北大别晚中生代火山岩的地球化学特征及对北大别构造属性的启示

GEOCHEMICAL CHARACTERISTICS OF LATE MESOZOIC VOLCANIC ROCKS IN THE HINTERLAND OF NORTH DABIE TERRANE AND THEIR TECTONIC IMPLICATIONS

北大别沙村地区发育一套由石英安山岩、粗面岩、粗面斑岩、晶屑凝灰熔岩和凝灰岩组成的晚中生代火山岩系,其K/Ar年龄变化于144～136 Ma。该火山岩系属偏铝质—弱过铝质高钾钙碱性岩石,LILE和LREE富集,Nb-Ta亏损明显,ε(Nd,t)=-18.73～-24.15,Ni(^(87)Sr)/Ni(^(86)Sr)=0.707 0～0.708 5。具有与北淮阳毛坦厂组火山岩(150～135 Ma)一致的元素-同位素地球化学特征。其元素-同位素变化主要受控于源区差异,岩浆源自10％～20％扬子陆块下地壳物质加入的富集岩石圈地幔。北大别与北淮阳构造带晚中生代火山岩有着相似的岩浆源区,暗示华北—扬子陆块俯冲碰撞的岩石圈边界可能并不与晓天—磨子潭断裂壳内缝合线叠合,而可能在水吼—五河断裂或其以南。

The Late Mesozoic volcanic sequence, which is composed of quartz-andesite, trachyte, trachyte-porphyry, tuff lava and tuff, was developed at Shacun area in the hinterland of North Dabie terra-ne. The typical samples of the sequence yielded the K-Ar age of 136-146 Ma. They are metaluminous and slightly peraluminous high-K calc-alkaline and characterized by LREE-LILE enrichment and HFSE (Nb-Ta) depletion with slight or no Eu anomalies. Age-corrected initial 87Sr/86Sr ratio spans a narrow range of 0. 707 0-0. 708 5 and e(Nd,t) of-18. 73--24. 15. These features are consistent with those of the Maotanchang volcanic sequence (150-135 Ma) of the North Huaiyang belt. The geochemical characteristics of the volcanic sequence can be explained by the source variation rather than the crustal contamination during the ascent and fractional crystallization. The magma likely represents partial melt-ing products of the enriched lithospheric mantle contaminated with 10%-20% Yangtze lower crust. The similarities of the elemental and isotopic variation between the Late Mesozoic volcanic sequences of North Dabie terrane and North Huaiyang belt suggest that they might have similar magma source and tectonic affinity. This paper also suggests that the lithospheric boundary may be defined by the Shuihou-Wuhe fault or other faults at the south, rather than the Xiaotian-Mozitan fault that defines the surface sutures between Yangtze and North China blocks in the recent studies.