青岛地区晚白垩世基性脉岩中麻粒岩捕虏体的成分特征及其温压条件

Compositional features and P-T conditions of granulite xenoliths from Late Cretaceous mafic dike.Oingdao region.

青岛地区晚白垩世基性脉岩中含有两类麻粒岩捕虏体,富辉石麻粒岩和富长石麻粒岩.富辉石麻粒岩的主要矿物组合为Opx＋Cpx＋Pl,具有细粒变晶结构;富长石麻粒岩主要矿物组合为Pl＋OpX＋Cpx十Q或Pl＋Opx/Cpx＋Q,具有中粗粒粒状变晶结构.麻粒岩捕虏体中辉石的Mg#较低（＜66）,普通辉石具有富Na贫Ca的特征,与岩浆底侵作用形成的麻粒岩特征接近.二辉石温度计的计算结果显示这些麻粒岩捕虏体的麻粒岩相变质平衡温度在847～910℃之间.结合富长石麻粒岩的地质压力计的计算结果（0.99～1.17GPa）和地温梯度推测出的＂岩石学＂Moho面深度在33～36公里之间,与＂地震＂Moho面深度基本一致.麻粒岩捕虏体中普通辉石的矿物成分和该地区活跃的岩浆活动暗示青岛地区下地壳麻粒岩不同于典型的古老下地壳麻粒岩地体或捕虏体.他们的形成可能是华北克拉通古老麻粒岩与岩浆底侵作用相互作用的结果.

Late Cretaceous mafic dike in Qingdao region, China contains two types of granulite xenoliths： the pyroxene-rich granulites and the plagioclase-rich granulites. The pyroxene-rich granulites are mainly composed of a mineral assemblage of Opx ＋ Cpx ＋ P1 and have fine-grained re-crystallized texture. The mineral assemblage for the plagioclase-rich granulites is dominantly Opx ＋ Cpx ＋ P1 ＋ Q or Opx/Cpx ＋ P1 ＋ Q and show medium- and coarse-grained texture. All granulites have relatively low Mg# in pyroxenes （ 〈 66）. The clinopyroxenes in these granilite xenoliths have chemical characteristics of high Na and low Ca, which is close to those for the granulites derived from magma underplating. Equilibrated temperatures calculated from the cpx-opx geothermometers are in a range of 847 -911℃. Combined with the geobarometry of the plagioclase-rich granulites and the geotherm, the petrological Moho was 33 36km at depths, broadly consistent with the seismic Moho. This shows that there was no obvious crust-mantle transition zone in the Qingdao region in the Late Mesozoic. The mineralogical characteristics of these granulite xenoliths and repeated magmatism in the Jiaodong region imply that the lower crust granulites were different from those of typical Archean lower crust granulite terrain or xenoliths. Their formation could be the result of the interaction between the old lower crust of the North China craton and the underplating magma.