【作者】 胡培远；

【导师】 李才；

【作者基本信息】 吉林大学 ， 构造地质学， 2014， 博士

【摘要】 菊花山－唐古拉－类乌齐火山岩浆弧沿龙木错－双湖－澜沧江板块缝合带分布，从藏北羌塘中部的冈玛错、菊花山，经唐古拉山，一直延伸到藏东类乌齐和吉塘地区。在藏北羌塘菊花山地区，菊花山－唐古拉－类乌齐火山岩浆主要表现为火山岩、花岗岩和闪长岩。其中，火山岩和闪长岩普遍具有Nb和Ti的负异常。在Nb－Y和Rb－Y+Nb图解上，样品投点几乎全部落入“火山弧花岗岩（VAG）”区，因而应当是形成于板块俯冲消减的环境。这些火山岩和闪长岩可以依据地球化学特征分为两类：埃达克岩（225~219Ma）和普通岛弧火山岩（217~205Ma），它们的岩浆源区和成因具有明显差别。埃达克岩主要包括宝户闪长岩（223Ma）和菊花山火山岩的一部分（225Ma、219Ma），这些样品普遍具有高的Sr含量（291~1367ppm，平均值为799ppm）以及低的Y含量（5.47~13.9ppm，平均值为10.2ppm）和Sr/Y比值（28~116，平均值为79）。在Sr/Y－Y图解上，样品投点落在埃达克岩区域。这些样品普遍具有富硅（SiO2=62.84~77.82wt.%）的特征，在MgO－SiO2图解上投点落入高SiO2埃达克岩（HSA）区域。在Sr/Y－La/Yb图解上，样品投点落入现代岛弧或陆缘弧的洋壳熔融成因埃达克岩区域，与青藏高原和大别地区的地壳增厚或拆离埃达克岩差别明显。因此，本文研究的埃达克岩与现代岛弧或陆缘弧埃达克岩类似，形成于俯冲大洋板片的部分熔融作用。这些埃达克岩同时具有正的锆石εHf(t)值（+3.2~+5.4）和负的全岩εNd(t)值（-2.6），表明它们形成于俯冲大洋板片与沉积物的部分熔融作用。羌塘菊花山地区的其他三叠纪火山岩显示出相对比较一致的地球化学特征。它们具有相对低的Sr含量（37~389ppm，平均值为186ppm）和Sr/Y比值（2~25，平均值为8）和变化的SiO2含量（60.30~80.90wt.%），同时还显示出高的初始87Sr/86Sr比值（0.708~0.714）和低的全岩εNd(t)值（-9.6~-7.9），可能是形成于羌北－昌都板块古老地壳物质的部分熔融作用。上述资料表明，在219Ma左右，羌塘菊花山地区火山岩有一个明显的地球化学特征变化，应当与俯冲大洋板片的断离有关。羌塘菊花山地区的花岗岩的研究程度较低，以219Ma为界可以分为两组。第一组为菊花山地区的绝大多数三叠纪花岗岩，形成时代为215~201Ma。其中，日湾茶卡东、果干加年山东和香桃湖地区的花岗岩为I型花岗岩，而本松错和戈木日地区的花岗岩为S型花岗岩。结合区域地质资料可知，这些三叠纪花岗岩与俯冲大洋板片的断离和软流圈上涌相关，代表了挤压环境向伸展环境的转化阶段。第二组仅包括果干加年山花岗闪长岩（225Ma），目前还没有可靠的地球化学资料，但是区域内已经识别出了同时代的埃达克质火山岩（225~219Ma），由此推测可能形成于俯冲消减环境。在藏东的唐古拉－类乌齐地区，菊花山－唐古拉－类乌齐火山岩浆主要表现为强变形的花岗岩，时代集中于251~236Ma。其中，吉塘和唐古拉地区的以及类乌齐地区的部分（241~236Ma）花岗岩为S型花岗岩，而察拉地区以及类乌齐地区的其余（249~244Ma）花岗岩为I型花岗岩。结合区域地质资料可知，这些花岗岩与羌塘菊花山地区的三叠纪花岗岩成因相似。沿板片断离窗上涌的软流圈地幔导致羌北－昌都板块的地壳物质发生广泛重熔，继而形成了该地区的三叠纪花岗质岩浆。其中，S型花岗岩的岩浆源区为羌北－昌都板块的古元古代地壳物质，而I型花岗岩具有相对年轻的火成岩源区，可能是新元古代新生地壳部分熔融的产物。综合前人报道和本文新测的年代学资料，菊花山－唐古拉－类乌齐火山岩浆弧的时代跨度为251~205Ma，主体为三叠纪，但是其年龄分布并不均一，具有西段年龄较年轻（225~205Ma）、东段年龄较古老（251~236Ma）的特点。由此推测龙木错－双湖－澜沧江洋的闭合时代应当具有穿时性特点。在羌塘中部地区，龙木错－双湖－澜沧江洋的俯冲消减一直持续到219Ma左右。随后发生俯冲板片断离，上涌的软流圈地幔导致羌北－昌都板块的地壳物质发生大规模重熔和高压变质岩石的折返。在藏东唐古拉－类乌齐地区，龙木错－双湖－澜沧江洋至少在早石炭世已经开始俯冲消减。羌南－保山板块在晚二叠世抵达海沟，俯冲板片断离可能发生在早－中三叠世。该地区251~236Ma的花岗岩应当是板片断离后软流圈地幔底侵的岩浆记录。与滇西昌宁－孟连板块缝合带的相关研究成果对比可知，滇西的临沧花岗岩基可能代表了菊花山－唐古拉－类乌齐火山岩浆弧的南向延伸，但是需要进一步研究工作来验证。更多还原

【Abstract】 The distribution of the Juhuashan–Tanggula–Leiwuqi volcanic arc is along theLongmu Co–Shuanghu–Lancangjiang suture zone, extending from Gangmacuo andJuhuashan areas of central Qiangtang, to the Tanggula, Leiwuqi, and Jitang areas ofeastern Tibet.In the Juhuashan area of Qiangtang, the Juhuashan–Tanggula–Leiwuqi volcanicarc consists of arc volcanic rocks, granites and diorites. The arc volcanic rocks anddiorites exhibit strong negative Nb, Ti, and Eu anomalies. These rocks fall in the‘‘volcanic arc granite’’(VAG) field in the Nb versus Y and Rb versus Y+Nb tectonicdiscrimination diagrams, which are comparable to those formed in a volcanic arctectonic setting. The arc volcanic rocks and diorites can be devided into two groups.The older group (225–219Ma) have high Sr (291–1367ppm, averaging799ppm) andlow Y (5.47–13.9ppm, averaging10.2ppm) contents with high Sr/Y ratios (28–116,averaging79). They all plot in the adakite field in the Sr/Y versus Y diagram. Thehigh SiO2contents (62.84–77.82wt.%) suggests that these rocks belong to high-SiO2adakites (HSA). In the Sr/Y versus La/Yb diagram, these rocks show features of themodern arc adakite formed by slab melting. They are significantly different from theTibetan and Dabie adakites formed by partial melting of the lower crust. Collectively,these rocks have the geochemical characters similar to the modern adakite, a rock typeoriginally defined as felsic magma produced by partial melting of subducted oceaniccrust. The negative εNd(t) value (-2.6) and positive εHf(t) values (+3.2to+5.4)suggested that these rocks were generated by the melting of subducted oceanic crustand associated sediments. The younger group (217–205Ma) have relatively low Sr contents (37–389ppm, averaging186ppm) and Sr/Y ratios (2–25, averaging8) withvarying SiO2contents (60.30–80.90wt.%). Moreover, these rocks display high initial87Sr/86Sr ratios (0.708–0.714), and distinguished negative εNd(t) values (-9.6to-7.9),indicating that these rocks were probably derived from anatexis of ancient continentcrust of the Northern Qiangtang–Qamdo terrane. A slab break-off model waspresented to interpret the compositional transition from ‘‘adakitic’’ to ‘‘non-adakitic’’magmatism at ca.219Ma.The studying degree of the predecessor is low to the Triassic granites in the theJuhuashan area of Qiangtang. These Triassic granites can also be devided into twogroups. The younger group (215–201Ma) are in the majority. Thereinto, the granitesof eastern Riwanchaka, eastern Guoganjianian, and Xiangtaohu are I-type, and thegranites of Bensongcuo and Gemuri are S-type. The granites of the younger groupwere related to the slab break-off of the Longmu Co–Shuanghu–Lancangjiang ocean.The older group (225Ma) includes only the Guoganjianian granodiorite. Consideringthe contemporaneous adakites (225–219Ma), this granodiorite was related to asubduction setting.In the Tanggula–Leiwuqi area of eastern Tibet, the Juhuashan–Tanggula–Leiwuqi volcanic arc mainly consists of deformated granites which were comparableto the Triassic granites in the the Juhuashan area of Qiangtang. These granites can besubdivided into S-and I-types. The S-type granites were generated by the melting ofthe Paleoproterozoic continent crust of Northern Qiangtang-Qamdo terrane; the I-typegranites were probably derived from a relatively young source of Neoproterozoicjuvenile crust.Collectively, the currently available high-quality geochronological data revealthat the Juhuashan–Tanggula–Leiwuqi volcanic arc was formed during Triassic(251–205Ma); the western section (225–205Ma) is younger than the eastern section(251–236Ma). Therefore, the closure of the Longmu Co–Shuanghu–Lancangjiangocean is probably diachronous. In central Qiangtang area, the Southern Qiangtang–Baoshan collided with Northern Qiangtang–Qamdo terranes at ca.237~230Ma andthe time of the oceanic subduction ranges from237to219Ma. The oceanic slab wassubsequently detached at ca.219Ma. In the meantime, the break-off induced the hotasthenosphere to underplate the North–Qiangtang subterrane and caused crustalanatexis in the ancient basement. Futhermore, asthenosphere upwelling also led to theuplifting and exhumation of the High-pressure metamorphic belt. In the Tanggula–Leiwuqi area of eastern Tibet, the Southern Qiangtang–Baoshan collidedwith Northern Qiangtang–Qamdo terranes at Late Permian and the related oceanicslab was probably detached at Early–Middle Triassic.Triassic granites are widespread in the Lincang area of the Changning–Mengliansuture zone in western Yunnan of China. We proposed that these Triassic granites areprobably the sourthern extension of the Juhuashan–Tanggula–Leiwuqi volcanic arc,though additional investigations and data will be required to test our arguments.更多还原