Sierra de Los Llanos, Malanz¨¢n and Chepes: Ordovician I and S-type magmatism in the Famatinian orogene

PANKHURST, R.J; RAPELA, C.W.; SAAVEDRA, J.; BALDO, E.G; DAHLQUIST, J.A.; PASCUA, I.

Buenos Airaes

Simposio; Simposio Internacional ¡°Proto-Andean Margin of Gondwana¡±, XIII Congreso Geol¨®gico Argentino; 1996

The sierras of southern La Rioja province constitute the most westerly block of the Eastern Sierras Pampeanas at 30-32¡ã S. They consist of a vast complex of granitoids, intruded into low-grade metasediments in the sierras of Los Llanos, Malanz¨¢n and Chepes, we have distinguished five principal igneous units of widely contrasting compositions, from metaluminous to strongly peraluminous. The Chepes Granodiorite is the dominant regionally-developed unit, with a penetrative foliation trending from NW-SE to NE-SW. This is a calc-alkaline series of hornblende and biotite granodiorites, locally tending to monzogranite or tonalite (62-70% Si02), with epidote and titanite+/-allanite as accessories. The granodiorites commonly contain darker fine-grained enclaves (60-62% Si02), aligned along the foliation. Some tonalites cut the main facies, demonstrating multiple injection of metaluminous magma. Rb-Sr data for the granodiorites he on common isochrons with enclaves and subsequent granites, confirming a generally cogenetic relationship. The Porphyritic Granodiorite is a coarse-grained biotite granodiorite (typically e. 69% S102), locally grading to monzogranite and tonalite. It shares the regional foliation of the Chepes granodiorite and has a similar paragenesis; it is principally distinguished by the presence of alkali-feldspar phenocrysts and fewer enclaves. It has inclusions of cordieritic gneiss. Separate exposures have been dated by Rb-Sr at 471 +1- 10 Ma (MSWD 0.8; initial 87Sr/86Sr= 0.7089) and 452 +1- 9 Ma (MSWD= 1.8; initial 87Sr/86Sr= 0.7111). The Los Llanos Diorite is well exposed in the NW of the region. Diorites and quartz diorites display mingling phenomena with the metaluminous granodiorites and there is a well-defined compositional gap between them. This unit was not amenable to Rb-Sr whole-rock dating, but it shares the same initial 87Sr/86Sr ratios as the other units. The Tuani Granite is the third largest unit, at least iii the Sierra de Chepes. It was intruded into the regional granodiorite, principally as a two-mica granite (72-75% Si02) and has tourmaline as an accessory. A later minor variant comprises small bodies of cordierite monzogranite, sometimes with sillimanite. These have lower Si02 contents (66-69%) and typically the lowest CaO contents of any granitoids in the region. The Rb-Sr age determined for the Tuani granite is 464 +1- 5 Ma (MSWD= 0.8; initial 87Sr/86Sr= 0.7089, up to 0.7 155 for the cordierite monzogranite). The fifth unit is termed El Elefante Granite, comprising small stock-like bodies throughout the region, but especially located along the western margin, intruding both granodiorites and metamorphic rocks. They range from monzogranite to syenogranite (74-77% Si02; aplites up to 78%). The rocks are leucrocratic, with scarce biotite and muscovite. Granites of this group have been dated at 469+/- 5 Ma, 459 +1- 5 Ma and 456 +1- 3 Ma, with initial 87Sr/86Sr ratios of 0.7088 to 0.7105. The evidence of this study is that the intense granitoid magmatism of these sierras was confined to early Ordovician times, mostly within the Llandeilo-Caradoc interval and terminating no later than mid-Caradoc. In this sense it is temporally equivalent to, and a southerly extension of, the granite magmatism of the Famatinian orogen. The trend from an extensive metaluminous granodiorite phase to more varied 1- and S-type granites is apparently ah encompassed within a brief period, and both magma types were available essentially simultaneously. Moreover, both types had elevated 87Sr/86Sr ratios and initial ¦ÅNd values of -5 to -7, signifying major melting of mid-Proterozoic (ca. 1700 Ma) continental crust. This is consistent with models for continent-continent collision at this time. This work was supported by European Community Project N¡ã CII -CT92-0088 and is a contribution to 1GCP Project 345 (¡°Andean Lithosphere Evolution¡±).