【作者】 王辉；

【导师】 刘贻灿；

【作者基本信息】 中国科学技术大学 ， 地球化学， 2019， 硕士

【摘要】 北淮阳带位于大别碰撞造山带北部,相对于南部三叠纪超高压变质带来说,通常被认为是一个相对低级变质的构造岩石单位。以商(城)一麻(城)为界,分为西段和东段。其中,东段未发现与西段相对应的古生代大洋俯冲记录,也缺乏相关的古生代岩浆作用和变质作用方面报道。因而严重地阻碍了对大别造山带形成和演化的深刻理解和准确认识。北淮阳带东段岩石类型丰富,主要包括三类(套)岩石:(1)变质岩,如变质花岗岩(花岗片麻岩)、(石榴)斜长角闪岩、大理岩、云母石英片岩和变质复理石等;(2)岩浆岩,主要为中生代花岗岩、正长岩和火山岩等;(3)盆地沉积岩,主要为中新生代沉积及少量石炭纪沉积。然而,该区一直未发现古生代变质作用的岩石学记录。为此,本论文对北淮阳带金寨县铁冲及商城县歪庙一带大理岩及相伴生的斜长角闪岩开展了系统的野外地质调查以及岩石学、元素-同位素地球化学和错石年代学等方面研究。岩相学观察和锆石U-Pb年代学研究表明,铁冲斜长角闪岩及大理岩经历了多阶段变质演化,包括355 Ma峰期变质作用以及～330和～310 Ma的退变质作用;斜长角闪岩具有石榴子石+斜长石+单斜辉石+切尔马克闪石+石英、低硅角闪石+斜长石+石榴子石+榍石+钛铁矿、普通角闪石+斜长石+榍石+钛铁矿及绿泥石+阳起石等4个变质阶段矿物组合。其中,前三个变质阶段温压范围依次为648～719℃/1.0～1.2GPa、660～690℃/0.71～0.90GPa、607～646 ℃/0.28～0.50GPa。歪庙斜长角闪岩及大理岩也经历了多阶段变质演化,包括～420 Ma峰期变质作用和～330 Ma的退变质作用;斜长角闪岩具有单斜辉石+斜长石+石英±角闪石、高铝角闪石+斜长石+石英+榍石+磷灰石、角闪石+斜长石+榍石+钛铁矿、阳起石+方解石+磁铁矿等4个变质阶段矿物组合。它们均显示近等温减压的顺时针变质P-T轨迹。首次证明北淮阳带东段存在高压变质作用以及石榴斜长角闪岩形成于晚古生代俯冲-碰撞环境。全岩的主微量元素和Sr-Nd-Pb同位素分析结果表明,(石榴)斜长角闪岩的原岩主要为亚碱性玄武岩。它们具有岛弧型的微量元素分布特征,即富集K、Sr、Ba、Rb、Th等元素,而高场强元素Ti、Nb、Ta、Zr、Hf等相对亏损;富集的Sr-Nd-Pb同位素组成,即高的初始87Sr/86Sr 比值(0.706208-0.708142),正的εNd值,207Pb/204Pb值随着206Pb/204Pb升高而升高,而208Pb/204Pb值随着206Pbp/204Pb值升高几乎无变化。这些元素和同位素特征表明,铁冲与歪庙斜长角闪岩的原岩主要为亚碱性玄武岩,形成于与古生代大洋俯冲相关的岛弧或弧后环境中,变质过程中有熔流体的加入,并造成某些元素和同位素发生明显的变化。更多还原

【Abstract】 In comparison with the Triassic ultrahigh-pressure metamorphic belt in the southern Dabie orogen,the Beihuaiyang zone(BZ)is generally regarded to be a relatively low-grade metamorphic unit located in the northern part of the orogen.The BZ can be subdivided into eastern and western segments separated by Shangcheng-Macheng fault zone,respectively.Among of them,the eastern segment of the BZ has no record related to Paleozoic oceanic subduction similar to those in the western segment.Also,there are rare reports on the Paleozoic magmatism and metamorphism in the region.As a result,these greatly have impeded the deeply understanding and precisely recognition on the formation and evolution of the Dabie orogen.The eastern segment of the BZ is mainly composed of three groups of rocks,i.e.(1)metamorphic rocks,e.g.,meta-granitoid(granitic gneiss),(garnet)amphibolite,marble,mica-quartz schist and meta-flysch;(2)igneous rocks mainly including Mesozoic granite,syenite and volcanics;(3)basin sediments mainly composed of Mesozoic-Cenozoic and subordinate Carboniferous rocks.However,up to now no petrological record of the Paleozoic metamorphism has been reported in the region.Therefore,this study carried out systematical petrology,element geochemistry,zircon geochronology study and field geological investigaition on the marble and amphibolite coexisting with marble from BZ.The results suggest that the studied rocks in Tiechong experienced a multistage metamorphic evolution with the peak at 355±5 Ma,and retrograde metamorphism at～330 and～310 Ma,respectively.The rocks have mineral assemblages of four metamorphic stage including Grt+Cpx+Pl+Ts+Qz,Hbl+P1+Grt+Ttn,Hbl+Pl+Mag and Chl+Act.The corresponding temperatures and pressures for the former three stages are 648～719℃/1.0～1.2GPa,660～690℃/0.71-0.90GPa and 607～646℃C/0.28～0.50GPa,respectively.The studied rocks in Waimiao also underwent a multistage metamorphic evolution with the peak at 420 Ma,and retrograde metamorphism at～330 Ma.The rocks have mineral assemblages of four metamorphic stage including Cpx+Pl+Qz±Hbl,Hbl+Pl+Ap+Ttn,Hbl+Pl+Ttn+Ilm and Act+Cal+Mag.They all suggest a clockwise P-T path with near-isothermal decompressional processes.It is for the first time documented that the high-pressure metamorphism occurred at the eastern segment of the Beihuaiyang zone and the amphibolites here formed in the late Paleozoic subduction-collisional setting.A combined study of whole-rock major and trace elements and Sr-Nd-Pb isotopes has been carried out for the rocks from the BZ,the results provide constraints on the origin of sub_alkaline basalt for the(garnet)amphibolites.They exhibit arc-like trace element distribution patterns,with enrichment in K/Sr/Ba/Rb/T but depletion in high field strength elements(IFSE),like Ti/Nb/Ta/Zr/Hf.The amphibolites have enriched Sr-Nd-Pb isotope compositions,with high initial 87Sr/86Sr ratios of 0.706208-0.708142,positive εΝd(t)values and elevated 207Pb/204Pb ratios at given 206Pb/ZO4Pb ratios,but The 208Pb/204Pb value hardly changed with the increase of 206Pb/204Pb value.These elements and isotopes indicate that the protoliths of Tiechong and Waimiao amphibolites are mainly subalkaline basalts formed in island arc or back-arc environments related to Paleozoic oceanic subduction,with the addition of melt/fluid during metamorphism,resulting in obvious changes in some elements and isotope compositions.更多还原