Acta Petrologica et Mineralogica

Oceanic plagioganites refers to felsic intrusions which can be generated in different tectonic setting including midocean ridge and subduction zone. Plagiogranite is volumetrically minor component of oceanic crust and ophiolites, but it is crucial to probe the evolution of oceanic lithosphere, subduction initiation and mechanism of oceanic-continental transformation. In this paper, we carried out an integrated study of petrology, geochemistry and zircon U-Pb chronology for the newly identified plagiogranite in the Tianshui area of the western part of the Shangdan suture zone. The whole rock geochemistry indicate that the plagiogranite have high content of SiO₂ and Al₂O₃ and low content of K₂O and TiO₂, and show characteristics of low potassium subalkaline, metaluminous to weak peraluminous. In terms of trace element composition, they are relatively enriched in Sr and depleted in Nb, Ta and Ti, and show a characteristic of REE trends is relatively flat. Generally, it has geochemical affinities to oceanic plagiogranite which formed via partial melting of subducted oceanic crust slabs in shallow level and developed in the forearc area. Zircon U-Pb datings of plagiogranite samples yield age of 526±4 Ma, 515±4 Ma and 517±6 Ma, respectively. Combined with previous studies data, we suggest that the Shangdan suture zone of the West Qinling records similar forearc magmatic rock associations as that of the Izu-Bonin-Mariana (IBM) subduction zone. The boninites, High-Mg andesites and plagiogranites generated almost simultaneously in forearc setting, reflecting the initial subduction of the Shangdan ocean at Early Cambrian. Combining the present study with regional geological data reveals that the northern Central China Orogenic system experienced a simultaneous subduction initiation of Proto-Tethys at Cambrian, similar to the scale in the IBM subduction zone.

West Qinling; Shangdan suture zone; plagiogranite; partial melting; subduction initiation