【作者】 唐俊； 郑永飞； 吴元保； 龚冰；

【Author】 TANG Jun ZHENG Yongfei WU Yuanbao GONG Bing CAS Key Laboratory of Crust-Mantle Materials and Environments,School of Earth and Space Sciences,University of Science and Technology of China,Hefei,Anhui,230026

【机构】 中国科学院壳幔物质与环境重点实验室中国科学技术大学地球和空间科学学院；

【摘要】 在苏鲁超高压变质带东北段威海地区皂埠镇发现锆石δ¹⁸O 值低至-7.8‰左右的变质花岗岩,对这些低δ¹⁸O 值变质花岗岩进行了锆石SHRIMP 法U-Pb 定年和系统的激光氟化法矿物氧同位素分析,结果对低δ¹⁸O 值锆石成因和变质花岗岩的原岩性质提供了制约。研究得到:①这些低δ¹⁸O 值锆石以新元古代岩浆锆石为主,但部分岩浆锆石在印支期超高压变质作用过程中发生了不同程度的重结晶作用。δ¹⁸O 值为～7.08‰的岩浆核锆石U-Pb 定年得到的变质花岗岩原岩谐和年龄和不一致线上交点年龄分别为760±49 Ma 和751±27 Ma,变质谐和年龄和不一致线下交点年龄分别为232±4 Ma 和241±33 Ma,指示其原岩为新元古代花岗岩并经历了印支期变质作用;②锆石δ¹⁸O 值在局部范围内变化于-7.76‰～5.40‰之间,低δ¹⁸O 值岩浆锆石表明它们是从新元古代低δ¹⁸O 值岩浆中直接结晶形成,锆石δ¹⁸O 值的局部变化表明其原岩岩浆的氧同位素组成具有不均一性,指示低δ¹⁸O 值岩浆源区物质曾经在地表与极度亏损δ¹⁸O 的大气降水发生过不同程度的高温水岩反应。威海皂埠镇地区和东海青龙山地区的花岗片麻岩在原岩时代、变质时代和氧同位素组成等方面基本相同,指示其应具有相同的原岩性质,并经历了相同的变质作用和水岩相互作用过程。因此,极度亏损δ¹⁸O 的新元古代双峰式基性-酸性岩浆岩可能分布于整个大别-苏鲁造山带。更多还原

【Abstract】 Extremely¹⁸O-depleted zircons in granitic gneisses,whose δ¹⁸O values are as low as-7.8‰,were found in Zaobuzhen ofWeihai area,the northeastern end of the Sulu orogenic belt.SHRIMP zircon U-Pb ages and oxygen isotope compositions were deter-mined for the low δ¹⁸O zircons.CL images reveal that the low δ¹⁸O zircons are primarily of igneous origin,but some grains have un-dergone metamorphic recrystallization.The igneous zircons from a granitic gneiss sample yield a concordant U-Pb age of 760±49 Maand an upper intercepted age of 751±27 Ma,indicating a protolith of middle Neoproterozoic age.Metamorphic zircons from the samesample yield a concordant U-Pb age of 232±4 Ma and a lower intercepted age of 241±33 Ma,pointing to the Triassic UHP meta-morphism.Most of the igneous zircons have unusually variable δ¹⁸O values of-7.76—5.40‰,implying that the gneiss protolithwas intruded as low δ¹⁸O magma generated by partial melting of altered rocks which had suffered intense water-rock interaction witha low δ¹⁸O fluid at high temperatures during Neoproterozoic.更多还原