长江中下游中生代安山质火山岩记录的新元古代大洋板片-地幔相互作用

大陆弧安山岩的形成是大洋板片向大陆边缘之下俯冲的结果，但是在具体形成机制上存在很大争议.针对这个问题，对长江中下游地区中生代安山质火山岩及其伴生的玄武质和英安质火山岩进行了系统的岩石地球化学研究，结果对大陆弧安山质火成岩的成因提出了新的机制.分析表明，这些岩石形成于早白垩世，它们不仅表现出典型的岛弧型微量元素分布特征，而且具有高度富集的Sr-Nd-Hf同位素和高的放射成因Pb以及高的氧同位素组成.通过全岩和矿物地球化学成分变化检查发现，地壳混染和岩浆混合作用对其成分的富集特征贡献有限，而其岩浆源区含有丰富的俯冲地壳衍生物质才是其成分富集的根本原因.虽然这些火山岩的喷发年龄为中生代，但是其岩浆源区形成于新元古代早期的华夏洋壳俯冲对扬子克拉通边缘之下地幔楔的交代作用.大陆弧安山岩地幔源区中含有大量俯冲洋壳沉积物部分熔融产生的含水熔体，显著区别于大洋弧玄武岩的地幔源区，其中只含有少量俯冲洋壳来源的富水溶液和含水熔体.正是这些含水熔体交代上覆地幔楔橄榄岩，形成了不同程度富集的超镁铁质-镁铁质地幔源区.在早白垩纪时期，古太平洋俯冲过程的远弧后拉张导致中国东部岩石圈发生部分熔融，其中超镁铁质地幔源区熔融形成玄武质火山岩，镁铁质地幔源区则熔融形成安山质火山岩.因此，大陆弧安山岩成因与大洋弧玄武岩一样，可分为源区形成和源区熔融两个阶段，其中第一阶段对应于俯冲带壳幔相互作用. 

Neoproterozoic Oceanic Slab-Mantle Interaction: Geochemical Evidence from Mesozoic Andesitic Rocks in the Middle and Lower Yangtze Valley

The generation of continental arc andesites is generally attributed to subduction of oceanic slabs beneath continental margins, but the petrogenetic processes of andesites remain widely debated. In order to address this problem, a series of integrated geochemical studies were performed for Mesozoic andesitic volcanics and associated basaltic and dacitic volcanics from the Middle and Lower Yangtze Valley, South China. The results lead to proposition of a new model for the generation of andesites. Laser-ablation inductively coupled mass spectroscopy (LA-ICPMS) zircon U-Pb dating yields consistent ages of Early Cretaceous for the formation of these volcanics, which are characterized by arc-like trace element distribution patterns showing significant enrichment in large ion lithophile element (LILE), Pb and light rare earth element (LREE) but depletion in high field strength element (HFSE) and heavy rare earth element (HREE). They also exhibit relatively enriched Sr-Nd-Hf isotope compositions, high radiogenic Pb isotope compositions and high zircon O isotope composition. Crustal contamination and magma mixing had insignificant contributions to the enriched compositions of these andesites. Instead, the enriched compositions were imparted by incorporating the subducted crust-derived materials into their magma sources. Despite their formation in the Late Mesozoic, their magma sources were generated through the crust-mantle interaction when the Cathaysian oceanic crust was subducted beneath the Yangtze craton in the Early Neoproterozoic. There are large amounts of subducted sediment-derived hydrous melts in the magma sources of continental arc andesites, in contrast to the limited amounts of aqueous solutions and hydrous melts in the magma sources of oceanic arc basalts. It is the hydrous melts that would chemically react with the overlying mantle wedge peridotite to generate mafic-ultramafic metasomatites. In the Early Cretaceous, these metasomatites underwent partial melting due to remote backarc extension owing to westward subduction of the Paleo-Pacific slab beneath the eastern China continent. Whereas partial melting of the ultramafic metasomatite produced basaltic melts, partial melting of the mafic metasomatite produced andesitic melts. In this regard, petrogenesis of both continental arc andesites and oceanic arc basalts shares two-stage processes, in which the first is the generation of mantle sources through subduction zone metasomatism and the second is the partial melting of mantle sources for mafic magmatism, with the first stage corresponds to the slab-mantle interaction in oceanic subduction zones.