| 北大巴山超基性、基性岩墙和碱质火山杂岩形成时代的新证据及其地质意义 |
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| 引用本文： | 向忠金,闫全人,宋博,王宗起.北大巴山超基性、基性岩墙和碱质火山杂岩形成时代的新证据及其地质意义[J].地质学报,2016,90(5):896-916. |
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| 作者姓名： | 向忠金 闫全人 宋博 王宗起 |
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| 作者单位： | 中国地质科学院地质研究所,中国科学院大学地球科学学院,中国地质调查局西安地调中心,中国地质科学院矿产资源研究所 |
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| 摘 要： | 北大巴山早古生代地层广泛出露一套超基性、基性岩墙和碱质火山杂岩(包括碱性玄武岩和粗面岩),为研究北大巴山早古生代构造演化提供了重要的载体。本文通过利用锆石U-Pb定年和金云母~(40)Ar/~(39)Ar同位素定年方法对它们的形成时代进行了系统研究,结合地球化学特征探讨它们的成因及构造意义。通过对岚皋县和镇坪县的两个辉绿岩墙开展锆石U-Pb定年,分别获得它们的年龄为399±1Ma和451±4 Ma,是研究区目前已报道的有关超基性、基性岩墙最年轻和最老的就位时代。地球化学分析结果表明,它们的主量元素与区内其他基性岩墙位于同一演化趋势线上,微量和稀土元素地球化学特征相似,类似于OIB的特征,表明它们来自相同的地幔源区。因此,研究区内基性岩墙侵入事件最早开始于晚奥陶世中期(约450Ma),结束于早泥盆世晚期(约400 Ma),期间经历了多次岩浆侵入活动。对与碱性玄武岩共生的火山碎屑岩中金云母晶屑进行~(40)Ar/~(39)Ar同位素定年,获得年龄446±3Ma,代表了火山喷发时间,表明碱性玄武质岩浆喷发活动与基性岩浆侵入事件大致同时发生。对粗面岩进行SHRIMP锆石U-Pb定年虽未获得准确的岩浆结晶年龄,但较年轻锆石年龄(165±3Ma和229±2Ma)的存在,暗示其形成时代可能属于中生代。系统的年代学研究表明,北大巴山地区早古生代地层中的超基性、基性岩墙和碱性玄武质火山杂岩为同期岩浆活动的产物,最早开始于450Ma,经历了多期火山喷发和岩浆侵入活动(450~400 Ma),结束于早泥盆世晚期(~400Ma)。而粗面岩与上述岩石不是同一期岩浆活动产物,研究区不存在双峰式火成岩组合,它们可能形成于中生代,属于南秦岭中生代岩浆活动的产物,为南秦岭、北大巴山中生代成矿作用提供物源。
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| 关 键 词： | 金云母 锆石U-Pb定年 碱性玄武岩 基性岩墙 北大巴山 |
| 收稿时间： | 2015/5/18 0:00:00 |
| 修稿时间： | 2015/8/22 0:00:00 |
New Evidence for the Ages of Ultramafic to Mafic Dikes and Alkaline Volcanic Complexes in the North Daba Mountains and Its Geological Implication |
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| xiang zhongjin,Yan quan ren,song bo and wang zong qi.New Evidence for the Ages of Ultramafic to Mafic Dikes and Alkaline Volcanic Complexes in the North Daba Mountains and Its Geological Implication[J].Acta Geologica Sinica,2016,90(5):896-916. |
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| Authors: | xiang zhongjin Yan quan ren song bo and wang zong qi |
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| Abstract: | A large volume of ultrabasic to basic dikes and alkaline volcanic complexes, widely occurring in the Early Paleozoic strata in the North Daba Mountains, provide a good opportunity to explore the nature of mantle and tectonic evolution of the South Qinling and the North Daba Mountains. The ages of these rock assemblages are still controversial. In this paper, zircon U-Pb dating and phlogopite 40Ar/39Ar isotopic dating are used to constrain the age of these rock assemblages. The Langao and Zhenping diabases are dated at 399±1 Ma and 451±4 Ma, respectively, using the zircon U-Pb dating technique, which so far are the youngest and oldest ages for the basic dikes. Geochemical data show that both Langao and Zhenping diabasses have OIB signature, enriched in LREE, LILE, Nb and Ta. They are plotted on the same linear variation tendency and have similar Primitive-normalized trace elements spider diagram indicate that they are originated from a same mantle source. Therefore, the emplacement of ultrabasic to basic dikes began at Katian of Late Ordovician (~450 Ma), ended at Emsian of Early Devonian (~400 Ma), with several peaks of magmatic intrusions. The phlogopites from the volcaniclastic rocks are dated at 446±3 Ma, using the 40Ar/39Ar isotopic dating method, which represent the age of volcanic activity. This age is consistent with the emplacement age of basic dikes within uncertainty, indicating that they started at the same time. Although the exact ages of the trachytes are not obtained, the younger zircon ages (165±3 Ma and 229±2 Ma) imply that they may be formed in Mesozoic. Systematic dating results show that the ultrabasic to basic dikes and alkaline basalts are originated from a same magmatic source, and magmatic activity started at 450 Ma, last till ~400 Ma. The trachytes, however, may be formed in Mesozoic and represent the products of Mesozoic magmatism in South Qinling orogenic belt. There is no Early Silurian bimodal volcanic rock in the North Daba Mountains. |
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| Keywords: | Phlogopite Zircon U-Pb dating Alkaline basalt Basic dike North Daba Moutain |
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