Chronology,Characteristics Of Early Jurassic Granites In Heihuashan Area,Heilongjiang Province And Tectonic Significance

The Mesozoic intrusive rocks are widely developed in Heihuashan Mo deposit area and nearby of the north Heihe area,Heilongjiang Province,but the formation age,rock assembleage and genesis of the intrusive rocks are still unclear.In this paper,the intrusive rocks in Heihuashan area have been studied in field,petrography,petrology,geochronology and petrochemistry,in order to discuss the formation age,petrogenesis and geotectonic background of these rocks,For two of monzonitic granites,the weighted average ages of zircon U-Pb dating by LA ICP-MS are 148.7 ± 0.60 Ma and 153.5 ± 0.56 Ma respectively,indicating that they were formed in the late Jurassic.The intrusive rock assemblage of tonalite(T1),trondhjemite(T2),and granite(G,granite in a narrow sense)indicates rock assembleage of T1T2 G in arc environment.The above granitoids are enriched in silicon,alkali,high potassium,peraluminium(A / CNK is greater than 1),and depleted in Fe2O3 T.The rocks belong to high potassium calcium-alkali series in the Si O2-K2 O diagram,low Fecalcium-alkali(LF-CA)series in Si O2-Mg O diagram and calcium alkali or alkali calcium in Si O2-(Na2O+K2O-Ca O)diagram.The distribution pattern of REE is right-leaning,with LREE relatively enriched and HREE depleted,giving(La/Yb)n=13.78 ~ 29.85 and no negative Eu anomaly or weak positive Eu anomaly or weak negative Eu anomaly.The granitoids are characterized by high Sr and low Y,with Sr /Y ratio ranging from 29.53 ~ 103.80,which are characteristic of "C-type" adakites.The above rock assemblages and their characteristics all indicate the properties of arc igneous rocks.Combined with the tectonic environment,it is speculated that the late Jurassic granitoids in this area were formed in the southwardly subduction environment of the Mongolia-Okhotsk Ocean.Magmas may derived from the partial melting of the thickened lower crust of Xing'an Arc.At this time,the underplating of mantle derived magma which reacts with mantle wedge produces extensive crust mantle interaction,forming granite batholith.After that,the batholiths of Heihuashangranite uplifted rapidly because of delamination and the magma of Heihuashan granite porphyry(including molybdenite and magmatic fluids)coming from deeper crust was intrusived into the batholiths of Heihuashangranite in the shallow crust,and then formed the Heihuashan granite porphyry and the molybdenum deposit.Thus,we suggest the Heihuashan molybdenite deposit was formed by post-batholith metallogenesis.