Peak Metamorphic Stage Research Articles

Laser Raman microspectrometry of metamorphic quartz: A simple method for comparison of metamorphic pressures

A Laser Raman microspectrometry method was applied to metamorphic quartz in quartz-eclogite-, epidote-amphibolite-, and amphibolite-facies rocks to assess the quantitative correlation between the Raman frequency shift and metamorphic pressure. Quartz crystals sealed in garnet and other phases have a higher frequency shift than those in the matrix. Furthermore, the quartz inclusions show a frequency shift speciÞ c to the individual host crystals in eclogites (garnet ≈ kyanite > omphacite ≈ epidote). These observations imply that the residual pressures retained by quartz inclusions depend on elastic parameters of the host crystals, as discussed by previous researchers. The Raman frequency shift of quartz inclusions in garnet systematically increases with increasing peak metamorphic pressures from the amphibolite facies (0.30–0.55 GPa/470–570 °C), through the epidote-amphibolite facies (0.8–1.1 GPa/470–635 °C) to the quartz-eclogite facies (2.1–2.5 GPa/660–710 °C). Calibrations based on experimental work suggest that the measured Raman frequency shifts signify residual pressures of 0.1–0.2, 0.4–0.6, and 0.8–1.0 GPa for these three groups of metamorphic rocks, respectively. Normal stresses (internal pressures) of quartz inclusions in garnet, numerically simulated with an elastic model, and inferred pressure-temperature conditions at peak metamorphic stage are compatible with the residual pressures estimated from the frequency shifts. Laser Raman microspectroscopic analysis of quartz is a simple and effective method for (1) comparison of pressure conditions in metamorphic rocks formed under various pressure-temperature conditions, and (2) detection of a higher-pressure signature in metamorphic rocks extensively recrystallized during the subsequent exhumation and hydration stage.

Two‐stage metamorphic evolution of the Bemarivo Belt of northern Madagascar: constraints from reaction textures and in situ monazite dating

AbstractNew results on the pressure–temperature–time evolution, deduced from conventional geothermobarometry and in situ U‐Th‐total Pb dating of monazite, are presented for the Bemarivo Belt in northern Madagascar. The belt is subdivided into a northern part consisting of low‐grade metamorphic epicontinental series and a southern part made up of granulite facies metapelites. The prograde metamorphic stage of the latter unit is preserved by kyanite inclusions in garnet, which is in agreement with results of the garnet (core)‐alumosilicate‐quartz‐plagioclase (inclusions in garnet; GASP) equilibrium. The peak metamorphic stage is characterized by ultrahigh temperatures of ∼900–950 °C and pressures of ∼9 kbar, deduced from GASP equilibria and feldspar thermometry. In proximity to charnockite bodies, garnet‐sillimanite‐bearing metapelites contain aluminous orthopyroxene (max. 8.0 wt% Al2O3) pointing to even higher temperatures of ∼970 °C. Peak metamorphism is followed by near‐isothermal decompression to pressures of 5–7 kbar and subsequent near‐isobaric cooling, which is demonstrated by the extensive late‐stage formation of cordierite around garnet. Internal textures and differences in chemistry of metapelitic monazite point to a polyphasic growth history. Monazite with magmatically zoned cores is rarely preserved, and gives an age of c. 737 ± 19 Ma, interpreted as the maximum age of sedimentation. Two metamorphic stages are dated: M1 monazite cores range from 563 ± 28 Ma to 532 ± 23 Ma, representing the collisional event, and M2 monazite rims (521 ± 25 Ma to 513 ± 14 Ma), interpreted as grown during peak metamorphic temperatures. These are among the youngest ages reported for high‐grade metamorphism in Madagascar, and are supposed to reflect the Pan‐African attachment of the Bemarivo Belt to the Gondwana supercontinent during its final amalgamation stage. In the course of this, the southern Bemarivo Belt was buried to a depth of >25 km. Approximately 25–30 Myr later, the rocks underwent heating, interpreted to be due to magmatic underplating, and uplift. Presumably, the northern part of the belt was also affected by this tectonism, but buried to a lower depth, and therefore metamorphosed to lower grades.

Pressure–temperature evolution of blueschist facies rocks from Sifnos, Greece, and implications for the exhumation of high‐pressure rocks in the Central Aegean

AbstractThe blueschist and greenschist units on the island of Sifnos, Cyclades were affected by Eocene high‐pressure (HP) metamorphism. Using conventional geothermobarometry, the HP peak metamorphic stage was determined at 550–600 °C and 20 kbar, close to the blueschist and the eclogite facies transition. The retrograde P–T paths are inferred with phase diagrams. Pseudosections based on a quantitative petrogenetic grid in the model system Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O reveal coeval decompression and cooling for both the blueschist and the greenschist unit. The conditions of the metamorphic peak and those of the retrograde stages conform to a similar metamorphic gradient of 10–12 °C km−1 for both units. The retrograde overprint can be assigned to low‐pressure blueschist to HP greenschist facies conditions. This result cannot be reconciled with the (prograde) Barrovian‐type event, which affected parts of the Cyclades during the Oligocene to Miocene. Instead, the retrograde overprint is interpreted in terms of exhumation, directly after the HP stage, without a separate metamorphic event. Constraints on the exhumation mechanism are given by decompression‐cooling paths, which can be explained by exhumation in a fore‐arc setting during on‐going subduction and associated crustal shortening. Back‐arc extension is only responsible for the final stage of exhumation of the HP units.

In-situ trace element analyses of zircons from Dabieshan Huangzhen eclogite: Trace element characteristics of ec-logite-facies metamorphic zircon

The internal structures of zircons in eclogite from Huangzhen have been studied by cathodoluminescence (CL) microscopy. Two growth stages were distinguished: protolith magmatic cores and metamorphic overgrowth rims. These different domains were analyzed for trace elements using LAM-ICP-MS. The protolith and the overgrowth zircons have different trace elements characteristics. The trace element contents of protolith zircons are high and very variable. The overgrowth zircons show a typical trace element feature of equilibrium with garnet, e.g. low contents of HREE (132.2–197.6 μg/g) and small differential degree of HREE ((Yb/Gd)CN=8.6–11.9). The contents of Nb, Ta and the ratio of Nb/Ta are lower in the metamorphic domains (0.5–1.4 μg/g, 0.7–1.5 μg/g, 0.3–1.3, respectively) than in the protolith domains (3.8–19.7 μg/g, 2.7–12.1 μg/g, 1.0–4.6, respectively). This is the first time to give the evidence that the metamorphic zircon equilibrates with the rutile, which formed during the peak metamorphic stage. The REEs and other trace elements data demonstrate that the metamorphic zircons overgrow in the eclogite-facies conditions. The trace element composition of zircon can therefore give new ways to constrain their formation conditions.

Geochemical constraints of the eclogite and granulite facies metamorphism as recognized in the Raobazhai complex from North Dabie Shan, China

AbstractA combined study of major and trace elements, fluid inclusions and oxygen isotopes has been carried out on garnet pyroxenite from the Raobazhai complex in the North Dabie Terrane (NDT). Well‐preserved compositional zoning with Na decreasing and Ca and Mg increasing from the core to rim of pyroxene in the garnet pyroxenite indicates eclogite facies metamorphism at the peak metamorphic stage and subsequent granulite facies metamorphism during uplift. A P–T path with substantial heating (from c. 750 to 900 °C) after the maximum pressure reveals a different uplift history compared with most other eclogites in the South Dabie Terrane (SDT). Fluid inclusion data can be correlated with the metamorphic grade: the fluid regime during the peak metamorphism (eclogite facies) was dominated by N2‐bearing NaCl‐rich solutions, whereas it changed into CO2‐dominated fluids during the granulite facies retrograde metamorphism. At a late retrograde metamorphic stage, probably after amphibolite facies metamorphism, some external low‐salinity fluids were involved. In situ UV‐laser oxygen isotope analysis was undertaken on a 7 mm garnet, and impure pyroxene, amphibole and plagioclase. The nearly homogeneous oxygen isotopic composition (δ18OVSMOW = c. 6.7‰) in the garnet porphyroblast indicates closed fluid system conditions during garnet growth. However, isotopic fractionations between retrograde phases (amphibole and plagioclase) and garnet show an oxygen isotopic disequilibrium, indicating retrograde fluid–rock interactions. Unusual MORB‐like rare earth element (REE) patterns for whole rock of the garnet pyroxenite contrast with most ultra‐high‐pressure (UHP) eclogites in the Dabie‐Sulu area. However, the age‐corrected initial εNd(t) is − 2.9, which indicates that the protolith of the garnet pyroxenite was derived from an enriched mantle rather than from a MORB source. Combined with the present data of oxygen isotopic compositions and the characteristic N2 content in the fluid inclusions, we suggest that the protolith of the garnet pyroxenite from Raobazhai formed in an enriched mantle fragment, which has been exposed to the surface prior to the Triassic metamorphism.

Garnet zoning and the identification of equilibrium mineral compositions in high‐pressure–temperature granulites from the Moldanubian Zone, Austria

A detailed investigation of the compositional variation in garnet has been undertaken in a garnet–pyroxene‐bearing granulite from the high‐grade Gföhl Unit, Moldanubian Zone, Lower Austria. Textural observations, together with the interpretation of the preserved garnet chemistry, enables the recognition of both prograde core and peak metamorphic garnet mantle growth stages, an extremely rare feature in high‐P–T granulite facies rocks. Initial thermobarometric calculations undertaken across whole garnet zoning profiles show how correct interpretation of a zoning profile is essential if the maximum peak metamorphic P–T conditions are to be recovered. The effect of retrograde decompression‐ and cooling‐driven reactions on inclusion and host garnet compositions has also been assessed. The results indicate that caution should be exercised when utilizing inclusion and adjacent garnet compositions for the thermobarometric evaluation of peak metamorphic equilibration conditions. Peak P–T conditions were determined by the TWEEQU thermobarometric method, utilizing the core compositions of matrix phases combined with the interpreted high‐P–T garnet mantle composition, to give 15.6 kbar and 1090 °C, consistent with previously determined results for Moldanubian granulites. Similar high‐P–T estimates are also provided by a re‐evaluation of previously published results for a granulite sample from the same lithological unit, using a modified interpretation of garnet and plagioclase compositional data. The new estimates presented confirm the previously disputed idea that the Gföhl Unit underwent a high‐pressure granulite facies stage and is therefore distinctly different from the underlying tectonostratigraphic units. It is emphasized that any interpretation of the peak metamorphic conditions in high‐grade rocks must be based on detailed petrographic observations combined with a thorough understanding of the co‐existing equilibrium mineral compositions.

High-grade contact metamorphism of calcareous rocks from the Oslo Rift, Southern Norway

Shallow-level plutons caused extensive contact metamorphism of Lower Paleozoic shale and carbonate sequences in the Permian Oslo Rift. A >500 m long and 100 m wide shale-limestone xenolith embedded within monzonites belonging to the Skrim plutonic complex experienced high-grade contact metamorphism and generation of minerals and mineral assemblages rarely reported from metamorphic rocks. The peak metamorphic (Stage I) assemblages in calcite-saturated rocks include wollastonite, melilites, fassaitic pyroxenes, phlogopite, titanian grossular, kalsilite, nepheline, perovskite, cuspidine, baghdadite, pyrrhotite, and occasional graphite. Mineral reactions involving detrital apatite produced a series of silicate apatites, including the new mineral species Ca (sub 3.5) (Th,U) (sub 1.5) Si 3 O 12 (OH). This assemblage equilibrated at T = 820-870 degrees C with a C-rich, internally buffered pore-fluid (20-40 mol% CO 2 +CH 4 ). During cooling the shale-limestone xenolith experienced infiltration of C-poor ( 2 ) fluids, triggering the formation of retrograde (Stage II) mineral assemblages comprising monticellite, tilleyite, vesuvianite, grandite garnets, diopside, and occasional hillebrandite. Rare potassium iron sulfides (rasvumite and djerfisherite) formed at the expense of primary pyrrhotite. These assemblages probably formed near 700 degrees C. Formation of diffuse sodalite-bearing veinlets was associated with breakdown of nepheline and the replacement of kalsilite and wollastonite by potassium feldspar. The sodalite-bearing Stage III assemblage formed by the infiltration of saline brines at a maximum temperature of 550 degrees C. Low-temperature (Stage IV) retrogression of the Stage I-III assemblage produced scawtite, giuseppettite, hydrogrossulars, phillipsite, thomsonite, and three hitherto undescribed mineral species.

The microarea composition of garnet and biotite from the Ji’an Group in Tonghua district and its significance as an indicator of metamorphicP-T-t path

The garnet (Grt) and biotite (Bt) from gneisses of the Ji’an Group are characterized by diffusion zoning at the rim, but equilibrium composition of metamorphic peak is usually remained in extensive interior area. Garnet with growth zoning is also found in the kyanite zone. In the light of microarea compositional variation of Grt and Bt, the temperature and pressure at the progressive, peak and post-peak metamorphic stages are determined by correctly using Grt-Bt thermometer and GASP barometer. On this basis, a counterclockwiseP-T-t path can be constructed, which reflects the closing process of an ensialic rift belt in this region during the Early Proterozoic.

Interaction of aqueous fluids with calcareous metasediments during high‐T, low‐P regional metamorphism in the Qadda area, southern Arabian Shield

The S.W. Nabitah Mobile Belt, Saudi Arabia, contains a Proterozoic island‐arc complex. In the Qadda area, the metavolcanic‐dominated supracrustal sequence records amphibolite facies regional metamorphism of high‐T , low‐P type. Calcsilicate rocks and aluminous dolomitic marbles within the supracrustal sequence have been studied in detail to refine estimates of peak metamorphic P–T conditions and assess the role of fluids during prograde and retrograde metamorphism.Fluid‐independent thermobarometers (including the calcite–dolomite thermometer and P‐sensitive equilibria involving grossular, wollastonite, anorthite, meionite, quartz and calcite) yield peak P–T conditions of c. 650–660 °C, 4 kbar, both higher than previous estimates, giving a revised average thermal gradient of c. 45 °C km–1.The close match between the peak temperatures implied by calcite–dolomite thermometry and those recorded by univariant devolatilization equilibria suggests that the calcareous rocks were fluid‐bearing during late‐prograde and peak metamorphic stages. These fluids were essentially binary H2O–CO2 mixtures with low NaCl and HF concentrations. Most were H2O‐rich, with XCO2 between 0.02 and 0.2, but values of c. 0.6 are recorded by two samples. High modal abundances of the solid products of decarbonation reactions (e.g. c. 10–50% wollastonite) in many of the rocks that record low‐XCO2 equilibrium fluids implies infiltration of significant quantities of externally derived aqueous fluid during late‐prograde metamorphism, but not enough to exhaust the buffering capacity of the rocks. Calculated minimum time‐integrated fluid‐to‐rock ratios of five wollastonite‐bearing calcsilicate rocks range from 0.7±0.22 to 1.39±0.46 (1σ); those of six marbles range from c. 0 to 4±1.4. The latter variation occurs on a metre‐scale, implying focusing of fluid flow. Diopside‐rich rocks record fluid‐to‐rock ratios of up to 88±48. Penetrative wollastonite lineations indicate a temporal link between infiltration and distributed ductile deformation. Infiltrating fluids were probably derived both from the prograde dehydration of adjacent metabasalts and metatuffs and from crystallization of voluminous pretectonic granitoid intrusions. In general, fluid‐to‐rock ratios deduced for the metavolcanic‐dominated Qadda area are similar to those recorded by rocks in the metasediment‐dominated terrane of N. New England.The occurrence of post‐tectonic retrograde hydration textures in both carbonate‐bearing and carbonate‐free rocks otherwise lacking hydrous minerals testifies to infiltration of aqueous fluids during retrograde metamorphism in the absence of penetrative deformation. Minimum fluid‐to‐rock ratios calculated for secondary grossular reaction rims in some calcsilicates are c. 0.04. Later patchy hydration of scapolite probably utilized static, pore‐filling fluids remaining after the early retrograde infiltration.

Carbonic metamorphism of charnockites in the southwestern Indian Shield: A fluid inclusion study

Forming the southwestern segment of the Precambrian granulite facies terrain of the Indian shield, the Kerala region largely comprises charnockites, khondalites and migmatitic gneisses. Fluid inclusions in quartz from the charnockites show distinct distribution patterns consistent with three generations of inclusions. The early monophase type records entrapment of high-density CO 2-rich fluid (0.95–1.0 g cm −3). A subsequent monophase type with lower-density CO 2-rich fluid (0.65–0.75 g cm −3) coexists with CO 2H 2O inclusions having an average degree of filling of 0.2 (H 2O = 20%; CO 2 = 80%). Late aqueous biphase inclusions show coexistence with a second category of CO 2H 2O inclusions showing a degree of filling of 0.6 (H 2O = 60%; CO 2 = 40%). The CO 2-isochores for early carbonic inclusions yield a pressure range of 4.6–6.1 kbar at granulite facies temperatures of 650–800°C, depicting the entrapment of fluids present during or close to the peak metamorphic stage. A definite sequence of fluid evolution is traceable for the subsequent stages. Thus, the coexisting CO 2 and CO 2H 2O inclusions were entrapped at 510°C and 2.2 kbar, marking the waning of carbonic regime and the beginning of aqueous regime. At 330°C and 0.4 kbar, fluid unmixing occurred, leading to the simultaneous entrapment of mixed CO 2H 2O and H 2O inclusions along rehealed microfractures. The data presented indicate that the metamorphic fluids evolved from early high-density carbonic through mixed carbonic-aqueous to late aqueous types. The dry granulite mineral assemblage of charnockites is a result of metamorphic equilibration under water-deficient and high- P CO 2 conditions.