Geochemistry and zircon Hf isotopes of the Early Mesozoic intrusive rocks in the south Hunchun, Yanbian area, Northeast China: petrogenesis and implications for crustal growth

摘要

This paper presents major element, trace element, and new zircon Hf isotopic data for the Early Mesozoic intrusive rocks in the south Hunchun, Yanbian area, Northeast China. These data are used to constrain the petrogenesis of these intrusive rocks and their implications for the Phanerozoic continental growth of the Central Asian Orogenic Belt (CAOB). Combining geology, geochronology, and whole-rock geochemistry, we identify three distinct episodes of magmatism as Early-Middle Triassic (249-237Ma), Late Triassic (224-206Ma), and Early Jurassic (200-187Ma). The Early-Middle Triassic (249-237Ma) adakitic tonalite and granodiorite were produced by the partial melting of subducted oceanic slabs, and the melts were contaminated by mantle peridotite during their ascent, whereas the coeval non-adakitic diorite and monzogranite were most likely derived from partial melting of crustal material. The remarkably high zircon Hf isotopic signature (epsilon(Hf)(t)=+9.4 - +18.9), the enrichment in large-ion lithophile element and light rare earth elements, and the depletion in high field strength element suggest that these 224Ma gabbros were derived from the partial melting of depleted mantle modified by subduction-related fluids. The 212Ma monzogranite was most likely derived from juvenile material mixed with old crustal material as evidenced by their high SiO2, low MgO, and low Cr concentrations and variable epsilon(Hf)(t) values (-4.6 to +10.0). Except for the 197Ma tonalites with affinity to the high silica adakites, the overall geochemical evolution of Early Jurassic (200-187Ma) rocks was consistent with fractional crystallization from quartz diorite, granodiorite, and monzogranite to syenogranite. Both the Early Jurassic syn-subduction lateral continental growth by accretion of arc complexes and the Late Triassic post-collisional vertical continental growth by accretion of mantle-derived material played an important role in the Phanerozoic continental growth of the CAOB.