From subduction initiation to hot subduction: Life of a Neoarchean subduction zone from the Dengfeng Greenstone Belt, North China Craton
GEOLOGICAL SOCIETY OF AMERICA BULLETIN
We report a spatially and temporally linked arc magmatic association of 2.55-2.50 Ga mid-ocean ridge basalt (MORB)affinity gabbros, arc-affinity basalts, high-Mg basalts/basaltic andesites (HMBA), Nb-enriched basalts (NEB), and a tonalite-trondhjemite-granodiorite (TTG)-like granodiorite from the Dengfeng greenstone belt (DFGB), North China Craton. These are identical to arc magmatic rock suites from hot subduction in the present plate mosaic, and resemble suites formed during subduction initiation. Laser ablation-inductively coupled plasma-mass spectrometry zircon U-Pb dating indicates that gabbros and granodiorites intruded at ca. 2.55 Ga and the HMBAs formed at ca. 2.50 Ga, indicating a long duration of subduction zone magmatic activity of at least 50 m.y. in the DFGB. The gabbros are characterized by flat light-rare earth element (LREE) patterns without negative Nb and Zr anomalies, showing a MORB-like geochemical affinity. Combined with previously reported normal-MORB-type basalts, we suggest that they may represent magmatic products during intraoceanic subduction initiation where their mantle source was slightly metasomatized by subduction-derived fluids during the initiation stage. The arc-affinity basalts are characterized by slightly enriched LREE patterns with negative Nb and Zr anomalies. The HMBAs have higher contents of MgO, Ni, and Cr and display more fractionated rare earth element (REE) patterns and large negative Nb and Zr anomalies than the arc-affinity basalts. The NEBs are characterized by high absolute contents of Nb (9.9-14 ppm) and high ratios of Nb/La-primitive mantle (pm) (p 0.40-0.46) and Nb/Th-pm (0.44-0.54), and moderately fractionated REE and minor negative Nb anomalies with no Zr anomalies. One granodiorite sample is characterized by a highly fractionated REE pattern and negative Nb and Zr anomalies. Geochemical modeling suggests that the association of the normal arc-affinity basalt (NAB)-HMBA-NEB can be interpreted to be generated by hybridization of mantle wedge peridotites by slab-derived TTG-like melts. Positive values of whole-rock epsilon Nd (t) and zircon epsilon Nf (t) of the 2.55-2.50 Ga magmatic associations are consistent with a long-term depleted mantle source for the mafic assemblage. The 2.55-2.50 Ga magmatic association from MORB-affinity gabbros and basalts to arc-affinity NAB-HMBA-NEB may record a continuous Neoarchean geodynamic process from intraoceanic subduction initiation to mature arc magmatism. The NAB-HMBA-NEB association with regional sanukitoids and high-Al TTGs indicates that hot subduction may have played an important role in the production of arc-related magmatism during the Archean.