Magmatismo adakítico del Cretácico tardío en la parte centro–oriental de Sonora, México, y su relación con skarns de Cu–Zn–Ni–Co

Efrén Pérez–Segura^1,*, Eduardo González–Partida², and Víctor A. Valencia³

¹ Departamento de Geología, Universidad de Sonora, Rosales y Boulevard Luis Encinas, 83000 Hermosillo, Sonora, Mexico. * efrenpese@yahoo.com

² Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Junquilla, 76230 Querétaro, Qro., Mexico.

³ Department of Geosciences, The University of Arizona, Gould–Simpson Building, 1040 East Fourth St., 85721–0077 Tucson, Arizona, USA.

ABSTRACT

The Late Cretaceous–early Tertiary (Laramide) orogeny (90–40 Ma) in the northwestern part of Mexico produced an important calc–alkaline magmatism, accompanied by several associated porphyry copper and skarn deposits. In the Sierra Santo Niño, Sonora, the Bacanora batholith intrusive complex, composed oftonalite–granodiorite and biotite–hornblende granites intrudes both upper Paleozoic platform sedimentary rocks and Upper Cretaceous volcanic rocks, and is intruded by the San Lucas porphyry (quartz monzonite). The Bacanora batholith and the San Lucas porphyry yield crystallization ages between ~91 Ma and ~89 Ma, and represent the oldest Late Cretaceous intrusive event yet reported for east–central Sonora. Cu–Zn–Ni–Co skarn deposits were generated by this magmatism, and have not been previously described in Sonora. The Bacanora batholith and the San Lucas porphyry show average compositions of 65.1–65.3wt.% SiO₂and 15.7wt.% Al₂O₃; the main geochemical differences among both unitsare in 4.3wt.% FejO₃, 1.9wt.% MgO, 3.8wt.% CaO and 3.3wt.% K₂O for the Bacanora batholith versus 2.8wt.% Fe₂O₃, 1.7wt.% MgO, 2.1wt.% CaO and 5.1wt.% K₂O for the San Lucas porphyry. Both units show a LREE enrichment as well as HREE depletion. These compositions match with that of adakitic magmas produced by partial melting ofsubduced oceanic slab, leaving garnet and amphibole in the restite.

Key words: magmatism, geochronology, adakite, skarn, Sonora, Mexico.

RESUMEN

La orogenia del Cretácico Tardío–Terciario temprano o Laramide (90–40 Ma) en el noroeste de México produjo un importante magmatismo calcialcalino, acompañado por diversos yacimientos asociados de tipo pórfido cuprífero y skarn. En la Sierra Santo Niño, Sonora, el complejo intrusivo del batolito Bacanora, compuesto de tonalita–granodioritay granitos de biotita–hornblenda intrusiona tanto una plataforma de rocas sedimentarias del Paleozoico superior, como rocas volcánicas de Cretácico Superior, y es intrusionado a su vez por el pórfido de cuarzo–monzonita San Lucas. El batolito Bacanora y el pórfido San Lucas arrojan edades de cristalización entre ~91 May ~89 Ma, y representan el evento intrusivo del Cretácico Tardío más antiguo reportado hasta ahora en Sonora centro–oriental. Algunos skarns de Cu–Zn–Ni–Co fueron generados por este magmatismo y no habían sido descritos previamente en Sonora. El batolito Bacanora y el pórfido San Lucas muestran una composición promedio de 65.1–65.3% de SiO₂ y 15.7% A1₂O₃; las principales diferencias geoquímicas son 4.3% Fe₂O₃, 1.9% MgO, 3.8% CaO y 3.3% K₂0 para el batolito Bacanora versus 2.8% Fe₂O₃, 1.7% MgO, 2.1% CaO y 5.1% K₂0 para el pórfido San Lucas. Ambas unidades muestran un enriquecimiento en LREE así como una disminución en HREE. Estas composiciones concuerdan con un origen a partir de magmas adakíticosproducidos por fusión parcial de una placa oceánica subducida, en la cual queda una restita con granate y anfibol.

INTRODUCTION

The Late Cretaceous–early Tertiary (Laramide) orogeny is recognized in Sonora as a minor ductile compressive tectonic event (Rangin, 1982; Pubellier, 1987; Pubellier et al, 1995) responsible for an important magmatic event that occurred between 90–40 Ma (Damon et al, 1981; Damon, 1986). Subduction of the Farallón plate under North America (Coney and Reynolds, 1977; Damon et al, 1983a) produced a continental margin magmatic arc that migrated from west to east from the paleotrench over time. This magmatic arc lay parallel to the northwestern Mexico Pacific coast and originated the Sonoranbatholith (Damon et al., 1983b) and correlative volcanic rocks (McDowell et al., 2001; Roldan–Quintana, 2002) that crop out extensively along NNW trending ranges. The magmatism has been traditionally considered to be cale–alkaline (Roldan–Quintana, 1991, 1994, 2002; Valencia–Moreno et al, 1999, 2001, 2003). Most of the mineral deposits, including porphyry copper and associated skarns are spatially and temporally related to this magmatism (Clark et al., 1979,1982; Damon etal, 1981,1983a; Pérez–Segura, 1985; Barton et al, 1995; Staude and Barton, 2001; Valencia–Gómez, 2005; Barra et al, 2005).

This work provides new geochemical and geochrono–logical evidence of intrusive rocks in the Sierra Santo Niño (east–central Sonora), and the genetic relationship between intrusions and recently discovered Cu–Zn–Ni–Co mineralization (Pérez–Segura et al., 2004). The intrusive rocks of Sierra Santo Niño have an adakitic signature, as defined by Defant and Drummond (1990), and were emplaced at the beginning of the Late Cretaceous–early Tertiary orogeny.

GEOLOGICAL SETTING

The study area is located 150 km east of the city of Hermosillo in the Basin and Range Province of Sonora, northwestern Mexico (Figure 1). The morphology of the Sierra Santo Niño is defined by Tertiary extension, which occurred in Sonora at ca. 25–10 Ma (McDowell et al, 1997; Gans, 1997). The Sierra Santo Niño is a horst that is structurally bound at the east by the Bacanora semi–graben and at the west by the Río Yaqui graben (Figure 1).

The geology of the area (Figure 2) has been previously described in the Santa Teresa (H12D–47) and Bacanora (H12D–55) geological maps edited by the Servicio Geológico Mexicano (formerly Consejo de Recursos Minerales, 1996, 2005). The highest portions of the Sierra Santo Niño are composed of Paleozoic sedimentary platform rocks, also recognized at the southern part of this range (Araux and Vega, 1984) and at the Sierra Agua Verde, northwest of the study area (Ochoa and Sosa, 1993; Stewart et al, 1999). The sedimentary rocks in the Sierra Santo Niño represent a roof pendant resting on granitic bodies. In the northern part of the study area (at the Todos Santos mine), Paleozoic platform rocks are thrust over pre–batholithic andesitic rocks, which are correlated with the Tarahumara Formation (Wilson and Rocha, 1946; McDowell et al, 2001; Roldan–Quintana, 2002).

Paleozoic rocks

The Late Cretaceous andesitic pre–batholithic complex: Tarahumara Formation

The Tarahumara Formation was originally identified by Wilson and Rocha (1946) at the Cañón del Obispo, located 65 km south of Bacanora. The Tarahumara Formation consists of andesitic and dacitic lava flows, volcanic breccias and pyroclastic deposits (McDowell etal, 2001). The thickness is up to 2500 m. The Tarahumara Formation has been interpreted as the volcanic equivalent of the Sonoran batholith, and has been dated using U–Pb isotopes in zircons with ages that range from 90 to 70 Ma (McDowell et al., 2001; Roldan–Quintana, 2002).

Rocks correlated with the Tarahumara Formation in the study area form extensive andesitic outcrops, which are ubiquitous in east–central Sonora. They are massive green andesite, andesitic tuff and volcaniclastic rocks. Commonly, these rocks are hydrothermally altered to chlorite + epidote + calcite + quartz + pyrite, and are black in color due to the presence of abundant secondary biotite. The thickness of these rocks is estimated to be up to 1500 m. At the Rancho Aguajito (Figure 1), andesitic rocks have been altered by contact metasomatism to garnet + pyroxene. In the northern part of the Sierra Santo Niño (at the Todos Santos mine), the andesitic rocks are thrust by Paleozoic limestones and in northern Sierra Chiltepinby Neoproterozoic clastic rocks (Figure 2).

INTRUSIVE MAGMATISM IN THE SIERRA SANTO NIÑO

The Bacanora batholith and the San Lucas porphyry

Rocks of the Bacanora batholith crop out in three main areas: 1) along the Arroyo Moras, in a 14 km² band; 2) east of the Novillo town in a 10 km² strip; and 3) along the El Novillo–Bacanora highway (Figure 1). It represents about 25 % of the surface of Sierra Santo Niño observed in Figure 2. Morphology, geological relationships and geophysical observations compiled in the Servicio Geológico Mexicano maps, suggest that the observed outcrops are the surficial expression of a major batholithic body at depth. The Bacanora batholith likely comprises a complex intrusive suite; the predominant lithology is granodiorite, varying from tonalite to sensu–stricto granite. The common mafic minerals are hornblende and biotite, but pyroxene is locally present. Pegmatite, aplite and lamprophyric dikes commonly crosscut the batholith.

The San Lucas porphyry is a hypabisal intrusive to subvolcanic body with an elongated or amoeboid shape. It is exposed within a 5 km² area along the Arroyo San Lucas, between the Rancho San Lucas and the Rio Yaqui, and forms about 6 % of the Bacanora batholith outcrop. The San Lucas porphyry intrudes limestones, quartzites and conglomerates inferred to be late Paleozoic in age, and the Late Cretaceous Tarahumara andesite (Figure 2). Andesitic dikes intrude the Bacanora batholith in the Arroyo Las Moras and the metamorphosed sedimentary rocks in the Arroyo San Lucas.

Relation to Cu–Zn–Ni–Co skarns

The La Esperanza skarns are the most important of these prospects and have only recently been described by Pérez–Segura et al. (2004). A preliminary economic evaluation suggests the existence of more than 1 Mt of ore with values around 1 g/t Au, 1–2% Cu, 1–2% Zn, 0.1–0.2% Ni and 0.1–0.2% Co. Most noteworthy is that the La Esperanza deposit is the first Ni–Co bearing ore body reported in Sonora, suitable for exploration. The mineralization was developed in the endoskarn zone in the contact between the San Lucas porphyry and the Paleozoic cover. Skarns contain a prograded assemblage with garnet more abundant than pyroxene, and a discrete retrograde assemblage with chlorite + epidote + calcite + quartz + magnetite + pyrite + siegenite [(Ni,Co)₃S₄] + chalcopyrite + sphalerite + hematite. Garnet composition is andraditic, whereas pyroxenes are diopsidic. Siegenite is the main Ni–Co mineral and is earlier than the deposition of the sphalerite and chalcopyrite assemblage. The ore mineralogy coupled with andraditic garnet > diopsidic pyroxene, the presence of pyrite–magnetite–hematite, and the absence of pyrrhotite, indicate that La Esperanza may be classified as an oxidized skarn (Meinert, 1998). The spatial and temporal relationship between the skarns (endoskarn) and the San Lucas porphyry, and consequently with this magmatim, is evident.

Petrography of the Bacanora batholith and the San Lucas porphyry

Two main petrographic assemblages are recognized in the Bacanora batholith: a pyroxene + biotite tonalite–sy–enogranite and a hornblende–biotite–bearing monzodiorite–quartzmonzonite–granodiorite–monzogranite assemblage. The contacts among different facies were not observed in the field, suggesting a gradational transition.

The pyroxene+biotite tonalite is phaneritic, fine to coarse–grained (0.5–3 mm), and commonly hypidiomor–phic. Compositionally, the rocks range from a pyroxene+ biotite tonalite to a pyroxene syenogranite. Mineralogy was identified in thin section and percentages were estimated by volume; the anorthite content in plagioclase was estimated by using selected extinction angles on sections perpendicular to 010 cleavage. The modal composition is the following: quartz (5–10%), K–feldspar (5–56%), plagioclase (10–55%), pyroxene (15–20%), biotite (0–4%) and opaque minerals (3–10%). Accessory minerals are apatite, sphene and zircon. Quartz is always interstitial and is observed in myrmekitic intergrowth with feldspar in some cases. K–feldspar is orthoclase, with large crystals (~3 mm) commonly with Carlsbad twinning. The plagioclase composition is An_30–32. The pyroxene is most likely augite. Biotite is pleochroic and frequently has apatite and pyroxene inclusions. Sericite in plagioclase and actinolite in pyroxene are present as selective alteration minerals.

Hornblende+biotite–bearing intrusive rocks are phaneritic with grain size of 0.2 to 3 mm. They contain quartz (5–30%), K–feldspar (10–45%), plagioclase (30–60%), hornblende (5–10%) and biotite (0–10%). The accessory minerals are apatite, sphene, zircon, and opaques. K–feldspar is present as orthoclase and microcline. Quartz is interstitial. The plagioclase composition is An_24–34. There are traces of selective mineral alteration that consist of actinolite in hornblende, chlorite + epidote in biotite, and sericite in plagioclase.

The San Lucas porphyry contains plagioclase and mafic phenocrysts, inanaphanitic, typically pink, groundmass. Phenocrysts are 1 to 5 mm in length, increasing in size and abundance northwards in the Arroyo San Lucas outcrop. Phenocrysts are typically plagioclase, but hornblende and biotite phenocrysts have also been observed. Orthoclase phenocrysts and quartz eyes are less common. The phen–ocryst composition is plagioclase, biotite, hornblende, orthoclase and quartz. The groundmass is fine–grained felsitic, and commonly microlithic to trachytic in texture, suggesting a hypabysal to subvolcanic emplacement. Plagioclase crystals are idiomorphic as thin sheets, with polysynthetic twinning, zonal structure, and a composition of An_20–34(oligoclase–andesine); they exhibit corroded edges. Quartz eyes commonly show three–phase fluid inclusions withNaCl crystals. Hornblende is locally altered to fibrous actinolite. Biotite is dark, appearing to be iron–rich, and frequently is altered to chlorite+calcite+epidote+opaque minerals (propylitic alteration). The groundmass is predominately composed of K–feldspar. Among the accessory minerals apatite is present and zircon crystals are idiomorphic, with prismatic shapes with a length/width ratio >2/l. Magnetite is the most common opaque mineral, although pyrite is associated with chlorite. Alteration minerals are selective: plagioclase is altered to sericite, while ferromagnesian minerals are altered to chlorite+carbonate+epidote+sphene+rutile+ opaque minerals.

Geochronology

Two samples from the Bacanora batholith (03–107) and the San Lucas porphyry (03 –11) were selected for zircon U–Pb geochronological analysis. Mercator coordinates of these samples are: 3216 450 N, 642 249 E for 03–107, and 3215 270 N, 646 945 for 03–11 (Figure 1). Analysis were performed by laser–ablation multicollector–inductively–coupled plasma–mass–spectrometry (LA–MC–ICPMS) at the Arizona LaserChron Center, following procedures by Valencia et al. (2005). Zircons were separated from 5 kg rock samples using standard techniques of the University of Arizona. Around 50 euhedral inclusion–free zircon crystals were mounted along with SL2 standards (Gehrels et al., 2008) on centers of one inch phenolic O–rings, polished to expose grain interiors, and imaged with cathodoluminis–cence. Analysis were performed in crystal cores and tips. Each measurement cycle consisted of one 20 s integration on every centered peak with the laser off for background counts, twenty 1 s integrations during mineral blasting, and ~30 s of purging. Zircons were ablated with a New Wave Research DUV193 ArF Excimer laser (wavelength = 193 nm). The laser beam was operated at ~60mJ energy (at 23.5kV), a pulse rate of 8Hz, and a spot size of 35 μm. The generated pit was ~12 μm deep. Ablated material was carried in helium gas mixed with Ar gas into an Isoprobe ICPMS (GV Instruments). U, Th and Pb isotopes were analyzed simultaneously in static mode using Faraday collectors for ²³⁸U, ²³²U, ²⁰⁸Pb, ²⁰⁷Pb and ²⁰⁶Pb, and an ion counter for ²⁰⁴Pb. Contributions to the 204 mass by Hg were removed by subtracting background counts. Common Pb corrections were calculated from ²⁰⁴Pb measurements assuming an initial isotopic composition according to the Pb evolution curve of Stacey and Kramers (1975). Inter–element fractionation of Pb/U was generally ~20%, whereas fractionation of Pb isotopes is generally <2%. In–run analysis of fragments of a large zircon crystal (every fourth measurement) with a known age of 563.5 ±3.2 Ma (2–sigma error, Gehrels et al., 2008) was used to correct for this fractionation. The uncertainty resulting from the calibration correction is generally ~1% (2–sigma) for both ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁶Pb/²³⁸U ages.

Sample 03–107 is abiotite and pyroxene tonalite from the Bacanora batholith, whose chemical analysis is shown in Table 1. Sample 03–11, from the San Lucas porphyry, was taken from a quartz monzonite dike with biotite and hornblende hosted by skarns and banded hornfels. Dating results are shown in Figure 3 and in Table 2. Sample 03–107 has a weighted mean ²⁰⁶Pb/²³⁸U age of 90.6 ± 1.0 Ma and sample 03–11 has a weighted mean ²⁰⁶Pb/²³⁸U age of 88.7 ± 1.0 Ma. No inherited component was recognized in the 52 grains analyzed in cores and tips. The systematic error (age of standard, calibration correction from standard, composition of common Pb, decay constant uncertainty) during the session was 1.1% and was added quadratically to the random errors of 0.4 and 0.3 % to assign uncertainties to the age determinations. These Late Cretaceous ages are interpreted as crystallization ages of these rocks, and correspond to the beginning of the magmatic Laramide event. Previous work in the area reported a K–Ar age in whole rock of 53 Ma for an intrusive body located west of the town of Bacanora (no coordinates reported), and a second age of 64 Ma (whole rock) from the Sierra Chiltepin (Pubellier, 1987). These ages are more typical of the Sonoran batholith (Damon et al., 1981, 1983b), but it is likely that these K–Ar ages results are reset ages.

Samples from the Bacanora batholith and the San Lucas porphyry were selected for geochemical analysis at the geochemistry laboratory of the Centre des Recherches Pétrographiques et Géochimiques (CRPG) in Nancy, France. Samples were analyzed by ICP–MS following the procedure described by González–Partida et al. (2003a). Results are shown in the Table 1 and some typical diagrams are shown in Figures 4 (5,6)–7. Also, some summarized geochemical data from the Central Sonora batholith and Coastal Sonora batholith (Valencia–Moreno etal, 1999, 2001, 2003) are shown for comparison in the same figures.

Six samples from the Bacanora batholith show that the SiO₂ range content is 55–71 % and the major oxides A1₂O₃, CaO, MgO, TiO₂ and P₂O₅ decrease systematically with increasing SiO₂ inHarker diagrams (Figure 4), whereas there are a good positive correlation between SiO₂ versus K₂O. Highest values in CaO corresponds to the tonalitic petrographic facies and high content in MgO and TiO₂ is consistent in samples with more abundant ferromagnesian minerals. The rocks plot in the high K calc–alkaline field in the K₂O/SiO₂ diagram (Figure 5). According to La Roche's et al. (1980) classification all rocks, excepting one, plot in the granodiorite, tonalite, quartz monzonite and syenogran–ite fields. The behavior of some key trace elements such as Sr/Rb versus SiO₂, which agrees with the degree of mag–matic differentiation, suggest that these rocks originated from the same magma. A multi–element spider diagram shows a homogeneous behavior for the Bacanora batholith (Figure 6). In a REE plot, a LREE enrichment and a relative depletion in HREE is observed, followed by a very flat trend in the Dy–Lu segment (Figure 7). The (La/Yb)_N is 9.6 and no Eu anomaly is present.

For the San Lucas porphyry five rock samples were analyzed. The samples have very similar characteristics because they come from an homogeneous individual stock that is classified as a hypabysal to subvolcanic quartz monzonite. The geochemical analysis are shown in Table 1 and some average values are included in Table 3. The SiO₂ composition is restricted to 63–68 % and the most important chemical differences with respect to the Bacanora batholith is the enrichment in K₂O and depletion in CaO at similar SiO₂ values. The high K₂O average concentration (5.1 %), is due to the presence of K–feldspar in the rock matrix; low CaO concentrations are due to the mainly sodic composition (An<30) of plagioclase that is only present as phenocrysts. The San Lucas porphyry compositions generally plot in the shoshonite field (Figure 5). The rock is Fe₂O₃ and MgO poor, as biotite and hornblende are in low abundance. Depletion of MgO, TiO₂ and P₂O₅ occurs as SiO₂ increases. The porphyry composition plots within the quartz monzonite field according to La Roche's et al. (1980) classification. The characteristic mean content of trace elements is given in Table 3. Sr is in general depleted and exhibits an inverse behavior to K₂O, suggesting that plagioclase controls the Sr content (plagioclase decreases as K–feldspar increases). Ni and Cr concentrations are higher than in volcanic arc granodiorite (Table 3) compiled by Martin (1999), while Y and Yb concentrations are lower. As in the Bacanora batholith, the REE pattern shows a LREE enrichment and a HREE depletion ((La/Yb)_N > 10), with no Eu anomaly present.

In summary, geochemistry of major and some trace elements of the Bacanora batholith and of the SanLucas porphyry could be explained by mineralogical composition. The basic differences are in high K₂O, low CaO and restricted SiO₂ for the San Lucas porphyry, and different concentrations for Ba, Pb and Sr in the multi–element diagram (Figure 6); the REE patterns in both cases are overlapped (Figure 7), suggesting an origin from the same magma source as it is discussed later. Field relations indicate that the Bacanora batholith (90.6 Ma) is intruded by the San Lucas porphyry (88.7 Ma). The similar age for both intrusive rocks suggest a comagmatic origin, being the SanLucas porphyry the latest intrusive facies, probably more contaminated by continental crust (higher in K₂O).

Relation of the Bacanora batholith and the San Lucas porphyry with adakites

As defined by Defant and Drummond (1990), adakites are intermediate to acidic rocks, and include andesites, dacites and rhyolites. They are porphyritic lavas containing zoned plagioclase, biotite, hornblende and rarely pyroxene. Their general silica content is >56%, A1₂O₃ >15 %, Na₂O 3.5–7.5% and K₂0/Na₂0 ~0.42. Key discriminants for adakites are: (La/Yb)_NverswsYb_N (Martin, 1987,1999) and Sr/Yb versus Y (Defant and Drummond, 1990). Adakites are similar to the calc–alkaline series, but they generate in particular collisional geotectonic environments and have been interpreted as originated from the partial melting of a subducted oceanic slab that interacts with the mantle wedge (Martin et al, 2005).

In relation with key discriminant elements, the San Lucas porphyry has a low Sr concentration (229 ppm in average), whereas the Bacanora batholith rocks have an average of 475 ppm Sr; this element is controlled by feldspars proportion in the rocks. In the Sr/Y vs. Y and (La/Yb)_N vs. Yb_N diagrams, these rocks plot within or near the adakite field (Figure 8). From the multi–element diagram and REE plots, a similar pattern is observed for the Bacanora batholith and the San Lucas porphyry, which also supports a common origin (Figures 7 and 8), with an LREE enrichment and a HREE depletion and with (La/Yb)_N > 9.7–14.1. The HREE depletion can be interpreted as reflecting the presence of garnet + hornblende in the restite of the magma source (Martin, 1999). However, the Ni values (>24 ppm) and Cr (>3 6 ppm) values in the San Lucas porphyry are higher than typical adakites, which can be interpreted as an adakitic magma reacting with the peridotitic mantle wedge (Maury et al, 1996; Martin, 1999; Martin et al, 2005). The most important argument supporting a co–magmatic origin for the Bacanora batholith and the San Lucas porphyry are the similar crystallization ages (~90 Ma).

DISCUSSION AND CONCLUSIONS

The U–Pb isotopic analysis inzirconforthe Tarahumara Formation in the Tecoripa–Tónichi–La Dura–Suaqui Grande quadrangles (McDowell et al., 2001) yielded ages in the range from 90.1 to 69.7 Ma. This formation is overlain by unaltered volcanic rocks, with ages of 62–53 Ma, that are not considered part of the Tarahumara Formation (Roldán–Quintana, 2002). Volcanic rocks of 90–60 Ma have been identified in different areas of Sonora, e.g., Jacques–Ayala et al. (1993) report a 72 Ma age (K–Ar in hornblende) for andesites in the Sierra El Cháñate (19); Gastil and Krummenacher (1977) dated a 85.1 Ma (K–Ar in hornblende) andesite and a 64 Ma (K–Ar in biotite) dacite at Puerto Libertad; Meinert (1982) reported ages of 67 Ma (K–Ar) for the Mesa Formation at Cananea (20) and Wodzicki (1995) obtained a 69 Ma (Ar/Ar in biotite) age for the same formation. At Arivechi, closer to our study area, Pubellierei al. (1995) found ages of 66 Ma and 75 Ma (K–Ar in whole rock) forandesites, andPubellier (1987) reported a strongly potassic porphyritic andesite with an age of 83 Ma (K–Ar in whole rock).

The new U–Pb ages reported here show that the intrusive rocks of the Bacanora batholith, including the San Lucas porphyry, were emplaced as early as 90 Ma. Our geochronological data establish the oldest Late Cretaceous (Laramide) intrusive rocks in east–central Sonora, which intrude andesitic volcanic rocks that are older than the ones described by McDowell etal. (2001), about 50–75 km to the south of the study area. The age of thrusting of the Paleozoic platform strata over the pre–batholithic andesites is constrained to a period ranging from 100 Ma to 91 Ma; the limits are established in accordance with the evidence of the Bacanora batholith (91 Ma) and the San Lucas porphyry (90 Ma) intruding Paleozoic limestones and Cretaceous andesites; the oldest 100 Ma limit is fixed according to the thrusting of Neoproterozoic sedimentary rocks over Albian–Cenomanian limestones at Sierra Chiltepin, on the highway between Bacanora and Sahuaripa (Pubellier et al., 1995).

Several authors (Damon et al., 1981,1983b; Valencia–Moreno et al, 2001, 2003; Roldan–Quintana, 2002) have pointed out the calc–alkaline nature of the Sonoran batholith. In general, the rocks of the Sonoran batholith have higher Y and Yb_N values than the Bacanora batholith and the San Lucas porphyry, and plot mainly in the island arcs fields of the Drummond and Defand (1990) and Martin (1987) diagrams. The HREE values of the Sonoran batholith are less depleted compared to the analysis reported here for the Bacanora batholith and the San Lucas porphyry (Figure 7). The Sr isotopic data indicate ⁸⁷Sr/⁸⁶Sr ratios higher than 0.707 for the northern and central Sonora granitoids (Damone/a/., 1983b; Valencia–Moreno et al, 1999,2001), which have been interpreted as reflecting a continental crust contribution to the magmas (Valencia–Moreno etal, 2001).

The intrusive rocks of the Sierra Santo Niño (Bacanora batholith and San Lucas porphyry) have an adakitic signature and hence were probably generated by the partial melting of an eclogitic oceanic slab with garnet + amphi–bole in the restite. This melt may have reacted with the overlying peridotitic mantle wedge to produce the Cr and Ni anomalies observed in granitoid rocks and at the La Esperanza skarn deposits (Pérez–Segura et al., 2004). Other adakitic intrusive rocks that have been described in México include the Paleocene intrusions reported in Mezcala, Guerrero (González–Partida et al, 2003a, 2003b). Tullock and Krimbrough (2003) have also proposed a "high Sr/Y plutonic magmatism", which could be look like adakites in Baja California (La Posta suite), composed of hornblende + biotite bearing granitoids with ages between 99 and 92 Ma (Ar/Ar).

Defant and Drummond (1990) and Drummond and Defant (1990) proposed that adakitic magma formation requires the subduction of a young (<25 Ma) and hot oceanic crust, which does not fit in early Laramide geodynamic setting for this portion of North America. In contrast, some authors (Maury et al, 1996) have indicated that adakites can be considered as geodynamic markers for the interaction of young oceanic crust subducted underneath a continental plate. Thus, adakites appear to be linked to the beginning of subduction or, in particular, oblique subduction processes involving old crust (Maury et al, 1996; Martin et al, 2005). On a global scale, there is a close relationship between low angle subducting slabs and the production of adakitic magmas (González–Partida et al, 2003a), in the same way that a low angle subducting slab allows for an important modification in the thermal structure of the mantle wedge (González–Partida et al, 2003a). The adakite melt forms proximal to the edges of a subduction slab at depths of 25–90 km (Thorkelson and Breitsprecher, 2005), and adakitic magmas are more likely to trigger hydrothermal convection cells more easily than other types of magmas.

In Mexico, the Laramide orogeny was the consequence of a low angle and high speed subduction processes between the Farallón and the North America plates in Late Cretaceous – early Tertiary interval (Damon et al, 1981, 1983a, 1983b,). The relative changes between the Farallón and the North America plates that took place during the Late Cretaceous (Atwater, 1989), induced an inboard arc migration, caused by the shallowing of the subducting slab. The presence of adakitic rocks of ~90 Ma in east–central Sonora, more than 200 km inland from the paleosubduction zone, has the following possible implications:

1) Although not completely understood, factors such as the trench length (>6000 km) and the plate width beneath the continent (>400 km) could have provided heterogeneous thermal conditions appropriate for the generation of magmas by partial melting.

2) Geometry of the Laramide magmatic belt during the Late Cretaceous was different from its traditionally generally accepted interpretation and, therefore, subduc–tion was not parallel to the present northwestern coast of Mexico, but oblique.

3) Localized adakitic magmatism was not generated from melting from the Farallón oceanic plate, but from the remnants of the Mezcalera plate that was present beneath the continental crust, shortly after the Guerrero terrane accretion (Dickinson and Lawton, 2001).

ACKNOWLEDGMENTS

This work was part of Efren Pérez–Segura's doctorate research project. Pérez–Segura was supported by the University of Sonora and PROMEP grants. We are most grateful to Gilíes Levresse for whole rocks analysis. This work has benefited from the critical revision of Fred McDowell and Kenneth F. Clark. The authors are grateful with Alex Pullen, William (Bill) W. Atkinson Jr., Eva Lourdes Vega–Granillo, Fernando Barra, Jaime Roldan–Quintana, Thierry Calmus and Juan Carlos García y Barragán, who helped to improve the manuscript. Analytical support for geochronology was provided by NSF EAR–0443387 (Arizona LaserChron Center).

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