Early Cambrian U-Pb zircon age and Hf-isotope data from the Guasayán pluton, Sierras Pampeanas, Argentina: implications for the northwestern boundary of the Pampean arc

An Early Cambrian pluton, known as the Guasayán pluton, has been identified in the central area of Sierra de Guasayán, northwestern Argentina. A U-Pb zircon Concordia age of 533±4 Ma was obtained by LA-MC-ICP-MS and represents the first report of robustly dated Early Cambrian magmatism for the northwestern Sierras Pampeanas. The pluton was emplaced in low-grade metasedimentary rocks and its magmatic assemblage consists of K-feldspar (phenocrysts)+plagioclase+quartz+biotite, with zircon, apatite, ilmenite, magnetite and monazite as accessory minerals. Geochemically, the granitic rock is a metaluminous subalkaline felsic granodiorite with SiO2=69.24%, Na2O+K2O=7.08%, CaO=2.45%, Na2O/ K2O=0.71 and FeO/MgO=3.58%. Rare earth element patterns show moderate slope (LaN/YbN=8.05) with a slightly negative Eu anomalies (Eu/Eu*=0.76). We report the first in situ Hf isotopes data (εHft=-0.12 to -4.76) from crystallized zircons in the Early Cambrian granites of the Sierras Pampeanas, helping to constrain the magma source and enabling comparison with other Pampean granites. The Guasayán pluton might provide a link between Early Cambrian magmatism of the central Sierras Pampeanas and that of the Eastern Cordillera, contributing to define the western boundary of the Pampean paleo-arc.


Introduction
The Sierras Pampeanas were sub-divided into Western and Eastern sectors according to their dominant lithologies (Caminos, 1972) (Fig. 1), whereas modern geochronological studies demonstrate that they correspond to different geological histories, as summarized below.
The oldest and best-studied unit of the Pampean basement is the Puncoviscana Formation (originally defined by Turner, 1960), a thick sequence (>2,000 m) formed by very low-grade marine metasedimentary succession, deposited in the Late Ediacaran (Omarini et al., 1999), but mainly in the Early Cambrian, and folded still within the Early Cambrian (537-523 Ma, Escayola et al., 2011 and references therein).Based mainly on ages from detrital zircons, the metasedimentary rocks of the Eastern Sierras Pampeanas are considered to be higher grade equivalents of this formation, more intensely affected by the Pampean orogeny (Toselli, 1990;Willner, 1990;Rapela et al., 1998;Schwartz and Gromet, 2004;Escayola et al., 2007;Rapela et al., 2007;von Gosen and Prozzi, 2009;Rapela et al., 2015 and references therein).
The metasedimentary rocks of Sierra Norte and Ambargasta are intruded by metaluminous and peraluminous granitic rocks of Early Cambrian age forming the Sierra Norte-Ambargasta batholith (Lira et al., 1997;Iannizzotto et al., 2013).The Early Cambrian Tastil batholith has two main intrusive phases, gray granodiorite and red granite.Extensive outcrops of a porphyry dacite are also associated with the intrusive rocks.In the southern part of the batholith the gray granodiorite intruded folded turbidites of the Neoproterozoic Puncoviscana Formation (Hauser et al., 2011).Studies of the Early Cambrian granites (541-530 Ma) of the Sierras Pampeanas and Eastern Cordillera indicate that crystallized from metaluminous calc-alkaline subduction-related magmas (e.g., Hauser et al., 2011;Iannizzotto et al., 2013 and references therein).In Sierras Pampeanas, late peraluminous granites (530-520 Ma) were linked with a post-collisional event (e.g., Rapela et al., 1998;Iannizzotto et al., 2013).
In this paper, we present the first precisely defined age by U-Pb LA-MC-ICP-MS zircon dating, together with complete petrological characterization, and major and trace element data for the dated sample collected from the Guasayán granitic pluton.We also report the first in situ Hf isotope data for zircons that crystallized in Early Cambrian granites of the Sierras Pampeanas, in order to evaluate their source.
The geochronological, isotopical, petrological, and geochemical data of the GUA-1 sample from the Guasayán pluton is important in that: i) this is the first report of robustly dated Early Cambrian magmatism in Sierra de Guasayán, ii) it is the north westernmost example of Early Cambrian magmatism in Sierras Pampeanas and thus might represents the link between the Pampean magmatism of Sierras Pampeanas and Eastern Cordillera in the northwest of Argentina.

Petrological and whole-rock chemical characteristics of the Guasayán pluton
The Guasayán pluton is located in the central area of Sierra de Guasayán (Figs. 1 and 2).It was emplaced discordantly in a metamorphic complex.It is mainly composed of interbedded phyllites, metapsammites and scarce calc-silicates layers, displaying a mean NNE-SSW S 1 metamorphic foliation subparallel to primary foliation (S 0 ).Localized hornfels is developed in the contact with Guasayán Pluton.The hornfels has fine-grained granoblastic texture with biotite+plagioclase+K-feldspar+quartz and probable andalusite (pseudomorphs of muscovite), and zircon, apatite and opaque minerals as accessory minerals.This subsolidus mineral assemblage with andalusite is typical of low-pressure conditions, suggesting a relatively superficial emplacement of the Guasayán pluton.
A porphyritic granitic rock is the dominant facies in the Guasayán pluton, and it crops out in the studied area.It is located in the western area of the pluton (Fig. 2) as a porphyritic biotitic granodiorite, with 3.0×1.5 cm to 2.0×1.0 cm K-feldspar megacrysts in an equigranular matrix formed of quartz, plagioclase and biotite.K-feldspar crystals are oriented and define a dominant N-S magmatic foliation.Biotitic clusters and centimetric basement fragments are recognized in the granite.Similar porphyritic biotitic granites have been recognized in our field works in the southern Sierra de Guasayán (La Punta village).
Petrographic studies of sample GUA-1 reveal that the modal magmatic assemblage is plagioclase (33.0%), quartz (34.3%),K-feldspar (12.3%), biotite (17.5%) with apatite and zircon as accessory minerals (∼1.85%).The modal data in the Streckeisen (1976) diagram indicate that the granitic rock is a granodiorite.Chlorite is a common secondary mineral associated with biotite.Scarce secondary epidote occurs associated with biotite and chlorite.Two main varieties of plagioclase were recognized: a) coarse-grained (Pl a , 8.0×5.0 mm; minerals abbreviation after Whitney and Evans 2010 and size of individual grains from Hibbard 1995), tabular and mostly subhedral crystals, showing optical zoning and polysynthetic twinning; b) medium-grained: Pl b , ranging from 5.0×2.0 mm to 4.0×2.0mm; and Pl c , ranging from 2.0×2.0 mm to 2.4×1.2,forming tabular and mostly subhedral crystals with polysynthetic twins.Pl a is scarce whereas Pl b and Pl c occurs evenly distributed throughout the rock.Systematic analysis using electron microprobe reveals compositions ranges from An 47.0 to An 33.2 , An 33.2 to An 23.6 , and An 21.5 to An 20.3 , for Pl a , Pl b , and Pl c , respectively.The alkali feldspar is perthite, medium-grained, ranging from 3.0×1.5 mm to 4×2 mm, and forms tabular or irregular subhedral-anhedral crystals, with distinctive perthitic texture.Feldspar is interstitial to the crystalline framework.The composition of alkali feldspar is uncertain since the abundance of perthite prevents determination of the original Na 2 O content, leading to a low total.
Biotite has variable size, ranging from 3.0×1.0mm to 1.3×1.0mm, and is frequently found forming clusters.It occurs as subhedral plates or irregular sections with light-to-dark brown pleochroism and abundant inclusions of apatite and zircon.In terms of Al IV versus (XFe=[Fe 2+ /(Fe 2+ +Mg)]) the biotites show relatively high siderophyllite-eastonite content (average Al IV =2.53 atom/formula unit and XFe=0.65)together with moderate to low F contents (average F=0.38 wt%, n=5).
Fluorapatite is an abundant accessory mineral; it is euhedral to subhedral with dominant hexagonal and short prismatic forms.It is variably fine-grained, 0.3 to 1.0 mm of diameter or 1.1×0.3mm (short prismatic), and is commonly observed as inclusions in biotite.
Systematic analysis using the electron microprobe (determining Zr and LREE data) reveals the presence of two radioactive minerals, zircon and monazite, as well as the occurrence of small-scale ilmenite and magnetite crystals (no greater than 100 µm), all mostly included in biotite.
In the spider-diagram (figure not included, data in Table 1) the sample GUA-1 shows depletion in Nb, Ti and Sr and enrichment in Ba, Rb, Th and K, typically characteristic of Pampean calc-alkaline magmatism (Iannizzotto et al., 2013).The total rare earth element (REE) total abundance is 204 ppm (Table 1), with moderate slope (La N /Yb N =8.05) and slightly negative Eu anomaly (Eu/Eu*=0.76).

U-Pb geochronology and Hf isotopes data
A representative sample (GUA-1) from Sierra de Guasayán was analysed for whole-rock major and trace elements using a ThermoARL sequential X-ray fluorescence spectrometer, following the procedure described by Johnson et al. (1999).Trace element compositions were determined using an Agilent 7700 ICP-MS, following the procedure described in http://cahnrs.wsu.edu/soe/facilities/geolab/technotes/icp-ms_method/ (last visit 20-11-2015).The data is included in Table 1.LA-MC-ICP-MS U-Pb analysis of separated zircons from GUA-1 (Table 2) sample was carried out at the Geochronological Research Center, Sao Paulo University, Brazil, using a 193 nm excimer laser (Photon Machines) coupled to a Neptune multicollector, double-focusing, magnetic sector ICP-MS.Operating procedures and parameters are discussed by Sato et al. (2011).Fractionation in the plasma was corrected by normalizing U-Pb and Pb-Pb ratios of the unknowns to those of zircon standards (GJ 1, 206 Pb/238 Pb age by IDTIMS = 599.8±2.4Ma).
In-situ LA-MC-ICP-MS Lu-Hf isotope analyses were conducted at Geochronological Research Center, Sao Paulo University, Brazil using Photon laser system (Sato et al., 2010) coupled to a Thermo-Finnigan Neptune MC-ICP-MS with 9 Faraday collectors.Lu-Hf isotopic analyses reported here were performed on the same zircon domains that were previously dated (Table 3).Laser operating conditions are reported in Table 3.
In-situ LA-MC-ICP-MS Lu-Hf isotope analyses were conducted at Geochronological Research Center, Sao Paulo University, Brazil using a Photon laser system (Sato et al., 2010) coupled to a Thermo-Finnigan Neptune MC-ICP-MS with 9 Faraday collectors.Lu-Hf isotopic analyses reported here were performed on the same zircon domains that were previously dated.The laser spot used was 47 μm in diameter with an ablation time of 60 seconds and a repetition rate of 7 Hz, and He was the carrier gas (Sato et al., 2009(Sato et al., , 2010)). 176Hf/ 177 Hf ratios were normalized to 179 Hf/ 177 Hf=0.7325.The isotopes 172 Yb, 173 Yb, 175 Lu, 177 Hf, 178 Hf, 179 Hf, 180 Hf, and 176 (Hf+Yb+Lu) were collected simultaneously.A 176 Lu/ 175 Lu ratio of 0.02669 was used to calculate 176 Lu/ 177 Hf.Mass bias corrections of Lu-Hf isotopic ratios were according to variation in the GJ1 standard (Basei et al., 2013).Laser operating conditions and results are reported in Table 3.Early Cambrian Guasayán pluton, siErra pampEanas The combined SEM-CL and optical images reveal that the zircons separated from GUA-1 are mostly elongate prismatic grains with oscillatory zoning and subhedral to euhedral-terminations.Analysis spots were mostly located in the outer oscillatory zoning and the majority of the zircon ages are concentrated at about 533 Ma (see Table 2).Twelve data points yield a Tera-Wasserburg Concordia age (Ludwig, 2003) of 533±4 Ma (2σ confidence limits, allowing for the uncertainty in U-Pb calibration).This is considered the best estimate for the crystallization of the host felsic granodiorite (Fig. 3).Two inheritance ages ("Grenvillian" and "Brazilian") were obtained from zircon core (966 Ma and 616 Ma, Table 2 and Fig. 3).
The Early Cambrian zircons have variable ε Hft (t=533 Ma) values ranging from -0.12 to -4.76.The average model age is calculated as 1.56 Ga (Table 3 and Fig. 4).

Discussion
Petrological and whole-rock geochemical indicate that GUA-1 is a subalkaline porphyritic biotite monzogranite.Frost et al. (2001) and Frost and Frost (2011) have provided exhaustive analysis and classification of granitic suites based on a range of major element indices.According to the Frost et al. (2001) classification scheme, GUA-1 is dominantly magnesian-type and it is projected close to the boundary separating calcic and calc-alkalic series in the modified alkaline-lime index of Peacock (1931) (figures not included, data in Table 1).This composition is those reported in classical calcalkaline magmatism (Frost et al., 2001, Frost andFrost, 2011).As mentioned in the Section 2, sample GUA-1 shows depletion in Nb, Ti, Ta and Sr and enrichment in Ba, Rb, Th and K typical of magmas formed in Pampean subduction zone.González and Toselli (1974) reported a Cambrian K-Ar age in biotite (541±7 Ma) for a granitic rock collected in the eastern flank of the Sierra de Guasayán, near Santa Catalina (Santiago del Estero province), located 25 km to the southeast of the studied area.To the best of our knowledge, no further ages have been reported from the Guasayán pluton.
In the Eastern Cordillera, northwest of Argentina, the metaluminous to slightly peraluminous, Tastil batholith yield two U-Pb LA-MC-ICP-MS zircon ages of 534±7 Ma and 541±4 Ma, for the grey and red granitic facies, respectively (Hauser et al., 2011), and can be also assigned to the early Pampean magmatism.Thus, the crystallization age obtained from the Guasayán pluton is similar to those reported for Pampean granites of Sierra Norte-Ambargasta and Tastil batholith.
The Guasayán pluton might provide a link between Early Cambrian magmatism of the central Sierras Pampeanas and that of the Eastern Cordillera, contributing to define the western boundary of the Pampean paleo-arc (Fig. 5).
Additionally, the studied area is relevant because the Famatinian and Pampean magmatic belts are close to each other in this area.In the Sierra de Ancasti (located to the west of the Sierra de Guasayán; Fig. 1), a precise Famatinian U-Pb age in zircon using LA-ICP-MS was reported by Dahlquist et al. (2012) but Pampean magmatism is absent.Conversely, Famatinian magmatism is not recognized in the Sierra de Guasayán where Pampean magmatism is now known (Fig. 1).
The ε Hf t values for magmatic zircons are reported in Table 3 and Fig. 4. As in the general study reported by Kemp et al. (2007), an important feature of the Hf isotope data is the significant range of ε Hft values exhibited by zircons within the same sample (up to 10 ε units).Such scatter can be in part because zircon can crystallize very early and retain vestiges of the original (e.g., juvenile) Hf isotope signature (Kemp et al., 2007;Dahlquist et al., 2013), whereas the 176 Hf/ 177 Hf ratio of the melt from which the zircons precipitated might change from early to late crystallization stages due to progressive assimilation of crustal material.The ε Hf t values reported here suggest interaction between juvenile and continental material, although the latter appear to be dominant.Thus, a petrogenetic model invoking interaction between a juvenile magma and crustal material of probable Early Mesoproterozoic age (see T DM in Table 3) could explain evolution of the parental magma of the Guasayán pluton.Alternatively, a heterogeneous crust could be assumed as the source of the parental magma (Villaros et al., 2012).
Hf isotope data were reported by Hauser et al. (2011) for Early Cambrian granites of the Tastil batholith with ε Hf t (t=534 Ma) ranging from +1.1 to -6.9 (n=7) although most are in the range -1.5 to -1.8, with an average T DM =1.45 Ga.Thus, the Hf isotope zircons data suggest that the Guasayán and Tastil granites have significant crustal participation in the source.
Hf isotope zircons data are unavailable for the granites of Sierra Norte-Ambargasta batholith.Initial 87 Sr/ 86 Sr ratios and ε Ndt values (Iannizzotto et al., 2013) are notably variable ranging from 0.706 to 0.710 and -1.8 to -5.9, respectively.In some cases inherited 600 Ma and 970 Ma zircon were founded, similar to the isotopic and zircon provenance seen in the metamorphic host rocks (Iannizzotto et al., 2013).The variable initial 87 Sr/ 86 Sr ratios and ε Ndt values could be explained assuming a hypothetical interaction between dominant crustal and juvenile material or heterogeneous crust, although Hf isotope data are required to establish a robust petrogenetic model.

Conclusions
1.The whole-rock chemical data from GUA-1 suggest that this pluton is a metaluminous subalkaline felsic granodiorite.The first precise U-Pb LA-MC-ICP-MS zircon age from GUA-1 indicate that the Guasayán pluton was emplaced in Early Cambrian time, 533±4 Ma.The Guasayán pluton is the north westernmost outcrop of the Early Cambrian magmatism in the Sierras Pampeanas.2. Based on Hf in zircon isotope data, an interaction between dominant crust of hypothetical Early Mesoproterozoic ages and juvenile magmas could be applied to the generation of the parental magma of the Guasayán pluton.Alternatively, a heterogeneous crust could be assumed as the source of the parental magma.Hf in zircon isotope data reported for the granites of the Tastil batholith suggest a similar petrogenetic process involving significant crustal participation.3. The new age from Guasayán pluton might contribute to define the western boundary of the Pampean paleo-arc between the Sierras Pampeanas and the Eastern Cordillera.
FIG. 2. A. Geological map of Guasayán pluton with the location of the dated GUA-1 sample; B-D.Outcrops photography of the Guasayán granite.

FIG. 3
FIG. 3. U-Pb LA-MC-ICP-MS zircon dating of GUA-1 sample from Guasayán pluton.The main Tera-Wasserburg plot shows most analyses plotting between 526 and 540 Ma and the inset shows a Concordia age of 533±4 Ma.Selected zircon images are also shown.Data reported in Table2.