HR: 14:25h
AN: T23D-04    [Abstracts]
TI: Synchronous Sm-Nd Mineral Ages from HP Terranes on Both Sides of the Motagua Fault of Guatemala: Convergent Suture and Strike Slip Fault?
AU: * Brueckner, H K
EM: hannes@ldeo.columbia.edu
AF: School of Earth and Environmental Sciences, Queens College and The Graduate Center, Flushing, NY 11367 United States
AU: * Brueckner, H K
EM: hannes@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 United States
AU: Hemming, S
EM: sidney@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 United States
AU: Sorensen, S
EM: sorena@volcano.si.edu
AF: Dept. Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560 United States
AU: Harlow, G E
EM: gharlow@amnh.org
AF: Dept. Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024 United States
AB: Plate tectonic reconstructions and the large scale of the Motagua Fault Zone (MFZ) of Guatemala suggest it is now an active plate boundary containing a series of left-lateral lateral faults that have accommodated displacements of more than 1000 km. Curiously, a major strand of the MFZ, the Motagua Fault (MF) exposes adjacent serpentinite mélange on both sides that contain HP/LT eclogites, as well as jadeitites, albitites and other unusual rocks. Ar-Ar ages from white micas of 116-125 Ma from HP/LT rocks south of the fault are significantly older than the 53-100 Ma ages of comparable rocks north of the fault suggesting the unlikely possibility that two similar eclogite-bearing terranes of different ages were coincidentally juxtaposed by strike-slip motion along the fault. However, two Sm-Nd mineral isochrons from eclogites south of the fault give ages of 131.7 ± 1.7 and 132 ± 4.6 Ma, which are identical to an age of 130.7 ± 6.3 Ma from an eclogite sampled north of the fault. It is even less likely that strike-slip motion would juxtapose two synchronous and nearly identical HP/LT terranes that originally were more than 1000 km apart. Either the MF has not undergone significant displacement since the lower Cretaceous, implying that pre-Cretaceous displacement was accommodated by movement along other faults (e.g. the Polochic fault to the north), or the MF has developed strike-slip duplexes that have shifted HP/LT rocks to the other side of the fault. There are complications to both models since the eclogites south of the MF are different from those in the north. Southern eclogites contain lawsonite (an indicator of lower T), give MORB major and trace element patterns, are isotopically depleted (87Sr/86Sr ratios of 0.70374 and 0.70489, Epsilon Nd of +8.6 and +9.2) and give the older Ar-Ar ages. Eclogites from north of the fault lack lawsonite and generally show an amphibolite overprint. One surviving northern eclogite has a significantly more enriched signature (87Sr/86Sr = 0.70536; Epsilon Nd = -2.1) and associated rocks give the younger Ar-Ar ages. The MF apparently represents a complex suture with eclogites derived from depleted mantle to the south and enriched mantle to the north. Explaining these features demands a complex origin, both tectonically and petrogenetically.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1115 Radioisotope geochronology
DE: 8010 Fractures and faults
DE: 8111 Continental tectonics: strike-slip and transform
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005