青藏高原北部可可西里地区新生代钾质火山岩地球化学特征及成因

Geochemistry and petrogenesis of Cenozoic potassic volcanic rocks in the Hoh Xil area, northern Tibet plateau.

青藏高原北部可可西里地区分布的中新世钾质火山岩(7.77～17.82Ma)主要为粗面安山岩、粗面岩和少量次火山相的流纹斑岩。主量、微量元素及Sr-Nd-Pb同位素地球化学研究表明,该套钾质火山岩强烈富集大离子亲石元素和轻稀土元素,明显亏损Nb-Ta-Ti元素,具有较高的^(87)Sr/^(86)Sr:0.707346～0.714915,较低的ε_(Nd)值:-3.70～-6.97,和较高的放射性成因Pb同位素组成(^(207)Pb/^(204)Pb=15.65～15.76,^(208)Ph/^(204)Pb=38.98～39.35,^(206)Pb/^(204)Pb=18.67～18.78)。上述特征指示岩浆源区可能是与古俯冲消减物质有关的EMⅡ型富集地幔。三大岩类的地球化学成分变异表明:该钾质火山岩系列是富集地幔(金云母-尖晶石二辉橄榄岩和金云母-石榴石二辉橄榄岩)低度部分熔融产生的母岩浆经过较强结晶分异形成的,其中流纹斑岩在岩浆后期可能经历了更为复杂的地壳混染和结晶分异过程。

Miocene potassic volcanic rocks （7.77 - 17.82Ma） in the Hoh Xil area of northern Tibet Plateau are mainly of three types： trachyandesite, trachyte and some subvolcanic rhyolitic porphyry. Major- and trace-elements as well as Sr-Nd-Pb isotope geochemistry researches show： these potassic rocks are strongly enriched in large-ion lithophile elements and LREE, and evidently depleted Nb-Ta-Ti. They have relatively high ^87Sr/^86 Sr： 0. 707346 - 0. 714915, low εNd ： - 3.70 - - 6.97, and relatively high radiogenic Pb （^207Pb/^204 Pb = 15. 65 - 15. 76, ^208Pb/^204 Pb = 38. 98 - 39. 35, ^206Pb/^204 Pb = 18.67 - 18. 78）. The above characteristics indicate that the source of the magmas is an enriched EMII mantle which possibly resulted from earlier episodes of oceanic slab subduetion. Geochemical variation of the three major rocks indicate this series of rocks are formed by fractional crystallization of parental magmas which is generated by low degree partial melting of enriched mantle （ phlogopite-spinel lherzolite and phlogopite-garnet lherzolite ）, and the rhyolitic porphyry may undergo much more complicated process of crustal assimilation and fractional crystallization.