Amphibolite Facies Shear Zones Research Articles

Crystal plasticity and fluid availability govern the ability of titanite to record the age of deformation

Here, we study the relationships of titanite-hosting microdomains, intragrain chemical variations, microstructure and fluids with the aim of deciphering the reliability of titanite U–Pb dating to constrain the age of deformation in mylonitic rocks. We investigate these relationships in a post-Variscan amphibolite-facies shear zone developed in the mid-low continental crust (Ivrea-Verbano Zone, Southern Alps, Italy). Quantitative orientation analyses along with textural imaging of titanite are combined with trace-element analyses and U–Pb age dating. Titanite is studied in mm- to cm-scale layered rocks showing compositional variation consisting of alternating ‘amphibole-rich’ (i.e., amphibolites) and ‘clinopyroxene/plagioclase-rich’ domains (i.e., calc-silicates). Titanite from amphibole-rich domains shows predominance of crystal–plastic deformation features, as abrupt or progressive core-to-rim structures characterized by increasing lattice distortion and local dislocation density, associated with the development of abundant subgrains and rare newly nucleated grains. We suggest that these microstructures form while interacting with small amounts of fluids circulating along the grain boundaries. Consequently, locally the chemistry of titanite is changing. In the clinopyroxene/plagioclase-rich domains, titanite is mostly undeformed and rarely shows bending localized in discontinuous narrow rims/tips. In these domains, fluid-mediated replacement reactions are either rare or absent, as also indicated by weak chemical variations across and among grains. These observations suggest different reactivities with respect to the same P-T-fluid conditions of the two compositional domains, which coexist within the same sample at the thin section scale. U–Pb data show correlations with chemical and microstructural domains that differ as function of the composition of the microdomain. This correlation is more apparent within amphibole-rich domains where microstructures characterized by high lattice distortion/dislocations and/or subgrains show significant variations of REE, Zr, Y, Nb, U with respect to the low deformed portion of grains. These titanite domains define an isotopic population providing the youngest (Jurassic) lower intercept age. A less clear correlation between titanite chemistry and microstructures is observed in clinopyroxene/plagioclase-rich domains. Here, the rare titanites showing lattice distortion and minor Sr depletion define a population providing a similar Jurassic lower intercept age. Therefore, our results demonstrate that microstructurally and chemical calibrated U–Pb dating of titanite provides realistic ages of shear zone activity, only in case of predominance of crystal-plastic processes and of local interaction of titanite with small amounts of fluids focused along grain boundaries. Finally, the different footprints recorded by titanite grains strongly depend on the composition of cm- or mm-scale interlayered domains in which titanite occurs.

40Ar/39Ar constraints on the tectonic evolution of the central part of the Mesoproterozoic Sveconorwegian orogen

Structural data combined with 40Ar/39Ar geochronology of hornblende, muscovite, biotite and plagioclase from 37 localities along three transects allow the time-constrained reconstruction of the Sveconorwegian deformation and cooling history of the Kongsberg lithotectonic unit and of the boundary zone to the adjacent Telemark lithotectonic unit in southern Norway. The Kongsberg lithotectonic unit consists of pervasively deformed, steeply dipping amphibolite-to granulite-facies gneisses, in which Mesoproterozoic 40Ar/39Ar ages of c. 1090-1070 Ma record cooling and exhumation. Ages scattering around 1000 Ma are interpreted as resetting of the K/Ar systems related to sinistral strike-slip deformation along c. N–S trending greenschist-to amphibolite facies shear zones and associated large-scale folding. Later, Neoproterozoic top-to-the-E normal shearing accommodated by the newly discovered Prestfoss detachment selectively exploiting the Kongsberg-Telemark boundary zone caused the exhumation of amphibolite-facies rocks of the Telemark lithotectonic unit and their juxtaposition against amphibolite-facies gneisses of the Kongsberg lithotectonic unit, which had cooled earlier in the orogenic history. 40Ar/39Ar ages constrain shearing and associated cooling to between c. 940 Ma and c. 900 Ma. Finally, a Silurian (420 ± 11 Ma) 40Ar/39Ar age may reflect localized partial to complete resetting due to Caledonian tectonics, and a Permian (288 ± 1 Ma) 40Ar/39Ar age constrains the intrusion of an Oslo Rift granite.

Seismic properties across an amphibolite- to greenschist-facies strain gradient (Neves area, eastern Alps, Italy): New considerations for shear zone imaging

Detecting the presence and geometry of crustal shear zones by geophysical methods relies on our understanding of the intrinsic parameters controlling the seismic properties of these deformed rocks, over the range of pressure-temperature conditions expected in the Earth's crust. To this end, we aimed to track changes in P-wave propagation velocity (VP), anisotropy (AVP) and S-wave splitting (AVS) across a natural shear zone using experimental and electron backscatter diffraction methods. Five samples were collected across a meter-scale shear zone developed under amphibolite- to greenschist-facies in the Neves metagranodiorite (Tauern Window, Eastern Alps). With increasing strain, seismic properties show a non-linear evolution resulting from interactions among structural, textural and mineralogical changes. Geometry and magnitude of seismic anisotropy are controlled by the interference of strongly anisotropic phyllosilicates with less anisotropic but more abundant quartz and plagioclase. The amphibolite-facies shear zone margin (> 500 °C) is free of major mineral transformations, and rather characterized by the development of S-C structure, inducing an orthorhombic symmetry to the VP distribution and up to 12 % AVP, with the fast axis parallel to lineation. AVS is maximized (6 %) in the two directions parallel to the C-planes. Further deformation under greenschist-facies (450–500 °C) induces significant crystallization of muscovite, chlorite, and albite. A dominant hexagonal symmetry develops, with a fast VP plane and a fast S-wave polarization plane along the C-planes. AVP and AVS exceed 10 %. The computed elastic tensor of the whole shear zone emphasizes the importance of the strain gradient on the seismic signature of the overall structure. In our model, the bulk seismic properties are dominated by the orthorhombic symmetry of the protomylonitic zone rather than by the ultramylonite core with hexagonal symmetry (C-plane), which may explain why seismic studies of shear zones commonly involve a fast velocity plane oblique to the layer boundaries.

Kinematics, strain pattern and geochronology of the Salem-Attur shear zone: Tectonic implications for the multiple sheared Salem-Namakkal blocks of the Southern Granulite terrane, India

Structural mapping, strain analysis, and a variety of geochronological studies were carried out to determine the tectonothermal evolution of the Salem-Attur shear zone in the Southern Granulite terrane of South India. The Salem-Namakkal blocks containing the shear zone consisted of quartzofeldspathic gneiss, charnockite and mafic granulite, and had undergone multiple phases of magmatism spanning over a period of 3.2–0.5 Ga. The rocks were deformed by four phases of deformation D1–D4. The D1 deformation was characterized by isoclinal and recumbent NE-SW trending F1 fold with a pervasive subhorizontal axial planar granulitic fabric, S1, and associated quartzofeldspathic leucosomes. Granulite metamorphism was dated at ca. 2.5–2.3 Ga. The F1 fold and S1 fabric were coaxially refolded by tight to isoclinal, upright to steeply inclined NE-SW trending F2 folds during D2 deformation. The D2 deformation was associated with F2 axial planar shear zones, crenulations and leucosomes, S2 fabric. Large-scale D2 shear zones characterized by high-temperature ductile shear fabric with a vertical flow host syntectonic syenite pluton which was dated at ca. 2.5–2.4 Ga. A P-T condition of 7 kb/600 °C was inferred for the D2 deformation. The D3 deformation was characterized by NW-SE to E-W trending F3 folds and the Salem-Attur shear zone. The shear zone was a greenschist to amphibolite facies shear zone being characterized by mylonitic foliation and dominantly down-dip stretching lineation defined by quartz, biotite and hornblende minerals and dated at ca. 2.0 Ga. It indicated N-NNE vergence of thrusting with the mean kinematic vorticity number, Wm, as 0.7 suggesting general simple shear strain with 50% pure shear component. The D4 deformation was manifested as NNE-SSW striking strike-slip faults and NW-SE striking extensional normal faults. Pseudotachylite veins having an age of 1.9 Ga injected during strike-slip faulting and granite-pegmatite veins showing age of 0.8–0.5 Ga intruded during normal faulting. The Salem-Namakkal blocks thus recorded a long-lived shearing history. We suggest that the Salem-Attur shear zone and other shear zones such as Palghat-Cauvery, Moyar, Bhavani, Karur-Kambam-Painavu-Trichur and Achankovil shear zones, were Paleoproterozoic intraterrane shear zones which were overprinted by Meso-Neoproterozoic-Cambrian ductile and brittle deformations.

Polyorogenic reworking of ore‐controlling shear zones at the South Range of the Sudbury impact structure: A telltale story from in situ U–Pb titanite geochronology

AbstractThe post‐impact orogenic evolution of the world class Ni–Cu–PGE Sudbury mining camp in Ontario remains poorly understood. New temporal constraints from ore‐controlling, epidote–amphibolite facies shear zones in the heavily mineralised Creighton Mine (Sudbury, South Range) illuminate the complex orogenic history of the Sudbury structure. In situ U–Pb dating of shear‐hosted titanite grains by LA‐ICP‐MS reveals new evidence for shear zone reworking during the Yavapai (ca. 1.77–1.7 Ga), Mazatzalian–Labradorian (1.7–1.6 Ga) and Chieflakian–Pinwarian (1.5–1.4 Ga) accretionary events. The new age data show that the effects of the Penokean orogeny (1.9–1.8 Ga) on the structural architecture of the Sudbury structure have been overestimated. At a regional scale, the new titanite age populations corroborate that the Southern Province of the Canadian Shield documents the same tectonothermal episodes that are recorded along orogenic strike within the accretionary provinces of the Southwestern United States.

Subduction or sagduction? Ambiguity in constraining the origin of ultramafic–mafic bodies in the Archean crust of NW Scotland

The Lewisian Complex of NW Scotland is a fragment of the North Atlantic Craton. It comprises mostly Archean tonalite–trondhjemite–granodiorite (TTG) orthogneisses that were variably metamorphosed and reworked in the late Neoarchean to Paleoproterozoic. Within the granulite facies central region of the mainland Lewisian Complex, discontinuous belts composed of ultramafic–mafic rocks and structurally overlying garnet–biotite gneiss (brown gneiss) are spatially associated with steeply-inclined amphibolite facies shear zones that have been interpreted as terrane boundaries. Interpretation of the primary chemical composition of these rocks is complicated by partial melting and melt loss during granulite facies metamorphism, and contamination with melts derived from the adjacent migmatitic TTG host rocks. Notwithstanding, the composition of the layered ultramafic–mafic rocks is suggestive of a protolith formed by differentiation of tholeiitic magma, where the ultramafic portions of these bodies represent the metamorphosed cumulates and the mafic portions the metamorphosed fractionated liquids. Although the composition of the brown gneiss does not clearly discriminate the protolith, it most likely represents a metamorphosed sedimentary or volcano-sedimentary sequence. For Archean rocks, particularly those metamorphosed to granulite facies, the geochemical characteristics typically used for discrimination of paleotectonic environments are neither strictly appropriate nor clearly diagnostic. Many of the rocks in the Lewisian Complex have ‘arc-like’ trace element signatures. These signatures are interpreted to reflect derivation from hydrated enriched mantle and, in the case of the TTG gneisses, partial melting of amphibolite source rocks containing garnet and a Ti-rich phase, probably rutile. However, it is becoming increasingly recognised that in Archean rocks such signatures may not be unique to a subduction environment but may relate to processes such as delamination and dripping. Consequently, it is unclear whether the Lewisian ultramafic–mafic rocks and brown gneisses represent products of plate margin or intraplate magmatism. Although a subduction-related origin is possible, we propose that an intraplate origin is equally plausible. If the second alternative is correct, the ultramafic–mafic rocks and brown gneisses may represent the remnants of intracratonic greenstone belts that sank into the deep crust due to their density contrast with the underlying partially molten low viscosity TTG orthogneisses.

Kiosk domain, Central Gneiss Belt, Grenville Province, Ontario: A Labradorian palimpsest preserved in the ductile deep crust

The Kiosk domain is a thrust sheet within a multi-component Grenvillian thrust stack, the polycyclic (evidence for pre-Grenvillian orogeny present) Bonfield-Kiosk–Algonquin stack. The stack is overlain to east and west by thrust sheets of predominantly monocyclic gneiss and uplifted along a ramp of Archean parautochthonous rocks. Central to the thrust stack, the Kiosk domain contains a homoclinal southeast-inclined core locally preserving granulite facies assemblages. Predominantly amphibolite facies shear zones, bounding and within the stack, and southeast trending folds overprint the margins of the core fabric. Finally, the Kiosk basal shear zone is locally reworked by extension. Monzonitic orthogneiss in Kiosk domain contain zircon with cores indicating protolith ages of ca. 1640Ma, and dark mantles yield an age of migmatization (and possibly of granulite facies) of ca. 1450Ma. However, metadiabase dykes containing granulite assemblages cut the ca. 1450Ma leucosome and share a southeast trending LS fabric with the country rock of the homoclinal core suggesting the possibility of an early Grenvillian, granulite event. Dating the amphibolite facies overprint is generally challenging, thin zircon rims yielding ages with large errors suggest an Ottawan (ca. 1090–1030Ma) age for stacking, consistent with a 1078±28Ma age for a shear zone which cuts the internal fabric of the stack. The Kiosk domain thus preserves successively younger Grenvillian events from core to rim interpreted to represent early thickening (homoclinal core LS fabric), retrogression-related softening (pegmatites), nappe flow (marginal shears and folds), and post convergence extension (localised shearing).The record of Paleo- early Mesoproterozoic igneous-metamorphic history preserved in the Kiosk core is complemented by detrital zircon data from a quartzite member of a metasedimentary assemblage. The detrital zircon display an age concentration spanning ca. 1500–2000Ma with a Penokean (ca. 1850Ma) maximum. Pre-Labradorian components of the age spectrum are similar to published spectra from the Penokean Upper Animikie sediments from the Lake Superior region and may represent reworking of these during early Pinwarian deposition on the Labradorian basement. We speculate that the Kiosk metasediments, metamorphosed at high grade with their hosts at ca. 1459Ma represent a pre-or early-Pinwarian back-arc (age of youngest detrital grain, ca. 1500Ma) in Labradorian crust inverted in the Pinwarian Orogeny and consistent with A-type plutonism of this age present in the Central Gneiss Belt and interpreted to have been emplaced in extended back-arc crust. The Kiosk domain is thus established as a Labradorian member of the peri-Laurentian Great Paleoproterozoic Accretionary Orogen (Condie, 2013), having received detritus from other members of the accretionary orogen as well as Laurentia.

Thermochronology of extensional orogenic collapse in the deep crust of Zealandia

The exhumed Fiordland sector of Zealandia offers a deep-crustal view into the life cycle of a Cordilleran-type orogen from final magmatic construction to extensional orogenic collapse. We integrate U-Pb thermochronologic data from metamorphic zircon and titanite with structural observations from >2000 km 2 of central Fiordland to document the tempo and thermal evolution of the lower crust during the tectonic transition from arc construction and crustal thickening to crustal thinning and extensional collapse. Data reveal that garnet granulite facies metamorphism and partial melting in the lower crust partially overlapped with crustal thickening and batholith construction during emplacement of the Western Fiordland Orthogneiss (WFO) from 118 to 115 Ma. Metamorphic zircons in metasedimentary rocks yield 206 Pb/ 238 U (sensitive high-resolution ion microprobe–reverse geometry) dates of 116.3–112.0 Ma. Titanite laser ablation split stream inductively coupled plasma–mass spectrometry chronology from the same rocks yielded complex results, with relict Paleozoic 206 Pb/ 238 U dates preserved at the margins of the WFO. Within extensional shear zones that developed in the thermal aureole of the WFO, titanite dates range from 116.2 to 107.6 Ma and have zirconium-in-titanite temperatures of ∼900–750 °C. A minor population of metamorphic zircon rims and titanites in the Doubtful Sound region yield younger dates of 105.6–102.3 Ma with corresponding temperatures of 740–730 °C. Many samples record Cretaceous overdispersed dates with 5–10 m.y. ranges. Core-rim traverses and grain maps show complex chemical and temporal variations that cannot easily be attributed to thermally activated volume diffusion or simple core-rim crystallization. We interpret these Cretaceous titanites not as cooling ages, but rather as recording protracted growth and/or crystallization or recrystallization in response to fluid flow, deformation, and/or metamorphic reactions during the transition from garnet granulite to upper amphibolite facies metamorphism. We propose a thermotectonic model that integrates our results with structural observations. Our data reveal a clear tectonic break at 108–106 Ma that marks a change in processes deep within the arc. Prior to this break, arc construction processes dominated and involved (1) emplacement of mafic to intermediate magmas of the Malaspina and Misty plutons from 118 to 115 Ma, (2) contractional deformation at the roof of the Misty pluton in the Caswell Sound fold-thrust belt from 117 to 113 Ma, and (3) eclogite to garnet granulite facies metamorphism and partial melting over >8 m.y. from 116 to 108 Ma. These processes were accompanied by complex patterns of lower crustal flow involving both horizontal and vertical displacements. After this interval, extensional orogenic collapse initiated along upper amphibolite facies shear zones in the Doubtful Sound shear zone at 108–106 Ma. Zircon and titanite growth and/or crystallization or recrystallization at this time clearly link upper amphibolite facies metamorphism to mylonitic fabrics in shear zones. Our observations are significant in that they reveal the persistence of a hot and weak lower crust for ≥15 m.y. following arc magmatism in central Fiordland. We propose that the existence of a thermally weakened lower crust within the Median Batholith was a key factor in controlling the transition from crustal thickening to crustal thinning and extensional orogenic collapse of the Zealandia Cordillera.

Contrasts in sillimanite deformation in felsic tectonites from anhydrous granulite- and hydrous amphibolite-facies shear zones, western Canadian Shield

The deformation behavior of crustal materials in variably hydrated metamorphic environments can significantly influence the rheological and seismic properties of continental crust. Optical observations and electron backscatter diffraction (EBSD) analyses are used to characterize sillimanite deformation behavior in felsic tectonites from two deformation settings in the Athabasca granulite terrane, western Canadian Shield. Under estimated conditions of 0.8–1.0 GPa, 725–850 °C in the Cora Lake shear zone, the data suggest that sillimanite deformed by dislocation creep with slip in the [001] direction accompanied by subgrain rotation recrystallization. Where sillimanite locally remained undeformed, strain was concentrated in surrounding weaker phases. Under hydrated conditions of 0.4–0.6 GPa, 550–650 °C in the Grease River shear zone, textures and cathodoluminescence imaging point to dissolution-precipitation creep as the major deformation mechanism for sillimanite, resulting in synkinematic growth of foliation-parallel euhedral sillimanite in a preferred orientation with [001] parallel to the lineation. The results suggest that temperature, fluid content, and modal mineralogy of the surrounding phases may all have significant influence on sillimanite deformation but that preferential alignment of sillimanite [001] parallel to the lineation persists regardless of contrasts in the conditions or mechanisms of deformation.

Textural Evolution of Plagioclase Feldspar across a Shear Zone: Implications for Deformation Mechanism and Rock Strength

Caledonian amphibolite-facies shear zones developed in granulitefacies anorthosites and anorthositic gabbros of the Bergen Arcs, western Norway, allow a detailed study of the relationships between fluid-infiltration, mineral reactions, the evolution of microstructure and deformation mechanisms. A sequence of rocks from the relatively pristine granulites into a shear zone has been studied by optical microscopy, electron microprobe microanalysis (EMPA), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM), focusing on the progressive development of microstructure in the plagioclase feldspars, leading up to their deformation in the shear zone. At the outcrop scale, fluid infiltration into the granulites is marked by a distinct colour change in the plagioclase from lilac^brown to white. This is associated with the breakdown of the intermediate composition plagioclase (� An50) in the granulite to a complex intergrowth of Na-rich and Ca-rich domains. EBSD analysis shows that this intergrowth retains the crystallographic orientation of the parent feldspar, but that the Ca-rich domains contain low-angle grain boundaries. Within the shear zone, this complex intergrowth coarsens by grain boundary migration, annihilating these grain boundaries but retaining the Na-rich and Ca-rich zoning pattern. Analysis of nearestneighbour misorientations of feldspar grains in the shear zone demonstrates that local crystallographic preferred orientation (CPO) is inherited from the parent granulite grain orientations. Random pair misorientation angle distributions show that there is no CPO in the shear zone as a whole, nor is there significant shape preferred orientation (SPO) in single grains. These observations are interpreted in terms of fluid-induced weakening and deformation by dissolution^ precipitation (pressure solution) creep.

Structure and timing of Neoarchean gold mineralization in the Southern Cross district (Yilgarn Craton, Western Australia) suggest leading role of late Low-Ca I-type granite intrusions

The formation of structurally controlled lode-gold deposits is a controversial subject with both magmatic and metamorphic fluids considered a feasible source for gold. Here we present a study from the Southern Cross district (Yilgarn Craton, Western Australia), where structurally controlled Neoarchean gold deposits are hosted in amphibolite facies greenstones. We combine geochronological data with structural analysis to constrain the timing and structural control of ore deposition. We show that gold mineralization took place between c. 2638 and 2630 Ma, contemporaneous with the onset of voluminous intrusions of Low-Ca, I-type granites, which are ubiquitous in Southern Cross and elsewhere in the Yilgarn Craton. We argue that the timing of the intrusions, their spatial association with gold deposits, their chemical composition, and isotopic signature are consistent with Low-Ca granites being a major fluid source for mineralization. We propose that the fluid release by cooling plutons was facilitated by transport along active amphibolite facies shear zones. As gold mineralization was synchronous with Low-Ca magmatism across large areas of the Yilgarn Craton, our model may be applicable to orogenic gold deposits in other parts of the Yilgarn Craton as well as other Archean granite-greenstone terrains with a ‘late granite bloom’.

Mid-crustal shear zone formation in granitic rocks: Constraints from quantitative textural and crystallographic preferred orientations analyses

Abstract This paper presents quantitative microstructural and crystallographic preferred orientation (CPO) analyses of an Alpine amphibolite facies shear zone developed in the Fibbia metagranite (Gotthard massif, Central Alps). The weakly deformed metagranite and orthogneiss at the margins of the shear zone are characterized by a bulk strain partitioning between harder coarse-grained monomineralic aggregates, derived from quartz and K-feldspar porphyroclasts, and softer fine-grained plagioclase-bearing shear bands. A characteristic feature is a dilatant fracturing of strong quartz and feldspar aggregates. CPOs and microtextures suggest that quartz and K-feldspar aggregates are dynamically recrystallized via dislocation creep while plagioclases show evidences of fluid-assisted diffusive mass transfer and grain boundary sliding. In the mylonite and ultramylonite shear zone core, the porphyroclasts-derived quartz and K-feldspar layers are broken-down to produce a polyphased matrix that is characterized by a homogeneous micron-scale grain size and regular/random distribution. Here, the deformation of the whole aggregate occurs via a fluid-assisted dissolution–precipitation creep and grain boundary sliding, referred as a fluid-assisted granular flow. We propose a model of shear zone formation associated with the nucleation of shear zone followed by lateral widening of the sheared domain. The lateral broadening of the shear zone is driven by (1) the increase in fluid pressure in permeable albite–oligoclase shear bands that results in expulsion of fluids to the shear zone margins and hydraulic fracturing of strong aggregates, and (2) the thermodynamic re-equilibration via metasomatic reactions of the shear zone walls.

Three‐stage evolution of lower crustal gneiss domes at Breaksea Entrance, Fiordland, New Zealand

AbstractNew structural analyses reveal the presence of several kilometer‐scale gneiss domes and many subdomes within an exhumed section of Cretaceous lower crust in SW Fiordland. The domes are defined by complex, nearly concentric trajectories of foliations and steep high‐strain zones in migmatitic, granulite, and eclogite facies orthogneiss. Structural patterns combined with evidence of deformation that was synchronous with partial melting and high‐grade metamorphism, steep mineral lineation orientations, sense of shear indicators, and near‐isothermal decompression suggest that buoyancy‐driven diapiric flow of hot, partially molten crust aided initial dome formation. Mafic‐intermediate magmatism appears to have enhanced this process. Superimposed on all high‐grade structures are upper amphibolite facies shear zones that record a change in the style and conditions of lower crustal flow. Lineation orientations, sense of shear, evidence of cooling with decompression, and other patterns suggest that the retrograde shear zones record bulk horizontal flow and crustal thinning driven by regional tectonic forces. A preferred sequence begins with the initial emplacement of mafic‐intermediate magma into the lower crust at depths up to 70 km (D1). Diapirism and initial dome formation occurred during a period of waning arc magmatism and episodic partial melting and high‐grade metamorphism (D2). Following melt crystallization, dominantly horizontal flow of a cooler, strain‐hardened lower crust occurred during Late Cretaceous continental extension (D3). This example highlights the interplay between mechanisms of horizontal and vertical flow at the root of a continental arc and illustrates their roles in the formation of gneiss domes.

Evidence for a Caledonian amphibolite to eclogite facies pressure gradient in the Middle Allochthon Lindås Nappe, SW-Norway

The Proterozoic anorthosite–mangerite–charnockite complex dominating the Lindås Nappe in the Scandinavian Caledonides was locally eclogitized in the southwestern part of the nappe during the Caledonian orogeny, whereas only amphibolite facies assemblages are recorded in the rest of the nappe. Sveconorwegian granulites of anorthositic to jotunitic composition in the northernmost eclogite-free exposures of the nappe exhibit large garnet phenoblasts (ca. 900°C) that are fractured and partly replaced by a Caledonian symplectitic amphibolite facies assemblage (ca. 515°C). Metamorphic zircon attributed to this garnet breakdown is dated by ID-TIMS U–Pb at 430 ± 3 Ma, suggesting that the amphibolite facies overprint was coeval with the formation of eclogite 30 km further south, probably implying that the section across the nappe represents a Caledonian pressure gradient. The rocks also preserve a complex Sveconorwegian history including an age of 969 ± 6 Ma, which we interpret as dating magmatic emplacement of jotunitic–anorthositic portions of the complex, 936 ± 12 Ma reflecting the granulite facies metamorphism, and 908 ± 16 Ma, representing a late generation of zircon best explained as having formed by metasomatic processes. Caledonian shearing severely deformed zircon grains in an amphibolite facies shear zone, resetting their U–Pb systems, and forming new ones, hereby also demonstrating a case of resetting and recrystallization of low-U zircon. Our data, gained from diverse lithologies, illustrate several processes involved in making and resetting zircon as well as indicate the contemporaneous evolution and similar origin of the Lindås Nappe and the Jotun Nappe Complex.

Monazite stability, composition and geochronology as tracers of Paleoproterozoic events at the eastern margin of the East European Craton (Taratash complex, Middle Urals)

The Precambrian Taratash complex (Middle Urals) is one of the rare windows into the Palaeoproterozoic and earlier history of the eastern margin of the East European Craton. Monazite from intensively deformed rocks within a major amphibolite-facies shear zone in the Taratash complex has been investigated by means of electron-probe microanalysis and laser-ablation SF-ICP-MS. Metamorphic and magmatic cores of monazite from metasedimentary and metagranitoid rocks yield U–Pb ages of 2244 ± 19 and 2230 ± 22 Ma (± 2 σ) and record a previously unknown pre-deformational HT-metamorphic event in the Taratash complex. Subsequent dissolution–reprecipitation of monazite, during shear zone formation under amphibolite-facies conditions, caused patchy zonation and chemical alteration of the recrystallised monazite domains, leading to higher cheralite and huttonite components. This process, which was mediated by a probable (alkali + OH)-bearing metamorphic fluid also caused a total resetting of the U–Pb-system. The patchy domains yield concordant U–Pb-ages between 2052 ± 16 and 2066 ± 22 Ma, interpreted as the age of the shear zone. In line with previously published ages of high grade metamorphism and migmatisation, the data may point to a Palaeoproterozoic orogenic event at the eastern margin of the East European Craton. Post-deformational fluid-induced greenschist-facies retrogression caused partial to complete breakdown of monazite to fluorapatite, REE + Y-rich epidote, allanite and Th-orthosilicate.The retrograde assemblages either form coronas around monazite, or occur as dispersed reaction zones, indicating that the REE, Y, and Th were mobile at least on the thin section scale. The greenschist-facies metamorphic fluid was aqueous and rich in Ca. Monazite affected by advanced breakdown responded to the retrogression by incorporating the cheralite or huttonite components during a fluid-induced dissolution–reprecipitation process. This event did not reset the U–Pb-system but caused partial Pb loss reflected by discordant U–Pb-dates.

Using zircon U–Pb ages and trace element chemistry to constrain the timing of metamorphic events, pegmatite dike emplacement, and shearing in the southern Parry Sound domain, Grenville Province, Canada

The integration of zircon U–Pb ages and trace element chemistry with structural and petrologic relations from a range of sample types provides important temporal constraints on the tectono-metamorphic evolution of the southern Parry Sound domain (PSD), Ontario, Canada, and the processes attending development of the underlying Twelve Mile Bay shear zone (TMBSZ). Intact granulites preserve ca. 1145 Ma ages, slightly younger than those farther to the north in the interior PSD, but similar to that of the underlying granulite-to-amphibolite facies Parry Sound shear zone. Weaker zircon HREE enrichment in sheared and retrogressed samples with partially resorbed garnet porphyroblasts (containing abundant zircon inclusions) suggests that the 1128–1143 Ma ages common to sheared rocks in the southern PSD record an earlier phase of metamorphism and deformation, and not the timing of shear zone development. Deformed pegmatite dikes bounded by sheared and retrogressed wall rocks from across the southern interior PSD consistently record ca. 1100 Ma ages, indicating synchronous pegmatite emplacement across the transect and constraining the bounding amphibolite-facies shear zones to ≤1100 Ma. TMBSZ samples retain evidence for (1) the older ( ca. 1145 Ma) high-grade metamorphic event, (2) pegmatite emplacement and shear deformation at ca. 1100 Ma, and (3) a later ( ca. 1070 Ma) shearing event. Combined with published structural and petrologic data, these ages indicate that pegmatite emplacement and shearing in the TMBSZ was synchronous with that in the southern interior PSD; further confirming published models for TMBSZ development. Additionally, the data suggest that the Twelve Mile Bay assemblage (within the TMBSZ) experienced high-grade metamorphism synchronously with the interior PSD, and may therefore be correlative with the basal PSD to the north.

Quartz microstructures and crystallographic preferred orientation: Which shear sense do they indicate?

Crystallographic preferred orientation (CPO) and shape fabrics of dynamically recrystallized quartz are currently used as shear sense indicators. We show that the CPO and shape fabric in a polyphase rock do not necessarily indicate the shear sense at the global scale of the shear zone but rather at the local scale of the deforming and recrystallizing quartz aggregates. In lower amphibolite facies shear zones in the Gran Paradiso metagranodiorite, magmatic quartz grains have recrystallized dynamically by subgrain rotation and grain boundary migration and deform inside a very fine-grained feldspar–mica matrix. The quartz CPO has a peripheral [c]-axis maximum inclined synthetically with the local shear sense. The orientation of the surface fabric is related to the orientation of the [c]-axis maximum and the microscopic shear sense in the quartz aggregate. The geometry of the surface fabric ranges from monoclinic to symmetric depending on the relative contribution of grain boundary migration. It is inferred that flow partitioning between the quartz aggregates and the matrix controls the local kinematics. CPOs are only reliable shear sense indicators in polyphase rocks if the most highly strained parts are analyzed where spin of the aggregates with respect to the shear zone boundary has ceased.

Lu–Hf garnet geochronology applied to plate boundary zones: Insights from the (U)HP terrane exhumed within the Woodlark Rift

High-pressure and ultra high-pressure (U)HP metamorphic rocks occur in many of the world's major orogenic belts, suggesting that subduction of continental lithosphere is a geologically important process. Despite the widespread occurrence of these rocks, relatively little is known about the timescales associated with (U)HP metamorphism. This is because most (U)HP terranes are tectonically overprinted and juxtaposed against rocks with a different history. An exception to this are the Late Miocene (U)HP metamorphic rocks found in active metamorphic core complexes (MCC) in the Woodlark Rift of southeastern Papua New Guinea. This region provides a rare opportunity to study the garnet Lu–Hf isotopic record of (U)HP metamorphism in a terrane that is not tectonically overprinted. In order to constrain the timing of garnet growth relative to the history of (U)HP metamorphism and the evolution of the Woodlark Rift, Lu–Hf ages were determined, in conjunction with measurements of Lu and major element zoning, for garnets from three metamorphic rocks. Garnets from the three samples yielded different ages that, instead of recording the spatial and temporal evolution associated with a single metamorphic event, provide information on the timing of three separate plate boundary events. The youngest Lu–Hf age determined was 7.1 ± 0.7 Ma for garnets in a Late Miocene coesite eclogite. The age is interpreted to record the time when a garnet-bearing partial melt of the mantle crystallized within subducted continental lithosphere at (U)HP conditions. The young Lu–Hf age from the coesite eclogite is in contrast to a 68 ± 3.6 Ma Lu–Hf age obtained on large (1–2 cm) garnet porphyroblasts, from within the Pleistocene amphibolite facies shear zone carapace bounding exposures of (U)HP rocks in the D'Entrecasteaux Islands. This older age records the growth of garnet in response to continental subduction and ophiolite obduction in the region north and east of Australia during late Mesozoic/early Cenozoic time. In order to relate the (U)HP metamorphic rocks to the larger-scale tectonic evolution of the Woodlark Rift, a third Lu–Hf age of 11.2 ± 2.1 Ma was determined for dynamically recrystallized garnet from the southeastern margin of the rift. While the tectonic implications of the Lu–Hf age from the southeastern-rifted margin are not fully known, this result suggests that the eastern and western parts of the Woodlark Rift might have a different metamorphic history. The garnet Lu–Hf results demonstrate that prior to rifting, the Woodlark Rift underwent a complex history of metamorphism involving garnet growth and recrystallization. The ability of the Lu–Hf system to date Late Miocene garnet growth, and at the same time preserve Latest Cretaceous garnets within Pleistocene shear zones in the Woodlark Rift demonstrate that Lu–Hf garnet geochronology is an essential component of investigations where the primary objective is documenting and understanding the full range of plate boundary processes recorded in metamorphic minerals.

Quartz grain size reduction in a granitoid rock and the transition from dislocation to diffusion creep

In the Gran Paradiso metagranodiorite (Western Alps) small scale lower amphibolite facies shear zones record the transition from a mylonite composed of polycrystalline mineral aggregates to a homogeneous ultramylonite with a grain scale phase mixture. Polycrystalline quartz aggregates in the mylonite deform by dislocation creep developing a crystallographic preferred orientation (CPO) and a monoclinic surface orientation distribution function (ODF). The polymineralic matrix of the mylonite and the ultramylonite deform by diffusion creep. In the ultramylonite the quartz CPO is randomized and the surface ODF becomes orthorhombic. The transition from mylonite to ultramylonite is accompanied by a grain size decrease and a disintegration of quartz aggregates, concomitant with the precipitation of K-feldspar (±biotite) between quartz grains. In quartz, reduction from the dynamically recrystallized grain size in the aggregates (110 μm) to the size of the dispersed grains in the ultramylonite (25 μm) occurs through the following processes: K-feldspar precipitates at opening sites along grain boundaries (strain incompatibility) pinning the grain size in quartz aggregates. Coalescence of K-feldspar leads to enhanced grain boundary sliding and disintegration of the quartz aggregates. Solution precipitation reduces the size of the dispersed grains to less than subgrain size (∼45 μm).

Initiation and development of the Twelve Mile Bay shear zone: the low viscosity sole of a granulite nappe

Structural, microstructural and petrological data have enabled determination of the mechanical and geochemical processes involved in dynamic weakening and fabric transposition along the margins of a granulite nappe (the Parry Sound domain (PSD)) during transport to mid-crustal levels of the Grenville Orogen. The data establish a genetic link between outcrop-scale structures in the southern PSD and the development of the underlying Twelve Mile Bay shear zone (TMBSZ). Following granulite facies metamorphism (� 11 kbar ⁄ � 850 � C) in the southern PSD, the emplacement of pegmatite dykes resulted in hydration reactions within adjacent wall rocks and the development of thin (<1 m) amphibolite facies (� 6.5 kbar ⁄ � 700 � C) shear zones. The shear zones exhibit bulk H2O and K2O enrichment and oxygen isotope values similar to the adjacent pegmatites, suggesting metasomatic alteration by pegmatite-derived fluids. Phase-equilibrium models indicate that the destabilization of the pre-existing pyroxene and garnet-bearing assemblages, as observed within discrete shear zones in the southern PSD and the TMBSZ, requires H2O-saturated conditions at these (amphibolite facies) P-T conditions. The spacing between discrete shear zones and the depth of hydration into the adjacent wall rock are of comparable length-scales (� metres), suggesting that this type of reworking process can be an effective means of hydrating kilometre-scale areas of crust relatively rapidly. Furthermore, considering the well- established effects of hydrous fluids on the creep strength of anhydrous minerals, a fracture-initiated, localized hydration-and-shearing process may be an efficient mechanism for weakening strong, dry rocks (e.g. granulites) in the middle to lower orogenic crust.