周爭(zhēng)艷 楊家喜 徐濤 聶麗娟 孫蘭蘭
摘要:西秦嶺天水地區(qū)早中生代草川鋪巖體內(nèi)發(fā)育一套似斑狀花崗巖,巖體主要由寄主似斑狀二長(zhǎng)花崗巖及閃長(zhǎng)質(zhì)微粒包體組成。SHRIMP鋯石UPb定年結(jié)果顯示,寄主巖體的侵位年齡為(210.9±17)Ma,平均標(biāo)準(zhǔn)權(quán)重偏差(MSWD)為1.74,屬于早中生代花崗巖體。巖石地球化學(xué)研究表明:寄主巖體屬于高鉀鈣堿性系列,具準(zhǔn)鋁質(zhì)特征,A/CNK值為0.99~1.07;稀土元素總質(zhì)量分?jǐn)?shù)為(128.13~196.56)×10-6,輕、重稀土元素質(zhì)量分?jǐn)?shù)比值為8.20~9.47,巖體富集輕稀土元素,微量元素特征表現(xiàn)為高場(chǎng)強(qiáng)元素貧化,但元素Ta、Nb明顯虧損。巖相學(xué)、地球化學(xué)和年代學(xué)特征表明,早中生代草川鋪巖體與南秦嶺糜署嶺巖體具有相似的巖相學(xué)特征和近一致的結(jié)晶年齡,并且二者的稀土、微量元素特征相似。因此,早中生代草川鋪巖體和糜署嶺巖體是同一套巖漿活動(dòng)的產(chǎn)物,前者雖然構(gòu)造位置處于北秦嶺造山帶內(nèi),但其物質(zhì)來(lái)源于南秦嶺造山帶,表明南秦嶺造山帶和北秦嶺造山帶在晚三疊世之前已經(jīng)碰撞。由于早中生代草川鋪巖體具高鉀鈣堿性埃達(dá)克巖地球化學(xué)特征,所以其是南秦嶺造山帶向北秦嶺造山帶俯沖熔融的產(chǎn)物。
關(guān)鍵詞:花崗巖;年代學(xué);早中生代;鋯石;UPb定年;地球化學(xué);西秦嶺
中圖分類(lèi)號(hào):P588.12+1;P595;P597+.3文獻(xiàn)標(biāo)志碼:A
Geochronological and Geochemical Characteristics of Early Mesozoic
Caochuanpu Granitoids in Tianshui Area of West Qinling
and Their Geological SignificanceZHOU Zhengyan, YANG Jiaxi, XU Tao, NIE Lijuan, SUN Lanlan
(School of Earth Science and Resources, Changan University, Xian 710054, Shaanxi, China)Abstract: Porphyaceous granitoid, which develops in Early Mesozoic Caochuanpu intrusion in Tianshui area of West Qinling, is mainly composed of host porphyaceous monzogranite and diorite microgranular enclave. SHRIMP zircon UPb dating shows that the emplacement age of host rock is (210.9±1.7)Ma with the mean standard weighted deviation (MSWD) of 1.74, so that the rock belongs to Early Mesozoic granitoid. Lithogeochemistry characteristics show that host rocks are highK calcalkaline series and metaluminous, and the values of A/CNK are 0.991.07; total mass fractions of rare earth elements are (128.13196.56)×10-6, the ratios of mass fractions of light and heavy rare earth elements are 8.209.47, so that the intrusions are rich in light rare earth element, high field strength elements are poor, and Ta and Nb are depleted. The petrology, geochemistry and geochronology show that the petrology, conformity, trace element and rare earth element characteristics of Early Mesozoic Caochuanpu intrusion are the same as that of Mishuling intrusion in South Qinling. Consequently, Caochuanpu and Mishuling intrusions are the products of the same magmatic activity. Tectonic setting of Caochuanpu intrusion is North Qinling orogenic belt, but the material source is South Qinling orogenic belt, so that the collision between North Qinling and South Qinling orogenic belts happens before Late Triassic. Because Early Mesozoic Caochuanpu intrusion has the characteristics of highK calcalkaline adakitic rock, it is the product of subduction and melting from South Qinling to North Qinling.
Key words: granitoid; geochronology; Early Mesozoic; zircon; UPb dating; geochemistry; West Qinling
0引言
西秦嶺造山帶是秦嶺構(gòu)造帶的主要組成部分。Zhang等對(duì)其各期巖體的源區(qū)性質(zhì)、巖漿演化、構(gòu)造環(huán)境等已做過(guò)大量的分析研究[124],為西秦嶺的形成與演化研究提供了豐富的基礎(chǔ)資料。研究區(qū)位于西秦嶺北部,北側(cè)毗鄰華北板塊,古生代變質(zhì)火山巖及中生代花崗巖、閃長(zhǎng)巖巖體廣泛分布,巖石類(lèi)型及巖相組合較為復(fù)雜。Zhang等認(rèn)為西秦嶺天水地區(qū)草川鋪巖體形成于早古生代(434 Ma),巖性由中細(xì)粒—等粒黑云母花崗巖、花崗閃長(zhǎng)巖和英云閃長(zhǎng)巖組成,具有中鉀鈣堿性過(guò)鋁質(zhì)花崗巖特征,是島弧環(huán)境下形成的花崗巖體[1]。然而,筆者發(fā)現(xiàn)的草川鋪巖體內(nèi)部還發(fā)育含暗色包體的似斑狀花崗巖體,與Zhang等研究的草川鋪巖體[1]不同,而與南秦嶺糜署嶺[26]、光頭山[25]、東江口[26]等巖體相似?;诖耍疚耐ㄟ^(guò)詳細(xì)的野外調(diào)查,結(jié)合巖相學(xué)、年代學(xué)及地球化學(xué)研究,對(duì)該巖體的結(jié)晶年齡、物質(zhì)來(lái)源、巖石成因等進(jìn)行研究,確定其巖石類(lèi)型、形成時(shí)代,闡明其構(gòu)造意義,以期為秦嶺造山帶的形成與演化提供參考。
1區(qū)域地質(zhì)概況與巖相學(xué)特征
研究區(qū)位于秦嶺造山帶西段,以古元古代變質(zhì)帶為界劃分為北秦嶺造山帶和南秦嶺造山帶。草川鋪似斑狀花崗巖分布于研究區(qū)的北部,屬北秦嶺造山帶。巖體周?chē)饕植己J河巖群變質(zhì)巖和古元古代秦嶺巖群變質(zhì)巖地層。區(qū)內(nèi)巖漿活動(dòng)強(qiáng)烈,主要分布有加里東期巖體和印支期巖體。本文所述巖體呈EW向展布,侵入于早古生代草川鋪花崗巖體內(nèi),兩巖體呈突變接觸關(guān)系。巖體只發(fā)育似斑狀花崗巖一種,在內(nèi)部發(fā)現(xiàn)暗色包體,包體形態(tài)各異,多為橢圓狀。
本文共采集樣品4件,其中3件為寄主花崗巖樣品,1件為暗色包體樣品。寄主巖體以樣品TSP31為代表,巖石鏡下鑒定為灰白色中細(xì)粒似斑狀二長(zhǎng)花崗巖,斑晶為石英、斜長(zhǎng)石和鉀長(zhǎng)石。石英斑晶(體積分?jǐn)?shù)約2%)粒度為2~5 mm,多具熔蝕結(jié)構(gòu);鉀長(zhǎng)石(體積分?jǐn)?shù)為15%)多為條紋長(zhǎng)石,呈板條狀、他形—半自形,粒度為2~20 mm;斜長(zhǎng)石(體積分?jǐn)?shù)為5%)呈他形—半自形、短柱狀,柱長(zhǎng)為1.5~5 mm,斜長(zhǎng)石An牌號(hào)小于30,部分發(fā)生絹云母化?;|(zhì)主要由石英、鉀長(zhǎng)石、斜長(zhǎng)石、黑云母等組成。石英(體積分?jǐn)?shù)為30%)呈他形、粒狀,粒徑為0.125~1 mm,一般為0.25~0.5 mm;斜長(zhǎng)石(體積分?jǐn)?shù)為38%)呈他形—半自形、板條狀,板長(zhǎng)為025~125 mm,發(fā)生強(qiáng)烈絹云母化;鉀長(zhǎng)石(體積分?jǐn)?shù)為25%)呈他形—半自形、板條狀,板長(zhǎng)為025~0.75 mm,發(fā)生絹云母化和高嶺土化;黑云母(體積分?jǐn)?shù)約6.5%)呈半自形—他形、片狀。副礦物有榍石、鋯石、褐簾石等。
包體(樣品TSP33)鏡下鑒定為石英二長(zhǎng)閃長(zhǎng)巖,具斑晶結(jié)構(gòu);斑晶主要為石英,其次為鉀長(zhǎng)石。石英斑晶(體積分?jǐn)?shù)約為3%)粒度為0.75~2.25 mm;鉀長(zhǎng)石斑晶(體積分?jǐn)?shù)為1%)呈板狀,粒度多為075 mm?;|(zhì)主要由石英、斜長(zhǎng)石、鉀長(zhǎng)石、普通角閃石、單斜輝石組成。石英(體積分?jǐn)?shù)為10%)分布于其他礦物顆粒之間,粒度多為0.25 mm;斜長(zhǎng)石(體積分?jǐn)?shù)為44%)呈他形、板條狀雜亂分布,An牌號(hào)小于30,為酸性斜長(zhǎng)石,柱長(zhǎng)為0.4~0.6 mm;鉀長(zhǎng)石(體積分?jǐn)?shù)為9%)呈他形、粒狀,多分布于斜長(zhǎng)石顆粒之間,粒度為0.1~0.3 mm;普通角閃石(體積分?jǐn)?shù)約20%)呈他形、粒狀,部分與輝石伴生,分布于單斜輝石邊緣;單斜輝石體積分?jǐn)?shù)為5%,黑云母體積分?jǐn)?shù)為12%,副礦物有榍石、磷灰石、鋯石等。包體形態(tài)多為橢圓狀和不規(guī)則狀,直徑為2~20 cm。
2分析方法
本次測(cè)年工作在中國(guó)地質(zhì)科學(xué)院北京離子探針中心完成,所選用樣品為草川鋪似斑狀二長(zhǎng)花崗巖,標(biāo)本編號(hào)為T(mén)SP3,采樣位置及巖體野外與鏡下照片可參見(jiàn)圖1、2。樣品鋯石顆粒的挑選在河北省區(qū)域地質(zhì)礦產(chǎn)調(diào)查研究所進(jìn)行。測(cè)年使用方法為現(xiàn)今國(guó)內(nèi)最先進(jìn)的SHRIMP IIeMC UPb技術(shù)。此方法使用的是比對(duì)測(cè)年方式,每種樣品首先以實(shí)驗(yàn)室標(biāo)樣開(kāi)始,每測(cè)完3個(gè)測(cè)試點(diǎn)后,再次進(jìn)行標(biāo)樣測(cè)定,以此來(lái)控制樣品和標(biāo)樣的誤差關(guān)系,樣品測(cè)試完畢必須以標(biāo)樣結(jié)束。通常情況每一次年齡測(cè)試操作耗時(shí)為15 min,樣品耗時(shí)為45 h。試驗(yàn)操作完畢后,數(shù)據(jù)由中國(guó)地質(zhì)科學(xué)院北京離子探針中心進(jìn)行最后校準(zhǔn)處理。
NCP為華北板塊;YZP為揚(yáng)子板塊;NQB為北秦嶺造山帶;SQB為南秦嶺造山帶;LLMF為洛南—欒川—明港斷裂;SDSF為商南—
丹鳳—舒家壩斷裂;BYFF為北川—洋縣—房縣斷裂;圖件引自1∶250 000天水幅地質(zhì)圖,有所修改
圖1西秦嶺花崗巖分布
Fig.1Distribution of the Granitoids in West Qinling寄主巖體樣品中的鋯石顆粒為自形晶,無(wú)色,陰極發(fā)光圖像顯示其大多較暗,巖漿生長(zhǎng)震蕩環(huán)帶明顯,鋯石粒徑為50~200 μm,長(zhǎng)寬比值多為2∶1或3∶1。12個(gè)測(cè)試點(diǎn)的w(Th)/w(U)值介于0.39~0.74之間,均大于0.1,且多數(shù)大于0.4,屬巖漿結(jié)晶產(chǎn)物[27]。圖3中鋯石UPb年齡諧和性較好,其中有效數(shù)據(jù)為11組,較為集中,介于205~218 Ma之間(表1)。測(cè)點(diǎn)號(hào)TSP311鋯石測(cè)得年齡為250 Ma,較其他點(diǎn)過(guò)大。其晶型和環(huán)帶特征表明,測(cè)點(diǎn)號(hào)TSP311鋯石可能為捕獲鋯石。其他10組測(cè)年數(shù)據(jù)經(jīng)過(guò)204Pb校正后的n(206Ph)/n(238U)加權(quán)平均年齡為(210.9±1.7)Ma(平均標(biāo)準(zhǔn)權(quán)重偏差(MSWD)為1.74),該年齡代表草川鋪似斑狀花崗巖的結(jié)晶時(shí)間為印支晚期三疊世。
圖2草川鋪巖體野外露頭照片和顯微照片
Fig.2Field Photographs and Micrographs of Caochuanpu Intrusion圖3草川鋪寄主巖體鋯石陰極發(fā)光圖像和UPb年齡諧和曲線(xiàn)
Fig.3CL Images and UPb Concordia Diagram of Zircon of Host Rock for Caochuanpu Intrusion3地球化學(xué)特征
3.1主量元素
寄主巖體的主量元素中,SiO2含量(質(zhì)量分?jǐn)?shù),下同)較高,為72.57%~74.13%,Al2O3含量較低,為13.57%~13.91%,K2O含量為464%~505%,w(K2O)/w(Na2O)值為1.35~147,顯示富鉀特點(diǎn);這些特征與鉀長(zhǎng)石似斑狀結(jié)構(gòu)等巖石學(xué)特征相符(表2)。樣品里特曼指數(shù)為2.00~2.40,A/CNK值為0.90~1.07,平均為1.04,A/NK值為1.25~1.29,在A/NKA/CNK圖解[圖4(a)]中投到了準(zhǔn)鋁質(zhì)與過(guò)鋁質(zhì)的過(guò)渡區(qū)域內(nèi),在K2OSiO2圖解[圖4(b)]中則投到了高鉀鈣堿性系列區(qū)域內(nèi)。
包體的主量元素中,w(SiO2)值為5707%,隸屬于中性巖(表1);樣品w(Al2O3)值為1481%,w(Na2O)值為388%,w(K2O)值為366%,w(K2O)/w(Na2O)值為0.94,顯示富鉀特點(diǎn);樣品A/CNK值為0.82,A/NK值為1.43,在A/NKA/CNK圖解中投入到準(zhǔn)鋁質(zhì)區(qū)域內(nèi),里特曼指數(shù)為3.77,顯示堿性特征;而樣品在K2OSiO2圖解中則投到高鉀鈣堿性與鉀玄巖系列的分界處。
3.2稀土元素和微量元素
從圖5(a)可以看出:草川鋪寄主巖體的球粒隕石標(biāo)準(zhǔn)化稀土元素配分模式呈右傾模式,具有較高的稀土元素總含量((128.13~196.56)×10-6);輕、重稀土元素含量比值為8.20~9.47,輕稀土元素相對(duì)富集,Eu異常為0.16~0.33,顯示強(qiáng)負(fù)Eu異常,說(shuō)明存在斜長(zhǎng)石的分離結(jié)晶作用。微量元素草川鋪寄主巖體顯示高Rb含量((205~687)×10-6)、Nb含量((22.9~39.3)×10-6)、Ta含量((2.4~3.9)×10-6)、Y含量((9.2~19.1)×10-6)。在原始地幔表1草川鋪巖體SHRIMP鋯石UPb定年分析結(jié)果
4討論
4.1測(cè)年數(shù)據(jù)的意義
Zhang等研究表明,位于西秦嶺天水地區(qū)草川鋪早古生代巖體(年齡為434 Ma)的巖性主要為中細(xì)?!攘:谠颇富◢弾r,部分巖性為花崗閃長(zhǎng)巖和英云閃長(zhǎng)巖[1]。這一結(jié)果與本文所研究的發(fā)育閃長(zhǎng)質(zhì)包體的似斑狀中細(xì)粒二長(zhǎng)花崗巖的特點(diǎn)不同。通過(guò)對(duì)草川鋪似斑狀花崗巖體寄主巖體鋯石SHRIMP UPb年齡的測(cè)定,草川鋪似斑狀花崗巖體寄主巖體的結(jié)晶時(shí)間為(210.9±1.7)Ma,屬于早中生代花崗巖,說(shuō)明草川鋪巖體不是同時(shí)期形成的一個(gè)簡(jiǎn)單巖體,而是由早古生代和早中生代兩期花崗巖形成的一個(gè)復(fù)式巖體。
4.2早中生代草川鋪巖體與糜署嶺巖體的關(guān)系
南秦嶺造山帶內(nèi)存在糜署嶺似斑狀花崗巖體,其與草川鋪似斑狀花崗巖體結(jié)晶年齡相似,因此,有必要討論二者之間是否存在成因關(guān)系。
從巖石學(xué)角度來(lái)看,Qin等認(rèn)為糜署嶺巖體主要由二長(zhǎng)閃長(zhǎng)巖、石英二長(zhǎng)巖、二長(zhǎng)花崗巖和閃長(zhǎng)質(zhì)微粒包體組成[26]。寄主巖體具有似斑狀結(jié)構(gòu),斑晶主要為肉紅色鉀長(zhǎng)石,結(jié)晶粒度較大,多為2~4 cm,基質(zhì)中暗色礦物主要為黑云母,偶見(jiàn)角閃石;閃長(zhǎng)質(zhì)微粒包體形態(tài)各異,大小不等,主要由斜長(zhǎng)石、鉀長(zhǎng)石、石英以及黑云母、角閃石、輝石等暗色礦物組成;角閃石和輝石含量不定,李永軍等根據(jù)包體基性程度和暗色礦物含量,將其分為以輝石為主的強(qiáng)基性包體和以角閃石、黑云母為主的弱基性包體[6]。草川鋪似斑狀花崗巖體由中細(xì)粒似斑狀二長(zhǎng)花崗巖和二長(zhǎng)閃長(zhǎng)質(zhì)微粒包體組成,其寄主巖體學(xué)特征與糜署嶺較酸性的二長(zhǎng)花崗巖相似,包體與糜署嶺巖體內(nèi)弱基性包體特點(diǎn)相同。
糜署嶺巖體數(shù)據(jù)引自文獻(xiàn)[3]
圖6草川鋪巖體與糜署嶺巖體元素含量對(duì)比
Fig.6Comparison of Element Contents Between
Caochuanpu and Mishuling Intrusions取草川鋪似斑狀花崗巖4件樣品的平均值與糜署嶺花崗巖樣品平均值對(duì)主量、微量、稀土元素做投圖對(duì)比,結(jié)果見(jiàn)圖6。在主量元素對(duì)比圖中,各主量元素投點(diǎn)皆落于等含量線(xiàn)兩側(cè),相對(duì)含量較高的元素趨近于等含量線(xiàn)之上,而含量相對(duì)較少的MgO、TiO等元素皆落于糜署嶺巖體區(qū)域內(nèi),表明兩巖體的主量元素分布具有相同的特點(diǎn)[圖6(a)]。在稀土元素對(duì)比圖中,投點(diǎn)皆落于等含量線(xiàn)附近,表明兩巖體的稀土元素具有相同的富集趨勢(shì)[圖6(b)],但投點(diǎn)多分布于糜署嶺巖體一側(cè),說(shuō)明糜署嶺巖體較為富集稀土元素。兩巖體的微量元素含量差距較為明顯[圖6(c)],表現(xiàn)為草川鋪似斑狀花崗巖較富Rb、Pb元素,貧Zr、Sr等元素,但兩寄主巖體的投點(diǎn)多分布于等含量線(xiàn)兩側(cè),說(shuō)明兩巖體微量元素的整體分配趨勢(shì)具有相似性。包體主量元素特征表現(xiàn)出明顯的相似性,微量、稀土元素稍有差異??紤]到本次對(duì)比所采用的糜署嶺寄主巖體的樣品多為偏中性的石英二長(zhǎng)巖,而本文所采樣品皆為偏酸性的二長(zhǎng)花崗巖,且暗色礦物特點(diǎn)不同,因此,糜署嶺巖體較之早中生代草川鋪巖體,表現(xiàn)出了貧Si,富Al、Ca等明顯特征。并且,結(jié)合二者圍巖、物質(zhì)遷移過(guò)程等的不同,發(fā)現(xiàn)二者在地球化學(xué)方面所表現(xiàn)出的差異性屬正常差異范圍。因此,二者的地球化學(xué)特征具有相似性實(shí)屬正常。
此外,糜署嶺巖體寄主巖體結(jié)晶時(shí)間為(212±5)Ma[2],與草川鋪似斑狀花崗巖結(jié)晶年齡((210.9±17)Ma)相近,同屬印支期晚三疊系花崗巖體。結(jié)合巖石學(xué)和地球化學(xué)對(duì)比,說(shuō)明草川鋪巖體和糜署嶺巖體具有極其相似的巖石學(xué)、地球化學(xué)和年代學(xué)特點(diǎn),因此,推測(cè)早中生代草川鋪花崗巖雖然位于北秦嶺造山帶內(nèi),但其物質(zhì)來(lái)源于南秦嶺造山帶,且與南秦嶺糜署嶺巖體為同一套巖漿活動(dòng)的產(chǎn)物。已有研究表明,南秦嶺造山帶在早中生代發(fā)生過(guò)強(qiáng)烈的巖漿活動(dòng),并在南秦嶺造山帶內(nèi)形成了大量印支期花崗巖體[26,1011,2526]。這些巖體以糜署嶺巖體為代表,多具有暗色包體,似斑狀寄主花崗巖發(fā)育普遍。因此,早中生代草川鋪似斑狀花崗巖的發(fā)現(xiàn)間接說(shuō)明,南、北秦嶺造山帶在早中生代時(shí)已經(jīng)碰撞拼合。
4.3巖石成因
早中生代草川鋪花崗巖體具有部分埃達(dá)克巖的地球化學(xué)特征。埃達(dá)克巖的成因一般有多種解釋?zhuān)鄶?shù)屬于大洋板片俯沖部分熔融或是下地殼熔融及底侵玄武巖成因。早中生代草川鋪巖體具有低Al2O3含量(13.57%~13.91%),高Rb含量((205~687)×10-6)、Nb含量((22.9~39.3)×10-6),而且w(K2O)>w(Na2O)(w(K2O)/w(Na2O)=1.35~1.47)[3334],樣品屬高鉀鈣堿性埃達(dá)克花崗巖。因此,早中生代草川鋪花崗巖又區(qū)別于大洋板片俯沖熔融或玄武巖底侵作用成因的富鈉型埃達(dá)克巖。前人對(duì)發(fā)育暗色微粒包體的高鉀鈣堿性埃達(dá)克巖的研究顯示,這類(lèi)巖石成因比較復(fù)雜,但在典型的后碰撞造山帶中多發(fā)育。早中生代草川鋪巖體的侵位時(shí)間為三疊紀(jì),剛好為華南板塊和華北板塊的碰撞時(shí)期,揚(yáng)子板塊向北俯沖到秦嶺構(gòu)造帶之下,而南秦嶺造山帶也沿著商丹縫合帶繼續(xù)向北俯沖,完成了南、北秦嶺造山帶的碰撞拼合[3538]。并且,早中生代草川鋪巖體雖然位于北秦嶺造山帶內(nèi),但其物質(zhì)來(lái)源于南秦嶺。因此,早中生代草川鋪花崗巖可能形成于大陸地殼俯沖熔融,是南秦嶺造山帶向北秦嶺造山帶俯沖碰撞后部分熔融形成的。
5結(jié)語(yǔ)
(1)西秦嶺天水地區(qū)草川鋪似斑狀花崗巖體寄主巖體SHRIMP鋯石UPb結(jié)晶時(shí)間為(210.9±1.7)Ma,屬于早中生代花崗巖,說(shuō)明草川鋪巖體不是同時(shí)期形成的一個(gè)簡(jiǎn)單巖體,而是由早古生代和早中生代兩期花崗巖形成的一個(gè)復(fù)式巖體。
(2)早中生代草川鋪花崗巖與南秦嶺糜署嶺巖體為同一套巖漿活動(dòng)的產(chǎn)物,表明其物質(zhì)來(lái)源為南秦嶺造山帶,間接表明南、北秦嶺造山帶在早中生代時(shí)已經(jīng)碰撞。
(3)早中生代草川鋪花崗巖形成于大陸地殼俯沖熔融,是南、北秦嶺造山帶俯沖碰撞后期的產(chǎn)物。參考文獻(xiàn):
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