Garnet Rims Research Articles

Early Cambrian eclogites in SW Mongolia: evidence that the Palaeo‐Asian Ocean suture extends further east than expected

AbstractNewly discovered eclogites, Early Cambrian carbonates and chloritoid‐bearing metapelites form the Tsakhir Uul accretionary wedge, which was thrust during the Early Cambrian over the Mesoproterozoic Dzabkhan‐Baydrag continent. The rock association of the wedge forms a tectonic window emerging through the hangingwall Khantaishir ophiolite unit, which preserves a typical Tethyan‐type ophiolitic sequence. The eclogites correspond geochemically to T‐MORB modified by fluid circulation. They are composed of garnet, omphacite, amphibole, rutile ±muscovite ±quartz ±epidote and exhibit well‐equilibrated matrix textures. Jadeite content of the omphacite reaches up to 45 mol.%, the Si content of muscovite is between 3.40 and 3.45 p.f.u., amphibole is winchite to barroisite, but reaches tschermakitic composition at some rims, and garnet composition is grs0.24–0.36, alm0.43–0.56, py0.05–0.18, sps0.00–0.18, . The peak assemblage, together with the composition of garnet rims, omphacite, amphibole and muscovite, correspond in a pseudosection to 20−22.5 kbar and 590−610 °C. The tschermakitic rim of amphibole is interpreted as partial reequilibration on decompression below 16 kbar and ∼600−630 °C. Two muscovite separates from the eclogite yielded an Ar–Ar plateau age of 543.1 ± 3.9 Ma (1σ) and a mean age of 547.9 ± 2.6 Ma (1σ), whereas muscovite from an interbedded garnet‐chloritoid micaschist yielded an Ar–Ar plateau age of 536.9 ± 2.7 Ma (1σ); these ages are interpreted as cooling ages. The P–T data, geochemistry of eclogites and cooling ages suggest an affinity between the Tsakhir Uul wedge and the Gorny Altai and the north Mongolian blueschist belt, which are believed typical for subduction of warm oceanic lithosphere and closure of small oceanic basins. Thus, the discovery of the Tsakhir Uul eclogites represents an important finding suggesting extension of the Early Cambrian subduction system of the Central Asian Orogenic Belt far to the east in a region where it was not expected.

Anatexis during High-pressure Crustal Metamorphism: Evidence from Garnet–Whole-rock REE Relationships and Zircon–Rutile Ti–Zr Thermometry in Leucogranulites from the Bohemian Massif

In situ mineral (garnet, zircon) trace element and garnet–rock REE distribution data obtained on leucogranulites from the Bohemian Massif are consistent with an origin of these now strongly deformed rocks as migmatitic leucogranites formed through dehydration-melting of muscovite-bearing protoliths at high pressures during the Variscan orogeny. Partial melting at P–T conditions of 900–940°C and 1·6–1·8 GPa consumed mica and feldspar to produce peritectic kyanite and high-Ca (20–24 mol % grossular), high-Eu garnet along with leucogranitic melt. DREE(Grt/rock) values calculated from this garnet and its host rock are consistent with garnet–melt equilibrium at these P–T conditions. Ternary feldspar and rutile (2000–3200 ppm Zr) crystallized from the leucogranite melts in the migmatites at minimum temperatures of >880–920°C. These formed along with apatite prior to and accompanying the crystallization of garnet rims and the majority of the garnet that occurs in the leucogranulites. This garnet is typified by strong depletion in Eu and DREE(Grt/rock) values consistent with garnet–melt equilibrium only after the fractionation of apatite (<0·3 wt %) and ternary feldspar (5–15 wt %). Zircons in one leucogranulite are dominated by oscillatory-zoned grains that yield pre-Variscan U–Pb ages (495 and 433 Ma) and crystallized, from Ti-in-zircon thermometry, at 700–810°C in their original host magmas. These xenocrystic grains are potentially preserved because of the short timespan of the high P–T Variscan event at 340 Ma and the moderate amounts of leucogranitic partial melt present in the rocks, estimated as at least 30 wt % melt from Zr mass balance. The production of new zircon associated with the Variscan event in this leucogranulite is limited to weakly zoned zircon rims characterized by flat heavy REE patterns that approach REE equilibrium with garnet rims. This post-peak zircon was formed at minimum P–T conditions of 840°C and 0·8–1·0 GPa along the decompression-cooling path defined by previous work for the Bohemian Massif granulites. The garnet–whole-rock REE relationships, observation of Eu depletion between early and later garnet growth phases, and Zr–Ti thermometric estimates indicating temperatures of >840°C for this evolution cannot be explained by the alternative interpretation of the leucogranulites as reworked and recrystallized pre-Variscan low-P granitoids, but support a model in which the Bohemian leucogranulites were high-P Variscan-age migmatites containing significant amounts of melt.

Evidence for multiple burial–partial exhumation cycles from the Onodani eclogites in the Sambagawa metamorphic belt, central Shikoku, Japan

AbstractEclogites from the Onodani area in the Sambagawa metamorphic belt of central Shikoku occur as layers or lenticular bodies within basic schists. These eclogites experienced three different metamorphic episodes during multiple burial and exhumation cycles. The early prograde stage of the first metamorphic event is recorded by relict eclogite facies inclusions within garnet cores (XSps 0.80–0.24, XAlm 0–0.47). These inclusions consist of relatively almandine‐rich garnet (XSps 0.13–0.24, XAlm 0.36–0.45), aegirine‐augite/omphacite (XJd 0.08–0.28), epidote, amphiboles (e.g. actinolite, winchite, barroisite and taramite), albite, phengite, chlorite, calcite, titanite, hematite and quartz. The garnet cores also contain polyphase inclusions consisting of almandine‐rich garnet, omphacite (XJd 0.27–0.28), amphiboles (e.g. actinolite, winchite, barroisite, taramite and katophorite) and phengite. The peak P–T conditions of the first eclogite facies metamorphism are estimated to be 530–590 °C and 19–21 kbar succeeded by retrogression into greenschist facies. The second prograde metamorphism began at greenschist facies conditions. The peak metamorphic conditions are defined by schistosity‐forming omphacites (XJd ≤ 49) and garnet rims containing inclusions of barroisitic amphibole, phengite, rutile and quartz. The estimated peak metamorphic conditions are 630–680 °C and 20–22 kbar followed by a clockwise retrograde P–T path with nearly isothermal decompression to 8–12 kbar. In veins cross‐cutting the eclogite schistosity, resorbed barroisite/Mg‐katophorite occurs as inclusions in glaucophane which is zoned to barroisite, suggesting a prograde metamorphism of the third metamorphic event. The peak P–T conditions of this metamorphic event are estimated to be 540–600 °C and 6.5–8 kbar. These metamorphic conditions are correlated with those of the surrounding non‐eclogitic Sambagawa schists. The Onodani eclogites were formed by subduction of an oceanic plate, and metamorphism occurred beneath an accretionary prism. These high‐P/T type metamorphic events took place in a very short time span between 100 and 90 Ma. Plate reconstructions indicate highly oblique subduction of the Izanagi plate beneath the Eurasian continent at a high spreading rate. This probably resulted in multiple burial and exhumation movements of eclogite bodies, causing plural metamorphic events. The eclogite body was juxtaposed with non‐eclogitic Sambagawa schists at glaucophane stability field conditions. The amalgamated metamorphic sequence including the Onodani eclogites were exhumed to shallow crustal/surface levels in early Eocene times (c. 50 Ma).

Phase equilibria and metamorphic evolution of glaucophane‐bearing UHP eclogites from the Western Dabieshan Terrane, Central China

AbstractGlaucophane‐bearing ultrahigh pressure (UHP) eclogites from the western Dabieshan terrane consist of garnet, omphacite, glaucophane, kyanite, epidote, phengite, quartz/coesite and rutile with or without talc and paragonite. Some garnet porphyroblasts exhibit a core–mantle zoning profile with slight increase in pyrope content and minor or slight decrease in grossular and a mantle–rim zoning profile characterized by a pronounced increase in pyrope and rapid decrease in grossular. Omphacite is usually zoned with a core–rim decrease in j(o) [=Na/(Ca + Na)]. Glaucophane occurs as porphyroblasts in some samples and contains inclusions of garnet, omphacite and epidote. Pseudosections calculated in the NCKMnFMASHO system for five representative samples, combined with petrographic observations suggest that the UHP eclogites record four stages of metamorphism. (i) The prograde stage, on the basis of modelling of garnet zoning and inclusions in garnet, involves P–T vectors dominated by heating with a slight increase in pressure, suggesting an early slow subduction process, and P–T vectors dominated by a pronounced increase in pressure and slight heating, pointing to a late fast subduction process. The prograde metamorphism is predominated by dehydration of glaucophane and, to a lesser extent, chlorite, epidote and paragonite, releasing ∼27 wt% water that was bound in the hydrous minerals. (ii) The peak stage is represented by garnet rim compositions with maximum pyrope and minimum grossular contents, and P–T conditions of 28.2–31.8 kbar and 605–613 °C, with the modelled peak‐stage mineral assemblage mostly involving garnet + omphacite + lawsonite + talc + phengite + coesite ± glaucophane ± kyanite. (iii) The early decompression stage is characterized by dehydration of lawsonite, releasing ∼70–90 wt% water bound in the peak mineral assemblages, which results in the growth of glaucophane, j(o) decrease in omphacite and formation of epidote. And, (iv) The late retrograde stage is characterized by the mineral assemblage of hornblendic amphibole + epidote + albite/oligoclase + quartz developed in the margins or strongly foliated domains of eclogite blocks due to fluid infiltration at P–T conditions of 5–10 kbar and 500–580 °C. The proposed metamorphic stages for the UHP eclogites are consistent with the petrological observations, but considerably different from those presented in the previous studies.

Metamorphic P–T paths of the Zanhuang amphibolites and metapelites: constraints on the tectonic evolution of the Paleoproterozoic Trans-North China Orogen

Garnet-bearing metapelites and amphibolites are exposed in the south and middle parts of the Zanhuang complex, which is located in the central segment of the nearly NS-striking Trans-North China Orogen. These rocks preserve three metamorphic mineral assemblages forming at the prograde, peak and post-peak decompression stages. The prograde metamorphic stage (M1) is represented by mineral inclusions within garnet porphyroblasts, the peak metamorphic stage (M2) is represented by garnet rims and matrix minerals, whereas the retrograde stage (M3) is represented by amphibole + plagioclase symplectite rimming garnet porphyroblasts in the amphibolites and biotite + plagioclase symplectite rimming garnet porphyroblasts in the metapelites. All garnet porphyroblasts in the metapelites preserve prograde chemical zoning except for the ubiquitous, quite narrow zones from the underwent post-peak decompression. It has been determined through thermobarometric computation that the metamorphic conditions are 650–710°C at 8.2−9.2 kbar for the M1 (inclusion) assemblages, >810°C at >12.5 kbar for the metamorphic peak M2 (matrix) assemblages, and 660–680°C at 4.4–4.5 kbar for the retrograde M3 (symplectite) assemblages. These rocks are thus determined to have undergone metamorphism with clockwise P–T paths involving nearly isothermal decompression (ITD) segments, which is inferred to be related to the amalgamation of the Eastern and Western Blocks to form the coherent basement of the North China Craton along the Trans-North China Orogen in the late Paleoproterozoic (1.88–1.85 Ga).

Zr-in-rutile thermometry in HP/UHP eclogites from Western China

Four Zr-in-rutile thermometry calibrations are applied to eclogites from Western China. Here, we show that if rutile grows in equilibrium with Qtz and Zrn, and is isolated inside garnet, it preserves its Zr composition and does not undergo compositional change due to cation exchange with the host garnet. It thus preserves the composition for the P-T conditions of its formation and the growth zoning of the host garnet. For the HP/UHP metamorphic temperature, the Tomkins et al. (J Metamorph Geol 25:703-713, 2007) calibration yields temperatures that agree well with previous studies, whereas the other three calibrations (Zack et al. in Contrib Mineral Petrol 148:471-488, 2004; Watson et al. in Contrib Mineral Petrol 151:413-433, 2006; Ferry and Watson in Contrib Mineral Petrol in 154:429-437, 2007), which do not include a pressure correction, give systematically lower temperatures. Zr contents of rutile inclusions within garnet show systematic decrease from garnet core to rim. The rutile inclusions in garnet rims contain the lowest Zr content, similar to that in the matrix. Analyses confirm that the pressure plays a significant role in modifying the primary temperature dependence of the Zr content of rutile. Rutiles trapped in garnets are unable to re-equilibrate easily during retrogression, but those in the matrix can do so, providing retrograde P-T path information.

Anti‐clockwise P–T paths in the lower crust: an example from a kyanite‐bearing regional aureole, George Sound, New Zealand

AbstractThe George Sound Paragneiss (GSP) represents a rare Permo‐Triassic unit in Fiordland that occurs as a km‐scale pillar to gabbroic and dioritic gneiss of c. 120 Ma Western Fiordland Orthogneiss (WFO). It is distinguished from Palaeozoic paragneiss common in western Fiordland (Deep Cove Gneiss) by SHRIMP and laser‐ablation U–Pb ages as young as c. 190 Ma and 176Hf/177Lu &gt;0.2828 for detrital zircon grains. The Mesozoic age of the GSP circumvents common ambiguity in the interpretation of Cretaceous v. Palaeozoic metamorphic assemblages in the Deep Cove Gneiss. A shallowly dipping S1 foliation is preserved in the GSP distal to the WFO, cut by 100 m scale migmatite contact zones. All units preserve a steeply dipping S2 foliation. S1 staurolite and sillimanite inclusions in the cores of metapelitic garnet grains distal to the WFO preserve evidence for prograde conditions of T &lt; 650 °C and P &lt; 8 kbar. Contact aureole and S2 assemblages include Mg‐rich, Ca‐poor cores to garnet grains in metapelitic schist that reflect WFO emplacement at ≈760 °C and ≈6.5 kbar. S2 kyanite‐bearing matrix assemblages and Ca‐enriched garnet rims reflect ≈650 °C and ≈11 kbar. Poorly oriented muscovite–biotite intergrowths and rare paragonite reflect post‐S2 high‐P retrogression and cooling. Pseudosection modelling in NCKFMASH defines a high‐P anti‐clockwise P–T history for the GSP involving: (i) mid‐P amphibolite facies conditions; preceding (ii) thermal metamorphism adjacent to the WFO; followed by (iii) burial to high‐P and (iv) high‐P cooling induced by tectonic juxtaposition of cooler country rock.

Linking P-T path with development of discontinuous phosphorus zoning in garnet during high-temperature metamorphism — an example from Lützow-Holm Complex, East Antarctica

Garnet porphyroblasts contained in the garnet-sillimanite gneiss from the Lutzow-Holm Complex at Skallevikshalsen, East Antarctica, have a phosphorus-poor core and phosphorus-rich rim. The core/rim boundary of the phosphorus zoning is discontinuous. The irregular shape of this core/rim boundary together with the pressure difference between the core and the rim inferred from the difference in aluminosilicate inclusions (kyanite in the core and sillimanite in the rim) suggests that this is the resorption/reprecipitation boundary. The difference between phosphorus concentrations in the garnet core and rim is accompanied by a change in phosphate inclusions in the garnet. Apatite and monazite are included in the phosphorus-poor garnet core, whereas monazite alone is included in the phosphorus-rich garnet rim. Utilizing the core/rim boundary as a contemporaneous surface when comparing different garnet grains, the timing of the discontinuous phosphorus-zoning formation (and thus, the garnet resorption) and change in the phosphate assemblage can be correlated to the pressure-temperature path of the garnet-sillimanite gneiss. The phosphorus-poor core of the garnet mainly formed during the prograde stage in the kyanite to sillimanite stability fields under which apatite probably buffered the phosphorus-content of garnet, and the phosphorus-rich garnet rim possibly crystallized from the melt at the retrograde stage near the vapor saturated solidus under which monazite alone (without apatite) probably did not buffer the phosphorus-content of garnet. Garnet resorption occurred during the decompression stage between these two garnet growth stages.

Polymetamorphism, zircon growth and retention of early assemblages through the dynamic evolution of a continental arc in Fiordland, New Zealand

AbstractThe Marguerite Amphibolite and associated rocks in northern Fiordland, New Zealand, contain evidence for retention of Carboniferous metamorphic assemblages through Cretaceous collision of an arc, emplacement of large volumes of mafic magma, high‐P metamorphism and then extensional exhumation. The amphibolite occurs as five dismembered aluminous meta‐gabbroic xenoliths up to 2 km wide that are enclosed within meta‐leucotonalite of the Lake Hankinson Complex. A first metamorphic event (M1) is manifest in the amphibolite as a pervasively lineated pargasite–anorthite–kyanite or corundum ± rutile assemblage, and as diffusion‐zoned garnet in pelitic schist xenoliths within the amphibolite. Thin zones of metasomatically Al‐enriched leucotonalite directly at the margins of each amphibolite xenolith indicate element redistribution during M1 and equilibration at 6.6 ± 0.8 kbar and 618 ± 25 °C. A second phase of recrystallization (M2) formed patchy and static margarite ± kyanite–staurolite–chlorite–plagioclase–epidote assemblages in the amphibolite, pseudomorphs of coronas in gabbronorite, and thin high‐grossular garnet rims in the pelitic schists. Conditions of M2, 8.8 ± 0.6 kbar and 643 ± 27 °C, are recorded from the rims of garnet in the pelitic schists. Cathodoluminescence imaging and simultaneous acquisition of U‐Th‐Pb isotopes and trace elements by depth‐profiling zircon grains from one pelitic schist reveals four stages of growth, two of which are metamorphic. The first metamorphic stage, dated as 340.2 ± 2.2 Ma, is correlated with M1 on the basis that the unusual zircon trace element compositions indicate growth from a metasomatic fluid derived from the surrounding amphibolite during penetrative deformation. A second phase of zircon overgrowth coupled with crosscutting relationships date M2 to between 119 and 117 Ma. The Early Carboniferous event has not previously been recognized in northern Fiordland, whereas the latter event, which has been identified in Early Cretaceous batholiths, their xenoliths, and rocks directly at batholith margins, is here shown to have also affected the country rock. However, the effects of M2 are fragmentary due to limited element mobility, lack of deformation, distance from a heat source and short residence time in the lower crust during peak P and T. It is possible that many parts of the Fiordland continental arc achieved high‐P conditions in the Early Cretaceous but retain earlier metamorphic or igneous assemblages.

Chloritoid-Bearing Mineral Assemblages in High-Pressure Metapelites from the Bughea Complex, Leaota Massif (South Carpathians)

High-Mg chloritoid (XMg ¼ 0� 40^0� 47) and relatively high-Mg staurolite (XMg ¼ 0� 25^0� 28) coexisting with kyanite and garnet were identified in a mica^garnet-rich rock associated with very high-pressure eclogites in the Bughea Complex of the Leaota Massif (South Carpathians). Major and trace element geochemical data for both fresh eclogites and associated rocks which represent a metasomatic or retrograde alteration rind of the eclogites, indicate a pelitic precursor. Magnesian chloritoid was found as inclusions in garnet as part of a chloritoid^kyanite^garnet assemblage which is indicative of high-pressure conditions. The host garnet shows a typically prograde chemical zoning pattern.The chloritoid-bearing assemblage is confined to the inner part of the garnet porphyroblasts, whereas the matrix assemblage in equilibrium with Mg-rich garnet rims has exceeded the thermal stability limit of chloritoid. Pressure^temperature pseudosections for simplified compositions approaching the rock bulk-chemistry show a high-pressure field for the identified chloritoid-bearing assemblage in good agreement with pressure^temperature estimates in the CFMASH and KCFMASH chemical subsystems using analysed mineral compositions. The derived pressure^temperature path is clockwise, indicating overprinting during exhumation from 1� 8G Pa and 5808 Ct o 1� 15 GPa and 6208C, at a water activity approaching aH2O ¼1.These conditions were attained � �

K-Ar ages of biotite and muscovite from Pangong metamorphic complex, Shyok suture zone, India: Implications for the youngest post-collision metamorphic event in Ladakh Himalaya

The garnet-kyanite-staurolite and garnet-biotite-staurolite gneisses were collected from a locality within Lukung area that belongs to the Pangong metamorphic complex in Shyok valley, Ladakh Himalaya. The kyanite-free samples have garnet and staurolite in equilibrium, where garnets show euhedral texture and have flat compositional profile. On the other hand, the kyanite-bearing sample shows equilibrium assemblage of garnet-kyanite-staurolite along with muscovite and biotite. In this case, garnet has an inclusion rich core with a distinct grain boundary, which was later overgrown by inclusion free euhedral garnet. Garnet cores are rich in Mn and Ca, while the rims are poor in Mn and rich in Fe and Mg, suggesting two distinct generations of growth. However, the compositional profiles and textural signature of garnets suggests the same stage of P -T evolution for the formation of the inclusion free euhedral garnets in the kyanite-free gneisses and the inclusion free euhedral garnet rims in the kyanite-bearing gneiss. Muscovites from the four samples have consistent K-Ar ages, suggesting the cooling age (∼ 10 Ma) of the gneisses. These ages make a constraint on the timing of the youngest post-collision metamorphic event that may be closely related to an activation of the Karakoram fault in Pangong metamorphic complex.

Metamorphic evolution of low‐T eclogite from the North Qilian orogen, NW China: evidence from petrology and calculated phase equilibria in the system NCKFMASHO

AbstractLow‐T eclogites in the North Qilian orogen, NW China share a common assemblage of garnet, omphacite, glaucophane, epidote, phengite, quartz and rutile with or without paragonite. Phase relations for the low‐T eclogites can be modelled well in the system NCKFMASHO with the updated solid‐solution models for amphibole and clinopyroxene. Garnet in the eclogite typically exhibits growth zonations in which pyrope increases while grossular somewhat decreases from core to rim, which is modelled as having formed mainly in the P–T conditions of lawsonite‐eclogite facies at the pre‐peak stage. Omphacite shows an increase in jadeite component as aegirine and also total FeO decrease in going from the inclusions in garnet to grains in the matrix, and from core to rim of zoned crystals, reflecting an increase in metamorphic P–T conditions. Glaucophane exhibits a compositional variation in X(gl) (= Fe2+/(Fe2+ + Mg)) and F(gl) (= Fe3+/(Fe3+ + Al) in M2 site), which decrease from the inclusions in garnet to crystals in the matrix, consistent with an increase in P–T conditions. However, for zoned matrix crystals, the X(gl) and F(gl) increase from core to rim, is interpreted to reflect a late‐stage decompression. Using composition isopleths for garnet rim and phengite in P–T pseudosections, peak P–T conditions for three samples Q5–45, Q5–01 and Q7–28 were estimated as 530–540 °C at 2.10–2.25 GPa, 580–590 °C at 2.30–2.45 GPa and 575–590 °C at 2.50–2.65 GPa, respectively, for the same assemblage garnet + omphacite + glaucophane + lawsonite (+ phengite + quartz + rutile) at the peak stage. The eclogites suggest similar P–T ranges to their surrounding felsic–pelitic schists. During post‐peak decompression of the eclogites, the most distinctive change involves the transformation of lawsonite to epidote, releasing large amount of water in the rock. The released fluid promoted further growth of glaucophane at the expense of omphacite and, in appropriate bulk‐rock compositions, paragonite formed. The decompression of eclogite did not lead to pronounced changes in garnet and phengite compositions. Peak P–T conditions of the North Qilian eclogite are well constrained using both the average P–T and pseudosection approaches in Thermocalc. Generally, the conventional garnet–clinopyroxene geothermometer is too sensitive to be used for constraining the temperature of low‐T eclogite because of the uncertainty in Fe3+ determination in omphacite and slight variations in mineral compositions because of incomplete equilibration.

Models of garnet differential geochronology

Rayleigh distillation models are developed to describe theoretical growth zoning of Lu, Hf, Rb, Sr, Sm, and Nd in typical garnet crystals from metapelites and metabasites. Effects of diffusion-limited transport within the matrix and intracrystalline diffusion are also considered qualitatively. Theoretical zoning profiles show strong depletions of Lu in garnet rims compared to cores, but virtually invariant Hf, Rb, Sr, Sm, and Nd profiles, generally consistent with natural profiles for Lu and Hf and previously published models. Theoretical isochron diagrams for Lu–Hf exhibit distinctive arcuate distributions and high MSWDs consistent both with Himalayan data, and with expectations that garnet growth durations exceed chronologic resolution by as much as an order of magnitude. Predicted isochron diagrams for Sm–Nd and Rb–Sr exhibit vertical arrays for garnet and high MSWDs that are generally lower than for Lu–Hf in metapelites. Inherent chronologic resolution for bulk separates is best for Lu–Hf in metapelites and Rb–Sr, but analytical considerations favor Rb–Sr or Sm–Nd for chronologic zoning studies. Diffusion-limited transport in the rock matrix strongly influences zoning profiles, but does not change the main trends on isochron diagrams. Intracrystalline diffusion will initially rotate Lu–Hf isochrons to steeper slopes, giving older apparent ages. The natural Himalayan data indicate growth of garnet in one rock from the Greater Himalayan Sequence at ∼34 Ma, consistent with previously measured monazite ages from the same rock. Data from another Himalayan rock suggest polymetamorphism that includes a Paleozoic component.

Lawsonite eclogites from the Pinchi Lake area, British Columbia—new P– T estimates and interpretation

Lawsonite eclogites from Pinchi Lake, British Columbia, occur as tectonic blocks associated with lawsonite, aragonite, and jadeite-bearing blueschists. One tectonic block from Tezzeron Lake shows extensive lower-temperature retrogression. The block from Beaver Lake Road (BLR) shows less retrogression and the garnet crystals contain abundant inclusions of lawsonite and omphacite suggesting these minerals were stable throughout most of the crystallization history. Compositional profiles and element maps of garnets show: (1) Mn enrichment in cores with decrease toward rims; (2) little change in Ca from core to rim; (3) general decrease in Fe/Mg from core to rim. Phase diagram sections for sample BLR-3 and garnet-core compositions suggest garnet-core growth at 505 °C and 24.3 kbar (1 kbar = 0.1 GPa). Temperature estimates based upon garnet–clinopyroxene Fe–Mg exchange geothermometry are, for inclusions in garnet at 25 kbar, 450 ± 30 °C (4 spots); 435 ± 15 (10); and 445 ± 25 (6). For garnet rims and matrix clinopyroxene the results are: 515 ± 50 °C (2); 440 ± 25 (5); 475 ± 20 (2). These results suggest very little if any statistically significant changes in temperature during crystal growth. Estimates of pressure are based upon several different calibrations of the garnet–clinopyroxene–phengite equilibria. Using the calibration of Krogh-Ravna we obtain pressures of: 24.2–26.8 kbar, compatible with that estimated for garnet-core growth in BLR-3. For the glaucophane = talc + jadeite geobarometer, the activity relations for glaucophane are not well constrained. At 450 °C a pressure of 26 kbar is calculated (one sample). A pressure of 25 kbar at 450 °C suggests depths of ~ 90 km, leading to a P– T gradient of ~ 5 °C km − 1 . This gradient passes through the inferred stability field of lawsonite eclogites and the calculated stability fields of lawsonite for Pinchi Lake samples. This is very near the P– T stability field of coesite, which is characteristic of Ultra-High-Pressure metamorphism.

광양-하동지역에 분포하는 편마암류의 지구화학 및 변성작용

The precambrian granitic gneiss and porphyroblastic gneiss are widely distributed in the Gwangyang-Hadong area of Korea. This study focuses on the geochemical properties and metamorphic P-T conditions of these gneisses. These gneisses are plotted according to granodiorite domain on an IUGS silica-alkali diagram. Geochemical properties of major elements suggest that these rocks are of the sub-alkalic rock series, and were farmed from S-type magmas which were generated in a syn-collision tectonic environment. The amounts of trace elements (Zn, Sc, Sr, V, etc.) decreased as concentrations increased. Almandine and spessartine mol%`s and XFe are higher in garnet rims, while pyrope mol%`s are higher in the garnet cores. This seems to be the result of garnet growth and retrogressive metamorphism. Metamorphic zones are divided into sillimanite-cordierite, sillimanite, garnet, and biotite zones. Metamorphic P-T conditions estimated from the gneisses indicate high temperature and low to medium pressure metamorphism (689-757, 5.0-5.6 kbar), followed by medium temperature, low pressure retrorade metamorphism (579-628, 3.1-4.5 kbar), and overprinted retrogade metamorphism (502-558, 1.6-2.3 kbar).

High-pressure garnet-bearing migmatites from the Chepelare area, Central Rhodope

The association of Ca-rich garnet and Na-rich plagioclase in migmatites is a proper indicator for high-pressure melting. Garnet-bearing leucosome and aplitoid gneiss from the Mechi Chal peak in the Chepelare area of the Central Rhodope comprise a mineral assemblage produced by high-pressure melting: Ca-rich garnet (grossular 22–33 %), Na-rich antiperthitic plagioclase (An15–20), perthitic K-feldspar, quartz, and minor back reaction biotite replacing garnet rims. The approximate conditions of melt crystallization are in the range 1.6–1.9 GPa/800–850°C that is well above the P–T estimates available for the Central Rhodope migmatites. The study focuses attention on the key role of the high-pressure melting in the Alpine metamorphic evolution of the Central Rhodope.

Synchronous peak Barrovian metamorphism driven by syn‐orogenic magmatism and fluid flow in southern Connecticut, USA

AbstractRecent work in Barrovian metamorphic terranes has found that rocks experience peak metamorphic temperatures across several grades at similar times. This result is inconsistent with most geodynamic models of crustal over‐thickening and conductive heating, wherein rocks which reach different metamorphic grades generally reach peak temperatures at different times. Instead, the presence of additional sources of heat and/or focusing mechanisms for heat transport, such as magmatic intrusions and/or advection by metamorphic fluids, may have contributed to the contemporaneous development of several different metamorphic zones. Here, we test the hypothesis of temporally focussed heating for the Wepawaug Schist, a Barrovian terrane in Connecticut, USA, using Sm–Nd ages of prograde garnet growth and U–Pb zircon crystallization ages of associated igneous rocks. Peak temperature in the biotite–garnet zone was dated (via Sm–Nd on garnet) at 378.9 ± 1.6 Ma (2σ), whereas peak temperature in the highest grade staurolite–kyanite zone was dated (via Sm–Nd on garnet rims) at 379.9 ± 6.8 Ma (2σ). These garnet ages suggest that peak metamorphism was pene‐contemporaneous (within error) across these metamorphic grades. Ion microprobe U–Pb ages for zircon from igneous rocks hosted by the metapelites also indicate a period of syn‐metamorphic peak igneous activity at 380.6 ± 4.7 Ma (2σ), indistinguishable from the peak ages recorded by garnet. A 388.6 ± 2.1 Ma (2σ) garnet core age from the staurolite–kyanite zone indicates an earlier episode of growth (coincident with ages from texturally early zircon and a previously published monazite age) along the prograde regional metamorphic T–t path. The timing of peak metamorphism and igneous activity, as well as the occurrence of extensive syn‐metamorphic quartz vein systems and pegmatites, best supports the hypothesis that advective heating driven by magmas and fluids focussed major mineral growth into two distinct episodes: the first at c. 389 Ma, and the second, corresponding to the regionally synchronous peak metamorphism, at c. 380 Ma.

Complex replacement patterns in garnets from Bergen Arcs eclogites: A combined EBSD and analytical TEM study

Eclogite-facies rocks within the Bergen Arcs, western Norway, have formed from granulites along shear zones and fluid pathways. Garnets that were inherited from granulite facies protoliths show different types of replacement patterns due to an incomplete eclogitisation process including concentric rim zoning, zoning along vein fillings and inclusion trails, and zoning bands without inclusions. The interfacial part between the granulitic core and the eclogitic rim of garnet as well as the microstructure of other relevant minerals (omphacite, plagioclase) has been analysed using analytical transmission electron microscopy (ATEM). In garnet, the interface is characterised by gradual changes in composition from X alm=0.31, X pyr=0.50 to X alm=0.54, and X pyr=0.25 within ≈20 μm and exhibits no distinct change in microstructure. Granulitic plagioclase shows exsolution lamellae of the Bøggild intergrowth. In omphacite, anti-phase domains (APDs) which potentially record the temperature of cation ordering after mineral growth have been observed and their size suggest eclogitisation at 600–700 °C. The electron backscatter diffraction (EBSD) analysis revealed that the lattice orientation of the granulitic feldspar is basically unrelated to tectonic axes whereas newly formed eclogitic minerals omphacite and kyanite show a crystallographic relation to the foliation. In garnet, no change in the basic crystallographic orientation between the eclogitic and granulitic garnet composition was confirmed. However, misorientation analysis suggests a cellular microstructure not more than 1° misorientation in the core of the garnets, which is missing in the eclogitic rim indicating textural equilibration of the latter. The heterogeneous replacement patterns are characteristic for dissolution and re-precipitation reactions in an open system limited to fluid availability. The appearance of the compositional profile in garnet is interpreted as a diffusional re-equilibration step after the time-limited, fluid-mediated eclogitisation event that apparently obscured the initially sharp interface within the further retrograde metamorphic history.

Geothermobarometry of the Dudatoli-Almora Crystallines, Garhwal, Kumaun Lesser Himalaya

The Garhwal and Kumaun regions of the Himalaya in Uttaranchal State of India are critical areas for studying the typical characteristics of the Himalayan fold-and-thrust belt, in contrast to the areas in close proximity of the northeast and northwest Himalayan syntaxes, where complications arise due to strike-slip faulting. In this region, the Lesser Himalaya is characterized by the occurrence of many crystalline bodies of varying dimensions, of which synformally disposed DudatoliAlmora Crystalline Zone (ACZ) is the largest. Dudatoli-Almora Crystallines (also called Almora Klippen) constitutes one of the most important tectonic units in the Garhwal-Kumaun Lesser Himalaya and it is the remnant of a large thrust sheet nappe that moved southward from the Higher Himalayan Crystalline Zone, to rest over the Lesser Himalayan Volcano-Sedimentary belt. The Almora klippen consists of amphibolite grade metapelites, quartzites, augen gneisses and granites belonging to the Precambrian Almora Group. At places, the greenschist grade metapelites of the Precambrian Ramgarh Group (Chails) occur underneath the Almora Group, forming a series of lower klippen. The North Almora Thrust (NAT) marks the northern boundary of the Almora klippen, its southern margin is bound by the South Almora Thrust (SAT). The metamorphic sequence of the Almora Klippen exhibits regional metamorphism of Barrovian type, which increases progressively upward in the sequence from peripheral part to the central part of the synform. In Champawat and Almora areas the metamorphism is largely restricted to the chlorite, biotite and garnet zones; however, in the Dudatoli-Bungidhar regions, the metamorphic grade reaches upto staurolite-kyanite and biotite-sillimanite zones. The EPMA data on garnet, biotite, muscovite and plagioclase were obtained on twenty samples collected from Chamoli, Pauri and Almora districts of Garhwal and Kumaun Himalaya by utilizing JEOL-8600 M super microprobe. In the rim of garnet, (X Mg /X Fe ) varies from 0.053 to 0.297, X Almandine varies from 0.488 to 0.780 and (Ca+Mn)/(Ca+Mn+Fe+Mg) varies from 0.065 to 0.423. In the core of garnet, (XMg/XFe) varies from 0.030 to 0.279, XAlmandine varies from 0.487 to 0.753 and (Ca+Mn)/ (Ca+Mn+Fe+Mg) varies from 0.118 to 0.461. Ti in garnets is significantly low, in the rim it varies from 0.004 to 0.012 and in cores 0.008 to 0.115. In the rim of biotites, (X Mg /X Fe ) varies from 0.513 to 2.245, (Al+Ti)/(Al+Ti+Fe+Mg) varies from 0.142 to 0.204 and XPhlogopite from 0.270 to 0.579. In the core of biotites, (XMg/XFe) varies from 0.547 to 2.198, (Al VI+Ti)/(AlVI+Ti+Fe+Mg) varies from 0.125 to 0.216 and XPhlogopite from 0.277 to 0.581. In all the samples from Dudatoli-Almora crystallines (Ca+Mn)/ (Ca+Mn+Fe+Mg) in garnets exceeds value of 0.2 or (AlVI+Ti)/ (Al+Ti+Fe+Mg) in biotite exceeds value of 0.15. P-T estimates have been done on the Dudatoli-Almora Crystallines in order to place quantitative constraints on the conditions attained during the regional metamorphism. P-T calculations were carried out using computer programs BGT (Jowhar 1999), GPT (Reche and Martinez 1996), TWQ (Berman 1991), WEBINVEQ (Gordon 1998). GPT computer program (Reche and Martinez, 1996) was used for simultaneous solution of P and T. For P estimation garnetplagioclase-muscovite-biotite-quartz geobarometer and for T estimation various models of garnet-biotite geothermometer were utilized. It is interpreted that in Dudatoli-Almora Crystallines T varies from 500 to 650 oC and P from 6 to 8 kbar. Spatial distribution of data indicates inverted metamorphism.

Age and duration of ultrahigh‐temperature metamorphism in the Anápolis–Itauçu Complex, Southern Brasília Belt, central Brazil – constraints from U–Pb geochronology, mineral rare earth element chemistry and trace‐element thermometry

AbstractIntegrated petrographic and chemical analysis of zircon, garnet and rutile from ultrahigh‐temperature (UHT) granulites in the Anápolis–Itauçu Complex, Brazil, is used to constrain the significance of zircon ID‐TIMS U–Pb geochronological data. Chondrite‐normalized rare earth element (REE) profiles of zircon cores have positive‐sloping heavy‐REE patterns, commonly inferred to be magmatic, whereas unambiguous metamorphic grains and overgrowths have flat to slightly negatively sloping heavy‐REE patterns. However, in one sample, a core of zircon interpreted as having formed prior to garnet crystallization and a metamorphic zircon formed within microstructures involving garnet breakdown both display elevated heavy‐REE (and Y) with positive‐sloping patterns. DREE(zrc/grt) partition coefficients suggest an approximation to equilibrium between zircon and garnet cores, although progressive enrichment in heavy REE towards garnet rims occurs in two of the samples investigated. Titanium‐in‐zircon thermometry indicates zircon growth during both the prograde and post‐peak evolution, but not at peak temperatures of the UHT metamorphism. By contrast, zirconium‐in‐rutile thermometry of inclusions armoured by garnet records crystallization temperatures, based on the upper end of the interquartile range of the data, of ∼890 to 870 °C and maximum temperature around 980 °C, indicating prograde and retrograde growth close to and after peak conditions. Rutile located in retrograde microstructures records crystallization temperatures of 850 to 820 °C. Rutile intergrown with ilmenite and included within orthopyroxene, which is associated with exsolved zircon, records temperatures ∼760 °C, consistent with Ti‐in‐zircon crystallization temperatures. ID‐TIMS U–Pb geochronological data from two of the four samples investigated define upper intercept ages of 641.3 ± 8.4 Ma (MSWD 0.91) and 638.8 ± 2.5 Ma (MSWD 1.03) that correlate with periods of zircon growth along the prograde segment of the P–T path. Individual zircon U–Pb dates retrieved from all samples range from 649 to 634 Ma, indicating a maximum duration of c. 15 Myr for the UHT event. This period is interpreted as recording modest thickening of hot backarc lithosphere located behind the Arenópolis Arc at the edge of the São Francisco Craton consequent upon terminal collision of the Parána Block with the arc during the amalgamation of West Gondwana.