敦煌地块南缘多坝沟花岗岩锆石U-Pb定年、地球化学特征及其地质意义
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摘要
多坝沟黑云二长花岗岩体出露于敦煌地块南缘,呈近东西向带状分布,侵位于太古宙—古元古代敦煌岩群。LA-ICP-MS锆石U-Pb年龄显示,该岩体形成于晚二叠世,~(206)Pb/~(238)U年龄加权平均值为252.1±2.3Ma。该岩体具高SiO_2含量(68.32%～72.82%)和全碱含量(7.76%～8.00%);A/CNK值为0.97～1.02,均小于1.1,为高钾钙碱性准铝质花岗岩。岩石具有明显的轻、重稀土元素分异和弱的负Eu异常,具高Sr/Yb和(La/Yb)_N值富集大离子亲石元素(K、Rb、Ba、Cs),显著亏损Nb、Ta、P和Ti,表明源岩具有经典岛弧岩石地球化学特征;(~(87)Sr/~(86)Sr)_i值为0.707083,ε_(Nd)(t)为-2.22,Nd模式年龄T_(DM2)=1102Ma。元素和同位素地球化学特征显示,多坝沟花岗岩形成于加厚下地壳中岛弧玄武岩的部分熔融。结合岩体地质特征、年代学和区域地质背景,初步认为敦煌地块在晚古生代晚期依然受古亚洲洋汇聚碰撞相关的造山活动的影响,可能是中亚造山带的最南缘组成部分。
The Duobagou biotite monzonitic granitic pluton, distributed as a nearly EW-trending belt, intruded into Archean –Paleoproterozoic Dunhuang complexes in southern Dunhuang landmass. Zircon LA ICP-MS U-Pb data revealed the crystallization age of 252.1 ± 2.3 Ma, suggesting that Duobagou pluton was formed in Late Permian. Geochemical characteristics of monzonitic granite are similar to those of typical arc rocks. For example, the granites have high content of SiO_2(68.32% ～ 72.82%) and total alkali(7.76% ～ 8.00%), and their A/CNK ratios are 0.97 to 1.02, which are less than 1.1, indicating that Duobagou granites are high potassium calcium alkaline aluminium. They have obvious differentiation of REEs and weak Eu negative anomaly, and are characterized by strong enrichment of K, Rb, Ba and Cs and depletion of Nb, Ta, P, and Ti, with high ratios of Sr/Yb and(La/Yb)N.The sample PM04-1 shows high initial Sr isotopic ratio(0.707083), relatively low ε_(Nd)(t)(-2.22) and Nd model ages(T_(DM2)=1102 Ma). The authors thus hold that the Duobagou biotite monzonitic granites may be the product of partial melting of island arc basalt in thickened low crust, and the Dunhuang landmass was likely to be involved into a series of orogenic events by the closing of the Palaeo-Asian Ocean at the end of Late Paleozoic, and possibly became the southernmost part of central Asian orogenic belt.
The Duobagou biotite monzonitic granitic pluton, distributed as a nearly EW-trending belt, intruded into Archean –Paleoproterozoic Dunhuang complexes in southern Dunhuang landmass. Zircon LA ICP-MS U-Pb data revealed the crystallization age of 252.1 ± 2.3 Ma, suggesting that Duobagou pluton was formed in Late Permian. Geochemical characteristics of monzonitic granite are similar to those of typical arc rocks. For example, the granites have high content of SiO_2(68.32% ～ 72.82%) and total alkali(7.76% ～ 8.00%), and their A/CNK ratios are 0.97 to 1.02, which are less than 1.1, indicating that Duobagou granites are high potassium calcium alkaline aluminium. They have obvious differentiation of REEs and weak Eu negative anomaly, and are characterized by strong enrichment of K, Rb, Ba and Cs and depletion of Nb, Ta, P, and Ti, with high ratios of Sr/Yb and(La/Yb)N.The sample PM04-1 shows high initial Sr isotopic ratio(0.707083), relatively low ε_(Nd)(t)(-2.22) and Nd model ages(T_(DM2)=1102 Ma). The authors thus hold that the Duobagou biotite monzonitic granites may be the product of partial melting of island arc basalt in thickened low crust, and the Dunhuang landmass was likely to be involved into a series of orogenic events by the closing of the Palaeo-Asian Ocean at the end of Late Paleozoic, and possibly became the southernmost part of central Asian orogenic belt.
引文
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[2]Zhao Y,Sun Y,Diwu C R,et al.The Dunhuang block is a Paleozoic orogenic belt and part of the Central Asian Orogenic Belt(CAOB),NW China[J].Gondwana Research,2016,30:207-223.
[3]张志诚,郭召杰,邹冠群,等.甘肃敦煌党河水库TTG地球化学特征,锆石SHRIMP U-Pb定年及其构造意义[J].岩石学报,2009,25(3):495-505.
[4]朱涛,王洪亮,徐学义,等.敦煌地块南缘石炭纪埃达克岩的发现及其地质意义[J].岩石学报,2014,30(2):491-502.
[5]刘东晓,王玉玺,王金荣,等.塔里木板块东南缘早二叠世角闪石岩的发现及大地构造意义[J].兰州大学学报(自然科学版),2016,52(2):167-178.
[6]王玉玺,第鹏飞,陈万峰,等.塔里木板块东南缘多坝沟金矿区花岗岩类年代学、地球化学特征及找矿意义[J].西北地质,2017,50(1):134-150.
[7]王玉玺,王金荣,周小玲,等.Columbia超大陆裂解:来自塔里木克拉通东南缘大红山A型花岗岩的证据[J].地质学报,2017,91(11):2369-2386.
[8]许志琴,杨经绥,张建新,等.阿尔金断裂两侧构造单元的对比及岩石圈剪切机制[J].地质学报,1999,73(3):193-205.
[9]Lu S N,Zhao G C,Wang H C,et al.Precambrian metamorphic basement and sedimentary cover of the North China Craton:a review[J].Precambrian Research,2008,160(1):77-93.
[10]辛后田,刘永顺,罗照华,等.塔里木盆地东南缘阿克塔什塔格地区新太古代陆壳增生:米兰岩群和TTG片麻岩的地球化学及年代学约束[J].地学前缘,2013,(1):240-259.
[11]赵燕,朱涛,王洪亮,等.敦煌三危山地区晚泥盆世斜长花岗岩的发现及其地质意义[J].岩石学报,2015,(7):1855-1869.
[12]郑荣国,吴泰然,张文,等.阿拉善地块北缘雅干花岗岩体地球化学,地质年代学及其对区域构造演化制约[J].岩石学报,2013,29(8):2665-2675.
[13]Zheng R G,Wu T R,Zhang W,et al.Late Paleozoic subduction system in the northern margin of the Alxa block,Altaids:Geochronological and geochemical evidence from ophiolites[J].Gondwana Research,2014,25(2),842-858
[14]Ge R F,Zhu W B,Wu H L,et al.The Paleozoic northern margin of the Tarim Craton:Passive or active?[J].Lithos,2012,142/143:1-15.
[15]Zong K Q,Zhang Z M,He Z Y,et al.Early Palaeozoic highpressure granulites from the Dunhuang block,northeastern Tarim Craton:constraints on continental collision in the southern Central Asian Orogenic Belt[J].Journal of Metamorphic Geology,2012,30(8):753-768.
[16]He Z Y,Zhang Z M,Zong K Q,et al.Metamorphic p-T-t evolution of mafic HP granulites in the northeastern segment of the Tarim Craton(Dunhuang block):Evidence for early Paleozoic continental subduction[J].Lithos,2014,196(5):1-13.
[17]王楠,吴才来,马昌前,等.敦煌地块三危山地区花岗岩体地球化学,锆石U-Pb定年及Hf同位素特征[J].地质学报,2016,90(10):2681-2705.
[18]Zhang J X,Yu S Y,Gong J H,et al.The latest NeoarcheanPaleoproterozoic evolution of the Dunhuang block,eastern Tarim craton,northwestern China:Evidence from zircon U-Pb dating and Hf isotopic analyses[J].Precambrian Research,2013,226:21-42.
[19]辛后田,赵凤清,罗照华,等.塔里木盆地东南缘阿克塔什塔格地区古元古代精细年代格架的建立及其地质意义[J].地质学报,2011,85(12):1977-1993.
[20]陈柏林,杨屹,王小凤,等.阿尔金北缘大平沟金矿床成因[J].矿床地质,2005,24(2):168-178.
[21]李月臣,陈柏林,陈正乐,等.阿尔金北缘红柳沟-拉配泉一带铜金矿床硫同位素特征及其意义[J].地质力学学报,2007,13(2):131-140.
[22]孟繁聪,张建新,相振群,等.塔里木盆地东北缘敦煌群的形成和演化:锆石U-Pb年代学和Lu-Hf同位素证据[J].岩石学报,2011,27(1):59-76.
[23]朱江,吕新彪,彭三国,等.甘肃北山小西弓金矿区石英正长斑岩LA-ICP-MS锆石U-Pb年龄和地球化学特征[J].地质通报,2015,34(8):1460-1469.
[24]Jackson S E,Pearson N J,Griffin W L,et al.The application of laser ablation-inductively coupled plasma-mass spectrometry to in situ U-Pb zircon geochronology[J].Chemical Geology,2004,211(1):47-69.
[25]Van Achterbergh E,Ryan C G,Jackson S E,et al.Data reduction software for LA-ICP-MS in the Earth Sciences[J].Mineralogical Association of Canada,2001,29:239-243.
[26]Anderson T.Correction of common lead in U-Pb analyses that do not report204Pb[J].Chemical Geology,2002,192(1):59-79.
[27]Ludwig K R.Mathematical-statistical treatment of data and errors for230Th/U geochronology[J].Reviews in Mineralogy and Geochemistry,2003,52(1):631-656.
[28]Maniar P,Piccoli P.Tectonic discrimination of granitoids[J].Geol.Soc.Amer.Bull.,1989,101:635-643.
[29]Collins W J,Beams S D,White A J K,et al.Nature and origin of a type granites with articular reference to southeastern Australia[J].Contrbutions to Mineralogy Petrology,1982,80:189-200.
[30]Sun S S,McDonough W F.Chemical and isotopic systematic of oceanic basalts:Implications for mantle composition and processes[C]//Saunders A D,Norry M J.Magmatism in the Ocean Basins.Geol.Soc.Special Publication,1989,42:313-345.
[31]Pearce J A,Harris N B W,Tindle A G.Trance element discrimination diagrams for the tectonic interpretation of granitic rocks[J].Journal of Petrology,1984,25:956-983.
[32]吴福元,李献华,杨进辉,等.花岗岩成因研究的若干问题[J].岩石学报,2007,23(6):1217-1254.
[33]张旗,李承东.花岗岩:地球动力学意义[M].青岛:海洋出版社,2012:1-81.
[34]Whalen J B,Currie K L,Chappell B W.A-type granites:Geochemical characteristics,discrimination and Petrogenesis[J].Contribution to Mineralogy and Petrology,1987,95(4):407-419.
[35]Defant M J,Drummond M S.Derivation of some modern arc magmas by melting of young subducted lithosphere[J].Nature,1990,347:662-665.
[36]张旗,王焰,熊小林,等.埃达克岩和花岗岩:挑战与机遇[M].北京:中国大地出版社,2008.
[37]Sajona F G,Maury R C,Pubellier M,et al.Magmatic source enrichment by slab-derived melts in a young post-collision setting,central Mindanao(Philippines)[J].Lithos,2000,54(3):173-206.
[38]Reagan M K,Gill J B.Coexisting calcalkaline and high-niobium basalts from Turrialba Volcano,Costa Rica:Implications for residual titanates in arc magma sources[J].Journal of Geophysical Research:Solid Earth,1989,94(B4):4619-4633.
[39]Gao S,Rudnick R L,Yuan H L,et al.Recycling lower continental crust in the North China craton[J].Nature,2004,432(7019):892-897.
[40]Rapp R P,Shimizu N,Norman M D,et al.Reaction between slab-derived melts and peridotite in the mantle wedge:experimental constraints at 3.8GPa[J].Chemical Geology,1999,160(4):335-356.
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