Geochemical contrasts between early Cretaceous ore-bearing and ore-barren high-Mg adakites in central-eastern China: Implications for petrogenesis and Cu–Au mineralization
Geochemical contrasts between early Cretaceous ore-bearing and ore-barren high-Mg adakites in central-eastern China: Implications for petrogenesis and Cu–Au mineralization
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DOI：
10.1016/j.gca.2010.09.003
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发表时间：
2010-12
影响因子：
5
通讯作者：
Sheng‐Ao Liu;Shuguang Li;Yongsheng He;F. Huang
中科院分区：
文献类型：
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作者：
Sheng‐Ao Liu;Shuguang Li;Yongsheng He;F. Huang
埃达克岩是世界上常见的斑岩型铜金矿床的伴生岩。中国中东部早白垩世两组埃达克岩分布广泛，但与成矿关系却截然不同：下扬子带（LYRB）的埃达克岩是中国最大的斑岩型铜金存款带之一，而与之相邻的南郯庐断裂（STLF）的埃达克岩均为贫矿。因此，这些埃达克岩提供了一个难得的机会，探讨的主要因素，控制埃达克岩和铜金矿化之间的成因联系。在这里，我们报告了新的年代学，元素和Sr-Nd-Pb同位素数据，并提出了一个全面的地球化学比较这两组埃达克岩。在SiO2一定的条件下，STLF埃达克岩比LYRB埃达克岩具有更低的Al 2 O3和更高的K2 O、K2 O/Na 2 O、MgO、Cr、Ni和Mg#。这些系统差异可能表明STLF埃达克岩的干玄武质来源和LYRB埃达克岩的富水玄武质来源。STLF埃达克岩具有高的Sr/Y和（La/Yb）N值，二者呈正相关，低的Sr/La和Ce/Pb值，而LYRB埃达克岩的（La/Yb）N值较低，但Sr/Y、Sr/La和Ce/Pb值较高。此外，LYRB埃达克岩具有高放射成因的Pb同位素组成特征，其206 Pb/204 Pb（t）高达18.8，明显不同于STLF埃达克岩的低放射成因Pb同位素组成（206 Pb/204 Pb（t）=15.8-16.4）。虽然两组埃达克岩的高Mg#表明在岩浆上升过程中与地幔橄榄岩反应，地球化学对比表明，STLF埃达克岩来自剥离榴辉岩下陆壳的部分熔融，而LYRB埃达克岩则来自早白垩世期间向LYRB俯冲的海水蚀变洋壳的部分熔融。因此，这两组高镁埃达克岩之间的岩石成因对比表明，大规模的铜，金矿化与大洋板熔融，而不是拆沉或回收的古老的下陆壳，如前所述。
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 Al2O3and 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)Nbut higher Sr/Y, Sr/La and Ce/Pb than the STLF adakites. Furthermore, the LYRB adakites are characterized by highly radiogenic Pb isotopic compositions with206Pb/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.