Staurolite Schist Research Articles

Crustal growth during back-arc closure: Cretaceous exhumation history of Cordillera Darwin, southern Patagonia

The Cordillera Darwin metamorphic complex is unique in the Andes in exposing kyanite–staurolite schist north of the Beagle Channel in southern Patagonia. Garnet in amphibolite facies pelitic schists from Bahia Pia has patchy textures whereby some grains consist of clear, grossular-rich garnet with fine-grained S1 inclusion trails truncated by regions of turbid spessartine–pyrope-rich garnet with biotite, muscovite, plagioclase and quartz inclusions. Micron-scale aqueous inclusions in turbid garnet are consistent with recrystallization facilitated by fluid ingress; S2 inclusion trails indicate this was broadly contemporary with the growth of kyanite and staurolite in the matrix. Pseudosection modelling in Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) is used to infer a P–T path dominated by decompression from 12 to 9 kbar at T ≈ 620 °C, coupled with garnet mode decreasing from ∼5% to <1%. U–Th–Pb in situ dating of S2 monazite indicates that staurolite and kyanite growth and thus exhumation was underway before 72.6 ± 1.1 Ma. Contact aureoles developed adjacent to late granite intrusions include sillimanite-bearing migmatites formed at P ≈ 6 kbar after 72 Ma. Metamorphism of southern Cordillera Darwin induced by continental underthrusting beneath the arc, related to closure of the Rocas Verdes back-arc basin, was terminated by thrusting-controlled exhumation, with the rocks at P ≈ 9 kbar by c. 73 Ma and 6 kbar by c. 70 Ma.

Insights into the metamorphic evolution of the Belt–Purcell basin; evidence from Lu–Hf garnet geochronology

We have determined Lu–Hf garnet ages from spatially separated garnet bearing localities in northern Idaho. The Lu–Hf ages are diverse and reflect a progression of Mesoproterozoic metamorphic events. The oldest Lu–Hf garnet age determined in this study is 1463 ± 24 Ma for garnet within a kyanite schist exposed in the core-zone of the Boehls Butte metamorphic complex. A garnet schist from the Priest River complex yields a well-defined age of 1379 ± 8 Ma. A garnet–staurolite schist, a garnet–mica schist, and a gem-grade Idaho star garnet sample all from the general vicinity of Clarkia, Idaho, yield ages of 1064 ± 10, 1081 ± 20, and 1078 ± 17 Ma, respectively. A garnet amphibolite, also collected from the Boehls Butte metamorphic complex, yields an age of 1151 ± 41 Ma for garnet porphyroblasts and 137 ± 25 Ma for diffuse overgrowths. The Hf in all of the Proterozoic garnets is extraordinarily radiogenic with εHf values ranging from +1210 to +10546, which attest to both their high Lu/Hf ratios and their great antiquity. All of the samples analyzed in this study are from the rocks of the Mesoproterozoic Belt–Purcell Supergroup or its basement. These data provide evidence of polymetamorphism in northwestern Laurentia during the Mesoproterozoic.

A buried Palaeoproterozoic spreading ridge in the northern Nagssugtoqidian orogen, West Greenland

Abstract Ophiolitic rocks on two small island groups in the northern Nagssugtoqidian orogen, West Greenland, shed new light on the eastern Laurentian Palaeoproterozoic plate-tectonic collage. The islands expose amphibolite-facies (520–550 °C, 2.6–3.0 kbar) tholeiitic pillow lava, black chloritic shale, manganiferous banded iron formation (BIF), podded chert, jasper, graded andalusite–staurolite schist with numerous sills, and terrigenous sandstone, an association that occurs today in oceanic spreading zones undergoing burial in forearc trenches. Deformation is weak, with upright folds preserving original younging. U–Pb detrital zircon ages indicate derivation from Archaean, 1890 Ma, and 1850 Ma sources. Occurring north of the Archaean Aasiaat domain that partly escaped Nagssugtoqidian deformation, the sequence pinpoints a new SSE-dipping Palaeoproterozoic subduction zone that links up in the NE with the Disko Bugt suture at the Nagssugtoqidian–Rinkian boundary. This solves an enigma of the previous plate-tectonic model, in which the Arfersiorfik magmatic arc in the central Nagssugtoqidian orogen was located within its own suture zone, and helps to explain why the Nagssugtoqidian–Rinkian system is so broad. It also explains previously recognized geochemical and Pb-isotopic compositional breaks in orthogneisses in the Kangaatsiaq region, and greatly simplifies correlation with adjacent eastern Canadian orogens in Baffin Island and Labrador.

P–T–t–D paths of Everest Series schist, Nepal

AbstractU(–Th)–Pb geochronology, geothermobarometric estimates and macro‐ and micro‐structural analysis, quantify the pressure–temperature–time–deformation (P–T–t–D) history of Everest Series schist and calcsilicate preserved in the highest structural levels of the Everest region. Pristine staurolite schist from the Everest Series contains garnet with prograde compositional zoning and yields a P–T estimate of 649 ± 21 °C, 6.2 ± 0.7 kbar. Other samples of the Everest Series contain garnet with prograde zoning and staurolite with cordierite overgrowths that yield a P–T estimate of 607 ± 25 °C, 2.9 ± 0.6 kbar. The Lhotse detachment (LD) marks the base of the Everest Series. Structurally beneath the LD, within the Greater Himalayan Sequence (GHS), garnet zoning is homogenized, contains resorption rinds and yields peak temperature estimates of ∼650 ± 50 °C. P–T estimates record a decrease in pressure from ∼6 to 3 kbar and equivalent temperatures from structurally higher positions in the overlying Everest Series, through the LD and into GHS. This transition is interpreted to result from the juxtaposition of the Everest Series in the hangingwall with the GHS footwall rocks during southward extrusion and decompression along the LD system. An age constraint for movement on the LD is provided by the crystallization age of the Nuptse granite (23.6 ± 0.7 Ma), a body that was emplaced syn‐ to post‐solid‐state fabric development. Microstructural evidence suggests that deformation in the LD progressed from a distributed ductile shear zone into the structurally higher Qomolangma detachment during the final stages of exhumation. When combined with existing geochronological, thermobarometric and structural data from the GHS and Main Central thrust zone, these results form the basis for a more complete model for the P–T–t–D evolution of rocks exposed in the Mount Everest region.

Thermo-tectonic evolution of the North Singhbhum Mobile Belt (eastern India): A view from the western part of the belt

The arcuate North Singhbhum Mobile Belt (NSMB) comprises multiply-deformed greenschist–amphibolite facies phyllites and schists flanking the centrally-located belt of the Dalma meta-igneous suite of rocks. The NSMB is separated from the Archaean Singhbhum Craton in the south, and the suite of granulite–amphibolite facies gneisses and foliated granites of the Chotanagpur Gneissic Complex to the north, by ductile shear zones. Three fabrics (S 1S, S 2S, and S 3S) formed due to D 1S, D 2S and D 3S deformation events and three metamorphic stages (M 1S: pre-S 2S; M 2S: syn-S 2S; and M 3S: syn/post-S 3S) were identified in phyllites and schists to the south of the Dalma meta-igneous in the NSMB. Reverse-sense transport along north-dipping imbricate thrusts (S 3S) emplaced the high-grade garnet–kyanite–staurolite schists centrally located in the orogen over low-grade muscovite–biotite–chlorite schists in the foreland. The M 1S stage (∼1.5 Ga; EPMA monazite date) in the high-grade gneisses characterized by high P/ T prograde metamorphism ( P > 10 kbar) was followed sequentially by post-loading prograde heating ( T max ∼650 °C) and exhumation – cooling at amphibolite facies (M 2S) and greenschist facies (M 3S: ∼1.3 Ga; EPMA monazite date) conditions. In the northern part of the NSMB, mica schists fringing the CGC document a northward increase in strain (sequential cleavage formation) and temperature (greenschist to amphibolite facies). The strain–temperature spatial gradient is correlated with crustal shortening and heating ( T ∼620 °C, P ∼7 kbar) synchronously with reverse-sense transport of schists along southerly-dipping thrust planes. The structural framework and thermal structure across the mobile belt is correlated with (a) collisional thickening of the orogen (>1.5 Ga), followed by convective removal of the lithospheric root resulting in post-burial heating and (b) syn-collision and exhumation of the orogen (∼1.3 Ga). The exhumation was caused by outward-directed extrusion of the crustal slices along imbricate foreland-vergent thrusts dipping towards the centre of the orogen.

Palaeoproterozoic metamorphism in the Jeori-Wangtu Gneissic Complex (JWGC), western Himalayas

The Jeori-Wangtu Gneissic Complex (JWGC) exposed as a tectonic window in the Lesser Himalayas represents one of the oldest Gneissic Complex of the Himalayas. Foliated granite and the metapelite constitute the dominant lithologies of the JWGC. The western margin of the JWGC is bounded by a brittle shear while in the east, the tectonic surface is a ductile shear zone. Kyanite schist, chloritoid schist, staurolite schist (St-1), garnet schist and staurolite schist (St-2) are present in a west to east sequence beginning from near to the Jhakhri thrust and up to the contact with the JWGC granite. Mica schist is intermittently present and is the dominant metapelite. Low to medium grade regional metamorphic conditions has been inferred for these rocks. Calc silicate enclaves within the JWGC granite preserve the contact metamorphic effects. These are reflected in development of narrow zones of disequilibrium assemblages of calcareous garnet (grs ∼53), clinopyroxene, K feldspar, calcic plagioclase (An 86), quartz, zoned sphene, zoned allanite, amphiboles, calcite and epidote. Recording of contact metamorphic assemblage of 1.80 Ga granite witin the enclave calc silicates and in the host metapelites over an earlier, relict low to medium grade assemblage indicates that the JWGC preserves palaeoproterozoic metamorphic imprints.

Monazite-epidote reaction in amphibolite grade blackwall rocks

Composite monazite-epidote grains in a staurolite schist protolith have reacted to form apatite, + thorite, with REE enrichment of the during amphibolite facies hydrothermal conditions of blackwall formation, Kvesjoen, east-central Norway. The simplified breakdown reaction is, \[1.8 {+} 1.5 epidote {+} 11.1 H_{2}O {+} 6 Fe_{aq} = 0.6 {+} 1.5 chlorite {+} 1.8 REE_{aq} {+} 11.1 H{+}\] with additional thorite (and a possible Y-oxide phase). The breakdown reaction is explained in terms of areas of utilisation (apatite) and destruction (thorite + chlorite) of the monazite phosphate structure in relation to several interrelated factors: coupled substitutions mainly involving P 5+ , REE 3+ , Th 4+ , U 4+ , Si 4+ , Ca 2+ , possibly (Fe,Al) 3+ ; dissolution (monazite) - reprecipitation (REE-poor apatite) with removal of REE by a fluid boundary layer. X-ray element distribution mapping shows that REE9s, Y, Th and U derived from monazite alteration probably remained within the boundaries of the original composite monazite-epidote grains. Magnesium, Fe, Mn, Cl (mainly in chlorite), F in apatite were introduced to the reaction site by way of a hydrous fluid; REE9s were largely fixed in the unreacted corona ( e.g. along grain boundaries); P, Th, U essentially remained at the reaction site in apatite and thorite. Radial cracks associated with REE-epidote and reacted epidote-monazite enclosed in corundum and staurolite of the blackwall resulted from volume expansion due to retrograde uptake of H 2 O (∼10 wt.%) into areas of inferred structurally damaged REE-epidote and/or formation of apatite + + thorite at monazite-epidote grain contacts. Decrease in bulk rock Ca and enrichment in Al during blackwall development favoured preservation of rather than monazite, and inhibited growth of new epidote.

PTt path in metamorphic rocks of the Khoy region (northwest Iran) and their tectonic significance for Cretaceous–Tertiary continental collision

Metamorphic rocks in the Khoy region are exposed between obducted ophiolites to the southwest and sedimentary rocks of Precambrian–Paleozoic age to the northeast. The Qom formation (Oligocene–Miocene) with a basal conglomerate transgressively overlies all of these rocks. The metamorphic rocks consist of both metasediments and metabasites. The metasediments are micaschist, garnet–staurolite schist and garnet–staurolite sillimanite schist with some meta-arkose, marble and quartzite. The metabasites are metamorphosed to greenschist and amphibolite facies from a basaltic and gabbroic protolith of tholeiitic and calc-alkaline rocks. Geothermobarometry based on the equivalence of minerals stability and their paragenesis in these rocks and microprobe analyses by several different methods indicate that metamorphism occurred in a temperature range between 450 and 680 °C at 5.5 and 7.5 kb pressure. Rims of minerals reveal a considerable decrease of pressure (<2 kb) and insignificant decrease of temperature. The PTt path of this metamorphism is normal. The MFG line passes above the triple junction of Al 2SiO 5 polymorphs, and the average geothermal gradient during metamorphism was from 27 to 37 °C/km, which is more concordant with the temperature regime of collision zones. We infer that crustal thickening during post-Cretaceous (possibly Eocene) collision of the Arabian plate and the Azerbaijan–Albourz block was the main factor that caused the metamorphism in the studied area.

Petrogenesis of andalusite–kyanite–sillimanite veins and host rocks, Sanandaj‐Sirjan metamorphic belt, Hamadan, Iran

AbstractQuartz‐rich veins in metapelitic schists of the Sanandaj‐Sirjan belt, Hamadan region, Iran, commonly contain two Al2SiO5 polymorphs, and, more rarely, three coexisting Al2SiO5 polymorphs. In most andalusite and sillimanite schists, the types of polymorphs in veins correlate with Al2SiO5 polymorph(s) in the host rocks, although vein polymorphs are texturally and compositionally distinct from those in adjacent host rocks; e.g. vein andalusite is enriched in Fe2O3 relative to host rock andalusite. Low‐grade rocks contain andalusite + quartz veins, medium‐grade rocks contain andalusite + sillimanite + quartz ± plagioclase veins, and high‐grade rocks contain sillimanite + quartz + plagioclase veins/leucosomes. Although most andalusite and sillimanite‐bearing veins occur in host rocks that also contain Al2SiO5, kyanite‐quartz veins crosscut rocks that lack Al2SiO5 (e.g. staurolite schist, granite). A quartz vein containing andalusite + kyanite + sillimanite + staurolite + muscovite occurs in andalusite–sillimanite host rocks. Textural relationships in this vein indicate the crystallization sequence andalusite to kyanite to sillimanite. This crystallization sequence conflicts with the observation that kyanite‐quartz veins post‐date andalusite–sillimanite veins and at least one intrusive phase of a granite that produced a low‐pressure–high‐temperature contact aureole; these relationships imply a sequence of andalusite to sillimanite to kyanite. Varying crystallization sequences for rocks in a largely coherent metamorphic belt can be explained by P–T paths of different rocks passing near (slightly above, slightly below) the Al2SiO5 triple point, and by overprinting of multiple metamorphic events in a terrane that evolved from a continental arc to a collisional orogen.

Chronology of deformation, metamorphism, and magmatism in the southern Karakoram Mountains

Research Article| November 01, 2001 Chronology of deformation, metamorphism, and magmatism in the southern Karakoram Mountains James E. Fraser; James E. Fraser 1Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK Search for other works by this author on: GSW Google Scholar Michael P. Searle; Michael P. Searle 1Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK Search for other works by this author on: GSW Google Scholar Randall R. Parrish; Randall R. Parrish 2Department of Geology, University of Leicester, and Natural Environment Research Council Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK Search for other works by this author on: GSW Google Scholar Stephen R. Noble Stephen R. Noble 3Natural Environment Research Council Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK Search for other works by this author on: GSW Google Scholar GSA Bulletin (2001) 113 (11): 1443–1455. https://doi.org/10.1130/0016-7606(2001)113<1443:CODMAM>2.0.CO;2 Article history received: 03 Dec 1999 rev-recd: 09 Aug 2000 accepted: 16 May 2001 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation James E. Fraser, Michael P. Searle, Randall R. Parrish, Stephen R. Noble; Chronology of deformation, metamorphism, and magmatism in the southern Karakoram Mountains. GSA Bulletin 2001;; 113 (11): 1443–1455. doi: https://doi.org/10.1130/0016-7606(2001)113<1443:CODMAM>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract U-Pb dating of metamorphic and igneous rocks from the Hunza Valley and Baltoro regions of the Karakoram Mountains in northern Pakistan addresses the thermal and magmatic evolution of the thickened Asian plate crust before, during, and after the collision of the Kohistan arc and the Indian plate. Crustal thickening and high- temperature, sillimanite-grade metamorphism in the southern Karakoram Mountains followed the collision and accretion of the Kohistan arc during the Late Cretaceous. U-Pb ages of metamorphic monazites from sillimanite gneisses in the Hunza Valley are 63.3 ± 0.4 Ma, ca. 50–52 Ma, and 44.0 ± 2.0 Ma, and monazites from a kyanite-grade schist from the Baltoro region are 28.0 ± 0.5 Ma. Metamorphic monazites from a highly graphitic garnet + staurolite schist from the Hunza Valley yield a crystallization age of 16.0 ± 1.0 Ma. Sillimanite gneisses from the Dassu dome have magmatic zircons of 1855 ± 11 Ma, reflecting a Proterozoic continental crustal source, and metamorphic monazites of 5.4 ± 0.2 Ma. Magmatism was also sporadic; early granodiorite, monzogranite, and leucogranite dikes yield zircon, monazite, and uraninite ages of 50–52 Ma and 35.0 ± 1.0 Ma. Widespread lower crustal melting during the latest Oligocene–early Miocene culminated with emplacement of the Baltoro Plutonic Unit in the Karakoram batholith that cuts deformation fabrics in the high-grade gneisses to the south. The youngest magmatic phase dated is the 9.3 ± 0.2 Ma Sumayar leucogranite pluton. On the basis of detailed structural field studies combined with U-Pb geochronology, sillimanite-grade metamorphism was either a protracted event lasting as long as 20 m.y. (64–44 Ma) or peaked at different times within the lower crust following collision of first, the Kohistan arc, and later, the Indian plate. We also present evidence for southward propagation of peak metamorphism and postmetamorphic thrusting and folding of isograds within the past 5 m.y. Detailed geochronology shows that deformation, metamorphism, and magmatism in the middle and lower crust of the south Asian margin has been occurring within the Karakoram metamorphic complex for more than 60 m.y. Similar processes may also have affected the unplumbed depths of the south Tibetan crust. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Old origin for an active mountain range: Geology and geochronology of the eastern Hindu Kush, Pakistan

Prior to accretion of the Kohistan island arc during the Late Cretaceous and final suturing of India with Asia at ca. 50 Ma, the Hindu Kush mountains along the border of Afghanistan and northwest Pakistan were situated on the active southern margin of Asia. Geology and geochronology of the eastern Hindu Kush range in Pakistan demonstrate that localized crustal melting and leucogranite intrusion took place in the Gharam Chasma area at ca. 24 Ma. More regionally developed and widespread deformation, metamorphism, and magmatism took place before collision between both India and the Kohistan island arc with Asia. Ca. 195 Ma U-Pb monazite ages on a deformed leucogranite dike from the upper Lutkho valley indicate an Early Jurassic phase of crustal melting. U-Pb monazite ages of 135–126 Ma on a staurolite schist from near Gharam Chasma are interpreted as a minimum age for staurolite-grade metamorphism. Within the Tirich Mir fault zone, pegmatite dikes crosscut the staurolite schists. U-Pb dating of uraninites from one of these pegmatite dikes reveals an age of 114 ± 2 Ma. Monazites from the same rock give ages of 125–121 Ma, possibly due to inheritance of older cores. These Jurassic–Early Cretaceous constraints on metamorphism and magmatism relate to subduction and accretion processes, perhaps including the suturing of the Karakoram and Hindu Kush terranes along the Tirich Mir fault. In general, high-temperature, low-pressure metamorphism and subduction-related granitoid magmatism in the eastern Hindu Kush suggest a high thermal gradient in an active-margin setting from Early Jurassic to Cretaceous time.

Compositional variations in metamorphosed sediments of the Littleton Formation, New Hampshire, and the Carrabassett Formation, Maine, at sub-hand specimen, outcrop, and regional scales

Rocks from several outcrops of low-grade Littleton Formation and laterally equivalent Carrabassett Formation were analyzed for major elements and Zr. Compositional variations well outside of analytical uncertainties were observed, in order of increasing magnitude, for the following: 10-gram subsamples from a 2-kg hand sample of visually homogeneous slate; 100-gram samples taken 10 meters apart in the same outcrop; 10-g subsamples from a visually homogeneous quartzite; samples of relict turbidite beds in the same outcrop; and single 100-g samples from seven outcrops. We regard these variations as inherited from the sedimentary protolith with little if any alteration over distance scales of centimeters or greater. All could be modeled successfully as mixtures mainly of clay, quartz, and chlorite-mica, although the compositions of the clay and the chloritemica components had to be adjusted for different sets of samples, within reasonable bounds for sedimentary-diagenetic minerals. Compositional variations and trends in 2-kg samples of higher-grade Littleton rocks are essentially the same as for the low-grade samples. For example, SiO2 and Zr correlate positively, as expected for sedimentary sorting of quartz and zircon from clay, but not for selective metamorphic mobilization of silica. Also found was centimeter-scale separation of K2O from Al2O3 and strong covariation of Zn with Al2O3 in ~10-g subsamples of a staurolite schist, compositional evidence of element transport over distances of a few centimeters. Because the protolith includes at least two major, Al2O3-bearing components in variable proportions, element ratios to Al2O3 can vary such that variation in metamorphic rocks need not be a sensitive indicator of element mobility relative to Al2O3 even if Al is immobile. The population of low-grade rocks encompasses a broad compositional range. Thus, it is not feasible to obtain a sample average with a small standard deviation. It would also be difficult to demonstrate conclusively whether a set of samples was representative of its metamorphic grade, or whether rocks of all metamorphic grades developed from the same combination of protolith compositions and physical characteristics.

Prograde reactions and garnet zoning reversals in staurolite schist, British Columbia: significance for thermobarometric interpretations

Abstract An outcrop of staurolite‐bearing pelitic schist from the Solitude Range in the south‐western Rocky Mountains, British Columbia, was examined in order to determine the nature of prograde garnet‐ and staurolite‐producing reactions using information from garnet zoning and inclusion mineralogy. Although not present as a matrix phase, chloritoid is present as inclusions in garnet and is interpreted to have participated in the simultaneous growth of garnet and staurolite by a reaction such as chloritoid + quartz = garnet + staurolite + H2O.A garnet zoning trend reversal, which is most pronounced with respect to almandine and grossular components, is present in the outer core of garnets. The location of the zoning reversal corresponds to the outer limit of chloritoid inclusions in garnet. As there is no evidence for polymetamorphism, the zoning reversal is interpreted to indicate continued garnet growth by prograde reaction(s) during a single metamorphic event after the exhaustion of chloritoid as a matrix phase.Metamorphic conditions recorded by mineral rim compositions are 550–600° C at 6–7 kbar. Because there is no evidence for partial resorption of garnet during production of staurolite, we interpret these results to represent peak conditions.

Metamorphic evolution of metapelites in the high-pressure terrane of the Rhodope Zone, northern Greece

The Rhodope zone of northern Greece has been involved in an Alpine metamorphic cycle that consists of a high-pressure (eclogite-facies) extensively overprinted under medium-pressure conditions in Eocene time. Abrupt differences in the grade of metamorphism within the Rhodope zone allows its subdivision into a lower and an upper tectonic unit. The pelitic rocks of the lower tectonic are typically gamet - chloritoid t staurolite schists. These rocks preserve the high-pressure paragenesis garnet+ chloritoid +chlorite +phengite + quartz+rutile. Minerals formedduringexhumation are staurolite, muscovite, Fe-rich chlorite and, rarely, biotite and andalusite. In the upper tectonic uni! the metapelites are generally gamet - biotite 1 kyanite gneisses and schists; the high-pressure paragenesis has been partly obliterated during extensive retrograde overprinting. Peak (minimum) conditions of pressure determined for the lower tectonic by use of the phengite geobarometer (for T = 550' to 600'C) are I 3-l 3.5 kbar. P-T estimates for peak of metamorphism of the upper tectonic are uncertain. Textural features and mineral chemical data provide information on metamorphic conditions for various stages of decompression of both units. The exhumation path of the lower tectonic was nearly isothermal to a depth of - I 2 km from the surface. ln the upper tectonic unit exhumation proceeded along a P-T path characterized by cooling. These differences are probably attributed to ttmsting oftle deeper lying, upper tectonic over the lower one at depth during unloading. ln this case, the upper acted as a warm shield and prevented the rocks of the undedying lower from losing heat. Since the P-T path of Rhodope shows important similarities in form with that of other high-pressure terranes (1a., the Westem Alps, the Seward Peninsula in Alaska, the Cyclades in the Aegean Sea), analogous constraints, such as rapid umoofing tectonics or continuous underthrusting ofcold material, can be invoked to explain the cooling that accompanied exhumation.

Early Namaqua low‐pressure metamorphism: deformation and porphyroblast growth in the Zoovoorby staurolite schist, South Africa

This paper describes the deformation and metamorphism recorded in the Zoovoorby staurolite schist, a sliver of pelitic supracrustal material in the 1.3–1.0 Ga eastern Namaqua Province, South Africa. The supracrustal Biesjepoort Group, of which the schist is a part, has undergone at least four phases of deformation (D1–D4). D1 and D2 are preserved in the pelitic schists; staurolite and garnet grew during D1, with staurolite growth persisting to the very earliest D2 crenulation. Andalusite, found in more Mg‐rich schists, grew during D2, overprinting both S1 schistosity and S0 banding. S2 has been rotated both with respect to S1 (preserved as parallel orientated inclusion trails in garnet and staurolite) and with respect to its original orientation (preserved as open D2 crenulations in staurolite). Staurolite is dissolved against S2 in zones of progressive shear. The pseudomorphing of staurolite and andalusite by cordierite, and the preservation of relic grains of both minerals in a wide range of garnet–cordierite pelites throughout the eastern Namaqua Province infers that what is preserved fortuitously in the Zoovoorby locality is representative of the early metamorphic history of a much larger terrane. The high thermal gradients needed to attain estimated conditions of 540–550° C and 1.6–2.4 kbar require substantial heat input. Large amounts of foliated (syn‐D2) granite amongst the supracrustal succession are inferred to be the result of delamination of a thickened crust at a destructive plate margin, generating an elevated thermal gradient during D1–D2 times.

ESTRUTURA GEOLÓGICA DA REGIÃO DE CAJAMAR-JORDANÉSIA, SP

The geological mapping of the Cajamar-Jordanesia region (Sao Paulo State) is outlined in five lithological units belonging to the Sao Roque Group (Upper Proterozoic). Geological, structural, and stratigraphical features support the setting. The structural trend of those units is ENE, with increasing metamorphic grade southwards . Slate and meta-arenite occur at the northern portion of the area and schists at the southern. The basal units are composed by micaschists and some porfiroblastic staurolite schists. The micaschists are in gradual contact with phyllites. Small bodies of amphibolites are found in the metapelites. The intermediate level of the lithological column is constituted by a carbonatic layer with few metapelitic inclusions. The top level is formed by metapsamit es with frequent metapelitic intercalations . Three generations of folds have been recognized. The second one is impressive in any scale of analysis. It is also associated to a zonal crenulation cleavage on metapelites and responsible for the general structure outline of rocks bodies at regional scale. This paper makes reference to the soil collapses which took place in August 1986, in Cajamar town. Those collapses were related to the karstic evolution of the studied area.

C–O–H–N fluids in quartz segregations from a major ductile shear zone: the Berzosa fault, Spanish Central System

Abstract The Berzosa fault is a major ductile shear zone, the Berzosa Shear Zone (BSZ), which separates the ‘Ollo de Sapo’anticline from the inner higher‐grade crystalline axis of the Iberian Hercynian Belt. This shear zone is the site of abundant early kinematic quartz (± Al‐silicates) segregations, rich in fluid inclusions. Host rocks are medium‐grade staurolite schists and sillimanite gneisses.Fluid inclusions in selected quartz segregations across the Berzosa shear zone have been studied by microthermometric methods as well as, in some instances, by Raman analysis. The recorded fluid inclusion history begins at the end of an intense secondary recrystallization period during late‐peak metamorphic conditions and lasts until late in the uplift history of the zone.Three types of inclusions have been found, which in a time sequence are: CO2± H2O; H2O+salt (B‐type); and, N2+CH4. Three types of B inclusion may be distinguished in turn, depending on whether they were trapped during an earlier dynamic‐recovery phase (B1‐type), formed later as intergranular trails (B2‐type), or were trapped apparently along with N2+CH4 in clusions from a heterogeneous fluid (B3‐type).Considerations from isochores confirm that CO2± H2O inclusions were trapped during late‐peak and high‐T retrograde metamorphic conditions (in the range 650–500°C and 5–2 kbar), whilst N2+CH4 inclusions, along with the B3‐type of inclusions, formed at low‐pressures (&lt;1 kbar) and temperatures (± 300°C). B2‐type inclusions were trapped chronologically between these two in a period in which strong inverse lateral thermal gradients developed in the zone. Inferred P‐T paths for the area are convex to the T‐axis.

Retrogression of staurolite schists and the sources of infiltrating fluids during metamorphism

Staurolite-kyanite schists from the Dalradian of Scotland and Ireland show two types of retrograde alteration. In the Irish examples staurolite and kyanite are replaced by muscovite and paragonite, often with margarite and sometimes chloritoid. Plagioclase in these samples may be albitized. In contrast one Scottish sample shows replacement of both staurolite and plagioclase by muscovite. Retrogression resulted from infiltration of fluid, but while in some samples the large number of retrograde phases internally buffered the fluid composition and cation metasomatism cannot be proven, in others the fluid composition was externally controlled and there was cation metasomatism indicating more extensive infiltration. It is demonstrated by the use of activity diagrams showing the relationships between Al-silicate, white mica and alkali feldspar that wholesale alteration to muscovite was most likely caused by fluids moving down-temperature from a granitic or arkosic source. In contrast, if growth of retrograde albite is actually accompanied by Na-metasomatism then the infiltrating fluids are likely to have been moving up-temperature through pelitic lithologies. Large fluid: rock ratios of around 10∶1 are needed to achieve significant alkali metasomatism except in extreme situations of large temperature changes and highly concentrated fluids.

Pressures and temperatures of metamorphism in the eastern Dalradian

Pressures and temperatures have been determined from locations in the eastern Dalradian using a number of different geothermometers and geobarometers. Temperatures range from 550°C in the low kyanite zone to about 800°C in the sillimanite–K feldspar zone. Pressures range from about 5.5 kbar in Glen Esk to 9–10 kbar in the Central Highlands. The barometric calibrations employed are consistent with the kyanite–sillimanite equilibrium. Evidence from mineral zoning suggests that rocks outside the Buchan area (in the Duchray Hill Gneiss, Glens Clova and Avon and in the Schichallion area) underwent a pressure decrease synchronous with the metamorphic peak. Regional temperature variations define a thermal high centred on the Cromar–Glen Muick–Duchray Hill Gneiss area. To the W of this high temperatures decrease rapidly. A sharp thermal break occurs between high grade sillimanite–K feldspar gneisses in Glen Muick and kyanite–staurolite schists to the W. This thermal contrast is attributed to a structural break which may be analogous to a number of other thermal features in the Dalradian.

Metamorphic conditions of staurolite schists of the Ryoke metamorphic belt in the Hazu-Hongusan area, central Japan

Metamorphic conditions of staurolite schists of the Ryoke metamorphic belt in the Hazu-Hongusan area, central Japan