Several Neoproterozoic granitic intrusions, including the Phin Ngan intrusion and some smaller ones from the Sin Quyen Cu mine, have been identified in the Phan Si Pan belt, Northwest Vietnam. Whole rock geochemical and Sm-Nd isotopic data and zircon U-Pb age and Hf-O isotopes have been determined for these intrusions in order to constrain their ages, petrogenesis and tectonic implications. The Phin Ngan intrusion, mainly composed of syenogranite, was emplaced at 824 +/- 4 Ma. Rocks from this intrusion have high SiO2 (72.1-73.5 wt.%), and K2O (5.14-5.52 wt.%), with A/CNK values ranging from 1.02 to 1.06. They have negative whole-rock epsilon(Nd)(t) (-8.4 to -5.2) and zircon epsilon(Hf)(t) values (-8.1 to -5.5), and high zircon delta O-18 values (9.7-10.9 parts per thousand). These geochemical features suggest that the Phin Ngan intrusion was derived from ancient, K-rich crustal sources. Granitic intrusions from the Sin Quyen mine occur as stocks or dykes intruding ores and ore-hosting rocks. They are composed of monzogranite and granodiorite, emplaced between 736 +/- 8 and 758 +/- 6 Ma. They have high SiO2 (68.2-76.7 wt.%) and (K2O + Na2O) (6.93-8.54 wt.%), and low MgO (0.19-0.87), with A/CNK values ranging from 0.91 to 1.14. Their whole-rock epsilon(Nd)(t) values range from -6.6 to 0, mostly clustered between -6.6 and -4.3. Their zircon epsilon(Hf)(t) values vary from -6.7 to +11.1, mostly lower than -2.0. Such geochemical data indicate that these intrusions were produced mainly through partial melting of ancient crustal materials, coupled with subordinate involvement of mantle-/juvenile crust-derived components. The Phin Ngan intrusion and intrusions from the Sin Quyen mine are sub-alkaline in nature, and have arc-like trace-element compositions. Compiled with other broadly synchronous igneous rocks with arc-like affinities in the region, the Neoproterozoic magmatism in the Phan Si Pan belt was generated in a subduction-related setting. The Neoproterozoic intrusions in the Phan Si Pan belt are geochronologically and geochemically correlated with the Neoproterozoic subduction-related igneous rocks along the western Yangtze Block. Thus, the Phan Si Pan belt represents the southern most part of the giant arc system of the western Yangtze Block. (C) 2017 Elsevier B.V. All rights reserved.