Metamorphic evolution of major tectonic units in the basement of the North China Craton: Key issues and discussion

Abstract

The basement of the North China Craton consists of three micro-continental blocks (Eastern Block, Yinshan Block and Ordos Block) and three Paleoproterozoic mobile belts ( Khondalite Belt, Trans-North China Orogen and Jiao-Liao-Ji Belt). These tectonic units are distinctly different in metamorphic time and P-T evolution. Metamorphism of the Late Archean basement rocks in the Eastern and Yinshan Blocks occurred at ∼2. 5Ga, characterized by anticlockwise P-T paths involving isobaric cooling, suggesting the origin of the metamorphism related to the underplating of numerous mantle-derived magmas. The major metamorphic event of the Khondalite Belt occurred at ∼ 1. 95Ga, and its metamorphic evolution is characterized by clockwise P-T paths involving near-isothermal decompression, which is interpreted as a result of collision between the Yinshan and Ordos Block to form the Western Block. The metamorphism of the Trans-North China Orogen occurred at ∼ 1. 85Ga, and is also characterized by clockwise P-T paths involving nearisothermal decompression, reflecting the final amalgamation of the united Western Block and the Eastern Block to form the coherent basement of the North China Craton. The metamorphism of the Paleoproterozoic Jiao-Liao-Ji Belt is featured by paired metamorphic zones of which the northwestern zone consists of the North Liaohe, Laoling and Fenzishan groups, metamorphism of which is characterized by medium-pressure-type clockwise P-T paths, whereas the southeastern zone consists of the South Liaohe, Ji'an and Jingshan groups, metamorphism of which is characterized by low-pressure-type anticlockwise P-T paths. Compared with the traditional 1 : 4,000,000 metamorphic map of China published in 1986, the newly compiled metamorphic evolution map of the North China Craton can be better used to reflect metamorphic histories of different tectonic units in the craton. Although magmatic arc, continental rifting and mantle plume models can all be applied to interpretations of the metamorphism of the Eastern Block with IBC-type anticlockwise P-T paths, a mantle plume model is favored because it can reasonably explain the presence of an 800km wide igneous province in which all rocks were nearly simultaneously emplaced at 2. 60 ∼2. 50Ga, numerous komatiites or komatiitic rocks, biomodal volcanic rocks, domiform structures, etc. In the North China Craton, the exposures of typical high-pressure basic granulites with garnet-clinopyroxen-plagioclase-quartz assemblage and high-pressure pelitic granulites with kyanite-Kf-feldspar assemblage are only restricted to the northern segment of the Trans-North China Orogen and the southern segment of the Jiao-Liao-Ji Belt, and their formation was related to subduction and collision operative in these Paleoproterozoic mobile belts. The local ultrahigh temperature ( UHT) metamorphism with anticlockwise P-T paths in the Khondalite Belt, represented by sapphirine granulites in the Daqingshan and Jining Complexes, most likely resulted from underplating of mantle-derived magmas during the post-collisional extension at ∼ 1. 92Ga, as indicated by the sapphirine granulites which are always in direct contact with ∼ 1. 92Ga basic dykes.

華北克拉通基底可分為三個太古宙微陸塊(東部陸塊、陰山陸塊和鄂爾多斯陸塊)和三個早元古宙活動帶(孔茲巖帶、華北中部帶和膠-遼-吉帶)。這些構造單元具有不同的變質作用時間和P-T演化特征。東部陸塊和陰山陸塊晚太古宙基底巖系的變質作用發生在～2.5Ga,變質演化以等壓冷卻(IBC)逆時針P-T軌跡為特征,反映變質作用的成因與大規模地幔巖漿底侵有關。孔茲巖帶主期變質作用發生在～1.95Ga,變質演化以近等溫減壓(ITD)順時針P-T軌跡為特征,反映陰山陸塊與鄂爾多斯陸塊碰撞形成西部陸塊的熱構造過程。華北中部帶變質作用發生在～1.85Ga,變質演化同樣以近等溫減壓(ITD)順時針P-T軌跡為特征,反映了西部陸塊和東部陸塊最終碰撞形成統一的華北克拉通基底的構造過程。早元古宙膠-遼-吉帶變質作用表現‘雙變質帶’特征:西北帶的北遼河群、老嶺群和粉子山群的變質作用以中壓順時針P-T軌跡為特征,而東南帶的南遼河群、吉安群和荊山群的變質作用以低壓逆時針P-T演化為特征。華北克拉通基底變質作用演化地質圖能更好地反映上述不同構造單元的變質作用演化特征。盡管巖漿弧、大陸裂谷和地幔柱模式都能解釋東部陸塊晚太古宙基底變質作用所具有的近等壓冷卻(IBC)逆時針P-T演化特征,地幔柱模式能夠更合理解釋東部陸塊所存在的寬達800千米而時代近于相同的晚太古代火成巖帶、大量科馬提質超鎂鐵質巖石和雙峰式火山巖、廣泛發育的穹窿構造等。華北克拉通變質基底中具有石榴石-單斜輝石-斜長石-石英組合的高壓基性麻粒巖和具有藍晶石-鉀長石組合的高壓泥質麻粒巖的出露只局限在早元古宙華北中部帶的北段和膠-遼-吉帶的南端;這些高壓麻粒巖形成在俯沖和陸-陸碰撞的構造環境中。西部陸塊孔茲巖帶含假藍寶石麻粒巖是碰撞后(～1.92Ga)拉伸引發地幔巖漿底侵導致局部地帶發生超高溫(UHT)變質作用的產物。