【作者】 汤华云；

【导师】 郑建平；

【作者基本信息】 中国地质大学 ， 矿物学、岩石学、矿床学， 2008， 博士

【摘要】 华北克拉通东部显生宙岩石圈破坏及其深部过程研究是我国固体地球科学的重要议题。近年的许多资料表明,将华北东部南缘岩石圈地幔演化与扬子大陆向华北地块的深俯冲进行有机联系可以为探讨华北克拉通的改造、破坏等深部过程提供有意义的约束。造山带内火成岩的地球化学特征是它们形成时地球动力学背景信息的最直接载体,能揭示出块体结合部位丰富而复杂的壳幔相互作用信息。本文以北苏鲁乳山市宫家辉长—闪长岩、上口辉石闪长岩及其中的基性岩石包体为研究对象,同时兼顾邻区同时代的三佛山花岗岩,在对它们进行详细的锆石U-Pb年代学、岩相学、地球化学和同位素地质学分析的基础上,细致讨论了它们的岩石成因、源区性质及所揭示的壳幔相互作用及岩石圈地幔演化信息,反演了华北东部南缘中生代—新生代岩石圈减薄等深部过程。获得了如下的主要认识:1.宫家辉长—闪长岩和上口辉石闪长岩是分别位于乳山市东南部宫家村和东北部上口村的两个独立小岩体。宫家岩体从早到晚由中—细粒黑云母辉长岩、中—粗粒黑云母辉长岩、细粒黑云母辉石闪长岩和中—细粒黑云母辉石闪长岩构成,以细粒黑云母辉石闪长岩为主体。上口岩体出露不完整,仅见到偏中性的辉石闪长岩单元。宫家岩体中的基性岩石包体类型包括黑云母橄榄辉石岩、含长单斜辉石岩、单斜辉石岩及黑云母辉长岩等四种类型,以黑云母辉长岩为主,主要分布在第三期侵位的细粒黑云母辉石闪长岩之中。2.宫家辉长—闪长岩中四个相带岩石的形成时代较接近,为114±1Ma～111±1Ma,上口辉石闪长岩的侵位结晶年龄为110.5±1Ma,它们同属早白垩世晚期岩浆活动的产物。宫家岩体中的黑云母橄榄辉石岩包体也获得了111.5±1Ma的²⁰⁶pb/²³⁸U加权平均年龄。3.宫家辉长—闪长岩和上口辉石闪长岩均显示出高钾钙碱性—钾玄岩系列的特点,具有高Al₂O₃、低MgO,富集轻稀土、大离子亲石元素和Pb,强烈亏损Nb、Ta、Zr、Hf,Ti等高场强元素和类似EMII型地幔的Sr-Nd同位素组成,这些特征相似于华北东部南缘中生代幔源火成岩。宫家和上口岩体与富集的大陆岩石圈地幔在软流圈上涌的伸展背景下减压熔融有关,并伴随着橄榄石、单斜辉石、磷灰石及铁钛氧化物等矿物的分离结晶。其地幔源区以相对富单斜辉石的尖晶石相为主,深度小于80kin,并具有亲华北南缘的特点。该地幔源区不同于华北古生代金伯利岩中橄榄岩捕掳体所代表的以方辉橄榄岩为主的难熔型地幔,也与新生代碱性玄武岩中橄榄岩捕掳体指示的主元素饱满、同位素亏损的新生大洋型地幔有差别,是遭受了来自俯冲的扬子大陆板片释放的熔/流体交代改造而演化的产物,但改造程度弱于华北南缘早白垩世早期方城玄武岩和沂南辉长代表的岩石圈地幔,显示俯冲的扬子陆壳物质对上覆岩石圈地幔的交代改造自下而上是不均匀的。4.宫家辉长—闪长岩中基性岩石包体的显微结构和岩石组合不同于邻区太古代或元古代超镁铁质岩块和华北克拉通东部中生代闪长岩中捕获的来自岩石圈地幔或壳幔过渡带的深源捕掳体。相反,它们具有的火成结构、与寄主岩相似的岩石化学和同位素组成,说明其成因与寄主岩浆在侵位上升时发生高压分异结晶或析离有关。四类包体各不相同的主元素、微量元素组成,显示其可能为寄主岩浆演化的不同阶段产物。5.对三佛山花岗岩的研究主要集中在王家口和潘家含斑二长花岗岩和尚岭正长花岗岩,它们形成于早白垩世晚期（～110Ma）,是一套高钾钙碱性系列的岩浆,具有高SiO₂、低FeO_t、MgO的特点,Na₂O/K₂O均小于1,富集轻稀土、大离子亲石元素（如Sr、Ba）,Sr-Nd同位素组成与邻区幔源岩石十分接近。它们是早白垩世软流圈上涌和华北东部岩石圈减薄的伸展背景下古老的陆壳物质发生重熔,形成的酸性熔体与幔源岩浆混合后再经历分离结晶的演化产物。它们具有中稀土下凹、重稀土富集的稀土配分模式,表明酸性岩浆的熔融源区残留相以含有角闪石为特征,基本无加厚地壳的必要矿物相石榴子石,揭示～110Ma时该地区地壳已恢复到正常厚度,暗示此时的岩石圈减薄已达到峰期。6.结合已有的研究成果和本文资料,华北克拉通东部南缘中—新生代岩石圈地幔演化的过程可能可以解释为:早中生代扬子大陆向华北深俯冲,破坏了华北南缘原有的地幔对流体系,加剧了软流圈的活动性能,引起岩石圈地幔的热—机械侵蚀和化学侵蚀减薄;苏鲁超高压变质带折返时释放的流体和俯冲板片熔融形成的硅质熔体不断地穿透并交代改造上覆华北古老岩石圈地幔楔,使之转变为高度富集的地幔;在中晚侏罗世时,软流圈继续抬升,引起原先的交代富集地幔熔融,形成的幔源岩浆或底侵在下地壳、壳幔过渡带,并同时诱发俯冲的扬子陆壳发生重熔形成花岗质岩浆;早白垩世郯庐断裂带的大幅左行平移走滑引起软流圈物质以郯庐断裂带为中心大幅上涌,导致上覆岩石圈地幔和地壳的强烈伸展,伴随岩石圈地幔的侧向挤出减薄,富集的岩石圈地幔同时发生大规模熔融而被消耗,岩石圈地幔减薄达到峰期,幔源岩浆同时烘烤地壳,诱使地壳发生重熔形成酸性岩浆,在合适的部位还形成壳幔混合花岗质岩浆。晚白垩世—新生代,太平洋板块快速向华北陆内斜向俯冲导致含深海远洋沉积物的俯冲洋壳软流圈发生混熔,加快了岩石圈地幔减薄,软流圈地幔于古近纪开始趋于冷却回沉,形成现今新生的大洋型地幔,实现对原有的富集型岩石圈地幔的置换。这一模式在华北东部南缘具有普遍意义。更多还原

【Abstract】 The nature and evolution of the North China lithosphere have attracted many attentions in the past few years,because of considerable losing of the Archean keels during the Phanerozoic. Recent published data indicated that the Dabie-Sulu orogenic belt has become one of the most ideal regions for the studies on deep subduction of continental lithosphere,and its affections to the destruction of North China Craton.Mesozoic igneous rocks from the Dabie-Sulu orogenic belt are considered to be the products of subduction-induced and post-collisional extension-related magmatism and thus can provide constraints on the processes involved in the lithosphere evolution.In this paper,we present geochronological and geochemical investigations for the Gonajia and Shangkou intermediate-mafic intrusive complex,and their mafic xenoliths, as well as the coeval Sanfoshan granites from the northern Sulu orogen,with the aim of constraining the petrogenesis and origin of these rocks and mantle processes involved in their generation,and will further discuss the crust-mantle interaction that occurred underneath the collision zone and its implication for the lithosphere evolution.In addition,the evolution crust-mantle interaction occurred underneath the southern wedge of the North China lithosphere have also been discussed.1.The Gongjia and Shangkou complexes are two independent small intermediate-mafic plutons that located at the southeastern and northeastern part of the Rushan city,respectively. The Gonajia complex comprises of four phases including（1）medium- to fine-grained biotite gabbro;（2）medium-grained biotite gabbro;（3）medium- to coarse-grained biotite gabbro;（4） fine-grained pyroxene-beafing diorite.The pyroxene-bearing diorite predominates in the outermost part of the Gongjia complex.The Shangkou pluton has relative poor occurrence and only the intermediate phase been found.Mafic xenoliths captured in the Gongjia complex include olivine-pyroxenite,plagioclase-bearing clinopyroxenite,clinopyroxenite and biotite-bearing gabbro,all of which mainly distributed in the third phase of the host rocks.2.The Gongjia complex has rather uniform zircon U-Pb ages of 114±1Ma-111±1Ma, whereas the Shangkou pyroxene-bearing diorite has an emplacement age of 110.5±1Ma, indicating that they were both intruded in the late of the Cretaceous.Zircons extracted from an olivine-pyroxenite xenoliths give a weighted mean ²⁰⁶pb/²³⁸U age of 111.5±1 Ma.3.The Gongjia and Shangkou complexes have high Al₂O₃ and low MgO,and show high-K calc-alkaline to shoshonitic affinities.They are highly enriched in large ion lithophile elements （LILE）and light rare earth elements（LREE）,depleted in high field strength elements（HFSE） and Pb,and exhibit EMII-like Sr and Nd isotopes.These complex are interpreted to originate from decompression melting of spinel-facies cratonic mantle that metasomatized by the subducted Yangtze crustal materials and the melts experienced fractionation of olivine, clinopyroxene,apatite and Fe-Ti oxides.Such mantle is not only different from the Palaeozoic refractory lithosphere,but also contrasts with the fertile one in Cenozoic.These complexes are similar in composition to other Early Cretaceous mafic-intermediate complexes from the southern margin of the North China Craton,implying the mantle source of the Rushan intrusion is tectonically affiliated to the southern wedge of the North China Craton.We infer that the Gongjia and Shangkou complexes formed in an extensional regime corresponding to the asthenosphere upwelling during gradual erosion and replacement of the cratonic mantle by the fertile lithosphere beneath the eastern North China Craton.4.The petrographic and rock assemblages of the mafic xenoliths are not only different from that of the Archean or Proterozoic ultramafic bodies from the adjacent regions,but also contrast with the deep-seated xenoliths entrained in the Mesozoic diorites from the eastern North China Craton.On the contrary,the magmatic textures and similar elemental and isotopeic compositions to the host rocks demonstrate that the xenoliths were most probably represented high pressure fractionational products of the host magmas.The different major and trace element compositions of the four kinds of xenoliths indicate that they might form during the different evolutional periods of the magma intrusion.5.For the Sanfoshan granites,we mainly focused on the Wangjiakou and Panjia monzonite granites,and Tongling alkali-granite.They were both emplaced in late Cretaceous（～110Ma）,and display high-K calc-alkaline affinities,have high SiO₂,low FeO_t and MgO,with Na₂O/K₂O ratios lower than 1.They are enriched in LILE（such as Sr and Ba）and LREE,and with Sr and Nd isotopic compositions very closed to the nearest mantle derived igneous rocks such as the Gonajia complex and lamprophyres.These Cretaceous granitic rocks were formed by crustal-derived acid magma mixed with mantle originated basic magma,and the mixed magmas experienced minerals fractionation during their followed evolution.The concave-up chondrite-normalized REE patterns suggest that the precursor magma from a garnet-free source and the depletion of the MREE may be an indication of amphibole as residual in the melting source of the acid magma.These imply that the crust was thinned to a normal thickness and the lithospheric mantle thinning peaked around 110Ma underneath the studied regions.6.The lithosphere evolution beneath the southern wedge of the eastern North China Craton during the Mesozoic to Cenozoic may have achieved in the following processes:The Early Mesozoic deep subduction of the Yangtze block at the southern part of the North China Craton destroyed the physical integrity of the North China lithosphere,and then triggered the thin of the North China lithosphere through thermo-mechanical erosion and chemical erosion caused by the asthenosphere upwelling.Those fluids and melts released from the subducted Yangtze slab would progressively migrate up,perturbate and metasomatize the overlying lithospheric mantle wedge and finally account for the formation of the highly enriched Mesozoic lithospheric mantle beneath the North China Craton.During the middle-late Jurassic,the pre-metasomatized enriched lithospheric mantle would begin to melt due to the upwelling of the hot asthenosphere,these mantle-derived magmas would underplate at the base of the lower crust or crust-mantle transition zone,causing the partial melting of the subducted Yangtze crust and giving the formation of the granitic rocks.The left-laeral strike-slip fault of the Tanlu fault in the Early Cretaceous triggered the strong upwelling of the asthenosphere,and simultaneously caused the intensive extension of the lithospheric mantle and crust,accompanied by splited apart of the lithosphere;the majority of the enriched lithospheric mantle was then considerably thinned by partial melting,these mantle-derived magmas may mixed with the crustal acid magmas and formed granitic rocks and certain environments.From late Cretaceous to Cenozoic,the oblique subduction of the pacific plate caused the oceanic crustal materials adequately mingled with the asthenosphere,and then the progresive asthenospheric cooling lowered the boundary between the lithosphere and the asthenosphere and created new oceanic-type lithospheric mantle.更多还原