USGS2023
Eugeoclinal metasedimentary and metavolcanic rocks in the Lane Mountain area, California, are considered part of the El Paso terrane, which is commonly thought to have been displaced several hundred kilometers (km) southeastward from its place of origin during late Paleozoic truncation of the North American continental margin. Uranium-lead dating of detrital zircons from this area was undertaken to limit the depositional ages of these nearly non-fossiliferous metamorphic rocks. Analysis of detrital zircons from 17 metasedimentary rock samples yielded a composite age distribution that ranges from Archean to Jurassic and has significant peaks at ~2,800 2,400 mega-annum (Ma), 2,100–1,600 Ma, and ~300–200 Ma. The Proterozoic and Archean ages indicate derivation from continental sources in ancestral North America, whereas the late Paleozoic and Mesozoic ages are interpreted as derived from a magmatic arc that began to develop along the continental margin in Permian to Triassic time. The 17 detrital zircon samples are from quartzitic and conglomeratic rocks of the Carbide, Williams Well, and Noble Well formations, which were informally named by T.H. McCulloh in 1960. The zircon data indicate that the oldest rocks in the Carbide formation are quartzites likely correlative with the Ordovician Eureka Quartzite of the Cordilleran miogeocline. These rocks lie structurally above the rest of the Carbide formation, different units of which yielded zircons that indicate maximum depositional ages ranging from middle Paleozoic to Late Triassic. Zircons from the Williams Well and Noble Well formations indicate maximum depositional ages of late Paleozoic and Early Jurassic, respectively. The Noble Well formation is interpreted to correlate with the lithologically similar, Early Jurassic, Fairview Valley Formation of the Black and Quartzite Mountain areas some 60 km to the southwest. The above interpretations depend on the presumption that the detrital zircons in these samples did not undergo extreme, postdepositional lead loss, which would result in misleadingly young ages. Although such lead loss is considered unlikely for these samples, further work could test the validity of this interpretation. Zircons from six additional samples were also analyzed: (1) a quartzite from which all the zircons are interpreted to have formed by Late Jurassic metamorphism; (2) three samples interpreted as albitized igneous rocks of Middle Permian age; and (3) two samples interpreted as fine-grained monzonite to diorite of Late Jurassic age. Both sets of igneous rocks were initially thought to be metasedimentary but were reinterpreted as igneous largely on the basis of the zircon data. Based on the interpretations presented here, this study demonstrates that the depositional, magmatic, and deformational history of the El Paso terrane was longer and more complex than previously thought and will require reevaluation of existing tectonic models involving this terrane.