Processes and sources during late variscan dioritic-tonalitic magmatism: Insights from plagioclase chemistry (Gesiniec Intrusion, NE Bohemian Massif, Poland)

Abstract

The Gesiniec Intrusion (Strzelin Massif, East Sudetes) (∼307-290 Ma) is composed predominantly of dioritic to tonalitic rocks with 87Sr/86Sr ratios ranging from 0.7069 to 0.7080 and εNd = -3.1 to -4.2, emplaced as post-collisional magmas following the Variscan orogeny. In situ Sr isotope and trace element analyses of plagioclase were carried out on five diorite-tonalite samples with variable whole-rock 87Sr/86Sr compositions to constrain magma sources and elucidate magma chamber processes. Plagioclase is characterized by complex zoning patterns, such as patchy zoning, asymmetrical zoning and strong resorption between An-rich cores and more albitic rims. The range of Sr isotopic compositions recorded in plagioclase is 0.7069-0.7091, greater than that of the whole-rocks. No change in isotopic composition is observed across resorbed core-rim boundaries, as would be expected if the resorption was caused by magma mixing. Two samples are close to Sr isotopic equilibrium between plagioclase and the whole-rock. Three samples are isotopically heterogeneous and exhibit the largest variations in Sr isotope ratios within high-An plagioclase cores, associated with only slight resorption and almost no change in An content. Consequently, we interpret the core resorption to be due to decompression during emplacement of phenocryst-bearing magmas in the upper crust rather than to magma mixing, whereas we interpret the isotopic heterogeneity in the high-An plagioclase cores to be due to open-system processes in the lower crust. The trace element and Sr isotopic compositions of the high-An plagioclase cores provide an insight into the lower crustal processes that took place prior to decompression. We show that different diorite-tonalite types cannot be related to each other by assimilation-fractional crystallization processes and consequently they probably evolved as separate magma batches. Processes recorded in plagioclase core compositions include contamination by high 87Sr/86Sr crustal material and interaction with water-rich magma. The studied samples represent magmas that probably formed either by continuous differentiation and crustal contamination of originally more mafic magmas or by remelting of ∼ 1.3-2.1 Ga basaltic materials in the lower crust. This study emphasizes the importance of integrating textural and in situ data to elucidate the processes that contribute to the formation of texturally and compositionally complex plagioclase crystals in plutonic rocks. © The Author 2008. Published by Oxford University Press. All rights reserved.