Abstract
New age determinations by Ar-Ar and U-Pb SHRIMP for the alkaline complexes of Banhadao and Itapirapua that occur in the Ribeira Valley are in agreement with the previously radiometric K-Ar ages available in literature. Ar-Ar ages on biotite for Banhadao range in the 106-110 Ma interval, and suggest a minimum age of 106 Ma for the complex. Ar-Ar data on biotite for Itapirapua are more uniform, indicating an age of 102 Ma for the complex. U-Pb SHRIMP determinations on titanite from Itapirapua melanite syenitic rocks yielded an age of similar to 106 Ma. Thus, the age interval of 100 and 110 Ma does exist indeed, suggesting an intermediate magmatic pulse between the already well-defined peaks of 80-90 Ma and 130-140 Ma for the alkaline rocks in southern Brazil, confirming the periodicity of the magmatism.
Highlights
The Ribeira Valley in Southern Brazil was the site of intense and repeated alkaline magmatic activity in Cretaceous times (Ruberti et al 2005, Gomes et al 2011)
The overall magmatism in the area is clearly controlled by the Ponta Grossa Arch, a prominent tectonic feature active since the Paleozoic, which extends more than 600 km and plunges toward the inner Paraná Basin, and whose evolution is associated with the reactivation of old deep structures (Ferreira 1982)
The radiometric ages of the alkaline rocks from Southern Brazil, mostly determined in the past by the K-Ar method, allowed Ulbrich and Gomes (1981) to draw attention to the wide age interval of 40 to 240 Ma showed by the numerous occurrences, with peaks of higher concentration of values clearly distinguished in the histogram
Summary
The Ribeira Valley in Southern Brazil was the site of intense and repeated alkaline magmatic activity in Cretaceous times (Ruberti et al 2005, Gomes et al 2011) It is characterized by NW-SE-trending swarms of tholeiitic dikes of the Serra Geral Group and magnetic alignments (Piccirillo et al 1990, Ussami et al 1994). As emphasized by Gomes et al (2011), the southern sectors of Guapiara and São Jerônimo-Curiúva are the main sites of the alkaline and alkaline-carbonatite magmatism in the region, with the intrusions being found on or close to these lineaments These alignements are geographically associated with alkaline bodies of two different ages, Lower Cretaceous (Guapiara: Jacupiranga, Juquiá, Pariquera-Açu; São Jerônimo-Curiúva: Banhadão, Barra do Itapirapuã, Itapirapuã, José Fernandes) and Upper Cretaceous (Guapiara: Cananeia; São Jerônimo-Curiúva: Barra do Teixeira, Mato Preto, Tunas) (Fig. 2). The São JerônimoCuriúva lineament, except for larger occurrences such as Tunas (22 km2), Banhadão (8 km2), Itapirapuã (4 km2), José Fernandes (~3 km, cf. Almeida et al 2017) and Barra
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