LREE distribution patterns in zoned alkali feldspar megacrysts from the Karkonosze pluton, Bohemian Massif - implications for parental magma composition
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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.Keywords:
Alkali feldspar
Fractional crystallization (geology)
Igneous differentiation
Massif
Trace element
Incompatible element
Abstract Quaternary calc-alkaline andesitic to dacitic lavas effusively erupted on top of about 30 km thick accreted continental crust at Methana peninsula in the western Aegean arc. We present new data of major and trace element concentrations as well as of Sr–Nd–Pb isotope ratios along with mineral compositions of Methana lavas and their mafic enclaves. The enclaves imply a parental basaltic magma and fractional crystallization processes with relatively little crustal assimilation in the deep part of the Methana magma system. The composition of amphibole in some mafic enclaves and lavas indicates deeper crystallization at ∼25 km depth close to the Moho compared with the evolved lavas that formed at <15 km depth. The presence of amphibole and low Ca contents in olivine suggest high water contents of ∼4 wt% in the primitive magmas at Methana. The compositions of andesitic and dacitic lavas reflect fractional crystallization, assimilation of sedimentary material, and magma mixing in the upper 15 km of the crust. The Methana magmas have fO2 of FMQ + 1 to FMQ + 2 (where FMQ is the fayalite–magnetite–quartz buffer) at temperatures of 1200 to 750 °C and the fO2 does not vary systematically from mafic to felsic compositions, suggesting that the mantle wedge was oxidized by sediment subduction. Amphibole is an important fractionating phase in the more evolved Methana magmas and causes significant changes in incompatible element ratios. Although xenocrysts and mineral compositions indicate magma mixing, the major and trace element variation implies only limited mixing between dacitic and basaltic melts.
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Semisopochnoi has erupted a tholeiitic suite of moderately evolved lavas (), whose bulk-rock chemistry and phenocryst mineralogy are generally compatible with an important role for fractional crystallization in controlling the overall evolution of the suite. Trace-element data, however, are not consistent with crystal fractionation operating alone. Instead, a variety of plots involving incompatible elements (or ratios among them) demonstrate that simple mixing between basaltic and silicic components can reproduce the essential aspects of the trace-element variations and must have been operative as well. We are unable to distinguish between a silicic end-member represented by residual dacitic liquid like one of the analyzed lavas (therefore implying magma mixing) and one consisting of pre-existing silicic crustal rocks (therefore implying assimilation). Variations in Sr and Nd isotopic compositions are very limited: and . With the exception of the Sr composition of one sample, the isotopic data are also compatible with a combined mixing-fractionation model. Coupled variations of and chondrite-normalized Ba/La ratios imply derivation of parental magmas from a source that has been enriched in incompatible elements, relative to MORB-sediment mixtures, perhaps by dehydration of the subducted slab and resulting metasomatism of the overlying mantle.
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