T41B-1957
Paragenetic Relationships among Staurolite and Aluminosilicate Polymorphs in the Barrovian Metapelites of the Imjingang Belt, Central Korea
Mineral parageneses, inclusion relationships and microstructures in the Barrovian metapelites of the Imjingang belt, Korea, were investigated to delineate the exhumation process of a crustal section thickened during the collisional orogeny. In particular, the timing of metamorphism was constrained by the in-situ SHRIMP dating of monazite grains in kyanite-zone schists. In a staurolite-zone metapelite, andalusite occurs as large poikiloblast containing biotite, garnet, staurolite and rare kyanite as inclusions, suggesting the low- P overprint after the Barrovian-type metamorphism. In the kyanite-zone schists, two types of inclusion assemblages are distinct: (1) high-P, staurolite-free assemblage of biotite and garnet within kyanite porphyroblasts; and (2) medium-P one of biotite, kyanite and garnet within staurolite. Kyanite is also present in andalusite-rich quartz veins of the kyanite zone. These veins are deformed or boudinaged on the outcrop scale. The coexistence of two aluminosilicate polymorphs is in contrast with the lack of sillimanite. Thus, the cooling path of the metapelites is characterized by a near-isothermal P-T path passing through the kyanite-andalusite boundary. The U-Th-Pb isotopic ages of monazite dated from three kyanite- zone schists define two distinct age groups. The 206Pb/238U ages of monazite were dated at 255 ± 5 Ma and 239 ± 4 Ma, respectively, and the 208Pb/232Th ages at 252 ± 3 Ma and 238 ± 5 Ma. All the above features in conjunction with available P-T-ometric and thermochronometric data suggest that the high-P peak metamorphism (~11 kbar and 670 °C) occurring at ~252 Ma was overprinted by the medium-P one (~6.5 kbar and 630 °C) at ~238 Ma. The subsequent cooling to 500 °C occurred at ~230-225 Ma. The latest Permian age of monazite is coeval with that of peak metamorphism, dated from the overgrowth rim of zircon in a paragneiss of the Imjingang belt (253 ± 2 Ma), but the heat and fluid sources for the Middle Triassic overprint are poorly constrained. The absence of paragonite and the low celadonite component of muscovite (less than ~3.1 Si / 11 O atoms) argue against the supply of fluid via the breakdown of hydrous minerals. On the other hand, Triassic igneous activities in the vicinity of the Imjingang belt, recently dated at 237-232 Ma, might have provided heat and fluid necessary for the growth of staurolite and monazite at mid- crustal depths. In summary, our result suggests that the Imjingang belt has experienced a near-isothermal decompression from ~11 to 6.5 kbar at ~252-238 Ma, and the formation of staurolite porphyroblasts enclosing biotite, kyanite and garnet is possibly attributed to the thermal flux associated with Middle Triassic magmatic activity.
T41B-1958
Tectonic erosion as a possible driving force of blueschist exhumation in the Cretaceous forearc of central Hokkaido, Japan.
Introduction In an oceanic subduction zone, where surface erosion was less effective and underlying slab was less buoyant, driving forces of unroofing and lifting-up for the blueschist exhumation are not still well understood. In the Kamuikotan Zone of central Hokkaido, Japan, ca. 125 Ma blueschist unit was exhumed and unconformably overlain by ca. 105-110 Ma forearc basin deposits. There is no trench-fill deposits coeval to the exhumation event, suggesting that the exhumation occurred in a non-accretionary stage. This paper presents a new idea to explain how the blueschist exhumation occurred in an non-accretionary margin, based on field observations of a unit underlying the blueschist. Geologic background The focused unit (Shizunai Unit) is an accretionary complex, which structurally underlies the blueschist facies metabasite unit. It is a structural pile of metabasites and pelagic to trench-fill sedimentary rocks. It wholly suffered a very low-grade HP metamorphism (lawsonite-albite facies or lower) dated as 105-115 Ma. Mudstones yield middle Early Cretaceous radiolarians (ca. 130-135 Ma). The forearc basin deposits unconformably overlying the exhumed rocks are fluvial to shallow-marine, whereas those in more trench-ward areas are deep-sea turbidites. Debris-flow and slump deposits are widespread over the basin, suggesting unstable slopes dipping to the trench, probably resulting from the uplift of the blueschists. Re-accretion of tectonically eroded rocks The Shizunai unit contains a nappe sub-unit consisting mostly of sedimentary rocks. It is an imbricate stack of sedimentary sequences from hemipelagic tuffaceous mudstone to terrigenous trench-fill turbidite. They represent upper horizons of trench strata, and lack rocks of the lower horizons (chert and metabasites). Stratigraphic upper parts of each sequence frequently intercalate debrites and olistostrome, implying syn- sedimentary imbricate thrusting. Deformation structures with bedding-parallel shortening at the earliest, unconsolidated stage are common in turbidites. These characteristics (imbricate upper trench strata, syn- sedimentary tectonics, and lateral compression just after sedimentation) suggest that the clastic nappe was originally formed as a frontal accretionary unit at the toe of the wedge. It then subducted to a depth of 15-20 km during the late Early Cretaceous. The subduction of a frontal accretionary unit means tectonic erosion. Whereas its incorporation into the high-pressure subduction complex means underplating re-accretion. Exhumation induced by tectonic erosion During the exhumation event, two types of material transport synchronously occurred. One is the rearward and downward transport along the base of the wedge as tectonic erosion and re-accretion, with mean vertical velocity of -0.5 to -1.3 mm/y. The other is upward transport at the rear of the wedge as the blueschist exhumation, with mean vertical velocity of 1 to 2 mm/y. These similar but opposite vertical velocities suggest that the two modes of material transport compensate by each other as a corner-flow transport. Steepening of the forearc slope is explained by removal of frontal wedge materials by tectonic erosion, and releasing them at the rear side. Unroofing by gravity slides inferably occurred under the oversteepened slope. If significant amounts of frontal materials are tectonically eroded and released at the depths, blueschists could thus be exhumed as a consequence of induced corner flow.
T41B-1959
Exhumation Rates and Processes of Ultrahigh-Pressure Metamorphic Rocks in the North Qaidam Terrane, China: Constraints from Sedimentary Rocks
Sediments deposited in contact with ultra-high pressure (UHP) rocks provide important constraints on the timing of their exhumation from depths of ca. 100 km, broadening the general understanding of deposition, subduction, and collision processes. This study focuses on sediments adjacent to UHP rocks in the Dulan- Chaka region of western China at the southeastern part of the North Qaidam UHP terrane in the northern Tibetan Plateau. The UHP rocks include ecoglite, ultramafic rocks, granitic gneiss, pelitic gneiss, schist, minor marble, and amphibolite. These UHP rocks are depositionally overlain by Devonian sediments, which include fine- to medium-grained sandstones as well as compositionally similar conglomerates dominated by quartz grains, few lithic fragments (mostly micas), and sporadic feldspathic grains. In general, thin sections show the samples to be poorly sorted, with angular to sub-angular grains, and abundant polycrystalline quartz grains, indicating immature sediments from a metamorphic source. Samples located in the NE part of the study area show greenschist-facies metamorphism as evidenced by suturing in quartz grains, growth of secondary chlorite, very low porosity, and deformation in outcrop. The samples in direct contact with the UHP rocks do not show metamorphism in field observations but development of foliation and pressure shadows are evident in thin sections. Detrital zircons from the sedimentary rocks were dated using the SHRIMP-RG U-Pb technique, revealing three major zircon populations: Meso-Archean to Mesoproterozoic (ca. 2.9-1.1 Ga), Early Neoproterozoic (ca. 900-1000 Ma), and Paleozoic (ca. 400-460 Ma). These ages are very similar to detrital zircon cores in UHP metasediments, igneous zircons in metagranites, and granitic and UHP rocks respectively. Eclogites at P-T conditions of 29-33 kbar and 631-746°C attained at depths of ca. 100 km have yielded ages of 459-422 Ma. A monazite age of 419 Ma records amphibolite-facies metamorphism at depths of ca. 20-25 km. Zircon rim ages and an 40Ar/39Ar muscovite age of 402 Ma record conditions of ca. 350°C and suggest depths of 10-15 km, signifying that UHP rocks moved to shallow crustal depths at about this time. Finally, the estimated maximum depositional age of the sediments is 389 Ma, obtained from the youngest zircon age in sandstones lying depositionally on the UHP rocks, which must have been exposed at the surface by around this time. These thermochronologic data imply exhumation rates of 15-20 mm/yr from 100 to ~20 km deep, 0.6-0.9 mm/yr from 20 to 10 km deep, and ~0.8 mm/yr from 10 km to the surface.
T41B-1960
Exhumation rates recorded by zircon and monazite, North Qaidam UHP terrane, NW China
Amphibolite-facies felsic gneisses near Dulan, at the southeast end of the North Qaidam terrane, enclose minor eclogite and peridotite. Field relations, and coesite inclusions in zircons from paragneiss suggest that felsic, mafic, and ultramafic rocks all experienced ultra-high pressure (UHP) metamorphism and a common amphibolite-facies retrogression. SHRIMP-RG U-Pb geochronology and trace element analyses of zircon and monazite from paragneiss constrain the ages of peak and retrograde metamorphism. Zircon rims in one sample yield a weighted mean age of 927 ± 6 Ma, contain Th/U ~ 0.1, and are enriched in HREE with negative Eu-anomalies, suggesting growth in the presence of plagioclase and in the absence of garnet. Monazite from this sample yields a weighted mean age of 433 ± 3 Ma, and is depleted in HREE, enriched in Sr (15,000-25,000 ppm), and negative Eu anomalies are small, suggesting growth in the absence of plagioclase and in the presence of garnet in the eclogite-facies. For the second sample, zircon rims and whole grains contain Th/U = 0.04-0.004, and yield a weighted mean age of 426 ± 4 Ma. Negative Eu-anomalies are absent, and HREE are depleted, suggesting eclogite- facies growth. Ti-in-zircon thermometry results cluster tightly at ~600°C. Monazite contains prominent negative Eu anomalies, less depleted HREE, and lower Sr (1000-2000 ppm) than the first sample, suggesting that the weighted mean age of 419 ± 2 Ma records retrograde amphibolite-facies metamorphism in the presence of plagioclase and less abundant garnet. Thermobarometry of this assemblage yields ~550°C, 6-7 kbar (20-25 km depth). Combined with a previously determined 423 ± 6 Ma age for coesite-bearing zircon, a 402 ± 2 Ma 40Ar/39Ar age inferred to reflect depths of 10-15 km, and our new 389 Ma constraint on deposition of sedimentary rocks on the UHP unit, these results suggest the following exhumation rates: 15- 20 mm/a from 100-20 km depth, 0.6-0.9 mm/a from 20-10 km depth, and ~0.8 mm/a from 10 km to the surface. These results are similar to those in other studies, and agree with models that predict initial, rapid, buoyancy-driven exhumation through the mantle, and slower, erosionally-influenced exhumation as the rocks near the surface.
T41B-1961
Temporal Constraints on Continental Rifting and the Exhumation of Pliocene Eclogites, SE Papua New Guinea
The youngest known HP-UHP rocks on Earth have been exhumed in the footwalls of the metamorphic core complexes (MCCs) of Goodenough and Fergusson Islands within the active Woodlark rift of southeast Papua New Guinea. What can the felsic and intermediate gneisses found in the lower plate of these metamorphic core complexes tell us about the thermal evolution of these HP-UHP rocks? Also, how does their exhumation during rifting temporally relate with the sea-floor spreading history of the Woodlark Basin? Ion-microprobe U-Pb zircon, 40Ar/39Ar amphibole, phengite, biotite, and K-feldspar analyses were performed on gneisses collected from the ductile shear zones of the MCCs of Goodenough and Fergusson Islands. The application of a broad range of thermochronometers permits the detailed determination of the thermal histories of these rocks from eclogite-facies conditions to their exhumation to middle and upper crustal levels. U-Pb zircon data from these lower plate gneisses indicate a period of zircon growth concordant with the timing of eclogite facies metamorphism as previously determined in mafic eclogites. This younger population of zircon occurs as rims on cores; the cores yield ages that range in age from Paleocene to Permian. These older ages indicate an inherited component likely reflecting the nature of the protolith of these gneisses. However, the abundance and degree of this inherited component is highly variable from sample to sample. New 40Ar/39Ar data from samples collected from the northern range-bounding Wakonai shear zone and the core of Goodenough Island MCC indicate rapid cooling of these rocks as recorded by Ar closure in amphibole, phengite, biotite, and K-feldspar (~500 to ~150 °C) at ~2-1.5 Ma. Samples from along the northern range front of the Goodenough MCC indicate a southeastward younging trend that parallels the active Wakonai normal fault from the northwest end of the island to the center of the island. However, no age gradient is apparent perpendicular to the strike of this fault. One sample of felsic gneiss from the lower plate shear zone of the Mailolo dome of northern Fergusson Island, located to the southeast of Goodenough Island, indicates cooling through Ar closure in phengite at ~2.6 Ma. Collectively, these data suggest that exhumation of eclogite-facies rocks within Goodenough Island and hence rifting in this area began at ≥2 Ma and occurred earlier towards the southeast in the area of northern Fergusson Island. Temporally correlated with this ~2 Ma cooling is the reorganization of the Woodlark spreading center and lithospheric rupture at segment 2, ~350 km to the east of the current location of Goodenough Island. These findings indicate that these shear zone fabrics formed during the Pliocene and rifting in the Woodlark Basin preceded lithospheric rupture and the arrival of the westward propagating seafloor spreading center rift tip at it's present location by at least 2 m.y.
T41B-1962
New views on 40Ar/39Ar dating of high pressure rock exhumation: an example from Oman
40Ar/39Ar dating has long been considered an unreliable method for determining cooling ages of high pressure terranes as a result of the common (though not ubiquitous) presence of excess argon. New advances in analytical techniques now allow 40Ar/39Ar spots and traverses to be measured within different zones of individual mica grains, thereby creating the possibility of measuring intra-grain age changes in order to determine spatial and temporal variations of excess argon in the micas. We will present phengite single grain fusion and intra-grain UV laser 40Ar/39Ar data from a variety of rocks of different composition and metamorphic grade from Saih Hatat, NE Oman. This high pressure terrane, subducted and exhumed during the Late Cretaceous obduction of the Semail Ophiolite Complex, provides ideal material for testing and refining models of subduction-exhumation mechanisms. Single-grain 40Ar/39Ar fusion data from the highest grade phengites fall into two distinct groups. Chemically growth-zoned phengites from the mafic eclogites and surrounding calcareous and quartz schists yield a spread of ages from 80-120 Ma within individual samples, equal to the complete range of previously reported bulk 40Ar/39Ar (mainly step-heating) ages. By comparison, chemically homogeneous phengites from the interlayered eclogite-facies pelitic schists yield a single age population, with mean ages of ca. 80 Ma, similar to the previously reported 79 Ma U-Pb zircon age. The difference between the 40Ar/39Ar cooling age and the U-Pb crystallisation age indicates either remarkably rapid cooling or the presence of some excess argon. However the tight grouping of ages suggests that the excess argon is distributed similarly in each grain. The combination of these data with zircon U-Pb ages and phengite major- element chemistry, allow us to begin quantifying the behaviour of argon in high pressure phengites, and potentially determine better constrained timescales for orogenic cycling.
T41B-1963
Elusive Himalayan eclogites: evidence that they were there from zircon U-Pb geochronology and trace element geochemistry
Mafic and pelitic granulites exposed in the eastern Himalayan kingdom of Bhutan preserve textural evidence for a precursor high-pressure metamorphic event, the precise conditions of which are generally unrecoverable due to the later high temperature overprint. As high pressure metamorphism is rare in the Himalayas, especially in the eastern parts of the orogen, their thermobarometrical and geochronological evolution place important constraints on the geodynamic evolution of the Himalaya in particular and continental collisions in general. We report SHRIMP-RG trace element (REE) and U–Pb zircon geochronological data, collected by the same instrument and on adjacent spots of the same crystal. These data suggest that zircons crystallized at 14-15 Ma over a temperature range of ca. 705-815 ° C. This age is interpreted to indicate the timing of HP metamorphism due to the lack of negative Eu anomaly, the depleted heavy REE signature and the low temperatures of crystallization. Zr-in-rutile electron microprobe data from similar rocks suggests that rutile crystallized over a larger and higher range of temperatures (ca. 620-870 ° C), interpreted as rutile growth during the eclogite-granulite transition and/or during granulite facies metamorphism. These data will help to establish better links between accessory mineral crystallisation and metamorphism and will also help to resolve the conditions and timing of eclogite facies metamorphism in the Himalaya.
T41B-1964
Evidence for a Mid-Crustal Continental Suture and Implications for Multistage (U)HP exhumation, Liverpool Land, East Greenland
The East Greenland Caledonides consists of a series of west-directed sheets that formed from 460-360 Ma as Baltica subducted westward beneath Laurentia, and offer an opportunity to study high- and ultrahigh- pressure exhumation in orogenic hangingwalls. The Liverpool Land (LL) gneiss complex, 100 km east of the nearest Caledonian gneisses, provides a window into the deepest levels of the Greenland Caledonides. From the bottom up, the LL tectonostratigraphy is comprised of the eclogite-bearing Tvaerdal orthogneiss and the granulite-facies Jaettedal paragneiss structurally below the top-N Hurry Inlet Detachment. We present new thermobarometry and U/Pb zircon and titanite geochronology from the LL gneisses to define the tectonostratigraphy, continental affinity, and exhumation histories of the LL gneiss complex. The Tvaerdal orthogneiss consists of felsic orthogneisses that host rare ultramafic bodies (Fo92) and mafic boudins that yield peak pressures of >25 kbar at 800°C. Host gneiss zircons dated using LA-MC- ICPMS yield 1676 ± 17 Ma (2s) cores with 403 ± 6 Ma (2s) rims that suggest Mesoproterozoic emplacement of the original intrusive body followed by late-Caledonian deformation. The Tvaerdal orthogneiss also includes voluminous decompression melts; one yielded a TIMS U/Pb titanite age of 387.5 ± 2.2 Ma (2s). The structurally higher Jaettedal paragneiss consists of pelitic gneisses interlayered with granodioritic-dioritic orthogneisses. The Jaettedal-Tvaerdal contact is petrologically abrupt and concordant to regional foliation and lacks sub-amphibolite-facies displacement. Aluminum silicate-bearing pelitic assemblages within the Jaettedal paragneiss yield peak metamorphic conditions of 10-11 kbar at 750- 800°C. U/Pb age maps made using LA-MC-ICPMS from three paragneisses reveal Mesoproterozoic- Archean detrital cores with Caledonian rim overgrowths that cluster between 439-434 Ma. An amphibolite restite from the Jaettedal paragneiss yielded a TIMS U/Pb titanite age of 413 ± 1 Ma (2s). This new data defines two distinct LL gneiss complexes beneath the Hurry Inlet Detachment and suggests the presence of a previously unidentified continental suture between the Tvaerdal and Jaettedal gneisses. Similar timing, metamorphic conditions, and detrital zircon signatures to units farther inland, as well as the presence of Archean detrital zircons indicate a Laurentian continental affinity for Jaettedal paragneiss. In contrast, ~400 Ma (U)HP metamorphism and Mesoproterozoic basement ages, which have not been identified in Laurentia, suggests correlation of the Tvaerdal gneiss with the Baltican-derived Western Gneiss Region. Furthermore, the suture between the Tvaerdal and Jaettedal gneisses, with kinematics that remain undefined, represents a structure responsible for the juxtaposition of the younger (U)HP Tvaerdal orthogneiss against the older mid-crustal Jaettedal paragneiss, and the initial stages of (U)HP exhumation from mantle depths to lower-middle crustal levels. This initial exhumation may have triggered subsequent displacement along the Hurry Inlet Detachment responsible for the final stages of (U)HP exhumation in the upper crust.
T41B-1965
Structural Relationships Between the Nordfjord-Sogn Detachment Zone and Ultrahigh- Pressure Rocks in the Nordfjord Region, Western Norway
The Nordfjord area of Western Norway hosts an archetypal prograde metamorphic transition within the root of the Scandian collisional orogen, from amphibolite facies to ultrahigh-pressure (UHP) eclogite facies. Although previous studies have correlated amphibolite-facies mylonites across the Nordfjord area with the regional, normal-sense Nordfjord-Sogn Detachment Zone (NSDZ), the large-scale structural relationships of the eclogite-bearing rocks, basement-allochthon tectonostratigraphy, and major zones of deformation remain poorly defined. We carried out a detailed structural, metamorphic and thermochronological analysis of the greater Nordfjord area, examining the structural character of the transition from lower pressure rocks westward to the UHP province. A >5 km thickness of shallowly W-dipping, normal-sense shear fabrics pervades structurally higher allochthonous units and fades downward into the Baltica basement; these mylonites are capped by greenschist-facies detachments. Eclogite and coesite-eclogite isograds cut across the tectonostratigraphy and were attenuated by mylonitic shearing and layer-parallel thinning; no evidence of a discrete high-strain contact with significant pressure difference was found within the UHP boundary zone. Top-W amphibolite-facies mylonitization occurred across a broad section of crust in the Nordfjord area, but was localized along two shear surfaces: 1) a major shear zone within the structurally- higher allochthons that defines the upper boundary of eclogitized crust, which we correlate with the NSDZ; and 2) a localized high-strain zone-informally named the Sandane Shear Zone-that follows an earlier thrust contact between the allochthonous units and the underlying basement. No structural features illuminate the mechanisms responsible for exhumation of the Nordfjord UHP rocks through the mantle; exhumation through lower- to mid-crustal depths was accomplished by normal-sense shear on the NSDZ. Muscovite 40Ar/39Ar cooling ages suggest regional amphibolite-facies shearing was finished by 399-395 Ma; subsequent unroofing was accomplished by higher-level greenschist-facies detachments.
T41B-1966
New Zircon U-Pb age Data From Midsund, Western Gneiss Region, Norway
New zircon U-Pb age data are reported for basement rocks in Midsund, Norway. These samples are located south of the Midsund mylonite, a major mylonite zone with subhorizontal shear that separates ultrahigh- pressure (UHP) rocks to the north from significantly lower pressure rocks to the south. Two samples of granodioritic basement gneiss are cut by mafic dikes similar in geochemistry to dikes within Saetra quartzites. Zircons from the gneiss reveal ~1650–1600 Ma protolith ages, slightly younger than the ~1680–1650 Ma (Gothian) protolith ages common to the area, as well as strong Sveconorwegian signatures, which have not been found in the surrounding Baltica basement. Both samples yield minimal evidence of the Caledonian (U)HP event. These ages contrast with those of a migmatitic, granodioritic basement gneiss separated from the former samples by a sliver of the Blåho Nappe. The migmatitic basement records the slightly older (Gothian) protolith age as well as a ~1600 Ma age but lacks Sveconorwegian ages, recording, instead, Scandian partial melting at 404 ± 3 Ma (MSWD = 1.09). A similar Scandian age of 403 ± 3 Ma (MSWD = 1.5) along with a ~1530 Ma age are recorded in zircons of a deformed pegmatite cross-cutting foliation within another sliver of Blåho Nappe.
T41B-1967
The Tectono-metamorphic Evolution of the Eclogite Zone, Tauern Window, Austria.
Eclogite-facies rocks pertaining to the Penninic realm of the Eastern Alps are exposed as a thin, fault- bounded sliver, juxtaposed against rocks of lower metamorphic grade, within the central Tauern Window, southwest Austria. Previous work suggests that the eclogites formed as a result of southward subduction of the European continental margin beneath the advancing Apulian complex during the Alpine orogeny. The Eclogite Zone itself consists of a diverse metasedimentary pile, dominated by calcareous schists, within which micro- to mega boudins of mafic eclogite are exposed. Peak conditions for both the eclogites and host metasediments are ~ 20 kbar at ~ 600° C. Structurally beneath the Eclogite Zone crystalline Pennine basement and associated cover units form the Venediger Complex, whereas the hanging-wall comprises basement slices (the Rote Wande Nappe) and an in--complete ophiolitic sequence - the Glockner nappe. This work presents results from a revised study of the tectono-metamorphic evolution of the Eclogite Zone and adjacent nappes. New pseudosections for eclogitic pelites show a well-constrained, clockwise P--T path, which culminates in peak conditions within the garnet-chloritoid-kyanite field. X-Ray mapping and REE profiles are employed to link the growth of allanitic epidotes to the rock's P--T evolution. Further pseudosections, which model rocks from both hanging and footwall units provide fresh constraints on the region's P--T evolution. Thermobarometric data are combined with new petrographic and field-based, structural datasets to form the basis of an isotope geochronology study, which aims to resolve conflicting views on the timing of eclogite- facies metamorphism in the Eastern Alps. A better understanding of the region's P--T--t relationship will have important implications for: 1. The geodynamic evolution of the Alpine chain as a whole; namely whether collision occurred contemporaneously along the eastern portion of the belt, and 2. The study of tectonic mechanisms and associated timescales thought to be operating during exhumation of high-pressure metamorphic terranes.
T41B-1968
The Subduction Channel and the Exhumation of HP-rocks: Evidence from the Ligurian Western Alps (Italy).
Recent numerical models predict the exhumation of HP-rocks inside subduction channels of low viscosity serpentinite where the different slices of HP-rocks should show variable PT-paths and ages of metamorphic re-equilibrations; the overall arrangement of the blocks and the matrix should reveal great structural complexity. These features fit the description of a tectonic mélange. In the Italian Western Alps (Voltri Massif) a serpentinite mélange crops out: it contains tectonic blocks, exotic with respect to the country rocks, floating in a metasomatic chlorite - amphibole matrix and displaying heterogeneous HP-metamorphic evolution. The structural evidence, P-T paths and ages of the different blocks suggest coupling of blocks and matrix in the blueschist facies. The geochronological data for the different HP blocks show diachronic metamorphic trajectories suggesting independent tectonic evolution of the different slices inside the channel, as predicted by the subduction channel model. This local-scale pattern occurs at larger scale in the Ligurian Alps, where the HP-mafic units inside country serpentinite may represent different slab fragments exhumed in a serpentinite (wedge) matrix. Available ages of such HP-mafic units reveal an heterogeneous timing of HP crystallization events; the eclogite stage ranging from 52.1 ± 0.5 Ma, to 49.00 ± 0.40 Ma, to 33.6 ± 1.0 Ma. These preliminary observations indicate that slices of subducted rocks were likely exhumed in large-scale serpentinite channels.
T41B-1969
Phase transformations in subducting and exhuming continental crust: Insight from geochemical modeling
Mass flow in orogenic belts is largely driven by differences and changes in density, and the rate of flow is controlled by rheology. During continental orogenesis, both are dictated by the rates and conditions under which quartzofeldspathic crust is eclogitized during subduction and back-reacted during exhumation. The ultrahigh-pressure (UHP) terrane of the Western Gneiss Region of Norway, where a continental margin was subducted to depths >100 km, is an ideal natural laboratory to these processes. The typical rock there is a biotite- and hornblende-bearing quartzofeldspathic gneiss with local muscovite and/or garnet that hosts rare blocks of eclogite. A few gneiss samples preserve (U)HP minerals formed during subduction, like the eclogites, but most show final equilibration at 9 to 13 kbar and ~700 ± 100°C during exhumation. This low-pressure overprint obscures processes that happened earlier. For example, what fraction of this gneiss transformed to eclogite-facies minerals during subduction? The best evidence for transformation at high pressure is the presence of fine-grained biotite + plagioclase symplectite formed from the breakdown of garnet + phengite during decompression. Oddly, however, some quartzofeldspathic gneisses have symplectite, whereas other nearly identical and interlayered gneisses do not. Because the presence of symplectite is essential in determining whether a sample transformed at (U)HP, it is critical to understand what controls the formation of symplectite. Interlayering of symplectite-bearing and symplectite-free quartzofeldspathic gneisses indicates that deformation or local fluid flow are not the controlling factors. Modeling of phase relations using PerpleX (Connolly) shows instead that minor differences in bulk composition (e.g., between alkali-feldspar granite and granite) are the controlling factor: symplectites occur preferentially in rocks with elevated Fe + Mg and Ca, factors that stabilize biotite + plagioclase and destabilize K-white mica. These data suggest that small differences in bulk composition -- even among outwardly similar quartzofeldspathic gneisses -- control phase changes during the subduction and exhumation of continental crust. Thus, the physical and chemical processes that change rock compositions -- even by small amounts -- may ultimately drive mass flow and control rheology in orogenic belts.
T41B-1970
The Zermatt-Saas Ophiolite: a Continuous Slice of Oceanic Lithosphere Detached at 80 km Depth in the Subduction Zone
The western Alps is a classic subduction-related collisional orogen, where large fragments of low-density continental crust (e.g. Dora Maira, Grand Paradis) were deeply subducted and then exhumed together with ophiolitic remnants of the Mesozoic Tethyan oceanic lithosphere (e.g. Monviso, Zermatt-Saas). Whereas the Monviso ophiolitic complex has been recognized as a paleao-subduction channel with tectonic blocks showing a wide range of pressure-temperature conditions, no comprehensive study has yet attempted to evaluate the metamorphic homogeneity of the extensive Zermatt-Saas ophiolite. Zermatt-Saas eclogitic assemblages are represented by omphacite-garnet-epidote-rutile +-lawsonite pseudomorphs +- glaucophane in "classical" MORB-derived metabasalts. Sea-floor hydrothermalized metabasalts are characterized by an unusual peak paragenesis characterized by garnet-chloritoid-talc +- lawsonite pseudomorphs +- glaucophane. Thermobarometric estimates with THERMOCALC and Raman Spectroscopy of carbonaceous material reveal homogeneous peak burial conditions at around 540 +- 20 °C and 23 +- 1kbar. These estimates are slightly lower (c.a. 50 °C - 4 kbar) than those from the rare, adjacent coesite-bearing metasediments, suggesting that most of the ophiolite detached from the slab at depths around 80 km. Our data indicate that the whole of the ophiolite, at least 50 km across, strikingly underwent similar tectonic patterns from burial to early exhumation. During exhumation, pervasive glaucophane recrystallization and later greenschist facies assemblages replaced the earlier eclogitic paragenesis. Early exhumation paths are homogeneous and characterized by nearly isothermal decompression between c.a. 23 and 10 kbar. The Zermatt-Saas ophiolite thus appears to be one of the world's largest oceanic lithosphere fragment exhumed from such depths. These results provide critical constraints on the migration of oceanic crustal slices along the subduction channel and on interplate coupling mechanisms.
T41B-1971
Is Absence of Evidence of UHPM Evidence of Absence: Did Conditions on Earth Before the Ediacaran Period Allow Formation of UHP Rocks but Only Rarely Their Exhumation?
UHPM provides petrologic evidence of transport of continental lithosphere to asthenospheric depth and return of some of these materials to crustal depth. The rock record registers UHPM since the Ediacaran Period, and studies of inclusion assemblages in zircon have increased the evidence of UHPM in Phanerozoic orogens and enabled an assessment of the real estate involved. Plots of apparent thermal gradient vs. age of metamorphism and P vs. age of metamorphism reveal two dramatic changes in inferred thermal environment and inferred depth of metamorphism from which continental lithosphere has been recovered during Earth evolution. First, from the Mesoarchean Era to the Neoproterozoic Era, sutures in subduction-to- collision orogens are marked by eclogite and high-pressure granulite metamorphism (characterized by apparent thermal gradients of 750-350 C/GPa). The P of metamorphism in sutures jumped from <1 GPa during the Eoarchean-Paleoarchean up to 2 GPa during the Paleoproterozoic. Second, from the Cryogenian- Ediacaran to the present, many sutures in subduction-to-collision orogens, and sometimes intracratonic sutures in the overriding plate, are marked by UHPM (characterized by apparent thermal gradients of <350 C/GPa) with P of metamorphism >2.7GPa. Given this pattern of secular change to colder apparent thermal gradients in sutures, the recent discovery of diamonds in zircons of crustal paragenesis in Neoarchean sedimentary rocks is surprising. Maybe UHPM has been possible since the Neoarchean but the evidence was rarely exhumed or if exhumed maybe the evidence was rarely preserved? The Appalachian/Caledonian-Variscide-Altaid and the Cimmerian-Himalayan-Alpine orogenic systems were formed by successive closure of short-lived oceans by transfer and suturing of ribbon-continent terranes derived from the Gondwanan side. Subduction of young ocean lithosphere followed by choking of the subduction channel by arc or terrane collision limited transport of water to the mantle wedge, and suppressed development of small-scale convection, arc magmatism and backarc formation. This allowed the retro- continental margin to remain strong, which favored efficient exhumation of UHPM rocks (Warren et al., 2008, EPSL). How should we interpret the presence of diamonds in detrital zircons (age range 3,050-4,260 Ma) from the Narryer terrane? Menneken et al. (2007, Nature) argue that the age range indicates repeated conditions for diamond formation (or recycling of ancient diamond) and that diamonds imply thick continental lithosphere and crust-mantle interactions since 4,260 Ma! This implies thermal environments and tectonics in the Hadean and Archean Eons similar to the Phanerozoic Eon. However, these ancient zircons originally crystallized from low-T melts (Watson and Harrison, 2006, Science) and the 'age' of the diamonds is only constrained to be > the age of deposition and <3,050 Ma. Williams (2007, Science) suggests that C was introduced as graphite precipitated from COH fluid in fractures/imperfections in zircon prior to deep burial to form diamond during a single event. COH fluid was involved in the formation of diamonds from Phanerozoic UHPM localities, so the hypothesis is viable if an appropriate tectonic model can be developed. I will present a model for the formation and exhumation of an overriding plate source terrane for the diamond-bearing detrital zircons that is consistent with periodic changes in the tectonic regime of Earth (Brown, 2006, Geology), and the geology and likely tectonic setting of the Narryer Terrane-Yilgarn Craton during the Neoarchean. Finally, I will speculate about UMPM during the Proterozoic and exhumation vs. relamination (Hacker et al., Eos, 2007).
T41B-1972
Microstructural and Lattice-Preferred Orientation Analyses of Ductile Shear Zones: Maggia Nappe, Switzerland
Microstructures from 24 samples and quartz lattice-preferred orientations (LPO) of 13 samples from the crystalline Maggia Nappe, Switzerland were measured in order to assess the deformation history, mechanisms, and conditions of ductile shear zone development related to nappe formation. Microstructures suggest dislocation creep-accommodated dynamic recrystallization as evidenced by grain size reduction and grain boundary area reduction (GBAR). However, weak LPOs suggest diffusion processes that destroyed previously developed crystalline fabrics. Large grain size (~100-300m) suggests a major component of diffusion creep-accommodated grain-boundary sliding (GBS) followed by grain size growth. Undulatory extinction in grains implies grain growth was not entirely post deformational and that fabrics are not purely magmatic. Assuming stain rates of 10-13s-1, the deformation history of the Maggia Nappe at conditions of 450° -650° C and 3-10 kbar is constrained as 1) development of a lineated magmatic fabric, 2) development of ductile shear zones and strong LPO through dislocation creep, 3) onset of diffusion creep- accommodated GBS after sufficient grain size reduction, 4) destruction of LPOs, and 5) syn- and post- deformational grain growth.
T41B-1973
Strain Analysis and Integration: Quantifying the Deformation of the Laghetti Area, Maggia Nappe, Switzerland
Fieldwork conducted in a nearly fully-exposed 2300 square meter field area in the Maggia Nappe of the northern Lepontine Alps aimed to test the validity of various strain analysis methods at varying scales and to gain a better understanding of the deformation expressed within the crystalline nappes of the Pennine Zone. Deformation in the Laghetti and surrounding areas exhibits conjugate sets of ductile shear zones with right- and left-lateral displacement sense and a penetrative lineation within the wallrock. Shear zones are generally curvilinear ranging in width from a few centimeters to 3.5 meters. Measured shear zone orientations are highly variable with those showing left-lateral separation striking between N3E and N80E and those with right- lateral separation striking between N60E and N70W. Estimates of strain across the shear zones were obtained using three independent techniques: the calculation of slip vectors based on the displacement of features offset by the shear zone, RfΦ analysis of biotite clots, and biotite LPO using electron back-scatter diffraction (EBSD). Results from all three methods (strain ellipsoids calculated from four slip vector measurements, thirteen RfΦ measurements, and ten LPO samples) are broadly consistent and indicate X/Y strain ratios ranging from 1-7 and Y/Z ratios ranging from 1-8. Integration of the data from all three methods indicates a bulk deformation that is nearly plane strain with a maximum shortening in the NNW-SSE direction, consistent with orogen perpendicular shortening acquired during Alpine orogenesis.