鄂爾多斯盆地長(zhǎng)城系碎屑鋯石SHRIMP U-Pb定年和Hf同位素研究*

白文倩 董春艷 頡頏強(qiáng) 萬(wàn)渝生

中國(guó)地質(zhì)科學(xué)院地質(zhì)研究所北京離子探針中心，北京 100037

古元古代晚期強(qiáng)烈構(gòu)造熱事件(呂梁運(yùn)動(dòng))導(dǎo)致華北克拉通最終穩(wěn)定化，形成統(tǒng)一的早前寒武紀(jì)變質(zhì)基底(Zhai and Santosh, 2011)。之后，華北克拉通構(gòu)造體制發(fā)生根本變化，從碰撞造山轉(zhuǎn)向伸展構(gòu)造。中元古代最早期熊耳群火山-沉積巖系形成之后(Zhaoetal., 2004)，長(zhǎng)城系蓋層在華北克拉通廣泛發(fā)育，之后沉積了中-新元古代薊縣系和青白口系。華北克拉通東部中新元古代沉積巖的碎屑鋯石測(cè)年工作表明，碎屑鋯石年齡峰值集中在新太古代晚期和古元古代晚期，表明中新元古代沉積巖的碎屑物質(zhì)主要來(lái)自華北克拉通變質(zhì)基底本身(Wanetal., 2011; 第五春榮等, 2011;翟明國(guó)等, 2014)。

鄂爾多斯盆地是華北克拉通的重要組成部分。長(zhǎng)城系碎屑鋯石定年初步研究表明，其具有與華北克拉通東部地區(qū)長(zhǎng)城系類(lèi)似的年齡分布模式(公王斌等, 2016)。長(zhǎng)城系物源區(qū)組成可為確定鄂爾多斯盆地基底性質(zhì)及該時(shí)期的大地構(gòu)造環(huán)境提供重要信息。然而，鄂爾多斯盆地長(zhǎng)城系缺乏鋯石SHRIMP U-Pb定年及Hf同位素資料，影響了對(duì)鄂爾多斯盆地的性質(zhì)及中元古代所處構(gòu)造環(huán)境的認(rèn)識(shí)。本文報(bào)道了鄂爾多斯長(zhǎng)城系碎屑沉積巖的碎屑鋯石SHRIMP U-Pb定年和Hf同位素分析結(jié)果，從鋯石外部形態(tài)和內(nèi)部結(jié)構(gòu)特征探討物源區(qū)性質(zhì)和外生作用過(guò)程，對(duì)沉積時(shí)代、物質(zhì)來(lái)源等相關(guān)問(wèn)題進(jìn)行了討論。

1 地質(zhì)背景

鄂爾多斯盆地位于華北克拉通西部，北臨孔茲巖帶，東臨中部造山帶，南臨秦嶺造山帶(圖1a)。由于中生代以來(lái)沉積蓋層廣泛覆蓋，對(duì)鄂爾多斯盆地基底的研究多通過(guò)地球物理、鉆孔及鄰區(qū)露頭進(jìn)行。鉆孔資料顯示，鄂爾多斯盆地基底主要由古元古代具孔茲巖系性質(zhì)的副變質(zhì)巖和片麻狀花崗質(zhì)巖石組成，也可能存在新太古代晚期侵入巖，在古元古代晚期遭受強(qiáng)烈構(gòu)造熱事件改造(Huetal., 2013; Wanetal., 2013; Wangetal., 2014; Zhangetal., 2015; Heetal., 2016)。鄂爾多斯盆地自古元古代晚期以來(lái)長(zhǎng)期穩(wěn)定，明顯不同于華北克拉通東部，后者遭受中生代以來(lái)的克拉通破壞，導(dǎo)致巖石圈明顯減薄。在鄂爾多斯盆地中南部，長(zhǎng)城系不整合覆蓋在早前寒武紀(jì)變質(zhì)基底之上，在東北部古生代-中生代地層直接覆蓋在變質(zhì)基底之上(公王斌等, 2016)。長(zhǎng)城系下部主要為石英巖夾板巖，上部為粉砂質(zhì)板巖、硅質(zhì)巖及含燧石條帶的白云質(zhì)灰?guī)r與石英砂巖。先前所進(jìn)行的長(zhǎng)城系LA-MC-ICPMS U-Pb定年，獲得1.60Ga、1.85Ga、1.95Ga、2.35Ga和2.50Ga碎屑鋯石年齡峰值(公王斌等, 2016)。本文采集樣品都來(lái)自緊鄰不整合變質(zhì)基底之上的(變質(zhì))沉積層位，為長(zhǎng)城系蓋層(中國(guó)長(zhǎng)慶油田，2011)。

2 樣品特征及分析方法

2.1 樣品特征

本文對(duì)鄂爾多斯盆地三口揭露到中元古代底層的鉆孔的4個(gè)(變質(zhì))沉積巖樣品進(jìn)行了碎屑鋯石SHRIMP U-Pb定年及Hf同位素分析。其中2個(gè)樣品(HT1-12、K1-2)來(lái)自合探1井和克1井，它們所屬地層為中元古代長(zhǎng)城系，對(duì)此前人已有討論(中國(guó)長(zhǎng)慶油田，2011；公王斌等, 2016)，這里不再述及。采樣位置見(jiàn)圖1b。

石英砂巖(HT1-12)采自鄂爾多斯盆地南部甘肅合水地區(qū)的合探1井。該井完鉆井深4140m。樣品采自4131～4132m，為肉紅色細(xì)粒石英砂巖(圖2a)。巖石幾乎都由石英組成(>95%，圖3a)，分選磨圓好，顆粒支撐，膠結(jié)物很少，部分石英顆粒有次生加大。

石英砂巖(K1-2)采自鄂爾多斯盆地西北部鄂托克旗地區(qū)的克1井。該井完鉆井深4080m。樣品采自4038～4046m，也為肉紅色細(xì)粒石英砂巖(圖2b)。巖石主要由石英(90%)組成，存在少量長(zhǎng)石(10%，圖3b)。與樣品HT1-12相比，分選和磨圓較差，顆粒支撐，膠結(jié)物充填在碎屑之間，存在具波狀消光的他型細(xì)粒石英集合體(圖3b)。部分石英顆粒發(fā)育次生加大。

變質(zhì)石英砂巖(ZT1-9A)采自鄂爾多斯盆地北部榆林地區(qū)的召探1井(N38°50′25″、E109°10′50″)。該井完鉆井深3602.3m。樣品采自3516～3519m，為肉紅色淺變質(zhì)石英砂巖(圖2c)。巖石主要由石英(90%)組成(圖3c)，分選磨圓好，顆粒支撐，膠結(jié)物充填在碎屑之間。膠結(jié)物轉(zhuǎn)變?yōu)榧?xì)片狀絹云母(10%)，表明巖石已遭受低綠片巖相淺變質(zhì)作用改造，但變形很弱。由于遭受變質(zhì)，原認(rèn)為形成時(shí)代為古元古代，與滹沱群相當(dāng)(中國(guó)長(zhǎng)慶油田， 2011)，變質(zhì)砂巖(ZT1-9B)與樣品ZT1-9A采自同一鉆井，位于其下部數(shù)米位置。巖石呈灰色(圖2d)，主要由長(zhǎng)石(55%)、石英(40%)和云母(5%)組成(圖3d)。長(zhǎng)石簾石化和絹云母化較強(qiáng)，但長(zhǎng)石顆粒仍可識(shí)別。為長(zhǎng)石石英砂巖淺變質(zhì)產(chǎn)物。巖石變形也弱。形成時(shí)代原也認(rèn)為是古元古代。

2.2 分析方法

鋯石分選和SHRIMP U-Pb定年都在北京離子探針中心完成。測(cè)年方法見(jiàn)Williams(1998)。一次離子流強(qiáng)度為4nA，束斑為30μm。標(biāo)準(zhǔn)鋯石91500(U=81.2×10-6)用于未知鋯石U、Th含量修正(Williams, 1998)，標(biāo)準(zhǔn)鋯石TEM(年齡=417Ma)用于未知鋯石年齡修正(Blacketal., 2003)。標(biāo)準(zhǔn)鋯石定年由5組掃描完成，未知樣品定年為3～5組掃描。TEM與未知樣品測(cè)定比例為1:3～1:5。數(shù)據(jù)處理采用SQUID和ISOPLOT 程序(Ludwig, 2001)，根據(jù)實(shí)測(cè)204Pb進(jìn)行普通鉛校正。由于鋯石年齡大于1.4Ga，使用207Pb/206Pb年齡。單個(gè)數(shù)據(jù)年齡誤差為1σ, 加權(quán)平均年齡誤差為95%置信度。

Hf同位素分析在中國(guó)地質(zhì)科學(xué)院礦產(chǎn)資源研究所LA-MC-ICPMS同位素實(shí)驗(yàn)室完成，使用儀器為T(mén)hermo Finnigan Neptune型多接受電感耦合等離子體質(zhì)譜儀(MC-ICP-MS)。分析方法見(jiàn)侯可軍等(2007)。采用靜態(tài)信號(hào)采集模式，背景采集時(shí)間30s，積分時(shí)間為0.131s，采集200組數(shù)據(jù)，總計(jì)約30s。激光能量密度為15J/cm2，束斑直徑為55μm。計(jì)算參數(shù)：176Lu衰變常數(shù)為1.865×10-11yr-1(Schereetal., 2001)，球粒隕石的176Hf/177Hf和176Lu/177Hf比值分別為0.0332和0.282772(Blichert-Toft and Albarède, 1997)，現(xiàn)今虧損地幔176Hf/177Hf比值為0.28325(Nowelletal., 1998)，用于計(jì)算2階段模式年齡(tDM2(Hf))大陸地殼的(176Lu/177Hf)平均值為0.015(Griffinetal., 2002)。

圖1 華北克拉通早前寒武紀(jì)構(gòu)造格局簡(jiǎn)圖(a，據(jù)Zhaoetal., 2002)和鄂爾多斯盆地及周緣地質(zhì)圖(b，據(jù)中國(guó)長(zhǎng)慶油田，2011[注]中國(guó)長(zhǎng)慶油田. 2011. 鄂爾多斯盆地與前寒武紀(jì)基底相關(guān)鉆孔簡(jiǎn)介. 內(nèi)部資料, 1-20)

A-陰山陸塊；B-孔茲巖帶；C-鄂爾多斯陸塊；D-中部造山帶；E-秦嶺造山帶

Fig.1 Simplified Early Precambrian tectonic framework of the North China Craton (a, after Zhaoetal., 2002) and geological map of the Ordos Basin and surrounding areas (b) A-Yinshan Block; B-Khondalite Belt; C-Ordos Block; D-Trans-North China Orogen; E-Qinling Orogen

3 鋯石特征和年齡

本文幾個(gè)樣品的鋯石都是碎屑鋯石。但從物源區(qū)角度，可把鋯石劃分為巖漿鋯石(MA)和變質(zhì)鋯石(ME)兩類(lèi)。巖漿鋯石具有明顯的巖漿震蕩環(huán)帶，變質(zhì)鋯石包括重結(jié)晶鋯石和變質(zhì)增生邊鋯石。在一些情況下，這兩類(lèi)鋯石不易區(qū)分。下面有關(guān)鋯石成因的論述都是相對(duì)于物源區(qū)而言的。鋯石定年結(jié)果見(jiàn)表1。

圖2 鄂爾多斯盆地鉆孔中元古代長(zhǎng)城系(變質(zhì))沉積巖巖芯照片F(xiàn)ig.2 Photographs of (meta-)sedimentary rocks of the Mesoproterozoic Changcheng System from drill cores in the Ordos Basin

圖3 鄂爾多斯盆地鉆孔中元古代長(zhǎng)城系(變質(zhì))沉積巖正交偏光鏡下巖相學(xué)照片Qz-石英;Fsp-長(zhǎng)石;Mca-云母Fig.3 Photographs under cross polarized light showing petrographic features of (meta-)sedimentary rocks of the Mesoproterozoic Changcheng System from drill cores in the Ordos BasinQz-quartz; Fsp-feldspar; Mca-mica

石英砂巖(HT1-12)的鋯石呈等軸狀或橢圓狀，陰極發(fā)光下鋯石具封閉環(huán)帶或板狀環(huán)帶， 一些鋯石發(fā)育核-邊結(jié)構(gòu)(圖4a)。對(duì)46顆鋯石進(jìn)行了47個(gè)數(shù)據(jù)點(diǎn)分析。除45.1ME外，其余數(shù)據(jù)點(diǎn)都位于諧和線上或附近(圖5a)。21個(gè)巖漿鋯石的U含量為19×10-6～211×10-6，Th/U比值為0.27～1.63，除11.1MA的年齡為～2.5Ga外，其余數(shù)據(jù)點(diǎn)的207Pb/206Pb年齡在1638～1941Ma之間。年齡小的3個(gè)巖漿鋯石數(shù)據(jù)點(diǎn)(6.1 MA、16.1 MA、38.1 MA)的加權(quán)平均207Pb/206Pb年齡為1779±29Ma (MSWD=1.03)。26個(gè)變質(zhì)鋯石的U含量為20×10-6～132×10-6, Th/U比值為0.27～1.63，207Pb/206Pb年齡在1512～2203Ma之間。

表1 鄂爾多斯盆地鉆孔中元古代長(zhǎng)城系(變質(zhì))沉積巖鋯石SHRIMP U-Pb 年齡

Table 1 SHRIMP U-Pb data for zircons from (meta-)sedimentary rocks of the Mesoproterozoic Changcheng System from drill cores in the Ordos Basin

測(cè)點(diǎn)號(hào)UTh(×10-6)Th/U206Pb*(×10-6)207Pb*206Pb*%207Pb*235U%206Pb*238U%誤差相關(guān)系數(shù)年齡(Ma)206Pb238U207Pb206Pb不諧和度(%)HTI-12石英砂巖1.1ME46440.98130.10022.94.604.10.33283.00.72185248162853-14 2.1ME59781.37140.09782.93.824.10.28312.90.70160741158254-2 3.1MA39340.90100.10024.04.065.10.29373.20.62166046162875-2 4.1MA49491.03130.10754.24.625.20.31193.00.581750461757770 5.1ME66400.62170.10652.44.463.60.30362.70.761709411741442 6.1MA93590.66270.10932.14.983.30.33082.60.78184242178738-3 7.1ME76550.75210.10482.24.633.50.32042.70.78179242171040-5 8.1MA63420.68170.10862.94.604.10.30702.80.701726431777533 9.1ME1321361.06320.09422.33.673.50.28242.70.76160338151243-6 10.1MA34270.8590.10683.44.704.80.31903.30.69178552174563-2 11.1MA861371.63350.16371.510.673.20.47302.80.882495582495250 12.1ME54330.63160.10613.04.864.20.33222.90.69184947173456-7 13.1ME45200.46130.10453.54.864.90.33703.40.69187255170665-10 14.1MA211560.27610.11901.35.492.70.33462.40.881861381941234 15.1ME67640.98190.10791.74.833.00.32492.50.83181440176431-3 16.1MA79500.65220.11162.24.953.70.32172.90.791798461826412 17.1ME46250.56140.10662.85.124.40.34903.30.76192855174352-11 18.1MA27180.7080.11473.55.214.90.32903.40.701835551876632 19.1(1)ME47380.84130.11072.94.804.20.31493.00.731765471810523 19.1(2)ME30441.53100.12021.36.482.80.39102.50.88212745196023-9 20.1ME31160.5490.10652.54.773.80.32472.90.75181345174046-4 21.1MA1451010.72370.10620.94.302.40.29372.20.921660321735174 22.1ME841091.33210.09651.33.872.70.29072.40.89164535155823-6 23.1ME32180.5690.10631.84.813.20.32782.60.82182841173833-5 24.1MA19160.8950.10242.64.153.90.29392.90.741661421668490 25.1MA40380.99100.10811.64.493.00.30102.50.831696371767304 26.1ME48440.94120.09851.83.913.00.28792.40.81163135159634-2 27.1ME20120.6360.10702.55.194.20.35203.40.80194457174946-11 28.1MA59520.92150.10411.54.092.80.28512.40.851617341699275 29.1ME50470.98150.10931.55.362.90.35552.50.86196143178827-10 30.1MA66661.04190.10591.34.832.60.33082.30.88184237173023-7 31.1MA56350.64160.10601.44.922.70.33682.40.86187138173125-8 32.1MA49290.60140.10701.74.812.90.32582.40.82181838174831-4 33.1MA59400.70180.10881.35.142.70.34252.40.88189939178024-7 34.1ME34230.69100.11221.95.373.20.34712.60.81192143183634-5 35.1MA57601.09160.11121.44.992.70.32532.40.871816371818250 36.1ME55380.73150.10631.54.693.00.32012.50.86179040173628-3 37.1ME57350.65160.11071.44.912.90.32182.50.871798401811261 38.1MA1191010.88330.10810.94.752.40.31872.20.92178434176817-1 39.1MA41360.92120.10621.65.143.10.35122.60.86194044173529-12 40.1ME33140.4590.10532.14.793.40.32992.60.77183841171939-7 41.1ME42220.56120.10782.84.953.70.33282.50.67185240176351-5 42.1ME24391.6660.09863.44.004.30.29432.80.64166341159863-4 43.1ME37290.8090.09832.03.773.20.27802.50.791581361592371 44.1MA61380.64170.10851.74.922.90.32882.40.82183338177430-3 45.1ME851321.60220.13800.85.782.50.30372.40.9517103622031422 46.1ME68330.51190.10761.34.802.80.32342.50.89180739176023-3

續(xù)表1Continued Table 1測(cè)點(diǎn)號(hào)UTh(×10-6)Th/U206Pb*(×10-6)207Pb*206Pb*%207Pb*235U%206Pb*238U%誤差相關(guān)系數(shù)年齡(Ma)206Pb238U207Pb206Pb不諧和度(%)K1-2石英砂巖1.1MA37330.91110.11043.15.234.10.34362.80.67190446180556-5 2.1MA63490.80180.11561.55.302.70.33252.20.841851361889262 3.1MA59801.40180.13812.16.923.10.36342.30.731998392203379 4.1ME1730.1750.12425.16.436.00.37603.30.54205758201790-2 5.1MA69691.04190.11112.04.843.00.31612.20.741770341817363 6.1MA1621420.91450.10841.04.772.20.31942.00.89178731177219-1 7.1MA57520.95160.10482.74.823.70.33392.50.68185741171051-9 8.1MA36270.76100.11702.75.293.70.32792.60.691828411910494 9.1ME103950.95290.11051.64.982.60.32692.10.79182333180729-1 10.1MA53430.84140.11371.94.923.00.31362.30.781759361859345 11.1MA62641.07170.11011.64.762.80.31382.20.811760351801292 12.1ME135730.56530.16270.910.222.20.45522.00.922418402484153 13.1ME60440.75170.10982.24.863.20.32072.30.721793361797400 14.1MA1141211.10310.10911.44.812.50.32012.00.831790321784250 15.1MA41391.00120.11213.15.224.00.33762.50.63187540183456-2 16.1MA67320.50270.16941.110.922.50.46702.20.892472462552193 17.1ME163760.48640.15970.810.032.10.45552.00.932420402452131 18.1MA83790.99240.11071.45.102.70.33412.30.86185837181125-3 19.1MA79911.19230.11111.45.152.60.33602.20.84186835181725-3 20.1MA48380.81200.15811.710.693.00.49002.40.81257351243529-6 21.1ME49290.61150.11441.75.513.00.34922.40.82193140187131-3 22.1MA40471.22110.11222.84.894.00.31602.90.721770451835504 23.1ME55611.15180.12542.16.403.10.36982.30.752029412035370 24.1ME120670.57330.11351.15.032.40.32142.10.891796331856203 25.1MA101850.87290.11191.55.082.50.32962.00.811836331831270 26.1MA65560.88280.16391.011.222.40.49702.20.91259946249617-4 27.1MA70630.93200.11011.45.002.60.32952.20.84183635180225-2 28.1MA42390.94120.11173.25.084.10.33012.50.61183939182758-1 29.1MA30311.0590.11822.85.674.10.34802.90.721923491930510 30.1ME45310.73130.11702.15.593.20.34632.40.751917391911380 31.1MA37300.84110.10853.05.253.90.35082.50.63193941177456-9 32.1MA51531.07150.11262.25.353.30.34482.50.76191042184239-4 33.1ME2140.1970.11664.45.945.30.37003.00.56202851190479-7 34.1ME72540.78200.11051.75.012.90.32862.30.80183137180831-1 35.1MA104850.85300.10901.94.972.80.33062.10.73184133178235-3 36.1MA66490.77190.11001.95.103.20.33612.60.80186842179935-4 37.1ME26311.26110.15732.110.313.80.47503.20.84250766242735-3 38.1MA60410.70260.16221.811.113.40.49702.90.85260162247830-5 39.1ME78470.62280.14951.68.513.00.41302.60.852228482341275 40.1MA69510.76200.11051.75.043.10.33062.60.84184142180830-2 41.1MA1311230.97360.10711.24.652.70.31512.40.89176637175023-1 42.1ME62210.34190.12281.56.132.90.36172.50.861990431998260 43.1MA1111761.63420.15021.39.102.80.44002.50.892349492348220 44.1MA901221.40270.11712.75.673.80.35112.60.70194044191249-1 45.1MA87670.81240.11322.05.073.30.32492.60.801814421852362 46.1MA931141.27250.11232.04.833.30.31212.60.791751401837365 47.1ME53410.80180.13412.17.183.50.38802.80.802115512152372 48.1MA981041.09270.10751.94.803.20.32412.60.80181040175735-3

續(xù)表1Continued Table 1測(cè)點(diǎn)號(hào)UTh(×10-6)Th/U206Pb*(×10-6)207Pb*206Pb*%207Pb*235U%206Pb*238U%誤差相關(guān)系數(shù)年齡(Ma)206Pb238U207Pb206Pb不諧和度(%)49.1ME12360.05470.15861.39.822.80.44902.50.892391502441212 50.1MA1581731.13430.10871.44.792.80.31972.40.86178937177725-1 51.1MA731011.43220.11612.35.623.50.35112.70.77194045189740-2 52.1MA75881.20220.11052.15.083.40.33312.60.79185443180837-3 53.1MA38481.32110.12042.65.844.30.35203.40.791943571963471 ZT1-9A變質(zhì)石英砂巖1.1MA4081260.321000.11580.54.562.30.28582.20.971620311893914 2.1ME519450.091130.11670.74.062.30.25212.20.9514492819071324 3.1MA106850.83410.16250.99.912.90.44202.70.952362542481165 4.1ME7913080.401430.10291.22.942.50.20742.20.8812152516762228 4.2ME5298861.73870.10821.12.842.80.19032.60.9211232617702037 5.1ME7535910.811060.11460.92.584.80.16354.80.989764318741648 5.2ME277821170.791260.08931.70.642.80.05192.20.78326.4714103377 6.1ME12053690.322150.10182.12.913.20.20722.40.7412142616584027 7.1ME14004340.321410.08691.71.382.80.11482.20.797011513583348 7.2MA7386320.881030.10751.32.392.60.16142.20.869642017572445 8.1ME3271320.42760.11780.84.372.40.26952.20.9415383019221420 9.1MA4314731.131260.11300.85.272.40.33852.20.95188036184814-2 10.1MA2181180.56630.11400.95.302.90.33692.70.951872451865160 11.1MA2041360.69520.11691.04.792.50.29702.30.9216763419091812 12.1MA7233800.541110.10860.92.682.40.17862.20.9210592217771740 13.1MA60601.03240.16271.510.563.00.47102.60.872486532484250 14.1ME10752180.211380.09833.32.004.00.14782.20.558891815916244 15.1MA65711.14180.10053.84.465.90.32204.50.77180071163270-10 16.1ME45220.51140.10205.65.096.90.36204.10.591993701659100-20 17.1MA108750.72310.10503.34.734.90.32703.60.74182357171360-6 18.1ME95550.60270.10262.84.664.50.33003.50.78183756167151-10 19.1MA2851550.56880.15460.97.613.40.35703.30.9719675623981518 20.1ME58681.22180.11282.55.595.20.36004.60.88198278184445-7 21.1ME7221050.151490.10880.93.593.40.23903.30.9713824117801622 22.1MA4082560.651020.12051.04.803.40.28853.30.9616344719641817 23.1MA5432060.391110.10721.33.473.70.23473.40.9313594217512422 24.1ME62440.74170.10314.04.595.40.32303.70.68180358168074-7 25.1ME6912330.351240.10881.33.093.50.20613.30.9312083617792432 26.1MA4115051.27870.11221.33.793.60.24533.30.9314144218352423 27.1ME94860.95380.14702.59.484.30.46803.50.82247572231142-7 28.1ME9262730.301480.09641.62.433.60.18303.30.9010843315562930 29.1MA5081700.351130.11151.13.973.50.25803.30.9414804418242119 30.1ME7931190.151610.10391.23.363.50.23453.30.9413584016952120 31.1MA4583040.691070.15631.05.833.40.27063.30.9515444524161736 32.1MA2492190.91490.11541.53.653.70.22963.40.9113324018862829 33.1ME11303410.311700.09241.82.193.70.17203.30.8810233114753331 34.1MA3801200.33910.11301.34.343.50.27873.30.9315854618482414 35.1MA4323100.741070.14901.35.863.60.28533.40.9316184823342331 36.1ME400320.08940.11091.54.163.60.27243.30.9215534618142714 37.1MA3822740.74740.11242.03.483.90.22473.40.8613074018383629 38.1ME8092760.351450.10071.92.853.80.20533.30.8712043716363526 39.1ME2911070.38660.11721.64.273.70.26393.30.9015104419152921 40.1ME6982870.431130.10871.72.803.70.18663.30.8911033417773138

續(xù)表1Continued Table 1測(cè)點(diǎn)號(hào)UTh(×10-6)Th/U206Pb*(×10-6)207Pb*206Pb*%207Pb*235U%206Pb*238U%誤差相關(guān)系數(shù)年齡(Ma)206Pb238U207Pb206Pb不諧和度(%)41.1ME 5732130.381220.11121.13.773.40.24613.30.9514184218181922ZT1-9B變質(zhì)砂巖1.1ME57350.64160.11142.65.083.60.33062.50.69184140182247-1 2.1MA64520.84180.10791.94.943.10.33222.40.78184939176435-5 3.1ME1692031.24460.15900.96.992.40.31902.20.9317853524451527 4.1ME51320.66130.11641.84.624.00.28803.50.8916325119013314 5.1MA88217842.09520.10651.01.002.40.06832.20.91423917411876 6.1MA3301330.421060.12471.26.422.60.37302.30.88204340202521-1 7.1MA6187991.34990.10721.12.742.70.18532.40.9210962517521937 8.1MA5618641.59690.11271.02.232.70.14362.50.938652018441853

注：(1)*代表放射成因Pb，測(cè)定204Pb值作普通鉛校正；(2)計(jì)算不諧和度的公式：D=[1-(206Pb/238U)年齡/(207Pb/206Pb)年齡] ×100；(3)MA代表巖漿鋯石，ME代表變質(zhì)鋯石

圖4 鄂爾多斯盆地鉆孔中元古代長(zhǎng)城系(變質(zhì))沉積巖的鋯石陰極發(fā)光圖像MA代表巖漿鋯石，ME代表變質(zhì)鋯石，均指其物源區(qū)的成因；圖5同F(xiàn)ig.4 Cathodoluminescence images of zircons from (meta-)sedimentary rocks of the Mesoproterozoic Changcheng System from drill cores in the Ordos BasinMA and ME mean magmatic and metamorphic zircons respectively, which refer to their origins in source region; also means in Fig.5

圖5 鄂爾多斯盆地鉆孔中元古代長(zhǎng)城系(變質(zhì))沉積巖的鋯石U-Pb諧和圖Fig.5 Concordia diagrams of zircon data for (meta-)sedimentary rocks of the Mesoproterozoic Changcheng System from drill cores in the Ordos Basin

石英砂巖(K1-2)的鋯石呈等軸狀或橢圓狀，個(gè)別鋯石具自形形態(tài)，陰極發(fā)光下具封閉環(huán)帶或板狀環(huán)帶，一些鋯石遭受不同程度重結(jié)晶改造(圖4b)。對(duì)53顆鋯石進(jìn)行了53個(gè)數(shù)據(jù)點(diǎn)分析，數(shù)據(jù)點(diǎn)都位于諧和線上或附近(圖5b)。37個(gè)巖漿鋯石的U含量為30×10-6～162×10-6，Th/U比值為0.50～1.63，207Pb/206Pb年齡在1774～2552Ma之間。16個(gè)變質(zhì)鋯石的U含量為17×10-6～163×10-6，Th/U比值為0.05～1.26，207Pb/206Pb年齡在1797～2484Ma之間。所有數(shù)據(jù)點(diǎn)均位于諧和線上或附近，可劃分為>2.3Ga和<2.3Ga兩個(gè)年齡段。

變質(zhì)石英砂巖(ZT1-9A)的鋯石呈渾圓狀或柱狀，陰極發(fā)光下鋯石具封閉環(huán)帶，部分具板狀環(huán)帶，許多鋯石都遭受重結(jié)晶改造，部分鋯石發(fā)育變質(zhì)增生邊(圖4c)。對(duì)41顆鋯石進(jìn)行了44個(gè)數(shù)據(jù)點(diǎn)分析。20個(gè)巖漿鋯石的U含量為60×10-6～768×10-6, Th/U比值為0.33～1.14，207Pb/206Pb年齡在1632～2484 Ma之間。24個(gè)變質(zhì)鋯石的U含量為45×10-6～2778×10-6，Th/U比值為0.09～1.73，207Pb/206Pb年齡在1358～1922Ma之間(圖5c)。許多鋯石存在不同程度鉛丟失，與鋯石U含量普遍較高有關(guān)。許多數(shù)據(jù)點(diǎn)分布于上交點(diǎn)年齡分別為～2.5Ga和～2.0Ga的不一致線上。

變質(zhì)砂巖(ZT1-9B)僅分選出很少鋯石顆粒。鋯石呈渾圓狀或柱狀，陰極發(fā)光下鋯石具封閉環(huán)帶或板狀環(huán)帶結(jié)構(gòu)，一些鋯石普遍遭受不同程度的重結(jié)晶改造，存在變質(zhì)增生邊(圖4d)。對(duì)8顆鋯石進(jìn)行了8個(gè)數(shù)據(jù)點(diǎn)分析。5個(gè)巖漿鋯石的U含量為64×10-6～882×10-6, Th/U比值為0.42～2.09，207Pb/206Pb年齡在1752～2025Ma之間。3個(gè)變質(zhì)鋯石的U含量為51×10-6～169×10-6, Th/U比值為0.64～1.24，207Pb/206Pb年齡在1822～2445 Ma之間。一些鋯石顯示強(qiáng)烈的鉛丟失， 數(shù)據(jù)點(diǎn)大致位于上交點(diǎn)年齡為～2.0Ga的不一致線上(圖5d)。

表2 鄂爾多斯盆地鉆孔中元古代長(zhǎng)城系(變質(zhì))沉積巖的鋯石Hf同位素組成

Table 2 Hf isotope compositions of zircons from (meta-)sedimentary rocks of the Mesoproterozoic Changcheng System from drill cores in the Ordos Basin

測(cè)點(diǎn)號(hào)年齡(Ma)176Yb177Hf176Lu177Hf176Hf177Hf2σεHf(0)εHf(t)2σtDM1(Ma)2σtDM2(Ma)2σfLu/HfHT1-12石英砂巖1.1ME16280.03240.00070.2814960.000022-45.13-9.750.78243960293597-0.982.1ME15820.04020.00090.2815960.000022-41.59-7.400.78231460275597-0.973.1MA16280.03060.00070.2816630.000023-39.21-3.770.812209622565100-0.984.1MA17570.03020.00070.2815020.000018-44.90-6.610.63242848283977-0.985.1ME17410.02750.00060.2815160.000022-44.43-6.410.77240559281495-0.986.1MA17870.03030.00070.2815700.000021-42.50-3.510.75233558267093-0.987.1ME17100.03820.00080.2815890.000018-41.85-4.740.62231948268877-0.988.1MA17770.03200.00070.2815740.000019-42.38-3.640.69233253267185-0.989.1ME15120.03220.00070.2815700.000020-42.50-9.650.72233755284189-0.9810.1MA17450.04000.00090.2815120.000020-44.57-6.750.72242655283889-0.9711.1MA24950.04550.00090.2813030.000019-51.942.400.67271751285082-0.9712.1ME17340.01950.00040.2816240.000020-40.59-2.510.72224855256889-0.9913.1ME17060.01730.00040.2815990.000019-41.47-3.960.68227952263784-0.9914.1MA19410.03780.00080.2816610.000016-39.302.940.55222043239069-0.9815.1ME17640.03570.00080.2815140.000021-44.49-6.150.74241857281691-0.9816.1MA18260.02580.00050.2815860.000025-41.96-1.960.902307692604111-0.9817.1ME17430.02430.00050.2815870.000020-41.92-3.750.70230454265287-0.9818.1MA18760.01980.00040.2814910.000022-45.31-4.070.78242959277396-0.9920.1ME17400.03340.00070.2815750.000020-42.32-4.420.70233054269187-0.9821.1MA17350.02270.00050.2815870.000020-41.91-3.860.70230153265286-0.9922.1ME15580.06420.00130.2815680.000025-42.57-9.350.882378682857108-0.9623.1ME17380.02160.00050.2816020.000021-41.36-3.230.76227958261694-0.9924.1MA16680.02080.00040.2815960.000021-41.58-4.990.76228658267194-0.99K1-2石英砂巖1.1MA18050.02120.00050.2815900.000019-41.79-2.150.68229552260084-0.992.1MA18890.05210.00110.2815830.000020-42.06-1.370.72234556261689-0.973.1MA22030.05550.00120.2814480.000028-46.830.571.002540782738123-0.964.1ME20170.02720.00060.2817110.000024-37.516.700.842140642216104-0.985.1MA18170.01590.00040.2815620.000017-42.81-2.780.60232845264874-0.996.1MA17720.02970.00060.2815580.000019-42.94-4.270.68235152270684-0.987.1MA17100.03190.00070.2815780.000019-42.23-4.990.67232651270382-0.988.1MA19100.01960.00040.2816140.000021-40.961.070.73226256248190-0.999.1ME18070.03410.00070.2816590.000019-39.350.020.68221753246785-0.9810.1MA18590.02840.00060.2815160.000019-44.41-3.790.67240752274383-0.9811.1MA18010.02500.00050.2816050.000019-41.26-1.810.67228051257683-0.9812.1ME24840.02850.00060.2813370.000021-50.753.880.76265058275293-0.9813.1ME17970.01820.00040.2815030.000019-44.87-5.380.67241151279383-0.9914.1MA17840.02990.00060.2815930.000018-41.68-2.740.63230349262178-0.9815.1MA18340.03030.00070.2816500.000020-39.680.370.72222655246790-0.98ZT1-9A變質(zhì)石英砂巖1.1MA18930.05120.00100.2816290.000020-40.410.530.71227455250288-0.972.1ME19070.03280.00070.2816520.000018-39.621.990.66222750242281-0.983.1MA24810.03180.00080.2813450.000024-50.463.890.842647642748103-0.984.2ME17700.14210.00260.2818400.000027-32.953.430.972069782227120-0.925.1ME18740.44500.00790.2820140.000032-26.805.081.1221281062205139-0.765.2ME14100.19410.00420.2817840.000028-34.92-7.621.002250852637124-0.876.1ME16580.08280.00170.2817270.000023-36.97-1.980.812180642477101-0.957.1ME13580.11430.00320.2817060.000022-37.69-10.520.78230364277896-0.907.2MA17570.05100.00120.2816400.000023-40.03-2.300.812269632573101-0.97

續(xù)表2

Continued Table 2

圖6 鄂爾多斯盆地鉆孔中元古代長(zhǎng)城系(變質(zhì))沉積巖的鋯石年齡直方圖Fig.6 Zircon age histograms for (meta-)sedimentary rocks of the Mesoproterozoic Changcheng System from drill cores in the Ordos Basin

圖7 鄂爾多斯盆地鉆孔中元古代長(zhǎng)城系(變質(zhì))沉積巖鋯石年齡-εHf圖解Fig.7 Zircon εHf(t) vs. U-Pb ages of Mesoproterozoic Changcheng System (meta-)sedimentary rocks from drill cores in the Ordos Basin

對(duì)4個(gè)樣品鋯石的207Pb/206Pb年齡進(jìn)行統(tǒng)計(jì)，年齡峰值為～1.8 Ga，在～2.5 Ga還有1個(gè)小的年齡峰值(圖6)。

4 鋯石Hf同位素特征

對(duì)62個(gè)碎屑鋯石進(jìn)行了Hf同位素分析，分析點(diǎn)位置與SHRIMP U-Pb定年位置相同或結(jié)構(gòu)類(lèi)似。Hf同位素分析數(shù)據(jù)見(jiàn)表2和圖7。35個(gè)巖漿鋯石的εHf(t)和tDM2(Hf)分別為-6.8～+5.2和2390～2850Ma，27個(gè)變質(zhì)鋯石的εHf(t)和tDM2(Hf)分別為-10.5～+6.7和2205～2935Ma。

5 討論

5.1 鋯石成因

本文定年樣品包括了2個(gè)變質(zhì)沉積巖，但變質(zhì)程度僅為低綠片巖相，不會(huì)對(duì)鋯石內(nèi)部結(jié)構(gòu)產(chǎn)生明顯影響，碎屑鋯石的內(nèi)部結(jié)構(gòu)都應(yīng)是物源區(qū)鋯石的特征。碎屑鋯石外部形態(tài)和內(nèi)部結(jié)構(gòu)特征可提供外生作用的重要信息。由于變質(zhì)作用通?？墒逛喪驁A的形態(tài)發(fā)生變化，以達(dá)到自由能最小。對(duì)于碎屑沉積巖中的變質(zhì)鋯石，其等軸狀和橢圓狀外形(如圖4a中的顆粒29，圖4b中的顆粒42)，不能作為其遭受強(qiáng)烈外生磨蝕的確切依據(jù)，巖漿鋯石的外形與內(nèi)部結(jié)構(gòu)的不協(xié)調(diào)切割關(guān)系(如圖4a中的顆粒3、11和38，圖4b中的顆粒29和39，圖4c中的顆粒9)則是外生磨蝕作用的結(jié)果(Wanetal., 2011)。對(duì)于具變質(zhì)增生邊或邊部發(fā)生重結(jié)晶的鋯石，鋯石內(nèi)部結(jié)構(gòu)與外形的不協(xié)調(diào)切割關(guān)系(圖4a中的顆粒20，圖4d中的顆粒1和2)，也是碎屑鋯石的典型特征。與其他樣品相比，石英砂巖樣品(HT1-12)膠結(jié)物最少，石英磨圓度最高，鋯石的磨圓度也最高。不過(guò)，這種強(qiáng)烈分選和磨蝕并不一定表明碎屑物質(zhì)經(jīng)過(guò)長(zhǎng)距離搬運(yùn)，也可能是在海岸線附近長(zhǎng)期海浪作用的結(jié)果。其余3個(gè)樣品(K1-2、ZT1-9A和ZT1-9B)中，一些鋯石呈長(zhǎng)柱狀或具自形形態(tài)(如圖4b中的顆粒36，圖4c中的顆粒9，圖4d中的顆粒6和7)，表明至少部分碎屑物質(zhì)來(lái)自近源區(qū)，并可能以巖屑形式搬運(yùn)沉積?？傮w上，與具巖漿結(jié)構(gòu)的碎屑鋯石相比，具變質(zhì)結(jié)構(gòu)的碎屑鋯石的Th/U比值變化更大，一些變質(zhì)鋯石的Th/U比值很低。

5.2 長(zhǎng)城系變質(zhì)作用

在華北克拉通東部，長(zhǎng)城系通常未遭受變質(zhì)，但原巖沉積時(shí)代可與長(zhǎng)城系對(duì)比的河北省平山縣東南的東焦群(主要由碎屑沉積巖組成)遭受了淺變質(zhì)作用(Wanetal., 2010)。在鄂爾多斯，長(zhǎng)城系未遭受變質(zhì)或遭受淺變質(zhì)(公王斌等，2016)。根據(jù)以往認(rèn)識(shí)，長(zhǎng)城系僅存在于鄂爾多斯盆地中南部，而在東北部缺失(公王斌等，2016)。但是，取自榆林西北部同一鉆孔相鄰的2個(gè)樣品(ZT1-9A、ZT1-9B)都為變形弱的淺變質(zhì)沉積巖，而根據(jù)鋯石結(jié)構(gòu)和年齡，它們不是早前寒武紀(jì)變質(zhì)基底巖石，而很可能屬于長(zhǎng)城系。因此長(zhǎng)城系的分布范圍應(yīng)比以往認(rèn)為的更廣，向東北至少到了召探1井的位置。長(zhǎng)城系沉積巖遭受變質(zhì)的時(shí)代還不清楚，但許多鋯石的分析數(shù)據(jù)點(diǎn)沿不一致線分布，下交點(diǎn)年齡為200～400Ma(公王斌等，2016；本文)，需進(jìn)一步研究來(lái)確定變質(zhì)作用是否發(fā)生在古生代-中生代。

5.3 長(zhǎng)城系底界年齡

中-新元古代蓋層是華北克拉通早前寒武紀(jì)變質(zhì)基底穩(wěn)定化之后的重要蓋層。長(zhǎng)城系的下限年齡有1.95Ga、1.8Ga和1.7Ga等不同認(rèn)識(shí)(王曰倫等，1980；陸松年和李惠民，1991；李懷坤等，1995；鐘富道，1997)。近年來(lái)，隨著對(duì)斑脫巖等的鋯石定年工作的開(kāi)展，華北克拉通東部中元古代蓋層年代學(xué)研究取得了重大進(jìn)展，確定長(zhǎng)城系底界年齡為1650～1680Ma(和政軍等, 2011; 李懷坤等，2011)。在對(duì)鄂爾多斯鉆孔長(zhǎng)城系碎屑沉積巖碎屑鋯石定年時(shí)，公王斌等(2016)獲得3個(gè)～1.6Ga的年齡數(shù)據(jù)，數(shù)據(jù)點(diǎn)位于諧和線上或附近。本次研究獲得更多位于諧和線及其附近的1.6～1.7Ga的碎屑鋯石年齡數(shù)據(jù)。盡管數(shù)據(jù)誤差較大及后期鉛丟失影響，最年輕碎屑鋯石年齡不可能明顯大于1.7Ga，支持了長(zhǎng)城系底界沉積時(shí)代小于1.7Ga的認(rèn)識(shí)，推測(cè)華北克拉通西部和東部長(zhǎng)城系沉積時(shí)代大致相同。鄂爾多斯盆地的中元古代長(zhǎng)城系主要由礫巖、砂巖、粉砂巖和泥質(zhì)巖組成，薊縣系主要由碳酸鹽巖和泥質(zhì)巖組成(中國(guó)長(zhǎng)慶油田，2011；公王斌等，2016；郭艷琴等，2019)，華北克拉通東部燕遼坳拉槽的中元古代長(zhǎng)城系和薊縣系也主要由這些類(lèi)型巖石組成(李懷坤等，2014及其中參考文獻(xiàn))。兩者巖石組合十分類(lèi)似，可以相互對(duì)比，都反映了陸相向?yàn)I海相，進(jìn)而向海相的海侵演化進(jìn)程。

5.4 物質(zhì)來(lái)源

4個(gè)樣品中都存在巖漿結(jié)構(gòu)和變質(zhì)結(jié)構(gòu)的碎屑鋯石，鋯石年齡峰值分別為～1.8Ga和～2.5Ga，以前者為主。物源區(qū)為以古元古代晚期巖漿構(gòu)造熱事件為主的早前寒武紀(jì)變質(zhì)基底。一些樣品中存在具波狀消光的細(xì)粒石英集合體表明它們來(lái)自變質(zhì)巖區(qū)。鄂爾多斯北部為古元古代孔茲巖帶(Zhaoetal., 2005)，2.0～2.3Ga巖漿作用和1.82～1.96Ga變質(zhì)作用發(fā)育(Wanetal., 2009; Dongetal., 2013; Liuetal., 2013;鐘長(zhǎng)汀等, 2014; 董春艷等，未發(fā)表資料)。鄂爾多斯盆地本身也廣泛存在古元古代晚期巖漿構(gòu)造熱事件(Huetal., 2013; Wanetal., 2013; Zhangetal., 2015)，都可以是長(zhǎng)城系碎屑沉積物的物源區(qū)。首次獲得的鄂爾多斯盆地長(zhǎng)城系(變質(zhì))沉積巖碎屑鋯石Hf同位素組成也支持這一認(rèn)識(shí)。鋯石的Hf同位素組成存在很大變化(εHf(t)=-10.5～+6.7)，具巖漿結(jié)構(gòu)的碎屑鋯石的εHf(t)值從-6.8 到+5.2，表明碎屑沉積物源區(qū)存在不同時(shí)期地幔添加或殼幔物質(zhì)混合作用。碎屑鋯石的內(nèi)部結(jié)構(gòu)、年齡譜和Hf同位素組成特征與孔茲巖帶和鄂爾多斯變質(zhì)基底的鋯石類(lèi)似(Dongetal., 2013; Huetal., 2013; Liuetal., 2013; Wanetal., 2013; Zhangetal., 2015)。長(zhǎng)城系碎屑沉積巖中存在少量～2.5Ga碎屑鋯石，也與變質(zhì)基底中新太古代晚期地質(zhì)體零星分布相一致。總體上，鄂爾多斯盆地長(zhǎng)城系與華北克拉通東部地區(qū)長(zhǎng)城系的沉積巖巖石組合和碎屑鋯石年齡譜及Hf同位素組成顯示了類(lèi)似的特征(Wanetal., 2011)，但是，一些樣品中含有更年輕的碎屑鋯石(王公斌等，2016)，表明不同物源區(qū)的存在。內(nèi)部結(jié)構(gòu)和年齡分布特征表明碎屑鋯石來(lái)自華北克拉通本身，支持了長(zhǎng)城系蓋層形成于大陸環(huán)境(陸內(nèi)裂谷、拗拉槽等)的認(rèn)識(shí)(馮娟萍等, 2018; 包洪平等，2019)。古元古代晚期變質(zhì)鋯石和巖漿鋯石在華北克拉通中元古代蓋層中以碎屑形式大量存在，與華北克拉通早前寒武紀(jì)變質(zhì)基底發(fā)育古元古代中-晚期構(gòu)造巖漿熱事件相吻合(Wanetal., 2006; 魏春景, 2018)。古元古代晚期克拉通化導(dǎo)致華北克拉通進(jìn)入穩(wěn)定階段，接受蓋層沉積。

6 結(jié)論

(1)鄂爾多斯盆地長(zhǎng)城系碎屑沉積巖的最年輕碎屑鋯石年齡為～1.7Ga，支持了長(zhǎng)城系底界時(shí)間小于1.7Ga的認(rèn)識(shí)。

(2)鄂爾多斯盆地長(zhǎng)城系與華北克拉通東部地區(qū)長(zhǎng)城系十分類(lèi)似，它們形成于類(lèi)似的構(gòu)造環(huán)境，長(zhǎng)城系碎屑沉積物來(lái)自其北部古元古代孔茲巖系和/或鄂爾多斯變質(zhì)基底本身。

(3)鄂爾多斯盆地長(zhǎng)城系碎屑沉積巖的碎屑鋯石年齡峰值為～1.8Ga和～2.5Ga，與華北克拉通變質(zhì)基底構(gòu)造熱事件的時(shí)間基本一致。具巖漿結(jié)構(gòu)鋯石的εHf(t)值變化范圍為-6.8到+5.2，表明物源區(qū)存在不同時(shí)期的地幔添加或殼幔物質(zhì)混合作用。

致謝 研究得到中國(guó)石油長(zhǎng)慶油田公司的大力支持。SHRIMP鋯石定年得到張玉海高級(jí)工程師和楊之青高級(jí)工程師的幫助；鋯石樣品靶由楊淳高級(jí)工程師制作；鋯石陰極發(fā)光圖像由李寧和周麗芹工程師完成；鋯石標(biāo)準(zhǔn)由Ian Williams和Lance Black提供；胡健民研究員、張成立教授、第五春榮教授和張健教授審閱了稿件，對(duì)提高論文質(zhì)量起了重要作用。在此一并深表謝意。