Zircon Reveals Diverse Trends of Magma Crystallization from Two Types of Early Post-Collisional Diorites (Variscan Orogen, NE Bohemian Massif)
Anna PietranikFederico FarinaKatarzyna DerkowskaUrs SchalteggerArkadiusz PrzybyłoCraig StoreyStéphanie LasalleBruno DhuimeMagdalena PańczykGrzegorz ZielińskiMałgorzata NowakKamil BulcewiczJakub Kierczak
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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.Keywords:
Amphibole
Massif
Fractional crystallization (geology)
Mercury
Disequilibrium
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APG2 is a computer application designed for amphibole-plagioclase geothermobarometry. It is the first updated version of APG and supports 4 thermometer models and 6 barometer models involving either amphibole-plagioclase or amphibole only. APG2 has capability to integrate all 4 thermometer models with 6 barometer models and produce 24 different states which user can export them all at once to an Excel table. APG2 works in both graphical and analytical way. APG2 is also able to calculate the H2O content and Oxygen fugacity (logfO2) of magma hosting amphiboles.
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Geothermobarometry
Thermometer
Fugacity
Maar
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Amphibole
Diopside
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Plagioclase phenocrysts from mafic magmatic enclaves and plagioclase crystals from host granitoids of some plutons of the central part of the Sierra Nevada Batholith are complexly zoned and commonly divided into three neatly distinct parts: an oscillatory, locally patchy zoned, andesine or more calcic core, a ring with dusty calcic plagioclase, and a normally zoned rim of sodic plagioclase. Although aspect of the calcic rings and width and zoning of the rims may slightly vary from the enclaves to the hosts, cores of both phenocrysts and large plagioclase crystals show similar zoning and composition. The andesine or more calcic cores are interpreted to have crystallized in the felsic or the mafic magma, respectively, and been incorporated into the coeval magma when the two magmas mixed. Introduction of the xenocrystic cores into a magma where they were not in equilibrium resulted in partial dissolution, development of abundant patchy zoning, and coating with dusty calcic plagioclase. In both granitoids and mafic magmatic enclaves, composition and zoning contrast between cores and rims of the plagioclase crystals reflect drastic changes in conditions of crystallization before and after the mechanical mixing event. Mixing of two magmas with contrasted compositions is suggested to be the major mechanism for generating complexly zoned plagioclase xenocrysts in granitoids and mafic magmatic enclaves. This hypothesis is consistent with many recent models in which mixing of two contrasting components is proposed to play a fundamental part in the generation of calc‐alkaline granitoids and mafic magmatic enclaves of the Sierra Nevada Batholith. Plagioclase xenocrysts may also provide information on the timing of the different mixing processes and on the magmatic evolution of the plutons.
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Batholith
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Igneous differentiation
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parallel to the macroscopic foliation. In the upper sheet, most of the magmatic amphiboles and plagioclases are recrystallized forming monomineral bands of 0.1 – 1 cm in size. The plagioclase-plagioclase grain boundaries are strongly serrated, while the amphibole-amphibole boundaries are mostly straight and equilibrated. The quantitative microstructural analysis shows in the lower gabbro sheet an important increase in shape preferred orientation (SPO) of amphiboles and slight increase of SPO of plagioclase with increasing deformation. Both minerals achieve higher aspect ratio but they do not exhibit change in grain size distribution with increasing strain intensity. On the contrary, the SPO in the upper gabbro sheet as well as the aspect ratio of amphiboles slightly decrease with increasing deformation, whereas these parameters in plagioclases remain unchanged. Moreover, the grain size of amphibole decreases, while that of plagioclase increases with progressive deformation.The electron backscatter diffraction (EBSD) measurements of crystal preferred orientation (CPO) reveal similar trends for both metagabbro sheets. Amphibole is marked by a relatively strong CPO already at lower deformation intensities, whereas plagioclase displays very weak CPO. With progressive deformation, the CPO of amphibole further strengthens and becomes entirely random for plagioclase. The quantitative microstructural analysis and the EBSD study suggest that the deformation on a microscale changes depending on temperature and degree of deformation. In the lower sheet, the magmatic grains of amphibole firstly rotate to the easy slip direction, which is represented by the (100)[001] glide system oriented parallel to the foliation and lineation. When this orientation is achieved, the dislocation creep on (100)[001] takes place together with activation of (110)[001] weak cleavage planes inducing a strong rock anisotropy at high deformation intensities. Plagioclase recrystallizes mostly by fracturing and nucleation of new grains occurring in the highly strained zones and to limited extent by mechanism of subgrain rotation. At high strains, the deformation mechanism switches to grain boundary diffusion creep, which is a grainsize sensitive process resulting in a random CPO. In the upper sheet, most of the longest axes of magmatic amphiboles are already oriented parallel to the foliation, and the predominant (100)[001] glide is active. Moreover, the dislocation glide is accompanied by chemically induced grain boundary migration, which is manifested by different composition of the new and old grains. On the contrary, the plagioclase recrystallizes by subgrain rotation mechanism. At the later stages, the dominant recrystallization mechanism is grain boundary migration, which is either chemically or strain induced. It is indicated by strongly serrated plagioclase-plagioclase grain boundaries as well as by important differences in the plagioclase compositions. The processes described above result in strong anisotropy of the whole rock.
Amphibole
Lineation
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