冀北崇礼高压-超高压榴英岩的发现及其矿物化学和锆石U-Pb年代学特征  

Discovery of HP-UHP garnet quartzite from Chongli County,North Hebei Province,and its mineral chemistry and zircon SHRIMP U-Pb chronology

作　　者：刘瑶 倪志耀[2] 牛腾 周新 潘登 田涛 LIU Yao;NI Zhi-yao;NIU Teng;ZHOU Xin;PAN Deng;TIAN Tao(China Railway Eryuan Engineering Group Co.Ltd,Chengdu 610000,China;College of Earth Sciences,Chengdu University of Technology,Chengdu 610059,China;Geothermal and Geological Party,Bureau of Mineral Resource Exploration and Development of Tibet,Lhasa 850000,China;Shaanxi Academy of Environmental Sciences,Xi'an 710000,China)

机构地区：[1]中铁二院工程集团有限责任公司,四川成都610000 [2]成都理工大学地球科学学院,四川成都610059 [3]西藏地质矿产勘查开发局地热地质大队,西藏拉萨850000 [4]陕西省环境科学研究院,陕西西安710000

出　　处：《岩石矿物学杂志》2023年第3期313-328,共16页Acta Petrologica et Mineralogica

基　　金：国家自然科学基金资助项目(41272087,41072050,40472041)。

摘　　要：首次在冀北崇礼晚古生代红旗营子变质表壳岩中发现了高压-超高压榴英岩,并对其进行了矿物化学研究及锆石U-Pb年代学限定。该高压-超高压榴英岩呈透镜状产出,直接围岩为石榴黑云斜长片麻岩,矿物成分主要为石榴子石和石英,有少量角闪石、斜长石和重晶石,副矿物可见磷灰石、钛铁矿以及铜铁硫化物等。石榴子石以高的Grs+And值(23.08%~29.38%)为特征,属铁铝榴石-钙铝榴石-镁铝榴石系列,铁铝榴石分子、钙铝榴石分子和镁铝榴石分子分别为51.70%~61.87%、23.08%~28.45%和11.31%~16.75%,另有少量的锰铝榴石分子(1.25%~2.45%)。磷灰石为氟磷灰石,F含量介于1.65%~3.29%之间,磷灰石包裹体周边石榴子石颗粒常具放射状减压膨胀缝,显示出高压-超高压变质岩的退变质特征。重晶石可能为磷灰石在高压-超高压榴英岩的折返过程中减压分解出溶的结果,该过程同时伴随有铜铁硫化物的形成。榴英岩形成的温压条件为667~710℃、1.189~1.279 GPa,与矿物学特征所显示的压力属性相一致。榴英岩锆石SHRIMP U-Pb定年获得两组不同的年龄数据,其中碎屑锆石的Th/U值较高(0.23~3.81),年龄介于485.5~304.9 Ma之间,主要为369.8~348.8 Ma,加权平均年龄为359.4±3.8 Ma,对应的原岩可能为大陆边缘的陆源碎屑岩,物源主要为晚古生代(约359 Ma)岩浆岩;变质锆石的Th/U值较低(0.05~0.18),年龄介于277.0~259.4 Ma之间,加权平均年龄为268.1±2.9 Ma,应该是高压-超高压榴英岩的峰值变质年龄,表明高压-超高压变质作用发生在晚古生代(约268 Ma),可能与古亚洲洋洋壳向南俯冲消减过程中部分陆壳物质的加入有关。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_(11.31~16.75)Sps_(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 ma

关 键 词：高压-超高压变质作用 晚古生代 榴英岩 古亚洲洋 冀北崇礼 

分 类 号：P588.34[天文地球—岩石学] P597.3[天文地球—地质学]