西藏冈底斯西段狮多地区火山岩的地质地球化学特征

西藏冈底斯西段狮多地区火山岩为一套高钾钙碱性和钙碱性系列岩石,分为古新世(59 7Ma)和中新世(17 2Ma)两个活动时期,早期的时代和层位大致相当于冈底斯东段的林子宗群典中组。与典中组火山岩相比较,该期火山岩的稀土元素总量(w(∑REE)=58 83×10-6)低得多,轻、重稀土分馏程度(w(La)N/w(Yb)N=3 44)较弱,分布曲线更为平缓;N(143Nd)/N(144Nd)的现在值(0 512275～0 512420)和初始值εNd(t)(-6 7～-4 0)也明显偏低,这反映冈底斯东、西段同一时期的火山活动及演化各具特色。根据Pb、Nd、Sr同位素的组成特征和微量元素构造环境判别推断,狮多地区火山岩形成于陆缘弧—陆 陆碰撞造山的发展演化环境,是新特提斯洋壳向大陆下俯冲作用过渡到陆 陆碰撞造山作用的产物。狮多铁 铜矿与古新世火山岩有成因联系。

GEOLOGIC AND GEOCHEMICAL CHARACTERISTICS OF VOLCANIC ROCKS IN SHIDUO,WESTERN GANGDISE,TIBET

Shiduo volcanic rocks in the western segment of Gangdise,Tibet belong to high potassium calc-alkali series and calc-alkali series. They are divided into two stages of volcanicism: Paleocene (59.7Ma) and Miocene (17.2Ma). In time and position, the former corresponds to Dianzhong Formation volcanic rocks of Linzizong Group in eastern Gangdise magmatic belt. Both of ∑REE (58.83×10~(-6)) and degree of fractionation between LREE and HREE ((La/Yb)_N=3.44) in the Paleocene volcanic rocks are much less than those in the Dianzhong Formation volcanic rocks.()~(143)Nd/()~(144)Nd (0.512 275 to 0.512 420) and ε_(Nd)(t)(-6.7　to　-4.0) in the Paleocene volcanic rocks are also smaller than those in the Dianzhong Formation volcanic rocks. These geochemical characteristics imply that volcanicism had some differences between western and eastern Gangdise magmatic belt in the same period. Based upon the Pb-Nd-Sr isotopes composition character and tectonic setting discrimination of trace element geochemistry, Shiduo volcanic rocks are formed in the environment of continental margin-arc—continental collision orogenic, and volcanicism originated from the transitional process from the subduction of Neo-Tethyan oceanic plate to continental collision orogenics. Minerogenesis of Shiduo iron-copper deposit is related to Paleocene volcanism.