华北克拉通怀安杂岩中早前寒武纪两期变质表壳岩的重新厘定:岩石学及锆石U-Pb年代学证据

Re-Definition of the Two-Stage Early-Precambrian Meta-Supracrustal Rocks in the Huai'an Complex,North China Craton:Evidences from Petrology and Zircon U-Pb Geochronology

华北克拉通怀安杂岩中早前寒武纪变质表壳岩具有变质程度深(麻粒岩相)、连续性差、变形及构造置换强烈等特征,地层划分及形成时代一直存在较大争议.通过详细的地质调查、岩石学及年代学研究,新厘定出新太古代桑干岩群铁英岩岩组、石榴黑云片麻岩岩组以及古元古代集宁岩群黄土窑岩组石榴高压基性麻粒岩-大理岩岩段和含石墨矽线石榴片麻岩-变粒岩-大理岩岩段4套变质表壳岩单元,并建立了相应的岩石学识别标志.新太古代变质表壳岩一般呈透镜体或带状体产出于新太古代变质深成岩中,且锆石年龄存在～2.5Ga和～1.8Ga两个峰值.铁英岩岩组原岩为一套基性火山岩夹磁铁石英岩建造,产出阿尔戈马型BIF铁矿,形成年龄为2 489±19Ma;石榴黑云片麻岩岩组主体岩石为条带状石榴黑云斜长片麻岩,原岩为杂砂岩建造,物源来自新太古代TTG岩石.古元古代石榴高压基性麻粒岩-大理岩岩段原岩为一套基性火山岩夹大理岩建造,形成年龄早于～2.03Ga;含石墨矽线石榴片麻岩-变粒岩-大理岩岩段野外呈带状构造岩片体产出,连续性较好,为一套"有层无序"的孔兹岩组合,碎屑锆石年龄介于2.30～1.99Ga,与区域上孔兹岩年龄结构一致,形成时代为古元古代晚期(1.99～1.95Ga).新太古代和古元古代变质表壳岩均卷入古元古代末期的造山过程(1.95～1.80Ga),遭受麻粒岩相变质与变形作用.区内孔兹岩可能是造山过程卷入的构造岩片.本研究进一步明确了晋冀蒙交界地区存在与孔兹岩不同的新太古代陆源碎屑岩和古元古代洋壳残片,这对重新认识本区早前寒武纪地质演化具有重要意义.

The Early-Precambrian meta-supracrustal rocks in the Huai’an Complex of the North China Craton are characterized by high-grade metamorphism(granulite-facies),poor continuity,strong deformation and structural transposition,etc.Divisionof the stratigraphic units and their formation ages are still controversial.Through detailed geological survey,petrology and geochronological studies,four meta-supracrustal rock assemblages are newly defined,which are the Neoarchean Sanggan Group magnetite-quartzite Formation(MQF),garnet-biotite-plagiogneiss Formation(GBPF)and Paleoproterozoic Jining Group amphibolites marble Formation(AMF),graphite-sillimanite-garnet gneiss Formation(GSGF).Their corresponding petrological marks are also established.Field investigation reveals that the Neoarchean meta-supracrustal rocks usually appear as lenticular inclusions or banded sheet outcropped in the Neoarchean metamorphic plutonites,and the zircon age shows two peaks at^2.5Ga and^1.8Ga.The protolith of the MQF is a suite of basic volcanic rocks with magnetite quartzite,which produced Algoma-type BIF iron ore and formed at 2 489±19Ma.The main rock type of GBPF is banded garnet-biotite-plagiogneiss and its protolith is greywacke,indicating that they originated from the Neoarchean TTG rocks.The protolith of the Paleoproterozoic AMF is a suite of basic volcanic rocks intercalated with marble,which formed earlier than^2.03 Ga.The GBPF,which is a suite of Khondalite,presents ribbon-like tectonic slices field outcrops and is characterized by good continuity and stratified but disordered structure.The detrital zircon age of the GBPF ranges from 2.30 to 1.99 Ga,which is regionally consistent with the age of the Khondalite,while its formation age is Late Paleoproterozoic(1.99-1.95Ga).The Neoproterozoic and Paleoproterozoic meta-supracrustal rocks were all involved by the Late Paleoproterozoic orogeny(1.95-1.80Ga)and underwent granulite-facies metamorphism and deformation.The Khondalites may be exotic tectonic slices during the Late Paleoproterozoic orogeny.This study clarifies that Neoarchean continental clastic rock and Paleoproterozoic oceanic crust exist at the border of Shanxi,Hebei,and Inner Mongolia provinces,which are different from the Khondalites.This may shed light on a new understanding of the geologic evolution of the Early Precambrian Huai’an Complex of the North China Craton.