北淮阳带东段铁冲石榴斜长角闪岩的变质演化和P-T轨迹

METAMORPHIC EVOLUTION AND P-T PATH FOR GARNET AMPHIBOLITE AT TIECHONG FROM EASTERN SEGMENT OF 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。本文对北淮阳带东段新发现的铁冲古生代变质的石榴斜长角闪岩开展了岩相学、矿物化学和热力学研究。结果表明,该类岩石具有石榴子石+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+钛铁矿等变质阶段矿物组合。其中,前三个变质阶段温压范围依次为648~719℃/1.0~1.3 GPa,660~686℃/0.71~0.90 GPa,607~647℃/0.27~0.48 GPa。因此,显示近等温减压的顺时针变质P-T轨迹。结合已发表的355±5 Ma峰期变质时代,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。

In comparison with the Triassic ultrahigh-pressure metamorphic belt in the southern Dabie orogen,the Beihuaiyang zone(BZ) can be subdivided into eastern and western segments separated by Shangcheng-Machenh fault zone and generally regarded as a relatively low-grade metamorphic unit in the northern part of the orogen.Integrated petrographic,mineralogical and thermodynamic investigation is carried out on the garnet amphibolites at Tiechong area, which are related to the Paleozoic metamorphism in the region.It shows that the rocks have varied mineral assemblages of multiple metamorphic stages,such as Grt+Cpx+Pl+Ts+Qtz,Hbl+Pl+Grt+Ttn and Hbl+Pl+Mag.The corresponding temperatures and pressures for the former three stages are 648-719 ℃/1.0-1.3 GPa,660-686 ℃/0.71-0.9 GPa and 607-647 ℃/0.27-0.48 GPa,respectively, suggestive of a clockwise P-T path with near-isothermal decompressional processes. Based on the published peak-metamorphic age of 355±5 Ma,it is considered that the high-pressure metamorphism occurred at the eastern segment of the Beihuaiyang zone and the garnet amphibolites were formed in the subduction-collisional setting in late Paleozoic period.