摘要：
鞍山是华北克拉通唯一保存有完整太古宙地质记录(3.81～2.5Ga)的地区.为了进一步了解本区太古宙早期花岗质地壳的形成及演化,本文对鞍山地区典型的太古宙杂岩——深沟寺杂岩进行了详细的岩石学、年代学及地球化学研究.根据岩石学及产状分析确定了始太古代奥长花岗质片麻岩和古太古代混合岩化片麻岩杂岩两个主要岩石单元:前者SIMS锆石U-P定年结果为3803 ±9Ma,代表始太古代古老的花岗质陆壳;后者年龄为3311～3335Ma,代表古太古代强烈构造-变质热事件.始太古代奥长花岗岩以高Si、富Na、贫K为特征;轻重稀土分异不明显,(La/Yb)N值介于6.76～35.96,具有弱的负铕异常(δEu=0.57～1.43);微量元素相对富集Rb、La、Hf和Zr等,亏损Nb、Ta、Sr和Ti等,且Sr/Y比值(8～29)、Mg# (21～30)、Cr(8.81×10-6～28.00×10-6)和Ni(0.881×10-6～18.55×10-6)含量较低,表明源区无地幔物质参与,且未与石榴子石发生平衡,斜长石可能残留源区,形成深度相对较浅.同时结合鞍山地区～3.8Ga锆石具有正的εHf(t)值,始太古代奥长花岗岩来源于地幔岩浆底侵导致地壳底部含水玄武质岩石(角闪岩)部分熔融形成的新生地壳.古太古代混合岩化片麻岩杂岩的主要矿物组合为石英+斜长石+黑云母,无不一致相矿物(钾长石、矽线石和石榴子石等),表明熔融过程中含水矿物未发生脱水反应,主要反应为Qtz+ Pl±H2O=melt.此外,混昆合岩化片麻岩杂岩中未见富铝矿物和碎屑岩结构,推断其为正片麻岩,原岩为奥长花岗岩或英云闪长岩;结合岩石中存在大量始太古代继承锆石,且～3.3Ga锆石εHf(t)多为负值,混合岩化片麻岩杂岩形成于始太古代奥长花岗质地壳的重熔.本文研究结果表明,鞍山地区始古太古代花岗质地壳演化可划分为两个主要阶段:始太古代奥长花岗质地壳生长(～3.8Ga)和古太古代花岗质地壳重熔(～3.3Ga).%Anshan is the only area in the North China Craton which preserves one complete set of Archean geological record (3.81 ～ 2.5 Ga).In this paper,detailed petrological,chronological and geochemical studies were carried out on the typical Archean Shengousi complex to further understand the formation and evolution of the early granitic crust.Petrologic and occurrence analysis of the Shengousi complex section determined two major rock units:Eoarchean trondhjemitic gneiss and Paleoarchean migmatitic gneiss complex.SIMS zircon U-Pb ages demonstrate that the trondhjemitic gneiss formed at 3803± 9 Ma,which represents the Eoarchean trondhjemitic crust.The migmatitic gneiss complex has an age of 3311～3335 Ma,representing a Paleoarchean tectonic-metamorphic thermal event.The Eoarchean trondhjemite is characterized by high Si and Na and poor K,weakly fractionated rare earth element (REE) patterns ((La/Yb)N =6.76～35.96)with negative Eu anomalies (δEu=0.57～1.43),and rich in Rb,La,Hf and Zr,poor in Nb,Ta,Sr and Ti,low Sr/Yratios (8～29),Mgnumbers (21～30),Cr (8.81×10-6～28.00×10-6) and Ni (0.881×10-6～18.55 ×10-6) concentrations,indicating that the magmas formed relatively shallow,no mantle material was involved,and not balance reaction occurred within garnet and plagioclase,with plagioclase remained within the source area.Combined with the fact that ～3.8 Ga zircon of the Anshan area has a positive εHf(t) value,the Eoarchean trondhjemite may derive from a juvenile crust by partial melting of aqueous basaltic rocks (amphibolite) at the bottom crust due to mantle magma upwelling.The main mineral assemblages of the Paleoarchean migmatitic gneiss complex are quartz+plagioclase+biotite,without non-uniform phase minerals (e.g.,potassium feldspar,sillimanite and garnet),indicating that no dehydration reaction occurred within hydrous minerals and the melting reaction is Qtz+ Pl±H2O=melt.Moreover,neither aluminous mineral nor clastic structure has been found in the migmatitic gneiss complex,indicating that it should be orthogneiss with protolith rock likely being trondhjemite or tonalite.A large amount of Eoarchean inherited zircons has been found in the Paleoarchean migmatitic gneiss complex,and εHf(t) of the ～3.3 Ga zircons are mostly negative,suggesting that it formed from remelting of the Eoarchean trondhjemitic crust.This study results suggest that evolution of the Eo-Paleoarchean granitic crust in the Anshan area can be divided into two main stages:Eoarchean trondhjemitic juvenile crust growth (～3.8 Ga) and Paleoarchean granitic crust re-melting event (～3.3 Ga).
