HR: 08:15h
AN: V41A-02    [PDF]
TI: Possible Melts of Delaminating Lower arc Crust Beneath Arcs
AU: * Hanghoj, K
EM: khanghoj@whoi.edu
AF: Woods Hole Oceanographic Institution, MS#8, Woods Hole, MA 02543 United States
AU: Kelemen, P B
EM: peterk@whoi.edu
AF: Woods Hole Oceanographic Institution, MS#8, Woods Hole, MA 02543 United States
AB: Theoretical models predict gravitational instability of dense igneous cumulates and orthogneisses overlying less dense upper mantle peridotites (e.g., Herzberg et al CMP 83; Kay \& Kay Geol Rundsch 91; Jull \& Kelemen JGR 01). This is especially likely when cumulates include pyroxenites, and when temperatures fall below $\sim$ $900\deg$C at pressures greater than $\sim$ 0.7 GPa, leading to transformation of high-Al arc gabbronorite to garnet granulite. This can induce viscous foundering or delamination of the base of the crust. Primitive cumulates and garnet granulites are present in small amounts near the base of the Jurassic Talkeetna arc section in south central Alaska. Recent work suggests that these are remnants of a much larger proportion of primitive cumulates, most of which is missing from the otherwise complete arc section, perhaps due to delamination (Greene et al J Pet submitted; Kelemen et al Treatise on Geochemistry, 03). If delamination of dense lower crustal rocks occurs in arcs, one might expect this component to participate in arc magmatism (Gromet \& Silver J Pet 87; Kay \& Kay, Tectonophys 93; Kay et al JGR 94). In the western Aleutian island arc, primitive andesites that are light REE enriched and heavy REE depleted, indicative of abundant garnet in their source, may form by partial melting of subducted eclogite, followed by reaction with the mantle during ascent into the arc crust (e.g., Kay, JVGR 78; Yogodzinski, JGR 95; Kelemen et al., AGU Monograph 03). However, Aleutian samples with this garnet signature have depleted isotopic compositions, with both $^{206}$Pb/$^{204}$Pb and $^{87}$Sr/$^{86}$Sr lower than in Pacific MORB sampled to date. Talkeetna pyroxenites and garnet gabbros generally have U/Pb and Th/Pb less than, and Sm/Nd and Lu/Hf greater than the MORB source, and with time, Talkeetna lower crust should evolve to isotope ratios more depleted than the MORB source. Anatexis of garnet-bearing lower crustal rocks will produce isotopically depleted, light REE enriched, heavy REE depleted partial melts. Interaction of these granitic melts with surrounding mantle peridotite will raise their Mg\# and Ni, and lower SiO$_{2}$ to produce andesitic or basaltic hybrid magmas. Moho temperatures in the western Aleutian arc may be relatively low, below $900\deg$C, due to slow convergence and low magma flux, enhancing formation of garnet granulite and subsequent delamination. Lower arc crust formed during early Aleutian magmatism, with the isotopic composition of MORB at 40 Ma and trace element patterns of Talkeetna pyroxenites and garnet granulites, will evolve to yield isotope ratios like those of Miocene and present day Aleutian andesites. Thus, melting of delaminated garnet granulites within the mantle wedge may be a viable alternative to partial melting of subducted eclogite, producing the light REE enriched, heavy REE depleted component in western Aleutian primitive andesites. Tectonic erosion of old arc basement from forearcs, and incorporation of these rocks into the mantle wedge, might have similar consequences.
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting