Petrogenesis and monazite EPMA geochronology of the Las Chacritas pluton (Sierra de Humaya, NW Argentina): peraluminous Ordovician magmatism in the Famatinian back-arc region

LARROVERE, MARIANO A.; ALASINO, PABLO H.; DE LOS HOYOS, CAMILO R.; WILLNER, ARNE P.

General Roca

Simposio; 3° Simposio sobre Petrología Ígnea y Metalogénesis Asociada; 2015

Universidad Nacional de Río Negro

The Famatinian belt is a NNW-SSE trending belt represented by Ordovician sedimentary-derived metamorphic and granitoid rocks formed during the Famatinian orogeny as result of a major subduction event along the continental margin of Gondwana. The Sierra de Humaya is an igneous-metamorphic basement block located in the north-eastern region of the Eastern Sierras Pampeanas, i.e. in the back-arc zone of the Famatinian belt. The basement is dominated by widespread metasedimentary rock sequences and small intrusive igneous bodies (González Bonorino 1950). The Las Chacritas pluton (LCP) is an elongated, nearly elliptical body in map-view exposed in the northern region of the Sierra de Humaya, which intruded into the medium grade metamorphic rocks of El Portezuelo Metamorphic?Igneous Complex. The pluton is about 4 x 1.5 km, with the long axis trending NNW. The main plutonic unit is a light gray, equigranular, medium-grained two-mica granodiorite to tonalite. On the margin of the pluton, a subordinate igneous unit is formed by leucocratic equigranular medium- to coarse-grained muscovite-bearing syenogranite to granodiorite. The host rocks are two-mica schist with a metamorphic foliation S2 (regional: strike 0°, dip 36°E; local: 330°/30W and 330°/42E). The contacts between the LCP and the wall rock are intrusive, i.e. injected-type. Mostly contacts are concordant, but locally, discordant contacts are observed. Xenoliths of wall rock occur within the pluton, mainly near the contacts. Disaggregation of the metasedimentary rocks is observed in the wall rock and xenoliths. The two-mica granodiorite to tonalite consists of Pl, Qtz, Kfs, Ms, Bt and Ap, Mnz, Zr and Ep as accessories. Textural evidence (large subhedral laths of Ms in the matrix) and low MgO (avg. 0.72 wt.%) and SiO2 (avg. 44.53 wt.%) contents in Ms support its primary magmatic origin (Miller et al. 1981). The leucocratic igneous unit consists of Kfs, Pl, Qtz, Ms and Bt, with Tur, Grt, Ox and Ap as accessories. The main igneous unit of the LCP is felsic or evolved in composition (SiO2 from 71 to 74 wt.%, n=6) and slightly peraluminous with ASI of 1.07-1.19. The leucocratic igneous unit is also felsic (SiO2 ≈ 73 wt.%, n=2) but moderately peraluminous (ASI=1.24-1.30). On the A-B diagram after Villaseca et al. (1998) the main igneous unit is distinguished as low to moderately peraluminous, whereas the leucocratic unit is felsic peraluminous.Field observations, petrography, mineral chemistry, and whole-rock compositions of the LCP suggest participation of the surrounding metasedimentary basement rocks in the genesis of the two distinct groups of silicic igneous rocks of the LCP. At first, the presence of disaggregated metasedimentary xenoliths directly indicates crustal contamination during magma emplacement. The main unit samples have concentrations of silica, alumina, and alkalis compatible with experimental melts produced by incongruent dehydration-melting of mica (Bt ± Ms) + quartz (e.g. Patiño Douce 1999). This interpretation is consistent with the presence of primary muscovite in them, which is an indicator of peraluminous magmas (Speer 1984). However, the calcium content is too high. On major element diagrams from Patiño Douce et al. (1999), these samples plot in a restricted zone between the fields of metagreywacke- and amphibolite-derived melts. On the A-B diagram, these samples plot without a clear trend. They fall between the trends of experimental melts and natural series (see Fig. 10 Larrovere et al 2015) suggesting that more than one source of melt were involved in the origin and evolution of the main unit of the LCP. Harker diagrams of the leucocratic granitoids of the LCP show that these diverge from the trend of the main igneous unit. Also their REE patterns are strongly LREE-depleted. According to their mineralogy and major element composition, they may correspond to pure crustal melts as a result of dehydration-melting of muscovite-rich metasediments, i.e. the peraluminous leucogranites of Patiño Douce (1999). LREE-depleted patterns in the leucocratic granitoids are consistent with disequilibrium melting, where monazite and perhaps other accessory minerals did not partake in the muscovite dehydration-melting reactions that produced LREE-depleted melts (Nabelek and Glascock 1995). Low Y contents in leucocratic granitoids suggest incomplete monazite and/or xenotime participation in the generation of the leucogranites. In summary, the origin and evolution of the LCP involved multiple melt sources. The main unit may be linked either with interaction of crustal rocks with mafic magmas or crystal-rich magmas that entrained residual phases. Conversely, leucocratic granitoids were derived exclusively from metasedimentary sources by local anatexis of the migmatitic basement underlying the emplacement level.One sample of the main unit was analyzed for in situ U-Th-Pb dating of monazite. The data set for this sample includes 28 spots measured in 16 monazite grains. Calculated dates spread from 522 to 461 Ma. Top-down approach monazite analysis strategy (Williams et al. 2006) based on the age histogram method (Montel et al. 1996) yielded two main date clusters with weighted means of 474.1 ± 4.3 Ma (2σ; n = 21) and 502.7 ± 10 Ma (2σ; n = 7). The first date cluster is considered the best estimate for the magma crystallization. The second date cluster is interpreted to be due to the existence of inherited crystals (xenocrysts or inherited cores). The newly obtained age of 474.1 ± 4.3 Ma (Lower Ordovician) is consistent with Ordovician ages (477-470 Ma) of the high-grade metamorphic rocks of the El Portezuelo Metamorphic?Igneous Complex (Larrovere et al. 2011) and thus indicates contemporaneous magmatism and metamorphism in the Famatinian back-arc region.