Post-collisional tectonic evolution of the north branch of the Proto-Tethys Ocean: evidence from Early Devonian I- and A-type granite at the eastern end of the North Qilian Orogenic Belt

The Early Paleozoic magmatic rocks in the North Qilian Orogenic Belt provide crucial insights into the deep crustal structure and tectonic evolution of the Proto-Tethys Ocean. This study presents geochronological, geochemical and Lu–Hf isotopic data for three representative plutons. The Pandaoshi (415 Ma) and Nantouhe (414 Ma) plutons are I-type granite with high SiO 2 contents (71.65–74.21 wt%, 73.09–74.24 wt%) and Mg # values (38.2–44.6, 42.3–51.3), and negative Eu anomalies (Eu/Eu* = 0.54–0.71, 0.55–0.71). The Pandaoshi pluton has evolved <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>ε</mml:mi> </mml:math> Hf ( t ) values (−6.8 to + 8.9), indicating that it formed from partial melting of ancient crust mixed with mantle-derived material. The Nantouhe pluton has positive <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>ε</mml:mi> </mml:math> Hf ( t ) values (+4.9 to +8.9), suggesting that it formed through partial melting of juvenile crustal material with addition of mantle-derived materials. The Guanshanhe pluton (411–412 Ma) is A 2 -type granite with high SiO 2 contents (72.33–75.22 wt%) and Mg # values (33.8–45.6). It exhibits negative <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>ε</mml:mi> </mml:math> Hf ( t ) values (−25.6 to −20.2), indicating that it formed from partial melting of the ancient crust. The three plutons were formed in a post-collisional orogenic phase. We propose the subduction of the Wushan–Shangdan Ocean, forming the Hongtubao back-arc limited ocean basin, and collision between the North Qinling Orogenic Belt and the North Qilian terrane owing to the closure of the Hongtubao Ocean.