K–Ar dating of authigenic minerals in siliciclastic sequences: an example from the south Sanfranciscana Basin (Western Minas Gerais, Brazil)

K–Ar dating was applied on authigenic potassic minerals which are abundant in sandstones from the south of the Sanfranciscana Basin, Western Minas Gerais State, central Brazil. The Quintinos Member fluvial sandstones (Três Barras Formation, Areado Group) contain significant amounts of authigenic K-feldspar as microcrystals of adularia and sanidine habits. The ages of these microcrystals cluster into three groups: 106.1 ± 2.2, 89.9 ± 1.9 and 88.8 ± 1.8 Ma (from Albian to Coniacian). The older age of 106.1 ± 2.2 Ma was obtained from the coarse fraction analysed (10–20 µm) that can contain a mixture of detrital potassic minerals (K-feldspar, muscovite, biotite and illite) and different authigenesis of K-feldspar (overgrowths and microcrystals). Thus, only the younger ages were interpreted as precipitation of K-feldspar microcrystals during the Late Cretaceous into the Quintinos Member sandstones. Moreover, these ages can document the formation of microcrystals within a few million years after deposition of the sandstones. The ages of authigenic illite from the Capacete Formation epiclastic sandstones (Mata da Corda Group) range from 88.5 ± 1.9 to 71.5 ± 1.9 Ma (Coniacian–Campanian). These results suggest the timing of the illitization event in these sandstones as well as a synchrony with K-feldspar authigenesis in the Quintinos Member sandstones. These results are well constrained and are in agreement with stratigraphic, biostratigraphic and radiometric ages previously reported for the Sanfranciscana Basin

Evaluating qualitative management research: towards a contingent criteriology

The term qualitative management research embraces an array of non-statistical research practices. Here it is argued that this diversity is an outcome of competing philosophical assumptions which produce distinctive research perspectives and legitimate the appropriation of different sets of evaluation criteria. Some confusion can arise when evaluation criteria constituted by particular philosophical conventions are universally applied to this heterogeneous management field. In order to avoid such misappropriation, this paper presents a first step towards a contingent criteriology located in a metatheoretical analysis of three modes of qualitative management research which are compared with the positivist mainstream to elaborate different forms of evaluation. It is argued that once armed with criteria that vary accordingly, evaluation can reflexively focus upon the extent to which any management research consistently embraces the particular methodological principles that are sanctioned by its a priori philosophical commitments