| Citation: |
Xu-xuan Ma, Li-E Gao, Zhong-bao Zhao, Xi-jie Chen, Hai-bing Li, 2021. Early Eocene leucocratic sill/dike swarms in the Gangdese belt, southern Tibet: Tectonic implications for Indo-Asian collision, China Geology, 4, 56-66. doi: 10.31035/cg2021019
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Early Eocene leucocratic sill/dike swarms in the Gangdese belt, southern Tibet: Tectonic implications for Indo-Asian collision
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Abstract
The timing of the initial Indo-Asian collision is a subject of debate for a long time. Besides, the magmatic trace of the collisional process is also unclear. In the present study, the authors report Early Eocene leucocratic sill/dike swarms in the northern edge of the Nymo intrusive complex of the Gangdese belt, southern Tibet. The Nymo intrusive complex was emplaced at ca. 50–47 Ma and surrounded by the metamorphosed Jurassic-aged Bima Formation volcano-sedimentary sequence along its northern side. At outcrops, the leucocratic sills/dikes intruded along or truncated the deformed foliations of the host Bima Formation, which has been subject to high-temperature amphibolite-facies metamorphism at ca. 50–47 Ma. Detailed cathodoluminescence image analyses reveal that the zircon grains of the leucocratic sills/dikes have core-mantle textures. The cores yield the Jurassic ages comparable to the protolith ages of the Bima Formation. In contrast, the mantles of zircon grains yield weighted mean ages of ca. 49–47 Ma, representing the crystallization timing of these leucocratic sills/dikes. The coeval ages for the Nymo intrusive complex, the high-temperature metamorphism, and the leucocratic sills/dikes indicate that a close relationship exists among them. The authors tentatively suggest that these leucocratic sills/dikes were generated from partial melting of the Jurassic-aged Bima Formation volcanic rocks, triggered by the high heat from the magma chamber of the Nymo intrusive complex. This Early Eocene tectono-thermal event of coeval magmatism, metamorphism and partial melting was most likely formed during the Indo-Asian collisional setting.
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Keywords:
- Leucocratic sill/dike swarm /
- Early Eocene /
- Indo-Asian collision /
- Gangdese /
- Tibet /
- China
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References
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Xu-xuan Ma, Li-E Gao, Zhong-bao Zhao, Xi-jie Chen, Hai-bing Li, 2021. Early Eocene leucocratic sill/dike swarms in the Gangdese belt, southern Tibet: Tectonic implications for Indo-Asian collision, China Geology, 4, 56-66. doi: 10.31035/cg2021019
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Figure 1.
Simplified geological map of southern Tibet showing Himalayan-Gangdese orogen (modified from Guillot S et al., 2008). Data in Himalaya: Annapurna (Kohn MJ and Corrie SL, 2011), Everest (Cottle JM et al., 2009), Jomolhari (Regis D et al., 2014), Kaghan (Kaneko Y et al., 2003), Kali Gandaki (Iaccarino S et al., 2015), Lopu Range (Laskowski AK et al., 2016), Mabja dome (Lee J and Whitehouse MJ, 2007), Namche Barwa (Zhang ZM et al., 2015), Sikkim (Rubatto D et al., 2013), Tso Morari (Donaldson DG et al., 2013), and Yardoi dome (Ding HX et al., 2016; Gao LE et al., 2012). Data in Gangdese: Lilong (Zhang ZM et al., 2014), Milin (Zhang ZM et al., 2010), and Nymo (Dong X et al., 2018). STDS–South Tibetan Detachment System; MCT–Main Central Thrust; MBT–Main Boundary Thrust; MFT–Main Frontal Thrust; MP–median pressure; HP–high-pressure; UHP– ultrahigh-pressure; MT–median temperature; HT–high temperature.
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Figure 2.
Simplified map for the central Gangdese belt, southern Tibet. Reported ages are collected from Dong X et al., 2018; Ji WQ et al., 2009; Ma XX et al., 2020a, 2020c; Meng YK et al., 2019; Mo XX et al., 2005a; Wang C et al., 2019; Wen DR et al., 2008; Xu WC et al., 2015; Zhang HF et al., 2007.
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Figure 3.
Cross-section of the Nymo intrusive complex showing the leucocratic sill swarm. The ages of the ca. 178 Ma granite, ca. 47 Ma diorite, ca. 42 Ma dikes, and ca. 34 Ma granite are referred to Ji WQ et al., 2009; Ma XX et al., 2016, 2020c; Zhang HF et al., 2007.
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Figure 4.
Field photos for the Nymo intrusive complex. a–deformed granite; b–undeformed diorite; c–garnet-biotite gneiss; d–plagioclase amphibolite; e–about 34 Ma granite; and f–leucocratic sill swarm within the amphibolitic rocks. Grt–garnet.
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Figure 5.
Field photos for the studied leucocratic sills. a, b–leucocratic sill swarm; c–stoped block warped by the sill; d–leucocratic sill intruded along the deformed foliation; e–necking of the sill, and; f–leucocratic sill truncated the deformed foliation.
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Figure 6.
Photomicrographs (cross-polarized light image) to illustrate petrographic features of the leucocratic sills (a, c, and e). Photomicrographs (plane-polarized light image) to illustrate petrographic features of the leucocratic sills (b, d, and f). Qtz–quartz; Bt–biotite; Pl–plagioclase; Mag–magnetite; Kfs–K-feldspar.
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Figure 7.
Representative cathodoluminescence (CL) images of the zircon grains and the corresponding dating spots. The CL images were taken at Institute of Geology, Chinese Academy of Geological Sciences.
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Figure 8.
U-Pb weighted mean ages of zircon grains from the leucocratic sills/dikes (a, c, e, and g) and field photos of sampling location (b, d, f, and h).
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Figure 9.
Tentative cartoon showing the Early Eocene tectonic regime of the Indo-Asian collision and the relationship between the Nymo intrusive complex and the leucocratic sill/dike swarm.