拉萨地块西段中新世查加寺钾质火山岩岩石成因*——岩石地球化学、年代学和Sr-Nd同位素约束

通常认为,青藏高原碰撞后钾质超钾质岩是交代富集上地幔低度部分熔融的产物,而最近的研究则表明,钾质火山岩也可以起源于下地壳源区.文章对青藏高原拉萨地块西段新识别出的查加寺火山岩进行了岩石地球化学、锆石SIMS U-Pb定年和Sr-Nd同位素研究,结果表明其岩石类型为碱性系列钾质粗面岩,锆石SIMS U Pb 年龄为(23.97±0.28) Ma,说明火山活动为中新世.钾质粗面岩显示出富集LREE及LILE(K、Rb、Ba、Th、U、Pb)、亏损HFSE(Ti、Nb、Ta、P),具有类似于埃达克质岩的稀土元素分布模式和微量元素蛛网图分布模式的特征；具有高的La/Yb比值(81～105)、较高的ω(Sr)(409×10-6～472×10-6)、较高的Sr/Y比值(28-37)、较低的ω(Y)(11.5×10-6～15.7×10-6)、明显亏损重稀土元素Yb(0.78×10-6～1.08×1)-6)、较高的w(Al2O3)(15.47％～16.7％)、较低的w(MgO)(0.63％～2.12％),无明显的Eu、Sr负异常,类似于典型的埃达克岩成分特征；具有高的w(K2O)(6.28％～6.97％)、高的Rb/Sr比值(0.94～1.03)和低的Ba/Rb比值(2.21～2.51),以及Na2O、K2O与SiO2无明显的相关关系,表明源区的富钾矿物是以金云母为主；具有较低的ω(Cr) 、ω(Ni)(分别为53.4×10-6～69.4×10-6,11.4×10-6～23.5×10-6),以及较低的εNd值(-12.6～-11.8)和较高的87Sr/86Sr比值(0.73207～0.73249).所有这些特征都表明,查加寺钾质粗面岩起源于拉萨地块增厚下地壳富钾物质的部分熔融.查加寺钾质粗面岩具有约145Ma、75 Ma和30 Ma等3组继承锆石年龄群.

Petrogenesis of Miocene Chajiasi potassic rocks in western Lhasa block, Tibetan Plateau: Constraints from lithogeochemistry, geochronology and Sr-Nd isotopes

It is generally believed that post-collisional potassic and ultrapotassic volcanic rocks in Tibetan plateau can be explained by low degrees of partial melting of a metasomatically enriched upper mantle,but recent studies show that some potassic rocks might have also come from the lower crust.This paper presents a series of lithogeochemical,zircon SIMS U-Pb dating and Sr-Nd isotopes data on the new-recognized Chajiasi volcanic rocks in western Lhasa block,Tibetan Plateau.The volcanic rock is named alkaline and potassic trachyte.Zircon SIMS U-Pb dating result shows that Chajiasi potassic trachyte was erupted at ≈24 Ma,suggesting that volcanism occurred in the Miocene.Similar to typical adakites in the chondrite-normalized REE patterns and primitive mantle-normalized trace element spider diagrams,the potassic trachyte is characterized by enrichment of LREE and large ion lithophile elements(LILE) such as K,Rb,Ba,Th,U and Pb and depletion of high field strength elements(HFSE) such as Ti,Nb,Ta and P.The potassic trachyte is characterized by high La/Yb ratios(81～105),high Sr content(409×10-6～472×10-6),high Sr/Y ratios(28～37),depletion of Y(11.5×10-6～15.7×10-6) and Yb(11.5×10-6～15.7×10-6),high Al2O3 content(15.47%～16.07%),low MgO content(0.63%～2.12%) and no obvious Eu and Sr negative anomalies,similar to those of typical adakites.This potassic trachyte is also characterized by high K2O content(6.28%～6.97%),high Rb/Sr ratios(0.94～1.03) and low Ba/Rb ratios(2.21～2.51),with no obvious correlation between Na2O and SiO2 and between K2O and SiO2,implying that K-rich mineral in the source region is predominated by phlogopite.In addition,this kind of rock has features of low Cr content(53.4×10-6～69.4×10-6) and Ni content(11.4×10-6～23.5×10-6),low εNd values(-12.6～-11.8) and high 87Sr/86Sr ratios(87Sr/86Sr=0.73207～0.73249).Based on the chemical features mentioned above,the authors hold that Chajiasi potassic trachyte was generated by partial melting of the relatively K-rich thickened lower crust beneath Lhasa block.The Chajiasi potassic trachyte shows three groups of inherited zircon ages(≈145 Ma,≈75 Ma and ≈30 Ma).