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

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 SiO 2, belonging to the tholeiitic to calc alkaline series with low K contents and demonstrating slight REE enrichment with insignificant Eu anomalies (Eu */Eu=0.99~1.04) and little fractionation within HREE (Dy/Yb CN =1.030~1.089). 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=0.81～1.01) for dacites but apparently negative Eu anomalies for rhyolites (Eu */Eu=0.65～0.76). 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 depletion, 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 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.