Late Mesozoic calc-alkaline volcanism of post-orogenic extension in the northern Da Hinggan Mountains, northeastern China

Late Mesozoic calc-alkaline volcanism in the northern Da Hinggan Mountains (NDHM), NE China, exhibits geochemical and Sr–Nd isotopic characteristics similar to those of Cenozoic calc-alkaline volcanism in the Basin and Range Province, USA. Whole-rock K–Ar dating results show that these volcanic sequences were erupted during 138–116 Ma, composed of basaltic andesites/trachyandesites (Group 1), hornblende andesites/trachytes (Group 2) and rhyolite lavas (Group 3). They are characterized by low MgO contents (≤4.20%), LILE, LREE enrichment and significant Nb–Ta depletion, as well as a little depleted to slightly enriched Nd and weakly enriched Sr isotopic ratios (Group 1: initial 87Sr/86Sr=0.70502–0.70572; ϵNd(t)=−0.78 to +0.91; Group 2: initial 87Sr/86Sr=0.70497–0.70518; ϵNd(t)=+0.86 to +1.26; Group 3: initial 87Sr/86Sr=0.70510–0.70635; ϵNd(t)=−0.41 to +0.25). The systematic variations in major and trace elements, homogeneous Sr–Nd isotope data and temporal consistency among three volcanic groups, indicate that they were derived from a similar mantle source metasomatized by fluids related to the closure of the paleo-Asian and/or Mongolia–Okhotsk Oceans, and were produced through different degrees of fractional crystallization of the primary melts. Group 1 basaltic rocks were formed through removal of olivine and pyroxene of the primary melts, while Group 2 trachytes, which contain the lowest LREE contents (e.g., La=24–28 ppm) and relatively less enriched Sr and higher Nd isotope ratios, were generated after removal of a few percent of LREE-rich minerals such as hornblende, clinopyroxene and apatite of melts like Group 1. Group 3 rhyolite lavas exhibiting the highest abundances of strongly incompatible elements such as Rb and K, moderate LREE contents (e.g., La=28–53 ppm) as well as apparently negative Eu and Sr anomalies, represent the final crystallized products following a plagioclase-predominant fractionation of melts like Group 2. The low MgO contents and evolved affinities of the volcanic rocks imply that beneath the NDHM there existed many crustal magma reservoirs throughout the eruption episodes, in which mantle-derived primary melts had experienced intense differentiation. These facts, in combination with the contemporaneous basin and range tectonic regime, suggest that the extensive calc-alkaline volcanism in the NDHM was attributed to post-orogenic diffuse extension rather than either an upwelling mantle plume or Mesozoic oceanic plate subduction.