甘肃省北祁连地区北大坂岩体锆石U-Pb年龄及其地质意义
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摘要
北大坂花岗岩体位于北祁连中东段,为了精确测定岩体形成年代,对岩体中的浅灰黑色中粗粒英云闪长岩锆石进行了阴极发光(CL)成像观察,并利用LA-MC-ICP-MS对锆石进行微区U-Pb同位素测年。锆石的加权平均年龄为(447. 2±2. 4) Ma,表明其属于晚奥陶世岩浆活动产物。岩石地球化学数据分析表明,浅灰黑色中粗粒英云闪长岩为准铝质-弱过铝质高钾钙碱性花岗岩,富集大离子亲石元素,亏损高场强元素,稀土元素配分曲线呈右倾型,轻、重稀土元素分馏较为明显,显示出了I型花岗岩的特征。样品在构造判别图上均落入后碰撞花岗岩区域,结合区域上其它花岗岩体形成的构造环境研究,反映了北大坂岩体形成于北祁连洋壳的北向俯冲作用导致的北祁连岛弧与阿拉善地块碰撞期结束后的构造环境,暗示着北祁连洋在奥陶世晚期已经闭合。
The Baidaban granite locates in mid-east of Northern Qinlian. In order to determine the exact formation age of the granite,cathodoluminescence( CL) imaging and LA-MC-ICP-MS Zircon U-Pb for the granite were carried out. TheU-Pb dating of these zircon yidlded a weighted mean age of( 447. 2 ± 2. 4) Ma. Geochemical data show that granodiorite is a aluminous graniteand belongs to the high-K calc-alkaline series. It is characterized by enrichment of LILE and depletion of HFSE,with REE patterns exhibiting the right-deviation type and strong fractionation with LREE enrichment. These show that the granites have characteristics of I-typegranite. These features suggest that the granodiorite has the characteristics of post-collisional granite. Combine the tectonic environment of granite body in adjacent region,the Baidaban granite body formed in a collision tectonic setting of the North Qilian island arc colliding with the Alaxa massif,which may be induced by the northward subduction of the North Qilian ocean crust. It implied that the North Qilian ocean was closed in Late Qrdovician.
The Baidaban granite locates in mid-east of Northern Qinlian. In order to determine the exact formation age of the granite,cathodoluminescence( CL) imaging and LA-MC-ICP-MS Zircon U-Pb for the granite were carried out. TheU-Pb dating of these zircon yidlded a weighted mean age of( 447. 2 ± 2. 4) Ma. Geochemical data show that granodiorite is a aluminous graniteand belongs to the high-K calc-alkaline series. It is characterized by enrichment of LILE and depletion of HFSE,with REE patterns exhibiting the right-deviation type and strong fractionation with LREE enrichment. These show that the granites have characteristics of I-typegranite. These features suggest that the granodiorite has the characteristics of post-collisional granite. Combine the tectonic environment of granite body in adjacent region,the Baidaban granite body formed in a collision tectonic setting of the North Qilian island arc colliding with the Alaxa massif,which may be induced by the northward subduction of the North Qilian ocean crust. It implied that the North Qilian ocean was closed in Late Qrdovician.
引文
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刘明强.2004.甘肃北祁连武威地区莲花山花岗岩体的单颗粒锆石U-Pb年龄及其地质意义[J].兰州大学学报(自然科学报),49(4):401-402.
孙宝璐,钱青,张建新.2017.甘肃大佛寺、金佛寺花岗岩体的锆石U-Pb年龄、Hf-O同位素和全岩地球化学特征及地质意义[J].岩石学报,33(10):3091-3107.
汤超,李志丹,陈军强,等.2015.内蒙古达茂旗阿拉格敖包地区英云闪长岩地球化学特征及构造背景[J].矿物岩石,35(2):51-59.
汤琳,张树明.2015.龙首山芨岭复式岩体岩石地球化学特征及构造环境探讨[J].东华理工大学学报(自然科学版),38(3):265-272.
王强,赵振华,熊小林.2000.桐柏-大别山造山带燕山晚期A型花岗岩的厘定[J].岩石矿物学杂志,19(4):297-306.
吴才来,徐学仪,高前明,等.2010.北祁连早古生代花岗质岩浆作用及构造演化[J].岩石学报,26(4):1027-1044.
吴才来,杨经绥,杨宏仪,等.2004.北祁连东部两类I型花岗岩定年及其地质意义[J].岩石学报,20(3):425-432.
吴福元,李献华,杨进辉,等.2007.花岗岩成因研究的若干问题[J].岩石学报,23(6):1217-1238.
夏林圻,夏祖春,徐学义.1995.北祁连山构造-火山岩浆演化动力学[J].西北地质科学,16(1):1-28.
熊子良,张宏飞,张杰.2012.北祁连东段冷龙岭地区毛藏寺岩体和黄羊河岩体的岩石成因及其构造意义[J].地学前沿:中国地质大学,19(3):214-227.
赵国斌,杨合群,任华宁,等.2013.北祁连黑石山花岗岩LA-ICP-MS锆石U-Pb年龄及其地质意义[J].地质通报,32(10):1575-1583.
Liegeios J P,Naver J,Hertogen J,et al.1998.Contrasting origin of postcollisional high-k calc-alkaline and shoshonitic versus alkaline and peralkaline granitoida[J].The use of sliding normalization Lithos,45:1-28.
Sun S S,Mc Donough W F.1989.Chemical and isotopic systematics of oceanic basalts:Implications for mantle composition and processes[M]//Saunders A D,Norry M J.Magmatism in Oceanic Basins.Geological Society,London,Special Publications:313-345.