Epidote Eclogites From Ross River Area, Yukon, Canada, Indicate Very High Pressure Metamorphism
Eclogites occur as tectonic inclusions in metasedimentary rocks at several locations within Yukon Territory. In the Ross River area the eclogites contain omphacite-garnet-Ca-Na amphibole-epidote-rutile-quartz. Phengite, paragonite, and titanite are local additional phases. The matrix of omphacite and Ca-amphibole is fine-grained and strongly deformed. Using bulk X-ray fluorescence analyses we have calculated isochemical phase diagram P-T sections (P-T pseudosections). The chemical system used is SiO2-TiO2- Al2O3-FeO-MgO-CaO-Na2O-K2O-H2O. We have not modeled MnO and ferric iron because of the lack of activity-composition data for minerals other than garnet. The P-T boundary of lawsonite- out provides approximate lower P and T limits on the stability of epidote with garnet and omphacite. The results are: 480-520°C at 20 kbar, near the lawsonite-eclogite/epidote-eclogite boundary at ∼ 500°C and 23 kbar. Garnet analyses indicate compositional zoning with Mn-rich cores. Intersections of the calculated grossular and pyrope isopleths (with observed compositions) provide estimates of P and T at the time of crystallization of garnet cores. The results range from 475-493°C and 19.7-23.5 kbar. Temperature estimates using the Zr in rutile geothermometer yield a T range of 521-557°C at 20 kbar. A metasedimentary host rock yielded three matrix results with 560 ± 54°C. These last results lead us to suggest that the host rocks experienced a similar T history to the eclogite inclusions. These eclogites were formed by subduction-related metamorphism, but did not involve continental collision. Ultrahigh pressures (coesite stability field) are usually associated with continental collision. The implied tectonic uplift of Ross River eclogite is greater than previously thought.
Fluids from the Deeply Continental Subduction Zone and the Metamorphic Chemical Geodynamics
The complex vein associations hosted in southern Sulu ultrahigh-pressure (UHP) eclogites contain quartz ¡À omphacite (or jadeite) ¡À kyanite ¡À allanite ¡À zoisite ¡À rutile ¡À garnet. These minerals have chemical compositions similar to those of host eclogites. Inclusions of polycrystalline quartz pseudomorphs after coesite were identified in vein allanite and garnet, and coesite inclusions were found in vein zircon. These facts suggest that the veins together with host eclogites have been subjected to synchronous UHP metamorphism. The vein minerals contain relatively high concentrations of rare earth elements (REE), high field strength elements (HFSE) and transition metal elements (TME). A kyanite-quartz vein has a whole-rock composition similar to adjacent UHP metamorphic granitic gneisses. Abundant primary multi-solid fluid inclusions trapped within UHP vein minerals contain complex daughter minerals of muscovite, calcite, anhydrite, magnetite, pyrite, apatite, celestite and liquid and gas phase of H2O with solids up to 30 to 70% of the inclusion volume. Presence of daughter mineral anhydrite and magnetite indicates the high oxygen fugacity in subduction released fluids, and provides a feasible interpretation to the high oxygen fugacity in convergent margins. These characteristics imply that the UHP vein minerals were crystallized from supercritical silicate-rich aqueous fluids that were in equilibrium with peak-UHP minerals, and that the fluids in deeply subducted continental crust may contain very high concentrations of silicate as well as HREE, HFSE and TME. Such fluids might have resulted in major fractionation between Nb and Ta, i.e. the UHP fluids have subchondritic Nb/Ta values, whereas the host eclogites after extraction of the fluids have suprachondritic Nb/Ta values. Therefore, voluminous residual eclogites with high Nb/Ta ratios may be the complementary suprachondritic reservoir capable of balancing the subchondritic depleted mantle and continental crust reservoirs.
Diamonds in ophiolitic mantle rocks and podiform chromitites: An unsolved mystery
In recent years ultrahigh pressure minerals, such as diamond and coesite, and other unusual minerals were discovered in chromitites of the Luobusa ophiolite in Tibet, and 4 new minerals have been approved by the CNMMN. These results have raised many questions£ºWhat are the occurrences of the diamonds, what is the source of their carbon and how were they formed? What is the origin of the chromites hosting the diamonds and at what depth did they form? What is the genetic relationship between the diamonds and the host chromitites? In what geological, geophysical and geochemical environments can the diamonds be formed and how are they preserved? The UHP minerals from Luobusa are controversial because they have not been found in situ and because ophiolites are currently believed to form at shallow levels above oceanic spreading centers in suprasubduction zone environments. More detailed study and experimental work are needed to understand the origin and significance of these unusual minerals and investigations of other ophiolites are needed to determine if such minerals occur elsewhere To approach these problems, we have collected two one-ton samples of harzburgite hosting chromitite orebodies in the Luobusa ophiolite in Tibet. The harzburgite samples were taken close to chromitite orebody 31, from which the diamonds, coesite and other unusual minerals were recovered. We processed these two samples in the same manner as the chromitites and discovered numerous diamonds and more than 50 other mineral species. These preliminary results show that the minerals in the harzburgites are similar to those in the chromitites, suggesting a genetic relationship between them. To determine if such UHP and unusual minerals occur elsewhere, we collected about 1.5 t of chromitite from two orebodies in an ultramafic body in the Polar Urals. Thus far, more than 60 different mineral species have been separated from these ores. The most exciting discovery is the common occurrence of diamond, a typical UHP mineral in the Luobusa chromitites. Other minerals include: (1) native elements: Cr, W, Ni, Co, Si, Al and Ta; (2) carbides: SiC and WC; (3) alloys: Cr-Fe, Si-Al-Fe, Ni-Cu, Ag-Au, Ag-Sn, Fe-Si, Fe-P, and Ag-Zn-Sn; (4) oxides: NiCrFe, PbSn, REE, rutile and Si-bearing rutile, ilmenite, corundum, chromite, MgO, and SnO2; (5) silicates: kyanite, pseudomorphs of octahedral olivine, zircon, garnet, feldspar, and quartz,; (6) sulfides of Fe, Ni, Cu, Mo, Pb, Ab, AsFe, FeNi, CuZn, and CoFeNi; and (7) iron groups: native Fe, FeO, and Fe2O3. These minerals are very similar in composition and structure to those reported from the Luobusa chromitites.
Recycling of crustal minerals into the upper mantle: evidence from ophiolites
A wide variety of ultrahigh pressure and crustal minerals has been recovered from podiform chromitites of the Donqiao and Luobusa ophiolites of Tibet, the Ray-Iz ophiolite of the Polar Urals and the Semail ophiolite of Oman. Microdiamonds are abundant in the Luobusa, Donqiao and Ray-Iz ophiolites, coesite and kyanite occur in Luobusa and moissanite is present in all four ophiolites. Numerous crustal minerals, including zircon, corundum, quartz, almandine garnet, rutile, and feldspar are also present. The diamonds are mostly euhedral grains,100-200 µm across, commonly containing metallic and Mg-Fe silicate inclusions. One small grain occurs as an inclusion in an Os-Ir alloy. Coesite and kyanite are intergrown with each other on the rim of a grain of Ti-Fe alloy, and the coesite has a prismatic form suggesting it may be pseudomorphic after stishovite. Moissanite is common in all four ophiolites and occurs as small colorless, green or blue, vitreous fragments. Zircon grains range from 20 to 300 µm, and are mostly well rounded with very complex internal structures. They commonly contain low-pressure inclusions of quartz, rutile, orthoclase, mica, ilmenite and apatite. 206Pb/238U SIMS and SHRIMP dates for the zircons are mostly Paleozoic and Precambrian, far older than the ophiolites. The grains of quartz, almandine garnet, corundum and feldspar range up to about 0.5 mm and are moderately to well rounded. Smaller, angular fragments of such grains are also present. The rounded morphology of these grains, as well as the zircons, strongly suggests derivation from sedimentary material, presumably transported into the mantle by subduction. The microdiamonds and moissanite could also have been derived from crustal materials recycled into the mantle. We suggest that the various minerals were picked up by melts from which the chromitites precipitated and carried to shallow crustal levels. The preservation of such minerals, particularly quartz and coesite, in the mantle implies isolation from the enclosing rock, perhaps in xenoliths. The recovery of essentially the same minerals from four widely separated ophiolites, processed in different laboratories, argues strongly against natural or anthropogenic contamination.
New Geological Settings for Ultra-High Pressure Rocks
This widely accepted that fragments of deeply subducted continental and/or oceanic crust may return back from depth of 200-300 km to the surface as UHPM rocks forming collisional orogenic belts. These rocks containing UHP minerals or microstructural relics of their decompressions, as well as mantle xenoliths and diamonds from kimberlitic pipes, are valuable fragments that provide snapshots of mantle mineralogy, melt, and fluids at depths of 100 to 1,700 km. Within them, diamond is a best indicator of the greater depth (up to 1,700 km) because it serves as almost ideal natural 'container' which is capable to preserve unchanged inclusions of UHP phases. Until recently it was known that diamonds occur only kimberlitic pipes and their derivates - alluvial placers, impact structures and UHPM belts related to continental collisions. Recently, unexpected evidences are collected that diamond formation is not restructed by the known occurrences. New geodynamic settings are recently discovered where diamond formation is closely linked to the high solubility of carbon in magmas, which are formed at higher pressures than it was previously thought. Some ophiolitic rocks contain puzzling relics of ultradeep mantle crystallization. Recently diamond was reported from an oceanic island, a mid-oceanic ridge ophiolite, and a forearc site, which are not suitable places for the diamond formation. The first finding of diamond in melt inclusions in mantle-derived grt-pyroxenite xenolith from Hawaii [1,2]. Diamonds of 20-nm-size occur within the Si-rich glass together with Fe Cu, FeS, FeS2, ZnS, AgS phases. Another diamond was discovered inside the OsIr inclusion from chromite pod from dunite-harzburgite section of the Luobasa ophiolite, Tibet. The polycrystalline coesite found in the same chromite ore suggests its replacement after stishovite. Other UHP phases TiN, c-BN, TiO2 II and coesite exsolution lamella are found in the Luobasa chromites suggesting depth of their possible origin of 400 km. Diamond in CPx from a Cenozoic lamprophyre dike was discovered in a forearc setting, Japan . A pressure of 5.5 GPa was calculated for the forearc diamond formation; it suggests that the diamond-bearing rocks rose to the forearc region from depths of about 160 km. This implies that mantle flow in convergent plate boundaries occurs on a larger scale than was previously recognized. We need a new understanding the interactions among magma generation and mantle convection beneath oceanic islands, forearcs, and the mid-oceanic floor.  Wirth & Rocholl (2003) EPSL 211, 357-369.  Frezotti & Peccerillo (2007) EPSL 262, 273-283.  Muzukami et al. (2008) Geology 36, 219-222.
Combined Experimentation and Microstructural Analysis in Evaluation of UHPM Rocks
Exhumation of (Ultra)High-Pressure Rocks in Extrusion Wedges. Examples from the Aegean and the Alps
The bulk of the exhumation of (ultra)high-pressure rocks usually occurs soon after these rocks were metamorphosed in the course of lithospheric convergence and deep underthrusting during early orogenic stages. A number of studies have demonstrated great exhumation rates during these early exhumation stages. It is poorly understood, however, how this early exhumation is being kinematically achieved. One possibility are extrusion wedges, which are characterized by a thrust-type shear zone at their base and a normal-sense shear zone at their top. The normal-sense shearing at the top of the wedge is a geometric effect only and must not be mistaken as an effect of net horizontal extension of the region. This is a very important characteristic of extrusion wedges. Therefore lithospheric extension is not required for very rapid early exhumation of (ultra)high-pressure rocks. For plausibly arguing for the existence of an extrusion wedge, it is critical to demonstrate that thrust-related mylonitic deformation in the footwall shear zone occurred contemporaneously with normal shearing in the hangingwall shear zone. We present three examples of fossil extrusion wedges from the European Alps and the Cycladic Blueschist Unit in the Aegean. The Eclogite Zone in the Tauern Window of the Eastern Alps was metamorphosed at 25-27 kbar and ca. 650°C at 31.5 Ma. Exhumation occurred very rapidly at rates exceeding 40 km Myr-1. Rb-Sr multimineral geochronology shows that the thrust at the base of the Eclogite zone operated at the same time as the oblique normal fault at the top of the Eclogite Zone. Early exhumation occurred in less than 2 Myr and accomplished about 50-70 km of exhumation of the Eclogite Zone. The two examples from the Cycladic Blueschist Unit in the Aegean are from Evia Island at the western side of the Aegean Sea, and Samos Island and the adjacent western Turkish mainland at the eastern limit of the Aegean Sea. In Evia, the extrusion wedge was active from 33 to 21 Ma and accomplished 15-30 km of blueschist exhumation. In Samos and western Turkey an extrusion wedge was active from 42 to 32 Ma and caused 30-40 km of blueschist exhumation. In both cases structural analysis reveals complementary senses of shear for the wedge-delimiting faults, with thrust-type shear zones at the base and normal-sense shear zones at the top of the extrusion wedges. In all three cases there is no evidence for regional-scale lithospheric extension or the development of contemporaneous extensional sedimentary basins. Instead, wedge extrusion is concurrent with the development of thrust belts in the footwall of the basal thrust of the extrusion wedges. We therefore conclude that all three extrusion wedges formed during overall lithospheric convergence.
Crustal Structure Linked to Ultra-High-Pressure Rock Exhumation: A Model for the Tso Morari Complex
Many syn-collisional ultra-high-pressure (UHP) complexes display an array of upper-crustal structures suggesting that doming accompanied by coeval normal and thrust faulting is closely linked to UHP exhumation processes. We present a geodynamical model that accounts for structural, metamorphic, and geochronological data from UHP terranes in terms of crustal burial and exhumation in a subduction channel below an accretionary wedge. Competition between down-channel shear traction and up-channel buoyancy forces, expressed as the exhumation number, E, controls crustal subduction and exhumation, leading to rapid up-channel flow when E > 1. In some cases, transient slab rollback can lead to incorporation of deep mantle peridotite into the subduction channel. Exhuming UHP material forms a nappe stack and structural dome as it penetrates and destabilises the overlying wedge, driving thrusting and extension. The Tso Morari complex, Ladakh, is a structural dome cored by UHP rocks that were metamorphosed and exhumed during the early stages of the Himalayan collision (ca. 55-45 Ma). Results from a high-resolution numerical model, in which convergence velocity decreases from 15 cm/a to 5 cm/a during the early stages of collision, are consistent with a wide range of structural, metamorphic, and geochronological data from Tso Morari. We conclude that the model offers a viable explanation for the geological evolution of syn-collisional UHP complexes. Moreover, the model demonstrates that buoyant exhumation from deep in the subduction channel is responsible for observed upper-crustal structures, which therefore hold important (but commonly overlooked) clues to UHP exhumation processes.