冀南綦村岩体斑状黑云母角闪闪长岩的成因矿物学研究

Formation and Mineralogy of Porphyritic Biotite Hornblende Diorite in Qicun Pluton,Southern Hebei Province

綦村岩体是冀南邯邢地区典型的中生代高镁闪长岩杂岩体,主要由斑状黑云母角闪闪长岩、角闪闪长岩、闪长岩、二长岩和少量辉长岩组成。对綦村斑状黑云母角闪闪长岩进行了较为详细的成因矿物学研究,探讨了其成因及其地质意义。研究发现,綦村斑状黑云母角闪闪长岩中的角闪石多为镁铝钙闪石和镁角闪石;黑云母主要为富镁黑云母;斜长石发育环带结构,核部以中长石和拉长石为主,边部多为更长石。角闪石矿物温压计估算结果显示,角闪石斑晶的形成温度为930~970℃,压力介于300~340 MPa之间,深度10~15 km,氧逸度为ΔNNO+0. 1^+2. 3,平衡熔融体中的水含量4. 8%~5. 5%;与黑云母平衡的角闪石形成温度为684~760℃,压力降低至42~61 MPa,熔体中水含量降至3. 1%~4. 3%,氧逸度为ΔNNO+0. 7^+2. 4。黑云母温度计和氧逸度计获得的结果与角闪石温压计估算结果一致。该结果表明,研究区高氧逸度的铁镁质岩浆在中地壳曾经驻留,并发生了以角闪石为主的结晶分异,富含挥发分的分异岩浆在后期快速侵位形成高镁闪长岩和矽卡岩型铁矿。

The Qicun pluton is a representative Mesozoic high-Mg diorite complex in the southern Taihang Mountains.It is mainly composed of porphyritic biotite hornblende diorite,hornblende diorite,gabbro-diorite and monzonite.In this study,formation and mineralogy of porphyritic biotite hornblende diorite in Qicun is studied in detail to reveal its petrogenesis and geological significance.We found that the hornblende belongs mainly to magnesiohornblende and magnesiohastingsite,and the biotite is magnesiobiotite.The core of the plagioclase is mainly andesine and labradorite,and the rim is mostly oligoclase.Hornblende thermobarometry shows that the hornblende phenocrysts were formed at 930-970℃and 300-340 MPa,corresponding to crustal depth of ca.10-15 km,with fO2 ofΔNNO+0.1 to+2.3 and magma water content of 4.8%-5.5%.Hornblende,associated with biotite that represents the final magma solidification,was formed at 684-760℃and 42-61 MPa,with magma water contents decreased to 3.1%-4.3%,and slightly elevated fO2(ΔNNO+0.7 to+2.4).Additionally,results of biotite thermometry are in good agreement with those of the hornblende thermobarometry,supporting that the estimation of magma-forming conditions are reliable.Consequently,we suggested that the high-Mg magma in Qichun area with high oxygen fugacity was temporarily stored in the middle crust,and underwent hornblende-dominated fractionation.The volatile-rich magma was then rapidly emplaced and formed the high-Mg diorite and the associated iron skarn deposits.