No abstract

A B S T R A C TThe Avila batholith of central Spain is composed of upper Carboniferous peraluminous granitoids that were preceded by volumetrically insignificant bodies of mafic-ultramafic hybrid magmas and postdated by several dike swarms of camptonitic lamprophyres. Rb-Sr dating indicates continuous magmatic activity from 350 Ma to 280 Ma, starting ∼ ∼ with the mafic precursors and a few midcrustal anatectic leucogranites, followed by massive autochthonous and allochthonous granodiorites and granites, and ending with the camptonitic lamprophyres. Early hybrid mafic magmas ( ; ) were produced in small batches during or immediately after the main deformation 340 Ma 340 Ma Sr ∼ 25 Nd ∼ Ϫ1.5 phase, probably by the partial melting of a mixture of 8 : 2 mantle and biotite-bearing crustal rocks at the crust-∼ mantle interface. These magmas were emplaced in the middle crust at 340 Ma, advecting a negligible amount of ∼ heat. The generation of crustal granites during the main deformation phase was very scarce and limited to highly fertile protoliths, rich in heat-producing elements, affected by strong shear zones. The generation of crustal granitoids on a batholithic scale took place from 330 Ma to 290 Ma, during the main extensional period. Granites ∼ ∼ ( -150; ) were produced by the partial melting of fertile crustal rocks 310 Ma 310 Ma Sr ∼ 45 Nd ∼ Ϫ2.1 to Ϫ 9 ( -218; ), characterized by high heat production ( 2.5-3 mW m Ϫ3 ). The zone of 310 Ma 310 Ma Sr ∼ 48Nd ∼ Ϫ2.2 to Ϫ 9 ∼ partial melting, 15-22 km in depth, was heated by thermal conduction from below after crustal thinning, but the ∼ contribution of radiogenic heat and the fertility of source rocks would have been essential for anatexis. The fast thinning of the crust from 310 Ma to 285 Ma released lithostatic pressure in the upper mantle and caused decom-∼ ∼ pressional melting of the metasome layer at 60-85 km in depth, producing camptonitic melts dated at 283 Ma. ∼ ∼ The existence of a fertile metasome layer implies that the lithospheric mantle beneath central Iberia was not actively involved in subduction during the Variscan orogeny.

No abstract

Dating the pre-Middle Ordovician metavolcanic rocks and metagranites of the Ollo de Sapo Domain has, historically, been difficult because of the small compositional variation, the effects of the Variscan orogeny and, as revealed in this paper, the unusually high fraction of inherited zircon components. The first reliable zircon data (U-Pb ion microprobe and Pb-Pb stepwise evaporation) indicate that the Ollo de Sapo volcanism spanned 495 ± 5 Ma to 483 ± 3 Ma, and was followed by the intrusion of high-level granites from 483 ± 3 Ma to 474 ± 4 Ma. In both metavolcanic rocks and metagranites, no less than 70-80 % of zircon grains are either totally Precambrian or contain a Precambrian core overgrown by a Cambro-Ordovician rim. About 80-90 % of inherited zircons are Early Ediacaran (602-614 Ma) and derived from calc-alkaline intermediate to felsic igneous rocks generated at the end of the Pan-African arc-continent collision. In the Villadepera region, located to the west, both the metagranites and metavolcanic rocks also contain Meso-Archaean zircons (3.0-3.2 Ga) which ultimately originated from the West African Craton. In the Hiendelaencina region, located to the east, both the metagranites and metavolcanic rocks lack Meso-Archaean zircons, but they have two different inherited zircon populations, one Cryogenian (650-700 Ma) and the other Tonian (850-900 Ma), which suggest older-than-Ediacaran additional island-arc components. The different proportion of source components and the marked variation of the 87 Sr/ 86 Sr init. suggest, at least tentatively, that the across-arc polarity of the remnants of the Pan-African arc of Iberia trended east-west (with respect to the current coordinates) during Cambro-Ordovician times, and that the passive margin was situated to the west.

The results of ~4000 LA-ICP-MS analyses in 152 thin sections from common crustal rocks reveal that several rock-forming minerals contain tens to a few thousand ppm of Zr. The highest concentrations of Zr are found in xenotime, followed by titanite, ilmenite, rutile, allanite, amphibole, clinopyroxene, garnet, magnetite and, less commonly, plagioclase, K-feldspar and orthopyroxene. Olivine, cordierite, biotite, muscovite, apatite, epidote and monazite have low levels of Zr (<5 ppm, generally <1 ppm). The minerals with the highest K D Hf /K D Zr are titanite (2.5), orthopyroxene (2.0), amphibole and clinopyroxene (1.8), and epidote and rutile (1.6-1.7). Ilmenite, magnetite, the feldspars and apatite have K D Hf /K D Zr ≈ 1, and values less than one were found in xenotime and zircon (0.8), garnet (0.7), and allanite (0.6). The most important implications of these results follow. First, the growth of a Zr-bearing phase during partial melting does not influence the Zr concentration of the melt, but increases the fraction of zircon that can be dissolved at a given temperature. This accelerates the disappearance of zircon from the protolith or, in melts already segregated, the dissolution of inherited zircon. The effect will be more marked in metaluminous magmas precipitating amphibole and titanite than in any other type of magma. Second, the presence of Zr-bearing phases has little influence of the zircon-saturation "geothermometer". It may cause somewhat higher (20-30°C) results in metaluminous rocks, but has no effect on peraluminous rocks. Third, the uptake of Zr by major minerals in crystallizing magmas may reduce both the concentration of Zr in the melt available to form zircon and the temperature at which zircon begins to precipitate. Mineral-melt reactions involving Zr-bearing phases may lead to zircon grains with complicated patterns of zoning and texturally discordant zones, apparently diachronous. Fourth, higher-than-chondrite Zr/Hf fractionates may arise from titanite, amphibole or clinopyroxene fractionation, but this requires very little or no crystallization of zircon. Significantly lower-than-chondrite Zr/Hf magmas only result from zircon fractionation. Lastly, two new examples of mineral reactions that involve the formation of a mass-balancing accessory phase, useful for high-resolution geochronology, are described: the formation of xenotime as a product of the breakdown of garnet in amphibolite-grade metapelites, and the subsolidus growth of a new rim on zircon included in Zr-bearing feldspars.

No abstract

No abstract

Pipe-like ultramafic bodies, hosting Ni-Cu-PGE sulphide deposits, INTRODUCTIONintrude the Main Gabbro and the roof metasediments of the Ivrea The Ivrea Zone, in NW Italy, is believed to represent an Zone, NW Italy. These bodies were emplaced at 287 ± 3 Ma exposed section of the lower continental crust (Mehnert, and represent the last mantle-derived melts associated Garuti et al., 1980;Percival et al., 1992) whose underplating event that largely drove the crustal evolution of this area evolution has been largely driven by the underplating during the late Carboniferous (>300-290 Ma). The pipes are of mantle-derived basic magmas (the Basic Complex), composed of volatile-rich ultramafic rocks and gabbros with an associated with the uplift of large fragments of mantle alkaline signature simultaneously enriched in both incompatible and peridotites (the Mantle Tectonite massifs of Baldissero, the most compatible elements but depleted in elements of intermediate Balmuccia and Finero), beneath the metamorphic basecompatibility. The isotope composition of these pipe rocks is 290 Ma (Nd) ment of the Southern Alps (the Kinzigitic Formation) >3•7 to −1•9 and 290 Ma (Sr) >0•8-26. In a 290 Ma (Nd) vs (Fig. 1). Although this area has been the object of 290 Ma (Sr) diagram they define a linear array between the unnumerous studies that have significantly contributed to metasomatized and metasomatized peridotites of Finero, but distinctly the understanding of the evolution of the lower conoblique with respect to the trend defined by Balmuccia peridotites. tinental crust [see Percival et al. (1992) and references The 34 S ranges from 0•0 to +0•9‰ and is indicative of a mantle therein], many aspects of the underplating process, espesource. We suggest that the pipes represent infiltration of melts derived cially the timing of emplacement of basic magmas and from a depleted mantle protolith flushed with alkaline metasomatic whether the environment was a mantle plume (e.g. fluids, probably of juvenile mantle origin, which underwent partial Shervais & Mukasa, 1991) or a subduction setting (e.g. melting as a consequence of the depression of the solidus owing to Hartmann & Wedepohl, 1993;Zanetti et al., 1999), are the increased activity of water and other volatiles. The similarity in still unresolved. age, trace-element, and isotopic signatures indicates that the pipes A particularly striking group of intrusions, whose study were probably produced in the course of the same metasomatic event may be relevant for understanding the geodynamic setting that affected the Finero ultramafic body. The overall geochemical and evolution of the Ivrea Zone, consists of pipe-like characteristics of the pipes are more consistent with magmatism related bodies formed of amphibole-rich peridotites and pyto a mantle plume than with a subduction setting.roxenites with minor hornblendites and gabbros, anomalously rich in volatiles and incompatible elements, which were emplaced into either the upper part of the