The Lower Silurian Osmundsberg K-bentonite. Part II: mineralogy, geochemistry, chemostratigraphy and tectonomagmatic significance

Abstract

The Lower Silurian Osmundsberg K-bentonite is a widespread ash bed that occurs throughout Baltoscandia and parts of northern Europe. This paper describes its characteristics at its type locality in the Province of Dalarna, Sweden. It contains mineralogical and chemical characteristics that permit its regional correlation in sections elsewhere in Sweden as well as Norway, Estonia, Denmark and Great Britain. The < 2 µm clay fraction of the Osmundsberg bed contains abundant kaolinite in addition to randomly ordered (RO) illite/smectite (I/S). Modelling of the X-ray diffraction tracings showed the I/S consists of 18 % illite and 82 % smectite. The high smectite and kaolinite content is indicative of a history with minimal burial temperatures. Analytical data from both pristine melt inclusions in primary quartz grains as well as whole rock samples can be used to constrain both the parental magma composition and the probable tectonic setting of the source volcanoes. The parental ash was dacitic to rhyolitic in composition and originated in a tectonically active collision margin setting.Whole rock chemical fingerprinting of coeval beds elsewhere in Baltoscandia produced a pronounced clustering of these samples in the Osmundsberg field of the discriminant analysis diagram. This, together with well-constrained biostratigraphic and lithostratigraphic data, provides the basis for regional correlation and supports the conclusion that the Osmundsberg K-bentonite is one of the most extensive fallout ash beds in the early Phanerozoic. The source volcano probably lay to the west of Baltica as part of the subduction complex associated with the closure of Iapetus. Figure 14. Results of preliminary studies of mineralogical ratios between the type Osmundsberg bed and its nearest correlative at Kallholn, some 20 km distant. Biotite and quartz are by far the most abundant non-clay phases present and this suggests that they may also constitute an additional physical parameter for stratigraphic usage.

Tectonic and eustatic control on the distribution of black-shale source beds in the Wufeng and Longmaxi formations (Ordovician-Silurian), South China

“…4). The analyses show that all the K-bentonites originated from explosive, felsic-intermediate volcanism in a syn-COLG-VAG to WPG setting, which supports a tectonomagmatic setting similar to that interpreted from various peri-Iapetus orogenies (Huff et al, 1992(Huff et al, , 1998, and the approximate mapping of bentonite distribution trends also shows that the volcanic vents were probably located to the southeast. Hence, bentonite research also supports the presence of a subduction-type orogeny in the southeast of China at the time.…”

Discussion on the PGE anomalies and source materials of K-bentonite (Bed 5) in the Lower Cambrian Meishucun section, Yunnan

“…In K-bentonite-relevant researches, Nb/Y vs. Zr/ TiO 2 diagrams established by Winchester and Floyd (1977) have been extensively employed to preliminarily discern the magmatic affinity of K-bentonite (Huff et al 1992(Huff et al , 1998Wan et al 2013;Zhou et al 2011Zhou et al , 2014. On a Nb/Y Table 3 Blank level (910 -9 g), detection limits (DL, 910 -9 ) and analytical results (910 -9 ) of PGEs for reference materials and MSC5-M The detection limit (910 -9 ) is calculated as three times the standard deviation of five individual procedural reagent blanks (910 -9 g), divided by the sample mass a Certified values followed Govindaraju (1994) b S = Standard deviation Chin.…”

Geochemistry of Çamardı Formation sediments, central Anatolia (Turkey): implication of source area weathering, provenance, and tectonic setting

“…The Al 2 O 3 /TiO 2 ratios of the Çamardl Formation sediments (both shale and sandstones) range from 18.8 to 45.8 (Table 2). In the case of most of the Çamardl Formation sediments (except CM-5; shale), a relatively high Al 2 O 3 / TiO 2 value (>20) can be derived from intermediate or felsic source rocks (Condie, 1993;Huff et al, 1997;Ekosse, 2001;Qiugen et al, 2005;Spaletti et al, 2008). The concentration of Zr is used for characterizing the nature and composition of source rocks (Hayashi et al, 1997;Paikaray, 2008).…”