Archean to paleoproterozoic crustal evolution in the Phan Si Pan zone, Northwest Vietnam: evidence from the U-Pb geochronology and Sr-Nd-Hf isotopic geochemistry

Abstract

ABSTRACT We studied zircon U-Pb geochronology, Sr-Nd-Hf isotopes, and whole-rock geochemistry of Mesoarchean-Paleoproterozoic igneous rocks in the Phan Si Pan zone, NW Vietnam. The Mesoarchean-Paleoproterozoic magmatism in the area can be divided into three phases as represented by ca. 2.9 Ga Mesoarchean tonalite-trondhjemite-granodiorites (TTGs), ca. 2.3 Ga Paleoproterozoic mafic dykes, and ca. 1.8 Ga Paleoproterozoic granites. The ca. 2.9 Ga Mesoarchean TTGs serve as part of the oldest basement rocks in SE Asia, which have zircon εHf(t) and whole-rock εNd(t) values ranging from −2.8 to +2.1. Their ancient zircon Hf and whole-rock Nd isotopic model ages (TDM2) of 3.4 Ga to 3.2 Ga suggest an origin from partial melting of ancient crustal rocks. The ca. 2.3 Ga mafic dykes were affected by post-tectonic thermal events at ca. 1.9–1.8 Ga. Their zircon εHf(t) (+1.4 to +5.9) and whole-rock εNd(t) (−4.1 to −1.6) and 87Sr/86Sr ratio (0.703459 to 0.719079) isotopic characteristics show that they originated from partial melting of mantle components with a minor input of Archaean continental crust. The Paleoproterozoic granites in the Phan Si Pan zone crystallized at 1803–1864 Ma, and show negative εHf(t) values of −17.9 to −14.8 and zircon Hf model ages (TDM2) ranging from 3.48 to 3.66 Ga, suggesting a derivation by partial melting of Paleoarchean continental crust during Paleoproterozoic time. These isotopic results shed light on the Precambrian tectonic evolution of NW Vietnam and its relationship with continental blocks in Yangtze and India.