摘要
华北克拉通1.8 Ga镁铁质岩墙群是我国规模最大的岩墙群之一。在晋冀蒙交界地区,岩墙群表现为NNW和EW向近直立具有冷凝边结构的岩墙,它们产出于统一构造体制之下。单条岩墙的长度达几十公里,宽度0.5～100m,通常20m左右。可以根据岩墙产状和岩石地球化学特征将这些岩墙群分成三组,简记为S-I、S-II和 S-EW,分别对应两组NNW向和一组EW向的岩墙群。岩墙的主要造岩矿物为单斜辉石和斜长石,其它矿物包括角闪石、Fe-Ti氧化物、黑云母、磷灰石、碱性长石和石英。S-II能见到橄榄石斑晶及围绕橄榄石和单斜辉石颗粒生长的斜方辉石和角闪石反应边结构,以及斜长石正环带结构。S-I相对低FeO(全)-TiO_2-P_2O_5,成分为相对高MgO拉斑玄武岩,而S-II高 FeO(全)-TiO_2-P_2O_5,由碱性到亚碱性高Fe玄武岩组成。S-EW包括高Fe拉斑玄武岩和安山岩。所有岩墙稀土总量(∑REE)和轻稀土含量(LREE)较高。它们相对富集Ba、K和P,并且亏损Nb和Ta。S-II和EW相对亏损Sr。分析表明,S-I表现为不同程度部分熔融的特征,内部又有一定的分离结晶作用和地壳混染过程。S-II表现为富铁贫硅(Fenner趋势)的分离结晶作用和明显的地壳混染。 而S-EW表现为富硅贫铁(Bown趋势)的演化过程,也有明显的地壳混染。我们认为岩墙群涉及两个或多个源区。
The 1.8 Ga dyke swarms in the North China Craton are among the largest ones in China. In the juncture of Shanxi, Hebei, and Inner-Mongolia, they presented as NNW and EW-oriented vertical to subvertical dykes with chilled margins from a uniform tectonic setting. The dykes are up to tens of kilometers in length, and 0.5 to 100 m in width. Three dyke swarms, S-I, S-II, and S-EW, are identified. They consist of clinopyroxene and plagioclase as rock forming minerals, and accessory amphibole, Fe-Ti oxides, biotite, apatite, alkaline-feldspar, and quartz as minor minerals. S-II is characterized by olivine phenocrysts and orthopyroxene + amphibole rim structures around olivine and clinopyroxene. S-I is tholeiite relatively high in MgO, with relatively low FeO (total) - TiO2 - P2O5; while S-II varies in composition from alkaline to sub-alkaline high-Fe basalt, with high FeO (total) - TiO2 - P2O5. S-EW includes high-Fe tholeiite basalt and andesite. All dykes have high total rare earth elements, and the light rare earth elements in dykes are enriched. They are relatively enriched in Ba, K, and P, and depleted in Nb and Ta in primitive mantle normalized diagrams. S-II and S-EW are depleted in Sr. S-I exhibits various degrees of partial melting, with slight fractionation and crustal assimilation. And S-II shows an iron-enriched and silica-poor trend ( the Fenner trend) and crustal assimilation. While S-EW performs a siliea-rich and iron-poor trend ( the Bowen trend) with strong crustal assimilation. We suggest that there are multiple sources, and the dyke swarms initiated the continental break-up, possibly associated with a plume.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2004年第3期439-456,共18页
Acta Petrologica Sinica
基金
国家自然科学重点基金(编号:40234050)
中国科学院重大创新项目(编号:KZCX1-07)资助
