HR: 16:15h
AN: T34B-02    [Abstracts]
TI: Petrology of upper mantle peridotites and gabbros at site 1277 (ODP Leg 210): Inherited high degrees of melting in a slow extensional system?
AU: * Muntener, O
EM: muntener@geo.unibe.ch
AF: University of Bern, Baltzerstr. 1, Bern, 3012 Switzerland
AU: Jagoutz, O
EM: jagoutz@geo.unibe.ch
AF: University of Bern, Baltzerstr. 1, Bern, 3012 Switzerland
AU: Manatschal, G
EM: manatschal@illite.u-strasbg.fr
AF: EOST, University of Strassbourg, 1, rue Blessig, Strassbourg, 67084 France
AB: ODP Site 1277 in the Newfoundland margin has penetrated about 80 m into a shallow basement high just on the oceanic side of magnetic anomaly M1. Cores from the upper part show a complex assemblage of basalt flows (50%) interleaved with sandstones (10%) and mass flows containing clasts of gabbroic rocks and serpentinized peridotite (20%) and variably deformed gabbroic rocks (20%). The lower 40m of the basement consists of serpentinized peridotite plus minor gabbroic veins (less than 5%). In pebbles from the mass flows between 100 to 140 m depth, the peridotites are strongly serpentinized, mylonitic harzburgites, which preserved an extreme chemical depletion ((Cr in spinel: 42 to 64, with calculated degrees of melting between 15 and 20%). Between 140 and 180 m depth, the in-situ basement consists of serpentinized, mylonitic harzburgites that are slightly less depleted (Cr in spinel: 34 to 51, TiO2 in cpx <0.09 wt% inversely correlated to Cr). Their chemistry indicates an evolution of progressive melt extraction, with 13 to 17% of partial melting. Ti in spinel is generally low (<0.16 wt%), except close to small gabbroic veins (TiO2 <0.6 wt%). The gabbro clasts in the upper part of the basement are mostly differentiated gabbros (cpx Mg# <80, An50-65) with minor oxide gabbros and rare anorthositic layers. A second type of intrusive rock occurring primarily in the lower 40m of the basement is monzonitic, containing two ternary feldspars (Oligoclase: An14-8, normally zoned; and microcline Or55-65), a Ba-rich hyalophane (Ce26-34) with a rim of nearly pure K-fspar, Mg-hornblende, biotite and rare zircon. An intriguing possibility is that these rocks are plutonic/subvolcanic equivalents of mafic sills intruding the uppermost aptian/base albian sediments at site 1276. It thus appears that the peridotite basement of the Newfoundland margin records high degrees of melt extraction, while the mafic rocks provide little evidence of being produced by high degrees of melting. We speculate that the mantle signature is recording a pre-rifting event (Variscan or older), which is also supported by the low equilibration temperatures.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3035 Midocean ridge processes
DE: 3614 Mid-oceanic ridge processes (1032, 8416)
DE: 3621 Mantle processes (1038)
DE: 8105 Continental margins: divergent (1212, 8124)
SC: Tectonophysics [T]
MN: Fall Meeting 2005