摘要
华北克拉通岩石圈减薄破坏及其深部地质过程是我国固体地球科学研究的热点之一。岩浆作用是其形成时深部地质过程信息的载体,可以为克拉通破坏的时限、机制、动力学背景和下地壳再造过程提供约束。论文选取华北北缘燕山造山带为切入点,对辽西三叠纪-侏罗纪水泉沟组、海房沟组和蓝旗组火山岩开展了详细的岩石学、地球化学、锆石U-Pb年代学和Hf同位素组成研究,初步修订了辽西及其邻区中生代火山-沉积地层的年代学格架,探讨了燕山造山带中生代岩浆作用的成因及其记录的壳幔相互作用信息,为华北北缘中生代克拉通破坏的时限、机制和动力学背景提供了约束。水泉沟组火山岩主要由粗面岩、粗安岩、安山质火山碎屑岩和凝灰岩组成。其下部粗面岩和辉石安山岩中的岩浆锆石给出了-220Ma的谐和年龄,表明水泉沟组火山岩形成于晚三叠世。它们具有高Sr(≥841ppm)、低Yb(≤1.3ppm)和Y (≤17ppm)及高Sr/Y(>72)等“埃达克质”地球化学特征,同时富集轻稀土元素和大离子亲石元素Rb、Ba和Pb,亏损高场强元素Nb、Ta和Ti,以及低的Ce/Pb(<4.3)、Nb/U(<4.8)和中等的GdN/YbN(2.8~3.9)。-2.5Ga的继承和捕获锆石指示新太古代地壳物质可能参与了成岩过程。结合这些火山岩演化的全岩Sr-Nd同位素组成(初始87Sr/86Sr=0.70529-0.70540、εNd(t)=-3.9~-9.9)以及中生代锆石演化的Hf同位素组成(εHf(t)=-8.6~-1.1),我们认为水泉沟组埃达克质火山岩可能是在古亚洲洋俯冲之后的伸展背景下,由底侵的玄武质岩浆诱发大陆下地壳部分熔融的产物。它们高的Sr/Y可能是继承了源区的组成,与较高的熔融深度(如榴辉岩相下地壳)无关。因此,大陆伸展背景下埃达克质岩浆可能来自古老大陆下地壳的部分熔融作用。海房沟组主要由沉积碎屑岩夹中酸性火山岩组成,是燕山的运动A幕的产物,其沉积地层中Schmeissneria和Xingxueanthus化石的发现为揭示被子植物起源提供了关键证据。该组火山岩主要由玄武质粗安岩、粗安岩、粗面岩、英安岩及相应的火山碎屑岩组成。粗面岩中岩浆锆石的206Pb/238U加权平均年龄为173.4±3.1Ma,属于中侏罗世,由此推断被子植物的起源至少可以追溯到中侏罗世,燕山运动A幕的时代为中侏罗世。海房沟组火山岩具有变化的Si02(51.7-66.8wt.%),高的Al2O3(14.8~18.4wt.%),中等的MgO(0.6~4.4wt.%)和低的Mg#(19~50)、TiO2(0.57~1.03wt.%)、Ni (13~33ppm)、Cr(7.2~61.8ppm),富集轻稀土和大离子亲石元素,亏损高场强元素Nb、Ta和Ti,以及与埃达克岩相似的高Sr(≥695ppm),低HREE(Yb≤1.65ppm)和Y(≤19.6ppm)等地球化学组成。它们具有EM1型的Sr-Nd同位素组成(初始87Sr/86Sr=0.70491-0.70542,ENd(t)=-16.3--7.3),锆石具有演化的Hf同位素组成(εHf(t)=-21.7~-12.7, Tcrust=2.02~2.58Ga),表明这些岩石主要来自古老下地壳物质的部分熔融。海房沟组火山岩形成于由板内伸展向陆内造山转换的阶段,与蒙古鄂霍茨克洋闭合对燕山造山带的远程效应有关。早中侏罗世时华北北缘富集岩石圈地幔部分熔融产生的玄武质岩浆,持续底侵至下地壳底部形成热区(壳幔过渡带),并诱发上覆古老下地壳部分熔融,这些壳源熔体在源区混合了少量幔源岩浆并经镁铁质矿物分离结晶,喷出地表形成了海房沟组高Sr/Y火山岩。蓝旗组火山岩主要由玄武岩、粗安岩、安山岩、粗面岩、英安岩、流纹岩和相应的火山碎屑岩组成。辽西北票常河营子蓝旗组上部安山质角砾熔岩的锆石LA-ICPMS U-Pb年龄分析结果表明,其结晶年龄为159.4±3.4Ma,属晚侏罗世。系统总结前人对蓝旗组/髫髻山组火山岩的同位素定年结果,并结合上下地层接触关系,认为蓝旗期火山作用时间的下限为166~168Ma,上限为152-154Ma。根据蓝旗组火山岩的岩性和地球化学组成,本文将其分为4个亚类:G1,以相对低Si02(<56wt.%)、LREE(La<35ppm)和较低的轻重稀土分异程度(LaN/YbN65wt.%)和ΣREE (171~280ppm)、低Sr/Y和显著负Eu异常(δEu=0.60--0.79)为特征的粗面岩-英安转-流纹岩,是下地壳长英质麻粒岩或片麻岩部分熔融产生的熔体经角闪石、斜长石分离结晶而成。蓝旗组火山岩形成于侏罗纪古太平洋板块向亚洲大陆快速、斜向俯冲形成的活动大陆边缘环境,是幔源岩浆底侵过程,强烈的壳幔相互作用引起不同源区物质熔融并相互作用的产物。对辽西及其邻区中生代火山-沉积地层的同位素年代学数据、构造沉积记录和地层对比资料进行了系统的总结,初步修订了研究区三叠纪-侏罗纪火山-沉积地层年代学格架。凌源水泉沟组、北票兴隆沟组和京西-冀北南大岭组火山岩分别形成于-220Ma、208-202Ma和-174Ma,不能作为相互对比的火山岩地层。京西-冀北杏石口组、辽西凌源-建昌邓杖子组和辽西北票羊草沟组形成于晚三叠世-早侏罗世。京西窑坡组-龙门组-九龙山组、冀北下花园组、辽西凌源郭家店组、辽西北票海房沟组主体是在中侏罗世形成的。蓝旗组/髫髻山组火山岩形成于166-153Ma,其上的土城子组/后城组沉积岩形成于153-137Ma,张家口组火山岩形成于135-126Ma,义县组火山岩形成于132-120Ma。燕山造山带中生代火山岩的成因研究表明,在华北克拉通岩石圈地幔中生代减薄破坏过程中,幔源岩浆的底侵引起了华北北缘多期(-220Ma、160-152Ma、130-80Ma)、连续的下地壳增生-再造事件。辽西三叠纪-侏罗纪高Sr/Y火山岩可能是正常厚度/略厚的古老下地壳部分熔融的产物,与榴辉岩部分熔融没有直接联系,其高Sr/Y是继承源区(华北古老下地壳)的特征,高Mg#、Cr、Ni来自幔源岩浆的贡献。因而,华北东部中生代高Sr/Y埃达克质火成岩可能不足以作为支持克拉通破坏拆沉作用模型的主要证据。软流圈地幔组分参与了华北北缘晚三叠世岩浆岩的形成过程,表明华北北缘岩石圈地幔破坏可能在晚三叠世已经开始,古生代-早中生代古亚洲洋闭合及后续碰撞改造了华北北缘岩石圈属性,对克拉通破坏起到了唤醒作用。蒙古-鄂霍次克洋自西向东的逐渐闭合和华北-蒙古联合板块与西伯利亚板块的碰撞为华北北缘中侏罗世构造变形提供了动力,引起燕山造山带南北向挤压和地壳缩短,但未引发华北北缘岩石圈属性的改变。晚侏罗世初期(166-153Ma)古太平洋板块向亚洲大陆快速、斜向俯冲改变了燕山造山带构造格局,诱发了进一步的克拉通破坏作用,同时导致郯庐断裂在中晚侏罗世时发生大规模的左行走滑运动,为软流圈地幔物质上涌并侵蚀岩石圈地幔提供了通道。早白垩世,太平洋板片由低角度斜向俯冲转变为高角度正交俯冲/回撤,引起了大规模软流圈上涌、强烈的壳幔相互作用和伸展构造,克拉通破坏达到峰期。因此,华北北缘克拉通破坏是古生代开始的多板块俯冲-碰撞和软流圈地幔沿深大断裂上涌并侵蚀岩石圈地幔的结果。
The destruction of cratonic lithosphere beneath the North China craton (NCC) is one of the central themes in the study of continental geodynamics. The widespread generation of Mesozoic igneous rocks accompanied the cratonic destruction, which provides some insights into the evolution of the craton during lithospheric thinning. Data on mineral chemistry, major and trace elements and Sr-Nd isotopes of whole rocks, and in-situ U-Pb age and Hf-isotope analyses of zircons are reported for Triassic-Jurassic volcanic rocks in the Western Liaoning, in order to investigate their sources, petrogenesis and implications for the Phanerozoic evolution of northern part of NCC.Late Triassic high-Sr/Y Shuiquangou lavas, mainly trachytes with minor pyroxene andesite and rhyolite, are found at Lingyuan, in the Yanshanian fold-and-thrust belt on the northern margin of the NCC. The Shuiquangou volcanic rocks with high Sr/Y (>72) and (La/Yb)N (>24) also show enrichment in light rare earth elements and large-ion lithophile elements (e.g., Rb, Ba and Pb), and depletion in high-field-strength elements (e.g., Nb, Ta and Ti). They have low Ce/Pb (<4.3) and Nb/U (<4.8) and moderate (Gd/Yb)N (2.8-3.9). U-Pb dating of zircons yields concordant and lower-intercept ages of~220Ma, indicating that they erupted during the late Triassic. Concordant grains and an upper intercept age of~2.50Ga suggest that Neoarchean materials may have been involved in their petrogenesis. The relatively low initial87Sr/86Sr (0.70529to0.70540) and negative εNd(t)(-3.9to-9.9) of the these rocks, and the negative εHf(t)(-8.6to-1.1) of their zircons, suggest that the magmas were derived by partial melting of the cratonic lower crust, induced by continuous magmatic underplating under an extensional regime following the southward subduction of the Palaeo-Asian Ocean. Their high Sr/Y is inherited from their source, and does not necessarily imply melting at great depths (e.g., garnet-bearing lower crust). We suggest that partial melting of the ancient lower crust may be important for the petrogenesis of "adakitic" magmas in a continental extensional setting.The Haifanggou volcanic rocks consist of basaltic andesite, trachy-andesite, trachyte, dacite and pyroclastic rocks. The zircon U-Pb age of the Haifanggou trachyte yield a middle Jurassic age (173.4±3.1Ma), indicating the origin of angiosperms could be as early as~173Ma and the time of phase A of Yanshanian movement is middle Jurassic. The Haifanggou volcanic rocks with variable SiO2(51.7-66.8wt.%), high Al2O3(14.8-18.4wt.%), moderate MgO (0.6-4.4wt.%) and low Mg#(19-50), TiO2(0.57-1.03wt.%), Ni (13-33ppm) and Cr (7.2-61.8ppm). The lavas with high Sr (≥695ppm) and low HREE (Yb≤1.65ppm), also show enrichment in LREE and LILEs (e.g., Rb, Ba and Pb), and depletion in HFSEs (e.g., Nb, Ta and Ti). The EMI type Sr-Nd isotopic compositions (initial87Sr/86Sr=0.70491to0.70542, εNd(t)=-16.3to-7.3) of these rocks, and the evolved Hf isotopic compositions (εHf(t)=-21.7to-12.7, Tcrust=2.02to2.58Ga) of their zircons, suggest the magmas were derived by partial melting of an ancient lower crust, induced by continuous magmatic underplating. They erupted in a switched tectonic regime from within-plate extension to orogeny, which is responded to the closure of Mongolo-Okhotsk Sea and following collision of an amalgamated North China-Mongolian plate with Siberian plate.The Lanqi volcanic rocks consist of basalt, basaltic andesite, trachy-andesite, trachyte, dacite, rhyolite and pyroclastic rocks. The zircon U-Pb age (159.4±3.4Ma) of the andesitic breccia lava indicates that they erupted during the late Jurassic. Combined with the pervious geochronology data and the relationship of strata, we propose the Lanqi Fm./Tiaojishan Fm. are formed between166Ma to152Ma. The Lanqi lavas can be subdivided into four group base on their geochemical compositions. Group1basalt and basaltic andesite, with low SiO2(<56wt.%), LREE (La<35ppm) and LaN/YbN (65%) dacite and rhyolite, with low Sr/Y and negative Eu anomalies (δEu=0.60to0.79), were produced by amphibole and plagioclase fractionation of melt derived from partial melting of felsic granulites and gneisses of the lower crust. The Lanqi volcanic rocks are the products of crust-mantle interaction, induced by rapid and oblique subduction of the Izanagi plate.The Mesozoic igneous rocks in Yanshan fold and thrust belt provide evidence for the continued reworking of lower crust of the northern part of NCC in Mesozoic time, which caused by episodic widespread magma underplating (-220Ma,160-152Ma,130-80Ma). Their petrogenesis also suggest that the igneous rocks with high Sr/Y "adakitic" signatures can be formed by partial melting of an ancient lower continental crust of normal thickness, without relationship to the melting of eclogite. Thus, they could not be used as an indicator of delamination of lower continental crust and the presence of Mesozoic plateau in the eastern NCC. The asthenospheric material involved in the generation of early Mesozoic magmatic rocks in the Yanshan belt, suggest the onset of the lithospheric destruction in the northern NCC may have occurred as early as the late Triassic, inducing by closure of Palaeo-Asian Ocean. The closure of Mongolo-Okhotsk Sea and following collision of an amalgamated North China-Mongolian plate with Siberian plate only caused the Mesozoic deformation without significantly modification of the lithospheric mantle beneath the North China craton. Pacific plate subduction resulted in extensive Jurassic magmatism and NE-to NNE-striking sinistral faults represented by the Tan-Lu fault zone which could be as channel of the upwelling asthenosphere. A shift from oblique shallow subduction of the Izanagi plate to orthogonal, steep subduction of the Pacific plate in early Cretaceous induced more intensive upwelling of asthenosphere and destruction of the North China craton. Thus, we propose that the destruction of the NCC lithospheric mantle was induced by subduction and collision of adjacent blocks in Phanerozoic.