Garnet Rims Research Articles

Origin of oscillatory zoned garnets from the Xieertala Fe–Zn skarn deposit, northern China: In situ LA–ICP-MS evidence

Oscillatory zoned garnets are able to record numerous geological processes including variations of physicochemical conditions and hydrothermal fluid evolution. In this contribution, we present the mineralogy, petrography, and systematic major, trace and rare earth element (REE) concentrations of zoned garnets from the Xieertala Fe–Zn skarn deposit. The in situ analytical results of oscillatory zoned garnets from core to rim demonstrate that garnet cores are remarkably HREE-rich, with relatively high ∑REE and HREE/LREE ratios as well as negative Eu anomalies. REE patterns of garnet rims are typically LREE-rich, with relatively low ∑REE and HREE/LREE ratios and positive Eu anomalies. Estimates of PT conditions suggest that the early fluid formed in relatively high temperature, pressure and salinity conditions, while the later fluid formed in relatively low temperature, pressure and salinity conditions. This transition in P, T and salinity conditions is interpreted to be caused by the dominance of magmatic and meteoric waters in early and late stages, respectively. Based on optical and textural characteristics, REE patterns, Eu anomalies and trace element variations in zoned garnets, it can be shown that during skarn formation, Xieertala hydrothermal fluids shifted from near neutral pH and oxidizing conditions with relatively high ∑REE, low LREE/HREE ratios, and U-poor and Y-rich characteristics to acidic and reducing conditions with relatively low ∑REE, high LREE/HREE ratios, and U-rich and Y-poor characteristics. We infer that variations of fluid compositions and physicochemical conditions may have a major control on incorporating trace elements and REEs into different sections of oscillatory zoned garnets.

Late‐stage orogenic loading revealed by contact metamorphism in the northern Appalachians, New York

AbstractPelitic schists from contact aureoles surrounding mafic–ultramafic plutons in Westchester County, NY record a high‐P (~0.8 GPa) high‐T (~790 °C) contact overprint on a Taconic regional metamorphic assemblage (~0.5 GPa). The contact metamorphic assemblage of a pelitic sample in the innermost aureole of the Croton Falls pluton, a small (<10 km2) gabbroic body, consists of quartz–plagioclase–biotite–garnet–sillimanite–ilmenite–graphite–Zn‐rich Al‐spinel. Both K‐feldspar and muscovite are absent, and abundant biotite, plagioclase, sillimanite, quartz and ilmenite inclusions are found within subhedral garnet crystals. Unusually low bulk‐rock Na and K contents imply depletion of alkalic components and silica through anatexis and melt extraction during contact heating relative to typical metapelites outside the aureole. Thermobarometry on nearby samples lacking a contact overprint yields 620–640 °C and 0.5–0.6 GPa. In the aureole sample, WDS X‐ray chemical maps show distinct Ca‐enriched rims on both garnet and matrix plagioclase. Furthermore, biotite inclusions within garnet have significantly higher Mg concentration than matrix biotite. Thermobarometry using GASP and garnet–biotite Mg–Fe exchange equilibria on inclusions and adjacent garnet host interior to the high‐Ca rim zone yield ~0.5 ± 0.1 GPa and ~620 ± 50 °C. Pairs in the modified garnet rim zone yield ~0.9 ± 0.1 GPa and ~790 ± 50 °C. Thermocalc average P–T calculations yield similar results for core (~0.5 ± ~0.1 GPa, ~640 ± ~80 °C) and rim (~0.9 ± ~0.1 GPa, ~800 ± ~90 °C) equilibria. The core assemblages are interpreted to record the P–T conditions of peak metamorphism during the Taconic regional event whereas the rim compositions and matrix assemblages are interpreted to record the P–T conditions during the contact event. The high pressures deduced for this later event are interpreted to reflect loading due to the emplacement of Taconic allochthons in the northern Appalachians during the waning stages of regional metamorphism (after c. 465 Ma) and before contact metamorphism (c. 435 Ma). In the absence of contact metamorphism‐induced recrystallization, it is likely that this regional‐scale loading would remain cryptic or unrecorded.

Modeling of zoning patterns in garnet: Thermodynamic and kinetic aspects

Results from the modeling of compositional zoning patterns in garnet porphyroblasts from the medium-grade metapelitic schist of northern Ladoga area are considered. The P-T pseudosections in the model KMnFMASH system were calculated for this purpose using THERMOCALC software (Powell et al., 1998). Particular emphasis is placed upon the effect of garnet growth kinetics on the model zoning profiles for Mn (Gulbin, 2013). They fit the observed profiles if intergranular diffusion-controlled growth is assumed for porphyroblasts. Additionally, a model of metamorphic fractional crystallization is used to characterize the oscillations in both the garnet core and rim. Starting from the assumption that a reservoir, where garnet grows, consists of chlorite, and that this mineral is intensely replaced with biotite and staurolite at the onset of crystallization, a partial release of Mn from the chlorite structure and the concentration of this component in intergranular space is inferred. In terms of the model under consideration, the coefficient of the Mn partition between garnet and reservoir temporarily increases at the early stage of garnet growth, giving rise to the enrichment of the intermediate zone of porphyroblasts in Mn. In addition to the modeling of garnet growth zoning, its subsequent diffusion modification is estimated on the basis of intracrystalline diffusion profile simulation. The reverse zoned, Mn-rich and Mg-poor garnet rims are related to retrograde growth of garnet at the late stage of porphyroblast formation. The data obtained are used to constrain metamorphic evolution and the P-T-t path of staurolite-bearing rocks in the northern domain of the studied area.

A Garnet–Zircon Oxygen Isotope Record of Subduction and Exhumation Fluids from the Franciscan Complex, California

Tectonic blocks of high-grade metamorphic rock hosted by the Franciscan me¤ lange commonly preserve textural and geochemical evidence for metasomatism and blueschist-facies overprinting within a subduction zone. These exhumation events can obscure information on the nature of subduction fluids at depth that might otherwise be preserved by these rocks in this tectonic setting. Cation and oxygen isotope zoning in garnets from eclogite and garnet^hornblende rocks with minimal textural evidence of retrogression reveals separate prograde and retrograde fluid histories. Garnet cores preserve a homogeneous prograde oxygen isotope history with both high (dO 11o) and low ( 4o) values in different samples probably recording altered ocean-crust protolith compositions. Multiple episodes of garnet resorption and regrowth record a dramatically changing bulk oxygen isotope ratio resulting in 6^7o neoformed garnet rims with sharp core^rim boundaries in both samples. Matrix omphacite and hornblende are in profound oxygen isotope disequilibrium with garnet cores but are in equilibrium with garnet rims despite the appearance of overall textural equilibrium in thin section.This cryptic metasomatism found in samples from different locations within the Franciscan Complex brings into question existing thermobarometry on non-inclusion phases for this complex, as well as interpretations based on whole-rock geochemical data. Zircon ages and oxygen isotope equilibrium with late-forming garnet are consistent with postmetasomatic formation of zircon.The ability of garnet and zircon to record multiple fluid events and tie them to P^T^t^fluid history is a powerful tool in gaining an improved understanding of the complex fluid environments within subduction zones.

The role of trace element partitioning between garnet, zircon and orthopyroxene on the interpretation of zircon U–Pb ages: an example from high-grade basement in Calabria (Southern Italy)

The recognition of the coeval growth of zircon, orthopyroxene and garnet domains formed during the same metamorphic cycle has been attempted with detailed microanalyses coupled with textural analyses. A coronitic garnet-bearing granulite from the lower crust of Calabria has been considered. U–Pb zircon data and zircon, garnet and orthopyroxene chemistries, at different textural sites, on a thin section of the considered granulite have been used to test possible equilibrium and better constrain the geological significance of the U–Pb ages related to zircon separates from other rocks of the same structural level. The garnet is very rich in REE and is characterised by a decrease in HREE from core to outer core and an increase in the margin. Zircons show core–overgrowth structures showing different chemistries, likely reflecting episodic metamorphic new growth. Zircon grains in matrix, corona around garnet and within the inner rim of garnet, are decidedly poorer in HREE up to Ho than garnet interior. Orthopyroxene in matrix and corona is homogeneously poor in REE. Thus, the outer core of garnet and the analysed zircon grains grew or equilibrated in a REE depleted system due to the former growth of garnet core. Zircon ages ranging from 357 to 333 Ma have been determined in the matrix, whereas ages 327–320 Ma and around 300 Ma have been determined, respectively, on cores and overgrowths of zircons from matrix, corona and inner rim of garnet. The calculated DREEzrn/grt and DREEopx/grt are largely different from the equilibrium values of literature due to strong depletion up to Ho in zircon and orthopyroxene with respect to garnet. On the other hand, the literature data show large variability. In the case study, (1) the D zrn/grt values define positive and linear trends from Gd to Lu as many examples from literature do and the values from Er to Lu approach the experimental results at about 900 °C in the combination zircon dated from 339 to 305 Ma with garnet outer core, and (2) D opx/grt values define positive trends reaching values considered as suggestive of equilibrium from Er to Lu only with respect to the outer core of garnet. The presence of a zircon core dated 320 Ma in the inner rim of garnet suggests that it, as well as those dated at 325–320 Ma in the other textural sites and, probably, those dated at 339–336 Ma showing depletion of HREE, grew after the garnet core, which sequestered a lot of HREE and earlier than the HREE rich margin of garnet. The quite uniform REE contents in orthopyroxene from matrix and corona and the low and uniform contents of HREE in the zircon overgrowths dated at about 300 Ma allow to think that homogenisation occurred during or after the corona formation around this age. The domains dated around 325–320 Ma would approximate the stages of decompression, whereas the metamorphic peak probably occurred earlier than 339 Ma.

Geochronology and trace element geochemistry of zircon, monazite and garnet from the garnetite and/or associated other high-grade rocks: Implications for Palaeoproterozoic tectonothermal evolution of the Khondalite Belt, North China Craton

It is well documented that the Khondalite Belt of the North China Craton formed by the collision of the Yinshan and Ordos Blocks during ca. 1.95Ga, but the onset of the post-collisional exhumation stage has not been well dated. The garnetite and garnet-bearing quartz-rich lens that are regarded as residue-melt phases occur occasionally in the foliated metasedimentary rocks in the Jining terrane of the Khondalite Belt. They are free of regional gneissosity, and may be generated during the post-collisional exhumation stage. Zircon U–Pb dating, monazite chemical dating and trace element analyses of zircon, monazite and garnet have been carried out on the garnetites and associated other high-grade metamorphic rocks. The geochronological results reveal a major age group of ca. 1890Ma from the zircon grains/overgrowths and inclusion-type monazites within the garnet poikiloblasts in the garnetites. The trace elements analyses show that zircon hosted in the mantle of the garnet poikiloblasts in the garnetite contains the highest HREE and Y contents, whereas zircon hosted in the garnet rims shows higher HREE and Y contents than matrix zircon grains. Inclusion-type monazite in the garnetite shows higher LaN/SmN ratio than matrix monazite grains. Large garnet poikiloblasts in the garnetite exhibit bell-shaped HREE and Y zoning profiles attributed to Rayleigh fractionation during their growth. Compared to garnet poikiloblasts, coronal garnet shows higher Gro, Cr and Li contents, but lower XMg, Sm, HREE, Zn and Zr contents and weaker Eu negative anomalies, reflecting their different formation environments. Based on evidences from microstructural analyses, REE concentrations and patterns, HREE partition coefficients and the Ti-in-zircon thermometer, it is proposed that most of the zircon grains/overgrowths in the garnetite formed coevally with garnet poikiloblasts. Consequently, ca. 1890Ma represents the timing of garnet poikiloblast growth during the partial melting of the protolith. Coronal garnet or Zn-rich spinel plus cordierite coupled with the matrix-type monazite formed after garnet poikiloblasts growth during near-isothermal decompression. Given the high metamorphic temperature and pressure revealed by the garnetite (820–850°C and up to 950°C; 8.5–9.5kbar), it is suggested that the ca. 1890Ma age group represents the beginning of extension/exhumation event in the Khondalite Belt, which is also recorded in the associated other high-grade rocks such as the garnet-bearing quartz-rich lenses, sillimanite–garnet-bearing quartzo-feldspathic gneisses, garnet-bearing quartzo-feldspathic gneisses and a pure quartz vein.

A window into the Early to mid‐Cretaceous infrastructure of the Yukon‐Tanana terrane recorded in multi‐stage garnet of west‐central Yukon, Canada

AbstractAmphibolite facies metasedimentary schists within the Yukon‐Tanana terrane in the northern Canadian Cordillera reveal a two‐stage, polymetamorphic garnet growth history. In situ U‐Th‐Pb Sensitive High Resolution Ion Microprobe dating of monazite provide timing constraints for the late stages of garnet growth, deformation and subsequent decompression. Distinct textural and chemical growth zoning domains, separated by a large chemical discontinuity, reveal two stages of garnet growth characterized in part by: (i) a syn‐kinematic, inclusion‐rich stage‐1 garnet core; and (ii) an inclusion‐poor, stage‐2 garnet rim that crystallized with syn‐ to post‐kinematic staurolite and kyanite. Phase equilibria modelling of garnet molar and compositional isopleths suggest stage‐1 garnet growth initiated at ~600 °C, 8 kbar along a clockwise P–T path. Growth of the compositionally distinct, grossular‐rich, pyrope‐poor inner portion of the stage‐2 overgrowth is interpreted to have initiated at higher pressure and/or lower temperature than the stage‐1 core along a separate P–T loop, culminating at peak P–T conditions of ~650–680 °C and 9 kbar. Stage‐2 metamorphism and the waning development of a composite transposition foliation (ST) are dated at c. 118 Ma from monazite aligned parallel to ST, and inclusions in syn‐ to post‐ST staurolite and kyanite. Slightly younger ages (c. 112 Ma) are obtained from Y‐rich monazite that occurs within resorbed areas of both stage‐1 and stage‐2 garnet, together with retrograde staurolite and plagioclase. The younger ages obtained from these texturally and chemically distinct grains are interpreted, with the aid of phase equilibria calculations, to date the growth of monazite from the breakdown of garnet during decompression at c. 112 Ma. Evidence for continued near‐isothermal decompression is provided by the presence of retrograde sillimanite, and cordierite after staurolite, which indicates decompression below ~4–5 kbar prior to cooling below ~550 °C. As most other parts of the Yukon‐Tanana terrane were exhumed to upper crustal levels in the Early Jurassic, these data suggest this domain represents a tectonic window revealing a much younger, high‐grade tectono‐metamorphic core (infrastructure) within the northern Cordilleran orogen. This window may be akin to extensional core complexes identified in east‐central Alaska and in the southeastern Canadian Cordillera.

Oxygen isotope zoning in garnets from Franciscan eclogite blocks: evidence for rock–buffered fluid interaction in the mantle wedge

The oxygen isotope compositions of eclogite and amphibolite garnets from Franciscan Complex high-grade blocks and actinolite rinds encasing the blocks were determined to place constraints on their fluid histories. SIMS oxygen isotope analysis of single garnets from five eclogite blocks from three localities (Ring Mountain, Mount Hamilton, and Jenner Beach) shows an abrupt decrease in the δ18O value by ~1–3 ‰ from core to rim at a distance of ~120 ± 50 μm from the rim in nine out of the 12 garnets analyzed. In contrast, amphibolite garnets from one block (Ring Mountain) analyzed show a gradual increase in δ18O value from core to rim, implying a different history from that of the eclogite blocks. Values of δ18O in eclogite garnet cores range from 5.7 to 11.6 ‰, preserving the composition of the eclogite protolith. The abrupt decrease in the δ18O values of the garnet rims to values ranging from 3.2 to 11.2 ‰ suggests interaction with a lower δ18O fluid during the final stages of growth during eclogite facies metamorphism (450–600 °C). We hypothesize that this fluid is sourced from the serpentinized mantle wedge. High Mg, Ni, and Cr contents of actinolite rinds encasing the blocks also support interaction with ultramafic rock. Oxygen isotope thermometry using chlorite and phengite versus actinolite of rinds suggests temperatures of 185–240 °C at Ring Mountain and Mount Hamilton. Rind formation temperatures together with the lower δ18O garnet rims suggest that the blocks were in contact with ultramafic rock from the end of garnet growth through low-temperature retrogression. We suggest a tectonic model in which oceanic crust is subducted at the initiation of subduction and becomes embedded in the overlying mantle wedge. As subduction continues, metasomatic exchange between high-grade blocks and surrounding ultramafic rock is recorded in low δ18O garnet rims, and later as temperatures decrease, with rind formation.

Growth, preservation of Paleoproterozoic-shear-zone-hosted monazite, north of the Western Dharwar Craton (India), and implications for Gondwanaland assembly

We examine the conditions and processes of growth and preservation of multiaged monazite in micaceous matrix and in garnet porphyroblasts in staurolite–kyanite mica schists hosted in a hitherto-undiscovered shear zone that limits the northern extent of the Western Dharwar Craton (WDC), India. Garnet in the footwall schists grew during mid-crustal (600 ± 40 °C, 7.3 ± 1.2 kbar) loading and cooling as a consequence of the northward transport of the WDC lithologies. U–Th–Pb (total) ages in monazites in the matrix and in post-tectonic garnets yield well-defined peaks at 2.5, 2.2 and 1.9 Ga. In garnet, 2.5 and 2.2 Ga monazite grains, and 2.2 Ga monazites with 2.5 Ga cores are commonly occluded, but monazites with 1.9 Ga mantles around older cores are rare. By contrast, in the matrix, 1.9 Ga monazite grains and monazite with 1.9 Ga mantles around older cores are prominent, but the peak age frequencies of the two older populations are significantly lower than for monazites hosted as inclusions in garnet. Both in the matrix and garnet, the low-Th, high-Y domains in monazites yield the two older peak ages, while the 1.9 Ga ages correspond to the high-Th, low-Y domains. The preponderance of older ages in monazite hosted as inclusions in garnet relative to matrix monazites is because garnets formed between 2.2 and 1.9 Ga shielded the older monazites from dissolution–precipitation at 1.9 Ga. A few 1.9 Ga monazites hosted as inclusions in the garnet rims suggest renewed garnet growth at post-1.9 Ga. Multiple Pb–Pb age populations (2.5, 2.25, 2.1 and 1.8 Ga) in detrital zircon in the Sahanataha Group north of the Paleoarchean Antongil-Masora block (NE Madagascar) are identical to the multiple monazites ages north of the WDC, inferred to share a similar history and to be contiguous with the Antongil-Masora block in pre-Jurassic reconstructions of the Gondwanaland. We suggest the newly discovered Paleoproterozoic tectonic zone continued westward into Madagascar north of the Antongil-Masora block and constituted the hitherto-unexplained basement for the multiaged detrital zircons in the Sahanataha quartzites (337).

Garnet geochemistry of tungsten-mineralized Xihuashan granites in South China

The Xihuashan complex intrusion in South China, which is emplaced at ca. 154Ma, mainly comprises medium-grained porphyritic biotite granite, medium-grained biotite granite, and fine-grained two-mica granite. Medium-grained biotite and fine-grained two-mica granites are important tungsten-bearing granites and contain an unusual amount of garnet. Garnets from this intrusion are dominated by almandine and spessartine, which constitute 94% to 99% of the total molecular composition of the garnet. These garnets display unusual compositional zoning. The cores of these garnets are rich in heavy rare earth element (HREE), Y, Ca, and contain abundant HREE- and Y-rich mineral inclusions. Their rims are free of mineral inclusions and have low of these elements. Two types of fluorite inclusions exist in garnet: Y fluorite and Y-poor fluorite. Garnet exhibits specific Mn zoning with a relatively Mn-poor core but a relatively Mn-rich rim, thus constituting a specific “spessartine inverse bell-shaped profile” that belongs to typical magmatic garnets. All analyzed garnets have high REE content and exhibit HREE-enriched and LREE-depleted patterns with strong negative Eu anomalies. The incorporation of REE into garnet is in part controlled by its crystal chemistry, with REE3+ following a coupled substitution of the type [Fe2+,Mn2+]−1VIII[REE3+]+1VIII[Si4+]−1IV[Z3+]+1IV. The texture and compositional zoning of garnet suggest that garnet growth is largely controlled by the pressure–temperature–composition condition of magmatic evolution, internal crystal-chemical parameters, and kinetics during mineral growth. The garnet core grows in near equilibrium with magmatic melt under a relatively high pressure–temperature (P–T) condition. By contrast, the garnet rim grows rapidly by the coexisting melt-fluid phase and CO2-rich volatile environment under a relatively low P–T condition, which is virtually unfavorable for the incorporation of REE into the magmatic garnet structure. Garnet fingerprints the magmatic-hydrothermal transition during crystallization of these granites.

Pressure–temperature and deformational evolution of high-pressure metapelites from Variscan NE Sardinia, Italy

Chloritoid schists crop out north of the village of Lula in the Inner Zone of the Variscan chain of Sardinia consisting of a variety of metamorphic rocks. The S1 and S2 foliations in these schists are defined by the orientation of muscovite, paragonite, and chloritoid. Chlorite is an additional mineral oriented along S2. Late margarite grew at the expense of chloritoid included in garnet. Garnet porphyroblasts, enclosing quartz, chloritoid, rutile, Fe-oxide, apatite and paragonite, show a progressive decrease of spessartine component from 17 to 7mol% and an increase of pyrope component from 4 to 6mol% from core to rim. The grossular content firstly increases from the inner (Grs~21) to the outer core (Grs~27) and then decreases towards the outermost rim (Grs~15). Compositional mapping of white mica also revealed zoning and a wide range in Si content (from 6.0 to 6.6pfu). The highest Si content is related to the highest Fe and Mg contents and the lowest Na content. P–T pseudosections were calculated in the system Na2O–K2O–CaO–FeO–MnO–MgO–Al2O3–TiO2–SiO2–H2O for compositions of chloritoid schists. The highest Si contents of K-white mica and the garnet core composition suggest pressures close to 1.8GPa and temperatures of 460–500°C. The garnet rim composition and low Si contents in K-white mica are compatible with re-equilibration at 540–570°C and 0.7–1.0GPa. These results suggest an HP-metamorphic imprint during the D1 deformation phase which occurred before the Barrovian amphibolite-facies metamorphism of NE Sardinia. D2 folding and shearing occurred at decreasing P–T conditions during the exhumation of the metamorphic complex.

Integrated pressure–temperature–time constraints for the Tso Morari dome (Northwest India): implications for the burial and exhumation path of UHP units in the western Himalaya

AbstractNorthward subduction of the leading edge of the Indian continental margin to depths greater than 100 km during the early Eocene resulted in high‐pressure (HP) quartz‐eclogite to ultrahigh‐pressure (UHP) coesite–eclogite metamorphism at Tso Morari, Ladakh Himalaya, India. Integrated pressure–temperature–time determinations within petrographically well‐constrained settings for zircon‐ and/or monazite‐bearing assemblages in mafic eclogite boudins and host aluminous gneisses at Tso Morari uniquely document segments of both the prograde burial and retrograde exhumation path for HP/UHP units in this portion of the western Himalaya. Poikiloblastic cores and inclusion‐poor rims of compositionally zoned garnet in mafic eclogite were utilized with entrapped inclusions and matrix minerals for thermobarometric calculations and isochemical phase diagram construction, the latter thermodynamic modelling performed with and without the consideration of cation fractionation into garnet during prograde metamorphism. Analysis of the garnet cores document (M1) conditions of 21.5 ± 1.5 kbar and 535 ± 15 °C during early garnet growth and re‐equilibration. Sensitive high resolution ion microprobe (SHRIMP) U–Pb analysis of zircon inclusions in garnet cores yields a maximum age determination of 58.0 ± 2.2 Ma for M1. Peak HP/UHP (M2) conditions are constrained at 25.5–27.5 kbar and 630–645 °C using the assemblage garnet rim–omphacite–rutile–phengite–lawsonite–talc–quartz (coesite), with mineral compositional data and regional considerations consistent with the upper P–T bracket. A SHRIMP U–Pb age determination of 50.8 ± 1.4 Ma for HP/UHP metamorphism is given by M2 zircons analysed in the eclogitic matrix and that are encased in the garnet rim. Two garnet‐bearing assemblages from the Puga gneiss (host to the mafic eclogites) were utilized to constrain the subsequent decompression path. A non‐fractionated isochemical phase diagram for the assemblage phengite–garnet–biotite–plagioclase–quartz–melt documents a restricted (M3) P–T stability field centred on 12.5 ± 0.5 kbar and 690 ± 25 °C. A second non‐fractionated isochemical phase diagram calculated for the lower pressure assemblage garnet–cordierite–sillimanite–biotite–plagioclase–quartz–melt (M4) documents a narrow P–T stability field ranging between 7–8.4 kbar and 705–755 °C, which is consistent with independent multiequilibria P–T determinations. Th–Pb SHRIMP dating of monazite cores surrounded by allanite rims is interpreted to constrain the timing of the M4 equilibration to 45.3 ± 1.1 Ma. Coherently linking metamorphic conditions with petrographically constrained ages at Tso Morari provides an integrated context within which previously published petrological or geochronological results can be evaluated. The new composite path is similar to those published for the Kaghan UHP locality in northern Pakistan, although the calculated 12‐mm a−1 rate of post‐pressure peak decompression at Tso Morari would appear less extreme.

Ultra-high temperature granulite-facies metamorphic rocks from the Mozambique belt of SW Tanzania

The metamorphic rocks in the Neoproterozoic (Pan-African) Mozambique belt of southwestern Tanzania, around the town of Songea, can be subdivided into one- and two pyroxene bearing charnockitic gneisses, migmatitic granitoid gneisses and amphibolite-facies metapelites. Lower-grade amphibolite-facies rocks are rare and can be classified as sillimanite- and/or garnet-bearing metapelites. Most of the studied charnockitic gneisses show excellent corona textures with large orthopyroxene grains rimmed by clinopyroxene, followed by quartz and well developed garnet rims due to the reaction Opx+Pl=Grt+Cpx+Qtz that formed during isobaric cooling. These and other charnockitic gneisses show symplectites of orthopyroxene and An-rich plagioclase that resulted from the breakdown of garnet during isothermal decompression due to the reaction Grt+Cpx+Qtz=Opx+Pl. Geothermobarometric calculations yield up to ~1050°C and up to ~12kbar for peak metamorphic conditions. These are higher temperature and slightly lower pressure conditions than reported for other granulite-facies terrains in the Mozambique belt of Tanzania. Single zircon Pb–Pb evaporation and U-Pb SHRIMP ages for magmatic zircons extracted from two charnockitic and two granitic gneisses cluster in two groups, one at ~750Ma and one at ~1150Ma with the older reflecting the time of emplacement of the igneous precursors, and the younger approximating the time of charnockitization. These protolith ages are similar to those farther east in the Masasi area of southern Tanzania, as well as in northern Mozambique and in southern Malawi, and suggest that the Mozambique belt consists of chronologically heterogeneous assemblages whose pre-metamorphic tectonic setting remains obscure.

Metamorphic pressure-temperature evolution of garnet-chloritoid schists from the Lake Zone, SW Mongolia

Garnet-chloritoid schists occur in the Chandman district of the Lake Zone, SW Mongolia, which is located within the central part of the Central Asian Orogenic Belt. The Chandman district is composed of Neoproterozoic ophiolites of the Khantaishir Formation, eclogite-bearing orthogneisses and micaschists of the Alag Khadny metamorphic complex, marbles of the Maykhan Tsakhir Formation, and the basement block of the Zamtyn Nuruu Formation. Garnet-chloritoid schists occur as lenses or layers within marbles of the Maykhan Tsakhir Formation, which lie in contact with eclogite bodies. They consist mainly of garnet, chloritoid, phengitic muscovite, chlorite and quartz, with minor amounts of kyanite, rutile, ilmenite, zircon and carbonaceous matter. The texture and mineral chemistry of the garnet-chloritoid schists record prepeak and peak stages of high-pressure intermediate type metamorphism. The pre-peak stage of epidote-amphibolite facies conditions (T = 500-510 °C and P = 7-8 kbar) is deduced from cores of garnet containing mineral inclusions of chloritoid, Fe-rich chlorite, phengitic muscovite, and quartz. The peak mineral assemblage of the garnet-chloritoid schists defined by garnet rims containing chloritoid, phengitic muscovite, Mg-rich chlorite, kyanite and quartz as inclusions indicates T = 560-590 °C and P = 10-11 kbar, falling in transitional conditions between the epidote-amphibolite and the eclogites facies. The pressure conditions of the garnet-chloritoid schists are distinctly lower than those of the eclogites (T = 590-610 °C and P = 20-25 kbar), whereas temperature conditions are similar. The eclogites are considered to have formed by subduction of oceanic crust. In contrast, the metamorphism of the garnet-chloritoid schists was probably related to collisional tectonic event in the Central Asian Orogenic Belt.

High-pressure garnet amphibolite from the Funaokayama unit, western Kii Peninsula and the extent of eclogite facies metamorphism in the Sanbagawa belt

Quantitative analyses of garnet-bearing epidote amphibolite (hereafter garnet amphibolite) from the Funaokayama unit, western Kii Peninsula of the Sanbagawa belt reveal a previously unrecognized high-grade part of the Sanbagawa metamorphism. Petrographic observations suggest that the garnet amphibolite underwent three stages of metamorphism (M1, M2 and M3). Records of M1 are preserved as syn-tectonic prograde-zoned garnet and its inclusions. Pseudosection modeling reproduces the observed M1 assemblage garnet + amphibolite + epidote + phengite + quartz and the growth zoning of garnet records the pressure (P)-temperature (T) evolution from 0.8 GPa, 570 °C to 1.3 GPa, 590 °C. Therefore, this garnet growth and associated deformation (D1) took place during subduction up to eclogite facies conditions. The garnet rim compositions and inclusions of epidote, titanite, rutile and quartz (5titanite + 2clinozoisite = 5rutile + 3grossular + 2quartz + H2O) give consistent peak-P estimates of 1.3-1.9 GPa. The garnet contains quartz inclusions retaining residual pressures of up to ∼ 0.7 GPa, which is as high as the values reported in well-characterized eclogite samples in central Shikoku. The inferred peak-P conditions in the eclogite facies is further supported by the occurrence of aragonite in associated pelitic schist. The assemblage hornblende + epidote + titanite + quartz + albite in the matrix and its equilibrium conditions of ∼ 0.8 GPa, 580 °C suggest decompression to the epidote-amphibolite facies (M2). The M2 minerals locally show replacement by the mineral assemblage actinolite + epidote + chlorite + titanite + calcite + quartz + albite, suggesting a partial re-equilibration in the greenschist facies (M3). M2 and M3 are synchronous with the main phase of ductile deformation during exhumation (D2). Despite the absence of the omphacite + quartz assemblage, it is likely that eclogite facies metamorphism in the Sanbagawa belt can be extended to western Kii Peninsula.

Metamorphism of ultrahigh‐pressure eclogites from the Kebuerte Valley, South Tianshan, NW China: phase equilibria and P–T path

AbstractEclogites from the Kebuerte Valley, Chinese South Tianshan, consist of garnet, omphacite, phengite, paragonite, glaucophane, hornblendic amphibole, epidote, quartz and accessory rutile, titanite, apatite and carbonate minerals with occasional presence of coesite or quartz pseudomorphs after coesite. The eclogites are grouped into two: type I contains porphyroblastic garnet, epidote, paragonite and glaucophane in a matrix dominated by omphacite where the proportion of omphacite and garnet is >50 vol.%; and type II contains porphyroblastic epidote in a matrix consisting mainly of fine‐grained garnet, omphacite and glaucophane where the proportion of omphacite and garnet is <50 vol.%. Garnet in both types of eclogites mostly exhibits core–rim zoning with increasing grossular (Xgr) and pyrope (Xpy) contents, but a few porphyroblastic garnet grains in type I eclogite shows core–mantle zoning with increasing Xpy and a slight decrease in Xgr, and mantle–rim zoning with increases in both Xgr and Xpy. Garnet rims in type I eclogite have higher Xpy than in type II. Petrographic observations and phase equilibria modelling with pseudosections calculated using thermocalc in the NCKMnFMASHO system for three representative samples suggest that the eclogites have experienced four stages of metamorphism: stage I is the pre‐peak temperature prograde heating to the pressure peak (Pmax) which was recognized by the garnet core–mantle zoning with increasing Xpy and decreasing Xgr. The P–T conditions at Pmax constrained from garnet mantle or core compositions with minimum Xgr content are 29–30 kbar at 526–540 °C for type I and 28.2 kbar at 518 °C for type II, suggesting an apparent thermal gradient of ∼5.5 °C km−1. Stage II is the post‐Pmax decompression and heating to the temperature peak (Tmax), which was modelled from the garnet zoning with increasing Xgr and Xpy contents. The P–T conditions at Tmax, defined using the garnet rim compositions with maximum Xpy content and the Si content in phengite, are 24–27 kbar at 590 °C for type I and 22 kbar at 540 °C for type II. Stage III is the post‐Tmax isothermal decompression characterized by the decomposition of lawsonite, which may have resulted in the release of a large amount of fluid bound in the rocks, leading to the formation of epidote, paragonite and glaucophane porphyroblasts. Stage IV is the late retrograde evolution characterized by the overprint of hornblendic amphibole in eclogite and the occurrence of epidote–amphibole facies mineral assemblages in the margins or in the strongly foliated domains of eclogite blocks due to fluid infiltration. The P–T estimates obtained from conventional garnet–clinopyroxene–phengite thermobarometry for the Tianshan eclogites are roughly consistent with the P–T conditions of stage II at Tmax, but with large uncertainties in temperature. On the basis of these metamorphic stages or P–T paths, we reinterpreted that the recently reported zircon U–Pb ages for eclogite may date the Tmax stage or the later decompression stage, and the widely distributed (rutile‐bearing) quartz veins in the eclogite terrane may have originated from the lawsonite decomposition during the decompression stage rather than from the transition from blueschist to eclogite as previously proposed.

MINERALOGY AND GEOCHEMISTRY OF PHOSPHATES AND SILICATES IN THE SAPUCAIA PEGMATITE, MINAS GERAIS, BRAZIL: GENETIC IMPLICATIONS

The Sapucaia pegmatite is located in the well-known Eastern Brazilian Pegmatitic Province (EBPP), Minas Gerais, Brazil. Detailed mapping of the pegmatite body revealed five zones, showing distinct mineral assemblages: a border zone (BZ), an external wall-zone (EWZ), an internal wall-zone (IWZ), an intermediate zone (IZ), and a quartz core (Q). Phosphate masses found in the pegmatite contains two different assemblages: assemblage I is formed under oxidizing conditions, and assemblage II is formed under less strongly oxidizing conditions. In both assemblages, triphylite is the only primary phosphate, and several secondary species are produced by its alteration. In association I, three main transformation stages are observed: (i) the progressive oxidation of triphylite accompanied by the leaching of Li leads to the successive crystallization of ferrisicklerite and heterosite; (ii) a hydration stage transforms triphylite into hureaulite, barbosalite, and tavorite, and (iii) a final stage, corresponding to meteoric alteration, the latest highly hydrated phosphate and oxide species. In assemblage I, ferrisicklerite is replaced by minerals like jahnsite s.l. and frondelite s.l. during the hydration stage, and in both assemblages, a second generation of Mn-rich triphylite (triphylite II) is observed. Two unusual petrographic textures were also observed, showing reactions between triphylite and albite to form montebrasite and garnet rims. These textures formed during the albitization stage, and correspond to the reactions triphylite + albite → ferrisicklerite + montebrasite + quartz, and triphylite + albite → almandine + quartz, respectively. Finally, the geochemical trends in phosphates and silicates indicate that Sapucaia is a weakly fractionated and geochemically primitive LCT-type pegmatite. The degree of differentiation of the pegmatite increases from the border to the core, as shown by a decrease of Fe/(Fe + Mn) in olivine-type phosphates, which decreases from 0.75 in the IWZ, to 0.71 in the IZ. Muscovite and tourmaline chemical compositions also show evidence of increasing degree of differentiation; for example, the Ga and Nb contents of muscovite increases from 57 and 49 ppm in the IWZ, to 86 and 80 ppm in the IZ, respectively.

Garnet-biotite geothermometer and estimation of crystallization temperature of zoned garnets from metapelites: I. Reconstruction of thermal history of porphyroblast growth

The possibility of using a garnet-biotite geothermometer to estimate the nucleation temperature of zoned crystals is considered. The data on the temperature evolution of mineral systems in the course of porphyroblast growth estimated with different methods are compared for a number of metamorphic provinces. The temperatures for garnet core and rim growth calculated by the program THERMOCALC are systematically overestimated compared to those derived using the garnet-biotite geothermometer (Kaneko and Miyano, 2004; Gulbin, 2011a, 2011b). The discrepancy reaches 40–70°C for the isopleth method and 60–90°C for the average PT method.

Metamorphic evolution of garnet amphibolite in the western Dabieshan eclogite belt, Central China: Evidence from petrography and phase equilibria modeling

The western Dabieshan terrane in central China involves numerous blocks of garnet amphibolite and eclogite enclosed in felsic gneiss and schist. Two representative garnet amphibolite samples SHS22 and BSW05 were selected for detailed study. They are composed of garnet, hornblendic amphibole, sodic plagioclase, epidote, quartz, biotite and/or muscovite, and small amounts of rutile, sphene and ilmenite. A few phengite flakes are found as inclusions in garnet in sample SHS22. Garnet varies in grain size and rim composition. The coarse-grained garnet porphyroblasts are mostly anhedral and exhibit compositional zoning with increasing pyrope and decreasing or constant grossular content from core to rim. Pseudosections calculated in the system NCKMnFMASHO suggest that this garnet zoning reflects prograde P–T paths of early-stage eclogite facies from 1.9GPa at 490°C to 2.3GPa at 555°C for sample SHS26 and from 1.8GPa at 450°C to 2.2GPa at 540°C for sample BSW05. The peak P–T conditions are constrained by the garnet rim compositions with the maximum pyrope (Xprp) and minimum grossular (Xgrs) contents. The Si contents in phengite from sample SHS22 were modeled to yield pressure conditions similar to that constrained by garnet. The other garnet rim compositions with decreasing Xprp and increasing Xgrs contents were modeled in P–M (H2O) pseudosections to reflect different degrees of the modification of eclogite facies to amphibolite facies in fluid-unsaturated conditions, indicating decreasing pressures from 0.7 to 0.5GPa and increasing water contents from 3.6 to 5.2mol.% at T=575°C for sample SHS22, and decreasing pressures from 0.8 to 0.55GPa and increasing water contents from 3.5 to 4.7mol.% at T=530°C for sample BSW05, respectively. The garnet rims with minimum Xprp and maximum Xgrs were predicted to be close to an equilibrium state with the matrix minerals involving hornblendic amphibole, plagioclase, epidote, biotite and quartz. The constituting assemblage was closely saturated with water. As it is hard to evaluate if the relevant minerals are in equilibrium or not in the garnet amphibolite with multi-generation assemblages, the conventional thermobarometers are not very useful for estimating the P–T conditions for this type of rock.

Evolution of a very deeply subducted metasediment from As Sifah, northeastern coast of Oman

Near the coastal village of As Sifah, NE Oman, eclogite-facies rocks occur in the Saih Hatat window. We investigated a metapelite from this area, which is composed of mm-sized garnet and greenish phengite and minor epidote, blue amphibole, paragonite, albite, quartz, rutile, opaque phases, barite, and carbonate. Garnet exhibits a chemical zonation with Gro17Alm66Pyr6Spe11, Gro22Alm72.5Pyr5Spe0.5, and Gro25Alm65Pyr8Spe2 as inner core, mantle and outermost rim compositions. Inner portions of phengite have Si contents of up to 3.6 per formula unit (pfu), whereas rims are poorer in Si (3.2–3.4 pfu). We constructed a P-T pseudosection in the system Na2O-K2O-CaO-FeO-O2-MnO-MgO-Al2O3-SiO2-TiO2-H2O for the bulk-rock composition of the studied metapelite and contoured it by isopleths of various parameters such as the molar fractions of garnet components. Based on this contouring a P-T path was derived that starts at ultrahigh-pressure conditions. Garnet began to form at 25kbar and 490°C. Subsequently, temperatures increased and pressures decreased to finally reach P-T conditions of 13kbar and 565°C at which low-Si phengite, the outermost rim of garnet, Na-amphibole, epidote, quartz, magnetite, and rutile were in equilibrium. The P-T path is related to events in a subduction channel where the top of subducted oceanic crust, including the studied metasediments, was involved in an upwards-directed mass flow, resulting in the release of about 3wt.% H2O by garnet formation from hydrous minerals such as chlorite and lawsonite. In order to get hints at the interaction of such hydrous fluids, we have analyzed the trace and minor elements in phengite. The contents of B, Rb, Cs, and Tl (20, 397, 6.7 and 1.7ppm, respectively) are nearly constant over the entire Si range of potassic white mica. In contrast, the contents of Ba and Sr increase from 900 and 0.5 to 10500 and 14ppm, respectively, with decreasing Si content in phengite. We hypothesize that this result reflects early leaching of mobile elements during subduction and the later approach of the rock to a barite deposit within a melange ascending in the subduction channel.