Origin of the Cretaceous ore-bearing granitoids in the Beihuaiyang Zone, northern margin of the Dabie Orogen, Eastern China

The Beihuaiyang Zone (BHY) is one of the most important Mo–Pb–Zn polymetallic metallogenic belts in China, and the BHY deposits are genetically and geographically associated with Cretaceous magmatic rocks. In this article, we present new zircon U–Pb ages and Hf isotope data, whole-rock major and trace-element analytical results, and Sr–Nd–Pb isotope data for the granite porphyry of the Shapinggou (SPG) Mo deposit and the quartz monzonite porphyry of the Gongdongchong (GDC) Pb–Zn deposit. The high contents of SiO₂, crust-like rare-earth-element and trace element patterns, and the enriched Sr–Nd–Pb–Hf isotopic compositions indicate that both porphyries originated from crustal melting. Inherited Neoproterozoic zircons are common in both porphyries, which implies that their crustal sources were the South China Block rather than the North China Block. Whole-rock <i>ε</i>_Nd(<i>t</i>) values (−10.8 to −9.8 for the GDC deposit, −12.9 to −12.4 for the SPG deposit) and zircon <i>ε</i>_Hf(<i>t</i>) values (−14.3 to −11.1 for the GDC deposit, −18.4 to −13.3 for the SPG deposit) for the ore-bearing rocks are significantly higher than the values found in the widespread and older ore-barren rocks, indicating that the magma sources of the ore-bearing rocks were younger than those of the ore-barren rocks. An integrated study of the Sr–Nd–Pb–Hf isotope contents shows that these younger source rocks are similar to the gneisses found in the South and Central Dabie units, which represent the upper crust of the subducted South China Block. Given the geochemical behaviour of molybdenum, a surface enrichment process would have been an essential prerequisite for the formation of the large Mo deposit. The early Paleozoic Mo–Pb–Zn-enriched black shales, which are widespread in the upper layers of the South China Block, might have been scraped off during Triassic subduction and then transported to deep-crustal levels below the BHY, thus forming an ideal source for the ore-bearing porphyries. An upper-crustal origin for the ore-bearing magmatic rocks is also consistent with the data for most other deposits distributed in the BHY of the Dabie Orogen.

北淮阳带（Beihuaiyang Zone, BHY）是中国最重要的钼-铅-锌多金属成矿带之一，该带内的矿床在成因与空间上均与白垩纪岩浆岩密切相关。本文报道了沙坪沟（Shapinggou, SPG）钼矿床花岗岩斑岩与龚家冲（Gongdongchong, GDC）铅锌矿床石英二长斑岩的全新锆石U-Pb年龄、Hf同位素数据，全岩主量与微量元素分析结果，以及Sr-Nd-Pb同位素数据。两类斑岩均具有高二氧化硅（SiO₂）含量、类地壳型稀土元素与微量元素配分模式，以及富集的Sr-Nd-Pb-Hf同位素组成，指示其起源于地壳熔融作用。两类斑岩中均普遍存在继承性新元古代锆石，这暗示其地壳源区为华南块体（South China Block）而非华北块体（North China Block）。含矿岩石的全岩εNd(t)值（龚家冲矿床为-10.8~-9.8，沙坪沟矿床为-12.9~-12.4）以及锆石εHf(t)值（龚家冲矿床为-14.3~-11.1，沙坪沟矿床为-18.4~-13.3）均显著高于区域广泛分布的古老无矿岩石，表明含矿岩石的岩浆源区比无矿岩石的源区更年轻。综合Sr-Nd-Pb-Hf同位素组成研究表明，这类更年轻的源岩与南大别和中大别地体中产出的片麻岩相似，而后者代表了俯冲华南块体的上地壳。考虑到钼的地球化学行为，地表富集过程是形成大型钼矿床的必要前提。广泛分布于华南块体上地壳层位的早古生代富钼铅锌黑色页岩，可能在三叠纪俯冲过程中被刮脱，并被搬运至北淮阳带下方的地壳深部，从而为含矿斑岩提供了理想的源岩。含矿岩浆岩的上地壳起源特征，也与大别造山带（Dabie Orogen）北淮阳带内多数其他矿床的地球化学数据相一致。