大别造山带北淮阳单元的变形历史与构造属性

It is limited in knowledge of the earlier collisional process in the Dabie orogenic belt (DOB), such as presence or absence of the microcontinent, island arc or accretionary wedge, which restricts understanding on all-around continent-continent collisional history between the South China block (SCB) and the North China block (NCB). The Beihuaiyang(BH) unit is the northern petrological-tectonic unit in the DOB, located between suture line of the SCB and the NCB and exhumation boundary of subduction continental crust, which is the key region for discovering the earlier collisional process in the DOB. It is exposed in the BH unit that the Foziling Group, the Luzhenguan Group and a few Carboniferous strata. The former works mostly emphasized no deep-subduction of the BH unit and were short of comprehending its deformation history and tectonic attribute. Based on detailed field observation, this project will take exhaustive structural analysis of the BH unit and thus obtain its deformation characteristics and stages. Synchronously, thermochronological works will be done about protolith ages of the Foziling Group and the Luzhenguan Group and their deformation or metamorphic time. Combined with these results, tectonic evolution history of the BH unit will be discovered. Simultaneity, this project will make provenance system analysis of the Mesozoic basin in the BH unit and the Hefei basin, which are used to look for arc magmatite or its existing evidences and thus find out tectonic attribute of the BH unit. Comprehensive analysis on deformation history and tectonic attribute of the BH unit and then contrasting to the previous research production, it is discovered that the earlier convergent mode and process of the DOB and so offering evidences for constructing a whole orogenic history.

大别造山带的早期碰撞过程，包括是否存在微陆块、岛弧或加积楔增生等方面，仍然了解有限，从而制约了对其陆－陆碰撞完整历史的认识。北淮阳单元是大别造山带北缘的构造－岩石单元，位于华南、华北板块缝合线与陆壳折返边界之间，是认识造山带早期碰撞过程的关键场所。该单元内出露佛子岭群、卢镇关群和少量石炭系地层。以往的研究主要强调该单元没有经历过深俯冲，但对其变形历史和构造属性仍然了解了限。本项目将通过野外观察，对北淮阳单元开展详细的构造分析，查明其变形特征与变形期次；同步开展热年代学工作，获得佛子岭群、卢镇关群原岩时代及变形－变质时间，查明北淮阳单元构造演化史。同时，在北淮阳单元内及北侧中生代盆地中开展物源年龄构成分析，查明是否存在岛弧岩浆活动及其构造属性。在此基础上，综合分析北淮阳单元的变形历史和构造属性，并与大别造山带已有成果进行对比，查明大别造山带早期汇聚方式和过程，为建立完整的造山历史提供证据。

北淮阳单元是大别造山带内最北缘的构造-岩石单元，向北与合肥盆地相接，向南以晓天-磨子潭剪切带与北大别单元相连。北淮阳单元主要由浅变质的绢云母石英片岩和石英岩为主的佛子岭群以及正片麻岩组成的卢镇关群构成。本项目以北淮阳单元佛子岭群为主要研究对象，通过详细的构造解析与年代学工作，查明了佛子岭群在大别造山带内主要分布于晓天-磨子潭剪切带以北，少量位于剪切带南侧，与北大别单元花岗岩表现为断层接触；佛子岭群总体表现为褶皱，轴线平行造山带方向，露头上其岩石面理产状稳定，局部发育小褶皱；利用合肥盆地详细的碎屑锆石年代学成果，获得了佛子岭群自大别造山带合肥后陆盆地形成以来，一直被其他岩石单元所覆盖而不作为合肥盆地南缘地层的物源区，直到古近纪以来才出露地表；首次确认了早古生代岩浆岩在大别造山带内的存在，获得了秦岭-桐柏-红安-大别造山带北缘可能存在统一的1.0-0.9 Ga的丝带状微陆块，华北板块向微陆块下俯冲形成佛子岭群复理石沉积。由于北大别单元与北淮阳单元直接相接，因而本项目也对北大别单元构造演化开展了构造解析与年代学工作，通过对北大别单元混合岩详细的显微构造鉴定及年代学工作，限定了北大别单元造山带尺度混合岩化事件发生于131 Ma左右。