威海地区超高压变质花岗片麻岩锆石U-Pb定年和氧同位素研究

在苏鲁超高压变质带东北端山东威海地区皂埠镇发现锆石δ18O值低至-7.8‰左右的花岗片麻岩, 与前人在苏鲁超高压变质带西南端江苏东海青龙山地区发现的锆石δ18O值为-7‰~-9‰左右的花岗片麻岩一致.对这些低δ18O值花岗片麻岩进行了锆石SHRIMP法UPb定年和系统的激光氟化法矿物氧同位素分析, 结果对低δ18O值锆石成因和花岗片麻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UPb定年得到的花岗片麻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18O的大气降水发生过不同程度的高温水岩反应; (3) 低δ18O值花岗片麻岩在印支期板块俯冲和折返过程中基本没有与外部发生显著的氧同位素交换, 在退变质作用过程中花岗片麻岩内部缓冲流体对原岩岩浆锆石的δ18O值影响不大.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, 指示它们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. 

A Study of Zircon U-Pb Dating and Oxygen Isotopes in UHP Granitic Gneiss from the Weihai Region in Northeast Sulu

Extremely 18O-depleted zircons from granitic gneisses, with δ18O values as low as -7.8‰, were found in the Zaobuzhen area in the Weihai region, at the northeastern end of the Sulu orogen. SHRIMP zircon U-Pb ages and the oxygen isotope compositions were determined for the Low-δ18O zircons. CL images reveal that the Low-δ18O zircons are primarily of igneous origin, but some grains underwent 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 Triassic UHP metamorphism. Most of the igneous zircons have unusually variable δ18O values of -7.76‰ to 5.40‰, indicating that the gneiss protolith was intruded as Low-δ18O magma that was generated by the partial melting of altered rocks, which suffered intensive water-rock interaction with a Low-δ18O fluid at high temperatures during the Neoproterozoic. The preservation of extreme 18O-depletion in the zircons suggests that there is no remarkable oxygen isotope exchange between the metagranite and the mantle during the processes of Triassic subduction and exhumation. The protolith nature, metamorphic timing and oxygen isotope compositions of the granitic gneisses in the Weihai region are similar to those of granitic gneisses in the Qinglongshan area in the southwestern part of the Sulu orogenic belt, indicating that the gneisses along the Sulu orogenic belt share the same nature of protolith origin, water-rock interaction and UHP metamorphism. The present study provides tight constraints not only on the origin of extremely 18O-depleted zircons, but also on the protolith nature of granitic gneisses in the Sulu orogenic belt.