弧后盆地玄武岩的成分变化及其成因

弧后盆地玄武岩(BABB)是弧后盆地扩张过程中岩浆作用的主要产物,其地球化学组成是认识弧后盆地演化的关键。现今弧后盆地主要集中在西太平洋地区。本文总结了该地区弧后盆地玄武岩的元素地球化学和同位素组成特征。总体而言,相对于开阔大洋洋中脊玄武岩(MORB),弧后盆地玄武岩的主量元素成分变化范围很大,在Al_2O_3-Mg O、Ti O_2-Mg O相关图上偏离了MORB的演化趋势,在Mg O相同的情况下表现出更高的Al_2O_3含量和更低的Ti O_2含量。弧后盆地玄武岩的微量元素特征一般介于MORB和弧玄武岩之间。一方面,它们与MORB一样在中、重稀土元素之间没有明显分馏;另一方面,与弧玄武岩一样富集大离子亲石元素Rb、Ba、Th、U、K,具有Pb的正异常和Nb、Ta的负异常等。其中,劳海盆、日本海海盆和冲绳海槽有部分样品具有Nb、Ta的正异常,表现出类似于E-MORB的微量元素特征。西太平洋地区弧后盆地玄武岩的Sr-Nd-Pb同位素组成变化范围较大,相对于MORB,其富集组分更常见,总体介于亏损地幔端元(DMM)、1型富集地幔(EM1)和2型富集地幔(EM2)三者之间。不同基底属性(大陆基底和大洋基底)和不同阶段的弧后盆地玄武岩的地球化学组成也有明显区别。弧后盆地玄武岩地球化学成分上的多样性主要受控于源区(地幔楔)的物质组成、熔融程度和岩浆上升过程中的变化等因素。地幔源区的不均一性主要体现在地幔楔自身的化学性质和俯冲板片的物质贡献差异。部分弧后盆地玄武岩具有异常高的地幔潜能温度、高的3He/4He比值以及E-MORB型的微量元素特征,说明其地幔源区还可能受到了地幔柱的影响。地幔潜能温度越高,俯冲流体贡献越多,地幔楔的熔融程度越大。此外,岩浆上升过程中发生的地壳混染、岩石圈中的熔体-岩石反应以及矿物的结晶分离都会改造岩浆的成分。

Geochemical variation of back-arc basin basalt and its genesis

Back-arc basin basalt (BABB) is the product of rifting and spreading ridge magmatism of back-arc basin. The geochemical compositions of BABB record the information of their genesis, which is the key to understand the tectonic evolution of back-arc basin. Back-arc basins are mainly distributed in the Western Pacific. In this study, we summarized the elemental and isotopic characteristics of BABBs from the Western Pacific. In general, the ranges of major elements of BABBs are much larger than those of mid-ocean ridge basalt (MORB). For a given MgO, BABBs have higher Al₂O₃ content and lower TiO₂ content than MORBs, deviating from the elemental trends of MORB on the plots of Al₂O₃ and TiO₂ versus MgO. The trace elemental characteristics of BABB are generally between MORB and arc basalts. On the one hand, BABB has no obvious elemental fractionation between medium and heavy rare earth elements, which is similar to MORB. On the other hand, BABB is rich in Rb, Ba, Th, U, K, with positive Pb anomalies and negative Nb, Ta anomalies, which is similar to arc basalts. Among BABBs, some samples from Lau Basin, the Japan Sea Basin, and the Okinawa Trough have positive anomalies of Nb and Ta, similar to those of E-MORB. The Sr-Nd-Pb isotopic compositions of BABBs in Western Pacific region vary in large ranges. Compared with MORB, the enriched components in mantle sources are more common from BABB. In general, the isotopic compositions of BABB are between the depleted mantle member (DMM), the type-1 enriched mantle (EM1), and the type-2 enriched mantle (EM2). Geochemical difference is also observed for BABB between different basement settings (continental basement versus oceanic basement) and different evolution stages of back-arc basin. The geochemical diversity of BABB is mainly controlled by mantle source (the mantle wedge) heterogeneity, the degree of partial melting, and the magmatic processes during magma transportation. The mantle source heterogeneity is reflected by the difference in chemical properties of mantle wedge itself and material contributions from subduction plate. The abnormally high mantle potential temperature, high ³He/⁴He ratio, and E-MORB-type trace elemental characteristics in some BABB further indicate that their mantle source could have been affected by mantle plume activities. The higher mantle potential temperature and the more contribution of subduction fluid would cause the greater partial melting degree of the mantle wedge. In addition, crustal assimilation in continental lithosphere, melt-rock interaction in oceanic lithosphere, and the complicated history of fractional crystallization during magma transportation would all modify the geochemical compositions of BABB melts.