Sillimanite Isograd Research Articles

Gneiss domes of the Danba Metamorphic Complex, Songpan Ganze, eastern Tibet

In this paper we address the formation and exhumation of the Danba Metamorphic Complex (DMC) that represents the deepest structural level of the Songpan Ganze terrane situated along the eastern margin of the Tibetan plateau. The DMC comprises a variety of gneiss domes and offers a unique opportunity to decipher their development during orogenic evolution. For that purpose, PTtD paths of metamorphic rocks sampled at different structural levels have been reconstructed. The DMC is composed of Triassic metaturbidites of the Xikang group, Paleozoic metasedimentary cover and basement of the Yangtze craton. The DMC is structurally marked by transposition of the upright S1 foliation of the Triassic metaturbidites into a NW-SE trending S2 composite foliation dipping to the NE. Transposition is associated with a localized top-to-the-northeast shear zone along the northeastern edge of the DMC and with pervasive top-to-the-southwest shearing from the core to the border of the complex. These structures are consistent with extrusion of the core of the DMC relative to the lower grade Triassic metaturbidites. The position of the biotite isograd overlapping the structural boundary of the DMC suggests that the Triassic metaturbidites have been affected by an increase in temperature as a result of extrusion. Within the DMC, the position of the metamorphic index minerals relative to the composite S2 foliation reveals that biotite, garnet, staurolite and kyanite grew before the transposition into S2, in contrast with sillimanite which crystallizes in the hinge of F2 folds and along the axial planar S2 schistosity. The sillimanite isograd delineates regional-scale overturned F2 folds and cross-cuts the staurolite and kyanite isograds consistent with an increase in temperature during D2. The melt-in isograd characterizes the deepest structural level of the DMC. PT conditions for these metamorphic rocks, determined using pseudosections and conventional thermometry, indicate a temperature increase from 400°C to more than 600°C from the edge to the core of the DMC for a relatively homogeneous pressure ranging from 5 to 6.5kbar suggesting that isograds and isotherms represent the syn-D2 thermal structure of the orogenic crust. Migmatites exposed in the deepest structural level of the DMC yield a pressure significantly lower than the surrounding metamorphic rock suggesting that they crystallized after D2 and after some exhumation of their hosts. Three different types of gneiss domes are distinguished on the basis of their position relative to the isograds, their structural characteristics, and their position relative to the margin of the Yangtze craton. Close to the craton and at the highest structural level, the Gezong dome represents a basement-rooted tectonic slice, in an intermediate position, the Gongcai dome corresponds to a basement-cored nappe, and further away and at the deepest structural level, the Bawang, Cunuchan and Qingaling domes are migmatite-cored domes. The presence at the current-day surface of this variety of gneiss domes reflects the difference in burial of the margin during the Mesozoic Indosinian orogeny.

Geology along the Arniko Highway between Barabise and Kodari (China-Nepal Boarder) area, central Himalaya

The area between Barabise and Kodari in central Nepal along the Arniko Highway is geologically located into Higher Himalayan Crystallines (HHC) and Lesser Himalayan Sequence (LHS) that is separated by the Main Central Thrust (MCT). The HHC consists of amphibolite facies rocks (pelitic schist, psamitic schist, pelitic gneiss and quartzite), while LHS is comprised by green schist to amphibolite facies rocks (phyllite, calcareous phyllite, garnet-mica schist, black schist, quartzite and augen gneiss) in uppermost section and carbonate (dolomite and limestone) with phyllite, and metasandstone in lower section. The MCT in the area is oriented in E-W direction with about 30° dip due north and S-C structure preserved in augen gneiss of LHS characterizes the top-to-south sense of shearing, which could be related to the movement along the MCT. Mineral lineation marked by stretched mica indicates N to NNE direction in both HHC and LHS.
 Metamorphism of inverse grade from biotite at stratigraphically lower most section of Kuncha Formation to garnet at the uppermost section having schist and augen gneiss is obvious close to the MCT in the section. However, the Kuncha Formation contains tiny crystals of garnet in the rocks of greenschist facies. Kyanite and sillimanite isograds are developed in pelitic and psamitic schists, and pelitic schists appeared at the basal part of HHC above the MCT. The transformation of garnet to chlorite at the margin and fractures and formation of chlorite within bulk rocks of the MCT zone and HHC are the indicators of traces of retrograde metamorphism because of dropping in pressure-temperature probably related to post deformation event.

Contrasting Grenville-aged tectonic histories across the Llano Uplift, Texas: New evidence for deep-seated high-temperature deformation in the western uplift

Models of doubly vergent orogens provide an excellent proxy for the Llano Uplift of central Texas, a Grenville-aged belt that consists of two portions with different structural styles, metamorphic grades, degrees of partial melting, and opposite directions of tectonic transport. Six phases of deformation at amphibolite-facies conditions are recorded in both portions of the uplift as a result of continent-continent and arc-continent collision. However, in the western uplift, field mapping and thin section analysis of microstructures and metamorphic mineral assemblages provide evidence for temperatures above the second sillimanite isograd and partial melting during both early and late deformation. These observations correlate well with numerical models for the Alps, which have identified a prowedge and retrowedge in the crust above a subducting slab. The western uplift is coincident with the retrowedge, located at greater depth in the orogenic pile, leading to greater temperatures and more melting as well as opposite vergence from the prowedge. A lack of discrete shear zones and opposite structural stacking and vergence in the western uplift, coupled with apparently greater temperatures and more widespread partial melting, suggest that the western uplift has a different tectonic history from the eastern uplift. Most notably, this study documents widespread and pervasive partial melting during uniformly distributed deformation, as well as abundant granitic intrusions during latest deformation in the western uplift. Analogue models readily accommodate these observations from both eastern and western parts of the uplift in the form of a bivergent orogenic wedge.

Age constraints on the tectonothermal evolution of the Selwyn Zone, Eastern Fold Belt, Mount Isa Inlier

The Selwyn Zone is characterized by multiple ductile deformation and metamorphic events. There is general consensus that early in the Isan Orogeny there were both extensional and thrust events that produced bedding-parallel foliation (S bp). We propose that these were followed by the D 1 event that resulted in a regional steep east–west foliation, and that the Snake Creek Anticline and the dominant S 2a regional foliation formed as upright north–south structures. Local overturning of D 2 structures was due to subsequent localized deformation (D 2b) that resulted in structures with flat-lying axial planes. D 3 and D 4 produced steep upright folds and crenulations with north–northwest and northeast trending axial surfaces. Metamorphism, largely controlled by the emplacement of mafic rocks, is dominantly low pressure, but the P–T–t path is complex. Early growth of cordierite (M 1) followed by andalusite, was succeeded by a clockwise medium pressure loop of ca. 500–600 MPa in the northern Snake Creek Anticline, as indicated by early D 2a kyanite growth. The metamorphic peak during D 2 was around 580 °C and 400 MPa at the sillimanite isograd and reached a maximum of around 670 °C and 400–500 MPa in the migmatitic sillimanite K-feldspar zone at the Cannington and Osborne mines. Previous age determinations for the metamorphic peak of ∼1580–1600 Ma are confirmed with new electron microprobe dating of monazite, whereas an age grouping of 1600–1630 Ma may represent the D 1 (M 2) event. Additional ages in the range 1640–1680 Ma are recorded in albitites from the Soldiers Cap Group, and it is suggested that they formed by infiltration of hydrothermal fluids during extension and the emplacement of mafic sills. This is supported by SHRIMP dating of zircon from a trondhjemite lens in a largely mafic sill complex in the Snake Creek Anticline that provided an age of 1686 ± 8 Ma, also indicating that at least part of the Soldiers Cap Group is older than previously indicated. A zircon SHRIMP age of 1527 ± 4 Ma was obtained for a sample of albitized Saxby Granite. This age also represents the D 4 (M 6) event, and is the timing of the extensive brecciation and albitization of calc-silicate rocks of the Corella Formation in the Selwyn Zone.

Isograd pattern and regional low-pressure, high-temperature metamorphism of pelitic, mafic and calc-silicate rocks along an east – west section through the Mt Isa Inlier

The southern Mt Isa Inlier is characterised by high-temperature/low-pressure metamorphism with metamorphic grade varying from greenschist to upper amphibolite facies. Metamorphism across the inlier occurred in a series of events during the Isan Orogeny (1610 – 1500 Ma), although some of these may be local in extent and related to various granitic intrusions. Localised events occurred prior to the Isan Orogeny during the emplacement of the Wonga and Sybella Batholiths, at ca 1740 Ma and 1672 Ma, respectively. The overall pattern is of a series of north – south amphibolite facies belts separated by zones of mainly greenschist facies. Isograd surfaces were initially shallowly dipping before being steepened by subsequent folding. Pelitic rocks change with increasing grade through chlorite, biotite, garnet, staurolite and/or andalusite, sillimanite, and sillimanite/K-feldspar zones. More magnesian-rich schists grew cordierite in place of garnet, with or without staurolite at higher grade. In the absence of pelitic rocks, regional metamorphic patterns in calc-silicate rocks were defined by the clinopyroxene isograd, which approximately corresponds to the sillimanite isograd. Estimated maximum P – T conditions for the highest grade zones are 400 – 500 MPa at >650°C with late (retrograde) kyanite + chlorite in some areas indicating anticlockwise P – T – t paths. Multiple stages of porphyroblast growth have been documented locally, suggesting a number of thermal events throughout the Isan Orogeny. The metamorphic peak occurred in most parts of the inlier between ca 1595 and 1580 Ma. Most workers correlate the metamorphic peak with regional D2. There is a broad-scale relationship between granites and amphibolite-facies metamorphic rocks, but with the exception of abundant migmatites in the sillimanite/K-feldspar zones, the granite bodies intruded before or after the metamorphic peak. A number of models have been proposed for metamorphism in the Isan Orogeny. Our preferred model implicates the intrusion of large volumes of tholeiitic magmas into the lower crust and advective transfer to the mid-crust via crustal melts that now comprise the migmatites of the sillimanite/K-feldspar zones. In addition, heat from older granites with relatively high values of heat-producing elements probably made a local contribution to the thermal budget.

Orogeny can be very short

In contrast to continent/continent collision, arc-continent collision generates very short-lived orogeny because the buoyancy-driven impedance of the subduction of continental lithosphere, accompanied by arc/suprasubduction-zone ophiolite obduction, is relieved by subduction polarity reversal (flip). This tectonic principle is illustrated by the early Ordovician Grampian Orogeny in the British and Irish Caledonides, in which a wealth of detailed sedimentologic, heavy mineral, and geochronologic data pin the Orogeny to a very short Arenig/Llanvirn event. The Orogeny, from the initial subduction of continental margin sediments to the end of postflip shortening, lasted approximately 18 million years (my). The collisional shortening, prograde-metamorphic phase of the Orogeny lasted 8 my, extensional collapse and exhumation of midcrustal rocks lasted 1.5 my, and postflip shortening lasted 4.5 my. Strain rates were a typical plate-boundary-zone 10(-15). Metamorphism, to the second sillimanite isograd, with extensive partial melting, occurred within a few my after initial collision, indicating that conductive models for metamorphic heat transfer in Barrovian terrains are incorrect and must be replaced by advective models in which large volumes of mafic/ultramafic magma are emplaced, syn-tectonically, below and into evolving nappe stacks. Arc/continent collision generates fast and very short orogeny, regional metamorphism, and exhumation.

PARTIAL MELTING OF THE ASSEMBLAGE SPHALERITE + GALENA + PYRRHOTITE + CHALCOPYRITE + SULFUR: IMPLICATIONS FOR HIGH-GRADE METAMORPHOSED MASSIVE SULFIDE DEPOSITS

The role of Cu, added as CuFeS 2 , and S, added as FeS 2 that decomposed to FeS + S during heating, on the melting behavior of Fe-Pb-Zn massive sulfide deposits has been investigated at 2 MPa. The solidus of the assemblage CuFeS 2 -PbS-FeS-ZnS-S is located at a temperature between 700° and 730°C, at least 70°C below that of the galena-pyrrhotite-sphalerite system. The lowest temperature melt that was present in an analyzable volume in this study (at 750°C) had the average composition of 68.8 at. percent Fe, 6.5 at. percent Cu, 1.7 at. percent Zn, and 23 at. percent Pb. The experimental charges underwent gravity-driven differentiation into two density-stratified crystal-rich zones (sphalerite- and pyrrhotite-rich, respectively) floating on a melt-rich zone. Vesiculation textures developed near the top of the melt-rich portion of the quenched charges, suggesting that the melts produced in this study contained sulfur at a concentration greater than the 1:1 ratio of sulfur to metal cations that typifies the sulfides that crystallized from the melt (pyrrhotite + galena + chalcopyrite). This study demonstrates that the assemblage CuFeS 2 -PbS-FeS-ZnS-S will melt close to the second sillimanite isograd in metapelites, within the upper amphibolite facies, and will produce initial melts that crystallize approximately equal proportions (wt %) of pyrrhotite and galena, with relatively little chalcopyrite and very little sphalerite. The apparent excess sulfur in the melts, in conjunction with the relatively low copper concentrations in the melts developed close to the solidus, suggests that the depression of the solidus relative to that of the galena-pyrrhotite-sphalerite system can be attributed to the solubility of a pyrite component in the melt. These findings indicate that sulfide mobility in a melt phase should be considered in the evolution of all massive sulfide bodies metamorphosed to upper amphibolite facies conditions or higher, and that the presence of a sulfur-rich fluid phase may promote melting during high-grade metamorphism.

GEOMETRY OF ISOGRADIC, ISOTHERMAL, AND ISOBARIC SURFACES: INTERPRETATION AND APPLICATION

An isograd represents a line on a map resulting from the intersection of an isogradic surface with the topography. It is inferred to represent a metamorphic reaction and is inherently diachronous because of the time required to move heat, fluid, and reactions through the rocks. Relief of 500 m or more or reconstruction of post-metamorphic faulting and folding will be essential to estimate the orientation of an isogradic surface. Ductile rocks require that isobaric surfaces dip gently during metamorphism, and they must do so in restored sections. Isotherms can be constructed into cross-sections because the angle between the isograds and isotherms are a function of the P–T slope of univariant reaction curves. Near Mica Creek, British Columbia, we can infer an 1100-bar difference in pressure along a kyanite–sillimanite (ky–sil) isograd, based upon structural relief. The experimentally determined ky–sil P–T curve suggests a P/T slope of ~20 bars/°C. We calculate the apparent variation in P and T along the isograd and reconstruct the isotherm geometry. Dehydration-reaction curves, at relatively high P, have steep P–T slopes, and the associated isogradic surfaces can approximate isothermal surfaces. Some dehydration isograds are “smeared out”, but for regional scales, these latter isogradic surfaces at high P still approximate the isothermal surface geometry. Data on isogradic surfaces can be used to set limits on parts of the P–T grids for pelitic rocks. Locally, bathozone-defining isobaric surfaces can be reconstructed; we suggest a ΔP of 2 kbar for the garnet – biotite – kyanite bathozone.

Distribution of boron in the rocks of central Nepal Himalaya

Boron content in the rocks of central Nepal Himalaya depends upon the lithology and the grade of metamorphism. The concentration of boron is abundant (up to 322 ppm) in the metasedimentary rocks of the Lesser Himalaya. There seems to be a rather good correlation between the boron content in the rocks and the grade of metamorphism. The boron content progressively increases from chlorite to garnet isograds, then it systematically decreases in the staurolite±kyanite, kyanite and sillimanite isograds, respectively. This trend may be related to the inverse metamorphism associated with movement along the Main Central Thrust.
 The Manaslu leucogranite contains very high amount of boron (950 ppm). The enrichment of boron in this rock may be due to the release of boron from the Lesser Himalayan rocks during the partial melting of the Higher Himalayan Crystallines (Tibetan Slab) as a result of the movement along the MCT. Tourmaline from the Manaslu Granite is also highly rich in boron (8460 ppm).

Carbon isotopic thermometry and geobarometry of sillimanite isograd in thermal aureoles: the depth of emplacement of upper crustal granitic bodies

Carbon isotope fractionation between coexisting calcite and graphite (ΔC ) has been studied in metamorphosed limestones from three thermal aureoles around Cretaceous granitic bodies (i.e., Tanohata, Tono, and Senmaya aureoles) in the Kitakami Mountains, Northeast Japan. ΔC in each aureole decreases toward the granitic bodies, and becomes virtually uniform near the sillimanite isograd for metapelites, although calcite has variable isotopic ratios reflecting the original sedimentary compositions. The relationships indicate that isotopic equilibrium has been attained in metamorphosed limestone of sillimanite grade. Estimated ΔC at the sillimanite isograd is similar in the Tanohata and Tono aureoles, but different in the Senmaya aureole with smaller carbon isotopic fractionations. From the temperature dependence of ΔC and the negative dP/dT of andalusite–sillimanite equilibrium, we conclude that the sillimanite isograd in the Senmaya aureole was under higher temperature and lower pressure than in the other two localities. Temperatures at the sillimanite isograd are estimated by using existing calibrations of carbon isotopic exchange between calcite and graphite, whereas pressures are estimated from carbon isotopic temperatures and the andalusite–sillimanite equilibrium (Holdaway and Mukhopadhyay 1993a). Consistency of the P–T estimates is examined in the light of phase equilibria in the pelitic system. The estimated pressures at the sillimanite isograd are at about 2.1–2.7(±0.2) kbar for the Tanohata and Tono aureoles and less than 1 kbar for the Senmaya aureole, respectively. Geobarometry of sillimanite isograd in thermal aureoles indicates a marked difference in the depth of solidification of upper crustal granitoids: the Senmaya pluton has intruded and solidified at a very shallow level of less than 4 km whereas the Tanohata and Tono plutons are more deep-seated (ca. 8–10 km). The method can also be an effective tool in studying low-pressure type metamorphism in which geothermobarometry using garnet is not always applicable.

Metamorphic, Thermal, and Tectonic Evolution of Central New England

Mountain, Skitchewaug and Big Staurolite nappes. Reactivation of A new, detailed tectonic model is presented for the Acadian orogenic this fabric during thrusting is recorded in some rocks of the Big belt of central New England (Vermont and New Hampshire) that Staurolite nappe by rotated garnets that grew during near-isothermal accounts for a wide range of petrological and structural observations. loading. Only the sillimanite isograd crosses the Fall Mountain– Three belts are considered: the Eastern Vermont, Merrimack, and Skitchewaug nappe boundary. Metamorphic breaks across the Skitintervening Bronson Hill belts. Specific observations in eastern chewaug–Big Staurolite nappe boundary, at the base of the Big Vermont that are accounted for in the model include the following. Staurolite nappe, and at the margin of the Keene and Alstead domes P–T paths are clockwise with maximum pressures near the Athens, require post-metamorphic thrusting when P–T conditions were in Chester, and Strafford domes of 8–11 kbar, but with maximum the greenschist facies. These observations can be explained by a pressures decreasing to 3–5 kbar at the boundary with the Bronson relatively simple model involving in-sequence thrusting from east to Hill belt. Differential exhumation of the Vermont domes relative to west commencing in central New Hampshire at 400–410 Ma. the rocks in easternmost Vermont is required by the recorded differences Preservation of the low-grade belt along the Vermont–New Hampin maximum pressure (5–6 kbar; 15–20 km) and the present-day shire border requires that crustal thickening in Vermont was not geographical separation (7–10 km). Specific observations in New caused by emplacement of New Hampshire nappes onto eastern Hampshire that are explained include the following. P–T paths in Vermont and that the nappes of western New Hampshire had time the Merrimack belt are counter-clockwise with maximum pressures to cool before final juxtaposition against the low-grade belt. Cooling of 4–5 kbar and are related to high regional heat flow and heat ages constrain this final juxtaposition to have occurred in the transfer by early Acadian plutons. P–T paths in the Bronson Hill Carboniferous, suggesting that the Acadian was a prolonged event belt are intimately associated with structural position. An early spanning as much as 100 Myr. contact metamorphism is evidenced in the Skitchewaug and Fall Mountain nappes near contacts with the early Acadian Bethlehem gneiss (>400–410 Ma). Peak metamorphic temperature rises upwards in the nappe sequence (an inverted metamorphic sequence)

Thermobarometric constraints on the tectonothermal evolution of the East Humboldt Range metamorphic core complex, Nevada

The East Humboldt Range of Nevada provides a record of deep-crustal tectonic processes in a classic Cordilleran metamorphic core complex located in the hinterland of the Late Cretaceous to early Tertiary Sevier orogenic belt. New constraints reported here on the metamorphic history of this terrane suggest an overall clockwise pressure-temperature-time ( P-T-t ) path that began with deep tectonic burial and metamorphism at kyanite + staurolite + garnet grade before Late Cretaceous time (possibly in Late Jurassic time?). Subsequently, higher-temperature Late Cretaceous peak metamorphism overprinted this event, resulting in widespread partial melting and leucogranite injection contemporaneous with emplacement of a large-scale recumbent fold (the Winchell Lake nappe). A new 207Pb*/206Pb* date of 84.8 ± 2.8 Ma on syntectonic leucogranite from the hinge zone of this fold constrains the age of this major phase of tectonism. Metamorphism at this time probably reached the second sillimanite isograd, at least at deep structural levels, with peak P-T conditions of 800 °C and >9 kbar. High-grade conditions persisted during extensional tectonic denudation throughout much of Tertiary time. In conjunction with previously published work, the petrologic and thermobarometric results reported here for the northern East Humboldt Range delineate a steeply decompressional P-T trend that extends from ∼9 kbar and 800 °C to 5 kbar and 630 °C. In the light of decompressional reaction textures, microstructural evidence, and previously published thermochronometric results, we interpret this trend as a P-T-t path for Late Cretaceous to Oligocene time. At least 2 kbar of this decompression (equivalent to at least 7 km of denudation) occurred in Late Cretaceous to early Tertiary time. This interpretation supports the idea that tectonic exhumation of deep-crustal rocks in northeastern Nevada began during or immediately after the closing stages of the Sevier orogeny. Finally, the steepness of the proposed P-T-t path implies a thermal evolution from a colder to a much hotter geotherm, a circumstance that probably requires plastic thinning of the lower plate in addition to brittle attenuation and removal of the upper plate during Tertiary extension.

北部北上山地，田野畑深成複合岩体接触帯の変成作用 白亜紀深成岩類マグマ溜りの深度の解明に向けて

The Tanohata plutonic complex, northern Kitakami Massif, Northeast Japan, is an early Cretaceous zoned pluton mainly composed of hornblende-biotite granodiorite. The complex is in a wall contact relation in its western margin and in a roof contact relation in the northern part of the complex, with surrounding sedimentary rocks of the Jurassic accretionary complex. The width of thermal aureole is about 5-7 km in the “western marginal area”. Sedimentary rocks in the “northern roof pendant area”, with an extension about 10×8 km, are thoroughly metamorphosed extensively. The aureole is divided into four progressive mineral zones, namely, the biotite, cordierite, cordierite-K-feldspar and garnet-cordierite zones, based on the paragenetic changes of pelitic rocks in the western marginal area. In the northern roof pendant area, only the cordierite-K-feldspar and garnet-cordierite zones are recognized, and the width of each zone is several times larger than that in the western marginal area. The distinct development of high-temperature zones in the northern roof pendant area indicates a selective upward heat transfer during cooling of a granodioritic magma chamber of the Tanohata complex. Conditions of metamorphism are estimated from the biotite-muscovite-cordierite-sillimanite-K-feldspar equilibrium at the sillimanite isograd, the Fe-cordierite-almandine-sillimanite equilibrium in the garnet-cordierite zone and the garnet-biotite geothermometry. The estimated temperatures are at about 560°C at the sillimanite isograd, and in a range of 650-750°C in the garnet-cordierite zone, at pressures of 2-3 kbar. The maximum temperatures of the Tanohata aureole are similar to those of the highest temperature zone of the Tono contact metamorphic aureole in the southern Kitakami Massif. The result implies that the thermal properties of magma and the surrounding wall rocks are quite similar between the Tanohata and Tono plutons. The estimated pressures of contact metamorphism are also quite similar between Tanohata and Tono aureoles, indicating that these two granodioritic bodies both emplaced and solidified in a deep part of upper crust (7-11 km) during early Cretaceous magmatism in the Kitakami Massif.

Quartz-sillimanite leucosomes in high-grade schists, Black Hills, South Dakota: A perspective on the mobility of Al in high-grade metamorphic rocks

In the Black Hills, South Dakota, anatectic migmatites that formed above the second sillimanite isograd comprise mesosomes dominated by quartz, biotite, sillimanite, plagioclase, and sporadic poikiloblasts of K-feldspar, melanosomes dominated by biotite and sillimanite, and leucosomes. Some leucosomes have near-minimum granite composition, but many lack alkali elements and have fibrolitic sillimanite instead of feldspar. The proportion of Si/Al is the same in both leucosome types. The sillimanite-containing leucosomes include fluorapatite, which indicates high activity of F. The lack of alkali feldspar in these leucosomes is attributed to its instability relative to sillimanite + quartz in the presence of fluids with high a H + / a K + ratios that evolved from the crystallizing leucosomes. It is suggested that quartz-aluminosilicate segregations and veins in other high-grade terranes may also represent pegmatitic melts in which alkali feldspar crystallization was inhibited by high activities of hydrogen, resulting in loss of alkali elements.

Comparative petrology of gneissic rocks in southwestern Newfoundland

In southwestern Newfoundland, pelitic migmatites of the Meelpaeg Subzone of the Gander Zone are separated by faults and plutons from metasedimentary rocks of the Port-aux-Basques gneiss complex (PBGC). The PBGC is a polymetamorphic sequence of amphibolite-facies, pelitic, semipelitic, and psammitic rocks (and associated metabasic dykes). Maximum metamorphic grade surpassed the first sillimanite (i.e., staurolite-consuming) isograd. Metamorphic conditions approached 650–700 °C at Pmax approximately 6.5–8.5 kbar (1 kbar = 100 MPa).The Meelpaeg gneisses also include sillimanite-grade, two-mica rocks, but they lack the Barrovian mineralogy (e.g., kyanite, staurolite, rutile) characterizing parts of the PBGC. The Meelpaeg rocks attained temperatures similar to those of the PBGC, but confining pressure was substantially lower (approx. 4 kbar), indicating uplift from relatively shallow structural levels.Both groups of paragneisses also differ in some aspects of their bulk chemistry (notably CaO/K2O ratios) and their lithologic associations. The Meelpaeg metapelites are less calcic and relatively potassic (mean CaO/K2O = 0.32) compared with their counterparts in the PBGC (mean CaO/K2O = 1.12), but both groups of rocks have similar bulk Fet/(Fet + Mg) ratios (mean XFe ≈ 0.75). In contrast with the PBGC, which contains abundant metabasites and thin coticule-like (garnet + quartz) seams, the Meelpaeg metapelites are associated with biotite + garnet "tonalitic" gneiss and, despite their relatively lime-poor composition, calc-silicate layers and pods.In terms of contrasting lithologic associations and bulk chemistry, paragneiss of the PBGC is distinct from gneissic rocks in the Meelpaeg Subzone. This underscores difficulties in relating rocks in the Port-aux-Basques area to well-established lithotectonic entities elsewhere in Newfoundland. Despite apparent differences in their protoliths and contrasts in metamorphic pressure, available U–Pb data suggest that high-grade metamorphism in both areas occurred during the middle Silurian.

Proterozoic low‐pressure/high‐temperature metamorphism and an anticlockwise P–T–t path for the Hazeldene area, Mount Isa Inlier, Queensland, Australia

In the Hazeldene area, situated in the Mount Isa Inlier, Queensland, the metamorphic grade changes from chlorite zone, through biotite and cordierite zones, to sillimanite/K‐feldspar zone.Microstructural studies of rocks near the sillimanite isograd demonstrate that cordierite grew early during the development of a steep foliation (S2), was replaced by biotite, andalusite and sillimanite at the metamorphic peak late in S2, and in turn by kyanite + chlorite adjacent to localized small post‐D2 shear zones. Although the anticlockwise P–T–t path is well defined, the precise P–T conditions are uncertain because of problems with experimental and thermodynamic data. The best estimate for the metamorphic peak for rocks close to the sillimanite isograd is around 600° C at 4 kbar.The metamorphism has been dated at 1544 Ma, and was synchronous with a major crustal shortening event. Because proposed extensional events occurred more than 60 Ma earlier, their contribution to the peak metamorphic thermal perturbation would have been insignificant. The syn‐metamorphic Mica Creek Pegmatites, the abundance of high heat‐producing elements in the nearby pre‐D2 Sybella Granite, and advective heat by fluids which caused considerable metasomatism in the Hazeldene area, may have each contributed to the thermal budget. However, the metamorphic thermal gradient may be 80°C km‐1 or higher, strongly suggesting a local magmatic control. As none are known in the area, such syn‐metamorphic plutons would have to lie beneath the exposed high‐grade rocks.

The Anvil aureole, an atypical mid-Cretaceous culmination in the northern Canadian Cordillera

The mid-Cretaceous Anvil batholith, in south-central Yukon near Faro, intrudes Upper Proterozoic to upper Paleozoic strata of the Cordilleran outer miogeocline. From previous work, it was unclear whether biotite, andalusite–staurolite, and garnet isograds near the pluton resulted from pre-Devonian regional metamorphism and subsequent arching in a structural culmination or from mid-Cretaceous instrusion. The present study has documented biotite, andalusite, staurolite, garnet, and sillimanite isograds concentric to the pluton. Prophyroblast–matrix relationships indicate that peak metamorphism occurred during intrusion, which took place under approximately 3 kbar (300 MPa) pressure and heated country rock to temperatures of 600°–620 °C. The metamorphism is thus compatible with a deep, mid-Cretaceous event. Regional uplift of 10 km is implied by the metamorphic minerals. From cogenetic relationships between some phases of the Anvil batholith and the nearby South Fork volcanic rocks, regional uplift appears to have been completed in a few million years in the mid-Cretaceous. The uncharacteristic aureole suggests that mid-Cretaceous events in this region are atypical of the Cordillera and may reflect a unique tectonic history or position in the orogen.

Unusual Preservation of Sedimentary Structures in Sillimanite-Bearing Metaturbidites of the Damara Orogen, Namibia

The Late Proterozoic Kuiseb Formation (Damara Sequence) of the Khomas Trough in Central Namibia consists of metapsammites (metagraywackes) and metapelites. Within the study area these rocks were metamorphosed under conditions above the first sillimanite isograd. The metamorphic grade is marked by assemblages containing sillimanite, staurolite, muscovite, and biotite in the pelitic schists, which indicate a temperature range of 600-660°C at 3-4 kbar water vapor pressure for the regional metamorphism. The structural setting of the investigated area is characterized by two phases of deformation with folds varying from open asymmetric through tight to isoclinal and with a heterogeneous distribution of strain. Notwithstanding strong deformation and metamorphic recrystallization delicate sedimentary structures have been discovered in several localities. These include cross-stratification, cross-lamination, load structures, ball-and-pillow structures, flame structures, scour surfaces, flute casts and rip-up clasts, associated with graded bedding. A high proportion of the sedimentary structures are arranged in Bouma sequences. The well-preserved sedimentary features, unusual in such high-grade metamorphic rocks, has allowed a detailed fades analysis within the Kuiseb schists, proving that the studied sequence has been deposited by turbidity currents. The organization of the Kuiseb schists into large-scale and small-scale cyclic successions of progradational and retrogradational character as well as the lateral extent of major sedimentary units indicate deposition within an elongate submarine fan system. Paleocurrents measured from flute casts and sets of cross-laminae indicate a sediment transport along the basin axis of the Khomas Trough from the ENE to the WSW. This supports the contention that sediment supply into the trough was derived from an easterly source.

Aluminum silicates in the Mount Raleigh pendant, British Columbia

ABSTRACTIn regionally metamorphosed pelites of the Mount Raleigh pendant, the fibrolite isograd occurs 5km downgrade from the sillimanite isograd. Fibrolite formed from the decomposition of biotite, a reaction that probably resulted from the late‐stage influx of acidic volatiles. In contrast, sillimanite formed by the direct,‘volume‐for‐volume’replacement of andalusite. Andalusite and sillimanite coexist in a 3 km‐wide zone above the sillimanite isograd. Electron probe analyses of these phases reveal low minor element contents and yield KD[=X] values close to unity; the low Fe2O3 contents are compatible with reducing conditions implied by the ubiquity of graphite. Because KD→ 1.0, the zone of coexisting andalusite + sillimanite cannot be attributed to multivariancy resulting from partitioning of minor elements between these phases. Rather, the metastable persistence of andalusite into the sillimanite P‐T stability field is suggested. The modal proportions of sillimanite versus andalusite imply that minimal (<5%) and alusitesillimanite reaction occurred in a zone 1.5km above the sillimanite isograd; in contrast, there was a marked increase in reaction progress immediately above this zone. With an estimated thermal gradient (in the plane of exposure) of approximately 20°C/km, the 1.5 km‐wide zone of nil reaction suggests that the andalusite‐sillimanite equilibrium boundary was overstepped by about 30 °C before significant reaction occurred. Inclusion‐rich areas in andalusite provided favourable sites for sillimanite nucleation; however, the growth of sillimanite may have been impeded by‘pinning’of sillimanite grain boundaries by inclusions.

Tectonic implications of metamorphism and gravity field in the Penokean Orogen of northern Michigan

Published geologic data, together with new metamorphic pressure and temperature data and gravity data, provide strong evidence for northward directed thrusting of Lower Proterozoic rocks in northern Michigan and Wisconsin. Metamorphic studies indicate pressures of 7 kbar at Watersmeet node and 4 kbar at the Peavy node. The Republic metamorphic node, located 30 km to the north of Peavy, attained pressures of 2–3 kbar. All three metamorphic highs reached temperatures in the range of 600°–650°C and are correlated with areas of negative gravity anomalies relative to surrounding regions. In the Peavy high the center of the gravity anomaly lies on the sillimanite isograd, and we suggest that the source of this negative anomaly may be remobilized underlying Archean gneiss. The highest grade rocks of the Republic node lie well within a residual gravity anomaly, which is interpreted to have a very shallow source, and the Watersmeet high lies above gneiss domes which show negative gravity anomalies. A north‐south geologic cross section constructed from a two‐dimensional gravity model, which extends northward from the Dunbar dome in the magmatic arc terrane of Wisconsin, across a terrane boundary interpreted as a suture and marked by the Niagara fault, and across the Peavy metamorphic node, suggests that the Dunbar dome was transported northward onto the continental foreland during the Penokean orogenic event. These north verging fold‐thrust sheets or nappes produced prominent tectonic thickening at the Peavy and Watersmeet, as indicated by the relatively high metamorphic pressures there. The thrust sheets evidently thinned to the north, as suggested by the relatively low pressures at Republic. We suggest that the Archean continental edge, defined by the steep gradient in the regional northeast trending gravity high, lies beneath the magmatic rocks of Wisconsin and thus extends south of the Niagara fault suture.