华北东南缘前寒武纪下地壳的生长和变质演化

Growth and metamorphic evolution of the Precambrian lower crust at the southeastern margin of the North China Block

华北陆块东南缘前寒武纪下地壳岩石主要以高级变质地体或麻粒岩地体和中生代闪长斑岩中（麻粒岩）捕虏体两种形式存在,它们为研究该区前寒武纪下地壳的形成和演化提供了极好的天然实验室。变质地体主要分布于霍邱和蚌埠地区,包括原＂霍邱群＂（霍邱杂岩）及＂五河群＂和＂凤阳群＂（五河杂岩）等。其中,霍邱杂岩主要由白云斜长片麻岩、石英岩、云母片岩、大理岩、变质砂岩、条带状铁建造（BIF）和斜长角闪岩等组成,但地表已被第四纪覆盖;五河杂岩主要含有石榴斜长角闪岩/榴闪岩、石榴麻粒岩、异剥钙榴岩、石榴斜长角闪片麻岩、花岗片麻岩、云母片岩、大理岩和变质砂岩等变质岩。相比较,五河杂岩大多出露地表,主要由变质的镁铁质和长英质火成岩以及表壳岩系组成,并伴生有古元古代片麻状钾长花岗岩和中生代花岗岩类,构成了＂蚌埠隆起＂。这些不同类型的变质岩常具有类似的峰期变质矿物,如石榴子石、单斜辉石、斜长石、金红石和石英等,结合其锆石U-Pb年龄,表明它们大多数都经历了古元古代高压麻粒岩相变质作用。综合的变质岩石学、岩石地球化学、Hf同位素及锆石U-Pb年代学研究表明,该区前寒武纪下地壳经历了幕式生长以及多阶段变质演化与改造。强烈的构造-热事件和变质改造时间主要集中于2.7～2.8Ga、2.5～2.6Ga、～2.1Ga、1.8～1.9Ga、390Ma和176Ma,而前寒武纪下地壳的形成时间≥2.1Ga。在综述、分析相关成果的基础之上,作者提出了华北东南缘前寒武纪下地壳变质岩石研究方面存在的重要科学问题与展望。

The Precambrian lower-crustal rocks at the southeastern margin of the North China Block （NCB） occur as high-grade metamorphic or granulite terrains and xenoliths in the Mesozoic dioritic porphyry. These rocks provide an excellent natural laboratory to study formation and evolution of the Precambrian lower crust in the region. The metamorphic terrains distribute over the Huoqiu and Bengbu areas, mainly composed of ＂ Huoqiu Group＂ （Huoqiu complex） and ＂ Wuhe Group＂ and ＂ Fengyang Group＂ （Wuhe complex）. The Huoqiu complex comprises muscovite-plagioclase gneiss, quartzite, mica schist, marble, meta-sandstone, banded iron formations （BIF） and amphibolite, which are mostly covered by Quaternary. In comparison, the Wuhe complex is chiefly exposed on the surface, and consists of the metamorphosed igneous mafic and felsic rocks and supracrustal rocks associated with the Paleoproterozoic foliated K-feldspar granites and Mesozoic granitoids, constituting the Bengbu uplift. Its main rock types comprise a variety of lithologies, including garnet amphibolite, garnet granulite, rodingite, garnet-plagioclase-amphibole gneiss, granitoid gneiss, mica schist, marble, quartzite and meta-sandstone. These various Precambrian metamorphic rocks generally contain similar peak minerals such as garnet, clinopyroxene, plagioclase, rutile and quartz. As a result, combined with their zircon U-Pb dating, it is suggested that they mostly suffered from the HP granulite-faeies metamorphism at Palaeoproterozoic. Integrated investigations on metamorphic petrology, geochemistry, Hf-isotope and zircon U-Pb geochronology of the rocks suggest that the Precambrian lower crust beneath the region experienced an episodic growth and multiple metamorphic evolution and modification. Intensive tectono-thermo events and metamorphic overprinting for them mainly occurred at several peaks such as 2.7 ~ 2. 8Ga, 2. 5 ~ 2. 6Ga, 2. 1Ga, 1.8 ~ 1.9Ga, 390Ma and 176Ma in the southeastern margin of the NCB. However, the formation time of the Precambrian lower crust was ≥ 2. 1Ga. Based on the related investigations, we propose the present scientific problems on the Precambrian lower-crustal metamorphic rocks at the southeastern margin of the NCB and their related prospective projects.