青藏高原西部赛利普中新世火山岩源区：地球化学及Sr-Nd同位素制约

Petrogenesis of Miocene volcanic rocks in the Sailipu area,Western Tibetan Plateau:Geochemical and Sr-Nd isotopic constraints

青藏高原拉萨地块西部赛利普地区新生代火山岩依据主量元素可划分为超钾质、钾质和钙碱性系列,主要的岩石类型为粗面安山岩、粗面岩,一个超钾质岩石的^(40)Ar-^(39)Ar年龄为17.58Ma,指示出火山活动为中新世。超钾质、钾质和钙碱性火山岩都显示出富集LREE及LILE(Th、U)、亏损HFSE(Nb、Ta、Ti)的特征。超钾质火山岩具有较高的K_2O(6.31%～8.55%)、MgO(6.75%～8.96%)、Cr(270.7×10^(-6)～460.4×10^(-6))、Ni(142.3×10^(-6)～233.9×10^(-6))含量,较高的(^(87)Sr/^(86)Sr)_i (0.71883～0.72732)和较低的ε_(Nd)(-14.78～-15.37),指示可能起源于一个前期亏损并经后期俯冲作用改造的富钾的方辉橄榄岩富集地幔源区。钾质火山岩具有比超钾质火山岩低的K_2O、MgO、Cr、Ni含量以及高的Ba、Sr含量,初始^(87)Sr/^(86)Sr为0.71553～0.71628.初始^(143)Nd/^(144)Nd为0.51197～0.51198,在空间上与超钾质火山岩共生,可能是前者母岩浆的演化产物。钙碱性火山岩具有较高的Sr(881.7×10^(-6)～1309.2×10^(-6))、Sr/Y比值(50～108)和较低的Y(12.05×10^(-6)～18.02×10^(-6)),明显亏损重稀土Yb(0.93×10^(-6)～1.30×10^(-6)),类似于典型的埃达克质岩成分特征但相对高钾,并具有相对低的(^(87)Sr/^(86)Sr)_i (0.70928～0.71374)以及高的ε_(Nd)(-7.90～-10.91),指示起源于富钾增厚下地壳物质的部分熔融。区域上拉萨地块超钾质岩、钾质岩与N-S向地堑系在空间上共存、时间上相吻合,由此本文认为拉萨地块中新世钾质-超钾质岩和南北向地堑系的形成可能与中新世早期北向俯冲的印度大陆岩石圈断离有关。

Cenozoic volcanic rocks distributed in the Sailipi area of the western Lhasa Block in the Tibetan plateau are divided into uhrapotassic, potassic and calc-alkaline series according to the content of major element composition. The main rock types are trachyandesite and trachyte. ^40Ar-^39Ar dating results show these rocks were erupted at ≈ 17.58Ma, confirming that volcanism occurred in the Miocene. Ultrapotassic, potassic, and clac alikaline volcanic rocks are characterized by enrichment in LREE and large ion lithophile elements （LILE） , such as Rb, Ba, Th and depletion in high field strength elements （HFSE） , such as Nb, Ta, Ti. The ultrapotassic volcanic rocks show relatively high K2O （6.3% - 8.55% ） , MgO （ 6.75% - 8.96% ） , Cr （ 270.7 × 10 ^-6 - 460.4 × 10^-6） , Ni （142.3 ×10^-6 -233.9 ×10^-6） , high initial ^87Sr/^86Sr （0.71883 -0.72732） and low εNd（ - 14.78 - - 15.37）. These geochemical and isotopic characteristics indicate that the ultrapotassic volcanic rocks were probably derived from a previously depleted harzburgite mantle source then potassium enrichment by metasomatism during early subduction events. Potassic volcanic rocks have lower K2O, MgO, Cr, Ni and higher Ba, Sr content than the ultrapotassic volcanic rocks, they show high initial ^87Sr/^86Sr （0. 71553 - 0.71628）, ^143 Nd/^144 Nd（0.51197 - 0.51198 ）, and spatially coexist with ultrapotassic volcanic rocks. These rocks were most likely differentiation products of the ultraptassic magmas. Similar to typical adakites in geochemistry, the clac alikaline volcanic rocks are characteristic of high Sr contents（881.7 × 10^-6 - 1309.2 × 10^-6 ）, Sr/Y （50 - 108 ） ratios, depletion of Y and Yb, no Eu anomaly and positive Sr anomalies, meanwhile these rocks show relatively higher K2O, MgO, lower initial ^87Sr/^86 Sr（0. 70928 -0.71374） and higher, εnd （ -7.90 --10.91 ） than the uhrapotassic volcanic rocks. Based on the chemical features mentioned above, we suggest that adakitic clac-alikaline volcanic magmas were generated by partial melting of the relatively K-rich marie lower crust beneath Lhasa block. It is in this region that ultrapotassic and potassic volcanic rocks in the Lhasa Block coexist with the N-S riffs on both space and time, therefore we conclude that the formation of the Miocene potassic-uhrapotassic volcanic rocks and N-S rift systems in the Lhasa Block were probably triggered by slab break-off of the India continent lithosphere.