Processes and Sources during Late Variscan Dioritic–Tonalitic Magmatism: Insights from Plagioclase Chemistry (Gęsiniec Intrusion, NE Bohemian Massif, Poland)
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Abstract:
The Gęsiniec 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.Keywords:
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Consistent core-to-rim decreases of 87Sr/86Sr ratios and coincident increases in Sr concentrations in plagioclase phenocrysts of varying size (∼1 cm to 2 mm) are reported from samples of the 1982 and pre-1982 (∼200 ka) eruptions of El Chichón Volcano. Maximum 87Sr/86Sr ratios of ∼0·7054, significantly higher than the whole-rock isotopic ratios (∼0·7040–0·7045), are found in the cores of plagioclase phenocrysts, and minimum 87Sr/86Sr ratios of ∼0·7039 are found near some of the rims. Plagioclase phenocrysts commonly display abrupt fluctuations in An content (up to 25 mol %) that correspond to well-developed dissolution surfaces. The isotopic, textural and compositional characteristics suggest that these plagioclase phenocrysts grew in a system that was periodically recharged by higher-temperature magma with a lower 87Sr/86Sr ratio and a higher Sr concentration. Rim 87Sr/86Sr ratios in plagioclase phenocrysts of rocks from the 200 ka eruption indicate that, at that time, the magma had already attained the lowest recorded 87Sr/86Sr value of the system (∼0·7039). In contrast, cores from plagioclase phenocrysts of the 1982 eruption, inferred to have grown in the past few thousand years, have the highest recorded 87Sr/86Sr ratios of the system. Collectively, the Sr isotopic data (for plagioclase and whole rock), disequilibrium textural features of the phenocrysts, known eruption frequencies, and inferred crystal-residence times of the plagioclases are best interpreted in terms of an intermittent magma chamber model. Similar processes, including crustal contamination, magma mixing, periodic recharge by addition of more mafic magma to induce plagioclase disequilibrium (possibly triggering eruption) and subsequent re-equilibration, apparently were operative throughout the 200 ky history of the El Chichón magma system.
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Various petrographic features and geochemical characteristics indicative of disequilibrium are preserved in plagioclase phenocrysts from basaltic to andesitic lavas in East Junggar, northwest China. These characteristics indicate that they crystallized in a magma chamber, which was replenished by less differentiated and high‐temperature magmas. The petrographic and geochemical features of the plagioclase phenocrysts are interpreted to record responses to changes in temperature, composition and mechanical effect during magma replenishment. Distinct rare earth element (REE) patterns between cores and rims of the same plagioclase crystal suggest derivation from two end‐member magmas. From core to rim, plagioclase phenocrysts commonly display sharp fluctuations of anorthite (An) content up to 20, which either correspond to reverse zoning associated with ovoidal cores and resorption surface (P1), or normal zoning with euhedral form and no resorption surface (P2). Plagioclase crystals with diverse textures and remarkably different An content coexist on the scale of a thin‐section. Cores of these plagioclases in each sample display a bimodal distribution of An content. From core to rim in P1, concentrations of FeO T and Sr increase remarkably as An content increases. During magma replenishment, pre‐existing plagioclase phenocrysts in the andesitic magma, which were immersed into hotter and less differentiated magmas, were heated and resorbed to form ovoidal cores, and then were overgrown by a thin rim with much higher contents of An, FeO T and Sr. However, pre‐existing plagioclase phenocrysts in the basaltic magma were injected into cooler and more evolved magmas, and were remained as euhedral cores, which were later enclosed by oscillatory zoned rims with much lower contents of An, Sr and Ba.
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Magmatic origin of large (up to 5 cm) phenocrysts of K-feldspar is indicated by mineralogical, chemical, and roentgenometric-structural analysis of the crystals. Perthites are found to be metasomatic products of replacement of plagioclase by K-feldspar. This reaction is accompanied by albitization of plagioclase and then by growth of albite after feldspar, in cleavage planes and elsewhere, or by spot-segregations of albite in places formerly occupied by plagioclase. — V.P. Sokoloff
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Summary The feldspar phenocrysts of some British and Icelandic Tertiary acid glasses have been studied optically and chemically. Six new analyses of the feldspar phenocrysts are presented together with an anorthoclase from a glassy pantellerite. The analysed feldspars and all the other feldspar phenocrysts that have been examined show high-temperature optics, and confirm the results of previous investigations of plagioclase feldspar determinative curves. The relationship of the feldspar phenoerysts to the plagioclase components in the analysed residual glasses conforms to the synthetic plagioelase system, and the crystallization of feldspar in natural acid melts is considered.
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The dacite of the Pebble Creek Formation (Mount Meager, BC, Canada) is an extraordinary occurrence of lavas containing coarse, sieve-textured plagioclase phenocrysts that appear to have reacted extensively with the melt. We record this unusual occurrence of a naturally reacted crystal/melt pair utilizing Nomarski Differential Interference Contrast (NDIC) imaging and electron microprobe data. Hieroglyphic textures are common in plagioclase of this study, and appear to represent older plagioclase remnants in sharp contact with new plagioclase forming a palimpsest texture outlining the occurrence of previous pools of melt. Throughout the processes affecting the phenocrysts, capillaries of liquid remained open to the external melt.
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