Abstract:

In the vicinity of Wujianfang Village, Lianggang Town within the Trans-North China orogen (TNCO) of the North China craton (NCC), we have discovered a set of garnet amphibolitesduring the 1/50 000 regional geologic survey. The garnet amphibolites were exposed as lenses in the paragneisses of Yuanfang Formation of the Fuping Group, showing typical “white-eye socket” structure, indicating that they were probablyretrograded eclogites or high-pressure mafic granulites. In order to gain a deeper understanding of the metamorphic genesis and geological significance of this suite of garnet amphibolites, detailed studies were conducted on their petrology, mineral chemistry, geochemistry, metamorphic evolution, and zircon U-Pb geochronology. The results suggest that they underwent three stages of metamorphic evolution: An early prograde metamorphic stage (M₁), a peak high-pressure granulite facies stage (M₂), and a retrograde stage (M₃). The early prograde stage features mineral assemblages of Am+Pl+Q inclusions within garnet, with metamorphic p-T conditions of approximately 756.0 ℃/ 6.0×10⁸ Pa; The peak high-pressure granulite facies stage shows a mineral assemblage of Grt+Cpx+Pl+Q+Melt, with p-T conditions of 884.1-987.5 ℃ / (12.2-14.2) ×10⁸ Pa; The retrograde stage is characterized by a “white-eye” structure around garnet and partial replacement of clinopyroxene by late amphibolewith residual ilmenite, forming a mineral assemblage of Am+Pl+Q+Ilm ± Grt and showing p-T conditions of 706.6-776.7 ℃/(3.7-4.2)×10⁸ Pa; The metamorphic evolution of these garnetamphibolites generally exhibits aclockwise p-T includinga near-isothermal decompression (ITD) process. LA-ICP-MS zircon U-Pb dating indicates a retrograde metamorphic age range of 1.86-1.84 Ga. The geochemical characteristics reveal that their protoliths were tholeiitic basalt formed under an island arc environment of convergent plate margin. Combined with previous studies, the garnetamphibolites exposed in the Lianggang region are retrograde products of high-pressure granulites,which are similar to the formation process of the Hengshan-Huai’an-Xuanhua-Chengde high-pressure granulite belt of the TNCO. This suggests they may also be a result of the collision between the eastern and western block that occurred at about 1.85 Ga. This understanding extends the high-pressure granulite and retrograde eclogite belt in the north-central part of the TNCO southeastward to the Yixian-Laiyuan area in Hebei Province.

Key words: Fuping complex')">

Fuping complex, garnet amphibolite, high-pressure granulite, geochemistry, geochronlogy')">zircon U-Pb geochronlogy, Trans-North China orogen of North China craton, Lianggang Town, Yixian County, Hebei Province ')">Lianggang Town, Yixian County, Hebei Province