Crystal mush rejuvenation triggered by gas sparging of injected mafic magma: Insights from Arxan Igneous Complex in the Great Xing’an Range, northeastern China

Abstract Crystal mush rejuvenation triggered by mafic magma injection is widely acknowledged, but the rejuvenation mechanisms are still not fully understood. We present an integrated petrological, mineralogical, and geochemical study of the Late Paleozoic Arxan Igneous Complex (311–310 Ma) in the Great Xing’an Range, northeastern China, which comprises syenogranite, porphyritic syenogranite, alkali-feldspar granite, monzogranite, dioritic enclaves, and hornblende gabbro. Zircons from the dioritic enclaves can be divided into dark-CL and light-CL groups with distinct Hf isotopic compositions. The dark-CL zircons have lower εHf(t) values (+3.98 to +7.73), similar to those from the alkali-feldspar granite, syenogranite, and porphyritic syenogranite (+4.45 to +11.55). In contrast, the light-CL zircons display higher εHf(t) values (+7.99 to +11.25), comparable to those of the gabbro (+7.29 to +14.52). The monzogranite shows a similarly wide εHf(t) range (+4.90 to +10.30). Reversely zoned plagioclases with sieved and dusty textures occur in both the dioritic enclaves and monzogranite and can be divided into high-An and low-An domains. In-situ Sr isotope analyses reveal that high-An domains have lower (87Sr/86Sr)i ratios (0.7037–0.7043), approaching those of the gabbro (0.7035–0.7048), whereas low-An domains have higher (87Sr/86Sr)i ratios (0.7048–0.7051). These mineralogical and isotopic features indicate that the dioritic enclaves and monzogranite formed through mafic–felsic magma mingling/mixing. The syenogranite, porphyritic syenogranite, and alkali-feldspar granite display relatively uniform Sr–Nd–Hf isotopic compositions, whereas their zircon δ18O values show clear differences. In the Arxan Igneous Complex, zircon δ18O values decrease from the syenogranite (9.57–10.33‰), porphyritic syenogranite (8.63–9.12‰), alkali-feldspar granite (7.99–9.25‰), monzogranite (7.85–8.89‰), and dioritic enclaves (7.28–7.75‰) to the gabbro (6.33–7.00‰). Our findings show that the main body syenogranite, with the highest zircon δ18O values, represents the solidified, unrejuvenated silicic mush, whereas the gabbro, with the lowest zircon δ18O values, represents the mafic recharge magma that triggered two key processes: proximal magma mingling/mixing and distal gas sparging. The injection of hydrous mafic magma induces magma mingling/mixing at the contact front between the mush and the mafic recharge magma, resulting in the formation of dioritic enclaves and monzogranite. This process is evidenced by Sr–Nd–Hf–O isotopic characteristics and disequilibrium between plagioclase compositions and whole-rock geochemistry in these rocks. Simultaneously, the upward percolation of low-δ18O volatiles from the hydrous gabbroic magma rejuvenates the near-solidus silicic mush, which accounts for the lower zircon δ18O values and elevated water contents in alkali-feldspar granite and porphyritic syenogranite compared to syenogranite, and the growth of K-feldspar megacrysts in porphyritic syenogranite. Meanwhile, gas sparging enhances the melt fraction and internal pressure of the silicic mush, resulting in the complementary geochemical compositions between alkali-feldspar granite and porphyritic syenogranite through interstitial melt extraction. These findings provide new insights into the roles of magma mingling/mixing and gas sparging, induced by mafic recharge magma, in mush rejuvenation and crystal–melt separation.