大兴安岭南段晚中生代双峰式火山作用

Petrogenesis of the late Mesozoic bimodal volcanic rocks in the southern Da Hinggan Mts, China.

大兴安岭南段晚中生代满克头鄂博组火山岩表现出双峰式特征 ,主要由玄武质安山岩、英安岩和流纹岩组成。基性火山岩属于低钾拉斑系列 ,轻微富集 L REE,Eu异常不明显 (Eu* / Eu=0 .99～ 1.0 4)和 HREE无明显分馏的特征 (Dy/Yb CN=1.0 30～ 1.0 89) ;富集大离子亲石元素 (L IL E)而亏损高场强元素 (HFSE) ,尤其是强烈亏损 Nb、Ta。英安岩和流纹岩为钙碱性系列 ,在 REE配分模式上为 L REE富集型 ,其中英安岩为 Eu弱负异常 (Eu* / Eu=0 .81～ 1.0 1) ,流纹岩的 Eu负异常明显 (Eu* / Eu=0 .6 5～ 0 .76 ) ;在微量元素蛛网图上 ,英安岩类似于基性火山岩 ,流纹岩除了具 L IL E富集和 HFSE亏损特征外 ,还显示出 Sr、P、Ti等元素的强烈亏损 ,可能与岩浆演化过程中斜长石、磷灰石的分离结晶作用相关。晚中生代双峰式火山岩的元素地球化学特征表明 ,基性火山岩源于受早期俯冲洋 /陆壳流体交代的地幔楔或亏损地幔 ,而英安岩则可能为基性岩浆分离结晶的结果。流纹岩表现出较高的 L a/ Sm比值和很高的 K/ P、K/ Ti比值 ,其成因可能与地壳混染作用或与大陆中、下地壳重熔作用有关。结合区域晚中生代盆岭构造格局特征 ,大兴安岭南段晚中生代双峰式火山岩形成于造山后阶段 。

The late Mesozoic volcanic rocks of Manketou' ebo Formation occurring in the southern Da Hinggan Mts exhibit a bimodal feature that was dominated by basaltic andesites, dacites and rhyolites. The basaltic andesites are relatively low in MgO and high in SiO2, belonging to the tholeiitic to calc-alkaline series with low K contents and demonstrating slight REE enrichment with insignificant Eu anomalies (Eu* /Eu = 0.99 similar to1.04) and little fractionation within HREE (Dy/Yb-CN = 1.030 similar to1.089). The primitive mantle (PM)-normalized spidergrams are characterized by LILE enrichment and HFSE depletion, especially for a significantly negative Nb-Ta anomaly. The dacites and rhyolites belong to the calc-alkaline series, displaying a steeply right-declined REE pattern with weakly negative Eu anomalies (Eu* /Eu = 0.81 similar to1.01) for dacites but apparently negative Eu anomalies for rhyolites (Eu* /Eu = 0.65 similar to0.76). The dacites occupy similar PM-normalized spidergrams to that of basaltic andesites while the rhyolites are significantly depleted in Sr, P and Ti as well as the LILE enrichment and Nb-Ta depiction, probably related to the fractional crystallization of plagioclases and apatites. The geochemical features of basaltic andesites suggest their origination from a metasomatized mantle wedge or depleted mantle, which had been modified during the previous subcontinental/oceanic subduction. According to the correlation between the dacites and basaltic andesites, it is inferred that the dacites were derived from the differentiated crystallization of basaltic magmas. On the other hand, the rhyolites possess a clear 'crustal' signature in that they demonstrated higher La/Sm, K/P and K/Ti ratios, possibly attributed to the crustal contamination during the magma ascent or anatexis of lower-middle crust in response to the basaltic underplating. Combining their geochemical characteristics with the regional basin and range tectonics, the late Mesozoic bimodal volcanism in the southern Da Hinggan Mts was formed in the post-orogenic stage in which a rapid lithospheric extension must have occurred, resulting in the decompressional melting of the pre-existent metasomatized lithospheric mantle.