LREE distribution patterns in zoned alkali feldspar megacrysts from the Karkonosze pluton, Bohemian Massif - implications for parental magma composition

Abstract

The elemental compositions of zoned alkali feldspar megacrysts from the Karkonosze pluton have been analysed and Pb isotope ratios determined using LA-ICP-MS, EMPA and TIMS. The results are used to interpret the magmatic environments within which they crystallized. Growth zones in the megacrysts show fluctuating trace element patterns that reflect a systematic relationship between incompatible LREE and compatible Ba. Chemical gradients between zones in the cores and rims of the megacrysts are not accompanied by significant variation in initial Pb isotope composition. The nucleation and crystallization of the megacrysts is interpreted as having occurred in an environment of magmatic hybridization caused by mixing of mantle and crustal components in which effective homogenization of the Pb isotope composition preceded the onset of megacryst growth. The concentrations of LREE in alkali feldspar zones were used to reconstruct hypothetical melt compositions. Some of the zones appear to have crystallized in an homogenous magmatic environment having clear geochemical affinities with end-member magmas in the Karkonosze pluton, whereas others crystallized in heterogeneous domains of magma hybridization. With the exception of Nd, zones crystallized in more homogeneous magma show LREE fractionation under near-equilibrium conditions. Trace element abundances of megacrysts grown in dynamic, homogeneous magmatic environments of the Karkonosze pluton occasionally deviate from the predicted patterns and show LREE impoverishment.

Chaotic three-dimensional distribution of Ba, Rb, and Sr in feldspar megacrysts grown in an open magmatic system

“…The Pb isotope signatures of the megacrysts are consistent with mixing between a mantle and a crustal endmember (Słaby et al 2007b). Using the Kober and Lippolt (1985) model for crust-mantle Pb isotope evolution, the amount of mantle-derived Pb in the feldspar composition can be estimated as 20-40% (Słaby et al 2007b).…”

“…If growth was driven by kinetically controlled processes, the uptake of compatible elements would have been decoupled from that of the incompatible elements (Morgan and London 2003). Some of the zones appear to have crystallized in a homogenous magmatic environment having clear geochemical affinities with the end-member magmas in the Karkonosze pluton (Słaby et al 2007b), whereas others crystallized in heterogeneous domains of hybridized magma. Geochemical modeling of megacryst growth revealed that zones that can be related to almost pure mafic end-member magma are rare and their width extremely narrow (Słaby et al 2007a).…”

“…However the relationship between compatible and incompatible elements argues against any widespread recrystallization process following dissolution. Even if zones of both types show fluctuating trace-element patterns, these patterns are characterized, as shown by Słaby et al (2007bSłaby et al ( , 2008a, by systematic and coupled relationships between incompatible (LREE, Cs) and compatible elements (Ba, Sr, Rb). Such coupling indicates near equilibrium growth conditions.…”

“…Using the Kober and Lippolt (1985) model for crust-mantle Pb isotope evolution, the amount of mantle-derived Pb in the feldspar composition can be estimated as 20-40% (Słaby et al 2007b). …”

Digital Concentration-Distribution Models – tools for a describing heterogeneity of the hybridized magmatic mass as reflected in elemental concentration of growing crystal

“…However, this method is of lower sensitivity and is usable only for an approximate assessment of crystal composition heterogeneity. The depiction of the crystal geochemical data obtained from measurements along a traverse consists of a Cartesian diagram that includes two variables: location of the measurement points along the traverse and the corresponding element concentration (Gagnevin et al 2005a;Ginibre et al 2002;Ginibre et al 2004;Słaby et al 2007a;Słaby et al 2007b). This methodology exhibits, only to a certain extent, the complexity of crystallisation process in a heterogeneous magmatic mass created by contamination, magma mixing and crystallization.…”