Bimetasomatic reaction zones between ultramafic rocks and trondhjemitic dykes in the Sierra Chica, Córdoba, Argentina: temporal variations and geochemical features

The emplacement of basic to acid magmas into ultramafic rocks represents a specialized scenario where bimetasomatic exchanges commonly occur. In the Sierra Chica of Córdoba, near the city of Alta Gracia, remmants of the intensely serpentinized upper mantle -tectonically emplaced into the metasedimentary sequence during the Neoproterozoic (pre-Pampean)- are intruded by dykes of tonalitic to granitic compositions. This contribution deals with some of these dykes of trondhjemitic composition that exhibit tabular shapes and relatively small dimensions (< 50 m long and 2 m thick). They develop penetrative reaction zones symmetrically at the contacts between serpentinized ultramafic rocks and trondhjemitic dykes, with regular thicknesses between 0.10 and 1.0 meter. Three distint mineralogical zones were identified, progressing from the dykes towards the serpentinite:

1.- Magnesio-hornblende zone (Mg-Hbl ± Pl ± Ttn ± Chl),

2.- Chlorite zone (Chl ± Mg-Hbl ± Zrn ± Ap)

3.- Anthophyllite zone (Ath ± Tr ± Chr ± Tlc ± Chl).

According to textural and mineralogical evidence, the following sequence of formation is proposed: 1) Ath zone, 2) Mg-Hbl zone and 3) Chl-zone.

Zircon U-Pb dating of the trondhjemitic dykes yields a concordia age of 528 Ma for the crystallization and emplacement of these rocks. Zircons extracted from the Chl-zone share similar petrographical features and ages, evidencing that they represent xenocrysts derived from the trondhjemitic magmas later affected by chlorite-forming reactions. The age of 528 Ma place the emplacement of the trondhjemitic dykes during the high-grade metamorphism, anatexis and magmatic events of the Pampean orogeny, occurred at 0.65 - 0.85 GPa and 700º- 850ºC. Anthophyllite, typically formed at relatively high temperature (>600 °C), is interpreted to have formed close to the time of trondhjemitic dyke emplacement and crystallization, when the whole enclosing region was undergoing high amphibolite-to-granulite facies at the main events of the Pampean orogeny. The conspicuous presence of chlorite (clinochlore) in all zones crosscutting the formerly crystallized minerals (e.g., Ath and Mg-Hbl), temperature ranges of 273º - 418ºC estimated by Chl geothermometers, along with pervasive low temperature deformation textures (kink folding) evidence that Chl was formed at lower P-T conditions.

Field and petrographic evidence, geochemical and geochronological data obtained in this work indicate that the reaction zones were formed in two different periods. Anthophyllite and Mg-Hbl zones resulted from the bimetasomatic ionic diffusion between the ultramafic rock and trondhjemitic dykes during the Pampean orogeny at high P-T conditions. Conversely, Chl crystallization should have occurred at lower P-T conditions during the Famatinian orogeny that took place from ~500-440 Ma in the Sierras de Córdoba mainly as deformational events. The chlorite zone is therefore proposed to have formed as a result of fluid infiltration in the already altered contact zones between the ultramafic rocks and the trondhjemitic dykes.