Greater Himalayan Crystalline Complex Research Articles

Top-to-south shear at the base of the eastern Tethyan Himalayan Sequence during the Eocene-Oligocene Himalayan orogeny

The early mountain building processes in the Himalayan orogen are still not clear because of extensive deformation and metamorphism since the Miocene. A large gently dipping ductile shear zone, referred as the Tethyan Himalayan Décollement (THD), is defined here as the sole décollement of the south-verging Tethyan fold-and-thrust belt in the Lhozag-Cuona area of the eastern Himalayan orogen. The ~4 km-thick THD is characterized by a top-to-south shear sense, moderate T/P Barrovian to high T/P Buchan type metamorphism and Eocene-Miocene partial melting. Zircon UPb dating of metasedimentary rocks and granitic gneisses from the THD yields protolith ages of the Late Cambrian to Early Ordovician. Based on structural analysis, zircon UPb ages, monazite UPb ages and mica 40Ar/39Ar thermochronology, the THD was the boundary shear zone at the top of the Greater Himalayan Crystalline Complex (GHC) and accommodated the persistent north-south shortening in the Tethyan Himalayan Sequence (THS) from ~50 Ma to 20–17 Ma. From ~20–17 Ma, the top-to-north South Tibetan Detachment System (STDS) was predominantly activated to juxtapose the unmetamorphosed or low-grade THS over the GHC. This tectonic transition can be attributed to the roof collapse in the eastern Himalaya (younger than that of the central-western Himalaya), which triggered rapid exhumation of the GHC and the northern Tethyan Himalayan gneiss domes. Hence, the THD was the predecessor of the STDS and a prolonged pathway for leucogranitic melts from the Eocene to early Miocene. The transition from the THD-controlled crustal thickening in the Eocene and Oligocene to the STDS-controlled extrusion in the Miocene shed insights on a new synthesis of the tectonic wedging model for the Himalayan evolution.

In-situ analysis of sphalerite trace elements and sulfur isotope of the Zhaxikang Pb-Zn-Sb-Ag deposit in southern Tibet: Implications for source and mineralization process

The metallogenesis of collisional orogen has been a significant research topic for comprehending the linkage between magmatism, metamorphism, hydrothermalism and ore deposition. In Southern Tibet, the Zhaxikang deposit is a super-large Pb-Zn-Sb-Ag polymetallic deposit with metal reserves of 2.367 Mt Zn + Pb (average grade of 6.43 %), 0.2552 Mt Sb (average grade of 1.14 %), and 2960.6 t Ag (average grade of 101.64 g/t), constituting the largest deposit within North Himalayan Metallogenic Belt. However, the metal sources of the ore-forming material and mineralization process in this deposit remain a subject of controversy due to previous studies primarily relying on a bulk analysis of sulfides, which record the mixed results of multistage mineralization and intergrown minerals of different stages. The objective of this study is to constrain the sources and mineralization process of the Zhaxikang deposit through in-situ analyses of trace elements and sulfur isotopes in sphalerite. Based on ore textures and mineral paragenesis, four mineralization stages and three generations of sphalerite deposited respectively in stage 1 (Sp1), stage 2 (Sp2), and stage 4 (Sp3), were identified. The Pb and Zn mineralization mainly occurred in stage 2, while Sb and Ag were dominantly in stage 4. The trace elements characteristics (Cd, Fe, In, Ga, Ag, Cu, Co, and Mn) observed in sphalerite indicate that the Zhaxikang deposit could be a hydrothermal vein-type deposit associated with magmatism. The δ34S values of sulfides (7.7–18.4 ‰) in the deposit generally exhibit a slight elevation compared to those (4.1–12.9 ‰) of the Jurassic Ridang Formation slate, indicating that the sulfur of the deposit primarily originated from seawater sulfate in the slate through thermochemical sulfate reduction, although a small amount of contribution from mafic rocks also existed in stage 4. The Pb isotopes in stage 2 range from 19.662 to 19.705, 15.852 to 15.865, and 40.214 to 40.283 for the ratios of 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb, respectively, indicating that the metals primarily from the Greater Himalayan Crystalline complex (GHC) with a minor contribution from host rocks. While, the Pb isotopes in stage 4 range from 19.687 to 19.849 for 206Pb/204Pb, from 15.86022 to 15.885 for 207Pb/204Pb, and from 40.261 to 40.414 for 208Pb/204Pb, respectively, indicating that the metals originated from sulfides in the earlier stage as well as the more radiogenic GHC.

Origin of Himalayan Eocene Adakitic Rocks and Leucogranites: Constraints from Geochemistry, U-Pb Geochronology and Sr-Nd-Pb-Hf Isotopes

Within the Himalayan collisional belt, granites occur along two subparallel belts, namely, the Tethyan Himalayan Sequence (THS) and the Greater Himalayan Crystalline Complex (GHC). In this study, Eocene adakitic rocks and leucogranite are found only in the northern Himalayas, so further research is required to constrain their origin. Here, we present zircon U–Pb and monazite U–Th–Pb ages, Sr–Nd–Pb and Hf isotopes, and whole-rock major and trace elements for Liemai muscovite granite in the eastern Himalayan region. The U–(Th)–Pb results show that Liemai muscovite granite was emplaced at 43 Ma, and that its geochemical characteristics are similar to those of adakitic rocks of the same age (Dala, Quedang, Ridang, etc.). Combined with previous studies, both Eocene adakitic rocks and leucogranite are high-potassium calc-alkaline peraluminous granites. The former is relatively rich in large-ion lithophile elements (LILEs), such as Ba and Sr, and relatively deficient in high-field-strength elements (HFSEs), such as Nb, Ta, Zr, and Y, with weak or no Eu anomalies, and the average light rare earth element (LREE)/heavy rare earth element (HREE) ratio is 17.8. The latter is enriched in LILEs (such as Rb) and U, Ta, and Pb, and depleted in HFSEs (such as Nb and Zr), La, and Nd, with obvious negative Sr, Ba, and Eu anomalies and a mean LREE/HREE ratio of 10.7. The 87Sr/86Sr of the former is in the range of 0.707517–0.725100, εNd (t) ranged from −1.2 to −14.7, the average is −11.6, εHf (t) ranged from −0.5 to −65, the average is −12.2. The average values of (206Pb/204Pb) i, (207Pb/204Pb) i and (208Pb/204Pb) i are 18.788, 15.712 and 39.221, respectively; The 87Sr/86Sr of the latter is in the range of 0.711049~0.720429, εNd (t) ranged from −9.8 to −13.8, the average is −12.3, εHf (t) ranged from −4.2 to −10, the average is −6.7. The isotopic characteristics indicate that adakitic rocks and leucogranites are derived from the ancient lower crust, and both may be derived from metamorphic rocks of the GHC. In this paper, the origin of the two is associated with the transformation of the Himalayan tectonic system during the Eocene, and it is inferred that the deep crust may have altered the tectonic environment (temperature and pressure), resulting in an obvious episodic growth trend of leucogranite and significant development of adakitic rocks from 51 to 40 Ma. From 40 to 35 Ma, the development of Eocene magmatic rocks was hindered, and adakitic rocks disappeared. It is proposed that the genetic difference is related to the transition from high to low angles of the subducting plate in the crustal thickening process.

Role of crustal radiogenic heating in ultra-high temperature metamorphism of the Greater Himalayan Crystalline complex: Phase equilibrium and numerical modelling

Ultra-high temperature (UHT) metamorphism is a significant feature of the Greater Himalayan Crystalline complex (GHC), which constrains the dynamics of continental subduction and subsequent continental collision. However, the mechanism of such kind of UHT metamorphism is widely debated, including slab tearing-induced mantle upwelling, mechanical heating, radiogenic heating, and others. To better understand the UHT metamorphism and the thermal budget of collisional orogens, we first conduct a forward phase equilibrium modelling coupled with geochemical modelling to explore the variation of radiogenic heat production (RHP) of felsic melt, indicating that it may range from 4 μW/m3 to 9 μW/m3. Then, we incorporate the variable RHP into a thermo-mechanical model to study its effect on the thermal evolution of the collision zone. The numerical models reveal that a reasonably increased radiogenic heat production of 4 μW/m3 for the partially molten felsic crust contributes to UHT metamorphism after the continental collision for 30–35 million years. The model results are consistent with the structural observations, geochemical signatures, and P-T paths of the GHC. After the comparison with other mechanisms (e.g., the effects of mechanical heating and anomalous mantle heating), we propose that radiogenic heating could be a predominant heat source for the rapid UHT metamorphism of the GHC, whereas mechanical heating and heat flux from the mantle play secondary roles.

The long-lived partial melting of the Greater Himalayas in southern Tibet, constraints from the Miocene Gyirong anatectic pegmatite and its prospecting potential for rare element minerals

The Cenozoic Himalayan leucogranite–pegmatite belt has been a hotspot for rare metal exploration in recent years. To determine the genesis of the pegmatite in the Himalayan region and its relationship with the partial melting of the Greater Himalayan crystalline complex (GHC), the Gyirong pegmatite in southern Tibet were chosen for geochronological and geochemical studies. The dating analyses indicate that the U‒Th‒Pb ages of zircon, monazite, and xenotime exhibit large variations (38.6-16.1 Ma), and the weighted average value of the four youngest points is 16.5 ± 0.3 Ma, which indicates that the final crystallization of the melt occurred in the Miocene. The age of the muscovite Ar–Ar inverse isochron is 15.2 ± 0.4 Ma, that is slightly later than the intrusion age, showing that a cooling process associated with rapid denudation occurred at 16-15 Ma. The εHf(t) values of the Cenozoic anatectic zircons are concentrated between -12 and -9 with an average of -11.4.

In-situ boron isotope and chemical composition of tourmaline in the Gyirong pegmatite, southern Tibet: Implications for petrogenesis and magma source

Leucogranitic rocks, mainly including leucogranite-pegmatite systems, have been found to be widely distributed in the South Tibetan Himalaya, and they have received considerable interest because of their significance in crustal evolution and associated rare-metal mineralization. Although the nature and geodynamic setting of the Himalayan leucogranites have been well documented by numerous studies, the pegmatites spatially associated with these leucogranites are still poorly understood. Tourmaline is a ubiquitous phase from the leucogranite to the pegmatite. We have therefore conducted in situ major and trace element and boron isotope investigations of tourmaline from the Gyirong pegmatite, synthesizing published data on the Gyirong leucogranite, to document the origin of tourmaline and its genetic implications. Two types of tourmaline (Tur-Ⅰ & Tur-Ⅱ) have been identified in this contribution and they are enriched in Fe, Si and Al but depleted in Mg and Ca, with Mg/(Mg+Fe) ratios ranging from 0.22 to 0.45. Accordingly, the tourmalines belong to the alkali group and have schorl composition. Trace elements, such as Zn, Ga, V, Sc, Li, Sn, Sr, and Co in the tourmalines are relatively enriched, whereas, other trace elements record low concentrations less than 10 ppm. The trace element concentrations of tourmaline are mainly controlled by melt composition. Morphological and geochemical characteristics reflect that the tourmalines from the Gyirong pegmatite are magmatic in origin. The Gyirong pegmatitic tourmalines have S-type granitoids and pegmatites boron isotopic signatures with a tight range of δ11B values between −11.8 and −9.7‰, which is consistent with the magmatic tourmalines (Mg-poor) of the Gyirong leucogranite. This study suggests that the Gyirong pegmatite was the product of crustal anatexis and that the crustal metapelitic rocks within the Greater Himalayan Crystalline Complex were the most likely source components.

Miocene granitic magmatism constrains the early E-W extension in the Himalayan Orogen: A case study of Kung Co leucogranite

Himalayan leucogranites record the tectonic evolution of the Himalayan Orogen and provide a window to explore the deep dynamic processes. In this study, we conducted petrological, geochronological, and geochemical investigations of the Kung Co leucogranites that intrude into the shallow Tethys Himalayan Sequence along the north-south trending Nyima-Tingri rift. Whole-rock major and trace elements, and SrNd isotope geochemical investigations of the Kung Co leucogranites revealed that they: (1) had high SiO2 (> 71.5 wt%) and Al2O3 (> 14.8 wt%), relatively high CaO (> 1 wt%) and TiO2 (avg. 0.2 wt%), high Na2O and K2O, and uniform K2O/Na2O ratios (0.98–1.17); (2) were enriched in large-ion lithophilic elements but depleted in high-field-strength elements, and had relatively high Sr (> 114 ppm), low Rb (< 249 ppm), and low Rb/Sr ratios (0.66–1.64); (3) had moderate total REEs, enriched light REEs, and flat heavy REEs patterns, and no or slightly negative Eu anomalies (Eu/Eu* = 0.65–0.86); and (4) had homogeneous initial 87Sr/86Sr ratios (0.738047 to 0.739784) and εNd(t) values (−14.4 to −15.3). Geochemical characteristics and comparisons with Himalayan magmatic sources indicated that the Kung Co leucogranites originated from the partial melting of metagraywacke in the Greater Himalayan Crystalline Complex. Zircon and monazite U-Th-Pb dating confirmed leucogranitic magmatism at ca. 16.2 Ma. Zircon saturation thermometry and monazite saturation thermometry showed that the Kung Co leucogranites had higher melting temperatures than the Miocene Great Himalayan Granites, suggesting an anomalous geothermal gradient. Combined with the previous model of the Miocene evolution of the Himalayan Orogen and geophysical data, we proposed that Himalayan lithospheric delamination and upwelling of the asthenosphere provided additional heat that enabled the partial melting of metagraywacke, while the lithospheric delamination caused east-west extension and associated rifting activity, with magma rapidly intruding upward along the rift to the shallow crust.

Genesis of the Bangbu gold deposit in the southern Tibet: Evidenced from in-situ sulfur isotopes and trace element compositions of pyrite

The Bangbu orogenic gold deposit in the North Himalaya of the southern Tibet contains more than 40 t Au at an average grade of 7.0 g/t. In this deposits, gold-bearing quartz veins were controlled by nearly E-trending Qusong-Cuogu-Zhemulang shear zone and occurred within the secondary faults which crosscut Late Triassic greenschist- facies rocks. To further understand the sulfur source and ore-forming process, we have conducted a compressive study of in-situ SIMS sulfur isotopes and LA-ICP-MS trace element compositions of two stages of pyrite at Bangbu. Early-stage pyrite (Py1) is coarse-grained (mostly 0.2–2 mm) and euhedral, and has gold concentrations of less than 0.3–54 ppm (mean of 20 ppm) and δ34S values of 1.6–5.1‰ (mean of 3.2‰). Late-stage pyrite (Py2) is generally fine-grained (mostly <50 μm to 1 mm) and subhedral to anhedral, and has gold concentrations of 3.6–115 ppm (mean of 30 ppm) and δ34S values of 0.9–5.2‰ (mean of 2.7‰). Gold occurs mainly as invisible refractory within Py1 and Py2, and to a lesser extent as native gold within quartz, pyrite, arsenopyrite and sphalerite. Sulfur for Bangbu gold mineralization was probably sourced from the Greater Himalayan crystalline complex. Release of ore-forming fluids was likely related to amphibolite-facies metamorphism during ~50–45 Ma. Ore fluids deposited Au-rich pyrite during early and late mineralization stage and precipitation of native gold was probably related to fluid boiling and/or remobilization of invisible gold within pyrite.

Activity of the south Tibetan detachment system: Constraints from leucogranite ages in the eastern Himalayas

Himalayan leucogranites are widely distributed in the North Himalayan gneiss dome (NHGD) and at the top of the Greater Himalayan Crystalline Complex (GHC) and are generally controlled by detachment faults. The ages of these prekinematic, synkinematic, and postkinematic leucogranites can be used to limit the activity of detachment structures (such as the South Tibetan Detachment System, STDS). Research on the STDS activity time in the eastern Himalayas is relatively sparse. In this study, the zircon and monazite U–Th–Pb geochronology of synkinematic and postkinematic leucogranites, which are affected by the STDS and NHGD, in four areas (Lhozhag, Kuju, Xiaozhan, and Cuonadong) in Shannan City, Tibet, China, was measured. The results show that the oldest synkinematic two‐mica granite from Lhozhag, which is affected by the STDS, is 24–25 Ma, so the time of STDS activity is at or slightly earlier than 25 Ma. The youngest synkinematic leucogranite is the garnet‐bearing muscovite granite in Cuonadong at 18.4 Ma. The oldest undeformed postkinematic leucogranite (not affected by the STDS) is the muscovite granite in Xiaozhan at 17.4 Ma. Therefore, the end of STDS activity can be limited to 18.4–17.4 Ma. The STDS includes three forms: detachment fault in the NHGD (northern extension of the STDS), the inner STDS between the GHC and Tethyan Himalayan Sequence, and the outer STDS at the bottoms of synformal klippes. In this paper, the active time limits of the above three kinds of detachment zones are comprehensively summarized. Based on this work, the northward extension (ductile deformation) time of the STDS in the region is considered to be 28–17 Ma. The exhumation of the GHC is mainly controlled by in‐sequence shearing: first, the South Tibet Thrust system (predecessor of the STDS) at the top of the GHC thrust southward at 45–28 Ma, then the High Himalayan Discontinuity fault in the middle of the GHC forms south‐vergent ductile thrusts at 28–17 Ma, and finally, the Main Central Thrust at the bottom of the GHC thrust southward at 17–9 Ma.

Geochronological and geochemical constraints on the Cuonadong leucogranite, eastern Himalaya

First comprehensive investigations of the Cuonadong leucogranite exposed in North Himalayan gneiss dome of southern Tibet are presented in this study. The SIMS U–Pb ages of oscillatory zircon rims scatter in a wide range from 34.1 to 16.0 Ma, and the Cuonadong leucogranite probably emplaced at 16.0 Ma. High-precision 40Ar/39Ar dating on a muscovite sample yields an essentially flat age spectrum with consistent plateau and isochron ages, indicating that the Cuonadong leucogranite cooled below 450 °C at 14 Ma. Based on the youngest zircon U–Pb age and muscovite 40Ar/39Ar age, the Cuonadong leucogranite experienced rapid cooling with a rate of 119 °C/Myr from 16 to 14 Ma. The geochronological data of this undeformed leucogranite also suggest that the ductile extension of the South Tibetan Detachment System in the eastern Himalaya ceased by ca. 14 Ma. Furthermore, the initial Sr–Nd isotopic compositions and Nd model ages demonstrate that the leucogranite was derived from metapelitic source within the Greater Himalayan Crystalline Complex. The distinct Ba depletion with high Rb/Sr ratios and negative Eu anomalies make it clear that the leucogranite melts were generated by breakdown of muscovite under fluid-absent conditions.

Monazite behaviour during isothermal decompression in pelitic granulites: a case study from Dinggye, Tibetan Himalaya

Monazite is a key accessory mineral for metamorphic geochronology, but interpretation of its complex chemical and age zoning acquired during high-temperature metamorphism and anatexis remains a challenge. We investigate the petrology, pressure–temperature and timing of metamorphism in pelitic and psammitic granulites that contain monazite from the Greater Himalayan Crystalline Complex (GHC) in Dinggye, southern Tibet. These rocks underwent isothermal decompression from pressure of >10 kbar to ~5 kbar at temperatures of 750–830 °C, and recorded three metamorphic stages at kyanite (M1), sillimanite (M2) and cordierite-spinel grade (M3). Monazite and zircon crystals were dated by microbeam techniques either as grain separates or in thin sections. U–Th–Pb ages are linked to specific conditions of mineral growth on the basis of zoning patterns, trace element signatures, index mineral inclusions (melt inclusions, sillimanite and K-feldspar) in dated domains and textural relationships with co-existing minerals. The results show that inherited domains (500–400 Ma) are preserved in monazite even at granulite-facies conditions. Few monazites or zircon yield ages related to the M1-stage (~30–29 Ma), possibly corresponding to prograde melting by muscovite dehydration. During the early stage of isothermal decompression, inherited or prograde monazites in most samples were dissolved in the melt produced by biotite dehydration-melting. Most monazite grains crystallized from melt toward the end of decompression (M3-stage, 21–19 Ma) and are chemically related to garnet breakdown reactions. Another peak of monazite growth occurred at final melt crystallization (~15 Ma), and these monazite grains are unzoned and are homogeneous in composition. In a regional context, our pressure–temperature–time data constrains peak high-pressure metamorphism within the GHC to ~30–29 Ma in Dinggye Himalaya. Our results are in line with a melt-assisted exhumation of the GHC rocks.

Geology, S–Pb isotopes, and 40Ar/39Ar geochronology of the Zhaxikang Sb–Pb–Zn–Ag deposit in Southern Tibet: implications for multiple mineralization events at Zhaxikang

Several Au, Sb, Sb–Au, Pb–Zn, and Sb–Pb–Zn–Ag deposits are present throughout the North Himalaya in southern Tibet, China. The largest Sb–Pb–Zn–Ag deposit is Zhaxikang (18 Mt at 0.6 wt% Sb, 2.0 wt% Pb, 3.5 wt% Zn, and 78 g/t Ag). Zhaxikang veins are hosted within N–S trending faults, which crosscut the Early–Middle Jurassic Ridang Formation consisting of shale interbedded with sandstone and limestone deposited on a passive continental margin. Ore paragenesis indicates that Zhaxikang mineralization occurred in two main phases composed of six total stages. The initial phase was characterized by assemblages of fine-grained Mn–Fe carbonate + arsenopyrite + pyrite + sphalerite (stage 1), followed by relatively coarse-grained Mn–Fe carbonate + Fe-rich sphalerite + galena + pyrite (stage 2). The second phase was marked by assemblages of quartz + pyrite + Fe-poor sphalerite and Ag-rich galena + tetrahedrite + sericite (stage 3), quartz + Sb–Pb sulfosalt minerals mainly composed of boulangerite and jamesonite (stage 4), quartz + stibnite ± cinnabar (stage 5), and quartz ± calcite (stage 6). Sulfides of stage 2 have δ34SV–CDT of 8.4–12.0‰, 206Pb/204Pb ratios of 19.648 to 19.659, 207Pb/204Pb ratios of 15.788 to 15.812, and 208Pb/204Pb ratios of 40.035 to 40.153. Sulfides of stage 3 have similar δ34SV–CDT of 6.1–11.2‰ and relatively more radiogenic lead isotopes (206Pb/204Pb = 19.683–19.792). Stage 4 Sb–Pb sulfosalt minerals have δ34SV–CDT of 5.0–7.2‰ and even more radiogenic lead (206Pb/204Pb = 19.811–19.981). By contrast, stibnite of stage 5 has δ34SV–CDT of 4.5–7.8‰ and less radiogenic lead (206Pb/204Pb = 18.880–18.974). Taken together with the geological observations that the Pb–Zn-bearing Mn–Fe carbonate veins were crosscut by various types of quartz veins, sphalerite and galena of stage 2 underwent dissolution and remobilization, and that Sb–Pb(−Fe) sulfosalts formed at the expense of Pb from stage 2 galena and of Fe from stage 2 sphalerite, we argue that the early Pb–Zn veins were overprinted by later Sb-rich fluids. Stage 2 fluids were likely acidic and oxidized and leached lead from high-grade metamorphic rocks of the Greater Himalayan crystalline complex (GHC) and sulfur from reduced rocks, such as slate of the Ridang Formation, along N–S trending faults, leading to precipitation of Pb–Zn sulfides and Mn–Fe carbonate and formation of solution collapse breccias. Later Sb-rich fluids leached Pb from the GHC and the pre-existing sulfides and deposited Fe-poor sphalerite, Ag-rich galena, tetrahedrite, Sb–Pb sulfosalts, and stibnite in quartz veins that cut pre-existing Pb–Zn-bearing Mn–Fe carbonate veins. The Sb-rich fluids also likely leached Pb from Early Cretaceous gabbro and formed stibnite at shallow levels where early Pb-Zn-bearing Mn-Fe carbonate veins are absent. A sericite 40Ar–39Ar plateau age of 17.9 ± 0.5 Ma from stage 3 veins represents the timing of the onset of stage 3 mineralization.

Anatomy of composition and nature of plate convergence: Insights for alternative thoughts for terminal India-Eurasia collision

The pattern and timing of collision between India and Eurasia have long been a major concern of the international community. However, no consensus has been reached hitherto. To explore and resolve the disagreements in the Himalayan study, in this paper we begin with the methodology and basic principles for the anatomy of composition and nature of convergent margins, then followed by an effort to conduct a similar anatomy for the India-Eurasia collision. One of the most common patterns of plate convergence involves a passive continental margin, an active continental margin and intra-oceanic basins together with accreted terranes in between. The ultimate configuration and location of the terminal suture zone are controlled by the basal surface of the accretionary wedge, which may show fairly complex morphology with Z-shape and fluctuant geometry. One plausible method to determine the terminal suture zone is to dissect the compositions and structures of active continental margins. It requires a focus on various tectonic elements belonging to the upper plate, such as accretionary wedges, high-pressure (HP)-ultra-high-pressure (UHP) metamorphic rocks, Barrovian-type metamorphic rocks and basement nappes, together with superimposed forearc basins. Such geological records can define the extreme limits and the intervening surface separating active margin from the passive one, thus offering a general sketch for the surface trace of the terminal suture zone often with a cryptic feature. Furthermore, the occurrence of the cryptic suture zone in depth may be constrained by geophysical data, which, in combination with outcrop studies of HP-UHP metamorphic rocks, enables us to outline the terminal suture zone. The southern part of the Himalayan orogen records complicated temporal and spatial features, which are hard to be fully explained by the classic “two-plate-one-ocean” template, therefore re-anatomy of the compositions and nature for this region is necessitated. Taking advantage of the methodology and basic principles of plate convergence anatomy and synthesizing previous studies together with our recent research, we may gain new insights into the evolution of the Himalayan orogeny. (1) The Yarlung-Zangbo ophiolite is composed of multiple tectonic units rather than a single terminal suture zone, and a group of different tectonic units were juxtaposed against each other in the backstop of the Gangdese forearc. (2) The Tethyan Himalayan Sequence (THS) contains melanges with typical block-in-matrix structures, uniform southwards paleocurrents and age spectra of detrital zircons typical of Eurasia continent. All of these facts indicate that the THS belonged to Eurasia plate before the terminal collision, emplaced in the forearc of the Gangdese arc. (3) The Greater Himalayan Crystalline Complex (GHC) and Lesser Himalayan Sequence (LHS) comprise complex components including eclogites emplaced into the GHC and the upper part of the LHS. Judging from the fact that HP-UHP metamorphic rocks are exhumed and emplaced in the upper plate, the GHC and the upper part of the LHS where eclogite occur should be assigned to the upper plate, lying above the terminal subduction zone surface. It is the very surface along which the continuous subduction of the India subcontinent occurred, therefore acting as the terminal, cryptic suture. From the suture further southward, the bulk rock associations of the LHS and Sub-Himalayan Sequence (Siwalik) show little affinity of melange, probably belonging to the foreland system of the India plate. By the anatomy of tectonic features of all the tectonic units in the Himalayan orogen as well as the ages of the subduction-accretion related deformation, we conclude that the terminal India-Eurasia collision occurred after 14 Ma, the timing of the metamorphism of the eclogites emplaced into the upper plate. The development of rifts stretching in N-S direction in Tibet and tectonic events with the transition from sinistral to dextral movements in shear zones, such as the Ailaoshan fault in East Tibet, can coordinately reflect the scale and geodynamic influence of the India-Eurasia convergence zone. By conducting a detailed anatomy of the southern Himalayas, we propose a new model for the final collision-accretion of the Himalayan orogeny. Our study indicates that the anatomy of structures, composition, and tectonic nature is the key to a better understanding of orogenic belts, which may apply to all the orogenic belts around the world. We also point out that several important issues regarding the detailed anatomy of the structures, compositions and tectonic nature of the Himalayan orogeny in future.

Spatial and temporal evolution of tectonometamorphic discontinuities in the central Himalaya: Constraints from P–T paths and geochronology

Recent studies evoke dispute whether the Greater Himalayan Crystalline Complex (GHC) was exhumed during more than one phase. This contribution investigates the timing of ductile shear along the South Tibetan Detachment (STD), Main Central Thrust (MCT), and hidden discontinuities within the GHC of the central Himalaya. New data from the Nyalam transect, southern Tibet, suggest that ductile shear along the STD was active from ~27–25Ma to ~17–15Ma, whereas ductile shear along the MCT was active from ~19–16Ma to ~10–8Ma. Pseudosection modeling results and published metamorphic ages indicate a P–T–t path discontinuity within the GHC and the upper GHC reached higher peak metamorphic temperatures (ΔT 100°C), similar peak pressures and older metamorphic ages (~5–10Myr), in favor of the presence of the Nyalam Discontinuity. Summarized results from literature suggest that timing of ductile shear along the STD was ~5–10Myr earlier than that along the MCT across the Nepal, Sikkim and Bhutan Himalayas. In addition, the GHC in Nepal can be separated into two portions by a regional-scale (>800km) in-sequence ductile shear zone — the High Himalayan Discontinuity. This shear zone was active coevally with the STD during 25–16Ma, but ~5–10Myr earlier than the MCT. The High Himalayan Discontinuity could possibly extend to the Sikkim Himalayas, but ends in Bhutan. The identified new architecture suggests that both in-sequence and out-of-sequence tectonometamorphic discontinuities are important components that formed the duplexing structure of the GHC in the central and eastern Himalaya, thus calling for more sophisticated numerical simulation than the current models such as the channel flow and critical taper.

Timing of Partial Melting and Cooling across the Greater Himalayan Crystalline Complex (Nyalam, Central Himalaya): In-sequence Thrusting and its Implications

The timing of crustal melting and cooling has been investigated across the migmatites of the Greater Himalayan Crystalline Complex (GHC) in the Nyalam region, central Himalaya. Monazite U–Pb ages vary from 32 to 14 Ma and are linked to metamorphic conditions on the basis of monazite internal zoning, mineral inclusions, and changes in heavy rare earth element and Y composition. Metamorphic temperatures were estimated by Zr-in-rutile thermometry and cooling rates were further constrained by rutile U–Pb ages. The results reveal two distinct blocks within the GHC of the Nyalam region. The upper GHC experienced higher peak metamorphic temperatures (730–750°C) and a higher degree of melting (15–25%). Partial melting was dominated by muscovite dehydration melting, which lasted from ∼32 to 25 Ma, possibly until ∼20 Ma. The lower GHC experienced lower peak metamorphic temperatures (640–675°C) and a lower degree of melting (0–10%) mainly via H2O-saturated melting from 19 to 16 Ma. At different times, both upper and lower blocks experienced initial slow cooling (rates 35 ± 8 and 10 ± 5°C Ma–1, respectively) followed by rapid cooling (100 ± 20°C Ma–1). The documented diachronous metamorphism implies the presence of the ‘High Himalayan Thrust’ that was active at ∼25–16 Ma within the GHC of the central Himalaya. Different degrees and durations of partial melting in the investigated section suggest that a channel flow process dominated the exhumation of the upper GHC migmatites at 25–16 Ma, whereas a critical taper process dominated the exhumation of the relatively lower-grade lower GHC rocks and cooled upper GHC migmatites at 16–10 Ma. We suggest that propagating thrusts along large tectonic boundaries together with low-viscosity lateral crustal flow could contribute to exhumation of high-grade metamorphic rocks in the Himalaya and other similar collisional orogens.

Structural kinematics, metamorphic P–T profiles and zircon geochronology across the Greater Himalayan Crystalline Complex in south‐central Tibet: implication for a revised channel flow

AbstractA specific question about the Himalayas is whether the orogeny grew by distributed extrusion or discrete thrusting. To place firm constraints on tectonic models for the orogeny, kinematic, thermobarometric and geochronological investigations have been undertaken across the Greater Himalayan Crystalline Complex (GHC) in the Nyalam region, south‐central Tibet. The GHC in this section is divided into the lower, upper and uppermost GHC, with kinematically top‐to‐the‐south, alternating with top‐to‐the‐north shear senses. A new thrust named the Nyalam thrust is recognized between the lower and upper GHC, with a 3 kbar pressure reversion, top‐to‐the‐south thrust sense, and was active after the exhumation of the GHC. Peak temperature reached ∼749 °C in the cordierite zone, and decreased southwards to 633–667 °C in the kyanite and sillimanite‐muscovite zones, and northwards to greenschist facies at the top of the South Tibetan Detachment System (STDS). Pressure at peak temperature reached a maximum value in the kyanite zone of 9.0–12.6 kbar and decreased northwards to ∼4.1 kbar in the cordierite zone. Zircon U‐Pb ages of a sillimanite migmatite and an undeformed leucogranite dyke cutting the mylonitized rocks in the STDS reveal a long‐lived partial melting of the GHC, which initiated at 39.7–34 Ma and ceased at 14.1 Ma. Synthesizing the obtained and collected results, a revised channel flow model is proposed by considering the effect of heat advection and convection by melt and magma migration. Our new model suggests that distributed processes like channel flow dominated during the growth of the Himalayan orogen, while discrete thrusting occurred in a later period as a secondary process.

Orogen‐parallel ductile extension and extrusion of the Greater Himalaya in the late Oligocene and Miocene

AbstractPredominant stretching structures in the Greater Himalayan Crystalline Complex (GHC) trend perpendicular to the belt and are linked to the southward exhumation or emplacement of the GHC between the South Tibet Detachment (STD) and the Main Central Thrust. However, our field investigations in southern Tibet reveal the widespread presence of gently dipping shear zones with a penetrative orogen‐parallel stretching lineation, which separates the Tethyan Himalayan Sequence and the underlying GHC. The shear zones are well preserved in the upper part of the GHC, south to and structurally lower than the STD. Field criteria, microstructures, and quartz fabrics indicate top‐to‐the‐east shearing in the Yadong shear zone (eastern GHC), coexistence of top‐to‐the‐east and top‐to‐the‐west shearing in the Nyalam shear zone (central GHC), but top‐to‐the‐west shearing in the Pulan shear zone (western GHC). Characteristic microstructures and slip systems of quartz in the high‐grade GHC rocks resulted from the lateral flow under upper amphibolite (up to 650–700 °C) to greenschist facies conditions. U‐Pb ages of metamorphic zircon rims by sensitive high‐resolution ion microprobe (SHRIMP) and laser ablation multi‐collector inductively coupled plasma mass spectrometry (LA‐MC‐ICP‐MS) analyses yield 28–26 Ma for the initiation of the Yadong and Nyalam shear zones and 22–15 Ma for the activation of the Pulan shear zone. In addition, 40Ar/39Ar cooling ages of biotite and muscovite suggest cessation of ductile sharing at 13–11 Ma on the Yadong shear zone, which is coeval with the activation of the STD. Combined with previous studies, we propose that initiation of orogen‐parallel extension marks the transition from burial/crustal thickening to exhumation of the GHC. Due to lateral crustal thickness gradients in a thickened crust, orogen‐parallel gravitational collapse occurred within the convergent Himalayan orogen in the late Oligocene‐Miocene. This tectonic denudation triggered and enhanced partial melting and ductile extrusion of the GHC in the Miocene.

Middle-Miocene transformation of tectonic regime in the Himalayan orogen

Understanding the multiple tectonic transformations during the Himalayan orogeny is significant in evaluating the evolution of Himalayan orogen. In the Gyirong area in south Tibet, deformed leucogranitic veins in the biotite-plagioclase gneisses of Greater Himalayan crystalline complex (GHC) constitute south-vergent asymmetric folds. The reconstruction of the veins shows that they experienced two generations of deformation under different tectonic regimes: an earlier top-to-north extension and a later top-to-south thrusting, implying a tectonic transformation from N-S extension to N-S shortening. Zircons LA-ICP-MS U-Pb dating of the leucogranite shows that it was emplaced during 21.03-18.7 Ma. The data suggest that the tectonic transformation occurred after 18.7 Ma. The chronological data of South Tibet detachment system (STDS) and North-South trending rift (NSTR) from Gyirong and other areas indicate that the Himalayan orogeny was in a period of tectonic transformation from N-S extension to N-S shortening during 19-13 Ma. The transformation of tectonic regime was probably controlled by the India-Asia convergence rate. An increase in the convergence rate resulted in N-S shortening of the orogen, thrusting and folding, with coeval formation of the NSTR in Tibet. A decrease in the convergence rate led to N-S extension and reactivation of the STDS.

Eocene–Oligocene granitoids in southern Tibet: Constraints on crustal anatexis and tectonic evolution of the Himalayan orogen

Tectonic models for the evolution of the Himalayan orogen interpret the Greater Himalayan crystalline complex (GHC) to be the result of either thick-skinned thrusting involved Indian basement, thin-skinned thrusting involving exotic terranes, middle-crustal ductile flow, or wedge extrusion of the Indian crust during India–Asia collision. Two key pieces of information needed to test the validity of these models is the temporal–spatial distribution of, and the identification of the dynamic mechanisms involved in, regional melting under southern Tibet. Here, we document an Eocene–Oligocene melting event in southern Tibet, which forms a 150-km-long, NW–SE-trending granitoid belt along the Zedong–Lhunze traverse between the Indus–Yarlung suture (IYS) and the south Tibetan detachment (STD). U–Pb dating of magmatic zircons indicates that this granitoid belt youngs northward from ∼46Ma (in Lhunze) to ∼30Ma (in Zedong). 40Ar/39Ar dating of deformed biotite within 42–46Ma granitoids constrains the timing of shearing to ∼39–41Ma.The granitoid belt of southern Tibet is dominated by Eocene two-mica granites in the Tethyan Himalaya, with minor ∼30Ma granodiorites along the IYS and ∼35Ma granites in the Yelaxiangbo dome, where Indian mid-crustal rocks are exposed. The ∼35Ma granites are characterized by variable Na2O/K2O ratios (1.03–4.44), relatively high Sr concentrations, and high Sr/Y (14.0–126.3) and La/Yb (11.1–42.8) ratios, which distinguish these granitoids from Miocene leucogranites in the Himalaya. Comparison of the Sr–Nd isotopic compositions of these granites with mid-crustal amphibolites exposed in the Yelaxiangbo dome suggests that the granites were derived from melting of the amphibolites at ∼880°C and ∼10kbar. The ∼30Ma granodiorites and ∼42–46Ma two-mica granites are Na-rich and peraluminous, and are adakitic. They contain inherited Proterozoic zircons, and have a much wider range in εNd(t) of –14.9 to –2.5 and (87Sr/86Sr)i of 0.7062–0.7188, and have a Nd isotopic model age of 1486–1978Ma, indicating that these magmas were derived from a thickened Indian lower crust and were subsequently mixed with amphibolite-derived granite melts or were contaminated by the middle crust under southern Tibet. An apparent northward-younging age trend and shearing of the Eocene–Oligocene granitoids requires the southward migration of slices of middle crustal material, in which the Eocene granitoid magmas were emplaced and stored. Our data, along with structural, metamorphic, and intrusive histories of the Himalaya, lead us to propose a model for crustal anatexis and tectonic evolution of the Himalayan orogen, controlled by a number of large-scale events, such as slab break-off, buoyancy-driven uplift, lateral movement, and subsequent exhumation of slices of the subducted Indian crust during Indo-Asia collision at 55–40Ma.

Andean-type orogeny in the Himalayas of south Tibet: Implications for early Paleozoic tectonics along the Indian margin of Gondwana

The granitic gneisses in the cores of north Himalayan gneiss domes (NHGD) and at the crest of the high Himalayas form part of the greater Himalayan crystalline complex (GHC). These rocks are characterized by the assemblage of quartz, plagioclase, K-feldspar, biotite, muscovite and minor garnet. Here we present results from LA-ICPMS zircon U–Pb dating of six granitic gneiss samples, which reveal formation ages of ca. 500–473Ma and suggest a long-lived magmatism during early Paleozoic in the GHC. Geochemical data show that these rocks are characterized by high SiO2 (70.93–74.59wt.%), K2O (4.22–5.91wt.%), A/CNK values (>1.1) and low Na2O/K2O (0.42–0.83), and enrichment in Rb, Th, U, depletion in Ba, Nb, Ta, Sr, Zr, and strong negative Eu anomalies. These features suggest that the protoliths of the gneisses are high potassium calc-alkaline and peraluminous S-type granites derived from partial melting of crustal materials. The high initial 87Sr/86Sr (>0.706) and negative εNd(t) (−6.2 to −10.6) of the granitic gneisses compare with those of the metasedimentary rocks in the GHC, indicating that the granites were generated from partial melting of the sedimentary protoliths. Based on the geochronological, whole-rock geochemical and Sr–Nd isotopic data presented in this study, we suggest that the granites formed in a back-arc setting with a continental arc-affinity related to the subduction of the Proto-Tethyan Oceanic lithosphere. The mafic magmatic underplating, triggered by the subduction, rollback and the break off of Proto-Tethyan Ocean slab, resulted in partial melting of the crust in the GHC. Integrating results from previous studies, we propose an Andean-type orogeny along the margin of the Gondwana.