MESOZOIC METAMORPHIC AND MIDDLE TO LATE TERTIARY MAGMATIC EVENTS ON MAGDALENA AND SANTA-MARGARITA ISLANDS, BAJA-CALIFORNIA-SUR, MEXICO - IMPLICATIONS FOR THE TECTONIC EVOLUTION OF THE BAJA-CALIFORNIA CONTINENTAL BORDERLAND

Citation
Ja. Bonini et Sl. Baldwin, MESOZOIC METAMORPHIC AND MIDDLE TO LATE TERTIARY MAGMATIC EVENTS ON MAGDALENA AND SANTA-MARGARITA ISLANDS, BAJA-CALIFORNIA-SUR, MEXICO - IMPLICATIONS FOR THE TECTONIC EVOLUTION OF THE BAJA-CALIFORNIA CONTINENTAL BORDERLAND, Geological Society of America bulletin, 110(8), 1998, pp. 1094-1104
Citations number
54
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Geosciences, Interdisciplinary
ISSN journal
0016-7606
Volume
110
Issue
8
Year of publication
1998
Pages
1094 - 1104
Database
ISI
SICI code
0016-7606(1998)110:8<1094:MMAMTL>2.0.ZU;2-0
Abstract
An Ar-40/Ar-39 study of metamorphic and volcanic rocks from Magdalena and Santa Margarita Islands, Baja California Sur, Mexico, provides evi dence for at least four events that can be related to the tectonic evo lution of this portion of the southern Baja California continental bor derland prior to, and during, the transition from a convergent to a tr ansform plate boundary system. (1) Metamorphism of amphibolites and qu artz-mica schist from upper plate units occurred in Late Jurassic and Early Cretaceous time (similar to 158-138 Ma). Analogous metamorphic r ocks from upper plate rocks in the Cedros-Vizcaino-San Benito terranes of,vest-central Baja California have not yet been identified. However , the Santa Margarita amphibolites are similar in age to amphibolite b locks from melange of the Puerto Nuevo terrane, (2) Metabasite and met apelitic float blocks associated with serpentinized ultramafic rocks w ere metamorphosed and subsequently cooled in Early Cretaceous time (12 9-115 Ma). Their corresponding Ar-40/Ar-39 age spectra do not show pro nounced age gradients, and they are dissimilar in age, as compared to Late Jurassic (160-170 Ma) amphibolite and late Early Cretaceous (100- 115 Ma) blueschist blocks from melange in the Cedros-San Benito terran es, (3) A 30.7 hla lamprophyre dike that crosscuts normal faults withi n ophiolitic rocks indicates that synsubduction extension of the upper plate on Santa Margarita Island occurred prior to mid-Oligocene time. (4) Latest Miocene-early Pliocene age (5-6 Ma) adakites were erupted into serpentinite-matrix melange on Santa Margarita Island and indicat e derivation by partial melting of a young subducted slab prior to col lision of ancestral East Pacific Rise ridge segments. The discovery of adakites in the continental borderland of Baja California Sur provide s additional constraints for the Neogene thermal evolution of this con vergent margin.