地球化学
地毬化學
지구화학
GEOCHIMICA
2009年
6期
509-522
,共14页
包志伟%王强%资锋%唐功建%杜凤军%白国典
包誌偉%王彊%資鋒%唐功建%杜鳳軍%白國典
포지위%왕강%자봉%당공건%두봉군%백국전
A型花岗岩%微量元素%同位素%地球化学%元古宙%Columbia超大陆
A型花崗巖%微量元素%同位素%地毬化學%元古宙%Columbia超大陸
A형화강암%미량원소%동위소%지구화학%원고주%Columbia초대륙
A-type granite%trace elements%isotope%geochemistry%Proterozoic%Columbia
龙王(石童)花岗岩岩体产于华北克拉通南缘,岩石类型主要为黑云母钾长花岗岩,局部见有霓辉石花岗岩.岩体高硅(SiO_2=72.17%～76.82%)、富碱(K_2O+Na_2O=8.28%～10.22%,K_2O/Na_2O>1),碱性指数AI(agpaitic index)=0.84～0.95,分异指数DI=95～97,铝指数ASI(aluminium saturation index)=0.96～1.13.含铁指数高(FeO~*/(FeO~*+Mg)=0.90 ～0.99),岩石为准铝质至弱过铝质、碱性-碱钙性、铁质A型花岗岩.岩石富集大离子亲石元素,稀土元素含量很高(854～1572 μg/g);高场强元素(Nb、Ta、Zr、Hf)的富集程度明显低于大离子亲石元素,因此在微量元素蛛网图上呈相对亏损特征;岩石显著亏损Ba、sr、Ti、Pb;ε_(Nd)(t)=-4.5～-7.2,Nd模式年龄为2.3～2.5 Ga.ε_(Hf)(t)=-1.11～-5.26,模式年龄t(Hf1)=2.1～2.3 Ga,t(Hf2)2=2.4～2.6 Ga.黑云母钾长花岗岩中的锆石主要为无色透明柱状晶体,CL图像多数显示清晰的岩浆成因的韵律环带结构,锆石LA-ICPMS U-Pb年龄为(1602.1 4±6.6)Ma,MSWD=0.48.岩石化学、微量元素及Nd、Hf同位素地球化学特征表明,龙王(石童)岩体形成于板内伸展构造环境,其形成可能与约1.8 Ga古大陆碰撞拼合之后岩石圈地幔及下地壳拆沉和玄武质岩浆底侵有关,岩体可能主要为富集地幔部分熔融的玄武质岩浆经强烈结晶分异的产物,在其形成和上升过程中可能有地壳物质的混染.龙王(石童)岩体的形成与Columbia超大陆裂解过程有关,是华北克拉通古元古代末-中元古代早期已知最年轻的非造山岩浆活动,可能代表华北克拉通古元古代末期自Columbia超大陆裂解出来的时间.
龍王(石童)花崗巖巖體產于華北剋拉通南緣,巖石類型主要為黑雲母鉀長花崗巖,跼部見有霓輝石花崗巖.巖體高硅(SiO_2=72.17%～76.82%)、富堿(K_2O+Na_2O=8.28%～10.22%,K_2O/Na_2O>1),堿性指數AI(agpaitic index)=0.84～0.95,分異指數DI=95～97,鋁指數ASI(aluminium saturation index)=0.96～1.13.含鐵指數高(FeO~*/(FeO~*+Mg)=0.90 ～0.99),巖石為準鋁質至弱過鋁質、堿性-堿鈣性、鐵質A型花崗巖.巖石富集大離子親石元素,稀土元素含量很高(854～1572 μg/g);高場彊元素(Nb、Ta、Zr、Hf)的富集程度明顯低于大離子親石元素,因此在微量元素蛛網圖上呈相對虧損特徵;巖石顯著虧損Ba、sr、Ti、Pb;ε_(Nd)(t)=-4.5～-7.2,Nd模式年齡為2.3～2.5 Ga.ε_(Hf)(t)=-1.11～-5.26,模式年齡t(Hf1)=2.1～2.3 Ga,t(Hf2)2=2.4～2.6 Ga.黑雲母鉀長花崗巖中的鋯石主要為無色透明柱狀晶體,CL圖像多數顯示清晰的巖漿成因的韻律環帶結構,鋯石LA-ICPMS U-Pb年齡為(1602.1 4±6.6)Ma,MSWD=0.48.巖石化學、微量元素及Nd、Hf同位素地毬化學特徵錶明,龍王(石童)巖體形成于闆內伸展構造環境,其形成可能與約1.8 Ga古大陸踫撞拼閤之後巖石圈地幔及下地殼拆沉和玄武質巖漿底侵有關,巖體可能主要為富集地幔部分鎔融的玄武質巖漿經彊烈結晶分異的產物,在其形成和上升過程中可能有地殼物質的混染.龍王(石童)巖體的形成與Columbia超大陸裂解過程有關,是華北剋拉通古元古代末-中元古代早期已知最年輕的非造山巖漿活動,可能代錶華北剋拉通古元古代末期自Columbia超大陸裂解齣來的時間.
룡왕(석동)화강암암체산우화북극랍통남연,암석류형주요위흑운모갑장화강암,국부견유예휘석화강암.암체고규(SiO_2=72.17%～76.82%)、부감(K_2O+Na_2O=8.28%～10.22%,K_2O/Na_2O>1),감성지수AI(agpaitic index)=0.84～0.95,분이지수DI=95～97,려지수ASI(aluminium saturation index)=0.96～1.13.함철지수고(FeO~*/(FeO~*+Mg)=0.90 ～0.99),암석위준려질지약과려질、감성-감개성、철질A형화강암.암석부집대리자친석원소,희토원소함량흔고(854～1572 μg/g);고장강원소(Nb、Ta、Zr、Hf)적부집정도명현저우대리자친석원소,인차재미량원소주망도상정상대우손특정;암석현저우손Ba、sr、Ti、Pb;ε_(Nd)(t)=-4.5～-7.2,Nd모식년령위2.3～2.5 Ga.ε_(Hf)(t)=-1.11～-5.26,모식년령t(Hf1)=2.1～2.3 Ga,t(Hf2)2=2.4～2.6 Ga.흑운모갑장화강암중적고석주요위무색투명주상정체,CL도상다수현시청석적암장성인적운률배대결구,고석LA-ICPMS U-Pb년령위(1602.1 4±6.6)Ma,MSWD=0.48.암석화학、미량원소급Nd、Hf동위소지구화학특정표명,룡왕(석동)암체형성우판내신전구조배경,기형성가능여약1.8 Ga고대륙팽당병합지후암석권지만급하지각탁침화현무질암장저침유관,암체가능주요위부집지만부분용융적현무질암장경강렬결정분이적산물,재기형성화상승과정중가능유지각물질적혼염.룡왕(석동)암체적형성여Columbia초대륙렬해과정유관,시화북극랍통고원고대말-중원고대조기이지최년경적비조산암장활동,가능대표화북극랍통고원고대말기자Columbia초대륙렬해출래적시간.
The Longwangzhuang A-type granites occur in the southern margin of the North China Craton and mainly consist of biotite K-feldspar granite,and locally distributed aegirine granite.The granites are characterized by high silicon and alkaline contents(SiO_2=72.17%-76.82%,K_2O+Na_2O=8.28%-10.22%,K_2O/Na_2O>1),AI (agpaitic index)=0.84-0.95,DI=95-97,ASI(aluminum saturation index)=0.96-1.13,very high Fe~* number(FeO~*/(FeO~*+Mg)=0.90-0.99),thus,the granites are of metaluminous to weakly prealuminous,alkalic to talc-alkalic ferroan A-type granites.The A-type granites are rich in large ion lithophile elements(LILE),especially high concentrations of rare earth elements(854-1572 μg/g);whereas the enrichment of high strength field elements (Nb, Ta, Zr and Hf) is obviously less than those of the LILEs, exhibiting mild depletions on trace element sipder plots; and significantly depleted in Ba, Sr, Ti and Pb. The low ε_(Nd)(t) ( -4.5 to -7.2) and high model ages (2. 3 -2.5 Ga) of the granites as well as the low ε_(Hf)(t) ( - 1.11 to -5.26), high Hf model ages ( t_(Hf1), = 2. 1 - 2. 3 Ga, t_(Hf2) = 2.4 ～ 2. 6 Ga) of zircons from the biotite K-feldspar granite suggest that the granites were derived from enriched mantle source and probably with the involvement of crustal materials. The zircons from the biotite K-feldspar granite are mainly colorless transparent crystals exhibiting well-developed oscillatory zonation on cathodoluminescence images. The LA-ICPMS zircon U-Pb age is (1602.1 +6.6) Ma (MSWD=0.48).Petrochemical, trace elements, as well as Nd and Hf isotopic compositions demonstrate that the granites were formed under within plate extensional tectonic regime related to the breakup of the Columbia supercontinent. The granites were most likely originated from highly differentiated basaltic magma resulted from partial melting of mantle sources, likely fertilized by recycling crustal materials triggered by the delamination of lithospheric mantle and lower crust following the ～ 1.8 Ga collision and amalgamation of the Columbia supercontinent. Nevertheless, the contamination of neo-Archean to Paleoproterozoic crustal materials during the upwelling and emplacement of the magma could not be excluded. Being the youngest known anorogenic magmatism on the southern margin of the NorthChina Craton related to Columbia breakup, the Longwangzhuang A-type granites might represent the break off of the North China Craton from the Columbia supercontinent by the end of Paleoproterozoic.
