天镇-怀安地区变质基性岩墙群:华北克拉通古元古代末期碰撞-伸展构造体制转换标志

古元古代是华北克拉通形成过程中重要的造山构造演化阶段，该阶段形成的基性岩墙群，为深入理解裂解-俯冲-碰撞-抬升的造山构造-岩浆过程提供了重要信息.本文报道了天镇-怀安地区广泛分布于新太古代-古元古代变质基底中的变质基性岩墙（二辉麻粒岩），野外产状与区域主期构造面理协调一致，主要由单斜辉石、斜方辉石、斜长石和少量角闪石组成.LA-MC-ICPMS锆石U-Pb同位素定年获得变质基性岩墙的变质年龄为1 820~1 834 Ma，与区内麻粒岩相变质事件一致，结合区域基性岩墙年龄记录，推测其原岩形成年龄为1.95~1.91 Ga.根据岩石地球化学特征可将变质基性岩墙划分为高Mg低Ti型和低Mg高Ti型两类，两者经历了不同程度的橄榄石、单斜辉石和斜长石的分离结晶.两类基性岩墙均亏损高场强元素（如Nb、Ta、Ti、Zr和Hf），结合锆石Hf同位素分析，研究表明基性岩墙来源于俯冲流体交代的岩石圈地幔或者受到过地壳物质的混染.华北克拉通古元古代存在2.16~2.04 Ga和1.97~1.83 Ga两期基性岩墙侵位事件:早期代表在初始克拉通基础上发生的板内裂解过程，晚期记录了由俯冲碰撞到伸展的转换过程，即碰撞造山构造体制由水平挤压转变为垂向抬升，构造转换时限大致介于1.95~1.91 Ga. 

Metamorphic Mafic Dykes from Tianzhen-Huai'an Area: Transformation Criteria of the Late Paleoproterozoic Collision to Extension in the North China Craton

Paleoproterozoic is an important orogenic tectonic evolution stage during the formation of the North China Craton. The mafic dyke swarms formed during this period provide important information to understanding the tectonic-magmatic process of rifting-subduction-collision-exhumation of the orogeny. This study reports the metamorphic mafic dykes (two-pyroxene granulite) widely distributed in the Neoarchean-Paleoproterozoic metamorphic base in the Tianzhen-Huai'an area. These dykes occur consistently with regional main tectonic foliation in the field and are mainly composed of clinopyroxene+orthopyroxene+plagioclase+amphibole. The metamorphic age of the mafic dyke is 1 820-1 834 Ma obtained by LA-MC-ICPMS zircon U-Pb isotope dating, which is consistent with the granulite facies metamorphic event in the study area. Based on the analyses of regional mafic dyke age data, we consider that the emplacement age is around 1.95-1.91 Ga. This study documents two types of metamorphic mafic dykes: high Mg low Ti type and low Mg high Ti type. They have experienced different degrees of fractional crystallization of olivine, clinopyroxene and plagioclase. Both types of mafic dykes show negative anomalies in high-field-strength elements (such as Nb, Ta, Ti, Zr, and Hf). According to the zircon Hf isotopic and the geochemical features, we suggest that the mafic dykes originated from a lithospheric mantle metasomatized by subduction fluids, or/and they were contaminated by the crust. Two episodes of metamorphosed mafic dykes are identified in the North China Craton: 2.16-2.04 Ga and 1.97-1.83 Ga. The early stage represents an intra-plate rifting process that occurred on the basis of the initial Craton; in contrast, the later period records transformation from subduction-collision to extensional setting, that is the collisional orogenic tectonic regime changed from horizontal compression to vertical uplift, and the tectonic transition time is roughly around 1.95-1.91 Ga.