Resumen

SANCHEZ NAVARRO, Teresita et al. Geology and geochronology of the Magdalena-Madera metamorphic core complex lower plate in the sierras Las Jarillas-El Potrero, northern Sonora. Rev. mex. cienc. geol [online]. 2025, vol.42, n.2, pp.93-112.  Epub 31-Oct-2025. ISSN 2007-2902.  https://doi.org/10.22201/igc.20072902e.2025.2.1838.

The area of sierras Las Jarillas-El Potrero in northern Sonora is interpreted as a portion of the lower plate of the Magdalena metamorphic core complex. Rock units that characterize this locality range in age from Jurassic to Eocene. Jurassic units comprise a small area of metasedimentary rocks that is probably correlative with the regional Middle Jurassic La Jojoba metasandstone and, is intruded by the La Cebolla granite of 158.1 ± 1 Ma (U-Pb, zircon). A younger succession consists from base upwards of a lower metasedimentary unit, a metalimestone unit, and an upper metasedimentary unit. The lower and upper units have similar lithologies of metapelites, metasandstones, and metavolcanic rocks for which maximum depositional ages (U-Pb, zircon) of ca. 119 to ca. 112 Ma were obtained. The intermediate unit consists of recrystallized limestone with interbedded metapelite and metasandstone. These three units that probably correlate with nearby unmetamorphosed outcrops of the Bisbee Group are intruded by the two-mica and garnet leucocratic mylonitic granites Las Jarillas, dated (U-Pb, zircon) at 60.1 ± 0.9 Ma (MSWD= 7), and La Yegua, dated at 46.9 ± 0.3 Ma (MSWD= 0.8). At the outcrop scale, Las Jarillas granite is fine-grained and saccaroid, whereas the La Yegua granite is medium- to coarse-grained, with common pegmatitic zones and abundant xenoliths of quartzo-feldspathic schists.

Mylonitic foliation in the area predominantly strikes NNW-SSE and dips gently NE and SW, with a NE65°SW trending lineation and average plunge of 16° NE and SW. The mylonitic granites record ductile to brittle-ductile deformation with foliation defined by bands of dynamically recrystallized quartz that envelop feldspar porphyroclasts. The metasedimentary and metavolcanic rocks show continuous and/or spaced cleavage defined by alignments of muscovite, biotite and epidote and subordinate tourmaline, titanite, garnet, hornblende, staurolite, actinolite, zoisite, and opaque minerals. These minerals suggest upper greenschist to lower amphibolite facies metamorphism for the rocks in the area. Mica-fish and S-C foliation kinematic indicators consistently indicate a top-to-the-SW tectonic transport direction. A concentrate of muscovite and quartz from a metasandstone sample yielded a cooling age of 24.14 ± 0.57 Ma (40Ar/39Ar), similar to muscovite ages reported by Wong et al. (2010) for the nearby Magdalena-Madera metamorphic core complex. We consider Sierras Las Jarillas-El Potrero to be part of the Magdalena-Madera complex that was exhumed in the footwall of the regional Magdalena-Tubutama detachment fault, which is inferred to pass directly southwest of the study area. A secondary, west-dipping detachment fault in the study area separates the Jurassic and Lower Cretaceous footwall successions near Cerro Picacho. The identified Jurassic through Eocene units, structural overprint, and new geochronologic results are consistent with large-magnitude late Oligocene-early Miocene extension that has affected the region.

Palabras llave : Magdalena-Madera metamorphic core complex; U-Pb geochronology; ⁴⁰Ar/³⁹Ar thermochronology; Cenozoic; Sonora.

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