Magma-mixing in the genesis of Hercynian calc-alkaline granitoids: an integrated petrographic and geochemical study of the Sazava intrusion, Central Bohemian Pluton, Czech Republic

Janousek, V., Braithwaite, C.J.R. , Bowes, D.R. and Gerdes, A. (2004) Magma-mixing in the genesis of Hercynian calc-alkaline granitoids: an integrated petrographic and geochemical study of the Sazava intrusion, Central Bohemian Pluton, Czech Republic. Lithos, 78, pp. 67-99. (doi: 10.1016/j.lithos.2004.04.046)

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The Devonian–early Carboniferous (354.1±3.5 Ma: conventional zircon U–Pb age) Sázava intrusion (biotite-amphibole quartz diorite, tonalite and granodiorite) of the Central Bohemian Pluton (CBP) associated with bodies of (olivine, pyroxene-) amphibole gabbro, gabbrodiorite, (quartz) diorite and rare hornblendite, gives an opportunity for a comparative study of a rather shallow, calc-alkaline magma-mixing zone at two levels, separated by a vertical difference of approximately 1 km. The deeper section (Příbram) displays the direct evidence for the existence of a long-lived, periodically tapped and replenished, floored magma chamber (MASLI). The contacts between the subhorizontal sheet-like basic bodies and the surrounding, commonly cumulus-rich, Sázava granitoid, are arcuate, and cut by a series of veins and ascending pipes. Shallow-dipping swarms of strongly elongated and flow-aligned mafic microgranular enclaves (MME), concordant with the contacts of the basic bodies, are commonplace. The higher level (Teletín) section shows relatively independent basic intrusions, some of them distinctly hybrid in character and mainly of quartz dioritic composition, surrounded by relatively homogeneous, nearly cumulus-free Sázava tonalite rich in texturally variable MME. Larger quartz microdiorite bodies and the MME, both interpreted as hybrids, contain varying proportions of highly heterogeneous plagioclase megacrysts with complex zoning, particularly well shown by cathodoluminescence (CL). Most often the megacrysts have cores of labradorite-anorthite, partly resorbed and overgrown by andesine rims but some are strongly brecciated and fragments have been annealed by rim growth. Also characteristic are long prisms of apatite, oikocrysts of quartz and K-feldspar and zoned amphibole. The latter has brown pargasite and magnesiohastingsite cores, resorbed and overgrown by magnesiohornblende, compositionally similar to the amphibole in the Sázava tonalite. The brown cores are probably relics of a higher P–T stage, with temperature exceeding 900 °C. Plots of major-element analyses of the Teletín quartz microdiorite and the MME fall close to the line joining the compositions of the evolved Sázava granodiorite-tonalite and the associated gabbros. The estimated minimal proportions of the acidic end-member range from ca. 15% to 20% for the quartz microdiorite and ca. 40% to 60% for the enclaves. Field relationships, macroscopic and mineralogical features and whole-rock geochemistry are consistent with a hybrid origin of the quartz microdiorite masses and enclaves. Unzoned calcic plagioclase cores are interpreted as having crystallized from a basic magma prior to its injection into a high-level acidic magma chamber. The sudden decrease in Ca activity and introduction of water, due to magma-mixing with tonalitic melt, led to their resorption and brecciation. Quartz ocelli that are abundant locally in some basic bodies are most likely xenocrysts introduced from the felsic magma. In the basic melt, they were resorbed and became a substrate for heterogeneous nucleation of the amphibole or pyroxene coronas. Strong undercooling of the basic magma resulted in the growth of long prismatic apatite together with dendritic, boxy cellular plagioclase. Both types of plagioclase cores were subsequently overgrown by more sodic mantles. At the same time, small unzoned plagioclase crystals nucleated in the groundmass. Following thermal re-adjustment, crystallization of the hybrid magma ended with considerably slower growth of quartz and K-feldspar oikocrysts.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Braithwaite, Dr Colin and Bowes, Professor Donald
Authors: Janousek, V., Braithwaite, C.J.R., Bowes, D.R., and Gerdes, A.
College/School:College of Science and Engineering > School of Geographical and Earth Sciences
Journal Name:Lithos
Publisher:Elsevier BV
ISSN (Online):1872-6143

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