2017, Vol. 33
冈底斯岩浆岩带形成于特提斯洋俯冲和印度-亚洲大陆碰撞过程中，其中包含了大量的地球动力学与成岩成矿的信息。因此，研究冈底斯岩浆岩对于了解青藏高原的深部过程，印度-亚欧大陆碰撞过程都有着十分重要的意义(朱弟成等, 2009a; 侯增谦等, 2006a)。前人对冈底斯岩浆岩进行了多方面卓有成就的研究(Zhang et al., 2016; Dong et al., 2005; 莫宣学等, 2005)，但是以往的工作主要针对基性超基性岩、埃达克质火山岩、超钾质火山岩、淡色花岗岩等岩石类型的成因和地球动力学进行探讨，而对高分异花岗岩的成因类型、源区特征和演化过程研究较少。
除此之外，众多学者就岩浆岩的地球化学、同位素等特征重点展开特提斯洋俯冲、印度-亚洲大陆碰撞时限的讨论，初步得出约260Ma，特提斯洋板块开始北向俯冲到冈底斯板块之下(黄丰等, 2015; 朱弟成等, 2009a)。100~70Ma，特提斯洋板片处于俯冲阶段末期(Ma et al., 2013a, b; Zheng et al., 2014)。到65~50Ma左右，印度大陆与亚洲大陆发生碰撞(Zhu et al., 2015; 莫宣学等, 2005, 2007)。随后进入后碰撞阶段。其中，前人对印度亚洲大陆碰撞的时限讨论较多(约为65Ma)(侯增谦等, 2006b; 莫宣学, 2011; 莫宣学等, 2005, 2007)，但对碰撞之前板片俯冲的动力学机制讨论较少且争论较大，主要观点有：(1) 平板俯冲或者低角度俯冲，随后发生拆沉，回转和断离(Chung et al., 2005; Wen et al., 2008a, b; Lee and Anderson, 2015)；(2) 正常角度俯冲，随后发生回转断离(王莉等, 2013)或洋脊俯冲(管琪等, 2010; Zhang et al., 2010; Zheng et al., 2014)。本文通过研究冈底斯带南缘67~66Ma桑日花岗岩的岩相学、锆石U-Pb年代学、Hf同位素、岩石地球化学特征，确定了桑日花岗岩的成因类型为高分异I型花岗岩。同时，结合冈底斯南带120~10Ma岩浆岩特征进行对比讨论，认为在70~65Ma之前，特提斯洋板片为低角度俯冲，而在70~65Ma左右，俯冲板片在重力的作用下发生回转。
2 地质背景和样品冈底斯地体位于班公湖-怒江缝合带(BNSZ)以南，雅鲁藏布缝合带(IYZSZ)以北，呈近东西方向延伸，自西向东构造结逐渐变窄直至转变为近南北走向。冈底斯地体又称为拉萨地体，以洛巴堆-米拉山断裂(LMF)，和狮泉河-纳木错蛇绿混杂岩带(SNMZ)为界，由南向北可以将拉萨地块划分为南部拉萨地块(SL)、中部拉萨地块(CL)、北部拉萨地块(NL)(图 1a)。冈底斯的沉积盖层主要由上古生界(石炭系、二叠系)和中生界沉积组成(中国地质科学院, 1990)。其中，南部拉萨地块的沉积盖层主要集中在东部，包括奥陶纪弱变质碎屑岩、碳酸盐岩层、桑日群-碎屑岩地层和白垩纪灰岩-碎屑岩地层。中部拉萨地块的古老基底主要被大范围的晚侏罗纪-早白垩纪的海陆交互相沉积地层和火山岩夹层覆盖，除此之外还可见到少量碳酸盐地层及双峰式火山岩地层。北部拉萨地块东部主要覆盖着三叠纪-侏罗纪碎屑岩沉积地层，而白垩纪含火山岩夹层的海相沉积地层则遍布整个北部拉萨地块(Mo et al., 2008; Zhu et al., 2012, 2013; Ding et al., 2005; Wu et al., 2010)。
由于特提斯洋和印度-亚洲大陆长期俯冲碰撞过程，拉萨地块上大面积出露中新生代火山岩和冈底斯花岗岩岩基带。拉萨地块中-新生代花岗岩可以分为南、北两带。北带的中生代花岗岩和晚侏罗纪-早白垩纪火山岩主要分布在北部和中部拉萨地块之上，被认为是班公湖-怒江新特提斯洋壳岩石圈向南俯冲消减最终发生板片断离的产物。而南部拉萨地块上呈东西向展布的I型花岗岩带即冈底斯岩基带。南部冈底斯岩基被认为是雅鲁藏布新特提斯洋岩石圈向北低角度俯冲，拆沉，回转和断离过程中的产物(Zhu et al., 2011, 2013; Ji et al., 2009; Green and Falloon, 2015; Wen et al., 2008a, b)。冈底斯花岗岩带的主体位于南拉萨地块雅鲁藏布缝合带北侧，隆格尔-工布江达断隆带以南的地区，主要是由岩基及大的复合岩体构成，并伴有同时代大规模的中酸性火山岩，这二者共占冈底斯岩浆岩带总面积的60%以上。该带主要由俯冲型花岗岩类、同碰撞花岗岩类、同碰撞-后碰撞强过铝花岗3类岩石构成，其中，俯冲花岗岩和同碰撞花岗岩类的出露最为广泛(图 1b)。
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图 1 藏南桑日地区地质简图和采样位置 (a)青藏高原构造单元划分地质简图；(b)拉萨地体中-新生代岩浆活动分布简图(据Zhu et al., 2009)；(c)采样区地质简图. JSSZ：金沙江缝合带；BNSZ：班公湖-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；IYZSZ：雅鲁藏布缝合带；NL：北拉萨地块；CL：中拉萨地块；SL：南拉萨地块 Fig. 1 Simplified geological map of Sangri in southern Tibet and the location of sampling (a) tectonic subdivision of the Tibetan Plateau; (b) simplified Mesozoic-Cenozoic magmatic activity distribution of Lhasa Terrane (after Zhu et al., 2009); (c) simplified geological map of sampling area. JSSZ: Jinsha suture zone; BNSZ: Bangong-Nujiang suture zone; SNMZ: Shiquanhe-Nam Tso Mélange Zone; LMF: Luobadui-Milashan Fault; IYZSZ: Indus-Yarlung Zangbo Suture Zone; NL: Northern Lhasa Terrane; CL: Central Lhasa Terrane; SL: Southern Lhasa Terrane |
本文样品采自南冈底斯东部的桑日县附近，该地区中新生代岩浆岩大片侵入到中生代的林布宗组、楚木龙组、设兴组、塔克那组及新生代的林子宗群(图 1c)。新生代侵入岩散布于整块区域，并多侵入于中生代侵入岩中。桑日花岗岩野外呈浅灰白色，花岗结构，块状构造，多为黑云母花岗岩，部分为二云母花岗岩。矿物成分由石英(30%~40%)、斜长石(1%~5%)、钾长石(20%~35%)、黑云母(3%~5%)、白云母( < 1%)、磁铁矿( < 1%)等组成见(图 2)。其中，石英为他形粒状，粒度不等，粒径为2~7mm(图 2a)。斜长石为自形-半自形板状，多有聚片双晶或卡纳复合双晶，表面偶见绿泥石化。斜长石亚族中的中长石内部可见明显的环带结构，部分可见卡钠复合双晶(图 2b)。钾长石为自形-半自形板状或粒状，表面可见高岭土化而成土褐色，多见卡式双晶。黑云母粒度变化较大，粒度由0.2~2mm不等，多呈半自形-他形片状(图 2c, d)。白云母粒度较小，为0.1~0.5mm，呈半自形-他形(图 2a)。磁铁矿粒度较小，多与黑云母伴生(图 2c)。
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图 2 桑日花岗岩显微照片 (a) SR1201-1；(b) SR1202-3；(c) SR1202-3；(d) SR1203-1.左为单偏光，右为正交光.Pl:斜长石; MS：白云母；Q：石英；Pth：条纹长石；Bi：黑云母；Mt：磁铁矿.比例尺为400μm Fig. 2 Photomicrographs of the Sangri granites Left: plane-polarized light; right: cross-polarized light. Pl: plagioclase; MS: muscovite; Q: quartz; Pth: perthite; Bi: biotite; Mt: magnetite. The scale is 400μm |
本文中全岩主量与微量元素分析均在中国地质大学(武汉)地质过程与矿产资源国家重点实验室(GPMR)进行。全岩主量元素含量测定利用XRF分析完成，分析精度优于5%。全岩微量元素含量利用Agilent 7500a ICP-MS分析完成，分析精度优于5%~10%，详细的样品消解处理过程同Liu et al. (2008)。
锆石微量元素含量和U-Pb同位素定年在中国地质大学(武汉)地质过程与矿产资源国家重点实验室(GPMR)利用LA-ICP-MS同时分析完成。激光剥蚀系统为Geo Las 2005，ICP-MS为Agilent 7500a。对分析数据的离线处理采用软件ICPMSDataCa 9.11完成。详细的仪器操作条件和数据处理方法同Liu et al. (2010)。锆石样品的U-Pb年龄谐和图绘制和年龄权重平均计算均采用Isoplot 3完成。
锆石Hf同位素测试在中国科学院地质与地球物理研究所MC-ICP-MS实验室进行, 本文选择已经调试过的锆石U-Pb定年分析的剥蚀坑的位置进行Hf同位素分析。束斑直径为40μm，剥蚀速率8Hz条件下，标准锆石91500的180Hf的信号强度约为5V左右。详细的测试流程和数据校正方法见Wu et al. (2006)。
4 结果 4.1 锆石U-Pb年龄和Hf同位素本文对样品SR1201-2和SR1203-1进行了锆石LA-ICPMS U-Pb年龄分析，分析结果见表 1。本文用于定年的锆石多为浅黄白色，柱状或长柱状，具有较好晶型，颗粒较大且多数具有震荡环带(图 3a-1, 1b-1)。文中所测锆石的Th/U比值范围为0.52~1.43，为典型的岩浆成因锆石(Hoskin and Schaltegger, 2003)。SR1201-2共测试18个锆石颗粒，其中12个锆石颗粒的206Pb/238U的值靠近谐和线，得到206Pb/238U加权平均年龄为66.4±1.6Ma(MSWD=2.1)(图 3a-2)。样品SR1203-1同样测试18个锆石颗粒，其中11个锆石颗粒的206Pb/238U的值靠近谐和线，得到206Pb/238U加权平均年龄67.3±1.0Ma(MSWD=2.4)(图 3b-2)。
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表 1 桑日花岗岩锆石U-Pb年龄数据 Table 1 Zircon U-Pb isotopic data of the Sangri granites |
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图 3 桑日花岗岩锆石阴极发光图像和U-Pb年龄谐和图 (a) SR1201-2; (b) SR1203-1 Fig. 3 CL images and U-Pb concordia diagrams of the Sangri granites |
本文对已测锆石年龄的SR1201-2和SR1203-1进行了Lu-Hf同位素分析，测试结果见表 2。
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表 2 桑日花岗岩锆石Hf同位素数据 Table 2 Zircon Hf isotopic data of the Sangri granites |
样品SR1201-2的锆石176Hf/177Hf比值为0.282860~0.283041，εHf(t)值为+4.6~+10.9，地幔模式年龄为303~562Ma，地壳模式年龄tDMC为439~888Ma。样品SR1203-1的锆石176Hf/177Hf比值为0.282888~0.282998，εHf(t)值为+5.6~+9.5，地幔模式年龄为373~526Ma，地壳模式年龄tDMC为537~785Ma。
4.2 全岩地球化学桑日花岗岩全岩主微量元素测试结果见表 3。桑日花岗岩的SiO2含量为74.26%~76.93%，在TAS地球化学判别图解中显示为花岗岩和碱性花岗岩(图 4)。岩石总体具有较高的K2O含量(4.47%~4.96%)及K2O/Na2O比值(1.31~1.38)、全碱含量偏低Na2O+K2O(7.87%~8.56%)，里特曼指数(σ=(K2O+Na2O)2/(SiO2-43))为1.89~2.33，在SiO2-K2O的地球化学判别图解中，样品显示为高钾钙碱性系列的花岗岩(图 5)。Al2O3含量(13.00%~14.19%)，铝饱和指数A/CNK为(1.08~1.16)。P2O5(0.02%~0.04%)、TiO2(0.12%~0.19%)、MgO(0.1%~0.35%)含量较低，且随着SiO2的含量增加而呈减少趋势。
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表 3 桑日花岗岩主量(wt%)和微量(×10-6)元素成分 Table 3 Whole-rock major (wt%) and trace (×10-6) elements compositions of the Sangri granite |
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图 4 桑日花岗岩TAS图解(底图据Le Bas et al., 1986) 参考文献数据引自Wu et al., 2003a, b; Zhou et al., 2016; Zhu et al., 2015b; 薄军委等, 2016; 蒋孝君等, 2016; 李碧乐等, 2016; 廖圣兵等, 2016; 刘群等, 2015; 马国祥等, 2016; 王永文等, 2015; 张建芳等, 2015; 章荣清等, 2016; 朱弟成等, 2009b. 图 5~图 10、图 13文献数据来源同此图 Fig. 4 Fig. 4 TAS diagram of the Sangri rocks (after Le Bas et al., 1986) The references data of highly fractionated granites from Wu et al., 2003a, b; Zhou et al., 2016; Zhu et al., 2015b; Bo et al., 2016; Jiang et al., 2016; Li et al., 2016; Liao et al., 2016; Liu et al., 2015; Ma et al., 2016; Wang et al., 2015; Zhang et al., 2015; Zhang et al., 2016; Zhu et al., 2009b. The references data sources in Fig. 5~Fig. 10 and Fig. 13 are same as this figure |
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图 5 桑日花岗岩K2O-SiO2图解(底图据Peccerillo and Taylor, 1976) Fig. 5 SiO2 vs. K2O diagram of the Sangri rocks (after Peccerillo and Taylor, 1976) |
微量元素组成上，桑日花岗岩的稀土总量较低(ΣREE=90.7×10-6~122.0×10-6)。在球粒陨石标准化图解(图 6)中，桑日花岗岩显示轻重稀土元素分馏，轻稀土元素相对富集(ΣLREE=77.8×10-6~91.9×10-6)，重稀土元素亏损(ΣHREE=12.3×10-6~19.9×10-6)，(La/Yb)N为4.62~6.68，(Ce/Yb)N为3.91~5.05，铕负异常较为明显(δEu=0.26~0.56)。在原始地幔标准化微量元素蜘蛛图解(图 7)中，样品显示富集大离子亲石元素Rb、Ba、Th和K，亏损高场强元素Nb、P、Ti。
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图 6 桑日花岗岩球粒陨石标准化稀土元素配分曲线图(标准化值据Boynton, 1984) Fig. 6 Chondrite-normalized REE patterns of the Sangri granites (normalization values after Boynton, 1984) |
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图 7 桑日花岗岩原始地幔标准化微量元素配分曲线图(标准化值据Sun and McDonough, 1989) Fig. 7 Primitive mantle-normalized trace-element spidergrams of the Sangri granites (normalization values after Sun and McDonough, 1989) |
桑日花岗岩主量元素显示高硅(SiO2：74.26%~76.93%)，高碱(K2O：4.47%~4.96%；Na2O：3.4%~3.62%)，高铝饱和指数(A/CNK：1.08%~1.16%)的特点。同时具有低磷(P2O5：0.02%~0.04%)、低镁(MgO：0.1%~0.35%)、低钛(TiO2：0.12%~0.19%)等特征。在微量元素中，桑日花岗岩具有较为明显的铕负异常(δEu=0.26~0.56)，富集Rb等大离子亲石元素，相对亏损Nb等高场强元素。这些特征使得桑日花岗岩既显示有A型花岗岩富硅，富钾，贫磷的特点，又同高硅、过铝质的S型花岗岩相似，同时又具有I型花岗岩富钠，高εHf(t)的特征。因此，桑日花岗岩的成因类型便成为一个值得讨论的问题。
通常情况下，按照花岗岩的源岩性质将花岗岩分为I型、S型、A型花岗岩。各类花岗岩之间的区分特征，前人多有研究(Eby, 1992; Champion and Chappell, 1992; Chappell and Stephens, 1987; Chappell and White, 1992, 2001; Hine et al., 1978; King et al., 2001; Yang et al., 2006; 吴福元等, 2007)。其中A型花岗岩在矿物学上，通常含有钠闪石-钠铁闪石、霓石-霓辉石、铁橄榄石等标志性碱性暗色矿物(Chappell, 1999)；在化学成分上，通常富硅、富钾、富Ga、Zr、Nb、Ta等高场强元素以及具有高的成岩温度，属于高温花岗岩(King et al., 2001; Watson, 1979; Watson and Harrison, 1983)。桑日花岗岩在薄片中没有观察到典型的A型花岗岩矿物，且根据全岩主量元素含量及Zr含量，桑日的全岩锆石饱和温度为718~735℃，明显低于A型花岗岩平均温度833℃(刘昌实等, 2003)。在微量元素上，桑日花岗岩具有相对较低的Ga、Zr、Nb等高场强元素，在10000Ga/Al和Zr+Nb+Ce+Y值的范围均低于A型花岗岩的下限值。同时，在FeO*/MgO-(Zr+Nb+Ce+Y)和(K2O+Na2O)/CaO-(Zr+Nb+Ce+Y)的判别图解上(图 8)，桑日花岗岩的数据也均落入了非A型花岗岩类的范围中，而是大多落在了分异的长英质花岗岩的区域。由此，可以判断桑日花岗岩不属于A型花岗岩，而应该属于分异的I型或者S型花岗岩。
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图 8 桑日花岗岩地球化学分类图解(据Whalen et al., 1987) (a) FeO*/MgO-(Zr+Nb+Ce+Y)；(b) (K2O+Na2O)/CaO-(Zr+Nb+Ce+Y). FG：分异的长英质花岗岩；OGT：未分异的M、I、S花岗岩 Fig. 8 The geochemical classifacation diagrams of Sangri granites (after Whalen et al., 1987) FG: Fractionated felsic granites; OGT: unfractionated M-, I-and S-type granites |
分异的I型或者S型花岗岩往往具有相似的主量元素特征和矿物组合，其成因类型难以准确识别(吴福元等, 2007)。但是Pichavant et al. (1992)指出，磷在强过铝质的熔体中，具有高的溶解度，并随着分异程度的增加含量升高；而在偏铝质或弱过铝质的熔体中，具有很低的溶解度，且随分异程度的增加含量降低。在保证演化样品属于同一岩套的前提下，P2O5和SiO2的关系可以作为一种有效的判别手段。桑日花岗岩具有很低的P2O5含量，且随着SiO2含量增加而降低(图 9)；桑日花岗岩Th、Y含量较高，且与Rb呈正相关关系(图 10)，因此桑日花岗岩源区很可能为偏铝质或弱过铝质。桑日花岗岩具有较高的Na2O(>3.2%)，SiO2与Ba、Zr、La具有正相关关系，与典型的S型花岗岩特征截然不同(Hine et al., 1978; Chappell, 1999)。在矿物学上，除了样品SR1201-1和SR1201-2含有少量白云母之外，其他样品均未见到典型的S型花岗岩富铝矿物如刚玉、电气石、堇青石、石榴石等(Zen, 1986)。此外，桑日高分异I型花岗岩同高分异的淡色花岗岩相比，具有环带清晰的新生锆石，同时明显不同于淡色花岗岩地球化学特征及分类(Gao et al., 2017; 吴福元等, 2015)，因此本文认为桑日花岗岩为高分异I型花岗岩。
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图 9 SiO2-P2O5图解 Fig. 9 SiO2 vs. P2O5 variation diagram |
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图 10 Rb-Th图解 Fig. 10 Rb vs. Th variation diagram |
前人资料显示，高分异的I型花岗岩可能有以下两种成因：(1) 由于幔源分异的岩浆底侵，导致地壳物质发生部分熔融形成高分异I型花岗岩(Richards, 2011; Wang et al., 2014; Wu et al., 2003a, 2003b)；(2) 幔源分异的基性岩浆底侵到下地壳并与壳源长英质岩浆混合在浅源形成混源岩浆房并在后期发生分离结晶作用形成的(邱检生等, 2008; Champion and Chappell, 1992)。
本文倾向于将桑日花岗岩成因解释为岩浆混合和后期分离结晶的共同作用。证据如下：在岩相学上，桑日岩体具有相似微量特征的基性岩脉出露(SR1201-3，SiO2含量为50.88%，坐标为N 29°38′38.8″，E 92°24′18.4″，未发表数据)；邻近的朱拉岩体同样发现65Ma左右的镁铁质暗色包体，这些野外证据均反映桑日地区在65Ma左右曾发生过岩浆混合作用(李奋其等, 2012)。镜下鉴定中，SR1202-3中出现斜长石斑晶(图 11a)且核部存在黑云母小包体，这可能是由于高温的基性岩浆注入酸性岩浆，导致长石再次生长，并将基性暗色矿物包裹其中(李胜荣等, 2006)，同时还发现了淬冷的标志性矿物针状磷灰石(图 11b)。
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图 11 桑日花岗岩矿物不平衡现象 (a) SR1202-3黑云母包体；(b) SR1202-3针状磷灰石. Ap：磷灰石 Fig. 11 Mineral disequilibrium of the Sangri granites (a) SR1201-1 biotite inclusion; (b) SR1202-3 needle-like apatite. Ap: apatite |
在地球化学上，桑日花岗岩与南冈底斯60~70Ma的侵入岩在哈克图解中，TiO2、MgO、Fe2O3T、Al2O3、CaO、MnO、P2O5均呈现良好线性关系，桑日花岗岩可能经历了岩浆混合作用(图 12)。桑日花岗岩锆石的Hf同位素具有明显的不均一性，每个样品的εHf(t)值的变化均大于4ε。这种Hf的不均一可能是由岩浆混合或者部分熔融过程中锆石的差异溶解造成的(Tang et al., 2014)。但是，锆石差异溶解主要发生高Zr(>100×10-6)的地壳深熔过程中，上文已提到，桑日高分异I型花岗岩的源区为偏铝质或弱过铝质，Zr含量较低，因此发生部分熔融的可能性较低。因此Hf的不均一性也可能指示岩浆在演化过程中有新的源区加入(Cherniak et al., 1997a, b)。
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图 12 桑日花岗岩哈克图解 南冈底斯60~70Ma岩浆岩数据来源于Mo et al., 2008; Zhu et al., 2009, 2015a; 纪伟强, 2010; 李奋其等, 2012; 徐旺春, 2010 Fig. 12 Harker diagram of Sangri granites Southern Gangdese 60~70Ma magmatite data from Mo et al., 2008; Zhu et al., 2009, 2015a; Ji, 2010; Li et al., 2012; Xu, 2010 |
桑日花岗岩特殊的地球化学特征，反映了其源区岩浆混合后，还经历了高程度的结晶分异作用。其中微量元素Sr、Ba、Eu的亏损反映了岩浆冷却结晶的成岩过程中斜长石和钾长石发生了分离结晶(邱检生等, 2005)。根据Ba-Sr的地球化学图解(图 13)，桑日花岗岩微量元素含量的变化趋势，同造岩矿物钾长石和斜长石分离结晶趋势一致，因此，桑日花岗岩花岗质岩浆在演化过程中，主要经历了斜长石、钾长石的分异结晶，这些造岩矿物的分离结晶控制了桑日花岗岩主量元素以及Sr、Ba、Eu的变化。与此同时，桑日高分异I型花岗岩具有较低的Zr含量和Zr/Hf比值(24.2~27.5)，可能与锆石的分离结晶有关。因此本文认为桑日岩体的花岗岩先由基性岩浆与酸性岩浆混合，在钾长石、斜长石、锆石等矿物分离结晶的控制下，经历了高程度的分异形成的。
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图 13 桑日花岗岩造岩矿物Ba-Sr图(据Hanson, 1978) Fig. 13 The Ba vs. Sr fractional crystallization of rock-forming minerals of Sangri rocks (after Hanson, 1978) |
众所周知，桑日花岗岩所处的冈底斯岩浆岩南带是青藏高原岩浆作用最发育的地区。这些岩浆岩随着冈底斯带所处的构造环境的不同显示不同的地球化学和同位素特征(纪伟强, 2010)。前人资料显示，从约260Ma左右，自南向北岛弧花岗岩的出露，表明密度较大的特提斯洋板块开始北向俯冲到拉萨地块之下(侯增谦等, 2006a; Niu, 2016)；到100~70Ma左右，“软碰撞”阶段出现洋脊俯冲或者加厚下地壳拆沉；到65~50Ma左右，印度大陆与亚洲大陆发生“硬碰撞”，拉萨地块急剧缩短隆升，产生大量同碰撞花岗岩和碰撞后花岗岩(Zhu et al., 2015a; 莫宣学等, 2005; 莫宣学, 2011)。Hf同位素很少或不受岩石演化程度的影响，所以常用εHf(t)来反映花岗岩源区壳幔的组成，tDM1则反映岩浆初始熔融时源区的亏损地幔模式年龄。本文通过使用冈底斯南带各类岩性岩浆岩的εHf(t)值和tDM对其锆石U-Pb年龄投图(图 14a, b)，发现从65Ma开始，冈底斯南带岩浆岩的εHf(t)值变化范围开始变大，反映从那时起岩浆源区物质组成多样，既有大量地壳物质的参与，又有幔源物质的加入，因此判断印度-亚洲陆陆碰撞的初始启动时间在65Ma左右。
桑日花岗岩的形成年龄在70~65Ma。根据前人研究，70~65Ma之前特提斯洋板片俯冲状态可能为平板俯冲或正常俯冲。深部高压源区，例如加厚下地壳，矿物组合以由角闪石+石榴石为主，其熔体具有高Sr/Y和La/Yb比值特征(Zhu et al., 2017)；在浅部低压源区，矿物组合以由斜长石+单斜辉石为主，此时Sr大量与斜长石中的Ca发生类质同象，从而在熔体相中表现亏损(Feeley and Hacker, 1995; Moyen, 2009)，具有低的Sr/Y和La/Yb比值特征。因此，岩浆岩中Sr/Y和La/Yb的值往往可以代表地壳的厚度。使用冈底斯中酸性岩浆岩地球化学数据令Sr/Y和La/Yb对锆石U-Pb年龄作图(图 14c, d)，可以发现在80~70Ma冈底斯带均有出露高Sr/Y和La/Yb的岩浆岩，说明冈底斯在印度-亚洲大陆碰撞之前发生过类似安第斯山的隆升过程，到了70~65Ma，冈底斯岩基出露的岩浆岩开始变得稀少且不具备加厚下地壳的特征，根据以上证据，本文推论特提斯洋板片在70Ma之前俯冲状态为平板俯冲，且在70~65Ma左右，俯冲板片发生回转或者处于陆陆碰撞的初期(Tian et al., 2016; Li et al., 2016; Wen et al., 2008a, 2008b; 张泽明等, 2009)。含白云母花岗岩的产出标志着构造体制的转换(侯增谦等, 2006a)。Brun and Faccenna (2008)又指出板片回转通常发生在大洋俯冲的末期和陆陆碰撞的早期，且为随后印度大陆板块的深埋腾出空间，因此本文认为桑日花岗岩产自特提斯洋板片回转的初期。而前期研究中65Ma年左右的高温高压变质作用、印度亚洲大陆汇聚速率开始快速增大等证据都证明了在70~65Ma左右，特提斯洋板片启动回转(张泽明等, 2009; Lee and Lawver, 1995)。
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图 14 冈底斯南带花岗岩地球化学时间变化图 εHf(t)值(a)、tDM (b)、Sr/Y (c)和La/Yb (d) vs. U-Pb年龄.南冈底斯岩浆岩数据管琪等, 2010; Guan et al., 2012; Mo et al., 2008; Zhu et al., 2009, 2015a; 纪伟强, 2010; 李奋其等, 2012; 尚振, 2016; 王青, 2016; 徐旺春, 2010; 叶丽娟, 2013; Liu et al., 2017 Fig. 14 Time variation of granite geochemistry in the southern Gangdese εHf(t) (a), tDM (b), Sr/Y (c), La/Yb (d) vs. U-Pb ages.The Southern Gangdese magmatite data from Guan et al., 2010, 2012; Mo et al., 2008; Zhu et al., 2009, 2015a; Ji, 2010; Li et al., 2012; Shang, 2016; Wang, 2016; Xu, 2010; Ye, 2013; Liu et al., 2017 |
(1) 桑日花岗岩锆石U-Pb年龄为65.3±1.0Ma~67.3±1.0Ma，为冈底斯岩浆岩带南部晚白垩纪末期岩浆活动的产物。
(2) 白垩纪末桑日花岗岩的主要岩性为黑云母花岗岩和二长花岗岩，具有高SiO2(74.26%~76.93%)，高K2O(4.47%~4.96%)，铝过饱和(13.00%~14.19%)，低P2O5(0.02%~0.04%)，富集K、Rb、Th亏损Nb、Ta、P、Ti，锆石εHf(t)>0，且锆石Hf同位素不均一的特征，属于高分异的I型花岗岩。
(3) 桑日花岗岩的形成的地球动力学背景为在特提斯洋板块北向俯冲过程中，板片开始发生回转，俯冲洋壳脱水产生的流体进入地幔楔，引发地幔楔发生部分熔融产生镁铁质幔源物质并底侵上涌，导致浅部地壳发生部分熔融，并与上涌的幔源岩浆混合，从而在浅部形成混源岩浆房，最终在侵位与成岩后期经历高程度的分异演化形成的。
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