Geochemical contrasts between early Cretaceous ore-bearing and ore-barren high-Mg adakites in central-eastern China: Implications for petrogenesis and Cu-Au mineralization

Adakites are commonly associated with porphyry Cu-Au ore deposits worldwide. Two groups of early Cretaceous adakites occur widely in central-eastern China but their association with mineralization contrasts sharply: adakites from the Lower Yangtze River Belt (LYRB) host one of the largest porphyry Cu-Au deposit belts in China, whereas those from the South Tan-Lu Fault (STLF), which is adjacent to the LYRB, are all ore-barren. These adakites, thus, provide a rare opportunity to explore the main factor that controls the genetic links between adakites and Cu-Au mineralization. Here we report new chronological, elemental and Sr-Nd-Pb isotopic data and present a comprehensive geochemical comparison for these two groups of adakites. At a given SiO₂, the STLF adakites show lower Al₂O₃ and higher K₂O, K₂O/Na₂O, MgO, Cr, Ni and Mg# than the LYRB adakites. These systematic differences may indicate a dry basaltic source for the STLF adakites and a water-enriched basaltic source for the LYRB adakites. The STLF adakites have high Sr/Y and (La/Yb)_N, which are positively correlated, and low Sr/La and Ce/Pb, while the LYRB adakites show lower (La/Yb)_N but higher Sr/Y, Sr/La and Ce/Pb than the STLF adakites. Furthermore, the LYRB adakites are characterized by highly radiogenic Pb isotopic compositions with ²⁰⁶Pb/²⁰⁴Pb(t) up to 18.8, which are clearly distinct from the STLF adakites with low radiogenic Pb (²⁰⁶Pb/²⁰⁴Pb(t) = 15.8-16.4). Although the high Mg# of the two groups of adakites suggest reaction with mantle peridotites during magma ascent, the geochemical comparisons indicate that the STLF adakites were derived from partial melting of the delaminated eclogitic lower continental crust, while the LYRB adakites were derived from partial melting of the seawater-altered oceanic crust that was being subducted towards the LYRB during the early Cretaceous. The petrogenetic contrasts between these two groups of high-Mg adakites, therefore, indicate that the large-scale Cu-Au mineralization is associated with oceanic slab melting, not delamination or recycling of the ancient lower continental crust, as previously proposed.