Mylonitic Granitoid Research Articles

Geochemistry and geochronology of Permian plutonic rocks at the north‐western margin of Gondwana

The occurrence of Permian magmatic rocks in the Colombian Andes is restricted to a few localities. Previous works have focused mainly on explaining its tectonic setting, whereas petrogenesis has received less attention. This study closes this gap by reporting whole‐rock geochemistry, zircon U–Pb geochronology, trace elements and Hf isotopes from massive and mylonitic granitoids along central Colombia to constrain their age, source and petrogenesis. Our results show that the granitoids have a calc‐alkaline character, are rich in light‐rare elements (LREEs) and large‐ion lithophile elements (LILEs), present negative Nb and Ti anomalies, crystallized between ca. 276–265 Ma and show variable εHf(i) values ranging from −1.5 to +1.7. Combining our results with published geochemical, geochronological and isotopic data from this region, we suggest that the Middle Permian granitoids from the Tolima region were formed in a continental magmatic arc installed at the western margin of Gondwana, where partial melting of the mantle wedge and further crustal assimilation of the magmas with an old radiogenic and heterogeneous crust were essential processes for the modification of the source of the magmas that generated the granitoids. The Middle Permian may have reached the peak magmatic conditions (ca. 280–270 Ma), according to the recurrence of ages in this time interval. This period of magmatic activity may have been interrupted by the Triassic extensional period related to the beginning of the Pangea breakup.

Kinematic vorticity, finite strain, and deformation thermometry analyses of the exhumed mylonites in the Samen ductile shear zone (Sanandaj-Sirjan Metamorphic Belt, Iran)

Quantitative microstructural investigations involving finite strain and vorticity analyses of quartz-rich mylonites provide significant information regarding the kinematic characteristics of the Samen shear zone (SSZ) within the Sanandaj–Sirjan Metamorphic Belt, Zagros Mountain of Iran. A dominant top-to-the-SE sense shear in the study area is recognized by several kinematic indicators. The results of the finite strain analyses indicate approximately plane strain deformation conditions (Rxz = 2.46–3.53). The mylonitic granitoid rocks record general-shear deformation (Wm = 0.37–0.94) associated with a spatial increase in simple-shear component (23%–78%) from W to E. In the study area, kinematic analyses indicate spatial variations of finite strain, deformation temperature, and the kinematic vorticity numbers resulted in partitioned progressive deformation. These spatial variations reveal an increase of all values towards the Samen thrust sheet. Ductile SSZ developed under greenschist-amphibolite facies metamorphic conditions (464–619 °C), as indicated by the presence of quartz, feldspar, biotite, muscovite, hornblende mineral assemblages, opening angles (OAs) of quartz c-axis fabrics and recrystallization regimes. The dynamic and kinematic characteristics of the study area were generally controlled by ongoing oblique convergence of the Afro-Arabian plate and the Iranian plateau with estimation of a horizontal shortening of ∼20–38% measured perpendicular to the boundaries of the SSZ.

Zircon U--Pb ages of Au-bearing host rocks in Northwest Iran as evidence for gold mineralization in the Carboniferous, not Cenozoic time!

A few gold deposits occur in northwest Iran which are hosted by mylonitic granitoids and phyllonite. New U--Pb ages yield ages of 350–311 Ma for the mylonitic granitoid and 354–348 Ma for the phyllonite, which indicate Carboniferous magmatic activity in northwest Iran. The host granitoid and phyllonite have high contents of Zr (212, 246 ppm) and Y (26.8, 38.7 ppm). High 10,000 × Ga/Al ratios (2.1–5.1) and fractionation of the rare earth elements (REEs) pattern with depletion of Eu, Sr, and P may infer an A-type (anorogenic) granite affinity for the host rocks. Correspondingly, these rocks are characterized by high contents of Nb (33.1 to 39.2 ppm), Ta (1.68 to 2.40 ppm), Tb (0.84 to 1.17 ppm), and Th/Yb ratios with some similarity to OIB-like rocks. Whole rock compositions of these host rocks are consistent with both A1 and A2 type granites, which infer magmatic activity in an extensional tectonic regime. Perhaps the extension may have occurred during rifting of Neo-Tethys in northern Gondwana. Upwelling of the hot OIB-like melts during the opening of Neo-Tethys ocean was the main mechanism of this extension. The OIB signatures of the A-type granitoids controlled the gold mineralization during this process. Accordingly, we classify the gold mineralization in NW Iran as intrusion-related gold deposits. These mantle-derived magmas from the asthenosphere may support a source of both sulfur and gold. These elements were concentrated in hydrothermal fluids in the late stages of magma crystallization. Also, hot magma injection increased the geothermal gradient and caused extensive dehydration of the sedimentary rocks inin the roof of the magma chamber. Circulation of these hot fluids with injection into the hot crystallizing magmatic rocks and mixing with late-stage magmatic-hydrothermal fluids and meteoric water (?) seem to all have essential roles for transporting the gold and precipitating it within fractures in the semi-brittle to brittle zone. The gold precipitated as native or nanoparticles in pyrite in the inner part of the shear zone. This research reveals that mineralization occurred in the Late Paleozoic in an extensional tectonic regime and not above the subduction zone or collision zone which had been suggested before.

Quartz preferred orientation in naturally deformed mylonitic rocks (Montalto shear zone–Italy): a comparison of results by different techniques, their advantages and limitations

In the geologic record, the quartz c-axis patterns are widely adopted in the investigation of crystallographic preferred orientations (CPO) of naturally deformed rocks. To this aim, in the present work, four different methods for measuring quartz c-axis orientations in naturally sheared rocks were applied and compared: the classical universal stage technique, the computer-integrated polarization microscopy method (CIP), the time-of-flight (TOF) neutron diffraction analysis , and the electron backscatter diffraction (EBSD). Microstructural analysis and CPO patterns of quartz, together with the ones obtained for feldspars and micas in mylonitic granitoid rocks, have been then considered to solve structural and geological questions related to the Montalto crustal scale shear zone (Calabria, southern Italy). Results obtained by applying the different techniques are discussed, and the advantages as well as limitations of each method are highlighted. Importantly, our findings suggest that patterns obtained by means of different techniques are quite similar. In particular, for such mylonites, a subsimple shear (40% simple shear vs 60% pure shear) by shape analysis of porphyroclasts was inferred. A general tendency of an asymmetric c-maximum near to the Z direction (normal to foliation) suggesting dominant basal slip, consistent with fabric patterns related to dynamically recrystallization under greenschist facies, is recognized. Rhombohedral slip was likely active as documented by pole figures of positive and negative rhombs (TOF), which reveal also potential mechanical Dauphine twinning. Results showed that the most complete CPO characterization on deformed rocks is given by the TOF (from which also other quartz crystallographic axes can be obtained as well as various mineral phases may be investigated). However, this use is restricted by the fact that (a) there are very few TOF facilities around the world and (b) there is loss of any domainal reference, since TOF is a bulk type analysis. EBSD is a widely used technique, which allows an excellent microstructural control of the user covering a good amount of investigated grains. CIP and US are not expensive techniques with respect the other kind of investigations and even if they might be considered obsolete and/or time-consuming, they have the advantage to provide a fine and grain by grain “first round” inspection on the investigated rock fabric.

Geochemistry and thermodynamics of an earthquake: A case study of pseudotachylites within mylonitic granitoid

Pseudotachylites are melts produced by frictional heating during seismic slip. Understanding their origin and their influence on slip behavior is critical to understanding the physics of earthquakes. To provide insight into this topic, we conducted a case study in the proto-mylonitic to mylonitic Asbestos Mountain granitoid in the eastern Peninsular Ranges batholith (California), which records both ductile (mylonites) and brittle deformation features (pseudotachylites and ultracataclasites). U–Pb chronology and Zr thermometry of titanite porphyroblasts in the mylonites indicate that mylonitization of the plutons occurred at near solidus conditions (∼750°C) over a 10 Ma interval from 89 to 78 Ma. Mylonitization resulted in recrystallization of quartz, plagioclase and biotite, with the biotite concentrated into biotite-rich foliation planes. Subsequent brittle deformation is superimposed on the ductile fabrics. Micro-XRF elemental mapping and in situ LA-ICP-MS analyses on these brittle deformation products show that the pseudotachylites are more mafic (lower Si, but higher Fe) and K-rich than the host mylonite, while the ultracataclasites are intermediate between the host and the pseudotachylites. Inverse mass balance calculations show that both brittle deformation products are depleted in quartz but enriched in biotite, with the pseudotachylites showing the most significant enrichment in biotite, indicating preferential involvement of biotite during brittle deformation. We suggest that biotite-rich layers generated during ductile deformation may have been the preferred locus of subsequent brittle deformation, presumably because such layers represent zones of weakness. Frictional heating associated with slip along such planes results in melting, which causes a decrease in viscosity, in turn leading to further strain localization. During the short time span of an earthquake, frictional melting appears to be a disequilibrium process, in which the minerals are melted in order of their melting points, from biotite (∼800°C) to plagioclase (∼1400°C) and finally to quartz (∼1700°C), rather than by equilibrium melting, which results in silicic eutectoid melts at lower temperatures (∼650°C). Thus, with progressive slip, melt composition should evolve from mafic to felsic, eventually approaching the bulk composition of the host rock. The mafic composition of the pseudotachylites thus indicates that they formed between the melting point of biotite and plagioclase (800–1400°C). Our chemical and modeling analyses on the pseudotachylites suggest that the chemical composition of pseudotachylites can potentially be used to constrain the thermodynamic conditions in the shear zone as well as earthquake source mechanics.

Vorticity analysis of the Palmi shear zone mylonites: new insights for the Alpine tectonic evolution of the Calabria–Peloritani terrane (southern Italy)

New microstructural data on the mylonites from the well‐exposed Palmi shear zone (southern Calabria) are presented with the aim to shed light on both the kinematics and the geometry of the southwestern branch of the Alpine belt during Eocene. In the study area, located between the Sardinia–Corsica block and the Calabria–Peloritani terrane, previous large‐scale geodynamic reconstructions suggest the presence of strike–slip or transform fault zones dissecting the southwestern branch of the Alpine belt. However, there are no field data supporting the occurrence of these structures. This paper uses vorticity analysis technique based on the aspect ratio and the long axis orientation of rigid porphyroclasts in mylonitic marbles and mylonitic granitoids, to estimate the contribution of pure and simple shear of deformation during the movement of the Palmi shear zone. Porphyroclasts aspect ratio and orientation were measured on thin sections using image analysis. Estimates of the vorticity number, Wm, indicate that the Palmi shear zone recorded general shear with a contribution of pure shear of c. 65%. Then, the Palmi shear zone can be interpreted as a segment of a left‐lateral transpressive bend along the southern termination of the Eocene Alpine front. Copyright © 2015 John Wiley & Sons, Ltd.

Structural setting and age of the Partridge Island block, southern New Brunswick, Canada: a link to the Cobequid Highlands of northern mainland Nova Scotia

The Partridge Island block is a newly identified tectonic element in the Saint John area of southern New Brunswick, located south of and in faulted contact with Proterozoic and Cambrian rocks of the Ganderian Brookville and Avalonian Caledonia terranes. It includes the Lorneville Group and Tiner Point complex. The Lorneville Group consists of interbedded volcanic and sedimentary rocks, subdivided into the Taylors Island Formation west of Saint John Harbour and West Beach Formation east of Saint John Harbour. A sample from thin rhyolite layers interbedded with basaltic flows of the Taylors Island Formation at Sheldon Point yielded a Late Devonian – Early Carboniferous U–Pb (zircon) age of 358.9 +6/–5 Ma. Petrological similarities indicate that all of the basaltic rocks of the Taylors Island and West Beach formations are of similar age and formed in a continental within-plate tectonic setting. West of Saint John Harbour, basaltic and sedimentary rocks of the Taylors Island Formation are increasingly deformed and mylonitic to the south, and in part tectonically interlayered with mylonitic granitoid rocks and minor metasedimentary rocks of the Tiner Point complex. Based on magnetic signatures, the deformed rocks of the Tiner Point complex can be traced through Partridge Island to the eastern side of Saint John Harbour, where together with the West Beach Formation, they occupy a thrust sheet above a redbed sequence of the mid-Carboniferous Balls Lake Formation. The Tiner Point complex includes leucotonalite and aegirine-bearing alkali-feldspar granite with A-type chemical affinity and Early Carboniferous U–Pb (zircon) ages of 353.6 ± 5.7 and 346.4 ± 0.7 Ma, respectively. Based on similarities in age, petrological characteristics, alteration, iron oxide – copper – gold (IOCG)-type mineralization, and deformation style, the Partridge Island block is correlated with Late Devonian – Early Carboniferous volcanic–sedimentary–plutonic rocks of the Cobequid Highlands in northern mainland Nova Scotia. Deformation was likely a result of dextral transpression along the Cobequid–Chedabucto fault zone during juxtaposition of the Meguma terrane.

Geodynamic setting and emplacement of mylonitic granitoids within the North Golpayegan shear zone, Iran

The metamorphic complex of the North Golpayegan is part of the Sanandaj-Sirjan Zone. There are at least three distinct stages of deformation in this complex. Throughout the first stage, Paleozoic and Mesozoic sedimentary rocks have experienced regional metamorphism during Late Jurassic tectonic events related to the subduction of the Neo-Tethys oceanic lithosphere under the Iranian microcontinent. During the second deformation stage in the Late Cretaceous-Paleocene, the rocks have been mylonitized. The third stage of deformation in the region has led to folding and faulting superimposed on previous structures, and to exhumation of the metamorphic complex. This stage has determined the current morphology and N70E strike of the complex.

Geology and geochemistry of the synextensional Salihli granitoid in the Menderes core complex, western Anatolia, Turkey

The Miocene Salihli granitoid (SG) in the footwall of the Alasehir detachment in the Menderes core complex is a synextensional intrusion, providing important structural, geochronological, and geochemical constraints on the nature of late Cenozoic magmatism associated with crustal extension in the Aegean province. The NW–SE elongated pluton crosscuts the extensional mylonitic fabric in the metamorphic host rocks but is foliated, mylonitized, and cataclastically deformed in a ∼100‐m‐thick shear zone along the detachment. Mylonitic granitoids range from ultramylonites to protomylonites, and exhibit ductile deformation structures showing consistently top‐to‐the north shearing. Grain‐size reduction and dynamic recrystallization were most common deformational mechanisms producing the structural fabric of the mylonitic granitoid rocks. This earlier ductile fabric is overprinted by north‐dipping cataclastic foliation that developed sub‐parallel to the Alasehir detachment and its shear zone. Crystallization and cooling ages of the SG are nearly coeval with the documented early Miocene ages of metamorphism and deformation in its host rocks. The SG pluton ranges in composition from granite, granodiorite, and alkali‐feldspar granite, to monzogranite, and syenogranite; it consists of metaluminous to slightly peraluminous, high‐K calc‐alkaline rocks with silica contents between 62.36 and 73.95 wt‐%. The rocks are enriched in LILE and depleted in HFSE, and show strong negative anomalies in Ba, Nb, Sr, P, and Ti. These geochemical features suggest derivation of the SG melts from a subduction‐metasomatized, subcontinental lithospheric mantle. Invasion of the lower and middle crust by mantle‐derived melts triggered MASH‐type processes (melting, assimilation, storage, homogenization), resulting in the production of hybrid SG magmas. Asthenospheric upwelling caused by lithospheric delamination was a likely heat source that triggered the inferred partial melting of the mantle lithosphere. Continued extensional deformation and unroofing of the Menderes core complex and the synextensional SG in the mid to late Miocene was accompanied by the formation of an asymmetric supradetachment basin in the hanging wall of the Alasehir detachment.

Evolution of quartz microstructure and c-axis crystallographic preferred orientation within ductilely deformed granitoids (Arolla unit, Western Alps)

We have studied quartz microstructures and the c-axis crystallographic preferred orientations (CPOs) in four granitoid samples representative of increasing ductile shear deformation, from a weakly deformed granitoid (stage 1) to a mylonitic granitoid (stage 4). The quartz c-axis CPO measured in the mylonitic granitoid has been compared with the one observed in a fully recrystallized quartz mylonite from the same area. All the samples belong to the Austroalpine Arolla unit (Western Alps) and were deformed at greenschist facies conditions. The quartz c-axis CPO was analyzed using a U-stage and the optical orientation imaging technique. The magmatic plagioclase, forming more than 50% of the volume of the granitoid, is extensively replaced by a mica-rich aggregate even in weakly deformed samples of stage 1. These aggregates flow to form an interconnected weak matrix with increasing deformation, wrapping relatively less strained quartz grains that undergo dominantly coaxial strain. Recrystallization of quartz ranges from less than 1% in the weakly deformed granitoid to up to 85% in the mylonitic granitoid, with average grain strain of 41% and 64%, respectively. With increasing strain and recrystallization, quartz grains in the granitoids show a sequence of transient microstructures and CPOs. Crystal plastic deformation is initially accomplished by dislocation glide with limited recovery, and at 50% grain strain it results in a CPO consistent with dominantly basal 〈 a〉 slip. At 60% grain strain, recrystallization is preferentially localized along shear bands, which appear to develop along former intragranular cracks, and the recrystallized grains develop a strong c-axis CPO with maxima orthogonal to the shear band boundaries and independent of the host grain orientation. Within the granitoid mylonite, at an average quartz grain strain of 64%, recrystallization is extensive and the c-axis CPO of new grains displays maxima overlapping the host c-axis orientation and, therefore, unrelated to the bulk sense of shear. The host-controlled CPO is inferred to reflect pervasive recrystallization by progressive subgrain rotation. The switch from ‘shear band-control’ to ‘host-control’ on c-axis CPO occurred between 40% and 70% of recrystallization. In the quartz mylonite, the quartz c-axis CPO develops an asymmetric single girdle consistent with the bulk sense of shear and the synkinematic greenschist facies conditions. This study indicates that the CPO evolution of quartz may significantly differ in cases of polymineralic vs. monomineralic rocks under the same deformation conditions, if quartz in the polymineralic rock behaves as a ‘strong’ phase.

Deformational history of part of the Acatlán Complex: Late Ordovician–Early Silurian and Early Permian orogenesis in southern Mexico

The Paleozoic Acatlán Complex of southern Mexico comprises polydeformed metasedimentary, granitoid, and mafic–ultramafic rocks variously interpreted as recording the closure of the Iapetus, Rheic, and Ouachitan Oceans. The complex is tectonically juxtaposed on its eastern margin against Grenville-age gneisses (Oaxacan Complex) that are unconformably overlain by Lower Paleozoic strata containing fossils of Gondwanan affinity. A thick siliciclastic unit (Chazumba and Cosoltepec Formations) at the base of the complex is considered part of a Lower Paleozoic accretionary prism with a provenance that isotopically resembles the Oaxacan Complex. This unit is tectonically overridden by a locally eclogitic mafic–ultramafic unit interpreted as a westward-obducted ophiolite, the emplacement of which was synchronous with mylonitic granitoid intrusion at ca. 440 Ma. Both units are unconformably overlain by a deformed volcano-sedimentary sequence (Tecomate Formation) attributed to a volcanic arc of presumed Devonian age. Deformed granitoids in contact with this sequence have been dated at ca. 371 (La Noria granite) and 287 Ma (Totoltepec pluton). Three phases of penetrative deformation (D 1–3) affect the Cosoltepec Formation; the last two correlate with two penetrative deformational phases that affect the Tecomate Formation. D 1 is of unknown kinematics but predates deposition of the Tecomate Formation and likely records obduction at ca. 440 Ma (Acatecan orogeny). A folded foliation in the Totoltepec pluton appears to record both deformational phases in the Tecomate Formation, bracketing D 2 and D 3 between 287 Ma and the deposition of the nonconformably overlying Leonardian Matzitzi Formation. D 2 records north–south dextral transpression and south-vergent thrusting and is attributed to the collision of Gondwana and southern Laurentia (Ouachitan orogeny) at ca. 290 Ma, the kinematics being consistent with the northward motion of Mexico that is required by most continental reconstructions for the final assembly of Pangea. D 3, which produced broadly north–south, upright folds, is also attributed to this collision and likely followed D 2 closely in the latest Paleozoic.

Late Visean mylonitic granitoids in the Argentera Massif (western Alps, Italy): age and kinematic constraints on the Ferrière–Mollières shear zone

Abstract The Argentera Massif in the western Alps is made of up two medium–high-grade metamorphic complexes of Variscan age. They are separated by a regional-scale northwest-trending shear zone, the Ferriere–Mollieres Line, which consists of low-grade mylonitic rocks. Muscovite-bearing mylonitic leucogranites crop out as subvertical northwest-trending intrusions, in the northern portion of the shear zone. A Rb/Sr muscovite–wr age of 327±3 Ma on foliated leucogranite gives a lower limit for the age of mylonitic deformation for which kinematic indicators indicate a dextral sense of shear. This strike-slip tectonics is compatible with the extensional regime that occurred during Carboniferous in the whole western Europe. To cite this article: G. Musumeci, F. Colombo, C. R. Geoscience 334 (2002) 213–220.

Fishing power, fishing effort, density, fishing intensity, and fishing mortality : SANDERS M. J. and A. J. MORGAN, 1976. J. Cons. int. Explor. Mer, 37 (1): 36–40

The Paleozoic Acatlán Complex of southern Mexico comprises polydeformed metasedimentary, granitoid, and mafic–ultramafic rocks variously interpreted as recording the closure of the Iapetus, Rheic, and Ouachitan Oceans. The complex is tectonically juxtaposed on its eastern margin against Grenville-age gneisses (Oaxacan Complex) that are unconformably overlain by Lower Paleozoic strata containing fossils of Gondwanan affinity. A thick siliciclastic unit (Chazumba and Cosoltepec Formations) at the base of the complex is considered part of a Lower Paleozoic accretionary prism with a provenance that isotopically resembles the Oaxacan Complex. This unit is tectonically overridden by a locally eclogitic mafic–ultramafic unit interpreted as a westward-obducted ophiolite, the emplacement of which was synchronous with mylonitic granitoid intrusion at ca. 440 Ma. Both units are unconformably overlain by a deformed volcano-sedimentary sequence (Tecomate Formation) attributed to a volcanic arc of presumed Devonian age. Deformed granitoids in contact with this sequence have been dated at ca. 371 (La Noria granite) and 287 Ma (Totoltepec pluton).Three phases of penetrative deformation (D1–3) affect the Cosoltepec Formation; the last two correlate with two penetrative deformational phases that affect the Tecomate Formation. D1 is of unknown kinematics but predates deposition of the Tecomate Formation and likely records obduction at ca. 440 Ma (Acatecan orogeny). A folded foliation in the Totoltepec pluton appears to record both deformational phases in the Tecomate Formation, bracketing D2 and D3 between 287 Ma and the deposition of the nonconformably overlying Leonardian Matzitzi Formation.D2 records north–south dextral transpression and south-vergent thrusting and is attributed to the collision of Gondwana and southern Laurentia (Ouachitan orogeny) at ca. 290 Ma, the kinematics being consistent with the northward motion of Mexico that is required by most continental reconstructions for the final assembly of Pangea. D3, which produced broadly north–south, upright folds, is also attributed to this collision and likely followed D2 closely in the latest Paleozoic.