Autori: Saleeby, J, Ducea M.N., and Clemens-Knott, D

Editorial: Tectonics, 22, doi:10.1029/2002TC001374, 2003.

Rezumat:

Eclogites are commonly belived to be highly susceptible to delamination and sinking into the mantle from lower crustal metamorphic environments. We discuss the production of a specific class of eclogitic rocks that formed in conjunction with the production of the Sierra Nevada batholith; and further discuss the removal and sinking of these rocks into the mantle. These high-density eclogitic rocks, however, formed by crystal-liquid equilibria, and thus contrast sharply in their petrogenesis and environment of formation from eclogite facies metamorphic rocks. Experimental studies show that when hydrous mafic to intermediate composition assemblages are melted in excess of 1 GPa the derivative liquids are typical of Cordilleran-type batholith granitoids, and garnet+clinopyroxene which is an eclogitic mineralogy, dominate the residue assemblage. Upper mantle-lower crustal xenolith suites that were entrained in mid-Miocene volcanic centers erupted through the central Sierra Nevada batholith are dominated by such garnet clinopyroxenites, which are shown by geochemical data to be petrogenetically related to the overlying batholith as its residue assemblage.
Petrogenetic data on garnet pyroxenite, and associated peridotite and granulite xenoliths in conjunction with a southward deepening oblique crustal section and seismic data, form the basis for the synthesis of a primary lithospheric column for the Sierra Nevada batholith. Critical aspects of this column are the dominance of felsic batholithic rocks to between 35 and 40 km depths, a thick (~ 35 km) underlying garnet clinopyroxenite residue sequence, and interlayered spinel and underlying garnet peridotite extending to ~ 125 km depths. The peridotites appear to be the remnants of the mantle wedge from beneath the Sierran arc. The principal source for the batholith was a polygenetic hydrous mafic to intermediate composition lower crust dominated by mantle wedge derived mafic intrusions. Genesis of the composite batholith over an ~ 50 m.y. time interval entailed the complete reconstitution of the Sierran lithosphere.
Sierra Nevada batholith magmatism ended by ca. 80 Ma in conjunction with the onset of the Laramide orogeny. The greater Sierra Nevada batholith ceased its magmatism at that time, and the underlying mantle lithosphere was cooled conductively. Disruption of the corner flow circulation of asthenosphere into the wedge environment by the Laramide slab is suggested to have been the prime factor in the termination of magma genesis. In the southernmost Sierra-northern Mojave Desert region the sub-batholith mantle lithosphere, including the garnet pyroxenite residue sequence, was mechanically delaminated by a shallow segment of the Laramide slab, and was replaced by underthrust subduction accretion assemblages. These relations record a segmentation of the Laramide slab beneath the southernmost Sierra, similar to that observed beneath the modern Andes.
Despite these Laramide events the greater Sierra Nevada mantle lithosphere for the most part remained intact throughout much of Cenozoic time. A pronounced change in xenolith suites sampled by Pliocene-Quaternary lavas to garnet absent, spinel and plagioclase peridotites, whose thermobarometry define an asthenosphere adiabat, as well as seismic data indicate that much of the remaining sub-Sierran lithosphere was removed in Late Miocene to Pliocene time. Such removal is suggested to have arisen from a convective instability related to high-magnitude extension in the adjacent Basin and Range province, and to have worked in conjunction with the recent phase of Sierran uplift and a change in regional volcanism to more primitive compositions. In both the Mio-Pliocene and Late Cretaceous lithosphere removal events the base of the felsic batholith was the preferred locus of separation. This is suggested to have resulted from the sharp rheological contrast between weak quartzofeldspathic deep batholithic rocks, and the stronger and much denser residue sequence. Such phenomena may be of global importance in continent edge arcs, and responsible for the common Moho depths of 30 to 40 km observed in the derivative batholithic belts. This process is also suggested to help fractionate and isolate the Earth’s felsic crust over geologic time.

Cuvinte cheie: eclogites, continental arcs, delamination, Sierra Nevada