Discovery of Middle Jurassic volcanic rocks in the Duobaoshan area of Heilongjiang Province and constraints on the influence area of the Mongolian-Okhotsk Ocean closure

The Duobaoshan area of Heilongjiang Province is located in the eastern part of the Central Asian Orogenic Belt and the southeastern margin of the Xing'an Block, where large areas of Early-Middle Jurassic intrusive rocks are developed, but no contemporaneous volcanic rock has been reported there. During the exploration of gold deposits in the Duobaoshan area, volcanic rocks of the same period were discovered for the first time and they were identified as a suit of dacite, rhyolite, and trachyte. Zircon U-Pb dating and petrogeochemical analyses were carried out on these volcanic rocks to further clarify their formation age and tectonic significance. The U-Pb dating of zircons from the volcanic rocks yielded ages of 167.1~169.3 Ma, indicating that these rocks erupted during Middle Jurassic. The petrogeochemical analysis shows that the volcanic rocks are characterized by high alkali (Na₂O+ K₂O=3.70%~7.66%) and aluminum (Al₂O₃=11.42%~19.00%), and are peraluminous (A/CNK=1.08~3.73) with high potassium calcium alkali. The rare earth elements are characterized by the enrichment of light rare earth elements and right-leaning of heavy rare earth elements depletion, with slightly negative Eu anomalies (δEu=0.53~0.79).Trace elements are enriched in Rb, Ba, K, Th, U and Pb, and depleted in Nb, Ta, Ti, P and Sr.It is generally shown that the Mid-Jurassic volcanic rocks were derived from the partial melting of newly continental crust material. The discriminant diagrams of Ta-Yb and Nb-Y indicates that the Middle Jurassic volcanic rocks were formed in the compressional background. Combined with the regional tectonic setting and evolution characteristics, it is concluded that the Middle Jurassic volcanic rocks should be the product of the continent-continent collision caused by Mongolian-Okhotsk Ocean closure.Indicatively, the collision affected the southeastern margin of the Xing'an Block.