Formation and Evolution of a Neoproterozoic Continental Magmatic Arc

Unlike many Archean diorites and granitoids that arguably formed in different geodynamic settings, their post-Archean counterparts are commonly regarded to have formed at convergent margins, although in detail their petrogenesis remains contentious. Here we present new whole-rock data and zircon Hf-O isotope analyses from dioritic (750-730 Ma), granitic (810-790 Ma) and tonalite-trondhjemite-granodiorite (TTG)-like intrusions (800-740 Ma) from the Panxi and Hannan regions, which form part of an extensive Neoproterozoic convergent margin exposed in South China. The dioritic rocks from the Panxi region exhibit high zircon ϵHf(t) (+10.1 to +13.1) and sub-mantle to mantle-like δ18O (3.1-6.3 ‰) values, whereas those from the Hannan region preserve low ϵHf(t) (+4.1 to +8.1) and high δ18O values (5.9-6.6 ‰), indicating that the dioritic melts were derived from subduction-modified lithospheric mantle sources and experienced variable degrees of lower crustal contamination. Zircons within granite and TTG from the Panxi region show a narrow range of Hf isotopic compositions generally spanning 2-4 ϵHf units (+3.1 to +7.9 for most felsic intrusions). By contrast, those from the Hannan region show a much wider range of zircon ϵHf(t) values spanning almost 10 ϵHf units (+1.1 to +10.9). Based on their O-Hf-Nd isotopic signatures, we propose that the granite and TTG from both areas were derived through partial melting of hydrated basaltic rocks in the arc root, and that the isotopic variability between the intrusions mirrors spatial and temporal chemical variations in these deep crustal source rocks. In both regions, the granites, along with mantle-derived mafic-ultramafic and intermediate rocks, show a coupled evolution associated with increasing ϵNd(t) and ϵHf(t) and decreasing δ18O with decreasing ages, whereas the TTGs formed during late-stage arc magmatism and preserve relatively homogeneous Nd-Hf isotopes and mantle-like δ18O values. Combined, these data record continuous crustal thickening through underplating of juvenile magmas and a progressive increase in the depth of melting, along with a decrease in the degree of interaction between the melts and basement rocks within the arc root. Our results suggest that slab melting was not required to produce post-Archean TTG signatures. Further, we suggest that the variability in the Hf-O-Nd isotopic compositions of metaluminous (I-type) granites mostly does not reflect a heterogeneity in upper mantle signatures, and that there is no conclusive evidence for the involvement of partial melts of subducted sediment based on Hf-O isotope signatures in zircon.