China
Africa
Explore chapters and articles related to this topic
The origin conditions of primary and detrital scheelite related to the Variscan granites in the Central Sudetes (SW Poland)
Published in Adam Piestrzyński, Mineral Deposits at the Beginning of the 21st Century, 2001
The studied area (Fig. 1) is situated in Central Sudetes within the northern margin of the Bohemian Massif. Sudetes, an exposed Variscan orogen, is the easternmost part of the European Hercynian Belt. In this area Late Hercynian igneous rocks formed plutons different in shape, size and mineral composition and were emplaced in the continent-continent collision zones and the active margins of the continental plates above subduction zones. In Central Sudetes the Variscan granites are represented by the late-tectonic Kłodzko–Złoty Stok pluton built of I-type granites and the vein-type body of the syntectonic Jawornik granite. The magmatic rocks, derived from pre-collisional magmas and mantle fractionates (Lorenc 1994), comprise metaluminous calc-alkaline igneous rocks of cafemic association rich in biotite and hornblende. Late magmatic and post-magmatic activity formed dykes of spessartites, vogesites, pegmatites and quartz veins. This intrusion is interpreted as an Andean-type pluton within the European Hercynides (Lorenc 1994). The Kłodzko–Złoty Stok intrusion was formed during the Asturian phase of the Hercynian orogeny (K-Ar dating - 298 Ma; Depciuch 1972) and is about 35 Ma younger than the Jawornik granite. The roof part of the intrusion contains irregularly distributed relics of the Late Proterozoic to Early Paleozoic volcanic and sedimentary rocks comparable to the members of the Orlica-Śniežnik metamorphic complex adjacent to the intrusion in the east. The metamorphic complex is composed of Lower Paleozoic metasedimentary rocks intercalated with mafic and felsic metaintrusives and metavolcanics of the amphibolite facies.
Magmatism in the Context of the Present-Day Tectonic Settings
Published in O.A. Bogatikov, R.F. Fursenko, G.V. Lazareva, E.A. Miloradovskaya, A. Ya, R.E. Sorkina, Magmatism and Geodynamics Terrestrial Magmatism Throughout the Earth’s History, 2020
O.A. Bogatikov, V.I. Kovalenko, E.V. Sharkov, V.V. Yarmolyuk
Plate-collision zone magmatism is also dominated by rocks of the calc-alkaline series. These are mainly andesites, dacites, rhyolites and comagmatic granitiods, often with high 87Sr/86Sr ratios.
Age and tectonic significance of the Louth Volcanics: implications for the evolution of the Tasmanides of eastern Australia
Published in Australian Journal of Earth Sciences, 2018
R. C. Dwyer, W. J. Collins, A. C. Hack, R. Hegarty, H.-Q. Huang
The calc-alkaline group is characterised by several apparently comagmatic dolerite and gabbro intrusives from MINOL21 and MINOL33 that are noticeably different from the alkaline group (Figure 3c, d). Granular equant to tabular pyroxene appear as 3–8 mm phenocrysts, but range to groundmass size, whereas plagioclase appears as an interlocking network of equant to tabular laths. Opaque minerals (magnetite/ilmenite) are commonly altered to leucoxene. Clinopyroxene crystals show only faint pale brown pleochroism diagnostic of augite, and are thus distinct from the pink-purple pleochroism of titanaugite in the alkaline rock group (Figure 3a, b). Deformation and alteration are more intense in calc-alkaline, compared with alkaline, rocks. Localised shearing occurs in a clinopyroxene-phyric ankaramite at the base of MINOL21 and provides a reasonably equivalent sample to compare with the pristine ankaramite from PANGBH5. Mylonitic S–C fabrics are developed in the ankaramite from MINOL21, whereas phenocrytic augite and the groundmass within this unit are highly altered. Elsewhere in associated tuffs, sedimentary rocks and field samples, bedding parallel and axial planar fabrics were observed (MINOL21-05, MINOL30-04, MINOL33-06, RDT7). Unlike the alkaline rocks, alteration assemblages partially to completely replace the primary igneous mineral assemblages and in some cases obscure the original calc-alkaline igneous texture. See Supplementary Papers (Table SM2) for petrographic summaries of key samples.
Geochemistry and origin of a Mesozoic ophiolite: the Pounamu Ultramafics, Westland, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2018
Alan F Cooper, Richard C Price, Anthony Reay
One line of evidence supporting an east- to south-dipping subduction zone beneath southeast Zealandia in the Late Cretaceous is the presence in inland Canterbury (Mt Somers, Mt Alfred, Malvern Hills) and at Charteris and Governors Bays, Banks Peninsula, of Cretaceous volcanic rocks dated at 99–81 Ma (Barley et al. 1988; Tappenden 2003; van der Meer et al. 2017). Undated Cretaceous volcanic rocks also occur in the J.D.George-1 drill hole near Ashburton and in subsurface rocks offshore (Forsyth et al. 2008). These volcanic rocks have a calc-alkaline affinity and are subduction-related (Timm et al. 2009). Because subduction at the Chatham Rise had ceased by this time, this volcanism may relate to the southeast-directed subduction proposed here (Figure 6).