浙江新昌复合式火成杂岩的岩石学、地球化学及其地质意义

Petrology and Geochemistry of the Xinchang Composite Igneous Complexes, Zhejiang and Their Geological Implication

浙江新昌复合式火成杂岩包括复合火山岩流和复合侵入杂岩。复合火山岩流由玄武岩和流纹岩/流纹质熔结凝灰岩组成,有的含少量安山质-英安质岩流和岩石包体;复合侵入杂岩由辉绿岩和花岗岩复合而成,含闪长质-石英闪长质岩石包体。它们在空间上紧密伴生,同位素年龄为96~113Ma;地球化学上,它们统属钙碱性岩系,具有轻稀土元素和大离子亲石元素富集、高场强元素亏损等特点,稀土元素、微量元素配分型式基本相同,Sr=0.7069~0.7079,εNd(t)=-2.3^-5.3,指示它们的微量元素和同位素发生过充分的交换。这些特点与浙闽沿海其他复合岩流和复合侵入杂岩的特点基本一致。它们形成在伸展构造背景,由起源于受消减作用影响的岩石圈地幔部分熔融而产生的玄武岩岩浆底侵,并与深熔的壳源花岗岩浆发生不同程度的岩浆混合而形成。新昌复合式火成杂岩的研究,为更深入研究中国东南部沿海地区晚中生代地球动力学环境变化和构造-岩浆作用提供了一个典型实例。

The Xinchang composite igneous complexes include several composite lava flows and composite intrusive massifs. The composite lava flows consist of basalt and rhyolites/rhyolitic ignimbrite, and may accommodate partly a small amount of intermediate lavas and enclaves. The composite intrusive massifs consist of diabase and granite, and contain dioritic enclaves. Geochemically, these rocks are of calc-alkaline series, enriched in LREE, LILE （e.g. K, Rb and Ba）, but depleted in HFS elements （e.g. Ta, Nb, Zr and Hf）, and with similar REE patterns and trace elements spidergrams. They have limited range of ISr values from 0.7069 to 0.7079 and eNa（t） values from -2.3 to -5.3, implying that a full exchange of trace elements and isotopic compositions took place among them. The overall elemental and Sr-Nd isotopic characteristics of the Xinchang composite igneous complexes are similar to those of the basalt-rhyolite composite lava flows and intrusions in the coastal area of Zhejiang and Fujian provinces. It is proposed that the Xinchang composite igneous complexes were formed under a extension tectonic setting and derived from the basaltic magma produced by partial melting of sub-continental lithospheric mantle which could have been influenced by early subduction processes, and underplated and interacted with deep crust melted granitic magma. This research, based on the composite igneous complexes, provides a typical case to better understand the evolution of Late Mesozoic geodynamic environment and tectono-magmatic activities in the coastal area of southeastern China.