Eastern Tethyan Himalaya Research Articles

Paleomagnetic constraints on the link between the Comei-Bunbury large igneous province and the Kerguelen mantle plume

The causal link between the Comei-Bunbury large igneous province (CBLIP) and the Kerguelen mantle plume (KMP) remains a topic of debate. Here, a new paleomagnetic study was conducted on the Zhela Formation basalts and sandstones (ca. 147–140 Ma) in the Taga area of the eastern Tethyan Himalaya (TH). The sandstone specimens had erratic demagnetization patterns, and reliable characteristic remanent magnetization (ChRM) directions could not be isolated. Positive fold and reversal tests indicate that the ChRM directions of the Zhela Formation basalts should be the primary magnetization. The tilt-corrected site-mean direction from 18 sites is D/I = 87.8°/70.0° (α95 = 6.8°), yielded a paleopole of 21.6°N/130.2°E (A95 = 12.0°) and a paleolatitude of 54.7° ± 12.0°S for the study area (28.4°N, 91.8°E). Comparison with robust latest Jurassic and Early Cretaceous paleomagnetic, geochemical, and geochronological data from the eastern TH, northeastern Indian craton, and southwestern Australia reveal that (1) the eruption periods of the Comei igneous rocks are similar to the Bunbury basalt (and the Naturaliste Plateau volcanic rocks); (2) the original erupted positions of the Comei-Bunbury (CB) igneous rocks were in agreement with the reconstructed KMP; and (3) the CBLIP originated from the KMP.

Extensional tectonics of the Indian passive continental margin in the Middle and Late Jurassic: Constraints from detrital zircon ages in the eastern Tethyan Himalaya

Knowledge the initial extension in the Indian continent during the Jurassic is important for understanding the dynamics of its breakup from Eastern Gondwana. The absence of Jurassic magmatic activity in the eastern Tethyan Himalaya hinders the understanding of this process. We report a provenance and tectonic setting study on the Middle Jurassic Zhela and Late Jurassic Weimei Formations sandstone in Gyangze, eastern Tethyan Himalaya. Detrital zircons of Jurassic sediments indicate four major age peaks: ∼500 Ma, ∼820 Ma, ∼950 Ma, and ∼2450 Ma, which reflect the affinity between the Tethyan Himalaya and India. The differences between the crystallization age and depositional age of isolated detrital zircons from the Middle Jurassic Zhela and Late Jurassic Weimei Formations, indicate that they were formed in an extensional continental margin and deposited on the passive continental margin of India. Combined with regional geological information, our results show that extensional tectonics was dominant on the northern margin of the Indian continent during the Jurassic. The source-sink sedimentary system and the topography remained stable at this time. After the extension event in the Middle and Late Jurassic, the Kerguelen mantle plume commenced activity, indicating the transition from the lithospheric thinning process to active rifting. The mantle plume activity in the Early Cretaceous led to large-scale uplift in the southeast part of the Tethyan Himalaya, that ultimately led to the breakup of India from Eastern Gondwana.

Formation and evolution of a magmatic system in the Indian passive margin: Insights from the Triassic Yumai Complex from the eastern Tethyan Himalaya

Abstract The Indian passive margin has preserved several pulses of magmatism during and after the disassembly of Gondwana since the late Paleozoic, providing valuable insights into the long-term magmatic evolution of various passive margins, including the Indian passive margin. In the Yumai Complex, eastern Tethyan Himalaya, a pulse of Late Triassic alkaline volcanism (ca. 227–216 Ma) is evident. The Late Triassic volcanic rocks are mildly alkaline to tholeiitic basalts with minor ultrabasic rocks, similar in geochemistry to within-plate flood basalts. The TiO2 contents (1.46–3.38 wt%, mainly >2 wt%), (La/Yb)N values (4.05–7.50), εNd (t) values (+4.86 to +6.98), and results from partial-melt modeling suggest that the basalts likely originated from garnet peridotite. Elemental and Sr-Nd systematics of magmatic rocks emplaced during the Triassic indicate oceanic island basalt (OIB) components in the magma source, interpreted as enriched mantle components rather than crustal contamination products. Spatiotemporal and geochemical patterns of magmatism reveal that the bulk compositions of the basalts changed from enriched OIB-like to depleted mid-ocean-ridge basalt (MORB)–like compositions. This phenomenon likely resulted from the evolving nature of the rifting basin, changing from a nascent continental setting to a mature ocean basin. The Triassic magmatism in the Tethyan Himalaya can be attributed to remnant lithospheric instability arising from the prolonged rifting of eastern Gondwana, leading to the formation of a magmatically passive margin.

Hydrothermal alteration, not metamictization, is the main trigger for modifying zircon in highly evolved granites

The question of whether the high U and Th concentrations in zircon are primary or secondary is often difficult to resolve, and a clear understanding of the modification processes of secondary U- and Th-rich zircon is lacking. Zircon crystals from two well-studied, highly evolved granites, the Jiangjunshan muscovite granite in the Chinese Altai Mountains and the Cuonadong leucogranite in the Eastern Tethyan Himalaya, have been investigated and classified into two types. Type I exhibits typical igneous growth zoning, and type II has “diffuse” or “spongy” internal structures. These textures, along with compositional data, indicate that the type II zircon crystals formed through hydrothermal modification of magmatic zircon (type I) by infiltrating hydrothermal fluids. During hydrothermal modification, the U and Th concentrations increase from type I to type II in the Jiangjunshan muscovite granite but decrease from type I to type II in the Cuonadong leucogranite. The Raman spectra of type II zircon crystals from Jiangjunshan muscovite granite have broader peaks (i.e., measured as the full width at half maximum, FWHM) with decreased intensities than their type I counterparts, which indicates that the former are affected by significant accumulated radiation damage. However, the preserved radiation damage in both the type I and II zircon measured by Raman spectroscopy is less than that expected from the total dose of alpha particles calculated from the U and Th contents, which indicates variable degrees of annealing. We propose that late magmatic-hydrothermal alteration was responsible for the modification of magmatic zircon grains in highly evolved granites and resulted in the enrichment or redistribution of U and Th. The calculated radiation dose of the Cuonadong leucogranite zircon is far below that required for metamictization, which indicates that metamictization is not always responsible for diffuse and spongy textures.

The early Late Triassic Yumen mafic rocks in the eastern Tethyan Himalaya: A record of the progressive opening of the Yarlung Zangbo Neo-Tethyan Ocean

The timing of the opening of the Yarlung Zangbo Neo–Tethyan Ocean (YZNTO) is debated. In this contribution, geochemical and geochronological analyses of diabase, pillow basalt and layered basalt from the Upper Triassic Yumen tectonic mélange (T3Y) of the eastern Tethyan Himalaya in South Tibet are used to elucidate the timing and the magmatic evolution preceding the development of the YZNTO. Zircon U-Pb geochronology indicates that the Yumen mafic rocks crystallized in the Late Triassic (diabase, c. 225 Ma). These rocks display alkaline basalt affinities and have variable enrichment in Ba, U, Pb, Sr and Ti and depletion in Rb, Th and Nb. The diabase and pillow basalt samples have compositions transitional between enriched mid–ocean ridge basalt and ocean island basalt (OIB); layered basalt is more compatible with an OIB affinity. Whole–rock εNd (t) ratios of the mafic rocks range from +5.5 to +11.5 and zircons from a diabase have εHf(t) values varying from – 1.1 to +10.0. Whole–rock trace element concentrations indicate that the parental magmas of the Yumen mafic rocks were derived from the mantle at variable depths extending from the garnet to spinel stability fields. The Yumen mafic rocks originated from partial melting of a heterogeneous magma source composed of dominant depleted asthenospheric magmas and the addition of some components of an ancient and enriched lithospheric mantle; negligible crustal contamination occurred. Combined with previous studies, we conclude that the Yumen mafic rocks were probably produced in an intercontinental rift setting with extremely thin continental lithosphere in the early Late Triassic prior to the opening of the eastern YZNTO. The YZNTO progressively opened along the northern edge of the Indian plate from west to east in the Phanerozoic.

Boron and Molybdenum Isotope Evidence for Source‐Controlled Compositional Diversity of Cenozoic Granites in the Eastern Tethyan Himalaya

AbstractThe origins of Cenozoic granites in the Himalaya are key to understanding the evolution of the Himalayan orogen. However, it is unclear whether these granites represent primary melts, and the nature of their magma source is controversial. Here, we present a systematic element and Sr–Nd–B–Mo isotope study of Cenozoic granites from the Yardoi area in the eastern Tethyan Himalaya, China. These granites can be divided into two groups: mid‐Eocene porphyritic two‐mica granites with low SiO2 contents (65.9−69.6 wt.%) and adakitic geochemical signatures, and mid‐Eocene to Miocene equigranular granites with high SiO2 contents (71.6−75.5 wt.%). The high‐SiO2 granites (HSG) have similar Sr−Nd isotope compositions to the low‐SiO2 granites (LSG), but they have distinct δ11B values of −19.4‰ to −11.4‰ and −10.6‰ to −6.89‰. This indicates that the two groups have different sources, with the LSG derived by partial melting dominantly of metamafic rocks at thickened lower crustal conditions, and the HSG generated by partial melting of the mid‐crust metasedimentary rocks with less enriched Nd isotope compositions. The δ98/95Mo of the LSG and HSG are highly variable with values of −0.68‰ to 0.12‰ and −1.13‰ to 0.46‰, respectively. δ11B values of the HSG correlate positively with δ98/95Mo and Sr/Y values and correlate negatively with K2O, Rb, Zr, and Rb/Sr, reflecting the addition of external metamorphic fluids during anatexis of the metapelites. The B–Mo isotope data robustly suggest source‐controlled compositional diversity of the Himalayan granites, which could provide clues to the physical and geochemical responses during the evolution of a large orogen.

Formation of Eocene−Miocene felsic magmatic rocks along N-S−trending Yardoi-Kongbugang mountain ranges in the eastern Himalaya: New insights into surface uplift and the initiation of E-W extension in southern Tibet

Large-scale N-S shortening induced by India-Asia convergence caused the formation of numerous E-W−trending mountain ranges in Tibet. However, the mechanism(s) of formation of N-S−trending mountain ranges remains elusive. We report on a felsic magmatic belt located along the N-S−trending Yardoi-Kongbugang mountain ranges on the flank of the Cona rift in the eastern Tethyan Himalaya. Zircon and monazite geochronology and whole-rock geochemistry revealed three epochs of middle- to lower-crustal anatexis beneath the Cona rift at ca. 47−42 Ma, 35−34 Ma, and 24−14 Ma. The mid-Eocene and early Oligocene granitoids show adakitic signatures indicating continuous crustal thickening, while the formation of Miocene leucogranites and N-S−trending dacitic dikes was related to ductile crustal extension. Silicic melts were exposed along the whole rift since the early Oligocene, suggesting that the early Oligocene could be regarded as a transitional epoch from tectonic compression to orogen-parallel extension. Widespread mid-Eocene and Miocene magmatism in the Himalaya, together with coeval metamorphic anatexis, represents two phases of crustal weakening. The weakened crustal zones under continued India-Asia convergence may have favored uplift and subsequent lateral flow of the weak zones, which initiated E-W extension. Finally, significant upwelling of the weak zones evolved into magma extrusion and formed the N-S−trending mountain ranges. This study provides new insights into the mechanisms of surface uplift and E-W extension and challenges the common view of initiation of E-W extension in southern Tibet not earlier than the early Miocene.

Early Jurassic Mafic Magmatism in the Eastern Tethyan Himalaya, Southern Tibet

Early Jurassic Mafic Magmatism in the Eastern Tethyan Himalaya, Southern Tibet

Role of the Kerguelen mantle plume in breakup of eastern Gondwana: Evidence from early cretaceous volcanic rocks in the eastern Tethyan Himalaya

To better understand the breakup mechanism of eastern Gondwana, U Pb Zircon geochronological and whole-rock geochemical studies were performed on the Zhela and Weimei formations volcanic rocks in the Zhuode area of the eastern Tethyan Himalaya. Sensitive high-resolution ion microprobe (SHRIMP) U Pb dating of zircons yielded 206 Pb/ 238 U ages of ~135–130 Ma, indicating that the volcanic rocks from Zhela and Weimei formations erupted during the Early Cretaceous. Whole rock analyses of major and trace elements show that the volcanic rocks are characterized by high content of TiO 2 (1.84 wt%–4.03 wt%) and P 2 O 5 (0.29 wt%–0.61 wt%), highly fractioned in LREE/HREE [(La/Yb) N = 9.12–14.53], and most of them have no obvious anomaly of Eu. The characteristic, highly enriched HFSE with no obvious anomaly of Nb and Ta, is similar to those of alkali basalts originated from the Kerguelen mantle plume. Our geochemical and geochronological results, combined with recent paleomagnetic results show that the original erupted position of the Zhela and Weimei formations volcanic rocks was just located in the center of the presently Kerguelen mantle plume, and support that the Kerguelen mantle plume led to the breakup of eastern Gondwana. • The volcanic rocks from Zhela and Weimei Formation (Fm) erupted at ~130–135 Ma. • The volcanic rocks from Zhela and Weimei Fm originated from the Kerguelen plume. • The Kerguelen plume played an active role in the breakup of eastern Gondwana.

The Tethyan Himalaya Igneous Province: Early Melting Products of the Kerguelen Mantle Plume

AbstractThe Kerguelen large igneous province (LIP) has been related to mantle plume activity since at least 120 Ma. There are some older (147–130 Ma) magmatic provinces on circum-eastern Gondwana, but the relationship between these provinces and the Kerguelen mantle plume remains controversial. Here we present petrological, geochronological, geochemical, and Sr–Nd–Hf–Pb–Os isotopic data for high-Ti mafic rocks from two localities (Cuona and Jiangzi) in the eastern Tethyan Himalaya igneous province (147–130 Ma). Zircon grains from these two localities yielded concordant weighted mean 206Pb/238U ages of 137.25 ± 0.98 Ma and 131.28 ± 0.78 Ma (2σ), respectively. The analyzed mafic rocks are enriched in high field strength elements and have positive Nb–Ta anomalies relative to Th and La, which have ocean island basalt-like characteristics. The Cuona basalts were generated by low degrees of melting (3–5 %) of garnet lherzolites (3–5 vol% garnet), and elsewhere the Jiangzi diabases were formed by relatively lower degrees of melting (1–3%) of garnet lherzolite (1–5 vol% garnet). The highly radiogenic Os and Pb isotopic compositions of the Jiangzi diabases were produced by crustal contamination, but the Cuona basalts experienced the least crustal contamination given their relatively low γOs(t), 206Pb/204Pbi, 207Pb/204Pbi, and 208Pb/204Pbi values. Major and trace element geochemical and Sr–Nd–Hf–Pb–Os isotope data for the Cuona basalts are similar to those for products of the Kerguelen mantle plume head. Together with high mantle potential temperatures (>1500 °C), this suggests that the eastern Tethyan Himalaya igneous province (147–130 Ma) was an early magmatic product of the Kerguelen plume. A mantle plume initiation model can explain the temporal and spatial evolution of the Kerguelen LIP, and pre-continental breakup played a role in the breakup of eastern Gondwana, given the >10 Myr between initial mantle plume activity (147–130 Ma) and continental breakup (132–130 Ma). Like studies of Re–Os isotopes in other LIPs, the increasing amount of crustal assimilation with distance from the plume stem can explain the variations in radiogenic Os.

Cenozoic deformation in the Tethyan Himalaya, SE Tibet: Insights from magnetic fabrics and structural analysis of Upper Triassic flysch

Anisotropy of magnetic susceptibility (AMS) analyses and structural observations of Triassic flysch in the Tsetang area, SE Tibet, were carried out to assess the Cenozoic tectonic evolution and kinematics of deformation of the eastern Tethyan Himalaya Thrust Belt. We analysed a total of 202 standard-sized specimens from four sample sites across the Renbu–Tsetang Thrust from north to south. Rock magnetism and the detailed mineralogy demonstrated that ferromagnetic and paramagnetic minerals both contribute to the AMS. However, the paramagnetic contribution is dominant in the study area. The AMS results, together with field-based structural investigations and analyses of microstructures, revealed that layer-parallel shortening (LPS) was accommodated mainly by isoclinal folds, cleavage, and thrust faults. Shear fabrics (K1 axes oriented parallel to the direction of local tectonic transport) were found mainly in the north thrust fault zone and were linked to fault movement. Microstructural analyses of quartz and metamorphic minerals such as muscovite (sericite) and chlorite in slates and sandstones imply that the peak temperature during the deformation did not exceed 300 °C. The shear AMS fabric in north Qonggyai, near the thrust zone, shows that the strain variation is related to the main activity (11–19 Ma) of the Renbu–Tsetang Thrust. The magnetic lineation that plunges towards the east in the southern Qonggyai area indicates extension that was linked to an E–W extensional event during the early Miocene. Based on the available geochronological data, two stages of tectonic deformation can be defined: (a) syncollisional layer-parallel shortening at ~40 Ma ago, and (b) E–W extension and continuing compaction resulting in shearing at 11–19 Ma.

Fingerprints of the Kerguelen Mantle Plume in Southern Tibet: Evidence from Early Cretaceous Magmatism in the Tethyan Himalaya

Early Cretaceous magmatism suggested to be related with the Kerguelen mantle plume has been reported in both eastern and western Tethyan Himalaya (TH). Coeval magmatism has been recently discovered in the central TH. This magmatism is recorded by an intrusion next to the Xiongma township. This intrusion is composed of gabbro and dolerites. The Xiongma gabbros are characterized by significant enrichment of light rare earth elements and prominent Na-Ta depletion in the primitive mantle–normalized spider diagrams. The εNd(t) ratios of the gabbros are between −4.33 and −2.20, and initial 87Sr/86Sr ratios are 0.707807–0.708557. Lead isotopes 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb of the gabbros range from 39.042 to 39.630, 15.646 to 15.709, and 18.455 to 18.833, respectively. Zircons from the gabbros and dolerites yielded three U-Pb ages of 138.5±1.6, 133.6±2.5, and 136.4±2.2 Ma, respectively. The magmatic zircons are exclusively characterized by negative εHf(t) values. Through geochemical modeling, we concluded that the Xiongma magmatic rocks have experienced combined crustal assimilation (10%–15% for the gabbros and 28%–32% for the dolerites) and fractional crystallization processes. The gabbros are derived from an Sp+Grt facies (Grt>Sp) enriched mantle source. The Xiongma magmatism is geochemically and geochronologically comparable with those in the western and eastern TH. Through comparison study between oceanic island basalt–like rocks in both the Yarlung Zangbo Suture Zone and TH, we propose a possibility that these geochemically and geochronologically comparable magmatic rocks may be genetically related with the Kerguelen mantle plume, and thus represent the dispersed fingerprints of the Kerguelen plume preserved in southern Tibet, China.

Fingerprints of the Kerguelen Mantle Plume in Southern Tibet: Evidence from Early Cretaceous Magmatism in the Tethyan Himalaya

AbstractEarly Cretaceous magmatism suggested to be related with the Kerguelen mantle plume has been reported in both the eastern and western Tethyan Himalayan terrane. Coeval magmatism (133–138 Ma) recorded by hypabyssal intrusive rocks have been recently discovered in the central Tethyan Himalaya (TH). The hypabyssal intrusions are dominated by OIB‐like basaltic rocks intruded by later porphyritic/ophitic intermediate rocks and are characterized by strongly light rare earth element enrichment and prominent Na‐Ta depletion and Pb enrichment. The basaltic rocks have low 143Nd/144Nd ratios ranging from 0.512365 to 0.512476 but relatively high 87Sr/86Sr ratios ranging from 0.708185 to 0.708966. The ∊Nd(t) ratios of the basaltic rocks are between −4.33 and −2.20 and initial 87Sr/86Sr ratios are 0.707807 to 0.708557. Geochemical data demonstrate that these rocks have experienced combined crustal assimilation and fractional crystallization processes. Magmatic zircons from the hypabyssal rocks exclusively have negative ∊Hf (t) values ranging from −0.7 to −12.7, suggestive of assimilation of crustal material. Zircons from these hypabyssal rocks have U‐Pb ages ranging from 130 to 147 Ma. Inherited zircons have U‐Pb ages from 397 to 2495 Ma. All the zircons are characterized by negative ∊Hf(t) values. The Jiding ocean island basalt (OIB)‐like magmatism is geochemically and geochronologically comparable with that in the western and eastern Tethyan Himalaya, indicating widespread OIB‐like magmatism in the northern margin of Greater India during the Cretaceous. Collectively, these rocks can be correlated with other early Cretaceous magmatism in western Australia and northern Antarctica. Considering the similarities, we suggest that the Jiding hypabyssal rocks are also genetically related to Kerguelen plume. Within the Yarlung Zangbo Suture Zone (YZSZ), there are also numerous occurrences of OIB‐like rocks derived from mantle sources different from those of N‐MORB‐like magmas. The OIB‐like magmatism in the YZSZ is nearly coeval with that in the TH, and the two are geochemically similar. We suggest that the OIB‐like magmatism in the Neo‐Tethyan ocean and the northern margin of Greater India may represent the dispersed fingerprints of the Kerguelen plume preserved in southern Tibet, China.

Miocene Sn polymetallic mineralization in the Tethyan Himalaya, southeastern Tibet: A case study of the Cuonadong deposit

The Cuonadong deposit is located in the Cuonadong gneiss dome of the eastern Tethyan Himalaya and is characterized as a large- to superlarge-scale Sn polymetallic deposit in the Himalaya orogen. The mineralization pattern includes skarn W-Sn mineralization in the strong shear zone of the dome mantle, hydrothermal Sn-W veins controlled by north-south-trending rifts (NSTRs), and Be-Rb mineralization in pegmatitic leucogranite. The results of this study show that the zircon U-Pb age of undeformed (postkinematic) leucogranites is 15.5 ± 0.1 Ma (n = 28, MSWD = 1.4), the cassiterite U-Pb age of the vein-type Sn-W orebody is 14.3 ± 0.5 Ma (n = 40, MSWD = 3.2), and the muscovite Ar-Ar age of beryl-bearing pegmatitic leucogranite is 15.2 ± 0.2 Ma (MSWD = 1.5). The isotopic dating results demonstrate that the Cuonadong Sn-W-Be-Rb mineralization has a close genetic relationship with the Miocene (ca. 16–14 Ma) leucogranite magmatic hydrothermal activity in the Cuonadong dome. The deformed (synkinematic) two-mica granite (18 Ma) was cut by the Sn-W ore veins (14 Ma) faulted by NSTRs, indicating that the E-W-trending extension of the NSTRs initiated at ca. 18–14 Ma. The Himalaya leucogranite belongs to a highly fractionated granite with high Sn content (>15 ppm). In conclusion, the Himalaya leucogranite is considered to be tin granite in this study, and therefore, the Himalaya presents high potential for Sn polymetallic mineralization prospecting.

Baddeleyite and zircon U-Pb ages of the ultramafic rocks in Chigu Tso area, Southeastern Tibet and their constraints on the timing of Comei Large Igneous Province

ABSTRACT A suite of ultramafic and mafic rocks developed in the Chigu Tso area, eastern Tethyan Himalaya. Baddeleyite and zircon U-Pb ages acquired by SIMS and LA-ICP-MS from olivine pyroxenite rocks in the Chigu Tso area are 138.9±3.0 Ma and 139.0±1.9 Ma, respectively. These two Early Cretaceous ages are similar with the ages of the more abundant mafic rocks in the eastern Tethyan Himalaya, indicating that this suite of ultramafic and mafic rocks in the Chigu Tso area should be included in the outcrop area of the Comei Large Igneous Province (LIP). These ultramafic rocks provide significant evidence that the involvement of mantle plume/hot spot activities in the formation of the Comei LIP. Baddeleyite U-Pb dating by SIMS is one reliable and convenient method to constrain the formation time of ultramafic rocks. The dating results of baddeleyite and zircon from the olivine pyroxenite samples in this paper are consistent with each other within analytical uncertainties, suggesting that baddeleyite and zircon were both formed during the same magmatic process. The consistency of baddeleyite U-Pb ages in the Chigu Tso area with zircon U-Pb ages for a large number of Early Cretaceous mafic rocks in the eastern Tethyan Himalaya further support that zircon grains from such mafic rocks yielding Early Cretaceous ages are also magmatic in origin.

Crustal thickening prior to 43 Ma in the Himalaya: Evidence from lower crust‐derived adakitic magmatism in Dala, eastern Tethyan Himalaya, Tibet

The Himalayan–Tibetan orogen is the world's largest active orogen with the thickest continental crust on Earth. However, the timing of crustal thickening of this orogen remains controversial. In recent years, magmatic rocks with adakitic affinities have been widely used to constrain the crustal thickness. Here, we present zircon U–Pb ages, geochemical and Sr–Nd–Pb–Hf isotopic data for the Dala two‐mica granites (eastern Tethyan Himalaya) that may shed light on this issue. Zircon U–Pb dating shows that the Dala two‐mica granites were emplaced at ca. 43 Ma. The granites display adakitic signatures, including high SiO2, Al2O3 and Sr contents, low Y and Yb contents with high Sr/Y (27−67) and La/Yb (35−99) ratios. High K2O and Th contents, low Nb/U, Ce/Pb, Ti/Eu and Nd/Sm ratios and low MgO, Mg#, Cr and Ni contents suggest that the Dala two‐mica granites were derived from partial melting of a thickened lower crust. Low negative zircon εHf(t) values (−10.4 to −0.5) with old two‐stage Hf model ages (1.1–1.8 Ga), high (87Sr/86Sr)i (0.715861–0.717665) with low εNd(t) (−12.5 to −11.3) and high Pb isotopes [(206Pb/204Pb)i=18.783–18.861, (207Pb/204Pb)i=18.783–18.861 and (208Pb/204Pb)i=39.052–39.285], together with previous literature data, indicate that the magma source for the Eocene granites in the Yardoi area was composed of ancient lower crust consisting mainly of garnet‐amphibolite and probably subordinate metapelites of the High Himalayan Crystalline Sequence. The residual mineral assemblage of garnet + rutile + plagioclase ± amphibole in the source region demonstrates that the crust beneath the Himalaya could have been thickened to ﹥50 km in the Eocene. Combining with previous studies, we propose that the Eocene magmatic rocks in the Tethyan Himalaya (e.g. ca. 43 Ma old Dala two‐mica granites) can be best interpreted as the consequence of breakoff of the Neo‐Tethyan oceanic slab.

Defining the Limits of Greater India

AbstractGreater India comprises a part of the Indian plate that subducted under Asia to help form the Tibetan Plateau. Defining the size of the Greater India is thus a key constraint to model the India‐Asia collision, growth of the plateau, and the tectonic evolution of the Neo‐Tethyan realm. We report Early Cretaceous paleomagnetic data from the central and eastern Tethyan Himalaya that yield paleolatitudes consistent with previous Early Cretaceous paleogeographic reconstructions. These data suggest Greater India extended at least 2,675 ± 720 and 1,950 ± 970 km farther north from the present northern margin of India at 83.6°E and 92.4°E, respectively. An area of lithosphere ≥4.7 × 106 km2 was consumed through subduction, thereby placing a strict limit on the minimum amount of Indian lithosphere consumed since the breakup of Gondwanaland.

Paleomagnetic and Geochronological Results From the Zhela and Weimei Formations Lava Flows of the Eastern Tethyan Himalaya: New Insights Into the Breakup of Eastern Gondwana

AbstractThe breakup of eastern Gondwana is among the hottest topics in the Earth sciences because of its effect on global climate during the Jurassic‐Cretaceous, its influence on the evolution of life, and its importance to paleogeographic reconstruction. To better constrain the Jurassic and Cretaceous paleogeographic position of the Tethyan Himalaya and the breakup of eastern Gondwana, a combined paleomagnetic and geochronological study was performed on the Zhela and Weimei Formations lava flows, dated at ~138–135 Ma, in the Luozha area of the eastern Tethyan Himalaya. Both positive fold and reversal tests together with a maximum grouping at 100% unfolding indicate that the characteristic remanent magnetization directions are primary magnetizations acquired before folding. The tilt‐corrected directions yielded a paleopole at 0.9°N, 293.4°E with A95 = 7.0° and a corresponding paleolatitude of 53.5°S ± 7.0°S for the Luozha sampling area (28.9°N, 91.3°E), validating that the original erupted position of the Zhela and Weimei Formations lava flows was located in the center of the Kerguelen mantle plume. Our new results, together with the published paleomagnetic, geochronological, and geochemical results, demonstrate that the Comei‐Bunbury large igneous province originated from the Kerguelen mantle plume. The temporal and spatial relationships between the Comei‐Bunbury large igneous province and the Kerguelen mantle plume indicate that eastern Gondwana initially rifted at ~147 Ma and that the Indian Plate fully separated from the Australian‐Antarctic Plate before ~124 Ma.

Reply to the comment titled “The usage of 238U/207Pb vs 206Pb/207Pb linear regressions for the LA-ICP-MS U-Pb dating of cassiterite” on the manuscript “Geological, geochemical, and geochronological characteristics of Weilasituo Sn-polymetal deposit, Inner Mongolia, China”

The Cuonadong deposit is located in the Cuonadong gneiss dome of the eastern Tethyan Himalaya and is characterized as a large- to superlarge-scale Sn polymetallic deposit in the Himalaya orogen. The mineralization pattern includes skarn W-Sn mineralization in the strong shear zone of the dome mantle, hydrothermal Sn-W veins controlled by north-south-trending rifts (NSTRs), and Be-Rb mineralization in pegmatitic leucogranite. The results of this study show that the zircon U-Pb age of undeformed (postkinematic) leucogranites is 15.5 ± 0.1 Ma (n = 28, MSWD = 1.4), the cassiterite U-Pb age of the vein-type Sn-W orebody is 14.3 ± 0.5 Ma (n = 40, MSWD = 3.2), and the muscovite Ar-Ar age of beryl-bearing pegmatitic leucogranite is 15.2 ± 0.2 Ma (MSWD = 1.5). The isotopic dating results demonstrate that the Cuonadong Sn-W-Be-Rb mineralization has a close genetic relationship with the Miocene (ca. 16–14 Ma) leucogranite magmatic hydrothermal activity in the Cuonadong dome. The deformed (synkinematic) two-mica granite (18 Ma) was cut by the Sn-W ore veins (14 Ma) faulted by NSTRs, indicating that the E-W-trending extension of the NSTRs initiated at ca. 18–14 Ma. The Himalaya leucogranite belongs to a highly fractionated granite with high Sn content (>15 ppm). In conclusion, the Himalaya leucogranite is considered to be tin granite in this study, and therefore, the Himalaya presents high potential for Sn polymetallic mineralization prospecting.

Components and structures of the eastern Tethyan Himalayan Sequence in SW China: Not a passive margin shelf but a mélange accretionary prism

The Tethyan Himalaya Sequence between the Yarlung‐Zangbo Suture and South Tibetan Detachment is a fundamental component of the Himalayan orogenic belt, traditionally considered to belong to the passive continental margin of India. However, the eastern Tethyan Himalaya Sequence is poorly understood, compared with that in the central and western Himalaya. To improve general understanding, we undertook north‐south geological‐structural profiles across all the litho‐tectonic units of the eastern Tethyan Himalaya Sequence and produced new isotopic‐geochemical data from all units, to constrain their mode of occurrence, structure, tectonic setting, and time of formation. The whole Tethyan Himalaya Sequence consists of a thrust‐imbricated mélange that contains blocks of ophiolites (such as Lang County, Yumen, and Lhunze), oceanic islands (e.g., Comei) with ocean island basalts‐type basalts, mafic greenschists, limestone olistoliths, and cherts in a turbiditic matrix of greywackes, siltstones, and slates in south‐directed thrust slices. Our findings suggest that the eastern Tethyan Himalaya Sequence represents the evolution of an accretionary prism, rather than a passive margin of the Indian Plate. The prism was created during northward subduction of Neo‐Tethys between ~130 and ~50 Ma. These data allow us to produce a new tectonic model for the structural development of the eastern Himalayan orogenic belt in relation to the India‐Asia collision.