Geochemistry and Petrogenesis of the Central Bohemian Pluton, Czech Republic

Janousek, Vojtech (1994) Geochemistry and Petrogenesis of the Central Bohemian Pluton, Czech Republic. PhD thesis, University of Glasgow.

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Abstract

An integrated study of the petrography, mineral chemistry, whole-rock major, trace and Sr-Nd isotope geochemistry of selected intrusions of the Hercynian Central Bohemian Pluton (CBP) has revealed compositional and genetic diversity within this igneous complex. This composite pluton covers 3200 km2 and has intruded in early Carboniferous times the boundary between two major units, the Tepla-Barrandian Unit and the Moldanubian (s.s.) Unit; parts of its roof are preserved as the so-called Metamorphic Islet Zone. At least five petrogenetic suites can be distinguished in the CBP: Sazava (gabbro, quartz diorite, tonalite, trondhjemite and granodiorite) Blatna (quartz monzonite, granodiorite, granite), Certovo bremeno (melasyenite and melagranite), Ricany (granite) and S-type anatectic granitoids. The Sazava and Blatna suites contain hornblende and relatively Fe-rich biotite, unlike the Ricany and Certovo bremeno suites with (secondary) actinolite and actinolitic hornblende and Mg-rich biotite. Based on the Al in hornblende geobarometry, the majority of the amphiboles studied (Sazava, Blatna, Kozarovice intrusions) have not crystallized deeper than 5-10 km. In contrast, brown cores of the amphiboles from Teletm quartz diorite may have originated at much greater depths, possibly up to 30 km. The abundance of mafic microgranular enclaves, syn-plutonic dykes and the presence of specific microtextural phenomena in many of the studied intrusions (like mantled plagioclases, amphibole-biotite clots, quartz and K-feldspar xenocrysts, quenched textures) point to contemporaneous intrusion of acid and basic magmas and their interaction (magma mixing). Besides the classic petrographic methods, cathodoluminescence has been proven to be a valuable tool for studying these features, in particular resorption and overgrowth features in the feldspar crystals. Each of the suites shows a distinctive range of the major, trace and Sr-Nd isotopic data. The total range of (87Sr/86Sr)330 = 0.7051 - 0.7129 and epsilon330Nd = 0.,2 - -8.9 is comparable with granitoids of other parts of the Hercynian belt. Two-stage TNdDM model ages ranging from 1,05 Ga (Sazava suite) to 1.65 Ga (Certovo bfemeno and fticany suites) are interpreted as reflecting a mixed source, rather than a major crust-formation event. In the CBP, there is a conspicuous trend towards isotopically more evolved compositions with time (in succession Sazava, Blatna, Ricany and Certovo bremeno suites) that is interpreted as being due to enrichment of the mantle by an incompatible-element-enriched and, or high 87Sr/86Sr component, possibly associated with an earlier (late Proterozoic ?) subduction environment. In the northern CBP, the Sazava suite may have originated by re-melting of adjacent metabasites of the Jilove Zone or the Metamorphic Islet Zone, by partial melting of a mantle source with an isotopic composition similar to Bulk Earth, or mixing of magmas derived from both sources. This is compatible with an abundance of mafic microgranular enclaves and a paucity of both surmicaceous enclaves and metasedimentary xenoliths. The basic rocks, associated with, and at least partly contemporaneous with, the Sazava suite have probably acted as a heat source as well as one of the major components of the Sazava magma. The geochemical modelling implies extensive fractional crystallization of mainly amphibole and plagioclase, with some biotite and orthite. The peraluminous Pozary intrusion, with low SigmaREE and high positive Eu anomalies, may have been derived from the Sazava parent largely by fractional crystallization or by small degrees of melting of a metabasic parent. Mixing tests performed on the Teletin quartz diorite are compatible with its origin by hybridization of the Sazava tonalite with a gabbroic melt. Overall, the processes which produced the most mafic members of the Blatna and Certovo bremeno suites appear to have been similar. (Abstract shortened by ProQuest.).

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Donald R Bowes
Keywords: Geology, Geochemistry, Petrology
Date of Award: 1994
Depositing User: Enlighten Team
Unique ID: glathesis:1994-76389
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 14:46
Last Modified: 19 Nov 2019 14:46
URI: http://theses.gla.ac.uk/id/eprint/76389

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