V31E-01 INVITED
Exhumation of Continental Ultrahigh-Pressure Terranes
Exhumation of continental ultrahigh-pressure (UHP) terranes is one of the most enigmatic processes associated with UHP metamorphism for at least two reasons: 1. the structures that must have been active during the UHP and high pressure part of the decompression path are overprinted by lower grade structures and are thus cryptic, and 2. felsic crust subjected to UHP conditions should become denser than lithospheric mantle and sink; however, we observe that slivers of UHP continental crust are buoyant enough to be exhumed to Earth's surface. Even when deformational fabrics demonstrably formed at UHP conditions are preserved, their geometric and kinematic significance are uncertain due to likely rotation during exhumation and subsequent deformation. In light of the partial structural record and limited understanding of what controls exhumation versus permanent subduction of continental material, models for exhumation must be based on alternate observations. Some common features of exhumed UHP terranes are: a tabular geometry with thicknesses on the order of 10 km; location in the lower plate of a subduction channel; formation and exhumation early in the collision history; and a two-stage process with rapid rise to the lower-middle crust and slower exhumation to the surface. Popular Chemenda-type analog models, where slab break-off of the subducting lithosphere leads to buoyant extrusion of UHP slivers at least to lower crustal levels, provide one mechanism for exhumation; but this type of model needs refinement to account for a wider range of geological circumstances under which UHP terranes form and are exhumed. This talk is meant to review and highlight current thinking on the exhumation of UHP terranes incorporating these alternate settings. Emphasis will be placed on the variation in the initial configuration of the subduction channel, the relative strengths of the colliding lithospheric margins, and large scale melt-enhanced exhumation by diapiric mechanisms. UHP metamorphic rocks that form late in a collision may be exhumed by a change in plate motion vectors from convergent to divergent with geometries quite different from a typical subduction channel.
V31E-02
Polyphase Formation and Exhumation of HP-UHP Rocks in Continental Subduction Zone: Numerical Modeling
High- to ultrahigh-pressure (HP-UHP) metamorphic rocks commonly form and exhume during the early continental collision, with the protoliths mainly derived from subducted upper and middle continental crust. While the geodynamic significance of HP-UHP complexes is widely recognized and their appearance in the Neoproterozoic is considered as a "hallmark" for establishing modern plate tectonic styles, many questions related to their origin still remain unresolved. Of particular importance is the poly-metamorphic origin of many HP-UHP terranes composed of tectonic units having strongly variable ages, peak metamorphic conditions and P-T paths. In order to address this issue we conducted 2D high-resolution thermomechanical numerical modeling of the continental subduction associated with formation and exhumation of the HP-UHP rocks, with testing different geometrical configurations, rheological properties and varied width of subducting continental margins, convergence velocity, sedimentation and erosion rates. Most of our experiments confirm poly-phase origin of HP-UHP terranes and predict existence of several consequent episodes of (U)HP rocks exhumation related to the inherently cyclic origin of continental crust subduction-detachment-exhumation process. Periodicity of formation of rheologically weak zones (thrusting faults) controlling HP-UHP rocks exhumation processes depends on the competing effects of downward directed subduction drag and upward directed crustal buoyancy forces. The buoyancy forces and related deviatoric stresses accumulate in the subduction channel due to subduction of low-density crustal rocks and are then reset back during rapid exhumation episodes. Numerical modeling suggest that UHP rocks may remain in the sub-lithospheric channel for several million years being heated to 800-900°C by the surrounding hot mantle. At the later stage upward extrusion of such hot partially molten rocks may exhume high-temperature (HT) UHP complexes toward the surface. Therefore, sub-lithospheric channel formation and extrusion processes may provide plausible explanation for occurrence of UHP-HT rocks in nature.
V31E-03 INVITED
Continental Crust Recycling at Collision Zones: Insights From Numerical Modeling and Observations
In the recent years, an increasing number of geochemical evidences of continental crust (cc) contributions in OIB lavas have been reported worldwide, suggesting that part of the lithospheric continents have sunk and were recycled in the deep mantle. So far, crustal recycling processes have been identified as eclogitization of the lower crust, foundering and subduction of crustal rocks and erosion of continents at ocean-continent and ocean-ocean margins activated by the subducting slab. But what is about continental crust recycling occurring during continental collision? In order to investigate this process, we perform 2D numerical models of subduction under a fix continent of a 1) retreating and 2) advancing oceanic slab attached to a continental passive margin to test this last process. 1) If the slab is retreating, when collision occurs and the continents are poorly coupled, asthenospheric mantle wedges between the continents triggering the retreating and delamination of the converging continental plate. A discrete volume of cc (mainly the lower crust) is then dragged into the deep mantle indicating that this is an efficient process for crustal recycling. Examples of active retreating collisional zone are the Northern Apennines and the Carpatians. 2) If the plates are coupled and convergence does not stop after collision, a large volume of lower cc is transported in the deep mantle. This models is representative of active advancing collisional zone such as the Himalayas where has been calculated that about 2x106 km3 of continental lower crust have been subducted In both cases, the rheologically weak part of the crust (upper crust and sediments, Wet Quartzite flow law) is scraped off and accreted to the accretionary wedge. Results then show that cc recycling occurring after collision is a considerable process that has to be taken into account for both crustal mass balancing and crustal growth models.
V31E-04 INVITED
Depth of diamonds formation: a novel spectroscopic approach to the 3-D mapping of stress-patterns
Diamond is a unique geological material carrying inside fluid and solid inclusions which are pristine witnesses of diamond crystallization media. Yet, residual stress patterns formed around inclusions in diamond are traditionally used to determine depth of kimberlitic diamond formation. Since diamond discoveries within ultrahigh pressure metamorphic terranes in metasedimentary rocks of continental affinities – the understanding to what depth can continental crust be subducted becomes a question of a great importance. Because diamonds from UHPM terranes contain a lot of fluid and solid crystalline inclusions, the residual stress-patterns technique traditionally developed for kimberlitic diamonds, may be also applied to the diamonds formed within metamorphic rocks during continental collisions. Recently, we have developed a new modification of the Raman spectroscopic technique which visualizes three-dimensional stress distribution around an inclusion in diamonds. Raman frequency is sensitive to pressure in materials and is useful to detect internal stress in a crystalline material. We have established a three-dimensional Raman mapping system with a real-time energy-calibration function for detecting stress distributions from subtle frequency shifts in Raman spectra; a high stability during long measurements was achieved. By applying curve fitting, positions of the Raman line can be determined with precision of about 0.05 cm-1 corresponding to stress of 0.2 kbar in diamond. Because micro-Raman spectroscopy has a high spatial resolution on the order of 1 micrometer or less, the new technique may be applied to diamonds of smaller size than the kimberlitic ones. We present here our result of measurements and 3D mapping of the stress patterns around inclusions of olivine and chromite in diamond specimen from the Internationalnaya Pipe, Yakutia, Russia. A new method of the simultaneous estimation of both pressure and temperature reflecting conditions at which olivine and chromite inclusions were trapped by diamond is under discussion. Our new method opens a new window into deep mantle processes recorded in diamonds of any geological formations.
V31E-05 INVITED
Nontraditional 'Deliverers' of UHP Rocks from Earth's Deep Interior to the Surface
It is widely accepted that fragments of deeply subducted continental and/or oceanic crust may return from depths ~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 the best indicator of the greater depth (up to 1,700 km) because it serves as an almost ideal natural 'container' which is capable to preserve unchanged inclusions of UHP phases. Recently, diamond was reported from an oceanic island, a mid-oceanic ridge ophiolite, and a forearc geodynamic structure, which are not suitable places for diamond formation and its transportation to the Earth's surface. This is because magmas associated with the mentioned geodynamic environments are thought to originate at shallow depth where pressure is too low and oxygen fugacity too high for diamond stabilisation. The first finding of nm-sized diamond in melt inclusions in mantle-derived garnet pyroxenite xenolith from Salt Lake Crater (Hawaii) was only possible using foils for cut TEM investigations by Focused Ion Beam (Wirth and Rocholl, 2003). Diamonds associated with native Fe and Cu, FeS, FeS2, ZnS, AgS, and several Ti-, Nb-, Zr-, Ir-, In-, and Pd-rich phases occur within Si-rich glass, forming small domains filled with dozens of ~20-nm-size crystals. Another microdiamond was discovered inside an OsIr inclusion from a chromite pod included in a mantle section of the Luobasa ophiolite formation of Tibet which marks the geological boundary between Asia and India (Yang et al., 2007). Prismatic domains of polycrystalline coesite found in the same massive chromite ore suggest pseudomorphic replacement of stishovite. Other UHP phases TiN, c-BN, TiO2 II and coesite exsolution lamellae are also found in the Luobasa chromites (Dobrzhinetskaya et al., 2007; Yamamoto et al., 2007), suggesting depth of their possible origin over 400 km. Solid-state transport within massive chromitite appears the only vehicle capable of transporting these phases and preserving their low-fO2 environment at the very high temperatures of oceanic spreading centers. Similar findings have now been made in other ophiolites. Microdiamond in clinopyroxene from a Cenozoic lamprophyre dike was discovered in a forearc setting on Shikoku island, Japan (Mizukami et al., 2008). Studies provide a pressure constraint of 5.5 GPa, which suggests that the diamond-bearing rocks rose to the forearc region from depths of ~160 km. This implies that mantle flow in convergent plate boundaries occurs on a larger scale than was previously recognized. The discoveries of UHP minerals in rocks located within 'forbidden' geological settings indicate that a new understanding is needed of the interactions between mantle convection and oceanic islands, forearcs, and oceanic spreading centers, as well as the depths of their magma formation.
V31E-06
Geochemistry of Coesite-Bearing Pyrope Quartzites and Related Rocks From the Dora Maira Massif, Western Alps: New Results and the Enigma of the Jadeite-Rocks
In contrast to the extensive petrological and geochronological work on the various ultrahigh-pressure (UHP) metamorphic rocks of the coesite-bearing unit of the Dora Maira Massif, there is still a deficiency of basic geochemical data. A complete suite of geochemical data for pyrope quartzites, various intercalations of phengite-schists and jadeite-bearing rocks, as well as country rock gneisses from different localities within the southern Dora Maira Massiv is now available, which was studied in detail in order to establish the nature of the different protoliths and their primary relationships (Schertl and Schreyer, 2008; see also Chopin, 1984 and Compagnoni and Hirajima, 2001). Typically, the pyrope quartzites are high in Mg and strongly depleted in Na, Ca, Fe, Cu, P, Rb, Ba, Sr against their country rock gneisses which essentially exhibit a granitic bulk composition. The country rocks have a peraluminous chemistry; they generally are corundum normative and best attributed to S-type granites. Trace element contents of phengite-schist inclusions in pyrope quartzite confirm their close relationship to the granitic country rocks. Internal variations of Na, Ca versus K, Mg are matched by Rb, Ba and Sr, which is in line with some phengite-schists to contain higher amounts of phengite or higher amounts of jadeite-pseudomorphs, respectively. The origin of the jadeite-rich rocks is still a matter of debate. Jadeite-bearing layers and jadeite quartzite forming conformable bands and boudins within pyrope quartzite differ generally by their lower contents in K, Mg, Rb and higher contents in Na, Fe, Ca, Mn, P and Zn. Earlier suggestions that these layers represent former melts seem unlikely in the view of their almost constant mass behaviour for SiO2 and Al2O3 relative to the surrounding pyrope quartzites. The present study indicates that the pyrope quartzites were formed metasomatically whereas an evaporitic nature of the protolith can be ruled out. Discrimination plots demonstrate that the whiteschist-type pyrope quartzites can be clearly distinguished chemically from metasedimentary whiteschists of former evaporite environments. Field relations of trails of pyrope quartzites, now forming lensoid inclusions within a former granite intrusion, would also make a metasedimentary origin rather unlikely. On the other hand, consistent genetical relationships between pyrope quartzites and their adjacent granitic country rocks were obtained, which virtually match those of Mg-metasomatic leucophyllites from the Eastern Alps occurring along shear zones within granitic gneisses. Similar Mg-rich rocks to be found within the Tauern Window and the Monte Rosa and Gran Paradiso Massivs of the Western Alps indicate that throughout the entire range of the Alps local processes of Mg-metasomatism occurred. References Chopin, C. (1984): Coesite and pure pyrope in high-grade blueschists of the Western Alps: a first record and some consequences. Contrib. Mineral. Petrol., 86, 107-118. Compagnoni, R. and Hirajima, T. (2001): Superzoned garnets in the coesite-bearing Brossasco-Isasca Unit, Dora-Maira massif, Western Alps, and the origin of the whiteschists. Lithos, 57, 219- 236. Schertl, H.-P. and Schreyer, W. (2008): Geochemistry of coesite-bearing "pyrope quartzites" and related rocks from the Dora-Maira Massif, Western Alps. Eur. J. Mineral., in press.
V31E-07
Exsolution Phenomena of UHP Type in Garnets From Western New England, USA
A population of garnet megacrysts, some euhedral and up to 5 cm diameter, from high-grade cover rocks of the Taconian Shelburne Falls arc in western Massachusetts (Karabinos et al. 1998) contains abundant exsolved apatite(OH), rutile, and ilmenite in multiple generations. The host is a restitic kyanite-phlogopite- garnet-rutile-quartz schist with retrograde cordierite or staurolite developed according to bulk Mg# and degree of metamorphic overprint of the matrix. The garnets are significantly Mg-rich (Py25), and retain strong compositional zonation in Mn and Ca (> Sp10, > Gr15 in the cores) consistent with first-cycle metamorphism of a pelitic protolith. The coarsest (up to mm-scale) exsolution is distributed in concentric zones, with apatite(OH) in the garnet cores, a broad mantle containing apatite(OH) + ilmenite and a rim containing apatite(OH) + rutile. Common bilaterally-symmetrical rutile twins and complex multiply-twinned 3-D rutile structures may indicate an anomalous nucleation environment for the coarsest rutile exsolution. The coarse exsolution is difficult to understand as decompressional and instead may have occurred near maximum pressure in accord with mechanisms proposed by Smith (Smith, DC 2006). A pervasive second generation of exsolved submicron apatite(OH) crystals and fine rutile needles and minor euhedral ilmenite plates envelops the coarse phases and extends nearly to the rims. The generally even distribution of this generation of apatite(OH) and the crystallographic orientation of the rutile needles across several cm diameter of euhedral garnet monocrystal requires exsolution from garnet. The garnets retain up to 0.18 wt% P2O5 similar to the highest reported, 0.17 wt% P2O5 from a rutile- and apatite-exsolved UHP crustal garnet from Sulu (Ye et al 2000). With only 0.03 wt% Na2O despite the high P2O5 content, a substitution mechanism other than Na + P must be sought, such as compensatory Mg in VI or vacancy in VIII after Smith (2006). These megacrysts correspond most closely to a well-defined UHP crustal garnet population at Bingara, Australia (Barron et al. 2005) that those authors identify with similar garnets from Sulu and from metapelites from the Greek Rhodope (Mposkos and Kostopoulos, 2001). If exsolution of apatite(OH) from garnet as from Bingara, Sulu, and elsewhere, and from mantle eclogites (Haggerty et al. 1994) is a reliable indicator of UHP metamorphic conditions, then these garnet megacrysts record a previously unrecognized UHP event in the Taconian of western New England.
V31E-08
Retrograde phase transitions of majoritic garnets included in diamonds: a case study of subcalcic Cr-rich majoritic pyrope from a Snap Lake diamond, Canada
Majoritic garnets, containing pyroxene solid solution were initially discovered in diamonds from Monastery mine, South Africa (Moore, Gurney, 1985). They are very rare both in limited number of kimberlite pipes and alluvial sources (e.g. Stachel et al., 2005). Most of them are eclogitic, but some peridotitic (U/P-type) of lherzolitic (L) and wehrlitic (W) assemblages were also found (e.g. Sobolev et al., 1997; 2004). Significant percentage (40%) of majoritic garnets among subcalcic Cr-pyrope inclusions in diamonds was discovered in Snap Lake kimberlites (Pokhilenko et al., 2004). We present the results from a revised study of a harzburgitic (H) garnet with highest majorite content (up to 16 mol.%) by TEM techniques using FIB prepared foils (Wirth, 2004). Fine-grained symplectite consisting of low Ca orthopyroxene, clinopyroxene, Cr-spinel and coesite was detected with TEM and confirmed by XRD in the inner part of the garnet grain forming a sharp interface with the host. EMPA showed identical chemical composition of the nanometer-sized symplectite and the garnet. Further polishing of the garnet grain removed the symplectite, which possibly was present as a thin lense within the garnet. Only pyroxene exsolutions from majorite garnet have been documented up to date (Wilding, 1990). The remaining garnet is completely homogeneous and contains unusually high Ni (118 ppm) and very depleted REE patterns, less than 0.8-0.4 Grt/C1-chondrite for MREE and HREE. This demonstrates very high temperature of its origin (1380°C) and pressure about 11 GPa. The detected symplectite represents partial retrograde phase transition of the examined garnet, which was probably caused by plastic deformation of diamond at high temperatures within the Earth's mantle (e.g. Stachel et al., 2005). In this particular sample such plastic deformation and retrograde reaction occurred within coesite stability field at depths no less than 100 km. Wehrlitic garnet containing very high majoritic component (29 mol.%) was previously studied by single crystal XRD (Sobolev et al., 2004), and appeared to be completely homogeneous after checking it by TEM/AEM. Since EMPA was the only tool used in previous studies of rare majoritic garnets, it is possible that the presence of partial (or even complete) phase transitions could have been missed without additional XRD and/or TEM/AEM studies and masked by identical results of chemical analyses of garnets and possible nanometer sized retrograde symplectite.