Mary L. Leech

Arrested orogenic development: eclogitization, delamination, and tectonic collapse

Abstract 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.

Graphite pseudomorphs after diamond? A carbon isotope and spectroscopic study of graphite cuboids from the Maksyutov Complex, south Ural Mountains, Russia

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

Petrotectonic Evolution of the Maksyutov Complex, Southern Urals, Russia: Implications for Ultrahigh-Pressure Metamorphism

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...

Low-temperature microdiamond aggregates in the Maksyutov Metamorphic Complex, South Ural Mountains, Russia

Abstract 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 late exhumation history of the ultrahigh‐pressure Maksyutov Complex, south Ural Mountains, from new apatite fission track data

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.

H2O Recycling During Continental Collision: Phase-Equilibrium and Kinetic Considerations

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.

Continuous Metamorphic Zircon Growth and Interpretation of U-Pb SHRIMP Dating: An Example from the Western Himalaya

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 coesite-bearing inclusions within dated zircon prove conclusively that zircon dates record UHP metamorphism. U-Pb SHRIMP dating of zircon for both the TMC and the Sulu belt demonstrate that zircon continues to crystallize at temperatures 400° ± 50°C based on 40Ar/39Ar dating yielding the same ages.

Age and origin of granites in the Karakoram shear zone and Greater Himalaya Sequence, NW India

The crustal-scale Karakoram shear zone structurally distinguishes the western Himalaya from—and provides an opportunity to compare to— the central and eastern portions of the orogen. To evaluate the tectonic evolution of the western Himalaya, this paper presents granite U/Th-Pb ages and zircon Hf isotopic signatures along the two major structures in northern India: the Karakoram shear zone and the Zanskar shear zone, the westernmost limb of the South Tibetan detachment system. Leucogranites in Zanskar crystallized 27-20 Ma and exhibit Precambrian to Paleozoic inheritance and predominantly negative e Hf (t) values typical of the Greater Himalayan Sequence. Karakoram shear zone leucogran- ites have igneous crystallization ages over a prolonged period from 22 Ma to <13 Ma, contain Late Cretaceous through Paleocene inherited cores, and have e Hf (t) values from +1 to +9. These inherited ages and mostly positive e Hf (t) values compare closely to the adjacent Ladakh batholith, but low e Hf (t) values along the Karakoram shear zone suggest an input of older crustal material from the proximal Karakoram terrane or subducted Indian crust. The Zanskar Greater Himalayan Sequence contains two suites of Paleozoic granites: (1) Pan-African Cambrian- Ordovician granites at the cores of gneiss domes and (2) Mississippian-Permian granites related to magmatism associated with the Panjal Traps. Monazite ages record peak through retrograde metamorphic conditions from 27.3 ± 1.2 Ma to 17.2 ± 0.9 Ma concurrent with anatectic leucogranite crystallization. Cenozoic partial melting in the Greater Himalayan Sequence occurred contemporaneously across the Himalayan orogen, but lower degrees of partial melting and ubiquitous doming distinguish the westernmost Greater Himalayan Sequence in Zanskar.

Mass balance during retrogression of eclogite-facies minerals in the Rongcheng eclogite, eastern Sulu ultrahigh-pressure terrane, China

Abstract 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. QuantiÞ cation 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 H2O, and by loss of Mg and Fe2+ 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 H2O 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.

Evidence of former majoritic garnet in Himalayan eclogite points to 200-km-deep subduction of Indian continental crust: This article has been retracted by the Geology editors

Relict majorite, recognized petrographically and confirmed by in situ laser-ablation microprobe analyses, is reported from eclogites of the Tso Morari Complex, India, and represents the first record of relict majorite in the Himalaya. Previous studies have established that the Tso Morari Complex eclogites represent continental metamorphosed basic rocks within the sequence of paragneisses and metapelites interbedded with granitic augen gneisses. Our study provides evidence for subduction of the Indian continental crust to majorite-forming depths (∼200 km), and even more significant, its exhumation from such depth without being completely overprinted. This estimated depth is almost double the previous depth approximations and hence has important implications on the estimation of the angle of subduction and average rates of exhumation of the Indian plate in the Himalayan collisional zone.