Acta Petrologica et Mineralogica

For the first time, the HP-UHP(high-pressure to ultrahigh-pressure) garnet quartzite has been discovered from the Late Paleozoic Hongqiyingzi metamorphic supracrustal rock characterized by amphibolite facies paragneisses in Chongli County, North Hebei Province. The garnet quartzite occurs as separate tectonic lenses within garnet biotite plagioclase gneisses. It mainly consists of garnet and quartz, with a small amount of amphibole, plagioclase and barite,and possesses accessory mineral assemblage of apatite, ilmenite and copper-iron sulfide. Garnets are characterized by a high Grs+And value (23.08%~29.38%) with the end-member components of Alm_51.70~61.87 Grs_23.08~28.45 Prp_11.31~16.75Sps_1.25~2.45, belonging to almandine-pyrope-grossular garnet series. Compared with the HP granulites of North China Craton and retrograded eclogite in Chicheng County, the garnets of studied samples are richer in Grs+And and poorer in Prp than the former, and poorer in Prp than the latter. Apatite is fluorapatite with the contents of fluorine from 1.65% to 3.29%. Garnets surrounding the apatite inclusions often have radial decompression expansion joints, which is interpreted as the result of retrogression of HP-UHP metamorphic rock. Barite may be the result of decompression and dissolution of apatite during the exhumation of HP-UHP garnet quartzite, which is accompanied by the formation of copper iron sulfide. Thermobarometric computation shows that the temperature and pressure conditions for the formation of garnet quartzite are 667~710℃ and 1.189~1.279 GPa, respectively, which are consistent with the pressure properties shown by mineralogical characteristics. Zircon SHRIMP U-Pb isotopic dating obtains two different groups of ages which from detrital zircons and metamorphic zircons separately. The former has higher value of Th/U from 0.23 to 3.81, which indicates that typical of magmatic crystallized zircon, ranging from 485. 5 to 304.9 Ma and mainly from 369.8 to 348.8 Ma with a weighted mean age of 359.4±3.8 Ma. The protolith of garnet quartzite may be the terrigenous clastic rock at the continental margin, and the clastics chiefly from Late Paleozoic (~359 Ma) magmatic rocks. The latter has lower value of Th/U from 0.05 to 0.18, ranging from 277.0 to 259.4 Ma, with a weighted average age of 268.1±2.9 Ma, which is interpreted as the peak metamorphic age of HP-UHP garnet quartzite, indicating that HP-UHP metamorphism occurred in the late Paleozoic (~268 Ma), and may be related to the addition of some continental crust materials during the southward subduction of the ancient Paleo-Asian oceanic crust.

HP-UHP metamorphism; Late Paleozoic; garnet quartzite; Paleo-Asian Ocean; Chongli County; North Hebei Province