HP-LT Metamorphic Rocks Research Articles

Integrating X-Ray Computed Tomography With Chemical Imaging to Quantify Mineral Re-crystallization From Granulite to Eclogite Metamorphism in the Western Italian Alps (Sesia-Lanzo Zone)

Metamorphic transformations and fabric evolution are the consequence of thermo-dynamic processes, lasting from thousands to millions of years. Relative mineral percentages, their grain size distribution, grain orientation, and grain boundary geometries are first-order parameters for dynamic modeling of metamorphic processes. To quantify these parameters, we propose a multidisciplinary approach integrating X-ray computed microtomography (µ-CT) with X-ray chemical mapping obtained from an Electron MicroProbe Analyzer (EMPA). We used a metapelitic granulite sample collected from the Alpine HP-LT metamorphic rocks of the Mt. Mucrone (Eclogitic Micaschists Complex, Sesia-Lanzo Zone, Western Alps, Italy). The heterogeneous Alpine deformation and metamorphism allowed the preservation of pre-Alpine structural and mineralogical features developed under granulite-facies conditions. The inferred granulitic mineral association is Grt + Bt + Sil + Pl + Qtz ± Ilm ± Kfs ± Wm. The subsequent pervasive static eclogite-facies re-equilibration occurred during the Alpine evolution. The inferred alpine mineral association is Wm + Omp ± Ky + Qtz+ Grt though local differences may occur, strongly controlled by chemistry of microdomains. X-ray µ-CT data extracted from on centimeter-sized samples have been analyzed to quantify the volumetric percentage and shape preferred orientation for each mineral phase. By combining tomographic phase separation with chemical variation and microstructures (i.e., different grain-size classes for the same phase and morphology of different pre-Alpine microdomains) the pre-Alpine mineralogical phases from the Alpine overprint have been distinguished and quantified. Moreover, the sample preserves 100% of the pre-Alpine granulite fabric, which surprisingly corresponds to less than 22% of the corresponding pre-Alpine metamorphic assemblages, while the Alpine eclogitic static assemblage corresponds to 78% though no new fabric is developed. This contribution demonstrates that the combined use of EMPA X-ray chemical mapping with the X-ray µ-CT shape analysis permits a dynamic approach to constrain the chemistry of the mineral phases linked to the development of metamorphic-related static and dynamic fabrics.

Two contrasting accretion v. collision orogenies: insights from Early Paleozoic polyphase metamorphism in the Altun–Qilian–North Qaidam orogenic system, NW China

Abstract The Altun–Qilian–North Qaidam (AQQ) orogenic system in northern Tibet is considered to be the northernmost orogenic collage of the Prototethyan domain. It is regarded as resulting from collisions between various continental terranes derived from the northern margin of Gondwana, although the AQQ orogenic system also includes abundant ophiolites, arc magmatic rocks and subduction–accretion complexes. Some researchers regard the orogenic system of north Tibet as a typical accretionary orogen built by the development of an evolving arc–accretion complex growing southwards along the margin of the Tarim and North China cratons during the Paleozoic. We propose, based on both published data and our new data, that two distinct accretion and collision orogenies developed in the AQQ during the early Paleozoic. The diagnostic marks are HP–LT metamorphic rocks in the North Altun–North Qilian Mountains and UHP metamorphic rocks in the South Altun–North Qaidam Mountains. A review of metamorphic, geochronological, geochemical and structural data indicates that the North Altun–North Qilian HP–LT metamorphic belt is related to early Paleozoic subduction–accretion and, together with ophiolite mélanges and arc metamorphic–magmatic complexes, forms an early Paleozoic accretionary orogen. By contrast, the South Altun–North Qaidam UHP metamorphic belt is associated with continental subduction and collision, accompanied by Barrovian-type metamorphic overprinting and collision-related magmatism, reflecting an early Paleozoic collisional orogeny.

The Tell-Rif orogenic system (Morocco, Algeria, Tunisia) and the structural heritage of the southern Tethys margin

The Tell-Rif (Tell in Algeria and Tunisia; Rif in Morocco) is the orogenic system fringing to the south the West Mediterranean basins. This system comprises three major tectonic-palaeogeographic zones from north to south: (1) the internal zones (AlKaPeCa for Alboran, Kabylies, Peloritan, Calabria) originating from the former northern European margin of the Maghrebian Tethys, (2) the “Flyschs zone” regarded as the former cover of the oceanic domain and (3) the external zones, forming the former southern Maghrebian Tethys margin more or less inverted. The Tell-Rif is interpreted as the direct result of the progressive closure of the Maghrebian Tethys until the collision between AlKaPeCa and Africa and, subsequently, the propagation of the deformation within Africa. This gives a consistent explanation for the offshore Neogene geodynamics and most authors share this simple scenario. Nevertheless, the current geodynamic models do not completely integrate the Tell-Rif geology. Based on the analysis of surface and sub-surface data, we propose a reappraisal of its present-day geometry in terms of geodynamic evolution. We highlight its non-cylindrical nature resulting from both the Mesozoic inheritance and the conditions of the tectonic inversion. During the Early Jurassic, we emphasize the development of NE-SW basins preceding the establishment of an E-W transform corridor connecting the Central Atlantic Ocean with the Ligurian Tethys. The Maghrebian Tethys developed just after, as the result of the Late Jurassic-Early Cretaceous left-lateral spreading between Africa and Iberia. By the Late Cretaceous, the occurrence of several tectonic events is related to the progressive convergence convergence between the two continents. A major pre-Oligocene (pre-35 Ma) compressional event is recorded in the Tell-Rif system. The existence of HP-LT metamorphic rocks associated with fragments of mantle in the External Metamorphic Massifs of the Eastern Rif and Western Tell shows that, at that time, the western part of the North-African margin was involved in a subduction below a deep basin belonging to the Maghrebian Tethys. At the same time, the closure of the West Ligurian Tethys through east-verging subduction led to a shift of the subduction, which jumped to the other side of AlKaPeCa involving both East Ligurian and Maghrebian Tethys. Slab rollback led to the development of the Oligo-Miocene back-arc basins of the West-Mediterranean, reworking the previous West Ligurian Tethys suture. The docking of AlKaPeCa against Africa occurred during the Late Burdigalian (17 Ma). Subsequently, the slab tearing triggered westward and eastward lateral movements that are responsible for the formation of the Gibraltar and Tyrrhenian Arcs respectively. The exhumation of the External Metamorphic Massifs occurred through tectonic underplating during the westward translation of the Alboran Domain. It resulted in the formation of both foredeep and wedge-top basins younger and younger westward. The lack of these elements in the eastern part of the systems signs a different evolution dominated by frontal accretion. In the discussion, we precisely address the origin of the non-cylindrical behavior of the orogenic system and question the mechanisms explaining at large scale the phases of coupling/uncoupling between the major plates.

Late Palaeozoic 40Ar/39Ar ages of the HP-LT metamorphic rocks from the Kekesu Valley, Chinese southwestern Tianshan: new constraints on exhumation tectonics

abstractAlthough numerous ages have been obtained for the Chinese southwestern Tianshan high pressure/ultrahigh pressure-low temperature (HP/UHP-LT) metamorphic belt in the past two decades, its exhumation history is still controversial. The poor age constraint was related to the appealing low metamorphic temperatures and excess Ar commonly present under HP/UHP conditions. This study aims to provide new age constraints on the orogen’s exhumation by obtaining 40Ar/39Ar mica ages using the conventional step-heating technique, with emphasis on the avoidance of excess Ar contamination. From a cross section along the Kekesu Valley, four samples, three from the HP-LT metamorphic belt (TK050, TK051, and TK081) and one from the southern margin of the low pressure metamorphic belt (TK097), were selected for 40Ar/39Ar dating. Phengites from garnet glaucophane schist TK050 and the surrounding rock garnet phengite schist TK051 yield comparable plateau ages of 321.4 ± 1.6 and 318.6 ± 1.6 Ma, respectively, while epidote mica schist TK081 gives a younger plateau age of 293.3 ± 1.5 Ma. Considering the chemical compositions of phengites, mineral assemblages, and microstructures in the thin slices, we suppose that the former represents the time the HP rocks retrograded from the peak stage (eclogite facies) to the (epidote)-blueschist facies, whereas the latter reflects greenschist facies overprinting. Biotite and muscovite from two-mica quartzite TK097 give similar plateau ages of 253.0 ± 1.3 and 247.1 ± 1.2 Ma, interpreted to date movement on the post collisional transcrustal South Nalati ductile shear zone. By combining our new ages with published data, a two-stage exhumation model is suggested for the Chinese southwestern Tianshan HP/UHP-LT metamorphic belt: initial fast exhumation to a depth of about 30–35 km by ~320 Ma was followed by relatively slow (~1 mm year–1) uplift to ~10 km by ~293 Ma.

Trace element and isotopic fingerprints in HP–LT metamorphic rocks as a result of fluid–rock interactions (Ile de Groix, France)

Whole rock trace element and isotopic compositions of different HP–LT metamorphic rocks of the Ile de Groix were analysed to characterise geochemical fingerprints during subduction and exhumation in a late Palaeozoic HP metamorphic terrain. Massive metabasites of hydrothermally altered enriched mid-ocean ridge basalt (E-MORB) origin are in association with banded metabasic rocks of volcano-sedimentary origin and metapelites. Fluid-rock interactions that likely occurred during seafloor hydrothermal alteration and early subduction metasomatism increased δ18O values, as well as K2O, Na2O, MgO, and LILE contents and decreased CaO contents of metabasites. Most metabasites have retained their early-subduction and pre-HP trace element and isotopic composition, even for rocks metamorphosed to lower eclogite-facies P–T conditions. Micaschists also preserved apparent pelitic protolith trace element values and oxygen isotopic compositions. During retrograde metamorphism related to the exhumation, metabasites were rehydrated by fluids in equilibrium with the host rock compositions, which were likely derived from the basic rocks. This style of fluid–rock interaction formed a greenschist facies mineral assemblage. Metabasites that underwent pervasive alteration by seafloor hydrothermal and metasomatism processes prior to peak metamorphism, show greater effects of retrogression and albitisation, probably because they were richer in H2O and Na2O. The variety of metamorphic assemblages on the Ile de Groix is thus directly related to the pre-HP rock composition. The extent of retrogression in the western part of the Ile de Groix primarily reflects stronger metasomatic intensities than in the eastern part.

Geochemical data and zircon ages for rocks in a high-pressure belt of Chu-Yili Mountains, southern Kazakhstan: Implications for the earliest stages of accretion in Kazakhstan and the Tianshan

The mechanism and age of Palaeozoic accretion in the Central Asian Orogenic Belt remain poorly constrained. One of the most complex belts extends from the Kokchetav area in northern Kazakhstan to the Kyrgyz northern Tianshan. It represents an assemblage of small blocks with Palaeoproterozoic continental crust, intervening slivers containing early Palaeozoic ophiolites and/or deep-marine sediments, and a number of HP and UHP metamorphic complexes. The HP–LT metamorphic rocks provide important clues for reconstructions of the overall structure and evolution of the accretionary collage. This study is aimed to constrain the metamorphic age and tectonic implications of HP garnet pyroxenites and enclosing mica schists in the Anrakhai area of the Chu-Yili Mountains of southern Kazakhstan. The HP belt is located in the central part of the Anrakhai metamorphic block and extends NW–SE between Neoproterozoic gneisses in the SW and undated ophiolites and granodiorites in the NE. Garnet pyroxenites and their retrograded equivalents form tectonic boudins and blocks in garnet–muscovite and muscovite schist of sedimentary origin. Metamorphic zircons from two garnet pyroxenite samples yielded a SHRIMP mean 206Pb/ 238U age of 489.9 ± 3.1 Ma. This age is interpreted to reflect the time of Early Ordovician peak metamorphism and ongoing subduction in the area as constrained by geological data and suggests that HP metamorphism was related to subduction. Exhumation of the HP rocks occurred between 490 and ∼475 Ma, based on the Early Ordovician age of overlap assemblages. Detrital zircons from a garnet–muscovite schist enclosing the pyroxenites were dated by LA-ICP-MS and range in age from 694 to 2557 Ma. They suggest a maximum late Neoproterozoic age for deposition of the sedimentary protolith and derivation from a continental source including Neoproterozoic to Archaean crustal components. Granodiorites with chemical characteristics of island arc rocks intruded into Proterozoic gneisses in the NE of the HP belt, and magmatic zircons from one sample yielded a SHRIMP 206Pb/ 238U mean age of 508.3 ± 3 Ma. This signifies the existence of a Cambrian magmatic arc. The early Palaeozoic age of the HP garnet pyroxenites indicates that the Anrakhai block is not part of a extensive Precambrian microcontinent, as previously thought, but represents a package of tectonically interlayered slivers, made up of Precambrian basement and early Palaeozoic rocks. Stacking of these heterogeneous domains may be due to subduction of continental crust, mutual underthrusting of continental and ophiolitic rocks, wedge extrusion of HP rocks, and strike-slip deformation in a subduction and/or collision setting. The Anrakhai collision zone is part of larger accretionary belt which formed by the end of the Early Ordovician and may extend from the Kyrgyz northern Tianshan to the Kokchetav area in northern Kazakhstan.

Kinematic vorticity flow analysis and 40Ar/ 39Ar geochronology related to inclined extrusion of the HP–LT metamorphic rocks along the Zagros accretionary prism, Iran

The Zagros accretionary prism in southwestern Iran is exposed along the NW–SE trending of the Zagros Thrust System and inclined Zagros transpression zone. This accretionary prism consists of two units: the upper sedimentary mélange unit on top and the high-pressure metamorphic mélange unit at the bottom. Both units show characteristics of a tectonic wedge. The upper unit consists of type-I, II and III mélanges which display S-, C- and C′-type shear-band cleavages, quartz ribbons and rectangular or fish-head boudins. The lower unit fabrics display σ-, and δ-type porphyroclasts and quartz ribbon mylonites. These fabrics formed from a combination of 60.5% simple shear and 39.5% pure shear. Both components were involved in a lateral exhumation of the high-pressure/low-temperature metamorphic rocks in an inclined transpression wedge-shaped geometry. The estimated kinematic vorticity number ( W k ) was calculated from quartz c-axis patterns, rotation of porphyroclasts and orientation of finite strain with respect to shear zone boundaries. Using the mean estimated W m value of 0.84, the inclination angle for the thrust wedge on top of the NE-subducting Neo-Tethyan oceanic lithosphere is 18°. The 40Ar/ 39Ar plateau ages of early generations of biotite from the lower metamorphic mélange are 119.95 ± 0.88 Ma and 112.58 ± 0.66 Ma. This late Aptian age is related to early thrusting and formation of HP–LT metamorphic rocks. The dating of two amphibole samples from the amphibolite yields a weighted mean age of 91.23 ± 0.89 Ma. This Turonian–Cenomanian age suggests a later metamorphic event associated with subduction and obduction of the Neyriz ophiolite and later lateral extrusion of HP–LT metamorphic rocks along the inclined Zagros accretionary prism.

Diachronous exhumation of HP–LT metamorphic rocks from south‐western Alps: evidence from fission‐track analysis

AbstractNew fission‐track ages on zircon and apatite (ZFT and AFT) from the south‐western internal Alps document a diachronous cooling history from east to west, with cooling rates of 15–19 °C Ma−1. In the Monviso unit, the ZFT ages are 19.6 Ma and the AFT ages are 8.6 Ma. In the eastern Queyras, ZFT ages range from 27.0 to 21.7 Ma and AFT ages from 14.2 to 9.4 Ma. In the western Queyras, ZFT ages are between 94.7 and 63.1 Ma and AFT ages are between 22.2 and 22.6 Ma. The Chenaillet ophiolite yields ages of 118.1 Ma on ZFT and 67.9 Ma on AFT. The combination of these new FT data with the available petrological and geochronological data emphasize an earlier exhumation in subduction context before 30 Ma, then in collision associated with westward tilting of the Piedmont zone.

Raman spectroscopic carbonaceous material thermometry of low-grade metamorphic rocks: Calibration and application to tectonic exhumation in Crete, Greece

We present new Raman spectra data of carbonaceous material (CM) to extend the range of the Raman spectra of CM thermometer (RSCM) to temperatures as low as 100 8C. Previous work has demonstrated that Raman spectroscopy is an excellent tool to describe the degree of graphitization of CM, a process that is independent of pressure but strongly dependent on metamorphic temperature. A linear relationship between temperature and the Raman parameter R2 (derived from the area of the defect band relative to the ordered graphite band) forms the basis of a previous thermometer. Because R2 shows little variability in low-temperature samples, 330 8C serves as a lower limit on the existing thermometer. Herein, we present Raman spectra from a suite of low-temperature (100 to 300 8C) samples from the Olympics Mountains and describe other aspects of the Raman spectra of CM that vary over this range. In particular, the Raman parameter R1 (the ratio of heights of the disordered peak to ordered peak) varies regularly between 100 and 350 8C. These data, together with published results from higher-temperature rocks, are used to calibrate a modified RSCM thermometer, applicable from 100 to 700 8C. Application to low-grade metasediments in the Otago region in the South Island of New Zealand gives temperatures consistent with previous estimates, demonstrating the reliability of the modified RSCM thermometer. We apply the modified RSCM thermometer to 53 samples from Crete to evaluate the role of the Cretan detachment fault in exhuming Miocene high pressure/low-temperature metamorphic rocks exposed there. The metamorphic rocks below the detachment (the Plattenkalk and Phyllite–Quartzite units) give metamorphic temperatures that range from 250 to 400 8C, consistent with previous petrologic estimates. We also demonstrate that the Tripolitza unit, which lies directly above the detachment, gives an average metamorphic temperature of about 260 8C. The modest break in metamorphic temperature in central Crete indicates that the Cretan detachment accounts for only 5 to 7 km of exhumation of the underlying HP–LT metamorphic rocks, which were initially accreted at ~35 km. We argue that the bulk of the exhumation (~28 km out of 35 km total) occurred by pervasive brittle stretching and erosion of structural units above the detachment. D 2005 Elsevier B.V. All rights reserved.

THE HP-LP META-OPHIOLITIC UNIT AND VERRUCANO OF THE CALA GRANDE AREA IN THE ARGENTARIO PROMONTORY (SOUTHERN TUSCANY, ITALY): STRUCTURAL-METAMORPHIC EVOLUTION AND REGIONAL CONSIDERATIONS

Ophiolite-bearing HP-LT metamorphic rocks (Schistes Lustres or Calcschists with ophiolithes Auctt.) are present in southern Tuscany and Tuscan Archipelago (Mt. Argentario Promontory, Giglio Island, Elba Island and Gorgona Island). They are interpreted as fragments of oceanic crust which experienced an initial HP-LT metamorphic evolution during the closure of the Ligurian ocean and successively, mooving eastwards, were enclosed together the Ligurian and Tuscan Units in the Apennine chain. The HP-LT ophiolitic succession of the Mt. Argentario is that of the Cala Grande Unit which is constituted by grey-green to red-violet metapelites, marble, calc- schists, metacherts and associated metamorphic ophiolitic rocks (metaserpentinites, metagabbros and metabasalts). The petrographical-structural studies performed by the authors allow to reconstruct the complex metamorphic and tectonic evolution of such successions. In the metabasites, the primary femic magmatic minerals and those of the oceanic metamorphism are overprinted by: 1) a polystadial HP-LT event in the blueschist facies (Dp, peak conditions P = 7-8kb for T = 340-350°C); 2) a greenschist facies event(s) characterized by the chloritization of the Na-amphiboles, development of a main penetrative schistosity and of top-to-NE and top-to-SW shear bands. In the associated metasediments, the HP-LT event is testified by the schistosity of the NE-vergent Dp event which is overprinted by two greenschists tectono-metamorphic events (NE-vergent Dgs1 and SW-vergent Dgs2). The tectonic slices of Verrucano (Triassic quartz-rich metasediments of the Tuscan Monticiano-Roccastrada Unit) intercalated in the Cala Grande Unit show a similar structural-metamorphic evolution, but with different orientation of the mineralogic lineations within the D1 foliations. This data point to a post-D1 emplacement of the Verrucano slices onto the Cala Grande Unit, probably during the D2 event of the Northern Apennines tectogenesis. The structural-metamorphic evolution of the studied area is also compared to that of other metamorphic successions of the Tuscan Archipelago and of the Southern Tuscany.

3 - SOUTHERN TUSCANY - A - ARGENTARIO PROMONTORY

The Argentario Promontory consists of a complex stack of Tuscan, Ligurian and Ligurian-Piedmontese Units (Lazzarotto et al., 1964; Decandia and Lazzarotto, 1980). From bottom to top they are (Fig. 1): 1. Monticiano-Roccastrada Unit. It comprises, from bottom to top: a- Mt. Argentario Formation. Graphitic phyllites with metasiltstone interbeds grading upwards into grey-whitish quartzites and metaconglomerates, with local graphite-rich phyllitic intercalations. This formation has been correlated to the Upper Carboniferous-Lower Permian successions cropping out along the Mid Tuscan Ridge (e.g. the St. Lorenzo Schists of the Pisani Mts. and Iano Schists and Sandstones) and in eastern Elba (Rio Marina Fm.). b- “Verrucano”. Alternating violet and greenish phyllites with local carbonate nodules and levels (caliche), quartzites and quartzitic white-pink metaconglomerates of Late Ladinian?- Carnian age. This succession, which shows an overall fining- and thinning-upward trend (including many metric/decametric positive cycles), is related to continental braided rivers passing upward to a coastal plain environment (Uncini, 1985). In the syntectonic quartz-rich segregations of the “Verrucano” rocks, Theye et al. (1997) found Mg-carpholite which testifies a HP-LT event (P> 8kb T = 300-400°C). c- Tocchi Formation. It consists of vacuolar, polymictic breccia made up of yellowish carbonatic and grey to green phyllitic clasts. These diagenetic and/or tectonic breccias derive from Carnian siliciclastic-carbonatic lithotypes which constitute the transition from the continental “Verrucano” and the Norian carbonatic shelf deposits. 2. Tuscan Nappe. This non-metamorphic unit is constituted by the “Calcare Cavernoso”: grey and dark grey, vacuolar calcareous and calcareous-dolomitic breccias with local karst structures. At places, these breccias include boulders and slices of stratified calcareous-dolomitic successions. Cataclastic textures are frequently recognisable. 3. Cala Piatti Unit. It is made up of Triassic recrystallised carbonate succession: a. Lower?-Middle Triassic stratified grey-pinkish limestones, b. Middle-Upper Triassic stratified black limestones, c. Upper Triassic? massive or poorly bedded grey dolostones. This succession represents one of the rare evidence of Early?-Middle Triassic marine sedimentation in the Northern Apennines (see also the Punta Bianca-Massa succession: Martini et al., 1986). 4. Cala Grande Unit. It is made up of HP-LT metamorphic rocks. In particular, two lithologic associations are recognisable: a. Varicoloured (reddish and grey-green) phyllites with grey and dark grey calcschists and recrystallised limestones; b. Metaophiolites (gabbros, basalts, serpentinites and ophiolitic breccias). Particularly, in b. HP-LP minerals are present: glaucophanitic amphibole (Mgriebeckite to crossite)+lawsonite+phengitic mica±Na-pyroxene (Gottardi, 1957; Ricci, 1972; Azzaro et al., 1977; Theye et al., 1997). The presence of Fe-carpholite relics within syn-tectonic quartz veins of the metasedimentary lithotypes (Theye et al., 1997) suggests T= 300-350°C and P>7 kb peak metamorphic conditions in agreement with the Blueschists facies minerals of the Metaophiolites. This unit was correlated to the “Schistes Lustres” of the Alpine Corsica (Ricci, 1972; Ricci and Serri, 1975). Polymictic tectonic breccias, mainly made up of elements and slices of “Calcare Cavernoso” and/or “Verrucano” (but locally also of the two uppermost units), separate the Tuscan Nappe from the overlying Cala Piatti Unit and this latter from the Cala Grande Unit. The complex tectonic stack of Mt. Argentario is related to the Oligo-Miocene compressional stage (which produced the piling up of the “Schistes Lustres” onto the Tuscan Units and the metamorphism of the Monticiano-Roccastrada Unit), but also to the post-compressional events (Burdigalian- early Tortonian) linked to the extensional tectonics of the Apennines and the Tyrrhenian Sea area (Bartole, 1995; Carmignani et al., 1995, cum bibl.). During the extensional events, the tectonic lamination of the Tuscan Nappe through low-angle normal faults, and the formation of the Mt. Argentario polymictic breccias occurred.

Pressure solution in siliciclastic HP-LT metamorphic rocks — constraints on the state of stress in deep levels of accretionary complexes

The magnitude of differential stress in deep levels of accretionary complexes along convergent plate margins is poorly constrained by theoretical models and cannot be directly measured. The only direct evidence to infer rheology and state of stress for such crustal settings is the microstructural memory of high-pressure/low-temperature metamorphic rocks recorded during their crustal evolution. Microfabrics in HP-LT metamorphic ( T = 400 ± 50°C, P = 10 ± 2 kbar) phyllites and quartzites of the Phyllite-Quartzite Unit on the island of Crete, southern Aegean, reveal (1) that progressive deformation was by pressure solution (dissolution precipitation creep), (2) that clastic quartz grains in the phyllites show no evidence for crystal plastic deformation during burial and exhumation, (3) that the unilaterally rational (001) mica quartz phase boundaries were sites of strongly enhanced dissolution, and (4) dislocation creep was restricted to a minor role in quartzites poor in mica. Consequently, the magnitude of differential stress in relation to temperature in the phyllites has remained below the level required to drive dislocation creep throughout the history of burial, to a depth of more than 30 km, and subsequent exhumation. Currently available flow laws for quartzite indicate that the differential stress in the phyllites remained well below 15 MPa at temperatures around 400°C at a depth of 30 km. This implies that the effective viscosity in deep crustal levels in forearc settings is much lower than that predicted by conventional models based on flow laws for dislocation creep.