【摘要】：大興安嶺是中國東北部陸相火山巖發(fā)育的地區(qū)之一,其不僅分布廣泛,而且空間巖相組合變化較大、成因復(fù)雜。筆者對大楊樹火山盆地西緣噴發(fā)就位在花崗巖帶的小莫爾可地區(qū)中生代火山巖的地質(zhì)、巖相學(xué)、年代學(xué)、元素和Hf同位素地球化學(xué)進(jìn)行了研究。結(jié)果表明,該區(qū)出露火山巖可劃分3個(gè)階段,第一階段主要為英安質(zhì)火山碎屑巖夾英安巖組合,呈爆發(fā)式火山噴發(fā)特征;第二階段為玄武質(zhì)粗面安山巖、粗面安山巖組合,呈溢流式火山噴發(fā)特征;第三階段是玄武粗面安山質(zhì)熔結(jié)火山碎屑巖(部分含角礫)。各階段代表性火山熔巖以及熔結(jié)火山碎屑巖的元素地球化學(xué)特征揭示:均為富堿(w(Na_2O+K_2O)=4.88%~7.12%),屬高鉀鈣堿性系列;具有相似的痕量元素地球化學(xué)分餾特征,即明顯富集Ba、K、LRRE等大離子親石元素,虧損Nb、Ta、Ti、HREE等高場強(qiáng)元素;稀土元素分餾程度中等(LREE/HREE=8.4~8.5)、Eu負(fù)異常不明顯(δEu為0.91~1.02)。這些特征表明它們是同源巖漿房結(jié)晶分異演化的產(chǎn)物,巖漿源區(qū)性質(zhì)呈現(xiàn)地幔與殼�；旌线^渡類型的屬性,或呈現(xiàn)E-MORB性質(zhì)的源區(qū)或交代洋殼性質(zhì)的源區(qū)。鑒于獲得的英安質(zhì)巖屑晶屑凝灰?guī)r(HSY-1)和玄武粗面安山質(zhì)巖屑晶屑熔結(jié)凝灰?guī)r(P2121)單顆粒鋯石U-Pb同位素定年分別為(124.8±1.0)、(123.3±1.3)Ma,ε_(Hf)(t)為1.7~9.7、T_(DM)=705~407 Ma、T_(DM)~C=1 464~748Ma,并結(jié)合區(qū)域地球動(dòng)力學(xué)研究進(jìn)展,可進(jìn)一步得出:火山作用背景應(yīng)屬于中生代古太平洋板塊向中國東部大陸俯沖的大陸邊緣巖漿弧環(huán)境,適值東北地區(qū)大規(guī)模巖漿底侵、巖石圈拆沉作用的峰期(約120 Ma),初始巖漿應(yīng)是古太平洋板塊深俯沖作用過程形成的埃達(dá)克質(zhì)巖漿,而呈現(xiàn)埃達(dá)克質(zhì)巖漿與島弧性質(zhì)的過渡巖石學(xué)、地球化學(xué)特征,可能是在巖漿底侵、巖石圈拆沉過程與地殼物質(zhì)發(fā)生一定程度的混染作用;并從成礦元素的相容性角度確認(rèn)該期巖漿作用具有提供Mo、Cu和Ag成礦流體的可能。
[Abstract]:Daxinganling is one of the areas where continental volcanic rocks are developed in northeast China. It is not only widely distributed, but also has great changes in spatial lithofacies assemblages and complex Genesis. The geological, petrographic, chronological, element and Hf isotopic geochemistry of the Mesozoic volcanic rocks in Xiaomoke area, which is located in the granite belt, is studied in this paper. The results show that the exposed volcanic rocks in this area can be divided into three stages, the first stage is mainly Yingan pyroclastic rock assemblage, showing explosive volcanic eruption characteristics, the second stage is basaltic rough andesite assemblage, showing overflow volcanic eruption characteristics, and the third stage is Black Tortoise rough andesite molten pyroclastic rock (part containing breccia). The element geochemistry characteristics of representative volcanic lava and molten pyroclastic rock in each stage show that they are alkali-rich (w (Na_2O K 鈮，

本文編號：2503414