Rutile petrochronology and titanium isotope compositions record multiple melt-fluid-rock interactions in a continental subduction zone

Subduction zones act as pivotal engines for global element cycling. Titanium (Ti) isotopes, exhibiting mass-dependent fractionation, emerge as a robust geochemical tracer for deciphering complex processes in these dynamic settings. However, the behavior of Ti and its isotopes during metamorphic dehydration and partial melting of deeply subducted continental lithosphere remains poorly constrained. This study addresses this issue through an integrated investigation of age, elemental signatures, and Ti isotopic compositions of rutile from quartz-bearing and granitic felsic veins, eclogites, paragneisses, and orthogneisses within the North Qaidam orogen—a paleo-continental subduction zone where eclogite boudins are embedded in gneissic matrices. Petrochronological U-Pb dating reveals distinct temporal records: rutile in eclogites and gneisses yields metamorphic ages of 439 ± 3 Ma to 430 ± 11 Ma, aligning with regional eclogite-facies metamorphism and implicating rutile growth during dehydration of subducted continental crust. In contrast, rutile from quartz- and felsic veins documents protracted melt/fluid activities spanning 433 ± 3 Ma to 408 ± 1 Ma, reflecting melt/fluid generation under eclogite-facies conditions and subsequent exhumation. High-resolution Ti isotope and trace element analyses demonstrate minimal intra-sample isotopic variability among rutile grains within individual eclogites or gneisses, suggesting negligible Ti isotope fractionation during metamorphic dehydration. Similarly, rutile in eclogite-hosted felsic veins exhibits δ⁴⁹Ti values indistinguishable from their host eclogites or adjacent gneisses, further negating significant isotopic fractionation during partial melting. However, inter-sample δ⁴⁹Ti variations correlate systematically with whole-rock geochemical proxies: negative correlations with εNd(t) and positive correlations with (⁸⁷Sr/⁸⁶Sr)i highlight protolith heterogeneity as the dominant control on Ti isotopic signatures. These findings collectively demonstrate that Ti isotopic compositions in deep subduction-related systems primarily inherit protolith characteristics rather than reflecting process-driven fractionation. Consequently, Ti isotopes serve as powerful tracers for identifying melt/fluid sources in subduction zones. Notably, cold subduction regimes promote localized Ti mobility via eclogite-derived melts/fluids, while warmer settings facilitate widespread Ti activation through partial melting of gneiss-eclogite mixtures, as evidenced by abundant rutile-bearing veins spanning 427–408 Ma. The study underscores that continental subduction zones—spanning thermal gradients from cold to warm—exhibit melt-mediated Ti mobilization influenced by melt abundance, source heterogeneity, and prolonged melt-crystal interaction. These insights from the North Qaidam orogen advance our understanding of Ti cycling in continental subduction systems globally, emphasizing the interplay between protolith inheritance and tectonic thermal regimes in governing element redistribution.