Paleoproterozoic (ca. 2.1–1.9 Ga) dioritic-granitic magmatism in the Dunhuang Terrane, Northwestern China: Implications for petrogenesis and tectonic evolution

The implications of petrogenesis and tectonics of Paleoproterozoic (ca. 2.1-1.9 Ga) granitoid rocks in Dunhuang Terrane have hitherto received less attention. According to data from zircon U-Pb dating, the ages of dioritic gneiss, granodioritic gneiss and potassic granite are 2045 +/- 44 Ma, 1998 +/- 15 Ma and 1950 +/- 38 Ma, respectively. The dioritic gneiss has MgO = 1.87-2.93 wt% (Mg# = 46.8-50.9), Yb <= 1.9 ppm and Y <= 18 ppm, with variable epsilon(Hf)(t) (-2.8 - +5.5) and epsilon(Nd)(t) (-5.6 - +1.3) values, indicating that the mantle wedge source above the subducted slab contributed with subduction fluids and ocean sediment melt in an arc setting. The granodioritic gneiss has relatively high MgO (3.00-1.83 wt%), variable Cr (13.5-106 ppm) and Ni (12.2-72.9 ppm), negative epsilon(Hf)(t) (-20.4 to 10.1) and epsilon(Nd)(t) (-15.2 to 12.8) values, and high P2O5 and TiO2 saturation temperatures (similar to 800 degrees C). These data indicate magma source of late Paleo- to early Mesoarchean (similar to 3.2-3.1 Ga) continental crust rocks. The potassic granite has high K2O (5.52-6.79 wt%) and low MgO (0.10-0.19 wt%) concentrations, low Mg# (37.7-44.6) values, and negative epsilon(Hf)(t) (-12.8 to 2.5) and epsilon(Nd)(t) (-10.6 to 4.8) values. These results suggest that pre-existing Neoarchean crustal rocks were partially melted possibly under transitional stage from syn- to post-collision. Compared with the adjacent areas, Paleoproterozoic (ca. 2.1-1.8 Ga) evolutionary period related to assembly of Columbia supercontinent in Dunhuang Terrane is comparable with that in northeastern Tarim Craton and southwestern Alxa Block, but terminated similar to 100 Ma later than that in southeastern Tarim Craton (Aketashtage Terrane).