A petrologic investigation of deep-crustal and upper-mantle xenoliths from the Sierra Nevada, California ; constraints on lithospheric composition beneath continental arcs and the origin of cordilleran batholiths

摘要

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.ududThere are few direct observations constraining the vertical extent of the large Cordilleran granitic batholiths and their composition at depths greater than 30 km. I present evidence for a ~ 100 km thick sequence of cogenetic rocks which together comprise the Sierra Nevada batholith of California (SNB). The SNB magmatism produced tonalitic and granodioritic magmas which reside in the Sierra Nevada upper- to mid-crust as well as deep crust/upper mantle mafic-ultramafic cumulates. Samples of the mafic-ultramafic sequence are preserved only as xenoliths in Miocene volcanics which erupted through the central part of the batholith. Sm-Nd and Rb-Sr mineral geochronologic analyses were performed on fresh, cumulate textured, garnet pyroxenite, eclogite and gabbroic xenoliths with large grainsize. All samples equilibrated between ~35 and 100 km beneath the batholith and yield Sm-Nd mineral ages between 81 and 136 Ma, broadly coincident with the previously established period of most voluminous batholithic magmatism in the Sierra Nevada.ududThe whole rock initial [...] and the [...] ratios of the igneous xenoliths are similar to the ratios published for the outcrops of the central SNB. I interpret these xenoliths to be magmatically related to the upper- and mid-crustal granitoids, as cumulates and/or restites. This more complete view of the vertical dimension in a batholith demonstrates that a large mass of mafic-ultramafic residue at depth complements the predominantly granitic batholiths, as predicted by mass balance calculations and experimental studies. The SNB magmatism was a large scale process responsible for efficiently segregating a ~30-35 km thick column consisting predominantly of granitoids from a ~ 70 km mafic-ultramafic, mainly eclogite facies root.ududTrace element data indicate that these garnet-rich assemblages were in equilibrium at depths of 45 km or more with SNB-like tonalitic-granodioritic melts. I propose that the petrologic mechanisms responsible for "distillation" of large scale granitoids in the central Sierra Nevada are similar to the ones which led to the extraction of the voluminous granitoids that make up the cratonic nucleii of the continents. The large magnitude isotopic heterogeneity observed in the xenoliths, and the similar range of isotopic ratios measured in surface granitoids, suggest that the compositional variability observed in the surface granitods is primarily inherited from the source rocks in the deeper parts of the lithosphere. The isotopic ratios of the deep-seated residues/cumulates require the existence of three source components for the batholith: (1) a young supracrustal component, represented by Mesozoic accreted arc rocks, (2) a Proterozoic lower crustal (+old lithospheric mantle) component, and (3) a Phanerozoic depleted mantle component. Xenolith data support the hypothesis of lithospheric scale thrusting of accreted masses over the autochtonous crust of western North-America prior to the generation of large volume granitic magmatism in the Sierra Nevada region.ududBatholith cumulates/residues resided under the batholith as eclogite facies rocks for at least 70 My after magmatism shutoff. However, mantle xenoliths sampled in younger, Pliocene volcanic rocks contain spinel peridotites which equilibrated over the same depth interval as the garnet pyroxenite assemblages from Miocene pipes. Futhermore, no garnet-bearing samples have been recovered from the Pliocene volcanics. Silica-rich glass inclusions, trapped along grain boundaries of peridotites from Pliocene volcanics, have isotopic compositions similar to the eclogitic keel. These geologic observations as well as geophysical constraints suggest that the thick and dense "eclogitic" root may have foundered in the mantle between Mid- Miocene and Pliocene.ud