Antonio Castro

Understanding Granites: Integrating New and Classical Techniques

Granite magmatism represents a major contribution to crustal growth and recycling and, consequently, is one of the most important mechanisms to have contributed to the geochemical differentiation of the Earth’s crust since the Archaean times. The important role of granites has been acknowledged from the times of James Hutton, being an important part of Hutton’s (1785) work The Theory of the Earth. Since that time, advances in Earth Sciences and the development of analytical instruments have produced a vast amount of data from granite terrains around the world (see, for instance, the Proceedings of the Hutton symposia, 1988, 1992, 1996). However, many problems still remain unsolved. There have been several important controversies concerning granites during the last 50 years. One of the best remembered was that led by H. H. Read and N. L. Bowen around the 1950s (Read 1958). The end of this obscure controversy, full of misunderstandings and with some rhetoric, ended with the publication of the classic memoir by O. F. Tuttle & N. L. Bowen (1958) entitled Origin of granites in the light of experimental studies in the system NaAlSi3O8-KAlSi3O8-SiO2-H2O. The main conclusion of this study was that granitic liquids may be generated from melting of quartzo-feldspathic rocks in excess water, at depths of ‘… 12–21 km in geosynclinal areas where the initial gradient is on the order of 30°C/km’ (Tuttle & Bowen 1958, p. 2). Detailed phase relationships in the granite system were determined in this seminal work. However, the conditions for the generation of

Rayleigh fractionation of heavy rare earths and yttrium during metamorphic garnet growth

We present laser-ablation microprobe inductively coupled–mass spectrometry analyses of Y, Yb, Er, Dy, and Gd in garnet crystals from high-grade metamorphic and migmatitic rocks from the Sierras Pampeanas of Argentina. These rocks contain large garnet porphyroblasts (≥4 mm) that are compositionally zoned. Rare earth element concentrations in the rims are typically one order of magnitude lower than those in the cores. A notable feature of this zonation is the inversion of Yb/Er and Yb/Dy ratios, from >1 in the cores to <1 in the rims. We show quantitatively that the spatial distribution of these trace elements in garnet can be most simply and effectively explained as arising from Rayleigh fractionation during garnet growth. We also analyzed a small garnet crystal (<2 mm) representative of a garnet population that is associated with migmatitic leucosomes. This crystal displays uniformly low concentrations of all trace elements that are virtually identical in both absolute and relative magnitudes to those in the rims of the larger garnet porphyroblasts. The small crystals, and perhaps part of the porphyroblast rims, are likely to be peritectic products of incongruent melting reactions. We argue that melts formed from garnet-bearing sources may generally be unable to equilibrate with metamorphic garnet cores and may thus be more strongly depleted in heavy rare earth elements and Y than modeling based on bulk source-rock abundances of these elements would indicate.

Determination of the fluid–absent solidus and supersolidus phase relationships of MORB-derived amphibolites in the range 4–14 kbar

Abstract Experiments on a MORB (Mid Ocean Ridge Basalt)-derived amphibolite have been conducted in order to determine fluid-absent solidus and supersolidus phase relationships. The solidus has been determined in the range 4-14 kbar by using the incremental heating technique, tested against classical procedures. Peritectic clinopyroxene was produced below 10 kbar and epidote was the stable peritectic phase at pressures greater than 10 kbar for temperatures near the solidus. Garnet was the peritectic phase for temperatures above 800 ∞C and pressures of 10 to 14 kbar. The shape of the solidus was similar to previous estimations but the temperature interval between the high- and low-pressure regions was strongly reduced. With these new estimations, the possibilities for melting of the oceanic crust at P below 10 kbar are increased and, consequently, the number of tectonic scenarios needed for magma generation from amphibolite sources is enlarged. In the absence of free water, the solidus in the pressure region above 10 kbar is at higher T than previous estimations.

Post-collisional polycyclic plutonism from the Zagros hinterland: the Shaivar Dagh plutonic complex, Alborz belt, Iran

The petrological and geochronological study of the Cenozoic Shaivar Dagh composite intrusion in the Alborz Mountain belt (NW Iran) reveals important clues to decipher complex relations between magmatic and tectonic processes in the central sectors of the Tethyan (Alpine–Himalayan) orogenic belt. This pluton is formed by intrusion at different times of two main magmatic cycles. The older (Cycle 1) is formed by calc-alkaline silicic rocks, which range in composition from diorites to granodiorites and biotite granites, with abundant mafic microgranular enclaves. The younger cycle (Cycle 2) is formed by K-rich monzodiorite and monzonite of marked shoshonitic affinity. The latter form the larger volumes of the exposed plutonic rocks in the studied complex. Zircon geochronology (laser ablation ICP-MS analyses) gives a concordia age of 30.8 ± 2.1 Ma for the calc-alkaline rocks (Cycle 1) and a range from 23.3 ± 0.5 to 25.1 ± 0.9 Ma for the shoshonitic association (Cycle 2). Major and trace element relations strongly support distinct origins for each magmatic cycle. Rocks of Cycle 1 have all the characteristic features of active continental margins. Shoshonitic rocks (Cycle 2) define two continuous fractionation trends: one departing from a K-rich basaltic composition and the other from an intermediate, K-rich composition. A metasomatized-mantle origin for the two shoshonitic series of Cycle 2 is proposed on the basis of comparisons with experimental data. The origin of the calc-alkaline series is more controversial but it can be attributed to processes in the suprasubduction mantle wedge related to the incorporation of subducted melanges in the form of silicic cold plumes. A time sequence can be established for the processes responsible of the generation of the two magmatic cycles: first a calc-alkaline cycle typical of active continental margins, and second a K-rich cycle formed by monzonites and monzodiorites. This sequence precludes the younger potassic magmas as precursors of the older calc-alkaline series. By contrast, the older calc-alkaline magmas may represent the metasomatic agents that modified the mantle wedge during the last stages of subduction and cooked a fertile mantle region for late potassic magmatism after continental collision.

Variscan intra-orogenic extensional tectonics in the Ossa-Morena Zone (Évora-Aracena-Lora del Rı́o metamorphic belt, SW Iberian Massif): SHRIMP zircon U-Th-Pb geochronology

Abstract Following a Middle–Late Devonian (c. 390–360 Ma) phase of crustal shortening and mountain building, continental extension and onset of high-medium-grade metamorphic terrains occurred in the SW Iberian Massif during the Visean (c. 345–326 Ma). The Évora–Aracena–Lora del Rı́o metamorphic belt extends along the Ossa–Morena Zone southern margin from south Portugal through the south of Spain, a distance of 250 km. This major structural domain is characterized by local development of high-temperature–low-pressure metamorphism (c. 345–335 Ma) that reached high amphibolite to granulite facies. These high-medium-grade metamorphic terrains consist of strongly sheared Ediacaran and Cambrian–early Ordovician (c. 600–480 Ma) protoliths. The dominant structure is a widespread steeply-dipping foliation with a gently-plunging stretching lineation generally oriented parallel to the fold axes. Despite of the wrench nature of this collisional orogen, kinematic indicators of left-lateral shearing are locally compatible with an oblique component of extension. These extensional transcurrent movements associated with pervasive mylonitic foliation (c. 345–335 Ma) explain the exhumation of scarce occurrences of eclogites (c. 370 Ma). Mafic-intermediate plutonic and hypabyssal rocks (c. 355–320 Ma), mainly I-type high-K calc-alkaline diorites, tonalites, granodiorites, gabbros and peraluminous biotite granites, are associated with these metamorphic terrains. Volcanic rocks of the same chemical composition and age are preserved in Tournaisian–Visean (c. 350–335 Ma) marine basins dominated by detrital sequences with local development of syn-sedimentary gravitational collapse structures. This study, supported by new U–Pb zircon dating, demonstrates the importance of intra-orogenic transtension in the Gondwana margin during the Early Carboniferous when the Rheic ocean between Laurussia and Gondwana closed, forming the Appalachian and Variscan mountains.

A study of inherited zircons in granitoid rocks from the South Portuguese and Ossa-Morena Zones, Iberian Massif: support for the exotic origin of the South Portuguese Zone

Abstract Trace element and U–Pb isotopic analyses of inherited zircon cores from a sample of Gil Marquez granodiorite (South Portuguese Zone, SPZ) and Almonaster nebulite (Ossa-Morena Zone, OMZ, in the Aracena Metamorphic Belt) have been obtained using laser ablation-inductively coupled plasma-mass spectrometry. These data reveal differences in the age of deep continental crust in these two zones. Inherited zircon cores from the Ossa-Morena Zone range at 600±100 Ma, 1.7–2 Ga and 2.65–2.95 Ga, while those from the South Portuguese Zone range at 400–500 and 700–800 Ma. These data support the “exotic” origin of the South Portuguese Zone basement relative to the rest of Iberian Massif. The young ages of inherited zircon cores and Nd model ages of magmatic rocks of the South Portuguese Zone are comparable to results from granulite facies xenoliths and granitic rocks from the Meguma Terrane and Avalonia and support a correlation between the basement of the southernmost part of the Iberian Massif and the northern Appalachians.

Pluton accommodation at high strain rates in the upper continental crust. The example of the Central Extremadura batholith, Spain

Abstract Emplacement in the tensional bridge of a stepped dextral shear zone system is proposed for the Central Extremadura batholith (Spain). The country rocks show a pervasive anisotropy that conditioned the style of the structures developed as a consequence of the transference of displacement from the stepped shear zones to the releasing area. The kinematic evolution of the resulting megakink fold provided the volume increase necessary for the granite emplacement. Thermal and kinematic models suggest that the growth of individual plutons took place in periods of no more than several hundred to a few thousand years. Fast strain rates (10 −10 –10 −11 xa0s −1 ) must concentrate in local structures (e.g. initiation of kink folds) even in zones deforming as a whole under typical strain rates (10 −14±1 xa0s −1 ). Granite plutons might be used as strain-rate gauges for syn-plutonic structures.

Granite intrusion by externally induced growth and deformation of the magma reservoir, the example of the Plasenzuela pluton, Spain

Abstract The Plasenzuela pluton in the Central Extremadura batholith in the southern Iberian Massif, is an example of permissive emplacement in relation to the tectonic development of extensional fractures in the upper continental crust. Paradoxically, this pluton has a concordant structural pattern which is classically attributed to diapirism or ballooning. This pattern consists of the following elements: (a) nearly elliptical shape in the horizontal section; (b) conformity of the pre-existing aureole structures to the shape of the pluton contacts; and (c) development of a crenulation cleavage, parallel to the contacts, in the vicinity of the pluton walls. All these features have been interpreted in many plutons as resulting from the pushing-aside of the country rock structures due to the expansion of the pluton. However, the detailed structural relationships in the aureole do not favour a forceful emplacement mechanism. By contrast, these relationships constitute prime evidence of permissive intrusion in extensional fractures. According to this interpretation, the concordant shape of the pluton was acquired by syn-plutonic opening of a mixed tensional-shear fracture, parallel to the main foliation in the host rocks, and by folding of the fracture walls together with the previous anisotropy of the country rocks. This is a growth–deformation process that can operate at local conditions in the upper continental crust giving rise to concordant syn-tectonic plutons.

Tectonometamorphic evolution of the Aracena metamorphic belt (SW Spain) resulting from ridge-trench interaction during Variscan plate convergence

[1]xa0The Aracena metamorphic belt is a high-temperature/low-pressure (HT/LP) band located at the southern end of the European Variscan chain. It marks a suture between Armorica and Avalonia. This belt is characterized by (1) the presence of mid-ocean ridge basalt-derived metabasites that were affected by an inverted HT/LP metamorphism related to SW verging thrusting, whose thermal peak diachronously migrated eastward; (2) the occurrence of a HT/LP metamorphism, related to an extensional event, affecting continental rocks belonging to the Ossa Morena zone with peak temperatures that are ∼150°C higher than that recorded in the oceanic metabasites; and (3) the presence of near-trench magmatism with high-Mg andesite composition. These and other characteristics can be interpreted in terms of a ridge-trench-trench triple junction that migrated along the Armorican margin during Variscan oblique convergence.

The source of granites: inferences from the Lewisian complex

Synopsis The origin of granitic rocks has been the subject of intense debate. It is broadly accepted that granites are generated in the continental crust. However, the composition of the source rocks and the processes controlling melting reactions remain controversial. Fluid-absent melting is a plausible mechanism for granite generation but is not supported by experimental data. Fluid-present melting is favoured by experiments but the presence of a free water phase is very unlikely in the deep crust. Isotopic ratios are also paradoxical because Nd isotopes indicate a crustal (recycled) source but the low 87Sr/86Sr ratios displayed by many granite batholiths require a substantial mantle contribution. Furthermore, mantle–crust hybridization mechanisms are difficult to apply in terms of magma mixing and assimilation according to models of the physical/rheological behaviour of silicic magmas. Here I present a new, source-based, genetic approach for granitic rocks in which all these paradoxes are satisfactorily explained. The model is supported by new constraints from laboratory experiments and observations of field relationships in the Lewisian complex of northern Scotland. The process starts with the invasion of crustal regions by water and K-rich monzodiorite (andesite) magmas derived from hybridized regions of the subduction-modified mantle wedge. These intermediate magmas supply the water and potassium necessary for crustal melting and granite batholith generation.