Magmatic-hydrothermal processes and porphyry Cu fertility of the Tieshan Complex in Fujian Province, southeastern China: Evidence from zircon and garnet U-Pb geochronology and geochemistry

The Tieshan Complex, located within the coastal volcanic rocks belt of southeastern China, had experienced pervasive hydrothermal alteration and hosts widespread Cu-Mo-Au-Pb-Zn polymetallic mineralization, indicating significant potential for porphyry Cu mineralization. New zircon and garnet U-Pb geochronology, integrated with published age data from the Tieshan Complex and surrounding plutons, mainly reveals three discrete episodes of magmatic-hydrothermal activity. The Tieshan Complex initially underwent a prolonged magmatic episode at ca. 254–236 Ma over a period of ~18 m.y., accompanied by skarn-forming hydrothermal mineralization at ca. 243–235 Ma. This was followed by two subsequent magmatic pulses at ca. 174–153 Ma and ca. 118–95 Ma, which were coeval with hydrothermal mineralization events at ca. 176 Ma and ca. 104–95 Ma, respectively. Zircons from Middle Jurassic magmatic rocks exhibit elevated Eu/Eu* values (0.24–0.37), 10000×(EuN/Eu*)/Y ratios (2.82–6.73), (Ce/Nd)/Y ratios (0.020–0.029), ZCFI values (3.19–4.68), and ΔFMQ values (0.12–0.69), together with low Dy/Yb ratios (0.23–0.24). In contrast, zircons from mid-Cretaceous magmatic rocks display higher Eu/Eu* values (0.27–0.58), 10000×(EuN/Eu*)/Y ratios (2.77–9.13), (Ce/Nd)/Y ratios (0.006–0.025), ZCFI values (4.57–7.79), and ΔFMQ values (0.39–2.20), along with low Dy/Yb ratios (0.27–0.35). These geochemical signatures suggest that, relative to Middle Jurassic magmas, mid-Cretaceous magmas are more oxidized and water-rich, conditions favorable for the formation of porphyry Cu deposit. Collectively, the three episodes of magmatism and associated hydrothermal mineralization are consistent with Mesozoic regional metallogenic periods in the southeastern China and occurred during tectonic transitions from crustal compression to extension, which promoted the generation and upward migration of fertile magmas at deep crustal levels conducive to porphyry Cu formation.