LA-ICP-MS zircon U-Pb geochronology, geochemical characteristics and tectonic significance of Early Devonian volcanic rocks in Malasu area, western Juggar Basin

The early Devonian strata in Malasu area is a set of littoral-neritic facies volcanic sedimentary rocks. This paper reports the zircon age and geochemical characteristics of the volcanic rocks. A lot of Neo-Archean and middle Neo-Proterozoic zircons indicate that this area contains ancient continental crust material. According to the values of major elements (SiO2 52.38%~69.6%, Na2O 2.80%~4.85%, K2O 0.16%~0.96%, TiO2 0.5%~1.96%, Al2O3 14.62%~18.18%, MgO 1.08%~5.75% ) and the values of Mg# (22.92~38), the volcanic rocks of Malasu area are characterized by high content of sodium and low content of potassium, and thus belong to low-potassium tholeiite series and calc-alkaline series. The volcanic rocks have significant negative Eu anomalies (0.83~0.83), total rare earth element values of 73×10-6~115×10-6, and LREE/HREE values of 2.66~3.25. The geochemical characteristics of basaltic andesite samples are similar to those of typical volcanic arc basalts, being rich in large ion lithophile elements K, Rb, Ba, Sr and depleted in the high field strength elements Nb, Ta, Zr, Hf. The same characteristics of enrichment of LILE and depletion of HFSE for dacite porphyry samples reflect the addition of lots of crust-derived materials. The chondrite-normalized REE and primitive mantle-normalized alteration-resistant trace element patterns for the dacite porphyry are similar to the patterns of basaltic andesite, which suggests that they belong to the comagmatic evolutionary series. Comprehensive research shows that the volcanic rocks have the characteristics of island arc volcanic rocks, and were formed in the volcanic island arc environment in the process of plate subduction.