Origin of adakitic intrusives generated during mid-Miocene east–west extension in southern Tibet
Origin of adakitic intrusives generated during mid-Miocene east–west extension in southern Tibet
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DOI：
10.1016/s0012-821x(04)00007-x
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发表时间：
2004-03
影响因子：
5.3
通讯作者：
Z. Hou;Yf Gao;Xiaoming Qu;Zongyao Rui;X. Mo
中科院分区：
文献类型：
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作者：
Z. Hou;Yf Gao;Xiaoming Qu;Zongyao Rui;X. Mo
埃达克岩是一种中长英质岩石，具有低K、高Al、Na和Sr的特征，并贫Y和HREE，通常产于与大洋板片俯冲有关的弧环境中。本文报道了西藏南部印支-亚洲大陆碰撞造山带中钾质埃达克岩的产出。这些埃达克质侵入体产于中新世含铜斑岩带中，是新近纪东西向伸展的产物，该斑岩带沿着与雅鲁藏布缝合线平行的冈底斯弧发育，但局部受南北走向正断层系统控制。藏南埃达克质侵入岩及其类似物的岩浆活动时间约为10-18 Ma，形成于后碰撞拉张环境。地球化学数据表明，这些埃达克质侵入岩为钾玄岩，具有钙碱性成分，高K，高Sr/Y和La/Y，低Y和HREE，类似于板片熔融形成的埃达克岩。然而，对于Nd（t），（−6.18至+5.52），初始87 Sr/86 Sr（0.7049-0.7079），207Pb/204Pb（15.502-15.626）和208 Pb/204 Pb（38.389-38.960），以及高K2 O含量（2.6-8.6重量%）和相对高的Mg#值（0.32-0.74）表明这些埃达克质岩浆是由一个复杂的机制形成的，包括在增厚的下地壳中部分熔融的镁铁质物质，以及富集的地幔和/或上地壳成分的输入。没有负Eu异常，极端亏损的Y，Nb和Ti，和变量高Sr/Y和La/Yb比值表明，下地壳源可能是含水角闪石榴辉岩或石榴角闪岩，在西部和东部喜马拉雅构造带在青藏高原折返。下地壳的部分熔融很可能是由板片断裂或地幔减薄形成的幔源超钾质岩浆作用（17-25 Ma）触发的。在原始埃达克质熔体的形成和迁移过程中，超钾质岩浆和上地壳物质的额外输入可以解释藏南大多数埃达克质侵入体所观察到的Nd-Nd-Sr同位素特征和高Rb/Sr、K和Mg#特征。
Adakite is an intermediate to felsic rock with low K, high Al, Na and Sr, and depleted in Y and HREE, usually occurring in arc settings related to subduction of an oceanic slab. Here we report the occurrence of potassic adakites from south Tibet in an orogenic belt produced by the Indo–Asian continent collision. These adakitic intrusives, as a product of Neogene east–west extension, occur in a Miocene Cu-bearing porphyry belt, which developed along the Gangdese arc paralleling the Yarlung–Zangbo suture, but is locally controlled by NS-striking normal faulting systems. Available age data define a duration of magmatism of 10–18 Ma for the adakitic intrusives and related extrusive analogues in south Tibet, which occur in a post-collisional extensional setting. Geochemical data indicate that these adakitic intrusives are shoshonitic and exhibit calc-alkaline composition with high K, and high Sr/Y and La/Y coupled with low Y and HREE, similar to adakites derived from slab melting. However, a wide range for ϵNd(t)(−6.18 to +5.52), initial87Sr/86Sr (0.7049–0.7079),207Pb/204Pb (15.502–15.626), and208Pb/204Pb (38.389–38.960), as well as high K2O contents (2.6–8.6 wt%) and relatively high Mg# values (0.32–0.74) indicate that these adakitic magmas were formed by a complex mechanism involving partial melting of mafic materials in a thickened lower crust with input of enriched mantle and/or upper crust components. Absence of a negative Eu anomaly, extreme depletion in Y, Nb and Ti, and variable high Sr/Y and La/Yb ratios suggest that the lower crustal source is probably a hydrous amphibole eclogite or garnet amphibolite, as exhumed in the western and eastern Himalayan syntaxes on the Tibetan plateau. Partial melting of the lower crust was most likely triggered by mantle-derived ultra-potassic magmatism (17–25 Ma) formed by slab breakoff or mantle thinning. During the formation and migration of pristine adakitic melts, additional input of ultra-potassic magmas and upper crustal materials could account for the observed ϵNd–ϵSrsignatures and high Rb/Sr, K and Mg# characteristics for most of the adakitic intrusives in south Tibet.