Silurian-devonian adakites and granitic pegmatitic dikes in North Qinling, China: Multi-stage magmatism and implications for the tectonic evolution

Abstract

The petrogenesis and tectonic setting of early Paleozoic adakites and granitic pegmatites in the North Qinling orogenic belt are highly debated issues. Zircon UPb dating revealed that the Huichizi pluton is composed of the Early Silurian granodiorites, Late Silurian granodiorites, and Early Devonian granitic pegmatite dikes. The average εHf(t) value of pegmatite dikes was −2.8, which is lower than those of the Early Silurian (4.8) and Late Silurian granodiorites (5.1). It is worth noting that the Early Silurian and Late Silurian granodiorites exhibited adakitic characteristics and were formed through the partial melting of a thickened juvenile mafic lower crust. In addition, the Early Devonian pegmatite dikes displayed a substantial fractional crystallization. They were derived from the partial melting of mixed sources, including the crustal and mantle materials. The early Paleozoic adakitic rocks in the North Qinling orogenic belt can be categorized into four groups according to their c geochemical characteristics. The adakitic rocks of the first group (496–458 Ma) were rich in Na and exhibited extremely high εHf(t) values. They were formed by the partial melting of the Erlangping back-arc oceanic crust. The adakitic rocks of the second group (454–420 Ma) exhibited enriched Na or K contents and varying εHf(t) values. They were derived from the partial melting of a thickened lower crust with various mantle components, underplated by mafic magma formed during the subduction of the Shangdan Ocean. The adakites of the third (420–410 Ma) and fourth (410–400 Ma) groups were formed by the partial melting of a thickened lower crust under collisional and post-collisional settings, respectively.