藏东南措美大火成岩省中OIB型镁铁质岩的全岩地球化学和锆石Hf同位素

摘要

西藏南东部新识别出来的措美大火成岩省的地幔柱头部物质成分尚未得到很好的约束.为探讨此问题,在全岩地球化学数据基础上,本文首次报道了藏东南措美大火成岩省中机布淌、打隆、措美和哲古错OIB型镁铁质岩的锆石Hf同位素和微量元素数据.本文报道的OIB型镁铁质岩包括碱性(组Ⅰ)和亚碱性(组Ⅱ)系列的辉长岩和辉绿岩,以岩墙或岩床的形式产出.这些镁铁质岩具有高的TiO2(2.61％ ～4.07％)和P2O5(0.32％ ～0.51％)含量,富集轻稀土元素和高场强元素,地球化学特征类似于OIB.全岩微量元素地球化学指标显示组Ⅰ样品没有或很少受到地壳混染,组Ⅱ样品经历了较高程度的地壳混染.组Ⅰ中一件样品(JBT03-1)具有变化范围大的锆石εHf(t)值(-4.8～ +5.3),可能暗示其受到了地壳和/或岩石圈地幔物质的混染.本文结果表明锆石Hf同位素比全岩地球化学数据能够更为有效地识别基性岩浆是否受到地壳和/或岩石圈地幔物质混染.措美大火成岩省中的OIB型镁铁质岩样品(组Ⅰ和组Ⅱ),具有不同于俯冲带镁铁质岩和洋壳镁铁质岩的锆石稀土元素配分型式和锆石Ti温度,这可能是岩浆源区温度和成分不同的结果.综合考虑全岩地球化学和锆石Hf同位素指标,本文提出未受到地壳或岩石圈地幔混染的打隆镇辉长岩体(以组Ⅰ中的DL01样品为代表)很可能代表了措美大火成岩省纯的地幔柱头部物质成分[87Sr/86 Sr(t)=0.7047,εNd(t)=+1.5,εHf(t)=+2.1～+5.7].这些成分与代表白垩纪Kerguelen地幔柱头部物质的Site 1138和Bunbury Casuarina玄武岩非常相似,可能指示措美大火成岩省中的OIB型镁铁质岩本身就是Kerguelen地幔柱头部物质发生减压熔融的产物.%The plume head composition of the newly recognized Comei Large Igneous Province (LIP) in southeastern Tibet has not yet been well constrained. This paper first reports zircon Hf-isotope and trace elemental data of OIB-type mafic samples from Jibutang, Dalong, Comei and Chigu Tso located within the Comei LIP. These data, combined with the whole-rock geochemical data, are used to explore this is9ue. The OIB-type mafic rocks reported in this paper include alkaline (Group I) and sub-alkaline (Group II) gabbros and diabases, which outcropped as dikes and sills. These mafic rocks have high TiO2 (2. 61% -4. 07% ) and high P20s (0. 32% -0.51%) contents, and are enriched in light rare earth elements (LREEs) and high field strength elements (HFSEs), similar geoehemieally to Ocean Island basalt (OIB). Geochemical signatures of whole-rock trace element indicate little or no appreciable crustal contamination for Group I samples and significant crustal contamination for Group II samples, respectively. One sample in Group I (JBT03-1) display a wide range of zircon em(t) values ( -4.8- ±5.3), indicating the involvement of component from crust and/or sub-continental lithospheric mantle. This result suggests that zircon Hf-isotope data can be used as an effective geochemical fingerprint superior to whole-rock geochemistry to determine whether mafic rocks contain components derived from crust and/or subcontinental lithospheric mantle or not The OIB-type mafic samples in the Comei LIP (Group I and Group II) display distinct zircon rare earth element patterns and Ti-in-zircon temperature from those of mafic rocks from subduction zone and oceanic crust Such difference is most likely controlled by the different temperature and composition of their magma source regions. Considering the whole-rock geochemical and zircon Hf-isotope signatures, this paper proposes that the Dalong gabbroic pluton (represented by sample DL01 in Group 1) without contributions from crust and/or sub-continental lithospheric mantle likely represents the pure compositions of plume head of the Comei LIP [namely: "Sr/*Sr(1) =0.7047, eNd(t) = ±1.5, zircon eM(t) = ±2. 1 - ±5. 7]. These compositions are very similar to those of the basalts from Site 1138 and Bunbury Casuarina in SW Australia that were interpreted to represent the Cretaceous Kerguelen plume head materials. This similarity indicates that the OIB-type mafic rocks in the Comei LIP may be the product of decompression melting of the Kerguelen plume head materials.