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Mantle and Crustal Sources in the Génesis of Late-Hercynian Granitoids (NW Portugal): Geochemical and Sr-Nd Isotopic Constraints
Introduction
La generación de magmas está asociada en espacio y tiempo con el crecimiento de la corteza continental, (rather than just recycling). Los procesos de hibridación que envuelven la corteza (coeval) y los componentes del manto han sido sugeridos para
The generation of granitic magmas is associated in space and time with growth of the continental crust, rather than just recycling. Hybridization processes involving coeval crustal and mantle components have been suggested to account for field, petrographic, chemical and isotopic features of granodiorites and monzogranites. The mafic microgranular enclaves and basic rocks, frequently associated with calc-alkaline and subalkaline granitic series, are normally interpreted as representing the mantle component associated with the genesis of the granitoids. However, they rarely preserve the primitive character and therefore the establishment of the nature of this component as well as its homogeneity/heterogeneity during a tectonic event is difficult to establish. Although it is generally admitted that the ascent of mantle magmas in the continental crust creates a thermal anomaly which induces partial melting of different crustal levels, with consequent formation of granitic magmas, there is no unanimous opinion concerning varous petrogenetic processes that lead to the formaion of composite massifs, associating basic to intermediate rocks and granitoids.
In the Central Iberian Zone (CIZ), nothern Portugal, large volumes of granitoids were emplaced during the post-collisional stage of the Hercynian orogeny (syn-to post-D3, the last ductile deformation phase). This was the main period of successive generation of granites which exhibit large compositional variability as described below.
(1) Syn-D3 granitoids, 313-319 Ma: perluminous biotite granodiorites to monzogranites and highly perluminous two-mica leucogranites of calc-alkaline and aluminopotassic affinities;
(2) Late-D3 biotite-dominant granitoids, 306-311 Ma: mainly as composite massifs displaying a wide compositional range from gabbroic to granitic and subalkaline affinity, being metaluminous to peraluminous. Other plutons exhibit characteristics of aluminopotassic associations;
(3) Late- to post-D3 granitoids, ca. 300 Ma: highly peraluminous, two-mica leucogranits of aluminopotassic affinity;
(4) Post-D3 granitoids, 290-296 Ma: slightly metaluminous to peraluminous monzogranites os subalkaline ferro-potassic affinity, occurring as zoned plutons.
Syn- and late-D3 biotite-rich granodiorites and monzogranites are the most abundant granitic rocks in the CIZ, spatially associated with mafic microgranular enclaves and minor bodies of basc to intermediate rocks. They represent an important period of Hercynian crustal growth and their origin is still a matter of debate.
Several composite plutons in the CIZ (nothern Portugal), which represent whis type of granitoid, were selected for the mineralogical, chemical and Sr-Nd isotopic study, complementing the earlier studies on the Braga pluton. They include coeval gabbro-norite to granodiorite stocks and/or mafic microgranular enclaves, which show evidence of interaction between felsic and mafic magmas. This presentation intends to provide constraints on the origin of these granitoids, as well as on the nature (homogeneity/heterogeneity) of the parental reservoirs.
Geology and Geochronology
The studied granitic plutons are located in the CIZ nothern Portugal, and their distribution is controlled by the Vigo-Régua ductile shear zone (Fig. 1). They are related to two different emplacement periods, relative to the third Hercynian tectonic phase: (1) syn-D3 biotite granitoids: Ucanha Vilar, Lamego, Felgueiras, Sameiro and Refoios do Lima Plutons; (2) late-D3 biotite-dominant granitoids: Braga pluton (Braga granite, associated gabbro to granodioritic bodies and Gonça granite), Agrela pluton and Celorico de Basto pluton (Celorico de Basto grnite and associated gabbro to granodioritic stocks).
Syn-D3 biotite granitoids
This group is represented, from south to north, by the Ucanha-Vilar, Lamego, Felgueiras, Sameiro and Refoios do Lima plutons. The Ucanha-Vilar granite is spatiall associated with hm-sized granodioritic stocks. Mafic microgranular enclaves occur dominatly in the Ucanha-Vilar, Lamego, Felgueiras and Sameiro plutons, decreasing in frequency from south to north, and become rare in the Refoios do Lima pluton. The granitic plutons present an essentially magmatic structure, marked by the orientation of K-feldspar phenocrysts and biotite. As proposed by Simoes, the fabric is the result of magma deformation by flattening with a reduced rotational component in a transpressive tectonic regime dominated by a NE-SW sub-horizontal compressive component, compatible with the D3 deformation phase. Based on structural and geological features, the plutons are considered early syn-D3 plutons. Zircon and monazite U-Pb geochronological dat from the Ucanha-Vilar, Lamego, Sameiro and Refoios do Lima granites show a narrow range of crystalization ages between 313 and 321 Ma, yielding the almost concordant monazites very similar ages, 317+-3 Ma, 317+-5 Ma, 318+-2 Ma, 317+-3 Ma, respectively (Table 1). The narrow range of age values is interpreted as representing an almost synchronous emplacement of the different plutons, at about 317 Ma. These age data obtained by Martins (1997) for the Lamego granite. For the Ucanha-Vilar granite, available K-Ar biotite ages in the range 294-306 Ma are interpreted as cooling ages.
Late-D3 biotite-dominant granitoids
This group is represented by the Braga, Agrela and Celorico de Basto plutons. They present magmatic structures (preferntial orientation of K-feldspar phenocrysts and partially of biotite) in conformity with D3 fabric.
The Braga massif is composite, associating two granitic units: the Braga granite which includes abundant mafic microgranular enclaves and hm-sized bodies of gabbroic to granodioritic rocks (gabbros, monzodiorites, quartz monzodiorites and granodiorites); the Gonça granite which is devoid of mafic rocks and contains mica-rich enclaves and metasedimentary xenoliths. The Braga biotite granodiorite/monzogranite is slightly porphyritic, fine to medium-grained. The Agrela biotite granodiorite/monzogranit is porphyritic, medium-grained and includes abundant mafic microgranular enclaves. The observed gradational contacts between the Braga-Gonça granites and Braga-Agrela granites, as well as the sharp but lobated contacts between the Braga granite and the basic intermediate bodies, indicate a synchronous emplacement of the different plutonic units. Conventional U-Pb analysis of multi-grain zircon and monazite fractions, carried oyt on four samples from the Braga, Gonça and Agrela granites and a quartz monzodiorite from the Braga composite pluton, have yielded similar ages in the range 307-311 Ma (Table 1). The narrow age range is agreement with the geological interpretation (late-D3 emplacement) and indicates coeval emplacement of the four units. The U-Pb ages are concordant with the Rb-Sr whole-rock age of 310+-10 Ma and 307+-10 Ma obtained for the Braga and Agrela granites, respectively.
The Celorico de Basto pluton comprises the Celorico de Basto biotite monzogranite, containing rare mafic microgranular enclaves, which is spaially associated with hm- to km-sized (maximum 3 km) gabbroic to granodioritic bodies. Detailed mapping (1:50000 scale) was undertaken on the bodies presenting greater petrographic variability (Fig 2). They reveal a zoned structure, from gabbro to quartz-diorite, quartz-monzodiorite and granodiorite. The contact relationships are typically magmatic, with gradational or lobate and crenulate sharp contacts. The outermost facies is a porphyritic biotite granodiorite with a heterogranular groundmass (fine- and coarse-grained), that passes gradually to the host-rock, the Celorico de Basto biotite monzogranite. This is a porphyritic (k-feldspar phenocrysts) coarse-grained granite. The field observations reveal syncrhonous emplacement of different units that comprise the Celorico de Basto massif. U-Pb isotope analysis was undertaken for one fraction (0.17 mg) of euhedral limpid monazite, extracted from one sample of Celorico de Basto monzogranite. The fraction is almost concordant (-0.41 % discordancy) and provides a 207 Pb / 235 U age of 308+-4 Ma, considered as the crystallization age of the granite. Due to the synchronou character of the different units of the Celorico de Basto massif, this result may be interpreted as representing the emplacement age of the massif, in the same age bracker of 306-311 Ma, proposed as the afe of late-D3 plutonism in the CIZ by Duas et al. This age is also in accordance with the age results for the equivalent granitic facies: Celeirós granite (Braga region), Rb-Sr 308+-6 Ma, U-Pb 306+-2 Ma; Vieira do Minho granite, U-Pb 311+-2 Ma.
Petrography and Mineralogy
Syn-D3 biotite granitoids
The syn-D3 granitoids (Table 2) are porphyritic (euhedral orthoclase phenocrysts) medium-grained biotite granodiorites/monzogranites and contain quartz + plagioclase (An 19-42) + perthitic orthoclase + biotite + zircon + monazite + apatite + ilmenite + muscovite + allanite + (+ cordierite + sillimanite +- garnet in the Refoios do Lima granite). The granodioritic bodies associated with the Ucanha-Vilar granite are fine-grained biotite-rich rocks that have compositions transitional between granodiorite and quartz-monzodiorite. They present the following mineralogical association: quartz + plagioclase (An 22-57) + K-feldspar + biotite + apatite + zircon + ilmenite +- allanite +- titanite. The pretrographic characteristics of the different granites and associated granodiorites are detailed in Simoes (2000).
Biotites from different granites are alominous (Al total = 3.01 3.54 apfu) and ferric (X Mg = 0.36 0.47), typical of biotites from calc-alkaline to aluminopotassic granites (Fig. 3). Biotite compositions show a continuous increase in Al total and decrease in Mg from southern to northern plutons, i.e., from the Ucanha-Vilar to the Refoios do Lima granites.
A morphological and geochemical study of zircon populations was undertaken by Simoes. The typology of zircon populations indicates that they are identical to zircon populations of calc-alkaline granites or aluminous monzogranites-granodiorites. For the Refoios do Lima granite, the typological evolutionary trend (TET) of zircon populatiions is typical of granites derived from crustal melts. For the other granites to the crustal granites, in contrast with what would be expected from a simple evolutionary crystallization process.
Electron microprobe analyses and observations on the internal structure enable the distinction of different stages in the evolutionary history of the zircon populations: an inherited phase, a magmatic phase and a late magmatic phase. In the Refoios do Lima granite, inherited zircon cores are abundant and the magmatic phase presents HfO2 > 1.2% predominance of U over Th and low ThO2 contents (< 0.5%). This is the signature of granites derived from crustal anatexis. Zircons from the other granites reveal the following features: the magmatic phase presents a HfO2 compositional discontinuity and resorption structures between two magmatic stages; HfO2 < 1.2 % and predominance of Th over U in the early magmatic stage, indicating a calc-alkaline signature; HfO2 > 1.2% in the later magmatic stage; rarity of the inherited phase. These features reveal de occurrence of a non-evolution of initial calc-alkaline magmas. The late magmatic phase, rich in Hf and U, is not very expressive in any of the granites, which indicates a H2O non-satured character of the magmas.
Late-3D biotite-dominant granitoids
The Braga, Gonça, Agrela and Celorico de Basto granodiorites-monzogranites present the following mineralogical association (Table 2): quartz + perthitic k-feldspar + plagioclase (An 15-42) + biotite (modal abundance 6-19 %) + muscovite (modal abundance 0-7 %) + apatito + zircon + ilmenite + monazite (+- andalusite +- sillimanite in the Gonça granite; +- cordierite in the Celorico de Basto granite). The higher muscovite contents (6-7 %) are found in the Gonça monzogranite and the lower biotite contents (6-12 %) have been noticed in the Gonça and Celorico de Basto granites. These biotites are high-Al (Al total = 3.35 - 3.57 apfu) and low-Mg (Mg = 1.18 1.74 apfu), typical of biotites from auminopotassic associations (Fig. 3). Biotites from the Braga and Agrela granites are less aluminous (Al total = 2.78 3.44 apfu) and more magnesian (Mg = 1.46 2.23 apfu). For a detailed petrographic study of Braga, Gonça and Agrela granites see Veloso.
The gabbroic rocks associated with the Braga and Celorico de Basto granites have similar petrographic characteristics. They are medium-grained gabbro-norites with labradorite (An 50-60), olivine (Fo 63-72), orthopyroxene (hypersthene-bronzite, En 60-75), clinopyroxene (augite, Wo 42-47, En 41-47), magnesio-hornblenda, phlogopite (X Mg = 0.64-0.75), apatite, magnetite and ilmenite (Table 2). In the quartz-diorites, monzodiorites and quartz-monzodiorites, plagioclase is mainly andesine (An 26-53), orthopyroxene and olivine are absent and mafic minerals amphibole (magnesio-horblende to actionlite), high-Mg biotie (X Mg = 0.52 0.65) and accessory clinopyroxene (Wo 46-50, En 35-38). They are melanocratic to mesocratic rocks, medium to fine-grained, with predominance of amphibolite over biotite in the quartzdioritic compositions. In the fine- to medium-grained, sometimes porphyritic biotite granodiorites the dominant mafic mineral is biotite (X Mg = 0.47 0.53) with rare amphibole (magnesio hornblende). They are leucocratic with perthitic K-feldspar and plagioclase of oligoclase-andseine composition (An 17-41).
The evolution of biotite compositions from the Celorico de Basto composite pluton in the Al total vs Mg diagram of Nachit et al. (1985) is marked by a two-step continuous trend, as pointed out by Dias and Leterrier for the Braga pluton (Fig 3). Biotites from the less evolved rocks (gabbro, quartz diorite, monzodiorite and quartz monzodiorite) have similar compositions in the two plutons and show n evolution characterized by a sharp decrease in Mg whereas total Al has remained almost constant. In contrast, biotites from the granodiorites and from the host granites are distributed along an almost vertical trend (increase in Al whereas Mg remains almost constant). According to Nachit et al., the biotite compositions from a given plutonic associaion generated by fractional crystallization follow an oblique trend parallel to the vector a in figure 3 (increase in Al total and decrease in Mg). Therefore, the observed biotite compositions rule out an origin by a simple fractional crystallization process for the Braga and Celorico de Basto plutons.
Geochemistry
Representative whole-rock chemical compositions from different units of the studied plutons are given in table 3. All the analyzed samples are plotted on variation diagramas of selected major and trace elements vs. Fe+Mg+Ti parameter (calculated in millications per 100 g of rock and proportional to its mafic mineral content) used as a differentiation index (Fig. 4). Figure 5 shows the chondrite-normalized REE patterns of the representative samples.
Sr and Nd isotopic data are given in table 4. Accounting for the geochronological data and for field data showing the synchronous emplacement of the different units from each composite pluton, the 87 Sr / 86 Sr initial ratios and epsilon Nd CHUR values were calculated for the following ages; syn-D3 granitoids, 317 Ma; late-D3 granitoids, 310 Ma, 307 Ma and 308 Ma for Braga, Agrela and Celorico de Basto plutons.
Major and trace elements
Syn-D3 biotite granitoids
The syn-D3 granitoids included in this study are slightly to moderately peraluminous [A = Al-(K+Na+2Ca) = 0-45; Fig. 4]. The lowest values of the A parameter are found in the Ucanha-Vilar granite and the Refoios do Lima granite is distinctly peraluminous (A = 34-45). In general, an increase in the peraluminous character is observed from the southern no northern plutons, i.e., from the Ucanha-Vilar to the Refoios do Lima granites. The SiO2 content ranges from 62 to 70 % and noteworthy characteristics of these granitoids are: rather high Ba (720-2181 ppm) and REE (La = 77-167 ppm) contents; highly fractionated REE patterns [La N/Yb N = 32-78]; and moderate negative Eu anomalies (Eu/Eu* = 0.52 0.72) (Figs 4 y 5).
Differences in the chemical composition are evident between the Ucanha-Vilar and the Refoios do Lima granites and, in general, the other granites have an intermediate composition. For the same values of the parameter Fe+Mg+Ti, the Ucanha-Vilar granite shows higher Ca, Mg, P, Ba, Sr, REE, La N/Yb N and lower Fe.
Both mineralogical and whole-rock data show that all the four plutons define separate evolutionary trends. The whole-rock chemical evolutions are characterized by a decrease in Al, Ca, P, Ba, Sr, Zr, Y, La and La N/Yb N, and an increase in Si and Na with the decrease of the Fe+Mg+Ti parameter. The Sameiro pluton is an exception and does not show significant chemical evolution.
The granodiorites from the mafic bodies associated with the Ucanha-Vilar granite are slightly metaluminous (A from -11 to 1) and higher Al, Ca, Na, Fe, Mg, Ti, Zr, Y and lowe Si, K, Rb contents in comparison to the granite (Fig. 4).
The data reveal lack of geochemical continuity among all the granitoids, indicating that the hypothesis of various intrusive units originated from magmas with distinct compositions that evolved independently must be envisaged.
Late-D3 biotite-dominant granitoids
Different units of the Braga and Celorico de Basto plutons display a wide compositional range (SiO2 = 48.5-71.4%, MgO = 0.6-12.4%, CaO = 1.2-9.1%). The basic to intermediate series are dominantly metaluminous, reaching A = -102 for gabbroic rocks (Fig 4). The Braga granite reveals a slightly peraluminous character (A = 0-32) and the Gonça and Celorico de Basto granites are moderately peraluminous (A = 25-52). As it was previously evidenced for the Braga pluton, the Celorico de Basto pluton presents an evolutionary chemical continuity, without significant compositional gaps, from gabbro to the granite. This continuous evolution indicates a genetic link between different units of each pluton. With the decrease in the Fe+Mg+Ti parameter, three types of evolution are observed in the two plutons: (i) increase of Si, Na, K and Rb contents; (ii) decrease of Ca, Sr, Cr and Eu/Eu*; (iii) curvilinear trends with initial increase (for the basic to intermediate compositions) and later decrease in Ti, Ba, Zr, and La (Figs. 4 and 5). On the R1-R2 diagram different units of the Braga and Celorico de Basto series are distributed in the subalkaline domain, along regular curves with a curvature to quartz-monzodiorite compositions. Such evolutionary trends are typical of subalkaline (monzonitic) associations and identical to the trend observed in the plutonic series of the Ballons massif. These curvilinear trends may be explained by a process of mineral fractionation, with significant variation in mineral phases that fractionate (from calcium-rich plagioclase + olivine + orthopyroxene - clinopyroxene to more sodic plagioclase + amphibole biotite + zircon) at different evolution stages of the plutonic series.
The gabbro-norite from the two series show a primitive character [(Mg/(Fe+Mg) = 0.68-0.72 and Cr = 439-626 ppm] and has a chemical signature typical of shoshonitic basalts, characterized by higher contents of Al, K, Ba, Sr, Th and LREE and distinctive low Ti, Nb and HREE contents (Table 3). A similar pattern is also displayed by the gabbro from the plutonic series of the monzonitic Ballons massif in France.
Comparing the two granitic units of the Braga pluton, the Gonça monzogranite presents lower contents in compatible elements such as Ti, Zr, Y and REE (mainly HREE) as well as steeper REE patterns (La N/Yb N = 33-66) (Table 3, Figs 4 y 5). The Agrela granite is identical to the Braga granite in both mineralogical and chemical composition (Table 2 and 3, Figs 3, 4 y 5). The geochemical data rule out any compositional relation of these granites with the Celorico de Basto granite, which is comparatively enriched in Si and depleted in Ca, Fe, Mg, Ti, Sr, Ba and REE, with less fractionated REE patterns
Sr-Nd isotopic data
Syn-D3 biotite granitoids
These granitoids display a general negative correlation between 87 Sr/ 86 Sr initial ratios (Sr i) and epsilon Nd values that vary in the range 0.7072 0.7106 and -4.4 to -6.3, respectively (Table 4, Fig 6A). The available dat suggest Sr and Nd isotopic homogenization within each granitic pluton. The Ucanha-Vilar and Refoios do Lima granites present the most extreme Sr I and epsilon Nd values (Sr i = 0.7072-0.7073, epsilon Nd = -4.4 to -4.8 and Sr i = 0.7104-0.7106, epsilon Nd = -6.0 to -6.3, respectively), while the other granites reveal an intermediate but similar isotopic composition (Sr I = 0.7082-0.7085, epsilon Nd = -4.6 to -5.4).
The isotopic signatures show progressive enrichment from South to North.
The granodioritic bodies associated with the Ucanha-Vilar granite reveal less-enriched Sr-Nd isotopic compositions (Sr I = 0.7064, epsilon Nd = -3.3) when compared to the granite (Table 4, Fig 6A).
Late-D3 biotite-dominant granitoids
The late-D3 granites show different isotopic compositions relative to the syn-D3 group. The Braga and Agrela granites have similar isotopic compositions (Sr i= 0.7064.7075 epsilon Nd = -5.0 to -6.2) (Table 4, Fig 6A). Within each granite unit the Sr-Nd data display small variations. The Gonça and Celorico de Basto granites show distinct compositions of more enriched isotopic signature (Sr i = 0.7093, epsilon Nd = -6.8 and Sr i = 0.7089-0.7090, epsilon Nd = -5.6 to -5.7, respectively).
Three gabbro-norite samples from the Braga and Celorico de Basto plutons reveal identical isotope composition (Sr i = 0.7050, epsilon Nd = -2.5 and Sr i = 0.7049-0.7053, epsilon Nd = -2.1 to -2.4, respectively) (Fig. 6). The isotope compositions of the other facies from the gabbro-granodioritic stocks are in the range 0.7053-0.7080 for Sr I and -2.6 to -5.5 for epsilon Nd, values intermediate between those obtained for the gabbro-norites and the corresponding host granites. In the epsilon Nd vs Sr I diagram (Fig 6A) the samples from these two plutons define distinct trends and are distributed close to hyperbolic mixing curves. These isotopic data do not support the hypothesis of a simple fractionation process relating all the members of each series.
From the integration of the isotopic data the following points can be emphasized (Fig. 6B):
(1) A clear relationship exists between isotope signatures and geochemical affinities for granites: (i) syn and late-3D moderately peraluminous granitoids of aluminopotassic affinity show the most enriched isotopic compositions (Sr i = 0.7089-0.7106, epsilon Nd = -5.6 to -6.8), (ii) syn- and late -3D slightly perluminous granitoids of calc-alkaline and subalkaline affinities display a more depleted isotopic signature (Sr i = 0.7064-0.7085, and epsilon Nd = -4.4 to -6.2);
(2) The gabbros have similar isotopic compositions (Sr i = 0.705, epsilon Nd = -2);
(3) In the syn- and late-D3 groups, alc-alkaline and subalkaline series display continuous trends of increasing Sr I and decreasing epsilon Nd from metaluminous (gabbros, quartzdiorites, monzodiorites, quartz monzodiorites) to peraluminous (granodiorites, monzogranites) rocks whitin the range Sr I = 0.7049 to 0.7106 and epsilon Nd = -2.1 to -6.8.
Granite Protoliths A Discussion
The geological, petrographic, mineralogica, chemical and isotopic data suggest that the studied plutons include three distinct genetic groups: mantle, crustal and hybrid rocks. From the integration of these data, potential magma sources can be discussed and characterized.
Depleted versus enriched mantle
The Braga Celorico de basto gabbronorites show chemical signature of shoshonitic rocks. Although having a primitive mantle-derived character [(Mg/(Fe+Mg) = 0.68-0.72 and Cr = 439626 ppm], they do not reveal a depleted mantle signature (Sr I = 0.7049 0.7053, epsilon Nd = -2.1 to -2.5). It must be noted that the chemical and isotopic compositions of the gabbros are similar, revealing compositional homogeneity of the mantle component involved in the genesis of the Braga and Celorico de Basto plutons (Fig. 6B).
Sr-Nd isotope characteristics of the gabbros may be inherited from the protolith (enriched mantle) or result from a depleted mantle-derived magma affected by extensive assimilation or mixing with a geochemically more evolved crustal component. However an extensive crustal contamination would have induced high crystallization and fractionation of the basic magma, a process that is incompatible with the primitive chemical character of the studied gabbros. Therefore, we propose that they were probably derived from an enriched mantle which suggests the existence of a sub-Iberian enriched mantle domain during the Hercynian event. The enriched nature of sub-continental mantle domains within the Hercynian orogen is also referred in other sectors of the Iberian massif, in the French Central Massif and in the Bohemian Massif.
As proposed by Wenzel and Janousek for K-rich intrusions in the Bohemian Massif (Central European Hercynides), a subduction-related volatile influx may have been responsible for the enrichment in incompatible elements of the lithospheric mantle, whose later partial melting generated basic shoshonitic magmas with enriched isotopic compositions. A comparable origin can be envisaged for the studied gabbros.
Crustal protoliths
Some of the studied syn- and late-D3 granites show mineralogical and geochemical features that are characteristic of crustal-derived melts (Refoios do Lima, Gonça and Celorico de Basto granites). These are moderately peraluminous monzogranites (A = 25-52) with typical aluminopotassic affinity, in which the mafic microgranular enclaves are rare or absent. Low contents of Al- and Fe-rich biotites as well as Al-rich mineral phases (cordierite, andalusite, sillimanite) occur in these granites. As described above, the typological and geochemical characteristics of zircon populations from the Refoios do Lima granite, together with the presence of abundant relict cores, indicate a crustal origin for this granite. The three granites present the most evolved chemical compositions when compared to the other studied granites (lowest Ba, Sr and REE whole-rock contents; more aluminous and less magnesian biotites), as well as a more enriched Sr-Nd isotopic signature (Sr I = 0.7089-0.7106, epsilon Nd = -5.6 to -6.8, Fig 6B). When comparing the Refoios do Lima, Gonça and Celorico de Basto granites, it must be noted that they reveal different mineralogical, chemical and isotopic compositions (Figs. 3, 4 y 6A). Therefore an origin by partial melting of a compositionally heterogeneous continental crust is assumed, in which the contribution of a mantle-derived component is very small or even absent.
The composition of these monzogranites is different from that of melts produced experimentally from politic protoliths, rather suggesting a major involvement of metaigneus protoliths or metasedimentary sources derived from immature sediments. Crustal protoliths with an appropiate Sr-Nd isotopic composition were checked from the available isotopic data on the pre-Hercynian basement of the Iberian Massif. Felsic peraluminous granulites from the lower crust that occur as metaigneous xenoliths in the Spanish Central System (Iberian Massif; Sr 305-320 = 0.706 to 0.713, epsilon Nd 305-320 = -1 to -8) are potential sources. The Nd isotopic compositions of felsic metaigneous rocks from the middle crust are similar to that of these monzogranites, but their Sr isotopic compositions are more radiogenic (Sr 305-320 > 0.715, epsilon Nd 305-320 = -4 to -7). The high Ba and Sr contents as well as the high K/Rb and La N/Yb N ratios, mainly evident in the Refoios do Lima and Gonça monzogranites, imply a feldspar-rich protolith and a residue relatively poor in K-feldspar or biotite and rich in garnet. We therefor propose that the corresponding magmas were produced in the granulite facies by partial melting of felsic igneous or greywacke sources.
A similar Nd-depleted mantle model afe of 1.4 Ga was obtained for Refoios do Lima, Gonça and Celorico de Basto granites. T DM values in the range 1.2-1.6 Ga are usually found in peraluminous granites from the European Hercynides. These data could indicate the reworking of a continental crust generated in Mid-Proterozoic times. However there is no evidence of a major crust-forming event during this period in the Hercynian belt, thus the model ages rather indicate average crustal residence ags involving mixed crustal components of Archean and late Proterozoic sources.
Highly peraluminous leucogranites do not occur in the studied plutons. These leucogranites display highly evolved chemical and isotopic signatures (Sr I > 0.711, epsilon Nd < -7) and are largely represented in the CIZ, mainly as syn- to late-3D magmatism, thus with synchronous emplacement relative to the less peraluminous biotite monzogranitic plutons. They are not genetically related. An origin by partial melting of upper crustal levels with a major contribution of aluminous metasedimentary sources is proposed for the highly peraluminous leucogranites.
Hybrid rocks
Hybridization processes between coeval crustal and mantle-derived magmas have been proposed to explain the field, petrographic, chemical and isotopic features of granodiorites and monzogranites in the European Hercynides and particularly in the Iberian Massif. In our study several evidences suggest interaction between coeval and contrasting magma types:
(1) The hybrid granites are associated with coeval basic to intermediate rocks and/or abundant mafic microgranular enclaves;
(2) Presence of net-veining structures and disequilibrium textures in the hybrid rocks (such as quartz ocelli surrounded by a reaction rim of biotite or amphibole; irregular cores of plagioclase, surrounded by dendritic and more calcic rims; zircon crystals presenting resorption structures and HfO2 compositional discontinuity between two magmatic stages);
(3) Zircon populations from the hybrid syn-D granites show typological evolutionary trends that cross the domain of the calc-alkaline granites to the crustal granites, in contrast with what would be expected from a simple evolutionary crystallization process;
(4) For each series, the granites and associated mafic rocks define continuous and regular trends in major and trace element diagrams (Fig 4) and are distributed close to hyperbolic mixing curves in the epsilon Nd vs. Sr i diagram (Fig 6A). In these diagramas the hybrid rocks plot between the gabbro-norites and the crustal monzongranites, displaying intermediate chemical and isotopic (Sr I = 0.7053-0.7085 and epsilon Nd = -2.6 to -6.2; Fig 6B) compositions.
(5) Biotites from the Braga and Celorico de Basto series show curved evolutions which cross-cut the granitic typological fields from the subalkaline/calc-alkaline domains to the aluminopotassic one in the Al total vs Mg diagram of Nachit (Fig 3).
The above evidences argue against the following petrogenetic processes; (i) pure crustal origin for the slightly peraluminous biotite granodiorites/monzogranites, (ii) metasomatic transformation of solid basic material associated with granitic magma and (iii) simple fractional crystallization associating all the units in each granitoid series. The evidences rather suggest hybridization processes between coeval and contrasting liquids, which generated hybrid magmas, represented by the slightly peraluminous biotite granodiorites/monzogranites (Ucanha-Vilar, Lamego, Felgueiras, Sameiro, Braga and Agrela granites) and by the associated diorite-monzodioritic-granodioritic rock series. The felsic component is represented by the monzogranites of crustal origin (Refoios do Lima, Gonça and Celorico de Basto granites). The studied gabbros represent the basic component for the late-D3 composite plutons. Syn-D3 gabbroic rocks were not found at the present-day level of exposure, therefore the nature of the basic component for the syn-D3 plutons is not well constrained. However, the mixing line in the epsilon Nd vs Sr i, Sr i vs 1/Sr and epsilon vs 1/Nd diagramas (Fig 6A) extrapolates to isotopic compositions similar to those of the late-D3 gabbroic rocks. It must be noted that the post-collisional stage of the Hercynian orogeny /syn- to post-D2, 300- 320 Ma) was the main period of successive generation of granites without any significant geochoronological gap (syn-D3, 313-320 Ma; late-D3, 306-311 Ma; late- to post-D3, ca. 300 Ma). Therefore, a basic component similar to the sutdied gabbros is not precluded for the genesis of the syn-D3 hybrid biotite granites.
The composite plutons show curved evolutions for some major and trace elements (Ti, Ba, Zr, La; Fig 4), indicating that mineral fractionation was a main petrogenetic process. On the other hand, isotopic data are compatible with mixing processes between two contrasting magma types. Therefore, the chemical and Sr-Nd isotopic compositions of the studied hybrid rocks can be explained by a complex petrogenetic process combining fractional crystallization and magma mixing between a mantle-derived magma (represented by the gabbros) and felsic crustal magmas (represented by the Refoios do Lima, Gonça and Celorico de Basto granites). An AFC model was applied to the Braga composite pluton which suggests that fractional crystallization was the predominant process in the initial evolution of the basic magma, with increasing degree of contamination towards more acidic compositions. From this model the generation of the hybrid Braga granite would imply a high rate of fractional crystallization (72%) and a moderate rate of crustal-melt contamination (34%).
The three evolutionary trends ain the epsilon Nd vs. Sr i diagram (Fig 6A) are ascribed to distinct crust-derived melts, generated from a heterogeneous source region in the lower crust, but the mantle-derived melt inputs had a similar enriched isotopic composition. Thus the differences in the isotopic composition of various hybrid rocks can be related to the heterogeneity of the crustal magmas and to the variable degree of crustal contribution. For the studied syn-D3 plutons the crustal contribution progressively drecreases from north to south, in accordance of mafic microgranular enclaves. These features have led us to postulate the existence of independent reservoirs, in which distinct magma batches evolved independently by fractional crystallization and magma mixing processes.
Hybrid granitoids and crustal growth
Hybrid peraluminous granodiorites/monzogranites are largely represented in the CIZ of the Iberian Massif, being generated by complex petrogenetic processes combining fractional crystallization and hybridization between coeval mantle- and crust-derived magmas. Therefore, these processes played a significant role in the granitoid production during the Hercynian event, implying an important input of juvenile mantle magmas into the crust, as also emphasized by Castro. These authors describe melting-assimilation experiments carried out at 1000ºC and 4,7 or Kb and using a proportion of 50% gabbro and 50% gneiss. These experimetns account for such input and for the generation of high proportion (> 50 vol. %) silica-rich liquids whose compositions closely resemble those of the hybrid peraluminous granodiorites.
The abundance of hybrid syn- to late-D3 granitic rocks in the Iberian Massif, produced by mantle-crust interactions, indicates that a major episode of crustal growth took place during the late Paleozoic times of the Hercynian orogeny. This is in agreement with the proposal of Patinho Douce that generation of granitic magmas is almost always associated in space and time with growth, rather than just recycling, of the continental crust.
In summary, the following conclusion can be started:
(1) In the Hercynian Central Iberian Zone of the Iberian Massif large volumes of granitoids were emplaced during the last ductile deformation phase (D3, 300-320 Ma). This was the main period of successive granite generation, which exhibit composition variation, the biotite-dominant granitoids being the most abundant. They are frequently associated with gabbro-granodioritic stocks and mafic microgranular enclaves and show evidence of interaction between felsic and mafic magmas.
(2) In the syn- and late-D3 periods (320-305 Ma) moderately peraluminous monzogranites of aluminopotassic affinity were produced, which display the most enriched isotopic compositions. An essentially crustal origin by partial melting of a heterogeneous crust is proposed. Potential protoliths are metasedimentary (immature sediments) and/or felsic meta-igneous lower crustal material (Sr 320-305 = 0.709 to 0.711, epsilon Nd 320-305 = -6 to -7).
(3) A large amount of hybrid magmas were generated by the interaction between these crust-derived liquids and a mantle-derived magma of shoshonitic affinity (equivalent to the gabbroic rocks) yielding an enriched isotopic signature. Slightly peraluminous monzongranites/granodiorites of calc-alkaline and subalkaline affinities were produced by crystallization of these hybrid magmas. Chemical and Sr-Nd isotopic data on the composite plutons (including basic to intermediate stocks) provide the evidence that the hybrid granitoids were generated by a complex petrogenetic process combining fractional crystallization and mixing between coeval mantle- and crustal-derived magmas (at variable degrees) in independent magma reservoirs.
(4) Mantle-derived mel inputs have a similar enriched isotopic composition, but crustal magmas are heterogeneous in composition. Thus the differences in the isotopic composition of various hybrid rocks are related the heterogeneity of the crustal source region in lower crust and also to variable degrees of crustal contamination.
(5) In the Iberian Massif an extensive crustal recycling event occurred at the post-collisional stage of the Hercynian orogeny. The abundance of hybrid granitoids, implying a significant input of juvenile mantle magma, indicates the occurrence of an important crustal growth episode together with the recycling processes,
Posted by cualvirtual at 1 de Febrero 2005 a las 06:59 PM