Geology, geochronology, and geochemistry of Isla Maria Madre, Nayarit, Mexico
Por:
Pompa-Mera, V, Schaaf, P, Hernandez-Trevino, T, Weber, B, Solis-Pichardo, G, Villanueva-Lascurain, D, Layer, P
Publicada:
1 abr 2013
Categoría:
Geology
Resumen:
Islas Marias archipelago is located 110 km NW of San Blas, Nayarit in
the mouth of the Gulf of California. The archipelago is formed by San
Juanito, Maria Madre, Maria Magdalena, and Maria Cleofas islands. The
position of these islands represents a key point for paleogeographic and
paleotectonic reconstructions of northwestern Mexico and of the tectonic
evolution of Baja California Peninsula. Maria Madre is the largest
island and covers an area of 145 km(2). This study presents the first
detailed geological map of the island together with geochemical and
geochronological data of its lithological units.
Isolated basement rocks are exposed along the western coast in the form
of migmatites and orthogneisses of granodioritic to granitic
compositions and middle Jurassic ages (163 - 170 Ma). In the
west-central part, a metasedimentary sequence, with biotite +/- garnet
paragneisses and folded calcsilicate rocks with or without garnet bands
of unknown ages, is exposed as a roof pendant of the underlying
Cretaceous intrusions (80.8 - 83.4 Ma) of tonalitic to granitic
compositions. These granitoids are cut by mafic and pegmatitic dikes. We
named the overall assemblage of metamorphic and plutonic rocks as
``Papelillo Complex'' due to the predominant outcrops in the
homonymous canyon. The Papelillo Complex is overlain by ignimbrites,
volcanic breccias, and lava flows of rhyolitic compositions. Ar-40/Ar-39
geochronology in sanidines from these rocks yielded Cretaceous
(71.6-80.6 Ma) and Tertiary (55.4 Ma) ages, suggesting contemporaneous
volcanic and plutonic activity. This inference is confirmed by similar
REE patterns in both units with typical distributions for a
subduction-related magmatic arc environment.
Towards the east, the igneous sequence is covered by Late Miocene to
Pleistocene? marine and shallow marine deposits and towards the south by
an Early Miocene? clastic continental sequence (informally named Isla
Magdalena sandstone). Marine and shallow marine sediments were
informally named ``Ojo de Buey sequence'' and subdivided into a lower
and an upper member Detrital zircon ages from both units display major
peaks at ca. 83 Ma, indicating sedimentation mainly from plutonic and
volcanic rocks of the same age. In the Ojo de Buey sequence, 120 zircons
younger than Late Cretaceous were found. The Isla Magdalena sandstone,
instead, shows a prominent peak at similar to 22 Ma, suggesting detritus
from a different igneous source.
The lithologies of Isla Maria Madre are similar to those from the Los
Cabos Block in Baja California Sur and from central Sinaloa. The overall
area forms part of a Cretaceous plutonic belt, which definitely rules
out large latitudinal displacements for the southern Baja California
Peninsula.
Filiaciones:
Pompa-Mera, V:
Univ Nacl Autonoma Mexico, Inst Geofis, Mexico City 04510, DF, Mexico
Schaaf, P:
Univ Nacl Autonoma Mexico, Inst Geofis, Lab Univ Geoquim Isotop LUGIS, Mexico City 04510, DF, Mexico
Hernandez-Trevino, T:
Univ Nacl Autonoma Mexico, Inst Geofis, Lab Univ Geoquim Isotop LUGIS, Mexico City 04510, DF, Mexico
Solis-Pichardo, G:
Univ Nacl Autonoma Mexico, Inst Geol, Dept Geoquim, Lab Univ Geoquim Isotop LUGIS, Mexico City 04510, DF, Mexico
Villanueva-Lascurain, D:
Univ Nacl Autonoma Mexico, Inst Geofis, Mexico City 04510, DF, Mexico
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