Leucocratic Veins Research Articles

Contrasting melt percolation and melt-rock reactions in the Parece Vela back-arc oceanic lithosphere, Philippine Sea: A mineralogical perspective

The accretion and evolution of oceanic lithosphere beneath back-arc spreading ridges remain less understood compared with mid-oceanic ridges (MOR). Here, we investigate mantle peridotites and gabbros recovered from the southernmost side of the Parece Vela back-arc basin, Philippine Sea. The peridotites contain multiple types of clinopyroxenes (Cpx) and amphiboles (Amp) which record contrasting melt percolation and melt-rock reaction processes. Type-1 Cpx shows primary texture and light rare-earth elements (LREE)-depleted pattern, which can be reproduced by ~8–10% fractional melting of depleted MORB mantle. Type-2 Cpx occurs in the deformed peridotites (SY224-1280-2a & -2b) and shows less depleted LREE at similar middle to heavy REE (M-HREE) contents compared with type-1 Cpx. Such features reflect re-equilibration with MORB-type percolating melts, which is further evidenced by elevated TiO 2 and Cr# in coexisting spinels and crystallization of associated MORB-type gabbros. Type-3 Cpx occurs in the undeformed peridotites (SY225-1255-1 & -2) and is texturally replaced by type-1 Amp. They show variably enriched large ion lithophile elements (LILE) and LREE at similar M-HREE contents compared with type-1 Cpx, which reflect an open-system (flux) melting and reaction process with a melt flux enriched in LILE and LREE. Type-1 Amp is almost magnesio-hornblende and shows trace-element pattern characteristic of supra-subduction amphiboles, which supports the hydrous melt that formed type-3 Cpx probably originated from subducted slab. In contrast, type-2 Amp occurs in a leucocratic vein in sample SY225-1255-1 and is characterized by nearly flat M-HREE pattern at 600–200 × CI, LREE depletion and marked negative Eu anomaly. These features are consistent with crystallization from an amphibole-saturated melt formed by extreme differentiation of MORB-type melts. Our data, together with previous observations, suggest co-occurrence of hydrous MORB-type and slab-derived melt percolations beneath the Parece Vela spreading ridge, which may affect the chemical evolution of back-arc oceanic lithosphere different from MOR. • Multiple types of clinopyroxenes and amphiboles occur in the PVB peridotites. • They reflect open-system melting and reaction with hydrous MORB and slab melts. • Hydrous open-system mantle processes are features of back-arc oceanic lithosphere.

Late Neoproterozoic extended continental margin development recorded by the Seve Nappe Complex of the northern Scandinavian Caledonides

The Scandinavian Caledonides comprise nappe stacks of far-travelled allochthons that record the opening and closure of the Iapetus Ocean, culminating with the collision of Baltica and Laurentia. The Seve Nappe Complex (SNC) of the Scandinavian Caledonides comprises relics of the outermost Baltoscandian passive margin that were subducted to mantle depths during ocean closure. Subduction of these rocks overprinted much of the Neoproterozoic record for Iapetus Ocean formation, and as a result, much of the work conducted in the SNC has focused solely on the Caledonian orogenic history.In this study, we combined petrological and geochronological work to expand the knowledge about the Neoproterozoic metamorphic history of the Baltoscandian margin in the understudied Váivančohkka-Salmmečohkat region of the northern Scandinavian Caledonides. The work focused on rocks that belong to the upper gneiss unit, which constitutes part of SNC in the region. The unit comprises migmatitic paragneisses and garnet-mica schist containing metamafic bodies. U-Pb zircon and Th-U-total Pb monazite dating of the migmatitic paragneiss yielded consistent age of metamorphism in 602 ± 5 Ma and 599 ± 3 Ma, respectively. A similar U-Pb age of 604 ± 8 Ma was obtained for the zircon from the leucocratic vein transecting the amphibolite within the studied gneiss. Interestingly, no Caledonian ages were identified. Likewise, no evidence for high or ultra-high pressure conditions was found, neither in the gneisses/schists nor in the metamafic rocks. Petrographic observations and calculated metamorphic P-T conditions indicate that rocks belonging to upper gneiss unit underwent upper-amphibolite facies metamorphism in a melt stability field; 8.0–10.5 kbar at 750–790 °C.Since the studied rocks underwent high-grade metamorphism in the Ediacaran and lack obvious evidence for Caledonian high-pressure metamorphism, the studied part of the SNC offers an extraordinary insight into the Late Neoproterozoic history of the hot, extended Baltoscandian margin.

Partial melting due to breakdown of phengite and amphibole in retrogressed eclogite of deep Precambrian crust: An example from the Algonquin terrane, western Grenville Province, Canada

Characterizing partial melting of eclogite in Precambrian orogens is critical to understanding the petrological processes that operate in the deep orogenic crust. Retrogressed eclogite from the Algonquin terrane in the western Grenville Province, Canada, was taken as an example to study such processes. The studied samples contained the assemblage of garnet, omphacite, amphibole, phengite, biotite, and rutile at peak pressure. Various exhumation- and melt-related textures formed show evidence for a complex post-peak pressure history. Rutile is only found in the core of garnet, whereas ilmenite-bearing assemblages predominate in the garnet rim and the matrix. Symplectite of diopside + plagioclase + amphibole replaced former omphacite. Symplectite of amphibole and plagioclase formed as a corona around garnet. Polymineralic inclusions of plagioclase + K-feldspar ± ilmenite ± biotite ± amphibole ± apatite ± sulfides in garnet crystallized from melt, which was captured during garnet growth and reacted with the host. Cusps of plagioclase into the boundaries of other phases (e.g. garnet and clinopyroxene) mimic the original melt-solid interface. Neoblasts of garnet with euhedral crystal faces were formed as a peritectic phase during eclogite melting. Leucocratic veins without external connection, present in the sampled mafic pod at outcrop scale, are interpreted as crystallized pockets of melt which derived internally. Pseudosection modeling of the studied samples along with compositional isopleths of chemically zoned garnet yielded an exhumation path from 1.7 ± 0.1 GPa, 680 ± 50 °C to 1.3 ± 0.1 GPa, 920 ± 50 °C. Along this path, the modal content of melt increased in response to the breakdown of phengite and amphibole along with involvement of omphacite and rutile. The melt crystallized during cooling and equilibrated at 0.6–0.8 GPa, 710–750 °C, estimated using empirical thermobarometry. This study shows that metabasite from Proterozoic collisional orogens can be partially melted on the exhumation and heating path through phengite- and amphibole-dehydration melting.

Petrographic, structural and textural analysis of the Atuba Complex, Southern Ribeira Belt: Case study in the Greca quarry

The Greca quarry is located in the Colombo municipality, close to the Curitiba Shear Zone, south of the Lancinha Shear Zone in the State of Paraná. It is inserted in the Atuba Complex (Curitiba Terrane) and comprises the meridional portion of the Ribeira Belt. This work was carried out by means of field work, petrography, electron backscattered diffraction analysis and strain analysis using the Fry, Polar and Rf/φ methods. Thus, it aims to analyze meso and micro scale strain in tectonites in a key area of the Atuba Complex. The heterogeneous ductile strain observed at a microscopic and macroscopic scale resulted in stromatic, protomylonitic and mylonitic metatexites, of submillimetric to metric widths. The occurrence of quartz and leucocratic veins of centimetric to metric thickness is rare, either discordant or concordant to the foliation of migmatites. Two deformation phases were found, both derived from ductile shear: the first is associated with the thrusting tectonics (Dn-1); and the second has a sinistral transcurrent-transpressive nature (Dn). The presence of new quartz grains observed by subgrain rotation (SGR) recrystallisation and new grains by bulging (BLG) recrystallisation in the feldspar crystals indicates temperatures above 450 °C (lower amphibolite facies) during the shear of both phases. The strain analysis was generated based on the investigation of quartz crystals of 2 samples and 4 thin sections (XZ and YZ planes). The analyzes indicated approximate strain ratios (R) for all methods used in the XZ and YZ planes. Quartz presented values between 2.250 and 1.304 (XZ section) and 2.237 to 1.110 (YZ section). The ellipsoids are mainly oblate and are equivalent to the apparent flattening group. The tectonites are predominantly of S or SL type. According to the EBSD data, it was possible to identify that the strain occurred by dislocation creep mechanisms, associated with the dynamic recrystallisation processes. The samples analyzed by EBSD indicated that the feldspar crystals have poles (010) in the <001> direction and the quartz crystals show the activation of the basal sliding system with the aid of the rhombohedral system. The results also indicate low temperatures, probably due to the retrometamorphic event that occurred in the last strain stage of the Atuba Complex.

Timing of deformation along theTan‐LuFault Zone in eastern China: Constraints from zircon U–Pb geochronology of the Malongshan Shear Zone

The Tan‐Lu Fault Zone is a major fault system in eastern China, and its timing of activity has been the focus of much research. We studied a ductile segment of the Tan‐Lu Fault Zone, the Malongshan Shear Zone, which is located in the Feidong Complex (the southern part of the Zhangbaling Uplift). A dark biotite‐adamellite mylonite has been recognized in the Malongshan Shear Zone, and the mylonitic foliation strikes NNE, consistent with the regional orientation of the Tan‐Lu Fault Zone. The occurrence of abundant syn‐tectonic leucocratic veins within the biotite‐adamellite mylonite indicates that ductile shearing was accompanied by hydrothermal activity. To constrain the timing of deformation along the Malongshan Shear Zone, we dated the biotite‐adamellite mylonite and the leucocratic veins using zircon U–Pb geochronology. The weighted mean ages of zircon grains from the biotite‐adamellite mylonite (159.0 ± 5.5 Ma) and the captured Group 2 (156.3 ± 3.7 Ma and 173.2 ± 4.2 Ma) of leucocratic veins are interpreted as the ages of protoliths of the biotite‐adamellite mylonite during the Jurassic. Additional weighted mean age of the hydrothermal zircon grains (130.9 ± 3.6 Ma) represents the timing of crystallization of the veins. Therefore, the results indicate that the Malongshan Shear Zone (and hence the southern segment of the Tan‐Lu Fault Zone) was active during the Early Cretaceous (~131 Ma). Based on the present results and existing data, we consider that sinistral shearing of the Tan‐Lu Fault Zone during the Early Cretaceous was controlled by oblique subduction of the Pacific Plate at a high angle.

Significance of viscous folding in the migmatites of Chotanagpur Granite Gneiss Complex, eastern India

ABSTRACTTo understand the physico-chemical processes associated with migmatisation is an interesting petrological problem. New developments in microfluidics and chaotic mixing experiments have helped us to better perceive these processes from the migmatic rocks of the Proterozoic Chotanagpur Granite Gneiss Complex (CGGC), eastern India. The migmatic rocks of CGGC have preserved folded leucocratic veins in amphibolites representing viscous folding. The viscous folding phenomenon occurred due to the interaction between leucosome and melanosome. Based on textural features and mineral chemical data interpretations, we infer that when granitic and pegmatitic magmas intruded the gneissic rocks and amphibolites of our study area, diffusion of heat and volatiles from the hotter felsic magmas to the colder country rocks initiated partial melting in the amphibolites, forming melanosomes. After their formation, the highly viscous felsic magmas veined into the melanosomes, by progressively melting them and then interacting, leading to chaotic mixing dynamics. The development of chaotic mixing allowed the leucosome to venture into the melanosome as veins by stretching and folding dynamics. As the leucocratic veins or leucosome traversed through the partially molten rock or melanosome due to advection, the veins underwent viscous folding owing to the exertion of compressional stress brought about by the viscosity difference between the two mediums. The occurrence of viscous folding exponentially increased the contact area between the leucosome and the melanosome, eventually leading to enhanced diffusion and augmented mixing between the two mediums. Evidence of mixing through elemental diffusion is well documented by the compositions of amphibole and biotite occurring in the leucosome and melanosome. These minerals show substitution of magnesium and ferrous ion that show linear variation between the endmember compositions.

Early Neoproterozoic assembly of the Yangtze Block decoded from metasedimentary rocks of the Miaowan Complex

The present Yangtze Block was proposed to be formed as a unified block in the Mesoproterozoic-Neoproterozoic transition, but the detailed timing and process are poorly constrained. Here we report integrated in situ U–Pb and Hf–O isotopic analyses of detrital zircons from the Xiaoxikou metamorphosed sedimentary rocks of the Miaowan Complex in the Yangtze Block, as well as U–Th–Pb and Nd–O isotopic analyses of magmatic monazites from an undeformed and unmetamorphosed leucocratic vein intruding these metasedimentary rocks. U–Pb dating results for detrital zircons from the metasedimentary rocks display two main age peaks at ca 970 and 900 Ma. The 970 Ma zircons are dominantly characterized by mantle-like δ18Ozir values (4.5–5.8‰) and highly depleted εHf(t) values (+5 to + 14). The 900 Ma zircons mostly have O and Hf isotopic features similar to the 970 Ma zircons, except relative small proportion of grains with high δ18O values (6.0–9.2‰) and variable εHf(t) values (–33 to + 7). Monazites from the vein are dated at ca 850 Ma and have uniform δ18O values (5.4–6.6‰), but significantly enriched εNd(t) values (–14.5 to –12.6). Based on the new isotopic results and immature lithology, the Xiaoxikou metasedimentary rocks are interpreted to be deposited between 900 and 850 Ma in a fore-arc basin, and clastic materials derived from ca 970–900 Ma arc rocks were related to the northward subduction (present-day coordinates) of the Miaowan oceanic plate. The 850 Ma leucocratic vein shows geological and isotopic characteristics consistent with local 860–840 Ma post-tectonic granitic plutons, and may represent a fractionated product of local coeval granitic magmas. We propose that the closure of the Miaowan oceanic basin and formation of the current unified Yangtze Block occurred between 900 and 860 Ma, i.e., later than the deposition of the fore-arc metasedimentary rocks while earlier than the emplacement of the post-tectonic plutons.

Reconciliation of discrepant U–Pb, Lu–Hf, Sm–Nd, Ar–Ar and U–Th/He dates in an amphibolite from the Cathaysia Block in Southern China

We present a combined garnet Lu–Hf and Sm–Nd, zircon and monazite U–Pb, hornblende Ar–Ar, and zircon U–Th/He geochronology study on a garnet-bearing amphibolite from the Cathaysia Block in Southern China. Pseudosection modeling and garnet–clinopyroxene thermometry indicate that the amphibolite attained peak metamorphic conditions of approximately 0.9 GPa and 780 °C. The Lu–Hf garnet date of 402.7 ± 1.0 Ma and Sm–Nd date of 331.6 ± 3.6 Ma define a ~ 70 Myr difference for the same garnet fractions. Reconciling the large discrepancies between these three geochronological systems with the observed major element diffusion profiles in garnet requires that REEs and Hf were fully re-equilibrated at peak temperatures and that subsequent rapid cooling prevented any disruption of the Lu–Hf or Sm–Nd systems throughout cooling from high temperatures. The Sm–Nd garnet age was partially reset during a thermal pulse associated with intrusion of leucocratic veins, which is inferred to have occurred at ca. 226 Ma based on the coinciding hornblende Ar–Ar (222.3 ± 0.9 Ma) and monazite U–Pb (226.2 ± 2.3 Ma) dates. The T–t path of a reheating event at 226 Ma necessary to produce both the major divalent cation diffusion profiles in garnet and partial resetting of the Sm–Nd garnet age, such that it records an apparent age of ~ 332 Ma, requires an effective diffusion radius of 45–60 μm in garnet during reheating, which is in agreement with the observation of pervasive chlorite-filled fractures within single garnet grains. Diffusion kinetics of Lu and Hf in garnet are sufficiently slow that this reheating episode would not impact the Lu–Hf system, and the 402.7 ± 1.0 Ma garnet age likely reflects the onset of rapid cooling from peak P–T conditions. The zircon U–Pb date of 434.8 ± 2.7 Ma predates the Lu–Hf garnet date by approximately 30 Myr and is interpreted to reflect zircon crystallization during prograde metamorphism. The ~ 226 Ma reheating event is contemporaneous with the intrusion and crystallization of the widespread Triassic A-type granites and alkaline syenites across the Cathaysia Block, indicating an extensional environment. We interpret the zircon U–Th/He date of 143.1 ± 4.2 Ma as reflecting a third thermal pulse based on the concurrence with the early Cretaceous volcanic rocks and granitoids, which overlies the amphibolite.

The Origin of Garnets in Anatectic Rocks from the Eastern Himalayan Syntaxis, Southeastern Tibet: Constraints from Major and Trace Element Zoning and Phase Equilibrium Relationships

AbstractAmphibolite- and granulite-facies metamorphic rocks are common in the eastern Himalayan syntaxis of southeastern Tibet. These rocks are composed mainly of gneiss, amphibolite and schist that underwent various degrees of migmatization to produce leucogranites, pegmatites and felsic veins. Zircon U–Pb dating of biotite gneiss, leucocratic vein and vein granite from the syntaxis yields consistent ages of ∼49 Ma, indicating crustal anatexis during continental collision between India and Asia. Garnets in these rocks are categorized into peritecitc and anatectic varieties based on their mode of occurrence, mineral inclusions and major- and trace-element zoning. The peritectic garnets mainly occur in the biotite gneiss (mesosome layer) and leucocratic veins. They are anhedral and contain abundant mineral inclusions such as high-Ti biotites and quartz, and show almost homogeneous major-element compositions (except Ca) and decreasing HREE contents from core to rim, indicating growth during the P- and T-increasing anatexis. Peak anatectic conditions at 760–800°C and 9–10·5 kbar are well constrained by phase equilibrium calculations, mineral assemblages, and garnet isopleths. In contrast, anatectic garnets only occur in the vein granite. They are round or subhedral, contain quartz inclusions, and exhibit increasing spessartine and trace-element contents from core to rim. The garnet–biotite geothermometry and the garnet–biotite–plagioclase–quartz geobarometry suggest that the anatectic garnets crystallized at ∼620–650°C and 4–5 kbar. Some garnet grains show two-stage zoning in major and trace elements, with the core similar to the peritectic garnet but the rim similar to the anatectic garnet. Mineralogy, whole-rock major- and trace-element compositions and zircon O isotopes indicate that the two types of leucosomes were produced by hydration (water-present) melting and dehydration (water-absent) melting, respectively. The leucocratic veins contain peritectic garnet but no K-feldspar, have lower whole-rock K2O contents and Rb/Sr ratios, higher whole-rock CaO contents and Sr/Ba ratios, and show homogeneous δ18O values that are lower than those of relict zircons, indicating that such veins were produced by the hydration melting. In contrast, the vein granite contains peritectic garnet and K-feldspar, has higher whole-rock K2O contents and Rb/Sr ratios, lower whole-rock CaO contents and Sr/Ba ratios, and shows comparable δ18O values with those of relict zircons, suggesting that this granite were generated by the dehydration melting. Accordingly, both hydration and dehydration melting mechanisms have occurred in the eastern Himalayan syntaxis.

Complexity of characterizing granitoids in high-grade terranes: An example from the Neoarchean Verbaard granitoid, Limpopo Complex, Southern Africa

Identifying the spatial distribution or extent of individual granitoids in Archean high-grade terranes can be complicated, with the effect of overprint events resulting in localized varieties. The case study presented here highlights this issue from the Limpopo Complex high-grade terrane in southern Africa. The Beit Bridge Complex segment preserves a variety of temporally and spatially associated Neoarchean granitoids. Of the different granitoids, the ~2.65–2.62 Ga biotite-bearing Verbaard granitoid is characterized by a conspicuous network of garnet-bearing leucocratic veins. Unlike these leucocratic veins with more or less straight line outline, garnet-bearing leucosomes with irregular outline are associated with surrounding migmatitic metapelites and have biotite-bearing melanocratic margins. Garnet from the Verbaard granitoid and migmatitic metapelite have comparable mineral chemical characteristics. The most common variety is characterized by low XCaGrt = 0.03–0.04 and the less common one has higher XCaGrt (0.05–0.06: granitoid; 0.08: migmatitic metapelite). No rim-core-rim zoning is observed in garnets from the host Verbaard granitoid and metapelite. However, garnets in the leucocratic veins and leucosomes exhibit distinct zoning with a decrease of XCa from 0.08 to 0.03. Phase equilibria modeling using PERPLE_X software reveals that such variation in grossular content of garnets reflects decompression from ~9.5 to ~6.5 kbar at 760–800 °C for both rocks. During decompression, melts were segregated trapping garnet including the high XCa variety. Both the Verbaard granitoid and migmatitic metapelite were affected by the segregated melts. Difference in composition between the modeled melts and leucosome/leucocratic vein is accounted in terms of different degree of a melt loss. Whole-rock geochemical characteristics of garnet- and leucocratic vein-free domains of the Verbaard granitoid indicate that they are similar to medium-pressure TTGs derived from the melting of metabasalts and are comparable to those of the Alldays granitoid. Thus, the Verbaard granitoid represents a local manifestation of the voluminous (widespread) Alldays granitoid magmatism in the specific (Verbaard) area.

Continuous zircon growth during long-lived granulite facies metamorphism: a microtextural, U–Pb, Lu–Hf and trace element study of Caledonian rocks from the Arctic

Microtextural, U–Pb, trace element and Lu–Hf analyses of zircons from gneisses dredged from the Chukchi Borderland indicate a long-lived, Cambrian–Ordovician, granulite facies metamorphism. These results reveal a complete prograde, peak and cooling history of zircon growth during anatexis. Early increasing temperatures caused modification and Pb-loss of Precambrian zircons by recrystallization and dissolution/re-precipitation of existing grains. Small variations in initial 176Hf/177Hf results (0.282325–0.282042) and flat HREE patterns of these zircons indicate that they grew by dissolution/re-precipitation in the presence of garnet. Zircons subsequently crystallized from a partial melt during peak to post-peak metamorphism from 530 to 485 Ma. A broad range of initial 176Hf/177Hf ratios (0.282693–0.282050) and mineral inclusions within zircons suggest that this phase of growth incorporated Zr and Hf obtained from the breakdown of Zr-enriched phases. Microtextural evidence along with trace element and isotopic data suggests that final growth of metamorphic rims on zircon occurred during slow cooling and crystallization of residual partial melts during the early Ordovician (485–470 Ma). Younger, late Ordovician–Silurian (420–450 Ma) euhedral, oscillatory-zoned, trace element-enriched zircons crystallized within leucocratic veins that intrude the gneisses. Their age corresponds to granitoids dated from this same dredge. The intrusives and veins provide evidence that the Chukchi Borderland rifted from a position near Pearya and northwest Svalbard, which represent the northern continuation of the Caledonian orogen. Evidence for earlier Cambrian metamorphism has not been reported from this region. The age of granulite facies metamorphism reported here represents the earliest phase of deformation in the Arctic Caledonides.

Valle d'Aosta section of the Sesia Zone: multi-stage HP metamorphism and assembly of a rifted continental margin

The spectrum characteristic of the EMC ranges from eclogites (containing omphacite and/or jadeite, garnet, phengite, glaucophane, zoisite, chloritoid, rutile) to phengite schists, calcschists, and marbles, as well as a variety of orthogneisses. Despite the intense polyphase deformation and HP-metamorphic recrystallization, it is possible in some locations to recognize pre-Alpine characteristics in some of the protoliths. For instance, two types of felsic orthogneiss can be distinguished in the Aosta Valley, one derived from Permian granitoids (with local preservation of intrusive contacts, magmatic inclusions, leucocratic veins and other magmatic structures; Stop 3), the other derived from pre-Variscan leuco-monzogranite, such as the building stone mined at the “Argentera” quarry near Settimo Vittone / Montestrutto (Stop 2; so-called “Verde Argento” contains jadeite, phengite, K-feldspar, quartz). Polycyclic and more rarely monocyclic metasediments contain evidence of a complex Alpine PTDt-evolution, locally including relics of their prograde history from blueschist, one or more stages at eclogite facies. Recent petrochronological studies have dated this HP-evolution of the Sesia Zone in some detail. In the area visited, clear evidence of HP-cycling has been identified in one km-size tectonic slice (Stop 1), but not in adjacent parts of the EMC, indicating “yo-yo tectonics”. Partial retrogression and attendant ductile to brittle deformation of the HP-rocks is evident in one of the outcrops (Stop 4). Apart from the four localities in the Sesia Zone, a final outcrop introduces HP-rocks of the adjacent Piemonte oceanic unit, specifically calc-schists and ophiolite members of the “Zermatt-Saas” zone. The hilltop outcrop (Stop 5) displays foliated antigorite schist with peridotite relics (clinopyroxene, spinel) containing lenses derived from doleritic dykes. These fine-grained metarodingites and the folded veins containing Mg-chlorite and titanoclinohumite within serpentinite once again indicate equilibration under low-temperature eclogite facies conditions. However, these units reached that HP stage more than 20 Ma after the youngest eclogite facies imprint recognized in the Sesia Zone. Despite nearly half a century of intense study in the Sesia Zone, the complex assembly of its HP-terranes and their relation to more external parts of the Western Alps remains incompletely understood. This field guide merely introduces a few of the classic outcrops and discusses some of the critical evidence they contain, but it could not incorporate details on each stage of the evolution recognized so far.

Late magmatic healed fractures in granitoids and their influence on subsequent solid-state deformation

Late magmatic fractures developed during final stages of magma crystallization are recorded in many granitoid plutons by planar trails of magmatic biotite and/or leucocratic veins but have not been previously described. Such biotite-rich trails, with lengths of a few centimetres to several tens of centimetres, widths of a few millimetres, and a variable orientation, are ubiquitous in meta-granitoids of the Neves area (Tauern Window, Eastern Alps, Italy). These pre-Alpine granitoids were metamorphosed to upper amphibolite facies and deformed during the Alpine orogeny, allowing the effect of planar healed magmatic fractures on the nucleation of subsequent solid-state ductile shear zones to be directly assessed. Numerical models considering power-law viscous materials predict that planar arrays of a weaker mineral (e.g. biotite) oriented at around 45° to the shortening direction should nucleate and localize viscous deformation. However, no discernible localization on the natural small-scale biotite trails is observed, even when they delineate planes that were well-oriented for shear reactivation. In contrast, heterogeneous shear zones nucleated on larger scale (>1–10 m long) planar compositional or structural boundaries such as joints, dykes, quartz veins, alteration layers surrounding veins, and zones of magma mingling outlined by densely packed clusters of basic enclaves, irrespective of their orientation relative to the imposed shortening direction. Clearly some crucial aspect is missing from purely viscous numerical models. We propose that this missing aspect is the observed interplay between fracture and flow, with new fractures developing and localizing shear at bulk strains too low for discernible localization to occur on the pre-existing healed magmatic fractures. Natural granitoids have a truly elasto-plasto-viscous rheology even at low differential stress (as in the Neves example) and for high grade metamorphic conditions considered as typical of wet “ductile” middle crust.

SHRIMP data on zircons from the Wanzi series: Constraints on the rock formation time and implications of migmatization at 2.1–2.0 Ga in the Fuping Complex, North China Craton

In most of the rocks of the Fuping Complex in the Trans-North China Orogen, a large quantity of leucocratic veins, dykes and granitic intrusions are present as migmatites. The incongruent partial melting of biotite to hornblende suggests the water-fluxed anatexis of the migmatization of which the melt is manifested by the euhedral crystallization of some quartzofeldspathic minerals and presence of albite rim on earlier plagioclase or K-feldspar. Importantly, some melt may be segregated in the sillimanite gneisses or extracted and injected as faint dispersed melt into the competent metasedimentary quartzite of the Wanzi series rocks. The quartzite has plenty of zircon ages cluster at c 2.5Ga and some at c 2.1–2.0Ga, the former may be detrital zircon age, while the latter represents the age of migmatization or crystallization of the injected melt, not detrital zircon ages as conventionally believed. The data have constrained that the sedimentation time of the protolith of the quartzite of the Wanzi series was between c 2.5Ga and c 2.1–2.0Ga, not younger than c 2.1–2.0Ga as once proposed. Therefore it must be cautious in applying detrital zircon ages in constraining the time of sedimentation if the rocks have undergone middle-high grade metamorphism, especially when migmatization was involved in the metamorphism process.

Unraveling the metamorphic history at the crossing of Neoproterozoic orogens, Sør Rondane Mountains, East Antarctica: Constraints from U–Th–Pb geochronology, petrography, and REE geochemistry

The Sør Rondane Mountains in East Antarctica preserve two stages of tectonothermal events, at c. 650–600Ma and c. 560–550Ma, each of which is also widely preserved elsewhere in Gondwana within tectonic suture zones/orogens, although both ages of tectonothermal events are only rarely documented together in these other parts of Gondwana. A geochronological and petrogenetic study of highly retrogressed garnet–sillimanite–biotite-bearing pelitic gneiss, along with three generations of associated sub-concordant to discordant leucocratic felsic veins, which together are exposed in the southern Austkampane area in the central part of the Sør Rondane Mountains, provides important new constraints on the geological and age relationships between these two regional tectonothermal events. U–Pb SHRIMP dating of zircon grains isolated from the host garnet–sillimanite–biotite gneiss yields at least four distinct zircon age populations of c. 2800–720Ma, c. 700Ma, 640–630Ma, and ∼500Ma. Chondrite-normalized REE patterns indicate that a pooled age of 637±6Ma represents the timing of recrystallization of metamorphic HREE-depleted zircon, during which the zircon was in equilibrium with garnet. The earliest emplaced leucocratic vein (LV-1), which is partly intermingled with the host garnet–sillimanite–biotite gneiss (and enclosing garnet-bearing pelitic enclaves), contains a population of zircon grains that yielded a crystallization age of 635±4Ma, which is almost identical to the age of zircons found within the host pelitic gneiss. The second-generation leucocratic vein (LV-2) occurs as a part of a set of sub-concordant veins/leucosomes comprising mostly biotite and muscovite. Although these particular leucosomes do not contain any zircon, they were found to contain abundant monazite, which yielded U–Pb ages of 640–630Ma along with some older (relict) ∼700Ma ages. The third-generation leucocratic vein (LV-3) was sampled from a suite of pegmatitic veins that discordantly crosscut both the host pelitic gneisses and the older two sets of leucocratic veins. These third-generation veins contain magmatic zircons that exhibit HREE-enriched patterns typical of magmatic zircon crystallization, and which yielded a crystallization age of 550±3Ma. The data imply that a major high-grade metamorphic event (possibly at granulite facies) took place in this region along with a subsequent late-stage re-hydration event, all within a relatively narrow time interval of <10m.y. at ∼635Ma. A subsequent hydration event that took place much later on is recorded by the intrusion of a suite of discordant pegmatitic veins at ∼550Ma that are observed to crosscut and alter both the pre-existing pelitic gneiss and the other two older sets of felsic veins. These two ages of tectonothermal events in East Antarctica are coeval with the two major stages of Gondwana assembly. Hence, these results provide new insights into the development of metamorphic-fluid regimes within these major continent-transecting Neoproterozoic orogens.

Element mobility during continental collision: insights from polymineralic metamorphic vein within UHP eclogite in the Dabie orogen

AbstractMetamorphic dehydration and partial melting are two important processes during continental collision. They have significant bearing on element transport at the slab interface under subduction‐zone P–T conditions. Petrological and geochemical insights into the two processes are provided by a comprehensive study of leucocratic veins in ultrahigh‐pressure (UHP) metamorphic rocks. This is exemplified by this study of a polymineralic vein within phengite‐bearing UHP eclogite in the Dabie orogen. The vein is primarily composed of quartz, kyanite, epidote and phengite, with minor accessory minerals such as garnet, rutile and zircon. Primary multiphase solid inclusions occur in garnet and epidote from the both vein and host eclogite. They are composed of quartz ± K‐feldspar ± plagioclase ± K‐bearing glass and exhibit irregular to negative crystal shapes that are surrounded by weak radial cracks. This suggests their precipitation from solute‐rich metamorphic fluid/melt that involved the reaction of phengite breakdown. Zircon U–Pb dating for the vein gave two groups of concordant ages at 217 ± 2 and 210 ± 2 Ma, indicating two episodes of zircon growth in the Late Triassic. The same minerals from the two rocks give consistent δ18O and δD values, suggesting that the vein‐forming fluid was directly derived from the host UHP eclogite. The vein is much richer in phengite and epidote than the host eclogite, suggesting that the fluid is associated with remarkable concentration of such water‐soluble elements as LILE and LREE migration. Garnet and rutile in the vein exhibit much higher contents of HREE (2.2–5.7 times) and Nb–Ta (1.8–2.0 times) than those in the eclogite, indicating that these normally water‐insoluble elements became mobile and then were sunken in the vein minerals. Thus, the vein‐forming agent would be primarily composed of the UHP aqueous fluid with minor amounts of the hydrous melt, which may even become a supercritical fluid to have a capacity to transport not only LILE and LREE but also HREE and HFSE at subduction‐zone metamorphic conditions. Taken together, significant amounts of trace elements were transported by the vein‐forming fluid due to the phengite breakdown inside the UHP eclogite during exhumation of the deeply subducted continental crust.

The stabilization of self-organised leucogranite networks—Implications for melt segregation and far-field melt transfer in the continental crust

Net-structured leucocratic vein systems are ubiquitous in high-grade and migmatite terrains and are interpreted to represent interconnected former magma-bearing structures that accommodated the migration of granitic magmas out of their anatectic source regions. Net-structured leucogranite vein systems in high-grade gneisses from the Damara Belt in Namibia describe a size-based hierarchy and spatial distribution that indicates self-organisation of the magma transfer system. The internal geometry and connectivity of the net-structured leucogranites are controlled by variations in melt pressure, whereas regional strains control the orientation of the leucogranites. Foliation-parallel (stromatic) and shear-band-hosted granite veins accommodate transport at relatively low melt volumes and account for the metre-scale segregation of melt close to the anatectic source. During times of higher melt supply, the formation of extensional fractures that cross-cut stromatic and shear-band-hosted melt sheets at high angles is promoted. Extensional fracturing and associated dilatancy create gradients in magmastatic head that cause melt to be drained from pervasive stromatic and shear-band-hosted vein networks into spaced dilatant fractures. This has the potential to initiate buoyancy-driven far-field melt transfer along suitably orientated extensional fractures as vertical dykes or self-propagating hydrofractures. Our results suggest that melt transfer systems are able to adjust to variations in melt supply by forming conduits that accommodate varying melt volumes and will tend to develop towards a state of self-organisation.

Petrological Implication of the Albite Rims in the Felsic Gneisses of the Fuping Complex

Abstract:The albite rim is present in most felsic gneisses of the Fuping Complex. The presence of the rim indicates the coexistence of plagioclase and K‐feldspar in the rock. The rim is formed immediately after the myrmekite, and both textures were derived from the alteration of K‐feldspar. The difference is that that there is no quartz present in the rim, and the rim is nearly albite and the anorthite content of the rim plagioclase is substantially lower than that of the myrmekite plagioclase. Formed at 400–500°C the albite rim was derived from the K‐feldspar composition adjustment in the late or post‐magmatism stage. As the temperature decreased, the equilibrium between K‐feldspar and plagioclase could be maintained, and reactions between the minerals occurred. The leucocratic veins in the complex show distinguished magma or migmatitic characteristics. The rim might form in the late magma or deuteric stage. The formation of the rim implies obvious granitic magma‐ or melt‐injection activity. Typical metamorphic rocks cannot produce the rims. Anatexis after medium‐high grade metamorphism might be subordinate. If present, the anatexis is water‐present, but the rim texture cannot be taken as the symbol of anatexis.

Caracterização geoquímica e estrutural do Granodiorito Cruzeiro do Sul: magmatismo shoshonítico pós-colisional neoproterozoico em zona de transcorrência, região de Quitéria, RS

The Cruzeiro do Sul Granodiorite is located in the Eastern portion of the Sul-rio-grandense shield. It forms one ENE-WSW elongate body of approximately 4 km² within a sinistral strike slip shear zone. The rocks were originally considered as part of the Arroio dos Ratos Complex, but they were recently mapped as a separate unit because igneous textures are widely preserved and they are intrusive in the gneissis complex rocks. The Cruzeiro do Sul Granodiorite is thus a porphyritic horblende-biotite granodiorite in a mafic-rich (M' ~ 20), medium-grained matrix. The magmatic foliation is marked by the shape alignment of feldspar megacrystals and mafic minerals. A solid-state, mylonitic structure is well-developed mainly near the contacts. Leucocratic veins are found both concordant and discordant to the main foliation, and they may give rise to a composite banding within high-strain zones, together with elongate mafic microgranular enclaves and synplutonic dykes of tonalitic to dioritic composition. The mylonitic foliation trends EW to ENE-WSW with medium to steep dip, and contains a shallow plunging stretching lineation, which indicates a transcurrent kinematic for this shear zone. Structural evidences, such as the parallelism and progressive evolution of mylonitic foliation relative to the magmatic one, as well as the elongated form of granodioritic bodies and the concordant character of planar structures with the ENE shear zone, are interpreted as an indication of syntectonic emplacement. Moreover, the absence of metamorphic foliation reworked by mylonitic shows that the Cruzeiro do Sul Granodiorite registers only one phase of deformation, which would have occurred during their emplacement. Thus, we have concluded that the Cruzeiro do Sul Granodiorite has no relation with the EW transversal event, and low dip angle responsible for the deformation of Arroio dos Ratos Complex, but is linked to the transcurrent event of Dorsal de Canguçu Shear Zone. The structural and compositional features of Cruzeiro do Sul Granodiorite magmatism are consistent with its post-collisional character, and the shoshonitic affinity is given by its high Sr contents, regular rare earth element chondrite-normalized patterns and the abundance of light rare earth regarding the heavy rare earth elements. Patterns of trace elements with enrichment in Ba and Rb, and impoverishment of High-Field Strength in relation to Large Ion Lithophile Elements is also an important feature of the rocks of shoshonitic affinity. Their medium- to high-K content, and metaluminous to slightly peraluminous characters are possibly indicative of some crustal contamination during their differentiation. The conditions of P and T, calculated with the geobarometer Alt-Hb and Plg-Hb thermobarometer were estimated at about 4.3 to 5.3 kbars and crystallization temperatures on the order of 720 to 760°C. The feldspar microstructures suggest deformation temperatures consistent with the amphibolite facies.

Alteration of the Oceanic Lower Crust at a Slow-spreading Axis: Insight from Vein-related Zoned Halos in Olivine Gabbro from Atlantis Massif, Mid-Atlantic Ridge

Incipient-stage alteration products in relatively fresh oceanic gabbros from deep boreholes provide critical information on hydration processes in the oceanic lower crust and their effect on lithosphere dynamics.We present the results of a petrographic study on the alteration of olivine-bearing gabbroic rocks recovered from the deeper parts of Integrated Ocean Drilling Program (IODP) Hole U1309D in the Atlantis Massif near the Mid-Atlantic Ridge at 308N. In these rocks, alteration is localized in proximity to fluid-infiltration veins or igneous contacts. It is most conspicuous in halos surrounding amphiboleþ chlorite veins or leucocratic veins in olivine-bearing gabbros, where coronitic fringes of tremolite, chlorite and talc occur around discrete olivine grains. Many of the halos exhibit a zonal pattern with systematic changes in mineral assemblage, generally consisting of three zones: tremoliteþ chlorite around relict olivine^ plagioclase contacts; talc pseudomorphs after olivine; and tremolite pseudomorphs after olivine. The tremoliteþ chlorite assemblage appears in increasing amounts and talc grows unevenly with increasing thickness toward the veins.The alteration minerals have highly magnesian compositions, reflecting the compositions of the precursor igneous phases.Within the zone closest to the veins, green hornblende with a relatively high Al content occurs, showing textures suggestive of its later formation than the coronitic tremolite and chlorite. Considering the mode of occurrence and chemical composition of the minerals combined with thermodynamic calculations of silica and water activities in a simplified system, we conclude that the zoned halos were caused by metasomatism owing to protracted or sequential infiltration of hydrothermal fluids at amphibolite-facies conditions (450^7508C, 1·5^2 kbar). Textural relationships clearly indicate that zoned halos formed earlier than serpentinization and clay mineral formation, and suggest that the high-temperature, amphibolite-facies alteration took place in a near-axis region before the exhumation of the lower crustal rocks. Recent results of seafloor drilling have provided supporting evidence for the predominance of gabbroic rocks in oceanic core complexes.The similarity in mineral association between zoned halos and schistose fault rocks suggests that preferential formation of talc and/or chlorite, rather than serpentine, at contacts between gabbroic rocks and peridotite plays an essential role in detachment faulting and tectonic exhumation of oceanic core complexes from lower crustal levels.