Granite Fabrics Research Articles

Granites: petrology, structure, geological setting, and metallogeny

1. What is a granite? 2. Origin of granitic magmas 3. Segregation of granitic melt 4. Genesis of hybrid granitoids: mingling and mixing 5. Transport of granitic magma 6. Emplacement and shape of granite plutons 7. Thermomechanical aspects in the country rocks around granite plutons 8. Crystallization of granitics magmas 9. Microstructures and fabrics of granites 10. Magnetic fabrics in granites 11. Zoning in granite plutons 12. Granites and plate tectonics 13. Precambrian granitic rocks 14. Granite metallogeny

Fabric evolution at basement–cover interfaces in a fold-and-thrust belt and implications for décollement tectonics (Autochthon, Lower Allochthon, central Scandinavian Caledonides)

Microstructural and magnetic investigations (anisotropy of magnetic susceptibility, AMS) on sections across basement–cover interfaces (BCI) revealed a complex evolution in the crystalline basement rocks beneath and in the basal units of the Caledonian fold-and-thrust belt: (1) Pre-Caledonian mylonitic fabrics in basement granite relate to steep shear zones. (2) Palaeoweathering formed smectite and illite at the expense of feldspar and mica. Secondary Fe-bearing clay minerals and the intensity of the chemical weathering control the bulk susceptibility. Changing susceptibility and AMS relate to a (time) sequence from primary magnetite to secondary paramagnetic clay to pyrite and ferrimagnetic pyrrhotite. (3) Burial compaction with BCI-parallel fabrics. (4) Caledonian cleavage, overprinted by decollement zones with S–C–C′ fabrics. Decollement cataclasis overprinted pre-existing magnetic fabrics and produced horizontal magnetic lineations and subhorizontal foliations defined by the S–C–C′ fabrics. Clay mineral enrichment, together with subsequent, BCI-parallel compaction fabrics, decreased the shear strength in the basement rocks beneath the BCI. Detachments initiated at such low-strength zones and produced allochthonous units with their footwall within crystalline basement rocks, an observation of general importance for orogenic fold-and-thrust belts.

Noncoaxial K‐feldspar and AMS subfabrics in the Land's End granite, Cornwall: Evidence of magmatic fabric decoupling during late deformation and matrix crystallization

A comparative study of the anisotropy of magnetic susceptibility (AMS) and K‐feldspar phenocryst fabrics in the Land's End granite demonstrates that the AMS fabric predominantly reflects late magmatic deformation. Tensor analysis of the K‐feldspar fabric shows a complex pattern, characterized by meter‐scale variations in orientation, symmetry, and intensity, mainly related to heterogeneous flow of the phenocryst‐rich magma during emplacement. In contrast, the AMS fabric is predominantly homogenous, subhorizontal, and oblate, and is stable at the pluton scale. Quantitative microstructural analysis suggests that the AMS fabric is controlled by deformation of the partially crystallized matrix, resulting from the combined effects of late internal adjustment within the pluton and regional deformation. A general model of fabric development associated with a vertical pure‐shear overprint on a variable vertical fabric is evaluated by numerical modeling. The study demonstrates how the memory of different fabric elements may be dependent upon their grain size, crystallization sequence, and recorded previous strain.

Fabric transpositions in granite plutons — An insight from non-scaled analogue modelling

Investigations on a set of experimental models of highly viscous intrusions were carried out in order to study the internal strain pattern during vertical ascent and emplacement of granite intrusions. The strain pattern was determined by means of anisotropy of magnetic susceptibility (AMS) resulting from the orientation of magnetite particles in a liquid plaster medium. The modelled intrusions show distinct fabrics reflecting the flow of a rheologically complex, non-Newtonian material. During the vertical growth of the intrusion, constrictional vertical fabrics are transposed into flattening fabrics, and along with the development of low-intensity fabric domains are passively transported upwards. Vertical growth takes place along subvertical thrust shear zones that satisfactorily explain the discordant magmatic fabrics in granites along intrusion sides. The resulting complex fabric patterns suggest that the vertical movement of material in ascending intrusions is accommodated by various flow mechanisms operating simultaneously.

Magnetic fabric of granitic composite pluton of the Velká Fatra Mountains (Western Carpathians, Slovakia): A Variscan remnant within the Alpine edifice?

The AMS study has been performed on various types of the basement – Variscan granitic and surrounding – Mesozoic sedimentary rocks in the Velká Fatra Mountains, Tatric Superunit of the Central Western Carpathians. The Velká Fatra Mts. provides good opportunity for AMS study because of composite S-type and I-type granite character of pluton and clear relations to Mesozoic sedimentary rocks in the cover and nappe positions. The granitic massif consists of the three types of weakly magnetic peraluminous granites (350 – 340 Ma in age), ranging from two-mica granites to biotite granodiorites in composition and carrying accessory monazite and ilmenite; whereby they resemble common S-type and/or Ilmenite Series granite. This pre-existing granitic body was intruded by relatively young (304 Ma old) metaluminous to subaluminous, strongly magnetic (due to magnetite) tonalitic intrusion of the I-type and/or Magnetite Series granite. In all S-types investigated as well as in the I-type tonalite body, the magnetic fabrics are not uniform, but slightly variable within a body and differing from body to body. The magnetic fabrics in all granitic rocks can be classified as mostly magmatic in origin, only subordinately affected by ductile deformation. The Alpine overprint of the magnetic fabric of the Variscan granite frequent in the central areas of the Central Western Carpathians was only weak in the Velká Fatra Mts. and the magnetic fabrics of these granites thus mostly comprise the original Variscan magmatic fabrics. On the other hand, in the marginal parts of the Velká Fatra Mts. the magnetic fabrics in granites are locally conformable to the deformational magnetic fabrics in surrounding sedimentary rocks (Mesozoic in age) thus indicating at least local effects of the Alpine deformation. The magnetic fabrics in Mesozoic sedimentary rocks covering the crystalline basement are partially (Cover Formation) to entirely (Nappe Units) deformational in origin.

Emplacement setting of the granite sheeted pluton of Esperança (Brasiliano orogen, Northeastern Brazil)

Anisotropy of magnetic susceptibility (AMS) and isotopic (U–Pb, Sm–Nd) data were combined to study the emplacement setting of the granite sheets that constitute the Esperança pluton in the Borborema Province (Northeastern Brazil). The sheets dip moderately to the SE along the contact zone between the Paleoproterozoic basement rocks and Early Neoproterozoic orthogneisses and metasediments. Granite fabrics were determined mainly using AMS in 136 sites distributed within the central and western part of the pluton. The sheets normally have susceptibility lower than 0.35 mSI but, locally, where a Ti-poor magnetite appears with titanite, the susceptibility increases up to 5 mSI. Comparison between the silicate fabric and AMS showed inconsistencies between the shape of mineral and magnetic ellipsoids despite of their orientations that fit fairly well to each other. AMS indicated the deformation was partitioned between the lower (tonalite, syenogranite) and upper (leucogranite and coarse porphyritic granite) sheets. In the lower sheets the curvilinear lineation trajectory is attributed to a dominant heterogeneous pure shear event that flattened laterally the still molten tonalite and syenogranite into the regional foliation. The associated microstructures are typically magmatic. Zircon U/Pb data of the syenogranite yielded a crystallization age of 592 ± 5 Ma. In the upper sheets the fabric recorded a component of simple shear deformation that displaced the coarse porphyritic granite and the top gneissic host rocks to the southwest. Microstructures are mostly of post-full crystallization type. T DM model ages and ɛ Nd ( t = 0) values indicate that the magma contaminated by partial melting of the regional host rocks. Sheet propagation at the emplacement level would have exploited the contact zone between crustal blocks of different rheologies when the melt pressures would be able to tensionally fail the anisotropy of the host rocks.

Timing and kinematics of the Colenso Fault: The Early Paleozoic shift from collisional to extensional tectonics in the Pan-African Saldania Belt, South Africa

The Colenso fault is a major northwest to southeast trending fault zone in the Pan-African Saldania Belt of the Western Cape Province in South Africa that is spatially closely associated with granitoids of the ~550 to 510 Ma Cape Granite Suite. Most of these granites were previously considered to be largely post-tectonic intrusions, but structural data presented in this study demonstrate the synkinematic emplacement of granitoids into, and along, the Colenso Fault. The kinematic analyses of shear zones and granite fabrics together with previously published and new geochronological data are combined to provide constraints on the complex kinematic history of the fault and the tectonic evolution of the hitherto poorly understood Saldania Belt. Early, strongly gneissose granitoids of the composite Darling batholith (547 ± 6 Ma) were emplaced during sinistral strike-slip movement along the Colenso fault. Both the timing of emplacement and penetrative deformation of the Darling batholith suggest an intrusion of the pluton during the main Pan-African collisional event in the Saldania Belt. The younger Trekoskraal granite intrudes synkinematically into dextral strike-slip faults related to deformation along the Colenso fault. Single-zircon ages from synkinematic aplites constrain the timing of dextral strike-slip shearing to 539 ± 4 Ma. The emplacement of the late-kinematic Cape Columbine granite during dextral strike-slip faulting indicates that dextral strike-slip kinematics along the Colenso fault continued at least until ~520 Ma. These results point to a reversal of strike-slip motion along the Colenso fault at ~540 Ma that coincides with the onset of uplift of rocks of the Saldania Belt. The final exhumation of the belt at ~515 to 520 Ma is marked by the near-surface emplacement of the last phases of the Cape Granite Suite, related subaerial volcanism, sedimentation of the coarse-clastic, fault-bounded Klipheuwel Group, and the overlying fluvial to shallow-marine sequence of the Mid-Cambrian Cape Supergroup. The temporal and spatial overlap between igneous activity and rift-type sedimentation indicates that a substantial part of the Cape Granite Suite was emplaced in an overall transtensional and/or extensional setting. During this time, the voluminous plutonism of the Cape Granite Suite most likely represented a significant heat input that also contributed to a thermal weakening of the crust. In view of the Early Paleozoic extensional setting suggested here, we interpret Ar-Ar mineral ages of ~500 Ma and post-orogenic plutonism that are widely documented from Pan-African belts throughout southwestern Africa to reflect a thermal event related to crustal thinning and associated mantle upwelling that follows the main phase of Pan-African collisional tectonics.

Granite fabrics and regional‐scale strain partitioning in the Seridó belt (Borborema Province, NE Brazil)

Fabrics of the Brasiliano‐age granitoid plutons of the Seridó belt (northeastern Brazil) were organized following the partitioned regional strain field which shaped larger areas of the basement and its metapelitic cover. Nine plutons located at the central and western portions of the belt have been investigated by means of anisotropy of magnetic susceptibility. These plutons, emplaced between 580 and 575 Ma, are mainly potassic calk‐alcalic magnetite‐bearing monzogranites. The presence of primary coarse‐grained magnetite as the main magnetic mineral is indicated by optical and electronic microscopy, magnetic susceptibilities, hysteresis ratios, and thermomagnetic curves. The plutons show a strong internal coherence of magnetic fabric, with foliation poles distributed around a zone axis parallel to the gently plunging mean lineation. The magnetic fabric was used to trace the strain partitioning along the belt. In the central transpressive shear belt the magnetic lineations are parallel to north trending regional stretching. On the other hand, in the western basement block a voluminous granitic magmatism was emplaced as a consequence of a transtensional/extensional deformation. Lineations in these granites trend in a NE direction, which is parallel to the transport, inferred from kinematic indicators along the shear zones. A crustal‐scale east trending dextral shear zone connects both the transtensional and transpressional domains.

Anisotropie de susceptibilité magnétique et fabrique des granites

The principle of anisotropy of magnetic susceptibility measurement, or AMS, and elements of magnetic mineralogy applied to granitic rocks, are presented. AMS measurement allows the study of the magnetic fabric of magnetite-free, paramagnetic granites in which the iron-bearing silicates carry the signal, and ferromagnetic granites dominated by magnetite. The case of mixed magnetic mineralogies is also evoked. Several examples demonstrate that crystal fabrics in granites, as deduced from AMS studies, are remarkably homogeneous from the sample scale to the scale of a whole pluton. This was totally unforeseen up to now. Origin of crystal fabrics in granites is briefly discussed in terms of preferred orientation of crystals acquired in the deforming magma before its full crystallisation. Kinematic and rheologic consequences of these fabrics are evoked and their perspective leading to a renewal of geological studies in basement rocks dominated by granitic rocks is stressed.

Relationships between syn-tectonic granite fabrics and regional PTtd paths: an example from the Gander-Avalon boundary of NE Newfoundland

Syn-tectonic granitic plutons and their host rocks not only provide a potential wealth of information about the timing and nature of regional kinematics, but also record information about thermal conditions. Successive syn-tectonic plutons emplaced into the Gander Zone of NE Newfoundland preserve fabric overprinting formed during cooling to regional ambient thermal conditions. Ambient conditions have been approximated for each pluton by careful analysis of microstructures and consideration of potential cooling histories, and have illustrated regional exhumation during a tectonic reversal from Silurian-Devonian sinistral transpression to Devonian dextral transpression. Because the time period of emplacement is relatively short by comparison to orogenic events (typically by more than an order of magnitude), individual plutons may record a ‘snap-shot’ of the regional history. By combining information from a number of plutons emplaced sequentially during a period of regional orogenesis, a picture may be built up defining time-temperature-deformation (Ttd) paths. Such information may compliment regional metamorphic PT studies in helping to establish orogenic PTtd paths.

Granite emplacement during contemporary shortening and normal faulting: structural and magnetic study of the Veiga Massif (NW Spain)

The Veiga Massif belongs to the calc-alkaline series of Hercynian granitic rocks of the Ibero-Armorican arc The Veiga granodiorite intruded during the Upper Carboniferous into the core of the WNW-ESE N-verging ‘Ollo de Sapo’ antiform, formed by Precambrian and Palaeozoic metasediments. Internal fabrics show that magma intrusion was contemporary with shortening. Measurements of feldspars orientations and anisotropy of magnetic susceptibility (AMS) throughout the granite are consistent and indicate a foliation striking WNW-ESE (parallel-to-folding), with a constant dip of 75–85 °N. The zonation of bulk low-field susceptibility is related to mineral content and indicates a more basic composition at the southern and western borders. The difference in elevation between outcrops (more than 600 m) allows us to infer the three-dimensional attitude of granite fabrics throughout the Massif. Syn-magmatic fabric folds are preserved in the inner part of the igneous body. The highest degree of magnetic anisotropy is observed in areas located near the bottom and top of the intrusion. At the scale of the Massif, foliation is convergent toward the bottom of the intrusion, along a line located at its northern border, where the magma source is interpreted to be located. In the western border of the Massif, the presence of C and S structures indicates that magma cooling was coeval with movement of the Chandoiro fault, a N-S striking normal fault with a N290E hanging wall displacement direction. These results indicate that emplacement of the Veiga granite is coeval with NNE-SSW shortening and with an WNW-ESE extension direction, parallel to the trend of the late folds.

Overprinting of magnetic fabrics in granites by small strains: numerical modelling : K. Benn, Tectonophysics, 233(3–4), 1994, pp 153–162

Overprinting of magnetic fabrics in granites by small strains: numerical modelling : K. Benn, Tectonophysics, 233(3–4), 1994, pp 153–162

Overprinting of magnetic fabrics in granites by small strains: numerical modelling

Abstract The anisotropy of magnetic susceptibility (AMS) is used to determine weakly defined mineral preferred orientation fabrics in granites. However, the effects of small strains superimposed on the low-intensity AMS fabrics typical of these rocks is not well understood. Numerical simulations are presented which investigate the effects of small superimposed strains on the shapes, magnitudes and orientations of pre-existing AMS fabrics, which are similar to those reported in the literature for undeformed granites. A computer-generated biotite petrofabric is subjected to progressive strain, using March's strain response model, for pure shear (plane strain), axially symmetric shortening and simple shear regimes. Results indicate that a pre-existing AMS fabric might be significantly modified or overprinted by small strains (≈10–15% strain in the pure shear and axially symmetric shortening regimes, 0.5 ⩽ γ ⩽ 1.0 for simple shear). A new AMS fabric may develop with an orientation similar to that expected for shearing of an initially isotropic petrofabric. At low strains, the evolution of the shape of the AMS ellipsoid depends strongly on the orientation of the initial petrofabric with respect to the strain framework; the AMS ellipsoid may evolve towards either oblate or prolate shapes and the evolution in the ellipsoid shape may be reversed during progressive strain. This modelling suggests that small strains affecting a pluton following its emplacement could mask AMS fabrics developed earlier. The results also suggest that, in simple shear, the orientation of the AMS ellipsoid may serve as a shear-sense indicator for small strains even if the earlier fabric is not isotropic.

Sinistral transpression and hydrothermal activity during emplacement of the Early Proterozoic Julianehåb batholith, Ketilidian orogenic belt, South Greenland

The first systematic investigations of the central part of the Early Proterozoic Ketilidian orogen in the vicinity of Søndre Sermilik in the early 1960s suggested that this part of the orogen comprised a mixture of the Julianehåb granite, altered supracrustal rocks and older orthogneisses. Recent field work has shown that the area consists only of a variably deformed suite of granitic to dioritic plutonic rocks and a range of hornblende-bearing dykes of the appinite suite which all belong to the Julianehåb batholith. Steep to vertical shear zones with widths from a few centimetres to more than one kilometre are a significant element of the structure. The principal shear zones trend north-east and they are parallel to the schistosity and subhorizontal linear structures in the granitoid rocks. Kinematic indicators in many of the shear zones indicate sinistral transcurrent displacements. The relationships between granite fabrics, shear zones and mafic dykes suggest that the Julianehåb batholith was emplaced during subduction from the south towards the Archaean craton in the north-west in a sinistral transpressional system. Effects of hydrothermal alteration, mainly in the form of quartz veining, silicification, chloritisation, epidotisation and pyritisation, are common within and adjacent to the largest shear zones. These effects are believed to be related to late stages of the evolution of the batholith. Gold anomalies appear to be closely tied to the hydrothermal phenomena.

The Flamanville Granite (Northwest France): An unequivocal example of a syntectonically expanding pluton

AbstractThe Flamanville pluton was emplaced during the upper Carboniferous in Palaeozoic sedimentary cover forming one limb of the Siouville syncline, a Hercynian kilometre‐scale fold in Normandy, Northwest France. We review the geological environment of the pluton with special emphasis on the Siouville syncline. The features described include gravity data, metamorphic environment, fold patterns in the contact metamorphic aureole, trajectories of principal strains in and around the pluton, finite strain and anisotropy of magnetic susceptibility within the pluton, and physical and kinematic aspects of pluton fabrics. The results emphasize that (1) the Flamanville pluton is syntectonic, (2) its emplacement involves lateral expansion of magma rather than spherical ballooning, and (3) granite fabrics do not reflect magmatic flow but are strain‐controlled irrespective of grain‐scale deformation processes and rheological state.The example of the Flamanville granite further suggests (1) that grain‐scale deformation features are not critical in distinguishing tectonic from magmatic origin of granite fabrics and (2) that pluton formation within the soft sediments of the shallow upper crust most likely results from lateral expansion of magma injected through the brittle crust rather than from ballooning of a diapiric body.

Granite emplacement mechanisms and tectonic controls: inferences from deformation studies

ABSTRACTThis paper is a structural and tectonic approach to the emplacement and deformation of granitoids. The main methods available in structural geology are briefly reviewed and this emphasises that (a) a wealth of data, particularly strain and shear sense, which pertain to these problems, can be determined in and around plutons; (b) given the nature, unlike many other crustal rock types, of granites to crystallise from isotropic through weakly anisotropic crystal suspension fluids, that deformation which has occurred in these states may not be well preserved; and (c) it is entirely possible, using this methodology, to separate deformation resulting from externally originating tectonic stresses from that which is associated with internal magma-related stresses. It is also recommended that the genetically-based Cloosian classification of granite fabrics and structures into “primary” (magmatic flow/magmatic flow current) and “secondary”, be abandoned and that a more observationally-based approach which classifies granite deformation fabrics and structures according to their time of occurrence relative to the crystallisation state of the congealing magma, be adopted (i.e. pre-full crystallisation deformation and crystal plastic strain deformation).Examples of recent, structurally based, studies of emplacement mechanisms of plutons within tectonic settings are described and these show that, in general, space for magma can be created by the combination of tectonically-created cavities and internal magma-related buoyancy. This occurs in both transcurrent and extensional tectonic settings and there is no reason to doubt that it can happen in compressive-contractional regimes. It is concluded that transient and permanent space creation, such as may be exploited by available magmas, is a typical feature of the tectonically stressed and deforming lithosphere and this, in combination with the natural buoyancy and ascending tendency of magmas, can generate the varied emplacement mechanisms of granites.