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      東天山卡拉塔格地區(qū)鎂鐵質(zhì)巖體年代學、巖石地球化學研究*

      2019-11-13 01:30:48周國超王玉往石煜解洪晶李德東郭博然
      巖石學報 2019年10期
      關(guān)鍵詞:東天山塔格輝長巖

      周國超 王玉往 石煜 解洪晶 李德東 郭博然

      1. 中國地質(zhì)大學地球科學與資源學院,北京 1000832. 北京礦產(chǎn)地質(zhì)研究院,有色金屬礦山深部資源勘查工程技術(shù)研究中心,北京 1000121.

      新疆東天山地區(qū)發(fā)育有大量的二疊紀鎂鐵-超鎂鐵質(zhì)巖體,形成了多個巖漿型銅鎳硫化物礦床及釩鈦磁鐵礦礦床(秦克章, 2000; 毛景文等, 2002; Maoetal., 2008; Tangetal., 2012; Suetal., 2011, 2012; 王玉往等, 2009, 2010, 2013; 石煜等, 2017a, b; Shietal., 2018),這些巖體和礦床沿區(qū)域斷裂分布在多個構(gòu)造單元內(nèi),是我國重要的正巖漿礦床集中產(chǎn)區(qū)(王京彬和徐新, 2006; 劉德權(quán)等, 2005; Qinetal., 2003)。前人研究在巖漿源區(qū)、巖漿演化、成礦作用及構(gòu)造背景等方面取得了許多重要的成果,認為其成巖成礦時代集中在268~300Ma(韓寶福等, 2004; 三金柱等, 2010; 李錦軼等, 2006; 唐冬梅等, 2009; Qinetal., 2011; Sunetal., 2013a, b),母巖漿主要來自虧損地幔(鄧宇峰等, 2011; Tangetal., 2012, 2013; 趙云等, 2016),在演化過程中發(fā)生了不同程度的巖石圈物質(zhì)混染,混染作用導致了巖漿中硫達到飽和而成礦(錢壯志等, 2009; 孫濤等, 2010; Hanetal., 2013; Maoetal., 2016)。對于該期巖漿活動的構(gòu)造背景認識分歧較大,主要有:地幔柱成因(Zhouetal., 2004; Pirajnoetal., 2008);俯沖作用所致(毛啟貴等, 2006; Hanetal., 2010, 2013);后碰撞階段幔源巖漿底墊作用所導致的地殼伸展作用的結(jié)果(韓寶福等, 2004; 王京彬和徐新, 2006; Wangetal., 2008);后碰撞伸展作用和地幔柱疊加(Qinetal., 2011; Suetal., 2011)。

      另一方面,關(guān)于區(qū)內(nèi)銅鎳硫化物礦床的成因,一些學者認為,東天山地區(qū)的銅鎳硫化物礦床形成于巖漿通道成礦系統(tǒng)(Maoetal., 2014a; Zhaoetal., 2016),該系統(tǒng)上部的同時期玄武巖和輝綠巖脈等可能由于大規(guī)模的剪切走滑作用而在隨后的地質(zhì)歷史中被剝蝕掉,只保留了下部賦礦的巖漿通道,形成造山帶背景下特殊的巖漿通道成礦系統(tǒng)(Qinetal., 2011; Maoetal., 2008),有別于保留了厚層玄武巖的Noril’sk礦床(Naldrett, 1992)。前人研究在大南湖島弧帶和吐哈盆地發(fā)現(xiàn)了~280Ma的輝長巖和玄武巖,且地球化學特征與東天山地區(qū)鎂鐵-超鎂鐵質(zhì)巖相似(周鼎武等, 2006; 唐冬梅等, 2017; Maoetal., 2014b),暗示這可能是該區(qū)域巖漿通道成礦系統(tǒng)未被剝蝕的上部層位??ɡ竦貐^(qū)月牙灣銅鎳硫化物礦床的發(fā)現(xiàn)(毛啟貴等, 2018; 孫燕等, 2018)進一步說明東天山二疊紀銅鎳硫化物成礦作用在大南湖島弧帶北緣也有響應。筆者及北京礦產(chǎn)地質(zhì)研究院項目組在卡拉塔格地區(qū)發(fā)現(xiàn)了一系列鎂鐵質(zhì)巖體,成巖時代為早二疊世,主要巖石類型為輝長巖、橄欖輝長巖、橄長巖、淡色輝長巖和輝綠巖,是研究東天山二疊紀巖漿演化與成礦作用新的切入點。本文對卡拉塔格地區(qū)鎂鐵質(zhì)巖體進行巖石學、年代學、巖石地球化學及同位素等方面的研究,以探討其構(gòu)造背景、巖漿演化及其與東天山鎂鐵-超鎂鐵巖體和吐哈盆地玄武巖的關(guān)系。

      1 區(qū)域地質(zhì)背景

      1.1 東天山區(qū)域地質(zhì)特征

      中亞造山帶位于西伯利亞克拉通和塔里木-華北克拉通之間(圖1a),是顯生宙以來最大的增生造山帶之一(Jahnetal., 2000; Gaoetal., 2009; Xiaoetal., 2013)。東天山地區(qū)位于中亞造山帶南緣,在長期的演化過程中經(jīng)歷了極其復雜的裂解與拼合,構(gòu)造環(huán)境多樣(秦克章等, 2012; Maoetal., 2008; Xiaoetal., 2004, 2009),自北向南依次劃分為博格達-哈爾里克構(gòu)造帶、吐哈盆地、覺羅塔格構(gòu)造帶和中天山地塊(秦克章等, 2012; 王玉往等, 2006),其中覺羅塔格構(gòu)造帶由北向南劃分為大南湖-頭蘇泉島弧、康古爾剪切帶和雅滿蘇島弧(Xiaoetal., 2004, 圖1b)。區(qū)域內(nèi)出露的地層較全,以康古爾塔格深大斷裂為界,以北出露大柳溝組、大南湖組、頭蘇泉組和企鵝山群,為一套鈣堿性島弧火山巖和內(nèi)碎屑巖建造;以南出露梧桐窩子組、干墩組和雅滿蘇組,主要為濱淺海相火山-沉積巖系建造組合(劉德權(quán)等, 1992; 姬金生等, 1994; Zhangetal., 2017)。區(qū)域斷裂構(gòu)造主要呈EW向展布,沿康古爾塔格斷裂和阿奇克庫都克-沙泉子斷裂及其次級斷裂發(fā)育跨構(gòu)造單元分布的眾多時代一致的小型鎂鐵-超鎂鐵質(zhì)巖體,并形成多個銅鎳礦床集中區(qū),如黃山-鏡兒泉成礦區(qū)(三金柱等, 2010; 夏明哲等, 2008; 王玉往等, 2009; 秦克章等, 2012; Sunetal., 2013a)、白鑫灘-路北成礦區(qū)(趙冰冰等, 2018; Fengetal., 2018)和白石泉-天宇成礦區(qū)(唐冬梅等, 2009; Tangetal., 2011; 毛啟貴等, 2006)。

      圖1 東天山及臨區(qū)構(gòu)造位置圖(a, 據(jù)Jahn et al., 2000)和構(gòu)造單元劃分(b, 據(jù)Xiao et al., 2004)及鎂鐵-超鎂鐵質(zhì)巖體分布圖(據(jù)Mao et al., 2016修改) ①沁城斷裂;②康古爾塔格斷裂;③苦水斷裂;④阿齊克庫都克-沙泉子斷裂;⑤星星峽斷裂;⑥卡瓦布拉克斷裂Fig.1 The map of tectonic position (a, after Jahn et al., 2000) and tectonic units (b, after Xiao et al., 2004) and distribution of mafi-ultramafic intrusions (modified after Mao et al., 2016) in eastern Tianshan ① Qincheng fault; ② Kanggurtag fault; ③ Kushui fault; ④ Aqqikkuduk-Shaquanzi fault; ⑤ Xingxingxia fault; ⑥ Kawabulak fault

      圖2 卡拉塔格地區(qū)地質(zhì)構(gòu)造及鎂鐵質(zhì)巖體分布圖(底圖據(jù)毛啟貴等, 2010; Deng et al., 2016修改)Fig.2 Geological map and locations of mafic intrusions in Kalatag area (modified after Mao et al., 2010; Deng et al., 2016)

      1.2 卡拉塔格地質(zhì)特征

      卡拉塔格地區(qū)位于大南湖-頭蘇泉古生代島弧帶北緣(圖1b),又被稱為東天山晚古生代構(gòu)造“天窗”(秦克章等, 2001; 李文鉛等, 2006),整體為一個隆起帶。卡拉塔格隆起帶核部發(fā)育奧陶系-志留系火山碎屑巖,邊部由內(nèi)向外發(fā)育連續(xù)的泥盆系-二疊系的火山-沉積巖地層(秦克章等, 2001; 毛啟貴等, 2010)。區(qū)內(nèi)主要構(gòu)造為NW向斷裂,與區(qū)域構(gòu)造線展布方向一致,早期為張性,晚期轉(zhuǎn)變?yōu)閴盒?,是控制卡拉塔格地區(qū)火山作用、巖漿侵入和成礦作用的主干斷裂??ɡ竦貐^(qū)侵入巖十分發(fā)育,加里東期侵入巖以卡拉塔格巖基為代表,產(chǎn)出花崗閃長巖、二長花崗巖等;海西期侵入體以閃長巖、石英斑巖、二長斑巖巖枝或巖株產(chǎn)出(毛啟貴等, 2010, 2018; Dengetal., 2016)。此外,在卡拉塔格地區(qū)發(fā)育多個鎂鐵質(zhì)巖體 (圖2),主要巖性為輝長巖、橄欖輝長巖、淡色輝長巖和輝綠巖等,部分鎂鐵質(zhì)巖體發(fā)育銅鎳硫化物礦化。

      表1卡拉塔格地區(qū)鎂鐵質(zhì)巖體特征

      Table 1 Charactersitics of mafic intrusions in Kalatag area

      巖體K1K2K3K4K5K6月牙灣洪湖灣形態(tài)橢圓形脈狀橢圓形橢圓形近圓形橢圓形月牙狀圓形走向北西北西北北西南北北西南北北西南北長(m)100~300200100~400500~100020050~1002000300寬(m)50~1004050~200200~50010030~60100~500300面積(km2)0.020.0060.060.40.020.0050.80.1圍巖玄武巖安山巖安山巖安山巖安山巖安山巖玄武巖玄武巖巖性輝長巖、輝綠巖輝綠巖橄欖輝長巖、橄欖輝綠巖、淡色輝長巖淡色輝長巖、輝長巖、橄欖輝長巖橄長巖、偉晶橄長巖橄欖輝長巖、偉晶橄欖輝長巖橄欖輝長巖、橄長巖、淡色輝長巖淡色輝長巖、輝長巖、輝綠巖特征中心粗,邊緣細,發(fā)育鈦鐵礦化輝綠巖脈中心粗,邊緣細,圍巖發(fā)育烘烤邊橄欖輝長巖侵位晚于輝長巖,圍巖發(fā)育烘烤邊巖性單一,見偉晶狀囊體巖性單一,見偉晶狀囊體巖體北傾、底部橄長巖中發(fā)育銅鎳礦化中心粗,邊部細,發(fā)育鈦鐵礦化

      2 巖體地質(zhì)

      2.1 巖體特征

      卡拉塔格地區(qū)鎂鐵質(zhì)巖體分布廣泛,由東向西分別為K2、K1、K3、K4、K5、K6、月牙灣和洪湖灣等8個巖體,其中洪湖灣、K4巖體見銅鎳礦化,月牙灣巖體賦存有小型銅鎳硫化物礦床(孫燕等, 2018)。筆者對上述巖體進行了詳細調(diào)查(圖3、表1)。這些鎂鐵質(zhì)巖體分布在卡拉塔格斷裂和卡北斷裂之間,受北西向斷裂構(gòu)造控制,侵位于大柳溝組、大南湖組和臍山組地層之中。巖體形態(tài)多為北西、北北西走向的橢圓形,巖體規(guī)模較小,長100~2000m,寬50~500m,出露面積<1km2,單個巖體巖性較為單一,以輝長巖、橄欖輝長巖、輝綠巖和淡色輝長巖為主。巖體的圍巖主要為中基性火山巖,其中K2、K3、K4、K5和K6等巖體圍巖為安山巖(圖4a),K1、月牙灣和洪湖灣巖體圍巖為玄武巖(圖4b),。K1、K3和洪湖灣巖體顯示中心粗邊部細的單期巖漿冷凝結(jié)晶的特征;K2巖體為一條輝綠巖脈;K4巖體至少發(fā)育兩期巖漿作用,橄欖輝長巖相侵位晚于輝長巖相;K5和K6巖體巖性單一,發(fā)育偉晶巖囊體;在K1、K3、K4和洪湖灣巖體輝長巖相中發(fā)育<10%的鈦鐵礦,偶見橄欖輝長巖中發(fā)育微量黃鐵礦和黃銅礦(<1%)。月牙灣巖體形態(tài)特殊,呈向北西拖尾的月牙狀,巖體北傾,由北向南依次發(fā)育淡色輝長巖、橄欖輝長巖和橄長巖,在橄長巖與橄欖輝長巖接觸部位發(fā)育銅鎳礦化。

      圖3 卡拉塔格地區(qū)鎂鐵質(zhì)巖體地質(zhì)簡圖Fig.3 Sketch geological maps of mafic intrusions in Kalatag area

      圖4 K4巖體(a)和月牙灣巖體(b)與圍巖接觸關(guān)系Fig.4 The contacting relation of wall rock and intrusion in K4 (a) and Yueyawan (b)

      2.2 巖石特征

      卡拉塔格地區(qū)鎂鐵質(zhì)巖體的巖石類型有輝長巖、橄欖輝長巖、橄長巖、淡色輝長巖和輝綠巖。各巖類分布情況見圖3,輝長巖為K1、K4和洪湖灣巖體的主體相;橄欖輝長巖在K3和K6巖體主體、K4巖體邊部及月牙灣巖體中部發(fā)育;橄長巖主要發(fā)育在K5巖體和月牙灣巖體南部;淡色輝長巖以局部囊體形式在K3、K4和洪湖灣巖體出露,在月牙灣巖體北部出露面積較大;輝綠巖主要發(fā)育在K1、K3和洪湖灣巖體邊緣。各巖石類型具體特征如下:

      輝長巖:淺灰綠色,普遍發(fā)生弱蝕變,粒狀結(jié)構(gòu)、變余輝長結(jié)構(gòu),塊狀構(gòu)造。主要由斜長石和單斜輝石組成(圖5a),斜長石為中-拉長石,粒徑0.5~2mm,含量40%~45%,發(fā)生高嶺土化、鈉黝簾石化蝕變;單斜輝石為普通輝石和透輝石,粒徑0.5~2mm,含量20%~30%,部分發(fā)生陽起石化、透閃石化、綠泥石化等蝕變;一般含褐色普通角閃石,粒徑0.5~1mm,含量約10%,不均勻分布于斜長石和單斜輝石顆粒之間;見鈦鐵礦,含量5%~10%,發(fā)育在蝕變的輝石顆粒中。

      橄欖輝長巖:灰綠色,巖石較新鮮,包橄結(jié)構(gòu)、嵌晶結(jié)構(gòu)、粒狀結(jié)構(gòu)等(圖5b),條帶狀構(gòu)造、塊狀構(gòu)造。斜長石為拉長石,自形長板狀,粒徑1~3mm,含量40%~60%;單斜輝石為普通輝石,半自形-他形粒狀,粒徑1~3mm,含量20%~30%,充填在長石間隙;橄欖石半自形-他形粒狀,粒徑0.5~1mm,含量10%~25%,發(fā)育在斜長石和單斜輝石晶粒之間或被單斜輝石包裹。發(fā)育少量(<5%)黑云母和鈦鐵礦等副礦物和微量磁黃鐵礦、黃鐵礦、黃銅礦等硫化物。礦物顆粒粗大者呈偉晶橄欖輝長巖,斜長石顆粒達10mm,交織結(jié)構(gòu)(圖5c),單斜輝石和橄欖石發(fā)育在斜長石晶粒間隙。

      橄長巖:灰黑色,較新鮮,中-細粒似斑狀結(jié)構(gòu)(圖5d),塊狀構(gòu)造。斑晶和基質(zhì)均由斜長石和橄欖石組成,斑晶斜長石為中-拉長石,自形板狀,含量60%~70%,斑晶粒度2~4mm,基質(zhì)粒度<0.5mm;橄欖石他形粒狀,含量25%~30%,斑晶顆粒1~2mm,基質(zhì)顆粒<0.2mm。含5%~10%單斜輝石,他形粒狀,粒徑0.5~1mm,充填在斜長石顆粒的間隙或包裹橄欖石,也見微量鈦鐵礦和黃銅礦微小顆粒發(fā)育。橄長巖礦物顆粒粗大者可形成偉晶狀橄長巖,交織結(jié)構(gòu),斜長石顆粒可達5~10mm,橄欖石2~4mm,局部皂石化、綠泥石化蝕變。

      淡色輝長巖:灰白色,蝕變中粗粒結(jié)構(gòu),交織結(jié)構(gòu),塊狀構(gòu)造。主要由斜長石、單斜輝石、角閃石組成。斜長石為中-拉長石,部分更長石,粒徑1~2mm,含量60%~70%,發(fā)生高嶺土化、絹云母化、鈉黝簾石化等蝕變,斜長石殘余晶形完整,呈凈邊結(jié)構(gòu)(圖5e)。單斜輝石多發(fā)生透閃石和綠泥石化蝕變,粒徑1~2mm,含量10%~15%。含10%~20%的褐色普通角閃石,微量黑云母、鈦鐵礦、磁鐵礦等,偶見石英。

      輝綠巖:灰綠色,中細粒輝綠結(jié)構(gòu)、交織結(jié)構(gòu)(圖5f)、包橄結(jié)構(gòu),塊狀構(gòu)造。主要由中基性斜長石、單斜輝石、橄欖石和普通角閃石等礦物組成。斜長石為針柱狀、交織狀,有時具有定向性,長軸<2mm,含量50%~60%;單斜輝石呈他形微粒狀,粒徑<1mm,含量10%~20%,充填在斜長石間隙,綠簾石化蝕變;橄欖石他形微粒狀,粒徑<1mm,含量0~10%,大多充填在斜長石和輝石礦物顆粒之間,少量被輝石和角閃石包裹;角閃石(約10%)發(fā)育在其它礦物間隙,多陽起石化和綠泥石化蝕變;含少量(5%~10%)鈦鐵礦和微量(<1%)黃銅礦。

      3 樣品采集及分析方法

      3.1 樣品采集

      本次研究挑選了新鮮、有代表性的樣品進行鋯石U-Pb定年、全巖主微量和Sr-Nd-Pb同位素分析測試。巖石樣品均采自巖體地表露頭,用于鋯石U-Pb定年的樣品分別來自洪湖灣輝長巖(H56-9)和K4輝長巖(K43-7),采樣坐標分別為42°45′10.59″N、91°18′60.01″E和42°39′59.48″N、91°43′52.40″E,采樣位置見圖3。

      3.2 分析方法

      全巖主微量分析在核工業(yè)北京地質(zhì)研究院分析測試研究中心完成。主量元素測試方法為X射線熒光光譜法,采用AxiosmAX X射線熒光光譜儀,分析精度優(yōu)于1%。微量元素分析采用酸溶法制備樣品并在ELEMENT XR電感耦合等離子質(zhì)譜儀上測試,分析精度優(yōu)于3%。詳細的分析方法見Gaoetal. (2002)。

      Sr-Nd-Pb同位素分析在中國科學院地質(zhì)與地球物理研究所完成。鉛同位素測試采用美國Thermofisher公司Triton Plus型熱電離質(zhì)譜儀。將純化好的樣品用微量鹽酸溶解,并加1微升硅膠和磷酸混合發(fā)射劑于錸燈絲表面,質(zhì)譜測量溫度為1250±50℃。國際標樣NIST981被用于監(jiān)控質(zhì)譜儀狀態(tài),鉛同位素質(zhì)量分餾校正系數(shù)為每質(zhì)量單位1.2‰。Sr-Nd同位素測試采用Thermofisher Triton Plus多接收熱電離質(zhì)譜儀,全流程本底Sr和Nd分別小于250pg和100pg,用88Sr/86Sr=8.375209和146Nd/144Nd=0.7219對Sr和Nd同位素比值進行校正,用國際標樣NBS-987和JNdi-1對儀器穩(wěn)定性進行監(jiān)測,詳細的分析方法見Lietal. (2012, 2015)。

      4 分析結(jié)果

      4.1 鋯石U-Pb年齡

      洪湖灣巖體輝長巖中的鋯石均呈透明的長柱-短柱狀,自形-半自形晶體;鋯石長100~200μm,長寬比1.5~3;鋯石陰極發(fā)光圖像較暗,巖漿結(jié)晶環(huán)帶一般不明顯,部分可見環(huán)帶狀結(jié)構(gòu)。鋯石U、Th、Pb含量分別為90×10-6~376×10-6、100×10-6~491×10-6和28×10-6~145×10-6;,Th/U為1.1~1.96(表2),表明所測樣品均為巖漿鋯石(Rubatto, 2002)。LA-ICP-MS測試結(jié)果顯示23個有效數(shù)據(jù)點206Pb/238U加權(quán)平均年齡為281.8±1.2Ma (MSWD=1.05),所有數(shù)據(jù)點均分布在諧和曲線上或附近,諧和年齡為282.2±0.6Ma (MSWD=1.05)(圖6a)。

      表2洪湖灣巖體和K4巖體輝長巖LA-ICP-MS鋯石U-Pb測試結(jié)果

      Table 2 LA-ICP-MS U-Pb age of zircons from gabbros of Honghuwan and K4 intrusions

      測點號含量(×10-6)UThPbTh/U同位素比值年齡(Ma)207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σHh56-9(洪湖灣輝長巖)-01223310851.390.0534800.0008490.3281940.0055720.0444920.000379350.130.6288.24.3280.62.3-022714841251.790.0523860.0007060.3202960.0045360.0443480.000389301.929.6282.13.5279.72.4-032243761041.680.0532570.0010490.3268660.0057060.0447540.000559338.944.4287.24.4282.23.5-042183681031.690.0529270.0006510.3273910.0052830.0447670.000379324.127.8287.64.0282.32.3-05155224601.440.0531830.0008950.3244390.0057860.0442710.000432344.538.9285.34.4279.22.7-0690100281.100.0530240.0011000.3217420.0070460.0440180.000427331.546.3283.25.4277.72.6-07160207581.300.0520360.0008210.3173760.0052600.0442450.000362287.137.0279.94.1279.12.2-083404811341.420.0526120.0006170.3280010.0045160.0451850.000367322.327.8288.03.5284.92.3-102243701011.650.0542700.0009730.3357490.0063410.0448570.000354383.438.9293.94.8282.92.2-11172197581.150.0525950.0008270.3189430.0072950.0446490.000474322.335.2281.15.6281.62.9-12187352961.880.0511040.0007480.3165570.0053410.0449580.000483255.633.3279.34.1283.53.0-13137197551.440.0518170.0007100.3290880.0055680.0460620.000502276.026.9288.94.3290.33.1-14133182511.360.0524170.0009130.3278390.0059440.0454110.000411301.938.9287.94.5286.32.5-151933791011.960.0529020.0007370.3271460.0049720.0448760.000415324.133.3287.43.8283.02.6-16143191551.330.0537540.0018030.3255590.0063860.0445090.000513361.275.9286.24.9280.73.2-172904901341.690.0527100.0006760.3253720.0044560.0447880.000364316.729.6286.03.4282.42.2-183124361251.400.0517220.0005990.3207940.0037150.0450560.000407272.321.3282.52.9284.12.5-19184259731.410.0527990.0012610.3208220.0052420.0444660.000538320.453.7282.54.0280.53.3-203024651331.540.0532850.0007720.3279720.0048830.0447740.000527342.731.5288.03.7282.43.2-21249325921.310.0534410.0015080.3242080.0047530.0445490.000502346.463.0285.13.6281.03.1-223764911451.310.0510970.0008720.3157390.0077810.0447000.000601255.638.9278.66.0281.93.7-23159218611.380.0528410.0007480.3265530.0048820.0448530.000414320.426.9286.93.7282.82.6-25190233681.230.0520800.0008590.3171370.0055190.0441830.000418287.138.9279.74.3278.72.6

      續(xù)表2

      Continued Table 2

      測點號含量(×10-6)UThPbTh/U同位素比值年齡(Ma)207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σK43-7(K4巖體輝長巖)-018069200.860.0504910.0011580.3070840.0074930.0441660.000479216.784.2271.95.8278.63.0-02127137391.080.0526060.0007960.3258840.0055550.0450040.000487322.335.2286.44.3283.83.0-03160226611.410.0526100.0009190.3164790.0059650.0436880.000436322.338.9279.24.6275.72.7-047360180.820.0529210.0011590.3202090.0072760.0440610.000560324.150.0282.15.6278.03.5-05135182491.340.0524950.0006920.3167920.0042860.0438670.000426305.634.3279.43.3276.82.6-06132118340.890.0542030.0009920.3296970.0062000.0441730.000376388.940.7289.34.7278.72.3-07144191511.330.0525650.0006380.3180880.0041700.0439490.000387309.330.6280.43.2277.32.4-08108110301.020.0521780.0009830.3159150.0070860.0438370.000430300.147.2278.85.5276.62.7-09116105300.910.0526120.0010050.3183700.0063560.0439990.000470322.310.2280.64.9277.62.9-10120109310.910.0528030.0010600.3179800.0064480.0437720.000379320.446.3280.35.0276.22.3-117868200.870.0510520.0010810.3095810.0068430.0440510.000433242.752.8273.95.3277.92.7-12130121340.930.0543680.0011680.3288460.0076110.0439110.000422387.145.4288.75.8277.02.6-13160193521.210.0528230.0008090.3230500.0055680.0443810.000437320.439.8284.24.3279.92.7-142204911312.230.0531080.0006990.3287220.0056230.0448960.000533344.529.6288.64.3283.13.3-15123175471.420.0521480.0008710.3158570.0059010.0439020.000373300.137.0278.74.6277.02.3-16108125341.160.0529790.0009780.3211050.0062330.0440130.000426327.840.7282.84.8277.72.6-17125122340.970.0521430.0008750.3158450.0051290.0440230.000362300.137.0278.74.0277.72.2-18119109310.920.0518140.0009370.3144740.0056100.0441180.000409276.040.7277.64.3278.32.5-19147196541.330.0528030.0008850.3269790.0064560.0448560.000435320.441.7287.34.9282.92.7-20145106320.730.0519360.0009170.3226840.0064370.0450700.000511283.440.7284.04.9284.23.2-219985250.860.0517940.0009100.3144520.0064880.0439760.000468276.040.7277.65.0277.42.9-22105105300.990.0533810.0010570.3217400.0063980.0437700.000415346.446.3283.24.9276.22.6-2310899280.920.0527810.0008620.3157900.0055220.0434140.000431320.441.7278.74.3274.02.7-24139168451.210.0509740.0010160.3075170.0063120.0437740.000388239.044.4272.34.9276.22.4-259899281.010.0526910.0010620.3220410.0073890.0443030.000511316.713.9283.55.7279.43.2

      表3卡拉塔格地區(qū)鎂鐵質(zhì)巖石主量元素(wt%)和微量元素(×10-6)分析結(jié)果

      Table 3 Major (wt%) and trace (×10-6) elements composition of mafic intrusions in Kalatag district

      樣品號K3-1K4-8K4-12K6-1K6-2K4-16H-4K1-5K1-8K5-1K5-3K5-4H-2K3-2K4-2K3-8H-9K2-3巖性橄欖輝長巖輝長巖橄長巖淡色輝長巖輝綠巖SiO247.9547.8747.7546.1148.0147.2351.3645.4745.0849.1947.0347.9957.4264.4353.2548.5949.6549.35TiO21.261.731.531.141.551.662.700.200.490.810.640.641.831.000.672.011.731.49Al2O318.5917.2917.7217.0519.4816.9714.8621.7622.6722.2919.0020.7614.9912.2321.7116.4516.8216.07Fe2O3T10.7711.2411.0012.5612.2410.4111.457.659.687.2310.148.778.285.764.2312.2810.329.37FeO8.247.538.239.908.015.076.305.524.705.728.397.474.913.862.728.335.805.37MnO0.150.160.160.170.140.150.170.140.130.100.140.120.170.120.060.180.200.14MgO7.917.247.7510.945.635.364.156.415.165.0810.408.512.852.051.016.925.297.23CaO8.888.788.858.027.829.526.9813.3511.919.288.259.084.707.768.408.456.909.40Na2O3.593.593.493.154.084.314.641.731.954.173.163.455.564.385.553.794.372.76K2O0.260.280.250.190.250.281.080.340.500.340.250.181.390.071.430.451.351.20P2O50.210.150.220.180.180.160.430.030.060.170.110.120.720.240.310.370.350.24LOI0.371.631.220.460.623.932.142.902.351.290.830.352.071.913.370.463.032.75Total99.9499.9699.9499.9699.9999.9899.9599.9899.9899.9599.9599.9699.9999.95100.0099.95100.00100.00Mg#65.5365.5665.0968.6358.1967.6756.6069.6968.4963.7571.0569.2953.4751.2642.3762.1964.3672.72Li6.209.609.146.285.957.4412.703.523.896.726.615.0713.002.455.185.8322.804.64Be0.830.660.690.650.550.561.360.120.220.600.410.402.080.931.131.221.050.75Sc20.8027.1024.6017.2013.8038.4032.5042.3031.409.5010.509.7222.0020.608.6830.2024.9044.20V16021417814946726026518741792847913410665228219283Cr15517713910618157741413349463244100105279Co52464771533627343234635213129483432Ni1101031001794732102831531381483126914369Cu5182623641554674106292423244124685939Zn1061201101061081171585571657465149925514615291Ga18.1018.2017.5015.7018.6018.5019.7015.6017.7018.1014.9015.8021.7014.1022.0019.8018.1017.10Rb2.262.042.201.291.673.1011.804.835.142.541.871.3214.500.5421.104.6522.5015.90Sr47239734845560941474439345256548553347959601422802305Y22.1023.3029.3017.5015.3024.0033.904.405.8315.0010.3010.7054.8039.3031.7035.8026.6028.60Mo0.350.550.420.270.310.370.700.090.200.290.200.221.140.470.450.620.540.81Cd0.100.070.100.090.060.110.160.050.060.050.060.050.240.140.070.140.160.11In0.060.070.060.050.050.070.090.040.040.040.030.030.100.100.040.080.070.08Sb0.060.050.040.050.060.120.100.080.090.050.030.040.070.230.050.070.320.19Cs0.090.220.180.380.140.140.120.130.170.270.150.180.110.030.260.170.460.09

      續(xù)表3

      Continued Table 3

      樣品號K3-1K4-8K4-12K6-1K6-2K4-16H-4K1-5K1-8K5-1K5-3K5-4H-2K3-2K4-2K3-8H-9K2-3巖性橄欖輝長巖輝長巖橄長巖淡色輝長巖輝綠巖Ba88857359101812868010471535435628322149273327La6.364.635.955.275.655.6416.401.632.964.953.503.5528.1011.8010.4011.4011.8013.90Ce17.0012.8016.6013.9014.1014.7039.903.205.9012.908.939.2066.0028.9025.9029.9029.5029.30Pr2.752.192.822.262.172.425.840.500.862.081.421.499.904.423.964.664.443.94Nd13.6011.7014.6011.6010.7012.5027.602.454.1010.306.997.3546.5021.3019.5022.9021.7017.90Sm3.533.444.192.972.593.576.710.720.952.541.731.8210.805.534.995.935.274.36Eu1.491.681.561.351.401.412.170.300.411.361.011.053.301.381.952.081.821.47Gd3.183.103.772.512.213.135.540.600.852.151.511.649.044.864.315.124.253.99Tb0.690.700.850.550.480.721.130.120.170.470.320.341.791.080.921.110.900.86Dy3.853.954.783.032.614.076.070.780.992.511.791.859.386.105.195.974.734.81Ho0.820.901.080.660.580.901.250.170.230.560.390.401.951.431.161.331.001.08Er2.152.252.821.711.472.333.120.460.571.411.001.045.043.762.983.402.512.78Tm0.380.430.520.310.270.420.530.080.110.250.190.190.880.710.530.620.440.52Yb2.412.613.141.921.722.633.340.530.751.621.101.185.414.553.263.852.693.24Lu0.350.380.460.280.240.380.470.080.110.230.170.160.770.650.440.540.370.45Pb1.941.401.521.491.501.833.340.580.611.381.390.997.643.923.503.313.613.24Bi0.010.000.000.00<0.0020.000.000.000.010.000.010.000.020.060.000.010.010.02Th0.220.150.210.140.230.280.810.200.320.120.100.091.351.100.590.470.581.42U0.090.100.110.070.100.140.300.110.200.080.050.060.600.670.230.200.220.44Nb2.751.992.392.312.452.086.770.290.621.961.311.3210.803.802.024.955.3018.60Ta0.190.160.160.150.160.150.440.020.030.140.090.090.680.250.160.350.331.18Zr11710012292839415881379515531815692185104117Hf2.672.563.042.071.902.443.950.270.431.811.211.247.284.192.294.382.522.89∑REE58.5650.7663.1448.3146.1954.82120.111.6218.9543.3330.0431.26198.996.4785.5098.8191.4288.60LREE44.7336.4445.7237.3536.6140.2498.628.8015.1934.1323.5824.46164.673.3366.7076.8774.5370.87HREE13.8314.3217.4210.969.5814.5821.462.823.769.206.466.8034.2623.1418.8021.9416.8917.73LREE/HREE3.232.542.623.413.822.764.603.124.043.713.653.604.803.173.553.504.414.00(La/Yb)N1.891.271.361.972.361.543.522.192.842.192.282.163.731.862.292.123.153.08δEu1.331.541.181.471.741.261.061.361.361.731.871.820.990.801.251.131.141.06

      注:Mg#=100×Mg2+/(Mg2++Fe2+)

      表4卡拉塔格地區(qū)鎂鐵質(zhì)巖石Sr、Nd、Pb同位素分析數(shù)據(jù)

      Table 4 Sr, Nd, Pb isotopic data of mafic intrusion in Kalatag district

      樣品號K3-1K4-12K6-2K4-16H-4K1-5K51-1H5-4H-2K3-2K4-2K3-8H-9巖性橄欖輝長巖輝長巖橄長巖淡色輝長巖輝綠巖Rb (×10-6)2.262.201.673.1011.804.832.541.3214.500.5421.104.6522.50Sr (×10-6)47234860941474439356553347959.360142280287Rb/86Sr0.01390.01830.00790.02170.04590.03560.01300.00720.08770.02650.10170.03190.081287Sr/86Sr0.7034160.7032000.7035110.7034500.7037960.7046330.7032700.7033520.7042330.7047170.7038100.7035270.7041482σ0.0000120.0000140.0000120.0000130.0000110.0000150.0000130.0000100.0000140.0000110.0000120.0000110.000011(87Sr/86Sr)i0.7033600.7031300.7034800.7033600.7036100.7044900.7032200.7033200.7038800.7046100.7034100.7034000.703820Nd (×10-6)13.6014.6010.7012.5027.602.4510.307.3546.5021.3019.5022.9021.70Sm (×10-6)3.534.192.593.576.710.722.541.8210.805.534.995.935.27147Sm/144Nd0.15690.17350.14630.17270.14700.17670.14910.14970.14040.15700.15470.15660.1468143Nd/144Nd0.5129880.5130380.5129610.5130120.5128650.5129470.5129720.5129680.5128700.5129480.5129720.5129510.5128822σ0.0000200.0000100.0000090.0000090.0000080.0000090.0000090.0000100.0000080.0000080.0000100.0000080.000009(143Nd/144Nd)i0.5127000.5127200.5126930.5126960.5125960.5126230.5126990.5126940.5126130.5126600.5126890.5126640.512613εNd(t)8.258.648.118.176.226.748.238.136.557.478.037.546.55U (×10-6)0.090.110.100.140.300.110.080.060.600.670.230.200.22Th (×10-6)0.220.210.230.280.810.200.120.091.351.100.590.470.58Pb (×10-6)1.941.521.501.833.340.581.380.997.643.923.503.313.61206Pb/204Pb17.979417.880017.994617.978918.275418.301017.947917.939218.193318.583318.001518.030118.12452σ0.00290.00180.00220.00180.00240.00290.00270.00230.00150.00170.00310.00160.0022207Pb/204Pb15.482015.453215.484215.468615.521215.563315.479415.477815.497615.501815.460815.487815.49382σ0.00250.00150.00200.00170.00220.00260.00250.00250.00120.00120.00250.00150.0017208Pb/204Pb37.695837.544237.715537.633337.974338.108937.669937.664237.847537.900537.631837.742237.77692σ0.00570.00380.00490.00380.00490.00650.00640.00570.00300.00300.00680.00380.0042(206Pb/204Pb)i17.845017.680017.810017.767018.025017.779017.780017.786017.976018.103017.821017.867017.9590(207Pb/204Pb)i15.475015.443015.475015.458015.508015.536015.471015.470015.486015.477015.451015.479015.4850(208Pb/204Pb)i37.592037.423037.579037.497037.755037.801037.593037.587037.688037.645037.482037.616037.6320

      注: Rb、Sr、Sm、Nd同位素計算參數(shù): λ(Sr)=1.42×10-11a-1, λ(Nd)=0.654×10-11a-1, (87Sr/86Sr)CHUR=0.7045, (143Nd/144Nd)CHUR=0.512638; Pb初始同位素計算參數(shù): λ1=1.55125×10-11a-1, λ2=9.8485×10-11a-1, λ3=0.49475×10-11a-1,t=280Ma

      圖5 卡拉塔格地區(qū)鎂鐵質(zhì)巖石顯微照片 (a)輝長巖(單斜輝石陽起石、綠泥石化蝕變,正交偏光);(b)橄欖輝長巖(斜長石嵌晶結(jié)構(gòu),正交偏光);(c)偉晶橄欖輝長巖(偉晶交織狀斜長石,正交偏光);(d)橄長巖(似斑狀結(jié)構(gòu),正交偏光);(e)淡色輝長巖(斜長石凈邊結(jié)構(gòu),單偏光);(f)輝綠巖(斜長石交織結(jié)構(gòu),正交偏光). Cpx-單斜輝石;Pl-斜長石;Ol-橄欖石;Tre-透閃石;Act-陽起石;Chl-綠泥石;Di-透輝石;Ilm-鈦鐵礦Fig.5 Mircophotographs of mafic intrusions in Kalatag area (a) gabbro (actinolitization and chlorophyllization alterated clinopyroxene, crossed polar); (b) olivine gabbro (poikiloblastic texture of plagioclase, crossed polar); (c) olivine gabbro pegmatite (pegmatitic interlaced texture of plagioclase, crossed polar); (d) troctolite (porphyritic texture, crossed polar); (e) leucogabbro (metasomatic edulcoration border texture of plagioclase, plane polar); (f) diabase (interlaced texture of plagioclase, crossed polar). Cpx-clinopyroxene; Pl-plagioclase; Ol-olivine; Tre-tremolite; Act-actinolite; Chl-chlorite; Di-diopside; Ilm-ilmenite

      圖6 洪湖灣巖體輝長巖(a)和K4巖體輝長巖(b)鋯石U-Pb諧和年齡圖Fig.6 U-Pb concrodia diagrams for zirons from gabbros in Hoghuwan (a) and K4 (b) intrusions

      K4巖體輝長巖中的鋯石呈透明的長柱狀或短柱狀,少量不規(guī)則狀,呈自形-半自形晶體;鋯石長50~200μm,長寬比1~3;鋯石陰極發(fā)光圖像較暗,巖漿結(jié)晶環(huán)帶一般不明顯,部分可見環(huán)帶狀結(jié)構(gòu)。鋯石U、Th、Pb含量分別為73×10-6~220×10-6、60×10-6~491×10-6和18×10-6~131×10-6,Th/U為0.73~2.23(表2),表明所測樣品均為巖漿鋯石(Rubatto, 2002)。LA-ICP-MS測試結(jié)果顯示25個有效數(shù)據(jù)點206Pb/238U加權(quán)平均年齡為278.2±1.2Ma (MSWD=0.95),所有數(shù)據(jù)點均分布在諧和曲線上或附近,諧和年齡為278.3±0.6Ma (MSWD=0.9)(圖6b)。

      4.2 主量元素

      卡拉塔格地區(qū)8個鎂鐵質(zhì)巖體主要巖石類型的主、微量分析數(shù)據(jù)見表3。其中淡色輝長巖SiO2含量介于53.25%~64.43%之間,屬中性巖類;其它巖石類型SiO2含量介于45.08%~51.36%之間,屬于基性巖類;各巖石類型Mg#介于42.4~72.7之間。與東天山二疊紀鎂鐵-超鎂鐵質(zhì)巖(黃山巖體SiO2=36.9%~53.9%,Mg#介于60.7~84.2,Dengetal., 2015;葫蘆巖體SiO2=29.67%~56.14%,Mg#介于42.2~87.4,Hanetal., 2013)相比,卡拉塔格地區(qū)巖體基性程度普遍較低(或酸度較高),巖漿演化程度較高。主量元素圖解顯示卡拉塔格地區(qū)鎂鐵質(zhì)巖體與東天山鎂鐵質(zhì)巖地球化學特征相似,且向同時期吐哈玄武巖過渡(圖7)。在SiO2-(Na2O+K2O)圖解上,只有1個淡色輝長巖樣品落在亞堿性區(qū)域,其余樣品均落在堿性和亞堿性分界線上(圖8)。

      圖7 卡拉塔格地區(qū)鎂鐵質(zhì)巖體主要氧化物與SiO2相關(guān)性圖解 東天山鎂鐵質(zhì)巖石數(shù)據(jù)來源:①據(jù)Deng et al. (2014)和趙冰冰等(2018);②據(jù)趙冰冰等(2018)和毛啟貴等(2006);③據(jù)Chai et al. (2008);④據(jù)Han et al. (2013)和夏明哲等(2008);⑤據(jù)唐冬梅等(2017);圖9數(shù)據(jù)來源同此圖Fig.7 Diagrams of oxides versus SiO2 of mafic intrusions in Kalatag area Data of mafic rocks in eastern Tianshan are from follows: ① after Deng et al. (2014) and Zhao et al. (2018); ② after Zhao et al. (2018) and Mao et al. (2006); ③ after Chai et al. (2008); ④ after Han et al. (2013) and Xia et al. (2008); ⑤ after Tang et al. (2017) ; data sources inFig.9 are same as thig figure

      圖8 卡拉塔格鎂鐵質(zhì)巖體SiO2-(Na2O+K2O) 圖解(底圖據(jù)Irvine and Baragaer, 1971)Fig.8 Diagram of SiO2 vs. (Na2O+K2O) (base map after Irvine and Baragaer, 1971) for mafic intrusions in Kalatag area

      4.3 微量元素

      卡拉塔格地區(qū)各巖石類型稀土元素總體特征相似,顯示輕稀土輕微富集的緩右傾型稀土配分曲線(圖9a),∑REE變化范圍為11.62×10-6~198.9×10-6,LREE為8.80×10-6~164.6×10-6,HREE為2.82×10-6~34.26×10-6,輕重稀土元素分餾不明顯,∑LREE/∑HREE=2.54~4.80,(La/Yb)N為1.27~3.73,無銪異?;蜉^弱的銪正異常(δEu=0.80~1.87)。稀土配分型式與東天山鎂鐵巖相似并向同時期玄武巖過渡,并顯示與OIB相似的特征(圖9c)。

      樣品微量元素普遍高于原始地幔值,總體表現(xiàn)為Rb、Ba、Sr、K正異常,Th、Nb、Ta負異常(圖9b)。區(qū)域內(nèi)同時期鎂鐵質(zhì)巖石顯示Sr正異常,玄武巖顯示Sr負異常,卡拉塔格地區(qū)鎂鐵質(zhì)巖石主體為Sr正異常,也有一些樣品顯示Sr負異常(圖9d),說明卡拉塔格鎂鐵質(zhì)巖石具有過渡性質(zhì)。

      4.4 Sr、Nd、Pb同位素

      Nd、Sr、Pb同位素分析數(shù)據(jù)見表4??ɡ竦貐^(qū)鎂鐵質(zhì)巖體(87Sr/86Sr)i=0.70313~0.70461,εNd(t)=+6.22~+8.64,(206Pb/204Pb)i=17.68~18.103,(207Pb/204Pb)i=15.443~15.536,(208Pb/204Pb)i=37.423~37.801,具有高Nd、低Sr、低Pb,且變化范圍較窄的特征。Sr-Nd同位素組成與二疊紀塔里木地區(qū)玄武巖和鎂鐵-超鎂鐵質(zhì)巖明顯不同,樣品點投影在洋島玄武巖(OIB)、島弧玄武巖(GVAB)和活動大陸邊緣(ACM)區(qū)域內(nèi)(圖10a),(206Pb/204Pb)i-(87Sr/86Sr)i圖解中樣品點投影在MORB和OIB范圍內(nèi)(圖10b)。Pb同位素組成位于地球等時線右側(cè)并落在MORB范圍內(nèi)(圖10c, d)??傮w來看,卡拉塔格地區(qū)鎂鐵質(zhì)巖同位素特征與黃山、黃山東等東天山鎂鐵-超鎂鐵質(zhì)巖及吐哈盆地玄武巖相似,顯示虧損型地幔源區(qū)特征。

      5 討論

      5.1 時代及構(gòu)造背景

      本文厘定卡拉塔格地區(qū)洪湖灣巖體和K4巖體輝長巖年齡分別為282.2±0.6Ma和278.3±0.6Ma,本文討論的該區(qū)其它巖體年齡為275~285Ma(毛啟貴等, 2018)。孫燕等(2018)報道了月牙灣巖體橄欖輝長巖成巖年齡為281.3±2Ma,Maoetal. (2014b)測定沙爾湖巖體輝長巖成巖年齡為286.5±2.1Ma,顯示成巖時代均為早二疊世,與東天山地區(qū)廣泛發(fā)育的銅鎳礦化鎂鐵-超鎂鐵質(zhì)巖石形成時代一致(韓寶 福等, 2004; 三金柱等, 2010; 李錦軼等, 2006; 唐冬梅等, 2009; Qinetal., 2011; Sunetal., 2013b)。

      圖9 卡拉塔格地區(qū)鎂鐵質(zhì)巖體及區(qū)域鎂鐵質(zhì)巖石球粒隕石標準化稀土元素分布型式圖(a、c)和原始地幔標準化微量元素蛛網(wǎng)圖(b、d) (MORB、OIB及標準化值引自Sun and McDonough, 1989)Fig.9 Chondrite-normalized REE patterns (a, c) and PM-normalized trace elements spider diagram (b, d) of mafic rocks in Kalatag and adjacent region (data for MORB, OIB and normalizing values after Sun and McDonough, 1989)

      圖10 卡拉塔格地區(qū)鎂鐵質(zhì)巖體Sr-Nd-Pb同位素圖解 數(shù)據(jù)來源:吐哈盆地玄武巖引自夏林圻等(2006);黃山東引自Sun et al. (2013a);黃山引自夏明哲(2009);虧損地幔(DM)引自Zindler and Hart (1986);活動大陸邊緣(ACM)引自Hawkesworth (1982);全球島弧玄武巖(GVAB)引自http://www. petdb. Org;塔里木鎂鐵-超鎂鐵質(zhì)巖引自Zhou et al. (2009);塔里木玄武巖引自Yuan et al. (2012)Fig.10 Plots of Sr-Nd-Pb isotopes for the mafic intrusion in Kalatag area Data sources: the Tuha basalts from Xia et al. (2006); the Huagnshandong intrusion from Sun et al. (2013); the Huagnshan intrusion from Xia (2009); a depleted mantle-derived melt (DM) from Zindler and Hart (1986); active continental margin (ACM) from Hawkesworth (1982); global volcanic arc basalts (GVAB) from a public database (http://www.petdb.org); Permian Tarim mafic-ultramafic rocks from Zhou et al. (2009); Permian Tarim basalts from Yuan et al. (2012)

      圖11 卡拉塔格地區(qū)鎂鐵質(zhì)巖體εNd(t)對(87Sr/86Sr)i (a)、(Th/Nb)N圖解(b)和SiO2對εNd(t) (c)、(87Sr/86Sr)i圖解(d) 虧損地幔(DMM)和原始地幔標準化數(shù)據(jù)引自Sun and McDonough (1989);上地殼(UC)和下地殼(LC)數(shù)據(jù)引自Rudnick and Gao (2003);早二疊世A型花崗巖數(shù)據(jù)引自汪傳勝等(2009)Fig.11 Diagrams of εNd(t) vs. (87Sr/86Sr)i (a), εNd(t) vs. (Th/Nb)N (b), SiO2 vs. εNd(t) (c) and SiO2 vs. (87Sr/86Sr)i (d) for mafic intrusions in Kalatag area Data for the depleted mantle-derived melt (DMM) and normalizing values from Sun and McDonough (1989). Data for the upper crust (UC) and Lower crust from Rudnick and Gao (2003). Data for the coeval A-type granite in the region from Wang et al. (2009)

      圖12 卡拉塔格地區(qū)鎂鐵質(zhì)巖體La/Nb-La/Ba圖解(a,據(jù)Fitton et al., 1991)和 (206Pb/204Pb)i-(143Nd/144Nd)i 圖解(b,據(jù)Deckart et al., 2005)Fig.12 Diagrams of La/Nb vs. La/Ba (a, after Fitton et al., 1991) and (206Pb/204Pb)i vs. (143Nd/144Nd)i (b, after Deckart et al., 2005) for mafic intrusions in Kalatag area

      圖13 卡拉塔格地區(qū)鎂鐵質(zhì)巖體Th/Yb-Ba/La (a)和εNd(t)-(Nd/Hf)N (b)圖解 虧損地幔(DMM)和原始地幔標準化數(shù)據(jù)引自Sun and McDonough (1989);蝕變洋殼流體(AOCF)數(shù)據(jù)引自Hauff et al. (2003)和Kelley et al. (2003);全球俯沖沉積物(GLOSS)數(shù)據(jù)引自Chauvel et al. (2009)和Plank and Langmuir (1998);俯沖沉積物流體(SSF)數(shù)據(jù)引自Chauvel et al. (2009)和Johnson and Plank (2000);板片流體(SF)相應值為90% AOCF和10% SSF混合Fig.13 Plots of Th/Yb vs. Ba/La (a) and εNd(t) vs. (Nd/Hf)N (b) for the mafic intrusion in Kalatag area Data for the depleted mantle-derived melt (DMM) and normalizing values from Sun and McDonough (1989). Data for the altered oceanic crust fluid (AOCF) from Hauff et al. (2003) and Kelley et al. (2003). Data for the global subducting sediment (GLOSS) from Chauvel et al. (2009) and Plank and Langmuir (1998). Data for the subducted sediment fluid (SSF) from Chauvel et al. (2009), Johnson and Plank (2000). Data for the slab fluid (SF) are the mixture of 90% AOCF and 10% SSF

      圖14 東天山地區(qū)二疊紀幔源巖漿侵位模式圖Fig.14 A brief model for Permian mantle-derived magma emplacement in eastern Tianshan

      關(guān)于新疆東部二疊紀幔源巖漿活動構(gòu)造背景的爭論主要在于其是否與塔里木地幔柱有關(guān)(夏林圻等, 2006; Qinetal., 2011; Maoetal., 2008; Pirajnoetal., 2008),或者為碰撞后伸展環(huán)境(Zhouetal., 2004; 王玉往等, 2009; Wangetal., 2008; 鄧宇峰等, 2012)。Qinetal. (2011)認為地幔柱疊加作用為東天山和北山鎂鐵-超鎂鐵質(zhì)巖的侵位及形成銅鎳礦床提供了的熱源。Xieetal. (2014)研究認為俯沖板片撕裂導致的軟流圈上涌也可以形成與巖漿硫化物成礦有關(guān)的玄武質(zhì)巖漿,并不需要地幔柱提供熱源;而且地幔柱能量的影響范圍可能有限(宋謝炎等, 2018)。前文研究表明卡拉塔格地區(qū)鎂鐵質(zhì)巖石主量元素(圖7)、稀土元素和微量元素特征(圖9)與二疊紀塔里木玄武巖明顯不同,且Sr、Nd、Pb同位素具有顯著區(qū)別(圖10a, b),說明卡拉塔格地區(qū)鎂鐵質(zhì)巖的形成可能與塔里木地幔柱活動無關(guān)。石煜(2018)總結(jié)了東天山地區(qū)εNd(t)隨時間變化規(guī)律,認為隨著時間變化虧損軟流圈物質(zhì)穩(wěn)定上涌,其過程中并未出現(xiàn)地幔柱物質(zhì)的劇烈加入過程,因此可能也不存在地幔柱疊加的過程。

      汪傳勝等(2009)、陳希節(jié)和舒良樹(2010)、陳希節(jié)等(2016)研究認為哈爾里克地區(qū)早二疊世(297~285Ma)的基性巖墻群、鉀長花崗巖和堿性花崗巖及雙峰式火山巖組合是碰撞后拉張階段的典型標志;康古爾-黃山構(gòu)造帶銅鎳硫化物礦區(qū)的A型花崗巖可能侵位于造山晚期的伸展環(huán)境(孫赫等, 2010);吐哈盆地早二疊世玄武巖微量元素特征顯示與區(qū)域性伸展作用密切相關(guān)(周鼎武等, 2006; 唐冬梅等, 2017);白建科等(2018)研究認為吐哈盆地南緣的企鵝山群玄武巖(314±3.5Ma)形成于伸展環(huán)境下,且捕獲有中天山地塊的碎屑鋯石。以上研究從多個方面說明北天山洋已于晚石炭世早期之前閉合消失,東天山地區(qū)早二疊世為后碰撞伸展環(huán)境(韓寶福等, 2004; 王京彬和徐新, 2006; 顧連興等, 2006)。

      綜上所述,本文認為在早二疊世時期,北天山洋已經(jīng)閉合,增生造山主碰撞過程已經(jīng)結(jié)束,包括卡拉塔格在內(nèi)的東天山地區(qū)鎂鐵質(zhì)-超鎂鐵質(zhì)巖體形成于早二疊世后碰撞伸展環(huán)境,而與塔里木地幔柱活動無關(guān),因此,可能不存在地幔柱疊加俯沖碰撞階段或者疊加后碰撞伸展階段的作用過程。

      5.2 巖漿源區(qū)

      一般來說,巖漿源區(qū)、同化混染和結(jié)晶分異是影響巖石地球化學特征的重要因素,在討論巖漿源區(qū)性質(zhì)時,首先要排除巖漿上升過程中同化混染和結(jié)晶分異作用的影響(李全忠等, 2008)。分別以虧損地幔(DMM)、上地殼(UC)和下地殼(LC)為端元進行的模擬表明母巖漿經(jīng)歷了<5%的上地殼混染(圖11a),加入同時期A型花崗巖端元的模擬結(jié)果表明上地殼混染程度約5%(圖11b)。對同化混染作用很敏感的Sr同位素和207Pb/204Pb比值沒有明顯升高(表4),在鋯石的CL圖像上沒有發(fā)現(xiàn)核幔結(jié)構(gòu),也沒有發(fā)現(xiàn)古老的年齡(圖5),卡拉塔格地區(qū)各巖石的全巖εNd(t)值和(87Sr/86Sr)i值與SiO2含量之間沒有相關(guān)性(圖11c, d)。以上證據(jù)說明卡拉塔格鎂鐵質(zhì)巖體的初始巖漿上升過程中沒有發(fā)生明顯的同化混染作用。結(jié)晶分異作用可以影響巖石中元素的含量,但同位素組成和不相容微量元素比值不因結(jié)晶分異而變化(Dengetal., 2015; Tangetal., 2012)。因此,卡拉塔格地區(qū)巖石同位素組成和不相容微量元素比值可以指示其巖漿源區(qū)。

      卡拉塔格地區(qū)鎂鐵質(zhì)巖體各類巖石具有與MORB和OIB相似的Sr-Nd-Pb同位素特征(圖10),微量元素Zr/Nb=6.29~51.04,Sm/Nd=0.23~0.29,分別與MORB對應比值一致(Anderson, 1994),表明其巖漿源區(qū)為虧損地幔(Saundersetal., 1992)。Sr-Nd同位素組成具有與弧火山巖相似的特征(圖10a),相對較低的Th/U(平均2.08)和較高的La/Nb(平均2.84)、Ba/Nb(61.87)比值與俯沖成因的巖漿巖(如阿拉斯加型巖體)相似(鄧宇峰等, 2011),表明巖漿源區(qū)遭受了俯沖事件的改造。巖石微量元素顯示Nb、Ta的強虧損和Ti弱虧損(圖9),表明巖漿源區(qū)混合了富集地幔組分。La/Nb-La/Ba圖解顯示卡拉塔格地區(qū)鎂鐵質(zhì)巖體母巖漿主要來自俯沖交代的巖石圈地幔并有軟流圈地幔的加入(圖12a)。利用虧損型地幔端元(N-MORB)和富集型巖石圈地幔(EMⅡ, EMⅠ, Deckartetal., 2005)兩端元模擬Sr、Nd同位素組成,表明卡拉塔格地區(qū)鎂鐵質(zhì)巖石母巖漿主要來自虧損地幔源區(qū),并有大約10%的EMⅡ型富集地幔組分加入(圖12b)。前人研究表明,EMⅡ型地幔端元可能是由古板塊俯沖帶入的陸源沉積物通過殼-幔交代作用形成(Hart, 1988),或者是有俯沖洋殼析出流體交代地幔楔形成(李曙光, 1994)。

      卡拉塔格地區(qū)鎂鐵質(zhì)巖石(Th/Yb)N和(Th/Nb)N分別為0.33~2.54和0.49~5.75,遠高于MORB的相應值(平均值分別為0.17和0.43, Sun and McDonough, 1989),可能是地幔源區(qū)俯沖板片流體的交代作用影響所致(Songetal., 2011; Sunetal., 2013b),Th/Yb-Ba/La圖解也表明巖漿源區(qū)存在流體交代改造作用(圖13a)。以虧損地幔(DMM)、全球俯沖沉積物(GLOSS)、蝕變洋殼流體(AOCF)和板片流體(SF)進行模擬投圖,模擬方法見Tangetal. (2013),結(jié)果表明,母巖漿在地幔源區(qū)遭受了5%~15%的板片流體交代(圖13b),結(jié)果與遭受10%左右的富集地幔組分交代的模擬一致(圖12b),表明前文所述的富集地幔組分可能是通過板片流體交代作用加入到虧損地幔之中。綜上所述,卡拉塔格地區(qū)鎂鐵質(zhì)巖體源區(qū)主要為虧損地幔,并有遭受了5%~15%俯沖流體交代而形成的富集地幔的加入。

      5.3 侵位模式

      前文討論表明,卡拉塔格地區(qū)鎂鐵質(zhì)巖石形成于早二疊世后碰撞伸展環(huán)境之下,巖漿源區(qū)為虧損地幔,并受到俯沖板片流體的交代,與東天山地區(qū)鎂鐵-超鎂鐵質(zhì)巖體和吐哈玄武巖的構(gòu)造環(huán)境和巖漿源區(qū)相似(夏明哲等, 2008; Tangetal., 2011, 2012, 2013; 唐冬梅等, 2017),說明它們形成于同一構(gòu)造巖漿系統(tǒng)。在地球化學特征方面,卡拉塔格地區(qū)鎂鐵質(zhì)巖體的主量元素和微量元素具有吐哈盆地玄武巖和東天山鎂鐵-超鎂鐵質(zhì)巖體中鎂鐵質(zhì)巖相過渡的特征(圖6、圖8),Sr、Nd、Pb同位素特征一致(圖9)。前人對東天山地區(qū)含銅鎳礦的鎂鐵-超鎂鐵質(zhì)巖體母巖漿成分進行了研究,結(jié)果表明黃山南和天宇鎂鐵-超鎂鐵質(zhì)巖石的母巖漿MgO含量分別為12.4%(Maoetal., 2016)和12.61%(Chaietal., 2008),而卡拉塔格地區(qū)月牙灣巖體母巖漿MgO含量為6.5%(本人未發(fā)表數(shù)據(jù)),吐哈盆地二疊紀玄武巖母巖漿MgO含量可能更低(唐冬梅等, 2017)。因此,卡拉塔格地區(qū)鎂鐵質(zhì)巖體與吐哈玄武巖和東天山鎂鐵-超鎂鐵質(zhì)巖體具有相同的巖漿源區(qū),原始巖漿在上升過程中發(fā)生分異形成了各自的母巖漿,并經(jīng)歷不同的地質(zhì)過程侵位或噴出地表(圖14)。在早二疊世,構(gòu)造巖漿活動進入碰撞后伸展階段,被俯沖板片流體交代改造過的巖石圈地幔被上涌的軟流圈物質(zhì)加熱而部分熔融,形成東天山二疊紀構(gòu)造-巖漿-成礦系統(tǒng)的巖漿源區(qū),原始巖漿在上升到地殼中的階段性巖漿房中后發(fā)生了分異:吐哈盆地玄武巖可能是整個構(gòu)造-巖漿-成礦系統(tǒng)的前導性噴發(fā)巖流,其母巖漿來自階段性巖漿房分異的上部位置;東天山鎂鐵-超鎂鐵質(zhì)巖體是巖漿成礦的終端巖漿房,母巖漿來自階段性巖漿房的下部位置,MgO含量較高,母巖漿侵位速度較慢,遭受了較高程度的地殼物質(zhì)混染而使硫化物發(fā)生熔離成礦;卡拉塔格地區(qū)鎂鐵質(zhì)巖體具有過渡性質(zhì),可能是前導性侵入巖體,母巖漿侵位速度較快,地殼混染程度較低(月牙灣巖體含礦,地殼混染程度較高)。因此,由巖體侵位模式和現(xiàn)今保存特征可以推測,從吐哈盆地到康古爾剪切帶,剝蝕厚度可能逐漸加大,在剝蝕較淺的區(qū)域存在形成銅鎳硫化物礦床的潛力。近年來在大南湖島弧帶北緣發(fā)現(xiàn)了月牙灣銅鎳礦床,其南緣發(fā)現(xiàn)了白鑫灘和路北銅鎳礦床(圖1),其成礦特征與東天山黃山、香山等典型銅鎳礦相似。綜上所述,卡拉塔格鎂鐵質(zhì)巖體、吐哈玄武巖和東天山鎂鐵-超鎂鐵質(zhì)巖體形成于同一構(gòu)造巖漿系統(tǒng),在剝蝕較淺的大南湖島弧帶具有形成銅鎳硫化物礦床的潛力。

      6 結(jié)論

      (1)東天山卡拉塔格地區(qū)出露多個鎂鐵質(zhì)巖體,面積出露面積<1km2。主要巖石類型有輝長巖、橄欖輝長巖、橄長巖、淡色輝長巖和輝綠巖,2個輝長巖LA-ICP-MS鋯石U-Pb年齡為分別為282.2±0.6Ma和278.3±0.6Ma。

      (2)卡拉塔格地區(qū)鎂鐵質(zhì)巖石SiO2含量介于45.08%~64.43%之間,Mg#介于42.4~72.7之間;輕稀土輕微富集,輕重稀土元素分餾不明顯,無銪異?;蜉^弱的銪正異常;富集Rb、Ba、Sr、K,虧損Th、Nb、Ta;Nd、Sr、Pb同位素組成顯示巖漿源區(qū)為俯沖交代的巖石圈地幔,并被軟流圈物質(zhì)混染。母巖漿在源區(qū)經(jīng)歷了俯沖板片流體的交代(5%~15%),上地殼混染較弱(<5%)。

      (3)卡拉塔格地區(qū)鎂鐵質(zhì)巖體的形成可能與塔里木地幔柱作用無關(guān),而是形成于后碰撞伸展的構(gòu)造環(huán)境,其與東天山二疊紀銅鎳礦化鎂鐵-超鎂鐵質(zhì)巖體和吐哈盆地玄武巖形成于同一構(gòu)造巖漿系統(tǒng),在剝蝕較淺的大南湖島弧帶具有形成銅鎳硫化物礦床的潛力。

      致謝本文野外工作得到哈密紅石礦業(yè)公司張銳總經(jīng)理的支持與協(xié)助;同位素分析得到了中國科學院地質(zhì)與地球物理研究所李潮峰和李友連老師的指導;全巖主、微量分析得到了核工業(yè)北京地質(zhì)研究院劉牧老師的幫助;審稿人對本文提出了建設性的修改意見;作者在此致以誠摯的謝意。

      在肖序常院士90華誕之際,謹以此文表示衷心的祝福!

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