蔡芃睿 王濤 王宗起 李龍明 王銘乾 賈建亮 吳彥佳

1. 中國(guó)地質(zhì)科學(xué)院地質(zhì)研究所，北京 1000372. 中國(guó)地質(zhì)科學(xué)院礦產(chǎn)資源研究所，北京 1000373. 合肥工業(yè)大學(xué)資源與環(huán)境工程學(xué)院，合肥 230009

中亞造山帶(CAOB)夾持在華北板塊和西伯利亞板塊之間，地處東北亞大陸東緣濱太平洋構(gòu)造域溝-弧-盆體系形成演化的重要位置，不但記錄了華北和西伯利亞兩大古板塊之間古亞洲洋的閉合過(guò)程，而且記錄了環(huán)太平洋構(gòu)造域與古亞洲洋構(gòu)造域的疊加與轉(zhuǎn)換過(guò)程。作為全球顯生宙大陸地殼生長(zhǎng)最為顯著的增生造山帶之一，一直受到學(xué)者們的廣泛關(guān)注(eng?retal., 1993；Xiaoetal., 2003, 2009; Xuetal., 2013; Eizenh?feretal., 2015; Liuetal., 2017)。它經(jīng)歷了一個(gè)復(fù)雜的弧-陸碰撞過(guò)程，以發(fā)育許多殘留的微大陸或地殼塊體、蛇綠巖帶、藍(lán)片巖帶、構(gòu)造相關(guān)的巖漿弧和弧前或弧后盆地為特征。關(guān)于古亞洲洋的閉合時(shí)限，部分學(xué)者認(rèn)為古亞洲洋東段洋殼在早古生代發(fā)生雙向俯沖，并在泥盆紀(jì)-早石炭世完全閉合形成興蒙造山帶(Xuetal., 2013; Tongetal., 2015; Zhaoetal., 2017; 邵濟(jì)安等，2017)；而大部分學(xué)者認(rèn)為古亞洲洋東部的雙向俯沖啟動(dòng)于早古生代，期間發(fā)生一系列島弧或微陸塊的拼合，俯沖一直持續(xù)到晚古生代末，并在二疊紀(jì)樂(lè)平世到早三疊世發(fā)生閉合(Li, 2006; Miaoetal., 2008; Wuetal., 2011; Eizenh?feretal., 2015)。

陸源碎屑巖中包含大量的源區(qū)信息，通過(guò)研究盆地內(nèi)砂巖的碎屑組成、重礦物組合、及U-Pb鋯石年代學(xué)可以系統(tǒng)地研究物源區(qū)特征及沉積盆地屬性及演化歷史，并可據(jù)此反演盆-山之間的耦合關(guān)系(閆臻等，2007)。鋯石的U-Th-Pb同位素體系具有較高的封閉溫度,以及較強(qiáng)的抗風(fēng)化和抗干擾性，它通?？梢员４嬖磪^(qū)原巖的年齡記錄。利用碎屑鋯石進(jìn)行物源分析，對(duì)了解盆地演化過(guò)程、與造山帶相對(duì)位置及相互作用方面意義重大，已成為區(qū)域大地構(gòu)造研究的重要手段(Wanetal., 2005)。

弧前盆地發(fā)育于匯聚板塊邊界，位于火山弧和增生楔之間，是俯沖帶溝-弧體系中重要組成部分?；∏芭璧爻练e作用與島弧火山作用、巖漿作用及俯沖雜巖的變質(zhì)變形作用是同時(shí)發(fā)生的(閆臻等，2008；李繼亮等，2013)，主要的沉積物為與島弧相關(guān)的碎屑沉積，通常情況下其主要組分為斜長(zhǎng)石和火山巖巖屑，為長(zhǎng)石巖屑砂巖-巖屑長(zhǎng)石砂巖組合(Dickinson, 1974; Dickinson and Valloni, 1980)，其重礦物組合以中酸性及中基性火成巖中的副礦物為主，砂巖結(jié)構(gòu)和成分成熟度都較低，體現(xiàn)出近源沉積的特征。

樂(lè)平世-早三疊世在東北地區(qū)的區(qū)域演化史上具有相當(dāng)重要的研究?jī)r(jià)值，它是承上啟下的關(guān)鍵地質(zhì)時(shí)期，被認(rèn)為是古亞洲洋構(gòu)造域向古太平洋構(gòu)造域轉(zhuǎn)換的關(guān)鍵時(shí)期(Zhouetal., 2009; Wuetal., 2011)。三疊系在東北地區(qū)出露較為局限，僅在索倫、扎魯特旗、龍江、嫩江等地出露，故目前對(duì)該時(shí)期地層的研究較為局限。因此本文從樂(lè)平統(tǒng)林西組及下三疊統(tǒng)老龍頭組重礦物組成及碎屑鋯石U-Pb年代學(xué)入手，試對(duì)研究區(qū)樂(lè)平世-早三疊世沉積地層物源信息、賀根山洋的最終關(guān)閉時(shí)間等問(wèn)題進(jìn)行探討。

1 地質(zhì)概況及樣品描述

研究區(qū)位于黑龍江省西部龍江縣孫家溝村附近，地處大興安嶺中段，大地構(gòu)造位置介于備受爭(zhēng)議的黑河-賀根山縫合帶與西拉木倫縫合帶之間(圖1a)，古亞洲洋洋殼于晚古生代-早中生代在該地區(qū)最終消亡，故研究區(qū)地層勢(shì)必保存了該洋盆地不同演化時(shí)期的沉積記錄(趙英利等，2016)。

研究區(qū)出露地層包括二疊系、三疊系及侏羅系。其中二疊系出露較為局限，僅在研究區(qū)孫家溝及架子山出露樂(lè)平統(tǒng)林西組(圖1b)，而下三疊統(tǒng)老龍頭組則在研究區(qū)廣泛出露。為了研究林西組及老龍頭組地層的沉積、構(gòu)造特征，故對(duì)龍江地區(qū)孫家溝村西山進(jìn)行了剖面實(shí)測(cè)(圖2)，其中林西組真實(shí)厚度>300m，老龍頭組真實(shí)厚度>100m。林西組主要為一套灰黑色及灰綠色泥巖、粉砂巖、砂巖組成的碎屑沉積巖, 含豐富的淡水瓣鰓類(lèi)和植物化石(圖3a, b)，水平層理發(fā)育，指示當(dāng)時(shí)的沉積環(huán)境為陸相還原環(huán)境。老龍頭組以紫紅色河、湖相沉積為特征，巖性主要為灰綠色砂巖、紫紅色砂礫巖、砂巖及雜色礫巖，平行層理、板狀交錯(cuò)層理、楔狀交錯(cuò)層理、沖刷面等沉積構(gòu)造發(fā)育(圖3c, d)，指示當(dāng)時(shí)沉積環(huán)境為陸相氧化環(huán)境。林西組與老龍頭組地層巖性連續(xù)且遞變，界面上下產(chǎn)狀基本一致，故認(rèn)為兩套地層整合接觸。

樂(lè)平世砂巖18SJG-06為中粒長(zhǎng)石巖屑砂巖(圖3e)，碎屑顆粒分選中等，磨圓較差，多為棱角狀-次棱角狀，顆粒支撐，接觸式膠結(jié)， 顆粒成分主要由石英、 長(zhǎng)石、巖屑及少量綠簾石及褐鐵礦，石英含量35%～45%，主要為單晶石英。長(zhǎng)石含量較少約5%～15%，為斜長(zhǎng)石，具聚片雙晶，未見(jiàn)鉀長(zhǎng)石。巖屑含量為20%～35%，且成分復(fù)雜，主要為火山巖巖屑，含少量沉積巖巖屑及變質(zhì)巖巖屑。

圖1 研究區(qū)地質(zhì)構(gòu)造簡(jiǎn)圖(據(jù)黑龍江省革命委員會(huì)地質(zhì)局區(qū)域地質(zhì)測(cè)量隊(duì)，1972(1)黑龍江省革命委員會(huì)地質(zhì)局區(qū)域地質(zhì)測(cè)量隊(duì).1972.中華人民共和國(guó)地質(zhì)圖(1:20萬(wàn)扎賚特旗幅)；黑龍江省地質(zhì)區(qū)域地質(zhì)測(cè)量第二隊(duì)，1976(2)黑龍江省地質(zhì)區(qū)域地質(zhì)測(cè)量第二隊(duì).1976.中華人民共和國(guó)地質(zhì)圖(1:20萬(wàn)華安公社幅))

Fig.1 Simplified geological map of the study area

圖2 龍江地區(qū)孫家溝村樂(lè)平統(tǒng)林西組-下三疊統(tǒng)老龍頭組實(shí)測(cè)剖面圖Fig.2 Measured geological section of the Upper Permian Linxi Formation and the Lower Triassic Laolongtou Formation in Sunjiagoucun Village, Longjiang area

圖3 林西組及老龍頭組砂巖野外和鏡下照片(a)林西組植物化石；(b)林西組瓣鰓類(lèi)化石；(c)老龍頭組砂巖中楔狀交錯(cuò)層理；(d)老龍頭組砂巖中沖刷構(gòu)造；(e)林西組砂巖鏡下照片(+)；(f)老龍頭組砂巖鏡下照片(+). Qtz-石英；Pl-斜長(zhǎng)石；Ffs-鉀長(zhǎng)石；Ser-絹云母Fig.3 Field and microscopic photographs of sandstone in Linxi and Laolongtou formations(a) plant fossils in Linxi Formation; (b) Lamelli branchia fossils in Linxi Formation; (c) wedge-shaped cross bedding in sandstone of Laolongtou Formation; (d) scour structure in sandstone of Laolongtou Formation; (e) microscopic photo of sandstone of Linxi Formation (+); (f) microscopic photo of sandstone of Laolongtou Formation(+). Qtz-quartz; Pl-plagioclase; Kfs-K-feldspar; Ser-sericite

三疊紀(jì)砂巖18SJG-09、18SJG-16和18SJG-24為中-粗粒長(zhǎng)石巖屑砂巖(圖3f)，碎屑顆粒分選較差，磨圓差，多為棱角狀-次棱角狀，顆粒支撐，接觸式膠結(jié)，顆粒成分主要由石英、長(zhǎng)石、巖屑及少量綠簾石及褐鐵礦，以及少量長(zhǎng)石蝕變后的絹云母。石英含量20%～35%，主要為單晶石英。長(zhǎng)石含量較少約10%～20%，主要為斜長(zhǎng)石，具聚片雙晶，見(jiàn)少量具卡斯巴雙晶的泥化鉀長(zhǎng)石。巖屑含量較高為30%～40%，成分復(fù)雜，主要為火山巖巖屑，含少量沉積巖巖屑及變質(zhì)巖巖屑。云母類(lèi)礦物含量<5%，多為絹云母。

圖4 龍江地區(qū)林西組及老龍頭組砂巖部分碎屑鋯石CL圖像及U-Pb年齡Fig.4 Representative CL images and U-Pb ages of sandstone detrital zircons from Linxi Formation and Laolongtou Formation in Longjiang area

2 分析方法

重礦物分選由廊坊市科大巖石礦物分選技術(shù)服務(wù)有限公司完成，具體過(guò)程如下：(1)經(jīng)無(wú)污染粉碎至10目(最大巖塊粒徑約2mm),利用標(biāo)準(zhǔn)干篩篩選40～80目,約0.45～0.18mm粒級(jí)(近乎大于砂巖分析樣品的最小平均粒度),以保證分離出的重礦物為碎屑成因顆粒;(2)通過(guò)重液、精淘分離和電磁分離出電磁、無(wú)磁和強(qiáng)磁三部分重礦物;(3)通過(guò)雙目鏡鑒定分離出不同的重礦物;(4)采用高精度天平完成計(jì)量,最終通過(guò)重礦物密度(平均)值(Deeretal., 1992)將質(zhì)量百分比換算為體積百分比。

鋯石U-Pb定年分析在北京燕都中實(shí)測(cè)試技術(shù)有限公司進(jìn)行，本次測(cè)試鋯石微量元素含量和U-Pb同位素定年利用LA-ICP-MS同時(shí)分析完成。激光剝蝕系統(tǒng)為New Wave UP213，ICP-MS為德國(guó)耶拿M90。激光剝蝕過(guò)程中采用氦氣作載氣、氬氣為補(bǔ)償氣以調(diào)節(jié)靈敏度，二者在進(jìn)入ICP之前通過(guò)一個(gè)Y型接頭混合。每個(gè)時(shí)間分辨分析數(shù)據(jù)包括大約20～30s的空白信號(hào)和50s的樣品信號(hào)。對(duì)分析數(shù)據(jù)的離線(xiàn)處理(包括對(duì)樣品和空白信號(hào)的選擇、儀器靈敏度漂移校正、元素含量及U-Th-Pb同位素比值和年齡計(jì)算)采用軟件ZSkits完成。

鋯石微量元素含量利用SRM610作為多外標(biāo)、Si作內(nèi)標(biāo)的方法進(jìn)行定量計(jì)算(Liuetal., 2010)。這些NIST玻璃中元素含量的推薦值據(jù)GeoReM數(shù)據(jù)庫(kù) (http://georem.mpch-mainz. gwdg.de/)。U-Pb 同位素定年中采用鋯石標(biāo)準(zhǔn)91500作外標(biāo)進(jìn)行同位素分餾校正，每分析5～10個(gè)樣品點(diǎn)，分析2次91500，并對(duì)Plesovice分析一次作為監(jiān)控，91500及Plesovice標(biāo)樣均符合推薦值(Wiedenbecketal., 1995；Slmaetal., 2008)。鋯石樣品的U-Pb年齡諧和圖繪制和年齡權(quán)重平均計(jì)算均采用Isoplot/Ex_ver3 (Ludwig, 2003)完成，普通鉛校正使用Andersen (2002)方法完成。本次測(cè)試剝蝕直徑根據(jù)實(shí)際情況選擇30μm。

表1孫家溝林西組及老龍頭組砂巖重礦物組成(%)

Table 1 Heavy mineral composition (%) of Linxi and Laolongtou sandstone in Sunjiagou

樣品號(hào)鋯石磷灰石金紅石角閃石綠簾石重晶石赤褐鐵礦白鈦石黃鐵礦其他18SJG-0638.650.600.800.220.771.9642.95--14.0418SJG-0962.201.470.630.540.39-10.539.130.5814.5318SJG-1615.70-0.79-0.04-79.36--4.1118SJG-244.840.160.07-0.280.1590.74--3.77

3 分析結(jié)果

3.1 重礦物組合

砂巖的重礦物組成對(duì)于物源的變化有極其敏感的指示意義，不同類(lèi)型的母巖經(jīng)后期剝蝕搬運(yùn)后沉積形成的碎屑巖往往擁有不同的重礦物組成特征。然而由于沉積物中重礦物特征受到母巖、風(fēng)化搬運(yùn)和沉積成巖等復(fù)雜地質(zhì)條件的影響(Morton and Hallsworth, 1994)，單一重礦物特征在物源分析過(guò)程中往往由于多因素的復(fù)雜影響, 而不能真實(shí)反映物源信息, 因此, 重礦物組合和單礦物原位分析等方法逐漸興起, 并得到了廣泛應(yīng)用(Garzantietal., 2012, 2013)。

本次選取林西組砂巖18SJG-06和老龍頭組砂巖18SJG-09、18SJG-16和18SJG-24四件樣品進(jìn)行重礦物含量測(cè)試，識(shí)別出來(lái)的重礦物包括鋯石、磷灰石、金紅石、綠簾石、赤褐鐵礦等(表1)。主要重礦物為赤褐鐵礦和鋯石，磷灰石、金紅石及綠簾石次之。黃鐵礦多為沉積過(guò)程中自生形成，赤褐鐵礦則由黃鐵礦氧化形成，它們不具有源區(qū)巖石類(lèi)型的指示意義，僅表明沉積環(huán)境為還原環(huán)境(張英利等，2018)。林西組砂巖樣品18SJG-06重礦物組合為鋯石+磷灰石+金紅石+角閃石+綠簾石+重晶石，其中磷灰石+鋯石+角閃石+金紅石的組合指向原巖為中酸性巖漿巖，而少量的綠簾石和重晶石的存在指示原巖可能有少部分為變質(zhì)巖及沉積巖。老龍頭組砂巖重礦物組合與林西組砂巖相近似，這說(shuō)明它們具有相近似的物源組成，但相較林西組砂巖重晶石含量明顯減少，說(shuō)明再旋回沉積巖對(duì)物源的貢獻(xiàn)程度降低。

3.2 碎屑鋯石U-Pb年代學(xué)

樣品18SJG-06中碎屑鋯石多為短柱狀，長(zhǎng)短軸比<2:1，CL圖像中大部分鋯石發(fā)育韻律環(huán)帶(圖4a)，Th/U比值為0.1～1.96，多數(shù)>0.4(表2)，呈現(xiàn)出典型的巖漿鋯石的特征。本次共獲得有效年齡48個(gè)(不協(xié)和度絕對(duì)值小于5%)，碎屑鋯石U-Pb年齡分布在278±3Ma～1966±15Ma之間，年齡峰值集中在284～310Ma及394～434Ma兩個(gè)區(qū)間(圖5a, b)。

樣品18SJG-09中碎屑鋯石多為短柱狀，長(zhǎng)短軸比<2.5:1，CL圖像中大部分鋯石發(fā)育韻律環(huán)帶(圖4b)，Th/U比值為0.26～0.98，多數(shù)>0.4(表2)，呈現(xiàn)出典型的巖漿鋯石的特征。本次共獲得有效年齡20個(gè)(不協(xié)和度絕對(duì)值小于5%)，碎屑鋯石U-Pb年齡分布在247±3Ma～812±7Ma之間，年齡峰值集中在247～303Ma及437～538Ma兩個(gè)區(qū)間(圖5c, d)。

樣品18SJG-16中碎屑鋯石多為短柱狀，長(zhǎng)短軸比<3:1，CL圖像中大部分鋯石發(fā)育韻律環(huán)帶(圖4c)，Th/U比值為0.17～1.29，多數(shù)>0.4(表2)，呈現(xiàn)出典型的巖漿鋯石的特征。本次共獲得有效年齡65個(gè)(不協(xié)和度絕對(duì)值小于5%)，碎屑鋯石U-Pb年齡分布在243±4Ma～1512±23Ma之間，年齡峰值集中在243～312Ma及359～478Ma兩區(qū)間(圖5e, f)。

樣品18SJG-24中碎屑鋯石多為短柱狀，長(zhǎng)短軸比<3:1，CL圖像中大部分鋯石發(fā)育韻律環(huán)帶(圖4d)，Th/U比值為0.15～1.19多數(shù)>0.4(表2)，呈現(xiàn)出典型的巖漿鋯石的特征。本次共獲得有效年齡67個(gè)(不協(xié)和度絕對(duì)值小于5%)，碎屑鋯石U-Pb年齡分布在237±3Ma～812±7Ma之間，年齡峰值集中在237～257Ma及277～329Ma兩個(gè)區(qū)間(圖5g, h)。

4 討論

4.1 地層時(shí)代

碎屑鋯石 U-Pb 年齡信息為碎屑巖的形成時(shí)代提供信息，可用來(lái)判斷地層時(shí)代的下限。Dickinson and Gehrels (2009)通過(guò)研究科羅拉多高原中生代的鋯石數(shù)據(jù)發(fā)現(xiàn)，約60%的樣品中，最年輕的單個(gè)碎屑鋯石晶粒年齡在沉積年齡的10Ma范圍內(nèi)。樣品18SJG-06的最年輕碎屑鋯石年齡為278±3Ma，表明該地層沉積時(shí)間應(yīng)晚于烏拉爾世。地層中多處產(chǎn)出的大量淡水瓣鰓類(lèi)、成層產(chǎn)出的植物化石及碎屑巖顏色為灰黑色和灰綠色表明該地層沉積于陸相還原環(huán)境,植物化石包含美羊齒、副蘆木、蘇鐵類(lèi)等，為典型的安加拉植物群組分，指向地層沉積時(shí)代為樂(lè)平世(鄧勝徽等，2009)，且從鋯石年齡對(duì)比圖(圖6)中可以看出，老龍頭組地層除最年輕的鋯石年齡峰值外與林西組有著相近似的年齡峰值，說(shuō)明兩套地層物源具有繼承性，結(jié)合前人研究，表明該套地層與上覆紫紅色碎屑巖地層整合接觸(劉兵等，2014)，故認(rèn)為該地層沉積于樂(lè)平世，為林西組地層，其沉積年齡可能為前人研究的254Ma(劉兵等，2014)。

表2林西組及老龍頭組砂巖樣品碎屑鋯石LA-ICP-MSU-Pb測(cè)試結(jié)果

Table 2 LA-ICP-MS U-Pb results for the detrital zircons from the Linxi Formation and Laolongtou Formation

測(cè)點(diǎn)號(hào)UTh(×10-6)Th/U同位素比值年齡(Ma)207Pb206Pb1σ207Pb235U1σ206Pb238U1σ207Pb206Pb1σ207Pb235U1σ206Pb238U1σ不協(xié)和度林西組砂巖18SJG-06-011781991.120.06390.00111.03670.01670.11750.000973938722871650.83%-021921070.550.05250.00120.36660.00820.05070.00063075331763194-0.63%-03341390.110.09460.00173.23700.04820.24810.00221519331466121429112.54%-043241320.410.05410.00120.49960.01110.06690.00073765241184174-1.47%-0510402020.190.05240.00070.33210.00400.04590.000330229291329020.56%-06156750.480.05430.00180.33780.01090.04530.000638576295828633.33%-07132430.330.05650.00160.52060.01440.06700.0008471634261041851.72%-0878400.520.05380.00190.33490.01090.04570.000536479293828831.74%-094603760.820.05240.00110.32880.00670.04560.000430250289528730.42%-11157780.490.05370.00140.47980.01330.06470.00073595839894044-1.53%-1290300.340.05660.00150.53230.01430.06880.000747459433942940.97%-132311910.830.05270.00110.33140.00700.04580.000431847291528830.77%-142431070.440.05200.00110.34630.00760.04840.00052874930263053-0.96%-15112620.560.05270.00160.35880.01220.04940.000731471311931140.24%-16442630.140.05420.00080.51760.00930.06890.00063813542464303-1.48%-174331670.390.05500.00080.52020.00870.06840.00044103442564273-0.33%-182831690.600.05550.00100.48330.00930.06320.000643240400639541.37%-192271030.460.05700.00130.51740.01300.06580.000949351423941152.91%-201131161.030.10050.00113.84040.05650.27630.00231633211601121573111.77%-2111154040.360.05290.00110.34750.00730.04750.000632346303629941.13%-2276801.050.06600.00201.08880.03460.11960.00178066474817728102.61%-232134171.960.05390.00150.36230.00940.04900.000436762314730931.71%-24539560.100.09520.00103.45980.05270.26310.00331533211518121506170.82%-261641100.670.05370.00130.39710.01020.05370.000535957340733730.66%-273613611.000.05180.00110.32410.00710.04540.00042774728552862-0.34%-282562020.790.05140.00100.32040.00720.04510.00042594528262853-0.81%-2998840.860.05260.00180.32860.01110.04550.000631278288828730.52%-302673771.410.05430.00240.36080.01350.04890.00113851013131030861.63%-31145580.400.05230.00160.32350.00940.04530.00052987128572853-0.30%-32154780.510.05450.00180.32980.01050.04410.000539075289827833.90%-3390690.760.11590.00145.17190.07250.32250.00281893221848121802142.50%-342422300.950.05480.00150.38950.01140.05140.000640663334832343.16%-35229690.300.05200.00140.32390.00780.04540.00052856028562863-0.46%-363061390.450.05550.00090.52040.01040.06790.000643137425742440.44%-372031090.540.05430.00140.47920.01300.06390.00073845839793994-0.44%-38119440.370.06440.00150.98830.02410.11130.0015755506981268092.50%-39137730.540.05580.00150.37590.01040.04900.000544561324830834.83%-403072310.750.05290.00100.35210.00720.04820.000432342306530331.00%-411301401.070.05270.00150.32840.00900.04550.000531865288728730.51%-42129510.400.05730.00150.54250.01420.06870.000650558440942942.61%-433372590.770.05300.00140.35870.00900.04920.000532758311731030.54%-44146830.570.05730.00110.61010.01280.07710.000750344484847941.02%-452231380.620.05340.00140.41940.01140.05730.00083466035683595-1.03%-462881660.580.05390.00110.42250.00930.05690.000736847358735740.36%-472201440.660.12070.00105.49110.06390.32970.00311966151899101837153.27%-481451501.030.05460.00180.36510.01210.04850.000639674316930643.33%

續(xù)表2ContinuedTable2測(cè)點(diǎn)號(hào)UTh(×10-6)Th/U同位素比值年齡(Ma)207Pb206Pb1σ207Pb235U1σ206Pb238U1σ207Pb206Pb1σ207Pb235U1σ206Pb238U1σ不協(xié)和度-49126310.250.05380.00200.35330.01360.04820.0010362833071030361.20%-503061160.380.05590.00080.53700.00930.06970.000844933436643450.44%老龍頭組砂巖18SJG-09-015635170.920.05410.00070.46120.00690.06170.00063753138553864-0.21%-024471500.340.05460.00140.52840.01310.07000.00083975943194365-1.23%-03170800.470.05110.00150.27790.00830.03940.00042456724972493-0.11%-042892390.830.05260.00130.34670.00870.04800.000631058302730240.11%-052601020.390.05220.00140.34810.01040.04820.00062936030383034-0.04%-062562230.870.06640.00091.23290.01830.13420.001281827816881270.50%-071981100.550.05210.00180.34510.01170.04800.00052897930193023-0.41%-083463020.870.05170.00130.27830.00630.03910.000427256249524730.72%-095162560.500.05710.00150.64130.01960.08100.0012494595031250270.19%-103321020.310.05710.00080.63890.01040.08100.00084943150265025-0.06%-112981450.490.05090.00110.27750.00670.03940.00032375124952492-0.19%-12113480.420.05470.00180.32540.01120.04320.000540273286927234.77%-131421400.990.05160.00170.27700.00870.03920.000526874248724830.19%-144062110.520.05130.00110.27660.00600.03910.000425248248524720.31%-1572450.620.05180.00220.34530.01580.04800.000727899301123025-0.34%-164613280.710.05120.00090.27860.00510.03940.000324939250424920.14%-175993850.640.05330.00130.34740.00810.04710.000434154303629731.92%-183911010.260.05590.00130.54160.01250.07000.000844851439843650.69%-191821220.670.05620.00100.58380.01190.07530.00064604146784684-0.26%-202771270.460.05300.00160.34630.00950.04740.000533070302729831.14%-212621470.560.05790.00170.60900.02040.07570.0010525634831347162.55%-222981760.590.05160.00100.27590.00640.03880.000527045247524630.76%-24105460.440.05550.00170.50760.01590.06650.0009432704171141550.43%-25136990.730.05220.00270.27670.01330.03900.00092961192481124760.48%-2781690.850.05280.00350.34180.02160.04750.0008320152299162995-0.23%-283924351.110.05300.00100.34590.00920.04750.000932945302729960.83%-292001430.720.05120.00180.27850.01000.03940.000724981249824940.06%-3099650.660.05180.00210.27460.01050.03900.00062789124682473-0.06%-311861190.640.05720.00110.64180.01570.08170.0016499445031050610-0.56%-322241370.610.05900.00120.56110.01160.06930.000956645452843254.43%-332661810.680.09310.00103.05410.03350.23720.0021149120142181372113.44%-34113760.670.05370.00170.42450.01350.05750.000835870359103605-0.25%-351441330.920.05340.00260.34560.01600.04730.00083471123011229851.17%-3678520.680.05330.00230.37970.01640.05190.0007342973271232640.11%-37222960.430.05390.00120.39420.00920.05350.000736650337733640.46%-381791610.900.05260.00120.34220.00760.04760.00063135329963004-0.30%-393983510.880.05500.00110.38140.00770.05020.000441144328631633.75%-4076580.770.06140.00170.81360.02210.09650.0012654606041259471.77%-413211630.510.05070.00220.27950.01200.03980.0007226102250102524-0.51%-42143720.500.10750.00104.52110.04440.30460.0026175717173581714131.20%-436883110.450.05130.00130.27720.00670.03920.000325257248524820.27%-44192830.430.06210.00090.86040.01480.10020.001067833630861662.33%-464785911.240.05080.00120.27510.00660.03930.00032305424752482-0.69%-48290700.240.05560.00100.53350.01040.06940.000743840434743340.34%

續(xù)表2ContinuedTable2測(cè)點(diǎn)號(hào)UTh(×10-6)Th/U同位素比值年齡(Ma)207Pb206Pb1σ207Pb235U1σ206Pb238U1σ207Pb206Pb1σ207Pb235U1σ206Pb238U1σ不協(xié)和度-492451070.440.06950.00091.46700.01930.15290.00119142691789176-0.03%-505262110.400.05640.00080.58710.00920.07530.000646930469646840.23%-511271361.060.05310.00160.34570.00980.04780.000533266301730130.15%-521541250.810.05140.00160.30790.01000.04330.00042587127382733-0.36%-534133190.770.05270.00120.31580.00820.04320.000531851279627332.09%-542191000.460.05140.00120.27820.00680.03930.000425852249524930.24%-55284550.190.06880.00101.27740.02030.13450.001289231836981372.66%-561661050.630.05750.00140.52870.01440.06660.0010510524311041663.54%-57116590.510.05180.00230.27620.01250.03890.00052751022481024630.57%-582781120.400.05710.00130.54540.01190.06940.000849750442843352.12%-592351880.800.05560.00110.52910.01030.06910.000643745431743140.16%-6096570.590.05300.00150.36000.01030.04980.00063286631283134-0.41%老龍頭組砂巖18SJG-16-012121200.570.05210.00110.29090.00600.04060.000428847259525730.96%-0295480.510.05520.00160.35020.01010.04620.000542264305829134.58%-032722120.780.05360.00150.28260.00790.03840.000635362253624343.85%-043031000.330.05200.00100.31030.00690.04340.000628446274527440.25%-0595570.600.05250.00220.33940.01350.04740.000830995297102985-0.57%-073541690.480.05370.00160.33020.00930.04460.000635965290728142.85%-082042451.200.05350.00150.34000.00920.04640.000734862297729341.53%-093491380.390.05340.00240.29110.01200.03940.0006348102259924943.93%-10146870.600.05530.00160.35930.01070.04720.000842566312829754.58%-1164360.570.05640.00250.56740.02630.07320.0013470984561745580.19%-1275300.400.05730.00300.48230.02450.06150.00085031144001738553.74%-133222020.630.05470.00160.33740.01040.04480.000740064295828244.40%-141401380.990.05320.00160.34570.01200.04720.000833870302929751.41%-152901330.460.05330.00100.34190.00660.04660.000534141299529331.75%-1656230.400.05270.00230.28250.01250.03930.00063161012531024841.75%-183172280.720.05300.00100.28770.00610.03920.000433043257524833.38%-194055231.290.05040.00090.27210.00600.03900.00042124124452473-0.98%-202943791.290.05490.00170.47850.01350.06330.000840970397939650.37%-2191520.570.05220.00150.32130.00970.04520.00062936828372854-0.77%-222491430.570.05490.00130.46870.01120.06230.001340852390839080.19%-232551310.510.05200.00140.28180.00820.03930.000528660252724931.39%-242621220.470.05230.00120.28120.00670.03910.000529653252524831.61%-2595670.710.05580.00200.48130.01660.06310.0009445803991139551.10%-27204860.420.05070.00150.27030.00860.03870.00062267024372454-0.78%-282601930.740.05030.00140.27200.00870.03940.00072106624472494-1.99%-29162970.600.05720.00140.49470.01270.06290.000850053408939353.68%-30196970.490.05360.00120.31610.00810.04280.000635552279627033.07%-312021440.710.05060.00170.27700.01100.03960.00082228024892505-0.77%-322181110.510.05160.00110.28140.00700.03970.000626650252625140.28%-33121670.560.05620.00130.58020.01300.07560.00104595046584706-1.13%-3492500.550.05730.00160.53730.01460.06870.0009502634371042861.87%-351231110.900.05230.00180.33690.01100.04680.00063007829582954-0.05%-361261100.880.05490.00170.36350.01130.04790.000640869315830244.17%-371431000.700.05350.00150.28610.00850.03910.000434965255724733.14%

續(xù)表2ContinuedTable2測(cè)點(diǎn)號(hào)UTh(×10-6)Th/U同位素比值年齡(Ma)207Pb206Pb1σ207Pb235U1σ206Pb238U1σ207Pb206Pb1σ207Pb235U1σ206Pb238U1σ不協(xié)和度-38155800.520.05370.00130.38460.01030.05200.000635756330832731.01%-392541920.760.05940.00130.73210.01870.08920.0008581495581155151.23%-40114870.770.05220.00170.34080.01150.04730.00062937529892984-0.09%-41137850.620.05240.00190.34190.01170.04760.00063048229993004-0.42%-42157730.470.05330.00140.41980.01060.05730.00063425935683594-0.92%-431931190.620.05330.00140.35550.00980.04860.000634261309730640.98%-4476370.490.05190.00200.27790.01060.03940.00052819024982493-0.05%-451621100.670.05400.00170.36360.01140.04910.000637271315930941.78%-46102740.720.05470.00170.36840.01130.04940.000640170318831142.45%-47154920.600.05430.00150.34670.00940.04660.000538463302729432.82%-485003250.650.05360.00110.29120.00570.03940.000435644259424923.99%-49124540.430.05490.00190.36590.01210.04910.000740777317930942.37%-50143660.460.05230.00170.32430.01030.04540.00053007528582863-0.29%-512211060.480.05390.00160.35120.01160.04730.001036669306929862.43%-5262370.590.05480.00250.35500.01570.04750.00074041023081229943.04%-532581230.470.05060.00150.27830.00820.03990.00042216924972522-1.29%-542411050.440.05110.00170.33220.01150.04730.00102477629192986-2.25%-55191920.480.05240.00160.34530.01080.04790.00073037030183024-0.20%-56159880.550.05230.00180.34790.01310.04820.000929880303103035-0.07%-572971650.560.05300.00110.36320.00840.04960.000533046315631230.88%-582221660.750.05610.00130.59010.01280.07630.00054555047184743-0.67%-5970130.190.05720.00200.59510.02160.07610.0012498764741447370.22%-60114660.580.05200.00160.33950.01130.04740.00072867229792994-0.66%-612991880.630.05370.00140.35420.01040.04790.000735960308830242.02%-621541220.790.05880.00130.73040.01830.09040.001355949557115588-0.19%-6386290.340.09420.00123.34350.04780.25710.00221512231491111475111.08%-642001890.940.05400.00130.34560.00840.04660.000537153301629432.48%-65194330.170.05230.00120.32400.00780.04510.000529951285628530.15%-665202910.560.05230.00090.33890.00600.04700.000329741296529620.13%-67124580.470.05300.00170.36200.01150.04950.000533071314931230.64%-684072380.580.05140.00160.28970.00900.04080.000725870258725840.23%-69124450.370.05660.00230.59530.02260.07700.0016475904741447810-0.85%老龍頭組砂巖18SJG-24-013562740.770.05260.00100.31910.00600.04410.000531142281527831.09%-02145990.680.05200.00130.35060.00890.04870.00052865530573073-0.50%-052581650.640.05210.00150.27250.00740.03820.000529266245624131.32%-06122570.470.05320.00280.27250.01320.03780.00073361192451123942.36%-07135520.390.05400.00180.29490.01050.03960.000637376262825144.50%-08158690.430.05540.00150.47040.01340.06160.000842961391938551.58%-092011000.500.05140.00150.27070.00740.03840.000526066243624330.16%-104062480.610.05500.00090.39710.00760.05230.000641338340632933.25%-11144700.490.05220.00190.27350.01010.03810.000529583245824131.75%-124742570.540.05260.00110.27360.00580.03780.000531149246523932.57%-134001370.340.05100.00110.26920.00640.03820.000524249242524130.22%-147534510.600.05090.00090.26750.00520.03810.00052364124142413-0.05%-15167880.530.05250.00130.36760.00870.05090.00063065831863204-0.64%-161391010.720.04970.00200.25820.01050.03760.00051829323382383-2.03%-173431510.440.05130.00130.26860.00670.03800.000525660242524030.58%

續(xù)表2ContinuedTable2測(cè)點(diǎn)號(hào)UTh(×10-6)Th/U同位素比值年齡(Ma)207Pb206Pb1σ207Pb235U1σ206Pb238U1σ207Pb206Pb1σ207Pb235U1σ206Pb238U1σ不協(xié)和度-189616840.710.05130.00330.27470.01520.03930.0007254148246122484-0.77%-192751070.390.05300.00140.38450.01000.05240.000633061330732940.35%-202061440.700.05090.00170.28420.00930.04030.00052387625472553-0.40%-252481690.680.05090.00160.26870.00810.03830.00042377324262433-0.35%-263841790.470.05150.00110.33820.00730.04750.00062635029662994-1.18%-272492601.040.05110.00150.27460.00760.03890.000524769246624630.08%-282691240.460.05240.00190.28520.00960.03970.000830284255825151.47%-293201300.410.05060.00210.27360.01010.03930.00052229624682493-1.26%-303944471.140.05810.00100.59720.01040.07430.000853336475746252.82%-31140710.510.05340.00230.28980.01160.03970.000734499258925142.84%-32268930.350.05360.00290.29110.01610.03950.00073531222591325043.69%-334001540.390.05540.00110.50580.00970.06600.000842845416741250.85%-347852660.340.05110.00060.26880.00370.03800.000424629242324120.49%-354111750.430.05750.00070.60090.00790.07570.000850925478547151.51%-3696300.310.05180.00220.26850.01150.03810.000627998241924140.11%-371872221.190.05560.00150.46180.01210.06050.000943558386837851.84%-38166860.520.04970.00160.27520.00900.04080.00051797624772583-4.48%-392161350.620.05200.00120.26800.00650.03750.000428454241523731.53%-40246840.340.05170.00120.28150.00680.03960.000527053252525130.46%-412881400.480.05260.00130.28530.00740.03950.000531156255625031.97%-425013480.700.05250.00080.27210.00430.03770.000430734244323832.45%-4393400.420.05450.00180.36160.01190.04880.000739373313930741.92%-44158930.590.05520.00130.57000.01580.07470.001141852458104656-1.44%-453681370.370.05220.00110.31720.00820.04410.000729547280627840.57%-4689470.520.05250.00240.27010.01240.03800.00073081052431024041.07%-474421840.410.05130.00080.31210.00590.04390.00052563627652773-0.49%-48143530.370.05190.00230.27080.01200.03840.00082801032431024350.27%-49152610.400.05560.00160.51020.01650.06660.0012437664191141670.64%-505864200.720.05560.00060.56740.00790.07390.00104372445654606-0.75%-513562260.630.05160.00110.28120.00740.03950.000626849252625030.76%-5297520.540.05370.00130.51670.01280.07030.00113575342394387-3.55%-535082010.400.05180.00100.28490.00620.03990.000527844255525230.93%-546052860.470.05080.00090.26620.00550.03800.00042344224042403-0.28%-557003450.490.05170.00090.27310.00470.03830.000527138245424231.16%-56166760.460.05320.00170.36770.01160.05050.000733570318931840.11%-57132600.450.05010.00140.26140.00750.03820.00051986623662423-2.51%-58107570.530.05210.00220.27180.01070.03840.000628898244924340.46%-595331170.220.05190.00120.28470.00690.03990.000528253254525230.94%-60151470.310.05460.00180.43210.01540.05730.0008395723651135951.53%-612751300.470.05120.00130.26870.00690.03820.000424960242624230.04%-622301620.710.05160.00150.28020.00840.03940.000526865251724930.62%-631721150.670.05200.00130.28120.00730.03940.000428758252624931.07%-644411910.430.05380.00120.28810.00650.03890.000536152257524634.20%-653943140.800.05100.00090.28150.00530.04010.00042394125242532-0.60%-662171420.660.05450.00110.36590.00720.04880.000439144317530733.05%-67183730.400.05210.00210.27360.01010.03840.000629092246824341.08%-682321010.430.05210.00130.28200.00710.03950.000528957252625031.01%-69600760.130.05820.00090.67480.01110.08410.001053634524752060.61%

圖5 龍江地區(qū)林西組及老龍頭組碎屑鋯石U-Pb年齡譜圖及協(xié)和曲線(xiàn)圖Fig.5 U-Pb histograms and concordia diagrams of the detrital zircon U-Pb ages of the Linxi Formation and Laolongtou Formation in the Longjiang area

圖6 龍江地區(qū)林西組與老龍頭組碎屑鋯石U-Pb年齡對(duì)比圖Fig.6 U-Pb age comparison of detrital zircons from Linxi Formation and Laolongtou Formation in Longjiang area

老龍頭組樣品18SJG-09、18SJG-16和18SJG-24中，最年輕的鋯石年齡分別是247±3Ma、243±4Ma及237±3Ma，這限定該套灰綠色及紫紅色碎屑巖沉積于早三疊世到中三疊世，而非原來(lái)認(rèn)定的早三疊世(劉兵等，2014)。

4.2 物源分析

在四個(gè)砂巖樣品的研究中，所測(cè)試的233顆碎屑鋯石分別記錄了五期構(gòu)造熱事件，分別為237～258Ma、270～329Ma、357～558Ma、680～1633Ma及1893～1966Ma，其中兩個(gè)主要峰值年齡為247Ma和305Ma，次要峰值為358Ma、394Ma、429Ma、476Ma、558Ma、717Ma及1535Ma。

(1)237～258Ma這組年齡僅出現(xiàn)于老龍頭組樣品中，共有73粒碎屑鋯石，占老龍頭組測(cè)試的碎屑鋯石數(shù)量的39.5%。研究表明，在研究區(qū)南部分布有大量的酸性火山巖、花崗巖、閃長(zhǎng)巖、基性火山巖、基性超基性侵入巖等(張曉暉等，2006；李錦軼等，2007；張連昌等，2008；葉栩松等，2011；王子進(jìn)等，2013；徐久磊等，2013；劉希雯等，2015)。此外，近年來(lái)在測(cè)繪沙巴爾吐等六幅1:5萬(wàn)地質(zhì)礦產(chǎn)圖的過(guò)程中，發(fā)現(xiàn)了大量的三疊紀(jì)花崗質(zhì)侵入巖(內(nèi)蒙古自治區(qū)地質(zhì)調(diào)查院,2018(3)內(nèi)蒙古自治區(qū)地質(zhì)調(diào)查院.2018.中華人民共和國(guó)地質(zhì)圖(1:5萬(wàn)沙巴爾吐等六幅))。雖然部分學(xué)者認(rèn)為這些火成巖形成于古亞洲洋閉合后的造山后階段，但早-中三疊世鐵鎂質(zhì)火成巖都具有較低的Ta含量(普遍低于3×10-6)和較高的Th/Ta比值(普遍大于3)的特點(diǎn)，長(zhǎng)英質(zhì)火成巖具有較高的Th/Ta(普遍大于10)比值的特點(diǎn)，且都富集大離子親石元素，這些特點(diǎn)都是島弧巖漿巖所特有的(Gorton and Schandl, 2000；Wood, 1980；Sun and McDonough,1989)。該組鋯石年齡可能與古亞洲洋洋殼消亡前的俯沖增生過(guò)程相關(guān)。

(2)270～329Ma這組年齡共有86粒碎屑鋯石，其中林西組樣品中存在24顆，占總數(shù)的50%，老龍頭組樣品中存在62顆，占總數(shù)的33.5%。該峰值年齡與區(qū)域內(nèi)報(bào)道的賀根山-黑河縫合帶東南側(cè)廣泛發(fā)育的大石寨組弧火山巖(Zhangetal., 2008, 2017; Yuetal., 2017)及扎蘭屯-嫩江一線(xiàn)的島弧花崗巖(Wuetal., 2011；崔芳華等，2013)基本吻合。

(3)357～558Ma這組年齡擁有多個(gè)峰值，分別為～358Ma、～394Ma、～429Ma、～476Ma及～552Ma，這幾個(gè)峰值年齡又可以分為兩組，其中357～493Ma的年齡與賀根山-黑河縫合帶以西分布著的一套早古生代-晚古生代早期代表了古亞洲洋洋殼向北俯沖開(kāi)始的巖漿弧(Fengetal., 2015, 2018; Geetal., 2015; Shietal., 2015; Wuetal., 2015)的年齡一致，而520～558Ma的峰值年齡對(duì)應(yīng)于興安地塊泛非期變質(zhì)基底，即扎蘭屯變質(zhì)雜巖的峰值年齡(周建波等，2014；那福超等，2018)。

(4)碎屑鋯石年齡中680～1633Ma的新元古代及中元古代年齡組分與大興安嶺北部廣泛分布的前寒武變質(zhì)基底興華渡口群的夕線(xiàn)石榴片麻巖和斜長(zhǎng)角閃巖的巖漿鋯石峰值年齡(Miaoetal., 2007; 周建波等，2011；表尚虎等，2012)及花崗質(zhì)雜巖(Wuetal., 2012)、錫林浩特變質(zhì)雜巖的碎屑鋯石年齡(Chenetal., 2009)及花崗片麻巖的鋯石年齡(孫立新等，2013a)、溫都爾廟群中捕獲鋯石年齡(徐備等，2016)可作對(duì)比。

(5)盡管目前在東北地區(qū)已經(jīng)發(fā)現(xiàn)了許多的～1800Ma的古元古代結(jié)晶基底的鋯石年齡(裴福萍等，2006；王穎等，2006；孫立新等，2013b)，但絕大部分學(xué)者認(rèn)為在內(nèi)蒙古東部地區(qū)中二疊統(tǒng)-早三疊統(tǒng)碎屑巖中普遍存在的擁有～1800Ma及～2500Ma的峰值年齡的碎屑鋯石直接或間接來(lái)自于華北克拉通(Shietal., 2003；Hanetal., 2012)，這暗示古亞洲洋洋殼的向南俯沖開(kāi)始于樂(lè)平世之前。

圖7 龍江地區(qū)林西組及老龍頭組碎屑鋯石年齡的構(gòu)造分類(lèi)圖解(據(jù)Cawood et al., 2012)A-會(huì)聚背景; B-碰撞背景; C-拉伸背景Fig.7 Tectonic classification diagram of the detrital zircon U-Pb ages of the Linxi Formation and Laolongtou Formation in the Longjiang area (after Cawood et al., 2012)A-convergent setting; B-collisional setting; C-extensional setting

4.3 盆地性質(zhì)

前人對(duì)于內(nèi)蒙古東部地區(qū)發(fā)育巨厚碎屑沉積的瓜德魯普世-樂(lè)平世沉積盆地的性質(zhì)進(jìn)行了大量的研究，目前存在著兩種意見(jiàn)：一種認(rèn)為是統(tǒng)一大陸背景下形成的陸內(nèi)裂谷盆地(徐備等，2014；耿元生等，2016；徐嚴(yán)等，2018)，而另外一種認(rèn)為板塊持續(xù)會(huì)聚擠壓背景下的殘留洋盆或殘余弧后盆地(李福來(lái)等，2016)

近幾年，一種通過(guò)碎屑鋯石年齡譜判斷盆地構(gòu)造類(lèi)型的方法已經(jīng)提出(Cawoodetal., 2012)，該方法把構(gòu)造環(huán)境分為以下三種：會(huì)聚、碰撞以及伸展環(huán)境。會(huì)聚環(huán)境包括位于俯沖帶環(huán)境中的盆地，從海溝延伸至弧后盆地。碰撞環(huán)境包括大陸碰撞期間和之后形成的盆地，如前陸盆地。伸展環(huán)境包括裂谷和裂谷后被動(dòng)大陸邊緣盆地以及內(nèi)部盆地。不同樣品(和環(huán)境)中鋯石年齡的總體范圍大致相似，但在最年輕的和與更老巖漿事件相關(guān)的鋯石年齡比例方面，不同環(huán)境之間存在顯著差異。會(huì)聚背景下的盆地的年齡接近沉積物年齡的碎屑鋯石比例較高(一般大于50%)，一些弧前盆地和海溝盆地具有單峰碎屑鋯石譜，其年齡接近地層的沉積年齡，而弧后盆地則增加了來(lái)自相鄰克拉通的舊碎屑的輸入。大陸碰撞形成的盆地(例如前陸盆地)通常只含有少量的鋯石，其年齡與沉積物的沉積年齡接近，含有相當(dāng)比例的鋯石年齡(50%至10%)在地層沉積時(shí)間的150Myr范圍內(nèi)。這一模式反映了同碰撞巖漿作用產(chǎn)生的鋯石碎屑量的變化、海洋閉合有關(guān)的先前存在的巖漿弧以及反映造山帶和克拉通前陸中捕獲的單元的一系列較老的年齡。伸展盆地主要由比巖石年齡大得多的碎屑鋯石年齡所控制，小于5%的鋯石年齡在沉積年齡的150Myr以?xún)?nèi)。接近沉積年齡的鋯石年齡可能反映了裂谷相關(guān)的巖漿活動(dòng)，但由于其主要的鎂鐵質(zhì)成分和由此產(chǎn)生的鋯石產(chǎn)量較低，這只是年齡譜的一個(gè)次要組成部分。

從圖7中可以看出，以林西組沉積年齡254Ma、老龍頭組沉積年齡237Ma來(lái)計(jì)算，前30%年輕的碎屑鋯石年齡與地層沉積年齡之差都小于100Ma，說(shuō)明碎屑鋯石記錄具有會(huì)聚背景盆地的特點(diǎn)，而非眾多學(xué)者所認(rèn)為的陸內(nèi)裂谷盆地，而它們的結(jié)晶年齡-沉積年齡累計(jì)概率曲線(xiàn)從形態(tài)上更接近于弧前盆地，其年輕鋯石的百分比含量相比于弧內(nèi)和弧后盆地更高，卻未達(dá)到海溝的程度。而通過(guò)對(duì)于碎屑鋯石組成的研究，發(fā)現(xiàn)盆地的物源主要來(lái)自于盆地西部及南部與古亞洲洋雙向俯沖相關(guān)的古生代弧火山巖，盆地位于島弧遠(yuǎn)離大陸邊緣(烏里雅斯太大陸邊緣和華北克拉通大陸邊緣)的一側(cè)，結(jié)合砂巖的碎屑組成及碎屑重礦物組合特征，認(rèn)為研究區(qū)樂(lè)平世-中三疊世沉積盆地類(lèi)型為弧前盆地。

5 結(jié)論

(1)林西組及老龍頭組重礦物主要由鋯石、磷灰石、金紅石、綠簾石、角閃石、重晶石及赤褐鐵礦組成，重礦物組合說(shuō)明原巖主要為中酸性巖漿巖，并有少部分的變質(zhì)巖及沉積巖。

(2)研究區(qū)林西組砂巖最年輕的鋯石年齡為278±3Ma，結(jié)合地層沉積特征，認(rèn)為其沉積時(shí)代為樂(lè)平世。而老龍頭組砂巖最年輕鋯石年齡分別為247±3Ma、243±3Ma及237±3Ma，表明其沉積于早三疊世到中三疊世。

(3)砂巖碎屑鋯石LA-ICP-MS U-Pb年齡指示，林西組物源主要來(lái)自研究區(qū)西部270～329Ma大石寨組火山巖及其同期侵入巖和357～493Ma沿賀根山-黑河縫合帶分布的早古生代-晚古生代早期巖漿弧，并有少量的前寒武變質(zhì)基底及華北板塊物源供給。老龍頭組除了有和林西組年齡峰值組成相近似的物源組成之外，還有大量樂(lè)平世-中三疊世(237～258Ma)在研究區(qū)西部沿賀根山-黑河縫合帶、南部地區(qū)沿溫都爾廟-西拉木倫縫合帶發(fā)育的與古亞洲洋洋殼俯沖相關(guān)的一系列弧巖漿巖。

(4)結(jié)合對(duì)于研究區(qū)林西組及老龍頭組砂巖碎屑組成、重礦物組合及盆地與火山弧位置關(guān)系的研究，我們認(rèn)為研究區(qū)樂(lè)平世-中三疊世沉積盆地具有會(huì)聚背景，為弧前盆地。

致謝本文碎屑鋯石U-Pb測(cè)試工作在北京燕都中實(shí)測(cè)試技術(shù)有限公司完成，感謝張晗老師的幫助。兩位審稿人老師及本刊編輯對(duì)文章提出了寶貴的修改意見(jiàn)，在此表示衷心的感謝。