Geofísica Internacional ISSN: 0016-7169 [email protected] Universidad Nacional Autónoma de México México Gross, Essa L.; Stewart, John H.; Gehrels, George E. Detrital zircon geochronology of Neoproterozoic to Middle Cambrian miogeoclinal and platformal strata: Northwest Sonora, Mexico Geofísica Internacional, vol. 39, núm. 4, october-december, 2000, pp. 295-308 Universidad Nacional Autónoma de México Distrito Federal, México Available in: http://www.redalyc.org/articulo.oa?id=56839401 How to cite Complete issue Scientific Information System More information about this article Network of Scientific Journals from Latin America, the Caribbean, Spain and Portugal Journal's homepage in redalyc.org Non-profit academic project, developed under the open access initiative Geofísica Internacional (2000), Vol. 39, Num. 4, pp. 295-308 Detrital zircon geochronology of Neoproterozoic to Middle Cambrian miogeoclinal and platformal strata: Northwest Sonora, Mexico Essa L. Gross1, John H. Stewart2 and George E. Gehrels3 1 Department of Geosciences, University of Arizona, Tucson, Arizona, USA 2 United States Geological Survey, Menlo Park, California, USA 3 Department of Geosciences, University of Arizona, Tucson, Arizona, USA Received: May 28, 1999; accepted: May 09, 2000. RESUMEN Ochenta y cinco circones detritales provenientes de estratos sedimentarios del Mesoproterozoico y/o Neoproterozoico al Cámbrico Medio del noroeste de Sonora, México, han sido analizados para determinar los terrenos fuente y delimitar las edades de deposición de las unidades. Los conjuntos de circones de la Formación El Alamo y la unidad El Aguila del Mesoproterozoico y/o Neoproterozoico tienen edades entre 1.06 Ga y 2.67 Ga, con predominio de edades de 1.1 a 1.2 Ga. Los circones de la Cuarzita Bolsa del Cámbrico Inferior y Medio muestra grupos de edades desde 525 Ma a 1.63 Ga, con un grupo dominante de 1.1 a 1.2 Ga. Minerales con edades más antiguas que 1.2 Ga, son probablemente provenientes del basamento y de granitos de ~1.4 Ga del suroeste de Estados Unidos y noroeste de México. También es posible que los sedimentos hayan sido transportados desde el sur, aunque posibles fuentes con la edad adecuada no están expuestas actualmente al sur del área de estudio en el norte de México. Existen tres posibilidades para explicar la fuente de los circones dominantes con edades de 1.1 a 1.2 Ga: (1) afloramientos del cinturón Grenvilliano en el sur de Norte América, (2) cuerpos de granito locales con edades de 1.1-1.2 Ga o (3) una fuente al sur, como el terreno de Oaxaca que experimentó posteriormente transporte tectónico. Estudios de unidades adicionales en el occidente de Estados Unidos y norte de México pueden ayudar a resolver la ambigüedad sobre la fuente de los circones de 1.1 a 1.2 Ga. PALABRAS CLAVE: Circones detritales, geocronología U-Pb, noroeste de México. ABSTRACT Eighty-five detrital zircon grains from Mesoproterozoic and/or Neoproterozoic to Middle Cambrian sedimentary strata in northwest Sonora, Mexico, have been analyzed to determine source terranes and provide limiting depositional ages of the units. The zircon suites from the Mesoproterozoic and/or Neoproterozoic El Alamo Formation and El Aguila unit yield ages between 1.06 Ga and 2.67 Ga, with predominant ages of 1.1 to 1.2 Ga. Zircons from the Lower? and Middle Cambrian Bolsa Quartzite show age groups from 525 Ma to 1.63 Ga, with a dominant population of 1.1 to 1.2 Ga grains. Grains older than 1.2 Ga in the samples were most likely derived from basement terranes and ~1.4 Ga granitic bodies of the southwest U.S. and northwest Mexico. It is also possible that the sediments were transported from the south, although source rocks of the appropriate age are not presently exposed south of the study area in northern Mexico. Three possibilities for the dominant 1.1 to 1.2 Ga grains include derivation from: (1) exposures of the Grenville belt in southern North America, (2) local 1.1-1.2 Ga granite bodies, or (3) a southern source, such as the Oaxaca terrane, that was subsequently rifted away. Sampling of additional units in the western U.S. and northern Mexico may help resolve the ambiguity surrounding the source of the 1.1 to 1.2 Ga grains. KEY WORDS: Detrital zircons, U-Pb geochronology, northwest Mexico. INTRODUCTION several samples of Ordovician and younger miogeoclinal and eugeoclinal rocks (Gehrels and Stewart, 1998). U-Pb geochronological analyses of detrital zircons have been completed on two Mesoproterozoic and/or The Mesoproterozoic to Cambrian is an important time Neoproterozoic feldspathic sandstones and one Lower? and in the tectonic development of western North America. From Middle Cambrian quartzite in northwestern Sonora. This about 1.1 to 0.7 Ga, western North America is considered to study was undertaken to determine source terranes for the have been a part of the supercontinent of Rodinia, which three units and to provide limiting depositional ages on the was an assembly of most continents into one conterminous two units suspected to be of Mesoproterozoic and/or plate (Moores, 1991; Hoffman, 1991; Dalziel, 1992; Neoproterozoic age. Previous detrital zircon studies in Sonora Karlstrom et al., 1999). In most reconstructions, the south- included one sample of Cambrian miogeoclinal quartzite and western part of the United States and northern Mexico are 295 E. L. Gross et al. adjacent to the Antarctic continent and the Kalahari region listed in Table 1. These projected model ages are interpreted of Africa. In the reconstruction of Karlstrom et al. (1999), to be more reliable than 207Pb*/206Pb* ages, as 80 Ma is the the western United States is adjacent to Australia, and Oaxaca likely age of isotopic disturbance in the region. (southern Mexico, in part comparable to Oaxaquia of Ortega- Gutiérrez et al., 1995) lies between Laurentia and Australia. Grain color and morphology have the potential to dis- These tectonic reconstructions are important for this study tinguish, in general, zircon populations from different sources, because the Las Viboras Group and the El Aguila unit may however, no relationship between age and either color or have accumulated during the existence of Rodinia, and some morphology is present in our samples. detrital zircons in these units may have been derived from the vast continental area of Rodinia to the south. STRATIGRAPHY Rodinia is considered to have existed until about 0.7 The three samples we studied are from (1) the Ga, when it was fragmented by rifting (e.g. Burchfiel et al., Mesoproterozoic and/or Neoproterozoic El Alamo Forma- 1992; Lawlor et al., 1999; Karlstrom et al., 1999). This rift- tion of the Las Viboras Group, (2) the informally named ing established Laurentia as a separate continent and led to Mesoproterozoic and/or Neoproterozoic El Aguila unit, and deposition of fringing Neoproterozoic and Paleozoic (3) the Lower? and Middle Cambrian Bolsa Quartzite. As miogeoclinal deposits that include the Cordilleran described below, each of these units has a different strati- miogeocline of western North America. The Cordilleran graphic setting. Little information has been published on the miogeocline extends along much of western North America, stratigraphic nomenclature, lithology, age, and tectonic set- including the Caborca area of northern Sonora. Inland of the ting of the El Alamo Formation and El Aguila unit, which miogeoclinal fringe, the cratonal area of Laurentia was cov- are the subject of ongoing studies by Stewart and Amaya- ered by relatively thin platformal strata. One of our samples, Martínez (2000). In addition, uncertainty exists about the true the Bolsa Quartzite, comes from this cratonal cover. age and tectonic setting of these two units. We provide a summary of existing information about the El Alamo For- We use the term “miogeocline” as defined by Dietz and mation and El Aguila unit so that the reader can better inte- Holden (1966) for ocean-ward thickening, wedge-shaped, grate the detrital zircon studies we present here with infor- continental-terrace deposits along continental margins. They mation on Mesoproterozoic and/or Neoproterozoic arenites considered the Cretaceous and Cenozoic deposits underly- elsewhere in North America. ing the continental shelf of eastern North America as a typi- cal example. In more recent interpretations, miogeoclinal El Alamo Formation deposits form when rifting and continental separation forms a new continental margin, and an ocean-ward thickening The El Alamo Formation was collected from the east wedge of sediment is deposited along the margin as a conse- side of Cerro Minas de Los Gambusinos at Lat. 29° 35.5’ N, quence of post-rift thermal contraction. Long. 110° 35.6’ W, about 4 km southeast of Cerro de Oro and 75 km northeast of Hermosillo (Figure 1). It is approxi- U-PB GEOCHRONOLOGY mately 770 m thick and consists of reddish brown cross-strati- fied arkosic sandstone with minor amounts of interbedded Zircons were separated from ~20 kg samples taken from siltstone. It is laminated to thin bedded with abundant small- a narrow, stratigraphic interval in each unit. The samples were scale trough cross strata. Stewart and Amaya-Martínez (2000) crushed, pulverized, separated on a Wilfley table, passed report that the strata were apparently deposited by a braided through a Frantz LB-1 magnetic separator, and separated river that spread out across a broad coastal plain and en- further with heavy liquids. The zircons were separated into croached on the intertidal zone. different size fractions, and individual crystals were analyzed by isotope dilution-thermal ionization mass spectrometry The El Alamo Formation lies between the El Tápiro after abrasion to approximately 2/3 of their original diam- Formation and the Año Nuevo Formation within the Las eter. The grains chosen for analysis were selected from all Víboras Group (Figure 2).