中条山中生代镁铁质侵入岩年代学与地球化学:对华北克拉通南部岩石圈地幔性质的制约

Geochronology and Geochemistry of Mesozoic Mafic Intrusive Rocks in Zhongtiao Mountain Area: Characterizing Lithospheric Mantle of Southern North China Craton

镁铁质岩石作为幔源岩浆产物,其成因研究有助于探讨华北克拉通深部地幔性质及其演化过程.对中条山地区镁铁质侵入岩开展了系统的锆石U-Pb年代学、全岩主、微量元素、全岩Sr-Nd同位素和锆石Hf同位素研究,揭示了晚三叠世(217±2 Ma)和早白垩世(121±2 Ma)两期镁铁质岩浆活动.晚三叠世镁铁质侵入岩SiO_(2)含量低至中等(46.03%~53.87%),MgO(14.37%~18.61%)、Ni(282×10^(-6)~433×10^(-6))、Cr(619×10^(-6)~1847×10^(-6))异常富集,亲岩浆元素丰度较低,上凸型稀土配分模式发育显著,指示岩体为堆晶成因.大量原生角闪石的发育表明晚三叠世杂岩的母岩浆高度富水,所有样品具有近平行的微量元素配分模式指示其轻度分异的LILE-HFSE(大离子亲石元素-高场强元素)组成反映地幔源区固有属性,母岩浆可能起源于经大洋板片沉积物熔/流体交代的地幔楔部分熔融.早白垩世镁铁质侵入岩SiO_(2)含量介于49.23%~54.99%,MgO和Fe_(2)O_(3) T含量分别为4.29%~7.17%和9.70%~14.79%,大离子亲石元素(Rb、Ba、K、Pb)和轻稀土元素富集,高场强元素(Nb、Ta、Zr、Hf、Ti)和重稀土元素亏损,可能为经俯冲陆壳衍生熔体交代的岩石圈地幔部分熔融形成.晚三叠世杂岩可能响应于扬子板块与华北克拉通碰撞引起的造山后岩石圈拆沉,早白垩世侵入岩则可能与古太平洋板块西向俯冲后回撤所引起的弧后岩石圈伸展有关.此外,二者均具有相对典型克拉通富集地幔亏损的全岩Nd(ε_(Nd)(t)=-18.56~-12.64)和锆石Hf(ε_(Hf)(t)=-20.2~+10.4),指示华北克拉通中南部岩石圈地幔性质自晚三叠世以来发生了显著改变,早白垩世克拉通破坏可能延及华北中部.

The mafic rocks originating from deep earth are probes for lithospheric mantle evolution.In this paper,it presents a synthesis study of zircon U-Pb chronology,whole-rock elemental and Sr-Nd isotopic geochemistry,and zircon Hf isotope of the mafic intrusive rocks in the Zhongtiao Mountain area.Two periods of magmatism during the Late Triassic(217±2 Ma)and the Early Cretaceous(121±2 Ma)are unraveled.The Late Triassic samples are characterized by low to intermediate SiO_(2) contents(46.03%-53.87%),high MgO(14.37%-18.61%),Ni(282×10^(-6)-433×10^(-6))and Cr(619×10^(-6)-1847×10^(-6))concentrations,low magmatophile element abundances,and convex rare earth element distribution patterns,indicating a cumulate origin.The existence of a large number of original amphiboles indicates that the parental magma is highly water-rich.All samples have nearly parallel trace element distribution patterns,which implies that their mild LILE-HFSE(large ion lithophile element-high field strength element)differentiation reflects the inherent attributes of mantle source region,and their parental magma probably originated from partial melting of the mantle wedge metasomatized by subducted sediment melt/fluid.The SiO_(2) content of Early Cretaceous mafic intrusive rocks lies between 49.23%-54.99%,while the contents of MgO and Fe_(2)O_(3) T are 4.29%-7.17%and 9.70%-14.79%,respectively.Meanwhile,these rocks are enriched in LILEs and light rare earth elements(LREEs),and depleted in HFSEs and heavy rare earth elements(HREEs).Their formation is ascribed to partial melting of lithospheric mantle metasomatized by subducted continental crust-derived melt.The Late Triassic complex may be related to post-orogenic oceanic slab breakoff caused by collision between the Yangtze plate and the North China craton(NCC),while the Early Cretaceous intrusive rocks may be linked with back arc lithospheric extension triggered by the retreat of Paleo-Pacific plate during its westward subduction.In addition,the relative depletion of whole-rock Nd(ε_(Nd)(t)=-18.56 to-12.64)and zircon Hf(ε_(Hf)(t)=-20.2 to+10.4)isotopic compositions in samples compared with that of typical craton lithospheric mantle indicates that the lithospheric mantle nature of the central and southern part of the NCC have changed significantly since the Late Triassic,and the Early Cretaceous craton destruction extends to the central part of the NCC.