Compositional Evolution of the Variscan Intra-Orogenic Extensional Magmatism in the Valencia del Ventoso Plutonic Complex, Ossa-Morena Zone (SW Iberia): A View from Amphibole Compositional Relationships

: The Ossa-Morena Zone (OMZ), SW Iberia, has numerous Lower Carboniferous compo-sitionally zoned plutons that formed in a Variscan intra-orogenic extensional setting. This magmatism shows a wide compositional variation comprising alkaline, transitional, and calc-alkaline suites. The calc-alkaline suite was produced by hybridization of alkaline magmas with felsic melts generated by crustal anatexis related to the intrusion of mafic magmas in the middle crust. In this work, we present a textural and mineralogical study of the Variscan Valencia del Ventoso main pluton from the OMZ to track the compositional evolution of magmas during hybridization using constraints from amphibole compositions and to determine the P-T conditions of emplacement using amphibole-based thermobarometry. This pluton exhibits reverse zoning with an inner facies containing alkaline dolerites, gabbros, and quartz diorites, an intermediate facies with transitional diorites, and an outer facies with calc-alkaline quartz diorites to monzogranites. Magmas from the intermediate and border facies crystallized under oxidizing conditions at relatively low temperatures (range: 640–760 °C) and ca. 280–300 MPa, implying near H 2 O-saturated conditions. These rock facies show mineralogical evidence of hybridization between alkaline to mildly alkalic and calc-alkaline magmas. The former is inferred from the occurrence of antecrysts of labradorite-andesine, high-Ti pargasite-hastingsite, and biotite with deficiency in tetrahedral-site occupancy, a distinctive feature of biotite from the inner facies alkaline dolerites. This contrasts with later crystallization from the calc-alkaline magma of andesine-oligoclase, low-Ti magnesiohornblende-edenite, and biotite with full tetrahedral-site occupancy. Constraints from amphibole-melt compositional relationships in antecrystic high-Ti amphibole suggest that the alkaline magmatic component could have a high- to ultra-K affinity.


Introduction
An assessment of the contribution of the mantle in the magmatism of continental petrogenetic settings is important to understanding the dynamics of crust-mantle interactions and the mechanisms of crustal growth. In collisional orogens, this is a difficult task because mantle-derived magmas must cross a thickened hot continental crust that experiences anatexis and devolatilization generating melts and fluids that can modify their composition significantly as discussed in Bea et al. [1].
However, the composition of phenocrysts (crystallized directly from the magma), xenocrysts (extraneous to the whole magma plumbing system), and antecrysts (recycled in their own active magma plumbing system by various events) can provide valuable constraints on the nature of the components involved in hybrid magmas (e.g., [2][3][4][5][6][7][8][9]). Amphibole is very useful for this purpose because it can record the compositional changes in host magma from early to late crystallization stages. Molina et al. [2] showed that the TiO2 content of magmatic amphibole is sensitive to the alkalinity of the liquid from which it crystallized. These authors also derived expressions based on the relationships between melt silica content and amphibole-melt K2O and Na2O-K2O partitioning that allow an estimation of alkali contents as a function of silica in amphibole-saturated melts (see also [3][4][5]10]).
In this work, we present a mineralogical study of the Valencia del Ventoso pluton, which has large compositional variability, including dolerites and diorites to monzogranites [3,4]. Amphibole compositions are used to track the compositional evolution of magmas during hybridization. We also determine the P-T conditions at which the hybridization took place using the amphibole-plagioclase barometer from Molina et al. [15], the amphibole-plagioclase thermometer from Molina et al. [16], and the Al-in-hornblende barometer from Mutch et al. [17].

Geological Setting
The OMZ is a Gondwana continental block, located in the westernmost end of the European Variscan Belt, between the Central Iberian Zone (CIZ) to the north, and the South Portuguese Zone (SPZ) to the south. It comprises an Ediacaran to early Carboniferous continuous stratigraphic succession ( Figure 1A,B) affected by the Cadomian, late Neoproterozoic, and Variscan, Early Carboniferous, orogenies, and by the Cambro-Ordovician extension (see Cambeses et al. [4,18] and references therein).

The Valencia del Ventoso Plutonic Complex
The Valencia del Ventoso plutonic complex is exposed over an area of 150 km 2 in the central part of the Olivenza-Monesterio antiform ( Figure 1C) where intruded Ediacaran and Lower Cambrian sedimentary formations cutting also the contacts of Cambrian granitoids ( Figure 1C, see also [4] and [42]). Its shape is elliptical in map view with the long axis in the northeast to southwest direction.
The VVP displays a reverse zoning with an inner facies containing dolerites, gabbros, and quartz diorites that is partially surrounded by an intermediate facies ranging from diorites to granodiorites, being both facies encircled by an outer border facies varying from quartz diorites to granodiorites and monzogranites ( Figure 1C). It shows compelling evidence for hybridization between mafic and felsic magmas (see Figure 3H,I in [4]).
SHRIMP U-Pb zircon dating yields an age for the emplacement of the various intrusive bodies ranging from 334 to 320 Ma (i.e., Visean-Bashkirian [4]).
Systematic point analyses were carried out in resorbed cores, overgrowths and rims of plagioclase, as well as biotite and amphibole grains. Mineral formulas were normalized to 8 oxygen atoms for plagioclase, 22 oxygen atoms for biotite, and 23 oxygen atoms for amphibole (Table S1).
The P-T conditions of crystallization of the intermediate and border facies from the VVP were estimated in samples VAAW-35, VAAW-37, and VAAW-38 using the amphibole-plagioclase barometer from Molina et al. [15] (expression B; precision ca. ±150-200 MPa) and the amphibole-plagioclase thermometer from Molina et al. [16] (expressions A1, A2 and B2; precision of ca. ±50 °C). For sample VAAW-38, we also used the Al-in-hornblende barometer from Mutch et al. [17] (assumed precision of ±100 MPa) as this rock sample bears the buffering assemblage quartz + plagioclase + K feldspar + amphibole + titanite (see below) that is required to buffer the amphibole composition [17]. For this purpose, we analyzed amphibole-plagioclase contacts and selected amphibole and plagioclase compositions from grain cores and overgrowths that encompass the whole compositional range of variation. The amphibole formula was calculated assuming 13 cations exclusive of Ca, Na, and K (13 eCNK method) for application of the amphibole-plagioclase barometer and the average Fe 3+ method of [47] modified by [48] for the Al-in-hornblende barometer and the amphibole-plagioclase thermometer. The selected compositional data and the estimated P-T conditions are available in Table S2.
The K2O and Na2O contents of melts in equilibrium with amphibole were estimated as a function of the melt silica content, in the present work from 55 to 70 wt.%, from amphibole composition using the expressions for amphibole-melt partition coefficients for K2O and Na2O-K2O derived from Molina et al. [2]:
In the studied samples, it also appears scarce chlorite after biotite, actinolite replacing locally magmatic amphibole, and saussurite replacing the anorthite-rich cores of plagioclase.

Amphibole
The compositions of amphibole in studied VVP samples are classified following [49] and displayed in Figure 3. The amphibole compositional data published by [44] for rocks from this pluton are also plotted and show similar trends.

Thermobarometric Estimations
The amphibole-plagioclase thermometer from [16] gave no reliable results for amphibole and plagioclase overgrowths of samples VAAW-35 and VAAW-37 because their compositions lie outside the limits of application of the calibrations (see [16] for details). Therefore, for these samples we only report the P-T conditions of crystallization of antercrystic amphibole and plagioclase estimated with the expressions by Molina et al. [15,16]. Three groups were distinguished for each sample whose compositional ranges are indicated in Figure 5.
For sample VAAW-38, with nearly constant plagioclase overgrowth composition, calculations were performed for a fix anorthite content of 0.27 and a wide range of amphibole compositions that were grouped into two types with compositional limits shown in Figure 5. The amphibole-plagioclase barometer from [15] did not yield good results for the amphibole-plagioclase pairs from this sample because their compositions are not suitable for the calculations as shown in Figure 5A,B. For that reason, the P-T conditions were estimated using the amphibole-plagioclase thermometer from [16] and the Al-in-hornblende barometer from [17].
The estimated P-T conditions are displayed in Figure 5C and reported in Table S2. The temperature estimates for the various groups distinguished in sample VAAW-35 are relatively close, ranging from 704 to 790 °C. For sample VAAW-37, they are more variable ranging as follows: 694-818 ºC in GA, 707-836 °C in GB, and 718-852 °C in GC. Temperature estimates for GB from sample VAAW-38 are subsolidus lying outside the temperature range of the calibration (c.f., [16]), whereas those for GA, with the highest amphibole Ti contents of the sample, cluster around 621-653 °C. It is interesting to note that, despite the compositional pairs depart slightly from the optimal region of use (see [16] for details), the temperature estimates do not show systematic variations with amphibole Ti occupancy ( Figure 5D), whose recommended values are <0.4 apfu.
Pressure estimates for GA from sample VAAW-38 are close to 290-310 MPa, whereas in samples VAAW35 and VAAW-37, they are more variable ranging, respectively, from 200 to 390 MPa and from 130 to 370 MPa.

Conditions of Magma Crystallization
Spear [52] demonstrated that for a fixed bulk composition the Mg/(Mg + Fe 2+ ) ratio of amphibole shows a strong positive dependence on fO2. Accordingly, the increase of the Mg/(Mg + Fe 2+ ) ratio of amphibole with increasing Si occupancy in the three studied samples ( Figure 3) implies an increase of fO2, which was relatively high (Figure 7); this is consistent with the presence of ilmenite + magnetite in the three rock samples (c.f., [53,54]). This amphibole compositional trend is characteristic of an oxidation process during magmatic differentiation in plutonic rocks as proposed by Czamanske and Wones [55]. These authors indicated that it is produced because H2O acts as an oxidizing agent, via dissociation and loss of H2, after its separation from the melt, providing an indirect evidence for a relatively high H2O content in the magma. This is consistent with the relatively low temperatures estimated for the various rock types that are close to those for the wet saturated solidus for granodiorites and diorites ( Figure 6). P-T estimates for Variscan alkaline gabbros and anatectic metasediments from the OMZ also shown (after [3]). Wet solidus and plagioclase-in and amphibole-in reactions in synthetic tonalite and granodiorite systems as experimentally established by [56], and wet solidus for metaluminous granodiorite and diorite as calculated by [57] using rhyolite-MELTS [58].

Magma Compositional Evolution in the Valencia del Ventoso Main Pluton: Constraints from Biotite and Amphibole Compositional Relationships
The reverse zoning exhibited by the Valencia del Ventoso main pluton suggests that the magma feeding system evolved from calc-alkaline to transitional and alkaline compositions. Cambeses et al. [4,5] indicated that the Variscan calc-alkaline magmas from the OMZ were formed by hybridization of mantle-derived mafic alkaline magmas with crustal-derived felsic magmas generated at depth due to crustal anatexis caused by heat transferred from the IRB intrusion in the middle crust (Figure 7).
In order to seek evidence for the alkaline component in the Valencia del Ventoso main pluton, below we discuss the compositional relationships of amphibole from the VVP samples and estimate the Na2O and K2O contents as a function of silica of the melts that were saturated in amphibole using the expressions (1) and (2) derived by [2].
For using this method, we must discriminate first between primary and secondary amphibole. We screened out secondary amphiboles using constraints from relationships of TiO2 with NaA (A-site Na) + K and Al IV as proposed by [2] (Figure 8).
In agreement with these authors, the supersolidus amphibole from an experimental database compiled by Molina et al. [16] defines compositional fields for magmatic amphibole in the NaA + K versus TiO2 and Al IV versus TiO2 diagrams (Figure 8). The limits of these fields suggest that amphibole with <0.7 wt.% TiO2 and both <0.5 apfu NaA + K and <0.9 apfu Al IV can likely be secondary.  [51] of compositional data of synthetic supersolidus amphibole compiled by [16]. The diagrams show a well-defined compositional field for magmatic amphibole thus permitting a discrimination between secondary and primary amphiboles (see further discussion in [2]). The arrows represent the main compositional trends of amphibole compositions. Compositional data of amphibole from [44] (smaller lighter symbols) are shown for comparison.
The relationships of TiO2 with Al2O3, K2O, and Na2O for the selected primary compositions and the calculated melt alkali contents are displayed in Figures 9 and 10, respectively.
Compositional relationships for anhedral brown grain cores of high-Ti amphibole from transitional diorite VAAW-37 suggest a melt with alkaline affinity (Figure 9). Accordingly, the calculated melt compositions have an alkaline affinity with high-to ultra-K and K2O/Na2O ratios >1.5 ( Figure 10). Amphibole rim compositions for this sample lie in the non-discriminating alkaline + subalkaline-melt field; however, the calculated compositions for melt silica contents <70 wt.% are subalkaline, but still high-K, with K2O/Na2O ratios >0.5.
The compositions of anhedral grain cores of amphibole from calc-alkaline quartz diorite VAAW-35, with slightly lower Ti contents, lie in the non-discriminating alkaline + subalkaline-melt field of the Al2O3-TiO2 and Na2O-TiO2 diagrams and in the alkaline-melt field of the K2O-TiO2 diagram (Figure 9). The calculated melt compositions for silica contents <65 wt.% are subalkaline, but straddling the alkaline-subalkaline boundary, with medium-to high-K affinities ( Figure 10). Both amphibole and calculated melt compositional relationships for rim compositions are consistent with a subalkaline-melt affinity with medium-to high-K and a K2O/Na2O ratio higher than that for the whole-rock composition. It is interesting to note that the composition of the more evolved monzogranite VAAW-38 from the same rock unit shows total alkali and K2O contents consistent with the melts calculated for amphibole grain cores of sample VAAW-35, but a K2O/Na2O ratio that is more consistent with melts calculated for rim compositions. This discrepancy suggests that the monzogranite magma could not be derived by simple crystal fractionation of a quartz dioritic parental magma. Accordingly, the crustal signature of radiogenic isotopes is higher in monzogranite VAAW-38 than in quartz diorite VAAW-35 (Table 1 in [4]).
Amphibole compositions from monzogranite VAAW-38 lie in the subalkaline-melt or in the non-discriminating alkaline + subalkaline-melt compositional fields of diagrams from Figure 9. Calculated melt compositions are mostly subalkaline with a high-or ultra-K character and a K2O/Na2O ratio of >1 for all amphibole compositions excepting Ti-richer grain cores that are alkaline to transitional ( Figure 10). and Na2O (wt.%) versus TiO2 (wt.%) with compositional fields for alkaline and subalkaline melts as proposed by [2]. Compositional data of amphibole from [44] (smaller lighter symbols) are shown for comparison. The resorption of biotite by amphibole observed in the transitional diorite VAAW-37 and the calc-alkaline quartz diorite VAAW-35 from, respectively, the intermediate and border facies could be explained by some of the following mechanisms: (1) reversal from the Bowen's discontinuous reaction series that is characteristic of potassic magmas [2,59,60] (i.e., primocryst, closed system behavior); (2) assimilation of country-rocks (i.e., xenocryst, open system behavior); and (3) cannibalization of phenocrysts coming from a different magma (antecryst, open system behavior). The medium-K affinity of the calc-alkaline quartz diorite VAAW-35 precludes a primocrystic origin for biotite (compare with the ultra-K affinity of alkaline dolerite VAAW-20; Figure 10B), being more likely an open sys-tem behavior. Accordingly, the similar Si-Al deficiency shown by resorbed grains of biotite from these samples and from dolerite VAAW-20 ( Figure 4) is more consistent with the entrance into the calc-alkaline and transitional magmas of biotite grains derived from the alkaline magma, thus favoring an antecrystic origin. In the same way, the inferred compositions for melts saturated in high-Ti amphibole from samples VAAW-35 and VAAW-37 are also consistent with derivation from an alkaline to mildly alkalic magma. Therefore, these relationships suggest that anhedral biotite, high-Ti amphibole, and labradorite-andesine from diorites and quartz diorites could represent antecrysts derived from an alkaline magma that were entrapped into a subalkaline magma. The similar pressure estimates for amphibole from monzogranite VAAW-38 and for antecrystic amphibole-plagioclase pairs from diorite VAAW-37 and quartz diorite VAAW-35 further suggest that hybridization did most likely occur at the depth of magma emplacement. Therefore, according to the sequence of intrusion implied by the reverse zoning of the VVP, the hybrid transitional diorites and subalkaline quartz diorites could be produced during replenishment of the magma chamber by the alkaline magma. The involvement of the alkaline component in the calc-alkaline monzogranite from the border facies was probably minor as there is not clear textural evidence of mineral resorption in sample VAAW-38, except for scarce plagioclase grains with anhedral cores. However, some amphibole compositions predict high-K alkaline to transitional melt compositions as in quartz diorite VAAW-35.

Conclusions
The VVP from the OMZ is a reversely zoned intrusion that consists of an inner facies with alkaline dolerites, gabbros, and quartz diorites, partially surrounded by an intermediate facies with transitional diorites to granodiorites that are encircled by an outer border facies with calc-alkaline quartz diorites, granodiorites, and monzogranites.
The magmas crystallized under oxidizing conditions at relatively low temperatures (range: 640-760 °C) and ca. 280-300 MPa, implying near H2O-saturated conditions. Replenishment of a calc-alkaline magma chamber by a biotite + high-Ti pargasitehastingsite + labradorite-andesine laden alkaline magma caused hybridization and dispersion of the phenocryst load into the calc-alkaline magma.
Reaction of the antecrystic minerals with the hybrid magma caused resorption and overgrowth of hornblendic amphibole on biotite and high-Ti pargasite-hastingsite, and of andesine-oligoclase on labradorite-andesine.
Amphibole-melt compositional relationships in antecrystic high-Ti amphibole reveal the interaction of calc-alkaline magmas with high-to ultra-K alkaline to mildly alkalic magmas.

Acknowledgments:
We thank the three anonymous reviewers and the academic editor for the thorough revision and the insightful comments.

Conflicts of Interest:
The authors declare no conflict of interest.