大别造山带超高压变质岩折返隆升的地层学证据——毛坦厂组榴辉岩砾石的启示

Stratigraphic Evidence of the Exhumation and Uplift of Ultrahigh-Pressure Metamorphic Rocks in the Dabie Orogenic Belt: Enlightenment from Eclogite Gravels in the Maotanchang Formation

大别山北缘地区发育厚达近万米的中新生代碎屑岩,它们记录着大别造山带自侏罗纪以来的演化历史。在安徽省六安地区毛坦厂组中,发现多块榴辉岩砾石。砾石新鲜,质地坚硬,表明属于第一旋回砾石。榴辉岩由石榴子石、多硅白云母、绿帘石、石英、金红石等组成。具有明显的退变质作用,发育以钠长石和闪石类组成的后成合晶、以及石榴子石周边的次变边。石榴子石以铁铝榴石和钙铝榴石为主,属于C类榴辉岩。根据其特征应该属于大别山南部和北部超高压榴辉岩。毛坦厂组层位确切,古生物化石和同位素年龄都表明以晚侏罗世为主。这一发现表明,在晚侏罗世,以榴辉岩为代表的造山带根部物质,即:超高压变质岩已经出露地表,并作为毛坦厂组的物源。因此,大别造山带超高压变质岩折返到地表,最迟在晚侏罗世。

Mesozoic-Cenozoic clastic rocks of ten thousands metres thick, on the northern flank of the Dabie Mountains recorded the evolution of the Dabie orogenic belt since the Jurassic. Gravels composed of eclogites were founded in the Late Jurassic Maotanchang Formation in the Lu'an region, Anhui Province. The gravels are hard and without weathering, and thus belong to the first cycle. The eclogites are composed of garnet, phengite, epidote, quartz, rutile, etc. , which marked retrogressive process and developed symplectites composed of albite and amphibole, etc. , and symplectites around garnet crystals. The garnets consist mainly of almandine and grossularite, and belong to C-type eclogite. Petrologic features suggest that the eclogites came from ultrahigh-pressure metamorphic rocks developed in the South and North of Dabie Mountains. The Stratigraphic age of the Maotanchang Formation is certain and mainly belongs to the Late Jurassic based on fossils and isotopic ages. The discovery indicates that eclogites at the base of the Dabie orogenic belt emerged on the earth's surface in the Late Jurassic and became the provenance of the Maotanchang Formation. So, it is in the Late Jurassic that the ultrahigh-pressure metamorphic rocks in the Dabie orogenic belt exhumed, uplifted to the surface, and were subject to violent denudation.