Geology and geochronology of granitic batholith complex, Sinaloa, México: Implications for Cordilleran magmatism and tectonics

Abstract

Most of southern Sinaloa is underlain by a large, composite batholith, a continuation of the better-known Cordilleran batholiths of California and Baja California. Field relations and extensive K-Ar and U-Pb dating within a 120-km-wide and 120-km-deep transect show that the Sinaloa batholith formed in several stages. Early layered gabbros have hornblende K-Ar ages of 139 and 134 Ma, although whether these record emplacement age, cooling from metamorphism, or excess Ar is unresolved. A group of relatively mafic tonalites and granodiorites were emplaced before or during an episode of deformation and are restricted to within 50 km of the coast. These plutons, referred to here as syntectonic, show dynamic recrystallization textures that suggest deformation between 300° and 450 °C. A U-Pb zircon date on a probable syntectonic intrusion is 101 Ma. Hornblende K-Ar ages on definite syntectonic intrusions range between 98 and 90 Ma; these may record cooling soon after emplacement or following regional metamorphism. Numerous posttectonic intrusions crop out from within ~20 km of the coast, where they intrude syntectonic rocks, to the eastern edge of the Sierra Madre Occidental, where they are covered by middle Cenozoic ash-flow tuffs. Posttectonic rocks are dominantly more leucocratic granodiorites and granites. Three samples were analyzed by both U-Pb and K-Ar methods. Their biotite and hornblende ages are concordant at 64, 46, and 19 Ma and agree within analytical uncertainties with U-Pb zircon ages of 66.8, 47.8, and 20 Ma. These data and field relations demonstrate that posttectonic intrusions were emplaced at shallow depths and cooled rapidly. Therefore, concordant K-Ar age pairs and hornblende ages in discordant samples approximate the time of emplacement. Discordance of biotite-hornblende age pairs is largely if not entirely a result of reheating by younger plutons. The combined age data indicate that posttectonic intrusions were emplaced nearly continuously between 90 and 45 Ma. One intrusion is 20 Ma. Based on outcrop area, volumes of intrusions were relatively constant through time. The combined geochronological data indicate that posttectonic magmatism shifted eastward between 1 and 1.5 km/Ma. Whether syntectonic magmatism also migrated is uncertain. Ages of middle and late Tertiary volcanic rocks indicate that magmatism shifted rapidly (10-15 km/Ma) westward from the Sierra Madre Occidental after ca. 30 Ma. The Sinaloa batholith is borderline calc-alkalic to calcic. SiO_2 contents of analyzed rocks range from 47 to 74%; the lower limit excluding two gabbros is 54%. Syntectonic rocks are more mafic on average than posttectonic rocks. SiO_2 contents of seven out of nine analyzed syntectonic rocks range narrowly between 59 and 62%, with one each at 65 and 67%. The posttectonic rocks show a wider range from 54 to 74% SiO_2, but only border phases and small intrusions have SiO_2 less than ~63%. Combined with their distribution, these data indicate that intrusions become more silicic eastward. The fact that the 20 Ma intrusion is relatively mafic (61 % SiO_2 ) and lies near the coast with syntectonic rocks indicates that composition is related to location rather than to age. The Sinaloa batholith shows both marked similarities and differences from batholiths of the Peninsular Ranges, Sonora, Caho San Lucas (Baja California Sur), and Jalisco. The greatest similarities are in types of intrusions, a common sequence from early gabbro through syntectonic to posttectonic rocks, and general eastward migration of magmatism. However, the end of deformation recorded by syntectonic rocks may be different in each area. Sinaloa rocks show a similar wide range of compositions as rocks of the Peninsular Ranges and Sonora but are more potassic than the calcic Peninsular Ranges. Rare earth element patterns are most like those of the eastern part of the Peninsular Ranges and central part of Sonora, both areas that are underlain by Proterozoic crust or crust with a substantial Proterozoic detrital component. However, southern Sinaloa lies within the Guerrero terrane, which is interpreted to be underlain by accreted Mesozoic crust. The greatest differences are in distance and rate of eastward migration. Published data show that magmatism migrated eastward at ~10 km/Ma across the Peninsular Ranges and Sonora and from Jalisco southeast along the southwestern México coast. The area of slower eastward migration roughly correlates with the location of the Guerrero terrane and of possibly accreted oceanic crust that is no older than Jurassic.

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