含榴花崗片麻巖Lu-Hf年代學初探

張 超, 程 昊*, J D Vervoort

(1. 同濟大學 海洋地質國家重點實驗室, 上海 200092; 2. School of Earth and Environmental Sciences, Washington State University, Washington 99164, USA)

0 引 言

20 世紀 90 年代 MC-ICP-MS 的出現[1–2]使 Hf同位素的化學分離與質譜測定大為簡化[3], Lu-Hf同位素研究由此進入了快速發展的時期。其中以定年為主要目的的 Lu-Hf同位素年代學研究更是取得了長足的發展。Lu-Hf同位素定年對象涉及隕石[4]、巖漿巖[5]、變質巖[6]和沉積巖[7]。定年的主要對象也從早期的全巖和石榴子石為主, 擴展到了磷灰石[7]、輝石[6]和硬柱石[8]等。由于石榴子石常具有較高的母子體176Lu/176Hf比值, 往往能夠構筑高質量的等時線, 石榴子石因此成為Lu-Hf年代學的首選礦物。近年來,石榴子石 Lu-Hf年代學[9]在造山帶的研究中體現出了其獨特的價值, 獲得了許多其他定年體系所沒能揭示出的造山帶演化的信息[10–25]。國內外研究者對大別-蘇魯造山帶這類花崗質片麻巖的成因、形成時代及其構造意義等重要的科學問題還存在較大的分歧, 其中一個核心的爭論則是其是否經歷了超高壓變質作用。詮釋該論題的一個直接辦法就是對該類巖石進行精細的年代學研究, 并與超高壓巖石的變質演化史進行對比。西大別屬于大別-蘇魯造山帶的西緣, 該區出露的含榴花崗質巖石的年代學工作寥寥無幾[26–29], 且僅限于鋯石U-Pb定年。單一的定年體系給出的信息往往有所局限, 常需要結合多個同位素體系以期提供更多的時間信息來厘定其精細的演化歷程。另一方面, 對于造山帶中廣泛出露的花崗質巖石, 其狹窄的 Lu/Hf比值變化范圍使得構建高精度全巖 Lu-Hf等時線極為困難, 國際上尚未有該類巖石的石榴子石Lu-Hf年齡報道?？紤]到石榴子石親Lu排Hf的特性, 這類特殊的巖石是可能給出高精度的石榴子石 Lu-Hf等時線的。因此, 本文嘗試對西大別一個典型的含榴花崗片麻巖進行Lu-Hf年代學研究, 結合鋯石U-Pb年代學、巖石學和礦物學研究, 給出首條高精度的花崗質巖石的Lu-Hf等時線, 并對含榴花崗片麻巖的石榴子石成因及Lu-Hf/U–Pb年齡所代表的地質意義進行討論。

1 區域地質背景和樣品

大別-蘇魯造山帶是位于華北和揚子兩大陸塊間的復雜碰撞造山帶。西大別又稱紅安地區, 為大別-蘇魯造山帶的西延部分, 東側以商麻斷裂與東大別造山帶分開, 西側由大悟斷裂與桐柏相隔(圖1)。大量的構造、巖石、年代學和地球化學研究表明西大別記錄了秦嶺-大別-蘇魯造山帶的多期演化歷史, 為一典型的復合造山帶。這一地區以新縣高壓-超高壓變質地體為核心呈一個構造穹窿, 南北兩翼由高壓至中低壓變質帶組成[26]。根據巖石構造特征, 從北到南可分為: (南灣)復理石帶、(八里畈)構造混雜巖帶、(滸灣-新縣-紅安)高壓-超高壓-高壓變質帶以及(木蘭山)藍片-綠片巖帶等六個相帶[29](圖1)。該區榴輝巖出露面積大, 白堊紀巖漿活動較弱, 并出露大量的藍片巖-綠片巖相變質巖石, 是研究大別-蘇魯造山帶構造演化理想的地區。該地區主要出露巖石為花崗質片麻巖, 榴輝巖以透鏡體狀產出在這些巖石中。少部分花崗質片麻巖含有石榴子石, 這類含榴花崗片麻巖主要零星分布于高壓-超高壓變質帶內。所研究的含榴花崗片麻巖樣品采自湖北省麻城市四道河村附近的一個采石場(31°20.712′N,114°3.051′ E, 圖1)。該花崗片麻巖巖體內部可見殘留的退變質榴輝巖透鏡體(圖 2a)。樣品為灰白色, 中細粒結構, 弱面理化。石英+斜長石+鉀長石組合占95%以上, 副礦物有黑云母(約1%)、石榴子石(約2%)、磁鐵礦和榍石等。石榴子石離散分布于巖石中(圖2b)。前人對該地區的含榴花崗片麻巖鋯石 U-Pb定年結果給出一致的約227 Ma[27–29]的諧和年齡。

圖1 西大別四道河含榴花崗片麻巖采樣圖[29]Fig.1 Simplified geologic map of the West Dabie Mountains, modified after Liu et al.[29], showing the sample locality for the Sidaohe granitic gneiss

2 分析方法

樣品經機械粉碎和磁選, 在雙目顯微鏡下挑選鋯石及不含可見包裹體的石榴子石。石榴子石和全巖的Lu-Hf同位素分析在美國華盛頓州立大學完成, 化學流程及儀器狀態與文獻[12]一致。石榴子石微量元素和鋯石U-Pb測試在中國地質大學(武漢)地質過程與礦產資源國家重點實驗室用LA-ICP-MS法完成。激光束斑直徑 32 μm。具體分析條件及流程詳見文獻[30]。石榴子石主要元素成分分析在該實驗室JXA 8100電子探針儀上完成, 分析條件為加速電壓15 kV , 電流為20 nA , 束斑直徑為2 μm。結果校正采用標準ZAF方法, 使用天然礦物作為標樣。

3 結 果

圖2 西大別四道河含榴花崗片麻巖Fig.2 Field photographs showing granitic gneiss and enclosed retrograded eclogite lenses (a) and granitic gneiss with eyeball-shaped garnet crystal (b) from Sidaohe, the West Dabie Mountains

圖3 西大別四道河含榴花崗片麻巖鋯石典型CL圖及U-Pb諧和圖Fig.3 Zircon U-Pb concordia diagram with selected CL images

四道河含榴花崗片麻巖中鋯石為透明短柱狀, 自形程度較差, 陰極發光(CL)結構表現為弱分帶和霧狀分帶[27–28](圖3)。對這些鋯石進行了U-Pb年齡測定,206Pb/238U年齡范圍是216.5～228.5 Ma (表1和圖3), 加權平均值為(223.2±1.1) Ma (2σ, MSWD = 1.8)。此年齡應為這些鋯石形成時間的最佳估計值。四道河含榴花崗片麻巖中石榴子石呈他形變晶結構, 粒徑 0.2～1.0 mm。主要元素環帶不明顯, 僅在顆粒邊部較窄的區域出現Mn和Ca含量升高、Fe含量降低的變化趨勢(表2和圖4a、4b)。重稀土元素則呈單邊遞增/減的變化趨勢(表3和圖4c～4f)。石榴子石具有極高的母子體同位素比值(176Lu/177Hf = 約300)。兩個石榴子石分析點和一個全巖分析點構筑的 Lu-Hf等時線給出(212.2±0.7) Ma (2σ, MSWD = 0.1)的高精度年齡(表 4和圖5)。

表1 四道河含榴花崗片麻巖鋯石LA-ICP-MS U-Pb同位素分析結果Table 1 Zircon U-Pb isotopic data for Sidaohe granitic gneiss

表2 四道河含榴花崗片麻巖中石榴子石代表性電子探針數據(%)Table 2 Representative electron microprobe analysis data (%) for garnet from Sidaohe granitic gneiss

圖4 西大別四道河含榴花崗片麻巖石榴子石的Mg-Fe-Ca-Mn成分環帶(a和b)、稀土元素分布模式(c和d)及Y-Lu成分分帶(e和f)Fig.4 Mg-Fe-Ca-Mn elemental zoning (a,b), REE patterns (c,d) and Y/Lu zonation (e, f) for two garnet grains from Sidaohe granitic gneissPrp– 鎂鋁榴石; Alm– 鐵鋁榴石; Grs– 鈣鋁榴石; Sps– 錳鋁榴石。石榴子石邊界接觸礦物為長石-綠簾石-石英-黑云母組合。Prp– pyrope; Alm– almandine; Grs– grossular; Sps– spessartine. Mineral assemblage of plagioclase-epidote-quartz-biotite is in contacted with the garnet grains.

4 討 論

盡管對大別造山帶內出露的含榴花崗片麻巖是否經歷過高壓/超高壓變質作用尚存在爭議[26–30,33–35],尚未在四道河含榴花崗片麻巖中找到超高壓/高壓變質作用的礦物學的直接證據, 然而, 四道河含榴花崗片麻巖中石榴子石不具Eu異常(圖 4)可能說明石榴子石與長石結晶的非同時性。石榴子石為富鈣鐵鋁榴石(Grs15-22Alm36-63Sps8-36Prp6-9), 與阿爾卑斯的高壓變質花崗巖和高壓正片麻巖[36]以及大別東部超高壓榴輝巖和超高壓變質花崗質正片麻巖所含的石榴子石類似[33]。利用鋯石中Ti的含量估算該花崗巖中巖漿鋯石和變質鋯石的形成溫度, 分別為約786 ℃和約674 ℃[28]。后者和與之直接接觸的榴輝巖峰期溫度約 692 ℃[28]一致。前人獲得的(227±2)Ma (16 個點)[28]和(227±5) Ma (4 個點)[27]的變質鋯石U-Pb年齡雖略高于我們獲得(223.2±1.1) Ma (26個點)的年齡, 但都具有極低的 Th/U 比值(0.01～0.03),且與直接接觸的榴輝巖年齡相近[28]。

表3 四道河含榴花崗片麻巖中石榴子石LA-ICP-MS稀土元素分析數據(μg/g)Table 3 LA-ICP-MS rare earth element data (μg/g) for garnet from Sidaohe garnet-bearing granitic gneiss

表4 四道河含榴花崗片麻巖石榴子石和全巖Lu-Hf同位素分析結果Table 4 Lu-Hf isotopic data for Sidaohe garnet-bearing granitic gneiss

圖5 西大別四道河含榴花崗片麻巖Lu-Hf等時線Fig.5 Garnet-whole rock Lu-Hf isochron for Sidaohe garnet-bearing granitic gneiss

正確解釋石榴子石Lu-Hf等時線/擬合線最有效的途徑就是厘定石榴子石生長歷程。Cheng et al.[14,37]在研究高壓榴輝巖中的珊瑚礁狀石榴子石時, 發現具有完好生長環帶的石榴子石Lu-Hf的等時線年齡實際代表了退變質階段流體活動的時間。四道河含榴花崗片麻巖中的石榴子石主要元素成分分帶非常微弱, 僅在最邊緣部分呈Mn和Ca含量升高、Fe含量降低的變化趨勢, 指示溶蝕-再吸收的過程。用Crank的一維擴散模型[38]和Carlson[39]的自擴散系數可以估算出石榴子石邊部約 100 μm的溶蝕-再吸收邊的 Ca-Fe-Mn-Mg成分環帶的均一化時間為9.2×106～5.3×107a, 指示石榴子石重結晶后的快速冷卻。結合單邊遞減/增的非中心對稱的微量元素分帶特征(圖4)以及石榴子石內包體的缺乏, 我們認為該石榴子石經歷的是一個典型的溶蝕-再結晶的過程[14,37,40-44]。石榴子石邊部的簾石、黑云母和石英可能就是退變質過程中流體沿裂隙活動的產物, 一種可能的機制是通過石榴子石+白云母+H2O→綠簾石+黑云母+石英的反應, 類似的石榴子石也在挪威的西部片麻巖(WGR)的超高壓變質巖中發現過[45,46]。

放射性同位素定年的等時線法必須滿足同源、同時和封閉這三個基本條件。不滿足這些前提的回歸擬合線都不是嚴格意義上的等時線。然而, 當相對于這些條件的偏差是可以忽略或者偏差本身具有明確的地質意義的情況下, 雖無法確定出嚴格意義上的等時線, 但對礦物和全巖的分析點的回歸剖析卻能提供諸多有益的地質信息[10–24]。我們把四道河含榴花崗片麻巖的石榴子石-全巖擬合線解釋為等時線是因為: (1) 石榴子石經歷的流體改造過程(溶蝕-再結晶)保證不同母子體比值石榴子石的同源和同時的條件; (2) 全巖極低的176Lu/177Hf比值(表 1)決定了其176Hf/177Hf比值非常接近體系的初始同位素比(圖 5); (3) 由于石榴子石快速生長期間造成基質大量Lu虧損及176Lu較長的半衰期, 石榴子石重結晶期間全巖的初始同位素比值變化可以忽略; (4) 石榴子石-全巖擬合線的平均標準權重偏差(MSWD = 0.1)遠小于1個自由度(3個數據點)時2.0的臨界值[47]。

由于稀土元素在石榴子石中的較慢擴散速率[48],一般地質條件下石榴子石的Nd封閉溫度在700～750℃[49]。石榴子石的Lu-Hf體系封閉溫度一般被認為要高于700 ℃[50]。結合四道河含榴花崗片麻巖中石榴子石的微量元素成分分帶, 約212 Ma的石榴子石Lu-Hf年齡代表的應該是石榴子石重結晶年齡, 對應一期流體活動時間。這個年齡比約223 Ma的鋯石U-Pb年齡要晚約10 Ma, 說明石榴子石和鋯石生長不是同期的, 這與鋯石陡立的重稀土富集型的稀土元素分布模式[28]是一致的。該年齡與大別山東緣的片麻狀變質花崗巖約 215 Ma 的高壓榴輝巖相重結晶時間以及約212 Ma的蘇魯-大別榴輝巖中石英脈的形成時間一致[51–53], 可能指示了一期相退變質作用流體活動[54]。含榴花崗片麻巖中石榴子石極高的母子體同位素比值也對Hf-Lu混合稀釋劑的配比提出了特殊的要求。

野外采樣由王超完成, 采樣過程中得到鄭永飛老師的悉心指導; 鄭曙老師、吳元保老師和陳璐同學在電子探針和 LA-ICP-MS測試過程中和數據處理上給予了指導和協助; 吳元保老師還提供了巖石薄片, 在此一并致謝。

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