The kinzigitic rocks from Santa Rosa River revisited: structure, petrology and CHIME age, Eastern Pampean Ranges, Argentina

MARTINO, ROBERTO DONATO; KRONZ, A.; GUERESCHI, ALINA BEATRIZ; SIEGESMUND, S.

Brasilia

Simposio; Granulites & Granulites 2006; 2006

Universidad de Brasilia

In the Sierras de Córdoba, Eastern Pampean Ranges of Argentina, a regional thermal axis of early Cambrian granulitic rocks is recognized. This axis is extended by 140 km in length and 40 km in width oriented N 330°, following the main structural trend of the ranges. The Achala batholith (100 km x 35-40 km), elongated in N 20° direction, interrupts it. In the southeastern tract of this thermal axis, the Yacanto Group (50 km x 15 km), composed by cordieritic diatexites and stromatitic metatexites (Qtz-Kfs leucosomes, Bt-Sil-Grt mesosomes), crops out. Between these rocks, the San Miguel Group is intercalated, constituted by mafic and ultramafic rocks, garnetiferous gneisses, forsteritic marbles and the body of the ‘Santa Rosa kinzigite’. This body crops out near the Santa Rosa river and is mining with the commercial name of ‘Azul Tango’. It is composed by Qtz + Grt + Crd + Pl + Bt + Mag ± Ath, distinguishing from the surrounding stromatitic metatexites by the lack of K feldspar and the scarce biotite. The kinzigitic rocks was interpreted like a restite, product of partial melting, where the quartz-feldspatic portion of granodioritic composition has migrated. This body represents the deepest crustal levels of the Pampean orogen at this latitude, with thermobarometric values of P = 8 Kb and T = 800ºC. Within the kinzigitic body, xenolithes of amphibolites and piroxenites are recognized. The kinzigitic rocks form a lens-like body with an S3 coarse foliation oriented N 350° and dipping steeply (> 50°) to the East. A metamorphic layering product of the general flattening of the previous S2 foliation represents S3. S2 is recognized like relict isoclinal folds in amphibolite xenolithes within S3. The same relation is identified in the stromatitic metatexites surrounding the kinzigitic body. The S3 foliation is locally overprinted by an intense mylonitization that generated an S4 foliation, mainly concentrated at contacts between the kinzigitic body and the surrounding rocks. Within the kinzigitic rocks it is evidenced by intracrystalline postmetamorphic ductile deformation, which increase its effects to the west. The identification of an S-C fabric in metatexites and marbles, and the presence of strechting lineations represented by sillimanite in the metatexite mesosome and by biotite clusters in the kinzigitic rocks, allow to deduce an extensional kinematic with dextral movements on map. Later on, a transgressive shear producing centimetric mylonitic bands of reverse kinematic is recognized. The S3 foliation and three joints groups allow the quarrying. Using the CHIME method (chemical Th-U-total Pb isochron method), an age of 587 ± 69 Ma (Phi-Rho-Z correction) to the Santa Rosa kinzigite has been determined. At the moment of this report, we have collected sample rocks to U/Pb SHRIMP dating to constrain this age. However, this neoproterozoic age is the only known data of this unique and important body, being one of the oldest granulitic rocks of the Sierras de Córdoba.