摘要
本文结合构造地质、盆地演化和深部结构探测的最新成果,重点研究大别造山带及邻区的燕山期岩浆岩的构造岩浆序列、成因类型、深部作用及构造背景,分析岩浆作用与区域构造演化关系及大别造山带地球动力学演化过程。取得了以下主要进展:1、建立了区域构造演化模式,将研究区中生代构造演化划分为印支期俯冲碰撞、侏罗纪陆内挤压—隆升、早中白垩世热窿伸展和早白垩世晚期—晚白垩世断陷伸展等四个阶段。2、结合同位素资料及岩体的空间展布、构造变形和相互穿切关系,划分了岩浆活动期次和构造—岩浆组合,建立了构造—岩浆事件序列。认为大别造山带燕山期主要有中侏罗世(166-154Ma)、晚侏罗世(138-135Ma)、早白垩世早期(134—126Ma)、早白垩世早中期(130—120Ma)和早白垩世中晚期等几期岩浆活动。3、研究了不同类型侵入岩的成因类型,认为高Sr低Y型侵入岩主要特征与埃达克岩相似,为增厚地壳底部部分熔融的产物;高Sr高Y型侵入岩是增厚地壳部分熔融岩浆与富集型岩石圈地幔低程度部分熔融形成的碱性基性岩浆混合作用产物;富Mg的高Sr低Y型中酸性侵入岩是拆沉下地壳部分熔融作用产物,并在岩浆上升过程中受幔源橄榄岩的交代、混染;低Sr型花岗岩是正常深度中下地壳部分熔融的产物;过碱性正长岩则是低压条件下幔源碱性玄武质母岩浆分离结晶作用的结果。4、分析了Sr、Nd同位素组成,认为大别造山带从中侏罗世到晚侏罗世,侵入岩幔源物质贡献的明显增高,白垩纪岩浆源区从早到晚则可能由含较多幔源物质的下地壳向陆壳物质占优的中上地壳迁移。5、中侏罗世因俯冲板片的断离作用导致热的软流圈上涌,诱发造山带增厚地壳底部部分熔融;新提出幔源岩浆底侵作用始于晚侏罗世,与下地壳底部部分熔融产生的岩浆发生混合。早白垩世早期(135-130Ma)底侵作用达到高峰,由上涌软流圈地幔及富集岩石圈地幔部分熔融产生的熔体底侵在壳幔过度层,并与下地壳相互作用;早白垩世早中期(130—120Ma)发生了大规模拆沉作用,导致造山带伸展、崩塌和热的软流圈物质上涌,造成大规模低Sr型花岗质岩浆和镁铁—超镁铁质岩浆作用。6、论证了大别造山带燕山期存在独立的造山作用旋回,燕山期陆内造山作用叠加在印支期俯冲碰撞造山带之上。7、提出印支期陆间造山带是冷板块俯冲—碰撞结果,地温梯度低,属一种“冷”的造山带。燕山期陆内造山作用可能与软流圈物质上涌有关,造山带向“热”的方向演化。8、燕山早期的陆内挤压造山仍受古特提斯构造体系控制,早白垩世早期造山带已开始向滨太平洋构造域转换,燕山期大别陆内造山带由挤压向伸展转化实质上是两大构造体制域转换的结果。
Integrated with the lastest achievements in tectonic, basin evolution and deep structural detection, this paper mainly studies tectono-magmatic sequence, genetic types and tectonic setting of the Yanshanian magmatites within Dabie orogen and its neighbouring areas, and then discusses the relationship between magmatism and regional tectonic evolution and geodynamic process of Dabie orogenic belt. The main progresses in this paper are as follow.1. Established a regional tectonic evolution model. The Mesozoic tectonic evolution in study area is divided into four stages, namely, the Indosinian subduction-collision stage; the Jurassic intracontinental compression and uplift stage; the early Cretaceous extension stage of the thermal dome and the late early cretaceous- late Cretaceous rifting extension stage.2. Combined with isotopic data, spatial distribution, tectonic deformation and cutting across relationship of intrusive bodies, this paper discusses the magmatic activities and tectono-magmatic compositions and establishes tectono-magmatic event sequences. Five periods of magmatic activity in the Yanshanian orogenic belt have been recognised: middle Jurassic (166-154 Ma); late Jurassic (138-135 Ma); early period of the early Cretaceous (134-126 Ma); early-middle period of the late early Cretaceous (130-120 Ma) and middle-late period in the early Cretaceous .3. Discussed the genesis of different types of intrusive rocks. The author suggests that high Sr, low-Y (Yb)-type intrusive rocks produced by partial melting at the bottom of the thicken crust are similar to adakites with main characteristics. High Sr, Y (Yb)- type intrusive rocks are mixtures of magmas derived by partial melting in the thickened crust with alkali-basaltic magmas produced by low degree partial melting in the enriched lithospheric mantle. Mg-rich, high Sr and low Y (Yb)-type intermediate- acidic intrusive rocks are generated by partial melting in the delaminated lower continent crust, and metasomatized and contaminated with mantle peridotites during ascent of the magma. Low Sr-type granites are produced by partial melting in the middle-lower crust at normal depths whereas the peralkali syenites are results of fractional crystallization of the upper mantle-derived alkali-basaltic parental magmas under the condition of low pressures.4. Based on the Sr, Nd isotopic composition of the intrusive rocks, the author shows that the contributions of mantle material in intrusive rocks evidently increased in Dabie orogen from middle Jurassic to late Jurassic. Magma source may migrate from the lower crust with more mantle material to the middle-upper crust with dominated continental crust material from early to late Cretaceous.5. Break-off of the subducted slab resulted in thermal asthenosphere upwelling and induced partial melting at the bottom of the thickened crust in the orogenic belt in the middle Jurassic. The author firstly puts forward that underplating of the mantle-derived magmas initiated in the late Jurassic, and mixed with the ones produced by partial melting at the bottom of the lower crust. In the early period of the early Cretaceous, underplating reached its peak, and magmas, which derived by partial melting in the upwelling asthenospheric mantle and enriched lithospheric mantle, underplated in the crust-mantle transition zone and interacted with the lower crust. Large-scale delamination occurring in the early-middle period of the early Cretaceous gave rise to the extension, collapse and hot asthenosphere upwelling in the orogenic belt and the large low-Sr-granitic and mafic-ultramafic magmatism.6. Demonstrated the existence of the independent Yanshanian orogenic cycles in Dabie orogenic belt. The Yanshanian intracontinental orogen overlapped on the Indosinian subduction orogenic belt.7. The Indosinian intercontinental orogeny, a "cold" one with low geothermal gradients, was produced by subduction-collision of the cold plates whereas the Yanshanian orogeny may be related to asthenosphere upwelling, and evolve to become hot.8. The intracontinental orogenic belt in the Early Yanshanian was still controlled by paleotethys tectonic system, and initiated to conversion to marginal Pacific tectonic domain in the early period of the early Cretaceous. The transformation from compression to extension of the Yanshanian Dabie orogenic belt is essentially the result of conversion of two tectonic system domains.