印度大陆与亚洲大陆早期碰撞过程与动力学模型——来自西藏冈底斯新生代火成岩证据

Early Processes and Tectonic Model for the Indian—Asian Continental Collision:Evidence from the Cenozoic Gangdese Igneous Rocks in Tibet

为了揭示青藏高原的形成演化及其隆升历史,本文主要立足于西藏冈底斯带新生代岩浆岩,研究了印度—亚洲大陆碰撞早期阶段的关键岩石记录、详细碰撞过程和深部动力机制。西藏新生代火山-岩浆活动贯穿于主碰撞造山过程的始终,形成规模巨大的冈底斯火成岩浆岩带,其中,火山活动形成著名的林子宗第三纪火山岩系(64~43Ma),岩浆作用则形成3个时间连续、但组合不同的岩浆序列,即:1壳源花岗岩组合(65~50Ma)、2正εNd花岗岩-辉长岩组合(52~47Ma)和3幔源玄武质次火山岩-辉绿岩组合(53~42Ma)。林子宗第三纪火山岩系形成于印度—亚洲大陆对接碰撞之后(~65Ma),不整合覆盖于中生代褶皱构造层之上,中下部钙碱性—高钾钙碱性火山岩显示岛弧/陆缘弧地球化学特征,主要来自于洋壳板片流体交代的地幔楔形区,上部钾玄岩系火山岩则更多地显示壳源特征。壳源花岗岩主要侵位于冈底斯东段腾冲地区,成因类型为白云母过铝花岗岩和富钾钙碱性花岗岩,其高(87Sr/86Sr)i(>0.710)和低εNd(<-7)同位素组成反映其源于碰撞加厚的砂泥质地壳的深熔作用。正εNd值(+2^+5)花岗岩和辉长岩沿冈底斯带成对侵位,花岗岩具有埃达克岩与弧花岗岩过渡特征,其形成有较多的幔源物质贡献;辉长岩正εNd值特征(+2.5^+7.0)、REE平坦型或弱富集型以及亏损大部分不相容元素(Nb,P,Ti,U,Th,LREE)特征,反映软流圈地幔对岩浆形成产生重要贡献。幔源玄武质次火山岩主要为钙碱性岩系,REE平坦型,低(87Sr/86Sr)i(<0.7060)、高εNd(高达+4.3),同位素组成接近于MORB,证明其来源于亏损的软流圈地幔。根据这些构造-岩浆事件的时空分布、岩石组合特征、岩石地球化学以及岩浆演变序列,提出了一个青藏高原大陆碰撞的四阶段演化模式。这个模式强调了170~60Ma,新特提斯洋板片回转,印度大陆与亚洲大陆发生碰撞(≥65Ma),并导致加厚地壳深熔;260~54Ma,印度大陆板片向北陡深俯冲,下地壳缩短加厚,地壳深熔作用持续;353~41Ma,新特提斯洋板片发生断离,并向下拆沉。软流圈物质透过板片断离窗上涌,诱发地幔楔、上覆加厚的镁铁质下地壳熔融;4陡深俯冲的印度大陆板片因特提斯洋板片断离而发生折返,开始低角度俯冲(<40Ma),导致高原内部的陆内俯冲、走滑剪切与地壳缩短,造成冈底斯岩浆间断(40~26Ma)和拉萨地体初始抬升。因此,在青藏高原碰撞造山过程中,主碰撞期造山(65~41Ma)的动力机制主要是印度大陆板片的陡角度俯冲和特提斯洋板片断离,晚碰撞期造山(40~26Ma)的动力机制主要为印度大陆板片的低角度俯冲。

The Tibetan plateau is characterized by the thickest continental crust on Earth （60-80 km） with a hot and soft lower crust, due to Indo--Asian continental collision since Paleocene. However, when and how this continental collision happened has long been a subiect of debates. Three suites of the Paleocene--Eocene igneous rocks developed along the Gnagdese belt in Tibet provide new constraints on them. These three igneous suites include （1） about 5000m thick, Linzizong volcanic successions （LVS）, uncomformably covering the Cretaceous sedimentary sequences, and syn-collisonal crust-derived granites near the eastern Himalayan syntax, （2） gabbro and granitoid intrusions, both formed two parallel belts along the Gnagdese, and （3） basaltic subvolcanic districts associated with LVS. The LVS rocks are calc-alkaline and shoshonitic, andesite-daciterhyolite, with depletion in HFSE （Nb, Ta, Hf, Zr, P） and enrichment in LREE and LILE （K, Rb）, but low-abundance of Ba and Sr. Sr-Nd-Pb systematics suggest a wedge mantle source metaomatied by fluids from the Neo-Tethyan oceanic-slab subduction, with minor lower-crustal contribution. ^40Ar/^39Ar dating yielded a range of erupting age of 65-44 Ma for the LVS. The syn-collisional granites are composed of muscovite granite with ^40Ar/^39Ar age of 65-58 Ma and monzogranite and syenitic granites with peaking at 53 Ma and 42 Ma, respectively. The muscovite granites have high ASI （1. 02-2. 63） and high Rb/Sr （257-404） and Rb/Ba ratios （13-40）, and high （^87Sr/^84Sr）,（〉0. 710） and low εNd（〈-7）, suggesting a crustal anatexis related to Indo--Asian collision at -65 Ma. The monzogranite and syenitic granites are potassic calc-alkaline, according to Barbarin （1990）, reflecting a stress regime transformed from compression （65- 54 Ma） to relaxation （53-42 Ma）. The gabbro and granitoid plutons intruded the Gangdese arc granite batholiths, along which they extend in EW for 1500 km. Available dating data gave an age range of 41- 52 Ma for granitoid plutons, whereas SHRIMP U-Pb age of zircons from six gabbro intrusions yielded an identical, but narrower range of 47.0-52.5 Ma, than that of associated granitoid. The granitoid is characterized by hosted numerous MME similar to gabbro in compositions and positive SNd values （2- 5）, whereas gabbros have similar εNd signature to MORB. The granitoids are transitional between adakite and arc granites geochemically, suggesting a large contribution of mantle components. While, gabbros do not exhibit arc geochemical characteristics, such as deposition in HFSE and enrichment in LILE, suggesting a subcontinental lithospheric mantle source for the mafic magmas. They are interpreted by the break-off of the subducted Indian slab at about 53 Ma; the latter also resulted in UHP rocks and exhumation in the NW Himalaya （Leech et al. , 2005）. Although no enough data define spatial distribution of the basaltic subvolcanic rocks in the Gangdese belt, an ^40Ar/^39Ar age of 42 Ma indicates that these mafic rocks are the last-epoch products, and mark the end of the Indo--Asian main collision. These rocks are tholeiitic and calc-alkaline, a few of them have high Mg^n values （0.6-0. 75） with high Cr （400×10^-6-750× 10^-6） and V （220×10^-6-320×10^-6）, with flat REE patterns, a typical feature of primary basaltic magmas. Trace element abundance patterns normalized by primitive mantle show strongly depletion in LILE （K, Rb, Ba）. （^87Sr/^86Sr）（0. 7045-0. 7065） and εNd（0. 0-＋4. 8）, similar to those of MORB, suggest that these basaltic magmas were derived from depleted upper mantle. A plausible interpretation is upwelling of the athenospheric materials through slab-window, accompanying the delamination of the Indian slab at about 42 Ma, which in turn led to initiation of low-angle subduction by the end of the main-collision between Indian and Asian continents （65-41 Ma）.