| The Dahalajunshan formation are widely distributed in West Tianshan in Xinjiang, and they are the host rocks of many deposits, so the research of their formation, geotectonic background and evolutionary pattern become much more important. On the basis of the previous work, a systematic petrographic observation was carrid out, and then their major element, trace element and Sr-Nd isotopic composition were analyzed. The petrologic characteristics show that the first member of the formation is marine volcanics, the second member is paralic volcanics, the third member is continental volcanics, overall, Dahalajunshan formation were changed from sea facies into continental facies, and they are a complete rock series contained rock types of basalt, basaltic andesite, andesite, dacite and rhyolite. Trace element show that they are a un-alkalic rock series, and cal-alkaline series are their main body, only a small component for tholeiite series. +εNd (t) proved that the magama came from a loss source region, the high K2O values, high Al2O3 values, high (87Sr/86Sr)0 values and negative Nb anomalies proved the continental crust and oceanic crust maybe subducted into the magma source area. The negative Eu anomalies show us that the fractional crystallization played an important role in magmatism. Different lithology show different features in HREE, that is a proof of magmatic differentiation. And as a result of the rapid upwelling magma, rocks avoid contamination of wall rock, but in the latter, part of the rocks had suffered the effects of seawater alteration. High-Ti, high initial 87Sr/86Sr ratio show that the tectonic setting of volcanic rocks in this area is the active continental margin, and the bimodal volcanic rocks of the upper section proved the idea that they were changed into extensional tectonic environment, thus Dahalajunshan volcanic rocks reflect the change from extruded of the main phase to extensional environment, such a change may be caused by a more steep angle of subduction.
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