岩石学报  2020, Vol. 36 Issue (2): 443-454, doi: 10.18654/1000-0569/2020.02.07   PDF    
引用本文
于云鹏, 王明, 解超明, 范建军, 罗安波, 曾孝文. 2020. 西藏拉萨地块北部晚白垩世晚期基性岩墙的成因:来自锆石U-Pb年代学及地球化学的制约. 岩石学报, 36(2): 443-454, doi: 10.18654/1000-0569/2020.02.07  
Yu YP, Wang M, Xie CM, Fan JJ, Luo AB and Zeng XW. 2020. Zircon U-Pb geochronology and geochemisty: Constraints on petrogenesis of the lately Late Cretaceous mafic dykes in northern Lhasa terrane, Tibet. Acta Petrologica Sinica, 36(2): 443-454(in Chinese with English abstract)  
西藏拉萨地块北部晚白垩世晚期基性岩墙的成因:来自锆石U-Pb年代学及地球化学的制约
于云鹏1, 王明1,2, 解超明1,2, 范建军1, 罗安波1, 曾孝文1     
1. 吉林大学地球科学学院, 长春 130061;
2. 东北亚矿产资源评价自然资源部重点实验室, 长春 130061
2019-01-01 收稿; 2019-10-02 改回.
基金项目: 本文受中国地质调查局区域地质调查项目（DD20160026）和国家自然科学基金项目（41602230、41702227）联合资助
第一作者简介: 于云鹏, 男, 1993年生, 博士生, 矿物学、岩石学、矿床学专业, E-mail:309382058@qq.com
通讯作者: 王明, 男, 1984年生, 博士, 副教授, 从事青藏高原大地构造与区域地质研究, E-mail:wm609@163.com.
摘要: 拉萨地块北部阿索地区新发现的晚白垩世晚期基性岩墙对于讨论班公湖-怒江洋盆闭合后的碰撞过程具有重要研究意义。本文报道了阿索地区目思基性岩墙的全岩地球化学组成和LA-ICP-MS锆石U-Pb年龄。研究表明，目思基性岩墙中一组最年轻锆石的U-Pb加权平均年龄为74Ma。岩墙内的辉绿玢岩SiO2含量介于51.90%~53.55%之间，MgO含量介于3.98%~4.97%之间，Mg#为50.0~57.5，低Cr（51.30×10-6~79.48×10-6）和Ni（55.94×10-6~74.17×10-6）含量。岩墙具有轻稀土元素富集的特征，明显的负Eu异常。在微量元素方面，富集Ba、Th、U、K等大离子亲石元素富集，亏损Nb、Ta、Ti等高场强元素，并具有Pb的正异常。La/Sm-Sm/Yb投图结果显示基性岩墙来自于尖晶石+石榴石二辉橄榄岩地幔源区1%~5%的部分熔融，并在演化过程中发生了橄榄石、单斜辉石及斜长石的结晶分异，同时在岩浆上升过程中受到地壳混染。结合区域地质背景，拉萨地块与羌塘地块碰撞后下地壳增厚并发生榴辉岩化，导致拆沉作用。目思基性岩墙的锆石U-Pb年龄表明拆沉作用持续到晚白垩世晚期。
关键词: 拉萨地块北部    晚白垩世晚期    基性岩墙    拆沉    
Zircon U-Pb geochronology and geochemisty: Constraints on petrogenesis of the lately Late Cretaceous mafic dykes in northern Lhasa terrane, Tibet
YU YunPeng1, WANG Ming1,2, XIE ChaoMing1,2, FAN JianJun1, LUO AnBo1, ZENG XiaoWen1     
1. College of Earth Science, Jilin University, Changchun 130061, China;
2. MNR Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Changchun 130061, China
Abstract: The lately Late Cretaceous mafic dykes newly discovered in the Asa area in the northern Lhasa terrane have important research significance for discussing the collision process after the closure of the Bangong-Nujiang ocean basin. This paper reports whole-rock major and trace element compositions and LA-ICP-MS zircon U-Pb ages from Musi mafic dykes in Asa area. Our aim was to identify the petrogenesis of Musi mafic dykes and to constrain the time limit of the delamination. LA-ICP-MS zircon U-Pb isotopic dating indicates that the Musi mafic dykes emplaced at 74Ma. The diabase porphyrites from Musi mafic dykes were characterized by their high content of SiO2 (51.90%~53.55%), MgO (3.98%~4.97%), Mg# (50.0~57.5), Cr (51.30×10-6~79.48×10-6) and Ni (55.94×10-6~74.17×10-6). They are characterized by light rare earth elements (LREE) enriched, and show a strong negative Eu anomaly. The large ion lithophile elements (LILEs) such as Rb, Th, U, K are enriched, and the high field strength elements (HFSEs) such as Nb, Ta, Ti are depleted, with a positive Pb anomaly. The discrimination diagram of La/Sm-Sm/Yb shows that Musi mafic dykes were formed by the magmas derived from 1%~5% partial melting of the spinel+garnet lherzolite mantle. These magmas fractionated mafic minerals such as olivine, clinopyroxene and plagioclase during the evolution process, and assimilated of crustal material during the uplifting. After the collision between the Lhasa terrane and the Qiangtang terrane, the lower crust was thickened and eclogites were formed, resulting in the delamination. Our data, in combination with published geochronological and geochemical data, show that the model of delamination is the small-scale gradual delamination, rather than the whole delamination of the lower crust with mantle. The zircon U-Pb ages of Musi mafic dykes suggest that the delamination lasted until the lately Late Cretaceous.
Key words: Northern Lhasa terrane    Lately Late Cretaceous    Mafic dykes    Delamination    

班公湖-怒江缝合带(后简称班-怒带)横贯青藏高原中部，其北部为羌塘地块，南部为拉萨地块。作为青藏高原众多缝合带中重要的一条，班-怒带的性质、大洋俯冲极性、演化过程与闭合机制过去一直受到众多学者的研究(Yin and Harrison, 2000; Kapp et al., 2003; 潘桂棠等, 2006; 张玉修, 2007)。目前主流的观点根据沉积及岩浆的记录，认为羌塘地块与拉萨地块于早白垩世期间沿班-怒带发生碰撞(Kapp et al., 2007; Sui et al., 2013; Zhu et al., 2016)，随后的持续碰撞导致缝合带附近的地壳在晚白垩世发生了明显的增厚缩短(Guynn et al., 2006; Kapp et al., 2007)。近年来在大规模区域地质调查的基础上，大量岩石年代学、岩石地球化学以及同位素数据的应用，使得拉萨地块北部的晚白垩世岩浆岩演化过程得到明确。拉萨地块中北部94~79Ma的埃达克岩及花岗质岩石为碰撞后加厚下地壳部分熔融及拆沉的产物(Zhao et al., 2008; 余红霞等, 2011; 王保弟等, 2013; 李华亮等, 2014; 雷鸣等, 2015; 雷鸣, 2016; 刘涵等, 2015; Sun et al., 2015; Yi et al., 2018; Liu et al., 2019)，80Ma左右的花岗质岩石则被解释为后碰撞环境下的岩浆作用产物(高顺宝等, 2011; 马蓁, 2013)，而在班公湖地区发现的88Ma辉绿岩脉、79Ma花岗斑岩脉以及措勤地区87Ma双峰式火山岩被认为是伸展环境下的产物(辛洪波和曲晓明, 2006; 江军华等, 2011; 张硕等, 2014)。这些晚白垩世岩浆活动不仅记录了班公湖-怒江洋(后简称班-怒洋)闭合后的碰撞过程：碰撞-地壳加厚-拆沉(伴随伸展)，同时也是认识碰撞后地壳的生长与组成的重要突破口。但由于以往对碰撞后岩浆岩的研究主要集中在晚白垩世早期，而对于晚白垩世晚期(< 80Ma)的基性岩浆事件鲜有报道(Zhu et al., 2016)。因此本文在以往研究的基础上，结合拉萨地块北部晚白垩世晚期基性岩墙的锆石U-Pb年代学及岩石地球化学数据，以期进一步确定拉萨地块北部碰撞后的地壳演化过程。

1 地质概况及样品描述

拉萨地块位于班公湖-怒江缝合带和印度-雅鲁藏布江缝合带之间，并被狮泉河-纳木错蛇绿混杂岩和洛巴堆-米拉山断裂带由南至北分为北、中、南三个块体(图 1a)。冈底斯岩浆带位于拉萨地块内部，其主体为白垩纪和古近纪的岩浆活动产物(莫宣学等, 2005; Harrison et al., 2000; Ji et al., 2012; Zhu et al., 2011; Zhang et al., 2010; Sui et al., 2013)。研究区位于北拉萨地块尼玛县阿索乡一带，在阿索乡以南50km的目思地区出露有一系列近NE-SW走向的晚白垩世基性岩墙(图 1b)。

图 1 西藏南部冈底斯岩浆带白垩世火山岩分布(a, 据Zhu et al., 2011修改)和目思地区地质简图(b) 年龄数据来自：Zhao et al., 2008; 余红霞等, 2011; 刘涵等, 2015; 雷鸣, 2016; Sun et al., 2015; Yi et al., 2018; Liu et al., 2019 Fig. 1 Distribution of the Cretaceous volcanic rocks on Gangdese belt, South Tibet (a, modified after Zhu et al., 2011) and simplified geological map of the Musi area (b)

目思地区出露的地层主要为晚古生代地层，包括中二叠统昂杰组及上石炭统-下二叠统拉嘎组(图 1b)。其中昂杰组主要以杂砂岩、变质砂岩为主，拉嘎组以含生物碎屑灰岩、泥晶灰岩为主，反映地层的形成环境为滨浅海相。为了进一步明确该区的晚白垩世岩浆事件，并探究基性岩墙的成因，对上述岩体进行样品采集并对其中一个岩体进行剖面测制。剖面的详细位置见图 1b，剖面起始坐标为北纬31°39′36″、东经85°57′00″。

在目思地区测制的基性岩墙剖面可反映岩墙的野外产状及围岩性质(图 2)。岩墙侵出产状近直立，宽约6~15m，长度约130m，其围岩主要为上石炭统-下二叠统拉嘎组(图 3a)，包括二段变质砂岩、变质长石石英砂岩、变质复成分砾岩及含砾板岩。岩墙岩性为典型的辉绿玢岩，风化程度较强烈，部分岩石的斑晶被剥蚀形成孔洞且辉石矿物破碎严重(图 3b, d)。岩石组构为似斑状结构，块状构造。斑晶的主要成分为斜长石，其次为单斜辉石。镜下明显可见岩石发生弱蚀变，板柱状斜长石斑晶发育聚片双晶，长度可达5mm(图 3c)。岩石基质由大量斜长石、少量单斜辉石及微量的角闪石、黑云母微晶组成(图 3d)，粒径在0.1~0.7mm之间。副矿物为少量锆石及磷灰石。

图 2 西藏尼玛县阿索乡北部目思基性岩墙剖面 1-变质砂岩；2-变质长石石英砂岩；3-变质复成分砾岩；4-含砾板岩；5-辉绿玢岩. C2P1l2-上石炭统-下二叠统拉嘎组二段；K2βμ-晚白垩世辉绿玢岩；N17T53-基性岩墙年龄及化学样品采样点 Fig. 2 Section of Musi mafic dyke in north of Asa Village, Nyima County, Tibet

图 3 目思基性岩墙野外产状及岩相学特征 (a)目思基性岩墙；(b)岩石近景照片；(c、d)目思基性岩墙岩相学特征(样品N17T51、N17T56，正交偏光). Pl-斜长石；Cpx-单斜辉石 Fig. 3 Occurrences and petrographic features of the Musi mafic dyke
2 分析方法

在目思基性岩墙中共采集9件辉绿玢岩地球化学样品用于全岩地球化学测试分析(表 1)。为了避免弱蚀变对样品全岩地球化学分析的影响即确保实验结果的可信程度，样品在野外采集过程中均挑选最新鲜岩石并去除风化面。之后用＜5% HNO3和去离子水清洗样品，待样品干燥后将样品放入无污染玛瑙球磨机粉碎至200目，以上实验过程均在河北省廊坊市区域地质矿产调查研究院无污染实验室完成。所有粉碎的样品送到中国地质大学(北京)地学实验中心用于全岩地球化学主微量元素分析。全岩地球化学主量元素测试在等离子体发射光谱仪(ICP-OEC)实验中进行，测试仪器为PS-950等离子体光谱仪；微量元素的化学预处理采用两酸(HNO3+HF)高压反应釜溶样法，测试方法为ICP-MS法，测试仪器为Agilent-7500a电感耦合等离子质谱仪，实验过程中采用国际标准参考样品AGV-2和GSR-3进行校对(Govindaraju, 1994)。

表 1 目思基性岩墙主量(wt%)和微量(×10-6)元素分析结果 Table 1 Analytic results of major (wt%) and trace (×10-6) elements of the Musi mafic dyke

此外在目思基性岩墙中共采集3件年龄样品，采样坐标见图 1b及表 2。定年样品的U、Th和Pb同位素元素分析在中国地质大学(北京)的实验室完成，分析仪器为193nm激光剥蚀进样系统UP 193SS和Agilent 7500a型四级杆等离子体质谱仪构成的激光等离子体质谱仪(LA-ICP-MS)。激光束直径为36μm，以He作为剥蚀物质载气。国际标样91500作为外标校对同位素分馏，U、Th含量采用NIST610中的29Si作为内标校正仪器引起的信号漂移。详细的方法和分析流程见Yuan et al. (2004)。锆石定年数据处理采用Isoplot (version 3.0)和Glitter (version 4.4)软件(Ludwig, 2003)。

表 2 目思基性岩墙锆石U-Pb同位素测试结果 Table 2 Zircon U-Pb isotope analytic results of the Musi mafic dyke
3 分析结果 3.1 全岩地球化学

目思基性岩墙中采集的辉绿玢岩样品的主量与微量元素数据如表 1所示。样品的烧失量(LOI)在3.84%~5.16%之间，烧失量较高可能与岩石斑晶剥蚀的孔洞有关(图 3c)。样品的主量元素均具有较窄的分布范围为：SiO2(51.90%~53.55%)、TiO2(1.45%~1.74%)、Al2O3(14.44%~16.23%)、Fe2O3T(8.34%~9.28%)、MnO(0.12%~0.15%)、MgO(3.98%~4.97%)、CaO(7.08%~8.19%)、Na2O(2.73%~3.34%)、K2O(0.95%~1.25%)、P2O5(0.46%~0.58%)。Mg#介于50.0~57.5之间。在Nb/Y-Zr/TiO2×0.0001图解中(图 4a)，目思基性岩墙为一套化学成分相近的亚碱性玄武岩；在AFM图解中(图 4b)，样品落入钙碱性系列区域。

图 4 目思基性岩墙Nb/Y-Zr/TiO2×0.0001图解(a, 底图据Winchester and Floyd, 1977)和AFM判别图解(b, 底图据Irvine and Baragar, 1971) Fig. 4 Nb/Y vs. Zr/TiO2×0.0001 (a, base map after Winchester and Floyd, 1977) and AFM (b, base map after Irvine and Baragar, 1971) discriminant diagrams for the Musi mafic dyke

在球粒陨石化稀土元素配分曲线图中，目思基性岩墙呈现出轻稀土元素富集的右倾曲线(图 5a)，(La/Yb)N(9.60~10.46)、(Ce/Yb)N(7.85~8.79)同样呈现出较高的比值。在原始地幔标准化微量元素蛛网图中(图 5b)，目思基性岩墙呈现出明显的大离子亲石元素富集(如Ba、Th、U和K)，亏损Nb、Ta、Ti等高场强元素，并具有Pb的正异常。同时，Eu具有明显的负异常(Eu*=0.81~0.84)。

图 5 目思基性岩墙球粒陨石标准化稀土元素配分曲线图(a)和原始地幔标准化微量元素蛛网图(b)(标准化值据Sun and McDonough, 1989) Fig. 5 Chondrite-normalized REE patterns (a) and primitive mantle normalized muti-element diagrams (b) for the Musi mafic dyke (normalization values after Sun and McDonough, 1989)
3.2 锆石U-Pb年龄

目思基性岩墙样品中获取的锆石的形态并不规则，在阴极发光图像中(图 6)，锆石多呈半自形长柱状，部分呈他形粒状，粒径范围100~300μm，长宽比1:1~3:1。锆石晶面较光滑，弱环带结构，Th/U比值为0.52~1.45，均大于0.3，表明锆石为岩浆成因。

图 6 目思基性岩墙代表性锆石阴极发光(CL)图像 Fig. 6 Representative cathodoluminescence (CL) images of zircons from the Musi mafic dyke

35个测点206Pb/238U年龄值变化在69~81Ma，锆石年龄较为分散，存在72~76Ma和76~80Ma两个峰值年龄(图 7a)。对两组年龄分别进行投图，在207Pb/235U-206Pb/238U谐和曲线图中，两组年龄样品的测点均分布于谐和线上(图 7b, c)，加权平均年龄分别为73.5±0.6Ma(MSWD=0.62, N=13)和77.3±0.5Ma(MSWD=1.1, N=17)。其中较为年轻的年龄73.5±0.6Ma最有可能代表岩体的形成时代，指示目思基性岩墙的岩浆结晶年龄为晚白垩世。

图 7 目思基性岩墙锆石U-Pb谐和图 Fig. 7 U-Pb concordia diagrams of zircons from the Musi mafic dyke
4 讨论 4.1 岩石成因

在讨论岩浆来源之前，应首先考虑岩浆在上升过程中受地壳混染程度。相较于活跃的大离子亲石元素，稳定富集的高场强元素可以用于讨论岩浆是否与地壳混染(Winchester and Floyd, 1977)。Nb/Yb-Th/Yb图解和(La/Yb)PM-(Th/Nb)PM图解能够有效判别地壳物质的贡献(Xu et al., 2002)。目思岩墙的岩石样品全部落入岩浆混染区域中(图 8a)，而在(La/Yb)PM-(Th/Nb)PM图解中同样呈现出受大陆地壳混染的趋势(图 8b)。岩石中的La/Ta和La/Sm受岩浆混合或地壳混染的影响较大，受地壳混染后的玄武岩La/Ta会在25以上，La/Sm会在5以上(王明等, 2010)。目思基性岩墙La/Ta介于39.46~43.40之间，La/Sm介于4.95~5.16之间。结合Nb/Yb-Th/Yb图解和(La/Yb)PM-(Th/Nb)PM图解的投图结果，认为岩浆在上升过程中受到了地壳物质的混染。

图 8 目思基性岩墙Nb/Yb-Th/Yb图解(a, 底图据Pearce, 2008)和(La/Nb)PM-(Th/Nb)PM图解(b, 底图据Frey et al., 2002) 原始地幔标准化数据引自Sun and McDonough (1989)，下地壳与中地壳数据引自Rudnick and Gao (2003) Fig. 8 Nb/Yb vs. Th/Yb (a, base map after Pearce, 2008) and (La/Nb)PM vs. (Th/Nb)PM (b, base map after Frey et al., 2002) plots of Musi mafic dyke (base map after Xu et al., 2002)

现有研究表明，玄武质岩浆一般来源于地幔橄榄岩的部分熔融。由尖晶石二辉橄榄岩部分熔融的玄武质岩浆通常具有平缓的稀土元素分布模式，而由石榴子石二辉橄榄岩低程度熔融生成的岩浆，通常重稀土元素显著分馏(Hart and Dunn, 1993)。目思基性岩墙具有轻微的重稀土元素分馏(图 5a)，同时La/Sm-Sm/Yb的投图结果显示，初始岩浆来自于尖晶石+石榴石二辉橄榄岩地幔源区1%~5%的部分熔融，尖晶石与石榴石比值接近1:1(图 9a)。在Ta/Yb-Th/Yb图解(Pearce, 2008)中，目思基性岩墙落入钙碱性玄武岩区域中，同样落入地幔序列以外(图 9b)。结合目思基性岩墙中富集Ba、Th、U、K等大离子亲石元素，亏损Nb、Ta、Ti等高场强元素的地球化学特征，推断目思基性岩墙来源于富集地幔部分熔融。

图 9 目思基性岩墙La/Sm-Sm/Yb图解(a, 底图据Aldanmaz et al., 2000)和Ta/Yb-Th/Yb图解(b, 底图据Pearce, 2008) Fig. 9 La/Sm vs. Sm/Yb (a, base map after Aldanmaz et al., 2000) and Ta/Yb vs. Th/Yb (b, base map after Pearce, 2008) plots of Musi mafic dyke

目思基性岩墙中的Mg#值以及Cr、Ni质量分数(Mg#=50.0~57.5、Cr=51.30×10-6~79.48×10-6、Ni=55.94×10-6~74.168×10-6)均低于原生玄武质岩浆(Mg#=68~75、Cr=300×10-6~500×10-6、Ni=300×10-6~400×10-6; Frey et al., 1978; 朱弟成等, 2005)，表明其经历了一定程度的结晶分异作用。在La/Yb-La和La-La/Sm中同样呈现出分离结晶的趋势(图 10a, b)。而在Hacker图解中，Mg#与Cr、Ni呈现出明显的正相关(图 10c, d)，表明岩浆在演化过程中发生了橄榄石、单斜辉石及角闪石等镁铁质矿物的结晶分异作用。同时目思基性岩墙具有Eu负异常(Eu*=0.81~0.84)的特征表明岩石还发生了斜长石的分离结晶。

图 10 目思基性岩墙La/Yb-La图解(a, 底图据Chung et al., 2009)、La-La/Sm图解(b, 底图据陈玲等, 2013)、Mg#与Cr、Ni元素相关图(c、d) Fig. 10 La/Yb vs. La plot (a, base map after Chung et al., 2009), La vs. La/Sm plot (b, base map after Chen et al., 2013), correlation diagram of Mg# and Cr, Ni (c, d) of Musi mafic dyke

综上所述，目思基性岩墙的源区为地幔尖晶石+石榴石二辉橄榄岩，在岩浆上升过程中遭受地壳物质混染。在岩浆演化的过程中，发生了明显的橄榄石、单斜辉石及斜长石矿物的结晶分异。

4.2 构造环境

以往的研究认为羌塘地块与拉萨地块于早白垩世沿闭合的班-怒带发生碰撞(Kapp et al., 2007; Sui et al., 2013; Zhu et al., 2016)。伴随着班-怒洋的洋壳陆下断离(113Ma)，拉萨地块中北部与羌塘-拉萨碰撞带进入陆内碰撞环境(Zhu et al., 2009; Zhu et al., 2011)。板片的断离引起软流圈地幔的上涌，于莫霍面附近形成广泛分布的镁铁质岩浆，从而增加了地壳的厚度(Zhu et al., 2017)。Kapp et al. (2007)根据尼玛盆地中地层的沉积特征，认为拉萨地块中北部于125~95Ma发生了明显的地壳增厚缩短，竟柱山组磨拉石建造与下覆海相地层的不整合进一步表明地壳最有可能在113~93Ma发生南北向的增厚缩短。而碰撞后末期的岩石圈增厚正是发生岩石圈拆沉作用的必要条件(Bonin, 2004)。随着拉萨中北部地壳的两次增厚，下地壳的深度与压力逐渐增大，成分逐渐由角闪岩相向榴辉岩相转变，密度逐渐增大(Wang et al., 2014)。张旗等(2006)认为该阶段的榴辉岩下地壳的密度小于下伏地幔，拆沉作用的发生仍需要大规模的岩浆活动使低密度的中酸性岩浆岩移出下地壳。而区域内大规模的同期(94~79Ma)埃达克质岩浆活动为拆沉模式提供了强有力的支持(Zhao et al., 2008; 余红霞等, 2011; 王保弟等, 2013; 李华亮等, 2014; 雷鸣等, 2015; 刘涵等, 2015; Sun et al., 2015; Chen et al., 2015; Yi et al., 2018; Liu et al., 2019)。

考虑到下地壳的拆沉作用受到空间及密度的制约，软流圈只能经受有限的岩浆上涌与部分熔融，大部分的拆沉模式并非下地壳+岩石圈地幔的大规模整体拆沉，而是下地壳如同冰块融化般的小规模逐渐拆沉(Houseman and Molnar, 1997; 张旗等, 2006)。区域内94~79Ma小规模岩浆活动埃达克质和富镁的地球化学特征全部与岩石圈拆沉有关(Zhao et al., 2008; 余红霞等, 2011; 雷鸣等, 2015; 雷鸣, 2016; 刘涵等, 2015; Sun et al., 2015; Yi et al., 2018; Liu et al., 2019)，这些独特的地球化学特征恰好反映了小规模岩石圈拆沉的结果。随着下地壳拆沉作用的进行，地壳逐渐减薄，部分地区由于地壳过薄发生垮塌，形成伸展环境，于中北拉萨地块内部形成了与拆沉环境有关岩浆活动(94~79Ma，图 1a)同时期的伸展背景下的岩浆活动(88~79Ma; 辛洪波和曲晓明, 2006; 江军华等, 2011; 张硕等, 2014)。本次研究于拉萨地块北部发现的基性岩墙具有74Ma的加权平均年龄，推测拉萨中北部的小规模拆沉作用持续到晚白垩世晚期。同时，由于小规模的拆沉模式导致基性岩浆的不发育(DeCelles et al., 2015)，镁铁质岩浆在产生过程中被同期大量的花岗质岩浆所消耗(Wang et al., 2014)。本次所研究的对象岩性更偏中性，同样是因为岩石圈地幔产生的镁铁质岩浆在上升过程中与下地壳物质发生混染(图 8)。

目思基性岩墙的构造成因可以总结如下：班-怒洋闭合后的后碰撞作用使拉萨地块增厚缩短，致使下地壳压力增大，形成榴辉岩相下地壳。下地壳发生岩浆上涌，拉萨地块中北部形成大规模埃达克质的岩浆活动，下地壳密度进一步加大。由于下地壳榴辉岩密度大于下伏橄榄岩地幔，重力的不稳定性引起下地壳发生拆沉。拆沉作用导致岩石圈地幔上涌，相关的地球化学特征显示幔源岩浆于源区内发生分离结晶，上升过程中与下地壳物质发生混染。与区域内同期拆沉成因岩浆岩的对比表明拆沉模式并非传统认为的下地壳+地幔整体拆沉，而是小规模的部分逐渐拆沉，该期拆沉作用沿羌塘-拉萨碰撞带持续至晚白垩世晚期。

5 结论

基于拉萨地块北部阿索地区目思基性岩墙的岩石学、全岩地球化学以及U-Pb年代学，可以得到以下几点认识：

(1) 目思基性岩墙中最年轻的一组锆石U-Pb谐和年龄为74Ma，表明其形成时代为晚白垩世晚期；

(2) 目思基性岩墙具有较高的SiO2、Mg#、Cr和Ni含量。这些地球化学特征表明岩浆来源于部分熔融的岩石圈地幔，并在上升过程中与拆沉的下地壳发生混染。

(3) 目思基性岩墙形成于班-怒洋闭合后的后碰撞拆沉环境中，结合目思基性岩墙的锆石U-Pb年龄表明羌塘-拉萨碰撞带的拆沉作用一直持续到晚白垩世晚期。

致谢      分析测试工作得到中国地质大学(北京)地学实验中心苏犁教授与张红雨老师的指导；两位审稿人提出了建设性的意见；在此一并致谢。

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