The ~1.97 Ga dioritic block in the Hong’an Terrane, central China: syn-collisional alkaline magmatism at the northern margin of the Yangtze Block

Abstract

It is uncertain whether the Yangtze Block was part of the Paleo- to Mesoproterozoic Columbia supercontinent and, if so, where it was located within the supercontinent. Our analysis of a dioritic intrusion near Tiepu village in the southern Hong’an Terrane of central China provides new constraints on the Paleoproterozoic tectonic evolution of the Yangtze Block. Three representative samples from the Tiepu diorite yield zircon U–Pb ages within analytical error of ~1970 Ma. The diorites have narrow ranges of SiO2 (53.5–57.7 wt%) and MgO (2.97–4.48 wt%). High contents of alkalis (K2O + Na2O = 6.70–10.2 wt%) and total rare-earth elements (∑REE = 570–803 ppm) suggest an affinity to intermediate alkaline rock. These rocks are generally characterized by arc-like trace-element patterns such as enrichment in large-ion lithophile elements and light REEs, and depletion in high-field-strength elements and heavy REEs. Zircons from the diorites display negative εHf(t) values (−12.5 to −3.1), and whole-rock samples show negative εNd(t) values (−6.53 to − 4.08). These isotopic characteristics are consistent with older crustal sources. Combining the new data and previous results from Paleoproterozoic rocks in the northern Yangtze Block, we propose a tectonic transition from pre- to post-collisional settings at ~2.0–1.95 Ga. The Tiepu alkaline diorite is interpreted as a syn-collisional magmatic intrusion that formed shortly after the cessation of oceanic plate subduction at ~2.0 Ga. The Tiepu alkaline diorite thus records the final tectonic amalgamation of the eastern and western Yangtze Block microcontinents during the assembly of Columbia.