科研之友
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摘要
Antigorite peridotite, Cr-magnetite dunite, chlorite harzburgite, and other ultramafic rock fragments included in the Green Knobs diatreme in NW New Mexico are used to investigate the evolution of the mantle wedge during low-angle subduction. The diatreme, part of the Navajo Volcanic Field, has a matrix of serpentinized ultramafic microbreccia (SUM). The meta-peridotite inclusions have been characterized by petrographic, electron microprobe, and bulk-rock analysis. The assemblage antigorite-diopside-olivine formed and was stable in the mantle. In a Cr-magnetite dunite, olivine compositions span the range Fa(9-3), and chlorite-rich pockets contain extraordinarily pure pyrope (Py(94 center dot 4)Alm(5 center dot 5)Gr(0 center dot 1)). The Cr-magnetite dunite is a product of prograde metamorphism of a brucite-bearing serpentinite. Second generations of olivine in that dunite and other samples formed during a sharp temperature increase caused by intrusion of the magma that triggered the eruption. Histories of lawsonite eclogite, garnetite, and other inclusions in SUM diatremes have been interpreted to provide context: some are from the lithospheric part of the mantle wedge, hydrated near the trench, tectonically eroded and transported some 700 km in a melange, and emplaced below the Colorado Plateau during low-angle subduction of the Farallon plate. The Cr-magnetite dunite was also derived from that melange, whereas some other inclusions represent mantle hydrated in place above the slab. Tectonic erosion of the Colorado Plateau mantle lithosphere, or serpentinite diapirism into that lithosphere, or both, accompanied the low-angle subduction. Serpentinization far from the trench and low-angle transport of parts of the lithospheric mantle wedge are consistent with models of the Laramide orogeny and may be common during low-angle subduction.
- 出版日期2010-6
