Genetic implications of new Sr and Nd isotopic data of the intrusive rocks from the Laramide Arc in Northern Sonora , Mexico

The Laramide Intrusive Arc constitutes a wide intrusive belt broadly parallel to the actual Sonora coastline. It was formed by the subduction of the Farallon Plate beneath the North-American Plate during the Late Cretaceous-Early Tertiary period. New isotopic data on rocks of this arc show initial 87Sr/86Sr and eNd isotopic values of 0.7066 to 0.7070 and -5 to -6, respectively, for two samples from Bacanora area; as well as 0.7074 to 0.7081 and -3 to -5.5 respectively, for five samples from Cananea, Mariquita and La Caridad areas. Isotopic ages were determined by U/Pb in zircons (95 Ma) and Ar/Ar in potassic feldspar (56 to 71 Ma) from a quartz monzonite porphyry, and by Ar/Ar in potassic feldspar (56 Ma) from another plutonic granodiorite, both from Bacanora. Initial 87Sr/86Sr and eNd values for samples reported in this study suggest that the Laramidic magmas had a large influence from the Proterozoic basement in northeastern Sonora. Four different isotopic zones are proposed for Sonora, according with Sr-Nd data of laramidic rocks and the substratum intruded.


Introduction
The Sr, Nd, and Pb radiogenic isotopes have been used to understand magma petrogenesis and also to define the composition of basement rocks through which the magmas raised to the upper crust (Kistler and Peterman, 1973;DePaolo, 1981;Farmer and DePaolo, 1983;Faure, 2001).In northwest Mexico (Fig. 1), Sonora is an important region to understand the paleotectonics of North America since Precambrian, because of the presence of remnants of hypothetical Rodinia continent suggested by Stewart et al. (2002), and also due to the probable connection between Laurentia and Gondwana during Paleozoic (Poole et al., 2005).The effect of the Basin and Range tectonics on this region is an inconvenience for the study of the intrusive bodies (Stewart and Roldán-Quintana, 1994), and also the presence of the voluminous Tertiary magmatism which covers the batholiths, mainly on the border between Sonora and Chihuahua (Lanphere et al., 1980;Ferrari et al., 2007).However, it is possible to identify a broad magmatic arc of cordilleran type that was active during the Late Cretaceous-Early Tertiary (Coney and Reynolds, 1977;Atwater, 1989), which produced abundant volcanic and intrusive rocks.
There are very few isotopic data from Sonora to understand the relationship between the basement and the Laramide magmas (Roldán-Quintana et al., 2000;Valencia et al., 2001;Housh and Mc Dowell, 2005).The origin of Tertiary magmatism in the Sierra Madre Occidental has been considered as mantle derived (McDowell et al., 1999;Lanphere et al., 1980) despite an important contribution from the upper crust has been also proposed (Verma, 1984).The aim of this study is to present new Sr and Nd isotopic data from some intrusive rocks of Laramide Arc to discuss their geochemistry, age, different magmas relationship, and the role and type of basement involved.

The pre-Laramide substratum
The pre-Laramide substratum is present in the northern Sonora area since the Precambrian.Paleoproterozoic metamorphic and intrusive rocks of 1.8 to 1.7 and 1.7 to 1.6 Ga have been identified at Caborca and Cananea regions, respectively (Anderson andSilver, 1977, 1979;Iriondo et al., 2005).Both assemblages had been juxtaposed during Jurassic probably due to displacements caused by the Mojave-Sonora Megashear (Anderson and Silver, 1979).

The Laramide Intrusive Arc
The Laramide Intrusive Arc (LIA) constitutes a broad discontinuous belt of intrusive rocks (Fig. 1), 300 km wide in a section from Bahía Kino to Moctezuma, without correcting for the tertiary distensive tectonics.The LIA represents the intrusive component of the Laramide Magmatic Arc (Roldán-Quintana, 2002;Roldán-Quintana et al., 2009), which is the product of subduction of the Farallon Plate underneath the North America Plate, from the Late Cretaceous to Early Tertiary (Coney and Reynolds, 1977;Atwater, 1989).At surface, these rocks are expressed by several NNW-SSE oriented sierras, which represent lifted blocks during the Upper Tertiary Basin and Range tectonics.To the south of the 28º 30' N, their outcrops are more dispersed, always following a NNW-SSE trend, even if the sierras are less conspicuous.
The batholiths along the coast are composed of tonalite to granodiorite (Valencia-Moreno et al., 2001, 2003), whereas inland they are composed of tonalite-granodiorite-granite (Roldán-Quintana, 1991, 2002).Two magmatic suites are often recognized in the Mazatán, Aconchi and Magdalena sierras: calc-alkaline series and per-aluminous series (Nourse, 1990;Richard, 1991;Roldán-Quintana, 1991).The key minerals present in the first case are hornblende and biotite, whereas the second one is defined by the presence of muscovite with or without biotite (Damon et al., 1983).There is evidence of multiple intrusions, which have been identified by the presence of basic xenoliths in the calc-alkaline granitoids (i.e.El Jaralito and Sierra La Madera batholiths), and by the intrusion of per-aluminous granitoids into the calc-alkaline cortege.The temporal definition for the LIA is generally accepted for the 90-40 Ma period, as it has been defined by Damon et al. (1983), but the magmatic period overlaps the Late Cretaceous, since the volcanic component of this arc, i.e.Tarahumara Formation, has been dated up to 100 Ma (McDowell et al., 2001).One temporal-spatial evolution has been frequently postulated, with a diminish in age from the coast to inland (Damon et al., 1981;Clark et al., 1982;Damon, 1986;Valencia-Moreno et al., 2006).But new ages on the intrusive rocks from Central-East Sonora, as old as 90 Ma (Pérez-Segura et al., 2009 and this work), suggest that space-time evolution of the LIA is more complicate than the simplistic schematic model.

Samples of this study
Seven representative samples from Northern Sonora were selected for isotopic analyses (Fig. 1): two samples from the Bacanora area, four samples from the Cananea District and one sample from the La Caridad District.The coordinates of samples are given in Table 1.The initial 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, eSr, eNd values and the model ages for Nd in relation to depleted mantle (DM) were calculated using the equations published by DePaolo (1981) and Farmer and DePaolo (1983).For samples 1 and 2 new isotopic ages determined in this work were used; for samples 3 and 4 (Lucy and Can-12) ages were taken from the Cuitaca Granodiorite published by Anderson and Silver (1977); for samples 5 and 6 (IntrCan and IntrMaria, respectively), the ages are from the Wodzicki (1995) work; and for sample 7 (Car-5) the age used was from the granodiorite reported by Valencia et al. (2005) and Barra et al., 2005).
Sample 2 is quartz-monzonite porphyry related to skarn mineralization near the San Lucas Ranch (Fig. 3a).This rock intrudes propilitized andesites correlated with Tarahumara Formation (Late Cretaceous).The rock is made of quartz phenocrysts (5%), plagioclase (35%) and biotite  Samples 4 and 5 are from the Cananea District.The sample 4 was collected from the Puerto Cananea, which corresponds to the Cuitaca Granodiorite, whereas sample 5 was collected from one of the porphyries related with + hornblende (10%), disseminated in a felsitic groundmass made of potassic feldspar (50%).
Samples 3 and 6 are from Mariquita mineralized zone, and they were weakly affected by phyllic alteration.Sample 3 corresponds to the Cuitaca Granodiorite, which is mineralized at the Lucy open pit, and the sample 6 is  The Sr isotopic ratios of standards and samples were normalized to 86 Sr/ 88 Sr = 0.1194, whereas the Nd isotopic ratios were normalized to 146 Nd/ 144 Nd = 0.7219.The estimated analytical ± 2s uncertainties are similar to those reported in Ducea et al. (2002) and Otamendi et al. (2009).
mineralization at the Cananea mine.Sample 7 was collected from the La Caridad mine.This sample (Car-5) comes from a core of a diamond drill hole at a deeper level than the lowest bench (1245 m) of current open pit.The rock is granodiorite in a potassic alteration zone with rare anhydrite veinlets.

Analytical procedures
Sr and Nd isotopes were analyzed by Mihai Ducea at the geochemistry laboratory of the Geosciences Department, University of Arizona, following the methodology and standards described by Ducea et al. (2002).The Sr isotopic ratios of standards and samples were normalized to 86 Sr/ 88 Sr = 0.1194, whereas the Nd isotopic ratios were normalized to 146 Nd/ 144 Nd = 0.7219.The estimated analytical + 2s uncertainties are similar to those reported in Ducea et al. (2002) and Otamendi et al. (2009).The measured 87 Sr/ 86 Sr (Otamendi et al., 2009) for the SRM987 Sr standard were 0.710285±7 (n=10) and the measured 143 Nd/ 144 Nd for the La Jolla Nd standard were 0.511853±2 (n=10).We also note that our results were not adjusted to any accepted values for these standards.
U/Pb and Ar/Ar geochronology were performed by Alexander Iriondo.For U/Pb geochronology a sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) instrument at the Stanford University was used; dating techniques in zircons were those reported by Williams (1998) and Nourse et al. (2005). 40Ar/ 39 Ar dating was carried out at the USGS in Denver, following the methodology described by Iriondo et al. (2004).
Chemical analyses for major and trace elements (including REE) were carried out at the Centre de Recherches Pétrographiques et Geochimiques (CRPG) of Nancy, France.

Geochemistry
Analytical values in major and trace elements for samples 3, 5, 6 and 7 are presented in Table 2.The rare earth elements (REE) values were normalized using the data proposed by McDonough et al. (1991).The REE pattern for the studied rocks is observed on Figure 2a.Normalization to depleted mantle was made just to compare with the spectrum from Wodzicki (1995) for the Cananea rocks (Fig. 2b).The rocks analyzed indicate patterns enriched in LREE and depleated in HREE, which is a characteristic feature of continental arcs.It is important to mention that the sample 3 corresponding to the mineralized plutonic intrusive at Lucy and the number 6, belonging to the mineralized hypabysal porphyry at Maria open pit, show a very similar geochemistry behavior, suggesting a comagmatic origin.Sample 5 coming from a mineralized porphyry stock of Cananea is affected by a strong hydrothermal alteration, as it is evidenced by absence of CaO (Table 2) and a very high value of 12.7 % on Lose on Ignition (LOI).This sample show HREE depleted pattern with a concave curve and an apparent absence of negative Eu anomaly.Similar pattern is in the 8-110 mineralized porphyry reported by Wodzicki (1995), with a significant positive Eu anomaly (Fig. 2b).In general the REE pattern of Cananea fresh rocks suggests a comagmatic origin; depletion in HREE elements for the mineralized porphyries is probably caused by hydrothermal alteration.The behavior of REE pattern for the sample 7 (Car-5), corresponding to the La Caridad Granodiorite, shows a similar pattern to those from Lucy and Maria (3 and 6), but with a slightly impoverishment in LREE and an enrichment in HREE.

Geochronology
Sample 2 (03-116 in Figure 3a and coordinates indicated in Table 1) was dated by U/Pb and Ar/Ar.U/Pb in zircons yield an age of 95.2 ± 1.8 Ma (Fig. 3b and Table 3); but ages obtained by 40 Ar/ 39 Ar in potassic feldspar of porphyry groundmass goes from 53-71 Ma, for C-H hot steps (Fig. 3c and Table 4).
For the Sr and Nd isotopic calculation of Sample 1 (03-102) we used the age of sample 03-101 (Mercator coordinates: 3203 487 N and 650 739 E), located 17 km to the    west of Sample 1 (Fig. 4a).Sample 03-101 shows apparent ages (Fig. 4b, Table 4) very homogeneous for steps B-J in 56-57 Ma., and it is possible to plot an isochron line with B to F data yielding an age of 55.8 + 0.29 Ma (Fig. 4c).There is no evidence of older ages.

Sr and Nd isotopic data
Table 5 show the isotopic analysis and calculated data for Sr and Nd of our samples.A summary of the same data is also given in Table 1.

Geochronology
The age of 95 Ma for sample 2, is similar to other U-Pb ages from zircons on a dyke correlative with the same porphyry (Mercator coordinates: 3215270 N and 646945 E) which yield 88 Ma (Pérez-Segura et al., 2009).The 95 Ma age is interpreted as a crystallization age, whereas the Ar-Ar ages from 52 to 71 Ma for the same sample are interpreted as cooling ages.Variations in the range of the Ar-Ar ages, indicate that the rock was subjected long time above the blocking temperature of the potassic feldspar, or that the rock was reheated during different periods.
ranging from 88 to 91 Ma (Pérez-Segura et al., 2009) do not allow to discard the possibility that ages older than 56 Ma have been probably erased.

Bacanora area
The isotopic values of 0.7066 to 0.7070 for initial 87 Sr/ 86 Sr and -5.0 to -6.0 for εNd, as well as the model ages for Nd in relation to the depleted mantle of 1.11 to 1.04 Ga, are very similar to previous isotopic values reported in northern Sonora, which indicates the probable presence of a Proterozoic basement in the area; in fact, other geological evidences indicate the presence of Neoproterozoic rocks between Bacanora and Sahuaripa (Stewart et al., 1999(Stewart et al., , 2001(Stewart et al., , 2002)).Also the relative uniformity of the isotopic data for samples 1 and 2 could be interpreted as that the source of the magma in the area of Bacanora remained constant at least during the period between 95 and 56 Ma, if the last age was close to the age of crystallization of the plutonic rocks.

Cananea District
Seven samples have been reported from the Cananea District (3,4,5,6,78,79 and 80;Fig. 1).The first four samples refer to the data reported here, and the other samples were from Woodzicki (1995).All the initial 87 Sr/ 86 values range for the Cuitaca Granodiorite goes from 0.7069 to 0.7081 and eNd from -3.0 to -5.5, whereas for the porphyries the initial 87 Sr/ 86 Sr varies from 0.7074 to 0.7086 and eNd from -5.3 to -5.7.The very similar values in Sr and Nd for the Cuitaca Granodiorite and the mineralized porphyry in Maria mine indicate a co-magmatic origin for both units and that hydrothermal alteration did not have any influence on the isotopic behavior; but in the case of Cananea mine there are small differences in initial 87 Sr/ 86 Sr probably due to differences in the rock composition as it has been demonstrated for other igneous rock series (Verma, 2001).The co-magmatic origin for Cuitaca Granodiorite and mineralized porphyries is also supported by the REE behavior.It is possible to suppose that the magmas that gave origin to the intrusives in the area of Cananea and María were derived from the same source at depth.The origin of both could be due to melting of the Proterozoic lower crust, as it is suggested by the Sr and Nd data in xenoliths from Arizona and Northern México (Ruiz et al., 1988).However, considering the lower Sr and less negative Nd values in Sonora compared with the values reported in southern Arizona (Wodzicki, 1995;Lang and Titley, 1998;Valencia-Moreno et al., 2001), we do not exclude some mantle contribution.

La Caridad District
Four samples have been compiled for Sr isotopic data from this region which range from 0.7064 to 0.7080 (Damon et al., 1983, and this study).Other data from Sr in Bella Esperanza (localities 12 and 72 in figure 1) indicates 0.7062 and 0.7070, respectively.The only available Nd data from this region indicates a value of -4.5 (sample 4).In this regard, we have analyzed a granodiorite with potassic hydrothermal alteration (Sample 7 in Table 2).The sample shows very low CaO (0.11%) and high K 2 O (6.4%) with relatively high loss on ignition (LOI-2.6%).The value of initial 87 Sr/ 86 Sr in sample 7 as well as of sample 3 are somewhat higher compared to other samples reported from the area (Damon et al., 1983;Housh and McDowell, 2005; see also Table 2), differences could be caused by hydrothermal alteration as it has been observed in oceanic basalts and in the volcanic geothermal field of Los Azufres (Verma, 1992;Verma et al., 2005).The isotopic values of Sr and Nd from La Caridad and Cananea are very similar suggesting the same origin for the magmas in both districts.

Correlation with other areas of Sonora and with the pre-Laramide substratum
Values for initial 87 Sr/ 86 Sr and eNd grouped by different ranges are represented on Figures 5 and 6, trying to relate the distribution with geography.One problem for interpretation is that most of data are located in central and northern Sonora.In any event it is evident that initial 87 Sr/ 86 Sr upper than 0.7060 and eNd more negatives than -4 are located broadly north to the 28 o parallel and to the east of coastline from Kino Bay.South and west of the same line, values for initial 87 Sr/ 86 Sr are lower than 0.7060 and eNd less negatives than -4; as well as in central-eastern Sonora on the San Nicolás batholith (Roldán-Quintana, 2002).Some of the most important tectonic features are shown on the Figure 7, trying to relate different geological terranes with the isotopic values.Following this logic and using isotopic data published up to date, four isotopic regions can be proposed for Sonora:Zone 1.To the north of the limit of the Sonora Allochthonous (Poole et al., 2005) and with values of initial 87 Sr/ 86 Sr >0.7060 and eNd <-4.
Zone 2. Corresponding to the zone where the Sonora Allochthonous overlaps the inferred continental crust.In terms of isotopic values it is characterized by 87 Sr/ 86 Sr between 0.7060-0.7070and eNd from -6 to -3.
tween San Carlos and Punta Tepopa.The wide range of the model ages for Nd may indicate the heterogeneity in the composition of the Lower Crust.
Finally, in zone 3, the values of initial 87 Sr/ 86 Sr are in a wide range (0.7054 to 0.7080) and εNd are relatively constant (-4.1 to -3.7), similar to values in the area of Tómochic, Chihuahua (Mcdowell et al., 1999).This, allow us to speculate on the absence of Sonoran proterozoic basement in this zone and relate the isotopic data to a pre-Laramidic geologic history similar to that of central Chihuahua (Housh and Mcdowell, 2005).

Conclusions
New isotopic data on hydrothermally mineralized rocks as for the Maria and La Caridad mines indicate that the isotopic compositions may change with respect to the fresh rock values, as it has been documented by other authors (Verma, 1992;Verma et al., 2005).This is important for future interpretations taking into account that many of the published isotopic data come from mineralized areas (Damon et al., 1983;Sansores-Bolivar and Wayne, 1977;Mead et al., 1988).Another important aspect in the Bacanora and Cananea regions is that the isotopic signature of the magmatic source did not change during the Early to Late Cretaceous (95-55 Ma).
According with new isotopic data and those published so far, various isotopic zones can be delineated in Sonora related to the major pre-Laramide tectonic features.Two of these zones have also been suggested by other authors.The isotopic characteristics and relation with substratum for the different regions proposed by us are: Zones 1 and 2 located at North and Central Sonora (Fig. 7) are characterized by > 0.7060 initial 87 Sr/ 86 Sr and < -4 eNd values.They plot into the range values field of laramidic intrusions in Southern Arizona (Fig. 8).Proterozoic and Neoproterozoic rocks have been recognized in the region of zones 1 and 2 at Cananea, Caborca, Bacanora and Sahuaripa and the limit to the south coincide with the continental crust limit proposed by Poole et al. (2005).It means that the Proterozoic basement of North America underlies zones 1 and 2, as it has been suggested by other authors (Valencia-Moreno et al., 1999, 2001, 2006;Poole et al. 2005).We assume that the Sr and Nd isotopic data of the laramidic intrusions in zones 1 and 2 could have a large influence of the underlying Proterozoic crust, that does not crop out continuously due to the Tertiary tectonics of the Basin and Range province.We also emphasize that the Mojave-Sonora Megashear (Ander-The south isotopic limits follows a line trough the north of Tiburon Island.
Zone 3.This zone corresponds to the San Nicolas Batholith with variable 87 Sr/ 86 Sr but with a very constant eNd between -4.1 to -3.7.
Zone 4. Located to the south of the supposed as the limit of continental crust (Poole et al., 2005).Typical values for this zone are 87 Sr/ 86 Sr <0.7055 and eNd>-4.
Initial 87 Sr/ 86 Sr and eNd data are plotted in Fig. 8.For comparison, the fields for intrusive rocks in southern Arizona and other parts of Mexico are also included.
The data discussed in this paper are located in here called zone and most of the known analytical data are located in the zones 1 and 2. These zones are characterized by initial 87 Sr/ 86 Sr greater than 0.7060 and initial eNd < -4 and they roughly coincide to the south with the limit of the continental crust proposed by Poole et al. (2005); however, the limit to the west is parallel to the coast of Sonora and continues obliquely to north of Tiburón Island (Fig. 7).In both zones 1 and 2, the isotopic data allow us to presume the presence of Proterozoic basement at depth.
The zone 4, south and west of the previous one, shows few data, where we only have information along the coast between San Carlos and Punta Tepopa.The isotopic values although punctual, indicate initial 87 Sr/ 86 Sr < 0.7060 and eNd > -4.These data plus the model ages for Nd of 1.43 and 0.85 Ga allow us to interpret that the intrusive rocks were emplaced at the external border of the Proterozoic basement.The prelaramide geology in southern Sonora is very little known, this region belongs partially to the Cortés and Guerrero Terranes (Campa and Coney, 1983) or the Tahue Terrane (Centeno-García et al., 2008).It is known that metavolcanic rocks, and metasediments of lower Paleozoic are exposed in northern Sinaloa (Mullan, 1978;Centeno-García et al., 2008).Similar series are correlated by other authors with the eugeoclinal rocks from central Sonora related to Gondwana (Poole et al., 2005).In southern Sonora, there are many exposed rocks of the Late Triassic Sonobari Complex (Mullan, 1978;Centeno-García et al., 2008), these protoliths have been proposed as tholeiitic volcanic rocks originated in an oceanic rift (Keppie et al., 2006;Vega-Granillo et al., 2012).According to the data mentioned previously, the magmas which originated the granitoids south of the parallel 28° were not derived from a Proterozoic basement related to the North American Craton, instead of that it is derived from a source with an important mantle contribution, including contamination from the lower crust of the Tahue Terrane (Centeno-García et al., 2008).This could be valid also for the granitoids exposed along the coast of Sonora be-son and Silver, 1979) had no influence on the isotopic signatures in these areas.
The zone 4 with isotopic data of < 0.7060 initial 87 Sr/ 86 Sr and > -4 eNd is clearly separated from zones 1 and 2 (Fig. 8).We interpret that Laramide age intrusive rocks are related to magmas with a probable mantle contribution, or due to contamination from the Tahue Terrane in which tholeiitic volcanic rocks of Paleozoic and Mesozoic age are present (Vega-Granillo et al., 2012).
The zone 3 with a wide range in initial 87 Sr/ 86 Sr values from 0.7054 to 0.7080 and a very restricted eNd values of -4.1 to -3.7.The position in the Sr/Nd diagram (Fig. 8) between zones 3 and 4 suggests a different type of substratum.In this case the underlying basement must be the same of central Chihuahua, consisting of a Paleozoic arc accreted to the Proterozoic North American craton during Late Paleozoic.The variation in Sr isotopic data of laramidic intrusions can reflect a more complex petrology of this substratum.

Fig. 6 .
Fig. 6.-eNd data for all the samples of Table 1 (a), and main range of values by regions (b).Fig. 6.-Datos de eNd (a) para todas las muestras de la Tabla 1, y principales rangos de valores por regiones (b).

Fig. 8 .
Fig. 8.-Initial values of 87 Sr/ 86 Sr versus eNd for samples grouped by zones according to isotopic values and geological features (a), and comparison with other provinces in Mexico and Southern Arizona (b).Province of Northern Sinaloa defined by Valencia-Moreno et al. (2001).Values for bulk earth and for important values of 87 Sr/ 86 Sr and eNd indicated in figures 5 and 6 are shown.Fig. 8.-Valores iniciales de 87 Sr/ 86 Sr versus eNd para muestras agrupadas por zonas de acuerdo a los valores isotópicos y a características geológicas (a), y comparación con otras provincias en México y Sureste de Arizona (b).También se muestran los valores para bulk earth y valores importantes de 87 Sr/ 86 Sr y eNd indicados en las figuras 5 y 6.

Table 3 .
Nourse et al. (2005)dual U/Pb dating data in zircons for sample 03-116 using SHRIMP.Tabla 3.-Datos analíticos y dataciones individuales por U/Pb en zircones para la muestra 03-116 utilizando SHRIMP.204Pbcorrectedforcommonlead.#207Pbcorrected for common lead.All errors are at one sigma level expressed in %, i.e., they are relative standard deviation expressed in % (%RSD, seeVerma, 2005).For more details on SHRIMP results, seeNourse et al. (2005).Ages calculated assuming an initial 40 Ar/ 36 Ar = 295.5 + 0. All precision estimates are at the one sigma level of precision.Ages of individual steps do not include error in the irradiation parameter J.No error is calculated for the total gas age.