Ultrahigh-pressure Metamorphic Terrane Research Articles

Fluid Evolution during HP and UHP Metamorphism in Dabie Shan, China: Constraints from Mineral Chemistry, Fluid Inclusions and Stable Isotopes

The Dabie Shan ultrahigh-pressure (UHP) metamorphic terrane is INTRODUCTION located in the eastern part of the east–west-striking Qinling–Dabie The discovery of coesite and micro-diamond as phases orogenic belt in China. A major mylonitized contact zone of produced by deep metamorphism of continental crust 200–300 m width divides Dabie Shan into the South Dabie revolutionized our understanding of continental collision Terrane (SDT) and the North Dabie Complex (NDC). Combined zones and mantle dynamics attending subduction of investigation of major and trace element geochemistry, fluid inclusions, continental lithosphere. In general, we define metaand oxygen and hydrogen isotopes constrains the fluid history during morphic rocks with coesite and micro-diamond as ultrathe metamorphic evolution of the two metamorphic belts, which high-pressure (UHP) rocks, which have been increasingly differ in their fluid and metamorphic evolution. Fluid inclusions in recognized. So far more than a dozen UHP terranes rocks from the SDT are mainly aqueous with varying salinities, have been documented within the major continental whereas those from the NDC are dominated by CO2. Low O collision belts in the Eurasian Plate. values in the SDT rocks (−2·8 to 8·6‰) indicate meteoric The Dabie–Sulu belt in east–central China is the water–rock interactions before UHP metamorphism, whereas rocks largest among the UHP metamorphic terranes found from the NDC show ‘normal’ O values (6·7–9·0‰) with no worldwide. Research activities over the past decade have obvious meteoric water–rock signature. Whole-rock rare earth element documented a number of characteristic features of this (REE) contents correlate with oxygen isotope compositions: samples area, including rapid subduction followed by rapid initial from the SDT have higher REE contents and lower O values, uplift (e.g. Li et al., 1993; Xiao & Li, 1993; Eide et al., whereas samples from the NDC have lower REE contents and 1994), the abundance of hydroxyl-bearing UHP mineral higher O values. During retrograde metamorphism fluids with phases (Okay, 1994; Zhang et al., 1995), very low O different hydrogen isotope compositions interacted with the rocks from values ranging from −15 to 10‰ (Yui et al., 1995, 1997; the SDT. Zheng et al., 1996, 1998, 1999; Xiao et al., 1997; Rumble & Yui, 1998), and the world-record highest Nd (0) values ever measured for eclogite ( Jahn et al., 1996). The nature and mobility of fluids in high-pressure (HP)

Cogenetic relationship of the Yangkou gabbro-to-granite unit, Su-Lu terrane, eastern China, and implications for UHP metamorphism

The Yangkou gabbro-to-granite complex is a petrotectonic unit of the Su-Lu ultrahigh-pressure (UHP) metamorphic terrane in eastern China. It consists of lherzolites, metagabbro–eclogites, metagranodiorites and deformed granites. Whether or not the felsic members of the complex have undergone UHP metamorphism as is recorded in the metagabbro–eclogites is significant to understanding the relationship between eclogite and host rock. The metagabbro–eclogites have low ε Nd (220 Ma) values (−17 to −12) at the time of UHP metamorphism, indicating that their protoliths are old mafic intrusive rocks emplaced in a continental domain. Geochemical and isotopic studies for the Yangkou unit suggest that the protoliths of the unit (except eclogites) represent a cogenetic magmatic suite, produced by differentiation of a mafic parental magma. The parental magma was derived from melting of a long-term enriched mantle, and was emplaced into a crustal level during continental extension at c . 700–800 Ma. The cogenetic relationship for the protoliths of the unit suggests that the metagranodiorites and deformed granites were also subducted to extreme mantle depths along with the metagabbro–eclogites at c . 220 Ma. The lherzolites, however, are genetically not related to the mafic protoliths; they represent ‘mantle xenoliths’ of tectonic origin.

Ultrahigh-pressure (UHP) low-Al titanites from carbonate-bearing rocks in Dabieshan-Sulu UHP terrane, eastern China

Titanites previously reported from high-pressure rocks commonly contain high Al and F contents. This finding has led previous workers to conclude that high Al and F in titanite can expand its stability field to high-pressure and ultrahigh-pressure (UHP) conditions. In this study, a coesite inclusion was identified in titanite of aragonite- and jadeite-bearing gneiss from the Dabieshan UHP metamorphic terrane of eastern China. We also found UHP titanite in carbonate-bearing garnet clinopyroxenite from the Sulu UHP terrane. Interestingly, both types of the UHP titanite mentioned above contain low Al and F contents. Therefore, our data indicate that the high-Al and F contents are not necessary criteria for stabilizing titanite to UHP conditions. It is inferred that the UHP titanite formed via the reaction TiO 2 + CaCO 3 + SiO 2 = CaTiSiO 5 + CO 2 , and that the presence of the UHP assemblage aragonite + rutile + clinopyroxene + coesite in the host rock and a very low X CO₂ value in the coexisting fluid are crucial conditions for the formation of these UHP titanites

Coesite exsolution from supersilicic titanite in UHP marble from the Kokchetav Massif, northern Kazakhstan

Coesite exsolved from supersilicic titanite was discovered in an impure calcite marble at Kumdykol in the Kokchetav UHP (ultrahigh-pressure) metamorphic terrane, northern Kazakhstan. This impure marble consists mainly of calcite, K-feldspar, diopside, and symplectites of diopside + zoisite, with minor amount of titanite, phengite, and garnet. No diamond was found in the marble. Coesite and quartz, which have needle or platy shapes measuring about 20-60 mm in length, occur as major exsolved phases in the cores and mantles of titanite crystals with minor calcite and apatite. The strongest Raman band for the coesite needles and plates was confirmed at about 524 cm -1 with a weak band at about 271 cm -1 . To estimate the initial composition of the titanite before coesite exsolution, exsolved phases were reintegrated by measuring their area fractions on digital images. The highest excess Si in titanite was thus determined to be 0.145 atoms per formula unit (apfu). This composition requires a pressure higher than 6 GPa on the basis of phase relations in the system CaTiSiO 5 -CaSi 2 O 5 . This pressure is consistent with other evidence of high pressure in the same marble, such as 1.4-1.8 wt% K 2 O and over 1000 ppm H 2 O in diopside. Supersilicic titanite and coesite exsolution also indicate that SiO 2 exsolution occurred in the coesite stability field during exhumation of the UHP metamorphic unit.

Discovery and implication of P2 1/ n crystal structure on a nano-scale in single jadeite crystals

We present the results of high-resolution transmission electron microscope (HRTEM) studies on jadeite crystals of jadeite quartzite from the Dabie ultra-high-pressure (UHP) metamorphic terrane, China. Although jadeite is known to have only C2/ c crystal structure, [100], [110], [112], [010] diffraction patterns and HRTEM images reveal C2/ c and P2 1 /n structures in 500 nm microdomains of single jadeite crystals, that we recognized in all the above four orientations. The cell parameters of C2 /c crystal structure correspond to those of jadeite, whereas the cell parameters of P2 1/ n structure are equivalent to those of omphacite. The presence of the P2 1/ n structure shows that jadeite partially changed into omphacite on a nano-scale during retrograde metamorphism and that this reaction failed to reach equilibrium. It suggests uplift of the host rock during exhumation accompanied by complex non-equilibrium retrogressive process in the jadeite, with significant implications for the derivation of P– T paths and tectonic interpretation of the Dabie UHP metamorphic terrane.

Overpressures induced by coesite-quartz transition in zircon

More than 120 coesite and few polycrystalline quartz (coesite pseudomorph) inclusions were identified by Raman microspectroscopy in zircons of various kinds from gneissic rocks located in the Dabieshan and Sulu ultrahigh-pressure (UHP) metamorphic terranes in eastern China. The coesite inclusions have undergone, to various degrees, the coesite-quartz transition. Raman spectra of coesite and subsidiary quartz inclusions show various shifts, which are closely correlated to the extent of the transition. The coesite-pseudomorph inclusions in zircon show the highest Raman shift. Calibrations based on the experimental work of Hemley (1987) yielded inconsistent present-day overpressures (0–24 kbar) in coesite inclusions inside zircons. The availability of fluids, which resulted in the pervasive regional retrogression of the UHP gneisses during later exhumation stages, is regarded as the main factor controlling the extent of the coesite-quartz transition. This study underscores the need for the elastic model applied by previous researchers to explain the preservation of the coesite inclusions in rigid host minerals.

Kinetics and Dynamics of Development of the Dabie–Sulu Uhpm Terranes Based on Geophysical Evidences

AbstractThe Dabie–Sulu region, located in the east part of the central orogenic belt of China, has the largest ultrahigh pressure metamorphic (UHPM) zone in the world. Based mainly on geophysical data newly obtained in the region, this paper shows the sophistication of the evolution and dynamics of the Dabie–Sulu UHPM belt. The subduction of the Dabie–Sulu terranes after collision between the Sino‐Korean and Yangtze cratons induced the ultrahigh pressure metamorphism. A local extension occurred in Dabie‐Sulu region after the subduction, yielding good conditions for exhumation of the UHPM rocks. The local extension was the result of Yangtze rotation that was generated by the oblique impact between the two cratons. The Yangtze rotation might also provided a pair of shear forces on bottom of the lithosphere to make a breakoff event that is essential to the exhumation of the UHPM terranes. Owing to continuing convergence between the two cratons after the exhumation of Dabie‐Sulu UHPM terranes, Yangtze craton proceeded to subduct beneath the Dabie‐Sulu terranes and the Sino‐Korean craton. This intracontinental subduction carried tremendous mass of crustal materials into the upper mantle, causing later partial melting, mantle doming and rifting. Based on above‐mentioned analysis, the previous models for evolution of the Dabie–Sulu terranes are elaborated.

Ion micro-probe U–Pb zircon geochronology of peak and retrograde stages of ultrahigh-pressure metamorphic rocks from the Kokchetav massif, northern Kazakhstan

Sensitive high resolution ion micro-probe (SHRIMP) U–Pb analyses of zoned zircons were performed to constrain the age and geodynamic significance of the Kokchetav ultrahigh-pressure (UHP) metamorphic rocks, which recrystallized at P>60 kbar and T>1000°C. Cathodoluminescence images were correlated with micro-assemblages of mineral inclusions in zircons from diamond-bearing gneisses. These revealed UHP mineral-bearing cores and low-pressure mineral-bearing rims, some zircons also retain inherited cores. Our SHRIMP data identified four discrete stages: Middle Proterozoic protolith age, 537±9 Ma for UHP metamorphic conditions, 507±8 Ma for the late-stage amphibolite facies overprint, and 456–461 Ma for post-orogenic thermal events. Thus, we conclude that Middle Proterozoic supracrustal protoliths of the Kokchetav UHP–HP rocks were subducted to mantle depths in the Middle Cambrian, and exhumed to mid-crustal levels in the Late Cambrian. The exhumation rate calculated from the SHRIMP data and relative P– T conditions is approximately 5 km/Ma. The rapid exhumation rate for the massif is in the same range as other UHP metamorphic terranes, and suggests that slab decoupling may be a driving force for exhumation of UHP metamorphic rocks from mantle depths.

Arrested orogenic development: eclogitization, delamination, and tectonic collapse

Fluids are key in the process of eclogitization and delamination of crustal roots in collisional orogens, and this process is not solely constrained by pressure–temperature conditions. Partially eclogitized amphibolites, gabbros, and granulites from the Western Gneiss Region of Norway, the Marun-Keu Complex in the polar Urals, and the Dabie-Sulu belt in China demonstrate that fluid is required for complete eclogitization. Conventionally, orogeny proceeds in a cycle that progresses from collision and uplift, to metamorphism and delamination of the crustal root, to completion when the orogen undergoes tectonic collapse. The south Ural Mountains and the southern Trans-Hudson orogen are type examples of arrested orogenic development in which delamination and post-orogenic extensional collapse have not occurred. Because the eclogitization of crustal roots leads to delamination and tectonic collapse of orogens, it is likely that the base of the Uralian crust has not undergone major eclogitization and therefore is under fluid-absent conditions. The lack of post-orogenic tectonic collapse and extensional faulting of some ultrahigh-pressure (UHP) orogens has major implications for exhumation models of UHP metamorphic terranes. Extension on the Main Uralian fault in the south Urals did not play a important role in the exhumation of the UHP Maksyutov Complex; the dominance of quartzofeldspathic rock types in the Maksyutov Complex and widespread retrograde metamorphism indicate that buoyancy rather than extensional faulting was likely the dominant cause of exhumation in the south Urals.

Eclogite rheology: Implications for subducted lithosphere

Research Article| August 01, 2001 Eclogite rheology: Implications for subducted lithosphere Z.-M. Jin; Z.-M. Jin 1Institute of Geophysics and Planetary Physics and Department of Earth Sciences, University of California, Riverside, California 92521, USA, and Faculty of Earth Sciences, China University of Geosciences, Wuhan, 430074, China Search for other works by this author on: GSW Google Scholar J. Zhang; J. Zhang 1Institute of Geophysics and Planetary Physics and Department of Earth Sciences, University of California, Riverside, California 92521, USA, and Faculty of Earth Sciences, China University of Geosciences, Wuhan, 430074, China Search for other works by this author on: GSW Google Scholar H.W. Green, II; H.W. Green, II 1Institute of Geophysics and Planetary Physics and Department of Earth Sciences, University of California, Riverside, California 92521, USA, and Faculty of Earth Sciences, China University of Geosciences, Wuhan, 430074, China Search for other works by this author on: GSW Google Scholar S. Jin S. Jin 2Faculty of Earth Sciences, China University of Geosciences, Wuhan, 430074, China Search for other works by this author on: GSW Google Scholar Geology (2001) 29 (8): 667–670. https://doi.org/10.1130/0091-7613(2001)029<0667:ERIFSL>2.0.CO;2 Article history received: 06 Nov 2000 rev-recd: 19 Mar 2001 accepted: 30 Mar 2001 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Z.-M. Jin, J. Zhang, H.W. Green, S. Jin; Eclogite rheology: Implications for subducted lithosphere. Geology 2001;; 29 (8): 667–670. doi: https://doi.org/10.1130/0091-7613(2001)029<0667:ERIFSL>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 SocietyGeology Search Advanced Search Abstract Eclogite, the high-pressure equivalent of basalt, is common in ultrahigh-pressure metamorphic terranes and constitutes the oceanic crust after it is subducted into the mantle. We present here first measurements of the rheology of eclogite and its constituent minerals, garnet and omphacite. Eclogite with approximately equal amounts of these two minerals has the same strength as harzburgite, whereas pure omphacite is much weaker and pure garnetite is much stronger. The strength variation of different mixtures of the two phases is quantitatively consistent with theoretical predictions for a two-phase material. The strengthening effect of garnet should increase the viscosity of the lower part of the mantle transition zone, but it probably is not sufficient to allow delamination of the oceanic crust from the underlying mantle during subduction. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Extensional deformation of post ultrahigh-pressure metamorphism and exhumation process of ultrahigh-pressure metamorphic rocks in the Dabie massif, China

A detailed tectonic analysis demonstrates that the present observed regional tectonic configuration of the ultrahigh-pressure metamorphic terrane in the Dabie massif was mainly formed by the extension processes of the post-Indosinian continent-continent oblique collision between the Sino-Korean and Yangtze cratons and ultrahigh-pressure metamorphism (UHPM). The configuration is characterized by a regional tectonic pattern similar to metamorphic core complexes and by the development of multi-layered detachment zones. On the basis of the identification of compressional and extensional fabrics, it is indicated that the exhumation and uplift of ultrahigh-pressure (UHP) metamorphic rocks from the mantle depth to the surface can be divided into at least three different decompression retrogressive metamorphism and tectonic deformation stages, in which the subhorizontal crustal-scale extensional flow in the middle-lower crust under amphibolite facies conditions is an important geodynamic process in the exhumation of UHP metamorphic rocks. Moreover, the extensional flow is probably driven by delamination and magmatic underplating of thickened lithospheric mantle following the continental oblique collision.

The roots of the Dabieshan ultrahigh-pressure metamorphic terrane: constraints from geochemistry and Nd–Sr isotope systematics

The Dabieshan area between the Sino–Korean and Yangtze cratons in east-central China has become the focus of much recent attention because of the discovery of abundant coesite and rare micro-diamond inclusions in both eclogites and their enclosing country rocks. The Dabieshan metamorphic complex, previously regarded as Archean continental basement of the Yangtze craton, is mainly made up of Precambrian felsic orthogneiss, amphibolite, and migmatitic gneiss with minor eclogite, granulite, ultramafic rock and marble. Our geochemical analyses and Nd–Sr isotope data show that most Dabieshan orthogneisses are distinctly different from the nearby Kongling gneisses of the Yangtze basement, which are Archean high-Al TTG rocks with an average Nd model age of 3.3±0.2 Ga with volcanic arc granitic affinity. The protoliths of the Dabieshan orthogneisses are diverse, and three types of rocks are distinguished: (1) the majority of the felsic gneisses in the eclogite units display geochemical signatures of post-Archean granites and may have resulted from Neoproterozoic magmatism in a rift environment; (2) some of the felsic gneisses in the eclogite units have an affinity with Kongling gneiss, and were presumably derived from the Yangtze basement by tectonic extrusion during Mesozoic exhumation of the ultrahigh-pressure (UHP) metamorphic rocks; and (3) the felsic gneisses of the dome region show geochemical signatures of Archean granitoids and Nd model ages between 3.1 and 1.0 Ga, attributed to mixing between Neoproterozoic mantle-derived material and the Archean Kongling gneisses. Numerical modeling shows that mixing between mantle-derived melts and Kongling gneiss can account for the Nd–Sr isotopic variation of Mesozoic mafic monzodiorites in the UHP eclogite unit, implying that the Kongling complex was extended beneath the Dabieshan terrane possibly during early Mesozoic continental collision. We suggest that the dome region was originally part of the Yangtze craton, and was separated from it by Neoproterozoic rifting. The orogen was later significantly modified, especially by Jurassic–Cretaceous migmatization and magmatism.

Large areal extent of ultrahigh-pressure metamorphism in the Sulu ultrahigh-pressure terrane of East China: new implications from coesite and omphacite inclusions in zircon of granitic gneiss

Coesite and omphacite inclusions have been identified for the first time as minute inclusions in zircon from amphibolite-facies granitic orthogneiss in the Sulu ultrahigh-pressure (UHP) metamorphic terrane of eastern China by Raman spectroscopy and microprobe analyses. The occurrences of these minerals in the voluminous granitic gneiss of Sulu support a regional and pervasive UHP metamorphic event that predated regional amphibolitic retrogression. Taking into account the widespread discoveries of coesite in other lithologies, we thus conclude that a substantial crustal component in the Sulu UHP metamorphic terrane appears to have shared a common history of Triassic subduction to mantle depths and later exhumation.

Significance of allanite-(Ce) in granitic gneisses from the ultrahigh-pressure metamorphic terrane, Dabie Shan, central China

Abstract A recent study of granitic gneisses in the ultrahigh-pressure (UHP) metamorphic terrane from Dabie Shan, central China reveals that allanite-(Ce) is a widespread REE-bearing accessory phase. The microprobe analyses show that allanites-(Ce) have nearly uniform chemical compositions, with total REE contents ranging from 23 to 28 wt.% and a La/Ce ratio of 0.57–0.73. All allanites-(Ce) are replaced or rimmed by epidotes and commonly form a relatively REE-poor rim near the interfaces. The textural relations and mineral chemistry suggest that allanite-(Ce) crystallized prior to metamorphism and therefore was of igneous origin, whereas the surrounding epidote was the product of metamorphism. Granitic gneiss, composed chiefly of sodic plagioclase, microcline and quartz with minor amounts of Mn-rich garnet, biotite, low Si phengitic mica, epidote, chlorite and sometimes amphibole, was formed at 495°C and ∼5 kbar, corresponding to the retrograde metamorphism of UHP rocks. Accordingly, it is believed that granitic gneisses have not experienced UHP metamorphism. This implies that a large scale tectonic juxtaposition developed before the epidote-amphibolite facies retrograde metamorphism of UHP rocks during the late continental collision in Dabie Shan.

Significance of Allanite-(Ce) in Granitic Gneisses from the Ultrahigh-Pressure Metamorphic Terrane, Dabie Shan, Central China

A recent study of granitic gneisses in the ultrahigh-pressure (UHP) metamorphic terrane from Dabie Shan, central China reveals that allanite-(Ce) is a widespread REE-bearing accessory phase. The microprobe analyses show that allanites-(Ce) have nearly uniform chemical compositions, with total REE contents ranging from 23 to 28 wt.% and a La/Ce ratio of 0.57–0.73. All allanites-(Ce) are replaced or rimmed by epidotes and commonly form a relatively REE-poor rim near the interfaces. The textural relations and mineral chemistry suggest that allanite-(Ce) crystallized prior to metamorphism and therefore was of igneous origin, whereas the surrounding epidote was the product of metamorphism. Granitic gneiss, composed chiefly of sodic plagioclase, microcline and quartz with minor amounts of Mn-rich garnet, biotite, low Si phengitic mica, epidote, chlorite and sometimes amphibole, was formed at 495°C and ∼5 kbar, corresponding to the retrograde metamorphism of UHP rocks. Accordingly, it is believed that granitic gneisses have not experienced UHP metamorphism. This implies that a large scale tectonic juxtaposition developed before the epidote-amphibolite facies retrograde metamorphism of UHP rocks during the late continental collision in Dabie Shan.

Sinking intrusion model for the emplacement of garnet-bearing peridotites into continent collision orogens: Comment and Reply

Garnet- (and probably spinel-) bearing ultramafic rocks that form tectonic lenses within high-metamorphic-grade gneisses in mountain core zones may intrude by moving downward or laterally into deeply subducted continental crust from the overlying mantle wedge. This model is most credible for garnet peridotites associated with ultrahigh-pressure metamorphic terranes in mountains formed by continent-continent collision. The extremely high pressures implied by ultrahigh-pressure metamorphism indicate that continental crust was subducted to mantle depths >100–120 km, where the mantle is in the garnet peridotite facies. Exchange between mantle and crust seems likely, particularly the transfer of mantle from the hanging wall into the crustal footwall. The mantle wedge overlying the subducted crustal terrane creates a denser over lighter configuration that is gravitationally unstable. If the interface becomes hot or wet enough so that peridotite can deform by ductile flow, it would sink as mantle blobs into the underlying crustal rocks. If not, the exchange presumably occurs through a brittle mechanism. The intruded bodies will be transported to the surface passively as the host crustal terrane is exhumed buoyantly toward the surface. This mechanism is proposed in particular to explain the intrusion of garnetiferous peridotites in the ultrahigh-pressure Western Gneiss region of the Norwegian Caledonides during the Caledonian orogeny.

Introduction to geodynamics for high‐ and ultrahigh‐pressure metamorphism

Since the first workshop on ultrahigh‐pressure (UHP) metamorphism at Stanford in 1994 and the special issue of The Island Arc `Ultrahigh‐Pressure Metamorphism and Tectonics' published in December 1995, many symposia and special sessions specifically with regard to the UHP metamorphic terrane have been held. While we are still wondering how exhumation of UHP rocks from mantle depths to the surface takes place, the finding of possible records from the mantle transition zone at 300–400 km depths is astonishing. The study of the UHP regime has expanded to include input from mineral physics, experimental geochemistry and kinetics in addition to the petrochemical and tectonic study of a variety of HP–UHP rocks. It was with this theme that the second workshop for the task group III‐6 `Ultrahigh‐Pressure Metamorphism and Geodynamics in Collision‐type Orogenic Belts' of the International Lithosphere Program was held during the International Geological Congress in Beijing, 1996. The Symposium 8–9 `Dynamic Metamorphic Rocks and High‐ and Ultrahigh‐Pressure Metamorphism' (Cong Bolin &amp; J. G. Liou conveners) had more than 25 presentations in two oral sessions and 70 papers in one poster session. This second special issue of The Island Arc includes nine papers from this symposium and a few related contributions to the geodynamics of HP–UHP metamorphism and tectonics. It is our hope that The Island Arc will continue to publish a special issue on this increasingly recognized subject that is essential to our understanding of continental collision, mantle dynamics and geochemical + fluid cycles.

Sinking intrusion model for the emplacement of garnet-bearing peridotites into continent collision orogens

Research Article| July 01, 1998 Sinking intrusion model for the emplacement of garnet-bearing peridotites into continent collision orogens Hannes K. Brueckner Hannes K. Brueckner 1Queens College and the Graduate Center, City University of New York, Queens College, Flushing, New York 11367 and Lamont-Doherty Earth Observatory, Palisades, New York 10964 Search for other works by this author on: GSW Google Scholar Author and Article Information Hannes K. Brueckner 1Queens College and the Graduate Center, City University of New York, Queens College, Flushing, New York 11367 and Lamont-Doherty Earth Observatory, Palisades, New York 10964 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1998) 26 (7): 631–634. https://doi.org/10.1130/0091-7613(1998)026<0631:SIMFTE>2.3.CO;2 Article history First Online: 02 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 Hannes K. Brueckner; Sinking intrusion model for the emplacement of garnet-bearing peridotites into continent collision orogens. Geology 1998;; 26 (7): 631–634. doi: https://doi.org/10.1130/0091-7613(1998)026<0631:SIMFTE>2.3.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 SocietyGeology Search Advanced Search Abstract Garnet- (and probably spinel-) bearing ultramafic rocks that form tectonic lenses within high-metamorphic-grade gneisses in mountain core zones may intrude by moving downward or laterally into deeply subducted continental crust from the overlying mantle wedge. This model is most credible for garnet peridotites associated with ultrahigh-pressure metamorphic terranes in mountains formed by continent-continent collision. The extremely high pressures implied by ultrahigh-pressure metamorphism indicate that continental crust was subducted to mantle depths >100–120 km, where the mantle is in the garnet peridotite facies. Exchange between mantle and crust seems likely, particularly the transfer of mantle from the hanging wall into the crustal footwall. The mantle wedge overlying the subducted crustal terrane creates a denser over lighter configuration that is gravitationally unstable. If the interface becomes hot or wet enough so that peridotite can deform by ductile flow, it would sink as mantle blobs into the underlying crustal rocks. If not, the exchange presumably occurs through a brittle mechanism. The intruded bodies will be transported to the surface passively as the host crustal terrane is exhumed buoyantly toward the surface. This mechanism is proposed in particular to explain the intrusion of garnetiferous peridotites in the ultrahigh-pressure Western Gneiss region of the Norwegian Caledonides during the Caledonian orogeny. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Buoyancy-driven, rapid exhumation of ultrahigh-pressure metamorphosed continental crust.

Preservation of ultrahigh-pressure (UHP) minerals formed at depths of 90-125 km require unusual conditions. Our subduction model involves underflow of a salient (250 +/- 150 km wide, 90-125 km long) of continental crust embedded in cold, largely oceanic crust-capped lithosphere; loss of leading portions of the high-density oceanic lithosphere by slab break-off, as increasing volumes of microcontinental material enter the subduction zone; buoyancy-driven return toward midcrustal levels of a thin (2-15 km thick), low-density slice; finally, uplift, backfolding, normal faulting, and exposure of the UHP terrane. Sustained over approximately 20 million years, rapid ( approximately 5 mm/year) exhumation of the thin-aspect ratio UHP sialic sheet caught between cooler hanging-wall plate and refrigerating, downgoing lithosphere allows withdrawal of heat along both its upper and lower surfaces. The intracratonal position of most UHP complexes reflects consumption of an intervening ocean basin and introduction of a sialic promontory into the subduction zone. UHP metamorphic terranes consist chiefly of transformed, yet relatively low-density continental crust compared with displaced mantle material-otherwise such complexes could not return to shallow depths. Relatively rare metabasaltic, metagabbroic, and metacherty lithologies retain traces of phases characteristic of UHP conditions because they are massive, virtually impervious to fluids, and nearly anhydrous. In contrast, H2O-rich quartzofeldspathic, gneissose/schistose, more permeable metasedimentary and metagranitic units have backreacted thoroughly, so coesite and other UHP silicates are exceedingly rare. Because of the initial presence of biogenic carbon, and its especially sluggish transformation rate, UHP paragneisses contain the most abundantly preserved crustal diamonds.

Near-end-member magnesiochloritoid in prograde-zoned pyrope, Dora-Maira massif, western Alps

In the ultrahigh-pressure metamorphic terrane of the Dora-Maira massif, western Alps, magnesiochloritoid (up to 97 mol% end-member) occurs along with ellenbergerite, talc, chlorite or kyanite as prograde inclusions in a few pyrope megablasts. Whereas most pyrope megablasts in this terrane are relatively homogeneous (Prp90-Prp98), the magnesiochloritoid-bearing megablasts commonly preserve a clear prograde FeMg zonation pattern (from Prp70 to Prp98). The nearly homogeneous chloritoid inclusions also record this zonation pattern, their X Mg increasing from 0.70 to 0.96 from garnet core to garnet rim. These zonations are shown to be prograde and to record increasing P and T conditions up to about 700°C, 30 kbar. The occurrence of magnesiochloritoid in some garnets simply reflects either a slightly more Fe-rich system (general case), or a more Al-rich and originally corundum-bearing system (one unzoned near-end-member pyrope megablast). FeMg partitioning between chloritoid and zoned host-garnet changes from normal to reverse (i.e., X Mg Cld < X Mg Grt) from core to rim, or from rim to core in other megablasts of yet similar composition. In case of reverse partitioning, a diffusion zone of a few tens of μm wide exists in garnet around the inclusions, toward which X Mg Grt decreases and reaches a normal partitioning value at the contact. These features are interpreted in terms of disequilibrium and tentatively ascribed to slow diffusion in chloritoid with respect to garnet growth rate, or to two competing garnet-forming reactions, with the metastable one, which produces a more Mg-rich garnet, being kinetically favoured. The prograde inclusions of magnesiostaurolite show a consistent FeMg reversal with garnet, i.e. X Mg St < X Mg Grt, which is interpreted as an equilibrium feature at high pressure (and high X Mg). Magnesiochloritoid locally breaks down into symplectites of chlorite and an extremely low-Si magnesiostaurolite (down to 7.0 Si per formula unit), which is probably metastable with respect to magnesiostaurolite + corundum.