Zircon U–Pb age and Sr–Nd–Hf–O isotope geochemistry of the Paleocene–Eocene igneous rocks in western Gangdese: Evidence for the timing of Neo-Tethyan slab breakoff
Introduction
The Gangdese magmatic belt, formed by the northward subduction of Neo-Tethyan slab in the Mesozoic–Early Cenozoic and the consequent India-Asia collision starting at ~ 55–50 Ma (e.g. Chung et al., 2003, Chung et al., 2005, Guo et al., 2007, Guo et al., 2013, Harris et al., 1986; Hou et al., 2004; Lee et al., 2009, Mo et al., 2008; Wen et al., 2008a, Wen et al., 2008b, Williams, 2000, Zhang et al., 2013, Zhu et al., 2011), spans the length of the Himalayan mountain chain. This ~ 1600 km-long magmatic belt is 100–200 km wide, and features voluminous continental-arc and post-collisional volcanic rocks and coeval intrusions. The tempo-spatial distribution of these igneous rocks, their petrology and mineralogy, and their geochemical and isotopic compositions are indicative of the behavior of the Neo-Tethyan slab during subduction (such as slab rollback and slab breakoff: Chung et al., 2005, Gao et al., 2008, Ji et al., 2009, Ji et al., 2012, Lee et al., 2009, Lee et al., 2011, Wen, 2007, Wen et al., 2008a, Zhang and Santosh, 2013), and India-Asia collisional and post-collisional processes (Ding et al., 2003; Gao et al., 2007, Guo et al., 2007, Guo et al., 2013, Miller et al., 1999, Mo et al., 2005, Mo et al., 2007, Mo et al., 2008, Turner et al., 1996, Wang et al., 2014a, Wang et al., 2014b, Wang et al., in preparationa, Wang et al., 2014c, Williams, 2000).
Neo-Tethyan oceanic lithosphere subduction began in the Late Triassic–Early Jurassic (Chu et al., 2006), and produced Jurassic–Cretaceous calc-alkaline magmatism in the Lhasa terrane (Fig. 1; Harris et al., 1986, Ji et al., 2009, Ma et al., 2014, Mo et al., 2008, Wen, 2007, Zheng et al., 2014, Zheng et al., 2014, Zhu et al., 2009, Zhu et al., 2011). The most voluminous magmatism occurred in the Paleocene–Eocene, and is characterized by extensive I-type calc-alkaline to high-K calc-alkaline Linzizong volcanic rocks, and coeval plutons. A large amount of lithogeochemical and geochronological data has been published on the Paleocene–Eocene igneous rocks in the eastern Gangdese belt (east of ~ 89°E), however, less work has been done in the western Gangdese belt because of limited road access. The occurrence of Linzizong volcanic rocks and cogenetic intrusions (Mo et al., 2005, Mo et al., 2008, Wen, 2007, Wen et al., 2008a) in the Paleocene–Eocene is thought to be caused by the Neo-Tethyan slab rollback, which likely happened at ~ 69–58 Ma (Lee and Lawver, 1995, Chung et al., 2005). The time of sequent Neo-Tethyan slab breakoff is debated. Gao et al. (2008) suggested that the breakoff happened at ~ 42–38 Ma, however, most researchers (Chung et al., 2005, Wen et al., 2008a, Lee et al., 2009, Lee et al., 2011) have proposed that slab breakoff happened earlier at ~ 50 Ma.
We report new geochronological, geochemical, and isotopic (Sr–Nd–Hf–O) data of Paleocene–Eocene igneous rocks from the western Gangdese belt, and discuss their petrogenesis. We approach a resolution for the timing of Neo-Tethyan slab breakoff by synthesizing new data with existing data from the whole Gangdese belt. The geochemical and isotopic data agree with previous studies that the Neo-Tethyan slab broke off earlier at ~ 53–50 Ma.
Section snippets
Tectonic framework
The Himalayan-Tibetan orogen is composed of the Himalayan sequences, Lhasa terrane, Qiangtang terrane, and Songpan-Ganze complex, from south to north, separated from each other by the Indus-Yarlung Zangbu, Bangong-Nujiang, and Jinsha River sutures (Yin, 2006, Yin and Harrison, 2000; Fig. 1). The Lhasa terrane is divided into the northern Lhasa subterrane, central Lhasa subterrane, and southern Lhasa subterrane, which are bounded by the Shiquan River–Nam Tso Mélange zone and the
Sampling and analytical strategy
Large amounts of lithogeochemical and geochronological data have been published on Paleocene–Eocene igneous rocks from the eastern Gangdese belt (e.g. Harris et al., 1986, Harris et al., 1988, Mo et al., 2005, Mo et al., 2007, Mo et al., 2008, Chen, 2006, Dong et al., 2006a, Dong et al., 2006b, Wen, 2007, Wen et al., 2008a, Ji et al., 2009, Ji et al., 2012, Lee et al., 2009, Lee et al., 2011, Du et al., 2012, Wang et al., 2014a, Wang et al., 2014b, Wang et al., in preparationa, Wang et al.,
Major and trace elements
Samples for lithogeochemical analysis were prepared by crushing (using corundum plates) and grinding in an agate disk mill at the Hebei Geological Survey Laboratory, China. The expected contamination of Si and Al by these methods is estimated to be less than one percent (as assessed from analyses of standards and blanks). All samples were analyzed by Activation Laboratories (Ancaster, Ontario, Canada) using a combination of methods including instrumental neutron activation analysis and fusion
Zircon U–Pb results
Six intrusive rocks and one volcanic rock from the western Gangdese belt were selected for zircon U–Pb dating. A large number of U–Pb zircon dates exist for Cenozoic rocks from the eastern Gangdese belt, such that ages of most outcrops are well established. However, we have supplemented these data with analyses for two new samples from our suite.
Sr–Nd–Hf–O features of western Gangdese Paleocene–Eocene igneous rocks and the geological implication
Although Paleocene–Eocene igneous rocks from the whole Gangdese belt have similar mineralogy and geochemical compositions (Fig. 8, Fig. 9), their Sr–Nd isotopic compositions are slightly different (Fig. 10A, B). Published data show that most Paleocene–Eocene igneous rocks from the eastern Gangdese belt have similar Sr–Nd isotopic compositions to the Gangdese Cretaceous arc rocks ((87Sr/86Sr)i: 0.7032–0.7048, εNdi = + 0.2 to + 5.3; Wen, 2007, Zheng et al., 2014, Zheng et al., 2014; Fig. 10A),
Conclusions
Few data of Paleocene–Eocene igneous rocks exist from the remote western Gangdese belt. Seven intrusive and volcanic rocks from the western Gangdese belt were selected for zircon U–Pb dating. Their geochemical and Sr–Nd–Hf–O isotopic data provide more constrains on their petrogenesis. Similar to Paleocene–Eocene igneous rocks from the eastern Gangdese belt, the western Gangdese rocks have relative depletions in Nb, Ta, P, and Ti, and enrichments in Rb, Ba, Th, U, K, Pb, Zr, and Hf on a
Acknowledgments
This study was funded by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada to Jeremy Richards, the Ministry of Science and Technology of China (973 project 2011CB403100), the IGCP/SIDA-600 Project (Metallogenesis of Collisional Orogens in the East Tethyside Domain), the National Natural Science Foundation of China (41203032) to Fang An. Rui Wang was funded by a Chinese Scholarship Council award from China and Teaching Assistantship from University of
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Current address: Mineral Resources Flagship, CSIRO at Perth 6101, Australia.