LA–SF–ICP-MS U–Pb baddeleyite and zircon geochronology applied to Cretaceous alkaline rocks of the São Paulo state– Brazil

Abstract

The laser ablation sector field inductively coupled plasma mass spectrometry (LA–SF–ICP-MS) U–Pb geochronological method is frequently used to date minerals, in particular zircon. In turn, baddeleyite is the ideal geochronometer when it comes to silica-undersaturated rocks, such as alkaline rock types. In this study, we describe a series of protocols to separate baddeleyite crystals and date them by the LA–SF–ICP-MS method in the Isotopic Geology Laboratory of the Geoscience Institute (University of Campinas), starting from sample crushing and milling and the calibration of the Wilfley table. The method includes the use of Kovdor and Phalaborwa primary and secondary reference materials, respectively. Laser operation conditions were spot size of 25 μm and ablation time of 60 s (20 for local blank and 40 for baddeleyite signal), at a fluence of 20 L per minute. Cretaceous alkaline rocks associated with the Paraná-Etendeka magmatic province were analyzed, comprising Jacupiranga, Pariquera-Açu, and Cananéia intrusions, all related to the Guapiara lineament. The results yielded concordant baddeleyite ages of 115–120 Ma for Jacupiranga carbonatite, around 110 Ma for Pariquera-Açu shonkinite, and 81 ± 2 Ma and 83 ± 0.4, respectively for baddeleyite and zircon from the Cananéia syenite. Despite the difficulties in baddeleyite separation, the LA–SF–ICP-MS U–Pb ages have proven to be robust for the young alkaline complexes related to the Serra do Mar Province, such as those occurring along the São Jerônimo-Curiuva Lineament, avoiding the concerns about thermal events that could reset, e.g. K–Ar and Ar–Ar systems. Field relations show a hierarchy of intrusive pulses and our data show that, temporarily, the Jacupiranga Complex is active in a younger time interval, 115 to 120 Ma, than previously established.