Structural geology and 40Ar-39Ar geochronology of the Goldstone–Lane Mountain area, Mojave Desert, California

Abstract

Metasedimentary rocks near Goldstone, California, consist in ascending structural order of (1) a calc-silicate hornfels unit; (2) a quartzite, schist, and radiolarian chert unit; and (3) a calcareous and pelitic sequence of rocks that contains minor basaltic or andesitic composition dikes, sills, and flows(?). Unit 1 is of unknown, but probably pre-Mesozoic age; unit 2 is lower Paleozoic, in part Ordovician, and is correlative to lower Paleozoic eugeoclinal rocks in the El Paso Mountains north of the Garlock fault; unit 3 is probably correlative to upper Paleozoic (Permian) basinal sequences in the El Paso Mountains. These rocks were strongly deformed during a progressive deformational event that caused isoclinal to tight folding about northwest-southeast–trending fold axes and northeast-dipping axial planes. Tectonic transport during this deformation was in a southwesterly direction. Similarity of structural trends to those in the El Paso Mountains suggests that the deformation in these two areas is the same age and predates intrusion of Permo-Triassic plutons in the El Paso Mountains. Regional relationships suggest that deep-water metasedimentary rocks in the northwestern Mojave Desert and in the El Paso Mountains are allochthonous with respect to miogeoclinal and cratonal Paleozoic rocks elsewhere in the Mojave. The emplacement of the allochthonous rocks postdates deposition of Permian strata and predates Permo-Triassic to earliest Triassic deformation and plutonism. This deformation involved both eugeoclinal and miogeoclinal sequences and took place within an Andean arc setting. The subsequent Mesozoic history of these two parts of the Mojave Desert has been the same. 40 Ar- 39 Ar data on hornblende, biotite, and muscovite in the Goldstone and Lane Mountain quadrangles indicate that a plutonic complex consisting of gabbro, quartz diorite, and tonalite was intruded during the Late Jurassic, probably slightly earlier than 148 m.y. ago. Voluminous granitic rocks were intruded during the Late Cretaceous, about 85–90 m.y. ago. These younger plutonic rocks caused variable resetting of all K-Ar systems in older rocks. Cooling of Late Cretaceous plutons to about 280 ± 40 °C occurred by about 78.4 m.y. Regional uplift of these rocks to a crustal level where no further diffusive loss of argon took place occurred at about 45 m.y. ago.