内蒙古沙德盖花岗岩岩浆混合作用:岩相学、矿物化学和年代学证据

华北地台北缘乌拉山地区的沙德盖钾长花岗岩体中普遍发育以二长岩为主的暗色微粒包体,包体具塑性流变特征,与寄主岩的接触界线或为截然或为渐变过渡。岩相学观察表明,包体中发育多种反映岩浆混合作用的典型组构,如石英眼斑、环斑长石、镁铁质团块、钾长石巨晶的溶蚀、磷灰石的针柱状形貌、长石中的包体带以及钙长石的"针尖"结构等。造岩矿物的电子探针分析表明,岩浆混合在沙德盖岩体的形成中起了重要作用,寄主花岗岩浆主要来自下地壳,而暗色包体岩浆则主要为地幔来源。锆石LA-ICP-MS U-Pb同位素定年结果显示,沙德盖花岗岩及其暗色微粒包体的形成时代基本一致,分别为233.4±2.3Ma和229.7±1.5Ma(中三叠世),进一步佐证了该岩体是岩浆混合作用的产物。研究认为,当铁镁质岩浆与长英质岩浆混合时,早期基性岩浆的快速淬冷形成了边界清楚、具明显冷凝边且暗色矿物含量较高的包体;随着两种不同成分岩浆之间温差的减小以及组分的交换,进一步形成了颜色较浅、边界渐变过渡和无明显冷凝边的包体。

Magma mixing and mingling of the Shadegai granite in Inner Mongolia: Evidence from petrography, mineral chemistry and geochronology

The Shadegai K-feldspar granite in the Wulashan area at the north margin of the North China craton contains abundant mafic microgranular enclaves (MMEs) consisting mainly of monzonite. The MMEs have either a sharp or a gradual contact zone with their host granite, indicating different rheological properties of the contrasting magmas. Petrographic observation shows that the MMEs contain various typical textures whose origin can be explained in terms of magma mixing and mingling, including quartz ocelli, rapakivi feldspars, mafic clots, resorption of K-feldspar megacrystals, acicular apatite morphology, inclusion zones in feldspars, and anorthite "spikes" in plagioclase. Microprobe analyses on the rock-forming minerals suggest that magma mixing and mingling played an important role during the formation the Shadegai granite. The host granite magma may be derived from the lower crust, while magma for the MMEs originated from the mantle. LA-ICP-MS U-Pb isotopic dating on zircons from the Shadegai granite and its MMEs obtained similar ages of 233.4±2.3Ma and 229.7±1.5Ma, respectively, further indicating the existence of magma mixing and mingling during their genesis. It is thus suggested that the quench of mafic magama upon its incorporation into the relatively cooler felsic host magma resulted in the formation of MMEs with sharp contact zones, chilled margins and more abundant mafic minerals. Subsequent interaction of mafic and felsic magmas caused the decrease in temperature difference of magmas and the formation of MMEs that have gradual contact zone, more abundant felsic minerals and no evident chilled margins.