Geochemical contrast of early Cretaceous adakitic rocks in central-eastern China: implications for petrogenesis and mineralization

The early Cretaceous adakitic rocks in central-eastern China, especially those associated with Cu (Au, Mo) mineralization, have attracted much attention in the past decade. Here, we report elemental and radiogenic isotopic data of granitoid plutons along the southern Tan-Lu fault (STLF) and from the Lower Yangtze River Belt (LYRB) to reveal their petrogenesis. The samples from STLF, barren in mineralization, are characterized by their high Sr/Y and La/Yb ratios and have Sr-Nd-Pb isotopes with typical old lower crust affinity. Additionally, they exhibit high-Mg features, such as Mg# up to 65. These geochemical features are comparable very well to the barren high-Mg adakitic rocks from the Dabie orogen. Both can only be explained by adakitic melts/mantle interaction, most likely to be magmas from delaminated lower continental crust (LCC). In contrast, the samples from LYRB, fertile in mineralization, which were previously regarded as partial melts of delaminated LCC[Wang et al., 2007. Partial melting of thickened or delaminated lower crust in the middle of eastern China: Implications for Cu-Au mineralization. Journal of Geology, v. 115, p. 149-161], are different from those from STLF in several aspects: (1) the former mainly have ɛNd(t) > -9 and 206Pb/204Pb(t) > 16.7, up to 18.9, while the latter have ɛNd(t) mainly < -14 and 206Pb/204Pb(t) < 16.4; (2) the former generally have Mg# no more than 55, systematically lower than the latter, suggesting difference in melting temperature; (3) the former exhibit trends comparable to typical adakites in modern arcs in the diagrams of Ce/Pb vs. Ba/Th and Sr/Y vs. La/Yb, different from contemporaneous magmas from thick/delaminated LCC. These lines of evidence are not consistent with the LCC delamination model, but suggest magmas from subducting altered oceanic crust, which supports the ridge subduction model of [Ling et al., 2009. Cretaceous ridge subduction along the Lower Yangtze River Belt, eastern China. Economic Geology, 104(2): 303-321]. Difference in genesis between high-Mg adakitic rocks from STLF and those from LYRB, coupled with being barren or fertile in mineralization, indicates that Cu (Au, Mo) mineralization may be rarely generated by lower crustal delamination but closely related to slab subduction.