5.1. Mafic Magma Evolution
The subalkaline composition of gabbroic and diabasic samples from the MPGD and the trend of increasing Fe in the AFM diagram (
Figure 5A,B) are features typical of tholeiitic suites. The TiO
2 value under 1.5 wt% in the most primitive samples relates the composition of the studied mafic rocks with that of the low-Ti continental flood basalts (CFB) (e.g., [
48]).
The major element geochemistry of MPGD gabbros describes a moderate to strong compositional heterogeneity, characterised by a positive correlation between Mg and Ca, whereas Ti, Na and K display a moderate enrichment towards lower Mg contents (
Figure 6). Such behaviour is coherent with some degree of crystal fractionation. Al and Fe depict a different evolution pattern, defined by an initial Al enrichment at constant Fe, followed by an Fe increase at constant Al (starting at MgO ~6 wt%;
Figure 6). These latter features, together with the overall early depletion of Ca and Mg, likely imply that fractionation was initially dominated by Ca-rich clinopyroxene (augite). The low Cr and Ni concentrations of the most primitive samples (Cr < 300 ppm, Ni < 140 ppm;
Figure 7) imply that they cannot be considered primary magmas (Cr = 500–1000 ppm; Ni = 200–500 ppm) and that some degree of early olivine fractionation also took place. On the contrary, plagioclase likely acquired a more relevant role afterwards. The chemical variation in trace elements agrees with this possibility, as all incompatible elements increase in composition in the more evolved samples, with the exception of Sr, which reproduces the same pattern of Al (
Figure 7) and supports that the change in behaviour is controlled by an increase in the modal abundance of plagioclase during fractionation. The lack of a final depletion in Fe and Ti towards the most evolved gabbroic samples indicates that the degree of differentiation did not achieve fractionation of Fe-Ti oxides, typical of the tholeiitic evolution.
An equivalent fractionated mineral assemblage has been proposed for several mafic magmas of the CAMP (including samples from the Messejana–Plasencia dyke) based on chemical modelling using MELTS software [
10]. Their results indicate that plagioclase is an early crystallising phase, but their data also present an evolution pattern with increasing Al towards lower Mg, which seems to contradict the typical evolution of tholeiitic magmas. The stabilisation of plagioclase in crystallising tholeiitic melts at low pressures prior to augite has been described in experimental studies (e.g., Refs. [
49,
50]). However, the plagioclase/pyroxene ratio, which can change with small variations in parental melt composition, has a strong effect on the magma chemical evolution [
49] and can lead to kink patterns with an initial increase in the Al content followed by depletion [
50].
5.2. Origin of the Granophyric Melt
Two broad possible scenarios can be considered in the origin of these granophyres: they might represent felsic melts unrelated to the MPGD or they may be genetically linked with the mafic magma via extensive differentiation, as shown by experimental works focused on the formation of silicic melt due to crystal fractionation of basalt (e.g., [
51]). The following two main reasons argue against the possibility of MPGD granophyres being related to granitic magmas of crustal origin totally disconnected from the mafic melt: first, felsic magmatic events in central Spain ended about 85 Ma before the intrusion of the MPGD (e.g., [
45]), and second, the granophyres composition is not in accordance with that of minimum temperature anatectic granitic melts (
Figure 12). The granophyres are found exclusively as intrusive bodies within the gabbros at several scales including the following: small interstitial pockets within the gabbro minerals, small veins of irregular contact permeating the mafic rocks and, occasionally, as a large dyke of 1 km in length (
Figure 2). In all cases, these felsic bodies are only found within the mafic dyke, not in the wall-rock granites. This layout of the granophyric melts points to a direct connection between the felsic pockets within the gabbros and the small veinlets and dykes, which might be considered segregates formed due to the squeezing and extraction of a residual felsic melt during the last stages of crystallisation. The presence of similar small-volume granophyres within tholeiitic gabbros has already been described (e.g., [
52,
53,
54]), including the CAMP magmas [
11,
16], and explained as the result of mafic magma evolution. Accordingly, it is likely that the granophyres formed during the last stages of the mafic magma differentiation within the MPGD intrusion.
The major and trace element geochemistry of the felsic rocks associated with the gabbroic intrusion is heterogeneous and clearly separated from that of the mafic rocks (
Figure 6 and
Figure 7), independently of the size of these bodies (veinlets, small dykes or larger dykes). This is manifested by a compositional gap that is noticeable for SiO
2, Fe
2O
3, CaO, Rb, Ba and Sr, among other elements (
Figure 6 and
Figure 7). The absence of volcanic and plutonic rocks of intermediate composition, identified as the Daly gap, is not uncommon in highly differentiated mafic magmas, but their origin is controversial (e.g., [
57]). Some studies suggest that melts of intermediate composition may form during crystal fractionation of basaltic magmas and that their absence or scarcity would be the result of viscosity–density barriers [
58], fast evolution of intermediate liquids [
59] or other mechanical factors related to magma chamber dynamics (e.g., low crystallinity percentage and magma convection) [
60]. However, the presence of interstitial quartz–alkali feldspar-bearing micro-aggregates within the gabbro mineral groundmass (
Figure 2C–F) implies that the differentiation process leading to granophyre formation can occur in the dyke itself after its intrusion, rather than within a deeper magma chamber. The fact that the micro-aggregates, veinlets and dykes represent felsic compositions, with no intermediate terms, suggests that simple crystal fractionation would not be the main differentiation mechanism in the generation of these granophyres.
An alternative hypothesis proposed to explain the Daly compositional gap is liquid immiscibility (e.g., [
57]). This process has been associated with the presence of glass droplets in the mesostasis of basalts (e.g., [
17]), but it has also been identified in mafic plutonic intrusions, such as Skaergaard, Sept Iles, Bushveld and Duluth (e.g., [
61,
62,
63,
64,
65,
66]). These studies describe the coeval segregation of silica and iron-rich melts at the mm- to cm-scale.
Immiscibility along tholeiitic trends involves the unmixing of ferrobasaltic and rhyolitic melts (e.g., [
66]). The compositional space of such an immiscibility gap has been represented in triangular diagrams such as those of
Figure 13. In all these plots, the granophyres from the MPGD display a composition very close to that of the silica-rich immiscible liquids, represented either by glass droplets in basalts (
Figure 13A) or melts from immiscibility experiments in a tholeiitic system (
Figure 13B,C).
The bulk of the gabbroic samples plot in an intermediate position between the Si-rich and Fe-rich poles, within the wide one-liquid field represented by lavas from some major tholeiitic provinces (
Figure 13B,C). The mineral chemistry of the granophyres is slightly different from that of the gabbros (e.g., higher Na-Al in biotites, higher Na + K in apatite;
Figure 4), but mostly overlaps with the mineral geochemical evolution trends (
Figure 4). Experimental and empirical studies ([
66] and references therein) have established that the onset of immiscibility in tholeiitic mafic systems is associated with plagioclase with a composition in the range of An
36–60 and clinopyroxenes with Mg# < 64. Such values are reached in the studied MPGD gabbros (
Figure 4A), which agree with the evolved contents usually associated with liquid immiscibility. The evolution depicted by the MPGD mafic rocks is also in accordance with the liquid line of descent described by tholeiitic suites, which have experienced immiscibility, such as the slight increase in Ti, Fe and P in the gabbros (
Figure 6). The relative depletion in Eu, Sr and Ti in the granophyres (
Figure 8) is coherent with their generation during the last stages of differentiation of the mafic magma, after fractionation of plagioclase and Fe–Ti oxides. Moreover, other chemical features of the granophyres, such as the relative enrichment in Rb and elevated Ba/La (20–94) and Ta/Nb (0.12–0.31) ratios (
Figure 8), can be considered good indicators of liquid immiscibility in silicic rocks. Liquid immiscibility in tholeiitic systems can partition trace elements, e.g., favouring the incorporation of LILE to the felsic melt over REE, thus imposing these distinct geochemical imprints in the two immiscible melts [
68]. Such partitioning could explain the similar or lower REE contents of the granophyres when compared to the gabbros.
A possible weakness of this magma immiscibility model is the low ratio of iron-rich melts to silica-rich melts, the latter being represented by the minor granophyric bodies within the MPGD. Only one sample (103329) shows a particularly high Fe–Mn–Ti-rich and Si–Al–Na-poor composition (
Figure 6), which plots towards the Fe-rich pole of the experimental immiscibility fields (
Figure 13B,C). This sample might be representative of cumulate rocks derived from this Fe-rich immiscible counterpart and may have been undervalued during sampling due to their similar appearance to any other MPGD gabbro. In any case, the experiments suggest that the relative amount of Fe-rich melt produced during magma immiscibility is low [
66]. In summary, these data, together with the lack of intermediate compositions and the close textural relationship between gabbros and granophyres, support the likely origin of the MPGD granophyres as immiscible Si-rich melts generated during the last stages of differentiation of the mafic magma.
5.3. Magma Ascent and Contamination
The slightly evolved composition of the MPGD gabbros and their isotopic heterogeneity points to an open-system differentiation process, although it is not straightforward at which lithospheric level it took place. As stated above, the low Mg and Cr values of these gabbros are not characteristic of primary magmas and point to a certain degree of olivine–clinopyroxene–plagioclase fractionation. The fact that both plagioclase and clinopyroxene (instead of only plagioclase) are relevant early-crystallising minerals suggests that fractionation took place at moderate pressures. Fractional crystallisation experiments with tholeiitic melts at 0.7 GPa [
50], equivalent to the lower crust (~23–25 km), yielded trends similar to the MPGD gabbros (e.g., initial Al
2O
3 increment).
The entrainment of zircon xenocrysts are also relevant data. The presence of such zircons in mafic magmas is usually interpreted as the result of crustal assimilation. The gabbro sample 103324 includes relatively abundant zircon xenocrysts with an internal CL structure similar to that of metamorphic zircons of the SCS granulitic xenoliths (
Figure 10): small, rounded grains with a homogeneous dark CL response or dark mantles surrounding older inherited zones [
44,
46]. Moreover, the distribution of U–Pb zircon ages both in gabbro xenocrysts and zircons from granulites overlaps and displays a remarkable peak at about 280–290 Ma (
Figure 11). The study of these xenoliths [
44,
46,
69], which are included in Permian mafic magmas, has shown that (1) they were extracted from the SCS lower crust (~25–30 km), (2) this deep crustal level is abundant in felsic granulites and (3) the above ages represent the last stages of granulitisation in the SCS lower crust at the end of the Variscan cycle. These textural features and geochronology data of zircon xenocrysts suggest that the mafic magma likely interacted with the lower crust. Another possibility is that these zircons were incorporated from the mantle source. Examples of mantle zircons have been described in orogenic peridotites and mantle xenoliths, either associated with mantle metasomatism (e.g., [
70]) or recycling of crustal components (e.g., [
71,
72]). Such recycling is possible via lower crustal delamination into the mantle, which has been proposed for the central zone of the Iberian Variscan Belt [
73]. Unfortunately, we do not have solid arguments to support this possibility.
Accordingly, we think that a process of mafic magma stagnation, fractionation and contamination with lower crustal granulites is likely behind the evolution of the MPGD magmas. Such a process has already been proposed in previous studies [
7,
10], although the conclusions differ with respect to the nature of the assimilating component. The possibility of crustal contamination could also account for the variability in the Sr–Nd radiogenic ratios (
Figure 9). We tested this possibility with a crystal fractionation and assimilation (AFC) model (
Figure 14) based on the equations of [
74]. The model (see details on the parameters used in
Table 6) illustrates the isotopic and trace element variation in an initial mafic magma with a composition similar to the most depleted MPGD gabbros. The contaminant used in the model is the average of SCS lower crustal granulite xenoliths (grey area) [
69]. The fractionating phases are cpx, pl and ol, as interpreted at the beginning of the discussion. The model fits well with the chemical variation in the gabbros when using an assimilation/fractionation ratio (r) of 0.25, which implies assimilation of lower crustal rocks in the approximate range of 10–12%. A model considering the SCS granites as the contaminating component does not fit the isotopic variation in the gabbros due to their high radiogenic Sr values. Taking into account that the analysed rocks do not represent a primary composition, these crustal contamination rates could be considered minimum values.
The more radiogenic isotope composition of the granophyres with respect to that of the MPGD gabbros (
Figure 9), along with the inclusion of typically igneous Variscan zircons, points also to crustal contamination, although involving a different crustal protolith. These zircon xenocrysts display CL textures similar to those of Variscan granites or pre-Variscan meta-igneous rocks: large euhedral bipyramidal prisms with oscillatory zoning (
Figure 10). Their U–Pb ages also reproduce the distribution observed in zircons from Variscan granites (
Figure 11D), with a large peak at ~300 Ma (crystallisation age) and older (usually Cadomian) inheritances. Such features are characteristic of the SCS granites [
45] and some lower crustal granulites [
44,
47] and likely reflect a contamination process occurring at a different crustal level than that described above. As the MPGD granophyres are formed and segregated within the gabbros, the contamination process has to occur initially in the mafic magma. We applied an AFC model for Sr–Nd isotopes (
Figure 15) starting with the composition of the least primitive gabbros (see details on the parameters used in
Table 6) because the granophyres were derived from the more differentiated mafic magmas. The fractionating minerals used in the model are the most abundant phases in the gabbros (cpx, pl and opx), and the contaminant is a representative SCS granite taken from [
40]. An assimilation/fractionation ratio (r) of 0.25 is used. The resulting model only fits well for the least radiogenic granophyres, yielding assimilation rates up to 21%. The evolution line does not reach the more radiogenic samples from NE Sanchorreja, which would require a much more radiogenic contaminant and/or unrealistic assimilation rates.
Another process, apart from crustal assimilation, is necessary to explain the composition of the two granophyres with
87Sr/
86Sr values higher than 0.714. This increase in radiogenic Sr could be associated with the alteration of these rocks, caused by their interaction with meteoric or hydrothermal fluids and manifested in the variable transformation of the main igneous minerals to secondary phases. The principle behind it is based on the washing of radiogenic
87Sr from K-feldspar lattice sites due to an extensive and prolonged rock–fluid interaction, which could lead to preferential mobilisation of Sr vs. Nd. A link between hydrothermal fluid circulation and the disturbance of Rb–Sr isotope systematics has already been proposed for granitic rocks [
75,
76]. Other studies focused on the effects of weathering or hydrothermal alteration on the Rb–Sr isotopic system [
77,
78,
79] also suggest that an increase in the alteration degree can lead to higher
87Sr/
86Sr ratios. The presence of miarolitic cavities filled with low-T minerals (prehnite, chlorite, clay minerals) in these samples from the NE Sanchorreja granophyre dyke, is indicative of the involvement of late-stage fluids. Also, the region where the samples were collected has been subject to several hydrothermal events after the MPGD intrusion [
80]. Anyhow, the nature of this fluid and the way it may have interacted with the MPGD felsic rocks is not straightforward and is beyond the scope of this work. However, it should be noted that the higher K
2O contents and general depletion in HREE and most HFSE of the samples from NE Sanchorreja, when compared to other granophyre dykes, are compatible with potassic alteration [
81].
Table 6.
Parameters used in the AFC models for the Messejana–Plasencia Great Dyke gabbros and granophyres.
Table 6.
Parameters used in the AFC models for the Messejana–Plasencia Great Dyke gabbros and granophyres.
AFC model for the MPGD gabbros |
| 87Sr/86Sr | 143Nd/144Nd | Sr | Nd | Rb | Ba |
Initial melt composition | 0.70560 | 0.512443 | 150 | 10.5 | 14.5 | 122 |
Contaminant composition | 0.71216 | 0.511983 | 248 | 32 | 104 | 1037 |
Fractionating phases: Cpx (67%), Pl (27%) and Ol (6%) |
AFC model for the MPGD granophyres |
| 87Sr/86Sr | 143Nd/144Nd | Sr | Nd | | |
Initial melt composition | 0.70636 | 0.512271 | 214 | 20 | | |
Contaminant composition | 0.71915 | 0.512016 | 96 | 32 | | |
Fractionating phases: Cpx (50%), Pl (40%) and Opx (10%) |
Mineral/mafic magma partition coefficients |
Mineral | Element | Kd | Reference |
Clinopyroxene | Sr | 0.1 | [82] |
Clinopyroxene | Nd | 0.38 | [82] |
Clinopyroxene | Rb | 0.015 | [83] |
Clinopyroxene | Ba | 0.005 | [82] |
Plagioclase | Sr | 2 | [84] |
Plagioclase | Nd | 0.3 | [84] |
Plagioclase | Rb | 0.08 | [84] |
Plagioclase | Ba | 0.2 | [84] |
Olivine | Sr | 0.0094 | [83] |
Olivine | Nd | 0.008 | [85] |
Olivine | Rb | 0.0133 | [83] |
Olivine | Ba | 0.01 | [83] |
Orthopyroxene | Sr | 0.0026 | [82] |
Orthopyroxene | Nd | 0.013 | [82] |
The bulk of the analysed granophyres constitute a relatively heterogeneous group, as highlighted by the variable concentrations of some elements (e.g., Na, K, Rb, Ba;
Figure 6 and
Figure 7). This variability, which is also reflected in the mineral chemistry (
Figure 4), suggests that granophyres from each sector followed independent differentiation processes as disconnected magma bodies after their segregation from the mafic intrusion. The larger size of the NE Sanchorreja dyke, along with its highly radiogenic Sr ratios and the entrainment of zircon xenocrysts, calls for a more complex differentiation scenario (a larger magma body). Overall, the above data point to the joint participation of crystal fractionation, crustal rocks assimilation and posterior fluid interaction.