A Study On Geology And Geochemistry Of The Keluo Volcanic Cluster In Heilongjiang Province

The Keluo volcanic cluster, which covers an area of ca.350km², is locatedbetween the Greater Khingan Mountains and the Songnen Plain, HeilongjiangProvince, and in the north of the famous Wudalianchi volcanic cluster. The Cenozoicvolcanoes that primarily consist of alkaline basalts are distributed along the KeluoRiver. The Keluo, Wudalianchi and Erkeshan volcanic clusters are commonly knownas potassic volcanic rock belts. The activities and distribution of this volcanic clusterare largely controlled by the regional basement faults, and consistent with the specialtectonic setting of the NE-trending fault basin. The Strombolian-type central eruptionis the predominant eruptive type in this area. Volcanic activities in this area can bedivided into three periods, namely Pliocene period, Pleistocene period and Holoceneperiod.During the Pliocene, a series of central-overflow-type volcanoes, which aresituated on the edge of the basin, were formed along the NE-trending faults. However,except for some shield lava platform, most of the original volcanic geomorphicfeatures have been destroyed by weathering. The volcanic activities of earlyPleistocene were relatively quiet. But the eruption activities of the Mid-andLate-Pleistocene were also controlled by the NE-trending basement faults, whileeruptive center, type and intensity were all changed as well. These volcanoes aremainly composed of alkali-basaltic scoria cones and lavas. The Holocene volcanoes,as represented by Nanshan, have similar eruptive type and composition to the formerones, but their volcanic structures are well preserved. Some micro-morphologicalcharacteristics can be easily recognized at their outcrops. On the whole, the lavacovered the former sedimentary strata and changed the river flows attributed to thevolcanic activities of the Keluo volcanic cluster. As a result, the original geomorphyin the basin was totally altered.The Keluo volcanic rocks are basanite-phonotephrite-tephriphonolite incomposition, with SiO₂and K₂O contents varying from45to55wt%and from2.8to 6.9wt%, respectively. In addition, the potassium content is higher than the adjacentWudalianchi and Erkeshan volcanic clusters, with K₂O/Na₂O ratio>1and up to1.9.The whole potassic volcanic rock belts are located on the northern margin of theXing’an-Mongolia Orogenic Belt and the North-South Gravity Lineament. Bycomparing with the Nuomin River and Halaha-Chaoer River Quarternary volcanicrocks, a spatially evolutionary trend of rock types from sodium series to potassicseries of the Xing’an-Mongolia Orogenic Belt can be inferred.The chondrite-normalized REE patterns are highly differentiated and arecharacterized by high total REE concentrations and LREE enrichment. The primitivemantle-normalized trace element patterns are highly uniform and indicate that thesimilar magma source for the Keluo volcanic rocks and those from Wudalianchi andErkeshan volcanic rocks. The obvious enrichment of incompatible elements might bederived from metasomatic fluids.Isotopically, the Keluo potassic basalts are characterized by high⁸⁷Sr/⁸⁶Sr, low¹⁴³Nd/¹⁴⁴Nd and radiogenic Pb, which distinguish them from the depleted mantleunderlying the Hannuoba and Datong areas, as well as the primitive mantleunderlying the Changbaishan area. The obvious complementary characteristicsbetween the Keluo area and the Halaha-Chaoer River Quarternary volcanic rockssuggest an evolution relationship of the mantle components beneath the NE-trendingCenozoic volcanic clusters that are located in the Greater Khingan Mountains and theSongnen Plain. The transition from depleted mantle to EMI-mantle might be resultedfrom mantle metasomatism by fluids with high Sr and incompatible elementconcentrations.The mantle xenoliths entrained in Keluo volcanic rocks include spinel lherzoliteand phlogopite-bearing lherzolite. Their estimated equilibrium temperature （850°C¹116°C） is consistent with oceanic or continental rift type geothermal gradient of theWudalichi area and the high geothermal gradient of the Greater Khingan Mountainsarea.The conclusion that the Keluo, Wudalianchi and Erkeshan volcanic clusters havethe similar magmatic property, source and mantle composition can be reached from the analyses above. Through comparing with geochemistry studies of adjacent areas,it is found that the mantle compositions beneath the Greater Khingan Mountains haveevolution and heterogeneity from southwest to northeast, which might be resultedfrom heterogeneity mantle metasomatism beneath the Songliao basin.