U–Pb and ⁴⁰Ar/³⁹Ar geochronology of the coastal Sonora batholith: New insights on Laramide continental arc magmatism

 

Geocronología U–Pb y ⁴⁰Ar/³⁹ del batolito costero de Sonora: nuevas aportaciones al magmatismo laramídico de arco continental

 

Ernesto Ramos–Velázquez^1,2,*, Thierry Calmus¹, Victor Valencia³, Alexander Iriondo^4,5, Martín Valencia–Moreno¹, and Hervé Bellon⁶

 

¹ Estación Regional del Noroeste, Instituto de Geología, Universidad Nacional Autónoma de México, Apartado Postal 1039, Hermosillo, Sonora, 83000, México.

² Universidad Autónoma de Baja California Sur, Departamento de Geología, Apartado Postal 19–B, La Paz, B.C.S., 23080, México. * eramos@uabcs.mx

³ University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA.

⁴ Centro de Geociencias, Universidad Nacional Autónoma de México (UNAM), Campus Juriquilla 76230, Juriquilla, Querétaro, México.

⁵ Department of Geological Sciences, University of Colorado at Boulder, Boulder, Colorado 80309, USA.

⁶ UMR 6538, Domaines Océaniques, IUEM, Université de Bretagne Occidentale, 6, Av. Le Gorgeu, BP 809, F–29285, Brest Cedex, France.

 

Manuscript received: April 10, 2007
Corrected manuscript received: February 26, 2008
Manuscript accepted: March 11, 2008

 

ABSTRACT

The coastal Sonora batholith comprises a series of Cretaceous granitoids that intruded a metasedimentary basement of possible Mesozoic age. They are partially covered by Tertiary volcanic flows and pyroclastic rocks. In order to elucidate the crystallization and cooling history of the granitoids, nine rock samples –were collected from Bahía Kino to Punta Tepopa. Eight samples dated by U–Pb zircon geochronology show that the Coastal Sonora batholith was emplaced during the Late Cretaceous, between 90.1 ± 1.1 and 69.4 ± 1.2 Ma. The interval of 20 Ma between the different stages of crystallization indicate that magmatism was relatively static within coastal Sonora, although the magmatic arc recorded an eastward migration as a whole during Cretaceous and Paleogene. In addition, three of these samples were also dated by ⁴⁰Ar/³⁹Ar in biotite and K–feldspar separates. Ages vary from 74 to 67 Ma in biotite and from –68 to 42 Ma in K–feldspar. We interpret these ages as the cooling progression of the batholith, associated with exhumation of the region before the Basin and Range extension. Furthermore, these results show a local trend towards younger ages to the north of the batholith, and they are in good agreement with the model of a general eastward migration of the Cretaceous–Tertiary magmatic arc in northwestern Mexico. In general, the available ages suggest that the arc moved slowly across Baja California between 140 and 105 Ma, and continued its eastward migration across the eastern portion of Baja California and Sonora between 105 and 60 Ma. According to the isotopic ages, the Coastal Sonora batholith would be the westernmost part of the Laramide magmatic event (90 – 40 Ma). Thus, on the basis of new and available geochronologic, petrographic, and geochemical data, we propose that the Coastal Sonora batholith and the eastern portion of the Peninsular Ranges batholith belong to a single magmatic arc, which was separated during the continental breakup and rifting of the Gulf of California in the Tertiary.

Keywords: geochronology, U–Pb, ⁴⁰Ar/³⁹Ar, Cretaceous–Tertiary magmatic arc, Laramide, Coastal Sonora batholith, Mexico.

 

RESUMEN

El batolito Costero de Sonora comprende una serie de granitoides cretácicos que intrusionan un basamento metasedimentario de posible edad Mesozoica. Están cubiertos parcialmente por rocas piroclásticas y flujos volcánicos terciarios. Para conocer la historia de cristalización y enfriamiento de los granitoides se colectaron nueve muestras de granitoides desde Bahía Kino hasta Punta Tepopa. Ocho de ellas fueron fechadas usando geocronología U–Pb en circones y muestran que el batolito Costero de Sonora fue emplazado durante en Cretácico Tardío, entre 90.1 ±1.1 y 69.4 ± 1.2 Ma. Este rango de edades de cristalización (20 Ma) sugiere que el magmatismo fue relativamente inmóvil en la región costera actual de Sonora, aunque el arco magmático registró en conjunto una migración hacia el este durante el Cretácico y Paleógeno. Además, tres de estas muestras fueron también fechadas por ⁴⁰Ar/³⁹Ar en separados de biotitayfeldespato potásico para evaluar el enfriamiento de los cuerpos graníticos. Las edades ⁴⁰Ar/³⁹Ar varían de 74 a 67 Ma en biotita, y de 68 a 42 Ma en feldespato potásico. Interpretamos estas edades como el resultado del enfriamiento del batolito, asociado con una importante exhumación de la región costera de Sonora. A nivel del batolito Costero de Sonora, las intrusiones muestran edades más jóvenes en la parte norte del batolito, sin contradecir el modelo general de una migración hacia el este del arco magmático Cretácico–Terciario en el noroeste de México. En general, las edades disponibles sugieren que el arco migró lentamente a través de Baja California entre 140 y 105 Ma, y continuó a través de la porción oriental de Baja California y Sonora entre 105 y 60 Ma. De acuerdo con las edades isotópicas obtenidas en la región costera de Sonora, el batolito Costero de Sonora puede representar la parte más occidental del evento magmático Laramide (90 – 40 Ma). Así, con base en los nuevos datos obtenidos y en los datos geocronológicos, petrográficos y geoquímicos disponibles, proponemos que el batolito Costero de Sonora y la porción este del batolito de las Sierras Peninsulares pertenecen a un mismo arco magmático, el cual fue separado durante la ruptura continental y el rifting del Golfo de California en el Terciario.

Palabras clave: geocronología, U–Pb, ⁴⁰Ar/³⁹Ar, arco magmático Cretácico–Terciario, Laramide, batolito Costero de Sonora, México.

 

INTRODUCTION

The geologic evolution of northwestern Mexico since the Jurassic has been associated with the interaction between the North American and oceanic plates of the Pacific realm (Dickinson and Lawton, 2001). One of the main consequences of this interaction has been the magmatism along the continental margin that originated a series of batholiths, currently exposed in southwestern USA (California and Arizona) and northwestern Mexico (Baja California peninsula and in the states of Sonora and Sinaloa). As a whole, these batholiths are oriented NW–SE, subparallel to the paleotrench. In detail, Cretaceous plutons in Sonora display a widespread geographic distribution, due to the great extent of magmatism during Laramide time and to the disruption related to the Basin and Range extension. Nevertheless, it is possible to recognize a NNW–SSE distribution with distinctive petrographic and geochemical characteristics, as well as cooling ages.

In California and Baja California peninsula, the Peninsular Ranges batholith (PRB) is composed by two magmatic belts, known as the western Peninsular Ranges batholih (WPRB) and the eastern Peninsular Ranges batholith (EPRB). Both belts are subparallel to the long axis of the peninsula (NW–SE), and such division is based on geochemical, petrographic, geophysical and geochronological data (Gastil, 1975; Langenheim and Jachens, 2003). The WPRB was probably developed on oceanic lithosphere, and is composed mainly of gabbroic to monzogranitic plutonic rocks, with U–Pb zirconages ranging between 140 and 105 Ma (Silver et al, 1979; Silver and Chapell, 1988). The EPRB was developed on continental lithosphere, and is dominated by tonalite, trondjhemite, and K–poor granodiorite, with U–Pb zircon ages from 105 to 90 Ma (Silver et al, 1979; Silver and Chapell, 1988; Walawender et al, 1990). The potassium enrichment in the EPRB rocks, as well as the Ti enrichment, higher ^87/86Sr and higher eNd towards the east (Ortega–Rivera, 2003) suggest an interaction of magmas with preexisting crust.

The lithospheric differences of the substratum in which the granitoids from the WPRB and the EPRB were emplaced have been used to propose several tectonic models for the region. These models have been summarized by Wetmore et al. (2002) and are: 1) a single eastward migrating arc developed across a pre–Triassic suture between oceanic and continental lithospheres (Walawender et al., 1991; Thomson and Girty, 1994); 2) an exotic island arc accreted to the North American margin across a non–terminal suture (Johnson et al., 1999; Dickinson and Lawton, 2001); 3) the reaccretion of a rifted and fringing arc to the North American margin (Gastil et al., 1981; Busby et al, 1998). Wetmore (2003) showed that these models are inconsistent with the geologic evidence, because they do not consider the variations of the WPRB to the north and south of the Agua Blanca fault. Wetmore et al. (2002) proposed that the WPRB evolved during the Early Cretaceous within two different tectonic blocks: a continental magmatic arc to the north (Santiago Peak arc segment), and an island arc to the south (Alisitos arc segment). Later, these two arc segments were joined due to the accretion of the island arc at the end of the Early Cretaceous.

Within continental Mexico, mainly in Sonora and Sinaloa, numerous batholiths were formed between 90 and 40 Ma during the Laramide orogeny. These batholiths form a NW–SE belt, which is known as the Laramide plutonic belt (Valencia–Moreno et al., 1999). This belt consists mainly of granodioritic to tonalitic plutons and subordinated bodies of diorite and granite, with U–Pb zircon ages of 90 to 40 Ma, younging eastward (Damon et al., 1983b; Roldán–Quintana, 1991; Valencia–Moreno et al., 1999; Valencia–Moreno et al., 2001; Henry et al., 2003; Valencia–Moreno et al, 2006). The coastal Sonora batholith (CSB) and batholiths further east in Sonora and Sinaloa are interpreted to be a continuation of the EPRB, whichis presently separated fromBaja California by the Gulf of California. The EPRB and the CSB are related to the subduction of the Farallón plate beneath North America during the Late Cretaceous and early Tertiary. The eastward migration of the magmatic arc has been interpreted as a result of the increase in the convergence rate between both plates and of the gradual flattening of the slab (i.e., Coney and Reynolds, 1977; Gastil, 1983; Dickinson, 1989). In Sonora, Sinaloa, Durango and Chihuahua, the Laramide plutons form a well defined belt, with an overall NW–SE orientation (Barton et al., 1995; Valencia–Moreno et al., 2001; Henry et al, 2003), subparallel to the Pacific margin (Figure 1), which suggests that: 1) Laramide magmatism was associated to the subduction of Farallón plate, and 2) the trench geometry has been steady since Late Cretaceous.

Previous studies of the Mexican Laramide batholiths have focused mainly on their geochemical characteristics (Roldán–Quintana, 1991; Valencia–Moreno et al, 1999, 2001,2003; Vargas–Navarro, 2002), geochronology (Henry et al, 2003) and their relation to mineralization (Staude and Barton, 2001; Barton et al, 1995; Wodziki, 1995; Valencia–Moreno, 1998).

This study presents new geochronological data on three intrusive units from the CSB, between Bahía Kino and Punta Tepopa (Figure 1). Six samples were dated by U–Pb zircon analysis. Three of these samples were analyzed by ⁴⁰Ar/³⁹Ar geocronology on biotite and K–feldspar to constrain the thermal history from crystallization to cooling below the Ar–Ar closure temperature of K–feldspar. In addition a K–Ar analysis was conducted on one andesite whole rock sample.

 

PREVIOUS STUDIES

Several studies have been published on the magmatic and tectonic evolution of the coastal Sonora batholith (CSB) and its relationship with the geological framework of the North America Cordillera. Anderson and Silver (1969) reported in an abstract U–Pb isotopic ages from 100 ± 3 to 82 ± 3 Ma in zircons from several granitoids of the coast of Sonora. In coastal Sonora, metasedimentary basement rocks of possible Carboniferous or Jurassic age were intruded by Cretaceous to early Tertiary granitic rocks, and later covered by Tertiary volcanic rocks (Gastil andKrummenacher, 1977). These authors report nine biotite and hornblende K–Ar ages on plutonic rocks that range between 91.0 ±1.8 to 59.9 ± 1.2 Ma, including an age of 85.1 ± 1.7 Ma for an andesitic dike. A K–Ar age of 83 ± 2 Ma was reported by Mora–Alvarez (1992) for a granodiorite of the Bahía Kino region.

The geochemical compositions of the CSB have been presented elsewhere (Valencia–Moreno et al, 2001, 2003; Vargas–Navarro, 2002). Vargas–Navarro (2002) summarizes petrographic and isotopic variations of batholiths on both sides of the Gulf of California. The major–element contents of the CSB rocks are similar to those of the EPRB, but show a tendency to be more alkaline. Variations in the ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotopic ratios were interpreted as reflecting compositional changes in the Proterozoic and Paleozoic basement (Valencia–Moreno et al, 2001). Towards the west, the CSB shows less crustal contamination ⁸⁷Sr/⁸⁶Sr 0.706 and εNd >3.5) whereas, eastwards, the crustal contamination increases (⁸⁷Sr/⁸⁶Sr >0.706 and εNd <3.5). The ⁸⁷Sr/⁸⁶Sr = 0.706 ratio isopleth may reflect the southwestern limit of the Precambrian basement of North America (Valencia–Moreno et al, 2001). The contour of the 0.706 initial ⁸⁷Sr/⁸⁶Sr line is not well constrained north of the CSB, but is better defined in northern Baja California, where it follows a NNW direction from San Felipe, Baja California (Gromet and Silver, 1987), to the northeasternmost part of the Peninsular Range batholith. There, the 0.706 initial ⁸⁷Sr/⁸⁶Sr line coincides mostly with the Agua Caliente fault in the Perris block, and with the San Jacinto fault zone, bording the San Jacinto Mountains to the west (Kistler et al, 2003).

Isotopic ages of the Laramide intrusives in northwestern Mexico are numerous, and document its magmatic evolution. Near the coast of the Gulf of California, intrusive rocks yielded U–Pb ages on zircons ranging between 100 and 82 Ma (Anderson and Silver, 1969), and the oldest K–Ar hornblende ages, from 90 to 60 Ma (Gastil and Krummenacher, 1977; Mora–Alvarez and McDowell, 2000; Henry et al, 2003). Eastward, the ages range from 63 to 49 Ma as indicated by K–Ar ages on hornblende and biotite in eastern Sonora (Damon et al, 1983a; Shafiqullah et al, 1983; McDowell and Roldan–Quintana, 1993; McDowell andMauger, 1994). The U–Pb zirconages of central Sonora intrusive rocks range from 63 to 56 Ma, being younger than these from the CSB (McDowell and Roldan–Quintana, 1993; McDowell and Mauger, 1994).

Numerous ages for volcanic rocks of the Cretaceous–Tertiary interval have been obtained by the K–Ar method, an to a lesser extent by ""AiZ^Ar and U–Pb; they also show younging ages from west to east, supporting the interpretation of an eastward migration of the magmatic arc (Damon et al, 1983a; McDowell and Mauger, 1994; Gans, 1997; McDowell et al, 2001; Henry et al, 2003; Valencia–Moreno et al, 2006).

 

GEOLOGICAL FRAMEWORK

Rocks of the Sonora coastal region and Isla Tiburón consist of Mesozoic(?) metasedimentary rocks intruded by granitoid rocks and an unconformable cover of mid–Tertiary volcanic rocks. The regional structural trend and the strike of metamorphic units have a general orientation subparallel to the coast, forming horsts separated by basins filled with Pliocene to Quaternary sediments.

In the Kino Nuevo–Punta Tepopa area (Figure 2), the oldest unit corresponds to roof pendants of greenschistfacies metasandstone, marble and metaconglomerate. In the Cerro El Tordillo area (Figure 2), the thickness of this unit reaches 350 m. Metamorphic rocks display a strong metamorphic foliation parallel to bedding, but the strike of foliation is very heterogeneous. Locally, the metamorphic unit is affected by ductile shear zones, parallel to foliation. Close to the plutons of the CSB, metamorphic rocks exhibit contact metasomatism with formation of wollastonite and garnet in calcareous rocks, and hornblende, kyanite and muscovite in sandstone. Gastil and Krummenacher (1977) suggests a Mesozoic age for this metamorphic unit on the basis of lithological correlations with a similar unit located to the north of Desemboque, which contains Trigonia inexpectata, of probable Early Jurassic age.

The CSB can be divided in two belts: 1) an eastern NNW–SSE striking belt, from Punta San Nicolás to Puerto Libertad (Figure 1), that is 160 km long and 10 km wide and corresponds to a westward tilted block, almost perpendicular to the main direction of Tertiary extension; 2) a 70–km–long western belt composed of several plutons that crop out in Isla Tiburón and Punta Tepopa (Figure 2). To the east, the CSB is limited by the 100 km wide Hermosillo coastal alluvial plain, which separates the CSB from the plutonic rocks of the Hermosillo area.

The CSB consists mainly of granodioritic andtonalitic plutons, with subordinated bodies of diorite and granite, besides of granitic dikes with aplitic and pegmatitic textures, and andesitic hipabysal intrusions. The oldest intrusive unit is the Puerto Rico diorite (Figure 2), which crops out in the CSB as small isolated bodies. In the northern part of the CSB, the Puerto Rico diorite is dark gray, fine– to coarse–grained and consists mainly of plagioclase, biotite, hornblende, scarce quartz, with apatite and zircon as accessory minerals. A magmatic foliated texture is defined by a preferential orientation of biotite, hornblende and plagioclase.

The Kino granodiorite and Tepopa tonalite (Figure 2) intrude the diorite; in Figure 2 they are not differentiated because contacts are transitional and irregular. The Kino granodiorite crops out mainly in the southern part of the area, between Kino Nuevo and Punta Chueca. It is light gray and coarse grained, composed of plagioclase, quartz, K–feldspar, biotite and hornblende. In some samples, the granodiorite presents a slight ductile deformation, underlined by irregular mineral contacts and undulating extinction of quartz crystals, as well as deformed biotite altered to chlorite, and hornblende altered to epidote and subrounded K–feldspar. The Tepopa tonalite crops out mainly in the northern part of the area, between Punta Tepopa and the NE of Pico Johnson (Figure 2). The tonalite is light gray to almost white in color, and is composed of plagioclase, quartz, biotite and hornblende, with zircon, sphene and apatite as accessory minerals. The texture is fine grained and equigranular, with a higher content of plagioclase and biotite than the Kino granodiorite. Intrusive contacts of the Kino granodiorite and Tepopa tonalite within the diorite are very straight and sharp, and crosscut by dikes. A 5–10 cm wide alteration zone is observed within the granodiorite, along the contact, characterized by a parallel recrystalliza–tion of quartz and biotite. All these data suggest that the plutons were emplaced at relatively cold conditions, with low thermal contrast between the host rock and intrusive, as is showed in several localities.

The Rancho Nuevo granite is the youngest unit of the CSB. It crops out extensively in the southeastern area of the CSB, and, towards the north, only around the Rancho Doble I locality (Figure 2). The Rancho Nuevo granite is reddish in color and coarse – to very coarse–grained, and its mineralogy of consists mainly of K–feldspar phenocrysts as long as 5 cm, quartz, plagioclase, biotite and hornblende, with apatite and zircon as accessory phase. K–feldspar is commonly altered to sericite along intracrystalline fractures. The characteristic reddish color is due to the oxidation of biotite. Intrusive contacts are sharp and characterized by textural changes and by the presence of local foliation. For instance, in the KI–12–35 locality, near Rancho Doble I (Figure 2), the contact between Rancho Nuevo granite and Kino granodiorite consists of two 50–cm–wide zones with distinct textures, developed in the granite. The closest zone to the granodiorite displays a strong foliation along the contact. The second one, with porphyritic texture, is characterized by phenocrysts of K–feldspar, plagioclase, biotite and quartz, in a fine–grained matrix of quartz and plagioclase. The texture and foliation changes are restricted to a very thin zone along the contact, suggesting a subsolidus flow (i. e., Vernon, 2000) during the intrusion of the Rancho Nuevo granite into the Kino granodiorite. An aplitic and pegmatitic dike system, genetically related to the Rancho Nuevo granite, has a wide distribution in the area and cuts across all the granitic and metamorphic units. The width of dikes varies from 2–3 cm to about 1 m.

The Tordillo hypabisal andesite unit is spatially associated with the Kino granodiorite in the southern part of the area. The Tordillo andesite is gray and has a porphyritic texture characterized by phenocrysts of plagioclase in an aphanitic matrix, with sparce biotite crystals.

The CSB and metasedimentary are unconformably overlain by volcanic rocks that include rhyolite, andesite, tuff and basalt, associated with the activity of the Tertiary magmatic arc (Damon et al., 1983b), with a total composite thickness of around 400 m. The coastal zone of Sonora is partially covered by Pliocene–Quaternary sediments, which belong to deltaic deposits of the Río Sonora, dunes and marine terraces.

The CSB is affected by two sets of extensional faults. The less conspicuous set has a nearly E–W orientation, and includes the north dipping Tordillo normal fault (Figure 2). The more developed set is oriented NNW–SSE and controls the morphology of the coastal region; the west dipping Rancho Nuevo fault and the east dipping Tepopa fault belong to this last set. The Rancho Nuevo fault has a single normal slip, whereas the Tepopa fault presents two generations of superimposed kinematic indicators; the oldest one indicates a normal slip and the youngest one indicates a lateral slip, with a minor normal component (Figure 3). This structural information reflects the post–12 Ma opening of the Gulf of California in this part of its eastern margin. After the subduction stopped at 12.5 Ma, continental breakup occurred along a SW–NE to W–E strike of extension, and was followed, since 6 Ma, by a right–lateral motion along NW–SE to NNW–SSE striking faults along which the Pacific plate is moving northwestward with respect to the North America plate (Stock and Hodges, 1989).

The igneous and metamorphic Mesozoic rocks, as well as Tertiary volcanic rocks as young as 12 Ma (San Felipe tuff; Oskin, 2002), and the Hermosillo ignimbrite (Vidal–Solano et al., 2005) are faulted and tilted locally up to 70° to the east (i.e., west of Sierra Kunkaak, north of Bahía Kino). The relation between tilting and continental breakup is clearly evidenced by the deformation of Miocene volcanic rocks, but the structural pattern of the Gulf Extensional Province partially obliterates the previous Basin and Range extensional faulting pattern.

 

SAMPLING AND ANALYTICAL METHODS

To shed ligth on the crystallization and cooling history of granitoids from the CSB, six samples were collected in the southern area between the northern area of Punta Chueca and Kino Nuevo, and three samples in the northern area, from Punta Tepopa to Rancho Doble I (Figure 2). Three samples of the southern area are aligned along a NE–SW section, perpendicular to the direction of tilting. Eight of these samples were dated by U–Pb zircon geochronology, and three samples of the southern area were also dated by the ⁴⁰Ar/³⁹Ar method in biotite and K–feldspar separates. A whole rock sample of the Tordillo andesite (sample KI–04–08) was dated by the K–Ar method.

For each sample, 1–2 kg of fresh rock chips were collected. Samples were prepared using standard separation techniques, including magnetic separation and heavy liquids. For the U–Pb analysis, zircon crystals were separated by hand–picking under a binocular microscope, for subsequent epoxy mounting and polishing.

Single zircon crystals were analyzed in polished grain mounts with a VG Isoprobe multi–collector ICP–MS equipped with nine Faraday collectors, an axial Daly detector, and four ion–counting channels (Gehrels et al., 2006). The Isoprobe is equipped with an ArF Excimer laser, which has an emission wavelength of 193 nm. The analyses were conducted on 50–35 micron spots with an output energy of 32 mJ and a repetition rate of 10 Hz. Each analysis consisted of a background measurement (one 20–seconds integration on peaks with no laser firing) and twenty 1–second integrations on peaks with the laser firing. Any Hg contribution to the ²⁰⁴Pb mass was accordingly removed by subtracting the background values. The depth of each ablation pit was 20 microns. Total measurement time was 90 s per analysis.

The collectors were configured for simultaneous measurement of ²⁰⁴Pb in an ion–counting channel and ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³²Th, and ²³⁸U in Faraday detectors. Inter–element fractionation was monitored by analyzing fragments of SL–1, a large concordant zircon crystal from Sri Lanka (SL–1) with a known (ID–TIMS) age of 564 ± 4 Ma (2 sigma) obtained by George Gehrels (unpublished data). The reported U–Pb ages are based entirely on ²⁰⁶Pb/²³⁸U ratios because the errors of the ²⁰⁷Pb/²³⁵U and ²⁰⁷Pb/²⁰⁶Pb ratios were too large. This is due primarily to the low intensity (commonly <0.5 mV) of the ²⁰⁷Pb signal from these young, low–U grains. The ²⁰⁶Pb/²³⁸U ratios were corrected for common Pb by using the measured ²⁰⁶Pb/²⁰⁴Pb, a common Pb composition from Stacey and Kramers (1975), and an uncertainty of ±1.0 on the common ²⁰⁶Pb/²⁰⁴Pb.

For each sample, the weighted mean of –20–25 individual analyses was calculated according to Ludwig (2003). The measurement error was added quadratically to the systematic errors, which include contributions from the calibration correction, decay constant, age of the calibration standard, and composition of common Pb. The systematic errors are 1–2% for these samples. All the U–Pb zircon ages are reported at the 2–sigma level.

Biotite and K–feldspar mineral separates were dated by ⁴⁰Ar/³⁹Ar furnace step–heating and total fusion methods. The mineral concentrates ranged in size between 250 and 180 um and were obtained to a purity of >99% using heavy liquids and hand–picking techniques. Samples were washed in acetone, alcohol, and deionised water in an ultrasonic cleaner to remove dust and then re–sieved to < 180 um.

Aliquots of biotite and K–feldspar for the different granitic samples were packaged in copper and sealed under vacuum in quartz tubes. The samples were then irradiated in package number KD3 8 for 5 hours in the central thimble facility at the TRIGA nuclear reactor (GSTR) at the U.S. Geological Survey in Denver, Colorado. The monitor mineral used in the package was Fish Canyon Tuff sanidine (FCT–3) with a K–Ar age of 27.79 Ma (Kunk et al., 1985; Cebula et al., 1986) relative to MMhb–1 with a K–Ar age of 519.4 ± 2.5 Ma (Alexander et al., 1978; Dalrymple et al., 1981). The type of container and the geometry of sample and standards are similar to those described by Snee et al. (1988).

The samples were analyzed at the U.S. Geological Survey Thermochronology Laboratory in Denver, Colorado, by the⁴⁰ Ar/³⁹ Ar furnace step–heating and total fusion dating methods with a MAP 216 mass spectrometer fitted with an election multiplier. Biotite aliquots were fused in the furnace in a single heating step at l,450°C to produce a total fusion age. For additional information on the analytical procedure, see Mondo et al. (2003, 2004).

The argon isotopic data, reported at the la level of analytical precision, were reduced using the computer program Mass Spec (Deino, 2001). We used the decay constants recommended by Steiger and Jager (1977). Table 4 shows ⁴⁰Ar/³⁹Ar furnace step–heating and total fusion data and includes the identification of individual temperature steps, plateau, average, and total gas ages. An individual step age represents the apparent age obtained for a single temperature step analysis. A plateau age (not present for these samples) is identified when three or more contiguous steps in the age spectrum agree in age, within the limits of analytical precision, and contain more than 50% of the ³⁹Ar_K released from the sample (Fleck et al., 1977). Total gas ages represent the age calculated from the integration of all the ⁴⁰Ar/³⁹Ar step–heating age results for the sample into one single age value; the total gas ages are thus roughly equivalent to conventional K–Ar ages. No analytical precision is calculated for the total gas age because this analytical uncertainty in many cases does not represent the total geological uncertainty in the age of the mineral and/or rock.

 

GEOCHRONOLOGY RESULTS

Most of the dated samples are granodiorite–tonalite to granite (Table 1), generally with coarse to very coarsegrained textures, and large phenocrysts of K–feldspar. Zircons separated from all the samples have the well defined crystallographic forms, with well–developed and preserved faces, that characterize igneous zircons (Pupin, 1983). The absence of inherited zircon is indicated by the low dispersion in the distribution of the calculated ages of the samples (Figure 4), which yielded only Cretaceous ages (from 90.1 ± 1.1 to 69.4 ±1.1 Ma). This result is supported by the ⁴⁰Ar/³⁹Ar ages obtained in biotite, which are also Late Cretaceous (from 66.95 ± 0.28 to 73.61 ± 0.12 Ma), suggesting a regional cooling below the biotite closure temperature at this time (rough closure temperature estimate for biotite: 300±40 °C; e.g., McDougall and Harrison, 1999). The description of the geochronological results was separated in two zones (north and south), based on their geographic location. Geochronological data are presented on Tables 1–4. (Tables 1, 2, 3 and 4).

 

Southern area

Three samples were collected along a 9 km long NE–SW transect between Kino Nuevo and the northern Punta Chueca (Figure 2). The units sampled are the Kino granodiorite close to the shoreline (KI–03–07), the Rancho Nuevo granite (KI–03–03) in the middle and, to the east, the Kino granodiorite (KI–03–15). The samples collected to the east and north of Punta Chueca are: Kino granodiorite (KI–12–12) and Tepopa tonalite (KI–12–53). A sample of the Tordillo andesite was collected north of Kino Nuevo (KI–04–08). The five intrusive samples were dated by U–Pb, three of them were also dated by ⁴⁰Ar/³⁹Ar, while the andesite sample was dated by the K–Ar method.

The three ²⁰⁶Pb–²³⁸U zircon ages obtained for the Kino granodiorite are consistent with each other, indicating that crystallization occurred in Late Cretaceous time, between 90.1 ± 1.1 and 81.4 ± 0.8 Ma (Figure 4; Table 1). Younger crystallization ages of the Rancho Nuevo granite (74.0 ±0.7 Ma) and the Tepopa tonalite (70.8 ±1.8 Ma) are consistent with field relations showing that the Rancho Nuevo granite and Tepopa tonalite intruded the Kino granodiorite. The older sample of the Kino granodiorite (KI–03–15) shows ductile deformation whereas the younger samples (KI–03–07 and KI–12–12) are unaffected by this deformation, perhaps indicating that the intrusion began under syntectonic conditions. The youngest sample (KI–12–53) could represent the beginning of a tendency to younger ages to the north of the CSB.

The three samples of the NE–SW section (KI–03–03, KI–03–07 and KI–03–15) were also dated by ⁴⁰Ar–³⁹Ar geochronology to determine their thermal history between 300°C (±40) and 150°C (±40), the rough estimate for closure temperatures of biotite and low–temperature K–feldspar domains, respectively.

Biotite from the Rancho Nuevo granite (KI–03–03) yielded a total fusion age of 66.95 ± 0.28 Ma (Figure 5a and Table 4) that we interpret as the best approximation for the timing of cooling of the granite below 300°C (±40). However, the reliability of this age is poor because of the low percentage of radiogenic ⁴⁰Ar* (31%), which may be explained by the alteration of biotite to chlorite. The K–feldspar analysis yielded an age range of cooling between 62.8 and 60 Ma. We interpret that the upper age represents the K–feldspar domains with higher argon retention [rough estimate of high temperature K–feldspar closure temperature: 250°C (±40)], whereas the lower end represents the feldspar domains with lower retention properties (rough estimate of low temperature K–feldspar closure temperature: 150°C (± 40). These cooling ages combined with the zircon crystallization age of 74.0 ± 0.7 Ma indicate a relatively slow initial cooling through biotite and a slower cooling between biotite and low–T K–feldspar (Figure 6).

Kino granodiorite biotite sample KI–03–07 yielded a single step age of 72.41 ± 0.22 Ma (Table 4) that represents the time of cooling below biotite closure. The age spectrum for K–feldspar (Figure 5b) shows a large age gradient between 63 and 41.5 Ma implying a significantly slower cooling rate than that of Rancho Nuevo granite, although cooling in the early stages, between zircon (84.1 ±1.0 Ma) and biotite, is slightly slower than for Rancho Nuevo granite sample (Figure 6). A second Kino granodiorite sample (KI–03–15) was dated on a biotite separate that yielded a single–step age of 73.61 ± 0.12 Ma (Table 4). The age spectrum for the K–feldspar of this sample (Figure 5c and Table 4) displays a gradient between 68.4 and 59.5 Ma. The cooling curve for this last granodiorite sample indicate an intermediate cooling rate with respect to the previously described samples. Appartently, the older Kino granodiorite samples KI–03–15 and KI–03–07 needed more time to cool below K–feldspar closure, perhaps because the younger pulse of magmatism (Rancho Nuevo granite) created a regional thermal gradient that maintained the preexisting rocks above their closure temperatures for the Ar–Argeochronometers. The Rancho Nuevo granite sample (KI–03–03) cooled relatively rapidly and was not affected by any significant thermal pulse since cooling below K–feldspar closure [low retention domains at 150°C (±40) around 60 Ma (Figure 6)].

The Tordillo andesite (KI–04–08) yielded a K–Ar whole rock age of 62.5 ± 1.5 Ma (Table 2). The emplacement of this unit occurred at the time the Kino granodiorite and Rancho Nuevo granite cooled through 250°C (±40), the rough estimate for closure temperature of the high–retention domains of K–feldspar. The age of the Tordillo andesite is similar to other ages obtained further to the east, in andesites of the Tarahumara Formation (McDowell et al., 2001; Roldan–Quintana, 2002), suggesting that the Cretaceous–Paleogene volcanic activity related to the Laramide magmatic arc was not restricted to central Sonora but was also present along the western margin of the North America craton. We interpret the scarcity of outcrops of the Tarahumara Formation in western Sonora as a result of the intense uplift and subsequent erosion during Basin and Range faulting and pre–Gulf rifting.

 

Northern area

In the area between Rancho Doble I and Punta Tepopa, three samples were collected and dated by U–Pb zircon geochronology (Figure 2). ²⁰⁶Pb–²³⁸U zircon ages of the three samples correspond to Late Cretaceous crystallization, varying between 75.9 ± 0.8 and 69.4 ± 1.2 Ma (Table 1). They are distinctly younger than the Kino granodiorite in the southern area.

Sample KI–12–46 belongs to the Isla Tiburón–Punta Tepopa range, whereas the other two samples belong to the Kunkaak range (Figure 2). Sample KI–12–35 is part of the Rancho Nuevo granite and corresponds to a porphyric monzogranite, which presents a clear magmatic foliation. Samples KI–12–46 and KI–12–41 belong to the Tepopa tonalite, which is older than the granite, based on intrusion relationships from several localities.

The KI–12–35 sample is the oldest one, with a zircon ²⁰⁶Pb–²³⁸U age of 75.9 ± 0.8 Ma, and corresponds to the zone of magmatic foliation of the Rancho Nuevo granite near the intrusive contact with the Kino granodiorite. This age is in agreement with the age obtained in the southern area (KI–03–03), also intruding the Kino granodiorite.

Samples of the Tepopa tonalite (KI–12–41 and KI–12–46) yield similar ²⁰⁶Pb–²³⁸U ages that are in close agreement with that obtained for the Tepopa tonalite (70.8 ± 1.8 Ma) from the southern zone. With these results we can conclude that the Tepopa tonalite was emplaced in an time interval of about 3 Ma, and is the last intrusive unit of the CSB.

 

DISCUSSION

Crystallization and cooling of the Sonora coastal batholith

Petrological characteristics and ²⁰⁶Pb–²³⁸U zircon ages indicate that the CSB consists of several granitoid complexes that crystallized during Late Cretaceous, between 90 and 70 Ma (Figure 6). The CSB was emplaced in the continental active margin of North America, in a region characterized by the transition between two previously juxtaposed basements; a northern one related to the margin of the North America craton, and a southern one with oceanic basin affinities since the Paleozoic time (Valencia–Moreno et al., 2001). Nevertheless, the lack of inherited zircon in the analyzed samples does not provide any complementary information about the nature of the basement in which the CSB intruded in the studied area (Figure 4). Although the selection of individual zircon crystals was made by hand–picking, it is unlikely that this procedure has induced a lack of inherited zircons in all samples.

The crystallization of the CSB began with the Puerto Rico diorite (not dated), emplaced as small bodies around 90 Ma. Later, between 90 and 80 Ma, the Kino granodiorite was emplaced within a relatively cold Puerto Rico diorite; this is interpreted by the sharp and regular contacts that could be related to a relatively shallow level of emplacement. The weak ductile deformation observed in the locality of sample KI–03–15 (Figure 2) is correlated to local processes during the cooling of the pluton, because such deformation was not observed in other samples. The Rancho Nuevo granite and granitic dikes were emplaced at around 76–74 Ma. The youngest units on the studied area corresponds to the Tepopa tonalite, emplaced between 72 and 69 Ma, and the Tordillo andesite emplaced at 62.5 Ma (Table 1). While the volcanics are spatially related to the Kino granodiorite, our geochronologic results demonstrate that the bodies are not coeval.

The U–Pb isotopic zircon ages obtained for the Kino granodiorite, Tepopa tonalite and Rancho Nuevo granite display a clear difference in crystallization age (Figure 2). The U–Pb ages reported in this study are older than U–Pb zircon ages obtained by Anderson et al. (1980) in the Sierra Mazatán and Puerto del Sol (57 ± 3 and 58 ± 3 Ma), and by Poole et al. (1991) in Barita de Sonora mining district (62.0 ± 1 Ma). Furthermore, these U–Pb ages are older and younger than ages obtained by Anderson et al. (1980) to the north in the Sierra Guacomea and Rancho Los Alamos (74 ± 2 and 78 ± 3 Ma, respectively). The correlation between the U–Pb ages from this and previous studies shows a tendency for the crystallization ages to be younger to the east, toward the interior of Sonora. This tendency supports the idea of an eastward magmatic migration during the Laramide orogeny (Damon et al., 1983b).

⁴⁰Ar/³⁹Ar and K–Ar ages obtained in this study agree with previous K–Ar and ⁴⁰Ar/³⁹Ar results (Gastil and Krummenacher, 1977; Mora–Alvarez, 1992, and Valencia–Moreno et al., 2006) for the central part of the CSB and Isla Tiburón. North of the study area, near Puerto Libertad, Gastil and Krummenacher (1977) reported K–Ar ages in biotite and hornblende for intrusive granitoids of the CSB ranging from 70 to 60 Ma. In the region of Guaymas and San Carlos, further to the south, Mora–Alvarez (1992) and Roldán–Quintana (2002) reported K–Ar ages in hornblende between 83.0 ± 2.1 and 82.7 ± 1.7 Ma and in biotite between 81.1 ± 2.8 and 76.9 ± 2.8 Ma, which provides further indication that the CSB exposes older rocks to the south.

The spatial distribution of isotopic ages of plutons in Central Sonora between 30° and 32° N latitude (Figure 7) has been interpreted as evidence of progressive eastward migration of the Cretaceous–Tertiaty magmatic arc (Damon et al., 1983a). The ages obtained in the present study support this hypothesis for the arc migration. In addition, the sotopic age gradient from south to north within the CSB suggests that the orientation of the arc axis was oblique with respect to the present coastline of Sinaloa and Sonora, which is a morphotectonic feature controlled by the Tertiary history dominated by the Basin and Range extension and the opening of the Gulf of California.

We suggest that some zones exist, as the CSB, with a local magmatic evolution that does not follow the general chronological evolution and migration pattern of the entire magmatic arc. In the case of CSB, the magmatism remained static for about of 10 to 20 Ma, which is suggested by the geochemical magmatic differentiation from diorite to granite emplaced at the same place.

 

Correlation of the CSB with the Peninsular Ranges Batholith

The correlation of the batholiths located on both sides of the Gulf of California has been addressed by several authors (e.g., Gastil and Krummenacher, 1977), and is supported by similar chemical, petrographic and geochronological characteristics (e.g., Schaaf et al., 2000). Ages of granitoid rocks dated in the eastern margin of the Baja California peninsula support this interpretation: for example 78.4 ±2.9 Ma K–Ar (McFall, 1968) and 99 ± 2 Ma (Ledesma–Vazquez, 2000) ages in the Bahía Concepción area, and 91.2 ± 2.1 Ma in the Santa Rosalia area (Schmidt, 1975). Alternatively, the coherent belt of high–amplitude magnetic anomalies present along the Baja California peninsula does not exist in mainland Mexico, suggesting a geological limit located in the Gulf of California (Langenheim and Jachens, 2003).

The crystallization and cooling ages obtained in this study further support this correlation between granitoids of Baja California and the coastal region of Sonora. Batholiths in both areas are the product of continental arc magmatism active during Cretaceous to Tertiary time. This magmatic arc began its activity at around 105 Ma, forming first the eastern Peninsular Ranges batholith (EPRB), and migrating later further to the east where the CSB and younger plutons in central and eastern Sonora were generated (Roldan–Quintana, 1991; Valencia–Moreno et al., 2001; Valencia–Moreno et al., 2006). The arc migration has been associated with a change in the subduction angle, which was responsible for the synchronous Laramide orogeny in the region. The eastward migration is underlined by changes in the geochemical characteristics (Damon et al., 1983a). The westernmost plutons are slightly more mafic, and consist mainly of low–K granodiorite and tonalite, which are common in the CSB and EPRB, whereas eastern plutons are more alkaline (Valencia–Moreno et al., 2003). The similarity in the geochemical characteristics and isotopic ages of the CSB and EPRB, suggest a genetic relationship between them. The ages of crystallization in the EPRB are older than 95 Ma (Ortega–Rivera, 1997; Kimbrough et al., 2001), whereas the ages of the CSB are constrained between 90.1 and 69.4 Ma. In central and eastern Sonora, ages vary between 78 and 57 Ma (Anderson et al., 1980; Poole et al., 1991). If we restore the peninsula of Baja California to its paleogeographic position before the opening of the Gulf of California (Gastil and Krummenacher, 1977; Oskin, 2002; Ortega–Rivera, 2003), the isotopic age distribution define continuous isochrones from the Baja California peninsula to the interior of Mexico (Damon et al., 1983b; Ortega–Rivera, 2003).

Furthermore, the offset of the contour of the 0.706 initial ⁸⁷Sr/⁸⁶Sr isopleth on both sides of the Gulf of California confirms the magnitude of dextral shearing across the Gulf obtained from a comparison between Miocene volcanic rocks from San Felipe area and Bahía Kino areas (Oskin and Scott, 2002).

 

CONCLUSIONS

The new U–Pb zircon and ⁴⁰Ar/³⁹Ar biotite and K–feldspar geochronology allow us to constrain ages of crystallization and decipher the thermal evolution after emplacement of the successive intrusions of the CSB, and to propose a regional correlation with other plutons of the Laramide magmatic arc. From older to younger magmatic units, the CSB consist of the Puerto Rico diorite, the Kino granodiorite, the Puerto Nuevo granite, and the Tepopa tonalite, as well as the Tordillo andesite as an associated subvolcanic unit. ²⁰⁶Pb–²³⁸U zircon ages show that the interval between granodiorite and tonalite crystallization is 21 Ma (between 90.1 ± I.land69.4± 1.2 Ma), which suggests that during this time interval there was no migration of the magmatic arc along the present coast of Sonora. ⁴⁰Ar/³⁹Ar and K–Ar ages are in agreement with ²⁰⁶Pb –²³⁸U zircon ages, and corroborate previous results obtained in that region (Anderson and Silver, 1969; Gastil and Krummenacher, 1977; Mora–Alvarez, 1992; Roldan–Quintana, 2002); they indicate a relatively slow cooling rate of the CSB, between zircon crystallization and closure temperature of K–feld–spar. The spatial distribution of ages, older to the south and younger to the north, suggests that the CSB coincide with the transition of the 70–60 Ma isochrons in the model proposed by Ortega–Rivera (2003). The ages determined for the CSB are younger, and consecutive, to those of the EPRB, which is interpreted to belong to the same magmatic arc that migrated eastwards along the continental margin of North America.

 

ACKNOWLEDGMENTS

This research was supported by the Conacyt research grant 36225 T. We want to thank George Gehrels for access and facilities of Isotope Geochemistry Laboratory of University of Arizona, in Tucson. Also we thank Michael Kunk for access and close supervision of ⁴⁰Ar/³⁹Ar geochronology at the U.S. Geological Survey Thermochronology Laboratory in Denver, Colorado. We also thank Alice Kaminski, a former University of Colorado at Boulder student for careful mineral separations for the Ar/Ar geochronology studies. We thank Pablo Peñaflor–Escárcega for his help in sample preparation at ERNO geochemical laboratory in Hermosillo, Sonora, and René Delgado–González for his field assistance in Baja California. Finally we are grateful to Margarita López Martínez, Luis A. Delgado Argote and Marty Grove for their helpful review of the manuscript.

 

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