华北吕梁地区2.4Ga A型花岗岩的确定及地质意义

Determination of the 2.4Ga A-type granite in Lvliang area of the North China Craton and its geological significance

华北克拉通新太古代末与古元古代早期的构造转化对认识华北克拉通早期地壳演化具重要意义。该克拉通中部带吕梁地区盖家庄正长花岗岩获得2398~2408Ma的形成年龄,其成因研究将为探讨华北克拉通新太古代晚期与古元古代早期构造演化提供重要信息。该岩体富K,高硅、碱、Fe OT/Mg O,低Ca、Mg,富集Rb、Th、U等LILE、较高的HFSE,明显亏损Sr、Ba、P、Ti,具弱轻重稀土分异和强负Eu异常的"燕式"稀土分配模式,并具高的Ga/Al比值和高锆石饱和温度,显示了碰撞造山后A2型花岗岩的特征。岩体全岩εNd(t)=2.0~2.3,两阶段Nd模式年龄tDM2=2606~2629Ma,锆石εHf(t)=+2.2^+7.1,tDM2(2494~2791Ma)略高或接近其形成年龄,一致证明主要源自新太古代中晚期新生下部陆壳物质的部分熔融,并有少量幔源物质的添加。结合区域地质及同期钾质花岗岩类的广泛出现,盖家庄正长花岗岩与同期钾质花岗岩一同代表了华北克拉通~2.5Ga形成后,中部构造带于古元古代早期~2.4Ga发生的一次陆壳伸展拉张作用引发的地幔物质上涌基性岩浆上侵导致下部新生陆壳物质增温发生部分熔融构造岩浆事件,证明陆壳已转入伸展拉张构造环境。

The understanding the tectonic transformation of the North China Craton（ NCC） from Late Neoarchean to Early Paleoproterozoic is significant for discussing early crustal evolution in NCC. The Gaijiazhuang granitic pluton is outcropped in Lvliang area of the Trans-North China Orogen（ TNCO） in NCC. The ages of 2398 - 2408 Ma was obtained using LA-ICPMS zircon U-Pb dating. Thus its petrogenesis and forming tectonic setting can provide us important information for Early Paleoproterozoic evolution of TNCO. The Gaijiazhuang granite was mainly composed of syenogranite,and geochemically characterized by the high Si,K,alkaline and Fe OT/ Mg O,low Ca,Mg,enrichment of LILE such as Rb,Th,U and HFSE as well as depletion of Sr,Ba,P,Ti,with a＂seagull-type＂pattern of slight REE fractionation between LREE and HREE and strong negative Eu anomaly,showing the features of typical A2-type granite formed in a post orogenic setting. They have positive εNd（ t） of 2. 0 - 2. 3 and narrow raging of tDM2（ 2606 -2629Ma）. Their zircon εHf（ t） also show positive values varying from ＋ 2. 2 to ＋ 7. 1,and the two-stage model ages is from 2494 Ma to2791Ma,slightly higher or close to its formation age. All demonstrate that Gaijiazhuang syenogranite was mainly derived from the Midlate Neoarchean lower crustal materials,and involved minor mantle-derived component. Combined with regional geology and the widespread coeval K-rich granitiods in TNCO, the Gaijiazhuang syenogranite together with the K-rich granitiods represent a tectonothermal event occurred at 2. 4Ga due to the partial melting of juvenile lower crustal materials trigged by the mantle upwelling,which resulted from the crustal extension after the - 2. 5Ga blocks amalgamation of NCC,suggesting that the NCC has been entered into the extensional tectonic environment during the Early Paleoproterozoic of - 2. 4Ga.