HR: 0800h
AN: V51D-1520    [Abstracts]
TI: Assembly of lower arc crust: Natural and Experimental constraints
AU: Ulmer, P
EM: peter.ulmer@erdw.ethz.ch
AF: ETH Zurich, ETH-Zentrum, NO E26, Zurich, 8092 Switzerland
AU: * Muntener, O
EM: othmar.muntener@geol.unibe.ch
AF: University of Bern, Baltzerstrasse 3, Bern, 3012 Switzerland
AB: Exposed deep roots of island arcs (e.g. Kohistan arc) reveal that garnet- and amphibole-bearing plutonic rocks dominate arc lower crustal rocks. Pressure estimates for garnet-bearing rocks range from 0.8-1.8 GPa (25-60 km depth) reflecting conditions of mature island arc settings. Combined petrologic and experimental studies on the phase relations among garnet, amphibole, pyroxenes and plagioclase under conditions relevant for the genesis of the deep arc crust quantify the relative importance of pressure, temperature, water content and fO₂ on the crystallization sequence of lower arc crust cumulates. The Jijal ultramafic-mafic sequence forming the base of the Kohistan terrane shows locally cyclic cumulate units consisting of ultramafic cumulates (websterites, dunites, garnet clinopyroxenites), garnet hornblendites and garnet gabbros. Garnet hornblendites and garnet gabbros have preserved some igneous textures. Garnet, cpx, hbl and Fe-Ti oxides in the hornblendites form early cumulus phases that equilibrated at temperatures of 1050-900C, pressures exceeding 1 GPa, and fO₂ close to NNO. Garnet gabbros consist of garnet, hbl, plagioclase, ilm/ru, ±epidote, ±zircon and ±apatite as cumulus phases. The cotectic intergrowth of epidote + qtz + paragonite (±hbl ±grt ±alb) between the cumulus phases formed from trapped, peraluminous, tonalitic, interstitial liquid. We compare field results with high pressure experiments performed at 0.8-1.2 GPa and 800-1000C on an H₂O undersaturated andesite. Phase relationships indicate that garnet pyroxenites and garnet-pyroxene gabbros (at 4 wt% H₂O) and garnet hornblendites (at 6-8 wt% H₂O) crystallize at high-pressure (1.2 GPa) and high-temperature (900-1000C). Hornblendites, hornblende gabbros (plagioclase+hbl+oxides) formed at lower temperatures (900C). Garnet-bearing amphibole-dominated gabbros are formed at 0.8 GPa, 6-8 wt% H₂O and 800-950C. At 4 wt% H₂O, spinel-bearing amphibole gabbros crystallized. These results suggest that garnet and amphibole fractionation from water undersaturated andesititc liquids exert an important control on the formation of intermediate derivative liquids in the genesis of arc crust.
DE: 1020 Composition of the continental crust
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3640 Igneous petrology
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005