Growth Of Cordierite Research Articles

From source to surface: clues from garnet-bearing Carboniferous silicic volcanic rocks, Iberian Pyrite Belt, Portugal

This work investigates the relationships between partial melting, melt extraction, pluton growth and silicic volcanism in garnet-bearing felsic volcanic rocks that were extruded in the Iberian Pyrite Belt, at ca. 345 Ma. The garnets are of peritectic origin, displaying textural and chemical features of disequilibrium crystallization during partial melting reactions involving biotite at high temperatures (up to 870 °C) in the middle-lower crust. Major element composition suggests compositional equilibrium with the entrained and pinitized peritectic cordierite, but reveals some subsequent homogenization by diffusion. Trace element maps and spot analyses of garnet show, nonetheless, significant trace element variations, reflecting biotite and Y-REE-P-rich accessory phase breakdown during partial melting reactions. Peritectic garnet and cordierite growth resulted in the preservation of Th- and Y-rich prograde suprasolidus monazite, which constrains the timing of partial melting of the metapelitic protolith at ca. 356.8 ± 2.4 Ma. The zircon cargo further shows that a significant amount of zircon crystals from previously crystallized felsic melts were also remobilized and erupted. These were likely stored in an upper crustal pluton that grew episodically since ca. 390 Ma during voluminous melt generation periods within the middle to lower crust, which also resulted in voluminous volcanism. The geochemical trends of the felsic volcanic rocks reflect the entrainment of xenoliths of peritectic garnet, cordierite and feldspar, and as such, the garnet-bearing felsic volcanic rocks represent an erupted mixture of a lower-temperature (ca. 770 °C) silicic melt and autocrysts, and peritectic phases and zircon crystals from previously crystallized and stored felsic melts.

Pseudotachylytes in felsic lower-crustal rocks of the Calabrian Serre massif: A record of deep- or shallow-crustal earthquakes?

Pseudotachylytes (quenched frictional melts produced on a fault by seismic slip) in dry rocks exhumed from the mid-lower crust are potential indicators of earthquakes that either nucleated at, or propagated to, depths below the main shallow brittle-ductile transition zone. Establishing whether these pseudotachylytes effectively record deep-crustal earthquakes, or shallow-level earthquakes overprinting the mid-lower-crustal rocks during the exhumation path, may represent a major challenge. This challenge is mainly related to the fact that the mineral assemblage of a pseudotachylyte develops out of equilibrium during the coseismic thermal transient leading to melting and melt quenching. Here we investigate pseudotachylytes within peraluminous, sillimanite-garnet-rich, migmatitic paragneiss of the Serre Massif in Calabria (Southern Italy). These exhumed lower-crustal rocks experienced granulite-facies metamorphism (∼700–800 °C; ∼600–800 MPa), partial melting and dehydration during the late Variscan Orogeny (ca. 320–280 Ma). The crosscutting pseudotachylytes contain hercynite and sillimanite microlites, globular-shaped poikilitic cordierite and plagioclase, and rare cauliflower- to subhedral-shaped garnet. The pseudotachylytes are pristine, not affected by ductile deformation, recrystallisation or extensive alteration by fluid after their formation. A Rb-Sr isochron age of 51.4 ± 5.1 Ma is obtained for the pervasively kinked biotite in the host rock immediately adjacent to the pseudotachylyte and associated with earthquake damage, while an age of 105.3 ± 4.1 Ma is obtained for the undeformed host-rock biotite. This indicates that the granulites were cooler than the closing Rb-Sr temperature of biotite (ca. 300–400 °C) in the Cretaceous and that the studied pseudotachylytes formed by shallow seismic faulting. Therefore, sillimanite, hercynite, garnet, plagioclase, and cordierite all formed during quenching of the frictional melt well above the ambient temperature. Modelling of cordierite growth during melt quenching indicates that cordierite should have started to crystallise at T > 900 °C to achieve the grain size (up to 10 μm in diameter) observed in the pseudotachylyte. Modelling and microstructural observations allow the crystallisation sequence of microlites during melt cooling to be established. These microlites include cauliflower garnet which, in this case, did not develop in a deep-seated faulting context as commonly reported.

Effect of MgO/SiO2 ratio and Al2O3 content on crystallization behavior and properties of cordierite-based glass–ceramics

Cordierite-based glass-ceramics with non-stoichiometric compositions were prepared by powder sintering method. The effects of MgO/SiO2 ratio and Al2O3 content on the crystallization behavior, microstructure and thermal expansion coefficient of the glass-ceramics were studied. Results indicated that the decreasing Al2O3 content showed a greater effect on decreasing the melting temperature and liquid viscosity than the increasing MgO/SiO2. DTA results showed that, at low temperatures, the MgO/SiO2 ratio dominated crystallization of cordierite due to the lack of liquid. At a high sintering temperature of 1340 °C, the Al2O3 content dominated the crystal growth of cordierite due to its great effect on decreasing the liquid viscosity. The chemical composition close to the stoichiometric cordierite and the absence of quartz were two essential factors for obtaining low thermal expansion coefficients. The decreasing MgO/SiO2 ratio and Al2O3 content caused phase separation with an interconnected structure, which followed the crystallization of cordierite in the Al2O3-rich phase.

Evidence of Archaean metamorphism from the Yerrabali schist belt of Eastern Dharwar craton from EPMA dating of monazite

The Dharwar craton is divided in to the Western and Eastern Dharwar craton (WDC and EDC) by the N–S trending Chitradurga Shear zone. The EDC mainly consists of late Archean granites interspersed with N–S trending linear schist belts. The Yerrabali Schst belt (YSB) is located at the north-eastern corner of the EDC, near the Karimnagar granulite belt. The YSB is metamorphosed in middle-upper amphibolite facies and consists of Banded Magnetite Quartzite (BMQ), ferruginous quartzite, ultramafics, quartz–cordierite–gedrite ± garnet bearing quartzite, dolomite, amphibolite, deformed meta-pyroxenite, tremolite schist, sillimanite–garnet–cordierite schist, and dolerite dykes. We have dated monazites in the sillimanite–garnet–cordierite schist, exposed at the SW part of the YSB by EPMA chemical method. So far no dates are available from the YSB. The monazite chemical ages with three distinct populations, viz., 2658 ± 42, 2817 ± 19 and 3097 ± 34 Ma, probably correspond to three distinct metamorphic phases. The peak metamorphism at around 2800 Ma corresponds with the garnet and cordierite growth. A temperature of 673°C and a pressure of 4.7 kb have been estimated for peak metamorphism.

Petrology, phase equilibria modelling, and in situ zircon and monazite geochronology of ultrahigh-temperature granulites from the khondalite belt of southern India

The granulite facies metapelite (khondalite) belt in the Trivandrum Block of southern India has been central to investigations on extreme crustal metamorphism associated with the final assembly of the Gondwana supercontinent during late Neoproterozoic-Cambrian. Here we investigate garnet-sillimanite-cordierite-spinel-bearing metapelites from this khondalite belt using state-of-the-art petrologic, mineral phase equilibria modelling, and coupled zircon and monazite U-Pb geochronology to characterize the nature and timing of metamorphism and their tectonic implications. From textural studies and mineral phase equilibrium modelling, we infer that Sill + Grt + Crd representing prograde metamorphism was stable at 6–9 kbar and 760–790 °C. Equilibrium spinel-quartz assemblage suggests peak metamorphic conditions of 6.5–7 kbar and 1010–1030 °C consistent with ultra-high temperature metamorphism in the Trivandrum Block. Growth of cordierite and biotite at the expense of garnet correlates with retrograde metamorphism at 4.5–6.5 kbar and 770–950 °C. Our results allow an interpretation of the prograde, peak, and retrograde P–T conditions of the khondalites, where from the peak ultrahigh-temperature stage of >1000 °C, the rocks underwent isothermal decompression as indicated by garnet breakdown to cordierite, followed by isobaric cooling, corresponding to an overall clockwise P–T evolution. We present results from U-Pb geochronology of zircon as well as LA-ICPMS data on monazite grain separates. The zircon grains show two distinct age peaks, with Paleoproterozoic cores surrounded by Late Neoproterozoic-Cambrian rims or recrystallized domains. The younger ages around 550–560 Ma are similar to the lower intercept age, whereas the wide range of Paleoproterozoic ages fall along a discordia. The LA-ICP-MS monazite U-Pb data also show two distinct age populations at Paleoproterozoic and latest Neoproterozoic-Cambrian, although the Paleoproterozoic population is scarce. We correlate the late Neoproterozoic-Cambrian ages to the ultrahigh-temperature metamorphic event, associated with the assembly of the Gondwana supercontinent. The wide range of monazite ages indicate that high temperature metamorphism was long-lived.

Neoproterozoic evolution and Cambrian reworking of ultrahigh temperature granulites in the Eastern Ghats Province, India

AbstractThe time‐scales and P–T conditions recorded by granulite facies metamorphic rocks permit inferences about the geodynamic regime in which they formed. Two compositionally heterogeneous cordierite–spinel‐bearing granulites from Vizianagaram, Eastern Ghats Province (EGP), India, were investigated to provide P–T–time constraints using petrography, phase equilibrium modelling, U–Pb geochronology, the rare earth element composition of zircon and monazite, and Ti‐in‐zircon thermometry. These ultrahigh temperature (UHT) granulites preserve discrete compositional layering in which different inferred peak assemblages are developed, including layers bearing garnet–sillimanite–spinel, and others bearing orthopyroxene–sillimanite–spinel. These mineral associations cannot be reproduced by phase equilibrium modelling of whole‐rock compositions, indicating that the samples became domainal on a scale less than that of a thin section, even at UHT conditions. Calculation of the P–T stability fields for six compositional domains within which the main rock‐forming minerals are considered to have attained equilibrium suggests peak metamorphic conditions of ~6.8–8.3 kbar at ~1,000°C. In most of these domains, the subsequent evolution resulted in the growth of cordierite and final crystallization of melt at an elevated (residual) H2O‐undersaturated solidus, consistent with <1 kbar of decompression. Concordant U–Pb ages obtained by SHRIMP from zircon (spread 1,050–800 Ma) and monazite (spread 950–800 Ma) demonstrate that crystallization of these minerals occurred during an interval of c. 250 Ma. By combining LA‐ICP‐MS U–Pb zircon ages with Ti‐in‐zircon temperatures from the same analysis sites, we show that the crust may have remained above 900°C for a minimum of c. 120 Ma between c. 1,000 and c. 880 Ma. Overall, the results suggest that, in the interval 1,050 to 800 Ma, the evolution of the Vizianagaram granulites culminated with UHT conditions from c. 1,000 Ma to c. 880 Ma, associated with minor decompression, before further zircon crystallization at c. 880–800 Ma during cooling to the solidus. However, these rocks are adjacent to the Paderu–Anantagiri–Salur crustal block to the NW that experienced counterclockwise P–T–t paths, and records similar UHT peak metamorphic conditions (7–8 kbar, ~950°C) followed by near‐isobaric cooling, and has a similar chronology during the Neoproterozoic. The limited decompression inferred at Vizianagaram may be explained by partial exhumation due to thrusting of this crustal block over the adjacent Paderu–Anantagiri–Salur crustal block. The residual granulites in both blocks have high concentrations of heat‐producing elements and likely remained hot at mid‐crustal depths throughout a period of relative tectonic quiescence in the interval 800–550 Ma. During the Cambrian Period, the EGP was located in the hinterland of the Denman–Pinjarra–Prydz orogen. A later concordant population of zircon dated at 511 ± 6 Ma records crystallization at temperatures of ~810°C. This age may record a low‐degree of melting due to limited influx of fluid into hot, weak crust in response to convergence of the Crohn craton with a composite orogenic hinterland comprising the Rayner terrane, EGP, and cratonic India.

Migmatization and low-pressure overprinting metamorphism as record of two pre-Cretaceous tectonic episodes in the Santander Massif of the Andean basement in northern Colombia (NW South America)

The core of the Santander Massif in the northern Andes of Colombia is dominated by migmatitic gneisses with a <1.71Ga protolith and was affected by continuous interactions of oceanic plates to the west and the northwestern corner of the South American continental plate. The exposed metamorphic core of the massif offers a unique opportunity to understand the tectonic evolution of northwestern South America. We present new metamorphic petrology and geochemistry data from the Bucaramanga Gneiss in the Santander Massif to document part of this tectonic evolution from late Proterozoic to Jurassic times.Metapelitic migmatite gneiss, quartz-feldspathic gneiss, and amphibolite from the Bucaramanga Gneiss recorded metamorphic peak conditions in the range of 660–850°C at pressures of >7.5kbar. Lithologies are overprinted by low-pressure metamorphism, related to extensive Jurassic intrusions and linked with growth of cordierite and equilibration of low-pressure mineral assemblages, recorded metamorphic conditions are <750°C and <6.5kbar.Observed leucosomes display significant compositional variations and can be grouped in three groups: i) Group One leucosomes with high total REE content, high LREE/HREE, and negative Eu anomaly, ii) Group Two leucosomes with low total REE, low LREE/HREE, and positive Eu anomalies, and iii) Group Three leucosomes with relatively low LREE/HREE and strong positive Eu anomaly. Geochemical data support the interpretation that Group Two leucosomes crystallized from melts originated in a partial melting event affecting mostly pelitic and quartz-feldspathic lithologies with fluid-present melting reactions. The evaluation of mesosomes (amphibolite, pelitic and quartz-feldspathic rocks) as potential protoliths or restites indicates that at least two pelitic samples of the analyzed lithologies have characteristics consistent with the occurrence of fluid-present melting reactions involving quartz and feldspar. The leucosomes produced by crystallization of modified partial melts contrast with several other leucosomes that were injected; however, in some cases the melts crystallized as injected leucosomes show consistent geochemistry with partial melting of lithologies geochemically similar to the ones observed in the unit.The migmatization and the low pressure metamorphic overprint are related here to two main tectonic events: an early Paleozoic tectonic pulse produced by subduction of the oceanic crust of the Iapetus Ocean beneath northwestern Gondwana, and an Upper Triassic to Lower Jurassic tectonic pulse produced by subduction of oceanic crust of the proto-Pacific ocean beneath western Pangaea.

Multiphase melting, magma emplacement and P-T-time path in late-collisional context: the Velay example (Massif Central, France)

AbstractThe West European Variscan chain is a remarkable illustration of how partial melting marks out the geodynamic evolution of mountain belt through time. Here, we focus on the Late Carboniferous melting events reported in the southeastern French Massif Central (Velay dome), with emphasis on the modes of partial melting, relationships between partial melting and magma emplacement, transition between the melting episodes and related P-T-t path. Following nappe stacking events under medium pressure/temperature conditions (M1 and M2 events), three melting events are identified in the southern envelope of the Velay dome. A first melting episode (M3 event) occurred within the biotite stability field at 325–315 Ma (T ≈ 720°C and P = 0.5–0.6 GPa). It led to the complete disappearance of muscovite and to the formation of migmatites consisting of biotite ± sillimanite melanosome and of granitic/tonalitic leucosomes depending on protolith composition. It is interpreted as the result of internal heating mainly linked to decay of heat producing elements accumulated in a thickened crust. It resulted in the formation of a partially molten middle crust with decoupling between the lower and upper crust, late-collisional extension and crustal thinning.The second episode of melting (M4 event) occurred at ca. 304 Ma (T 800°C and P 0.4 GPa), synchronously with emplacement of the Velay granites and growth of the dome. It led to the breakdown of biotite and growth of cordierite (locally garnet or tourmaline), with formation of diatexites and heterogeneous granites. This high-T event synchronous with crustal extension is considered to result from intrusion of hot mantle-derived and lower crustal magmas triggering catastrophic melting in the middle crust. This event ends with local retrograde hydrous melting within the stability field of biotite close to the solidus in response to local input of water during temperature drop in the late stage of emplacement of the Velay dome.The last evidence of melting in this area (M5 event) corresponds to emplacement of late granites generated under conditions estimated at ≈850°C and 0.4–0.6 GPa. They may have been generated from melting of specific lithologies triggered by injection of mafic magmas. These granites emplaced in a partly cooled crust (medium-grade conditions). The emplacement age of these granites is not well constrained (305–295 Ma) though they clearly post-date the Velay granites.The melting episodes in the Velay area and generation of granites appear to correspond to the conjunction between (i) the effects of collision-related crust thickening and (ii) those related to slab break off and asthenospheric mantle decompression melting. The driving process is mainly the internal radiogenic heat in a first stage, relayed by the propagation of a thermal anomaly initially located in the lower crust (M3 event), but which subsequently rose to the middle and upper crustal levels through magma transfer (M4 event). Overall, the Velay example is a remarkable illustration of the progressive dehydration and sterilisation of a thickened crustal segment. It documents how large amounts of granitic magmas can be produced at shallow crustal levels in relation to the injection of mantle-derived magmas.

P–T–t–d evolution of orogenic middle crust of the Roc de Frausa Massif (Eastern Pyrenees): a result of horizontal crustal flow and Carboniferous doming?

AbstractStructural, petrological and textural studies are combined with phase equilibria modelling of metapelites from different structural levels of the Roc de Frausa Massif in the Eastern Pyrenees. The pre‐Variscan lithological succession is divided into the Upper, Intermediate and Lower series by two orthogneiss sheets and intruded by Variscan igneous rocks. Structural analysis reveals two phases of Variscan deformation. D1 is marked by tight to isoclinal small‐scale folds and an associated flat‐lying foliation (S1) that affects the whole crustal section. D2 structures are characterized by tight upright folds facing to the NW with steep NE–SW axial planes. D2 heterogeneously reworks the D1 fabrics, leading to an almost complete transposition into a sub‐vertical foliation (S2) in the high‐grade metamorphic domain. All structures are affected by late open to tight, steeply inclined south‐verging NW–SE folds (F3) compatible with steep greenschist facies dextral shear zones of probable Alpine age. In the micaschists of the Upper series, andalusite and sillimanite grew during the formation of the S1 foliation indicating heating from 580 to 640 °C associated with an increase in pressure. Subsequent static growth of cordierite points to post‐D1 decompression. In the Intermediate series, a sillimanite–biotite–muscovite‐bearing assemblage that is parallel to the S1 fabric is statically overgrown by cordierite and K‐feldspar. This sequence points to ~1 kbar of post‐D1 decompression at 630–650 °C. The Intermediate series is intruded by a gabbro–diorite stock that has an aureole marked by widespread migmatization. In the aureole, the migmatitic S1 foliation is defined by the assemblage biotite–sillimanite–K‐feldspar–garnet. The microstructural relationships and garnet zoning are compatible with the D1 pressure peak at ~7.5 kbar and ~750 °C. Late‐ to post‐S2 cordierite growth implies that F2 folds and the associated S2 axial planar leucosomes developed during nearly isothermal decompression to &lt;5 kbar. The Lower series migmatites form a composite S1–S2 fabric; the garnet‐bearing assemblage suggests peak P–T conditions of &gt;5 kbar at suprasolidus conditions. Almost complete consumption of garnet and late cordierite growth points to post‐D2 equilibration at &lt;4 kbar and &lt;750 °C. The early metamorphic history associated with the S1 fabric is interpreted as a result of horizontal middle crustal flow associated with progressive heating and possible burial. The upright F2 folding and S2 foliation are associated with a pressure decrease coeval with the intrusion of mafic magma at mid‐crustal levels. The D2 tectono‐metamorphic evolution may be explained by a crustal‐scale doming associated with emplacement of mafic magmas into the core of the dome.

Prograde and retrograde growth of monazite in migmatites: An example from the Nagercoil Block, southern India

Data from a migmatised metapelite raft enclosed within charnockite provide quantitative constraints on the pressure–temperature–time (P–T–t) evolution of the Nagercoil Block at the southernmost tip of peninsular India. An inferred peak metamorphic assemblage of garnet, K-feldspar, sillimanite, plagioclase, magnetite, ilmenite, spinel and melt is consistent with peak metamorphic pressures of 6–8 kbar and temperatures in excess of 900 °C. Subsequent growth of cordierite and biotite record high-temperature retrograde decompression to around 5 kbar and 800 °C. SHRIMP U–Pb dating of magmatic zircon cores suggests that the sedimentary protoliths were in part derived from felsic igneous rocks with Palaeoproterozoic crystallisation ages. New growth of metamorphic zircon on the rims of detrital grains constrains the onset of melt crystallisation, and the minimum age of the metamorphic peak, to around 560 Ma. The data suggest two stages of monazite growth. The first generation of REE-enriched monazite grew during partial melting along the prograde path at around 570 Ma via the incongruent breakdown of apatite. Relatively REE-depleted rims, which have a pronounced negative europium anomaly, grew during melt crystallisation along the retrograde path at around 535 Ma. Our data show the rocks remained at suprasolidus temperatures for at least 35 million years and probably much longer, supporting a long-lived high-grade metamorphic history. The metamorphic conditions, timing and duration of the implied clockwise P–T–t path are similar to that previously established for other regions in peninsular India during the Ediacaran to Cambrian assembly of that part of the Gondwanan supercontinent.

Separating metamorphic events in the Fosdick migmatite–granite complex, West Antarctica

AbstractThe Fosdick migmatite–granite complex in West Antarctica records evidence for two high‐temperature metamorphic events, the first during the Devonian–Carboniferous and the second during the Cretaceous. The conditions of each high‐temperature metamorphic event, both of which involved melting and multiple melt‐loss events, are investigated using phase equilibria modelling during successive melt‐loss events, microstructural observations and mineral chemistry. In situ SHRIMP monazite and TIMS Sm–Nd garnet ages are integrated with these results to constrain the timing of the two events. In areas that preferentially preserve the Devonian–Carboniferous (M1) event, monazite grains in leucosomes and core domains of monazite inclusions in Cretaceous cordierite yield an age of c. 346 Ma, which is interpreted to record the timing of monazite growth during peak M1 metamorphism (∼820–870 °C, 7.5–11.5 kbar) and the formation of garnet–sillimanite–biotite–melt‐bearing assemblages. Slightly younger monazite spot ages between c. 331 and 314 Ma are identified from grains located in fractured garnet porphyroblasts, and from inclusions in plagioclase that surround relict garnet and in matrix biotite. These ages record the growth of monazite during garnet breakdown associated with cooling from peak M1 conditions. The Cretaceous (M2) overprint is recorded in compositionally homogeneous monazite grains and rim domains in zoned monazite grains. This monazite yields a protracted range of spot ages with a dominant population between c. 111 and 96 Ma. Rim domains of monazite inclusions in cordierite surrounding garnet and in coarse‐grained poikiloblasts of cordierite yield a weighted mean age of c. 102 Ma, interpreted to constrain the age of cordierite growth. TIMS Sm–Nd ages for garnet are similar at 102–99 Ma. Mineral equilibria modelling of the residual protolith composition after Carboniferous melt loss and removal of inert M1 garnet constrains M2 conditions to ∼830–870 °C and ∼6–7.5 kbar. The modelling results suggest that there was growth and resorption of garnet during the M2 event, which would facilitate overprinting of M1 compositions during the M2 prograde metamorphism. Measured garnet compositions and Sm–Nd diffusion modelling of garnet in the migmatitic gneisses suggest resetting of major elements and the Sm–Nd system during the Cretaceous M1 overprint. The c. 102–99 Ma garnet Sm–Nd ‘closure’ ages correspond to cooling below 700 °C during the rapid exhumation of the Fosdick migmatite–granite complex.

Mimicking syntectonic growth: cordierite overgrowth of earlier rotated staurolite porphyroblasts, strain caps and deflected foliation

Monoclinic shape fabrics and inclusion trail geometries of porphyroblasts are regarded as reliable shear-sense indicators, provided that timing and sequence of growth can be established. In the southern Bossòst dome of the Central Pyrenees, staurolite and cordierite porphyroblasts in mica schists contain inclusion trails oblique to, but continuous with the external foliation, indicating porphyroblast rotation. Straight inclusion trails in staurolites record growth between formation of the main schistosity and subsequent shear, i.e. an intertectonic origin. Strain caps and deflection of foliation around porphyroblasts show a distinct asymmetry with uniform sense of shear. Cordierite porphyroblasts are significantly larger than staurolites and contain curved inclusion trails which would suggest syntectonic growth and a similar shear of sense. However, staurolite and cordierite do not belong to the same paragenesis and textural evidence, corrosion of staurolite rims and relict inclusions in cordierites, suggest partial consumption by cordierite. Complete overgrowth of staurolite with preservation of its inclusion trails and adjacent foliation deflection results in curved inclusion trails that mimic syntectonic growth of cordierite. Actually, the larger cordierites statically overgrew rotated staurolites during post-tectonic contact metamorphism.

CHIME dating and age mapping of monazite in granulites and paragneisses from the Hwacheon area, Korea: implications for correlations with Chinese cratons

The basement of the Hwacheon area in the northern part of the Gyeonggi massif in the Korean Peninsula comprises granulite and gneiss complexes that are separated by an extensive mylonite zone. Granulites distant from the mylonite zone show little evidence of a retrograde overprint beyond minor growth of biotite along garnet grain boundaries. Monazite grains enclosed in garnet show a core to rim decrease in Y, and those in the matrix show a peripheral enrichment of Y on low-Y interiors. Similarly, garnet grains decrease in Y from core to rim, with peripheral Yenrichment, which can be attributed to early to peak garnet growth and post-growth modification, respectively. The three chemical domains representing stages of monazite growth show no detectable age difference, together yielding a CHIME age of 1868 ± 24 Ma. Granulites near the mylonite zone show growth of cordierite and biotite aggregates around garnet grains, and yield Permo-Triassic monazite domains on and within predominantly Paleoproterozoic monazite grains. Mylonitized granulites have a metamorphic overprint characterized by the growth of the sillimanite and K-feldsper assemblage at ca. 241 Ma, subsequent to ductile shearing. Pelitic gneisses in the gneiss complex contain zircon grains with Neoproterozoic detrital cores and monazite grains with unequivocal metamorphic ages of ca. 245 Ma. Geological, petrological and chronological evidence suggests differing evolutional histories between two crustal domains prior to amalgamation. The widespread occurrence of Permo-Triassic paragneisses and mylonites in the Gyeonggi massif can be explained by detachment of the metamorphosed upper crust of the South China block, which was thrust over the Paleoproterozoic granulite complex at ca. 241 Ma during the Sulu orogeny. Permo-Triassic metamorphic overprints on the latter are attributed to contact metamorphism from the hot allochthon.

Exhumation of HT–LP Variscan metapelites from interlayered cordierite–orthoamphibole assemblages (MnNCFMASH System) Catalan Coastal Ranges, NE Iberia

Garnet–cordierite–orthoamphibole rocks crop out in centimetric lenses within the pre-Caradocian metapelites of the Osor complex in the sillimanite metamorphic zone of the HT-LP Variscan sequence from the Guilleries Massif in the Catalan Coastal Ranges (NE Iberia). The P– T path followed by the metapelites shows increasing T and P to a peak of 660 °C–5 kbar, then isobaric cooling and finally cooling and decompression. Reaction textures on amphibolite lenses show evidence of resorption of garnet and gedrite and growth of cordierite, anthophyllite and chlorite, which has led to the stable sub-assemblages: garnet + cordierite and gedrite + anthophyllite + cordierite ± chlorite. Petrography, mineral chemistry, and modelling of the low-Ca amphibolite reaction assemblages using a pseudosection approach in MnNCFMASH also suggest a decompressional path when generation of two effective bulk compositional microdomains during cordierite moat formation is taken into account.

Geodynamic evolution of Mt. Riiser-Larsen, Napier Complex, East Antarctica, with reference to the UHT mineral associations and their reaction relations

Abstract Mt. Riiser-Larsen is the largest outcrop in the Archaean–early Proterozoic Napier Complex, East Antarctica. The area is structurally divided into the Main and the Western Blocks by the subvertical Riiser-Larsen Main Shear Zone (RLMSZ) of about 200 m width composed of mylonite and pseudotachylite. Mineral parageneses including sapphirine+quartz and osumilite, diagnostic of ultrahigh-temperature (UHT) metamorphism, are found in Mg-rich aluminous, siliceous and quartzo-feldspathic gneiss layers in both the Main and the Western Blocks of the Mt. Riiser-Larsen area. Some of the sapphirine–quartz associations are accompanied by retrograde reaction textures, which include growth of cordierite and/or garnet between sapphirine and quartz in the Main Block, and of orthopyroxene+sillimanite in the Western Block. These textures indicate the reaction (1) sapphirine + quartz ( + orthopyroxene ) = cordierite and (2) sapphirine + quartz ( + orthopyroxene ) = garnet in the Main Block and (3) sapphirine + quartz = orthopyroxene + sillimanite in the Western Block. Phase equilibria and P – T pseudosections for sapphirine+quartz-bearing associations suggest that these three reactions took place during a temperature drop from 1100 °C to 1000 °C at pressures of 0.6–0.8 GPa in the Main Block and 0.8–0.9 GPa in the Western Block. The geological structure and distribution of the UHT rocks provide an insight into the vertical extent of the&gt;1000 °C UHT metamorphic zone: a minimum thickness of 4–5 km of the UHT-metamorphosed layers, which become deeper towards the west in the Main Block. The Western Block represents a c. 0.1–0.3 GPa ( c. 3–10 km) deeper structural level than the Main Block. In addition to the extent of the horizontal distribution of UHT metamorphism in the Napier Complex, our results on the vertical component provide new constraints for modelling the heat source and tectonic process of the unusually high-temperature regional metamorphism in the late Archaean–early Proterozoic. Electron microprobe monazite U–Th–Pb dating for hydrated and mylonitized sapphirine–quartz gneiss gave a wide spectrum of monazite age distribution between 2300 and 800 Ma, suggesting the tectonic uplift and juxtaposition of the two blocks in the Mt. Riiser-Larsen area later than the mid–late Proterozoic.

Age constraints on the tectonothermal evolution of the Selwyn Zone, Eastern Fold Belt, Mount Isa Inlier

The Selwyn Zone is characterized by multiple ductile deformation and metamorphic events. There is general consensus that early in the Isan Orogeny there were both extensional and thrust events that produced bedding-parallel foliation (S bp). We propose that these were followed by the D 1 event that resulted in a regional steep east–west foliation, and that the Snake Creek Anticline and the dominant S 2a regional foliation formed as upright north–south structures. Local overturning of D 2 structures was due to subsequent localized deformation (D 2b) that resulted in structures with flat-lying axial planes. D 3 and D 4 produced steep upright folds and crenulations with north–northwest and northeast trending axial surfaces. Metamorphism, largely controlled by the emplacement of mafic rocks, is dominantly low pressure, but the P–T–t path is complex. Early growth of cordierite (M 1) followed by andalusite, was succeeded by a clockwise medium pressure loop of ca. 500–600 MPa in the northern Snake Creek Anticline, as indicated by early D 2a kyanite growth. The metamorphic peak during D 2 was around 580 °C and 400 MPa at the sillimanite isograd and reached a maximum of around 670 °C and 400–500 MPa in the migmatitic sillimanite K-feldspar zone at the Cannington and Osborne mines. Previous age determinations for the metamorphic peak of ∼1580–1600 Ma are confirmed with new electron microprobe dating of monazite, whereas an age grouping of 1600–1630 Ma may represent the D 1 (M 2) event. Additional ages in the range 1640–1680 Ma are recorded in albitites from the Soldiers Cap Group, and it is suggested that they formed by infiltration of hydrothermal fluids during extension and the emplacement of mafic sills. This is supported by SHRIMP dating of zircon from a trondhjemite lens in a largely mafic sill complex in the Snake Creek Anticline that provided an age of 1686 ± 8 Ma, also indicating that at least part of the Soldiers Cap Group is older than previously indicated. A zircon SHRIMP age of 1527 ± 4 Ma was obtained for a sample of albitized Saxby Granite. This age also represents the D 4 (M 6) event, and is the timing of the extensive brecciation and albitization of calc-silicate rocks of the Corella Formation in the Selwyn Zone.

Two‐stage exhumation of midcrustal arc rocks, Coast Mountains, British Columbia

New geologic data from the Central Gneiss Complex along Douglas Channel help delineate the burial and exhumation of the Late Cretaceous to Eocene Coast Mountains magmatic arc. Arc plutonism was on going between ∼90 and 60 Ma, replaced by formation of dikes to 52 Ma. Supracrustal rocks were buried to midcrustal levels by 90 Ma, and garnet and kyanite grew from ∼90 through 70 Ma. Subsequent exhumation was nearly isothermal and took place in two stages. The first occurred from ∼70 to 59 Ma as the arc contracted obliquely and sillimanite replaced kyanite. Exhumation was slow (∼0.5 mm/yr), probably accomplished by erosion aided by coaxial crustal thinning. Exhumation rates about doubled in the second stage of exhumation, after 59 Ma and before ∼52 Ma. Penetrative deformation ended prior to intrusion of the Quottoon pluton at 59 Ma and ∼6.5 kbar. Production of cordierite rims on garnet at 4.5 kbar and ∼700°C signaled the end of near‐isothermal decompression prior to 52 Ma. Rapid cooling (100°C/106 years) of the Central Gneiss Complex followed cordierite growth; temperatures dropped to ∼250° by ∼48 Ma. The increase in exhumation rate and the subsequent rapid cooling are attributed to excision of &gt;6 km of crust on a detachment system on the northeastern side of the Central Gneiss Complex. Comparison to other parts of the Coast Mountains arc, the Sierra Nevada, and Fiordland, New Zealand, shows that the amount and tempo of exhumation vary greatly within and between arcs, suggesting that the processes accommodating exhumation vary significantly.

Polymetamorphism and ductile deformation of staurolite–cordierite schist of the Bossòst dome: indication for Variscan extension in the Axial Zone of the central Pyrenees

The Bossòst dome is an E–W-trending elongated structural and metamorphic dome developed in Cambro-Ordovician metasedimentary rocks in the Variscan Axial Zone of the central Pyrenees. A steep fault separates a northern half-dome, cored by massif granite, from an E–W-trending doubly plunging antiform with granitic sills and dykes in the core to the south. The main foliation is a flat-lying S1/2 schistosity that grades into a steeper-dipping slaty cleavage at the dome margins. Three major deformational and two metamorphic phases can be differentiated. S1/2 schistosity is an axial planar cleavage to W-vergent recumbent folding that probably occurred in mid-Westphalian time. Peak regional metamorphism M1 is characterized by static growth of staurolite and garnet following thermal relaxation of the previously thickened crust. Strong non-coaxial deformation recording uniform top-to-the-SE extension during D2a is preserved in staurolite–garnet schists in a 1.5 km thick, shallowly SE-dipping zone in the southeastern dome. A 500 m thick contact aureole (M2) was imprinted on the regionally metamorphosed rocks following the intrusion the Bossòst granite during D2b. More coaxial deformation prevailed during synkinematic growth of M2 phases in the inner part of the contact aureole around the northern part of the dome, where it obliterated D2a fabrics. Progressive non-coaxial deformation continued in the southeastern antiform and is recorded by late-synkinematic growth of cordierite. Successive overprinting of the M1 staurolite–garnet assemblage by andalusite and cordierite of M2 is preserved in the southern part. The assemblage muscovite+cordierite+staurolite+biotite is considered metastable, given the low Mn and Zn contents of staurolite and cordierite, and interpreted as the result of prograde metamorphism during decompression. P–T conditions during M2 were approximately 3 kbar and 600 °C. Pervasive crenulations and mesoscopic to regional southerly verging folds are the result of D3 NNE–SSW compression post-dating ductile deformation and contact metamorphism. Polymetamorphic assemblages of the Bossòst dome preserve a regionally confined zone of ESE-directed extensional shearing within an overall N–S compressional setting. Exact timing of extensional shearing is not known, but can be constrained by recumbent folding during the mid-Westphalian and granitic intrusions, which confine it to Late Carboniferous time (c. 305 Ma). Crustal-scale flat-lying extensional shear zones with similar orientation and time frame are observed in the Hospitalêt massif of the eastern Axial Zone. This suggests that crustal extension, though probably restricted by regional strain partitioning over orthogneiss or intruding granitic bodies within an overall compressive setting, was not uncommon in Late Carboniferous time in the Axial Zone of the Pyrenees.

Orthopyroxene–sillimanite–quartz assemblages: distribution, petrology, quantitative P–T–X constraints and P–T paths

AbstractGranulite facies magnesian metapelites commonly preserve a wide array of mineral assemblages and reaction textures that are useful for deciphering the metamorphic evolution of a terrane. Quantitative pressure, temperature and bulk composition constraints on the development and preservation of characteristic peak granulite facies mineral assemblages such as orthopyroxene + sillimanite + quartz are assessed with reference to calculated phase diagrams. In NCKFMASH and its chemical subsystems, peak assemblages form mainly in high‐variance fields, and most mineral assemblage changes reflect multivariant equilibria. The rarity of orthopyroxene–sillimanite–quartz‐bearing assemblages in granulite facies rocks reflects the need for bulk rock XMg of greater than approximately 0.60–0.65, with pressures and temperatures exceeding c. 8 kbar and 850 °C, respectively. Cordierite coronas mantling peak minerals such as orthopyroxene, sillimanite and quartz have historically been used to infer isothermal decompression P–T paths in ultrahigh‐temperature granulite facies terranes. However, a potentially wide range of P–T paths from a given peak metamorphic condition facilitate retrograde cordierite growth after orthopyroxene + sillimanite + quartz, indicating that an individual mineral reaction texture is unable to uniquely define a P–T vector. Therefore, the interpretation of P–T paths in high‐grade rocks as isothermal decompression or isobaric cooling may be overly simplistic. Integration of quantitative data from different mineral reaction textures in rocks with varying bulk composition will provide the strongest constraints on a P–T path, and in turn on tectonic models derived from these paths.

Heterogeneous growth of cordierite in low P/T Tsukuba metamorphic rocks from central Japan

AbstractThis paper examines the spatial statistics of matrix minerals and complex patterned cordierite porphyroblasts in the low‐pressure, high‐temperature (low P/T) Tsukuba metamorphic rocks from central Japan, using a density correlation function. The cordierite‐producing reaction is sillimanite + biotite + quartz = K‐feldspar + cordierite + water. The density correlation function shows that quartz is distributed randomly. However, the density correlation functions of biotite, plagioclase and K‐feldspar show that their spatial distributions are clearly affected by the formation of cordierite porphyroblasts. These observations suggest that cordierite growth occurred through a selective growth mechanism: quartz adjacent to cordierite has a tendency to prevent the growth of cordierite, whereas other matrix minerals adjacent to cordierite have a tendency to enhance the growth of cordierite. The density correlation functions of complex patterned cordierite porphyroblasts show power‐law behaviour. A selective growth mechanism alone cannot explain the origin of the power‐law behaviour. Comparison of the morphology and fractal dimension of cordierite with two‐dimensional sections from a three‐dimensional diffusion‐limited aggregation (DLA) suggests that the formation of cordierite porphyroblasts can be modelled as a DLA process. DLA is the simple statistical model for the universal fractal pattern developed in a macroscopic diffusion field. Diffusion‐controlled growth interacting with a random field is essential to the formation of a DLA‐like pattern. The selective growth mechanism will provide a random noise for the growth of cordierite due to random distribution of quartz. Therefore, a selective growth mechanism coupled with diffusion‐controlled growth is proposed to explain the power‐law behaviour of the density correlation function of complex patterned cordierite. The results in this paper suggest that not only the growth kinetics but also the spatial distribution of matrix minerals affect the progress of the metamorphic reaction and pattern formation of metamorphic rocks.