郭佩,劉池洋,王建強(qiáng),李長(zhǎng)志

1.西北大學(xué)大陸動(dòng)力學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室/西北大學(xué)地質(zhì)學(xué)系,西安 710069 2.中國(guó)石化勝利油田東勝精攻石油開(kāi)發(fā)集團(tuán)股份有限公司,山東東營(yíng) 257000

碎屑鋯石年代學(xué)在沉積物源研究中的應(yīng)用及存在問(wèn)題

郭佩1,劉池洋1,王建強(qiáng)1,李長(zhǎng)志2

1.西北大學(xué)大陸動(dòng)力學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室/西北大學(xué)地質(zhì)學(xué)系,西安 710069 2.中國(guó)石化勝利油田東勝精攻石油開(kāi)發(fā)集團(tuán)股份有限公司,山東東營(yíng) 257000

近年來(lái)，碎屑鋯石U-Pb定年技術(shù)的不斷突破極大地促進(jìn)了沉積盆地物源分析的發(fā)展，該方法被進(jìn)一步應(yīng)用于分析古地理環(huán)境、沉積盆地演化以及物源區(qū)剝蝕速率等。在廣泛應(yīng)用的同時(shí)，國(guó)際學(xué)者逐漸意識(shí)到諸多不確定因素可影響分析結(jié)果、導(dǎo)致偏差。在系統(tǒng)總結(jié)碎屑鋯石年代學(xué)在沉積物源研究中值得關(guān)注的幾個(gè)問(wèn)題的基礎(chǔ)上，分析結(jié)果認(rèn)為：①采樣過(guò)程中要注意采樣間隔和水動(dòng)力對(duì)鋯石的分選作用；②實(shí)驗(yàn)過(guò)程中合理地選擇U-Pb定年方法，根據(jù)特定科學(xué)問(wèn)題選定待測(cè)鋯石和確定分析數(shù)量；③物源對(duì)比過(guò)程中不應(yīng)簡(jiǎn)單地將沉積巖的碎屑鋯石年齡譜與周緣造山帶結(jié)晶巖體(包括巖漿巖和變質(zhì)巖)的年齡進(jìn)行對(duì)比，還應(yīng)關(guān)注較老沉積巖提供的再旋回鋯石、不同源巖的鋯石產(chǎn)出能力、剝蝕區(qū)適當(dāng)?shù)穆∩龝r(shí)間及碎屑鋯石的“滯后時(shí)間”等因素。

物源分析;碎屑鋯石;U-Pb定年;偏差;再旋回鋯石

0 引言

U-Th-Pb地質(zhì)年代學(xué)的研究已進(jìn)入新時(shí)代，測(cè)試方法達(dá)到前所未有的精度、空間分辨率和靈活性，應(yīng)用范圍日益廣泛。鋯石是應(yīng)用最廣泛的礦物，在巖漿巖、變質(zhì)巖和沉積巖中均普遍存在，對(duì)風(fēng)化及磨損具有相對(duì)強(qiáng)的抵抗力，可通過(guò)較多的同位素方法進(jìn)行定年[1-2]。碎屑鋯石U-Pb年代學(xué)的技術(shù)突破(尤其是近十多年LA-ICP-MS U-Pb定年技術(shù))，極大地推動(dòng)了沉積盆地物源分析的發(fā)展[3-6]。碎屑鋯石U-Pb定年技術(shù)進(jìn)一步的衍生應(yīng)用包括：①識(shí)別現(xiàn)代水系中各已知物源的貢獻(xiàn)率[7]及未知物源信息[8-11]；②重建古地理環(huán)境，恢復(fù)大型古水流系統(tǒng)[12-16]；③限定地層單元的絕對(duì)年齡，提供地層最大沉積年齡[6,17-20]；④進(jìn)行地層對(duì)比[21-24]；⑤恢復(fù)沉積盆地演化歷史[25-26]；⑥重現(xiàn)物源區(qū)構(gòu)造環(huán)境及演化[27-32]；⑦確定區(qū)域剝蝕速率[33-35]。

碎屑鋯石U-Pb年代學(xué)應(yīng)用的有效性是建立在“碎屑信息可以真實(shí)地復(fù)制并代表母巖的性質(zhì)”的假設(shè)基礎(chǔ)之上，任何潛在的偏差(bias)都可能導(dǎo)致該假設(shè)不成立。碎屑礦物經(jīng)過(guò)蝕源區(qū)的物理—化學(xué)風(fēng)化作用、河流或風(fēng)的搬運(yùn)作用，最后于沉積區(qū)沉淀下來(lái)，遭受埋藏和后期成巖作用，漫長(zhǎng)而復(fù)雜的地質(zhì)歷史過(guò)程存在諸多不確定的因素，使碎屑鋯石展現(xiàn)的年齡信息有所偏差。這些不確定因素包括[6-7,36-38]：①各物源貢獻(xiàn)率不均一；②排水系統(tǒng)的存儲(chǔ)、緩沖及再循環(huán)作用；③因盆地內(nèi)水動(dòng)力或沉積環(huán)境的變化導(dǎo)致沉積物的潛在封存。因此，全面了解碎屑鋯石年代學(xué)在物源分析過(guò)程中存在的問(wèn)題至關(guān)重要[2]。

國(guó)際上相關(guān)討論較多，涉及廣泛，總體偏向?qū)我蛩氐纳钊胙芯縖1-2,37,39-48]。國(guó)內(nèi)相關(guān)研究起步較晚，近五年來(lái)有增多趨勢(shì)，總體偏應(yīng)用，多限于沉積巖碎屑鋯石年齡譜與潛在物源區(qū)結(jié)晶巖體年齡的對(duì)比研究[49-52]，對(duì)其機(jī)理及應(yīng)用時(shí)應(yīng)注意的問(wèn)題鮮有討論。本文旨在總結(jié)國(guó)際期刊在此方面的討論，綜述采樣過(guò)程、分析方法及物源對(duì)比過(guò)程中值得注意的問(wèn)題，為國(guó)內(nèi)的相關(guān)研究提供借鑒。

1 采樣過(guò)程

1.1 采樣間隔及數(shù)量

碎屑鋯石U-Pb年代學(xué)的物源分析主要包括：剖面上，同一地區(qū)不同層位的樣品用于分析不同地質(zhì)歷史時(shí)期物源的聯(lián)系和變化，重現(xiàn)盆地演化歷史及造山帶剝蝕歷史；平面上，不同地區(qū)同一層位的樣品可分析不同地區(qū)物源的聯(lián)系和變化，再造古地理環(huán)境和恢復(fù)古水流方向。

目前多數(shù)研究，在平面上采樣間隔達(dá)千米甚至是上百千米，剖面上往往一個(gè)組僅一個(gè)或少量樣品，這意味著單個(gè)樣品將代表所采區(qū)域或所采層段的的整體情況。若分析結(jié)果顯示各樣品的碎屑鋯石年齡譜不同，則往往被解釋成沉積物源的重大轉(zhuǎn)變、地層的漸變或板塊范圍內(nèi)動(dòng)力學(xué)的轉(zhuǎn)變。單個(gè)樣品能否代表一個(gè)地區(qū)或一個(gè)層位的整體物源情況？

Hietpasetal.[44]采集美國(guó)東南部阿巴拉契亞山脈(Appalachian Orogen)的佛蘭西布羅德河及其支流中的不同河段的現(xiàn)代沉積物(可代表不同地區(qū)同一時(shí)期的沉積物)，進(jìn)行碎屑鋯石U-Pb定年，發(fā)現(xiàn)沿著河流方向樣品的年齡譜發(fā)生了很大的變化(圖1)。Zimmermannetal.[53]對(duì)西班牙北部坎塔布連山寒武系上、下層段分別采樣，兩層段均屬淺海相沉積，且?guī)r相可進(jìn)行對(duì)比，但碎屑鋯石年齡譜差異較大，反映寒武系沉積時(shí)物源已發(fā)生較大變化。

上述兩個(gè)典型例子說(shuō)明，平面上或剖面上單個(gè)樣品代表整個(gè)地區(qū)或整個(gè)層段的物源情況可能導(dǎo)致結(jié)果有所偏差，具有局限性，尤其對(duì)橫向上分區(qū)明顯、縱向上構(gòu)造演化復(fù)雜的地區(qū)，應(yīng)采集多個(gè)樣品。

1.2 沉積微環(huán)境

碎屑鋯石最初主要來(lái)源于巖漿巖或變質(zhì)巖，不同結(jié)晶巖產(chǎn)生的鋯石大小、形狀不同，從粗粉砂(>30 μm)到中砂(<500 μm)級(jí)別均有。在沉積物搬運(yùn)過(guò)程中鋯石的大小與水動(dòng)力的分選作用有關(guān)，如果不同粒徑的鋯石代表不同的結(jié)晶年齡，則在搬運(yùn)—沉積過(guò)程中可能使鋯石的年齡譜存在偏差。這種差異分選作用的影響一般是局部的，如因不同的流速和河床剪切應(yīng)力，沙丘向流面顆粒的分離；或沖刷坑內(nèi)重顆粒聚集，所以僅僅一個(gè)同粒度的樣品，不能代表一種沉積環(huán)境的整體情況。

Lawrenceetal.[37]在單個(gè)沙丘中采集了5個(gè)代表不同水動(dòng)力沉積微環(huán)境的樣品，砂巖粒度不同(圖2a)，進(jìn)行碎屑鋯石U-Pb定年，結(jié)果發(fā)現(xiàn)，這5個(gè)樣品碎屑鋯石展現(xiàn)明顯不同的年齡譜，不僅主年齡峰值不匹配，而且部分年齡組僅在部分樣品中出現(xiàn)(圖2b)。該作者認(rèn)為水動(dòng)力分選作用確實(shí)對(duì)碎屑鋯石年齡譜存在影響，采樣時(shí)應(yīng)考慮沉積巖粒度。解決的辦法之一即采樣時(shí)可挑選粒度分選最差的沉積體；若無(wú)分選差的巖體，則可對(duì)不同沉積微環(huán)境的巖石分別采樣(小樣)，并分開(kāi)挑選鋯石，最后按等比例將其年齡組合起來(lái)成一個(gè)樣品進(jìn)行分析。

圖1 美國(guó)東南部阿巴拉契亞造山帶南部簡(jiǎn)化地質(zhì)圖及所采樣品位置和對(duì)應(yīng)的碎屑鋯石年齡概率分布圖[44]Fig.1 Simplified geological map of Southern Appalachian Orogen in Southeastern USA and the location and detrital zircon probability density plots of collected samples[44]

圖2 a.同一大型沙丘不同沉積微相(A～E)的粒度分析圖；b.對(duì)應(yīng)碎屑鋯石年齡概率密度分布圖和餅狀圖[37]Fig.2 a.Grain-size distributions for different sedimentary microfacies(A～E) in the same large sand dune; b.Detrital zircon probability density diagrams and pie charts for the five dune samples [37]

2 分析方法及實(shí)驗(yàn)過(guò)程

碎屑鋯石U-Pb定年技術(shù)一般分三種，以下是簡(jiǎn)單介紹各種方法的特征及優(yōu)缺點(diǎn)，并進(jìn)行對(duì)比。

2.1 同位素稀釋—熱電離質(zhì)譜法(ID-TIMS)

ID-TIMS(Isotope dilution-thermal ionization mass spectrometry)分析要求對(duì)整個(gè)或大部分鋯石晶體進(jìn)行溶解，通過(guò)TIMS進(jìn)行同位素跟蹤(一般是205Pb和238U)、U和Pb的化學(xué)分離及同位素分析[54]。分析過(guò)程中化學(xué)溶解和分離過(guò)程需要耗費(fèi)大量的時(shí)間，并要求環(huán)境絕對(duì)干凈，以減少對(duì)Pb和U的污染。該種方法得出的結(jié)果具有很高的精度(0.1%，2σ)，在三種方法中具有最高的準(zhǔn)確度(accuracy)和精確度(precision)(表1)。該方法需要加入205Pb、233U(或235U、238U)同位素稀釋劑，技術(shù)難度高，中國(guó)目前還沒(méi)有達(dá)到相應(yīng)水平的實(shí)驗(yàn)室。在大多數(shù)情況下，碎屑鋯石物源分析時(shí)并不需要如此高的精度。

2.2 二次離子質(zhì)譜法(SIMS)

SIMS(Secondary ion mass spectrometry)分析，又稱離子探針，是在環(huán)氧基樹(shù)脂內(nèi)晶體拋光表面進(jìn)行，無(wú)法添加同位素追蹤器，所以年齡校正需用標(biāo)樣。低溫和高度真空的環(huán)境，Pb和U的背景值低，典型的剝蝕坑直徑在10～30 μm，深度1 μm，近似于無(wú)損表面分析；精確度和準(zhǔn)確度一般在1%～2%(2σ)，分析時(shí)間一般約15分鐘(表1)。在測(cè)定U-Pb年齡的同時(shí)，離子探針也可用于分析其他元素，比如碎屑礦物的Ti和Zr濃度，氧同位素等[55]。國(guó)內(nèi)目前不斷引進(jìn)大型二次離子質(zhì)譜儀器，如北京離子探針中心的SHRIMP Ⅱ和SHRIMP Ⅱe-MC、中國(guó)科學(xué)院地質(zhì)與地球物理研究所Cameca IMS 1280、Cameca IMS 1280HR和NanoSIMS 50L，大大提高了國(guó)內(nèi)原位微區(qū)研究分析的能力[56-57]。

2.3 激光剝蝕—等離子體質(zhì)譜法(LA-ICP-MS)

LA-ICP-MS(Laser-ablation inductively coupled plasma mass spectrometry)方法與SIMS在以下三方面是相似的：年齡通過(guò)標(biāo)樣確定，均在一個(gè)拋光的晶體表面進(jìn)行，年齡精確度和準(zhǔn)確度偏差約在1%～2%(2σ)。LA-ICP-MS優(yōu)勢(shì)在于分析時(shí)間更短(一個(gè)點(diǎn)分析時(shí)間約2分鐘)，若有足夠的分散器和接收器則可同時(shí)測(cè)定U和Pb同位素值；劣勢(shì)在于等離子體電離產(chǎn)生很高的氬氣流速，并在常規(guī)大氣壓和高溫環(huán)境下，這些環(huán)境導(dǎo)致較高的Pb和Hg的背景值。為提高信號(hào)的質(zhì)量，LA-ICPMS測(cè)試過(guò)程中需要高速地剝蝕晶體，消耗較大量的樣品(典型的剝蝕坑直徑為30 μm，深度10～20 μm)(表1)。LA-ICP-MS儀器也很適合分析Hf-Lu-Yb等其他元素[58-61]。

2.4 方法的選擇

U-Th-Pb定年方法的選擇取決于特定的科學(xué)問(wèn)題，應(yīng)考慮：①地質(zhì)過(guò)程的持續(xù)時(shí)間；②原始材料的大小和豐富程度；③樣品和地質(zhì)事件的復(fù)雜程度；④所需數(shù)據(jù)量[62]。方法的選擇還在于理解每種方法的優(yōu)點(diǎn)及局限，不可過(guò)度地解釋數(shù)據(jù)、量化不確定因素。對(duì)于側(cè)重時(shí)間尺度的研究，需要高精度和準(zhǔn)確度，推薦ID-TIMS。ID-TIMS可解決相對(duì)老的巖漿侵入事件的年齡問(wèn)題，對(duì)于晚更新世及更晚事件的時(shí)間確定，可選SIMS及LA-ICP-MS。對(duì)于需要最少的材料消耗、分析具細(xì)小生長(zhǎng)環(huán)帶的礦物，要求最高空間分辨率的研究，SIMS分析方法是最好的選擇[63]。對(duì)于需要統(tǒng)計(jì)大量的有效數(shù)據(jù)、偏差范圍在1%～2%左右、深度剖析更大的研究，LA-ICP-MS分析方法更好，該方法是碎屑物源分析和基礎(chǔ)調(diào)查研究的最好選擇[62]。

表1 三種U-Pb定年方法的優(yōu)、缺點(diǎn)對(duì)比[62]

2.5 實(shí)驗(yàn)過(guò)程

實(shí)驗(yàn)過(guò)程中面臨兩個(gè)最基本的問(wèn)題：分析數(shù)據(jù)量和鋯石顆粒的選擇。可分以下2種情況考慮：

(1) 若為了限制最大沉積年齡，則合理的策略是集中分析顏色最淺和磨圓最差的晶體，因?yàn)轭伾顪\的晶體最有可能是最年輕的[63]，晶體的磨圓度則隨搬運(yùn)距離和再旋回次數(shù)的增加而增加。顆粒數(shù)量的選擇取決于樣品中最年輕鋯石所占的比例及確定最大沉積年齡需要的鋯石數(shù)量。

(2) 若為了物源分析或地層對(duì)比，則得出的結(jié)果需反映碎屑鋯石真實(shí)年齡分布情況。樣品制作過(guò)程(如磁力分離、大小分選、或手工挑選)中盡量避免產(chǎn)生偏差；在分析過(guò)程中，需根據(jù)現(xiàn)有的鋯石晶體隨機(jī)挑選，不管大小、顏色、形狀、磨圓度等。遺憾的是，太小的顆粒無(wú)法分析；具裂縫的鋯石通常產(chǎn)生不可靠的年齡，因?yàn)槠溟g可能充填次生礦物、裂縫表面Pb容易丟失、或入射的激光或原離子束在裂縫處可能產(chǎn)生異常行為[63]。含包裹體的部分和混合年齡區(qū)域亦應(yīng)盡量避免。

理論上，物源分析的數(shù)據(jù)量取決于各年齡段的數(shù)量和所占的比例、晶體是否受Pb丟失的影響、分析方法的精確度及數(shù)量的可信度等。Anderson[64]使用標(biāo)準(zhǔn)二項(xiàng)概率公式，認(rèn)為若要使占顆粒數(shù)5%的年齡成分得到識(shí)別的概率達(dá)到95%，則至少需要60個(gè)分析數(shù)據(jù)。若要確保識(shí)別每一個(gè)5%的年齡成分，則至少需要117個(gè)分析數(shù)據(jù)[65]。實(shí)際上，上述學(xué)者仍低估了最低分析量，因?yàn)檫@是建立在每個(gè)數(shù)據(jù)都可靠的前提下，但實(shí)際情況并非如此。若使每個(gè)年齡集(age clustering)均可靠，數(shù)量最少的年齡集也均可以被識(shí)別，則需要更多的數(shù)據(jù)。Pullenetal.[66]分析和對(duì)比了4組均接近1 000個(gè)數(shù)據(jù)的碎屑鋯石，認(rèn)為以大量分析數(shù)據(jù)為基礎(chǔ)的實(shí)驗(yàn)具更高的可信度，可增加識(shí)別較少年齡成分的概率。一個(gè)合理的做法是：每個(gè)樣品均測(cè)試大約100個(gè)數(shù)據(jù)，過(guò)程中用一個(gè)不協(xié)和過(guò)濾器軟件保證“老年齡”不至于過(guò)少，并且分析解釋數(shù)據(jù)時(shí)主要關(guān)注年齡集[63]。

3 數(shù)據(jù)處理及解釋過(guò)程

碎屑鋯石U-Pb年齡數(shù)據(jù)分布有多種展示方法：①直方圖(Histograms)：展示落入一定年齡范圍的鋯石個(gè)數(shù)，較為直觀，但圖的面貌受如起始點(diǎn)和柱寬(bin width)等因素的影響較大；②概率密度分布函數(shù)(Probability Density Function，PDF)：通過(guò)每顆鋯石的年齡及偏差展示的正態(tài)曲線，這是較為常見(jiàn)的展示方法，形態(tài)受鋯石顆粒數(shù)及絕對(duì)年齡的影響；③累計(jì)分布函數(shù)(Cumulative Distribution Function，CDF)：是在PDF基礎(chǔ)上發(fā)展起來(lái)，隨年齡的增加概率累積曲線，較明顯地展示了鋯石小于某特定年齡的概率。PDF和CDF展示的信息相同，PDF表現(xiàn)“某一特定年齡的出現(xiàn)或缺失”方面更直觀；CDF展現(xiàn)“一系列年齡分布的相似或差別”方面更直觀(圖3)。

在對(duì)比年齡譜相似性方面，如不同地區(qū)同一組砂巖、同一地區(qū)不同組或不同層段、沉積巖與潛在物源區(qū)結(jié)晶巖體等之間的年齡譜對(duì)比，可用國(guó)際上較為流行的對(duì)比公式Kolmogorov-Smirnoff statistical test(柯?tīng)柲缏宸颉姑茁宸蛟囼?yàn))，簡(jiǎn)稱K-S test[40,48,67-69]。K-S test 是基于CDF的運(yùn)算公式，對(duì)比兩個(gè)CDF曲線的最大垂直差異D(圖4)，若觀察的差異值Dobs大于某一臨界值Dcrit，則這兩個(gè)樣品具有顯著性差異；臨界值Dcrit取決于樣本數(shù)量和理想置信度，一般情況下，一個(gè)樣品至少需要20個(gè)分析點(diǎn)才具有統(tǒng)計(jì)學(xué)意義。地質(zhì)學(xué)家將K-S test應(yīng)用于地質(zhì)分析，將其算法鑲嵌于excel表格中(具體軟件可于www.geo.arizona.edu/alc下載)，通過(guò)對(duì)比Dobs和Dcrit，返回P值。若P值大于0.05，則兩組年齡譜可能來(lái)自同一物源，P值越大，相似程度越大；若P值小于0.05，則兩組樣品不可能來(lái)自同一物源。P值與每個(gè)樣品分析點(diǎn)數(shù)、每個(gè)年齡段所占比例有關(guān)。在利用K-S test時(shí)，需同時(shí)輸入年齡值及相應(yīng)的偏差。這一方法排除了肉眼觀察的偏差，使結(jié)果更加量化，更加令人信服。

圖3 碎屑鋯石U-Pb年齡分布兩種展示方式(左：概率密度分布函數(shù)；右：累計(jì)分布函數(shù))[63]Fig.3 Two presentation ways of U-Pb age distribution (left: PDF; right: CDP) [63]

圖4 K-S test 分析原理：對(duì)比不同CDF曲線間的最大差異值Fig.4 The principle of K-S test: comparison of the maximum difference-D between several CDFs

數(shù)據(jù)的快速獲取導(dǎo)致忽視了對(duì)單顆碎屑晶體的特征分析，最明顯的例子是鋯石年齡往往比高倍陰極發(fā)光圖像應(yīng)用更廣泛。CL圖像展示的內(nèi)容豐富，尤其是鋯石的巖相學(xué)，可確定復(fù)雜多期的鋯石增生事件。若缺乏對(duì)CL圖像的認(rèn)真研究，激光剝蝕點(diǎn)或離子束可能跨多期生長(zhǎng)的環(huán)帶，產(chǎn)生無(wú)意義的“混合年齡”。同時(shí)鋯石的Th/U比值，亦有助于識(shí)別鋯石增生的巖漿或變質(zhì)事件[48]，應(yīng)同樣受到重視。

4 物源分析過(guò)程

利用碎屑鋯石U-Pb年代學(xué)分析物源的傳統(tǒng)做法是：將沉積巖的碎屑鋯石年齡譜與周緣造山帶結(jié)晶巖體(包括巖漿巖和變質(zhì)巖)的年齡進(jìn)行對(duì)比，若年齡匹配，則該造山帶可解釋成潛在物源之一。但是，物源分析遠(yuǎn)比上述做法復(fù)雜，因?yàn)椋孩偻粋€(gè)造山帶可能經(jīng)歷多期巖漿活動(dòng)；②構(gòu)造作用可導(dǎo)致不同時(shí)代巖體碰撞成一個(gè)物源區(qū)；③來(lái)自不同物源區(qū)的河流系統(tǒng)在沉積區(qū)匯合；④沉積物再旋回造成與年輕巖體的混合[46]。因此，物源分析時(shí)還需考慮以下問(wèn)題：

4.1 再旋回鋯石

鋯石的耐久性(durability)一方面能使其在沉積環(huán)境中經(jīng)歷了風(fēng)化、搬運(yùn)、埋藏后仍能保持晶體內(nèi)部U-Pb系統(tǒng)的穩(wěn)定，從而能記錄最初形成時(shí)的年齡[43]；另一方面，也暗示其在經(jīng)歷多次沉積旋回后仍能留于沉積物中，即較老沉積物中的碎屑鋯石可再次出現(xiàn)在年輕的沉積物中[9,70]。

Dickinsonetal.[43]在分析美國(guó)亞利桑那州東南部的下白堊統(tǒng)河流相石英砂巖物源時(shí)，發(fā)現(xiàn)碎屑鋯石年齡譜與科羅拉多高原東部中—上侏羅統(tǒng)的風(fēng)成石英砂巖的碎屑鋯石年齡譜幾乎一致，認(rèn)為下白堊統(tǒng)直接物源可能為北部隆起裂谷肩出露的侏羅系風(fēng)成石英砂巖；Dickinsonetal.[19]證明了科羅拉多高原上侏羅統(tǒng)的海相Curtis 砂巖物源直接來(lái)自于下伏侏羅系的Entrada風(fēng)成砂巖；Pereiraetal.[48]研究伊伯利亞半島西部Lusitanian盆地三疊系砂巖的沉積物源，碎屑鋯石年齡譜分析結(jié)果表明，沉積物最終來(lái)源于伊伯利亞半島中部的結(jié)晶基底，但之間的Bu?aco盆地沉積巖作為“中間沉積物倉(cāng)庫(kù)”(intermediate sediment repository)，是其直接物源(圖5)；即石炭紀(jì)時(shí)Bu?aco盆地接受來(lái)自伊伯利亞半島中部結(jié)晶基底的物源，于三疊紀(jì)期間該盆地隆起為L(zhǎng)usitanian盆地提供物源。

上述實(shí)例表明，物源對(duì)比時(shí)，不僅應(yīng)調(diào)查周緣造山帶結(jié)晶巖體的年齡分布，還應(yīng)調(diào)查早期沉積巖(尤其是下伏沉積巖)的碎屑鋯石年齡譜，否則可能導(dǎo)致錯(cuò)誤的結(jié)果[46,67]。

4.2 鋯石產(chǎn)出能力

在利用碎屑鋯石U-Pb年齡譜對(duì)比物源時(shí)，應(yīng)了解有些地質(zhì)事件是無(wú)法恢復(fù)的。不同類型巖漿巖產(chǎn)出鋯石能力不同，鋯石一般在SiO2含量大于60%的巖漿巖中結(jié)晶，在低硅質(zhì)含量的巖漿巖中僅發(fā)現(xiàn)少量例子[1]，因此有些巖漿巖可能只產(chǎn)生有限的鋯石，或在有些熱化學(xué)條件下不足以形成新的鋯石。Dickinson[42]對(duì)北美72套花崗巖類巖石的1 386個(gè)全巖Zr分析，得出不同種類的花崗巖的平均鋯石發(fā)育能力不同，且隨年齡變化而變化。變質(zhì)巖的變質(zhì)程度只有達(dá)到角閃巖相到麻粒巖相，才能改變鋯石的惰性，產(chǎn)生新的變質(zhì)鋯石[71-73]。從巖體碰撞、地殼增生、加熱和負(fù)載、熔融及最后遭受剝蝕的一整個(gè)地殼循環(huán)過(guò)程中，可能均沒(méi)有碎屑鋯石記錄[41]。而碎屑鋯石年齡譜可能主要受異常發(fā)育鋯石的熱事件的影響。

圖5 a.伊伯利亞半島西部簡(jiǎn)化層序地層圖；b.Lusitanian盆地三疊系沉積時(shí)古地理環(huán)境示意圖[48]Fig.5 a.Simplified stratigraphy of western Iberia strata；b.Paleogeographic context of the Triassic deposition within the Lusitanian Basin[48]

沉積物中碎屑鋯石的豐富程度還與源區(qū)鋯石的保存能力有關(guān)。受構(gòu)造環(huán)境的影響，不同巖漿巖的產(chǎn)量及保存幾率不同[74]：匯聚板塊邊緣構(gòu)造環(huán)境可產(chǎn)生大量的巖漿巖，但巖石保存的潛力相對(duì)較?。慌鲎脖尘跋聨r漿巖產(chǎn)量較少，主體以前期地殼的局部熔融為主[74]，但由于周圍超大陸的包圍，形成的巖漿巖被保存的潛力很大；伸展環(huán)境下巖漿巖的產(chǎn)量變化很大，但該階段形成的巖漿巖以鎂鐵質(zhì)為主，不可能產(chǎn)生大量鋯石[41]。所以，巖漿巖的年齡主體記錄超大陸的聚集事件，不是因?yàn)檫@是地殼產(chǎn)生的主要階段，而是因?yàn)槠涮峁┝吮４鎺r漿巖的場(chǎng)所[74]。

巖體發(fā)育鋯石和被保存的能力導(dǎo)致在利用碎屑鋯石分析物源時(shí)結(jié)果存在偏差，偏向于更富鋯石的巖體(Zr-rich lithologies)，而無(wú)鋯石記錄或鋯石保存條件差的巖體雖也提供沉積物源，但碎屑鋯石無(wú)法記錄[41]。

4.3 適當(dāng)隆升時(shí)間

無(wú)論是原始結(jié)晶巖還是再旋回沉積巖，若要提供物源，均需在合適的時(shí)間內(nèi)隆起遭受剝蝕。勞倫克拉通結(jié)晶基底在晚古生代時(shí)被古生代早、中期的沉積物覆蓋，不可能為上古生界提供物源[46]，鄂爾多斯盆地內(nèi)部的結(jié)晶基底自顯生宙以來(lái)均被沉積物廣泛覆蓋，鄂爾多斯地塊雖遭受數(shù)次抬升隆起剝蝕，但由于上覆巨厚沉積物的保護(hù)，并未使結(jié)晶基底露出地表，因此在分析鄂爾多斯盆地內(nèi)顯生宙沉積巖的物源時(shí)，無(wú)需考慮盆地內(nèi)部結(jié)晶基底。

在識(shí)別古物源區(qū)時(shí)，不能受現(xiàn)今地形隆坳格局的影響[46,67]。鄂爾多斯西北緣賀蘭山地區(qū)現(xiàn)今隆出地表，其主體隆升時(shí)間是晚侏羅世[75]，該區(qū)出露的孔茲巖系年齡(元古代晚期(1 850 Ma左右)，與西緣上三疊統(tǒng)、下—中侏羅統(tǒng)砂巖中50%～60%碎屑鋯石的年齡很吻合[76]，但考慮賀蘭山的隆升時(shí)間，前晚侏羅世的沉積巖的物源不可能來(lái)自于賀蘭山地區(qū)。

剝露的最終結(jié)果可能導(dǎo)致物源區(qū)某些巖體完全消失，從而造成某些年齡段的碎屑鋯石在現(xiàn)今地形條件下無(wú)法找到合適的物源。北美西部1 100 Ma的碎屑鋯石物源推測(cè)可能是一個(gè)元古代的硅質(zhì)火山巖區(qū)域，該區(qū)域隨后幾乎被剝蝕殆盡，僅殘留很少的花崗侵入巖體[77]。在一個(gè)造山帶，早期的碎屑鋯石可能來(lái)源于淺層的侵入巖體，現(xiàn)今剝露地表的可能僅是深部的結(jié)晶巖系。因此，在物源分析時(shí)，應(yīng)調(diào)查周緣造山帶的構(gòu)造演化歷史。

4.4 滯后時(shí)間

滯后時(shí)間(即鋯石出現(xiàn)于沉積物中的時(shí)間與晶體形成時(shí)間的間隔)，是控制碎屑鋯石年齡譜的關(guān)鍵因素之一[46]，這一點(diǎn)在利用碎屑鋯石分析沉積物最大沉積年齡方面表現(xiàn)尤為明顯。同期巖漿巖分為兩種，火山巖滯后時(shí)間短，但火山巖發(fā)育鋯石能力低[42]；侵入巖滯后時(shí)間較同期火山巖長(zhǎng)，主要取決于巖體侵入的深度和上覆圍巖剝蝕的速率。美國(guó)Appalachian前陸盆地上石炭統(tǒng)中普遍缺失Alleghanian年齡的鋯石，說(shuō)明造山帶內(nèi)的Alleghanian侵入巖較晚出露于地表[67]。鄂爾多斯西緣南側(cè)秦嶺造山帶內(nèi)廣泛發(fā)育印支期巖漿巖體，西緣上三疊統(tǒng)延長(zhǎng)組砂巖中普遍缺失印支期的碎屑鋯石[76,78-79]，直至中侏羅統(tǒng)延安組砂巖中才出現(xiàn)少量該期鋯石[76]，說(shuō)明秦嶺造山帶印支期花崗巖滯后時(shí)間較長(zhǎng)。

不同構(gòu)造背景下形成的巖體滯后時(shí)間差異較大，據(jù)統(tǒng)計(jì)[1](圖6)：匯聚邊緣盆地一般50%以上的碎屑鋯石結(jié)晶年齡接近地層沉積年齡，一些弧前盆地和海溝盆地碎屑鋯石年齡譜存在一個(gè)與沉積年齡接近的年齡單峰，而弧后盆地隨與相鄰克拉通的距離而展現(xiàn)不同的年齡譜；碰撞背景形成的盆地一般含少量(50%～10%)結(jié)晶年齡與沉積年齡接近的碎屑鋯石(100 Ma<滯后時(shí)間<150 Ma)；伸展背景下形成的沉積盆地碎屑鋯石滯后時(shí)間最大，僅少于5%的顆粒滯后時(shí)間小于150 Ma，因?yàn)樯煺贡尘跋滦纬傻膸r漿巖主體以鎂鐵質(zhì)為主[1]。

圖6 不同構(gòu)造環(huán)境下鋯石的結(jié)晶年齡與沉積年齡差異模式圖[1]Fig.6 Models of differences between crystallization and depositional ages of zircons in different tectonic settings[1]

由于沉積巖碎屑鋯石存在滯后性，在物源分析時(shí)，不可因剝蝕區(qū)的巖體的年齡(尤其是年齡稍大于沉積年齡的巖體)在碎屑鋯石年齡譜中并未表現(xiàn)，而否定該潛在物源，應(yīng)結(jié)合構(gòu)造背景共同討論。

5 結(jié)語(yǔ)

物源分析(無(wú)論是現(xiàn)代還是古代沉積物)是一項(xiàng)挑戰(zhàn)性的工作，尤其是對(duì)構(gòu)造演化歷史復(fù)雜、后期改造強(qiáng)烈的地區(qū)更是如此。盡管現(xiàn)代技術(shù)手段的飛速發(fā)展，改變了傳統(tǒng)方法定性分析的趨勢(shì)，使物源分析趨于量化，但鋯石U-Pb定年技術(shù)成本高、樣品少，測(cè)試過(guò)程本身存在諸多不確定因素，加之地質(zhì)過(guò)程的時(shí)空不均一性，導(dǎo)致所采的少量樣品不具代表性，或無(wú)法全面反映整體情況。因此，在利用碎屑鋯石U-Pb定年分析沉積物源時(shí)，應(yīng)結(jié)合傳統(tǒng)物源分析手段，盡量避免采樣、制樣和實(shí)驗(yàn)過(guò)程中存在的不確定因素，綜合考慮碎屑礦物在剝蝕、搬運(yùn)、埋藏、成巖或變質(zhì)過(guò)程中可能導(dǎo)致年齡偏差的各種因素。

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Considerations on the Application of Detrital-Zircon Geochronology to Sedimentary Provenance Analysis

GUO Pei1,LIU ChiYang1,WANG JianQiang1,LI ChangZhi2

1. State Key Laboratory of Continental Dynamics/Department of Geology, Northwest University, Xi’an 710069, China 2. Shengli Oilfield Dongsheng Jinggong Petroleum Development Group, Dongying, Shandong 257000, China

In recent years, the growing breakthroughs in detrital-zircon U-Pb dating technology have fueled the development of provenance analysis of sedimentary basin, and this approach is further used to analyze paleogeographic environment, basin evolution and unroofing rate of provenance areas. Simultaneously, a number of potential problems that will lead to bias in applying detrital-zircon U-Pb geochronology to provenance analysis have increasingly attracted international attentions. On the basis of summarizing international relevant discussions, this paper concludes such considerations as sample intervals and hydrodynamic fractionation during sampling; reasonable choices of U-Pb dating methods, specific zircons and analytical numbers according to specific science issues during experiments; multi-recycling zircons, zircon-fertility ability, appropriate uplifting time and lag time during provenance comparison, and so on. The last of which is traditionally and simply conducted by matching detrital-zircon age spectrum of sediment units with crystallization age of source rocks in adjacent orogenic belts.

provenance analysis; detrital zircon; U-Pb dating; bias; multi-recycling zircons

1000-0550(2017)01-0046-11

10.14027/j.cnki.cjxb.2017.01.005

2016-01-18；收修改稿日期： 2016-03-04

國(guó)家自然科學(xué)重點(diǎn)基金項(xiàng)目(41330315)；西北大學(xué)研究生自主創(chuàng)新項(xiàng)目(YZZ15018)；中國(guó)地質(zhì)調(diào)查局項(xiàng)目(12120113039900,12120114009201)[Foundation: National Natural Science Fund Project, No.41330315; Independent Innovation Project of Northwest University, No.YZZ15018; China Geological Survey Project, No.12120113039900,12120114009201]

郭佩,女,1990年出生,博士研究生,盆地分析與石油地質(zhì),E-mail：nwupeig@126.com

劉池洋,男,教授,E-mail：lcy@nwu.edu.cn

P597+<.3 P512.2 class="emphasis_bold">.3 P512.2 文獻(xiàn)標(biāo)識(shí)碼 A.3 P512.2

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