秦岭地区陡岭—小茅岭隆起带西段几个岩体的SHRIMP锆石U-Pb测年及其地质意义

SHRIMP U-Pb Ages of Zircons from the Intrusions in the Western Douling—Xiaomaoling Uplift and Their Geological Significances

通过对南秦岭与中秦岭之间陡岭—小茅岭隆起构造—岩浆岩带西段柞水—镇安—山阳之间的磨沟峡、黑沟和冷水沟等岩体的锆石U-Pb定年研究,获得磨沟峡闪长岩年龄743±12Ma、黑沟碱性花岗岩年龄686±10Ma、冷水沟辉长岩年龄680±9Ma;侵入冷水沟辉长岩的正长闪长斑岩年龄141.7±1.4Ma,确定这些岩体形成时代分别为新元古代中、晚期和早白垩世初。岩石化学分析结果表明,新元古代中、晚期岩体形成于大陆裂解环境,较早期的磨沟峡闪长岩,岩石地球化学具板内花岗岩的特征;较晚期的冷水沟和黑沟岩体,由超基性—基性岩和偏碱性花岗岩组成,具非造山双模式岩浆岩组合特征。侵入冷水沟辉长岩的正长闪长斑岩为高K-钙碱性岩浆岩,具后造山花岗岩的地球化学特征。新元古代岩浆岩为古中国地台裂解的产物,燕山期正长闪长岩则代表秦岭多旋回造山最终完成的时代。

In recent years, writers have carried out SHRIMP U-Pb dating for the zircons from several intrusions in the western Douling-Xiaomaoling uplift, located between the South Qinling Indosinian fold belt and the Central Oinling Variscan fold belt. The results yield four ages. 743 ± 12 Ma of diorite from Mogouxia ; 686 ± 10 Ma of alkaline granite from Heigou; 680±9 Ma of gabbro and 141.7±1.4 Ma of syenodiorite intruding into the gabbro in Lengshuigou. The first three belong to the Late Neoprotozoic in age, and the syenodiorite porphyry, to the earliest Early Cretaceous （Middle Yanshanian）. Lithogeochemical analyses show that the Neoproterozoic magmatites were all formed in the continental extension setting , and that the early stage intrusion of Mogouxia is mainly composed of diorite with intraplate granitic charateristics, while the intrusions of Lengshuigou and Heigou are composed of ultrabasic-basic and subalkaline granites with characteristics of anorogenic bimodal magmatics. The Middle Yanshanian syenite-diorite porphyry of Lengshuigou are high-K calcalkalic igneous rocks, by characteried by post-orogentic granites. The dating studies in combination with regional geology indicate that the Late Neoproterozoic magmatic rocks are as a result of the disintegration of the Chinese Protoplatform, and that the Middle Yanshanian granites represent the closing of the Qinling polycyclic orogeny.