LettersEocene–Oligocene granitoids in southern Tibet: Constraints on crustal anatexis and tectonic evolution of the Himalayan orogen
Graphical abstract
North–south sketch section through southern Tibet and Himalaya during late Eocene-early Oligocene, showing initial geometry of exhumation of the subducted Indian crustal mass.
Highlights
► The Eocene granites in the eastern Tethyan Himalayan show affinity with adakite. ► Zircon U–Pb data indicate the adakitic rocks formed in the period from 30 to 46 Ma. ► These granitoids define a magmatic trail southward becoming older from 30 to 46 Ma. ► Exhumation of the subducted Indian crustal mass was initiated at ∼46 Ma.
Introduction
The Himalaya mountain system, which is the largest active collisional orogen on Earth, was initiated by India–Asia collision at ∼55–50 Ma or earlier (Yin, 2006 and references herein); however, growth mechanisms within the orogen remain uncertain. The Himalaya collisional orogen has been subjected to intense Cenozoic deformation, high-grade metamorphism, and widespread syncollisional anatexis (cf. Le Fort, 1996; Yin, 2006), which has led to many uncertainties about the tectonic evolution of the orogen. More recent debate has focused on mass exchange between the Himalayan (Indian plate) and Tibetan crust (Asian plate) during collision (Yin et al., 2010b, Yin et al., 2010a), and on the origin of the Greater Himalayan crystalline complex (GHC), a high-grade metamorphic unit that forms the core of the Himalaya (Fig. 1A). Four potential models for the origin of the GHC and the formation of the Himalaya have been proposed: (1) Indian basement-involved thick-skinned thrusting (Le Fort, 1975, Yin, 2006, Yin et al., 2010b), (2) an exotic terrane-involved thin-skinned thrusting (DeCelles et al., 2000, DeCelles et al., 2001, DeCelles et al., 2002, Robinson et al., 2006), (3) large-scale horizontal channel flow of the Tibetan middle crust (Nelson et al., 1996, Beaumont et al., 2001, Beaumont et al., 2004, Searle et al., 2003, Godin et al., 2006), and (4) wedge extrusion and exhumation of a slice of subducted Indian crust during India–Asia collision (Chemenda et al., 1995, 1996, 2000).
Crust-derived granitoids can be generated by the distinctive dynamic processes that each of these above models predicts, but the timing, spatial relationships, source region, and dynamic mechanisms involved in crustal anatexis differ among the models (Harris and Massey, 1994, Thompson and Connolly, 1995, Harrison et al., 1997, Searle and Szluc, 2005, Harrison, 2006, King et al., 2007). Thus, constraining the onset and duration of regional melting under southern Tibet, and determining the spatial distribution and source region of anatectic magmas during collision, is a crucial test of the validity of these models. All of these variables can be constrained by studying the granitoids produced during India–Asia collision and that intruded various tectonic units in southern Tibet (Fig. 1).
Miocene granites within the parallel high Himalayan leucogranite (HHL) and the North Himalayan granite (NHG) belts have contrasting ages, petrogenetic histories, and emplacement styles (Harrison et al., 1997 and references therein). In addition, recent studies have reported an Eocene (∼42–44 Ma) regional melting event that formed two-mica granites in southern Tibet (Aikman, 2007, Aikman et al., 2008, Aikman et al., 2012a, Aikman et al., 2012b, Qi et al., 2008, Gao, 2009, Zeng et al., 2011). These studies promote an understanding of crustal anatexis related to continental collision, but a genetic link between crustal anatexis and the growth mechanisms involved in Himalayan orogenesis has not been well established.
In order to explore the relationship between crustal anatexis and Himalayan orogenic processes, and to critically compare the predictions of each of the models mentioned above with direct observations, a NW–SE-striking, 200-km-long tectonic traverse through the Indus–Yarlung suture (IYS), the north Himalayan antiform (NHA), the Tethyan Himalayan thrust–fold belt, and the south Tibetan detachment system terranes (STD; Fig. 1), was undertaken during this study, combined with an analysis of existing data (e.g., Zeng et al., 2011). Along this traverse, representative samples of granitoids, exposed within the IYS, the NHA, and the THS (Fig. 1), were taken for age dating and geochemical analysis to further constrain the temporal and spatial distribution, geochemical variation, and possible origin of these granitoids. This paper presents new ion microprobe U–Pb zircon and biotite 40Ar/39Ar dates, major and trace element compositions, and the Sr–Nd isotope systematics of granitoids from this traverse. These results constrain the onset and duration of crust-derived magmatism and the origin of Eocene–Oligocene granitoids of southern Tibet, and provide new insights into the exhumation of high-pressure metamorphic rocks and the development of the Himalayan orogen.
Section snippets
Major tectono-stratigraphic units
The Himalayan orogen can be loosely divided into three tectono-stratigraphic units, all juxtaposed by orogen-scale faults that are roughly parallel with the trend of the orogen (Fig. 1A; Gansser, 1964; Le Fort, 1975, Le Fort, 1996; Yin and Harrison, 2000; DiPietro and Pogue, 2004). From north to south, in decreasing stratigraphic height, these are: the Tethyan Himalayan sedimentary sequences (THS), a deformed package of predominantly Paleoproterozoic to Eocene metasediments bounded by the IYS
Eocene–Oligocene magmatism in southern Tibet
Unlike the Miocene granites that form the east–west-trending HHL and NHG, recently reported Eocene granitoids (Aikman, 2007, Aikman et al., 2008, Aikman et al., 2012a, Qi et al., 2008, Zeng et al., 2011) are restricted to the Yelaxiangbo dome and surrounding areas of south Tibet. Our field investigations indicate that this regional melting event forms a nearly N–S-trending Eocene–Oligocene granitoid belt, approximately orthogonal to both the HHL and the NHG, which extends ∼150 km from the IYS to
Age dating of the crystallization and deformation of granitoids
To further constrain the timing of the onset and duration of crystallization of the Eocene–Oligocene granitoid belt of south Tibet, zircon samples were separated from granitoid intrusions at Chongmuda (near Yajia), Dala, Quedang, and Yangxiong (near Lhunze) for U–Pb dating (Fig. 1; Suppl. Table 1). Age of shearing deformation is constrained by 40Ar/39Ar dating of biotites from the Eocene granitoids with typical evidence suggestive of deformation (Figs. 2 and 4; Suppl. Table 2).
Petrology and geochemistry of granitoids
Granitoids in southern Tibet range in composition (Suppl. Table 4) from ∼30 Ma granodiorite (SiO2=64.0–71.7 wt%) and ∼35 Ma granite (SiO2=72.0–76.1 wt%) to Eocene two-mica granites (SiO2=68.3–73.0 wt%) that contain plagioclase, quartz, biotite, K-feldspar, and rare garnet phenocrysts. The granodiorites at Yajia and Chongmuda, and the two-mica granites at Yelaxiangbo, Dala, Quedang, and Yangxiong, are relatively Na-rich, with Na2O/K2O ratios of 1.16–2.12, CaO rich (1.98–5.33 wt.%), high-alumina
Origin of ∼35 Ma granites
Several models have been proposed for the origin of Miocene granites in the NHG and HHL: (1) fluid-saturated anatexis (Le Fort et al., 1987), (2) decompression melting under fluid-absent conditions (Harris and Massey, 1994; Davidson et al., 1997), and (3) melting triggered by shear heating or high heat production during radioactive decay (Harrison et al., 1997, 1998). However, the majority of these models consider that the leucogranites with high Rb/Sr (>1.0) and high (87Sr/86Sr)i (>0.7300)
Acknowledgments
This study is supported by grants from the Ministry of Science and Technology of China (2011CB4031006), IGCP/SIDA-600, NSFC (40730419; 40425014), the Program of the China Geological Survey (1212011121255), and the Funds for Creative Research Groups of China (40921001) We are most grateful to the two anonymous reviewers for critical and constructive reviews of this manuscript.
References (110)
- et al.
Evidence for early (<44 Ma) Himalayan crustal thickening, Tethyan Himalaya, southeast Tibet
Earth. Planet. Sci. Lett.
(2008) - et al.
Age and thermal history of Eo- and Neohimalayan granitoids, eastern Himalaya
J. Asian Earth Sci.
(2012) - et al.
Remnants of a Cretaceous intra-oceanic subduction system within the Yarlung–Zangbu suture (southern Tibet)
Earth Planet. Sci. Lett.
(2000) - et al.
Mio-Pliocene adakite generation related to flat subduction in southern Ecaudor: the Quimsacocha volcanic center
Earth Planet. Sci. Lett.
(2001) - et al.
Improved 206Pb/238U microprobe geochronology by the monitoring of a trace-element-related matrix effect; SHRIMP, ID–TIMS, ELA-ICP-MS and oxygen isotope documentation for a series of zircon standards
Chem. Geol.
(2004) - et al.
Himalayan metamorphism and deformations in the North Himalayan belt, southern Tibet, China
Earth Planet. Sci. Lett.
(1984) - et al.
A mechanism for syn-collisional rock exhumation and associated normal faulting: results from physical modelling
Earth Planet. Sci. Lett.
(1995) - et al.
Continental subduction and a mechanism for exhumation of high-pressure metamorphic rocks: new modelling and field data from Oman
Earth Planet. Sci. Lett.
(1996) - et al.
Evolutionary model of the Himalaya–Tibet system: geopoem: based on new modelling, geological and geophysical data
Earth Planet. Sci. Lett.
(2000) - et al.
Multi-stage development of the southern Tibet detachment system near Khula Kanger, New data from Gonto La
Tectonophysics
(1996)