Surprises from the top of the mantle transition zone

Recent studies of chromite deposits from the mantle section of ophiolites have revealed a most unusual collection of minerals present as inclusions within the chromite. The initial discoveries were of diamonds from the Luobosa ophiolite in Tibet. Further work has shown that mantle chromitites from ophiolites in Tibet, the Russian Urals and Oman contain a range of crustal minerals including zircon, and a suite of highly reducing minerals including carbides, nitrides and metal alloys. Some of the minerals found represent very high pressure phases indicating that their likely minimum depth is close to the top of the mantle transition zone. These new results suggest that crustal materials may be subducted to mantle transition zone depths and subsequently exhumed during the initiation of new subduction zones—the most likely environment for the formation of their host ophiolites. The presence of highly reducing phases indicates that at mantle transition zone depths the Earth’s mantle is ‘super’-reducing.Uo

The great eclogite debate of the Western Gneiss Region, Norwegian Caledonides: The in situ crustal v. exotic mantle origin controversy

An entertaining debate arose in the latter half of the 20th century among scientists working on the spectacular eclogite facies rocks that occur within metamorphic rocks of the Western Gneiss Region (WGR) of the Norwegian Caledonides. It resulted in part from Eskola's influential publication “On the Eclogites of Norway” who concluded, incorrectly, that mafic eclogites within gneisses (external eclogites) and garnetiferous ultramafic rocks within peridotite lenses had a common origin. The debate featured two end‐member positions. One was that all these garnet‐bearing assemblages, regardless of association, had an exotic origin, where they recrystallized at extremely high pressures and temperatures (P–T) in the mantle and then were tectonically inserted upward into the crust. The other was the in situ origin where this recrystallization occurred within the enclosing gneisses during regional metamorphism. Garnet peridotites and pyroxenites have compositions identical to ultramafic xenoliths in kimberlites and define P–T conditions that are appropriate to the upper mantle. Therefore, peridotite lenses were generally (and correctly) interpreted to be mantle fragments. However, some extended this exotic origin to external eclogites, particularly coarse‐grained orthopyroxene‐ (and coesite‐) bearing eclogites, which also formed at extremely high P–T. They noted an apparent pressure and temperature disequilibrium between anhydrous eclogites and the surrounding amphibolite facies gneisses. It was generally accepted that eclogites could form only in “dry” environments (urn:x-wiley:02634929:media:jmg12314:jmg12314-math-0001 << Ptotal). Thus, eclogites had to form within the anhydrous mantle rather than the host hydrous crust. Finally, there was doubt as to whether the necessary P–T conditions could be generated in continental crust, even when tectonically thickened. The arguments for an in situ origin were based largely on external eclogites. Thin sections showed garnet cores with amphibolite facies inclusions and rims with eclogite facies minerals suggesting prograde metamorphism. Similarly, core to rim changes in mineral chemical composition were consistent with increasing P–T. Coesite and microdiamond were found in both eclogites and host gneisses. Finally, thermobarometry showed burial depths increased from SE to NW across the WGR. Breakthroughs occurred when old assumptions were discarded. Eclogite recrystallization actually can occur in the presence of water. Eclogites and garnet peridotite and pyroxenites had completely different histories. They give different ages, formed under different P–T conditions, and have different geochemical fingerprints. The debate was finally resolved when it became generally accepted that continental crust could subduct into the mantle. Thus, it could subduct to eclogite facies depths where, simultaneously, peridotites could be inserted from the overlying mantle wedge. Both sides of the debate were correct! However, eclogites recrystallized “in situ” only because the enclosing crust was deep in the mantle and garnet peridotites did invade continental crust as solids, but only because the crust was below a mantle wedge. The “Great Debate” was fierce at times, but it led to the modern understanding that continental subduction is a vital part of mountain building

Structural, petrological and chemical analysis of syn-kinematic migmatites: insights from the Western Gneiss Region, Norway.

International audienceEvidence of melting is presented from the Western Gneiss Region (WGR) in the core of the Caledonian orogen, Western Norway and the dynamic significance of melting for the evolution of orogens is evaluated. Multiphase inclusions in garnets that comprise plagioclase, potassic feldspar and biotite are interpreted to be formed from melt trapped during garnet growth in the eclogite facies. The multiphase inclusions are associated with rocks that preserve macroscopic evidence of melting, such as segregations in mafic rocks, leucosomes and pegmatites hosted in mafic rocks and in gneisses. Based on field studies, these lithologies are found in three structural positions: (1) as zoned segregations found in high-pressure (HP) (ultra) mafic bodies, (2) as leucosomes along amphibolite facies foliation and in a variety of discordant structures in gneiss, and (3) as undeformed pegmatites cutting the main Caledonian structures. Segregations post-date the eclogite facies foliation and predate the amphibolite facies deformation, whereas leucosomes are contemporaneous with the amphibolite facies deformation and undeformed pegmatites are post-kinematic and were formed at the end of the deformation history. Geochemistry of the segregations, leucosomes and pegmatites in the WGR defines two trends, which correlate with the mafic or felsic nature of the host rocks. The first trend with Ca-poor compositions represents leucosome and pegmatite hosted in felsic gneiss, whereas the second group with K-poor compositions corresponds to segregation hosted in (ultra) mafic rocks. These trends suggest partial melting of two separate sources: the felsic gneisses and also the included mafic eclogites. The REE patterns of the samples allow distinction between melt compositions, fractionated liquids and cumulates. Melting began at high pressure and affected most lithologies in the WGR before or during their retrogression in the amphibolite facies. During this stage, the presence of melt may have acted as a weakening mechanism that enabled decoupling of the exhuming crust around the peak pressure conditions triggering exhumation of the upward-buoyant crust. Partial melting of both felsic and mafic sources at temperatures below 800°C implies the presence of an H2O-rich fluid phase at great depth to facilitate H2O-present partial melting

Rapid Eocene erosion, sedimentation and burial in the eastern Himalayan syntaxis and its geodynamic significance

The lower Bomi Group of the eastern Himalayan syntaxis comprises a lithological package of sedimentary and igneous rocks that have been metamorphosed to upper amphibolite-facies conditions. The lower Bomi Group is bounded to the south by the Indus–Yarlung Suture and to the north by unmetamorphosed Paleozoic sediments of the Lhasa terrane. We report U–Pb zircon dating, geochemistry and petrography of gneiss, migmatite, mica schist and marble from the lower Bomi Group and explore their geological implications for the tectonic evolution of the eastern Himalaya. Zircons from the lower Bomi Group are composite. The inherited magmatic zircon cores display 206Pb/238U ages from ~ 74 Ma to ~ 41.5 Ma, indicating a probable source from the Gangdese magmatic arc. The metamorphic overgrowth zircons yielded 206Pb/238U ages ranging from ~ 38 Ma to ~ 23 Ma, that overlap the anatexis time (~ 37 Ma) recorded in the leucosome of the migmatites. Our data indicate that the lower Bomi Group do not represent Precambrian basement of the Lhasa terrane. Instead, the lower Bomi Group may represent sedimentary and igneous rocks of the residual forearc basin, similar to the Tsojiangding Group in the Xigaze area, derived from denudation of the hanging wall rocks during the India–Asia continental collision. We propose that following the Indian–Asian collision, the forearc basin was subducted, together with Himalayan lithologies from the Indian continental slab. The minimum age of detrital magmatic zircons from the supracrustal rocks is ~ 41.5 Ma and their metamorphism had happened at ~ 37 Ma. The short time interval (< 5 Ma) suggests that the tectonic processes associated with the eastern Himalayan syntaxis, encompassing uplift and erosion of the Gangdese terrane, followed by deposition, imbrication and subduction of the forearc basin, were extremely rapid during the Late Eocene

Late Cretaceous UHP metamorphism recorded in kyanite-garnet schists from the Central Rhodope Mountains, Bulgaria.

In this study, we report the first discovery of microdiamond inclusions in kyanite–garnet schists from the Central Rhodope Mountains in Bulgaria. These inclusions occur in garnets from metapelites that are part of a meta-igneous and meta-sedimentary mélange hosted by Variscan (Hercynian) orthogneiss. Ultra-high-pressure (UHP) conditions are further supported by the presence of exsolved needles of quartz and rutile in the garnet and by geothermobarometry estimates that suggest peak metamorphic temperatures of 750–800 °C and pressures in excess of 4 GPa. The discovery of UHP conditions in the Central Rhodopes of Bulgaria compliments the well-documented evidence for such conditions in the southernmost (Greek) part of the Rhodope Massif. Dating of garnets from these UHP metapelites (Chepelare Shear Zone) using Sm–Nd geochronology indicates a Late Cretaceous age (70.5–92.7 Ma) for the UHP metamorphic event. This is significantly younger than previously reported ages and suggests that the UHP conditions are associated with the Late Mesozoic subduction of the Vardar Ocean northward beneath the Moesian platform (Europe). The present-day structure of the RM is the result of a series of subduction–exhumation events that span the Cenozoic, alongside subsequent post-orogenic extension and metamorphic core complex formation