Elsevier

Gondwana Research

Volume 27, Issue 1, January 2015, Pages 326-341
Gondwana Research

Origin of Late Triassic high-K calc-alkaline granitoids and their potassic microgranular enclaves from the western Tibet Plateau, northwest China: Implications for Paleo-Tethys evolution

https://doi.org/10.1016/j.gr.2013.09.022Get rights and content

Highlights

  • We study three Mesozoic (215-209 Ma) granitic plutons in the W Tibet.

  • They were formed in a continental arc setting.

  • The closure of the Paleo-Tethyan ocean was not earlier than 209 Ma.

Abstract

The western Tibet Plateau comprises a series of crustal terranes that were successively accreted to the southern margin of Eurasia associated with the evolution of Paleo- and Neo-Tethys. This paper presents the first detailed SHRIMP zircon U–Pb chronology, major and trace element, and Sr–Nd–Hf isotope geochemistry of three Mesozoic plutons (South Kudi, Arkarz and Mazha) and their microgranular enclaves in the western Kunlun and Tianshuihai terranes. SHRIMP zircon U–Pb dating shows that the three plutons were emplaced in the Late Triassic (215–209 Ma) and show a southward-younging trend. The South Kudi pluton (215 Ma) is composed of high-K calc-alkaline granodiorite and monzogranite, with initial 87Sr/86Sr ratios of 0.7093–0.7099, εNd (T) of − 4.9 to − 5.4, and εHf (T) (in-situ zircon) of 0.3. Elemental and isotopic data suggest that the granitoids were generated by partial melting of the Precambrian metasedimentary–igneous basement in the normal lower-crust (< 40 km) of the western Kunlun terrane triggered by underplating of basaltic magma. The Arkarz pluton (213 Ma) consists of high-K calc-alkaline monzogranite, syenogranite and alkali-feldspar granite and contains abundant microgranular enclaves. The host granites have initial 87Sr/86Sr ratios of 0.7071–0.7085, εNd (T) of –3.7 to − 4.8, and εHf (T) of − 0.7, and were also generated by partial melting of the Precambrian metasedimentary–igneous basement in the normal lower-crust (< 40 km) of the western Kunlun terrane triggered by underplating of enclave-forming potassic magma. Fractional crystallization of these pure crustal melts may explain the more felsic end-member granite. The enclaves are mainly basic (SiO2 48.0–54.9 wt.%) with high K2O (1.4–3.8 wt.%). They have initial 87Sr/86Sr ratios of 0.7071–0.7080 and εNd (T) of − 2.6 to − 4.5. We interpret the enclave magmas as having been derived by partial melting of the metasomatized mantle wedge in the spinel–garnet transition zone (~ 60–80 km). The Mazha pluton (209 Ma) is composed of high-K calc-alkaline tonalite and granodiorite, with initial 87Sr/86Sr ratios of 0.7087–0.7097, εNd (T) of − 6.3 to − 6.5, and εHf (T) of − 2.8. Elemental and isotopic data suggest that the granitoids were generated by mixing of mantle-derived basaltic magma with felsic melt derived by partial melting of lower crust of the Tianshuihai terrane. Our new data suggest that during the Late Triassic (215–209 Ma) the Tianshuihai terrane and the southern part of the western Kunlun terrane were a continental arc in response to the northward subduction of the Paleo-Tethys between the Tianshuihai and Karakorum terranes; the collision of the Karakorum terrane with the Tianshuihai terrane is not earlier than the age of 209 Ma, and most likely occurred during the Early Jurassic.

Introduction

The Tibetan plateau, the largest uplifted structure on Earth, is considered to be formed in response to the India–Asia collision since the Paleozoic (e.g. Zhu et al., 2013). It comprises a series of crustal terranes that were successively accreted to the southern margin of Eurasia (e.g. Zhang and Santosh, 2012). In the western Tibet Plateau, these terranes from north to south are the western Kunlun, Tianshuihai, Karakorum, and Kohistan terranes (Fig. 1a). It is now generally accepted that such terrane accretions were associated with the evolution of Paleo- and Neo-Tethys (e.g. Pan, 1996). However, the exact timing of the collision between each terrane still remains controversial and uncertain. For example, there are at least three opinions about the time of the collision between the western Kunlun and Tianshuihai terranes, i.e. Late Permian (e.g. Pan, 1996), Late Triassic (e.g. Matte et al., 1996, Mattern and Schneider, 2000) and Late Jurassic (e.g. Xiao et al., 2005). It is also uncertain when the Karakorum terrane collided with the Tianshuihai terrane. There are also at least three opinions about this topic, i.e. Late Triassic (e.g. Dewey et al., 1988, Pan, 1996), Early Jurassic (e.g. Matte et al., 1996, Mattern and Schneider, 2000) and Late Jurassic (e.g. Xiao et al., 2005).

In western Tibetan plateau, magmatism was intensive and extensive, and granitoids are widespread and long-lived (Fig. 1a). Their petrogenesis can help constrain the evolution of the orogen. However, rugged topography, high elevation and glaciers have impeded detailed geochronologic, and element + isotope geochemical studies of most of the granitoids. More recently, Jiang et al. (2013) have investigated three Middle Triassic granitic plutons (Yuqikapa, Muztaga and Taer) and their microgranular enclaves in the western Kunlun terrane (Fig. 1a). The new data have revealed new insights into the tectonic evolution of western Tibetan plateau, which suggests that the Tianshuihai terrane most likely collided with the western Kunlun terrane in the Middle Triassic Anisian (~ 243 Ma). In this paper, we present the first detailed SHRIMP zircon U–Pb chronology, major and trace element, and Sr–Nd–Hf isotope geochemical investigations of three Late Triassic granitic pluton and their microgranular enclaves in the western Kunlun and Tianshuihai terranes. These new data allow us to explore the origin of the granitoids and enclaves and their relationship to the evolution of the Paleo-Tethys.

Section snippets

Geological setting

The western Tibet plateau, adjoining the Tarim Basin in the north and the Himalaya in the south, comprises four crustal terranes, which are bounded by five tectonic sutures (Fig. 1a). The northernmost terrane, i.e. western Kunlun terrane, exists a Paleoproterozoic to Mesoproterozoic metamorphic basement, similar to that of the Tarim Basin. The Paleoproterozoic basement consists mainly of gneisses, migmatites, amphibolites and schists, while the Mesoproterozoic basement is mainly composed of

Sampling and analytical methods

We collected 17 samples (11 host rocks and 6 enclaves) from surface exposures of three plutons and sample locations are shown in Fig. 1b. Each enclave sample was taken from the center of large (> 50 cm) enclaves. We also collected 5 samples of Precambrian metamorphic rocks (2 plagioclase amphibolites and 3 mica-quartz schists) from the western Kunlun terrane and sample locations are shown in Fig. 1a. All samples were crushed to 200-mesh using an agate mill for whole-rock geochemical analysis.

SHRIMP zircon U–Pb dating

SHRIMP zircon U–Pb ages have been determined for each pluton. The results are summarized in Table 2 and illustrated in Fig. 4. All zircons, with the length mostly between 100 and 200 μm, show regular oscillatory magmatic zoning. They have Th/U ratios of 0.30–0.73 (Table 2), consistent with those of magmatic zircon (Williams et al., 1996). Most U–Pb analyses of each sample plot in a group on the Concordia curve yield a weighted mean 206Pb/238U age of 215.0 ± 2.3, 212.7 ± 3.1 and 209.2 ± 3.7 Ma,

Origin of the Arkarz high-K calc-alkaline granites

The Arkarz host granites are high-K calc-alkaline and show similar Mg# to pure crustal partial melts except for the more felsic sample (X10-35-1) (Fig. 6), suggesting that they might be produced by partial melting of crustal rocks and that no significant mantle-derived magmas were mixed into the crustal melts during generation of these granites. This is further supported by compositional trends between the host granites and their enclaves. A magma mixing process should result in linear

Conclusions

The South Kudi, Arkarz and Mazha plutons were emplaced in the Late Triassic (215–209 Ma) and show a southward-younging trend. The South Kudi pluton (215 Ma) is composed of high-K calc-alkaline granodiorite and monzogranite. They were generated by partial melting of the Precambrian metasedimentary–igneous basement in the normal lower-crust (< 40 km) of the western Kunlun terrane triggered by underplating of basaltic magma. The Arkarz pluton (213 Ma) consists of high-K calc-alkaline monzogranite,

Acknowledgments

We are grateful to Hang-Qiang Xie and Jian-Hui Liu for his assistance with SHRIMP zircon U–Pb dating, to Wei Pu for her assistance with measurements of whole-rock Sr and Nd isotopes, and to Ke-Jun Hou for his assistance with measurements of zircon Hf isotope. We thank Zeming Zhang (Associate editor) and Wenjiao Xiao as well as one anonymous reviewer for their thoughtful reviews and constructive comments. This work was financially supported by the National Natural Science Foundation (40972040;

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