铜陵地区高钾钙碱性系列侵入岩锆石U-Pb年代学及其地质意义

摘要

High-K calc-alkaline intrusions from the Tongling district consist mainly of quartz monzodiorite (porphyrite) and granodiorite (porphyry), with lesser amounts of gabbro-diorite. In this paper, we report new zircon U-Pb ages and geochemical results for those rocks. The dating results show that Nanhongchong and Shatanjiao granodiorites, Xinqiaotou granodiorite porphyry and Miaojia quartz monzodiorite porphyrite have zircon SHRIMP U-Pb ages of (138.8±1.3) Ma, (144.1±1.2) Ma, (147.2±1.5) Ma and (142.8±1.6) Ma respectively. LA-ICPMS zircon U-Pb ages of the Shatanjiao and Miaojiao plutons are (144.1±1.5) Ma and (143.2±1.3) Ma, respectively, in good agreement with their SHRIMP zircon U-Pb ages. When combined with previous geochronological data, the results presented here indicate that the rocks with porphyrtic texture are older than those without porphyritic textures for the same type rocks. The same type rocks also have different ages, e.g., Nanhongchong granodiorite is younger than other ones from the area, which may reflect multistage intrusive magmatism. The intrusive sequence of the high-K calc-alkaline rocks can be generalized as:granodiorite porphyry/ the first intrusive granodiorite → gabbro-diorite / the first intrusive pyroxene monzodiorite → quartz monzodiorite porphyrite/quartz monzodiorite → the second intrusive granodiorite → the second intrusive pyroxene monzodiorite. High-K calc-alkaline intrusions in Tongling coincided temporally with the Late Jurassic to Early Cretaceous magmatic event widespread in the southeast China, indicating that the regional tectonics in southeast China switches from the Late Jurassic transpressive to the early Cretaceous extensional regimes.%　　铜陵地区高钾钙碱性系列侵入岩主要岩石类型为石英二长闪长(玢)岩、花岗闪长(斑)岩,其次为辉长闪长岩.本文报道该系列侵入岩最新锆石U-Pb定年结果及岩石地球化学数据.结果表明,南洪冲和沙滩脚花岗闪长岩、新桥头花岗闪长斑岩、缪家石英二长闪长玢岩的SHRIMP 锆石U-Pb年龄分别为(138.8±1.3) Ma、(144.1±1.2) Ma、(147.2±1.5) Ma和(142.8±1.6) Ma,沙滩脚岩体和缪家岩体的LA-ICPMS锆石U-Pb年龄分别为(144.1±1.5) Ma、(143.2±1.3) Ma,与SHRIMP定年结果一致.一般来说,区内具有斑状结构的岩石,其年龄大于同类无斑状结构的岩石,但狮子山矿田的南洪冲花岗闪长岩年龄比区域上其他的同类岩石偏小,可能反映了区域上此类岩浆具有多次活动的特点.铜陵地区岩浆侵位的大致顺序为花岗闪长斑岩/第一次花岗闪长岩→辉长闪长岩/第一次辉石二长闪长岩→石英二长闪长玢岩/石英二长闪长岩→第二次花岗闪长岩→第二次辉石二长闪长岩.本区高钾钙碱性系列岩浆侵入活动是中国东部晚侏罗-早白垩世期间岩浆事件的组成部分,反映了中国东部区域构造由晚侏罗世的挤压转变为早白垩世的伸展环境.