Mafic rocks of the Ordovician Famatinian magmatic arc (NW Argentina): New insights into the mantle contribution

We studied the petrogenesis of mafic igneous rocks in the Famatinian arc in the western Sierra Famatina (NW Argentina), an Early Ordovician middle-crustal section in the proto-Andean margin of Gondwana. Mafic rock types consist of amphibolite, metagabbro, and gabbro, as well as pod-and dike-like bodies of gabbro to diorite composition. Field relations together with geochemical and isotopic data for the mafic rocks of the western Sierra de Famatina (at 29°S) define two contrasting suites, which can be correlated with similar assemblages noted in other parts of the orogen. Amphibolite, metagab-bro, and gabbro bodies are mostly the oldest intrusive rocks (older than 480 Ma), with the host tonalite and post-tonalite mafic dikes being slightly younger. The older mafic suite is tholeiitic to calc-alkaline and isotopically evolved, except for most of the amphibolite samples. The younger suite is calc-alkaline, typically displaying subduction-related geo-chemical signatures, and it is isotopically more juvenile. Whole-rock chemical composition and isotopic analyses are compatible with a progressive mixing of different isotopic reservoirs. Pyroxenite (±garnet) was likely the dominant source of the older gab-broic magmas, whereas peridotite dominated in the source of the younger suite, implying that the mafic magma experienced a progressive shift toward more juvenile compositions though time (over 20 m.y.). Pyroxenite-derived melts could have been generated by lithospheric foundering followed by upwelling of primitive melts by adiabatic decompression of mantle wedge peridotite.


INTRODUCTION
Continental crust is the most prominent manifestation of silicate differentiation on Earth.Unlike the formation of oceanic crust by decompression melting of the upper mantle at mid-ocean ridges, the formation of continental crust at convergent margins of Andean/Cordilleran type remains a subject of debate.Although liquids generated by melting of the mantle require diversification processes such as differentiation and contamination in the crust to produce intermediate and felsic magmas, an understanding of the large-scale interaction between deep lithosphere and the underlying asthenospheric mantle is criti cal (e.g., Lee, 2014, and references therein).The removal of the deep lithosphere may be relevant to the initiation of voluminous magmatism in the overriding plate (e.g., Kay et al., 1994).The cold and negatively buoyant lithospheric mantle, composed of garnet-pyroxenite and melt-depleted peridotite (commonly metasomatized), would be unstable and could potentially founder or be subducted into the asthenospheric convecting mantle.In recent years, foundering processes have been invoked on the basis of regional geochemical shifts of magma compositions that indicate a more primitive asthenospheric source over time scales of tens of millions of years (e.g., Kay and Kay, 1993;Manthei et al., 2010;Gutierrez-Alonso et al., 2011;Putirka and Platt, 2012;Ducea et al., 2013).
The Early to Middle Ordovician Famatinian arc along the southwestern Gondwana margin between present-day Patagonia and Venezuela produced abundant magmatism in the Sierras Pampeanas in central and northwestern Argentina (e.g., Pankhurst et al., 1998).This arc consists of relatively abundant mafic and intermediate igneous rocks (Pankhurst et al., 1998(Pankhurst et al., , 2000;;Dahlquist et al., 2007Dahlquist et al., , 2008Dahlquist et al., , 2013;;Otamendi et al., 2009Otamendi et al., , 2012) ) and has been related to subduction along the western margin of Gondwana between ca.486 Ma and 463 Ma (e.g., Pankhurst et al., 2000;Dahlquist et al., 2008;Chernicoff et al., 2010;Ducea et al., 2010;Hongn et al., 2014).The granitoids display a wide range of isotopic signatures (e Nd [t] +4.8 to -6), indicating that this arc magmatism reworked old lithospheric sources (Pankhurst et al., 2000;Dahlquist et al., 2008Dahlquist et al., , 2013;;Rapela et al., 2008;Casquet et al., 2012a;Ducea et al., 2015).Our understanding of the petrogenesis of Fama tinian granitoids has improved over the last 15 yr (e.g., Bahlburg and Hervé, 1997;Pankhurst et al., 1998Pankhurst et al., , 2000;;Dahlquist et al., 2008Dahlquist et al., , 2013;;Otamendi et al., GSA Bulletin;July/August 2016;v. 128;no. 7/8;p. 1105-1120;doi: 10.1130 2009, 2012;Ducea et al., 2010Ducea et al., , 2015;;Alasino et al., 2014;and references therein), but many key aspects of the initiation and evolution of this arc are not well understood.In this contribution, we present an updated and expanded description of field relations, petrog raphy, and composition of mafic Ordovician rocks in the western flank of the Sierra de Famatina in the area around 29°S, one of the ranges where Famatinian magmatic rocks crop out extensively.These new data, along with a review of available information from the Sierra de Valle Fértil and Sierra de Los Llanos (see Fig. 1) and their comparison with some localities from the Puna region (see Table DR1 1 ), suggest a progressive geochemical shift with time of the Famatinian mafic magmatism toward more asthenospheric sources.This evidence is used to infer a large-scale mantle evolution under the Famatinian magmatic arc in northwestern Argentina.

Famatinian Arc
Along the 27°S-33°S flat-slab segment of the Nazca plate in the southern Andes (Fig. 1), present-day landforms result from basement uplift along Miocene to Holocene reverse faults located up to 900 km away from the trench (e.g., Jordan and Allmendinger, 1986).These basement exposures constitute the Sierras Pampeanas, which extend from the 2.0-2.2Ga (Paleoproterozoic) Río de la Plata craton in the east to the modern Andes in the west.The Sierras Pampeanas consist of igneous and metamorphic rocks of Mesoproterozoic to Ordovician age that record a complex paleogeographic and tectonic history extending in time from the Grenvillian orogeny to the early Paleozoic accretion to Gondwana (e.g., Casquet et al., 2012b;Rapela et al., 2015).Further, the region preserves a well-exposed remnant of the proto-Andean active margin of Gondwana represented by the Famatinian magmatic arc, with contemporaneous deformation and hightemperature-medium-pressure metamorphism, i.e., the Famatinian orogeny.U-Pb zircon ages constrain Famatinian magmatism to between 486 Ma and 463 Ma.The belt displays wellexposed sections across the transition from lower-to midcrustal levels of the arc (e.g., Pankhurst et al., 2000;Dahlquist et al., 2005aDahlquist et al., , 2007;;Grosse et al., 2011;Otamendi et al., 2012).A continuous section, ~15 km long, of pre-Triassic tilted arc crust in the Sierra de Valle Fértil (Fig. 1) shows inferred paleodepths of ~15-30 km (Otamendi et al., 2012).Here, granodiorite and tonalite dominate the middle crust down to ~20 km depth, tonalite and diorite from 20 to 25 km depth, and gabbro from 25 to 30 km depth (e.g., Tibaldi et al., 2013).The Sierra de Valle Fértil section of the arc records protracted intrusion of mafic magmas throughout the life of the magmatic arc (Otamendi et al. 2012).Field geological evidence further shows that arc activity began with mafic magmatism (gabbro) and that tonalite-granodiorite magmatism developed somewhat later.Host rocks are medium-pressure, upper-amphibolite-to granulite-facies metasedimentary rocks (6 ± 1 kbar, 800 ± 40 °C; Otamendi et al., 2008).To the west of the Sierra de Valle Fértil, high-pressure, upper-amphibolite-facies paragneiss (12 ± 1 kbar, 780 ± 45 °C; Casquet et al., 2012a) was recognized at the small outcrop of Las Chacras (Fig. 1).Although separated from the rest of the Sierra de Valle Fértil by shear zones and faults, the Las Chacras outcrop was interpreted as a subducted and forearc basin underplated to the arc (Casquet et al., 2012a).
Evidence for large-scale interaction between partially molten country rocks and metalumi-nous tonalitic magmas at midcrustal level is preserved in the western flank of Sierra de Famatina (Saavedra et al., 1992;Alasino et al., 2014), where the Cerro Toro igneous complex (Toselli et al., 1988;Saavedra et al., 1992) consists of a succession of steeply dipping N-S-trending sheets of tonalite and less abundant granodiorite.Host rocks are high-grade metasedimentary rocks (5 ± 1 kbar, 750 ± 40 °C; Alasino et al., 2014).The metamorphic grade increases eastward from amphibolite to granulite facies.Pelitic migmatite together with amphibolite and metagabbro occur as screens and stoped blocks in the igneous complex, and these rocks together constitute the medium-pressure-hightemperature Cerro Toro regional thermal aureole (Fig. 2).Emplacement of metaluminous magma and regional metamorphism were largely coeval (e.g., Alasino et al., 2014).U-Pb sensitive highresolution ion microprobe (SHRIMP) zircon dating of a hybrid rock (sample FAM7086) from the intermediate zone yielded an age of 481 ± 4 Ma with highly negative e Hf (t) values in zircon (average = -14.7),typical of a supracrustal component (Dahlquist et al., 2008(Dahlquist et al., , 2013)).We consider this to be the age of formation of the Cerro Toro aureole.
In places where shallower levels of the magmatic arc are preserved, such as in the Sierra de Los Llanos, the granitoids dominantly consist of granodiorite, with lesser biotite monzo- ´ S 9 0 0  granite, and highly peraluminous varieties including two-mica cordierite monzogranite (Pankhurst et al., 1998).Small, discontinuous outcrops of metamorphic rocks occur as stoped blocks and roof pendants in the granodiorite.The metasedimentary sequence predominantly consists of Cambrian metapelite, alternating with beds of metamorphosed siltstone and sandstone (e.g., Pankhurst et al., 1998;Collo et al., 2009;Cristofo lini et al., 2012;Verdecchia et al., 2014).The granitoids were emplaced at low pressure (2.7-3.6 kbar) and high temperature (780-850 °C); metamorphism was associated with emplacement of the metaluminous granitoids (Dahlquist et al., 2005a).We focus hereafter on the western Sierra de Famatina outcrops of Famatinian mafic rocks.

Mafic Rocks of Western Sierra de Famatina
Mafic rock types were distinguished on the basis of the field relations, texture, and mineralogy (Fig. 2).They are grouped as: (1) Amphibolite is part of the high-grade metamorphic series exposed mainly at Cerro Asperecito (part of the external zone of the Cerro Toro aureole) as lens-shaped or tabular bodies, meters to tens of meters thick (former sills), and concordant with the regional steeply dipping NNW-SSE metamorphic foliation.They also occur in the intermediate zone of the aureole as stoped blocks in tonalite and hybrid or leucogranitoid rocks (Fig. 3A).Contacts with the host are mostly sharp.Amphibolite consists mostly of amphibole and plagioclase with a pronounced planar structure.
(2) Metagabbro and gabbro form numerous sills and small bodies that intruded the highgrade metasedimentary rocks.These mafic rocks occur as rafts or stoped blocks in the intermediate and internal zones (Fig. 3A).Contact relationships vary along the regional steeply dipping NNW-SSE foliation; they can be sharp, or complex with multiple magma injections, stoping and mingling with hybrid rocks of the Cerro Toro aureole (Fig. 3B).Petrographically, the metagabbro may have olivine and pyroxene crystals and shows evidence of metamorphic changes at near-liquidus and lower temperatures (see section 4.2).Unlike most amphibolite rocks, metagabbro lacks a planar structure.Gab- bro also crops out as a small body (~300 m 2 ) to the southeast of Cerro Asperecito (Fig. 2).The rocks vary gradually from inequigranular, coarse-to medium-grained, to equigranular and fine-grained rocks.They are green-black in color and rich in amphibole.
(3) Pod-and dike-like bodies of gabbro and diorite composition commonly intrude tonalite.Their contacts are lobate where the tonalitic melt back-intruded the mafic bodies, and in places display hybridization.In other cases, exceptionally well-developed mechanical interactions, such as sinking structures, were observed (Fig. 3C).Here, mafic intrusions into the tonalite magma mush were dismembered, forming enclaves.They are characterized by dropshaped geometry indicating vertical movement, in some places showing trails (or "wakes") of mafic minerals (Fig. 3D).

SAMPLING AND ANALYTICAL METHODS
More than 30 samples were collected for petrography, of which 11 (six amphibolite samples; four metagabbro-gabbro samples; one dioritic dike) were selected for whole-rock majorand trace-element analyses using inductively coupled plasma-optical emission spectrometry (ICP-OES) and inductively coupled plasmamass spectrometry (ICP-MS) at ACTLABS, Canada (Table 1).Samples were first fused with lithium metaborate/tetraborate and then dissolved in nitric acid.Major elements, Be, Sc, V, Sr, Ba, and Zr were determined by ICP-OES; all other trace elements were determined by ICP-MS.Precision and accuracy for major elements were generally better than 2% (relative); for trace elements, they were generally better than ±6% when signals were 10 times above background.Additionally, two representative igneous samples (one metagabbro and one amphibolite) were analyzed by GeoAnalytical Laboratory, Washington State University, 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.
Mineral compositions were measured using a JEOL JXA 8230 microprobe at LAMARX-National University of Córdoba (WDS mode, 15 kV, 20 nA, 10 nA for plagioclase) on carboncoated polished mounts.The beam diameter was 3 mm (8 mm for plagioclase), with counting time of 10 s on the peak and 5 s at each Sr and Nd isotopic analyses of 12 samples, including 10 amphibolite samples, one metagabbro sample, and one gabbro sample, were carried out at the Geochronology and Isotope Geochemistry Center, Complutense University (Madrid, Spain), using an automated multicollector VG® SECTOR 54 mass spectrometer (Table 2).Errors are quoted throughout as two standard deviations from measured or calculated values.Analytical uncertainties were estimated to be 0.006% for 143 Nd/ 144 Nd and 0.1% 147 Sm/ 144 Nd, with the latter parameter determined by isotope dilution.Fifty-six analyses of the La Jolla Nd standard over 1 yr gave a mean 143 Nd/ 144 Nd ratio of 0.511846 ± 0.00003.Oxygen isotope analyses of three rock types (one olivine-pyroxene metagabbro, one mafic dike, and a small hornblende-gabbro body) were determined at the Servicio General de Análisis de Isótopos Estables, University of Salamanca, Spain, on whole-rock powders by laser fluorination (Clayton and Mayeda, 1963), employing a Synrad 25 W CO 2 laser (Sharp, 1990) and ClF 3 as reagent (e.g., Borthwick and Harmon, 1982).Two migmatite samples of the Cerro Toro aureole, one tonalite sample, and one hybrid rock sample were also analyzed for comparison (Table 3).Isotope ratios were measured on a VG-Isotech SIRA-II dual-inlet mass spectrometer.Both internal and international reference standards (NBS-28, NBS-30) were run to check accuracy and precision.Results are reported in d 18 O notation relative to the Vienna standard mean ocean water (V-SMOW) standard, using a d 18 O value of 9.6‰ for NBS-28 (quartz) for the mass spectrometer calibration.Long-term reproducibility for repeated determination of reference samples was better than ±0.2‰ (1s).

Metagabbro and Gabbro
Metagabbro, composed of plagioclase (46 modal %), amphibole (37%), and orthopyroxene (14%) as essential minerals, is medium grained (3-6 mm) with relics of primary (igneous) texture, consisting typically of large cumulate crystals of plagioclase and clinopyroxene, with smaller olivine crystals overprinted by late-to postmagmatic changes (metamorphism).The former two minerals were transformed into a recovered polygo-nal aggregate.Olivine in turn shows a typical multilayer coronitic texture along the contact.Olivine relics are enveloped by orthopyroxene, which in turn is separated from plagioclase by a metamorphic assemblage consisting of clinoamphibole showing symplectic intergrowth with a spinel-hercynite solid solution, and biotite.Amphibole occurs as large single crystals or transformed into polycrystalline disoriented aggregates.Olivine crystals are unzoned (Fo ~71) and have Cr and Ca below detection limits.Nickel is very low (up to 0.08 wt% NiO).Plagioclase crystals almost reach the Ca end member (An 98-99).Cores are partially altered to clay minerals and finegrained epidote-clinozoisite; plagioclase also hosts tiny amphibole and spinel solid solution (s.s.) grains.Orthopyroxene (En 72.9-74.9Fs 24.0-25.8Wo 0.2-0.6Ac 0.5-0.8 ) contains very few inclusions of ilmenite and magnetite.Amphibole can be classified as magnesiohornblende according to Hawthorne et al. (2012); the Mg/(Mg + Fe 2+ ) ratio varies between 0.58 and 0.83.Magnetite and ilmenite occur as euhedral to subhedral crystals, some of them skeletal, up to 100 mm The decay constants used in the calculations are the values λ 87 Rb = 1.42 × 10 -11 and λ 147 Sm = 6.54 × 10 -12 yr -1 recommended by the IUGS Subcommission for Geochronology (Steiger and Jäger, 1977) long.Grains of spinel-hercynite solid solution composition (with ~50% of each end member) form fine symplectic intergrowths with amphibole.Similar metagabbro samples were described from the nearby Sierra de Valle Fértil (Baldo et al., 1999;Gallien et al., 2012).
Gabbro grades from coarse to fine grained with an assemblage mainly formed of plagioclase (from 48 to 32 modal %) and amphibole (from 58% to 47%).Coarse-grained rocks have large (up to 5 cm) poikilitic anhedral to subhedral crystals of hornblende with inclusions of plagioclase.Cumulate texture is present in places.Fine-grained gabbro samples exhibit magmatic hypidiomorphic granular texture.Microprobe analyses show that amphibole is magnesiohornblende according to the scheme of Hawthorne et al. (2012).Plagioclase crystals range from An 94 to An 85 .The hornblendeplagioclase pairs analyzed in the coarse-grained type [Mg/(Mg + Fe 2+ ) = 0.72 and An = 93; average from six samples] and in the fine-grained rock [Mg/(Mg + Fe 2+ ) = 0.63 and An = 87; average from five samples] show slight variations in their compositions.Magnetite and ilmenite occur as euhedral to subhedral crystals in both varieties.

Pod-and Dike-Like Mafic Bodies
The typical rock in these bodies consists of plagioclase + quartz + biotite + hornblende and shows a magmatic hypidiomorphic granular texture.Titanite, epidote, and opaque minerals are accessories.No differences in mineral composition were found between the dikes and pod-like mafic bodies.Plagioclase is zoned, with slightly altered, Ca-rich cores (up to An ~ 70) and sodic rims (up to An ~ 50).Anhedral potassic feldspar grains (Or 93 Ab 5 An 2 ) are very scarce.Most amphibole grains are magnesiohornblende to pargasite according to Hawthorne et al. (2012).Titanite contains some Al and Fe 3+ (~1 wt% of each, as oxide).

Gabbro and Diorite Pod-and Dike-Like Bodies
Two samples were analyzed, one from a dike and one from a pod-like body (Table 1; Fig. 2).They have different SiO 2 contents (46 and 54 wt%) and low Mg# values (45 and 51).They plot in the gabbro field on the TAS diagram modified for plutonic rocks (not shown).They are metaluminous (ASI ≈ 0.75).

Rb-Sr and Sm-Nd Isotopic Compositions
A reference age of 481 Ma for calculating the isotope compositions of the samples at the time of metamorphism/magmatism is based on the weighted U-Pb SHRIMP zircon age of sample FAM7086, a hybrid rock from the intermediate zone of the Cerro Toro aureole (Dahlquist et al., 2008;Alasino et al., 2014).Ten amphibolite samples from Cerro Asperecito yielded 87 Sr/ 86 Sr t values between 0.7063 and 0.7122 and e Nd (t) values from +0.3 to -5.0 (Table 2; Fig. 2).The olivine metagabbro sample yielded 87 Sr/ 86 Sr t = 0.70816 and e Nd (t) = -2.2, and a sample from the small pod-like gabbro body yielded 87 Sr/ 86 Sr t = 0.7068 and e Nd (t) = -3.7 (Table 2; Fig. 2).Dahlquist and Galindo (2004) reported values of 87 Sr/ 86 Sr t = 0.70686 and e Nd (t) = -5.8 for a hornblende-gabbro body (VCA7037) from the same region (Fig. 2).
The negative e Nd (t) values indicate a crustal contribution to the parent magmas, which could have been incorporated in the source, or during ascent or emplacement.The calculated depleted mantle model ages (T DM ; Table 2) vary from 1.2 Ga to 1.8 Ga, indicating that the crustal source contributing the Nd was Mesoproterozoic or older; model ages assuming pre-Ordovician crustal residence are even more consistent at 1.2-1.6Ga.Such uniformity is unlikely to arise from random degrees of mixing or contamination with a much older component, suggesting that the crustal material involved was indeed Mesoproterozoic.

DISCUSSION
Field relations in the study area suggest that metagabbro, amphibolite, and gabbro are the oldest intrusive rocks, with the tonalite and post-tonalite mafic dikes being slightly younger.The older mafic rocks mostly form numerous sills that intruded the high-grade metasedimentary rocks.Moreover, they commonly occur as screens and stoped blocks in the tonalite (Fig. 2; section 2.2).A hybrid tonalite from the Cerro Toro aureole was dated at 481 ± 4 Ma (Dahlquist et al., 2008;Alasino et al., 2014).On the other hand, the pod-and dike-like bodies (of gabbro and diorite composition) that intrude and are commonly mingled with the tonalite are younger.A sample of hornblende gabbro (VCA1007) in the study area, showing mingling relationship with tonalite, was dated at 468 ± 4 Ma (Pankhurst et al., 2000).Similar timing relationships for mafic rocks also have been reported in other regions of the Sierras Pampeanas, such as Sierra de Los Llanos and Sierra de Valle Fértil (e.g., Pankhurst et al., 1998Pankhurst et al., , 2000;;Otamendi et al., 2009Otamendi et al., , 2012;;Ducea et al., 2010).U-Pb zircon ages from the Sierra de Valle Fértil (Pankhurst et al., 2000;Ducea et al., 2010) are consistent with our proposed age difference: The older mafic unit was dated at 478 ± 4 and 484 ± 8 Ma, and the pod-and dike-like bodies of hornblende gabbro in the tonalite unit were dated at 474 ± 4 Ma.The large tonalite intrusions are widespread and relatively restricted in time (mostly between ca.480 and 470 Ma; e.g., Ducea et al., 2010).Additionally, at Las Chacras, one of the most westerly parts of the Famatinian magmatic arc, garnet amphibolite is interbedded with paragneiss metamorphosed under high-pressure upper amphibolite facies (Casquet et al., 2012a).The paragneiss contains Ordovician (ca.468 Ma) igneous zircon grains with rims having similar metamorphic ages, pointing to an age of ca.468 Ma for the mafic protolith.Thus, in the Famatinian magmatic arc, we can distinguish: (1) an older (pre-to syntonalitic intrusion) mafic suite of amphibolite, meta gabbro, and hornblende-gabbro, and (2) a younger (syntonalitic intrusion) mafic suite dominated by dike-and pod-like bodies of gabbro and diorite composition and to a lesser extent amphibolite (e.g., Las Chacras area).Next, we explore the geochemical and isotopic characteristics of these groups of mafic rocks considering other outcrops in the western Sierras Pampeanas (i.e., Sierras de Famatina, Valle Fértil, and Los Llanos; these are summarized in Table DR1 [see footnote 1], with published sources).Additionally, data from some locations in the Puna region (e.g., the western Puna eruptive belt and the Pocitos igneous complex) are also considered.

Geochemical Constraints: Major-and Trace-Element Compositions
Discrimination diagrams such as SiO 2 versus FeO*/MgO (Fig. 4A) show that ~60% of the samples, dominated by the older mafic suite, are tholeiitic; the remaining samples are transitional into the calc-alkaline field (see also DeBari, 1994;Otamendi et al., 2012).Similarly the K 2 O versus SiO 2 diagram shows that the older mafic rocks are tholeiitic, and samples of the younger group fall in the calc-alkaline and high-K calc-alkaline fields (Fig. 4B).Majorelement versus Mg-number (Mg#) diagrams exhibit some correlations, including increasing TiO 2 and decreasing CaO with decreasing Mg#, suggesting a compositional continuum in the mafic samples (Fig. 5).Clearly, the older mafic suite is more primitive (higher Mg#) with lower Ti and higher Al contents than the younger suite.
Enrichment in LILEs is seen in many stoped blocks of both amphibolitic and gabbroic rocks hosted in the tonalite (Table 1), suggesting some transfer of these elements from the tonalitic magma.Moreover, the overall analyses (~50) have Ba/Nb (average = 38, minimum = 10, maximum = 265) and Th/Nb (average = 0.62; minimum = 0.05, maximum = 3.6) exceeding the typical ranges for normal (N)-type midocean-ridge basalt (MORB; Ba/Nb = 1.7-8.0 and Th/Nb = 0.002-0.06;Saunders et al., 1988).Involvement of upper-crustal and/or pelagic sediment in their petrogenesis could generate such enrichment.However, considering the Th and Nb contents (after normalization to Y content, to minimize the effect of fractionation in mantle-derived magmas; Saunders et al., 1988), most early gabbro and amphibolite samples exhibit low Th/Y ratio, suggesting that a crustal component was minimal in the source (Fig. 4C; e.g., Pankhurst et al., 1998).Additionally, the older mafic suite also shows relative depletion of Rb (Fig. 4D).On the other hand, the younger gabbroic samples show similar trace-element compositions to pelagic sediments, thus implying a larger crustal component (Fig. 4D).The same differences between both suites of mafic rocks are seen in the N-MORB-normalized trace-element diagram (Fig. 6).Younger gabbros on average show enrichment in LILEs and Th relative to the older mafic rocks; along with the differences in Nb-Ta patterns, this may correspond to a subduction signature (GLOSS [Global subducted sediment composition]; Plank and Langmuir, 1998).
while mantle-derived magmas typically have relatively high P 2 O 5 /K 2 O ratios.Decreasing P 2 O 5 /K 2 O values along with an increase in 87 Sr/ 86 Sr t ratio are expected for mafic magmas mixing with crustal materials.This behavior is not significant in the mafic samples of this study.Moreover, e Nd (t) values between +5 and -6 of the mafic samples are largely uncorrelated with the Mg# values (Fig. 7B), suggesting that crystal fractionation did not play a significant role in magma diversification.
Pelitic migmatites show higher d 18 O values (+10.6‰ and +13.8‰) typical of sedimentary rocks that have interacted with meteoric waters at low temperatures (Hoefs, 2009).All igne-ous samples from gabbro to tonalite analyzed here gave d 18 O values between +5.3‰ and +9.9‰, within the normal range for unaltered igneous compositions as reported by Hoefs (2009).The lowest value of +5.3‰ (FAM392, an old metagabbro) and that of +6.7‰ for a dike (i.e., young gabbroic rocks) are typical of juvenile magmas (Harmon and Hoefs, 1995), whereas the remaining samples (including one gabbro = 9.2‰, one diorite dike = 8.2‰, and one tonalite = 7.7‰) have higher d 18 O values typical of continental arc magmas.Moreover, one hybrid rock in the Cerro Toro thermal aureole yielded an oxygen composition close to those of the migmatites (+9.9‰), suggesting assimilation of metasedimentary rocks through partial melting during emplacement of tonalitic magmas (Alasino et al., 2014).
As shown already, the mafic samples do not show evidence for significant middle-or uppercrustal contamination, but they have a wide range of d 18 O values from +5.3‰ to +9.2‰.Involvement of slab-derived components in the mantle source do not seem applicable as both the higher and lower d 18 O values are in samples of the older gabbros, and only the younger mafic rocks show the chemical enrichments ascribable to subduction (see previous section).Moreover, peridotite xenoliths from modern subduction zones do not show d 18 O values higher than that of the upper mantle of 5.18‰ ± 0.28‰ (Mattey et al., 1994;Eiler, 2001;Chin et al., 2014;Liu et al., 2014).In conclusion, although some fractionation and interaction with the continental crust are clearly unavoidable, a metasomatized old subarc lithospheric mantle (±lower crust) with 18 O-enriched signatures (e.g., 8.03‰ ± 0.28‰; Liu et al., 2014) was probably involved in the origin of these magmas by mixing with melts generated by asthenospheric mantle upwelling.
Within the first suite, the role of mafic magmas in the generation of tonalite by mixing with supracrustal material is not straightforward.The mixing plot of 87 Sr/ 86 Sr against Sr (Fig. 8B), which includes gabbro and diorite, tonalitegranodiorite, and metasedimentary rocks (migmatite), reveals that the old gabbroic rocks were not involved in the generation of the intermediate magmas.Although old gabbroic rocks have mean 87 Sr/ 86 Sr values similar to a hypothetical mafic end member, they have lower Sr contents (mostly Sr ≤ 200 ppm; Fig. 8B).In fact, most of   the young mafic rocks (Sr ~330 ppm) plot close to the hypothetical mafic end member.Remarkably, the compositions of amphibole in the intermediate rocks (tonalite-granodiorite) are similar to those of the younger mafic rocks but clearly distinct from the composition of amphibole in the older gabbroic rocks (Fig. 9).

New Insights into the Evolution of Magmas in the Famatinian Arc
This work reveals that in the Early and Middle Ordovician, the crust of the proto-Andean margin of Gondwana experienced a progressive geochemical shift in the mafic magma compositions.The earlier suite consists mainly of tholeiitic to calc-alkaline rocks (with relatively high LREEs, Rb, Ba, Th, and U compared to N-MORB, and without a negative Eu anomaly); these rocks may represent near-primary liquids.The second suite is calc-alkaline with an arc signature enriched in LILEs, and it may indicate addition of a subduction component.
The Zn/Fe T ratio is insensitive to olivineand orthopyroxene-melt fractionation, but it is strongly sensitive to the involvement of garnet or clinopyroxene during melting (Lee et al., 2010).Our mafic samples (older and younger mafic suites) have a wide range in Zn/Fe T (×10 4 ) values (Fig. 10A).Some samples are similar to typical mantle peridotite and basaltic melts generated from sources dominated by olivine-orthopyroxene (~9 ± 1; Le Roux et al., 2010;Lee et al., 2010), while others have higher values (12-15, some even >30) expected for sources dominated by clinopyroxene ± garnet (Figs.10A and 10B).In fact, samples with Zn/Fe T (×10 4 ) <10 (mostly younger mafic rocks; Fig. 10B) have d 18 O values typical of MORB (d 18 O = 5.18‰ ± 0.28‰), whereas those with higher Zn/Fe T values (older gabbros) have higher d 18 O values (~9‰), reflecting sources with dominant clinopyroxene ± garnet (e.g., old lithospheric pyroxenite; e.g., Ducea et al., 2013;Lee, 2014;Liu et al., 2014;Murray et al., 2015).The Hf/Sm ratio is also related to the presence of garnet in the source (van Westrenen et al., 2011).Our rocks show a wide range of Hf/Sm values: The young gabbros and amphibolites have values that scatter around that of peridotite (0.78), whereas the older gabbroic rocks are closer to that of garnet pyroxenite (0.4; Fig. 10C), in agreement with low and high d 18 O values, respectively.This evidence suggests that a garnet pyroxenite source was dominant for the older mafic rocks (excluding most of the amphibolite samples), whereas peridotite was significant in the younger mafic suite.All the mafic rocks have a relatively restricted range of Sr (100-400 ppm) but highly variable Y (1-60 ppm), the latter being partitioned into garnet at high pressure; this leads to contrasting values of Sr/Y, with most of the old mafic rocks showing the higher values (Fig. 10D).
Finally, the more-evolved Nd-isotope compositions correlate with higher values of Zn/Fe T that can be attributed to mixing between pyroxenite-and peridotite-derived melts (Fig. 10E), whereas amphibolite samples with low Zn/Fe T values and isotopically less evolved Nd-isotopic composition were probably generated from peridotite-dominated sources.The high values of Zn/Fe T in some samples show no correlation with Nd isotopes; one sample (SVF-577) in this group with negative e Hf (t) in zircon (average of -5.3; Dahlquist et al., 2013) indicates crustal assimilation during or prior to zircon crystallization.

Tectonic Implications for the Famatinian Arc
A conceptual model for the possible origin of the Famatinian mafic magmas and their link with contemporaneous intermediate to silicic magmas throughout the arc is shown in Figure 11.Otamendi et al. (2012) envisaged a pre-arc crust ~30 km thick for the Famatinian orogen, with half of this consisting of sedimentary sequences, although this is probably a minimum overall inasmuch as the Mesoproterozoic basement that formed the root of the arc is not exposed.Earlier mafic magmas represented by numerous dikes and sills in these high-grade metasedimentary rocks (e.g., Mannheim and Miller, 1996;Otamendi et al., 2012;Alasino et al.,  2014) increased the geothermal gradient and promoted partial melting (migmatization) in the crust (Otamendi et al., 2012).Subsequently, the input of dioritic and tonalitic magmas promoted widespread anatexis of crustal protoliths, generating hybrid rocks (e.g., Otamendi et al., 2012;Alasino et al., 2014;Ducea et al., 2015).Granodioritic to monzogranitic plutons emplaced at shallow crustal levels formed through the interaction between crustal melts and tonalite (or their differentiates; e.g., Otamendi et al. 2012).Some silica-rich (>70% SiO 2 ) rocks cannot be related to this hybrid origin as they are isotopically less evolved and slightly younger (by a few million years) than the tonalites, suggesting that these uncontaminated magmas evolved from different magma batches.The younger mafic rocks were emplaced contemporaneously with the tonalitic intrusions, forming numerous dikeand pod-like bodies that mingled with the host granitoid magma.
The progressive shift from a pyroxenite subcontinental mantle to an asthenospheric peridotite mantle source through time (over 20 m.y.) produced melts that mixed in varied proportions.Partial melting of subcontinental pyroxenite was probably followed by upwelling of the asthenospheric mantle resulting from foundering of the former (Fig. 11).This process produced earlier melts that were isotopically evolved and had little contribution from juvenile peridotite melts (the latter only being evident in the amphibolite rocks), followed by isotopically depleted melts resulting from extensive melting of peridotite in the late stages of the arc.Pyroxenite starts melting ~35-40 km deeper than an upwelling peridotite diapir by adiabatic decompression (e.g., Pertermann and Hirschmann, 2003).A pos-sible model for magma formation would imply foundering of the subcontinental pyroxenite mantle and partial melting producing tholeiitic melts, with a progressive shift to calc-alkaline melts (with subduction signature and increasing H 2 O) resulting from adiabatic decompression of upwelling mantle wedge peridotite (Fig. 11).

CONCLUSIONS
Field, geochemical, and isotopic data for mafic rocks of the western Sierra de Famatina (at 29°S) define two contrasting suites, which can be correlated with similar suites elsewhere in the Sierras Pampeanas (e.g., Sierra de Vale Fértil and Sierra de Los Llanos): (1) Amphibolite, hornblende gabbro, and minor olivine-(ortho)pyroxene metagabbro formed sheeted plutons of varied thickness that intruded high-grade metasedimentary rocks.Mafic rocks of this suite are commonly found within the volumi nous tonalite intrusions, either as rafts or stoped blocks and in places mingled, indicating that they are pre-to syntonalite.The geochemical features of mafic stoped blocks were partially modified by interaction with more felsic magmas.(2) Dikes and minor mafic bodies intruded the tonalites.These usually show mingling relationships with the tonalite magma, indicating that they were contemporaneous.
Chemically, the first suite is tholeiitic to calcalkaline, whereas the second is calc-alkaline.Our new data, together with those previously published, suggest that during the Fama tinian orogeny, mafic magmatism underwent changes largely coeval with the intrusion of voluminous tonalite-granodiorite magmas (ca.480-470 Ma).The earlier mafic suite consisted of pre-to syntonalite mafic magmas of tholeiitic to calc-alkaline signature that were isotopically evolved.The later suite of syntonalite mafic intrusions was both calc-alkaline with subduction-related chemistry and isotopically more juvenile than the first.
Petrogenesis of the mafic rocks involved progressive mixing of different isotopic reservoirs.Pyroxenite (±garnet) was probably the dominant source of the earlier gabbroic magmas (with little contribution from peridotite melts represented mostly by amphibolite samples), whereas peridotite dominated in the younger ones.This change in source provides an explanation for the shift toward more juvenile compositions through time (over ~20 m.y.).We hypothesize that pyroxenite-derived melts, i.e., melts sourced in the subcontinental mantle of the arc, were followed by more primitive melts resulting from upwelling of the mantle wedge peridotite after lithospheric foundering.According to this interpretation, lithospheric foundering was coeval with the high-heat-flux event (i.e., tonalitic magmatism) in the arc.
Figure 3. (A) Mafic rafts and blocks of amphibolite and metagabbro surrounded by granitoid sheeted bodies.(B-C) Mingling relationships between granitoid and contemporaneous mafic dikes (in places leading to stoped blocks) in Sierra de Famatina.(D) Dismembered mafic intrusions in tonalite interpreted to have been magma mush, forming enclaves with mafic trajectories (or "wakes").Photograph was taken on a fallen block (not in situ).
Figure 6.Normal mid-oceanridge basalt (N-MORB)-normalized (Saunders and Tarney, 1984) trace-element variation plots showing the average compositions of mafic samples from the Famatinian arc (n = number of samples).GLOSS (dotted line) is the global average subducted sediment composition for large ion lithophile elements and Nb-Ta (Plank and Langmuir, 1998).Averages are from TableDR1(see text footnote 1).

TABLE 1 .
Pankhurst et al. (2000) COMPOSITIONS OF MAFIC SAMPLES FROM WESTERN SIERRA DE FAMATINA Total iron expressed as FeO total .Sample VCA1007 is fromPankhurst et al. (2000).position.Minerals and synthetic compounds were used as standards.Results are shown in TableDR2(see footnote 1).

TABLE 2 .
Rb-Sr AND Sm-Nd DATA FOR MAFIC SAMPLES FROM THE WESTERN SIERRA DE FAMATINA . *t-time used for the calculation of the isotopic initial ratios.For metagabbro and amphibolite, t = 480 Ma, and for gabbro, t = 475 Ma. † Epsilon-Nd values were calculated relative to a present-day chondrite: ( 143 Nd/ 144 Nd) today CHUR = 0.512638; ( 143 Sm/ 144 Nd) today CHUR = 0.1967, where CHUR is chondritic uniform reservoir.§ T DM is depleted mantle model age with average crustal Sm/Nd prior to emplacement at 470 Ma, following DePaolo et al. (1991).

TABLE 4 .
GEOTHERMOBAROMETRY FOR HORNBLENDE GABBRO OF THE CERRO ASPERECITO