A stable isotopic (δ¹⁸O, δD) study of the late-Hercynian granites and their host-rocks in the Central Iberian Massif (Spain)

Abstract

Stable isotopic ratios (mainly 18O/16O, but also D/H) have been measured for the three most important types of late-Hercynian granites, and their hosts, in the western area of the Central Iberian Massif (CIM), Spain. These granites are amphibole-bearing biotite granites, biotite granites and cordierite-bearing biotite granites. No intrusive relationships have been observed among them; the contact of each granite with the others is always gradational. Host-rocks are Precambrian/Cambrian metasediments, ranging from low-grade schists to migmatites (nebulites).Whole-rock δ18OSMOW values are as follows: amphibole-bearing biotite granites 8·9 ± 0·58% (1σ, n = 17); biotite granites 9·0 ± 0·35% (1σ, n = 11); cordierite-bearing biotite granites 9·6 ± 0·24% (1σ, n = 21). δ18O values for nebulites, into which some of these granites were emplaced, are significantly higher, at 11·1 ± 0·58‰ (1σ, n = 13). The Precambrian to Cambrian shales gave an average value of δ18O = 11·9 ± l·23‰ (lδ, n = 5). Whole-rock oxygen isotope ratios indicate that the origin of the granites was in neither purely sedimentary/metasedimentary rocks nor pristine mantle melts. δ18O values close to 9·0‰ require a crustal protolith, having an important recycled component.Oxygen isotope results are compatible with the cordierite-bearing granites being generated by assimilation of nebulite-like material by a biotite granite magma. However, 18O/16O of mineral separates obtained from the three different granites and the nebulite indicate that isotopic equilibrium, if ever reached, has not been preserved. The modified isotopic equilibrium is attributed to fluid activity, but mineral-pair δ-δ plots suggest that the granite system behaved as a closed system, and that the fluid was deuteric (magmatic) in origin. This implies that if assimilation did happen, it occurred at a temperature higher than the closure temperature of the different minerals to isotopic exchange. In a δ18O vs δD plot, hornblende and biotite separates from the granites plot within the igneous field. A simple mesocrustal anatectic origin for the peraluminous late Hercynian granites of the western area of the CIM is difficult to sustain on the basis of the stable isotope data, consistent with other field, petrographic and geochemical evidence. Cordierite in the cordierite-bearing granites is not “restitic” from a deep source area, but rather is xenocrystic from the high-grade metamorphic country rock (nebulites).

Zircon U-Pb and Hf isotopic constraints on the genesis of a post-kinematic S-type Variscan tin granite: the Logrosán cupola (Central Iberian Zone)

“…Similar δ 18 O values have been found in other granites from the southern CIZ (e.g., Castelo Branco Batholith: 12.2-13.7‰, Antunes et al, 2008). Nevertheless, these values are significantly higher than those found in the peraluminous S-type granites of the Spanish Central System, which mostly range from 8.3 to 10.2‰ (Villaseca and Herreros, 2000;Recio et al, 1992).…”

Late-Variscan emplacement and genesis of the Vieira do Minho composite pluton, Central Iberian Zone: Constraints from U–Pb zircon geochronology, AMS data and Sr–Nd–O isotope geochemistry

“…Mechanisms other than pure crustal melting could generate peraluminous magmas (e.g. Barbarin, 1996) (Villaseca and Herreros, 2000, modified); Nd and oxygen isotopic data of SCS granites and orthogneisses are from Villaseca et al (1998) and Villaseca and Herreros (2000); Nd and oxygen isotopic data of felsic granulites are from Villaseca et al (1999) and Villaseca and Herreros (2000); data of Upper-Precambrian Schist-Greywacke Formation (SGF) of the Central Iberian Zone are from Ugidos et al (1997); microgranular enclaves data are from Recio et al (1992), Pinarelli and Rottura (1995) and Moreno-Ventas et al (1995); other isotopic data are from Table 5. but it seems unlikely that they could generate large batholiths of peraluminous felsic granites.…”

300 Million years of episodic hydrothermal activity: stable isotope evidence from hydrothermal rocks of the Eastern Iberian Central System

“…They have b l O values between 7.3 and 10.4:100 (Aparicio et al 1986;Recio et al 1992). Granites s.s. have slightly heavier compositions, 10.1 to 11.5%0' Most isotopie eompositions are within the range typical of Variscan granites (9.5 to 13%0; Sheppard 1986a; see also Hoefs and Ernmermann 1983;Del � ado 1993).…”