Time lag between metamorphism and crystallization of anatectic granites (Córdoba, Argentina)

SHRIMP and LA-ICP-MS analyses carried out on zircons from the Río de los Sauces granite revealed their metamorphic and igneous nature. The metamorphic zircons yielded an age of 537±4.8 (2σ)Ma that probably predates the onset of the anatexis during the Pampean orogeny. By contrast, the igneous zircons yielded a younger age of 529±6 (2σ)Ma and reflected its crystallization age. These data point to a short time lag of ca. 8Myr between the High Temperature (HT) metamorphic peak and the subsequent crystallization age of the granite. Concordia age of 534±3.8 (2σ)Ma, for both types of zircon populations, can be considered as the mean age of the Pampean HT metamorphism in the Sierras de Córdoba.


INTRODUCTION
The Neoproterozoic to middle Cambrian Pampean orogen (Aceñolaza and Toselli, 2009), located at the north-central part of Argentina (Fig. 1), was part of the Terra Australis orogen at the western margin of Gondwana (Cawood, 2005). It was defined as a paired belt, with calcalkaline magmatic rocks in the eastern belt and medium-to high-grade metamorphic rocks with peraluminous granites in the western belt (Rapela et al., 1998;Schwartz et al., 2008). This calc-alkaline magmatism is widely exposed in the Sierra Norte de Córdoba and is usually referred to as the Sierra Norte-Ambargasta batholith (Iannizzotto et al., 2013;Lira et al., 1997;Schwartz et al., 2008;Von Gosen and Prozzi, 2010). The U-Th-Pb dating in granitoids from the Sierra Norte-Ambargasta batholith gave crystallization ages between 530 and 537Ma and were linked to the development of a continental magmatic arc in an eastward subduction of oceanic lithosphere down to the western margin of Gondwana (Casquet et al., 2018;Dahlquist et al., 2016;Iannizzotto et al., 2013;Rapela et al., 1998;Schwartz and Gromet, 2004;Schwartz et al., 2008;Weinberg et al., 2018).
The western belt of the Pampean orogen is formed mainly of sedimentary and metasedimentary rocks deposited during Neoproterozoic times. These sequences are represented by the low-grade metamorphosed turbidites of the Puncoviscana Formation, and their higher metamorphic grade counterparts (schists, gneisses and migmatites) located at the Sierras de Ancasti and Córdoba (Aceñolaza and Aceñolaza, 2007;Omarini, 1999;Rapela et al., 1998;Rapela et al., 2007;Schwartz and Gromet, 2004;Toselli, 1990;Turner, 1960). There is a lack of consensus regarding the tectonic setting of deposition of the Puncoviscana Basin (Casquet et al., 2018). This has been interpreted as either a 1,000km long passive margin along the western margin of Gondwana (Ježek et al., 1985;Schwartz and Gromet, 2004) or a fore-arc basin (Hauser et al., 2011;Rapela et al., 2007). The onset of subduction turned these sequences into an accretionary prism leading to medium to high-grade schists, gneisses, and migmatites (Weinberg et al., 2018 and references therein).
The Sierras de Córdoba are located at the southernmost part of the Eastern Sierras Pampeanas, central Argentina ( Fig. 1). They are constituted by N-S trending mountain ranges mostly composed of Neoproterozoic to Devonian metasedimentary and igneous rocks. The main tectonothermal event registered in the Sierras de Córdoba, during the Pampean orogeny, is the so called M 2 (Casquet et al., 2018;Guereschi and Martino, 2014;Rapela et al., 1998). This event was linked to an increase in the P-T conditions, due to crustal shortening with thrusts propagating into the fore-arc (Weinberg et al., 2018), leading to an upper amphibolite to granulite facies metamorphism and anatexis. The anatexis transformed the protoliths (gneisses and schists) into metatexites and diatexites, with associated peraluminous magmatism. The M 2 event reached temperatures above 800ºC at mid-crustal pressures of 8-9kb (Fagiano, 2007;Otamendi et al., 2004;Rapela et al., 1998;Weinberg et al., 2018). This crustal thickening was followed by adiabatic decompression leading to massive crystallization of cordierite in rocks that experienced subsequent partial melting after the first onset of anatexis (Casquet et al., 2018;Otamendi et al., 2004;Weinberg et al., 2018). The timing of these metamorphic events (crustal thickening and adiabatic decompression) has been bracketed between 550 and 515Ma, according to U-Pb SHRIMP dating in zircons and monazites from different gneisses and migmatites of the Sierras de Córdoba (Rapela et al., 1998;Siegesmund et al., 2010;Sims et al., 1998;Weinberg et al., 2018), thus concluding that the Pampean orogeny lasted for at least 35Myr.
Geological and geochronological studies dealing with regional metamorphism (M 2 ) and peraluminous granites derived from the anatectic events of the Pampean orogeny, in the Sierras de Córdoba, are concentrated in the north, mainly in El Pilón complex (Rapela et al., 1998;Rapela et al., 2002;Stuart-Smith et al., 1999). The HT metamorphism and the probably coeval magmatism, were dated in 522Ma (Rapela et al., 1998). Studies on other Pampean peraluminous granitoids generated during high-grade metamorphism in other parts of the Sierras de Córdoba, are scarce (Escayola et al., 2007;Tibaldi et al., 2008). The lack of geochronological data limits our knowledge on the age of the metamorphic climax, the anatexis, and their temporal relationships with the syntectonic granites of the studied region.
The temporal relationship between metamorphism and granite crystallization differ from orogen to orogen (Esteban et al., 2015;Keay et al., 2001;Vanderhaeghe et al., 1999;Whitney et al., 2003). Although the age of each of these processes can be obtained separately from migmatites and anatectic granites, Esteban et al. (2015) demonstrated that zircons extracted from one pluton may be enough to detect the time lag between the metamorphic peak and the magmatic activity in internal domains of orogenic belts. They found a time span of 7Myr between the metamorphic climax and the emplacement of the Lys-Caillaous pluton in the Axial Zone of the Pyrenees (southern France), by recognizing two populations of zircons, one with igneous and the other one with metamorphic affinities.
Here, we report a rather similar case in the Río de los Sauces granite, Sierra de Comechingones (Argentina), where two distinct zircon populations identified by LA-ICP-MS geochemistry and dated by U-Th-Pb SHRIMP analyses, allowed us to establish the time lag between the metamorphic climax and the crystallization of the granite.

FIELD RELATIONSHIPS AND PETROGRAPHY OF THE RÍO DE LOS SAUCES GRANITE
In this work, we defined a new granitic body, the Río de los Sauces granite, located two kilometers to the north of Río de los Sauces village ( Fig. 2A, B). The country rocks are garnet-rich gneisses and metatexite migmatites of the Calamuchita Metamorphic complex (Otamendi et al., 2004). These rocks show an ENE-WSW-trending foliation (S 2 ) with high angle dips (generally >70º) both to the south and the north. This foliation is parallel to the axial plane of the tight, isoclinal folds and is associated with constrictional non-coaxial strain  and references therein). The mineral assemblage of the gneisses is biotite+plagioclase+quart z+garnet, whereas the metatexites are mainly stromatites There are a few examples of leucosomes of stromatites that form phacoliths or patches within boudin necks, evidencing that melting took place during folding. Folded, layer-parallel leucosomes also merge continuously with leucosomes parallel to axial-plane foliation. Otamendi et al. (2004) described similar structures and interpreted them in the same way. Magma transfer through this fold-assisted leucosomes network has been described in other parts of the world (Weinberg and Mark, 2008). Finally, leucosomes coalesced giving rise to leucogranite and pegmatite sheets, and small plutons. Because of the continuous link from leucosomes to larger sheets and plutons (Fig. 2C), we interpret that leucogranites are autochthonous granites.
The Río de los Sauces granite was formed by the coalescence of numerous leucogranite sheets, which intruded concordantly the gneisses, close to the gneiss-metatexite boundary ( Fig. 2A, B). The resulting granite body is 1.2km long and 100-150m wide. The regional metamorphic foliation trend deflects around the Río de los Sauces granite, implying that after solidification, it also behaved as a competent body during continued deformation. This is a hypidiomorphic coarse-grained leucogranite with a mineral association of mi-crocline+quartz+plagioclase+biotite+muscovite+garnet±sillimanite, and accessories such as zircon, titanite and apatite. The modal proportion of main minerals is: microcline (35-50%) quartz (35-45%) and plagioclase (20-30%). It also shows late patches and pegmatite sheets. The Río de los Sauces granite is weakly foliated. The foliation is defined by oriented biotite and elongated quartz (Fig. 2D, E), and is parallel to the contacts and the gneiss structures.

U-Th-Pb SHRIMP GEOCHRONOLOGY
One sample from the Río de los Sauces granite (32º30'37.7''S/64º34'18.8'W) was processed according to routine zircon mineral separation (crushing, grinding, sieving under 250 µm, Wilfley table and methylene iodide) at the University of Río Cuarto (Argentina), in order to date the emplacement of the pluton. The selected zircons were mounted in epoxy resin together with the TEMORA 2 reference zircon (416.78Ma; Black et al., 2004), sectioned, polished and analyzed on a SHRIMP-IIe/MC at the Centro de Pesquisas Geocronologicas of the University of São Paulo (GeoLab-IGc-USP; Brazil). The obtained U-Pb ion microprobe data were processed with the SQUID and Isoplot/Ex 3.00 (Ludwig, 2003) software programs and are presented in Table 1. In order to select target areas, cathodoluminescence (CL) images were obtained by a FEI Quanta 250 Scanning Electron Microscope and XMAX CL detector, in the same laboratory. Further details on the sample preparation, analytical setup, acquisition and data processing are given in Sato et al. (2014).

TRACE AND RARE EARTH ELEMENT ANALYSES BY LA-ICP-MS
Trace and Rare Earth Elements (REE) on both zircon populations were analyzed by LA-Q-ICP-MS (Laser Ablation Quadrupole Inductively Coupled Plasma Mass Spectrometry) at the University of the Basque Country (SGIker) using a 213nm New Wave Nd:YAG laser, with a ~3.65J/cm 2 energy density at a repetition rate of 10Hz, coupled to a Thermo iCAP Qc quadrupole ICP-MS. The analytical spot size was 40µm in diameter, and in most cases, the zircons were completely pierced through during the 60 seconds of acquisition time. External calibration was performed to the NIST SRM 612 (Jochum et al., 2011). The internal standard was the stoichiometrycalculated 90 Zr, and data reduction was carried out by the Metamorphism and magmatism time lag 6 laboratory staff using Iolite 3.32 software (Paton et al., 2011;Paul et al., 2012).
In this work, the Th/U ratio and the observed compositional zircon zonings can be apparently correlated with ZP 1 (low Th/U ratios from 0.01 to 0.1) and ZP 2 (high Th/U ratios, from 0.1 to 1) ( Fig. 5A; Table 1), allowing to link them to a metamorphic or an igneous origin, respectively. However, due to its questionable validity, another geochemical parameter should be taken into account to link the zircons with their origin.
The Nb and Ta can also be used as discriminating elements between these two zircon populations, as ZP 2 have almost invariably higher Nb/Ta ratios than ZP 1 ( Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively. Error in Standard calibration was 0.46% (not included in above errors but required when comparing data from different mounts). * Common Pb corrected using measured 204 Pb.  l o g i c a A c t a , 1 8 . 1 7 , 1 -1 4 ( 2 0 2 0  Metamorphism and magmatism time lag 7 (Fig. 5C), a clear differentiation can be observed between the two zircon groups. ZP 2 zircons tend to plot towards higher Nb+Ta and lower Th+U values compared to ZP 1 .
In general, it is assumed that zircons with a metamorphic origin have higher U and Hf and lower Ce concentrations than the igneous ones (e.g. Hoskin and Schaltegger, 2003). Moreover, the metamorphically grown zircons usually have flatter HREE (Heavy Rare Earth Elements) patterns, relatively low contents of HREE (and Y), and MREE (Middle Rare Earth Elements), low (Lu/Gd) N , Nb/Ta, Y/Ho and Ce/Ce* ratios and high Eu/Eu* ratios than the igneous ones (e.g. Chen et al., 2010).
The chondrite-normalized REE patterns (Sun and McDonough, 1989) of the analyzed zircons (Fig. 6) are strongly enriched in HREE compared to LREE (Light Rare Earth Elements) and show prominent anomalies in Ce (positive) and Eu (negative). The ranges of normalized REE patterns of both zircon populations overlap (Fig. 6), but differ from normalized trends for the HREE. The ZP 1 zircons have flat HREE patterns [(Lu/Gd) N ratios up to 10] whereas ZP 2 shows steeper patterns [(Lu/Gd) N ratios up to 46]. In the LREE, MREE or HREE vs. Th/U plots (Fig. 5D-F) the REE contents do not show significant variations but increases slightly as the Th/U increases in ZP 2 .
In the Th/U vs. Hf plot (Fig. 5G), the Hf concentration decreases from ZP 1 to ZP 2 . The U/Ce vs. Th/U or Th plots can be considered as potential indicators of metamorphic and igneous zircons since  (Castiñeiras et al., 2011). In our study, the analyzed zircons are again split into two main populations: ZP 2 is characterized by high Th and little variation in the U/ Ce content, and the ZP 1 by low Th and highly variable U/Ce contents (Fig. 5H). The application of Grimes et al. (2007) diagrams agrees with zircons evolved from a continental crust (Fig. 7).

Ti-IN-ZIRCON GEOTERMOMETRY
The Ti-in-zircon content was used to calculate the approximate crystallization temperatures of the metamorphic and igneous zircons using the equation of Ferry and Watson (2007). As the studied samples are saturated in SiO 2 (a SiO 2 = 1), rutile is absent (a TiO 2 < 1) but titanite is present (a TiO 2 > 0.5), we have applied values of a SiO 2 = 1 and a TiO 2 = 0.5 to calculate the crystallization temperatures (Table 2; 3). Mean temperatures of 735±17 and 750±22ºC were obtained for the metamorphic (4.5ppm Ti) and igneous (5.5ppm Ti) zircons, respectively. The calculated temperatures are within the uncertainty of each other, although a slight increase towards the igneous zircons is predicted (Fig. 5I).

DISCUSSION
The zircons from the Río de los Sauces granite yielded uncommon geochemical signatures and SHRIMP results.
The following results stand out twofold as i) two zircon populations (ZP 1 and ZP 2 ) with different textures and geochemical signatures and ii) a bimodal age distribution in the geochemical two end-members, 537±4.8 and 529±6Ma, have been identified. According to presented geochemical features, zircon textural analysis and Ti-inzircon geothermometry we proposed that the identified ZP 1 and ZP 2 can be correlated with metamorphic and igneous zircons populations respectively, and do confirm the utility of the Th/U, U/Ce and Nb/Ta as potential ratios for zircon nature indicator. These new data aid in the understanding of the metamorphic evolution of the Calamuchita complex migmatites and help to constrain the time gap between the Pampean HT metamorphism and the crystallization of anatectic granites.
Melting products forming leucosomes in migmatites are commonly related to the formation of sheets like bodies of leucogranites and larger accumulations of magmatic rocks along the Sierra de Comechingones as leucosomes are texturally and compositionally similar to the leucogranitic bodies (Barzola et al., 2019) Sample  GRS_215  Source file  59  36  64  24  50  11  43  55  34  62  30  57  14  28  67  12  10  8  66  21  51  20  P  742  698 1060 1096  836  909 1004  785  716 1016  813 1113  921 1097  927 1041 1568  760  707  923  681  679  Sc  380  389  511  519  430  455  455  412  430  484  453  510  446  491  446  477  740  433  372  458  380  418  Ti  3  gneisses related to anatectic melt during the migmatization and can be considered as a syn-to the late-M 2 body. In this regard, the age of 529±6Ma obtained from high Th/U (ZP 2 ) zircons is interpreted as the crystallization age of the Río de los Sauces granite, and therefore it dates the vanishing stages of M 2 . Meanwhile, the age of 537±5Ma, obtained from low Th/U (ZP 1 ) metamorphic zircons, could represent the age of the Pampean HT metamorphism prior to the anatexis. This metamorphic event might be the origin of the solid-state growth of metamorphic zircons, which were further incorporated in the melts without dissolving until newly crystallization of igneous ones at the age of 529±6Ma. This age agrees well with the previously published ages for the Pampean metamorphism in other parts of the Sierras de Córdoba ; and references therein).

. The Río de los Sauces granite is a sheeted intrusion emplaced into
Although the temporal relationship between the metamorphism and the anatexis differ from orogen to orogen, time gaps of less than 10Myr between the syntectonic granites and regional metamorphism have been reported in other orogenic domains. For example, in the island of Naxos (Greece), a time lag of at least 5Myr between the peak of the Miocene metamorphism and the main period of magmatism was reported by Keay et al. (2001). While, a time span of 7Myr, has been established by Esteban et al. (2015) between the metamorphic climax of the HT/LP Variscan metamorphism (Late Carboniferous) and the final emplacement of the Lys-Caillouas pluton (Late Carboniferous-Early Permian) at middle crustal levels. The latter geochronological study presents many similarities with our study because in both cases zircons from only one granite body have been sufficient to detect the time gap between the two processes.
The presence of zircons with metamorphic affinity in the Río de los Sauces granite suggests the incomplete assimilation of the zircons derived from the metamorphic protoliths and opens an opportunity to study the timing of the metamorphism prior to the emplacement of the pluton. The mean temperatures of 735±17 and 750±22ºC determined by Ti-in-zircon in metamorphic and igneous zircons, respectively, are, although slightly lower, coherent to the calculated metamorphic conditions in the Sierras de Córdoba (Fagiano, 2007;Otamendi et al., 2004;Rapela et al., 1998). Since the crystallization temperatures calculated for igneous and metamorphic zircons are within the uncertainty of each other, a further increase in the regional temperature during anatexis cannot be inferred. Finally, the age of the climax of the Pampean metamorphism has been constrained between 537 and 529Ma, and the Concordia age of 534±4 (2σ)Ma ,   Sample  GRS_215  Source file  32  29  25  60  68  63  17  52  40  56  6  37  46  35  49  61  31  58  23  48  9  19  P  1174  1510  612  642  866  1390  469  830  822  920  396  342  980  402  1070  490  630  958  772  920  875  434  Sc  508  576   obtained from both types of zircons, has been considered as the mean age of the HT Pampean metamorphism in the Sierras de Córdoba.

CONCLUSIONS
The presence of zircons with metamorphic and igneous affinities in the Río de los Sauces granite, identified by LA-ICP-MS analyses, suggest their incomplete assimilation during the anatexis and open the opportunity to date both processes from the same sample.
U-Pb SHRIMP analyses from both kind of zircons suggest that the Río de los Sauces granite crystallized at 529±6 (2σ)Ma, whereas the age of 537.1±4.8 (2σ) Ma refers to the Pampean metamorphism prior to the emplacement of the pluton, and could be considered as a good approximation for the Pampean HT metamorphism of the study area.
A time gap of ≈8Myr is established between the metamorphism and the granite crystallization.
The Ti-in-zircon geothermometry yielded temperatures of ca. 750±22ºC for the granite crystallization and thus constraint the minimum temperature reached during the anatexis.
According to obtained and previously published data, we suggest that LA-ICP-MS and U-Th-Pb SHRIMP analyses should be performed together in order to achieve a well geochemical zircon characterization, previous to the granitoid dating.
The Th/U, U/Ce and Nb/Ta ratios can be considered as very useful markers to infer the igneous or metamorphic nature of the zircons.

ACKNOWLEDGMENTS
This work has been supported by grants PICT1754/16,