Search

This study combines field observations, mineral and whole-rock geochemistry, phase equilibrium modeling, and U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon geochronology to investigate sillimanite-bearing felsic migmatites exposed on Ledge Mountain in the central Adirondack Highlands (New York, USA), part of an extensive belt of mid-crustal rocks comprising the hinterland of the Mesoproterozoic Grenville orogen. Phase equilibrium modeling suggests minimum peak metamorphic conditions of 960–1025 °C and 11–12.5 kbar during the Ottawan orogeny—significantly higher pressure-temperature conditions than previously determined—followed by a period of near-isothermal decompression, then isobaric cooling. Petrography reveals abundant melt-related microstructures, and pseudosection models show the presence of at least ~15%–30% melt during buoyancy-driven exhumation and decompression. New zircon data document late Ottawan (re)crystallization at ca. 1047 ± 5 to 1035 ± 2 Ma following ultrahigh-temperature (UHT) metamorphism and anatexis on the retrograde cooling path. Inherited zircon cores give a mean date of 1136 ± 5 Ma, which suggests derivation of these felsic granulites by partial melting of older igneous rocks. The ferroan, anhydrous character of the granulites is similar to that of the ca. 1050 Ma Lyon Mountain Granite and consistent with origin in a late- to post-Ottawan extensional environment. We present a model for development of a late Ottawan migmatitic gneiss dome in the central Adirondacks that exhumed deep crustal rocks including the Snowy Mountain and Oregon anorthosite massifs with UHT Ledge Mountain migmatites. Recognition of deep crustal meta-plutonic rocks recording UHT metamorphism in a migmatite gneiss dome has significant implications for crustal behavior in this formerly thickened orogen.

The Maksyutov complex is a mid- to late-Paleozoic high- to ultrahigh-pressure (HP-UHP) eclogite-bearing subduction zone terrane in the south Ural Mountains. Previous reports of radial fractures emanating from quartz inclusions in garnet, omphacite, and glaucophane, cuboid graphite pseudomorphs after matrix diamond, and microdiamond aggregates preserved in garnet identified by Raman spectroscopy indicate that parts of the complex were subjected to physical conditions of ~600 °C and >2.8 GPa for coesite-bearing rocks, and >3.2 GPa for diamond-bearing rocks. Peak UHP eclogite-facies metamorphism took place at ca. 385 Ma, and rocks were exhumed through retrograde blueschist-facies conditions by ca. 360 Ma. Bulk analyses of 18 rocks reflect the presence of mid-oceanic-ridge basalt (MORB), oceanic-island basalt (OIB), and island-arc tholeiite (IAT) basaltic and andesitic series plus their metasomatized equivalents. To more fully constrain the petrotectonic evolution of the complex, we computed isochemical phase equilibria models for representative metabasites in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2 based on our new bulk-rock X-ray fluorescence (XRF) data. Both conventional Fe-Mg exchange thermometry and phase equilibrium modeling result in higher peak equilibrium temperatures than were previously reported for the complex. Pseudosection analysis provides minimum P-T conditions of 650–675 °C and 2.4–2.6 GPa for peak assemblages of the least retrogressed Maksyutov eclogites, whereas Fe-Mg exchange thermometry yields temperatures of 750 ± 25 °C for a pressure of 2.5 GPa. We interpret our new P-T data to reflect a thermal maximum reached by the eclogites on their initial decompression-exhumation stage, that defines a metamorphic field gradient; the relict coesite and microdiamond aggregates previously reported testify to pressure maxima that define an earlier prograde subduction zone gradient. The eclogitic Maksyutov complex marks underflow of the paleo-Asian oceanic plate and does not represent subduction of the Siberian cratonal margin.

The preservation of metastable diamond in ultrahigh-pressure metamorphic (UHPM) complexes challenges our understanding of the processes taking place during exhumation of these subduction zone complexes. The presence of diamonds in UHPM rocks implies that diamonds remained metastable during exhumation, and within thermodynamic stability of graphite for an extended period. This work studies the influence of pressure on the surface graphitization rate of diamond monocrystals in carbonate systems to understand the preservation of microdiamond during exhumation of UHP subduction complexes. Experiments were performed with 2–3 mm synthetic diamond monocrystals at 2–4 GPa in СаСО3 (1550°С) and К2СО3 (1450°С) melts using a high-pressure multi-anvil apparatus. The highest rate of surface graphitization took place at 2 GPa; diamond crystals were almost completely enveloped by a graphite coating. At 4 GPa, only octahedron-shaped pits formed on flat {111} diamond crystal faces. Our results demonstrate that the...

The Karakoram fault (KKF) is the 1000 km-long strike-slip fault separating the western Himalaya from the Tibetan Plateau. From geologic and geodetic data, the KKF is argued either to be a lithospheric-scale fault with hundreds of km of offset at several cm/a, or to be almost inactive with cumulative offset of only a few tens of kilometers and to be just the upper-crustal localization of distributed deformation at depth. Here we show 3He/4He ratios in geothermal springs along a 500-km segment of the KKF are 3–100 times the normal ratio in continental crust, providing unequivocal evidence that a component of these hydrologic systems is derived from tectonically active mantle. Mantle enrichment is absent along the Indus–Yarlung suture zone (ISZ) just 35 km southwest of the KKF, suggesting that the mantle fluids flow only within the KKF. Within the last few Ma, the KKF must have accessed tectonically active Tibetan mantle northeast of the “mantle suture” which we therefore locate vertically beneath the KKF, very close to the surface trace of the ISZ. Hence, in southwestern Tibet, Indian crust may not now be underthrusting substantially north of the ISZ, even though Miocene underthrusting may have placed Indian crust north of the ISZ in the lower half of the Tibetan Plateau crust. This is in significant contrast to central and eastern Tibet where underthrust Indian material not only forms the lower half of the Tibetan crust but is also currently underthrusting for ∼200 km north of the ISZ. Our new constraint on KKF penetration to the mantle allows an improved description of the continuously evolving Karakoram fault, as a tectonically significant yet perhaps geologically ephemeral lithospheric structure.

The Tan–Lu fault is a major strike-slip fault in eastern China that appears to offset the high-grade rocks of the Hong’an–Dabie–Sulu orogen left-laterally ∼540 km. We evaluate models for the collision between the South and North China blocks, published radiometric dates recording HP–UHP metamorphism and exhumation in the Hong’an–Dabie and Sulu terranes, and the timing of sinistral motion on the Tan–Lu fault to evaluate whether UHP rocks provide a piercing point for offset on the Tan–Lu fault. UHP metamorphism in Hong’an–Dabie was concurrent with Sulu based on U–Pb dating of coesite-bearing domains of zircon at 244 ± 5–226 ± 2 Ma for Hong’an–Dabie and 243 ± 4–225 ± 2 Ma for Sulu. Retrograde metamorphism began c. 220 Ma for both Hong’an–Dabie and Sulu, but retrograde zircon growth ended c. 214 Ma in Hong’an–Dabie and continued until c. 202 Ma in Sulu based on U–Pb dating of zircon domains external to coesite-bearing domains. Structures in Sulu are rotated 25° counter-clockwise from, but are broadly similar to, Hong’an–Dabie suggesting the two areas have a common Triassic orogenic history that pre-dates motion on the Tan–Lu fault, and that is consistent with paleomagnetic studies. We constructed a pre-Cretaceous restoration of the Hong’an–Dabie–Sulu belt that moves the Sulu terrane south, aligning the suture and the eclogite-facies isograd, and rotates Sulu c. 25° clockwise to re-align structures with Hong’an–Dabie. Our restoration is supported by published data and shows that the Hong’an–Dabie–Sulu orogen is a piercing point for post-collisional offset on the Tan–Lu fault and that these regions shared a common subduction–exhumation history. The Tan–Lu fault did not play a significant role in the Hong’an–Dabie–Sulu collision and likely developed later, in the Early Cretaceous.

The northward subduction and subsequent exhumation of the Sulu ultrahigh-pressure metamorphism terrane exposes a tilted coherent crustal section of the overriding North China Block (NCB), providing an opportunity to study three-dimensional structures of the subduction/collision process. Detailed geological mapping and structural measurements on the Liaodong peninsula, located at the southern margin of the NCB, reveal that the Neoproterozoic to Permian sedimentary cover of the NCB experienced Triassic northward-decreasing crustal shortening, displayed in variable fold interference patterns. The interference patterns are characterized by: (1) irregular distribution of domes or basins and non-cylindrical folds with curvilinear trends, without systematic interference geometry; (2) amoeboid and hair-pin outcrop patterns; and (3) folds with variable plunges and aberrant relationships to the axial plane cleavage. These features are consistent with the criteria for structures produced by constrictional deformation, suggesting the overriding plate experienced simultaneous orogen-normal and orogen-parallel shortening during the Sulu continent–continent collision. Such a strain and structural pattern may suggest that the Sulu continent–continent subduction/collision was transpressional with significant lateral sliding.

Variations in the volume and age of Miocene granites and in mid-crustal conductance from the northwest Himalaya to southeastern Tibet imply lateral differences in late orogenic processes. The change from west to east occurs near the Gurla Mandhata dome, where the Karakoram fault terminates and merges with the Indus–Yarlung suture zone. The ‘channel flow’ model, developed in southeastern Tibet, predicts that anatectic partial melts beneath the Tibetan plateau are gravitationally-driven south to a topographic erosional front and are exposed as leucogranites in the Greater Himalaya Sequence; upwellings of these channel granites occur as gneiss domes in the Tethyan Himalaya Sequence. Published magnetotelluric profiles show high conductivity 30–40 km deep beneath Tibet from c. 400 km north of the Main Frontal thrust south across the suture zone, beneath the Himalayan gneiss domes, and to the topographic front; this conductive middle crust corresponds to 2–4% partial melt in the northwest Himalaya and 5–12% melt in southeastern Tibet, sufficient in the latter case to weaken rock for flow. East of the Karakoram termination leucogranites are abundant and are as young as 7 Ma; west of the termination, channel granites are less abundant and no younger than 18 Ma. Middle Miocene (16–14 Ma) leucogranites are found in the Karakoram fault zone located north of the suture zone and south of the proposed anatectic melt source. The initiation of motion on the crustal-penetrating Karakoram fault at 25–21 Ma may have created a barrier to the southward flow of mid-crustal melts and acted as a vertical conduit for these same melts.

Ultrahigh-pressure (UHP) rocks in the northwest Himalaya are some of the youngest on earth, and allow testing of critical questions of UHP metamorphism and exhumation and the India-Asia collision. The Tso Morari Complex (TMC) is a UHP subduction-zone complex in eastern Ladakh in the western Himalaya, south of the Indus-Yarlung suture zone. U-Pb SHRIMP dating of zircon shows the TMC has a Proterozoic protolith, preserves a Pan-African magmatic history, and shows continuous metamorphic zircon growth during the Early to Middle Eocene, hence constraining the timing of collision, subduction, and exhumation in the western Himalaya. Zircon dating indicates that UHP metamorphism occurred at 53.3 ± 0.7 Ma, followed by 8 m.y. of continual zircon crystallization to amphibolite-facies metamorphic conditions at 45.2 ± 0.7 Ma. Similar continuous zircon growth during UHP metamorphism and through early exhumation to amphibolite-facies conditions occurs in other UHP subduction complexes, including the Sulu terrane, where ...

Ultrahigh-pressure (UHP) rocks in the NW Himalaya are some of the youngest on Earth, and allow testing of critical questions of UHP formation and exhumation and the timing of the India–Asia collision. Initial collision of India with Asia is widely cited as being at 55F1 Ma based on a paleomagnetically determined slowdown of India’s plate velocity, and as being at ca. 51 Ma based on the termination of marine carbonate deposition. Even relatively small changes in this collision age force large changes in tectonic reconstructions because of the rapid India–Asia convergence rate of 134 mm/a at the time of collision. New U–Pb SHRIMP dating of zircon shows that Indian rocks of the Tso Morari Complex reached UHP depths at 53.3F0.7 Ma. Given the high rate of Indian subduction, this dating implies that Indian continental crust arrived at the Asian trench no later than 57F1 Ma, providing a metamorphic age for comparison with previous paleomagnetic and stratigraphic estimates. India’s collision with Asia may be compared to modern processes in the Timor region in which initiation of collision precedes both the slowing of the convergence rate and the termination of marine carbonate deposition. The Indian UHP rocks must have traveled rapidly along a short, hence steep, path into the mantle. Early continental subduction was at a steep angle, probably vertical, comparable to modern continental subduction in the Hindu Kush, despite evidence for modern-day low-angle subduction of India beneath Tibet. Oceanic slab break-off likely coincided with exhumation of UHP terranes in the western Himalaya and led to the initiation of low-angle subduction and leucogranite generation. D 2005 Elsevier B.V. All rights reserved.

Eclogite from Rongcheng in the easternmost part of the Sulu ultrahigh-pressure metamorphic terrane has broken down (at least partially) with the formation of coronae of spinel + anorthite around kyanite, symplectites of pargasite + plagioclase around garnet, and symplectites of diopside + plagioclase after omphacite. Textural evidence and microprobe analysis indicate that this breakdown was generated by discontinuous reactions. Quantification of material transfer and textural information suggest that the reaction mechanism involved local exchange of components among the three major minerals (omphacite, garnet, and kyanite) during dissolution and discontinuous precipitation, coupled with exchange between eclogites and their host rocks. The symplectite formed after omphacite gained Al and minor Si and Na from its surroundings, and lost Mg, Ca, and minor Fe to its surroundings. Formation of the symplectite around garnet was accompanied by gain of Ca, Al, Si, Na, and H 2 O, and by loss of Mg and Fe 2 + to its surroundings. Coronae around kyanite gained Ca, Fe, Mg, and minor Si, and provided Al to their surroundings. Material exchange between eclogite and host rocks occurred as well: eclogite gained minor Al, Na, and H 2 O from the host rocks, and provided Mg, Fe, and Ca to the host rocks. Pressure and temperature estimation using THERMOCALC yielded a lower temperature (655 ′ 70 °C) and higher pressure (1.3 ′ 0.2 GPa), which values contrast with estimates made by other authors. Strong deformation may have created a path for ingress of hydrous fluids, then resulted in a localized lower T and/or higher P.

A guided inquiry exercise has been developed to help teach the geology of the U.S. This exercise is intended for use early in the school term when undergraduate students have little background knowledge of geology. Before beginning, students should be introduced to rock types and have a basic understanding of geologic time. This exercise uses three maps: the U.S. Geological Survey’s “A Tapestry of Time and Terrain” and “Landforms of the Conterminous United States” maps, and a geologic map of the United States. Using these maps, groups of 3 to 5 students are asked to identify between 8 and 12 geologic provinces based on topography, the age of rocks, and rock types. Each student is given a blank outline map of the contiguous U.S. and each group is given a set of the three maps and colored pencils; as a group, students work to define regions in the U.S. with similar geology. A goal of 8 to 12 geologic provinces is given to help establish the level of detail being asked of students. One member of each group is asked to present their group’s findings to the class, describing their geologic provinces and the reasoning behind their choices.

The Middle Paleozoic Maksyutov Complex is an important component of the Eurasian collisional orogeny. It consists of dominant mica-rich garnet schist and mica-poor quartzofeldspathic gneiss enclosing minor mafic eclogite boudins (unit no. 1). Employing Raman spectroscopy, we identified three cuboidal microdiamond inclusions (~2‐3 micrometers in diameter) in garnet hosts from two different mica-poor gneissic samples. Broad spectral bands and high magnification SEM images suggest that the cuboids are fine-grained nanocrystalline diamond aggregates characterized by limited long-range ordering. Their poor crystallinity is compatible with relatively low-temperature, solid-state growth in the absence of both melt and a C-O-H-N fluid. Poor crystallinity, and small grain size suggest that such aggregates may represent the lowest temperature microdiamonds yet identified in nature. Their formation required ultrahigh-pressures (UHP) at a minimum of 3.2 GPa, and a metamorphic temperature of ~650 ∞C. Blocky graphite up to 10+ mm across in the matrix of mica-rich carbonaceous garnet schist may represent pseudomorphs after much larger neoblastic diamonds. Thermobarometric calculations for analyzed coexisting garnet + omphacite + phengite from six Maksyutov unit no. 1 mafic eclogites indicate retrograde physical conditions of 610‐680 ∞C, 1.7‐2.6 GPa, slightly lower-pressure conditions than the coesite stability field. Complete conversion of diamond to blocky graphite in the mica-rich schists, and recrystallization of coesite to quartz in the schists, quartzofeldspathic gneisses, and eclogite pods reflect relatively slow exhumation from ~110 km depth to upper crustal levels over 60‐90 m.y. Phengite inclusions in zircon and garnet hint at modest activity of H2O during prograde UHP metamorphism of the eclogites and mica-poor gneisses. The latter have retained rare, tiny microdiamond inclusions in garnet on decompression. Abundant white mica in the carbonaceous garnet schists probably reflects a C-O-H-N fluid-mediated, kinetically enhanced prograde production of diamond, and efficient obliteration of this phase accompanying leisurely ascent of the subduction complex. In contrast, associated micapoor gneisses and eclogites were relatively dry during exhumation, so retained rare nanocrystalline microdiamond inclusions in garnet.

The Maksyutov Complex consists of three fault-bounded lithologic units: a quartzofeldspathic gneiss containing mafic . . eclogite boudins Unit a1 ; a metasedimentary blueschist-facies Yumaguzinskaya unit; and a meta-ophiolitic melange ´ . . Unit a2 . The geologic history of the high- to ultrahigh-pressure HP-UHP assembly of the Maksyutov Complex is complicated by several stages of prolonged retrograde metamorphism and deformation. The Sakmara River exposes all three units near the former village of Karayanova. A structuralrpetrologic cross-section through the area yields new quantitative data for the complex and, regionally, for the south Urals. Analysis of the Karayanova area has identified the major structures. Regional folding within the complex is parallel to the dominant foliation trending northeast-southwest. Stereonet data show that, during exhumation, this large-scale folding was refolded about axes trending southeast. Unit a1 and the Yumaguzinskaya are tectonically and petrologically distinct units juxtaposed by west-vergent thrusting and recrystallization within the same subduction zone. A shear zone developed later between Unit a2 and the Unit a1q Yumaguzinskaya tectonic package accompanying exhumation. Field relations and petrofabric demonstrate that blueschist-facies recrystalliza- tion overprinted an earlier eclogite-facies metamorphism. Thermobarometric measurements yield P-T values of 594-6378C, 1.5-1.7 GPa for eclogite, but these conditions may reflect annealing during the early-stage exhumation at ; 375 Ma. Cuboid graphite aggregates testify to precursor conditions for Unit a1 within the diamond stability field, if such textures are correctly interpreted. Measured 18 Or 16 O partitioning between pairs of coexisting phases yield three main recrystallization . . temperature ranges: 1 678 " 838C, attending Unit a1 eclogite-facies metamorphism; 2 453" 178C, during transitional . blueschistrgreenschist-facies metamorphism for the amalgamated Unit a1q Yumaguzinskayaq Unit a2 assembly; and 3 250" 688C, reflecting late-stage hydrothermal alteration and exhumation. Oxygen isotope data for Units a1 and a2 indicate that garnet, blue amphibole, and pyroxene crystallized in isotopic equilibrium, validating previous thermobarometric calculations for a Unit a1 retrograde metamorphic event. Variations in d 18 O values for phengites suggest the possibility of late metamorphic fluid infiltration. Retrograde recrystallization at high pressure in the presence of fluids and a calculated

Apatite fission track samples were collected from the ultrahigh-pressure (UHP) Maksyutov Complex, south Ural Mountains, in the foot wall of the Main Uralian fault (MUF) to constrain the low-temperature cooling history and to establish the late stage exhumation rate for the complex. Fission track samples were taken along a 70-km north-south transect and a 5-km east-west traverse through the Maksyutov Complex, with two samples from the hanging wall of the MUF. Apparent age and track length modeling results indicate that the Maksyutov Complex was exhumed and cooled to 110°C en masse in the Early Permian (300 ± 25 Ma). The east-west transect shows that no significant interunit movement occurred in the Maksyutov Complex after ∼315 Ma; on the basis of higher-temperature thermochronometers, the entire Maksyutov Complex must have been assembled between 335 and 315 Ma. Modeling for the north-south transect indicates that exhumation occurred contemporaneously in the north and south regions of the complex with cooling to 110°C between 375 and 315 Ma, coinciding with the onset of the Uralian orogeny. Comparison of modeling for Maksyutov samples and an Ordovician metasediment from the hanging wall of the MUF indicates that late movement on the MUF was minor and that the footwall and hanging wall had a similar cooling history after the late Carboniferous (∼300 Ma). Exhumation rates range from 0.3 to 1.5 mm yr¹ between a high-pressure metamorphic event at 375 and 315 Ma using current heat flow data. Our calculated exhumation rate for the Maksyutov Complex is consistent with the complex being a UHP terrane, even though coesite and diamond are not preserved.

Unusual cuboid graphite aggregates (up to 13 mm edge length) from the eclogitic gneiss unit of the Maksyutov Complex deflect a foliation defined by groundmass graphite and phengite, and pressure shadows have developed around these blocky aggregates. Carbon isotope ratios, d 13 C/ 12 C, for the cuboid graphite range from about 224 to 242‰, demonstrating that these rocks have retained an original biogenic carbon signature. X-ray diffraction, laser Raman spectroscopy, infrared spectroscopy, and transmission electron microscopy indicate that graphite is well-crystallized with minor defects; no relict organic compounds were detected. Comparisons of these cuboid aggregates with thin sections and scanning electron microscope images of proven graphitized diamonds from the Beni Bousera peridotite massif show that Maksyutov graphite is similar. Laboratory experiments by other workers on graphite demonstrate that this intriguing morphology could not be the result of deformation, because graphite returns to its original shape and size on stress release. Existing experiments on diamond graphitization do not adequately replicate the conditions of natural rocks being exhumed from subduction zones characterized by ultrahigh pressures, and thus cannot be applied with confidence to the Maksyutov Complex. Our spectroscopic and microscopic studies suggest that these cuboid aggregates probably are diamond pseudomorphs. Copyright © 1998 Elsevier Science Ltd

The Maksyutov Complex, situated in the southern Ural Mountains of Russia, is the first location where quartz aggregates within garnets exhibiting radial fractures were identified as coesite pseudomorphs (Chesnokov & Popov 1965). The complex consists of two tectonic units: a structurally lower eclogite-bearing schist unit and an overlying meta-ophiolite unit. Both units show evidence for multiple stages of metamorphism and deformation. The high-pressure metamorphism of the eclogite-bearing schist unit, discussed in this report, is suspected to be related to a collision between the Russian platform and a fragment of the Siberian continent during the early Cambrian. At least three stages of metamorphism (M1-3) and two stages of deformation (S1 and S2) were observed in thin sections: M1) garnet (Alm55-60, Prp22-28, Grs16-20) + omphacite (Jd46-56) + phengite (Si ≅ 3.5) + rutile; M2) garnet + glaucophane ± lawsonite + white mica; and M3) epidote + chlorite ± albite ± actinolite + white mica. Observed mineral parageneses define a retrograde P-T path for the eclogite. Mineral assemblages within the most representative eclogite from the lower unit of the Maksyutov Complex indicate minimum peak pressures of 15 kbar at temperatures of approximately 600°C. If the presence of coesite pseudomorph is confirmed, the peak ultrahigh-pressure metamorphism may be as high as 27 kbar at 615°C.

The Maksyutov Complex consists of two juxtaposed lithotectonic units—Unit #1 of probable Late Proterozoic formation age, and Unit #2, apparently generated in Cambro-Ordovician time. The eclogite-facies metamorphism of Unit #1 occurred prior to 370-380 Ma, when this unit was subjected to blueschist-facies overprinting. Unit #2 displays the effects of a somewhat similar blueschist- or high-pressure greenschist-facies recrystallization, indicating that it may have been metamorphosed contemporaneously with Unit #1. Our field work and geochemical studies have focused on the Sakmara River area. Preliminary conclusions are as follows: (1) Unit #1 was subjected to metamorphic temperatures of 620 ± 70° C and minimum pressures of 1.5 GPa, or 2.7 GPa if the previously reported interpretation of coesite pseudomorphs from similar rocks exposed near the village of Shubino, 75 km to the south (Chesnokov and Popov, 1965), is correct. Peak metamorphic pressures would have reached at least 3.2 GPa if blocky graphite descri...

Two-dimensional thermal modeling of the subduction and exhumation of the ultrahigh-pressure (UHP) Maksyutov Complex in the south Ural Mountains tests factors influencing the low modern heat flow in the Urals and the feeble preservation of UHP index minerals. Best-fit models are obtained with initial surface heat flow of 60 mW m 2 indicating that low modern heat flow in the Urals is a recent anomaly. Ultrahigh-pressure metamorphism was modeled at 388 Ma based on new U–Pb SHRIMP dating of zircon; the average modeled exhumation rate for the UHP eclogites to the mid-crust is ~5.0 mm/year, much slower than other UHP complexes. The model predicts that cooling during exhumation is strongly dependent on concomitant subduction/underthrusting in the footwall of the UHP unit and normal faulting in the hanging wall. Low radiogenic heat production in the crust and a relatively thin UHP slab (3–10 km) also favor cooling. For Maksyutov, the modeling shows that cooling was controlled by low radiogenic heat production and underthrusting during decompression to lower crustal levels; these cooling mechanisms accompany exhumation despite low exhumation rates (~5 mm/year) thereby denying the call for fast exhumation in order to preserve UHP index minerals. D 2004 Elsevier B.V. All rights reserved.

During the early stages of subduction of the lithosphere, anhydrous rocks become partially hydrated, and volatiles evolve from hydrous lithologies. Where present, aqueous fluid markedly enhances reaction rates. Phase-equilibrium studies demonstrate that, under typical subduction-zone P—T trajectories, clinoamphibole constitutes a major phase in deep-seated metamorphic rocks of MORB composition; other hydrous minerals are either absent or of relatively minor abundance. Clinoamphiboles dehydrate at pressures < 2 GPa, so blueschists and amphibolites expel H2O at great depth, and commonly achieve the stable eclogitic assemblage of garnet + omphacite + rutile ± phengite. Partly serpentinized mantle beneath the oceanic crust devolatilizes at comparable pressures. In contrast, for reasonable subduction-zone P—T gradients, white micas ± biotites remain stable to pressures substantially exceeding 3.5–4.0 GPa. Accordingly, when the micaceous lithologies that dominate the continental crust (granitic, pelitic, and quartzofeldspathic gneisses) are subducted to depths of ≥_100 km, they fail to evolve significant H2O, and may transform incompletely to the stable eclogitic assemblage of coesite + jadeite + K-spar + garnet + rutile + phengite. Thus, although all “juicy” rock types expel volatiles during compaction and shallow burial, the especially deep underflow of partly hydrated oceanic crust-capped lithosphere probably generates most of the ultrahigh-pressure volatile flux along and above a subduction zone prior to continental collision; as large volumes of sialic crust enter the convergent plate junction, volatile flux at deep levels severely diminishes.

Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.7