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Autori: D.L. Barbeau, M.N. Ducea, G.E. Gehrels, S. Kidder, J.B. Saleeby
Editorial: Geological Society of America Bulletin, 117 (3), p.466-481, 2005.
Salinia is an out-of-place granitic terrane in central coastal California, whose origin is critical to understanding the tectonic development of southwestern North America. Although Salinia most easily fits geologically and geochemically in the modern gap between the Sierra Nevada and Peninsular Ranges batholiths of the California arc, paleomagnetic and geologic arguments have been used to suggest Salinia may have originated much farther south, possibly in the latitudes of southern Mexico. To further address this problem, we report U-Pb detrital zircon geochronologic data collected by laser-ablation multi-collector inductively coupled plasma mass-spectrometry (LA-MC-ICPMS) methods from six Salinian metasedimentary framework (Sur Series) and sedimentary cover samples. Samples collected from the Sur Series have Late Archean (2.5-2.9 Ga), late Paleoproterozoic (1.6-1.9 Ga), Mesoproterozoic (0.9-1.5 Ga), Neoproterozoic (0.65-0.8 Ga), Paleozoic (250-450 Ma), and possibly Mesozoic detrital zircon U-Pb ages. Samples collected from Upper Cretaceous cover units have various age-peak distributions, which collectively include late Paleoproterozoic (1.6-1.8 Ga), early Mesoproterozoic (1.35-1.55 Ga), Permo-Triassic (220-290 Ma), and Jurassic-Cretaceous (80-190 Ma) peaks. Maximum depositional ages of the Sur Series and cover intervals are 280-360 Ma and 78-90 Ma, respectively, as indicated by the youngest age peaks of known detrital origin in each set of samples. Five of six key detrital-zircon age peaks identified in Salinian basement and cover units are present in western North America, and are nowhere more closely arranged than in the Mojave Desert-Peninsular Ranges region of Baja and southern Alta California. The presence of Late Archean and early Paleoproterozoic zircon suggest that uplift and erosion of ancient basins that were once adjacent to Archean-Paleoproterozoic terranes delivered sediment to the Salinian basement protolith. Broad late Paleoproterozoic and Mesoproterozoic age-peaks infer that Salinia was partially composed and covered by sediment derived from terranes associated with the polyphase development of the Rodinian supercontinent. The abundant pre-Mesoproterozoic detrital-zircon ages in sampled Sur Series and cover units preclude the possibility that Salinia originated in southern Mexico. Numerous Neoproterozoic zircon in one Sur Series sample suggest that while western and southern North America was adjacent to South America during Pangean time, dissection of terranes associated with the Braziliano orogeny may have delivered sediment to the western North American miogeocline. Uplift and erosion of these and associated rocks during Phanerozoic orogenesis along the western margin of North America is interpreted to have transferred the Neoproterozoic, Mesoproterozoic, Paleoproterozoic and Late Archean zircon to Salinia. Paleozoic and early Mesozoic detrital zircon in Sur Series and cover units have ages similar to many plutonic events that occurred along the western margin of North America, and do not allow unambiguous provenance interpretations with other considerations. However, overlap of a Permian age peak in one cover sample (ESALEN) with granitoid ages in the El Paso Mountains also favors a Mojave Desert origin over suggested candidates in southern Mexico. Collectively, these and other data suggest that Salinia resided in the Mojave Desert-Peninsular Ranges region from the late Paleozoic until the Late Cretaceous, after which it was rapidly exhumed, deposited upon, and then translated outboard and northward to its current position.
Cuvinte cheie: tectonics, Salinia, detrital zircon, provenance, California