The Characteristics And Significance Of Early Cretaceous Volcanic Rocks In Heihe Area

Mesozoic volcanic rocks are widely developed in Heihe Region of Heilongjiang Province.However,there are a lot of controversies about the association,genesis,magma source and tectonic setting of the volcanic rocks.The relationship between gold mineralization and volcanic rocks in the Region is still lack of in-depth study.In this paper,a detailed study of petrology,geochronology,and geochemistry,etc.on the volcanic rocks in the Region is carried out along with a supplementary study of the mineral deposits,and the following knowledge is obtained:1.It is determined that the formation age of the volcanic rocks is the Late Period of Early Cretaceous and the volcanic rocks are dominated by andesite-dacite-rhyolite,including the volcanic rock association of shoshonite,basaltic andesite,benmoreite,latite,trachydacite,and trachyte.2.The geochemical characteristics of the volcanic rocks show that they consist of a wide range of Si,Al,K,Na,Ca,Fe,Mg,etc.Volcanic rocks are mainly sub-alkaline series;the Na₂O/K₂O ratio of the rocks is generally larger than 1.2.And since the A/CNK ratio is around1,they are quasi-aluminous/peraluminous rocks.The SiO₂-TFe O/Mg O diagram shows that there are both calc-alkiline series?including low-Fe calc-alkiline series?and tholeiitic series.The SiO₂-K₂O diagram shows that there are mainly medium-K and high-K calc-alkiline series.The SiO₂-?Na₂O+K₂O-Ca O?diagram indicates that there are mainly calc-alkiline series,alkali-calcic series,and alkaline series,and the SiO₂-Mg O diagram mainly involves the local melting range of the experimental basalt and Mg andesite range.The spider diagram of trace elements shows that it is rich in large ion lithophile elements;K,La,Zr,Hf,Tb,and Tm are the peaks and Nb,Ta,Ti,Y,Yb and other high field strength elements are lost.The normalized distribution mode of REE chondrite meteorite is the slowly right-decline mode;light rare earth is relatively rich;the fractionation within the heavy rare earth is not obvious;and there is a weak negative Eu abnormity.Most rocks have high Sr and low Y and some rocks have the characteristics of adakite.3.Based on the volcanic rock association and geochemical characteristics of the volcanic rocks,considering the identification of tectonic setting,etc.,it is believed that the EarlyCretaceous volcanic rocks in the Region were formed in the tectonic setting related to the subduction of the ocean and a mature continental margin arc was developed in the Late Period of Early Cretaceous.It is speculated that the Early Cretaceous volcanic rocks in the Region are the products of the subduction of Mongol-Okhotsk Ocean to the south side;the possible magma sources are the subducted oceanic crust,the overlying mantle wedge,and the lower crust of the Xing'an arc.The development of Early Cretaceous arc volcanic rocks in Heihe Region indicates that during Early Cretaceous the Mongol-Okhotsk Ocean was still subduing to the microcontinent in the south;and Mongol-Okhotsk Ocean closed later than Late Period of Early Cretaceous?¹17Ma?.4.The study shows that the surrounding rocks?or parent rocks?of the gold deposits in the studied region are Early Cretaceous volcanic rocks.The north-east ductile shear zone in the Region may be a part of the subduction zone of Mongol-Okhotsk Ocean and manifestation of shallow source,and is the main part of regional lithology and ore controlling.The gold deposits in the Region are closely related with volcanic rocks equil to TTG,MA series,adakite and other subduction related rocks,which suggests that the formation of the gold deposits is closely related to the melting of the oceanic plate.There are two types of gold deposits related to Early Cretaceous volcanic rocks in the studied region: volcanic hydrothermal gold deposits related with andesite and altered rock type gold deposits related with diorite-porphyrite.The above two types of gold deposits were formed by the different depths of magma location because diorite porphyrite and andesite have the same formation period,same source and same geochemical properties.It is speculated that the altered rock type?ductile shear zone type?gold deposits may be found in the deep part of middle-low temperature volcanic hydrothermal gold deposits;there is good prospect of finding gold ore in the Early Cretaceous Andesitic volcanic rocks and subvolcanic rocks?diorite-porphyrite?distribution areas.