Genesis of pristine adakitic magmas by lower crustal melting: A perspective from amphibole composition

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 1GPa, and (2) low-Al amphiboles, as a late mineral phase at 220-400MPa, 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. Plain Language Summary Genesis of adakites in arc and nonarc tectonic settings plays an important role in magmatic processes and material recycling along convergent margins. In addition to the original slab melting scenario, petrogenesis of adakites remains highly controversial with two principal competing models. One involves melting of thickened mafic lower crust. The other involves high-pressure (e.g., similar to 1.0-1.5GPa) crystal fractionation of normal basaltic arc magmas. Here we present a detailed analysis of amphibole composition from postcollisional adakites in the core of Tianshan Orogen. Two distinct populations of amphibolesare 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 220-400MPa, 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 from amphibole compositions that can be used to constrain the geochemical characteristics of pristine adakitic magmas.