Abstract New laboratory experiments using granulite xenoliths support a dual origin for I-type granites as primary and secondary. Primary I-type granites represent fractionated liquids from intermediate magma systems of broadly andesitic composition. Fluid-fluxed melting of igneous rocks that resided in the continental crust generates secondary I-type granites. The former are directly related to subduction, with Cordilleran batholiths as the most characteristic examples. Experiments with lower crust granulite sources, in the presence of water, show that amphibole is formed by a water-fluxed peritectic rehydration melting reaction . Entrainment of only 10% of restites composed of amphibole, pyroxene, plagioclase and magnetite, is sufficient to account for discrepancies in aluminium saturation index and maficity in secondary I-type granites. Lower crust granulite xenoliths, attached to a sanukitoid containing 6 wt% water, have been used in two-layer capsules to test fluid-fluxed melting reactions as the origin of secondary I-type granites. It is proposed that sanukitoid magmas act as water donors that trigger extensive melting of the lower crust, giving rise to granodioritic liquids. Because primary granites are related to coeval subduction, and secondary ones are crustal melts from older subduction-related rocks, the distinction between both I-types is essential in tectonic reconstructions of ancient orogenic belts.

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.

Abstract Most Palaeozoic magmatic rocks in Spain were produced during the Variscan orogeny, and there are excellent and abundant examples of both volcanic and plutonic lithologies. Volcanic units include those in the world-famous Iberian Pyrite Belt, and plutonic rocks exposed in the Iberian Massif include some of the largest and best granite outcrops in the European Variscides. Magmatic rocks are present in all the Iberian tectonostratigraphic zones into which the Variscan orogen in Spain has been classically divided. In addition to these Variscan igneous rocks, there is also evidence for earlier magmatism, including widespread exposures of Neo-proterozoic–Cambrian (Cadomian) age, and the diatreme-like breccias linked to the origin of the remarkable mercury mineralization at Almaden. In this chapter we deal initially with Palaeozoic volcanic rocks, with special emphasis on the volcanism related to the generation of the Iberian Pyrite Belt. With regard to the Variscan granitoid rocks we have grouped these according to compositional features and relative age, rather than by tectonostratigraphic zones (the latter approach does not contribute to a better understanding of the magmatism because the emphasis is on differences and not on similarities). However, Variscan granites of the Iberian Massif are described separately from other granitic massifs in the Pyrenees and Catalonian Coastal ranges, because of their geographic separation and the lack of obvious direct links between them. Outcrops of distinctive mafic and ultramafic rocks, mostly related to granitoids of the appinite–granodiorite association (cf. Pitcher 1997 ), are treated separately, not least because of their importance in international

Abstract 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 NaAlSi 3 O 8 -KAlSi 3 O 8 -SiO 2 -H 2 O . 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 granite

Abstract Crustal melting occurs at the higher grades of metamorphism. Migmatites reflect crustal anatexis without necessary additional heat supply. Granites usually result from crustal melting resulting from ‘hot’ geotherms, or reflect an external heat supply. The commonest mechanisms of crustal melting are: (i) decompression of crust thickened into mantle in convergent orogens; (ii) asthenosphere upwelled beneth the crust in extensional orogens; (iii) massive invasion or underplating of crust by mantle magma generated by mantle decompression. This paper considers some of the processes governing timescales and lengthscales of these various magmatic events with particular view to determining the intensity and duration of crustal melting, and relationships between the rates of melt extraction, ascent and emplacement.

Abstract The viscosities of granitic melts play a crucial role in controlling the kinetics and dynamics of magma transport including generation, segregation, ascent, differentiation and emplacement. No general theory of liquid viscosity has been successfully applied to date to the description of granitic melt viscosities. The approach in the earth sciences so far has been empirical and experiments designed to measure the viscosity of granitic melts have made considerable progress in improving our picture of the composition-, temperature- and pressure-dependence of granitic melt viscosities. This chapter summarizes the recent progress in parameterizing the viscosity of granitic melts with respect to the most important compositional variables such as alkali/aluminium ratio, silica content, water content, anorthite content. The pressure-dependence of granitic melt viscosities is very slight. The temperature-dependence, in contrast, is a very strong and non-linear function of the reciprocal absolute temperature (i.e. non-Arrhenian). Indirect determinations of the viscosity of granitic melts have also contributed substantial information on the composition-, pressure- and temperature-dependence of melt viscosity. These measurements are particularly useful where direct viscosity determinations are difficult or impossible to perform. The outlook for a more complete picture of granitic melt viscosities in the next years is excellent. The combination of several factors, including conceptual advances in the quantitative understanding of relaxation in silicate melts, experimental advances in the measurement of viscosity of silicate melts, and the identification in the experimental studies to date of the areas of investigation where the effort should be concentrated in the future, give reason to expect a more or less complete picture, fully adequate for the purposes of modelling the dynamics of granitic magmas, in the near future.

Abstract This review deals with direct evidence (field statements and geochemistry) and indirect observations (modelling experiments, analogical models, and geophysics) on granite plutons to model their shape at depth. 3D modelling of granite plutons can be achieved using geophysical tools. Amongst these tools, heat-flow and heat-generation data used earlier to estimate the thickness of granitic plutons appear inadequate. Electrical methods are strongly influenced by near-surface heterogeneities and temperature, which minimize their effectiveness at depth. Magnetic surveys provide information on contacts between pluton and country rocks, since magnetic halos are appropriate to delineate surface contours, but the technique lacks the resolution to reveal deep boundaries. Anisotropy of magnetic susceptibility is particularly well adapted to determine the internal structures of plutons. Seismic profiles at usual frequencies (30–80 Hz) define the layered structure of the floor of several bodies but fail to show the rock-type variations and their internal fabrics, because of their low impedance contrasts. Conversely, high-resolution seismic reflection profiles reveal fault structures in granites but the pluton’s floor remains transparent. Gravity measurements have been widely applied in granites and owing to the 3D inversion of data, the shape of the pluton at depth and depth of its floor may be derived with confidence from density contrasts.

Abstract The origin of different kinds of granitic rocks is examined within the framework of experimental studies of melting of metamorphic rocks, and of reaction between basaltic magmas and metamorphic rocks. Among the types of granitic rocks considered in this chapter, only peraluminous leucogranites represent pure crustal melts. They form by dehydration-melting of muscovite-rich metasediments, most likely during the fast adiabatic decompression that results from tectonic collapse of thickened intracontinental orogenic belts. All other granitic rocks discussed here represent hybrid magmas, formed by reaction of basaltic melts with metamorphic rocks of supracrustal origin. These hybrid rocks include Cordilleran granites, formed at or near convergent continental margins, strongly peraluminous ‘S-type’ granites, alumina-deficient ‘A-type’ granites, and rhyolites associated with continental flood basalts. The differences among these types of granites reflect differences both in their source materials and in the pressures at which mantle-crust interactions take place. In turn, these variables are correlated with the tectonic settings in which the magmas form. Hybrid mafic cumulates are also produced by mantle-crust interactions, simultaneously with the granitic melts. These cumulates range from orthopyroxene + plagioclase-rich assemblages at low pressure to clinopyroxene + garnet-rich assemblages at high pressure, and are known to be important constituents of the lower continental crust. With the exception of peraluminous leucogranites, generation of granitic magmas is almost always associated in space and time with growth, rather than just recycling, of the continental crust.

Abstract Granite emplacement in the brittle crust can be modelled by means of the injection of a Newtonian fluid (low-viscosity silicone putty) into sandpacks. This paper describes dynamically scaled analogue models of granite intrusions in the upper crust under different tectonic regimes. Experiments analyse three boundary conditions: (1) static conditions, with different rheological profiles (single sand-layer system, two-layer silicone-sand system, three-layer sand-silicone-sand system and five-layer sand-silicone-sand-silicone-sand system), (2) extensional regime, including gravitational sliding and divergent basal plate, with both mobile and fixed velocity discontinuities and (3) strike-slip regime, induced by two mobile basal plates. The results obtained indicate that: (1) a soft level between two competent units is necessary for laccolith formation—the critical thickness of the soft layer necessary for laccolith formation decreases with increasing depth, (2) when there are two soft layers in the brittle crust, laccolith emplacement occurs in the deeper soft level, even when this is thin and the overburden does reach its critical thickness, (3) in extensional regimes the geometry of intrusions is mainly controlled by normal faults and at the same time intrusions determine the location of faults within the cover and (4) in strike-slip zones intrusions are elongate and their long axis tends to track the principal stretching direction associated with the strike-slip regime. Some natural examples of granitic bodies were considered to test the applicability of experimental results.

Abstract Granitic pluton emplacement and zonation are controlled, among others, by regional deformation and the rate of magma supply. The latter has consequences for the disposition of successively emplaced, more chemically evolved, batches of magma. Our general interpretation is based on a multidisciplinary approach combining field observations, gravity data, internal structures and geochemical variations. Magma feeders are identified in the plutons as the deepest zones, inferred from gravity measurements, when they also correspond to vertical lineations. Correlation of the root zone location with compositional zoning indicate how the magma evolved during emplacement. Two case studies of Hercynian granite plutons illustrate the interpretations: the normally zoned Cabeza de Araya pluton (Spain), and the multiphase Fichtelgebirge pluton (Germany) which displays both normal and reverse zoning. It is proposed that reverse zoning reflects discontinuous magma injection due to a tectonic rate slower than the rate of magma supply. Conversely, normal zoning can occur when magma injection is continuous in time, with successive magma batches entering within not yet crystallized magma. The two case studies illustrate how the understanding of compositional zoning and emplacement of granitic plutons can be improved by multidisciplinary approaches combining classical and modern techniques.

Abstract The opening of the South Atlantic ocean at 110 Ma triggered the inversion of the Casma basin and the switch from marine volcanicity to plutonism, which evolved through three distinct phases. The first was the intrusion of the Coastal Batholith which forms a well-defined linear structure over the whole coastal region and which, in the Lima segment, endured from 100 to 60 Ma, and was terminated by the formation of the ring complexes. The second was the post Incaic development of the andesitic terrestrial plateau volcanics, the Calipuy group with associated scattered plutons of tonalite and granodiorite, which extended from perhaps 50 Ma to 20 Ma, and the third was the emplacement of the high level stocks and associated ignimbrite sheets from 20 to 6 Ma. Of these the Cordillera Blanca batholith which is of trondhjemitic affinity is the most important, and it is the intrusives of this zone which are the most important economically. Many plutons within the Batholith were emplaced into the brittle crust by processes of magmatic stoping and some of the evidence for this process is presented.

Abstract Integrative gravity and structural modelling of Ordovician-Silurian granitoids in the Eastern Lachlan Fold Belt (southeastern Australia) revealed contrasts in emplacement mode and deformation style between coeval S- and I-type granites. The NNE-SSW directed contraction during the Benambran event of the Lachlan Orogen caused dextral movement along two major strike-slip faults (Carcoar Fault/Copperhannia Thrust) and simultaneous formation of both transtensional pull-apart and transpressional shear zones. The geometry and deformation style of the plutons and country rock, their spatial relationship at depth to adjacent faults and the structural history of both the granites and country rocks suggest a genetic linkage between magma emplacement and synmagmatic deformation. Synchronously, the Carcoar Granodiorite was emplaced into a transtensional pull-apart structure and the Barry Granodiorite and Sunset Hills Granite intruded transpressional shear zones. The I-type Carcoar and Barry granites are square to tabular, wedge-shaped bodies exhibiting a weak deformation; whereas the S-type Sunset Hills Granite is an elongated, tabular to sheet-like pluton showing a moderate deformation degree. The contrasts in 3D shape, emplacement mode and deformation style between the I- and S-type granites are due to differences in nearfield stress regime, geometry of the emplacement sites, intrusion level with respect to thermal and rheological conditions, and in their response to deformation. This response is in part controlled by the proportion of resistant/non-resistant minerals in the granite and host rock. This study demonstrates that distinctive emplacement modes can operate simultaneously in different parts of a fault system under contrasting deformation conditions.

Abstract The emplacement mechanisms of the Palaeoproterozoic Gåsborn granite, a satellite of the Transscandinavian Igneous Belt (TIB), are investigated using an integrated structural and geophysical approach. The pluton is discordant to c . 1.89 Ga folded supracrustal rocks that were deformed and metamorphosed at c . 1.85–1.80 Ga during the Svecokarelian Orogeny. Emplacement occurred at a depth of c . 10 km, within a regime of late Svecokarelian dextral transpression. Deformation of the pluton during cooling resulted in the formation of a variably developed foliation in the granite and deflection of less competent wall-rock units around its western and eastern contacts. Later E-W Sveconorwegian shortening resulted in the formation of shear zones that affect one of the pluton margins and may contribute to a component of the observed wall-rock distortion. The granite is situated above strong NNW-trending linear magnetic and negative gravity anomalies, which are interpreted to correspond to an important early Svecokarelian shear zone. The geophysical data indicate that the pluton is markedly asymmetric and modelling of the residual gravity field suggests that it consists of a deep root zone in the west and a thin sill-like body, which makes up most of the east and south parts of the body. Emplacement of the sill-like part occurred by lateral flow of magma from the root zone accommodated by downwarping of the underlying units. Intrusion of the thicker, discordant west part may have been accommodated by a combination of roof lifting and floor depression, aided by displacement on an active shear zone.

Abstract The Joshua Flat-Beer Creek Pluton (JBP) in the White Inyo Mountains, California, is part of the Inyo batholith, which intruded a Neoproterozoic to Lower Cambrian metasedimentary sequence at about 180–160 Ma ago. Contact metamorphism around the JBP reached hornblende-hornfels- to lower amphibolite-facies conditions. The intrusion consists of three distinct phases. Field relations suggest an intrusion sequence Marble Canyon Diorite-Joshua Flat Monzonite-Beer Creek Granodiorite as nested diapirs. In the Marble Canyon Diorite as well as the Joshua Flat Monzonite the subsequent intrusions led to brecciation and stoping, but also to mingling and mixing between the Joshua Flat Monzonite and the Beer Creek Granodiorite. Fabrics in the contact aureole document several emplacement mechanisms such as stoping and dyking, ductile downward flow, partial melting and magma chamber expansion. The relative importance of the different emplacement mechanisms through time is as follows: the Marble Canyon Diorite probably intruded as dyke/sill, whereas stoping and dyking, ductile downward flow together with assimilation acted during the emplacement of the Joshua Flat Monzonite. Magma chamber expansion represents only the latest stage of intrusion during the emplacement of the Beer Creek Granodiorite into the already existing magma chamber of the JBP. AMS, quartz c -axis and strain measurements support the field observations.

Abstract This paper reports field, petrological and structural data of the peraluminous cordierite-andalusite-bearing Campanario-La Haba granite. Crystallization age is constrained by Rb-Sr whole-rock dating at 309 ± 6 Ma (with ( 87 Sr/ 86 Sr) i = 0.70739 ± 0.00038) and took place during late Hercynian tectonic events. The pluton shows a petrographic zonation, although there are no marked differences in chemical compositions between the margin and the centre of the intrusion. Petrography, mineralogical data and geochemical modelling indicate melt generation by partial melting of a metasedimentary protolith, probably with some mantelic contribution as shown by its low Sr i value, followed by emplacement at P < 3 kbar. The internal structure of the pluton resulted from the lateral spreading in the stretching direction given by an N120–130E dextral strike-slip zone and the external geometry seems to be strongly conditioned by faults (Riedel R type fractures) formed in the host rocks. This emplacement model agrees with that defined for the adjacent Extremadura granitic plutons and for the Los Pedroches batholith suggesting the existence of a dextral regional shear-zone in the South branch of the Central Iberian Zone.

Abstract The magmatic structures appearing in the Puente del Congosto granitic outcrop (central Iberian Massif) are described in this work. They are interpreted as the result of a complex interplay between viscous (Newtonian) and brittle behaviour of granitic magma, which allowed the newer magma pulses to intrude and deform older magma batches. A model of Newtonian magma intruding a linear viscoelastic host rock may be extended with some confidence to the case of magma into magma emplacement. The resulting structures combine the characteristics of dykes and diapirs. The formation of large batholiths might be initiated or entirely accomplished by this process. In order to investigate the influence of the general stress conditions characteristic of a given tectonic regime on the strength of granitic magma, an over-simplified macroscopic model considering mixed Newtonian and brittle behaviour has been developed in this work. The brittle response is simulated by the Modified Griffith criterion, so that only rough estimates of the critical differential stresses for brittle magma behaviour can be gained. The results of this model suggest that the brittle response of viscous granitic magmas is possible for any type of tectonic regime (specially under contractional tectonics). A comprehensive, physically sound model for this viscous-brittle behaviour of granitic magma is not yet available. Integrated theoretical, experimental and field-based studies are the best way to arrive at such a complete model.

Abstract Microcline megacrysts in the Cathedral Peak Granodiorite and other parts of the Tuolumne Intrusive Suite were formed by textural coarsening (Ostwald Ripening) of earlier formed crystals. The early-formed crystals nucleated and grew in an environment of increasing undercooling, probably during the ascent of the magma. Emplacement of the magma into warm host rocks promoted textural coarsening. Crystals smaller than a certain size (the critical size) dissolved in the interstitial melt whilst large crystals grew. Microcline was most sensitive to this effect as the magma temperature was buffered close to its liquidus for a long period by the release of latent heat of crystallization. Positive feedback between textural coarsening and magma permeability channelled the flow of interstitial melt to produce a heterogeneous distribution of megacrysts. Megacryst growth was halted when cooling resumed at the end of the intrusive cycle. K-feldspar nucleation was then renewed and K-feldspar crystals grew to form part of the groundmass. It was the particular thermal history of this pluton that promoted textural coarsening—chemically similar plutons that lack megacrysts probably did not have the pause during cooling that was necessary for the development of this texture.

Abstract Granulite facies anatexis ( T ≈ 900 °C) in the Wuluma Hills region of the Arunta Inlier was synchronous with deformation. During D3 contractional deformation strain was partitioned into S3 shear zones, which alternate with lower strain domains containing F3 fold hinges. Subsequent D4 deformation was minor and in part extensional. Leucosomes in the S3 shear zones are principally veins oriented parallel, or subparallel, to the pervasive S3 foliation. Leucosomes in the F3 hinge domains are more complex, and occur parallel to anisotropy due to lithological layering, the pre-existing S1/2 foliation, S3 and fold axial planes (F3 and F4). Some leucosomes (generally high Na 2 O, low K 2 O and Rb/Sr) record melt migration paths, and other sites of melt accumulation. All the migmatites are residual and lost melt when deformation forced melt from matrix grain boundaries, through a network of small lensoid channelways to accumulation sites in fold hinges, there larger batches of magma developed. Leucosomes in accumulation sites develop a schlieric or diatexitic appearance because inflowing melt eroded the host rocks. Later increments of D3 contractional strain overpressured the accumulated granitic magma and it migrated again to other (more stable) low pressure sites through veins generally oriented parallel to S3. Magma/melt movement stopped when the solidus was reached, or the magma reached a structurally stable site (e.g. pluton).

Abstract The migmatites with Kfs-poor leucosomes from the Svecofennian domain of southern Finland are characterized by a suite of anastomosing leucosome veins and patches characterized by low and varying abundance of K-feldspar with or without cordierite. These leucosomes are generated by in situ melting reactions at different P-T-a H 2 O conditions. Microtextural analysis in conjunction with thermocalc calculations and geothermobarometry show that these rocks were metamorphosed under granulite facies conditions at 700–750°C, 4–5 kbar and a H 2 O = 0.4−0.7. The formation of cordierite coronas around garnet and the late crystallization of andalusite suggest that the final stage of the P-T history was characterized by decompression and cooling to within the andalusite stability field, estimated at 500–650°C and 3–4 kbar. U-Pb and Sm-Nd conventional analyses of monazite and garnet, respectively, from different parts of these migmatites (mesosomes and leucosomes) indicate that, within error limits, all leucosomes were formed at about 1878 Ma during a single tectonometamorphic event. Since there is no evidence of an earlier high-grade metamorphic event in this area, it is assumed that this is the approximate age of peak metamorphism and partial melting.