Multiple mantle metasomatism recorded by Triassic post-collisional ultrapotassic and tholeiitic magmatism in West Qinling, NW China

Mantle properties and metasomatic processes, together with interlayer interactions, in orogenic belts are still under debate. Post-collisional magmatic rocks could provide secondhand constraints on these issues. We have conducted an integrated geochronological, geochemical and isotopic study on post-collisional Zhanwa dolerites, the Dashui granodiorite–monzonite–monzogabbro complex and the Jiuzigou pyroxenite–syenite complex at West Qinling. The Zhanwa dolerites, being tholeiitic with flat trace element patterns, were mixtures of melts from asthenospheric and oceanic slab-fluid-metasomatized lithospheric mantle. The Dashui monzonite–monzogabbro suite is ultrapotassic with strongly right-inclined trace element patterns and evolved isotopic composition with ( 87 Sr/ 86 Sr) i = 0.7066–0.7068, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>ε</mml:mi> </mml:math> Nd ( t ) = −4.56 to −4.35, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>ε</mml:mi> </mml:math> Hf ( t ) = −3.14 to −2.35 and ( 206 Pb/ 204 Pb) i = 17.99–18.12. It was derived from melting of a mantle source metasomatized by continent-crustal materials. In contrast, the Dashui granodiorites are shoshonitic–calc-alkaline with more evolved isotopic compositions with ( 87 Sr/ 86 Sr) i , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>ε</mml:mi> </mml:math> Nd ( t ), <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>ε</mml:mi> </mml:math> Hf ( t ), and ( 206 Pb/ 204 Pb) i of 0.7074–0.7091, −7.56 to −5.51, −5.61 to −3.53 and 18.18–18.46, respectively. They represent hybrids of mantle-derived K-rich and crustal-derived felsic melts. The Jiuzigou complex has extremely high trace element contents. Its mantle source was metasomatized by both oceanic crustal fluids and sedimentary melts. Generation of these rocks ( c. 235–223 Ma) not only recorded multiple mantle metasomatism, but also captured collective asthenospheric and lithospheric magmatic responses in a post-collisional setting.