High-pressure Phengite Research Articles

Interplay between fluid circulation and Alpine metamorphism in the Monte Rosa whiteschist from white mica and quartz in situ oxygen isotope analysis by SIMS

Abstract In situ oxygen isotope compositions of white mica and quartz have been used to characterize the interplay of metamorphism and fluid events between a metasomatic whiteschist and its granite protolith in the Monte Rosa nappe, Western Alps. New natural muscovite and phengite reference materials were calibrated for in situ Secondary Ion Mass Spectrometry (SIMS) oxygen isotope measurement. White mica and quartz oxygen isotope compositions were measured in situ in one whiteschist and two metagranites. Based on microtextural observation, phengite composition of white mica, and phase petrology modeling, it is possible to identify two events of fluid infiltration and one event of fluid expulsion, all of which were responsible for forming this unique whiteschist occurrence and for tracing its metamorphic evolution from late Permian intrusion to Alpine subduction and finally to the present day, exhumed whiteschist. Metagranite samples contain three generations of white mica: igneous, high-P metamorphic, and late Alpine, retrograde compositions. In the whiteschist samples, we distinguish two distinct Alpine white mica generations: (1) prograde to peak generation and (2) retrograde generation. The δ18OVSMOW values of white mica and quartz from a whiteschist of 5.3 to 7.3‰ and 9.1 to 10.6‰ are significantly lower than in the metagranites, with 9.1 to 10.8‰ and 13.2 to 14.6‰, respectively. This indicates a complete recrystallization of the whiteschist protolith during intense fluid-rock interaction. Subsequent Alpine metamorphism transformed the protolith into the whiteschist. The isotopic composition of the whiteschist, fine-grained, retrograde white mica (5.3 to 6‰) is lower than that of the high-pressure phengite (6.2 and 7.5‰). The low δ18O values could be explained by infiltration of external fluids with δ18O values of 2 to 6‰. Such fluids would carry the isotopic signature of the serpentinites of the Piemonte-Liguria Ocean by either equilibration of fluids with or dehydration of serpentinites. Another, more simple explanation would be the infiltration of very small quantities of fluids leading to the breakdown of chloritoid. Local inheritance of the oxygen composition would then hide the origin of the fluids. Isotope exchange temperatures calculated from high-P phengite-quartz pairs in whiteschist give an average temperature of 440 ± 50 °C. These are lower than the best T-estimates from phase petrology of 570 °C, at 2.2 GPa. Igneous muscovite-quartz pairs in the metagranite yield 400 ± 40 °C. Only one high-P phengite-quartz pair was analyzed, resulting in 350 ± 40 °C. Greenschist facies, low silica phengites give an average temperature of 310 ± 10 °C. Propagation of analytical uncertainty suggests large errors of 60 to 120 °C, due to the relatively small T-dependence of the quartz-white mica fractionation factor for oxygen isotopes.

A short, sharp pulse of potassium-rich volcanism during continental collision and subduction

Abstract Potassic volcanic rocks are characteristic of collisional tectonic zones, with recycling of continental crust playing an important role in their generation. Potassium-rich partial melts and/or fluids derived from subducted continental material initiate and/or mix with mantle-derived melts and then erupt at the surface with varying degrees of interaction with the overlying lithosphere. The details of how continental material incorporates into mantle melts are, however, uncertain. In particular, the depths from which the potassium-rich fluids and/or melts are released from the continental material and then react with the mantle-derived melts remain a subject of debate. We have measured the boron isotope composition of volcanic rocks from Western Anatolia (Turkey) that erupted between 52 and 0.1 Ma, and span the lifetime of collisional events from initial arc-type eruptions to post-collisional volcanism. These data and other geochemical indices show that ultrapotassic volcanism was mainly confined to a narrow window between ca. 20 and 15 Ma, consistent with recycling of high-pressure phengite, with the timing of the potassic volcanism coincident with slab rollback and breakoff.

White mica trace element and boron isotope evidence for distinctive infiltration events during exhumation of deeply subducted continental crust

ABSTRACTPrevious study of subducted continental crust within the Luliang Shan terrane in Northwest China has documented metasomatic formation of thick, hydrated phengite + garnet-rich selvages at the interface between mafic eclogite blocks and quartzofeldspathic host gneiss. Whole rock concentrations of Cs and Ba within the selvage are enriched by two orders of magnitude relative to the eclogite blocks and host gneiss. We performed in situ ion microprobe analyses of Li, Be, B, Rb, Sr, Cs and Ba and δ11B of phengite within the Luliang Shane terrane to better constrain the source(s) of the infiltrating fluid. The phengite within the selvage are enriched in Li, Cs and Ba and yield δ11B values between −30‰ and −9‰, values that are lower than mantle values. High Ba/Rb, Cs/Rb coupled with low B/Be, B/Li and highly negative δ11B values indicate that the high-pressure fluid that formed the selvage was derived from highly devolatilized rocks within the subduction channel. In contrast, muscovite, which crystallized in the adjacent host gneiss during a subsequent lower pressure phase of fluid infiltration at approximately 0.9 GPa depths, has much lower Li, Cs and Ba relative to the high-pressure phengite. These retrograde muscovite have very high concentrations of B (up to 5500 ppm) and Be (up to 50 ppm) and high (−2 to +8‰) δ11B values that are consistent with crystallization from a fluid derived from shallower and less devolatilized regions of the subduction zone. Additional host gneiss samples, regionally distributed and kilometres away from the studied area lack the B-rich signature and indicate that the late stage fluids were likely localized to the region near the studied traverse.

Syn- to post-orogenic exhumation of metamorphic nappes: Structure and thermobarometry of the western Attic-Cycladic metamorphic complex (Lavrion, Greece)

The Lavrion peninsula is located along the western boundary of the Attic-Cycladic metamorphic complex in the internal zone of the Hellenic orogenic belt. The nappe stack is well exposed and made, from top to bottom, of (i) a non-metamorphic upper unit composed of an ophiolitic melange, (ii) a middle unit mainly composed of the Lavrion schists in blueschist facies, (iii) and a basal unit mainly composed of the Kamariza schists affected by pervasive retrogression of the blueschist facies metamorphism in greenschist facies. The middle unit is characterized by a relatively steep-dipping foliation associated with isoclinal folds of weakly organized axial orientation. This foliation is transposed into a shallow-dipping foliation bearing a N-S trending lineation. The degree of transposition increases with structural depth and is particularly marked at the transition from the middle to the basal unit across a low-angle mylonitic to cataclastic detachment. The blueschist facies foliation of the Lavrion schists (middle unit) is underlined by high pressure phengite intergrown with chlorite. The Kamariza schists (basal unit) contains relics of the blueschist mineral paragenesis but is dominated by intermediate pressure phengite also intergrown with chlorite and locally with biotite. Electron probe micro-analyzer chemical mapping combined with inverse thermodynamic modeling (local multi-equilibrium) reveals distinct pressure–temperature conditions of crystallization of phengite and chlorite assemblages as a function of their structural, microstructural and microtextural positions. The middle unit is characterized by two metamorphic conditions grading from high pressure (M1, 9–13kbar) to lower pressure (M2, 6–9kbar) at a constant temperature of ca. 315°C. The basal unit has preserved a first set of HP/LT conditions (M1–2, 8–11kbar, 300°C) partially to totally transposed-retrogressed into a lower pressure mineral assemblage (M3, 5–8.5kbar) associated with a slight but significant increase in temperature (∼350°C).These structural, petrologic and thermobarometric data document the tectonic evolution from construction to destruction of the Hellenic orogenic belt. The steep-dipping blueschist facies foliation, preferentially preserved at high structural level in the middle unit, is considered to reflect tectonic accretion marked by successive burial (D1M1) and syn-orogenic exhumation (D2M2) without thermal relaxation. The transposition-retrogression of the blueschist facies mineral assemblage into a shallow-dipping greenschist facies foliation associated with an increase in temperature corresponds to lateral flow of the thermally relaxed nappe stack (D3M3). The development of a low-angle detachment, accommodating post-orogenic exhumation of the orogenic root, is attributed to gravitational collapse of the Hellenic belt.

Petrology and geochronology of ‘muscovite age standard’ B4M

Abstract Muscovite B4M, distributed in 1961 as an age standard, was ground under ethanol. Five grain size fractions were obtained and characterized by X-ray diffraction. They display a mixing trend between a phengitic (enriched in the fraction <0.2 µm) and a muscovitic component (predominant in the fraction >20 µm). High-pressure phengite is preserved as a relict in retrograde muscovite. Electron microprobe analyses of the distributed mineral separate reveal at least four white mica populations based on Si, Al, Mg, Na, Fe and F. Rb/K ratios vary by one order of magnitude. Rb–Sr analyses link the mineralogical heterogeneity to variable Rb/Sr and 87 Sr/ 86 Sr ratios. The grain size fractions define no internal isochron. Relict fine-grained phengite gives older ages than coarse-grained retrograde greenschist facies muscovite. The inverse grain size–age relationship also characterizes 39 Ar/ 40 Ar analyses. Cl/K anticorrelates with step ages: Cl-rich coarse muscovite is younger than Cl-poor fine relict phengite. Sr and Ar preserve a similar isotopic inheritance despite peak metamorphism reaching 635±20 °C. A suitable mineral standard requires that its petrological equilibrium first be demonstrated. Relicts and retrograde reaction textures are a guarantee of isotopic disequilibrium and heterogeneous ages within single crystal at the micrometre scale. Supplementary material: Electron microprobe results on two grain mounts of the unprocessed B4M separate as distributed and on a whole-rock thin section of Brione gneiss from the teaching collection of the Universität Bern are available at: www.geolsoc.org.uk/SUP18590 .

Interpreting high-pressure phengite 40Ar/39Ar laserprobe ages: an example from Saih Hatat, NE Oman

New single grain fusion and core-rim 40Ar/39Ar laserprobe phengite data from the Saih Hatat high-pressure terrane in NE Oman show that individual samples yield a range of apparent ages which is similar to that previously reported from across the entire terrane. The majority of the determined ages are older than the previously reported U-Pb zircon peak metamorphic age. Core to rim age variations within individual grains range from no discernible difference across the grain to grains with older cores, or, rarely, older rims; some samples manifest all three patterns. Numerical diffusion modelling shows that due to the peak temperature of ca. 550°C, the measured apparent ages cannot be explained by simple cooling or by partial retention of crystallisation or detrital ages in an open system. The age variability is better explained by spatially and temporally variable open or closed system behaviour at the mm-cm scale coupled with pervasive and heterogeneously distributed excess argon. Anomalously old eclogite phengite 40Ar/39Ar ages are due either to internally derived 40Ar inherited from a K-bearing precursor, or externally derived 40Ar distributed by grain boundary fluids. Mica-rich schists within the eclogite boudins yield younger phengite ages, suggesting excess argon was absent or diluted. Pelites hosting the eclogite appear to have been affected by later fluid ingress during deformation and greenschist-facies overprint and yield very variable ages commonly with apparently older rims on younger cores. The grain- and sample-scale age variations measured in Saih Hatat indicate that the grain boundary network in eclogite pods was not an efficient transfer pathway for argon transport, whereas the grain boundary network in the surrounding pelites acted as a more efficient pathway on the timescale of the metamorphic cycle.

RbSr and 40Ar 39Ar laser probe dating of high-pressure phengites from the Sesia zone (Western Alps): underscoring of excess argon and new age constraints on the high-pressure metamorphism

The combined use of the RbSr and 40Ar 39Ar laser probe methods allows confirmation of the existence of excess argon in phengites from eclogites from the Sesia zone (Western Alps) despite systematic 40Ar 39Ar plateau ages. 40Ar 39Ar phengite ages from two different areas show integrated age spreads between 65.4 ± 0.3 Ma and 109.5 ± 0.4 Ma. The youngest 40Ar 39Ar plateau age (65.9 ± 0.4 Ma) is concordant with two RbSr phengite-whole-rock isochron ages from the same outcrop which display a mean age at 64.2 ± 2.5 Ma and older than a Rb-Sr biotite-whole-rock isochron age at 53.0 ± 1.0 Ma. This age concordance, being incompatible with the concept of isotopic closure temperature, is probably fortuitous and probably results from an excess argon contamination. However, the concordance of the RbSr phengite isotopic closure temperature with the temperature reached during the eclogite-facies event suggests that the age at 64.2 ± 2.5 Ma could be a crystallization age probably contemporaneous with the eclogite-facies event in the Sesia zone. When compared to recent age data from the Piemont zone and the Dora Maira Massif, the entire data set (which needs to be confirmed by extra measurements) support diachronous high-pressure (HP) events in the Western Alps.

Excess 40Ar transport scale and mechanism in high-pressure phengites: A case study from an eclogitized metabasite of the Dora-Maira nappe, western Alps

Application of the bulk-separate furnace heating, laser step-heating, and in situ laser agemapping 40Ar/ 39Ar techniques reveals that high-pressure phengites from an Alpine eclogite of the Dora-Maira basement nappe, western Alps, were pervasively contaminated with excess 40Ar to yield ages > 130 Ma. Spot-fusion dating of individual grains with the 40Ar/ 39Ar laserprobe reveals concentric radial patterns with core ages as high as 320 Ma, showing that this excess component was trapped at an early stage and subsequently remobilized by volume diffusion during a late greenschist thermal pulse. The single grains and population from this hand-sample retain major differences in apparent ages indicating that the extent of the excess 40Ar contamination varies from grain to grain. Attempt to model these differences in terms of grain-size-dependent fractional loss falls short and demonstrates that strong cm-scale argon isotopic gradients locally prevailed during syn-kinematic high-pressure metamorphism. On petrostructural grounds, a model of internal isotopic buffering is outlined, whereby the excess 40Ar is argued to have been internally derived from within the eclogite protolith itself. Low intergranular Ar diffusion rates due to reduced water activity (buffered below unity by the phengite assemblage) resulted in the local segregation and partitioning of the excess 40Ar along grain boundaries during ongoing ductile deformation. Supporting evidence for this model comes from the recent literature on fluid and stable isotope behaviour in Alpine eclogites and very-high-pressure metasediments, which shows that volatile recycling during high-pressure metamorphism may occur on a very limited scale.

Evidence for excess argon during high pressure metamorphism in the dora maira massif (Western Alps, Italy), using an ultra-violet laser ablation microprobe 40Ar-39Ar technique

Ultra-high pressure eclogite/amphibolite grade metamorphism of the Dora Maira Massif in the western Alps is a well established and intensively studied event. However, the age of peak metamorphism and early cooling remains controversial. The 40Ar-39 Ar step-heating and laser spot ages from high pressure phengites yield plateau ages as old as 110 Ma which have been interpreted as the time of early cooling after the high pressure event. Recent U/Pb and Sm/Nd results challenge this assertion, indicating a much younger age for the event, around 45 Ma, and hence a radically different timing for the tectonic evolution of the western Alps. In a new approach to the problem, samples from the undeformed Hercynian metagranite, Brossasco, were studied using an ultra-violet laser ablation microprobe technique for 40Ar-39 Ar dating. The new technique allowed selective in situ analysis, at a spatial resolution of 50 urn, of quartz, phengite, biotite and K-feldspar. The results demonstrate the frequent occurrence of excess argon with high 40Ar-39 Ar ratios (1000–10000) and a strong relationship between apparent ages and metamorphic textures. The highest excess argon ratios are always associated with high closure temperature minerals or large diffusion domains within single mineral phases. The best interpretation of this relationship seems to be that excess argon was incorporated in all phases during the high pressure event, then mixed with an atmospheric component during rapid cooling and retrogression, producing a wide range of argon concentrations and 40Ar/36Ar ratios. Step-heating analysis of minerals with this mixture would produce linear arrays on a40Ar/36Ar versus 40Ar/36 Ar correlation diagram, leading to geologically meaningless plateau ages, older than the true closure age. In the present case, some ages in the range 60–110 Ma could be explained by the presence of excess argon incorporated around 40–50 Ma ago. Similar results found in other high pressure terrains in the Alps may reconcile the argon geochronometer with other systems such as Rb/Sr, U/Pb or Sm/Nd. This study therefore calls for an increasing use of high resolution in situ sampling techniques to clarify the meaning of 40Ar/ 36 Ar ages in many high pressure terrains.

Formula omitted] and [(Mg,Fe)Si-Al 2] compositional control on argon behaviour in high-pressure white micas: A [formula omitted] continuous laser-probe study from the Dora-Maira nappe of the internal western Alps, Italy

High-pressure white micas from a restricted area in the Dora-Maira nappe of the western Alps (Italy) were analyzed by the conventional 40Ar 39Ar bulk-sample step-heating method and by the step-heating and the spot-fusion 40Ar 39Ar continuous laser-probe techniques on single grains. High-Si, Ferich phengites from fine-grained gneisses display homogeneous 35–40 Ma plateau and spot-fusion ages, consistent with the upper Eocene 40Ar 39Ar phengite ages documented over the whole internal western Alps. In contrast, phengite separates and single-grains of high-Si and Mg-rich to intermediate composition selected from metapelitic and Mg-rich mica schist samples yield older discordant apparent ages, even when sampled in the same outcrop as the fine-grained gneisses. These local age variations are interpreted to reflect primarily an increasing Ar retentivity with increasing Mg content in phengites. In metapelite-derived white micas, the isotopic discordance is marked by 1. (1) a characteristic bulk-sample upward-convex spectrum shape, 2. (2) single-grain disturbed age spectra, 3. (3) a systematic radially outward decrease in spot ages across individual mica cleavage planes and 4. (4) strong grain-to-grain spot-age variations in rock sections. 1. (1) In Mg-rich mica schists, the isotopic discordance is observed as a reproducible upward-convex bulk-sample age spectrum with high (140 Ma) intermediate apparent ages, 2. (2) upward-convex single-grain age spectra and 3. (3) irregular intra- and intergrain age gradients. On geochemical grounds, it is argued that the isotopic discordance occurring in the metapelites is mainly controlled by the compositional changes introduced by the [(Mg,Fe)Si-Al 2] celadonitic re-equilibration of the high-pressure phengites during greenschist overprinting. In Mg-rich phengites, this chemical re-equilibration may induce Ar loss below their presumed higher blocking temperature. A model of Ar behaviour in high-pressure white micas is proposed which explains the age variations recorded throughout the different lithologies in terms of a simple interaction between the Mg Fe compositional control on Ar retentivity and the [(Mg,Fe)Si-Al 2] modifications occurring in phengites during progressive overprinting.

K ? Ar age determinations on phengites from the internal part of the Sesia Zone, Western Alps, Italy

K-Ar age determinations have been carried out on various, well-defined phengite populations from a small area of the internal part of the Sesia Zone, lower Aosta valley. There, the rocks have suffered high-pressure metamorphism, attributed to early Alpine subduction, in the stability field of jadeite + quartz (P≥15±1 kbar at T=550±50° C). Coarse-grained phengites from well-preserved high-pressure parageneses, and phengites (re)crystallized early during decompression at still elevated temperatures in the stability field of albite, yield identical ages close to 80 Ma. In the most external sample high-pressure phengites yield 72 Ma. The ages around 80 Ma are interpeted as essentially undisturbed cooling ages; they are not notably influenced by paragenesis, chemical composition, polymorphism, grain-size, deformation, and recrystallization at higher temperatures. This part of the Sesia Zone has cooled down about 80 Ma ago to below the blocking temperature for the K-Ar system of white mica. Some of the samples show pronounced retrogression of the high-pressure assemblages, especially formation of albite and fine-grained phengite at the expense of jadeite, and are affected by intense late deformation at temperatures of about 300 to 350° C, estimated from the small grain-size of dynamically recrystallized quartz. The ages obtained from extremely kinked or sheared coarse high-pressure phengites scatter considerably, being partly higher, partly lower than 80 Ma. The fine-grained phengite fractions from these samples yield lowered ages down to 70 Ma. These values are interpreted as mixed ages resulting from variable mixtures of fragmented high-pressure phengites and new phengite replacing jadeite; the K-Ar data suggest that the new phengites have crystallized between 60 and 70 Ma b.p. All phengites formed at low temperatures at the expense of jadeite reveal high Si-contents; these range up to 6.9 for the coarser-grained earlier generations, and up to 6.7 for the very fine-grained last generation. Relatively high pressures are required to stabilize phengites so high in Si. This implies that cooling had not been achieved by uplift and erosion; obviously the thermal structure of the crust was still strongly perturbed by continuing subduction and thrusting during the late Cretaceous.