Cenozoic high-strontium andesites in the Eastern Cordillera of Northwestern Argentina, Central Andes

JOSÉ MARÍA VIRAMONTE; SUZAÑO, N.; PRESCOTT. C; BECCHIO, R; JOSÉ GERMÁN VIRAMONTE; ARNOSIO, M; PIMENTEL, M.

Nice

Simposio; 7th International Symposium on Andean Geodynamics (ISAG 2008, Nice),; 2008

ISAG

Cenozoic magmatism in the southern Central Andes occurs generally in the N–S trending volcanic arc and in NW–SE trending transverse volcanic belts (Viramonte et al., 1984). In the Eastern Cordillera of northwestern Argentina, near the Huachichocana town (24º-65º, Jujuy province; Figure. 1) crop out Cenozoic andesites with high Sr contents. These rocks are associated with the Lipez transverse belt (Figure 1) and were firstly described by Ramos et al., (1967) as “Huachichocana Andesite”. They are located ~590 Km from the trench in a back arc position and represent one of the easternmost magmatism between 23º and 25º LS with the Diego de Almagro Complex (Hauser, 2005) and Alemania-Pampa Grande Andesites (Figure. 1). The Huachichocana rocks occur as a 200m-thick and 700m-large sill along the “Tilcarica” unconformity between the Puncoviscana Formation (Upper Precambrian-Lower Cambrian) and the Meson Group (Mid-Upper Cambrian). This body present columnar jointing and have a porphyritic texture comprising by phenocrysts of hornblende, biotite, clinopyroxene, orthopyroxene and zoned plagioclase included in a cryptocrystalline matrix. Titanite occurs as an accessory mineral. Major and trace elements composition are presented in Table 1. They have 61-63 % wt. SiO2 and according to the SiO2 vs K2O (Figure 2) diagram Huachichocana rocks plot in the high-K andesite field. Figure 3 show that these rocks present the highest Sr contents (950-1200 ppm) comparing with other andesites of the southern Central Andes. On chondrite normalized diagram (Figure 4) these rocks display coherent REE patterns characterized by enrichment in LREE relative to HREE as indicated by La/SmN= 3.99 to 4.65 and Gd/YbN= 2.04 to 2.31. Also, the samples show very minor Eu anomalies (Eu/Eu*=0.85-0.91). Sm-Nd and Sr isotopic data for selected samples are presented in table 2. These rocks show low 87Sr/86Sr ratios (0.70591-0.70665), negative Nd values ranging from -2.2 to -5.4 and TDM model ages in the interval between 0.81 to 0.99 Ga. Following the ideas of Kay et al., (1999) the geochemical and isotopic data, we suggest a possible origin of the Huachichocana rocks through partial melting of lower mafic crust (Sunsas?) and subsequent contamination with upper crustal rocks during its ascent to shallow crust levels. The high Sr values as well as the Eu/Eu* values allow a little, if any, plagioclase in the restite. These rocks present some differences when comparing with typical andesites of the southern Central Andes indicating a different origin and evolution. So, further studies are carried out in order to better constrain the source and the evolution of the Huachichocana rocks and similar rocks outcropping southward in the Eastern Cordillera (Diego de Almagro Complex and Alemania-Pampa Grande Andesites).