西藏高原西北缘钾质火山岩地球化学特征及其地质涵义  

Geochemical Characteristics of the Potassic Volcanics in the Northwestern Tibet Plateau and Its Implications

作　　者：鲍佩声[1] 肖序常[1] 苏犁[2] 

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]中国地质大学实验中心,北京100083

出　　处：《地质学报》2006年第10期1578-1587,I0008,共11页Acta Geologica Sinica

基　　金：国土资源部国际合作与科技司项目(编号2001010205;2001010201);中国地质调查局项目(编号200313000059);国家科技部(MOST)(编号2001CB71001)项目资助的成果。

摘　　要：西藏高原西北缘钾质火山岩属橄榄玄粗岩（shoshonite）系列。其K-Ar同位素年龄变化于0．28～7．79Ma，属中新世末-更新世；其K2O／Na2O几乎均〉1，Mg^#值有较宽的范围（从0．51～0．64），且具有较低的Cr-Ni含量（均≤326×10^-6）。它们代表着经历了不同程度分异演化的岩浆结晶产物。研究区钾质火山岩（shoshonites）以高度富集LREE和LILE（K，Rb，Sr，Ba，Th）以及高度亏损HREE和HFSE（Nb，Ta，Zr，Hf，Ti）等元素为特征。（La／Yb）。高达29．82～84．94。钾质火山岩对球粒陨石标准化的REE配分型式均为向右陡倾的LREE富集型。对原始地幔标准化的痕量元素蛛网图均具有明显的Nb，Ta，Ti负异常。钾质火山岩具有较高的。^87Sr／^84Sr（0．707755～0．710426）和较低的^143Nd／^144Nd（0．51196～0．512439）比值。上述特征表明这些火山岩来源于富集的地幔源区。钾质火山岩较高的Ba／Nb比值（40．84～97．28）、较高的Th／Ta及Ce／Yb比值（131～366）、和较低的Nb／Y比值（0．9～3．3），结合Th／Yb—Ta／Yb及（Th×100）／Zr（Nb×100）／Zr判别图，提示了它们也形成于活动大陆边缘，但与其伴生的钙碱性火山岩为早期与俯冲作用有关的火山岩，而钾质火山岩则为晚期岩石圈拆沉作用的产物。The potassic volcanics of the Northwest Tibet plateau belongs to Shoshonite series. The K/Ar isotope age varies from 0. 28-7.79 Ma, being a part of Late Miocene-Pleistocene. The K2O/Na2O ratios are almost more than one and Mg＂ ratios have a wide range from 0.51-0.64 as well as have lower contents of Cr, Ni （all less than 326 × 10^-6）. They represent the products of magmatic differentiation of varying degree. The potassic volcanics of the Northwest Tibet plateau are characterized by highly enriched in LREE and LILE （K,Rb,Sr, Ba,Th）, and highly depleted in HREE and HFSE （Nb, Ta, Zr, Hf, Ti）. The （La/Yb）n is as high as 29.82 84. 94. The chondrite-normalized REE patterns are LREE-riched type of dipping steeply to right. The primitive-normalized spider diagrams have distinctly negative anomalies of Nb, Ta, Ti. Besides, the potassic volcanicse have higher ^87Sr/^86Sr ratios （ 0. 708101 to 0. 72002） and lower ^143Nd/^144Nd ratios （512192-0. 512439）. The geochemical characters mentioned-above indicate that potassic volcanics in study areas should be drived from enriched mantle sources. Their higher Ba/Nb （40.84-97.28）, Th/Ta and Ce/Yb（131-366） ratios and lower Nb/Y ratio （0. 9-3.3）, as well as other characters shown in discriminant diagrams of Th/ Yb-Ta/Yb and Th × 100/Zr-Nb × 100/Zr all suggest that the Cenozoic potassic volcanics are orogenic volcanics formed at active continental margin. Even though the calci-alkalic volcanics associated with potassic volcanics also formed at active continental margin, but the former are related to subduction in early stage of orogeny and the later are products formed by lithospheric delamination in later stage of orogeny.

关 键 词：西藏高原 钾质火山岩 富集地幔 活动大陆边缘 岩石圈拆沉 

分 类 号：P588.14[天文地球—岩石学]