Mylonitic Foliation Research Articles

Mineral fabrics unravel syn-magmatic deformation and constrain Permian and Jurassic tectonic regimes in arc plutons from the southern Colombian Andes

ABSTRACT Mineral fabric analyses of Permian La Plata and Jurassic Paez arc plutons provide information about the interplay of tectonic deformation and magma emplacement in the southern Colombian Andes. We combined field mapping, microstructural data, AMS fabrics, rock magnetic properties, vorticity analysis, and thermobarometry to study the internal fabrics of the plutons. Our results indicate that the studied plutons have well-developed NE magmatic and mylonitic foliations, with the latter being distinctive for the Permian pluton. These foliations are consistent with the N65°E/63°W Permian and N43°E/82°W Jurassic AMS magnetic fabrics. Magnetic lineations record subhorizontal (Permian) and vertical (Jurassic) magmas upwards in the NE–SW trajectories. The orientations of all the plutonic fabrics indicate structurally controlled magma emplacement consistent with compressional tectonics and bulk NW–SE crustal shortening. Microstructural features recorded in the plutons reveal magmatic to sub-magmatic and solid-state deformation developed at high- to low-temperatures during their syn-tectonic emplacement history. Thermobarometric calculations indicate similar shallow emplacement conditions (~10 km depth) in the La Plata and Paez plutons. The application of vorticity analysis shows that the magnetic fabrics and syn-emplacement of the Permian pluton developed by simple-shear dominated dextral transpression, whereas the AMS fabrics for the Jurassic syn-tectonic intrusion may have been associated with pure-shear contractional deformation.

Fracturing, comminution and grain-size-sensitive creep as a record of coseismic loading in the middle-crust: Insights from the Urtiga mylonitic pluton (NE Brazil)

Evidence for mid-crustal seismic slip is scarce due to the limited capacity of downward propagation of rupture damage imposed by the confining pressure. Nevertheless, structures indicative of elevated stresses occur throughout the crustal profile. To further investigate this setting, we have studied the fabrics of the Urtiga pluton, emplaced at the south Patos shear zone (northeast Brazil), a major Neoproterozoic crustal boundary. K-feldspar and plagioclase porphyroclasts are fractured, with fine-grained K-feldspar + plagioclase mixtures filling cracks. The edges of the clasts are rimmed by fine feldspar grains that form a fine-grained matrix. Quartz ribbons are parallel to the mylonitic foliation and show microstructures and a fabric indicating deformation via dislocation creep. Chemical compositions of feldspars are typically similar between porphyroclasts, fractures and matrix, with plagioclase grains locally being more albitic within fractures. Crystallographic preferred orientations (CPO) in grains within fractures are host-controlled by the adjacent porphyroclast, while fine grains rimming the clasts show a weak CPO and are mostly strain-free. These characteristics suggest that grain size reduction in the Urtiga mylonitic pluton occurred through fracturing and subsequent grain-size-sensitive creep under mid-crustal conditions, which were possibly attained via downward propagation of seismic rupture from the overlying seismogenic zone during transient, seismic slip episodes, giving rise to spatially related pseudotachylytes at the boundaries of the southern Patos shear zone.

Geochronology of the Paleoproterozoic Helanshan ductile shear zones: Insights into temporal framework of polyphase deformation in the Khondalite Belt, North China Craton

The Khondalite Belt has been regarded as a Paleoproterozoic collisional orogen in the northwestern North China Craton, but the geochronological framework of polyphase deformation in the Khondalite Belt still remains unclear, particularly the timing of orogen-parallel ductile shearing. In this study, we conducted field-based structural analysis and LA-ICP-MS zircon U-Pb geochronology on the Helanshan ductile shear zones (HDSZ) in the Khondalite Belt. The results show that three pre-kinematic leucocratic dykes have been reworked by the shear zones and yielded magmatic zircon ages of 1934 ± 13 Ma, 1924 ± 5 Ma and 1920 ± 24 Ma. Two syn-kinematic leucocratic dykes appeared along the mylonitic foliations and were dated at 1887 ± 21 Ma and 1871 ± 28 Ma. A post-kinematic leucocratic dyke cut the mylonitic foliations and gave a crystallization age of 1816 ± 28 Ma. These emplacement ages of shear zone-related dykes constrain the development of the HDSZ at some time between ∼ 1920 Ma and ∼ 1816 Ma. Meanwhile, zircon overgrowth rims from three intrusions that suffered the shear deformation exhibited metamorphic ages of 1904 ± 11 Ma, 1840 ± 13 Ma and 1826 ± 31 Ma. Similar metamorphic ages of 1901 ± 7 Ma, 1888 ± 8 Ma and 1849 ± 29 Ma were also obtained from high-temperature felsic, pelitic and calcitic mylonites. These results further unravel that the HDSZ most likely developed at ca. 1904–1826 Ma. Additionally, we have summarized available deformation-related geochronological data in the Khondalite Belt, and proposed that this belt underwent three major phases of deformation (D1-D3) in the Orosirian. D1 mainly produced overturned to recumbent isoclinal folds F1, penetrative transposition foliations S1 and mineral lineations L1 at ca. 1.97–1.93 Ga. Subsequently, D2 dominantly resulted in tight to open upright folds F2 at ca. 1.93–1.90 Ga. Later, D3 generated a series of NE- to E-trending orogen-parallel ductile shear zones at ca. 1.90–1.82 Ga. Together with multiple high-grade metamorphism and magmatism, the D1-D3 deformation recorded a complex protracted orogeny (>100 Myr) in response to the collision between the Yinshan and Ordos blocks.

Geology, geochronology and tectonic context of the Carreira Comprida Anorthosite in the evolution of the Goiás Massif, Porto Nacional Domain – Tocantins Province

The Carreira Comprida Anorthosite (CCA) is exposed in the Paleoproterozoic Porto Nacional High-Grade Metamorphic Terrene, in the north of the Goiás Massif, central Brazil, represented by an ancient pluton affected by late Neoproterozoic tectonics, with protoliths predominantly of anorthosite consisting of more than 85% labradorite (An60-65), with subordinate presence of clinopyroxene, quartz, hornblende and garnet. There is evidence of intrusive relationships in paragneisses country rocks of the Morro do Aquiles Formation, due to the effects of high-temperature contact metamorphism (∼750 °C) that reached the hornblende hornfels facies. Features of a magmatic nature are present as a cumulate texture formed by the accumulation of tabular plagioclase megacrystals by flotation associated with gravitational action due to the difference in magma density and magmatic flow, with clinopyroxene and interstitial (intercumulated) quartz, during the emplacement of the pluton and crystallization of magma. At the end of the Neoproterozoic, the body was affected by the tectonics of the transcontinental strike-slip system of the Transbrasiliano Lineament (Porto Nacional Transcurrent Shear Zone). U–Pb dating on igneous zircon crystals indicated an age of 2084 ± 4 Ma, which represents the age of emplacement of the Carreira Comprida Anorthosite in the Rhyacian, characterizing an event of gabbroic-anorthosite magmatism not known in the Goiano Massif. Lu–Hf isotopic data show variable εHf (2.08 Ga) values ranging from −1.3 to +4.5 and Hf-TDMC model ages with two crust formation episodes, one from the Siderian (2.46 - 2.38 Ga) and another in Neoarchean (2.74 - 2.59 Ga) which indicates that their parental magmas were derived from a depleted mantle source and range in composition from uncontaminated to variably contaminated by pre-existing older crust in the formation of the CCA. Considering that the ages of the Hf-TDMC model are similar to those of the orthogranulites from the Porto Nacional Complex, its indication as a possible source of the CCA is suggestive. Field relationships, primary structural features, petrographic and compositional data, geochronological and literature data suggest that the emplacement of the CCA is late-tectonic in relation to the evolution of the high-grade terrain of Porto Nacional, in the Rhyacian. After its crystallization, the CCA was affected by the tectonics of the dextral transcurrent system of the Transbrasiliano Lineament that implemented the Porto Nacional Shear Zone at the end of the Neoproterozoic, imposing strong deformation on the anorthosite and surrounding rocks, generating a variety of mylonites. These transformations generated a diversity of microstructures (stretch lineation, mylonitic foliation, tectonic banding, ribbon quartz, lentiform porphyroclasts, microshear, undulatory extinction, thinning of mechanical twinning, wedge twinning, deformation bands, mantle-core, subgrains, symplectites of garnet), which are suggestive of ductile tectono-metamorphic transformations that reached amphibolite facies conditions. Finally, the CCA is a rare example in Brazil of a massif-type anorthosite developed in the Paleoproterozoic (2.08 Ga). On the other hand, the Rhyacian granite bodies of the Ipueiras Suite (2.08 - 2.07 Ga) located very close to the anorthosite pluton are contemporary with the CCA, allowing them to be considered an association of the Anorthosite-Mangerite-Charnockite (AMC) type.

Timing and Tectonic Implications of the Development of the Orosirian Qianlishan Ductile Shear Zones in the Khondalite Belt, North China Craton

Orogen-parallel ductile shear zones are conspicuous structures in the Khondalite Belt, but the timing of shearing remains poorly understood. Here, we present field-based structural and zircon U-Pb geochronological studies on the newly discovered Qianlishan ductile shear zones (QDSZ) in the Khondalite Belt. Our results show that the nearly E-W-trending QDSZ are characterized by steeply S(SW)-dipping mylonitic foliations and mainly display a top-to-N(NE) sense of shearing. Two pre-kinematic intrusions yielded zircon crystallization ages of 2055 ± 17 Ma and 1947 ± 9 Ma, providing the maximum age limit for the QDSZ. Additionally, zircon overgrowth rims from three high-temperature mylonites gave metamorphic ages of 1902 ± 8 Ma, 1902 ± 26 Ma and 1884 ± 12 Ma, interpreted to record the timing of development of the QDSZ. Integrated with previous studies, we propose that the Qianlishan Complex suffered three phases of Orosirian deformation (D1–D3), of which the D3 deformation led to the development of the QDSZ. Deformation events D1, D2 and D3 are considered to have occurred at ca. 1.97–1.93 Ga, 1.93–1.90 Ga and 1.90–1.82 Ga, respectively. These events document that the Khondalite Belt underwent a protracted (>100 Myr) orogenic history in response to the collision between the Yinshan and Ordos blocks.

Fracture evaluation of the plutonic basement in the Upper Magdalena Basin: Implications for the development of naturally fractured reservoirs in the Northern Andes

Plutonic rocks typically have negligible matrix porosity and permeability. However, fractures and mineral alterations create storage space and flow pathways that turn plutonic rocks into fluid reservoirs. Despite significant hydrocarbon discoveries, naturally fractured reservoirs in plutonic rocks have been poorly studied. In most Colombian basins, the crystalline basement has undergone multiple deformational events and is thrust over the Cretaceous to Cenozoic source and reservoir rocks of the conventional petroleum system. This structural configuration is ideal for the migration of oil into a fractured basement. A multiscale fracture analysis, including field, petrographical and petrophysical techniques was conducted on the Permian and Jurassic plutonic basement of Upper Magdalena Basin in order to understand the controls on brittle deformation, the development of fracture networks and their potential to form hydrocarbon reservoirs. The results indicate that protolith textures and structures, including magmatic and mylonitic foliation, favours fracturing. Dykes exhibit higher fracture density (7–48 fractures/m), porosity (mean = 0.4%) and permeability (mean = 125,818.75 mD) than the host rock (2–25 fractures/m; 0.23%; 12,066.09 mD). Intersection zones from regional faults, are characterized by the highest fracture and lineament intensity. Our results suggest that dyke swarms and interacting damage zones can significantly enhance the reservoir quality of plutonic rocks by providing storage in fractures and fluid pathways to the host rock.

Effect of Water‐Rock Ratio on the Stable Isotope Record of Fluid‐Rock‐Deformation Interactions in Detachment Shear Zone

AbstractOxygen and hydrogen stable isotope analyses of quartz and muscovite veins from the footwall of the Raft River detachment shear zone (Utah) provide insight into the hydrology and fluid‐rock interactions during ductile deformation. Samples were collected from veins containing 90%–100% quartz with orientations either at a high angle or sub‐parallel to the surrounding quartzite mylonite foliation. Stable isotope analysis was performed on 10 samples and compared with previous quartzite mylonite isotope data sets. The results indicate that the fluid present during deformation of the shear zone was meteoric in origin, with a δ2H value of approximately −100‰ and a δ18O value of approximately −13.7‰. Oxygen stable isotope O18O depletion correlates with the muscovite content of the analyzed rocks. Many of the analyzed samples in this and other studies show an apparent lack of equilibrium between the oxygen and hydrogen isotope systems, which can be explained by hydrogen and oxygen isotope exchange at varying fluid‐rock ratios. Our results suggest that the Raft River detachment shear zone had a low static fluid‐rock ratio (<0.1), yet experienced episodic influxes of fluids through semi‐brittle structures. This fluid was then expelled out into the surrounding mylonite following progressive shearing, causing further 18O‐depletion and fluid‐related embrittlement.

Multistage mineralizing episodes of the Proterozoic world-class Volta Grande gold deposit, Amazonian Craton, northern Brazil: Implications for the Bacajá Domain metallogenesis

The easternmost region of the Amazonian Craton in northern Brazil has been the focus of several mining exploration surveys, which led to the identification of the world-class Volta Grande gold deposit (∼6.0 Moz@1.02 g/t). The deposit is inserted in the Bacajá Domain (2.24–2.0 Ga), and part of the mineralization occurs in Proterozoic mylonite, gneiss, and associated metamafic rocks that underwent greenschist to amphibolite-facies metamorphism. Electrum (Au–Ag) occurs as grains in cm-sized quartz high-grade veins and veinlets (up to 34.1 g/t in some rock intervals) that follow the mylonitic foliation and are associated with pervasive carbonate alteration, synchronous with the dynamic metamorphism. Disseminated electrum grains, pyrite, chalcopyrite, arsenopyrite, tellurides, and galena occur associated with potassic, argillic, propylitic, and silicic alteration types mainly in fracture-controlled or pervasive styles. These geological features as a whole suggest an orogenic high-tonnage orogenic gold system. Isotropic core samples along a stratigraphic profile reveal late andesite, dacite, rhyodacite, and rhyolite lava flows and dikes and plutonic rocks such as quartz monzonite, granodiorite, monzodiorite, and subordinate microgranite crosscutting the older metamorphic rocks. These rocks reveal potassic, propylitic, argillic, silicic, and sericitic alteration types. Electrum grains, possibly related to boiling deposition, occur along cm-sized quartz, calcite, and adularia high-grade veins (up to 29.5 g/t in some rock intervals) with chalcopyrite, pyrite, arsenopyrite, tetradymite, and tellurobismuthite. Such mineralogy is compatible with a low-sulfidation epithermal system. Data integration points to at least two distinct mineralizing events leading to the Volta Grande gold deposit – orogenic gold and low-sulfidation epithermal – which explains such large-tonnage and high-grade ore accumulation. It configures a new exploration guide for the Bacajá Domain of the Amazonian Craton, attested by other worldwide regions with similar metamorphic, magmatic, and hydrothermal alteration characteristics.

Topaz-bearing Lower Ordovician orthogneiss within the Ostrong Nappe System – The Laimbach Orthogneiss (Bohemian Massif, Lower Austria)

Abstract In the 1980s, large parts of the Bohemian Massif in Austria were explored for the occurrence of raw materials (Göd, 1988), and a trace content of topaz was discovered in channels draining the eastern slope of the Ostrong (Lower Austria). Orthogneiss bodies in the Ostrong Nappe System are shown to be the source of these topaz occurrences, and Raman spectroscopy indicates that topaz is fluorine rich. So far, this is the only occurrence of topaz-bearing orthogneiss within the entire Moldanubian Superunit. The orthogneiss shows a tectonic overprint and exhibits a weak to well-developed foliation, as well as local folding. Samples taken near the tectonic boundary of the Ostrong to the Drosendorf Nappe Systems show stronger shear deformation with a pronounced mylonitic foliation and stretching lineation. Detailed geo-chemical investigations of the major and trace elements indicate a classification as S-type granite with a high degree of differentiation and a peraluminous character. The orthogneiss has a high SiO2 content of 72.8–77.3 wt%, as well as a noteworthy high fluorine content of up to 2760 ppm. Mineral compositions show zoned plagioclase with an albite component of about 87–99 mol%, increasing towards the grain margin. Garnet occurs rarely, but consistently in those samples near the tectonic boundary of the Ostrong- to the Drosendorf Nappe Systems. Garnet is almandine-dominated, shows no zoning and is single-phased. Sillimanite is common and samples near the tectonic boundary also contain kyanite. To determine the previously unknown protolith age of the orthogneiss, U-Pb zircon dating was applied. Three zircon fractions from two samples yield concordia ages of 475.3 ± 1.0 Ma, 474.8 ± 2.9 Ma, and 473.5 ± 1.5 Ma, identical in assigned uncertainties, reflecting magmatic zircon growth. Furthermore, the short prismatic habit of zircon grains indicates a plutonic rather than volcanic origin of the protolith. Dating results also provide a minimum sedimentation age for the rocks of the Ostrong Nappe System within Austria. In comparison with other metagranitoids and orthogneisses of the Moldanubian Nappes, the investigated orthogneiss shows strong similarities with the Gföhl Gneiss and the Moldanubian Granulite. The orthogneiss therefore is considered as a more fractionated equivalent of the Gföhl Gneiss. In conclusion we suggest to name the studied orthogneiss Laimbach Orthogneiss in the rank of a lithodeme (NACSN, 2005), after the locality Laimbach am Ostrong (48°19′01″N; 15°07′19″E), which is located centrally with respect to the occurrences of this gneiss.

P–T path reconstruction in a multi‐layered garnet‐bearing schist: Evidence for 2.2–2.1 Ga near‐isothermal loading in the northern margin of the Western Dharwar Craton (India), and regional implications

P–T paths were reconstructed using P–T pseudosection modelling from garnets in a finely laminated mylonitic hornblende‐biotite schist with four sub‐cm wide mineralogically distinct layers separated by poly‐crystalline quartz layers from the northern margin of the Western Dharwar Craton (WDC). The almandine‐grossular‐rich garnets, syn/post‐tectonic with respect to the mylonite foliation, exhibit considerable variations in the zoning patterns of divalent cations in the different layers. By contrast, the compositions of the matrix minerals dominated by biotite, chlorite, hornblende and plagioclase are tightly constrained within and across the layers. The varying abundances of major element oxides in the layers reflect variations in the abundances of the minerals, rather than the mineral compositions. P–T conditions estimated using mineral thermo‐barometry in the garnet‐muscovite‐biotite‐plagioclase assemblages are 520–575°C and 6–8 kbar; somewhat higher P–T values (615–665°C, 7–9 kbar) are obtained from hornblende‐garnet‐plagioclase‐quartz assemblages. The P–T paths obtained from the multi‐mineralic layers using P–T pseudosection modelling yield near‐isothermal loading (4–7 kbar) at 500–550°C in all layers. The near‐isothermal loading experienced by the multi‐layered rock suggests rapid crustal thickening involving overthrusting followed by nucleation of steeply dipping transpressional shear zones due to oblique convergence of the WDC with a crustal domain in the north. U‐Th‐total Pb chemical dating of metamorphic monazites indicates the crustal convergence occurred at 2.2–2.1 Ga. The event does not have known equivalents in the neighbouring crustal domains eastwards in India as well as in Madagascar in the west. This suggests the crustal domain may be an exotic block.

Alpine Ductile Deformation of the Upper Iberian Collided Margin (Eaux‐Chaudes Massif, West‐Central Pyrenean Hinterland, France)

AbstractThe Eaux‐Chaudes massif provides keys to unravel the deep‐seated deformation of the Iberian rifted margin during the Alpine orogeny in the Pyrenees. The massif conforms to an inlier of upper Cretaceous carbonate rocks within the Paleozoic basement of the western Axial Zone, originally deposited in the upper margin shelf before the Cenozoic collision. New geological mapping and cross‐section construction lead to the description of the lateral structural variation from a km‐scale fold nappe in the west to a ductile, imbricate fold‐thrust fan in the east. The transition from a Variscan pluton to Devonian metasediments underlying the autochthonous Cretaceous induced this structural change. Recumbent folding, which involved upper Paleozoic rocks, was facilitated by a lower detachment in Silurian slates and an upper detachment in an overlying Keuper shale and evaporite thrust sheet. Remnants of this allochthonous sheet form shale and ophite bodies pinched within the upper Cretaceous carbonates, conforming unusual tertiary welds. Ductile shear in the overturned limb of the Eaux‐Chaudes fold nappe imparted strong mylonitic foliation in carbonate rocks, often accompanied by N‐S stretching lineation and top‐to‐the‐south kinematic indicators. The burial of the massif by basement‐involved thrust sheets and the Keuper sheet, along with their Mesozoic‐Cenozoic cover, account for ductile deformation conditions and a structural style not reported hitherto for the Alpine Pyrenees. A hypothesis for the tectonic restoration of this part of the Pyrenean hinterland is finally proposed.

Neoarchean synkinematic metamorphic peak in the Isua supracrustal belt (West Greenland)

Abstract We present petrological data and seven Lu-Hf garnet–amphibole–whole rock ages obtained from a single garnet-hornblende-mica schist sample from the Isua supracrustal belt (West Greenland). Garnets grew during prograde metamorphism toward regional amphibolite-facies peak conditions, and a mylonitic foliation formed during and after garnet growth. Garnet crystals show typical prograde zoning with no visible traces of a relict garnet generation. They do show various degrees of retrogression. While some crystals are perfectly euhedral with only minor chemical alteration along cracks, others are elongated in the foliation and either grew in this shape or were deformed. Six garnet splits were separated from crushed single crystals and one from a crushed bulk sample. Individual three-point garnet–hornblende–whole rock ages scatter between 2.603 ± 0.018 Ga and 2.432 ± 0.059 Ga for single garnets. The garnet split from the bulk sample defines an age of 2.463 ± 0.031 Ga, the data point farthest from the regression line for all data points (2.551 ± 0.074 Ga, mean square of weighted deviates = 25). We interpret these data to indicate partial retrogression of a Neoarchean garnet population not significantly older than the oldest obtained three-point age. Well-preserved garnet zoning, regional peak temperatures well below the closing temperature of the Lu-Hf system, and the small scatter of Lu-Hf ages preclude an interpretation of the observed metamorphism and deformation as being Eoarchean in age.

Polyphase deformation and deformation-magmatism interaction in the Pan-African 4°50′ shear zone (Hoggar, Southern Algeria)

The 4°50′Shear zone (SZ), one of the major structures of western Gondwana, formed in Hoggar during the Pan-African orogeny is still poorly studied; especially ages and deformation characteristics are lacking. We performed microstructural analysis and EBSD measurements on nine selected mylonites from the 4°50′SZ, and zircon U-Pb dating on two granitoid samples. The protolith of these mylonites are granitoids. Most samples display segregation of dominant feldspar and minor quartz in separate layers that underline the N-S subvertical mylonitic foliation. Feldspar grains display evidence of intracrystalline deformation, supporting dextral shear-sense. Plagioclase and K-feldspar are frequently corroded with embayments infilled by secondary K-feldspar and plagioclase respectively. Interstitial quartz grains disseminated in feldspar aggregates may infill fractures in feldspar grains. Quartz ribbons, usually parallel to the foliation, locally crosscut feldspar layers. Quartz is partially recrystallized and elongate crystals form a secondary foliation at ∼ 30° to the main foliation, supporting a later sinistral shearing. In some samples, amphibole is associated with quartz and oriented parallel to the foliation. In all samples, quartz CPO suggests a dominant prism-<a > slip-system, activated at medium/high temperature (∼500-700°c), but stress-induced oriented crystallization may have contributed to this CPO. K-feldspar CPO supports the activation of the [100](010) slip system while the plagioclase CPO points to activation of [100](001) system. These CPO support dislocation creep under amphibolite facies conditions. These new data, in addition to U-Pb dating, suggest the following evolution: 1) successive intrusions of granitoids in the SZ (∼661-∼639 Ma) due to partial-melting of the lower-crust and, possibly also of the lithospheric-mantle, 2) the 4°5 0′SZ was rooted in the lower-crust or the upper-mantle, and 3) dextral shearing lasted over ≥ 20My. This was followed by successive magma batches that intruded these rocks during a late migmatitic episode (∼623-∼609 Ma). After ∼ 609 Ma, a moderate sinistral reactivation of the 4°50′SZ occurred and was accommodated through dislocation creep mainly localized in quartz ribbons. During accretion of the Western Gondwana, the 4°50′SZ was active during subduction. During ≥ 20My, it was successively intruded by granitoids that cooled down slowly, and have been deformed in the submagmatic state then in the solid-state.

Making sense of shear zone fabrics that record multiple episodes of deformation: Electron backscatter diffraction–derived and crystallographic vorticity axis–enhanced petrochronology

Abstract We present a new method of linking microstructures, electron backscatter diffraction (EBSD)–derived crystallographic vorticity axis (CVA) analysis, and titanite petrochronology to directly link fabric development to specific deformation events in shear zone rocks with complex histories. This approach is particularly useful where overprinting is incomplete, such that it is unknown which fabric is being dated by the petrochronometer. Here, we compared single-phase CVA patterns of fabric-forming minerals with those of synkinematic petrochronometers (e.g., titanite) to associate the timing of fabric development with deformational events in the middle crust of the George Sound shear zone, Fiordland, New Zealand. The host rocks to the George Sound shear zone include the Carboniferous Large Pluton, where titanite petrochronology demonstrates an unequivocally Cretaceous age of metamorphic titanite growth within mylonitic foliation. However, the host rocks show two distinct CVA patterns: a transtensional deformation event recorded by quartz and plagioclase, and a pure-shear–dominated transpressional deformation event recorded by biotite and titanite. Therefore, the transpressional CVA pattern of the titanite, coupled with its Cretaceous age, shows that it cannot be used to date the quartz and plagioclase fabric developed in response to an older transtensional deformation event. These results demonstrate the necessity of combining EBSD and CVA analysis with petrochronology to demonstrate that synkinematic accessory phase petrochronometers show the same kinematic deformation geometry (i.e., CVA pattern) as the fabric being dated.

A nearly isobaric P-T-deformation evolution path of the Cerro Negro Paleoproterozoic mylonitic igneous suites: The exhumation of the Tandilia Belt of the Rio de la Plata Craton

Two high-grade mylonites from Cerro Negro hill were studied in detail to shed light on the igneous-metamorphic evolution of the northwestern extreme of the Azul Megashear Zone (AMSZ) of the Tandilia Belt, Rio de la Plata craton. Bulk-rock compositions of a garnet-bearing hornblende gneissic mylonite (ribbon mylonite) and a xenolith of garnet-bearing granite mylonite (striped gneiss) included in the former were modeled with the GeoPs software (Xiang and Connolly, 2021) to construct P-T pseudosections in the 11-component system Si–Ti–Al–Fe–Mn–Mg–Ca–Na–K–O–H by Gibbs energy minimization. Pseudosections were contoured for isopleths parameters and mineral isomodes of interest. Garnet, with a homogeneous composition of pyr5(gro + andr)18.5spes6.5alm70, present in the younger granite crystal mush (<8 vol% of melt for 0.6 wt% of H2O) formed at 820 °C and 7.2 kbar. These conditions were encompassed by biotite with 0.21–0.23 Ti pfu and amphibole with Mg# of 0.30–0.33. A nearly isobaric cooling is outlined during a high-grade deformational event (D1) until core with the composition of pyr5(gro + andr)10spes5alm80, in slightly zoned garnet of the granite xenolith (modeled with 0.3 wt% of H2O), yields conditions of 540–580 °C and 7.2–8.2 kbars. Rimwards, a subsequent cooling with exhumation was recorded in garnet composition of pyr3(gro + andr)17spes6alm72 to reach conditions of ∼420–440 °C and ∼6.6 kbar. The integrated P-T path holds conditions of middle crust realms (ca. 24–25 km) from the initiation of deformation (D1) coevally to, or shortly, after the younger granite crystallization with a waning apparent geothermal gradient to 20 C°Km−1. Consistent mechanisms of deformation in minerals (d) generated a subvertical E-W mylonitic foliation (D1) under granulite to amphibolite metamorphic facies conditions until greenschist metamorphic facies conditions (D2). Fragile events (D3 and D4) occurred at higher crustal levels concomitant with water influx and sporadic retrogression. Hitherto, the studied rocks experienced a somewhat different evolution path when compared to adjacent Paleoproterozoic areas around the AMSZ or from other places of the Tandilia Belt, which show gently partial exhumation P-T paths after their solidus.

On the crustal framework of the central Alto Pajeú Terrane (Borborema Province, NE Brazil): Geophysical and structural data constrain late Neoproterozoic transpression in Western Gondwana

The structural framework of the Alto Pajeú Terrane, Borborema Province (NE Brazil) records a complex tectonic evolution related to the Neoproterozoic amalgamation of Western Gondwana. Combined, multiscale geophysical and litho-structural datasets for the Central Borborema Province constrain the regional crustal architecture. Magnetic and radiometric lineaments mostly follow NE-SW and E-W trends that correlate to strike-slip shear zones and associated subvertical mylonitic foliations. Regional, outcrop-scale and microstructural kinematic indicators consistently assign sinistral and dextral motion sense for NE-SW and E-W shear zones, respectively. Four sectors with contrasting litho-geophysical and structural patterns are juxtaposed across major shear zones: the Alto Pajeú, Alto Moxotó and Piancó-Alto Brígida terranes, plus a small portion of the Northern Borborema Province. Expressive magnetic lineaments highlight the boundary between the Alto Pajeú and Alto Moxotó terranes, represented by a pair of sinistral strike-slip shear zones. At outcrop scale, relic thrust surfaces (top-to-NW vergence) with associated flat-lying foliation and down-dip mineral stretching lineations affecting Tonian orthogneisses and metapelites disclose a contractional episode of uncertain age. Transposition of the shallowly-dipping foliation in highly sheared domains imply it is older than the shear-related deformation. The shear zone system and associated structures record the transpressional regime that characterizes the late stages of the Brasiliano/Pan-African Orogeny in the region (ca. 575–565 Ma). Microstructures of a mylonitic orthogneiss from the Juru-Belém shear zone reveal greenschist to amphibolite facies deformational conditions, with increasing importance of subgrain rotation mechanisms at higher strain rates and/or temperature. Microstructural similarities between this rock and tectonites along the southern Patos lineament suggest that bordering and internal regions of the Alto Pajeú terrane were subject to similar thermodynamic conditions during the late Brasiliano transpression.

Arrested tectonic development: Preservation of high-temperature fabrics in the sinistral Caiçara shear zone (Borborema province, northeastern Brazil)

The protracted duration of shearing in large-scale shear zones and/or their reactivation may obscure the metamorphic conditions under which the mylonites initially formed. Here, we describe the case of the high-temperature Caiçara shear zone (Borborema Province, northeastern Brazil), which shows only limited overprint by low temperature fabrics. The NE-trending Caiçara shear zone is ∼35 km long and comprises a 2 to 5 km-wide mylonitic belt. The shear zone was characterized by using kinematic analysis, including field and microstructural observations and measurement of quartz c-axis. The mylonitic foliation is steeply NW-dipping and contains a shallowly SW-plunging stretching lineation. Kinematic criteria (e.g., asymmetric porphyroclasts, S–C fabrics, asymmetric folds, C′-type shear bands) are consistent with sinistral shear. The dominant microstructures (e.g., chessboard extinction in quartz, grain boundary migration and subgrain rotation recrystallization in quartz, and polygonal aggregates of feldspar around porphyroclasts) and application of the opening angle thermometer indicate deformation dominantly at temperatures higher than ∼ 600 °C. In contrast to the common situation in the central Borborema Province, the Caiçara shear zone is unconnected to other shear zones. We attribute preservation of high-T fabrics to cessation of shearing in an early stage of development, preventing continuous fabric development at decreasing temperature conditions.

Structural and geochemical studies of the Palaeozoic granites from Dalhousie region of Himachal Himalaya: Implications on petrogenesis and deformation of strongly peraluminous S‐type granites

The Palaeozoic granites forming the Dalhousie pluton occur as an elongated body intruded into the core of an antiform in Salkhala metasedimentary rocks and are referred to herein as Dalhousie granites (DG). The studied mesoscopic structures, for example, well‐developed and randomly distributed megacrysts of K‐feldspar at the core part of massif, show undeformed porphyritic nature, whereas augen along with the mylonitic foliation and parallelly aligned feldspar megacrysts are dipping towards north‐north‐east (NNE) in the marginal part. The presence of aplite veins is evidence of final stage crystallization, where these rocks rapidly crystallized from siliceous residual fluids/solutions that escaped along fractures in the granites. Moreover, the granites show intrusive as well as thrusted contact with the Chamba metamorphics. The microscopic study from the marginal parts shows the dynamic recrystallization of quartz along with the fractures filled with secondary material oriented opposite to the top‐to‐the‐SW. Whereas euhedral quartz grains with undeformed grain boundaries, compositional zoning in plagioclase, and magmatic (perthite) texture of K‐feldspar are present in the core of Dalhousie pluton. In addition, a new whole‐rock geochemical dataset of DG is presented and investigated to elucidate the petrogenesis and tectonic environment. The geochemical data shows that DG (dominantly monzogranites) are formed from a pelitic source‐derived, (molar Al2O3/CaO + Na2O + K2O &gt; 1.1) strongly peraluminous (S‐type) calc‐alkaline magma. This magma was generated by muscovite vapour absent dehydration melting. Harker bivariate plots indicate that fractional crystallization was a dominant process during the evolution of these granites. The tectonic discrimination diagrams suggest that DG were generated in syn‐collisional setting. Exceptionally high Pb in DG occurred due to primary melt generation at low‐temperature during partial melting (low‐degree melting) of the pelitic source rock. Trace and rare earth elements characteristics, such as positive anomaly of Zr show a notable amount of zircon as one of the accumulating phases. A pronounced negative Eu anomaly (EuN/Eu* = 0.04–0.66), along with high silica content and highly varied trace element ratios in these rocks, show that these are fairly evolved granites.

Deformation Mechanisms of Blueschist Facies Continental Metasediments May Offer Insights Into Deep Episodic Tremor and Slow Slip Events

AbstractExhumed fossil subduction zones are archives of the deformation conditions and mechanisms from depths not directly accessible. Microstructural analysis of samples exhumed therefrom offers insights into the micromechanics and deformation processes associated with subduction such as earthquakes, slow earthquakes, and aseismic creep. Subducted and exhumed continental metasediments of the Tuscan Metamorphic Units of the Italian Northern Apennines contain a mylonitic foliation and quartz and carpholite dilational hydroshear veins with crack‐and‐seal textures, both developed at blueschist facies conditions (350–400°C, ∼1 GPa). As shown by overprinting relationships and mineral assemblages, these two structure types formed broadly coeval within the stability field of carpholite. Metaconglomerates and metaquartzarenites deformed mainly by dissolution‐precipitation creep and secondary by dislocation creep. Microstructural and electron backscatter diffraction analyses of the veins suggest only limited recrystallization of quartz fibers by subgrain rotation recrystallization, with adjacent metapelite bands acting as decollement horizons, likely by slip on the basal plane of phyllosilicates. We estimated differential stresses of 43–55 MPa and strain rates between 10−13 and 10−14 s−1 from the vein recrystallized quartz fibers. We propose these microstructures and deformation mechanisms to represent a geological evidence of deep episodic tremor and slow slip events in subducted continental metasediments. Pore pressure cyclically reached supralithostatic values triggering tremors causing fracturing of all involved lithotypes. Likely, slow slip was accommodated preferentially by slip on phyllosilicate bands. Aseismic creep occurred mainly by dislocation creep with subgrain rotation recrystallization in vein quartz, slip on the basal plane of phyllosilicates, and dissolution and precipitation creep in the host rock.

Sinistral shear during Middle Jurassic emplacement of the Matancilla Plutonic Complex in northern Chile (25.4° S) as evidence of oblique plate convergence during the early Andean orogeny

Arc magmatism in a continental subduction zone facilitates rheological weakening of the rigid upper plate, and can accommodate the partitioned trench-parallel component of oblique subduction into an intra-arc shear zone. We document a shear zone at latitude 25.4° S near Taltal, Chile that was associated with intrusion of the Matancilla Plutonic Complex at ∼169 Ma to evaluate intra-arc deformation and possible tectonic plate configurations during this time period. Polyphase folding of Paleozoic metasedimentary rocks is overprinted by mylonitic fabrics that are most extensive in a zone up to 1.4 km wide in the thermal aureole of the granodioritic Matancilla pluton, where contact metamorphic andalusite porphyroblasts are synkinematic with fabric development. Mylonite in metasedimentary rocks is overprinted by a ∼130 Ma granodiorite (zircon U–Pb) and by ∼133 Ma postkinematic monazite (U–Pb). Within the Jurassic Matancilla granodiorite, pervasive ductile shear occurs along the intrusive contact while centimeter-scale discrete high-strain zones throughout the pluton are associated with focused hydrothermal alteration and reaction weakening. Mylonitic foliation in the metasedimentary rocks and within the pluton strikes N- to NE and dips steeply, while stretching lineations are subhorizontal on average. Kinematic indicators record dominantly sinistral shear, though some dextral or symmetric indicators and S > L fabrics suggest a component of coaxial strain and flattening. Sinistral strike-slip kinematics in the Matancilla shear zone may indicate that Middle Jurassic convergence had sinistral obliquity that was locally partitioned into the contemporaneous magmatic arc. Sinistral-oblique convergence would require the Phoenix-Farallon spreading center to be north of ∼25° S in the Middle Jurassic, providing a constraint to plate reconstructions during the early Andean orogeny.