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西藏林周盆地设兴组玄武岩地球化学特征及意义*

2015-07-21李晓雄江万梁锦海赵志丹刘栋莫宣学

岩石学报 2015年5期
关键词:白垩火山岩玄武岩

李晓雄 江万 梁锦海 赵志丹 刘栋 莫宣学

1.中国建筑材料工业地质勘查中心陕西总队,西安 710003

2.地质过程与矿产资源国家重点实验室,中国地质大学地球科学与资源学院,北京 100083

3.中国地质科学院地质力学研究所,北京 100081

4.黑龙江三道湾子金矿有限公司,黑河 164300

1 引言

岩浆岩作为地球深部活动最直接的记录者,对了解板块运动及地球深部过程有着十分重要的作用(莫宣学,2011)。青藏高原南部的拉萨地块,发育大规模中-新生代岩浆作用(Yin and Harrison,2000;潘桂棠等,2004,2006;Zhu et al.,2013),开展这些岩石的年代学、构造背景和成因研究,对揭示雅鲁藏布特提斯洋岩石圈的俯冲、印度与亚洲大陆的碰撞等构造事件有着重要意义。近年岩浆作用、古生物学与沉积地球化学研究普遍认为,在主碰撞带内的印度-亚洲大陆碰撞开始时间为70~60Ma(莫宣学等,2003,2007;Ding et al.,2005;Zhu et al.,2011;Wu et al.,2014),到40Ma 左右完成,这段时间为同碰撞阶段;40Ma 之后则称后碰撞阶段(Yin and Harrison,2000;Mo et al.,2002,2007,2008,2009;周肃等,2004;Dong et al.,2005;Zhao et al.,2009)。

林子宗火山岩(又称林子宗群,由下到上分为典中组、年波组和帕那组三个组)呈带状分布于拉萨地体南部、冈底斯岩基带北部,它呈角度不整合覆盖于强烈变形的上白垩统设兴组或更老地层之上(董国臣,2002),Ar-Ar 定年和锆石UPb 定年结果(周肃等,2004;李皓揚等,2007;Lee et al.,2009;He et al.,2007;Chen et al.,2014)较好揭示了林周盆地的林子宗火山岩各组的形成时代(典中组65~60Ma、年波组60~50Ma、帕那组50~44Ma)。不难看出,林子宗群代表的岩浆活动的时代恰好与目前认为的大陆碰撞的初始时间和碰撞过程重合,因而备受关注。莫宣学等人(周肃等,2004;莫宣学等,2009;莫宣学,2010,2011)将林子宗最底部典中组火山岩的年龄(约65Ma)作为印度与亚欧大陆碰撞的起始时间。

本文首次报道了在林周盆地典中村附近出露的设兴组顶部地层中,新发现的一套呈层状产出的玄武岩。林周盆地出露的林子宗标准剖面之下的白垩世设兴组地层,已有研究多认为是变质-弱变质的红色碎屑岩类,本文在其中识别出了多个玄武岩和安山岩夹层,并开展了详细的年代学和地球化学研究,试图揭示白垩纪设兴组沉积、浅变质沉积岩的最晚时代,确定林子宗群之下不整合的时间间断;判别设兴组玄武岩的构造环境,为大陆碰撞之前的新特提斯洋演化提供新证据。

2 地质背景与样品

拉萨地块为一东西向的狭长巨型构造岩浆带,夹持于班公-怒江缝合带与雅鲁藏布缝合带之间,是青藏高原岩浆作用最为发育的地区。其中中生代岩浆岩的面积达10.2 万平方千米,占整个拉萨地块面积的近20%(Zhu et al.,2011)。根据沉积盖层和基底性质的不同,以狮泉河-永珠-纳木错-嘉藜蛇绿混杂岩岩(SNMZ)和洛巴堆-米拉山断裂带(LMF)为界,将拉萨地块分为北(NL)中(CL)南(SL)三部分(图1a)。其中南部拉萨地块主要由白垩纪-古近纪冈底斯岩基和古新世-始新世林子宗火山岩以及部分侏罗系-白垩世沉积地层构成,可能主要是中生代岛弧侧向加积作用增生到具有基底的拉萨地块之上形成的(朱弟成等,2009)。

本文所研究的林周盆地位于拉萨东北约60km 处,是林子宗火山岩的命名地。林周盆地的南缘受冉木江逆断裂控制,北部则被洛巴堆-古鲁-左岗-唐加逆断层截切,整体呈不规则的椭圆形,长轴近东西向展布,面积约230km2,其中林子宗火山岩呈环状分布,岩层产状整体北倾,倾角较缓,一般20°~25°。盆地内只有上二叠统旁那组和上白垩统设兴组与林子宗火山岩呈不整合接触(莫宣学等,2009)。

在采样剖面(图1b),晚白垩世设兴组不整合于典中组火山岩之下,主要岩性为紫红色砂砾岩、灰绿色玄武岩和红褐色安山岩。其中设兴组火山岩岩性为玄武岩和安山岩,在红色砂砾岩中呈夹层产出,多层韵律分布,夹层厚度介于0.5~1.2m(图1c)。玄武岩和安山岩分布于不同的韵律层中。其中玄武岩层风化后呈灰绿色,细粒结构,岩石中暗色矿物较多,杏仁构造发育。玄武岩层产状为313°∠34°。岩石薄片显微镜观察表明,玄武岩呈斑状结构,斑晶为斜长石(较少)和辉石(约占10%)。其中辉石呈星散状分布,均为半自形柱状,被绿泥石、方解石交代,仍存假象,部分界限模糊不清或消失;斜长石呈板状,已绢云母化;基质为微晶斜长石和辉石;副矿物为磁铁矿和磷灰石等(图2)。

3 分析方法

图1 西藏林周盆地地质图及样品位置(a)青藏高原构造单元划分(据Zhu et al.,2013 和Meng et al.,2014 修改,其中标注的年龄见Meng et al.,2014 及其中的文献);(b)林周盆地地质图(据董国臣,2002;周肃等,2004 修改);(c)本文采样剖面的野外照片,所示岩脉就是本文采样点.BNSZ:班公-怒江缝合带;SNMZ:狮泉河-纳木错蛇绿混杂岩带;LMF:洛巴堆-米拉山断裂带;YZSZ:印度河-雅鲁藏布江缝合带Fig.1 Simplified geological map of Linzhou basin,Tibet,showing sample locations(a)tectonic units of the Tibetan Plateau (after Zhu et al.,2013;Meng et al.,2014 and references therein);(b)geological map of Linzhou basin(after Dong,2002;Zhou et al.,2004);(c)photography showing the sampling outcrop,samples were collected from the layers.BNSZ:Bangong-Nujiang suture zone;SNMZ:Shiquan River-Nam Tso Mélange Zone;LMF:Luobadui-Milashan Fault;YZSZ:Yarlung Zangbo Suture Zone

玄武岩的斜长石单矿物Ar-Ar 年代学分析在中国地质科学院地质研究所完成。选纯的矿物(纯度>99%)用超声波清洗。清洗后的样品被封进石英瓶中送核反应堆中接受中子照射。照射工作是在中国原子能科学研究院的“游泳池堆”中进行的,使用B4 孔道,中子流密度约为2.65 ×1013n·cm-2S-1。照射总时间为1444min,积分中子通量为2.30 ×1018n·cm-2;同期接受中子照射的还有用做监控样的标准样:ZBH-25 黑云母标样,其标准年龄为132.7 ±1.2Ma,K 含量为7.6%。样品的阶段升温加热使用石墨炉,每一个阶段加热30min,净化30min。质谱分析是在多接收稀有气体质谱仪Helix MC 上进行的,每个峰值均采集20 组数据。所有的数据在回归到时间零点值后再进行质量歧视校正、大气氩校正、空白校正和干扰元素同位素校正。中子照射过程中所产生的干扰同位素校正系数通过分析照射过的K2SO4和CaF2来获得,其值为:(36Ar/37Aro)Ca= 0.0002389,(40Ar/39Ar)K=0.004782,(39Ar/37Aro)Ca=0.000806。37Ar 经过放射性衰变校正;40K 衰变常数λ =5.543 ×10-10y-1;用ISOPLOT程序计算坪年龄及正、反等时线(Ludwig,v2.49,2001)。坪年龄误差以2σ 给出。详细实验流程见有关文章(陈文等,2006;张彦等,2006)。设兴组玄武岩的斜长石Ar-Ar 测试结果见表1。样品编号为1003-4,矿物为斜长石,称样质量为150.41mg,J=0.004468。

图2 设兴组玄武岩镜下显微照片图中玄武岩为斑状结构,斑晶为辉石(已蚀变为绿泥石等),基质为间粒结构,可见微晶斜长石、辉石(蚀变为绿泥石)和不透明的副矿物(磁铁矿等).(a)为单偏光,(b)为正交偏光Fig.2 The microscopic photographs of the basalt from Shexing Formation in Linzhou basinThe rocks are porphyry texture,with altered pyroxenes (Py)as phenocrysts.The groundmass shows intergranular texture,with plagioclase (Pl),Py,and other Fe-Ti oxides.(a)is under plane polarized light;(b)is under crossed polars

图3 林周盆地设兴组玄武岩斜长石40Ar-39Ar 年龄谱和等时线图Fig.3 Ar-Ar dating spectra and isochron age of plagioclase of the basalt from Shexing Formation in Linzhou basin

表1 西藏林周盆地设兴组玄武岩斜长石40Ar-39Ar 年龄测定结果Table 1 40Ar-39Ar analysis data of the plagioclase of the basalt from Shexing Formation in Linzhou basin

表2 林周盆地设兴组玄武岩的主量元素(wt%)和微量元素(×10 -6)数据Table 2 Whole-rock major(wt%)and trace elements (×10 -6)data of the basalts from Shexing Formation in Linzhou basin

表3 林周盆地设兴组玄武岩样品Sr-Nd-Pb 同位素数据Table 3 Sr-Nd-Pb isotope data of the basalts from Shexing Formation in Linzhou basin

主、微量元素测定在中国地质科学院国家地质实验测试中心完成。其中主量元素测试使用X 射线荧光光谱仪(2100),微量元素的测试使用等离子质谱仪(X-series)。样品测试误差普遍小于10%,稀土元素均小于5%。岩石主、微量结果见表2。Sr-Nd-Pb 同位素测定在核工业北京地质研究院测试完成。其中Rb-Sr 同位素在PHOENIX 仪器上进行;Sm-Nd 同位素测定在ISOPROBE-T 仪器上进行,Pb 同位素在ISOPROBE-T 仪器上进行。Sr-Nd-Pb 同位素测试结果见表3。

4 结果

4.1 玄武岩Ar-Ar 定年

林周盆地设兴组顶部玄武岩的斜长石Ar-Ar 定年结果表明(表1),斜长石坪年龄为90.6 ±1.8Ma;等时线年龄为84.9±6.4Ma,在误差范围内与坪年龄一致(图3)。因此,根据采样斜长石Ar-Ar 坪年龄结果,设兴组玄武岩的喷发年龄约为90.6Ma。

4.2 全岩主、微量元素

设兴组火山岩的主量元素数据列于表2 中。由于样品总体烧失量大(9.9%~11.85%),CaO 含量较高(8.14%~11.16%),在薄片中也可见碳酸盐化,因此岩石受到了后期碳酸盐蚀变。为确定原始岩石的成分,对数据进行了去碳酸盐化处理,按照全部烧失量为水和二氧化碳,应用CO2反算,从CaCO3中扣除与CO2等摩尔量的CaCO3然后再将主量元素重新百分化。计算结果表明,去碳酸盐化后岩石的SiO2含量52.25%~54.76%,Al2O3含量16.04%~17.85%,Mg#为59~91,平均值为66,因此原始的岩石应为玄武质或者部分为玄武安山岩成分。本文以下讨论中统称为设兴组玄武岩。

图4 林周盆地设兴组玄武岩(a)Nb/Y-Zr/TiO2分类图解(据Winchester and Floyd,1977);(b)Th-Co 图解(据Hastie et al.,2007);(c)稀土元素球粒陨石标准化图(球粒陨石据Boynton,1984);(d)微量元素原始地幔标准化图(原始地幔据Sun and McDonough,1989);桑日群数据引自Kang et al.(2014)Fig.4 Geochemical plots of the basic rocks of the Shexing Formation from Linzhou basin(a)Nb/Y-Zr/TiO2 plots (after Winchester and Floyd,1977);(b)Th-Co plots (after Hastie et al.,2007);(c)chondrite-normalized REE patterns (chondrite values after Boynton,1984);(d)primitive-mantle-normalized trace element patterns (primitive mantle values after Sun and McDonough (1989).Sangri Group data after Kang et al.(2014)

由于样品遭受后期蚀变,可能有活泼的碱性元素的带入带出,故使用抗蚀变的高场强元素和不活泼元素(Zr/TiO2-Nb/Y 图解)进行岩石成分识别(图4a),岩石为碱性玄武岩,与主量元素的成分一致。为避免镁铁质岩受Fe-Ti 氧化物堆晶作用影响而不能真实反映源区特征,本文利用Co-Th 分类图解(Hastie et al.,2007),岩石主要属于高钾钙碱性系列的玄武岩-玄武安山岩(图4b)。

设兴组玄武岩8 个样品具有非常均一的微量元素组成(图4)。岩石具有较高的稀土总量(ΣREE =291 ×10-6~310 ×10-6,岩石显示明显的轻重稀土分馏((La/Yb)N=28~34)。玄武岩具有弱的Eu 负异常(Eu/Eu*= 0.81~0.93)。在原始地幔标准化图解中(图4d),设兴组玄武岩表现出明显的Nb、Ta、Ti 等高场强元素的亏损和Ba、Th、U、Pb等大离子亲石元素的富集,呈现出与拉萨地块南缘的洋壳俯冲产生的岛弧型桑日群火山岩类似的弧火山岩的地球化学特征,但是微量元素含量明显高于桑日群。

4.3 Sr-Nd-Pb 同位素地球化学

3 个玄武岩样品的Sr-Nd-Pb 同位素组成列于表3。采用本文获得的Ar-Ar 年龄90.6Ma 进行校正。设兴组玄武岩均具有正的εNd值(+0.72~+4.75)和年轻的亏损地幔模式年龄(tDM<660Ma);87Sr/86Sr 为0.706633~0.706654。Pb 同位素校正后比值分别为,(206Pb/204Pb)t为18.649~18.675,(207Pb/204Pb)t为15.640~15.666,(208Pb/204Pb)t为39.055~39.108。

5 讨论

5.1 玄武岩时代与设兴组沉积时限

图5 设兴组玄武岩Ni-Mg#(a)和Cr-Mg#(b)图解Fig.5 Plots of Ni (a)and Cr (b),against Mg# of basalts from Shexing Formation in Linzhou basin

图6 设兴组基性岩石的Nb/La-SiO2图(a)和(Hf/Sm)N-(Ta/La)N图(b)(据La Flèche et al.,1998)Fig.6 Nb/La-SiO2(a)and (Hf/Sm)N-(Ta/La)N(b)plots (after La Flèche et al.,1998)of the basic rocks from Shexing Formation

王乃文等(1983)根据建组剖面上设兴组的第2~3 段所产的双壳类Amphidonte ostracina,以及在典中-那玛剖面上发现介形虫和孢粉化石并结合设兴组所处层位,认为整个设兴组沉积时代应为晚白垩世,设兴组上部的时代定为晚白垩世晚期。此后,许多研究结果(朱志文等,1981;王乃文等,1983;Besse et al.,1984;Molnar et al.,1988;Jaeger et al.,1989;Rage et al.,1995;Leier et al.,2007;Kapp et al.,2007)在进行相关的研究和总结时,都将设兴组置于晚白垩世(72~100Ma)。井天景(2014)通过马乡青藏线路西出露的设兴组与上覆林子宗群火山岩的剖面研究,获得设兴组顶部砂岩的最年轻的碎屑锆石年龄为81~88Ma,表明设兴组地层的沉积时代不早于81Ma。

本文获得林周盆地设兴组顶部地层中玄武岩夹层的斜长石Ar-Ar 等时线定年结果表明,设兴组玄武岩的年龄约为90.6Ma,该年龄结果与沉积学和古生物学获得的设兴组年龄相吻合,从岩浆岩方面进一步证实设兴组的形成时限为晚白垩世。

5.2 设兴组玄武岩的成因

设兴组玄武岩MgO(4.47%~8.41%)和Ni 含量(79 ×10-6~249 × 10-6)与原生玄武质岩浆范围比较(Hess,1992)变化范围较大,说明玄武岩浆形成后经历了分离结晶等后期演化过程。设兴组玄武岩的Ni 含量随Mg#的减小呈明显的下降趋势(图5a),表明岩浆在上升过程中经历了一定程度的橄榄石分离结晶作用。随着岩石CaO 含量从11.16%降低到8.14%,对应的CaO/Al2O3比值也逐渐从0.71 减小到0.53(表2),Cr 含量随Mg#的减小而降低,这些趋势被认为是单斜辉石结晶分离作用所致(图5b)(刘丛强等,1995)。玄武岩弱的Eu 负异常(Eu/Eu*= 0.81~0.93),表明斜长石的分离结晶作用不显著。镜下岩相学观察发现斑晶以单斜辉石为主,缺少斜长石斑晶,与以上的成分演化趋势一致(图2)。因此,从岩相学与元素变化规律看,设兴组玄武岩的岩浆演化中,早期可能是橄榄石的分离结晶作用为主,而后期主要是单斜辉石为主,而斜长石分离结晶作用不明显。

图7 设兴组玄武岩(143Nd/144Nd)-(87Sr/86 Sr)图解(a)、(207Pb/204 Pb)-(206 Pb/204 Pb)图解(b)和(208 Pb/204 Pb)-(206Pb/204Pb)图(c)图内主要地球化学端员引自Zindler and Hart (1986);林周盆地林子宗火山岩数据引自Mo et al.,2007,2008;董国臣,2002;其余数据引自Zhao et al.(2009)以及其中的文献Fig.7 (87 Sr/86 Sr)t-(143 Nd/144 Nd)t(a),(206 Pb/204 Pb)-(207Pb/204Pb)(b)and (208 Pb/204 Pb)-(206 Pb/204 Pb)(c)plots of the basalt from Shexing FormationThe main geochemical endmembers are from Zindler and Hart(1986),the Linzizong data form Linzhou basin from Mo et al.,2007,2008;Dong,2002;Others after Zhao et al.(2009)and the references therein

林周盆地设兴组玄武岩,随SiO2增高,Nb/La 比值变化范围不大(图6a),表明其受到地壳混染的可能性很小,这暗示玄武岩的成分可以反映其地幔源区的特征。在(Hf/Sm)N-(Ta/La)N图解中(La Flèche et al.,1998),林周盆地基性岩石具有较低的(Hf/Sm)N和(Ta/La)N值,表明其源区可能受到了俯冲流体的交代(La Flèche et al.,1998;Zhu et al.,2012)(图6b)。样品相对较高的Nb 含量(12.8 ×10-6~14.5 ×10-6)、低的Ce/Pb 比值(平均为11.4),也说明可能有俯冲板片熔体和流体的加入(Seghedi et al.,2004),样品已经偏离了原始地幔、MORB 和OIB 的Ce/Pb 比值(Ce/Pb=25,Sun and McDonough,1989)。与同样产出在拉萨地块南缘的桑日群玄武岩(189~195Ma)进行比较(图4c,d、图6),发现它们具有成分的相似性,说明本文样品也是起源于特提斯洋俯冲消减阶段产生的岛弧地幔楔部分熔融的产物(Kang et al.,2014),但是设兴组玄武岩具有更加富集的微量元素组成。另外,设兴组玄武岩的Sr-Pb 同位素成分靠近EMⅡ富集地幔端员(图7),也暗示源区有消减物质(洋壳携带的沉积物或洋壳板片熔体)加入到了地幔源区。此外,从Sr-Nd-Pb 特征看,设兴组玄武岩与拉萨地块南缘的与特提斯洋消减有关的岛弧岩浆作用具有一致的同位素地球化学特征。

5.3 玄武岩形成的构造环境

玄武质岩石的地球化学特征对构造背景非常敏感,可以有效的揭示岩石产出的构造背景(Pearce and Norry,1979)。本文主要选用玄武岩形成后蚀变过程中不活泼的高场强元素进行构造环境的判别。设兴组玄武岩Nb、Ta、Ti 三种元素显著亏损和所有样品Ta/Yb 比值近0.5,表明其源区岩浆为与俯冲有关的岛弧岩浆(Condie,1989);另外其大离子亲石元素和轻稀土元素明显富集,相对亏损高场强元素和重稀土元素,较高的La/Nb 比值(4.03~5.61),表明其岩浆可能来源于交代过的地幔,其形成与大洋板块俯冲作用有关(Condie,2003;Innocenti et al.,2005)。设兴组玄武岩在Ti-Zr 图解中全部落入火山弧区域(图8a);在玄武岩Hf-Th-Ta图解中,设兴组玄武岩投影点也较集中,与南部拉萨地块晚白垩世基性岩一样,数据处于CAB 火山弧(陆缘弧)玄武岩成分范围内(图8b),显示其形成于俯冲环境。研究表明,Ta/Yb 比值变化主要与地幔部分熔融及幔源性质有关,因此其对鉴别火山岩源区特征有着重要意义(Pearce and Cann,1973)。设兴组玄武岩在Th/Yb-Ta/Yb 图解中(图8c)均位于MORB-OIB 趋势线的上方,处于活动大陆边缘(陆缘弧)区域,这种地球化学特征也表明设兴组玄武岩形成于俯冲环境,并且与亏损地幔源区及陆壳物质参与有密切成因关系(Hergt et al.,1991)。另外,作者还发现同玄武岩互层产出的安山岩显示了俯冲洋壳部分熔融形成的埃达克质岩石特征(另文发表),进一步证明设兴组玄武岩形成于岛弧体制之下的俯冲构造背景。

此外,值得提出的是,除了本文大约90Ma 的设兴组玄武岩揭示的岛弧岩浆作用反映的特提斯洋正常安第斯型岛弧岩浆作用(Ji et al.,2009;Chu et al.,2006),在拉萨地块南缘晚白垩世时期也存在多种类型构造环境。例如,叶丽娟等(2015)认为在南木林南部存在大约94Ma 的弧后伸展构造背景;在拉萨地块南缘的卧龙镇(管琪等,2010;Guan et al.,2012)、里龙与米林一带(Zhang et al.,2010)存在洋脊俯冲的岩浆作用记录。

图8 玄武岩微量元素成分构造判别图(a)Zr-TiO2;(b)Hf/3-Th-Ta,原图引自Wood (1980);(c)Ta/Yb-Th/Yb 图解,引自Perfit et al.(1980).南部拉萨地块晚白垩世基性岩数据引自管琪等(2011)和叶丽娟(2013)Fig.8 Trace element discrimination diagrams showing the tectonic setting of the Shexing basalts(a)Zr/Y-Zr;(b)Th/Hf-Ta/Hf,after Wood (1980);(c)Ta/Yb-Th/Yb,after Perfit (1982).The late cretaceous rock data in southern Lhasa block from Guan et al.(2011)and Ye (2013)

5.4 南部拉萨地块白垩纪构造-岩浆演化

近年来在拉萨地块南缘,不断发现200Ma 以来的中生代岩浆作用,新特提斯洋岩石圈的北向俯冲消减于拉萨地块南缘,是解释这些中生代岩浆作用成因的最主要机制。目前主要有以下几种观点,(1)正常安第斯型岛弧岩浆作用(Ji et al.,2009;Chu et al.,2006);(2)雅鲁藏布新特提斯洋壳低角度或平板俯冲(Coulon et al.,1986;Wen et al.,2008a);(3)新特提斯洋脊俯冲(70~95Ma),在拉萨地块南缘存在的“埃达克质”岩浆作用,被解释为与特提斯洋脊俯冲有关,但该类岩浆作用分布的地区比较局限,主要位于拉萨地块南缘的卧龙镇(管琪等,2010;Guan et al.,2012)、里龙与米林一带(Zhang et al.,2010)。

安第斯型岛弧俯冲作用是解释中生代岩浆作用的普遍模型。Ji et al.(2009)结合其他研究成果,认为大约205Ma以来直到大约109~80Ma 的冈底斯带岩浆作用都与新特提斯洋北向俯冲的“安第斯型”稳定的岛弧俯冲作用有关,与其对应的是南部拉萨地块上发育的一系列具有岛弧性质的岩浆岩,如大面积发育的早侏罗世叶巴组火山岩、早白垩世桑日群火山岩都具有明显的俯冲性质,证明该时期俯冲作用的存在(陈炜等,2009;Zhu et al.,2008,2009;Kang et al.,2014)。其中在晚白垩世(约100~90Ma),北向俯冲的新特提斯洋壳俯冲角度变小,南部拉萨地块发生安第斯型造山作用(Wen et al.,2008b;Zhang et al.,2010),增厚的地壳部分熔融形成该时期一系列的埃达克质岩石。同时该时期的俯冲带前缘造山作用造成拉萨地块晚白垩时期地壳的缩短、抬升、变形事件的广泛发育(Murphy et al.,1997;Yin and Harrison,2000;Kapp et al.,2007;Leier et al.,2007)。而之后的晚白垩世(90~55Ma),之前低角度北向俯冲的新特提斯洋板片由于重力原因,在约90Ma 发生板块回转(rollback)(Chung et al.,2005;侯增谦等,2006),在这个过程中,上涌的对流软流圈提供了热量,诱使受俯冲的新特提斯大洋板片熔体及流体交代的上覆地幔楔发生部分熔融产生设兴组玄武岩。

本文在林周盆地设兴组新发现的玄武岩(90.6Ma)和埃达克质安山岩(另文发表),可能就是板块回转初始期的岩浆活动响应。俯冲的大洋板片发生回转,将拖拽大陆板片向下俯冲到较深位置,最直接的结果是导致陆-陆汇聚速率的加快(侯增谦等,2006)。在90.6~60Ma 左右,印度-亚洲两大陆汇聚速率达到最大(190.6mm/a),也证明了这个推论的正确性,同时造成在65Ma 左右印度-亚洲大陆的初始碰撞。之后在约60Ma 左右,板片回转过程被印度大陆板片的陡深俯冲取代,其结果导致板块汇聚速率出现骤减,在约55Ma 左右,向北俯冲的新特提斯洋板片发生断离,导致南部拉萨地块林子宗火山岩和大量花岗岩的侵位(赵志丹等,2011;侯增谦等,2006;莫宣学等,2005;董国臣等,2006,2008;Chung et al.,2005;Mo et al.,2005,2007,2008,2009;Wen et al.,2008a,b;Ji et al.,2009;Lee et al.,2009;Zhu et al.,2011)。

因此,本文研究的林周盆地设兴组玄武岩的形成可能是晚白垩世北向俯冲的新特提斯洋板块在回转初期的岩浆活动响应。

6 结论

(1)在西藏南部林周盆地设兴组顶部新发现了玄武岩夹层,玄武岩的斜长石Ar-Ar 年龄为90.6Ma;该玄武岩为碱性-高钾钙碱性系列,具有俯冲性质陆缘弧的地球化学特征。

(2)设兴组玄武岩可能为新特提斯洋板片在约90Ma 发生板片回转初期的岩浆活动响应,来源于俯冲带流体交代的地幔楔的部分熔融。

致谢 董国臣教授和戴紧根副教授审阅稿件并提出宝贵修改意见,在此表示感谢。

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