Partial melting of lower crust at 10–15 kbar: constraints on adakite and TTG formation
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Amphibole
Adakite
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Abstract Genesis of adakites in arc and nonarc tectonic settings plays an important role in magmatic processes and material recycling along convergent margins. However, little is known about characteristics of pristine adakitic melts due to late stage magma evolutionary processes that dilute or obscure primary melt features, and thus, the genesis of adakites remains controversial. Here we present a detailed analysis of amphibole composition from Early Permian Awulale postcollisional adakitic diorite and granodiorite porphyries in the core of Tianshan Orogen, the central Asian orogenic belt. Two distinct populations of amphiboles, with markedly different aluminum contents, are observed in the adakitic rocks. These are (1) high‐Al amphiboles, crystallizing as an early mineral phase at about 1 GPa, and (2) low‐Al amphiboles, as a late mineral phase at 220–400 MPa, estimated on the basis of experimental phase equilibria data. Trace element modeling indicates that melts in equilibrium with the high‐pressure amphiboles are of adakitic composition, suggesting that the Awulale pristine magmas already had an adakitic nature before the amphibole crystallization. This is consistent with the mafic lower crustal melting model, rather than a high‐pressure basaltic melt fractionation, for adakite petrogenesis. Our results lend new insights into how amphibole composition can be used to constrain the geochemical characteristics of pristine adakitic magmas.
Amphibole
Adakite
Petrogenesis
Fractional crystallization (geology)
Diorite
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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.
Amphibole
Geothermobarometry
Thermometer
Fugacity
Maar
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Thermometer
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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.
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