Survival of the Lhasa Terrane during its collision with Asia due to crust-mantle coupling revealed by ca. 114 Ma intrusive rocks in western Tibet
Survival of the Lhasa Terrane during its collision with Asia due to crust-mantle coupling revealed by ca. 114 Ma intrusive rocks in western Tibet
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拉萨地体在与亚洲碰撞期间由于壳幔耦合而得以幸存
DOI：
10.1016/j.lithos.2018.01.006
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发表时间：
2018-04
期刊：
影响因子：
3.5
通讯作者：
Zhao Zhi Dan
中科院分区：
文献类型：
--
作者：
Wang Qing;Zhu Di Cheng;Liu An Lin;Cawood Peter A;Liu Sheng Ao;Xia Ying;Chen Yue;Wang Hao;Zhang Liang Liang;Zhao Zhi Dan
拉萨地体在穿越特提斯洋和随后与亚洲碰撞期间的生存可能是由其古老的岩石圈地幔和上覆地壳之间的机械耦合维持的。这种耦合的证据是由地质年代学和地球化学数据从高镁闪长玢岩岩脉，侵入花岗闪长岩与闪长质包体内的尼雄岩在西段的拉萨subjecie，西藏南部。锆石LA-ICP-MS U-Pb定年表明闪长玢岩脉（113.9 ± 2 Ma）、闪长质包体（113.9 ± 1 Ma）和寄主花岗闪长岩（113.1 ± 2 Ma）同时侵位。含角闪石的花岗闪长岩为变铝质-弱过铝质（A/CNK = 0.95-1.05），属高钾钙碱性I型花岗岩。这些岩石具有低Mg#（37-43）、负锆石εHf（t）值（−6.8至−1.2）和负全岩εNd（t）值（−8.1至−5.4）的特征，表明是通过古老下地壳的深熔作用形成的。两个混合或污染最少的闪长玢岩岩脉样品具有高MgO（8.46 - 8.74重量%），高Mg#（69 - 70）和高丰度的相容元素（例如，Cr = 673 - 646 ppm，Ni = 177 - 189 ppm），与原始岩浆的含量接近。它们是高钾钙碱性的，全岩εNd（t）值为负值（−1.9至−1.2），表明这些样品很可能来自于被板状流体交代的古岩石圈地幔的部分熔融。闪长质包体样品为变铝质高钾钙碱性，全岩εNd（t）值为负值（−7.8 ~ −3.7），这被解释为古下地壳熔体与软流圈地幔（而非岩石圈地幔）熔体混合的结果。高镁闪长玢岩脉样品的Nd同位素地幔模式年龄（1.1 - 1.4Ga）接近Nd同位素两阶段模式年龄（1.3 - 1.6 Ga）和锆石Hf同位素地壳模式年龄（1.1 - 1.5 Ga）的古老下地壳衍生花岗闪长岩，表明拉萨中亚段西段岩石圈地幔与上覆地壳在~114 Ma时是机械耦合的。结合元古代地壳岩石记录在中部和东部段的拉萨subjecie，我们建议，这种耦合使其能够抵抗俯冲过程中增生到亚洲。
Survival of the Lhasa Terrane during its drift across the Tethyan Ocean and subsequent collision with Asia was likely maintained by mechanical coupling between its ancient lithospheric mantle and the overlying crust. Evidence for this coupling is provided by geochronological and geochemical data from high-Mg dioritic porphyrite dikes that intruded into granodiorites with dioritic enclaves within the Nixiong Batholith in the western segment of the central Lhasa subterrane, southern Tibet. Zircon LA-ICP-MS U-Pb dating indicates synchronous emplacement of dioritic porphyrite dikes (113.9 ± 2 Ma), dioritic enclaves (113.9 ± 1 Ma), and host granodiorites (113.1 ± 2 Ma). The hornblende-bearing granodiorites are metaluminous to weakly peraluminous (A/CNK = 0.95–1.05) and belong to high-K calc-alkaline I-type granite. These rocks are characterized by low Mg# (37–43), negative zircon εHf(t) values (−6.8 to −1.2), and negative whole-rock εNd(t) values (−8.1 to −5.4), suggestive of derivation through anatexis of ancient lower crust. The two least-mixed or contaminated dioritic porphyrite dike samples have high MgO (8.46−8.74 wt%), high Mg# (69−70), and high abundances of compatible elements (e.g., Cr = 673−646 ppm, Ni = 177−189 ppm), which are close to those of primitive magma. They are high-K calc-alkaline and show negative whole-rock εNd(t) values (−1.9 to −1.2), indicating that these samples are most likely derived from the partial melting of ancient lithospheric mantle that was metasomatized by slab-derived fluids. The dioritic enclave samples are metaluminous high-K calc-alkaline and have varying negative whole-rock εNd(t) values (−7.8 to −3.7), which are interpreted as the result of magma mixing between the ancient lower crust-derived melts and asthenospheric mantle- (rather than lithospheric mantle-) derived melts. The Nd isotope mantle model ages of the least-mixed or contaminated high-Mg dioritic porphyrite dike samples (1.1−1.4 Ga) are close to the Nd isotope two-stage model ages (1.3−1.6 Ga) and the zircon Hf isotope crustal model ages (1.1−1.5 Ga) of the ancient lower crust-derived granodiorites, indicating that the lithospheric mantle of the western segment of the central Lhasa subterrane is mechanically coupled to the overlying crust at ~114 Ma. In combination with the Proterozoic crustal rocks documented in the central and eastern segments of the central Lhasa subterrane, we propose that this coupling enabled it to resist subduction during accretion to Asia.
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