地学前缘 ›› 2009, Vol. 16 ›› Issue (2): 79-89.

• 论文 • 上一篇    下一篇

甘肃西秦岭两类新生代钾质火山岩:岩石地球化学与成因

喻学惠 莫宣学 赵志丹 黄行凯 李勇 陈延芳 韦玉芳   

  1. 中国地质大学 地质过程与矿产资源国家重点实验室, 岩石圈构造、深部过程及探测技术教育部重点实验室, 北京 100083
  • 收稿日期:2009-03-11 修回日期:2009-03-20 出版日期:2009-03-02 发布日期:2009-03-02
  • 作者简介:喻学惠(1944—),女,教授,博士生导师,主要从事岩石学及有关矿产资源教学与科研工作。E-mail: xhy532@yahoo.com.cn
  • 基金资助:

    国家自然科学基金项目(40572046)

Two types of Cenozoic potassic volcanic rocks in West Qinling, Gansu Province: Their petrology, geochemistry and petrogenesis.

  1. State Key Laboratory of Geological Processes and Mineral Resources and Key laboratory of Lithosphere Tectonics, Lithoprobing Technology of Ministry of Education, China University of Geosciences, Beijing  100083, China
  • Received:2009-03-11 Revised:2009-03-20 Online:2009-03-02 Published:2009-03-02

PDF (PC)

926

摘要：

甘肃西秦岭地区存在钾霞橄黄长岩和钾质粗面玄武岩(钾玄岩)两类钾质火山岩,出露在甘肃西秦岭礼县、宕昌县等,地理坐标大致相当于104°20′~104°50′E, 33°30′~34°10′N。钾霞橄黄长岩是一种不含斜长石,但普遍含有高钛金云母、黄长石、白榴石、霞石的岩石,全岩化学成分具低SiO2和Al2O3,富TiO2、CaO、MgO和高K/Na、高Mg#值的特征;钾质粗面玄武岩(钾玄岩)含有大量斜长石但是缺乏高钛金云母、黄长石、白榴石和霞石,全岩化学中SiO2、Al2O3明显高于前者,而TiO2、CaO、MgO、K/Na和Mg#值要比钾霞橄黄长岩低。钾霞橄黄长岩的全岩K/Ar和金云母单矿物的39Ar/40Ar同位素定年落在71~23 Ma,而钾玄岩的全岩39Ar/40Ar同位素定年落在9 Ma左右,因此它们同为中新世产物。两类钾质火山岩具有相似的富集不相容元素和轻稀土的特征。两类钾质火山岩的初始87Sr/86Sr分别在0704 03~0707 49和0704 12~0705 22;143Nd/144Nd 分别在0512 74~0512 94和0512 65~0512 76;εNd 分别在112~595和03~23。206Pb/204Pb、207Pb/204Pb和208Pb/204Pb分别落到18374 6~18998 6、15529~15669 3和38497 1~39414 4。在火山岩源区示踪的143Nd/144Nd87Sr/86Sr, 207Pb/204Pb206Pb/204Pb,208Pb/204Pb206Pb/204Pb, 143Nd/144Nd206Pb/204Pb, 87Sr/86Sr206Pb/204Pb 和Ba/NbLa/Nb图解中,一致显示两类钾质火山岩具有与OIB相似的地球化学特征,源区可能与地幔柱有关,并具有EM1、DMM和HIMU端员混合特征。结合前人对该区深部地球物理和断裂构造的研究,论证了火山岩的起源与成因,指出作为对印度—欧亚大陆强烈碰撞的吸收与调节,高原下软流圈地幔流沿400 km界面向北东方向的侧向流动以及西秦岭周边克拉通块体的阻挡,是形成西秦岭断裂系左行走滑特征和巨大拉分盆地的主要原因,也是导致西秦岭新生代两类钾质火山岩和碳酸岩起源与成因的动力学机制,较好地解释了西秦岭新生代岩浆作用起源深度大,具有地幔柱源的地球化学特征,岩石组合与地球化学有别于高原内部及其周边地区新生代钾质火山岩的原因。

关键词: 西秦岭;新生代;钾质火山岩;岩浆源区;动力学

Abstract:

Two types of Cenozoic potassic volcanic rocks, kamafugite and potassic trachybasalt, were recognized in Lixian and Dangchang counties in West Qinling, Gansu Province (N104°20′N104°50′, E33°30′E34°10′). The first type, kamafugite, is plagioclasefree, consisted of Tirich phlogopite, melilite, leucite and nephelite. They have lower contents of SiO2 and Al2O3, but have high TiO2, CaO, MgO contents and K/Na and Mg# ratio whereas the second type, potassic trachybasalt, is abundant in plagioclase and  contains no phlogopite, melilite, leucite and nephelite. They have higher SiO2, Al2O3, (Na2O+K2O), and lower TiO2, CaO, MgO, and K/Na and Mg#, in comparision to the first type of rocks. The whole rock K/Ar and phlogopite 39Ar/40Ar ages of kamafugite range from 71 to 23 Ma, and the potassic trachybasalt yields the whole rock 39Ar/40Ar age of ~9 Ma. They are both Miocene magmatism. Both kamafugite and potassic trachybasalt are enriched in incompatible trace elements and LREE. The kamafugite (87Sr/86Sr=0704 030707 49, 143Nd/144Nd=0512 740512 94, εNd=1125.95) and the potassic trachybasalt  (87Sr/86Sr=0.704 120.705 22, 143Nd/144Nd=0.512 650.512 76, εNd=1.125.95) have similar Pb isotopic compositions (206Pb/204Pb=8374 618.998 6, 207Pb/204Pb=15.5295.669 3, 208Pb/204Pb=38.497 139.414) and both show plumerelated OIBlike features, suggesting a possible mixture of the geochemical endmembers of EM1, DMM and HIMU. Combining the available regional geological and geophysical research, the petrogenesis of the magmatism was studied. As a response and absorption to the IndiaAsia collision, the northeastward flow of the asthenosphere beneath the Tibetan Plateau along the 400 km depth interface was blocked by the North China craton, and caused a systemic sequences in West Qinling Region, including the strikeslip fault systems and pullapart basins in the shallow crust and the origin of the twotype magmatism in this work in deep lithosphere. This explanation is consistent with the lithology and plumelike geochemical features of the kamafugite and potassic trachybasalt, which are significantly different from the potassic magmatism within the plateau and its adjacent regions.

Key words: West Qinling; Cenozoic; potassic volcanic rocks; magmatic source region; dynamics

中图分类号: