Abstract:

This paper deals with the geochemical data including major and trace elements and Sr-Nd isotopes of the volcanics from Yamansu Formation and Tugut Bulak Formation, occurring along the southern Juoluotage tectonic belt. Yamansu felsic lava is mainly of lowK tholeiite series, Mg# changes from 49 to 51. The values of ISr range from 0.70642 to 0.70768 and εNd(t) from 3.28 to 5.49. It is relatively enriched in LREEs and LILEs, and depleted in HREE and HFSE with moderate Eu anomalies(δEu=0.620.65). All these geochemical characteristics suggest that the formation of the Yamansu felsic lava is probably related to differentiation process of mantlederived mafic magma. The andesites from the Tugut Bulak Formation are mainly of calc-alkaline and high K calcalkaline series, Mg# changes from 53 to 57. Geochemical studies on trace elements demonstrate the total LREE enrichment and relative HREE depletion with slight negative Eu anomalies(δEu=0.630.84). The primordial mantlenormalized trace element patterns are characterized by LILE enrichment and HFSE depletion. SrNd isotopic data show that the ISr values range from 0.70371 to 0.70498, while εNd(t) changes from 6.48 to 6.83, indicating that the parent magma is originated from a juvenile lithospheric mantle wedge that was previously metasomatized by subduction zone fluids. Combining with previous studies, we suggest volcanics from the southern Juoluotage area were formed in island arc setting which was related to southward subduction of Paleo-Asian Oceanic lithosphere. Early Carboniferous volcanics from Yamansu Formation represent juvenile intra-oceanic arc, while Late Carboniferous volcanics from Tugut Bulak formation indicate that the previous juvenile arc had evolved to a relatively mature stage. The composition and temporalspatial distribution of the volcanics in the study area reveals the southward thickening of lithosphere during Carboniferous, which provides strong evidence for the southward subduction of the southern margin of Tuha oceanic crust.

Key words: Juoluotage, Yamansu Formation, Tugut Bulak Formation, volcanic arc, Paleo-Asian Ocean, tectonic evolution