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Most widespread is the use of the term granulite for light-coloured, quartzofeldspathic, high-grade metamorphic rocks. This meaning for the name was introduced by Weiss Granulite is a high-grade metamorphic rock in which Fe-Mg-silicates are dominantly hydroxyl-free; the presence of feldspar and the absence of primary muscovite are critical, cordierite may also be present. The mineral composition is to be indicated by prefixing the major constituents.

Changes in mineralogy depends very much on protolith, however, production of pyroxene, microcline and sapphirine are most characteristic. Amphiboles and micas both disappear in granulite facies rocks, hornblende dehydrates to form pyroxene and plagioclase. Kyanite and sillimanite are often produced from muscovite and biotite. The presence of orthopyroxene is essential, according to the original definition, hornblende and pyroxene being present in approximately equal amounts.

The presence of Opx is essential according to the original definition. This definition was modified by Mehnert , and plagioclase was added to the characteristic constituents of pyrigarnite. Biotite and cordierite may be present, as well as kyanite and sillimanite. Garnet-bearing granulite. The biotite is partially replaced by chlorite and the plagioclase is partially replaced by sericite.

The garnets are subhedral, fractured, and contain abundant inclusions of quartz, plagioclase, biotite, rutile, zircon, and rare, minute grains of ilmenite Fig. Patches of sericite and quartz with myrmekitic texture are common Fig. The rutile invariably occurs intergrown with chlorite and Zr concentration in rutile occurring within the matrix and as inclusions in garnets is generally below detection limits Figs.

For these reasons, we interpret the rutile as a retrograde product forming after ilmenite, and we believe the principal Ti phase at peak metamorphic conditions was ilmenite. Because of evidence for late re-equilibration of mineral compositions, we restrict our analysis of thermobarometric conditions of equilibration for this sample to inferences based on assemblage alone and not on individual mineral compositions. Garnet and staurolite are texturally early, suggesting that 06T10 records a melting event in which the pelitic rocks contained staurolite.

Allowing some Fe 2 O 3 would lower the activity of ilmenite and extend its stability to higher pressure. Both varieties occur in both the Moose Basin gneiss and in the Layered Gneiss. The amphibolites form generally concordant but locally cross-cutting bodies within the host gneisses and leucogranites; they all have well-developed fabrics.


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Fitz-Gerald evaluated pressures and temperatures for these rocks using the barometer of Kohn and Spear and the thermometer of Dale et al. These results are consistent with the presence of titanite in the assemblage and the absence of garnet in all but the most iron-rich amphibolites in the region. Based on the information presented above, the following observations must be taken into account in a tectonic interpretation of the Archean rocks of the northern Teton Range. Two contrasting gneiss units are present in the northern Teton Range:.

Moose Basin gneiss, composed of metapelitic and mafic rocks. These have reached granulite facies and have partially melted. The pelitic rocks contain zircon cores ca. The Moose Basin gneiss preserves evidence of both F1 and F2 deformation. The Layered Gneiss, composed of quartzofeldspathic paragneiss, interlayered amphibolite and areas of peridotite and gabbro.

Encyclopædia Britannica/Granulite - Wikisource, the free online library

The Layered Gneiss has relatively radiogenic Nd isotopic compositions consistent with derivation from juvenile sources. The Layered Gneiss preserves evidence of both F1 and F2 deformation. Amphibolites, some of which contain garnet, occur in both the Moose Basin gneiss and in the Layered Gneiss. Amphibolite dikes are generally concordant but locally cut the leucogranites and migmatites, suggesting that they were emplaced either during or after the emplacement of the leucogranites. These units are intruded by the Webb Canyon and Bitch Creek leucogranites.

These leucogranites did not experience the earliest folding event, F 1 , which is preserved in both the Moose Basin gneiss and Layered Gneiss. The thermobarometry shows that the rocks from the northern Teton Range record three distinct thermal regimes Fig. Significant melting accompanied both metamorphic events, and some unknown amount of melt was probably lost from the affected units.

From the P-T relations and the geochronology we infer that the Moose Basin gneiss underwent high-pressure granulite metamorphism and melting at This event also produced titanite in amphibolite and gabbro throughout the Teton Range. This event is interpreted as the time of accretion of crustal blocks, including the northern Teton Range, to the Wyoming craton Frost et al. This event may also be responsible for some of the extensive retrogression of the higher grade assemblages observed in the Layered Gneiss and in the Moose Basin gneiss.

In Moose Basin gneiss pelites, both kyanite and sillimanite occur as inclusions in garnets, but sillimanite clearly replaced kyanite as metamorphism and deformation progressed. Approximately 10 m. Migmatitic Layered Gneiss lacks evidence of the early granulite facies metamorphic event M1 , but clearly records evidence of the later amphibolite facies event M2. This clockwise P-T path is likely the product of rapid burial by thrusting, heating, uplift, and exhumation, all characteristic of continent-continent collisions in Phanerozoic time England and Thompson, ; Thompson and England, These dates indicate that the high-pressure metamorphism occurred around 10 m.

Extensive emplacement of leucogranites could have followed the Ma tectonic assembly for up to 10 m. The ages of the leucogranites fall within error of each other, but because some leucogranites show F 2 folding and some do not, Frost et al. The Webb Canyon and Bitch Creek leucogranites, although both trondhjemitic, have distinct geochemical characteristics. Frost et al. The geochemical characteristics of the Bitch Creek gneiss are consistent with water-excess melting in a collision-related overthrust, where a relatively cool, hydrous lower plate releases water into a hotter upper plate.

The Webb Canyon magmas formed by dehydration melting caused when dramatically thickened crust undergoes gravitational collapse and tectonic extension. Because the slope of the dehydration reaction is positive, melts formed by this process can migrate to shallower levels without intersecting their solidus.

The sheet-like form and F 2 fabrics of the Webb Canyon Gneiss are consistent with layer-parallel magma migration during orogenic collapse. Both water-excess and dehydration melting have been called upon to explain leucogranite of the Himalaya Le Fort et al. Five features in the gneisses of the northern Teton Range are characteristic of modern continent-continent collisional orogens. This is typical of high-pressure rocks from the Himalaya and Alps, where rocks subducted to great depths are exhumed as the subduction zone migrates prior to the final collision Rubatto et al.

Such a time scale is typical of the Himalayan orogeny Searle et al. Our results suggest that leucogranite emplacement accompanied tectonic assembly at Ma, and continued for as long as 10 m. The Nd isotopic evidence thus supports the interpretation that the Moose Basin gneiss represents detritus derived from a different, more ancient continental block than the Layered Gneiss, which is derived from relatively juvenile crustal sources, and that these were juxtaposed during collision at high-pressure granulite facies conditions.

Ultramafic rocks within the Moose Basin and Layered gneisses may represent remnants of oceanic crust caught up with the two sedimentary packages during collision. First, crustal rocks must be taken down to depth where they are metamorphosed under high pressure and temperatures conditions. Second, they must then be exhumed rather rapidly in order to preserve peak metamorphic conditions. One might predict that in a hotter Archean Earth, hotter slabs would be weaker and less likely to subduct.

INTRODUCTION

A hotter mantle early in Earth history is likely to have experienced a greater degree of partial melting and partial melting also may have taken place at greater depths. Because water in the source mantle is transferred almost completely into the melt phase, a thicker dehydrated lithosphere is formed, one which is stiffer and stronger than hydrated mantle.

As these thick plates become more dense with age they will reach neutral buoyancy, and then subduct. If Archean lithosphere is strong enough to subduct, why are Archean high-pressure granulites so rare? Thermomechanical numerical models indicate that the ultimate driver is the ratio of the intrinsic buoyancy of the subducted continental crust to side traction forces in the conduit Butler et al. Models by Husson et al. Moreover, when rollback is associated with a decrease in slab dip, the exhumation process becomes more efficient. These studies suggest that exhumation is not expected to accompany vertical tectonic processes that have been called on to explain many Archean terrains.

High-pressure granulites may instead be restricted to those areas where subduction enables exhumation rather than where sagduction or crustal overturn processes occur. We have argued above that the Archean rocks of the northern Teton Range are best interpreted in terms of a collisional orogeny in which fine-grained sediments and ultramafic rocks are transported along a clockwise P-T path to depths of 35 km or greater, then exhumed during orogenic collapse and intrusion of leucogranites.

This distinctly plate tectonic interpretation has not been universally invoked for the other Archean high-pressure granulite terrains shown on Table 1 and Figure In many cases, incomplete preservation or a subsequent metamorphic overprint precludes definitive identification of tectonic process.

For the oldest high-pressure rocks, both plate tectonic and pre-plate vertical tectonic interpretations have been proposed. The oldest high-pressure granulites occur within the 3. Some authors have interpreted these rocks to record a collisional orogeny Nutman et al. Likewise, the high-pressure amphibolites of the Inyoni shear zone, Barberton granite-greenstone terrane have yielded multiple tectonic interpretations.

Bibliography

Moyen et al. They interpreted synkinematic trondhjemites as the result of decompression melting during return flow. Alternatively, Van Kranendonk et al. They suggest that the greenstones were exhumed along a mylonite zone associated with an intrusive event at 3. If so, then exhumation was much later than expected by analogy with modern continent-continent collisions. Most Archean high-pressure granulites are Neoarchean, and one of the best studied also has disparate tectonic interpretations. The granulites of the Assynt block, Lewisian Gneiss Complex, record multiple periods of deformation and metamorphism, making the Archean high-pressure event difficult to interpret.

Johnson et al. However, the authors propose that a process of sagduction is also possible, in which mafic and ultramafic rocks sank into the deep crust due to their greater density compared to underlying, partially molten felsic orthogneisses. The authors suggest that downward flow would be arrested by increasing stiffness of the orthogneiss residua as partial melt was extracted.

In contrast to Archean terranes that have been affected by Proterozoic metamorphism and deformation, the last such events in the Wyoming province were Neoarchean. The lack of subsequent tectonism greatly simplifies the interpretation of Archean events in the northern Teton Range, allowing the peak conditions and clockwise P-T-t path to be identified and the 20 million year cycle of subduction and exhumation to be quantified.

We suggest that the Archean rocks of the northern Teton Range formed by a process involving lateral, collisional orogeny. The metapelitic rocks of the Moose Basin gneiss derived from a ca.

Felsic Granulite

The mafic and ultramafic rocks are interpreted as the remnants of ocean crust consumed prior to collision. Shortly thereafter and for as long as 10 million years following, leucogranites formed by water excess melting and dehydration melting were intruded. This history is analogous to Cenozoic continental collisions such as the Alps Engi et al. A number of authors, including Voice et al. These age peaks correlate with times of supercontinent formation. Brown and Johnson noted that ages of peak metamorphism also cluster at these times, and suggest that metamorphism records the amalgamation of continental fragments into supercontinents.

Neoarchean continent assembly includes the construction of Superior, Sclavia, and Vaalbara Bleeker, Collisional orogeny recorded in the Teton Range supports the hypothesis that plate tectonic processes were involved in the amagalmation of crustal blocks into larger continental masses by the Neoarchean.

Frost, C. Frost, and S.

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Frost and C. We extend sincere thanks to Dave Mogk and to one anonymous reviewer for extremely thorough and very helpful reviews; the paper is greatly improved thanks to their comments and suggestions.


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We also thank Al McGrew for his help as the managing editor for this submission. This manuscript was completed while C. Frost was serving at the National Science Foundation. Precambrian geology of Wyoming and adjacent states showing the location of the Teton Range and its relation to other Archean exposures in the Wyoming province.

Precambrian rocks are exposed in the cores of Laramide uplifts patterned. The southwestern extent of the province is uncertain. The northeastern boundary of the Wyoming province is after Worthington et al. Geologic map across the northern Teton Range showing the relations between the high-pressure granulites Moose Basin gneiss , leucogranites undifferentiated Webb Canyon and Bitch Creek gneisses of Frost et al.

Modified after Love et al. Numbers identify the samples cited in this paper. Two samples of Moose Basin gneiss analyzed for Sm-Nd isotopic compositions are located south of the map area 08T10 and 08T Photos of field relations in the Moose Basin and Layered gneisses. Moose Basin gneiss: A kyanite-bearing pelitic gneiss with leucosome interpreted as the result of partial melting during granulite-facies metamorphism; B granulite-facies assemblages surrounded by amphibolite in mafic gneiss; C partially melted mafic gneiss that contains garnet-bearing leucosomes. Layered Gneiss: D folded paragneiss with layers of leucosome.

Block diagram showing relations among the major units in the northern Teton Range. Key features: A The Moose Basin gneiss consists of pelitic schist and mafic gneiss with the mafic gneiss forming the cores of the F 2 antiforms. The margins of the mafic gneiss bodies are always foliated amphibolites but the core of the antiform may contain granulite assemblages. The F 1 folds occur only as isolated fold axes in the pelitic schist and in the Layered Gneiss.

Major foliation in the area is parallel to the limbs of the F 2 folds. Late amphibolite dikes have locally been boudinaged. All units have been folded into large F 3 folds. Structural data for the northern Teton Range. A Poles to the F 2 foliations in the Moose Basin area. B Lineations mostly L 2 lineations in the Moose Basin area. C Poles to foliations across the whole northern Teton Range. Anderson, A. Rollinson, H. Myers, J. Spencer, K.

Download references. Reprints and Permissions. Contributions to Mineralogy and Petrology Mineralium Deposita Journal of Earth System Science By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate. Article metrics. Advanced search. Skip to main content. Subscribe Search My Account Login. Abstract Pervasive flooding of CO 2 has been proposed as the cause of granulite facies metamorphism that is capable of producing many distinctive characteristics of the deep continental crust: reduced water activity, orthopyroxene-bearing assemblages, depletion of large-ion lithophile LIL elements, and dehydration 1—6.

Rent or Buy article Get time limited or full article access on ReadCube. References 1 Touret, J. Rights and permissions Reprints and Permissions. Nemchin , Martin J. Whitehouse , Simon A.