大别山北淮阳带东段石榴斜长角闪岩石炭纪变质作用的测定

Dating of Carboniferous Metamorphism for Garnet Amphibolite from the Eastern Beihuaiyang Zone in the Dabie Orogen

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)-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3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,对北淮阳带东段金寨县铁冲一带与大理岩相共生的石榴斜长角闪岩开展了锆石SHRIMP U-Pb定年和初步的岩石学研究.结果表明,该区石榴斜长角闪岩经历了355±5 Ma变质作用,因而首次发现研究区经历了石炭纪变质作用,为秦岭-桐柏造山带的东延以及大别山碰撞造山带的古生代构造演化过程提供了新的制约.

In comparison with the Triassic ultra-high-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 have greatly impeded the deep understanding and precise recognition on the formation and evolution of the Dabie orogen.The eastern segment of the BZ is mainly composed of three groups of rocks:(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,no petrological record of the Paleozoic metamorphism has been reported in the region.Therefore,SHRIMP zircon U-Pb dating and preliminary petrographic observations on the garnet amphibolite coexisting with marble from Tiechong,Jinzhai in the eastern segment of the BZ are carried out in this study.The results suggest that the studied rocks experienced a strong metamorphism with the peak at 355±5 Ma.It is reported for the first time on the Carboniferous metamorphism in the region,and thus provides new constraints on the eastern extension of the Qinling-Tongbai orogen and the Paleozoic tectonic evolution of the Dabie collisional orogen.