Garnet Rims Research Articles

Evaluating chemical equilibrium in metamorphic rocks using major element and Sm–Nd isotopic age zoning in garnet, Townshend Dam, Vermont, USA

A quantitative assessment of metamorphic chemical equilibrium derived from correlation of spessartine content and garnet Sm–Nd ages suggests that major element matrix equilibrium was maintained (to a first order) throughout a ca. 40cm-wide rock sample during garnet growth; however cm-scale Sm–Nd isotopic heterogeneity limits the Sm–Nd age precision required to evaluate more subtle age differences within individual garnet crystals. Central wafers from 1–3cm diameter garnet grains within a 1.21×104cm3 block of pelitic schist were used to document concentric growth zoning of major elements, with decreasing Mn and Ca and increasing Fe and Mg from cores to rims. Garnets also preserve growth zoning patterns for HREE and MREE and show evidence for resorption and partial recrystallization of the outermost rims. Similar garnet core compositions and identical garnet rim compositions for large like sized porphyroblasts throughout the sample suggest that garnet growth occurred at near equilibrium P–T–X conditions for major elements over the sample volume.Comparison of 28 rock Sm–Nd isotope values from the sample indicates substantial cm-scale heterogeneity, which precludes meaningful use of local garnet rock isotope pairs for isochron age calculation. Therefore, Sm–Nd isotopic compositions of thirty-eight concentric core to rim garnet segments from ten large (1–3cm) garnets and two small (1–4mm) bulk garnets, with narrow ranges of Mn content, are paired with sixteen matrix/whole-rock Sm–Nd isotopic compositions collected over the rock volume to define a range of isochron ages from 383.1±6.8Ma to 324.5±3.3Ma. Four of the garnets have anomalously young rims that likely result from post-growth alteration. Chlorite, quartz, and xenotime haloes around garnet suggest that anomalously young garnet rim ages reflect post-growth resorption/recrystallization effects. Excluding these young rims yields a range of ages from 383.1±6.8 (oldest core) to 374.9±1.8Ma (youngest rim). Sm–Nd age precisions >1.5m.y. (and high MSWD) result primarily from isotopic heterogeneity in the finely layered metasedimentary rock matrix. However, garnet cores with high Mn (n=7), mantles with intermediate Mn (n=14), and rims with low Mn (n=8; including the 2 smaller bulk garnet analyses), define three distinct multi-grain isochrons of 380.3±2.0Ma (n=23, MSWD=14), 377.3±1.4Ma (n=30, MSWD=18), and 376.5±1.0Ma (n=24, MSWD=18), respectively, yielding an average garnet growth duration of 3.8±2.2m.y. These three composite Mn-age zones define a Mn vs. age relationship that reflects depletion of Mn in the rock matrix as it is sequestered by growing garnet. Correlation of garnet major element compositions throughout the sample suggests that major element matrix equilibrium was generally maintained (to a first order) throughout the ca. 4m.y. duration of garnet growth.

P–T–t–D record of crustal‐scale horizontal flow and magma‐assisted doming in the SW Mongolian Altai

ABSTRACTThe Chandman massif, a typical structure of the Mongolian Altai, consists of a migmatite–magmatite core rimmed by a lower grade metamorphic envelope of andalusite and cordierite‐bearing schists. The oldest structure in the migmatite–magmatite core is a subhorizontal migmatitic foliation S1 parallel to rare granitoid sills. This fabric is folded by upright folds F2 and transposed into a vertical migmatitic foliation S2 that is syn‐tectonic, with up to several tens of metres thick granitoid sills. Sillimanite–ilmenite–magnetite S1 inclusion trails in garnet constrain the depth of equilibration during the S1 fabric to 6–7 kbar at 710–780 °C. Reorientation of sillimanite into the S2 fabric indicates that the S1–S2 fabric transition occurred in the sillimanite stability field. The presence of cordierite, and garnet rim chemistry point to decompression to 3–4 kbar and 680–750 °C during development of the S2 steep fabric, and post‐tectonic andalusite indicates further decompression to 2–3 kbar and 600–650 °C. Widespread crystallization of post‐tectonic muscovite is explained by the release of H2O from crystallizing partial melt. In the metamorphic envelope the subhorizontal metamorphic schistosity S1 is heterogeneously affected by upright F2 folds and axial planar subvertical cleavage S2. In the north, the inclusion trails in garnet are parallel to the S1 foliation, and the garnet zoning indicates nearly isobaric heating from 2.5 to 3 kbar and 500–530 °C. Cordierite contains crenulated S1 inclusion trails and has pressure shadows related to the formation of the S2 fabric. The switch from the S1 to the S2 foliation occurred near 2.5–3 kbar and 530–570 °C; replacement of cordierite by fine‐grained muscovite and chlorite indicates further retrogression and cooling. In the south, andalusite containing crenulated inclusion trails of ilmenite and magnetite indicates heating during the D2 deformation at 3–4 kbar and 540–620 °C. Monazite from a migmatite analysed by LASS yielded elevated HREE concentrations. The grain with the best‐developed oscillatory zoning is 356 ± 1.0 [±7] Ma (207Pb‐corrected 238U/206Pb), considered to date the crystallization from melt in the cordierite stability ~680 °C and 3.5 kbar, whereas the patchy BSE‐dark domains give a date of 347 ± 4.2 [±7] Ma interpreted as recrystallization at subsolidus conditions. The earliest sub‐horizontal fabric is associated with the onset of magmatism and peak of P–T conditions in the deep crust, indicating important heat input associated with lower crustal horizontal flow. The paroxysmal metamorphic conditions are connected with collapse of the metamorphic structure, an extrusion of the hot lower crustal rocks associated with vertical magma transfer and a juxtaposition of the hot magmatite–migmatite core with supracrustal rocks. This study provides information about tectono‐thermal history and time‐scales of horizontal flow and vertical mass and heat transfer in the Altai orogen. It is shown that, similar to collisional orogens, doming of partially molten rocks assisted by syn‐orogenic magmatism can be responsible for the exhumation of orogenic lower crust in accretionary orogenic systems.

Garnet geochemistry records the action of metamorphic fluids in ultrahigh-pressure dioritic gneiss from the Sulu orogen

A combined study of major and trace elements, water content and oxygen isotope composition was carried out for newly grown garnets in ultrahigh-pressure dioritic gneiss from the Sulu orogen. The results indicate the occurrence of a core–rim structure of garnet grains with growth of rims over cores via the mechanism of coupled dissolution–reprecipitation. This provides new insights into the action of geofluids on garnet growth during continental subduction-zone metamorphism. The garnet cores and rims show distinct geochemical compositions. The cores have high spessartine contents (4.2–19.4mol%), relatively low grossular contents (26.5–29.8mol%) and low δ18O values (5.0±0.2‰). The rims have relatively low spessartine contents (1.0–3.4mol%), high grossular contents (28.7–36.1mol%) and high δ18O values (6.1±0.3‰). Furthermore, the rims show much lower contents of Li and HREE, and considerably higher contents of Sc, V and Ga than the cores. In addition, the cores exhibit very low contents of structural hydroxyl in 32±10 to 48±20ppm H2O, whereas the rims show much higher contents of structural hydroxyl in 112±16ppm to 171±34ppm H2O. Although the boundary between core and rim is highly irregular in the garnet grains, the zoning in water contents, element and oxygen isotope compositions are all spatially correlated with each other. Taken all the observations together, it is suggested that the geochemical compositions of garnet cores and rims would be primarily dictated by two types of geofluids with distinctive compositions. Some garnet fragments, bounded by fractures, show the two types of domains with distinct water contents. This difference indicates insignificant exchange of structural hydroxyl by diffusion during growth of the rims over the cores. Mineral-pair O isotope thermometry yields consistent temperatures of ~550°C for the quartz-garnet rim and quartz-titanite, and titanite U–Pb dating yields a metamorphic age of 226±6Ma. Thus, the growth of garnet rims is linked to the action of geofluids (probably anatectic melts) during the exhumation of deeply subducted continental crust. Therefore, the compositions of garnet cores and rims are primarily controlled by the composition of attending fluids. As such, garnet provides a mineralogical record of the fluid action during subduction-zone metamorphism.

UHP kyanite eclogite associated with garnet peridotite and diamond-bearing granulite, northern Bohemian Massif

Kyanite eclogites enclosed in garnet peridotites may provide important information on P-T evolution of orogenic peridotites in deep subduction and collision zones. Kyanite eclogite interlayered with garnet peridotite occurs in the borehole T-7, in the Saxothuringian basement of the northern part of the Bohemian Massif. This orogenic peridotite of mantle origin is associated with felsic granulites, which contain diamond as a consequence of deep subduction of the continental crust. Here, we report on the metamorphic evolution of kyanite eclogite, which shows a well-preserved peak-pressure mineral assemblage of garnet, omphacite, kyanite and phengite. Conventional geothermobarometry, average PT method and thermodynamic modelling constrain the metamorphic conditions of this assemblage up to 3.5–4.5GPa at 900–1050°C. Two compositional types of garnet, i.e., Mg-rich and Ca-rich, have been recognised. Thermodynamic modelling shows that the composition of Ca-rich garnet with XCa (0.35–0.37) in the core corresponds to stability of garnet at 3.5–4.5GPa. Amphibole and zoisite are preserved as inclusions in garnet cores, and they are stable below 2.5GPa, indicating that garnet grew at the expense of these phases at increasing P-T conditions during the prograde evolution of the rock. A post-peak metamorphism decompression and cooling are recorded by decrease of Ca-Eskola end-member in omphacite, drop in XMg and XGrs at garnet rim and a very restricted formation of pargasitic amphibole in the matrix. The absence of symplectites after omphacite in the investigated eclogite may be due to a very low content of quartz and possibly also fluid in the rock. Our study suggests that kyanite-bearing eclogite underwent UHP metamorphism as a consequence of subduction, together with interlayered garnet peridotite. Both rocks were incorporated into the subducted continental crust (diamond-bearing granulites) during the Variscan orogeny.

Determination of Pressure-Temperature Conditions of Retrograde Symplectic Assemblages in Granulites and Amphibolites

Symplectites form during post-orogenic fast uplift processes in orogenic belts, and retrograde Symplectic assemblages mainly consist of plagioclase + quartz ± orthopyroxene ± clinopyroxene in mafic granulites or plagioclase + hornblende + quartz ± biotite in amphibolites, usually rimming relict garnet porphyroblasts. Such Symplectic assemblages resulted from retrograde reactions between garnet and other peak-metamorphic minerals by nearly isothermal decompression (ITD), so the Symplectic mineral assemblages could not be at equilibrium with the relict garnet rims and thus the so-called “local equilibrium” between garnet rims and the Symplectic assemblages does not exist. Thus, the P-T conditions of the Symplectic assemblages are best determined using only the Symplectic mineral compositions. This is best accomplished using garnet-free thermobarometers, i.e., two-pyroxene or hornblendeplagioclase thermometer in combination with the hornblende-plagioclase-quartz or applying the clinopyroxeneplagioclase-quartz geobarometer to minerals within the symplectites. Taking two mafic granulite samples and two amphibolite samples as examples, reasonable ITD P-T paths from the metamorphic peaks to retrogression stages have been derived, but P-T paths obtained using the relict garnet rims and the Symplectic minerals gave anomalous results. These examples demonstrate that the P-T conditions of the retrograde Symplectic assemblages cannot be estimated using chemical compositions of the decomposed, relict garnet rims in this case.

Decoding polyphase migmatites using geochronology and phase equilibria modelling

AbstractIn this study, in situ U–Pb monazite ages and Lu–Hf garnet geochronology are used to distinguish mineral parageneses developed during Devonian–Carboniferous and Cretaceous events in migmatitic paragneiss and orthogneiss from the Fosdick migmatite–granite complex in West Antarctica. SHRIMP U–Pb monazite ages define two dominant populations at 365–300 Ma (from cores of polychronic grains, dominantly from deeper structural levels in the central and western sectors of the complex) and 120–96 Ma (from rims of polychronic grains, dominantly from the central and western sectors of the complex, and from monochronic grains, mostly from shallower structural levels in the eastern sector of the complex). For five paragneisses and two orthogneisses, Lu–Hf garnet ages range from 116 to 111 Ma, c. 12–17 Ma older than published Sm–Nd garnet ages of 102–99 Ma from three of the same samples. Garnet grains in the analysed samples generally have Lu‐enriched rims relative to Lu‐depleted cores. By contrast, for three of the same samples, individual garnet grains have flat Sm concentrations consistent with high‐T diffusive resetting. Lutetium enrichment of garnet rims is interpreted to record the breakdown of a Lu‐rich accessory mineral during the final stage of garnet growth immediately prior to the metamorphic peak, and/or the preferential retention of Lu in garnet during breakdown to cordierite in the presence of melt concomitant with the initial stages of exhumation. Therefore, garnet is interpreted to be part of the Cretaceous mineral paragenesis and the Lu–Hf garnet ages are interpreted to record the timing of close‐to‐peak metamorphism for this event. For the Devonian–Carboniferous event, phase equilibria modelling of the metasedimentary protoliths to the paragneiss and a diatexite migmatite restrict the peak P–T conditions to 720–800 °C at 0.45–1.0 GPa. For the Cretaceous event, using both forward and inverse phase equilibria modelling of residual paragneiss and orthogneiss compositions, the P–T conditions after decompression are estimated to have been 850–880 °C at 0.65–0.80 GPa. These P–T conditions occurred between c. 106 and c. 96 Ma, determined from Y‐enriched rims on monazite that record the timing of garnet and biotite breakdown to cordierite in the presence of melt. The effects of this younger metamorphic event are dominant throughout the Fosdick complex.

Combined U-Pb, Lu-Hf, Sm-Nd and Ar-Ar multichronometric dating on the Bailang eclogite constrains the closure timing of the Paleo-Tethys Ocean in the Lhasa terrane, Tibet

The Lhasa terrane, the main tectonic component of the Himalayan–Tibetan orogen, has received much attention as it records the entire history of the orogeny. The occurrence of Permian to Triassic high-pressure eclogites has a significant bearing on the understanding of the Paleo-Tethys subduction and plate suturing processes in this area. An eclogite from the Bailang, eastern Lhasa terrane, was investigated with a combined metamorphic P–T and U–Pb, Lu–Hf, Sm–Nd and Ar–Ar multichronometric approach. Pseudosection modeling combined with thermobarometric calculations indicate that the Bailang eclogite equilibrated at peak P–T conditions of ~2.6GPa and 465–503°C, which is much lower than those of Sumdo and Jilang eclogites in this area. Garnet–whole rock–omphacite Lu–Hf and Sm–Nd ages of 238.1±3.6Ma and 230.0±4.7Ma were obtained on the same sample, which are largely consistent with the corresponding U–Pb age of 227.4±6.4Ma for the metamorphic zircons within uncertainty. The peak metamorphic temperature of the sample is lower than the Lu–Hf and Sm–Nd closure temperatures in garnet. This, combined with the core-to-rim decrease in Mn and HREE concentrations, the slightly U-shaped Sm zonation across garnet and the exclusive occurrence of omphacite inclusion in garnet rim, are consistent with the Lu–Hf system skewing to the age of the garnet core and the Sm–Nd system favoring the rim age. The Sm–Nd age was thus interpreted as the age of eclogite-facies metamorphism and the Lu–Hf age likely pre-dated the eclogite-facies metamorphism. 40Ar/39Ar dating of hornblende from the eclogite yielded ages about 200Ma, which is interpreted as a cooling age and is probably indicative of the time of exhumation to the middle crust. The difference of peak eclogite-facies metamorphic conditions and the distinct metamorphic ages for the Bailang eclogite (~2.6GPa and ~480°C; ca. 230Ma), the Sumdo eclogite (~3.4GPa and ~650°C; ca. 262Ma) and Jiang eclogite (~3.6GPa and ~750°C; ca. 261Ma) in the same (ultra)-high-pressure belt indicate that this region likely comprises different slices that had distinct P–T histories and underwent (U)HP metamorphism at different times. The initiation of the opening the Paleo-Tethys Ocean in the Lhasa terrane could trace back to the early Permian. The ultimate closure of the Paleo-Tethys Ocean in the Lhasa terrane was no earlier than ca. 230Ma.

Trace element distribution in mineral inclusions in zoned garnets from eclogites of the Atbashi Range (South Tianshan)

Ion microprobe data for minerals from the eclogites of the Atbashi Range (South Tianshan) constrain the distribution of trace (Rb, Sr, Ba, Cr, V, Zr, Hf, Nb, Ta, U, Th, and Y) and rare-earth elements (REE) in zoned garnets and mineral inclusions in them. This study showed that garnets from the Atbashi eclogites are the main hosts for heavy REE; epidotes are important hosts for REE, Y, Sr, Th, and U; and omphacites are depleted in almost all trace elements compared with the bulk-rock compositions. Garnet, as well as epidote and omphacite inclusions exhibit systematic rimward depletion in a number of trace elements, which is related to the depletion of the rock matrix in these elements during crystallization. Deviations from this trend, including the enrichment of garnet rims in HREE and strong variations in the REE contents of garnets and mineral inclusions, can be explained by metamorphic reactions involving the destabilization of REE-bearing minerals. Our data suggest that the mobility of trace elements under eclogites-facies conditions is mainly controlled by the stability of certain minerals.

Metamorphic P–T–t paths of the Zanhuang metamorphic complex: Implications for the Paleoproterozoic evolution of the Trans-North China Orogen

Garnet-bearing metapelites are cropped out in the Zanhuang metamorphic complex located in the central section of the N–S trending Trans-North China Orogen. Petrological studies indicate that these garnet-bearing metapelites preserve three distinct metamorphic assemblages (M1–M3). The cores of garnet porphyroblasts and fine-grained inclusions of plagioclase+quartz±biotite±apatite±magnetite±rutile±ilmenite define the prograde metamorphic stage (M1). The assemblage of the peak metamorphic stage (M2) consists of biotite+plagioclase+quartz±kyanite±hornblende±magnetite±ilmenite in equilibrium with the lowest-XMn garnet rims. The peak metamorphism was followed by a near-isothermal decompressional stage (M3) and the development of symplectitic coronas of biotite+plagioclase+quartz±hornblende±magnetite surrounding garnet porphyroblasts. Quantitative phase equilibria modeling and traditional thermobarometric estimations were applied to yield P–T conditions of 4.5–5.9kbar and 551–596°C for stage M1, 9.6–12.3kbar and 770–830°C for stage M2, and ∼4.7kbar and ∼635°C for stage M3. The combination of the mineral inclusions, mineral compositions, and metamorphic reactions in the Zanhuang metapelites defines a clockwise P–T path with nearly isothermal decompression, which is considered to have been related to the amalgamation of the Eastern and Western Blocks to form the North China Craton. Abundant U–Pb spot analyses of metamorphic zircons from the gneiss and amphibolite reveal two discrete, meaningful ages of metamorphism within the Zanhuang complex: the first age dates to ∼2507Ma, and the second age dates to ∼1839Ma. The first age group (∼2507Ma) was most likely associated with a regional Neoarchean granulite-facies metamorphic event within the North China Craton, whereas the second age group (∼1839Ma) is interpreted as the result of a Paleoproterozoic orogenic event that occurred in the Trans-North China Orogen (TNCO).

Garnet as a major carrier of the Y and REE in the granitic rocks: An example from the layered anorogenic granite in the Brno Batholith, Czech Republic

Garnet and other rock-forming minerals from A-type granite dykes in the Pre-Variscan Brno Batholith were analyzed to determine relative contributions of individual minerals to whole-rock Y and REE budget and to assess incorporation mechanisms of these elements in garnet. Minor to accessory garnet (<2 vol%) is the essential reservoir for Y+REE in the Hlina granite accounting ~84% Y and 61% REE of the total whole-rock budget. Zircon is another important carrier of REE with ~13% Y and ~11% REE. At least ~21% REE and 1% Y were probably hosted by Th- and U-rich monazite that has been completely altered to a mixture of secondary REE-bearing phases. The contribution of major rock-forming minerals (quartz and feldspars) is low (~1% Y; 10% LREE; ~1% HREE) excluding Eu, which is hosted predominantly by feldspars (~90%). Minor to accessory muscovite and magnetite incorporate ~1% Y and ~2% REE of the whole-rock budget. Magmatic garnet Sps41–46Alm28–44And0–13 Grs6–12Prp0–1 is Y- and HREE-rich (up 1.54 wt% Y; up ~1 wt% total REE), and the Y+REE enter the garnet structure via the menzerite-(Y) substitution. The Y and REE show complex zoning patterns and represent sensitive indicator of garnet evolution, in contrast to a homogeneous distribution of major divalent cations. General outward decrease of Y+REE is a common feature due to the strong partitioning of Y+HREE in the garnet relative to the other phases. REE underwent significant fractionation during growth of early garnet I; the YbN/NdN ratio generally decreases from the core to rim of garnet I. Higher Mn and Al, lower Ca, and Y+REE contents, as well as higher YbN/NdN ratio and more negative Eu anomaly in garnet II overgrowths indicate its crystallization from a more evolved melt. Application of zircon saturation geothermometry provides upper temperature limit of 734 ± 14 °C for the closed-system crystallization. Mineral equilibria reveal that crystallization started at QFM + 1.2, and preferential sequestration of Fe3+ into garnet and magnetite was responsible for progressively reducing conditions. Equilibrium between magnetite, garnet, quartz, and plagioclase, representing the final crystallization stage of the granitic magma, occurred at 658–663 °C and QFM 0 to + 0.8, hence at undercooling of ~75 °C.

PT-path reconstruction via unraveling of peculiar zoning pattern in atoll shaped garnets via image assisted analysis: an example from the Santa Lucia del Mela garnet micaschists (northeastern Sicily-Italy)

Garnet micaschists nearby Santa Lucia del Mela - Peloritani Mountains (North-eastern Sicily, Italy), were studied by integrating information obtained from petrographic and image-assisted analyses of mineral phase compositions by using X-ray maps and thermodynamic modeling. These rocks are characterized by the presence of atoll garnets, and preserve a continuous array of textures from pristine garnets, through peninsula or island textures, to atoll textures. Completely substituted garnets represent the final stage of the process. These textures result from the metamorphic evolution of the rocks, consisting of two main stages of recrystallization-deformation events. The sequence of mineral parageneses associated to successive deformation events is as follows: early D1 is characterized by the assemblage wm1 + chl1 + bt1 + pl1 + grt1 + qtz + ilm + ap; late D1 by grt2 + wm2 + chl2+ bt2 + pl2 + qtz + ilm + ap + hem; D2 (associated to garnet core transformation) by grt3 + bt3 + pl3 + wm3 + qtz + chl3 + ilm. On the basis of textural observations and mineral parageneses, the atoll formation is related to the substitution of garnet cores during the D2 stage, with the consumption of garnet cores (grt1) and production of new garnet rim (grt3) with wm, bi, qtz and pl. The recognition of the different stages of garnet growth and associated parageneses has been obtained using image analysis coupled with compositional maps, performed via multivariate statistical analysis of four selected X-Ray map arrays. Following the textural and mineral-chemical analyses, PT conditions associated to the three recrystallisation stages have been determined via pseudosection computations. Results indicate that the analyzed rock-types have recorded a clock-wise PT trajectory during the Variscan orogeny, defined by a prograde evolution, in amphibolite facies conditions, preserved in relic garnet cores, followed by an initial decompressional evolution toward lower amphibolite facies conditions associated to a partial heating. During this last stage the multi stadial atollization process took place, retrogradely evolving toward a green-schist facies conditions, constrained through the compositions of the pseodomorphyc mineral aggregates gradually substituting previously formed garnets.

Wealth of P–T–t information in medium-high grade metapelites: Example from the Jubrique Unit of the Betic Cordillera, S Spain

It is demonstrated here that medium-high grade metapsammopelites contain a wealth of information to derive a well-constrained P–T–t path relevant for the understanding of the geodynamics resulting in a collisional orogen. The presented example is related to three samples from the Migmatite gneiss zone of the Jubrique Unit, being part of the Alpujárride Complex half way between Malaga and Cadiz. The rocks were studied to determine their pressure (P)–temperature (T) evolution mainly on the basis of the zonation of mm-sized garnet and contoured P–T pseudosections calculated with PERPLE_X. In addition, monazite was dated with the electron microprobe. Ca-poor garnet cores yielded granulite-facies P–T conditions around 0.7 GPa and 800 °C. This metamorphic event occurred in late Hercynian times [287 ± 9 (2σ) Ma] according to monazite enclosed in such garnet cores. The cores are surrounded by a Ca-richer garnet zone, which points to a high-pressure (HP) event close to 1.3 GPa and temperatures above 650 °C. At this stage, melting occurred as indicated by abundant melt inclusions in the Ca-rich garnet zone. Subsequently, the rocks were exhumed and heated to peak temperatures of 740 °C. The final metamorphic stage is characterised by late sillimanite and an outermost garnet rim which is poor in Ca. The melting event is late Alpine confirmed by age dating on monazite in the rock matrix. Similar conditions of a late Alpine HP event were also reported from other areas of the Alpujárride Complex. As the studied rocks never reached P–T conditions of the diamond stability field, it is concluded that the previously reported nanodiamonds in these rocks either grew metastably or were artificially introduced into rock thin-sections.

Microstructures and petrology of melt inclusions in the anatectic sequence of Jubrique (Betic Cordillera, S Spain): Implications for crustal anatexis

Abstract We report a new occurrence of melt inclusions in polymetamorphic granulitic gneisses of the Jubrique unit, a complete though strongly thinned crustal section located above the Ronda peridotite slab (Betic Cordillera, S Spain). The gneissic sequence is composed of mylonitic gneisses at the bottom and in contact with the peridotites, and porphyroblastic gneisses on top. Mylonitic gneisses are strongly deformed rocks with abundant garnet and rare biotite. Except for the presence of melt inclusions, microstructures indicating the former presence of melt are rare or absent. Upwards in the sequence, garnet decreases whereas biotite increases in modal proportion. Melt inclusions are present from cores to rims of garnets throughout the entire sequence. Most of the former melt inclusions are now totally crystallized and correspond to nanogranites, whereas some of them are partially made of glass or, more rarely, are totally glassy. They show negative crystal shapes and range in size from ≈ 5 to 200 μm, with a mean size of ≈ 30–40 μm. Daughter phases in nanogranites and partially crystallized melt inclusions include quartz, feldspars, biotite and muscovite; accidental minerals include kyanite, graphite, zircon, monazite, rutile and ilmenite; glass has a granitic composition. Melt inclusions are mostly similar throughout all the gneissic sequence. Some fluid inclusions, of possible primary origin, are spatially associated with melt inclusions, indicating that at some point during the suprasolidus history of these rocks granitic melt and fluid coexisted. Thermodynamic modeling and conventional thermobarometry of mylonitic gneisses provide peak conditions of ≈ 850 °C and 12–14 kbar, corresponding to cores of large garnets with inclusions of kyanite and rutile. Post-peak conditions of ≈ 800–850 °C and 5–6 kbar are represented by rim regions of large garnets with inclusions of sillimanite and ilmenite, cordierite-quartz-biotite coronas replacing garnet rims, and the matrix with oriented sillimanite. Previous conventional petrologic studies on these strongly deformed rocks have proposed that anatexis started during decompression from peak to post-peak conditions and in the field of sillimanite. The study of melt inclusions shows, however, that melt was already present in the system at peak conditions, and that most garnet grew in the presence of melt.

Polyphase growth of garnet in eclogite from the Hong'an orogen: Constraints from garnet zoning and phase equilibrium

Abstract Major and trace element profiles as well as mineral inclusions were analyzed in garnets from massive and foliated eclogites from the low-T/high-P eclogite-facies zone in the Hong'an orogen, China. Garnets in the two types of eclogites show different core–rim zoning and mineral inclusions. At least two stages of garnet growth are evident for most garnet grains. Garnets in the massive eclogite contain abundant mineral inclusions such as quartz, chlorite, amphibole, rutile and phengite in cores, but only a few mineral inclusions such as quartz, rutile and titanite in rims. These garnet grains exhibit increasing MgO but decreasing CaO and REE contents from core to rim. Together with pseudosection calculations it is suggested that hydrous minerals such as chlorite, epidote and amphibole were probably the major reactants for the growth of garnet cores during prograde subduction, whereas the growth of garnet rims involves amphibole breakdown and/or dissolution of previously formed garnet. On the other hand, garnets in the foliated eclogite exhibit nearly constant MgO, decreasing MnO but increasing CaO and heavy REE contents (HREE) from core to rim. Along with pseudosection calculations it is inferred that the decomposition of epidote would consistently contribute to the growth of garnet core and rim. This is also supported by epidote zoning in matrix, with the occurrence of low-jadeite omphacite inclusions in the core and a few rutile inclusions in the rim. The enrichment of light REE (LREE) and depletion of HREE in the epidote rim relative to the core indicate that the epidote rim was recrystallized in equilibrium with the garnet rims. For protolith compositions, the foliated eclogite shows higher SiO2, Na2O, K2O and MgO contents but lower Al2O3, Fe2O3 and TiO2 contents than the massive eclogite. Along with phase equilibrium modeling, it is concluded that the differences in the protolith compositions of eclogites primarily dictate the differences in the garnet zoning patterns and the mineral assemblages of matrix and inclusions. The decomposition of hydrous minerals such as chlorite, amphibole and epidote is necessary for garnet growth during subduction-zone metamorphism.

Metamorphic P-T trajectory and multi-stage fluid events of vein-bearing UHP eclogites from the Dabie terrane: insights from compositional zonations of key minerals

The multi-stage metamorphic evolution and high-pressure (HP) to ultra-high-pressure (UHP) fluid events of vein-bearing eclogites from the Ganghe in the Dabie UHP terrane have been established based on a detailed study of compositional zonations in garnet, omphacite, and epidote. Comparison of mineral assemblages and zoned garnet compositions with the predictions of phase equilibrium modelling results in six stages of metamorphism in eclogites: (i) prograde low-temperature (LT)-HP eclogite-facies metamorphism (2.0–2.3 GPa, <540°C); (ii) peak UHP lawsonite eclogite-facies metamorphism (~3.1 GPa, ~650°C); (iii) decompressional breakdown of UHP lawsonite during initial exhumation (2.8–3.0 GPa, 650–680°C), as we have previously reported; (iv) middle-temperature (MT) and HP eclogite-facies metamorphism (~2.6 GPa, ~720°C); (v) retrograde eclogite-facies metamorphism (1.5–1.7 GPa, 610–670°C); and (vi) amphibolite-facies metamorphism (<1.1 GPa, <600°C). A heating event with decompression is confirmed from metamorphic stages (iii) to (iv), indicating that the Ganghe eclogites underwent a relatively sluggish exhumation during the stages from the UHP to HP conditions. During prograde metamorphic stages (i) and (ii), fluid activities in the eclogites are limited because lawsonite growth consumed large amounts of early-stage hydrous minerals and potential free water. Dehydration reactions during metamorphic stage (iii) triggered a significant liberation of UHP fluid. Intense fluid-eclogite interaction and dissolution-precipitation processes during metamorphic stages (iii) to (iv) led to the occurrence of patchy zones in epidote cores and the systematic compositional change of epidote and omphacite cores near the vein. A late-stage infiltration of HP fluid into eclogites occurred in metamorphic stage (v), leading to the final-stage growth (mineral rims) of garnet, omphacite, and epidote. This study presents detailed mineral zonation patterns and mineral compositions of the vein-hosting eclogites to link the complicated metamorphic P-T evolution with episodic multi-stage HP-UHP fluid events in deeply subducted continental crust.

Thermal evolution of the Tseel terrane, SW Mongolia and its relation to granitoid intrusions in the Central Asian Orogenic Belt

AbstractThe timing and thermal effects of granitoid intrusions into accreted sedimentary rocks are important for understanding the growth process of continental crust. In this study, the petrology and geochronology of pelitic gneisses in the Tseel area of the Tseel terrane, SW Mongolia, are examined to understand the relationship between igneous activity and metamorphism during crustal evolution in the Central Asian Orogenic Belt (CAOB). Four mineral zones are recognized on the basis of progressive changes in the mineral assemblages in the pelitic gneisses, namely: the garnet, staurolite, sillimanite and cordierite zones. The gneisses with high metamorphic grades (i.e. sillimanite and cordierite zones) occur in the central part of the Tseel area, where granitoids are abundant. To the north and south of these granitoids, the metamorphic grade shows a gradual decrease. The composition of garnet in the pelitic gneisses varies systematically across the mineral zones, from grossular‐rich garnet in the garnet zone to zoned garnet with grossular‐rich cores and pyrope‐rich rims in the staurolite zone, and pyrope‐rich garnet in the sillimanite and cordierite zones. Thermobarometric analyses of individual garnet crystals reveal two main stages of metamorphism: (i) a high‐P and low‐T stage (as recorded by garnet in the garnet zone and garnet cores in the staurolite zone) at 520–580 °C and 4.5–7 kbar in the kyanite stability field and (ii) a low‐P and high‐T stage (garnet rims in the staurolite zone and garnet in the sillimanite and cordierite zones) at 570–680 °C and 3.0–6.0 kbar in the sillimanite stability field. The earlier high‐P metamorphism resulted in the growth of kyanite in quartz veins within the staurolite and sillimanite zones. The U–Pb zircon ages of pelitic gneisses and granitoids reveal that (i) the protolith (igneous) age of the pelitic gneisses is c. 510 Ma; (ii) the low‐P and high‐T metamorphism occurred at 377 ± 30 Ma; and (iii) this metamorphic stage was coeval with granitoid intrusion at 385 ± 7 Ma. The age of the earlier low‐T and high‐P metamorphism is not clearly recorded in the zircon, but probably corresponds to small age peaks at 450–400 Ma. The low‐P and high‐T metamorphism continued for c. 100 Ma, which is longer than the active period of a single granitoid body. These findings indicate that an elevation of geotherm and a transition from high‐P and low‐T to low‐P and high‐T metamorphism occurred, associated with continuous emplacement of several granitoids, during the crustal evolution in the Devonian CAOB.

Metamorphic evolution of relict lawsonite‐bearing eclogites from the (U) HP metamorphic belt in the Chinese southwestern Tianshan

AbstractIn the Chinese southwestern Tianshan (U)HP belt, former lawsonite presence has been predicted for many (U)HP metamorphic eclogites, but only a very few lawsonite grains have been found so far. We discovered armoured lawsonite relicts included in quartz, which, on its part, is enclosed in porphyroblastic garnet in an epidote eclogite H711‐14 and a paragonite eclogite H711‐29. H711‐14 is mainly composed of garnet, omphacite, epidote and titanite, with minor quartz, paragonite and secondary barroisite and glaucophane. Coarse‐grained titanite occasionally occurs in millimetre‐wide veins in equilibrium with epidote and omphacite, and relict rutile is only preserved as inclusions in matrix titanite and garnet. H711‐29 shows the mineral assemblage of garnet, omphacite, glaucophane, paragonite, quartz, dolomite, rutile and minor epidote. Dolomite and rutile are commonly rimed by secondary calcite and titanite respectively. Porphyroblastic garnet in both eclogites is compositionally zoned and exhibits an inclusion‐rich core overgrown by an inclusion‐poor rim. Phase equilibria modelling predicts that garnet cores formed at the P‐peak (490–505 °C and 23–25.5 kbar) and coexisted with the lawsonite eclogite facies assemblage of omphacite + glaucophane + lawsonite + quartz. Garnet rims (550–570 °C and ~20 kbar) grew subsequently during a post‐peak epidote eclogite facies metamorphism and coexisted with omphacite + quartz ± glaucophane ± epidote ± paragonite. The results confirm the former presence of a cold subduction zone environment in the Chinese southwestern Tianshan. The P–T evolution of the eclogites is characterized by a clockwise P–T path with a heating stage during early exhumation (thermal relaxation). The preservation of lawsonite in these eclogites is attributed to isolation from the matrix by quartz and rigid garnet, which should be considered as a new type of lawsonite preservation in eclogites. The complete rutile–titanite transition in H711‐14 took place in the epidote eclogite facies stage in the presence of an extremely CO2‐poor fluid with X(CO2) [CO2/(CO2 + H2O) in the fluid] &lt;&lt;0.008. In contrast, the incomplete rutile–titanite transition in H711‐29 may have occurred after the epidote eclogite facies stage and the presence of dolomite reflects a higher X(CO2) (&gt;0.01) in the coexisting fluid at the epidote eclogite facies stage.

Phase relationships and P–T path in NCFMASHTO system of the eclogite from the Tighsi area (Egere terrane, Central Hoggar, Algeria)

The Tighsi area is situated in the northern part of the Egere-Aleksod terrane (Central Hoggar, South of Algeria). The eclogites and the garnet-amphibolites occur as lenses in marble, metapelite and quartzite. The petrological study shows that the high pressure paragenesis is characterized by the assemblage omphacite–garnet–rutile–quartz and epidote. Omphacite contains epidote, rutile and quartz inclusions. Due to the decompression, omphacite is the first mineral to destabilize into very thin symplectites of albite and clinopyroxene poorer of the jadeitic component. The paragenese of the lower pressure is represented by kelyphites and symplectites of amphibole–plagioclase separating garnet from omphacite and quartz. During this evolution, rutile transforms into ilmenite then, in the latest stage sphene replaces ilmenite. The thermodynamic modeling using THERMOCALC has confirmed these textural observations and suggests that the eclogites have experienced three stages of metamorphism. Stage I is the high pressure association, an H2O under-saturated pseudosection adapted to this stage has been constructed in NCFMASHTO system. The association of high pressure corresponds to the field where garnet is in equilibrium with omphacite including epidote-rutile and quartz; The P–T conditions of 19.6kbar and 694°C have been estimated using the garnet core compositions. Stage II is the isothermal decompression where the amphibole–plagioclase kelyphites occur at the expense of garnet, omphacite and quartz at 15.5kbar and 774°C (conditions estimated using the garnet rim and the plagioclase–amphibole kelyphites compositions). Stage III is the late retrograde evolution; a second pseudosection with H2O in excess has been constructed to model this stage and show the transformation of eclogites into garnet-bearing amphibolites and explains the occurrence of sphene. The P–T conditions of 9.3kbar and 695°C have been estimated using the compositions of the plagioclase with the maximum Xan and the amphibole with the minimum concentration of AlVI. Both pseudosections show a clockwise path for eclogite evolution to garnet-amphibolites. This path characterizes the exhumation that succeeded a subduction event; therefore, the P–T path shows an isothermal decompression. Then, the geothermic gradients became normal so the decrease of pressure is accompanied with a decrease in temperature.

Garnet oxygen analysis by SHRIMP-SI: Matrix corrections and application to high-pressure metasomatic rocks from Alpine Corsica

Garnet is a key mineral used to constrain pressure, temperature and age of metamorphic rocks. This contribution reports oxygen isotope measurements in garnet using the SHRIMP-SI ion microprobe. The reproducibility of oxygen isotope analyses on garnet standard UWG2 is ~0.3–0.4‰ (2σ) within and across sessions. The correlation between oxygen isotope measurements and the grossular and andradite components in garnet fits a second-degree polynomial with a maximum bias in δ18O of 2.4 and 8.3‰, respectively. This bias is similar to that determined for other large ion microprobes. Analysis of two additional Mn-rich garnet crystals allowed identification of a separate bias caused by the spessartine component, which can reach a maximum of 2.3‰.The standardisation and correction scheme proposed in this study are applied to garnet crystals from two samples from Alpine Corsica in order to link fluid evolution with the pressure–temperature–time path. The samples have experienced a polyphase metamorphic history, which includes Permian high temperature metamorphism, followed by late However, 92-W2 was re-analysed because of the variability observed in the previously published δ18O values (-0.29 and 0.53‰, Kohn and Valley, 1998). The δ18O value found in this study (0.81 ± 0.44‰, 2s, Table 1) agrees within error with the highest value previously published for this garnet. Eocene high pressure–low temperature metamorphism. Permian Ca-poor garnet cores have high δ18O values (9.9±0.6‰ and 11.1±0.5‰ 2σ, in two samples) with respect to garnet mantles (7.2±0.4‰) and rims (5.4±0.5‰ and 2.2±0.4‰). The dramatic decrease in δ18O from Permian garnet cores to Alpine rims in both samples reflects a combination of external fluid influx and change in sample mineralogy. The low δ18O of the garnet rims that formed at eclogite facies conditions indicates that the metasomatic fluid equilibrated with mafic or ultramafic rocks. This study illustrates that fluid-mediated mass transfer during subduction occurred at lithological contacts between felsic and ultramafic rocks.

Magmatic garnet in the Triassic (215 Ma) Dehnow pluton of NE Iran and its petrogenetic significance

The Triassic Dehnow pluton of NE Iran is a garnet-bearing I-type calc-alkaline metaluminous diorite-tonalite-granodiorite intrusion. The parental magma formed as the result of partial melting of intermediate to felsic rocks in the lower crust. Petrological and geochemical evidence, which indicates a magmatic origin for the garnet, includes: large size (~10–20 mm) of crystals, absence of reaction rims, a distinct composition from garnet in adjacent metapelitic rocks, and similarity in the composition of mineral inclusions (biotite, hornblende) in the garnet and in the matrix. Absence of garnet-bearing enclaves in the pluton and lack of sillimanite (fibrolite) and cordierite inclusions in magmatic garnet suggests that the garnet was not produced by assimilation of meta-sedimentary country rocks. Also, the δ18O values of garnet in the pluton (8.3–8.7‰) are significantly lower than δ18O values of garnet in the metapelitic rocks (12.5–13.1‰). Amphibole-plagioclase and garnet-biotite thermometers indicate crystallization temperatures of 708°C and 790°C, respectively. A temperature of 692°C obtained by quartz-garnet oxygen isotope thermometry points to a closure temperature for oxygen diffusion in garnet. The composition of epidote (Xep) and garnet (Xadr) indicates ~800°C for the crystallization temperature of these minerals. Elevated andradite content in the rims of garnet suggests that oxygen fugacity increased during crystallization.