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Received:October 08, 2011  Revised:March 28, 2012  点此下载全文
Cite This Paper:.2013.Magma Source of the A-Type Granite and Slab Break-Off in the Middle Segment of the Bangonghu-Nujiang Suture, Tibet Plateau[J].Acta Geologica Sinica,87(6):759-772
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Author NameAffiliationE-mail
曲晓明 中国地质科学院矿产资源研究所北京100037 xiaominqu@163.com 
辛洪波 中国地质科学院矿产资源研究所北京100037  
杜德道 中国地质科学院矿产资源研究所北京100037  
陈华 中国地质科学院矿产资源研究所北京100037  
中文摘要:近年来地质调查在班公湖 怒江缝合带中段发现了一批A 型花岗岩,岩体呈大小几百平方米的岩株产出,岩性上分为黑云母花岗岩和花岗闪长斑岩两种,侵入白垩系地层中。全岩元素地球化学分析显示这些A 型花岗岩相对富硅, SiO2含量在68.62%~75.36%之间,全碱(K2O+Na2O=8.03%~9.37%)和全铁(Fe2O3T=0.86%~5.39%)含量偏高,Al2O3(12.76%~15.54%)偏低,显示弱过铝质和亚铝质特征。微量元素中大离子亲石元素(LILE)Rb、Th、U、K、Pb明显富集,Ba、Sr明显亏损;同样在高场强元素(HFSE)中Nb、Ta、Ti亏损明显,但Zr、Hf 相对富集,这些与岛弧型花岗岩都是明显不同的,显示出了A 型花岗岩的特征。岩体稀土元素含量总体较高(∑REE=122.37×10-6~291.19×10-6,平均为201.31×10-6),相对富集轻稀土元素(LREE/HREE=4.89~9.58,平均为5.93),负Eu异常明显(〖QX(Y12#〗δ〖QX)〗Eu=0.14~0.54,平均为0.34),造成分配曲线呈向右缓倾的V 型分布型式。Nd、Sr、Pb同位素分析显示班公湖 怒江缝合带中段的这些A 型花岗岩富集放射成因同位素,其中87Sr/86Sr、87Rb/86Sr及Isr比值分别在0.719842~0.786395 、6.7171~48.8063和 0.706900 ~0.710378之间;143Nb/144Nb和147Sm/144Nd比值分别在0.512123~0.512392和0.0853~0.2847之间,〖QX(Y12#〗ε〖QX)〗Nd为较大的负值(在-3.37~-10.34之间),显示出地壳组分的重要影响。206Pb/204Pb、207Pb/204Pb和208Pb/204Pb比值分别在18.703~19.070、15.680~15.732、和39.121~39.576之间,在Zartman等(1981)的Pb构造模式图上位于上地壳线和造山带演化线之上,显示出异常高的富集组分。分析表明班公湖 怒江缝合带中段的A 型花岗岩是岩石圈地幔在洋壳俯冲过程中受到沉积物熔体的交代富集,之后早白垩世晚期(110Ma±)在碰撞后伸展阶段由于俯冲的洋壳板层断离形成了“板片窗”,软流圈物质借此上涌,引起富集的岩石圈地幔(接近EMⅡ)发生部分熔融形成的。
中文关键词:A 型花岗岩  Nd、Sr、Pb同位素  岩浆源区  板片断离  班公湖 怒江缝合带  青藏高原
Magma Source of the A-Type Granite and Slab Break-Off in the Middle Segment of the Bangonghu-Nujiang Suture, Tibet Plateau
Abstract:During the geological investigation of last several years, a group of A type granitic plutons has been found in the middle segment of the Bangonghu Nujiang suture. These granitic plutons occur as stocks with an exposed area of less than1 km2 and lithologically include two types of biotite granite and granodiorite porphyry, which intrude into Cretaceous sedimentary rocks. A systematic analysis of whole rock element geochemistry and Nd, Sr, Pb isotope for these A type granites indicates that they contain high SiO2 content varying from 68.62%~75.36%. Moreover they have relatively high K2O+Na2O(8.03%~9.37%)and Fe2O3T(0.86%~5.39%) and low Al2O3(12.76%~15.54%) content, appearing weak peraluminous and metaluminous features. Trace elements N MORB normalized spider diagram displays apparent enrichment in large ion lithophile element(LILE) Rb, Th, U, K, Pb with Ba and Sr of marked depletion. High field strenth element(HFSE) Nb, Ta and Ti are sharply depleted with Zr and Hf of relative enrichment. All of these are characteristic of A type granite and different from arc granite. These granites have high REE contents(∑REE=122.37×10-6~291.19×10-6,with an average of 201.31×10-6). Light REE are rich relative to heavy REE(LREE/HREE=4.89~9.58, with an average of 5.93)and negative Eu anormalies are sharp(〖QX(Y12#〗δ〖QX)〗Eu=0.14~0.54,with mean value of 0.34), making their chondrite normalized rare earth element patterns appear slightly right dipping V type. Nd, Sr and Pb isotope results indicate that the A type granites from the middle segment of the Bangonghu Nujiang suture are rich in radioactive isotope. Their 87Sr/86Sr and ISr ratio values vary from 0.719842~0.786395 and 0.706900~0.710378, respectively. 143Nd/144Nd ratio values are from 0.512123~0.512392, with large negativeεNd values(-3.37~-10.34), reflecting important involvement of crustal components. Pb isotope composition 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb display ranges of 18.703~19.070, 15.680~15.732 and 39.121~39.576, respectively, which plot on the upper crustal line and above the orogenic evolution line in the plumtotectonic framework diagram of Zartman and Doe (1981) and show anormaly high enrichment components. The study points out that the A type granites were derived from partial melting of enriched lithospheric mantle (approximating EMⅡ), which were metasomalized by sediment melt from slab during the oceanic crust subduction and later heat by upwelling asthenospheric materials through slab windows formed as a result of slab break off during the postcollisional extension of the orogenic zone.
keywords:A type granites  Nd, Sr, Pb isotope  magma sources  slab break off  Bangonghu Nujiang suture  Tibet plateau
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