威海地区超高压变质花岗岩成因：锆石U-Pb定年和氧同位素研究  

Zircon U-Pb Dating and Oxygen Isotopes of UHP Metagranite from Weihai Area, Northeast Sulu Orogenic Belt

作　　者：唐俊[1] 郑永飞[1] 吴元保[1] 龚冰[1] 

机构地区：[1]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,安徽合肥230026

出　　处：《地球学报》2005年第B09期112-114,共3页Acta Geoscientica Sinica

基　　金：本文为国家自然科学基金项目（编号：40334036）资助.

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δ^18O值低至-7．8‰左右的变质花岗岩，对这些低占δ^18O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，结果对低占δ^18O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这些低占δ^18O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。δ^18O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±49Ma和751±27Ma，变质谐和年龄和不一致线下交点年龄分别为232±4Ma和241±33Ma，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锆石占δ^18O值在局部范围内变化于-7．76‰～5．40‰之间，低占δ^18O值岩浆锆石表明它们是从新元古代低占δ^18O值岩浆中直接结晶形成，锆石占δ^18O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，指示低占δ^18O值岩浆源区物质曾经在地表与极度亏损δ^18O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δ^18O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。Extremely δ^18O-depleted zircons in granitic gneisses, whose δ^18O values are as low as - 7.8‰, were found in Zaobuzhen of Weihai area, the northeastern end of the Sulu orogenic belt. SHRIMP zircon U-Pb ages and oxygen isotope compositions were determined for the low δ^18O zircons. CL images reveal that the low δ^18O zircons are primarily of igneous origin, but some grains have undergone metamorphic recrystallization. The igneous zircons from a granitic gneiss sample yield a concordant U-Pb age of 760 ± 49 Ma and an upper intercepted age of 751 ± 27 Ma, indicating a protolith of middle Neoproterozoic age. Metamorphic zircons from the same sample yield a concordant U-Pb age of 232 ±4 Ma and a lower intercepted age of 241 ± 33 Ma, pointing to the Triassic UHP metamorphism. Most of the igneous zircons have unusually variable δ^18O values of - 7.76‰-5.40‰, implying that the gneiss protolith was intruded as low δ^18O magma generated by partial melting of altered rocks which had suffered intense water-rock interaction with a low δ^18O fluid at high temperatures during Neoproterozoic.

关 键 词：苏鲁造山带 变质花岗岩 锆石U—Pb定年 氧同位素 原岩性质 锆石U-PB定年 苏鲁超高压变质带 氧同位素分析 花岗岩成因 威海地区 

分 类 号：P588.345[天文地球—岩石学] P588.3[天文地球—地质学]