周伏順,林鑫,王郅睿,等. 閩北邵武地區(qū)玄武安山巖的成因和意義:來自年代學、地球化學及NdHf同位素的約束.吉林大學學報(地球科學版),2024,54(3):840861.doi:10.13278/j.cnki.jjuese.20230345.
Zhou Fushun,Lin Xin,Wang Zhirui,et al. Petrogenesis and Significance of Basaltic Andesite in the Shaowu Area, Northern Fujian: Constraints from Geochronology, Geochemistry and NdHf Isotopes. Journal of Jilin University (Earth Science Edition),2024,54(3):840861.doi:10.13278/j.cnki.jjuese.20230345.
摘要:
東南沿海地區(qū)中生代火山巖研究成果頗豐,但有關中侏羅世晚期基性火山巖的報道較少,限制了對該區(qū)域中生代構造巖漿活動及大地構造演化的深入理解。本文對江紹斷裂帶東南側邵武地區(qū)的中生代火山巖開展了系統(tǒng)的巖石學、鋯石UPb年代學、鋯石LuHf同位素、地球化學和SmNd同位素研究。結果顯示:邵武地區(qū)玄武安山巖噴發(fā)年齡為(161.0±2.0)Ma;同位素地球化學結果顯示,這些樣品中—晚侏羅世鋯石的εHf(t)值介于-14.33~-10.41之間,εNd(t)值較低(-9.2~-8.4),反映富集Nd同位素的特征;巖石地球化學結果表明,該套火山巖具有高w(Al2O3)、w(Na2O)、低w(MgO)、w(TFe2O3)等特征,稀土總量較低,稀土配分曲線為右傾型,且具弱的Eu負異常,大離子親石元素Rb、Ba和K相對富集,高場強元素Nb、Ta、Ti、P等相對虧損。綜合巖石學和地球化學研究結果,本文認為邵武玄武安山巖起源于交代巖石圈地幔的部分熔融,并經歷一定結晶的分異作用,其大地構造背景總體為板內環(huán)境。結合前人對區(qū)域構造巖漿活動的認識,本文認為在中侏羅世晚期,太平洋俯沖板片發(fā)生回撤、撕裂,導致幔源巖漿底侵并置換了古老殼源巖石,從而東南沿海地區(qū)雖整體處于擠壓背景,但仍存在局部拉張環(huán)境。
關鍵詞:
中侏羅世;地球化學;伸展背景;玄武安山巖;東南沿海
doi:10.13278/j.cnki.jjuese.20230345
中圖分類號:P588.145
文獻標志碼:A
收稿日期:20231221
作者簡介:周伏順(1997—),男,碩士研究生,主要從事地球化學方面的研究,E-mail:z913909811@163.com
通信作者:林鑫(1987—),男,副教授,主要從事應用地球化學及其與數據科學交叉領域方面的研究,E-mail: xinlin@chd.edu.cn
基金項目:陜西省重點研發(fā)計劃(2024GHZDXM26);國家重點研發(fā)計劃(2016YFC0600601);中央高?;痦椖浚?00102271206)
Supported by the Key Research and Development Program of Shaanxi (2024GHZDXM26), the National Key Research and Development Program(2016YFC0600601) and the Central University Fund Project (300102271206)
Petrogenesis and Significance of Basaltic Andesite in the Shaowu Area, Northern Fujian: Constraints from Geochronology, Geochemistry and NdHf Isotopes
Zhou Fushun,Lin Xin,Wang Zhirui,Shao Chengbo
School of Earth Sciences and Resources, Chang’an University, Xi’an 710054, China
Abstract:
Among the Mesozoic volcanic rocks along the southeast coast of China, Middle Jurassic rocks are seldom reported, yet they hold significant importance for understanding Mesozoic magmatic activities and tectonic evolution in this region. The Shaowu basaltic andesite, exposed in the southeast of the Jiangshao fault zone, provides an ideal opportunity for study. This paper systematically presents petrological, zircon UPb geochronological, LuHf isotopic, whole-rock geochemistry and SmNd isotopic analyses of the basaltic andesite, indicating an eruption age of (161.0±2.0) Ma. The isotopic data shows that all samples are characterized by εHf(t) values ranging from -14.33 to -10.41 and low εNd(t) values between -9.2-8.4. The geochemical results shows high w(Al2O3)" and w(Na2O) contents, along with low levels of w(MgO) and w(TFe2O3) etc. The total rare earth elements (REE) content is low, with chondrite-normalized REE patterns showing LREE enrichment and weak Eu negative anomalies. Furthermore, large ion lithophile elements such as Rb, Ba and K are enriched, while the high field strength elements such as Nb, Ta, Ti and P are relatively depleted. The geochemical and petrological characteristics of the basaltic andesite imply that it originated from partial melting of a metasomatic mantle wedge, underwent certain crystallization differentiation, and formed in an intraplate tectonic environment. Based on previous studies on regional tectonic-magmatic activities, it is proposed that Pacific subducted plate retreat and tearing, resulting in encroachment of mantle-derived magma and the replacement of ancient crust-derived rocks, occurred amidst local extension within the broader compressional background of the south-eastern coast of China during the Middle Jurassic.
Key words:
Middle Jurassic; geochemical; extensional setting; basaltic andesite; southeast coast of China
0" 引言
玄武安山巖直接導源于上地幔,受地幔源巖成分的制約,其巖漿的形成與全球構造,如裂谷擴張、板塊俯沖消減及地幔的深部作用(地幔對流、局部隆升、流體交代)等過程有關。因此,玄武安山巖研究對于反演地幔物質成分、分析構造環(huán)境及地球深部動力學均具重大意義[1],是國際火山巖研究的熱點[25]。
華南地區(qū)以大面積分布的巖漿巖[6]及金屬礦床[7]為特色,其內部可劃分出多個構造帶[8]及成礦帶[9],使得該地區(qū)成為國內外學者研究的熱點地區(qū)之一?,F有研究表明,這些大規(guī)模的巖漿及礦床多與晚中生代的火山作用密切相關,并被普遍解釋為是古太平洋板塊向歐亞大陸俯沖作用所造成的應力體制轉換的產物。然而,前人[1011]對于轉換的時限是否為中侏羅世目前還存在爭議。目前,對于該時期華南地區(qū)的應力狀態(tài)也相應存在不同的觀點,主要包括:由擠壓轉為伸展體制[10]、擠壓體制[11]以及整體擠壓體制下的局部伸展[6]。
玄武安山巖作為深部地質過程的記錄者,能夠為理解造山帶構造演化過程提供很好的窗口,也為解決上述爭議提供了新的思路。閩北地區(qū)侏羅紀火山巖廣泛發(fā)育,其成因及構造背景的界定對于確定區(qū)域構造格架及演化起著至關重要的作用。該地區(qū)火山巖以大規(guī)模中酸性火山巖為主[1217],而對于玄武安山巖的報道較少。本文以邵武地區(qū)采集的玄武安山巖為研究對象,通過對樣品進行巖石學、年代學、鋯石LuHf同位素、全巖地球化學以及SmNd同位素的研究,確定玄武安山巖的形成時代及其成因,闡明中侏羅世古太平洋板塊俯沖作用對閩北地區(qū)的影響。
1" 地質背景及樣品描述
華南板塊由揚子地塊、華夏地塊以及其間的縫合帶 — 江南造山帶共同組成[1821],南北方向上分別夾持于東南亞塊體和秦嶺造山帶之間,西鄰青藏高原,東側為太平洋板塊。自元古宙板塊拼合以來,華南板塊經歷了多階段的構造巖漿活動,成為研究大陸體制轉換的理想區(qū)域[2223]。其中,中生代板塊間構造體制的變化導致了區(qū)域巨量巖漿的侵位(圖1a),引發(fā)了華南大規(guī)模的成礦事件,成為國內外地質學家研究的焦點。
揚子地塊內部可劃分為同一蓋層的不同基底,新元古代以來拼合成為穩(wěn)定的克拉通[25];華夏地塊主要由大面積的新元古代碎屑巖夾火山巖、巖漿巖及碳酸鹽巖組成。新元古代末期,揚子和華夏地塊發(fā)生碰撞拼合,形成江南造山帶,沿該造山帶南界的江紹斷裂帶分布了大量冷侵位的蛇綠巖套、藍片巖
和具島弧屬性的巖漿巖[2627]。加里東期和印支期華南板塊經歷了兩期陸內造山運動,主要體現在揚子地塊東半部和整個華夏地塊,區(qū)域內巖石卷入強烈變質變形過程中[25]。尤其是加里東期造山運動使得前泥盆紀巖石大多變質成為片巖、片麻巖和混合巖,且伴隨強烈的巖漿活動,形成變質變形巖石花崗巖復合體系[27]。
邵武地區(qū)位于華夏地塊中北部,區(qū)域出露的地層由老到新依次為:震旦系、下古生界羅峰溪群、侏羅系(圖1b)。震旦系為黑云母斜長片麻巖、云母片巖夾斜長角閃巖,變質程度達角閃巖相,且發(fā)育強烈的混合巖化作用[2829]。下古生界羅峰溪群為灰綠色變砂巖、千枚巖、片巖,局部夾大理巖。侏羅系通過斷層與下古生界分隔,內部巖石單元包括:兜嶺群,下部為砂巖夾凝灰?guī)r及灰?guī)r透鏡體,上部為晶屑凝灰?guī)r;漳平群,為紅綠相間的粉砂巖及細砂巖;梨山群,分為下部的灰黑色砂礫巖、粗砂巖、粉砂巖及上部的砂巖夾灰?guī)r透鏡體。研究區(qū)南東側及北西側分別出露NE走向的政和—大埔斷裂帶及江紹斷裂帶,區(qū)內斷裂構造發(fā)育,包括NE向、NW向及EW向等多組,其中近NE向最為發(fā)育。區(qū)內巖漿巖主要為印支期和燕山期黑云母花崗巖,侵位于震旦系中,其次是玄武安山巖和閃長巖,零星分布于下古生界羅峰溪群中[30]。
本次研究的玄武安山巖出露于邵武市南側的羅峰溪群千枚巖中,規(guī)模較小,呈NE向延伸,長度約300 m,寬度約100 m。由于植被覆蓋,玄武安山巖與千枚巖接觸界面不易觀察,巖石表面風化較強,呈黃褐色,而人工揭露的斷面上巖石相對新鮮,呈深灰色,節(jié)理發(fā)育(圖2a),十分破碎。巖石表面可見氣孔構造,內部被方解石礦物充填(圖2b)。
鏡下觀察顯示,玄武安山巖具間粒間隱結構,塊狀構造,斑晶體積分數為35%~40%,多為斜長石,自形板狀,表面可見弱絹云母化,粒徑為0.2~0.5 mm,見少量石英,粒徑多為0.1~0.2 mm。局部可見杏仁體,主要由方解石和石英充填,大小為1.0~1.5 mm,體積分數為5%~10%?;|由細粒斜長石和輝石等組成,基質中見半自形粒狀的次生綠簾石和次生方解石(圖3)。根據巖石結構、構造及礦物成分組成,定名為玄武質安山巖。
2" 分析方法
本次工作采集了露頭上較新鮮的玄武安山巖樣品,采樣位置見圖1b。選擇其中1件樣品進行年代學及鋯石LuHf同位素分析,4件樣品進行全巖主微量及稀土元素、SmNd同位素測試。
鋯石單礦物挑選在河北省廊坊市尚藝巖礦檢測技術服務有限公司完成。將野外采集的樣品粉碎,通過常規(guī)重選法選出單礦物,在雙目鏡下挑選出形態(tài)較好的鋯石。鋯石制靶在南京宏創(chuàng)地質勘查技術服務有限公司完成。首先通過環(huán)氧樹膠將待測鋯石和標準鋯石固定在玻璃板上,然后拋光至鋯石暴露出
中心面,在偏光顯微鏡下對鋯石進行反射光和透射光拍照,確定鋯石的包體以及裂紋情況,最后進行陰極發(fā)光(CL)照相,以檢查鋯石的內部結構。
選擇晶形較好,無包體、裂紋的鋯石顆粒進行鋯石UPb測年分析。鋯石分析在中國地質科學院礦產資源研究所使用電感耦合等離子體質譜儀(LAMCICPMS)完成,詳細的儀器參數和分析流程見文獻[31]。儀器的型號和相應的激光剝蝕系統(tǒng)分別為Finnigan Neptune型LAMCICPMS和NewWave UP213 nm。實驗過程采用單點剝蝕,直徑為30 μm,采樣深度為20~40 μm,頻率為10 Hz,輸出能量約為2.5 J/cm2。實驗過程采用He作為載氣。鋯石年齡以GJ1為外標,元素質量分數以M217為外標。在測試過程中,每隔10個樣品點測試
2個GJ1和1個Plesovice標準鋯石。最后使用ICPMSDataCal軟件完成數據處理(信號選擇、漂移校正和年齡計算),通過Isoplot3.0軟件計算加權平均年齡。
鋯石LuHf同位素測定在北京科薈測試技術有限公司完成,儀器為激光剝蝕多接收器電感耦合等離子體質譜儀(Neptune plus),激光進樣系統(tǒng)為RESOlution SE固體激光器。根據鋯石照片選擇合適區(qū)域,利用激光剝蝕系統(tǒng)對鋯石進行剝蝕。測點位置為UPb年齡的同一測點或其附近。激光剝蝕的斑束直徑為30~38 μm,能量密度為6 J/cm2,頻率為6 Hz,激光剝蝕物質以高純He為載氣送入Neptune Plus(MCICPMS),積分時間為27 s。鋯石樣品GJ1用作分析的參考標準。在分析過程中,GJ1標準的176Hf/177Hf加權平均值為0.282 007±0.000 007(2σ,n=36),與誤差范圍內的報告值一致。采用179Hf/177Hf = 0.7325對Hf同位素比值進行指數歸一化質量歧視校正,采用173Yb/172Yb=1.35274對Yb同位素比值進行指數歸一化質量歧視校正。由于鋯石中176Lu/177Hf值通常小于0.002,因此鋯石中176Hf的同質異位素的干擾主要來自176Yb。在鋯石激光剝蝕過程中直接測定Yb信號,用剝蝕過程中βYb的平均值作為Yb的質量歧視校正系數來進行同質異位素176Yb的干擾校正。
巖石樣品主、微量及稀土元素測定在北京燕都中實測試技術有限公司完成。將巖石粗碎至厘米級的碎塊,選取無蝕變及脈體穿插的新鮮樣品用純化水沖洗干凈,烘干并粉碎至200目備用。主量元素測試:首先將粉末樣品稱量后加入Li2B4O7 (1∶8)助熔劑混合,然后使用融樣機加熱至1 150 ℃使其在金鉑坩堝中熔融成均一玻璃片體,最后使用XRF(Zetium, PANalytical)測試,測試結果保證數據誤差小于1%。微量及稀土元素測試:首先將200目粉末樣品稱量并置放入聚四氟乙烯溶樣罐并加入HF+HNO3,然后在干燥箱中將高壓消解罐保持在190 ℃溫度72 h,最后取出經過趕酸并將溶液定容為稀溶液上機測試。測試使用ICPMS(M90, analytikjena)完成,所測數據根據監(jiān)控標樣GSR2顯示誤差小于5%,部分揮發(fā)性元素及極低質量分數元素的分析誤差小于10%。
SmNd同位素測定在核工業(yè)北京地質研究院使用ISOPROBET熱電離質譜儀進行。詳細的化學制備、質譜法和標準樣品測定見文獻[32]。使用143Nd/144Nd=0.7219校正Nd同位素分析質量,測定過程中標準樣品的測量結果如下:基準物質JMC=0.521109±3.000000(143Nd/144Nd)。Sm和Nd的分析空白總量為5×10-11 g。
3" 分析結果
3.1" 鋯石年齡
玄武安山巖中的鋯石大多為短柱狀,粒徑50~150 μm,長寬比為1∶1~1.5∶1。在CL圖像中可見其具有清晰的振蕩環(huán)帶(圖4),具有巖漿鋯石的特征。選取29顆發(fā)育環(huán)帶的鋯石進行UPb同位素測試分析,測試結果見表1。結果顯示所有分析點的Th/U值均大于0.1,這也反映其鋯石為巖漿成因。其中一個測點的
206Pb/238U年齡
為234.4 Ma,結合其CL測點位置認為該數值代表了樣品中鋯石的混合年齡。其余鋯石分析年齡可以分為兩組,在鋯石年齡諧和圖上,7個年齡數據點的206Pb/238U年齡為458.0~443.2 Ma,其加權平均年齡為(449.6±5.3) Ma (n=7, MSWD=0.49)(圖4a);其余21個數據點206Pb/238U年齡為168.1~151.4 Ma,其加權平均年齡為(161.0±2.0) Ma (n=21, MSWD=2.5)(圖4b)。
3.2" 鋯石LuHf同位素
玄武安山巖中鋯石LuHf同位素分析結果見圖5及表2。樣品FJ115中的12顆鋯石加權平均
年齡為161 Ma,其(176Hf/177Hf)i值和εHf(t)值分別為0.282 28~0.282 39和-14.33~-10.41,對應于Hf地殼TDMC為2 096~1 847 Ma。
4顆鋯石的
加權平均年齡為450 Ma,其(176Hf/177Hf)i值為0.282 22~0.282 25,εHf(t)值為-10.08~-8.98,對應于2 040~1 977 Ma的TDMC。
3.3" 主、微量及稀土元素
玄武安山巖的主量元素分析結果見表3。由于采集的樣品發(fā)生了輕微蝕變,因此在進行元素分析時,盡量不采用對蝕變作用比較敏感的Rb、Ba、K、Si等活動性元素,而利用不活潑的Ti、Zr、Nb、Y、Ta、Hf及稀土元素來判別巖石類型[3334]。w(Th)w(Co)和Zr×0.0001/
TiO2Nb/Y判別圖(圖6)[3536]中,采集的樣品大多落入安山巖或玄武安山巖的范圍,綜合樣品巖相學中較高的石英體積分數、長石的寬板狀形態(tài)以及測年樣品中挑選出的鋯石形態(tài),認為樣品應屬于鈣堿性玄武安山巖。
玄武安山巖樣品的SiO2質量分數為50.80%~52.48%,MgO質量分數為5.40%~6.59%,TFe2O3質量分數為8.04%~8.54%,TiO2質量分數為0.92%~1.02%,Al2O3質量分數為14.50%~15.64%,Mg#值為55~59。主量元素分析中產生了較高的燒失量(6.70%~7.36%),因此,需要對后期蝕變作用是否對活動性元素產生影響進行判別[37]。活動性元素SiO2與
相對不活動的元素Ce之間明顯負相關,
而K2O、Rb、Th與其明顯
正相關(圖7),這些元素在巖石后期蝕變過程中受到的影響較小,其質量分數的變化是由巖漿作用產生的。另外,在稀土元素及微量元素圖解(圖8)中,4個樣品呈現出大致相同的變化規(guī)律,也指出玄武安山巖樣品的化學成分沒有受到后期蝕變作用的明顯改變。
玄武安山巖樣品稀土和微量元素質量分數分析結果見表4。稀土元素總量w(ΣREE)較低,為82.33×10-6~92.40×10-6。在球粒隕石標準化稀土元素配分型式圖上(圖8a),樣品總體富集輕稀土,呈現出輕稀土右傾而重稀土近平坦的樣式,輕重稀土之間分餾較弱(LREE/HREE=7.38~7.71,(La/Yb)N=7.69~8.21),表現為弱的負Eu異常,δEu為0.76~0.84。樣品微量元素組成特征見圖8b,整體富集大離子親石元素Rb、Ba和K,相對虧損高場強元素Nb、Ta、Ti、P。
3.4" SmNd同位素
玄武安山巖SmNd同位素組成相對均勻(表5),147Sm/144Nd為0.131 3~0.135 0,143Nd/144Nd為0.512 104~0.512 139,低于原始地?,F代值0.512 638[39]。采用樣品的形成年齡(161 Ma)進行計算,樣品的εNd(t)=-9.2~-8.4,這些特征表明在巖石形成過程中有大量地殼物質加入。在計算巖石Nd模式年齡時,SmNd同位素的分餾效應會產生明顯的影響,以致其模式年齡結果不合理,為了減少此類影響,本文采用兩階段模式計算巖石Nd同位素模式年齡,用以近似代表樣品源巖的地殼存留年齡。Nd單階段模式年齡(TDM)為2 020~1 890 Ma,與鋯石中Hf的模式年齡大體一致。Nd模式年齡值大多分布在古元古代,這也進一步表明巖漿形成過程中有新元古代地殼物質的參與。
4" 討論
4.1" 火山巖時代
1∶20萬順層幅區(qū)調報告中未將本次工作中涉及的玄武安山巖圈出,但在其周邊地區(qū)零星圈出了多個安山玢巖露頭,它們呈巖脈產出于羅峰溪群內,時代定為燕山早期[30]。本次地質考察通過野外和鏡下特征認為該地區(qū)巖石具有明顯的杏仁構造,且SiO2質量分數變化范圍為50.80%~52.48% (<
53%),該套巖石屬于典型的玄武安山巖。本文選取新鮮樣品的LAICPMS 鋯石UPb 定年結果表明,玄武安山巖內部包含了(449.6±5.3) Ma和(161.0±2.0) Ma兩組年齡。但玄武安山巖產于羅峰溪群內部,表明其噴發(fā)時代應該晚于地層,羅峰溪群內部的化
石表明其時代為晚奧陶世(440 Ma)[40]。
因此,玄武安山
巖中(449.6±5.3)" Ma的年齡不能代表其噴發(fā)時代。華南地區(qū)在早古生代經歷了強烈的構造熱事件,在華夏地塊尤為突出,華夏地塊的大多數巖石發(fā)生變質變形改造及混合巖化,同時也引發(fā)了大面積的巖漿活動[41]。玄武安山巖中(449.6±5.3) Ma的年齡與華南板塊加里東期發(fā)育的巖漿事件一致,代表了玄武安山巖形成過程中的巖漿鋯石捕虜晶。結合上述討論,(161.0±2.0) Ma的年齡應為邵武地區(qū)玄武安山巖的噴發(fā)時代,為晚侏羅世。
侏羅紀是華南板塊重要的構造巖漿活化期,該時期巖漿巖廣泛出露,包括中酸性及基性巖漿巖[42]。其中大量的A型花崗巖、雙峰火山巖、板內拉斑玄武巖和板內堿性玄武巖分布于南嶺構造帶[43]。而東南沿海地區(qū)則發(fā)育大量的中酸性巖漿巖,如邢光福等[6]報道了福安地區(qū)安山巖的年齡為(162.3±3.7) Ma,張偉等[13,44]認為浙江松陽毛弄組火山巖的時代為180~153 Ma。武夷山北緣侏羅紀巖漿活動也有大量的報道和記錄,如孟祥金等[4546]認為冷水坑地區(qū)的火山巖時代為160~146 Ma,Wang等[47]獲得了閩西北地區(qū)I型花崗巖的時代為161 Ma。本文年代學結果及上述數據表明,華南東部晚侏羅世大規(guī)模巖漿作用形成的火山侵入巖在閩北邵武地區(qū)也有體現,由于晚侏羅世—早白堊世處于華南構造域轉換的特殊時期,因此,邵武地區(qū)玄武安山巖的厘定對于進一步闡明華南大地構造演化具有很好的指示意義。
4.2" 巖石成因
玄武安山巖中相對低MgO、TFe2O3質量分數,低Mg#值(標準地幔的Mg#值為68~75[48]),貧不相容元素 Ni、Co、V,富Al2O3 、Na2O、Sr等,均顯示出演化巖漿的特點。微量元素蛛網圖(圖8b)中Sr、P、Ti元素的虧損可能與斜長石、磷灰石和FeTi氧化物的分離結晶有關。稀土元素配分曲線圖(圖8a)中,樣品中弱的Eu負異常(δEu=0.76~0.84)也暗示了可能存在角閃石或斜長石的分離結晶。w(Sm)w(Rb)圖(圖9a)中可見樣品主要分布于含角閃石的分離結晶趨勢線附近。根據 100(Fe+Mg+Mn)/Ti100Si/Ti圖解(圖9b),邵武地區(qū)玄武安山巖樣品數據分布于單斜輝石分離結晶線附近,同樣說明巖漿演化過程中發(fā)生了以單斜輝石為主的分離結晶作用。結合上述特征,筆者認為玄武安山巖巖漿演化過程中發(fā)生了一定的結晶分異作用。
玄武安山巖的SmNd同位素組成特征顯示,它們具有較小的εNd(t)值,從原始地幔標準化微量元素蛛網圖(圖8b)中可見,樣品顯示出 Nb、Ta強烈虧損,Pb 正異常和 Ti 弱負異常,造成這種特征的原因可能為:1)源區(qū)富集,即巖漿源區(qū)存在有因俯沖進入地幔的地殼物質組分;2)玄武安山巖在形成過程中受到了強烈的地殼物質混染。物質組成方面,Nb/Ta值在巖漿形成之后基本上能夠保持不變,對于反映巖漿源區(qū)有很好的指示意義。地殼物質相對于地幔熔體具有較低的Nb、Ta質量分數,較低的Nb/Ta值(地殼中Nb/Ta=8.33~13.33,地幔中Nb/Ta=18.27[49])和較高的Th質量分數;相比于原始巖漿,地殼混染后的巖漿具有較高的Th/Nb(>5)和Th/Ta(>10)值[48],而樣品中Nb/Ta值為14.98~23.89,Th/Nb和Th/Ta值分別為0.30~0.41和5.40~8.15。據上述特征,認為邵武玄武安山巖巖漿并未表現出明顯地殼混染的特征。另一方面,其他研究表明,高場強元素中Zr/Hf值對于巖漿源區(qū)的區(qū)分具有很好的指示意義,如地幔中Zr/Hf值約為50,而地殼中Zr/Hf值約為36[50]。樣品中Zr/Hf值為38.85~40.19,指示邵武玄武安山巖的形成過程又涉及到一定殼源物質加入。巖漿上升過程中,無論是上地殼還是下地殼的混染,在造成巖漿 Pb 正異常的同時,均能夠導致混染巖漿中 Ti 質量分數降低從而引起 Ti 的負異常[51],樣品中具有明顯的Pb 正異常和Ti負異常,表明有地殼物質的加入。稀土元素配分曲線圖(圖8a)中,樣品曲線與OIB曲線樣式相似,但其微量元素中卻表現為Nb、Ta負異常及Pb正異常等,顯示出富集型地幔的特征。另外,玄武安山巖內部鋯石εHf(t)值為很小的負值,表明其源區(qū)主要來自古老的地殼物質。樣品εHf(t)和εNd(t)均具有很小的負值,分別為-14.33~-8.98和-9.2~-8.4,這在東南沿海地區(qū)的鎂鐵質火成巖是很少見的[52]。相反地,這種情況在長英質巖石中較常見,如Zhao等[53]獲得了南嶺地區(qū)晚侏羅世桃山花崗質巖石較低的εHf(t)(-14.3~-4.8)和εNd(t)(-11.2~-8.8)值,并根據地球化學特征認為它們源于古老變沉積巖的部分熔融。Li等[54]得出晚侏羅世佛岡巖基的εHf(t)和εNd(t)值分別為(-11.5~-3.1)、(-12.2~-4.3),屬于古元古代中基性巖的部分熔融產物。劉高峰等[55]報道了遂川晚侏羅世花崗巖εHf(t)值為(-19.5~-8.5),代表了深部熱驅動下古老地殼熔融的產物。類似的巖石在紫金山、溈山等其他地區(qū)也均有報道[5658]。華夏地塊東部古老基底的εHf(t)和εNd(t)值分別在<-10和-16~-8。因此,結合本次研究得出的玄武安山巖εHf(t)和εNd(t)較小的負值,本文認為邵武玄武安山巖源區(qū)在發(fā)生部分熔融之前就已經發(fā)生了富集,幔源巖漿不斷底侵到中、下地殼,破壞并置換了原有的老地殼,兩者在源區(qū)發(fā)生相互作用,并通過部分熔融產生了母巖漿,同時也將古老地殼和地幔的部分地球化學信息傳遞到母巖中,隨后通過演化形成了玄武安山巖。在巖漿演化和發(fā)展過程中并未經歷明顯的地殼混染。
4.3" 大地構造意義
大量研究表明,在中—晚侏羅世,華南地區(qū)發(fā)生了擠壓到板內伸展的構造體制轉變[11,54,5960]。但對于具體構造轉換的時間仍存在爭議。Shi等[10]認為,隨著俯沖角度的變化,構造背景從火山弧轉變?yōu)橹匈_世(165~150 Ma)的弧后伸展,這對應著區(qū)域構造應力場從擠壓向伸展的轉變。余心起等[61]通過沉積盆地研究,指出中國東南部早—中侏羅世普遍為拉張裂陷沉積環(huán)境,在泛濫平原環(huán)境中有裂陷型盆地共生。而其他學者[6,10,1217,45,47,62110]認為伸展作用在晚侏羅世才開始,中侏羅世為擠壓環(huán)境(表6)。Su等[111]提出,晚侏羅世發(fā)生了有限的地幔上升流,暗示了從擠壓到伸展的過渡階段。Li等[112]指出,中—晚侏羅世太平洋板塊持續(xù)北西向俯沖造成了大面積的擠壓作用,在南嶺地區(qū)發(fā)生了板片撕裂,形成了近EW向的裂谷帶。舒良樹[27]也表示,在中侏羅世(190~160 Ma)期間東南沿海地區(qū)遭受了強烈擠壓,此時南嶺地區(qū)為拉張環(huán)境,主要發(fā)育雙峰式火山巖、層狀基性雜巖體、A型花崗巖等。
邵武地區(qū)玄武安山巖具有高Zr/Y、Th/Hf、Ta/Hf值,在構造環(huán)境判別圖中,主要位于板內玄武巖區(qū)和大陸拉張帶(初始裂谷)玄武巖區(qū)(圖10),指示出玄武安山巖是在中侏羅世晚期(161.0 Ma)板塊內部拉張環(huán)境下形成的。根據華南板塊內火成巖地球化學的差異性及變化規(guī)律,古太平洋板塊俯沖過程中,中生代經歷了板片的回撤過程[113117],而不均一的板片后撤能夠導致板片撕裂[113,118121],這就引發(fā)了明顯的伸展作用,并導致軟流圈物質上涌、底侵、置換華夏板塊古老地殼物質,從而形成富集源區(qū)。
結合前人研究成果,本文支持研究區(qū)在中侏羅世晚期屬于拉張環(huán)境的觀點,即區(qū)域伸展作用在中侏羅世晚期已經有所體現。但另一方面,構造地質學[11]、沉積學[26]、巖石學[13,122]等證據顯示出中侏羅世東南沿海發(fā)育強烈的擠壓應力場,結合前人數據及本次研究結果,認為該地區(qū)屬于整體擠壓背景下的局部伸展環(huán)境。邢光福等[6]指出中—晚侏羅世華南處于擠壓還是拉張環(huán)境這一爭議的原因在于,火山巖主要沿張性斷裂上升侵位或噴發(fā),但這種張性斷裂有可能是區(qū)域整體擠壓背景下局部存在拉張;基于金華地區(qū)花崗巖類的地球化學分析,高萬里[74]指出由于古太平洋板塊俯沖的后退,華南板塊經歷了從整體擠壓和局部伸展背景(中侏羅世)到大規(guī)模伸展背景(晚侏羅世)的轉變。因此,本文認為邵武地區(qū)火山作用發(fā)生于板內伸展,表明中侏羅世晚期在東南沿海的俯沖的背景下仍局部存在伸展環(huán)境,這源于板塊回撤引發(fā)的拉伸,同時也誘發(fā)了俯沖帶深部地幔物質與地殼的混合、置換等作用。
5" 結論
1)LAICPMS 鋯石 UPb 同位素定年結果表明,邵武地區(qū)玄武安山巖形成時代為中侏羅世晚期((161.0±2.0) Ma),指示出閩北地區(qū)存在中侏羅世晚期的一期巖漿作用。
2)邵武地區(qū)玄武安山巖為鈣堿性玄武巖,巖石富集輕稀土及大離子親石元素Rb、Ba 和K,高場強元素Nb、Ta、Ti和P相對虧損。地球化學特征指示其母巖漿應是由拉張環(huán)境下的富集型巖石圈地幔部分熔融產生,經歷一定結晶分異作用,其地殼混染作用不明顯。
3)邵武地區(qū)玄武安山巖指示中侏羅世晚期東南沿海地區(qū)俯沖背景下,板片發(fā)生回撤導致局部拉張。
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