites can be grouped into two rocktypes: 1) Ti-augite rich dolerites and 2) differentiated dolerites, with lower modal contents of Ti-augite, but having higher biotite and potassium feldspar contents.These two rock-types have a poorly developed chilled margin facies with a quick transition to their respective central facies which are dominant in volume.
The chilled margin facies consist, for both kinds of dolerites, of scattered Ti-augite phenocrysts or glomerophyric associations, surrounded by plagioclase microlites, opaque minerals and a cryptocrystalline groundmass.Olivine (always altered) is rare and only appears in the Ti-augite rich dolerite.
The Ti-augite rich dolerite (in central facies) is predominant in the sills of Valacloche whilst it is restricted to the margins of the sills in Camarena.It has dominant doleritic and occasionally subophitic textures.The average mineral assemblage is: 50- The biotite rich dolerite forms the internal parts of the Valacloche sills and is the main rock-type in the central sector of the Camarena sills, where crustal enclaves occur.Doleritic texture prevails and the average mineral assemblage is: 30-35 % augite + 40-60 % plagioclase + 7-10 % biotite + 5 % potassium feldspar + 2 % quartz + 2 % opa-que minerals + 1 % apatite ± accesory minerals (titanite).

Mineral composition
Mineral compositions were obtained with an electronic microprobe CAMECA SX-50 (Universities of Toulouse and Oviedo) operating under standard conditions (15 Kv, 10 nA, 1 llm beam diameter and 10 seconds integration time) except C1inopyroxene is augite-diopside, disp1aying a Ti enrichment (tab1e 1) from centre to border of the crysta1s, according with the observed optica1 zonation.Its Fe contents are higher in the Camarena do1erites when compared to those of Va1ac1oche (fig.7a).The pyroxene crystals that form the rims on quartzitic enclaves are slightly more evolved in composition than the two previous cases (fig.7b; ana1ysis 9 to 12 in table 1).This is coherent with their later crystallization according to the previously exposed emplacement scheme.
The high modal proportion of this mineral phase (45-60 %) allows to consider that the evolution of the liquid should be expressed in the compositional evolution of pyroxene [represented by the descent in mg* (=Mg/Mg+Fe z +)].Compositions obtained on this mineral show, from centre to border, a progressive decreasing in Si0 2 and Cr203 contents and a correlative increase in Al z 0 3 , FeO and TiO z contents (fig.7c).Pyroxene compositions for Camarena and Valacloche rocks are very similar for low Cr/AI ratios (fig.7b) but in the Camarena dolerites two compositional trends can be observed; one is similar to that of Valacloche do1erites and the other is characterized by a smaller initia1 mg* value.This significant difference supports an intrusive process in two different stages in the Camarena dolerites.
The pyroxene crystals forming the rims on quartzite enclaves indicate a more evolved magma compisition, with 10wer Cr/AI (fig.7b) and Ti (fig.7c) va1ues than those analysed in dolerites; this can be explained by the fractionation of Cr and Ti in pyroxene and biotite crystals in the less evolved dolerites.Taking into account the whole available data, the progressive Ti enrichment with nearly constant (Ca+Na) values, indicates the alkaline affinity of the liquid (Leterrier et al., 1982;fig. 7c).The late pyroxene crystals in Camarena are more evo1ved (richer in Ti) than those ana1ysed for the Valacloche do1erites.
Biotite crysta1s are more abundant (6-7 %) in the Camarena dolerites, and have higher Fe 2 + and Al contents (fig.9 and selected values in table 2) than the Valacloche ones.Tiny apatite and/or Ti-magnetite inclusions are common.
Accesory minerals are Ti-magnetite (table 10), apatite (table 4), and rare titanite (table 6).Greenish amphibole is rare and probably has a subsolidus origin.A noticeable aspect of the composition of early mineral phases (apatite and titanite) is their low REE content.These elements were probably enriched in opaque minerals (not analysed).

Geochemical composition
Petrologically representative samples were analysed at the X-RAL laboratory (Toronto, Canada) by XRF (major elements and Ba, Rb, Nb, Sr and Zr), INAA (Th, Cr, Ta, Hf and U) and ICP-MS (rest of the trace elements, including REE).A selection of thirteen analysis (with LOI < 4 %), comprising the entire range of rock-types in the studied area, is given in table 11.
According to their geochemical composition, these samples are relatively evolved alkali basalts (%Si0 2 vs. Nb/Y: fig.lOa); mg* ranges from 0.60 to 0.46 (being mg* =MgfMg+Fe 2 + and taking a 0.15 value for the Fe 3 +/Fe 2 +ratio).Geochemical differentiation is expressed by the gradual decrease in compatible elements (Co, Cr, V, Sc and Ni) and mg* parameter (Cr/Ni vs. mg*: fig.lOb).Linear-positive correlations are also observed for incompatible elements (Zr, Y, Nb, Th and Ce; g.e.La vs. Ta: fig.lOc) and ratios of strongly incompatible elements.
hips, together with structural constraints strongly suggest a main magma ascent conduit, related tothe intrusion to the S of Camarena (fig.6).The intruding magma was emplaced concordantly with the bedding planes, in the form of sheet-like injections in the Camarena outcrop or as individualized sills in the case of Valacloche.In both cases, fluidality structures and very low grade contact metamorphism, were favoured by the plastic character of damp sediments in Keuper facies prior to the dolomitic sedimentation of the Imón Fm.In these subvolcanic conditions and with an adequate stratigraphic control the magma could reach a wide lateral extension; the relative vicinity of doleritic sills suggests an initial1y reduced number of intrusions.
Crustal enclaves (quartzites, granitoids and high grade metapelites) are associated to the most evolved dolerite and occur in the internal part of the Camarena outcrop.The magmatic conduits were probably developed by pervasive fracturing, related to the onset of the Triassic rifting events, of the middle and upper crust, as the enclaves derive from different levels of the crust.Magma ascent probably took place in two successive stages.The first, corresponding to an unevolved basaltic magma is represented by the less evolved  The sedimentary and structural evolution of the Triassic sedimentary basins in the SE sector of the Iberian Chain, have been related to the process of triassic rifting that began in the Upper Permian and lasted until the Hettangian, and was the main external control on their sedimentary infill (Salas & Casas, 1993).In this context the beginning of the rifting stages was related to the late-variscan fracturation, which was responsible for the genesis of  (Leterrier et al., 1982).
subsident basins, that progressed with a constant tectonic subsidence rate until the end of the deposit of the Triassic supersequence (Salas & Casas, 1993).The progress of this Triassic rifting at a crustal scale is consistent with the emplacement of this magmatism, rich in crustal enclaves, in the highest levels of the upper Triassic sequence.The alkaline affinity of the studied rocks, close to an OlE geochemical signature, is compatible with the early stages of a passive rifting model, under a slighdy distensive regime affecting the crust and accordingly, a low partial melting ratio of the underlaying mande.
This magmatic activity was continued, as the rifting progressed, by another alkaline magmatism emplaced within Liassic sediments of the SE of the Iberian Chain (from Cubla to Segorbe fig.1B; MartÍnez el al., 1998).

Conclusions
The host sediments is common in all of the studied outcrops.
The alkaline affinity of these dolerites is supported by its mineral assemblage (Ti-augite and a variable content in accessory minerals such as apatite and titanite) and its geochemical composition (high Nb, P, and Y contents and moderate Ta contents).Spider diagram plots and REE pattems show close similarities with the composition of OIB-type basalts.
Fractionation is expressed in two rock-types (with different Ti-augite modal proportions) and is verified by the whole rock geochernical composition of these rocks (incompatible elements concentrations and normalized REE trends), as well as for the Ti-augite compositional variation.
Crustal enclaves are common, prevailing upper crust compositions over those of middle crust (high grade metapelites and granitQids).Enclave-magma reaction textures are not common which indicates a relatively quick magma ascent.The absence of higher grade metamorphic enclaves indicates a more effective fracturation at upper-middle crustal levels.
This magmatism was emplaced prior to the Hettangian, being the first expression of the Triassic rifting in this part of the Iberian Chain.This rifting was characterized by the development of subsident basins at upper crustal levels and by an increasing distension rate from the Permian to the end of the Triassic at deeper levels (Salas & Casas, 1993).Under these conditions, the reduced distension is represented by this alkaline magmatism, similar in  (Lago et al., 1999).lithostatic pressure and the poorly lithified character of these sediments favoured a wide areal spreading of the intrusions.In sorne cases, these emplacement conditions allowed for the development of fluidality structures at the top of the sills.Very low grade contact metamorphism, affecting a thin sheet of the composition to the üIB type.Finally, the continuity of the distension regime is expressed by the later liassic magmatism emplaced in the near sectors of Cubla and Segorbe (Martínez el al., 1997).
Dolerite sills, emplaced in Keuper facies sedimentary rocks in the SE sector of the Iberian Chain (fig. 1) are the expression of a Triassic magmatism, postdated by the Imón Fm. (pre-Hettangian age; Goy & y ébenes Fig. I.-Geological setting of studied outcrops (b) in the SE sector of Iberian Chain (a).Detail of the studied area.