V53A-01
Age and Petrogenesis of Garnet Peridotites in the Sulu Ultrahigh-Pressure Metamorphic Belt
Investigations of garnet peridotites in the Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt indicate that they have experienced subduction zone UHP metamorphism and mantle metasomatism. However, the formation age, origin and source of the garnet peridotites are still poorly constrained. Re-Os formation ages of 1.8 and 2 Ga are obtained for Raobozhai peridotites from the Dabie terrane and Xugou peridotites from the Sulu terrane, respectively [1,2]. The latter is interpreted to originate from the Yangtze craton. Although the basement beneath the Yangtze craton and the continental lithospheric mantle (CLM) of the eastern block of the North China craton prior to Triassic was Archean, no Archean formation ages of peridotites have been reported from the Dabie-Sulu UHP metamorphic belt. We report Re-Os isotopic results for garnet peridotites from the Pre-pilot hole 1 (PP1) of the Chinese Continental Scientific Drilling Project, located in the western part of the Sulu terrane. Samples from 147 to 245 m include harzburgite, lherzolite, dunite and serpentinite. They are fresher and less serpentinized than those in any other drill holes or field outcrops. Os and Re concentrations range from 0.0089-3.7345 ppb and 0.0049-0.2489 ppb, respectively, with 187Re/188Os varying from 0.0067-6.9925 and 187Os/188Os varying from 0.11163-0.15723. The lack of correlation between 187Os/188Os and 187Re/188Os indicates that the Re-Os isotope system has been disturbed by later metasomatism due to Re mobility. However, most PP1 samples have unradiogenic 187Os/188Os varying from 0.11163-0.11730, among the most depleted peridotites found in China. Re depletion ages (TRD) range from 1.22 to 2.43 Ga, indicating that the PP1 peridotites are at least 2.43 Ga in age. Re-Os model ages (TMA) range from 0.095-2.881 Ga, and three samples have Archean TMA, indicating that the PP1 peridotites could have originated from refractory Archean CLM beneath either the North China or Yangtze cratons. References [1] Jin, Y. et al. 2004 Chinese Science Bulletin 49, 508-513 [2] Yuan, H. et al. 2007 Chemical Geology 236, 323-338
V53A-02
Variable slab and subarc mantle signatures within dying arc setting-clues from the volcanology and geochemistry of Quaternary volcanic rocks from Armenia.
Armenian volcanoes occur within the active continental collision zone involving the Arabian and Eurasian plates. The volcanism is hosted by a chain of pull-apart basins, cumulatively forming an arc across Armenia and extending into Turkey and Iran. We collected fresh volcanic rocks from >100 volcanoes in proximity to the large calc-alkaline strato-volcano Mt.Ararat (Turkey) and the sub-alkaline shield-volcano Mt.Aragats (Armenia).The samples are trachybasalt-andesites o dacites (Aragats Volcanic Plateau) and trachybasalts to rhyolites (Arteni Volcanic Complex, Gegham Plateau and Lake Sevan regions).The major and trace element systematics of the Armenian volcanics reveal mixed arc-like and OIB-like signatures may accompany the transition from subduction to collision (Miocene-recent). Relative to N-MORB our samples show enrichments of fluid mobile elements,Th,U,LILE and LREE,and depletions of HREE and Hf, Nb, Ta and Zr.The lower 87Sr/86Sr ratios (0.7041 to 0.7051) compared to any known crustal material in the region, the regional mantle 144Nd/143Nd isotope ratios [0.5128-0.5129] and the absence of crustal xenoliths cause us to conclude that crustal assimilation did not play a significant role in the magmagenesis.We will report large mineral chemistry dataset and detailed textural observations revealing no significant mineral zoning.Based on mineral rim and groundmass chemistries and using variety of hygrothermometers, we calculated melt H2O contents ranging from 1.9 to 4.5 wt% and also elevated eruption temperatures [range= 1030- 1060°C].This calculations are in agreement with the generally anhydrous nature of the mineral assemblages [Pl+Opx+Cpx+Ol+TiMt] and with the ionprobe study of volatile contents in olivine hosted melt inclusions [H2O = 0.5-2.8 wt%; CO2 = 10-371 ppm; F= 1865-2905 ppm, S= 225-5122 ppm;Cl= 650-1013 ppm]. Although other mechanisms such as delamination and localized extension related to strike slip faulting might also contribute to magma generation we suggest that the unusual combination of anhydrous but fluid mobile element, LILE and LREE-enriched mantle source under the Armenia is due to long-lasting (Jurassic- Miocene) pre-collisional subduction modifications, followed by slab break-off and interaction with hot mantle asthenosphere [1]. Our new data confirms recent tomography scans showing heterogeneous hot mantle domain under the volcanic highlands of Armenia based on large and sharp low shear wave velocity anomaly correlated with long wavelength free-air gravity anomalies [2]. [1] Keskin, M. (2003). Magma generation by slab steepening and breakoff beneath a subduction-accretion complex: An alternative model for collision-related volcanism in Eastern Anatolia, Turkey, Geophys. Res. Lett.,30(24),8046. [2] Maggi, A. and Priestley, K. (2005). Surface waveform tomography of the Turkish-Iranian plateau, Geophys. J. Int.,160, 1068-1080.
V53A-03
A trace element perspective on the veined sub-arc mantle: Melts, fluids, or both?
Convergent margin magmas carry slab signatures, but it is not clear whether this signature is transferred from slab to mantle wedge via an aqueous fluid, a silicate melt, or a supercritical liquid, as the chemical characteristics of arc volcanics can be explained by either melt-mantle interaction or slab fluid additions. In the Kamchatka Arc, mantle xenoliths carried in lavas from Avachinsky volcano comprise refractory harzburgites dissected by amphibole-bearing wherlitic veins. The very low modal clinopyroxene content, the refractory mineral chemistry, and the low bulk Al2O3 and CaO contents in the harzburgites provide evidence for melt extraction. The veins contain orthopyroxene (En88-94), clinopyroxene (En53-48Wo44-49), and minor amounts of magnesiohornblende, but lack olivine. Primitive-mantle normalized trace element patterns of cpx and amphibole show enrichment of U over Th (UN/ThN > 2) and relative enrichments of Pb and Sr compared to elements with similar compatibilities (SrN/NdN > 2.7, PbN/CeN > 2.4). Furthermore, clinopyroxene is characterized by low HREE contents, relative depletions in HFSE, and a positive Li peak. In contrast, amphibole is enriched in HFSE, but has no Li anomaly. Trace element concentrations of bulk veins, calculated from modal mineralogy and mineral compositions, are characterized by relative enrichments in Sr, Pb and Li. These features are also characteristics of both basaltic and andesitic primitive island arc volcanic rocks and suggest a mantle wedge source affected by metasomatic processes that are also evident in the host lavas. The mantle-like Sr isotopic composition of the bulk vein material (0.7040-0.7048), along with the wehrlite vein - host harzburgite Li isotope equilibrium (δ7Li ~ 4 ± 1.5), support the idea of melt- dominated metasomatism. On the other hand, enrichment of the fluid-mobile elements, in particular Pb, and the elevated U/Th ratios are more consistent with fluid-dominated metasomatism.
V53A-04
Partitioning of deformation within high-strain metapelitic zones and preservation of early high- pressure metamorphic structures : Insights from phase relation modelling along a cold alpine geothermal gradient
The influence of shearing and fluid transfer on phase relationships is investigated in a alpine eclogitic massif of
metapelitic composition, deformed at different crustal levels, from blueschist to greenschist metamorphic
conditions. Phase diagram sections are commonly used to study phase relationships as a function of pressure
and temperature in closed-systems. Here, an open system model without mass element transfer is required to
discuss the interaction between fluid flow and mineral phase evolution occurring during deformation. Mapping
phase relations as a function of fluid flow variation, from underdeformed to mylonitic metapelitic sequences
provides a model coupling metamorphism, deformation and fluid circulation. Predictions of phase composition
and modal proportion of P-T-MH20 phase diagrams (using Perplex calculations) are in good agreement with
natural observations. They suggest that the distribution of shearing throughout the massif leads to the
preservation or destabilisation of the HP-assemblages it contains. Thermodynamic modelling of open systems
illustrates how the development of small-scale (mm-cm) textural heterogeneities in high-strain zones, such as
large porphyroblasts of albite, have preferentially been controlled by the amount of H20 brought by shearing,
rather than by the effective bulk rock composition. Such effects of Def versus MH20 would have a profound impact
on the control of rheological properties for the bulk rock, and ultimately the processes of deformation leading to
the exhumation of the Internal Briançonnais massifs.
Keywords: High-strain zone, strain partitioning, channelized fluid-flow, High-Pressure-metapelites, P-T-MH20
phase diagram, exhumation
http:www.lmtg.obs-mip.fr/