Transformation of two-pyroxene hornblende granulite to garnet granulite involving simultaneous melting and fracturing of the lower crust, Fiordland, New Zealand

Granulite facies gabbroic and dioritic gneisses in the Pembroke Valley, Milford Sound, New Zealand, are cut by vertical and planar garnet reaction zones in rectilinear patterns. In gabbroic gneiss, narrow dykes of anorthositic leucosome are surrounded by fine‐grained garnet granulite that replaced the host two‐pyroxene hornblende granulite at conditions of 750 °C and 14 kbar. Major and trace element whole‐rock geochemical data indicate that recrystallization was mostly isochemical. The anorthositic veins cut contacts between gabbroic gneiss and dioritic gneiss, but change in morphology at the contacts, from the anorthositic vein surrounded by a garnet granulite reaction zone in the gabbroic gneiss, to zones with a septum of coarse‐grained garnet surrounded by anorthositic leucosome in the dioritic gneiss. The dioritic gneiss also contains isolated garnet grains enclosed by leucosome, and short planar trains of garnet grains linked by leucosome. Partial melting of the dioritic gneiss, mostly controlled by hornblende breakdown at water‐undersaturated conditions, is inferred to have generated the leucosomes. The form of the leucosomes is consistent with melt segregation and transport aided by fracture propagation; limited retrogression suggests considerable melt escape. Dyking and melt escape from the dioritic gneiss are inferred to have propagated fractures into the gabbroic gneiss. The migrating melt scavenged water from the surrounding gabbroic gneiss and induced the limited replacement by garnet granulite.     

Trace element partitioning during high-P partial melting and melt-rock interaction; an example from northern Fiordland, New Zealand

Pods of granulite facies dioritic gneiss in the Pembroke Valley, Milford Sound, New Zealand, preserve peritectic garnet surrounded by trondhjemitic leucosome and vein networks, that are evidence of high‐P partial melting. Garnet‐bearing trondhjemitic veins extend into host gabbroic gneiss, where they are spatially linked with the recrystallization of comparatively low‐P two‐pyroxene‐hornblende granulite to fine‐grained high‐P garnet granulite assemblages in garnet reaction zones. New data acquired using a Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA‐ICPMS) for minerals in various textural settings indicate differences in the partitioning of trace elements in the transition of the two rock types to garnet granulite, mostly due to the presence or absence of clinozoisite. Garnet in the garnet reaction zone (gabbroic gneiss) has a distinct trace element pattern, inherited from reactant gabbroic gneiss hornblende. Peritectic garnet in the dioritic gneiss and garnet in trondhjemitic veins from the Pembroke Granulite have trace element patterns inherited from the melt‐producing reaction in the dioritic gneiss. The distinct trace element patterns of garnet link the trondhjemitic veins geochemically to sites of partial melting in the dioritic gneiss.     

Transport of low-aH2O dehydration products to melt sites via reaction-zone networks, Milford Sound, New Zealand

The gabbroic/dioritic Pembroke Hornblende Granulite (PHG) of Milford Sound displays a geometrically simple mesoscopic network of sub‐planar garnet reaction zones (GRZ) in which the meta‐igneous hornblende granulite has been depleted of Na, Si, and H₂O, and c. 25 vol.% almandine‐rich garnet has formed. Some studies postulate the initial presence of melt along the centres of all GRZ, explaining the frequent absence of feldspathic veins by selective melt loss. A more parsimonious model is necessitated by structural evidence and, together with chemical data, suggests a relationship between mid‐range metasomatic transport and anatexis. The Pembroke outcrops show a process of incipient melting of gabbro/diorite in an environment of relatively low aH₂O in lithologies that have limited free quartz. A non‐equilibrium steady state is proposed, in which a sodic dehydration fluid moves some distance via the GRZ network towards areas of partial melting. Only in these areas are Na and Si reconstituted as albite, with more garnet as byproduct, having avoided the need for melt percolation. The combined structural and chemical evidence directs a focus on mass transport in low‐aH₂O gabbroic environments. In subsequent events of shearing and complete transposition, both sets of garnet – the atypical GRZ residue and partial melt melanosomes – were inherited by the Milford Gneiss ‘facies’ of the PHG.     

Prograde and retrograde history of eclogites from the Eastern Blue Ridge, North Carolina, USA

Abstract The prograde metamorphism of eclogites is typically obscured by chemical equilibration at peak conditions and by partial requilibration during retrograde metamorphism. Eclogites from the Eastern Blue Ridge of North Carolina retain evidence of their prograde path in the form of inclusions preserved in garnet. These eclogites, from the vicinity of Bakersville, North Carolina, USA are primarily comprised of garnet–clinopyroxene–rutile–hornblende–plagioclase–quartz. Quartz, clinopyroxene, hornblende, rutile, epidote, titanite and biotite are found as inclusions in garnet cores. Included hornblende and clinopyroxene are chemically distinct from their matrix counterparts. Thermobarometry of inclusion sets from different garnets record different conditions. Inclusions of clinozoisite, titanite, rutile and quartz (clinozoisite + titanite = grossular + rutile + quartz + H₂O) yield pressures (6–10 kbar, 400–600 °C and 8–12 kbar 450–680 °C) at or below the minimum peak conditions from matrix phases (10–13 kbar at 600–800 °C). Inclusions of hornblende, biotite and quartz give higher pressures (13–16 kbar and 630–660 °C). Early matrix pyroxene is partially or fully broken down to a diopside–plagioclase symplectite, and both garnet and pyroxene are rimmed with plagioclase and hornblende. Hypersthene is found as a minor phase in some diopside + plagioclase symplectites, which suggests retrogression through the granulite facies. Two‐pyroxene thermometry of this assemblage gives a temperature of c. 750 °C. Pairing the most Mg‐rich garnet composition with the assemblage plagioclase–diopside–hypersthene–quartz gives pressures of 14–16 kbar at this temperature. The hornblende–plagioclase–garnet rim–quartz assemblage yields 9–12 kbar and 500–550 °C. The combined P–T data show a clockwise loop from the amphibolite to eclogite to granulite facies, all of which are overprinted by a texturally late amphibolite facies assemblage. This loop provides an unusually complete P–T history of an eclogite, recording events during and following subduction and continental collision in the early Palaeozoic.     

Local partial melting of the lower crust triggered by hydration through melt–rock interaction: an example from Fiordland,New Zealand

We investigate a low‐strain outcrop of the lower crust, the Pembroke Granulite, exposed in northern Fiordland, New Zealand, which exhibits localized partial melting. Migmatite and associated tschermakite–clinozoisite (TC) gneiss form irregular, elongate bodies that cut a two‐pyroxene–pargasite (PP) gneiss. Gradational boundaries between rock types, and the progressive nature of changes in mineral assemblage, microstructure and chemistry are consistent with the TC gneiss and migmatite representing modified versions of the PP gneiss. Modification is essentially isochemical, where partial modification involves hydration of the assemblage and mineral chemistry changes, and complete modification involves additional recrystallization and in situ partial melt production. Microstructures of quartz and plagioclase, including small dihedral angles, string of beads textures and films surrounding amphibole and garnet grains are consistent with the former presence of melt in modified rock types. The documented rock modification is attributed to melt–rock interaction occurring during porous melt flow of a dominantly externally derived, hydrous silicate melt. Microstructures indicate melt flow occurred along grain boundaries and field relationships show it was focused into channels tens of metres wide, with preference for following the pre‐existing foliation. Melt–rock interaction at the grain scale resulted in hydration and modification of the host PP gneiss, which resulted in localized partial melting. These relationships indicate prograde hydration during localized melt–rock interaction drove migmatization of the lower crust.     

Mineral evolution of a garnet-pyroxenite nodule within eclogite, eastern Sulu ultrahigh-pressure metamorphic terrane, East China

Detailed microtextural observations and bulk chemical analysis were undertaken on a garnet‐pyroxenite nodule within retrograde eclogites from the NE Sulu ultrahigh‐pressure metamorphic (UHPM) terrane. The results suggest that the protolith was a cumulate from a gabbroic body. The nodule consists primarily of coarse clinopyroxene grains with a very high content of the Ca‐Tschermakite molecule. Microscopic observations and back‐scattered electron images (BSE) demonstrate a complicated intergrowth of clinopyroxene, garnet and ilmenite, which represents the peak metamorphic assemblage. The primary clinopyroxene grains are armoured with a thin garnet corona up to 0.5 mm wide that forms an interconnected network. Within the clinopyroxene grains, four sets of garnet lamellae are distributed along crystallographic planes; locally, a vermicular intergrowth of garnet and diopside is developed. Besides the garnet, parallel arrays of ilmenite blebs are common within the clinopyroxene. Hydrous minerals such as amphibole, zoisite and titanite formed at later stages, and replaced diopside, garnet and ilmenite respectively. The P–T conditions determined for the formation of the garnet lamellae indicate that the garnet pyroxenite experienced UHP metamorphism at the same peak P–T condition as its host eclogite. The very high Ca‐Tschermakite content (31–34 mol.%) of the primary clinopyroxene indicates crystallization at about 9–17 kbar and 1250–1450 °C, and together with the microtextural observations, suggests that the protolith of the garnet pyroxenite was a cumulate from a former gabbroic body, in which case, the host eclogite might represent the gabbroic body.     

Gabbro-norite cumulates from strongly depleted MORB melts in the Alpine–Apennine ophiolites

Hundred-meter wide cumulate bodies and decimetric dykelets of gabbro-norites are widespread within the distal ophiolitic peridotites from the Jurassic Ligure-Piemontese oceanic basin, now emplaced in the Alpine–Apennine orogenic system. These peridotites derived from the sub-continental mantle of the pre-Triassic Europe–Adria lithosphere and underwent profound modifications of their structural and compositional characteristics via melt–rock interaction during diffuse percolation by porous flow of upwelling asthenospheric melts. Gabbro-norite cumulates show the peculiar association of high forsteritic olivine, high-Mg# clinopyroxenes and orthopyroxenes and high anorthitic plagioclase with respect to mineral compositions in common ophiolitic and oceanic MORB gabbros. Abundance and early crystallization of magnesian orthopyroxene suggests that parental magmas of the gabbro-noritic cumulates were relatively silica-rich basaltic liquids. Clinopyroxenes and plagioclase have anomalously low Sr and LREE, resulting in highly fractionated C1-normalized LREE patterns in clinopyroxenes and negatively fractionated C1-normalized LREE patterns in plagioclases.Modal mineralogy and mineral major and trace element compositions indicate that these gabbro-norites crystallized from MORB-type basaltic liquids that were strongly depleted in Na, Ti, Zr, Sr and other incompatible trace elements relative to any erupted liquids of MORB-type ophiolites and modern oceanic lithosphere. Computed melt compositions in equilibrium with gabbro-norite clinopyroxenes are closely similar to depleted MORB-type single melt increments after 5–7% of fractional melting of a DM asthenospheric mantle source under spinel-facies conditions.Present knowledge on the ophiolitic peridotites of Monte Maggiore indicate that they were formed by interaction of lithospheric mantle protoliths with depleted, MORB-type single melt increments produced by the ascending asthenosphere. Their composition was progressively modified from olivine-saturated to orthopyroxene-saturated by the early reactive melt–peridotite interaction (i.e., pyroxene dissolution and olivine precipitation).Gabbro-norite cumulates marked the change from diffuse porous flow percolation to intrusion and crystallization when cooling by conducive heat loss became dominant on heating by melt percolation. Progressive upwelling and cooling of the host peridotite during rifting caused transition to more brittle conditions and to hydration and serpentinization.The Monte Maggiore peridotite body was then intruded along fractures by variably evolved, Mg–Al- to Fe–Ti-rich gabbroic dykes. Computed melt compositions in equilibrium with clinopyroxenes from less evolved gabbro dykes are closely similar to aggregated MORBs. The event of gabbro intrusion indicates that aggregated MORB-type liquids: i) migrated through and stagnated in the mantle lithosphere and ii) underwent evolution into shallow ephemeral magma chambers to form the parental magmas of the gabbroic dykes and the basaltic lava flows of the Ligurian oceanic crust.     

Successive hydration and dehydration of high-P mafic granofels involving clinopyroxene–kyanite symplectites, Mt Daniel, Fiordland, New Zealand

Three texturally distinct symplectites occur in mafic granofels of the Arthur River Complex at MtDaniel, Fiordland, New Zealand. These include symplectic intergrowths of clinopyroxene and kyanite, described here for the first time. Pods of mafic granofels occur within the contact aureole of the Early Cretaceous Western Fiordland Orthogneiss batholith. The pods have cores formed entirely of garnet and clinopyroxene, and rims of pseudomorphous coarse‐grained symplectic intergrowths of hornblende and clinozoisite that reflect hydration at moderate to high‐P. These hornfelsic rocks are enveloped by a hornblende–clinozoisite gneissic foliation (S1). Narrow garnet reaction zones, in which hornblende and clinozoisite are replaced by garnet–clinopyroxene assemblages, developed adjacent to fractures and veins that cut S1. Fine‐grained symplectic intergrowths of (1) clinopyroxene and kyanite and (2) clinozoisite, quartz, kyanite and plagioclase form part of the garnet reaction zones and partially replace coarse‐grained S1 hornblende and clinozoisite. The development of the garnet reaction zones and symplectites was promoted by dehydration most probably following cooling of the contact aureole. Maps of oxide weight percent and cation proportions, calculated by performing matrix corrections on maps of X‐ray intensities, are used to study the microstructure of the symplectites.     

High-pressure ultramafics in the lower crustal rocks of the Pekul’ney complex, central Chukchi Peninsula. 1. Petrography and mineralogy

Dunites, peridotites, olivine and spinel pyroxenites, and metagabbroids have been described in the tectonic blocks of the Pekul’ney complex of the central Chukchi Peninsula together with garnet-hornblende-clinopyroxene and zoisite (clinozoisite)-garnet-hornblende rocks, which are indicative of high-pressure complexes. However, the interpretations of previous researchers on the composition, structure, setting, and processes of formation of this rock association are highly controversial. The petrographic and mineralogical results reported in this paper indicate that the blocks of the complex host bodies of cumulate ultramafics among metamorphic rocks. These relationships were supported by the finding of xenoliths and xenocrysts of metamorphic rocks in the ultramafics. The metamorphic country rocks are lower crustal amphibolites and schists with peak metamorphic parameters corresponding to the high-pressure portion of the epidoteamphibolite facies (610–680°C and 9–14 kbar). All the varieties of ultramafic rocks studied in the blocks of the complex are assigned to a single cumulate series (from dunite to clinozoisite-garnet hornblendite), and the compositions of their primary minerals show regular correlations similar to crystallization differentiation trends. Specific features of the ultramafics of the Pekul’ney complex are the early crystallization of hornblende (which is present already in peridotites), wide range of garnet crystallization (associating with clinopyroxene, ceylonite, and hornblende), presence of magmatic clinozoisite in the most evolved assemblages (with garnet, hornblende, and clinopyroxene), and absence of evidence for plagioclase crystallization. Clinopyroxene from the most evolved ultramafic rocks contains more than 15 wt % Al₂O₃. The classification of the rocks of the complex provides a basis for the interpretation of geological relations between them and the elucidation of the characteristics of the internal structure of the blocks of the complex and bodies of cumulate ultramafic rocks in them.     

Roles for fluid and/or melt advection in forming high-P mafic migmatites, Fiordland, New Zealand

A series of striking migmatitic structures occur in rectilinear networks through western Fiordland, New Zealand, involving, for the most part, narrow anorthositic dykes that cut hornblende‐bearing orthogneiss. Adjacent to the dykes, host rocks show patchy, spatially restricted recrystallization and dehydration on a decimetre‐scale to garnet granulite. Although there is general agreement that the migration of silicate melt has formed at least parts of the structures, there is disagreement on the role of silicate melt in dehydrating the host rock. A variety of causal processes have been inferred, including metasomatism due to the ingress of a carbonic, mantle‐derived fluid; hornblende‐breakdown leading to water release and limited partial melting of host rocks; and dehydration induced by volatile scavenging by a migrating silicate melt. Variability in dyke assemblage, together with the correlation between dehydration structures and host rock silica content, are inconsistent with macroscopic metasomatism, and best match open system behaviour involving volatile scavenging by a migrating trondhjemitic liquid.     

Types of Enclaves and Their Features and Origins in Intermediate-Acid Intrusive Rocks from the Tongling District, Anhui Province, China

Enclaves in intermediate-acid plutons from Tongling can be divided into three types: xenoliths, relics and magmatogenic enclaves. The magmatogenic enclaves consist of cumulates, micrograined dioritite mixtite and dioritic chilled border enclaves. Petrologically, relics with eyed and meta-poikilitic texture are characterized by high content of biotite (>80%) and low content of cordierite and grossular. The cumulates with accumulate texture consist of a great amount of pyroxene, hornblende and minor spinel and phlogopite. The micrograined dioritic mixtite is composed of more hornblendes and feldspar and less needle apatites and an ellipsoid basic core included in plagio-clase. The chilled border enclaves have the same mineral association, but more dark minerals than the host rocks consisting of plagioclase, quartz, alkaline feldspar, hornblende and biotite. Geochemically, the relics exhibit high REE content (455.8×10-6) and high ratio of LREE/HREE, more obvious Eu negative anomaly and are rich in Cr and     

Evidence for melt migration enhancing recrystallization of metastable assemblages in mafic lower crust, Fiordland, New Zealand

A major arc batholith, the Western Fiordland Orthogneiss (WFO) in Fiordland, New Zealand, exhibits irregular, spatially restricted centimetre-scale recrystallization from two-pyroxene hornblende granulite to garnet granulite flanking felsic dykes. At Lake Grave, northern Fiordland, the composition and texture of narrow (<10–20 mm across) felsic dykes that cut the orthogneiss are consistent with an igneous origin and injection of melt to form orthogneiss migmatite. New U–Pb geochronology suggests that the injection of dykes and migmatization occurred at c . 115 Ma, during the later stages of arc magmatism. Recrystallization to garnet granulite is promoted by volatile extraction from the host two-pyroxene hornblende granulite via adjacent dykes and the patchy development of garnet granulite is left as a marker adjacent to the melt migration path. New mineral equilibria modelling suggests that a two-pyroxene hornblende assemblage is stable at <11 kbar, whereas a garnet granulite assemblage is stable at >12 kbar, suggesting that garnet granulite may have formed with <5 km crustal loading of the batholith. Although the garnet granulite assemblages signify that the WFO experienced high- P conditions, the very local nature of these textures indicates widespread metastability (>90%) of the two-pyroxene hornblende granulite assemblages. These results indicate the strongly metastable nature of assemblages in mafic lower arc crust during deep burial and demonstrate that the degree of reaction in the case of Fiordland is related to interaction with migrating melts.     

Timing of high-pressure metamorphism and exhumation of the eclogite type-locality (Kupplerbrunn–Prickler Halt, Saualpe, south-eastern Austria): constraints from correlations of the Sm–Nd, Lu–Hf, U–Pb and Rb–Sr isotopic systems

Sm–Nd, Lu–Hf, Rb–Sr and SIMS U–Pb data are presented for meta‐gabbroic eclogites from the eclogite type‐locality ( Haüy, 1822 ) Kupplerbrunn–Prickler Halt and other areas of the Saualpe (SE Austria) and Pohorje Mountains (Slovenia). Mg‐rich eclogites derived from early gabbroic cumulates are kyanite‐ and zoisite rich, whereas eclogites with lower Mg contents contain clinozoisite ± kyanite. Calculated P–T conditions at the final stages of high‐pressure metamorphism are 2.2 ± 0.2 GPa at 630–740 °C. Kyanite‐rich eclogites did not yield geologically meaningful Sm–Nd ages due to incomplete Nd isotope equilibration, whereas Sm–Nd multifraction garnet–omphacite regression for a low‐Mg eclogite from Kupplerbrunn yields an age of 91.1 ± 1.3 Ma. The Sm–Nd age of 94.1 ± 0.8 Ma obtained from the Fe‐rich core fraction of this garnet dates the initial stages of garnet growth. Zircon that also crystallized at eclogite facies conditions gives a weighted mean U–Pb SIMS age of 88.4 ± 8.1 Ma. Lu–Hf isotope analysis of a kyanite–eclogite from Kupplerbrunn yields 88.4 ± 4.7 Ma for the garnet–omphacite pair. Two low‐Mg eclogites from the Gertrusk locality of the Saualpe yield a multimineral Sm–Nd age of 90.6 ± 1.0 Ma. A low‐Mg eclogite from the Pohorje Mountains (70 km to the SE) gives a garnet–whole‐rock Lu–Hf age of 93.3 ± 2.8 Ma. These new age data and published Sm–Nd ages of metasedimentary host rocks constrain the final stages of the eo‐Alpine high‐pressure event in the Saualpe–Pohorje part of the south‐easternmost Austroalpine nappe system suggesting that garnet growth in the high‐pressure assemblages started at c. 95–94 Ma and ceased at c. 90–88 Ma, probably at the final pressure peak. Zircon and amphibole crystallization was still possible during incipient isothermal decompression. Rapid exhumation of the high‐pressure rocks was induced by collision of the northern Apulian plate with parts of the Austroalpine microplate, following Jurassic closure of the Permo‐Triassic Meliata back‐arc basin.     

Petrogenetic relations among titanium‐rich minerals in an anatectic high‐P mafic granulite

The petrogenetic relations among Ti‐rich minerals in high‐grade metabasites is illuminated here through a detailed petrological investigation of an anatectic garnet–clinopyroxene granulite from the Grenville Province, Ontario, Canada containing rutile, titanite and ilmenite in distinct microtextural settings. Garnet porphyroblasts exhibit zoned Ti concentrations (up to 0.15 wt% TiO₂ in their cores), as well as a variety of rutile inclusion types, including clusters of small, variably elongate grains and thin (≤1 μm) oriented needles. Calcite inclusions in garnet, commonly observed surrounding garnet cores containing quartz and clinozoisite, indicate the presence of evolving C–O–H fluids during garnet growth and suggest that the rutile clusters may have formed from subsequent Ti diffusion and rutile precipitation within existing fluid inclusions. Titanite forms large subhedral crystals and typically occurs where the primary garnet–clinopyroxene assemblage is in contact with leucosome containing megacrystic hornblende, silvialitic scapolite and calcic plagioclase. Many titanite crystals exhibit marginal subgrains that correspond with sharp changes in their major and trace element composition, likely related to a dissolution–precipitation or recrystallization process following primary crystallization. Clinopyroxene–ilmenite symplectite coronas surround titanite in most locations, likely forming from reaction with the hornblende‐plagioclase matrix (±fluids/melt). Integration of multi‐equilibria thermobarometry and Zr thermometry in rutile and titanite with phase equilibrium modelling allows definition of a clockwise P–T path evolving to peak pressures of ~1.5 GPa at ~750°C during garnet and rutile growth, followed by peak temperature conditions of ~1.2 GPa and ~820–880°C associated with melt‐present titanite growth, and finally cooling and decompression to regional amphibolite facies conditions (~1.0 GPa and ~750°C) associated with the formation of clinopyroxene–ilmenite symplectites surrounding titanite. P–T pseudosections calculated for the pristine (leucosome‐ and titanite ‐free) metabasite bulk composition reproduce much of the prograde phase relations, but predict rutile as the stable Ti‐rich mineral at the peak thermal conditions associated with melt‐present titanite growth. The P–M(CaO) and T–M(CaO) models show that bulk CaO concentrations have a significant effect on the stability ranges of titanite and rutile. Increased bulk CaO tends to stabilize titanite to higher pressure and temperature at the expense of rutile, with a ≥15% increase in CaO producing the observed titanite‐bearing assemblage at high‐P granulite facies conditions. Thus, the model results are consistent with the textural observations, which suggest that titanite stability is associated with a chemical exchange between the host metabasite and a Ca‐rich melt.     

Deep subduction of mantle-derived garnet peridotites from the Su-Lu UHP metamorphic terrane in China

Garnet peridotites from the southern Su‐Lu ultra‐high‐pressure metamorphic (UHPM) terrane, eastern China, contain porphyroblastic garnet with aligned inclusions comprising a low‐P–T mineral assemblage (chlorite, hornblende, Na‐gedrite, Na‐phlogopite, talc, spinel and pyrite). Orthopyroxene porphyroblasts show fine exsolution lamellae of clinopyroxene and minor chromite. A clinopyroxene inclusion in garnet shows some orthopyroxene exsolution lamellae. Both the rims of porphyroblastic pyroxene and garnet and the matrix pyroxene and garnet crystallized at the expense of olivine. This is interpreted as a result of metasomatism of the peridotites by an SiO₂‐rich melt at UHP conditions. A chromian garnet further overgrew on the rims of the garnet. The X_Mg values (Mg/(Mg+Fe)) of porphyroblastic garnet decrease from core to rim and vary in different peridotite samples, while the compositions of both the porphyroblastic and the matrix pyroxene are similar in terms of Ca–Mg–Fe. The Mg‐rich cores of porphyroblastic garnet and orthopyroxene record high temperatures and pressures (c. 1000 °C, ≥5.1 GPa), whereas the matrix minerals, including the rims of porphyroblasts, record much lower P–T (c. 4.2 GPa, c. 760 °C). Sm–Nd data give apparent isochron ages of c. 380 Ma and negative ε_Nd(0) values (c.?9). These dates are considered meaningless due to isotopic disequilibrium between garnet cores and the rest of the rocks. The isotopic disequilibrium was probably caused by metasomatism of the peridotites by melt/fluids derived from the coevally subducted crustal materials. On the other hand, the Rb–Sr isotopic systems of phlogopite and clinopyroxene appear to have reached equilibrium and record a cooling age of c. 205 Ma. It is suggested that the garnet peridotites were originally emplaced into a low‐P–T environment prior to the c. 220 Ma continental collision, during which they were subducted together with crustal rocks to mantle depth and subjected to UHP metamorphism. An important corollary is that at least some of the coevally subducted crustal rocks in the Su‐Lu terrane have been subjected to peak metamorphism at P–T conditions much higher than presently estimated (≥2.7 GPa, ≤800 °C).     

Early Neoproterozoic granulite facies metamorphism of mafic dykes from the Vestfold Block,east Antarctica

Proterozoic mafic dykes from the southwestern Vestfold Block experienced heterogeneous granulite facies metamorphism, characterized by spotted or fractured garnet‐bearing aggregates in garnet‐absent groundmass. The garnet‐absent groundmass typically preserves an ophitic texture composed of lathy plagioclase, intergranular clinopyroxene and Fe–Ti oxides. Garnet‐bearing domains consist mainly of a metamorphic assemblage of garnet, clinopyroxene, orthopyroxene, hornblende, biotite, plagioclase, K‐feldspar, quartz and Fe–Ti oxides. Chemical compositions and textural relationships suggest that these metamorphic minerals reached local equilibrium in the centre of the garnet‐bearing domains. Pseudosection calculations in the model system NCFMASHTO (Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃) yield P–T estimates of 820–870 °C and 8.4–9.7 kbar. Ion microprobe U–Pb zircon dating reveals that the NW‐ and N‐trending mafic dykes were emplaced at 1764 ± 25 and 1232 ± 12 Ma, respectively, whereas their metamorphic ages cluster between 957 ± 7 and 938 ± 9 Ma. The identification of granulite facies mineral inclusions in metamorphic zircon domains is also consistent with early Neoproterozoic metamorphism. Therefore, the southwestern margin of the Vestfold Block is inferred to have been buried to depths of ~30–35 km beneath the Rayner orogen during the late stage of the late Mesoproterozoic/early Neoproterozoic collision between the Indian craton and east Antarctica (i.e. the Lambert Terrane or the Ruker craton including the Lambert Terrane). The lack of penetrative deformation and intensive fluid–rock interaction in the rigid Vestfold Block prevented the nucleation and growth of garnet and resulted in the heterogeneous granulite facies metamorphism of the mafic dykes.     

The effect of pre‐tectonic reaction and annealing extent on behaviour during subsequent deformation: insights from paired shear zones in the lower crust of Fiordland,New Zealand

Granulite facies pargasite orthogneiss is partially to completely reacted to garnet granulite either side of narrow (<20 mm) felsic dykes, in Fiordland, New Zealand, forming ~10–80 mm wide garnet reaction zones. The metamorphic reaction changed the abundance of minerals, and their shape and grain size distribution. The extent of reaction and annealing (temperature‐related coarsening and nucleation) is greatest close to the dykes, whereas further away the reaction is incomplete. As a consequence, grain size and the abundance of garnet decreases away from the felsic dykes over a few centimetres. The aspect ratios of clusters of S1 pyroxene and pargasite in the orthogneiss, which are variably reacted to post‐S1 garnet, decrease from high in the host, to near equidimensional close to the dyke. Post‐reaction deformation localized in the fine‐grained partially reacted areas. This produced a pattern of ‘paired’ shear zones located at the outer parts of the garnet reaction zone. Our study shows that grain size sensitive deformation occurs where the grain size is sufficiently reduced by metamorphic reaction. The weakening of the rock due to the change in grain size distribution outweighs the addition of nominally stronger garnet to the assemblage.     

Coseismic formation of eclogite facies cataclasite dykes at Yangkou in the Chinese Su‐Lu UHP metamorphic belt

Eclogite facies cataclasite is recognized at Yangkou in the Chinese Su‐Lu ultrahigh‐P metamorphic belt. The cataclasite dykes (5?15 cm wide) are bounded by mylonite/ultramylonite zones, cutting through unfoliated metagabbro and/or eclogite. The cataclasite veins (generally 2–4 cm wide) are free of mylonite boundary zones, cutting through the foliation of the high‐P host rock. The dykes and veins are dominated by eclogite fragments consisting of debris of omphacite, garnet, quartz, phengite and kyanite, in a matrix of variable amounts of a schist rich in quartz, phengite and kyanite. Garnet clasts in the fragments are welded and overgrown by more Ca‐rich garnet containing mineral inclusions different from those in the garnet cores. The micropoikilitic texture of garnet is typical of eclogitic pseudotachylytes. Crack‐sealing K‐feldspar veinlets in the cataclasite dykes also imply frictional or shock‐induced melting of K‐mica. The modal abundances in the cataclasite and the schist imply that the dykes formed by flow of the omphacite and garnet‐dominated cataclasites into the fractures during seismic faulting, while the lower density minerals (quartz, phengite and kyanite) were largely left in the ultramylonite boundary zones. The dykes have the same composition as their host rocks, except for slightly lower Si and large ion lithophile elements and higher Mg, Ca, Cr, Co and Ni. Chromite, probably spurted from the nearby ultramafic rock, is found as rare particles in the cataclasite fragments. This indicates that material exchange occurred by mechanical mixing between the dykes and the ultramafic rock during seismic faulting. The Cr‐rich eclogite minerals grown on the chromite are evidence for coseismic high‐P crystallization. Short‐lived crystal growth is implied by the fine grain sizes of the eclogite minerals and very limited element diffusion between the garnet clasts and their overgrowths. The fact that the host rocks are more hydrated implies that the dyke formation was not related to fluid infiltration. It appears, therefore, that stress was the key factor inducing the high‐P phase transformation in the dykes. Both stress and temperature were only transiently high in the dykes, which have been metastable since they were formed.     

K-rich fluid metasomatism at high-pressure metamorphic conditions: Lawsonite decomposition in rodingitized ultramafite of the Maksyutovo Complex, Southern Urals (Russia)

Fine grained rodingite‐like rocks containing epidote, clinozoisite, garnet, chlorite, phengite and titanite occur within antigorite serpentinite boudins from the high‐pressure metamorphic Maksyutovo Complex in the Southern Urals. Pseudomorphs after lawsonite, resorption of garnet by chlorite and phengite and stoichiometry suggest the reaction lawsonite + garnet + K‐bearing fluid → clinozoisite + chlorite + phengite, and define a relic assemblage of lawsonite + garnet + chlorite + titanite ± epidote as well as a later post‐lawsonite assemblage of clinozoisite + phengite + chlorite + titanite. The reaction lawsonite + titanite → clinozoisite + rutile + pyrophyllite + H₂O delimits the maximum stability of former lawsonite + titanite to pressures >13 kbar. P–T conditions of 18–21 kbar/520–540 °C result, if the average chlorite, Mg‐rich garnet rim and average epidote compositions are used as equilibrium compositions of the former lawsonite assemblage. These estimates indicate a similar depth of formation but lower temperatures to those recorded in nearby eclogites. The metamorphic conditions of the lawsonite assemblage are considerably higher than previously suggested and, together with published structural data, support a model in which a normal fault within the Maksyutovo complex acted as the major transport plane of eclogite exhumation. The maximum Si content of phengite and minimum Fe content in clinozoisite constrain the metamorphic conditions of the later pseudomorph assemblage to be >4.5 kbar and <440 °C. Rb–Sr isotopic dating of the pseudomorph assemblage results in a formation age of 339 ± 6 and 338 ± 5 Ma, respectively. These results support the recent exhumation models for this complex.     

P–T–t evolution of garnet amphibolites in the Wutai–Hengshan area,North China Craton: insights from phase equilibria and geochronology

Garnet amphibolites can provide valuable insights into geological processes of orogenic belts, but their metamorphic evolution is still poorly constrained. Garnet amphibolites from the Wutai–Hengshan area of the North China Craton mainly consist of garnet, hornblende, plagioclase, quartz, rutile and ilmenite, with or without titanite and epidote. Four samples selected in a south–north profile were studied by the pseudosection approach in order to elucidate the characteristics of their metamorphic evolution, and to better reveal the northwards prograde change in P–T conditions as established previously. For the sample from the lower Wutai Subgroup, garnet exhibits obvious two‐substage growth zoning characteristic of pyrope (X_py) increasing but grossular (X_gr) decreasing outwards in the core, and both X_py and X_gr increasing outwards in the rim. Phase modelling using thermocalc suggests that the garnet cores were formed by chlorite breakdown over 7–9 kbar at 530–600 °C, and rims grew from hornblende and epidote breakdown over 9.5–11.5 kbar at 600–670 °C. The isopleths of the minimum An in plagioclase and maximum X_py in garnet were used to constrain the peak P–T conditions of ~11.5 kbar/670 °C. The modelled peak assemblage garnet + hornblende + epidote+ plagioclase + rutile + quartz matches well the observed one. Plagioclase–hornblende coronae around garnet indicate post‐peak decompression and fluid ingress. For the samples from the south Hengshan Complex, the garnet zoning weaken gradually, reflecting modifications during decompression of the rocks. Using the same approach, the rocks are inferred to have suprasolidus peak conditions, increasing northwards from 11.5 kbar/745 °C, 12.5 kbar/780 °C to 13 kbar/800 °C. Their modelled peak assemblages involve diopside, garnet, hornblende, plagioclase, rutile and quartz, yet diopside is not observed petrographically. The post‐peak decompression is characterized by diopside + garnet + quartz + melt = hornblende + plagioclase, causing the diopside consumption and garnet compositions to be largely modified. Thus, the pesudosection approach is expected to provide better pressure results than conventional thermobarometry, because the later approach cannot be applied with confidence to rocks with multi‐generation assemblages. U–Pb dating of zircon in the Wutai sample records a protolith age of c. 2.50 Ga, and a metamorphic age of c. 1.95 Ga, while zircon in the Hengshan samples records metamorphic ages of c. 1.92 Ga. The c. 1.95 Ga is interpreted to represent the pre‐peak or peak metamorphic stages, and the ages of c. 1.92 Ga are assigned to represent the cooling stages. All rocks in the Wutai–Hengshan area share similar clockwise P–T morphologies. They may represent metamorphic products at different crustal depths in one orogenic event, which included a main thickening stage at c. 1.95 Ga followed by a prolonged uplift and cooling after 1.92 Ga.