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In situ garnet Lu‐Hf geochronology has the potential to revolutionise the chronology of petrological and tectonic processes, yet there is a paucity of well‐characterised reference materials to account for laser‐induced matrix‐dependant elemental fractionation. Here, we characterise two reference garnets GWA‐1 (Lu ~ 7.0 μg g−1) and GWA‐2 (Lu ~ 8.5 μg g−1) for in situ garnet Lu‐Hf geochronology. Isochron ages from isotope dilution Lu‐Hf analyses yield crystallisation ages of 1267.0 ± 3.0 Ma with initial 176Hf/177Hfi of 0.281415 ± 0.000012 (GWA‐1), and 934.7 ± 1.4 Ma with 176Hf/177Hfi of 0.281386 ± 0.000013 (GWA‐2). In situ Lu‐Hf analyses yield inverse isochron ages up to 10% older than the known crystallisation age due to matrix effects between garnet and reference glass (NIST SRM 610) under different instrument tuning conditions. This apparent age offset is reproducible for both materials within the same session and can be readily corrected to obtain accurate ages. Our results demonstrate that GWA‐1 and GWA‐2 are robust reference materials that can be used to correct for matrix‐analytical effects and also to assess the accuracy of in situ Lu‐Hf garnet analyses across a range of commonly encountered garnet compositions. [ABSTRACT FROM AUTHOR]

Deciphering the tectono‐metamorphic evolution of Precambrian terranes can be difficult due to reworking by later superimposed events. Whole‐rock elemental and isotopic geochemistry and zircon U–Pb geochronology are often employed in those studies, but these approaches are often not sensitive to the presence of multiple events and medium‐grade metamorphic episodes. The Rio Apa Terrane (RAT), an allochthonous fragment of the Amazonian Craton, is a crustal block with a well‐characterized crustal evolution but with no detailed thermal constraints for its tectono‐metamorphic evolution. In contrast to previous studies, we show the existence of four tectono‐metamorphic events at c. 1,780, c. 1,625, c. 1,420–1,340, and c. 1,300–1,200 Ma on the basis of apatite, titanite, and rutile U–Pb, in situ white‐mica Rb–Sr, and in situ garnet Lu–Hf geochronology combined with mineral chemistry and phase‐equilibria modelling. The c. 1,780 Ma event is recorded in the basement of the Western domain, representing an extensional event coeval with the development of its Eastern domain in response to the retreat stage of the accretionary system. This is followed by juxtaposition of the Western and Eastern domains along a major crustal boundary at c. 1,625 Ma, which is defined by the magnetic profiles and zircon U–Pb–Hf data across the boundary. The third and fourth events correspond to progressive high‐pressure/medium‐temperature (HP/MT) metamorphism, characterized by an anticlockwise P–T path, suggesting a convergent‐to‐collisional tectonic setting. The RAT was accreted to the adjoining Paraguá Terrane at c. 1,420–1,340 Ma under an isobaric P–T evolution spanning ~530°C to 600°C and ~10.0 kbar. Subsequently, the combined Rio Apa and Paraguá terranes collided with the SW Amazonian Craton at c. 1,300–1,200 Ma, reaching P–T conditions of ~560–580°C and ~10.9–11.7 kbar during crustal thickening. This study reveals for the first time the existence of a HP/MT metamorphic evolution related to the growth of the SW Amazonian Craton as part of an accretionary orogenic system during Rodinia assembly in the Palaeoproterozoic to Mesoproterozoic. [ABSTRACT FROM AUTHOR]