藏東玉龍斑巖銅礦帶扎拉尕含礦斑巖體鋯石U-Pb年齡及其地質意義

何國朝, 王廣強, 黃文婷, 鄒銀橋, 伍 靜, 梁華英, 張玉泉, Charllote M ALLEN

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藏東玉龍斑巖銅礦帶扎拉尕含礦斑巖體鋯石U-Pb年齡及其地質意義

何國朝1,2, 王廣強2,3, 黃文婷2,3, 鄒銀橋2,3, 伍 靜2*, 梁華英2, 張玉泉2, Charllote M ALLEN4

1. 廣西有色金屬集團 資源勘查有限公司, 廣西 南寧 530022; 2. 中國科學院 廣州地球化學研究所 中國科學院礦物學與成礦學重點實驗室, 廣東 廣州 510640; 3. 中國科學院大學, 北京 100049; 4. Research School of Earth Sciences, Australian National University, Canberra ACT0200, Australia

玉龍斑巖銅礦帶扎拉尕斑巖銅鉬礦床位于玉龍斑巖銅礦帶中北部, 賦礦巖體侵入下二疊統(tǒng)火山巖及三疊系砂泥巖中, 主要由早階段為二長花崗斑巖及晚階段正長花崗斑巖組成。分析了早階段二長花崗斑巖及晚階段正長花崗斑巖鋯石LA-ICP-MS U-Pb年齡。早階段二長花崗斑巖該年齡為(38.5±0.2) Ma, MSWD=1.12, 晚階段正長花崗斑巖該年齡為(38.5±0.2) Ma, MSWD=1.08, 早階段和晚階段含礦斑巖體鋯石U-Pb年齡完全一致。這表明早晚兩階段成礦巖體是在很短的時間間隔內形成的。扎拉尕賦礦斑巖體形成年齡為(38.5±0.2) Ma。據扎拉尕斑巖礦床形成時代及藏東地區(qū)在始新世至漸新世地質構造背景, 提出扎拉尕斑巖礦床和玉龍斑巖銅礦帶的形成與印度板塊-歐亞板塊碰撞在藏東地區(qū)形成的走滑構造活動誘發(fā)的巖漿活動有關, 為陸陸碰撞走滑構造環(huán)境的斑巖礦床。

斑巖銅鉬礦床; 成巖成礦時代; 碰撞與成礦; 西藏東部

0 引 言

西藏東部玉龍斑巖銅礦帶是世界上重要的碰撞造山環(huán)境斑巖銅礦帶, 該斑巖銅礦帶長約300 km, 寬10~30 km, 含一個超大型斑巖銅礦床(玉龍)、兩個大型斑巖銅鉬礦床(多霞松多和馬拉松多)、兩個中型斑巖銅礦床(扎拉尕、莽總)及數十個礦化點。自該礦帶發(fā)現(xiàn)以來, 國內外學者對玉龍斑巖銅礦帶及其南側沿哀牢山-紅河巨型走滑斷裂帶分布的新生代斑巖銅鉬(金)礦床開展了大量的研究工作[1–34]。目前, 對藏東喜山期富堿(鉀質)巖帶及玉龍含礦巖體形成的構造環(huán)境、巖漿來源及巖體的屬性看法不一, 主要有: (1) 印度板塊與亞洲板塊碰撞后, 軟流圈對流循環(huán)導致巖石圈減薄, 軟流圈上拱引致巖石圈地幔融熔形成[34]; (2) 古近紀以來陸內俯沖作用導致軟流圈上拱引起地?；旌蠈硬糠秩谌坌纬蒣16]; (3)玉龍銅礦帶含礦斑巖和俯沖作用有關, 為火山弧型, 成巖物質為殼幔混合物[3–4]; (4) 形成于板內非造山構造背景[6,7,22], 成巖物質來自交代富集地幔[6–7]。對含礦巖體屬性也存在不同的認識: 有的認為其屬于鈣堿性巖石系列[3–5], 有的則認為其屬堿性巖[6,7,22,26]。Hou.提出玉龍含礦斑巖有三個活動期, 與走滑斷裂有關[26]。陳文明提出玉龍含礦斑巖體及其中的斑晶并非巖漿直接結晶的產物, 而是深源的富堿硅熱水流體交代、熔融上部地殼含銅巖石形成的[24]。過去對玉龍斑巖銅礦帶的工作多主要集中在玉龍超大型斑巖銅礦床及多霞松多大型斑巖礦床上, 而對其他斑巖礦床的工作則較少。

扎拉尕斑巖體位于玉龍斑巖銅礦帶北部(圖1), 前人曾對扎拉尕賦礦斑巖體作過3個巖石樣品鉀長石和1個巖石樣品黑云母K-Ar測年, 所得的年齡變化較大, 在33.9~41.0 Ma之間[1,5,6]。為了深入分析藏東玉龍斑巖銅帶時空分布特征及成礦演化, 很有必要對玉龍礦帶一些中型規(guī)模礦床成礦巖體作精確的同位素定年。為此, 本文以扎拉尕含礦巖體為主要對象, 開展含礦巖體鋯石LA-ICP-MS U-Pb年代學研究并分析其形成背景。

圖1 藏東玉龍斑巖銅礦帶(a)及扎那尕斑巖銅鉬礦床地簡圖(b) (據文獻[4–5]修改)

1–下二疊統(tǒng)火山巖; 2–上三疊統(tǒng)砂巖; 3–巖脈; 4–扎拉尕礦化斑巖; 5–地質界線; 6–斷層。

1 扎拉尕斑巖礦床地質及礦化特征

扎拉尕斑巖銅鉬礦床位于西藏東部玉龍斑巖銅礦帶北部玉龍斑巖銅礦床南側(圖1), 含礦巖體侵入下二疊統(tǒng)火山巖和上三疊統(tǒng)砂泥巖中, 地表出露面積約0.6 km2。巖體的空間形態(tài)為巖株, 產狀陡立[5]。

扎拉尕斑巖銅鉬礦床含礦巖體具多階段活動特征, 張玉泉等[6–7]據在ZK3孔348.8 m和ZK4孔68 m見到正長花崗斑巖和二長花崗斑巖的侵入接觸關系而把巖體分為早晚兩階段: 早階段為二長花崗斑巖, 晚階段為正長花崗斑巖。扎拉尕含礦斑巖斑晶主要為鉀長石、斜長石、石英及云母和少量角閃石, 斑晶礦物粒度主要在0.31~3.0 mm之間, 基質為顯晶質, 基質礦物組成和斑晶的礦物組成相似。扎拉尕早階段及晚階段斑巖都發(fā)生了蝕變及礦化, 蝕變由內向外可分為鉀硅化帶、黃鐵絹英巖化帶、黏土化帶及青磐巖化帶[4–5]。鉀化帶主要分布于巖體內, 黃鐵絹英巖化帶主要分布于巖體內外接觸帶, 其余2個蝕變帶以接觸帶為中心依次向外展布, 各蝕變帶之間呈漸變過渡關系[4–5]。銅鉬礦化主要發(fā)育于鉀硅化帶和黃鐵絹英巖化帶疊加部位。Cu含量在垂向變化上變化不大, Mo則有由上往下增強的趨勢[4–5]。

扎拉尕斑巖礦床為中型礦床, 其Cu金屬量為0.3 Mt, Cu平均含量在0.36%左右, Mo含量約為0.03%, Au約0.03 μg/g[27]。礦化主要為細脈浸染狀發(fā)育于巖體和接觸帶圍巖中, 礦化體為柱狀, 鉆孔未穿透礦化巖體, 控制礦化厚度約540 m, 直徑約200 m[5]。主要金屬礦物黃銅礦、輝鉬礦、黃鐵礦、磁鐵礦、輝銅礦等, 偶見方鉛礦、閃鋅礦等。

2 分析方法

兩個鋯石LA-ICP-MS U-Pb定年樣品分選自巖芯, 早階段二長花崗斑巖采自ZK3孔345.3 m處, 晚階段正長花崗斑巖采自ZK4孔200 m處(圖1)。樣品經碎樣、磁選及重液選等選出鋯石, 鏡下挑選純后裝入環(huán)氧樹脂靶中并打磨拋光至鋯石內部結構充分暴露。根據光學顯微鏡透反射特征和掃描電鏡陰極發(fā)光(CL)照片綜合選出晶形較好, 沒裂紋及包裹體不發(fā)育的鋯石顆粒對其進行LA-ICP-MS U-Pb年代學測試。鋯石定年在澳大利亞國立大學地球科學研究院ICP-MS實驗室完成, 分析流程見文獻[35]。為了減少繼承鉛、鉛丟失等對年齡的影響, 在207Pb/235U-206Pb/238U圖中和諧度低于95%的年齡數據點在統(tǒng)計年齡時將被剔除。

3 分析結果

扎拉尕斑巖銅鉬礦床賦礦巖體早階段二長花崗斑巖和晚階段正長花崗斑巖鋯石U-Pb同位素組成特征見表1及表2, 扎拉尕賦礦斑巖礦床早階段二長花崗斑巖鋯石LA-ICP-MS U-Pb年齡比較集中, 分布在37.4~39.6 Ma之間, Th/U在0.28~1.26之間; 晚階段正長花崗斑巖鋯石LA-ICP-MS U-Pb年齡主要分布在至37.6~39.0 Ma之間, Th/U在0.32~0.79之間。

扎拉尕賦礦斑巖體鋯石CL圖韻律環(huán)帶發(fā)育(圖2), 具巖漿鋯石的一般特征, 此外, 扎拉尕斑巖鋯石Th/U比值較大, 在0.28~1.26之間, 也具巖漿鋯石的特征。因此, 可以認為分析鋯石為巖漿結晶作用過程中形成的鋯石, 鋯石U-Pb主群年齡代表巖漿侵位年齡。LA-ICP-MS U-Pb測年具有快速及相對較經濟的優(yōu)點, 為了獲得較精確的年齡值, 我們分析較多鋯石顆粒, 然后用累積概率統(tǒng)計圖分析鋯石年齡分布特征。正態(tài)分布數據在累積概率統(tǒng)計圖上呈一條斜率為正值的直線分布, 落在直線沿伸方向上方的較大年齡被解釋為繼承鋯石年齡, 而分布在直線沿伸方向下方的年齡被解釋為鉛丟失鋯石年齡。在累積概率統(tǒng)計圖上呈線性分布的年齡代表鋯石主群年齡[29,35]。

圖2 扎拉尕斑巖銅鉬礦床賦礦巖體鋯石CL圖

表1 扎拉尕斑巖早階段二長花崗斑巖鋯石LA-ICP-MS U-Pb年齡

4 討 論

4.1 扎拉尕斑巖體形成時代

我們對扎拉尕早階段二長花崗斑巖共做了31點鋯石U-Pb同位素組成分析, 所有分析點在207Pb/235U-206Pb/238U圖中和諧度都高于95%, 因此, 所有的分析點數據都符合計算年齡要求。31個分析點數據在累積概率統(tǒng)計圖中為直線分布(圖3a, 內插), 其統(tǒng)計年齡代表主群年齡。這31個分析點得出的統(tǒng)計年齡值為(38.5 ±0.2) Ma, MSWD = 1.12, 因此, 扎拉尕早階段二長花崗斑巖是在(38.5±0.2) Ma侵位形成的。

在扎拉尕晚階段正長花崗斑巖鋯石29個分析點中, 1個分析點在207Pb/235U-206Pb/238U圖中和諧度小于95%, 其余28個分析點諧度大于95%, 因此, 1個分析點在計算年齡時被剔除(表2)。其余28個分析點數據在累積概率統(tǒng)計圖中為直線分布(圖3b, 內插), 其統(tǒng)計年齡代表主群年齡。扎拉尕晚階段正長花崗斑巖鋯石28個分析點U-Pb統(tǒng)計年齡值為(38.5±0.2) Ma, MSWD = 1.08, 因此, 扎那尕晚階正長花崗斑巖是在(38.5±0.2) Ma侵位形成的。

表2 扎拉尕斑巖晚階段正長花崗斑巖鋯石LA-ICP-MS U-Pb年齡

注: (1) 141-14號分析點因和諧度低于95%而剔除。

雖然在鉆孔中見到早階段二長花崗斑巖與晚階段正長花崗斑巖呈侵入接觸關系[6–7], 但我們的分析結果表明, 扎拉尕賦礦斑巖早階段二長花崗斑巖和晚階段正長花崗斑巖鋯石LA-ICP-MS U-Pb年齡卻一致。這表明扎拉尕早階段二長花崗斑巖和階段正長花崗斑巖是在很短時間隔間內形成的, 目前同位素定年精度難以區(qū)分早晚兩階段巖漿侵位時代的差異, 扎那尕賦礦斑巖形成時代為(38.5±0.2) Ma。

我們獲得扎拉尕賦礦斑巖體鋯石LA-ICP-MS U-Pb年齡((38.5±0.2) Ma)和前人鉀長石和黑云母K-Ar年齡(33.9~41.0 Ma)[1,5,6]明顯不同。我們認為兩種同位素體系所得的同位素年齡不同, 主要原因是兩種同位素體系抗后期地質熱事件干擾能力不同。鉀長石或黑云母K-Ar同位素體系封閉溫度較低, 在300 ℃左右[36], 易受后期地質事件干擾。如前所述, 扎拉尕巖體發(fā)生了溫度多大于300 ℃的鉀化及硅化[32], 巖體中的鉀長石和黑云母K-Ar同位素封閉體系會在巖體發(fā)生鉀硅化蝕變時受到干擾, 發(fā)生放射性成因Ar的增加或丟失, 導致K-Ar同位素年齡發(fā)生變化而不能精確反映巖體形成年齡; 而鋯石U-Pb同位素系統(tǒng)封閉溫度在800 ℃左右[36], 抗后期干憂能力強, 后期鉀硅化及其他地質熱事件一般不會破壞鋯石U-Pb同位素封閉系統(tǒng), 因此, 扎拉尕賦礦斑巖鋯石LA-ICP-MS U-Pb法年齡能更精確地反映巖體形成年齡。

4.2 含礦巖體形成動力學背景分析

印度板塊與歐亞板塊在約65 Ma左右時發(fā)生的陸陸碰撞在藏東地區(qū)形成了紅河-哀牢山巨型左旋走滑斷裂[37–38]。沿紅河-哀牢山巨型左旋走滑斷裂系統(tǒng)及其北延斷裂發(fā)育一系列新生代鉀質堿性巖及和鉀質堿性巖相關的斑巖型銅鉬(金)礦床, 形成三江新生代與鉀質堿性巖有關的成礦帶。玉龍斑巖銅礦帶位于紅河-哀牢山巨型斷裂帶北延斷裂系統(tǒng)中[39]。目前對三江新生代鉀質堿性巖成礦帶形成動力學背景有不同的看法, Chung.[34]提出藏東新生代鉀質堿性巖與藏東地區(qū)40 Ma 左右發(fā)生的地幔對流導致巖石圈減薄有關; 張玉泉等[40]提出哀牢山-金沙江新生代富堿侵入巖形成于裂谷構造背景; Wang.[39]提出哀牢山-紅河始新世至漸新世富堿侵入巖和沿著紅河-哀牢山深大走滑深大斷裂活動產生的局部俯沖有關。

圖3 扎拉尕斑巖銅鉬礦床含礦巖體鋯石LA-ICP-MS U-Pb年齡(內插圖為累積概率統(tǒng)計圖)

紅河-哀牢山走滑斷裂帶兩側晚三疊紀到始新世陸相紅層發(fā)生褶皺及沿紅河-在哀牢山斷裂及其北延斷裂發(fā)育一系列始新世至早漸新世扭壓盆地[41], 表明紅河-哀牢山走滑斷裂系統(tǒng)在始新世至早中新世處于扭壓構造背景[32,39]。紅河-哀牢山巨型左行走滑斷裂帶水平錯距(700±200) km, 切穿巖石圈地幔[37–38]。切穿巖石圈地幔斷裂活動引起軟流圈上涌, 從而會導致巖石圈地幔部分熔融[38]。

玉龍斑巖銅礦帶主要賦礦巖體微量元素及Sr-Nd-Pb同位素組成特征[40]和沿紅河-哀牢山巨型左行走滑斷裂分布的鐵鎂質至硅鋁質鉀質堿性巖的相似[38,41–43], 都具交代富集地幔源區(qū)的地球化學特征[33,42,44,45], 顯示它們具相似的形成機制。前人成果表明, 沿著紅河-哀牢山左行走滑深大斷裂活動帶分布的鐵鎂質至硅鋁質鉀質堿性巖與該深大斷裂左行走滑活產生的局部俯沖有關[39]。玉龍斑巖銅礦帶位于紅河-哀牢山左行走滑深大斷裂北延斷裂系統(tǒng), 而玉龍斑巖銅礦帶從北西往南東五個主要賦礦巖體從玉龍(鋯石U-Pb年齡(41.3±0.2) Ma[12])、扎拉尕(鋯石U-Pb年齡(38.5±0.2) Ma)、莽總(鋯石U-Pb年齡(37.6±0.2) Ma[26])、多霞松多(鋯石U-Pb年齡(37.5± 0.2) Ma[30])至馬拉松多(鋯石U-Pb年齡(36.9±0.6) Ma[13])年齡逐漸降低, 顯示出左行斷裂構造控制的特征。因此, 有理由認為玉龍斑巖銅礦帶賦礦巖體的形成與印度板塊與歐亞板塊在65 Ma左右時發(fā)生的陸陸碰撞, 在藏東三江地區(qū)產生切穿巖石圈地幔的紅河-哀牢山左行走滑斷裂及其北延斷裂系統(tǒng)活動, 產生局部俯沖, 誘發(fā)地幔上涌, 發(fā)生強烈殼幔物質交換及引發(fā)的巖漿活動有關, 玉龍斑巖銅礦帶形成于陸陸碰撞走滑構造背景。

5 主要結論

(1) 扎拉尕銅鉬礦賦礦斑巖體主要由早階段二長花斑巖斑和晚階段正長花崗斑巖組成, 早階段二長花崗斑巖鋯石LA-ICP-MS U-Pb和晚階段長花崗斑巖鋯石LA-ICP-MS U-Pb年齡相同, 都是(38.5± 0.2) Ma。扎拉尕斑巖早晚兩階段巖體是成礦巖漿在很短時間隔間內侵位形成的。

(2) 玉龍斑巖銅礦帶斑巖銅鉬礦床與陸陸碰撞形成的走滑深大斷裂誘發(fā)的殼幔相互作用有關, 玉龍斑巖銅礦帶形成于陸陸碰撞走滑構造背景。

野外工作期間西藏地質礦產局第一地質大隊給予了大力協(xié)助; 澳大利亞國立大學地球科學研究院ICP-MS實驗室在實驗工作中給予了大力支持, 特此致謝!

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Zircon LA-ICP-MS U-Pb age of the Zalaga porphyry associated with Cu-Mo mineralization in the Yulong ore belt and its geological implication

HE Guo-chao1,2, WANG Guang-qiang2,3, HUANG Wen-ting2,3, ZOU Yin-qiao2,3, WU Jing2*, LIANG Hua-ying2, ZHANG Yu-quan2and Charllote M ALLEN4

1. Resource Exploration Co. Ltd of Guangxi Non-Ferrous Metal Group Co., Ltd, Nanning 530022, China; 2. Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China; 3. University of Chinese Academy of Sciences, Beijing 100049, China; 4. Research School of Earth Sciences, Australian National University, Canberra ACT0200, Australia

The Zalaga porphyry Cu-Mo deposit is located at the northern domain of the Yulong porphyry copper ore belt in the eastern Tibet. The porphyry emplaced into the lower Permian volcanic rock and Late Triassic sandstone and could be divided into early stage monzonite granite porphyry and late stage syenogranite porphyry. Both stages of the porphyry were dated, respectively, using zircon LA-ICP-MS U-Pb method. The early stage monzonite granite porphyry has zircon LA-ICP-MS U-Pb age of (38.5±0.2) Ma, MSWD=1.12 and the late stage syenogranite porphyry has zircon LA-ICP-MS U-Pb age of (38.5±0.2) Ma, MSWD=1.08. The early porphyry has the same zircon LA-ICP-MS U-Pb age as that of the late stage porphyry, suggesting that the early porphyry and late stage porphyry emplaced almost at the same time and that the Zalaga porphyry was emplaced during the Late Eocene. Based on the structural condition of the period during Eocene to Oligocene in eastern Tibet, it is concluded that the Zalaga porphyry Cu-Mo(Au) deposit and the Yulong porphyry copper ore belt has genetic relation to the activities of strike-slip fault zone which transverses the lithosphere mantle and triggered the magmatic activities caused by the collision between Indian and Asia continents.

porphyry Cu-Mo deposit; age of porphyry and associated mineralization; continental collision and mineralization; Eastern Tibet

P597

A

0379-1726(2014)04-0399-09

2013-04-08;

2013-12-09;

2013-12-23

中國科學院戰(zhàn)略性先導科技專項(B類) (XDB03010302); 國家自然科學基金(41272099, 41121002)

何國朝(1964–), 男, 教授級高級工程師, 主要從事礦產地質及勘查工作。E-mail: 124737253@qq.com

WU Jing, E-mail: wujing7808@163.com, Tel: +86-20-85290107