Garnet Rims Research Articles

Constraints on Sm-Nd Diffusion Rates from Laser Ablation ICP-MS Analyses and Conventional Dating of Prograde Garnets

Metamorphic garnets are important phases for obtaining both P-T and age information on metamorphic processes. Establishing the rates of cation diffusivity and closure temperatures of the relevant isotopic systems in garnet are therefore crucial for any P-T-t interpretations in metamorphic terranes. While the generally slow diffusion rates of major and trace cations in garnets do not allow a precise experimental determination of their respective diffusion data, calculation of diffusion parameters from the composition of natural garnets is often hampered by the lack of information on their prograde evolution and independent P-T-t constraints. Here we report results of a major element, REE and Sm-Nd isotopic study of prograde garnets from the HP-HT felsic granulites from the Variscan Bohemian Massif. The garnets (Aim 0.53, Prp 0.27, Grs 0.19, Sps 0.01, up to 2 mm in diameter) are zoned in Mg, Fe, Ca and Mn with Fe/(Fe+Mg) decreasing from core to rim. The outer rim (c. 200 microns) shows retrograde zoning with increasing Fe/ (Fe+Mg), decreasing Ca and a large increase of Mn content (Fig. 1). As garnet is the only Mn-bearing phase in the rock, the Mn increase in the rim can only be attributed to garnet resorption during the retrograde decompression path. Composition maps suggest that the zoning in the rim is not related to proximity of any other phase (e.g. biotite or plagioclase) in the matrix. Laser probe ICP-MS analyses from the identical grain show zoning in REE on a comparable scale to that of the major elements, with Sm and Nd concentrations of 10 and 6 ppm in the core and 5 and 1 ppm in the rim, respectively. The Sm/Nd ratio of 1.5 is constant across the grain, suggesting an infinite REE reservoir (i.e. efficient REE mobility) during garnet growth. However, it increases to 5.5 in the outer 200 microns of the profile. This is also likely to have resulted from the preferential uptake of Sm to Nd during garnet resorption. Modelling the garnet core and plagioclase compositions using THERMOCALC suggests that they equilibrated at c. 20 kbar and 1000~ but approx. 20% of the original garnet volume has been resorbed during decompression and exhumation to upper crustal levels. However, such P-T conditions contrast with biotite inclusions which are present as a prograde phase in the garnet. Simple diffusion modelling using the available diffusion parameters for major cations suggests that in order to preserve the residual growth zoning, the studied garnet must have spent only a short time (<<1 Ma) at the peak conditions. In such a short time span the heat could not have been conducted to the crust but it was rather heated through convection of mantle melts. A minimum final equlibration temperature of the garnet rim and matrix biotite of 750~ is calculated from the Fe-Mg exchange. A whole-rock-garnet SmNd isochron yields an age of 351 + 4 Ma. Because of the presence of the high Sm/Nd rim in the garnets, this data can only represent a minimum age for prograde garnet growth. The end of prograde evolution is further constrained by a series of U-Pb zircon and monazite data as well as Sm-Nd dating of other garnets and accessory phases. These mark the drop in temperature below c. 700~ following isobaric decompression and mostly yield ages in the range of 345-338 Ma. In addition, hornblende, muscovite and biotite Ar-Ar ages, which cluster within the range of 330-295 Ma, indicate a cooling rate of c. 9-10~ between 340 and 295 Ma. The studied compositional profiles show that both prograde and retrograde zoning of major cations and REE in the garnet have been preserved on the same (1 ram) scale. In addition, it can be inferred from the geochronological data that the garnet must have stayed above c. 700~ for a minimum of 10 Ma without resetting its Sm-Nd isotopic system.

The eclogites in the Monotonous Series of the Moldanubian zone and the theory of thermal pulses: a discussion

O’Brien and Vrana (1995) recently interpreted textural and paragenetic relations as well as garnet zoning occurring in the rare eclogites of the Monotonous Series as evidence of (a) prograde evolution from lower pressures, (b) eclogite facies overprint and (c) granulite facies overprint caused by a thermal pulse of less than 1-Ma duration. The thermal pulse was interpreted as the result of the convergence of the thermal patterns of a hot-rock association over the cooler Monotonous Series metabasites. Re-evaluation of the data presented shows that the postulated prograde evolution is not supported. Similarly, the Fe– Mg-partitioning relations at garnet rims and associated orthopyroxene in these rocks point to cooling rather than heating. The age of less than 1 Ma dates a certain stage during cooling of the rocks, rather than the duration of a thermal pulse. The rare metabasites of the Monotonous Series are re-interpreted as tectonically incorporated bodies derived from the higher Moldanubian units during the intra-Moldanubian nappe stacking.

Intracrustal subduction and gravity currents in the deep crust: Sm-Nd, Ar-Ar, and thermobarometric constraints from the Skagit Gneiss Complex, Washington

Isotopic and thermobarometric (pressure-temperature, P-T) data on a gneissic quartz diorite, combined with previous U-Pb ages and P-T data on wall-rock metapelites, define a precise P-T time series for a portion of the Skagit Gneiss Complex, a deep crustal crystalline complex within a zone of intracontinental collision. Concordant U-Pb and Sm-Nd ages on zircon indicate crystallization at 68 Ma, which preceded metamorphism of nearby pelitic wall rocks (garnet core assemblages) and the pluton (plagioclase-hornblende coronas on garnet) at ≈800–900 MPa, 700–800 °C. Following nearly isothermal decompression and sillimanite-cordierite metamorphism of wall rocks (garnet rims plus matrix) at 300–500 MPa, 650–700 °C, a Sm-Nd mineral isochron including two garnet separates records an initial phase of cooling of the complex at 60 Ma. Slow cooling (≈10 °C/m.y.) extends from approximately 60 Ma to 50 Ma. Hornblende and biotite Ar-Ar ages of 47 Ma and 45 Ma, respectively, define a second unroofing event; cooling rates during this period were approximately 100 °C/m.y. Late Cretaceous deep burial followed by two distinct unroofing episodes is also observed in some Cordilleran metamorphic core complexes and may suggest a common origin. We propose that regional shortening in the Cordilleran interior is accommodated by a process of intracrustal subduction, i.e., the upper crust is forced downward into a crustal asthenosphere, resulting in a cold, buoyant root. Heating and weakening of the root cause the formation of gravity currents, effective viscosity being in the range of 10^(16)–10^(19) Pa s, spreading outward below nonfluid upper crust. Spreading of the gravity current, which does not necessarily contribute to crustal thinning, is driven by the small density contrast between the root and its mid-crustal surroundings. Hence unroofing is neither a response to overthick crust nor controlled by the orientation of principal stress axes in the upper crust and upper mantle. The root then cools slowly until final unroofing to near-surface levels, which may occur via either extension or shortening with consequent erosion.

Garnet zoning and reaction textures in overprinted eclogites, Bohemian Massif, European variscides: a record of their thermal history during exhumation

Eclogites from different parts of the Variscan basement of the Bohemian Massif show a variation in temperatures recorded at peak pressures as well as during subsequent exhumation. These differences are reflected in the extent of modification of pre-, syn- and post-eclogite facies garnet compositions and by the types of reactions taking place at garnet rims. Münchberg Massif eclogites formed at conditions of 600–650°C, > 20 kbar, were mainly overprinted in the amphibolite facies field, and contain garnets with an old homogeneous core and often well preserved idiomorphic growth zoning to Mg-rich rims. In the Mariánské Lázně Complex the eclogite facies stage (640–715°C, < 19.5 kbar) was followed by a weak granulite facies stage, with the formation of orthopyroxene and sapphirine, and a more pervasive amphibolite facies overprint. Despite the slightly higher temperatures, strong compositional profiles in garnet have not been smoothed or homogenised. In the Moldanubian Zone (eclogite stage 700–800°C, > 15 kbar), many eclogites show well developed granulite facies overprints with widespread orthopyroxene growth in garnet and omphacite domains, spinel and/or sapphirine after kyanite, and in some cases even a low-pressure high-temperature overprint that produced Fe-rich olivine. The latter rocks clearly endured higher temperatures during breakdown but, remarkably, strong growth zoning and sharp overgrowth features are preserved in some cases. Diffusion modeling shows that the preservation of zoning in garnet of rocks that have experienced granulite facies temperatures is only possible if the time spent at these conditions is short ( < 1 Ma). This time scale is inconsistent with a smooth P-T-t path as expected from thermal modeling for erosion- or extension-driven exhumation and implies that these Variscan rocks experienced short-lived thermal events during their exhumation history.

Sm–Nd garnet dating of polyphase metamorphism: northern Coast Mountains, south‐eastern Alaska, USA

Sm–Nd ages of garnet from the northern Coast Mountains of south‐eastern Alaska, USA, constrain the timing of thermal events in polyphase metamorphic rocks of the western metamorphic belt and provide new data on the spatial extent of Cretaceous regional metamorphism. Bulk garnet–whole‐rock Sm–Nd ages for a sillimanite‐zone amphibolite (Taku Inlet) and a biotite‐zone metapelite (Tracy Arm) are 77±17 Ma and 59±12 Ma, respectively. Garnet core–whole‐rock (80±9 Ma), core–matrix (84±9 Ma), rim–whole‐rock (59±4 Ma) and rim–matrix (62±4 Ma) ages were obtained from a sample collected 200 m west of a Palaeocene Coast plutonic–metamorphic complex sill‐like pluton that separates medium‐grade metamorphic rocks from high‐grade metamorphic rocks and voluminous Tertiary plutons in the core of the orogen. The garnet core ages of c. 80 Ma indicate that the regional metamorphic grade reached garnet zone prior to the intrusion of the plutons and high‐grade metamorphism of rocks to the east. Similar ages for the younger plutons, the youngest garnets and the rim of a multistage garnet (c. 59 Ma) indicate a later episode of contact metamorphic garnet growth. Documentation of pre‐71 Ma garnet‐zone metamorphism along the western edge of the Coast plutonic–metamorphic complex confirms that Albian to Late Cretaceous metamorphism associated with crustal thickening affected this part of the orogen. The similarity of garnet Sm–Nd ages to independent age estimates for metamorphic events confirms that this technique provides useful estimates for the timing of Late Cretaceous to Tertiary thermal events. The c. 20 Myr difference between garnet core and rim ages suggests that the Sm–Nd isotope systematics of a single garnet grain can be used for distinguishing between multiple metamorphic events.

Metamorphic structure and P–T conditions of the metapelitic constituent of the ZEV

Bounded by mylonite zones and imbrications the Zone of Erbendorf-Vohenstrauss is built up by medium- and low-pressure amphibolite-facies metapelites associated with amphibolites and biotite–hornblende gneisses, and intruded by the Leuchtenberg granite. Pelitic index assemblages define the staurolite– kyanite-, the garnet–kyanite-, the garnet–sillimanite- and the garnet–kyanite–sillimanite zone of medium pressure metamorphism, which is estimated at P=5–9.6 kbar and T=580–730 °C, and the cordierite zone of subsequent low-pressure metamorphism, which is estimated at P<4.5 kbar and T≤700 °C. Both facies series probably are tectonically juxtaposed. Staurolite–kyanite- and garnet zones are related by the reaction: staurolite+muscovite+quartz=garnet+ aluminium-silicate+biotite+H2O. This relation defines the only observable trace of a field gradient in the ZEV. An early sillimanite generation (sillimanite I) is found in all mineral zones as mineral inclusions in garnet rims. Post-P–Tmax evolution is indicated by the inversion kyanite⇒sillimanite (sillimanite II), the reaction: garnet+muscovite⇒sillimanite (sillimanite III)+biotite+quartz and by XFe decreasing in garnet rims. Garnet is resorbed under the conditions of stable kyanite+sillimanite+muscovite+quartz. Some staurolite–kyanite zone garnets show two stages separated by resorption which may be ascribed to published Ordovician and Late Devonian radiometric ages. The second garnet generation formed near 9.7 kbar/ 620–660 °C. No evidence for high-pressure metamorphism was found. Comparison of the medium-pressure part of the ZEV with the Zone of Tepla-Domazlice (Czech Republic) and with garnet–kyanite rocks at the northern edge of the Black Forest seems possible. The low-pressure cordierite rocks are similar to metapelites framing the Winklarn eclogites towards southeast of the ZEV, but are in contrast to the Moldanubian-type cordierite gneisses.

Investigation into Partial Melting of Rocks by Means of Metamorphic Petrology

Recent discussions on partial melting in high-grade metamorphic rocks are briefly reviewed. Garnets in high-grade pelitic rocks commonly contain euhedral calcic plagioclase inclusions. In addition, they display compositional growth zoning for phosphorus as well as major elements (Fe, Mn, Mg and Ca) and other minor elements (e.g., Y), though modified to various degrees by volume diffusion. The phosphorus zonation is defined by a low-P core with a distinct high-P rim, and correlates with the presence or absence of euhedral calcic plagioclase inclusions. Apatite, xenotime and other phosphate minerals as well as K-feldspar are present in the rock matrix and as inclusions in the garnet rim, whereas these minerals are usually absent in the garnet core. This suggests phosphate undersaturation during the growth of the core. All these features are satisfactorily explained by the following partial melting reaction.Biotite + sillimanite + sodic plagioclase + K-feldspar + quartz + phosphates (+H2O) =granitic melt +garnet core+calcic plagioclase+rutile (or ilmenite).The garnet rim and the rock matrix may have formed from the granitic melt upon cooling.These data would be a new indicator for previous partial melting of pelitic gneisses and granulites.

The gabbro–eclogite transformation: an oxygen isotope and petrographic study of west Alpine ophiolites

Both magmatic and eclogitic parageneses are preserved in the gabbros of western Alpine ophiolites. Samples with relic magmatic mineralogies display partial transformation to eclogitic assemblages along cracks and grain boundaries. Gabbros with eclogitic mineralogies contain zoned pseudomorphs after olivine, comprising talc‐rich cores with kyanite, Mg‐chloritoid and omphacite in outer cores and garnet rims. The compositional zonation of these olivine pseudomorphs closely parallels that shown by olivines in hydrothermally altered ocean‐floor gabbros.The eclogitic gabbros are hydrous, containing paragonite, zoisite and other water‐bearing minerals, and it has been suggested that water was introduced during high‐pressure metamorphism. However, the similarity of olivine alteration patterns to those of ocean‐floor gabbros suggests that hydration and local metasomatism leading to the stability of aluminous minerals in olivine sites occurred during hydrothermal alteration prior to subduction. Oxygen‐isotope systematics are consistent with this proposal: Alpine gabbros with magmatic relics have a mean δ18O value of 5.7±0.7, similar to that of unaltered oceanic crust, whereas eclogitic gabbros have a mean δ18O value of 4.8±0.9.This statistically significant difference is consistent with the eclogitic samples having undergone high‐temperature ocean‐floor alteration. The preservation of magmatic and hydrothermal δ18O values in ocean‐floor gabbros that have been metamorphosed at 2–2.5 GPa (60–75 km) implies that the deeper levels of ocean crust have not experienced pervasive fluid flow during subduction or subsequent exhumation. Magmatic assemblages were preserved despite an overstep of eclogitization reactions by at least 0.6–1.1 GPa implying that equilibrium was not attained in undeformed parts of the system because of slow diffusion in water‐deficient rock volumes.

Garnet Sm–Nd data from the Saualpe and the Koralpe (Eastern Alps, Austria): chronological and P–T constraints on the thermal and tectonic history

Garnets from recrystallized, staurolite‐ and kyanite‐bearing mica schists from the central Saualpe basement, representing the host rocks of the type‐locality eclogites, give concordant Sm–Nd garnet–whole‐rock isochron ages between 88.5±1.7 and 90.9±0.7 Ma. The millimetre‐sized, mostly inclusion‐free grains show fairly homogeneous element profiles with pyrope contents of 25–27%. Narrow rims with an increase in Fe and Mn and a decrease in Mg document minor local re‐equilibration during cooling. According to phengite geothermobarometry, peak metamorphic conditions at 90 Ma were close to 20 kbar and 680 °C and similar to those recorded by the eclogites. The garnet rims record about 575 °C/7 kbar for the final stages of metamorphism. A phengitic garnet–mica schist, sampled at the immediate contact with the Gertrusk eclogite, gave a garnet–whole‐rock Sm–Nd age of 94.0±2.7 Ma.Garnet porphyroclasts separated from a pegmatite–mylonite of the Koralpe plattengneiss near Stainz are unzoned and show spessartine contents of 15%. Composition and Sm–Nd ages of close to 260 Ma point to a magmatic origin for these garnets.The garnet data from the Saualpe document an intense Alpine metamorphism for this part of the Austroalpine basement. The mica schists recrystallized during decompression and rapid exhumation, at the final stages of and immediately following a high‐P event. The Koralpe data show that high Alpine temperatures did not reopen the Sm–Nd isotope system, implying a closure temperature in excess of c. 600 °C for this isotopic system in garnet.

Geothermobarometry of Al2SiO5-bearing metapelites in the western Austroalpine �tztal-basement

The dominant amphibolite-facies Variscan event in the Austroalpine Otztal basement can best be studied in the northwestern part of the Otztal block. Further to the southeast it is overprinted by Alpine metamorphism. Metapelites with the assemblage garnet-staurolite-kyanite-sillimanite±andalusite-biotite-muscovite-plagioclase were used to reconstruct pressure and temperature conditions with exchange thermometry, net transfer equilibria and multi-equilibrium methods. Assuming kyanite as equilibrium Al2SiO5 polymorph, conditions of 570–640°C and 5.8–7.5kbar are derived using garnet rim compositions. Typical nonequilibrium textures are (1) continuous chemical zoning of garnets, (2) inclusions of kyanite and fibrolite in andalusite porphyroblasts and (3) the spectacular replacement of garnet by fibrolite and biotite. The latter two textures were used to decipher the retrograde part of the P-T path. Application of the differential thermodynamics approach (Gibbs method) indicates prograde garnet growth during pressure release. Addition of CaO to the KFASH-system allows the garnet breakdown within the staurolite stability field and its quantification, using the quartz-garnet-aluminosilicate-muscovite geothermobarometer, revealed temperatures of 530–630°C and 3.5–5.7kbar. Andalusite formation is thought to constrain the final stage of the P-T path. Textural and chemical data clearly indicate a continuous pre-Alpine metamorphic evolution.

Tectonometamorphic evolution of the Chuncheon amphibolite, central Gyeonggi massif, South Korea

Abstract The Chuncheon amphibolite, part of the Gubongsan Group which overlies the Yongduri gneiss complex, is interlayered with calc‐silicate rock, marble, quartzite, biotite schist and quartzofeldspathic gneiss in the central Gyeonggi massif, South Korea. Metamorphic pressures and temperatures estimated from the amphibolite are 5.5–10.6 kbar and 615–714°C. These P—T conditions are close to those defined by the reaction curve between kyanite and sillimanite, and suggest medium‐pressure‐type metamorphism of the Chuncheon amphibolite. For two metapelites intercalated with the amphibolite, temperatures are estimated to be 607–699° C, consistent with those obtained from the amphibolite. On the other hand, pressures estimated from these metapelites are significantly different, 4–6 kbar and 9–13 kbar, when rim and core compositions of garnet are, respectively, used. These P—T estimates obtained from the amphibolite and metapelite suggest a nearly isothermal decompression of 3–7 kbar during denudation. Rapid decompression is likely on the basis of the results of mineral chemistry, phase equilibria and geothermobarometer. Moreover, in conjunction with the occurrence of kyanite in the adjacent Gyeonggi gneiss complex, P—T estimates of the Chuncheon amphibolite and metapelite suggest a clockwise P—T—t path. This evolutionary path may be related to the amalgamation of continents during the late Proterozoic event which corresponds to the Jinningian orogeny in the Qinling belt of China.

High‐pressure amphibolite facies dynamic metamorphism and the Mesozoic tectonic evolution of an ancient continental margin, east‐central Alaska

Abstract Ductilely deformed amphibolite facies tectonites comprise two adjacent terranes in east‐central Alaska. These terranes differ in protoliths, structural level and cooling ages. A structurally complex zone of gently north‐dipping tectonites separates the two terranes. The northern, structurally higher Taylor Mountain terrane includes garnet amphibolite, biotite ± hornblende gneiss, marble, quartzite, metachert, pelitic schist and cross‐cutting granitoids of intermediate composition (including the Late Triassic to Early Jurassic Taylor Mountain batholith). Lithological associations and isotopic data from the granitoids indicate an oceanic or marginal basin origin for the Taylor Mountain terrane. 40Ar/39Ar metamorphic cooling ages from the Taylor Mountain terrane are latest Triassic to earliest Middle Jurassic. The southern, structurally lower Lake George subterrane of the Yukon‐Tanana terrane is made up of quartz‐biotite schist and gneiss, augen gneiss, pelitic schist, garnet amphibolite and quartzite; we interpret it to comprise a continental margin and granitoid belt built on North American crust. Metamorphic cooling ages from the Lake George subterrane are almost entirely Early Cretaceous.Geothermobarometric analysis of garnet rims and adjacent phases in garnet amphibolite and pelitic schist from the Taylor Mountain terrane and Lake George subterrane indicate peak metamorphic conditions of 7.5‐12 kbar at 555‐715° C in the northern part of the Taylor Mountain terrane, in which NNE‐vergent shear fabrics are preserved; 6.5‐10.8 kbar at 520‐670° C within the contact zone between the two terranes, in which NW‐vergent shear fabrics predominate; and 6.8‐11.8 kbar at 570‐700° C in the Lake George subterrane of the Yukon‐Tanana terrane, in which NW‐vergent shear is recorded in the northern part of the study area and SE‐vergent shear in the southern part. Where the two shear‐sense directions occur together in the northern Lake George subterrane and, locally, in the contact zone, fabrics that record NW‐vergent shear are more penetrative and preceded fabrics that record SE‐vergent shear.We interpret the pressure, temperature, kinematic and age data to indicate that the metamorphism of the Taylor Mountain terrane and Lake George subterrane took place during different phases of a latest Palaeozoic through early Mesozoic shortening episode resulting from closure of an ocean basin now represented by klippen of the Seventymile‐Slide Mountain terrane. High‐ to intermediate‐pressure metamorphism of the Taylor Mountain terrane took place within a SW‐dipping (present‐day coordinates) subduction system. High‐ to intermediate‐pressure metamorphism of the Lake George subterrane and the structural contact zone occurred during NW‐directed overthrusting of the Taylor Mountain, Seventymile‐Slide Mountain and Nisutlin terranes, and imbrication of the continental margin in Jurassic time. The difference in metamorphic cooling ages between the Taylor Mountain terrane and adjacent parts of the Lake George subterrane is best explained by Early Cretaceous unroofing of the Lake George subterrane caused by crustal extension, recorded in its younger top‐to‐the‐SE fabric.

A laser microprobe, mass spectrometric study of Ar, Kr, K, Cl and Br in an “unconformity garnet”, associated fluid inclusions, staurolite and micas from Vermont, U.S.A.

Garnet prophyroblasts in paragonite schist of the Pinney Hollow Fm. on the west limb of the Athens mantled gneiss dome in Vermont exhibit apparent angular unconformities. Cores of garnets contain inclusions of chloritoid, rutile and ilmenite that define a foliation which is truncated at a high angle by the foliation in the rims. Electron microprobe traverses across a garnet indicate that the core-rim boundary is the beginning of a steep compositional gradient in Ca, Mg, Mn and Fe that characterizes the rim. Following electron microprobe study, a sample was irradiated in a nuclear pile and then studied by laser microprobe noble gas mass spectrometry. Ar isotopes were measured in garnet and in adjacent micas and staurolite, and Ar plus Kr isotopes were measured in fluid inclusions (FI) in quartz. In garnet and in FI, trapped 40 Ar 36 Ar varies between ∼ 300 and 2000, with no simple correlations indicated between trapped Ar isotope ratios and K, Cl and 84Kr or petrographic features. Similar trapped Ar occurs in the core and rim of garnet and in FI in quartz. Most FI indicate 40 Ar e Cl > 10 −3 and Cl 36 Ar < 10 6 (atom ratios). Br abundances in FI are usually too low to be measured, indicating Br Cl < ∼ 2 · 10 −3 (atom ratio). 84 Kr 36 Ar in FI varies between ∼ 0.02 and ∼ 0.08. Gases extracted from laser craters in white micas in schist outside the garnet indicate mean 40 Ar 39 Ar ages of 362 ± 5 Ma and in biotites 332 ± 7 Ma, with K-bearing inclusions in the garnet rim yielding ages close to 360 Ma. In the garnet core, interpretation of 40 Ar 39 Ar ages is ambiguous due to the combination of low concentrations of in situ radiogenic 40Ar and comparatively large amounts of trapped 40Ar e poorly correlated with other parameters measured.

Reactions and textures in wollastonite-scapolite granulites and their significance for pressure-temperature-fluid histories of high-grade terranes

Reaction textures in wollastonite-scapolite calc-silicates provide important evidence for pressure-temperature-fluid histories in granulites. Pressure-temperature, T-aCO2 and P-aCO2 diagrams, calculated for appropriate mineral compositions using the internally consistent dataset of Holland and Powell (1990), are presented and used to provide a framework for interpretation of the reaction textures. Garnet rims replacing wollastonite and scapolite in calc-silicate gneisses is a common texture in many granulite terranes. This texture can plausibly be developed in post-peak P-T histories dominated by cooling (e.g., near-isobaric cooling) and does not generally imply the infiltration of hydrous fluids. Examples from the Arunta Complex, Australia, and the Northern Prince Charles Mountains, Antarctica, are consistent with cooling without fluid influx, at 7–8 kbar and aCO2 of 0.3–0.5. Contrasting textures involving the replacement of garnet by wollastonite+scapolite or wollastonite+plagioclase symplectites, and the growth of wollastonite+plagioclase rims or coronas on scapolite-quartz boundaries, can be interpreted as essentially closed-system features and modelled through modal analysis of reaction products. In the case of the Rauer Group, Antarctica, near-isothermal decompression from ca. 8 to 6 kbar at 850–800°C and aCO2 in the range 0.35–0.45 is indicated by these textures, based on calculated P-T-aCO2 grids in the CaOAl2O3SiO2CO2 (CASV) and more complex systems. Calculated reactions producing wollastonite+scapolite from garnet-bearing assemblages do not involve decarbonation, but may consume CO2 with increasing temperature in the absence of calcite. Hence, these calc-silicates may act as CO2 “sinks” on the prograde segments of clockwise P-T paths. Conversely, scapolite-wollastonite granulites will act as sources of post-peak CO2-rich fluids liberated upon cooling and potentially trapped elsewhere as post-peak carbonic fluid inclusions.

Symplectites in upper mantle peridotites: Development and implications for the growth of subsolidus garnet, pyroxene and spinel

Silicate-oxide symplectites in complex mineral intergrowths are relatively common in upper mantle xenoliths and in xenoliths in the Jagersfontein Kimberlite, South Africa.Harzburgites of olivine and high-Al (1.9–3.6 wt%), Ca (0.6–0.9 wt%) and Cr (0.3–0.9 wt%) enstatite contain symplectites of spinel and diopside, or spinel, diopside and lower-Al (0.8–2.2 wt%), Ca (0.1–0.4 wt%) and Cr (0.02–0.8 wt%) enstatite. From textures and mineral chemistries these symplectites are interpreted to have formed by mineral unmixing and migration from Al−Ca−Cr discrete enstatite to adjoining mineral interfaces.Garnet harzburgites are composed of large (0.5–1 cm) olivine, equally large discrete low-Al (0.6–1.1 wt%), Ca (0.1–0.5 wt%), and Cr (0.1–0.3 wt%) enstatite and smaller interstitial garnet, diopside, and high-Cr and low-Al spinel. Symplectites are composed of either spinel+diopside+garnet, or garnet+spinel. Spinel diopside garnet symplectites have cores of spinel+diopside, resembling symplectites inharzburgites, but surrounded by rims of garnet or garnet+undigested globular spinel. From textures and chemistries we suggest that the spinel+diopside cores formed from Ca-Al-Cr-rich orthopyroxene initially as a nonstoichiometric homogeneous single phase clinopyroxene enriched in Fe, Cr and Al. This was followed by decomposition of the clinopyroxene to diopside+spinel, and subsequent garnet formation in a prograde reaction with olivine or enstatite. In bothharzburgites andgarnet harzburgites the metastable cellular structures may also have formed by the simultaneous precipitation of pyroxene and spinel. In all cases there is a strongly preferred embayment of symplectite bodies into olivine. Olivine appears to have activated adjacent

P-T-t Evolution of the Valhalla Complex, British Columbia, Canada

The main assemblage in the petitic paragneiss is garnet + biotite + sillimanite + K-feldspar + plagioclase + quartz + ilmenite ruffle. Garnet is feebly zoned with rimward increases in Ca, Mn, Fe and Fe/(Fe + Mg), and a decrease in Mg. Zoning in Fe, Mg and Mn is smooth as a result of diffusion whereas Ca zoning is discontinuous as a result of garnet production by reaction (2) below and slower diffusion. Peak pressures from GASP, GPBQ and GRAIL barometry provide consistent values of 8 1 kbar at 800~ (Fig. 1). A peak temperature of 820 20~ is constrained by dehydration and vapor-absent melting reactions (T quoted at 8 kbar): (1) Ms + P1 + Qtz = Sil + K.fs + Liquid (constrains T > 700~ (2) Bt + Sil + P1 + Qtz= Grt + Kfs + Liquid (constrains T > 7500C) (3) Ms = Kfs + Cm + H20 (constrains T > 800~ (4) at + Qtz = Opx + Kfs + Liquid (constrains T 5 wt.%) are found as inclusions in garnet and yield apparent temperatures of 830 to 655~ which are closure temperatures resulting from Fe-Mg exchange between garnet and biotite. The initial apparent temperature for these inclusions in garnet would have likely been T ~ 870~ and the smaller inclusion re-equilibrated to lower temperature. Following the metamorphic peak at T ~ 820~ minor back reaction of (2) or the similar reaction (5) Bt + Sil + Pl + Qtz = Grt + Kfs + H20 produced garnet rims and matrix biotites that have higher Fe / (Fe+Mg) than at the peak, and produced the observed garnet zoning and matrix biotite in_homogeneity in Fe/(Fe+Mg) and Ti. Garnet core + matrix biotite temperatures range from 800~ which may reflect near peak equilibration, to 1050~ which reflects biotites with higher Fe /Mg than ' peak ' bioti tes. Thermodynamic modeling suggests that reaction (2) or (5) took place over a 25-100~ interval (depending on H20 availability) with the net transfer reaction shutting down at approximately 700~ consistent with the matrix biotite + garnet rim temperatures of 700-750~ The maximum amount of water required is approximately 2 % volume, which may have exsolved from melts produced by (2) during crystallization.

Contrasting zoning profiles in high-grade garnets: evidence for the allochthonous nature of a Grenville province terrane

In the central part of the Grenville Province, Proterozoic metasediments of the Réservoir Cabonga Terrane arc underlain by gently dipping Archean migmatites. A major mylonite zone, located within the migmatites, is the base of a large thrust sheet made up of the Réservoir Cabonga Terrane and a migmatitic sole. Garnets from meta-aluminous assemblages in Proterozoic metasediments, mylonites and migmatites display contrasting types of Ca zoning patterns. Garnets with Ca-rich cores are found along the western edge of the Réservoir Cabonga Terrane, whereas garnets with flat Ca profiles are found in the remaining Proterozoic metapelites, in the mylonites and in the Archean migmatites. Considering samples in which an infinite reservoir of Ca ions is warranted by abundant plagioclase in the matrix, pressures well in excess of 1200 MPa, at temperatures between 700° and 800°C are indicated for a metamorphic event recorded in the cores of garnets from the western edge of the Réservoir Cabonga Terrane. The inner rim of garnet records pressures of around 1000 MPa for the same temperature range. In homogeneous garnets, a single P-T set, around 800 MPa and 750°C, is recorded in the migmatites. Some of the mylonites initiate their recrystallization under relatively high pressures, which is continued during decompression, and record the lowest P-T values. These metamorphic signatures suggest that the Proterozoic metapelites were already metamorphosed under relatively high pressure when emplaced on to Archean migmatites and that the entire area was subsequently subjected to metamorphism at a lower pressure.

Monazite controlled [formula omitted] fractionation in leucogranites: An ion microprobe study of garnet phenocrysts

Rare earth element (REE) abundances in four garnet phenocrysts from two monazite-bearing leucogranites from southern British Columbia have been measured with an ion microprobe. This study examines the effect of monazite crystallization on the LREEs by using garnet as a monitor of elemental variations in the liquid. This approach virtually eliminates uncertainties associated with source-induced LREE fractionation. The garnets examined are spessartine-rich (Alm 70–75Sps 18–22Pyp 3–4Grs 3–4) phenocrysts that show primary igneous zoning and lack monazite inclusions. Garnet rims have zoning profiles (i.e., Eu to Ce) that are depleted by factors of five to ten relative to cores. Of the three models investigated to constrain processes controlling LREE zoning in garnet— 1. (1) variations in the garnet/melt distribution coefficients, 2. (2) liquid diffusion controlled partitioning and 3. (3) crystal diffusion controlled partitioning—the latter model best explains the observed zoning. This model is consistent with experimental studies suggesting that monazite has a low solubility in granitic melts. These results suggest that Nd model ages (calculated using measured 147Sm 144Nd ) in leucogranites should be viewed with caution, as is demonstrated by neodymium isotopic data from southern British Columbia and the central Nepalese Himalaya.

High-temperature metamorphism in a major strike-slip shear zone: the Ailao Shan—Red River, People's Republic of China

Petrographic and thermobarometric analysis provides constraints on thePT path of the mylonitic gneisses of the left-lateral Ailao Shan-Red River shear zone which has accommodated the lateral extrusion of Indochina during the Tertiary. Two different paragenesis, P1 and P2, are coeval with this deformation and correspond respectively to the amphibolite and greenschist facies. Microprobe analysis reveals that P1 garnets bear a chemical zonation from core to rim. This zonation indicates a temperature increase during garnet growth. Conditions of formation of garnet rims (P1b), which are estimated using biotite-garnet and plagioclase-garnet thermobarometers, are close to the granitic solidus (710 ± 70°C and4.5 ± 1.5 kbar). P2 conditions are estimated to be approximately 500°C and < 3.8 kbar. Both P1b and P2 conditions correspond to much higher temperatures than expected for their depths in the continental crust, suggesting a perturbed geothermal gradient during strike-slip deformation along the Ailao Shan-Red River shear zone. Thermochronology results suggest that cooling between P1b and P2 was fast ( ≈ 100°C/Ma) and may have been associated with significant uplift. Uplift during the left-lateral shearing may have resulted from a slight reverse or, more probably normal, component of movement along the strike-slip fault. A simple numerical model suggests that the high temperatures in the shear zone at the time of deformation may be explained by shear heating in the more competent upper mantle and by advection of this heat along the shear zone by ascent of magmas and/or fluids. In this hypothesis, the medium-pressure and temperature schists bounding the mylonitic gneisses to the southwest previously interpreted as resulting from collision-related metamorphism result instead from ‘contact’ metamorphism of the shear zone at mid-crustal depths.

The origin of correlated variations in in-situ [formula omitted] and elemental concentrations in metamorphic garnet from southeastern Vermont, USA

CO 2 laser-heating in a fluorinating atmosphere was used to obtain 18O 16O analyses of coexisting garnet, staurolite, muscovite, chlorite, and quartz from a sample of the Gassetts schist, southeastern Vermont, USA. Garnet and quartz δ 18 O v- smow values vary on a millimeter scale while values for other minerals are uniform within analytical uncertainties. Garnets exhibit 18O 16O zoning with core δ 18 O values of 10.9 ± 0.3‰ and rim values of 10.1 ± 0.2. The depletion of 18O in garnet rims correlates with reversals in cation zonation and intracrystalline textural unconformities. Quartz plucked from a garnet core yields a δ 18 O of 14.8‰ while vein quartz δ 18 O values vary from 14.3 ± 0.2 in centers to 13.8 ± 0.1 at margins. Staurolite, muscovite, and chlorite have mean δ 18 O values of 10.8, 11.9, and 10.3, respectively. Two parageneses are distinguished on the basis of the mineral δ 18 O and expected isotope partitioning and on the basis of textures. One consists of garnet cores, staurolite, muscovite, and quartz included in garnet. Garnet rims, chlorite, and vein quartz comprise the other. Quantitative models which explain the lower bulk δ 18 O of the garnet-rim assemblage show that garnet textural unconformities represent resorption during infiltration of externally derived aqueous fluid. Chlorite grew by this hydration reaction and was the principal storage site for low 18O 16O oxygen. Low- δ 18 O garnet rims later grew at the expense of chlorite with little change in modal abundances of staurolite and muscovite. Retention of pre-infiltration δ 18 O in muscovite precludes significant diffusion of oxygen in this phase and indicates that the total duration of external fluid flow was ≤ 10 5 years.