南祁连土尔根达坂巴龙贡噶尔组火山岩地球化学及锆石U-Pb年代学研究

广泛分布于南祁连的志留纪巴龙贡噶尔组的形成时代和构造归属存在很大分歧,为了说明土尔根达坂巴龙贡噶尔组英安岩-流纹岩形成时代及地质意义,本文选择代表性岩石进行了主微量元素和LA-ICP-MS锆石U-Pb年代学分析。地球化学结果表明,巴龙贡噶尔组火山岩为高钾钙碱性过铝质岩石,在成岩过程中经历了一定的结晶分异作用。火山岩微量元素表现出不同程度的富集K、Pb和轻稀土元素(LREE),相对亏损高场强元素(HFSE)Zr、Ce、Nb,具有明显的P、Ti负异常。LREE/HREE之比为6.98-10.13,δEu介于0.40-0.54显示负异常,表明稀土元素分馏明显;各样品稀土曲线具有相似的右倾型配分模式,反映为同源岩浆产物。三个样品LA-ICP-MS锆石U-Pb谐和年龄分别为438.1±2.2Ma, 403.0±2.6Ma, 404.9±2.8Ma,说明火山岩的形成地质时期为早志留世-早泥盆世。综合对比得出原志留纪巴龙贡噶尔组应进行解体及重新认识,土尔根达坂地区巴龙贡噶尔组火山活动与南部柴达木花岗岩岩浆活动近于同期,两者可能都与柴达木陆块与中南祁连板块碰撞造山带上不同块体之间的伸展、滑塌等一系列岩浆侵入活动相关。

Geochemical and zircon U- Pb chronology of the volcanic rocks from the Balonggonggaer Formation in Tuergendaban, southern Qilian Mountain

Widely distributed in Southern Qilian, the Silurian Balonggonggaer Formation(B.F) differs greatly in its formation age and tectonic properties. To illustrate the age and geological significance of the dacite-rhyolite from the B.F in the Tuergendaban, representative rocks were selected for the main and trace elements, LA-ICP-MS zircon U-Pb Chronological analysis. Geochemical results indicate that volcanic rocks of the B.F are high potassium, calcium alkaline and peraluminous, and experienced certain cystallization differentiation during diagenesis. Enrichment in K, Pb and LREE, relatively depleted HFSE, such as Zr、Ce and Nb, with obvious P, Ti negative anomaly. The ratio of LREE/HREE is 6.98-10.13, δEu between 0.40-0.54 with negative anomaly, indicating that REE fractionation is obvious. The right-inclined distribution pattern of REE in each sample is basically similar, suggesting that they have similar magmatic sources and processes. The three accordant ages through LA-ICP-MS zircon U-Pb were 438.1±2.2 Ma, 403.0±2.6 Ma, and 404.9±2.8 Ma, indicating that the geological period of the formation of volcanic rocks was Early Silurian-Early Devonian. The comprehensive comparison shows that the former Silurian B.F should be disintegrated and re-understood. The volcanic activity of the B.F in Tuergendaban area are close to the same period of the southern Chaidamushan granite magma activity, both may be related with the collision, which the Qilian block from the north to the south thrusted over the Qaidam block, form a series of continuous magmatic intrusion activities.