GeocherrHcal And Petrogenesis Studies Of Jurassic Diorite In Helong Of Yanbian Area, NE China

Yanbian area in Jilin province locates on the landward edge of northeast China as well asbetween the plots of Kamusze and Huabei. It is a superposition compound tectonic zone camethrough the evolution of Paleo-Asian Ocean and the dubduction of Paleo-Pacific Plate inMesozonic, in which distributes vest granitic rock dating from late hercynian period,indo-china period and Yanshanian. Among them are mainly monzonitic granite, granodioriteand partly diorite. In recent years, lots of experts have done some researches for the graniteand volcanic in Yanbian area and gotten huge of chronology information. While compareswith the former, the detailed researches for lithogeochemistry are quite less, and theresearches on medium ultrabasic invade rock put much emphasis on the periods ofNeopaleozocic and Mesozoic, even lead to the restriction of going depth into theunderstanding of the influence to spatial and temporal pattern, the evolution of lithosphericmantle and its dynamic mechanics. On this condition, this paper does some researches for theMesozoic dirorite on the respect of chronology and geochemistry, in order to discuss thegeodynamics backdrop of its factors and formation, as well as provide new gist for regionaltectonic evolution.1. Combination and formation age of DioriteDiotite in Jurassic period consists of quartz diorite and gabbro diorite. Zircon U-Pbisotopic dating shows that the diorite in Helong area in Jurassic period is made of173-172Ma:quartz diorite ages174-173Ma, and gabbro diorite ages175Ma, the former kind of rock’szircon crystals age is entirely later than the latter.2. Geochemical characteristics of Diorite rock Element geochemical characteristics show that local diorite rock in Jurassic periodw(SiO2)=52.77%~58.0， w(Al2O3)=15.87%~18.93%， w(MgO)=1.96%~6.45%，w(TiO2)=0.99%~1.08%， w(CaO)=5.38%~8.26%， w(TFe2O3)=6.59%~9.64%，w(Na2O)=3.37%~4.88%，w(K2O)=0.94%~2.30%，and the content of alkli w(Na2O+K2O) is4.43%~6.76%，Na2O/K2O=1.70~4.16，sodic type; A/NK percent number is1.88~2.46(molecule), A/CNK percent number is0.73~0.95(molecule)，Quasi aluminum rock;Mg#(37~59），all the w (MgO) of gabbro diorite is bigger than4%， while the Mg#is biggerthan45，basically in the line of the characteristics of Mg andesite.Diorite rare earth element REE pattern shows that europium has no abnormalities andweak positive anomalies (δEu=0.96~1.27) for the right type, which may has a relationshipwith the existing of massive felspathic. The rare earth element contents low in the two kindsof diorite, the∑REE is between81.7×10-6and162.6×10-6, LREE/HREE=2.25~4.29, thefractionation in light and heavy rare earth is weak,(La/Yb)N=5.18~14.31，LREE is relativerich, while HREE is vice versa. Chondrite standardized graphic shows that rare earthelement REE pattern for the right type has no abnormalities and weak positive anomalies(δEu=0.96~1.27) may has a relationship with the existing of massive felspathic. On theprimitive mantle standardized charter, rare elements show local diorire are rich of large ionlithophile element (LILE) Rb、Th、U、K,etc, lack of high field-strength element Zr、Hf、Y、Yb, and extremely lack of Nb、Ta、P、Ti and so on.3. Petrogenesis and structural setting of dioriteDiorite in Helong area has its geochemical characteristics to contain slightly of SiO2,relatively high of Mg#(37~59) and K-rich Na (Na2O/K2O=1.7~4.2) and rich of transitionelements (Sc、V、Co、Cr、Ni and so on), which definitely shows its magmatic source is uppermantle. The rare elements’ specific value of Nb/Ta(15.70~19.89, average number:7.56) isentirely equal of earth mantle (17.5), and that’s also one of the characteristics of mantle,while the specific value of Rb/Sr is (0.03~0.09) a little higher than that of mantle (0.034) andextremely higher than that of earthcrust (0.35). Rare elements are rich, heavy rare earthelements are rare and high field-strength element are extremely short. In conclusion, thispaper thinks diorite coming from the melted enriched mantle caused by subduction zone’s fluid metasomatism under the circumstance of the action that the pacific plate diving to thewest.