藏北火车头山新生代火山岩的岩石特征与时代

Petrology and ages of the Cenozoic volcanic rocks in the Huochetoushan,North Tibet,China

火车头山地区的新生代火山岩位于藏北火山岩带的中段,火山岩主要以熔岩被、平顶山、火山锥的形式产出,熔岩厚约20 ～300 m.对其矿物组合、矿物化学和岩石化学的研究表明,为一套强碱性的碱玄岩-响岩组合.根据火山岩的接触关系、K-Ar和40Ar/39Ar同位素年龄测定结果,可分为三期.第Ⅰ期(年龄为34～32 Ma)火山喷发较弱,形成小范围分布的白榴碧玄岩;第Ⅱ期(年龄为29～26 Ma)火山喷溢最为强烈,形成大面积厚层白榴石响岩、响岩质碱玄岩和粗斑状霞石响岩;第Ⅲ期(年龄为24～19 Ma)火山喷发较弱,岩性为白榴碱玄岩和白榴碱玄质响岩.

The Cenozoic volcanic rocks in Huochetoushan are distributed over the middle segment ot the Cenozoic volcanic rock belt in the northern Tibetan plateau, occurring mainly as lava sheets, mesas and volcanic cones, about 20-300 m in thickness. The studies of petrography, mineralogical chemistry and petrochemistry show that the lavas formed a tephrite-phonolite suit. Three stages of volcanic activity are recognized in the Huochetoushan area on the basis of geological contact relation among lavas and K-Ar and ^40 Ar ^39 Ar isotopic dating. In Stage Ⅰ （34-32 Ma）, the volcanic eruption was relatively weak, and only small-scale of leucite basanite formed. In Stage Ⅱ （29-26 Ma）, the volcanic eruption reached its peak, resulting in the formation of widespread thick beds of leucite phonolite, phonotephrite and macro-porphyritic nepheline phonolite. In Stage Ⅲ （19-24 Ma）, the volcanic eruption was relatively weak again, and leucite tephrite and leucite tephriphonolite formed.