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The geological origin of building stones
Published in John A. Hudson†, John W. Cosgrove, Understanding Building Stones and Stone Buildings, 2019
John A. Hudson†, John W. Cosgrove
During the movement of the Earth’s tectonic plates, subduction can occur, i.e., one plate can be forced beneath another, see the subduction zones at the convergent plate margins in Figure 2.6. The subducting plate is heated up and begins to melt, producing large volumes of magma which can be injected into the overlying rocks where it either cools slowly at depths as granite masses or cools quickly as a result of extrusion at the Earth’s surface as a volcano. These granite masses and volcanoes form along the line of the mountain belt, i.e., parallel to the boundary between the two converging plates. Two such chains of granites, linked to ancient collisions between plates, can be seen in Figure 2.11. One runs NE–SW from John O’Groats SW into Northern Ireland. This is linked to the closing of an ancient ocean, the Iapetus ocean, which occurred in the Palaeozoic Era during the Cambrian and Ordovician periods ~500–450 million years ago, and which produced the Caledonian mountain belt.
Has the tectonic regime of the Baltic Shield always remained the same?
Published in GFF, 2022
The Mylonite Zone (Figs. 6 and 7) separates the Klarälven and Ätran terranes from the younger western terranes. In the north-west, the Zone is hidden below the Caledonides and cut by the formation of the Iapetus Ocean. It runs as a steeply dipping zone in a south-eastern direction turning towards the south (Lindh 1974); it may be interpreted as a large sinistral shear zone (Fig. 6). A number of small younger faults offsets the major faults (Lindh et al. 1998). To the south of Lake Vänern, it gradually becomes less steep (e.g., Samuelsson 1978) and turns sharply westwards shortly north of the eclogite-bearing area (Figs. 6 and 7). The east-west striking part of the mylonite zone slopes gently northwards (Möller 1998), suggesting a change in mutual relations between the Idefjorden and Ätran terranes, from a strike-slip to a thrust movement. The younger age and the totally different lithologies of the Idefjorden block compared to neighbouring areas in the east hint to a plate movement along a transform fault replaced by a collision zone including subduction in the south. This replacement involves an almost 90° turn of the zone. Its western continuation is hidden below the Skagerrak and the Danish platform sediments. Movements along the much younger Tornquist-Teisseyre zone (Fig. 6) further complicates the pattern.
Niobium mineralization of sedimentary carbonates, Lewisian Complex, UK
Published in Applied Earth Science, 2021
John Parnell, Ryan Michie, Eleanor Heptinstall, John S. Still
The Lewisian and Moinian rocks are tectonically inter-sliced (Peacock 1975; Soper 2009) and intruded by numerous granitoid plutons above the subduction zone on the Laurentian margin of the Iapetus Ocean (Oliver et al. 2008). The plutons include two alkaline complexes, at Loch Loyal (426 Ma) and Loch Borralan (429 Ma) in the Northern Highlands terrane (Goodenough et al. 2011). Carbonatite occurs in the Loch Borralan Complex (Young et al. 1994). The terrane also includes plutons dated to the end-Neoproterozoic, which possibly relate to an early stage of Iapetus history. The early plutons include the Carn Chuinneag granite (594 Ma), which was mineralized with the tin (Sn) ore cassiterite (Gallagher et al. 1971). Vein mineralization at the southern end of Loch Shin, in Lewisian, Moinian and Caledonian granitic rocks (within a few kilometres to the south of the marble locality: Figure 1), includes the tungsten (W) ore scheelite, Sn-rich galena and fluorite (Gallagher 1970; Gallagher and Smith 1976).
Timing of deformation, metamorphism and leucogranite intrusion in the lower part of the Seve Nappe Complex in central Jämtland, Swedish Caledonides
Published in GFF, 2021
Yuan Li, David G. Gee, Anna Ladenberger, Håkan Sjöström
From Jämtland, the SNC can be followed westwards into Trøndelag, where it attenuates and is partly excised, but is present in the Tømmerås and Trollheimen antiforms and west of Trondheimsfjord (Gee 1978; Gee et al. 1985). From southern Jämtland, fragments of the SNC occur in southeastern Trøndelag as far as Heidal near Otta (Jakob et al. 2019; Corfu & Heim 2020). Possible correlatives include the Lindås Nappe (Austrheim 1991; Bingen et al. 2004) near Bergen and, farther south, the Refsegg Nappe (Roffeis & Corfu 2014) and Jaeren Nappe (Smit et al. 2010) near Stavanger. Thus, this complex affected by subduction-related metamorphism may well have extended along the entire length of the Baltoscandian outer margin during closure of the Iapetus Ocean (Gee et al. 2020). The ophiolites, volcanic arc and fore- and back-arc terranes in the lower parts of the overlying Köli Nappe Complex (Fig. 1), were probably accreted to Baltica during this early Caledonian subduction of the outermost parts of the Baltoscandian margin (Sturt et al. 1991; Gee 2020).