Crustal growth and evolution in convergent margin: Evidence from three Paleozoic granitic pulses in the junction zone between Qinling and Qilian orogenic belt

Abstract

Paleozoic igneous rocks in the junction zone between the western Qinling and Qilian orogenic belts preserve important message about the deep crustal structure and their tectonic relationship. By means of zircon U Pb dating, Lu Hf isotope, bulk-rock geochemistry for a suite of Paleozoic granites from the western Qinling and Qilian orogenic belt, three main granite pulses were identified: 1) First pulse (~500 to 450 Ma) quartz diorite with typical island-arc affinity, i.e., intermediate SiO 2 (56.42 to 61.44 wt%) contents, variable Na 2 O/K 2 O ratios of 0.9 to 1.7, high MgO contents (3.14 to 4.40 wt%) with high Mg# values of 55.6 to 60, moderate Sr/Y (22 to 29) ratios and insignificant Eu*/Eu anomalies (0.77 to 0.84), zircons from the quartz diorite have positive ε Hf (t) values of +2.2 to +5.5; all these features indicate that the quartz diorite was formed by hydrous melting of depleted mantle wedge in subduction setting; 2) Second pulse (ca. 440 to 430 Ma), monzogranite (e.g., the Dangchuan and Caochuanpu monzogranite), they display higher SiO 2 (71.23 to 73.91 wt%) contents, K 2 O/Na 2 O (1.10 to 2.26) ratios, lower MgO (0.34 to 0.43 wt%) contents, significant negative Eu anomalies of 0.36 to 0.48, evolved Sr Nd isotopic compositions, and variable zircon Lu Hf (ε Hf (t) = −5.0 to +10.6) compositions, suggesting dehydration melting of heterogeneous crust; 3) Third pulse (ca. 420 to 410 Ma), biotite granite that display typical A-type affinity, i.e., high FeO T /MgO and Ga/Al (10,000 × Ga/Al = 2.7 to 3.0) ratios, high Zr (230 to 313 ppm), Nb (3.49 to 34.6 ppm) and Zr + Nb + Ce + Yb (351 to 512 ppm) contents, most zircons from these biotite granites have negative ε Hf (t) values of −2.7 to −26.6, in combination with their higher Zr saturation temperatures (822 to 842 °C), suggesting high temperature melting of matured continental crust in extensional setting. These results indicate that the western Qinling and Qilian orogenic belts experience a tectonic switch from subduction to extension in Early-Paleozoic (ca. 460 to 410 Ma). All these granites record the crustal evolution and switch of melting regime in convergent margin. • Three Paleozoic magmatic pulses in the junction zone between North Qinling and Qilian. • Hydrous melting of depleted mantle wedge formed the first pulse. • Dehydration melting of juvenile mafic crust formed the second pulse. • High temperature melting of heterogeneous crust in extensional setting formed the third pulse.