Zircon Reveals Diverse Trends of Magma Crystallization from Two Types of Early Post-Collisional Diorites (Variscan Orogen, NE Bohemian Massif)

Author:

Pietranik Anna1,Farina Federico2,Derkowska Katarzyna13,Schaltegger Urs4,Przybyło Arkadiusz1,Storey Craig5,Lasalle Stephanie5,Dhuime Bruno6,Pańczyk Magdalena3,Zieliński Grzegorz3,Nowak Małgorzata1,Bulcewicz Kamil1,Kierczak Jakub1

Affiliation:

1. University of Wrocław , Institute of Geological Sciences, Borna 9, 50-204, Wrocław, Poland

2. University of Milano Department of Earth Sciences, , Via Botticelli, 23, 20133 Milano, Italy

3. Polish Geological Institute , Rakowiecka 4, 00-975, Warsaw, Poland

4. University of Geneva Department of Earth Sciencesa, , rue des Maraîchers 13, CH-1205 Genève, Switzerland

5. University of Portsmouth , School of Earth and Environmental Sciences, Burnaby Building, Burnaby Road, PO1 3QL, Portsmouth, UK

6. University of Bristol Deptertment of Earth Sciences, , Wills Memorial Building, Queen’s Road, BS8 1RJ, Bristol, UK

Abstract

Abstract Amphibole- and clinopyroxene-bearing monzodiorites were emplaced at 340 Ma (CA-ID-TIMS zircon age), suggesting the formation of hydrous and dry magmas closely related in space and time in the NE Bohemian Massif. Hafnium and oxygen isotopes of zircon in less evolved rocks (<55 wt% SiO2) are similar between Amp and Cpx monzodiorites (εHf = −3.3 ± 0.5 and − 3.5 ± 0.8; δ18O = 6.4 ± 1.0 and 6.8 ± 0.7, respectively), consistent with a common source—a contaminated mafic magma derived from an enriched mantle. At the same time, the conditions of crystallization are distinct and zircon appears to be an excellent tool for distinguishing between hydrous and anhydrous crystallization conditions, a process that may be more ambiguously recorded by whole rock and major mineral chemistry. In particular, elements fractionated by either amphibole or plagioclase crystallization, such as Hf, Dy, and Eu, differ in zircon from amphibole- and clinopyroxene-bearing rocks, and Zr/Hf, Yb/Dy, and Eu/Dy are therefore useful indices of crystallization conditions. We show that the composition of zircon from hydrous dioritic magmas is not comparable with that of typical zircon from dioritic-granitic suites worldwide, suggesting a specific process involved in their formation. Here, we propose that fluid-present remelting of a mafic underplate is necessary to explain the rock textures as well as the composition of the whole rock, zircon, and other minerals of amphibole-bearing monzodiorites and that a similar process may control the formation of amphibole-rich dioritic rocks worldwide, including appinitic suites. Overall, we show that dioritic rocks represent snapshots of differentiation processes that occur in the early stages of magma evolution before the magma is homogenized into large-scale batholiths.

Publisher

Oxford University Press (OUP)

Subject

Geochemistry and Petrology,Geophysics

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