The early continental crust is composed predominantly of Archean tonalite-trondhjemite-granodiorite (TTG); however, the tectonic regime and melting conditions for TTG magmas have been debated. In this study, we report field and microscopic evidence for the partial melting of arc-related metagabbro and its products, including TTG-like melt, volumetrically significant plutons evolved from melt, and the associated granulitic residua during continental collision in the North Qaidam Mountains, China. Migmatite shows successive stages of initial intragranular or droplet-like melt along grain boundaries, which grew into a three-dimensional interconnected intergranular network, segregated, and accumulated in pressure shadow areas, and merged to form melt channels and sheets that finally combined to form a TTG-like tonalite pluton. Pressure-temperature (P-T) calculations indicate high-pressure granulite-facies metamorphism and the crystallization of leucosome at P = 15.5−18.5 kbar and T = 850−950 °C. Based on zircon U-Pb dating and petrological analyses, partial melting and magmatic crystallization occurred 438−430 m.y. ago, which is slightly younger or temporally overlaps with ultrahigh-pressure metamorphism in the region. The metagabbros exhibit a subduction-related arc signature with slightly positive εNd(t) values of 2.1−4.2. The felsic leucosomes and tonalite plutons are characterized by high Na, Sr, Sr/Y, and La/Yb values and low heavy rare earth element values, with εNd(t) values of 0.1−4.3, similar to typical TTGs. The geological context, geochemistry, and timing of the TTG-like melt formation observed in this study differ from the prevailing models; however, our observations and documentations demonstrate that melting of arc-like metagabbro under high-pressure granulite-facies conditions during continental collision may make important contributions to crustal growth and differentiation.
Linking high-pressure mafic granulite, TTG-like (tonalitic- trondhjemitic) leucosome and pluton, and crustal growth during continental collision
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Shengyao Yu, Sanzhong Li, Jianxin Zhang, Deyou Sun, Yinbiao Peng, Yunshuai Li; Linking high-pressure mafic granulite, TTG-like (tonalitic- trondhjemitic) leucosome and pluton, and crustal growth during continental collision. GSA Bulletin doi: https://doi.org/10.1130/B31822.1
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