Geology and whole-rock geochemistry of the Las Chacritas pluton, Sierra de Humaya, Catamarca, Argentina.

LARROVERE, MARIANO A.; ALASINO, PABLO H.

Conferencia; 22° International Lateinamerika-Kolloquium; 2011

The Sierra de Humaya (Sierras Pampeanas, NW Argentina) is an igneous-metamorphic basement block uplifted during the Andean tectonic event. The basement is dominated by widespread metasedimentary rock sequences metamorphosed under different metamorphic grades, such as migmatites, gneisses, and schists, and small intrusive igneous bodies, all grouped under the name of El Portezuelo Metamorphic–Igneous Complex. The high T/P metamorphism took place during the Famatinian orogen, between 477 to 470 Ma (ID-TIMS U-Pb in monazite; Larrovere et al., in press). In the northern region of the Sierra de Humaya, the Las Chacritas pluton (LCP), an elongated sub-ellipsoidal body (about 4 x 2 km) is intruded into the mentioned metamorphic rocks. This body is concordant with the main regional foliation (NNW-SSE /N-S strike and ENE/E dip). Internal foliation of the body is nearly parallel to the country-rock foliation. The contacts between the LCP and the wall rock (mostly schists) are sharp and straight. In the border zone, mainly concordant dykes (up to 5 m thick) and thin quartz-rich veins (1-5 cm) are observed into the country rock. In some dykes wall rock fragments (xenoliths) with angular shapes are presents. The main plutonic facies is a light gray equigranular medium-grained two-mica granodiorite to tonalite. It consists of plagioclase (31-44%), quartz (30-46%), microcline (0-14%), muscovite (6-12%), biotite (5-11%) and less than 2% (modal) of accessory minerals (apatite, zircon and epidote) (n=4). On the eastern margin of the pluton, leucocratic equigranular muscovite-bearing dikes are associated to the main plutonic facies. They are white, medium- to coarse-grained, and consist of microcline (13-37%) plagioclase (18-33%), quartz (26-43%), muscovite (13-25%) (n=4) and biotite (less than 1%), with tourmaline, garnet, oxides and apatite as accessories. The main facies of the LCP is felsic or evolved in composition (SiO2 ≈ 73 wt.%) and slightly peraluminous (ASI=1.12-1.16). The leucocratic dykes are also felsic (SiO2 ≈ 73 wt.%) but slightly to moderately peraluminous (ASI=1.18-1.23). On the Na2+K2O vs. Si2O classification diagram, all samples (main facies and dykes) fall in the field of granite and on the subalkaline trend, whereas on the A-B diagram after Villaseca et al. (1998) they are discriminated as moderately peraluminous and felsic peraluminous respectively. Chondrite-normalized REE patterns show that the main granitic facies has relatively high LREE and low HREE values ([La/Yb]N=7.7-8.3), with moderately negative Eu-anomalies (Eu/Eu*=0.57-0.65). Leucocratic dyke samples are depleted in REE, with a flat pattern ([La/Yb]N=1.7-1.9) and show strong to moderate negative Eu-anomalies (Eu/Eu*=0.26-0.70) suggesting previous plagioclase fractionation in the magma or retention of this phase in the source. On the Y-Nb tectonic discrimination diagram all studied rocks plot in the field of volcanic arc-granites + syn-collisional granites. In conclusion, we report a new magmatism in the Sierra de Humaya, Las Chacritas pluton, composed by peraluminous granitoids and Ms-Tur-Grt bearing dykes associated. The field and whole-rock geochemical data suggest that this magmatism was originated by anatexis of metasedimentary rocks probably during the Famatinian orogen.