Gangdese Arc Research Articles

Tectonic Implications of Early Permian Arc Rocks and Their Cretaceous to Early Cenozoic Reworking in Southern Lhasa Terrane, Tibet

AbstractThe Lhasa terrane in southern Tibet occupies a central position in Asian tectonics, yet its pre‐Mesozoic petrologic and tectonic evolution is poorly constrained, especially the Southern Lhasa subterrane (SLS). Here, new zircon U–Pb ages, zircon trace element and Hf isotopic compositions, and whole‐rock geochemical data for mafic meta‐igneous rocks from the SLS distinguish three tectono‐thermal events at ∼290 Ma, ∼126 Ma and ∼49 Ma. Whole‐rocks and zircons with ages of the two older events have arc magma geochemistry, but Hf isotopes are distinct from Mesozoic Gangdese arc magmas. Zircon cores and, arguably, whole rocks instead derive from ∼290 Ma magma formed during southward subduction of Paleo‐Tethys beneath the SLS. These rocks later underwent Early Cretaceous (∼126 Ma) remelting and early Cenozoic (∼49 Ma) metamorphism with P–T conditions of ~800°C and 15.3 kbar, and record a retrograde P–T path characterized by exhumation with cooling, consistent with a collisional origin. These data suggest Permian igneous rocks underwent reworking during both the Early Cretaceous and the early Cenozoic, reaching crustal thicknesses of at least ~50 km during the early Eocene. Combined with regional data, Paleo‐Tethys evidently experienced early Permian double‐sided subduction within the Lhasa terrane, with back arc basins forming between the SLS and the Indian margin to the south. These back arc basins ultimately widened to form the Neo‐Tethys Ocean, which then subducted during the Mesozoic, leading to Cretaceous arc magmatism and overprinting, followed by early Cenozoic metamorphism and final reworking during collision with India.

Tectonic Evolution and Sedimentary Responses of Palaeocene–Eocene Tethys Himalayan Foreland Basin in Southern Tibet

ABSTRACTThe tectonic evolution of the Palaeocene–Eocene Tethys Himalayan foreland basin plays a crucial role in reshaping the collisional orogenic process of the Yarlung–Tsangpo oceanic basin. However, studies examining the sedimentary response during the tectonic evolution of the foreland basin are lacking. In this study, through a detailed field investigation and analysis of Palaeocene–Eocene strata in the Tingri area, we clarified the evolution of the Tethys Himalayan tectonic regions and its sedimentary response. Carbon and oxygen isotopes, geochemistry and detrital zircon U–Pb dating demonstrated that the lower Palaeocene Jidula Formation and upper Palaeocene–lower Eocene Zongpu Formation were deposited in a coastal–shallow marine environment, with the detritus sourced from the northern Indian passive margin. The upper Eocene Pengqu Formation was deposited in a deltaic environment, with its detritus sourced from the Gangdese arc and the Yarlung–Tsangpo suture zone at the active continental margin. Combined with the nearshore subaqueous fan branch channel of the Jidula Formation and the slump deformation of the Zongpu Formation, the Palaeocene–Early Eocene southern Tethys Himalaya in the Tingri area was located in the forebulge of the peripheral foreland basin. The marine–continental interactive delta of the Pengqu Formation and its provenance from the Gangdese magmatic arc indicate that the Tingri area was situated in the foredeep of the peripheral foreland basin during the Late Eocene. The study provides valuable insights into the collisional orogenic processes between the Indian and Eurasian plates.

Detrital zircon provenance record of the northern shelf in the Rakhine Basin, Bay of Bengal

The Cenozoic strata in the Rakhine Basin play a crucial role in hydrocarbon exploration; however, the provenance of these sedimentary rocks remains poorly investigated. In this study, we employed detrital zircon geochronology and geochemistry in the northern Rakhine shelf to investigate the provenance of sedimentary rocks and, combined with the previous studies, to identify their spatial and temporal variations and to elucidate the provenance evolution since the middle Miocene. The geochemical results of the zircons indicate that the primary source rocks of the sedimentary rocks are magmatic and metasedimentary rocks, particularly those of continental arc and post-collision granites. The origin of these rocks can be attributed to the Gangdese arc magmatic belt in the north, as well as the Indo-Burma Ranges in the east. The sedimentary rocks in the northern Rakhine yield zircon U-Pb age peaks primarily at 0-150 Ma, 400-600 Ma, 600-1000 Ma, and 1600-2100 Ma, with minor populations at 150-400 Ma and >2100 Ma. The detrital zircon provenance study shows that the zircons in the Miocene sedimentary rocks were primarily derived from the paleo-Brahmaputra River, with contributions ranging from 68.7% to 77.0%. During the Pliocene, the paleo-Brahmaputra River and the Chittagong-Tripura Fold Belt were the most significant sediment suppliers. Since the early Pleistocene, the northern Rakhine coastal region has played a significant role in their composition. The Brahmaputra River was the predominant contributor to upper Pleistocene rocks, and the Rakhine coastal region’s contribution to the shelf intensified throughout the Pleistocene. In conjunction with regional tectonic evolution, our study underscores the importance of the Indian Plate subduction as a pivotal factor influencing the provenance transitions in the northeastern Bay of Bengal.

Paleolatitude of Mafic Dykes in the Xiugugabu ophiolite: Implications for the intraoceanic Trans-Tethyan subduction zone and multistage India-Eurasia collision

An intraoceanic Trans-Tethyan subduction zone has been identified in both the Kohistan-Ladakh arc and the West Burma Terrane. This has significant implications for the India-Eurasia collision. Concurrently, the dismembered ophiolites within the Yarlung-Tsangpo Suture Zone likely originated from the intraoceanic Trans-Tethyan subduction zone or the Andean-type southern Eurasian continental margin. A paleomagnetic study was conducted on the Lower Cretaceous (∼120–130 Ma) mafic dykes in the Xiugugabu ophiolite to resolve the uncertainty of its origin. The characteristic remanent magnetization (ChRM) obtained through stepwise thermal demagnetization successfully passed consistency/fold and reversal tests. After tilt correction, the overall mean direction of the ChRM was D = 296.9°, I = −25.5°, k = 47.6, α95 = 4.5°, and N = 22, indicating a paleolatitude of 13.4°N/S and a paleopole at 15.0° N, 326.6°E with A95 = 3.8°. Compared with previous paleomagnetic data from the Trans-Tethyan subduction zone, our findings strongly support the involvement of the Xiugugabu ophiolite in the intraoceanic Trans-Tethyan subduction zone. This finding reinforces the hypothesis that there were two distinct subduction zones in the Neotethyan Ocean during the Early Cretaceous. One subduction zone was situated on the southern margin of the Gangdese Arc. The second was the intraoceanic subduction zone, located approximately 3500 km from the southern margin of Eurasia, in the southern hemisphere. Our results also support a multistage India–Eurasia collision process involving continental plates, intraoceanic arcs, and terranes within the Neo-Tethyan Ocean.

Amphibole fractionation as a key driver for oxidation of magmas in convergent margins

During the process of differentiation, the magmas in convergent margins undergo an increase of oxidized nature, accompanied by a decreased Fe content and concentration of heavy Fe isotopes. Garnet and amphibole are both Fe-rich minerals, which can be responsible for this phenomenon through fractional crystallization. One prevailing hypothesis suggests that Fe2+-rich garnet cumulates in the arc root as a "crustal redox filter." However, the stability of garnets is highly dependent on pressure conditions. In contrast, amphibole can crystallize under a broader range of temperature and pressure conditions and is a more common mineral phase in magmas. As such, the contribution of amphibole might have been underappreciated. Here, we conducted elemental composition, zircon trace element, and high-precision Fe isotope analyses on Miocene magmatic rocks from the Gangdese arc to trace the evolution of magmatic oxidation. The results indicate that the enrichment of heavy Fe isotopes in these magmas is primarily controlled by amphibole-dominated fractional crystallization rather than garnet. This also implies that amphibole fractional crystallization may play a role in enhancing the oxygen fugacity of the magmas. Taking a global perspective, we found a pervasive correlation between amphibole fractional crystallization and Fe isotope fractionation in magmatism at convergent plate margins, indicating its widely applicable influence on oxidation. The influence of garnet cannot be entirely neglected in some specific scenarios, such as within thickened continental arcs, but its impact is generally limited. Continuous amphibole fractional crystallization increases oxidation, facilitating the mobilization and concentration of Cu within the magma, thereby enhancing the potential for porphyry deposit formation. This impact is especially notable in spatiotemporally related magmatic events and could be decisive in determining the magmatic mineralization potential.

Zircon Zr[sbnd]Hf isotope disequilibrium during crustal anatexis: A record from the Mesozoic migmatite in the eastern Gangdese arc, South Tibet

Stable zirconium (Zr) isotopes have become the focus of attention as potential tracers of magma crystallization and differentiation processes. However, the effects of partial melting, especially disequilibrium melting, on Zr isotope fractionation remain unclear. Here we report in-situ zircon UPb ages, major and trace elements, and ZrHf isotope data for the migmatites in the eastern Gangdese arc, South Tibet. The leucosomes from the metatexite and diatexite migmatites are generally concordant with the foliation of the paleosome and display diffuse and gradational contacts with the melanosomes, indicating that they are in-source leucosomes. Zircon in the paleosome, the protolith of the migmatites, have UPb age of 361 ± 2 Ma and low 176Hf/177Hf ratios of 0.282399–0.282482. They exhibit intragrain variation in δ94/90Zr values, increasing from the core (−0.10 ‰ to +0.36 ‰) to the rim (+0.14 ‰ to +0.55 ‰) domains. The negative correlation between δ94/90Zr values and Zr/Hf ratios suggests preferential incorporation of light Zr isotopes during the crystallization of the zircon in the protolith. Most zircon in the leucosomes from the metatexite and diatexite migmatites exhibit distinct overgrowth rims around the inherited cores. The inherited cores have consistent UPb ages (ca. 360 Ma), trace elements, and Hf isotopic compositions (0.282362–0.282449) with zircon in the paleosome, indicating in situ partial melting. The inherited cores display δ94/90Zr values ranging from −0.12 ‰ to +0.31 ‰, consistent with the zircon core domains in the paleosome. The overgrowth rims have UPb ages of ca. 102 Ma and display higher 176Hf/177Hf ratios (0.282592–0.282900) and δ94/90Zr values (+0.01 ‰ to +0.63 ‰) than their inherited cores. The elevated 176Hf/177Hf for the overgrowth rims resulted from the retention of unradiogenic 177Hf in the residue due to limited zircon dissolution and more release of radiogenic 176Hf to the melts because of the garnet breakdown, indicating disequilibrium melting. The elevation of δ94/90Zr for the overgrowth rims relative to the inherited cores was not caused by the disequilibrium melting of other Zr-bearing minerals but was mainly controlled by the Zr isotopic composition and dissolution degree of the zircon in the protolith. The preferential melting of isotopically heavy zircon rims would elevate the δ94/90Zr values in the melts relative to their zircon-bearing protoliths. Crustal anatexis may be another important mechanism contributing to the Zr isotope fractionation during crustal differentiation.

Sedimentary Record of the middle Cretaceous uplift across the Gangdese magmatic arc system in Southern Tibet

AbstractSedimentary basins adjacent to subduction‐related continental arcs provide important archives for deciphering the intricate history of convergent plate margins. The east‐west trending Gangdese magmatic arc was one of the most predominant topographic features located at the southern margin of Tibet before the arrival of the Indian plate. However, the detailed Cretaceous growth and evolution across the arc system remains ambiguous. Stratigraphy of the adjacent Xigaze forearc basin provides a well‐preserved and well‐exposed record of the tectonic and magmatic evolution of the arc throughout the Cretaceous period. We report new stratigraphic, sedimentological, geochronological, and provenance analyses of the Quarry Ridge sandstone in the Xigaze forearc basin along with compiled zircon U‐Pb ages (n = 9674) and Lu‐Hf isotopic signatures (n = 3389) from the Gangdese arc, the Xigaze forearc basin, and the Linzhou retroarc foreland basin to reconstruct the Early to middle Cretaceous magmatism and uplift of the Gangdese arc and concurrent sedimentary responses within both basins. Exhumation of the arc initiates at around 113 Ma suggested by arc detritus first arriving in both basins. Another episode of inferred uplift occurs at around 108 Ma, which resulted in coarse‐grained sedimentation in adjacent basins, preventing Central Lhasa detritus from reaching the Xigaze forearc basin further south and a facies and provenance change within the Linzhou basin. Finally, a third episode at around 101 Ma is reflected by deposition of the progradational Quarry Ridge clastic succession and marks the initiation of a substantial coarse‐grained depositional stage in the Xigaze forearc basin. Our study emphasizes the connection between coarse‐grained deposition in the forearc basin and arc magmatism and uplift. This study also provides an orogen‐scale assessment of the history of arc magmatism, uplift, and sedimentation across the Gangdese magmatic arc system, which supports interpretations that Tibet was already characterized by complex and substantial topographic relief during the Cretaceous before the collision between the Indian and Eurasian plates.

Crustal evolution of a continental magmatic arc from subduction to collision: A case study in the Gangdese arc, southern Tibetan Plateau

Magmatic arcs are the main environment where continental crust is created on the post-Archean Earth; however, how juvenile arc crust evolves into mature continental crust is still controversial. In this study, we report new bulk-rock major and trace elements, Sr-Nd isotopes, and zircon U-Pb ages and Hf isotopes from a large suite of granites collected from the eastern segment of the Gangdese arc, southern Tibetan Plateau, which record a complete history of arc crust evolution from Mesozoic subduction to Cenozoic collision. These new data show that Gangdese crust-derived granites generated during the subduction to collisional stages record significant geochemical changes with age, indicating that the bulk composition, lithological makeup, and thicknesses of the arc crust evolved over time. Here, we propose that the Gangdese arc had a thick juvenile crust with a small volume of ancient crustal components during late-stage subduction of the Neo-Tethys Ocean, a thin juvenile crust with heterogeneously distributed ancient crustal materials during early collision, and a thick juvenile crust with minor proportions of ancient rocks during late collision. This implies that the arc experienced episodes of crustal thickening during the Late Cretaceous and Eocene, interspersed by periods of thinning during the Paleocene and Miocene, and several discrete episodes of partial melting in the lower arc crust, and cycling or recycling of juvenile and ancient crustal materials within the arc crust and between the crust and mantle. We suggest that shallow subduction of the Neo-Tethys during the Late Cretaceous promoted tectonic thickening of the arc crust, partial melting of lower crust, and formation of high Sr/Y granites. After the onset of the Indo-Asian collision, breakoff of the subducted Neo-Tethyan oceanic slab during the Paleocene/early Eocene allowed thinning of the overlying arc crust and generation of granites derived from juvenile and ancient crustal sources. Continued underthrusting of the Indian continental crust and subsequent delamination of thickened lithospheric mantle led to thickening and thinning of the arc crust, respectively, and partial melting of thickened lower crust and generation of high Sr/Y granites during the Oligocene and Miocene. Using the Gangdese as an analogue for post-Archean continental margins, we suggest that the repeated thickening and thinning of arc crust, and associated multistage remelting of the lower arc crust, and material cycling or recycling within the crust and between the crust and mantle from subduction to collision are common processes that drive maturation of juvenile arc crust.

The Timescale and Carbon Flux Recorded by Skarn Garnet From Gangdese Arc, Southern Tibet

AbstractContact metamorphism of carbonate rocks in response to fluid infiltration releases carbon dioxide (CO2), potentially affecting Earth's carbon budget over geological timescales. This process was notably active in the Cretaceous when the closure of the Neo‐Tethys Ocean led to extensive arc magmatism in southern Tibet, coincident with a greenhouse climate interval. These arc plutons intersected carbonate sequences and formed widespread calcsilicate rocks. The mineral assemblages of these rocks record both the progression and duration of the CO2 release. In this study, we focus on a representative aureole in southern Tibet that was metamorphosed by a ∼64 Ma granitoid pluton of the Gangdese Batholith. The aureole presents lithologic zonation, with the inner garnet‐wollastonite zone indicating equilibrium with a water‐rich fluid at ∼525°C. Large garnet porphyroblasts feature cross‐cutting veins of secondary garnets that illustrate several stages of interface‐coupled dissolution‐reprecipitation processes facilitated by fluid infiltration. Diffusion modeling for Mg concentration profiles across vein‐host garnet yields a brief high‐temperature stage (25–150 kyr)—a duration still significantly overestimated considering factors that may accelerate diffusion in garnet, such as lattice parameters and hydration. Such short timescale aligns with conduction modeling of a cooling pluton, which, combined with the bulk‐rock mass balance analysis, indicates a local carbon flux comparable to fluxes at other modern tectonic settings. The metamorphic decarbonation, potentially episodic, in the continental arc may have been an important endogenic carbon source in the late Cretaceous.

Early Cretaceous intrusive rocks in the Gangdese arc of southern Tibet do not support shallow subduction of the Neo-Tethyan oceanic lithosphere

The subduction geometry of the Neo-Tethyan oceanic lithosphere during the Early Cretaceous (i.e., shallow vs. steep subduction) remains in dispute due to the limited occurrence of the Early Cretaceous igneous rocks in the Gangdese arc. This study reports petrographical, zircon UPb geochronological, zircon Hf isotopic, whole-rock geochemical, and mineral geochemical data for the ultramafic-mafic and intermediate-felsic rocks in the eastern Gangdese arc, southern Tibet. All these rocks were emplaced at ca. 112–105 Ma (i.e. ca. 110 Ma). The ultramafic-mafic rocks (including hornblendite and hornblende gabbro) display field, petrographic, and geochemical features of cumulates and can be explained by the accumulation of hornblende + plagioclase from an andesitic parental magma. The intermediate-felsic rocks (including quartz diorites and granodiorites) show adakitic features and are most likely related to melts that experienced high-pressure hornblende fractionation followed by plagioclase accumulation. Hornblende thermobarometer reveals that these ultramafic-mafic and intermediate-felsic rocks formed at 837–907 °C and 6.6–11.1 kbar, indicating a rapid decompression condition that is most likely related to an extension setting. The data reported in this study and the literature reveal that the Early Cretaceous magmatism was active in the southern margin of the Gangdese arc, suggesting that the Neo-Tethyan oceanic lithosphere most likely experienced a steep, rather than a shallow subduction beneath the Lhasa Terrane during the Early Cretaceous.

Timing of India-Asia suturing: Evidence from a remnant peripheral foreland basin in Xigaze, South Tibet

The absence of a fully preserved foreland basin system in the central-eastern Yarlung-Zangpo Suture Zone (YZSZ) of Tibet has hindered the understanding of the initial timing of the India-Asia collision along strike. To address this issue, remnant foredeep and wedge-top deposits of the foreland basin system are described from the Tethyan Himalaya sequence and the YZSZ in the Xigaze area. The foredeep is defined by the olistostrome of the Zongzhuo Formation, which consists of matrix and blocks, including chert, limestone, sandstone and volcanic rocks. Based on the matrix properties, the Zongzhuo Formation is divided into two units: a lower member (Zhilong unit) with a matrix composed of black shale and thin layers of siltstone and an upper member (Yongla unit) with a matrix composed of distinctly red shale or yellow-green siliceous mudstone with interbedded siltstone and limestone. The Zongzhuo Formation is dated to 65–57 Ma by detrital zircon U - Pb chronology and radiolarian assemblages. In the YZSZ, the Puxia and Jiabi units are interpreted as wedge-top deposits consisting of mudstone matrix and exotic blocks, such as sandstone, chert and limestone. The maximum depositional ages (MDAs) of the sandstone blocks from both units are ca. 65–59 Ma. Detrital zircon provenance analysis and trace element data exclude an intraoceanic arc origin and point to a source from the Gangdese arc and recycled accretionary wedge in the suture zone. Therefore, we propose that the initial collision between India and Asia occurred at 65 Ma in the central-eastern YZSZ.

Exploring hindered decarbonation in contact metamorphism: A glimpse into marble aureoles in Southern Tibet

Fluid-driven contact metamorphism in the Gangdese Arc, resulting from Late Cretaceous intrusions, led to a diverse range of aureoles in carbonate sequences in southern Tibet. These include extensive carbon loss in calc-silicate rocks with minor calcite remnants and minimal carbon loss in predominantly pure marble formations. Many studies connecting contact metamorphism and decarbonation flux focus on metacarbonate aureoles comprising calc-silicate assemblages, while marble formations represent a less prominent endmember. To investigate the factors determining decarbonation extent, we analyzed the thermobarometric, geochemical, and isotopic signatures of two representative marble aureoles in southern Tibet. Our findings suggest that isobarically exsolved magmatic fluid from a neighboring ∼64 Ma granodiorite pluton led to limited fluid-rock interaction in the marble aureole. Based on the petrologic observations, we present a conceptual model to elucidate this process. The heating and fluid infiltration were brief, likely due to short-lived intrusion events. Consequently, a thin ductile layer was formed adjacent to the pluton, while the majority of the country rock remained brittle. The lithostatic fluid in the ductile layer likely escaped through structurally weak zones, creating veins along the boundary between the pluton and marble. The fluids traversed the brittle-ductile transition experienced a sudden pressure drop from lithostatic in the ductile rock to hydrostatic in the brittle regime. The boiling fluid escaped by brecciating the brittle country rock and resulted in brecciation and stockwork of veins. In both cases, most fluid escaped through pathways, preventing infiltration into the country rock and decarbonation reactions. In contrast, a longer-lived heat source would have produced more extensive ductile country rocks, allowing for widespread fluid infiltration and calc-silicate formation. Thus, the thermal history of the intrusion and the rheology of the country rock significantly influence the extent of decarbonation in contact aureoles.

Oligocene Orogen‐Parallel Extension in Southern Tibet During Indian Continental Subduction

AbstractThe controversial history of Indian subduction beneath Asia is crucial to understand the Himalayan orogeny and more in general the geodynamic process of continental subduction. New key information is here presented from the Oligocene‐Pliocene Shiquanhe Basin located in the southwestern Tibetan Plateau. The alluvial‐fan, lacustrine, and braided‐river sediments of the Oligocene Rigongla Formation were non‐conformably deposited onto the Upper Cretaceous Gangdese granitoid rocks and fed from erosion of the batholith itself and of associated Paleogene Linzizong volcanic rocks. Stratigraphic evidence testifies to the development of an orogen‐parallel intracontinental rift along the retro‐side of the Gangdese arc in the Oligocene, at the same time as the Kailas basin formed along the pro‐side of the Gangdese arc. The subsidence of these twin basins may have been caused by steepening of the subducting Indian continental slab or by the passage of a wave of dynamic topography during continuing subduction.

Fe–Mg Isotopes Trace the Mechanism of Crustal Recycling and Arc Magmatic Processes in the Neo‐Tethys Subduction Zone

AbstractThere has been intense debate on the mechanism of crustal recycling in subduction zones, and Fe–Mg isotopes may provide new constraints on this issue. This study reports Fe–Mg isotope data for mafic plutonic rocks from the eastern and central Gangdese arc, along with their associated trench sediments in southern Tibet. The δ26Mg values of the eastern Gangdese arc rocks show negative correlations with (87Sr/86Sr)i and (206Pb/204Pb)i values, but positive correlations with εNd(t) and εHf(t) values. Conversely, the δ56Fe values of the eastern Gangdese arc rocks show positive correlations with (87Sr/86Sr)i and (206Pb/204Pb)i values, but negative correlations with εNd(t) and εHf(t) values. The Mg and Fe isotopic compositions of the central Gangdese arc rocks are comparable with those of the eastern ones, but they do not correlate with Sr–Pb–Nd–Hf isotopic compositions. Notably, the Fe–Mg isotopic compositions of most arc rocks fall between those of local trench sediments and the mantle wedge. Combined qualitative analyses and quantitative simulations suggest that: (a) the Fe–Mg isotope variations observed in the eastern Gangdese arc rocks highlight the important role of source mixing between sediment‐derived melts and peridotite, whereas (b) the Fe–Mg isotope variations observed in the central Gangdese arc rocks reflect the superposition of carbonated serpentinite‐derived Mg‐rich fluids‐peridotite source mixing and source melting. The strong correlations between Fe–Mg isotope ratios and traditional geochemical tracers provide further evidence for the recycling of crustal materials in subduction zones via various types of slab‐derived fluids and melts.

Disclosing the most intense magmatic flare-up in southern Tibet: Insights from study of the Pangduo volcanic-plutonic complex

The Gangdese arc underwent the most intense magmatic flare-up of southern Tibet in the early Eocene (peaked at ∼50 Ma), generating widespread plutonic and volcanic rocks both along and across the arc. However, the dynamic mechanism for the magmatic flare-up is mysterious. This paper systematically analyzed the petrology, mineralogy, geochronology, and geochemistry of the early Eocene volcanic and intrusive rocks in the Pangduo basin in the eastern Gangdese arc. These rocks yielded identical zircon UPb ages of ∼50 Ma and similar isotopic compositions. The low-silica (< 60 wt%) rocks, including basalt andesite and trachyandesite, result from low degree (2–3%) partial melting of garnet-bearing lithospheric mantle. Based on textural observations and geochemical studies, coupled with Rhyolite-MELTS modeling and zircon trace elements Rayleigh fractionation models, the following research results were obtained. The high-silica rhyolite, ignimbrite, and granite porphyry are the product of limited melt extraction, while the quartz monzonite belongs to the associated residual silicic cumulates. The quartz monzonitic porphyry and trachydacite show a widespread dissolution structure and high formation temperature of ∼850 °C. Whole-rock geochemical mixing model results indicate that they are generated by mixing products of mafic magma, high-silica melts, and remelting melts of quartz monzonite. The volcanic and intrusive rocks in the Pangduo basin are considered as a volcanic-plutonic complex, resulting from the synergistic effect of melt extraction and cognate magma mixing. Both the complex magmatic processes and the early Eocene magmatic flare-up are attributed to mafic magma injection.

The India-Asia collision results from two possible pre-collisional crustal configurations of northern Greater India

Interpretations of pre-collisional configurations of Greater India are highly controversial and predict distinct processes during the India-Asia collision. To better determine the possible pre-collisional configuration(s) of Greater India, we conduct a mass-balance analysis combined with previously published geologic, paleomagnetic, and geodynamic evidence. The mass-balance analysis determines the magnitude of northern Greater India (NGI) width needed to provide sufficient crustal accretion to form the Tethyan-Greater Himalaya orogenic wedge in Cenozoic time. Applying endmember crustal thicknesses of 10–40 km to a mass-balance equation yields a broad range of plausible pre-collisional NGI widths of ∼3016±1000 km and ∼956±283 km, respectively, which we further assess considering contrasting models/evidence. The integrated evidence requires a thin NGI continental crust to form 1) continuous Tethyan-Greater Himalayan crustal thickening, 2) a narrow foredeep width of Himalayan foreland basin, 3) continuous Gangdese arc magmatism with oceanic-subduction-style mantle wedge, and 4) low-magnitude exhumation in the North Himalaya and Gangdese arc-forearc from ∼60-30 Ma. Adding the structurally restored ∼740 km wide southern Greater India, the synthesized analyses yield two possible configurations: 1) an ∼1350±440 km wide and ∼23-30 km thick NGI indicating an ∼2080±450 km wide Greater India with ∼500-1000 km wide oceanic basin systems in both Asia and NGI; and 2) a ≥1815±630 km wide and ∼10-23 km thick Zealandia-type NGI indicating a ≥2550±640 km wide pre-collisional Greater India without or with limited ∼500-1000 km Xigaze back-arc oceanic basin. The former is conditionally consistent with the integrated evidence by assuming no Cenozoic oceanic subduction initiation within NGI and predicts multi-stage collision since ∼60 Ma. The latter is consistent with the integrated evidence and predicts an approximate-single-stage collision at ∼60 Ma. Both configurations predict significant post-collisional NGI crustal shortening that may have been accommodated by the Eocene-Oligocene Greater Himalayan structural discontinuities.

Mobilization of Cu in the continental lower crust: A perspective from Cu isotopes

Recent studies have shown that Cu-rich sulfide accumulates in the lower continental crust and serves as a critical reservoir to balance Cu depletion in the upper crust. Recycling of Cu in the lower crust is also assumed to be a major metal source for non-arc setting porphyry Cu deposits. To test this hypothesis and further explore the behavior of Cu in the lower crust, we analyzed the elemental and Cu isotopic compositions of lower crustal rocks from different geological domains. The collected samples include hornblendites from the Kohistan arc, granulite xenoliths and hornblendites from the Gangdese arc, hornblendites and gabbros from the Laiyuan complex in the North China Craton, and hornblendite xenoliths from the western margin of the Yangtze Craton. These lower crustal rocks have experienced varying degrees of primary or secondary sulfide accumulation, with significantly varied Cu contents (11.2 to 145 ppm) and δ65Cu (−1.05‰ to 1.40‰). Petrography and geochemistry reveal varying degrees of metasomatism and fluid interaction in these rocks, and based on this, they can be further divided into three groups: Group I includes the Gangdese granulites and Yunnan hornblendites, which perhaps experienced significant metasomatism. This suite of rocks shows enrichment of 65Cu (δ65Cu = 0.01‰ to 1.40‰), positively correlated with metasomatism (δ65Cu vs. Ce/Pb). We suggest the secondary sulfides which transformed from sulfates during the interaction between lower crust and arc magma are dominant in these rocks, so the feature of heavy isotope enrichment is inherited. Group II includes Laiyuan hornblendites and gabbros, derived from the same parental magma and emplaced at different depths (hornblendites, 23.3 – 28.1 km; gabbros 8.4 – 11.1 km). The Cu isotopic compositions are strongly fractionated between these two kinds of rocks, with low δ65Cu in the hornblendites (0.00‰ to 0.28‰) and highly polarized δ65Cu in the gabbros (−1.05‰ to 0.81‰). Geochemical indicators and mineral assemblages reveal that fluid interaction is most likely responsible for this feature. Primary sulfides were decomposed by fluids and reprecipitated at shallower depths. Since this process involves multiple redox reactions, the Cu isotopic composition in the shallowed emplaced gabbros was large fractionated. Group III includes the Gangdese hornblendites and Kohistan hornblendites which show negligible impacts of subduction-like metasomatism and fluid interaction. The Gangdese hornblendites show a homogeneous and unfractionated Cu isotopic composition (−0.09‰ to 0.18‰) and Cu content (83.4 to 128 ppm), suggesting insignificant Cu migration and isotope fractionation. In contrast, the Cu isotopic composition of the Kohistan hornblendites is strongly fractionated (−0.36‰ to 1.27‰). Geochemistry and modeling results suggest partial melting plays a role in the Cu isotope fractionation. The light Cu isotopes are preferentially distributed into sulfide melts and removed from the source region during partial melting of the lower crust, resulting in a decrease in Cu content and enrichment of heavy Cu isotopes in residues. Results suggest that partial melting and fluid interaction are two efficient mechanisms that encourage Cu migration in the lower crust.

Late Cretaceous‐Palaeocene arc magmatism in Bumeicun, Gangdese, southern Tibet: Products of slab rollback of Neo‐Tethyan Ocean?

The most intense arc magmatism in the Gangdese arc occurred during the Palaeocene–Eocene, which is widespread across the entire southern Lhasa subterrane with Palaeocene–Eocene I‐type intrusive rocks and Linzizong volcanic successions. What drives the voluminous magmatism remains enigmatic. New geochronological and geochemical data on Late Cretaceous‐Palaeocene magmatic activity from Bumeicun, southern Tibet enables recognition of this distinct magmatic flare‐up event that we ascribe to Neo‐Tethyan Oceanic slab rollback. Bumeicun, located in the southern margin of Gangdese magmatic arc, exposes volcanic and plutonic rocks with close spatial–temporal links, and a wide range of silica content (53.0–74.7 wt.%). Zircon U–Pb dating result shows that the rocks form at 65.24 ± 0.77–67.65 ± 0.31 Ma. The Bumeicun igneous rocks are characterized by relative enrichment of large‐ion lithophile elements, depletion of high field‐strength elements (HFSEs) and enrichment of light rare earth elements, showing typical arc features. These rocks also show positive zircon εHf(t) value (+8.70 to+19.68), which are ascribed to the partial melting of arc mantle wedges. Followed by a succession of melt extraction and fractional crystallization, the coeval intermediate‐felsic volcanic and plutonic rocks in Bumeicun are formed. Previous researches note that Gangdese magmatic arc had a period of magmatic quiescence in the Late Cretaceous, and a subsequent burst of magmatism occurred in the P‐E. Our proposal of slab rollback model is consistent with the distinctly high zircon saturation temperatures (Tzr: 730–827°C) of Bumeicun magmatism. In addition, the slab rollback led to the Gangdese arc magmatism migration from north to south from the Cretaceous to the Eocene.

Using Zircons to Disentangle Back-Veining and Hybridization of Diorite Dykes: an Example From the Gangdese Arc, Tibet

AbstractThermo-chemical modeling and chemical data suggest that the genesis of arc melts commonly involves re-melting of older intrusions, triggered by the injection of mantle-derived melts, followed by magma mixing. Remelting and mixing may lead to complex zircon populations, which can be used to gain insights into the conditions of mixing. This paper investigates a case where such processes can be studied through the compositional and thermal record provided by zircons preserved in a diorite dyke swarm that remelted host rock tonalites in the Gangdese Batholith in Tibet. Autocrystic zircons from the diorite yield consistent ages of 46 to 47 Ma even though they can be altered, having highly enriched trace elements, reaching ~1 wt % in Fe, Ca, Y, U, Th, and anomalously high values of LREE and Ti. Granitic magmas derived from the tonalite, back-veined the dykes and mixed with the dioritic mush, transferring small quantities of 77–79 Ma zircon xenocrysts. The xenocrysts are euhedral with little evidence for resorption, indicating that they were apparently stable during the process of tonalite anatexis and transfer to the diorite magma. This requires that: (i) tonalite melting occurred at low temperatures with minimal zircon dissolution, and (ii) the diorite either cooled rapidly before significant resorption of the grains or was already saturated in zircon when mixing occurred. Zircon saturation temperatures of the diorite are relatively low, indicating that xenocrystic zircons were transferred to a highly crystalline dioritic mush. This requires either liquefaction by injection of the back-veining felsic magma to allow for mixing, or pervasive throughflow of the diorite mush by the felsic magma leaving behind zircon xenocrysts. The findings suggest that the dykes triggered low-T, water-fluxed melting of the host tonalite and that these anatectic melts invaded the diorite mush initially through the interstices leading to mixing. This may have caused the breakdown of the crystalline framework leading to liquefaction and renewed magma flow. Dyking and assimilation of wall-rock through back-veining as recorded in these rocks could be common in transcrustal arcs. However, this process could be hidden due to a combination of the similarity in the isotopic and chemical nature of arc rocks and resorption of zircon xenocrysts during mixing. This process may explain some complex chemistry of arc magmatic rocks and their minerals that are not easily explained by endmember models, such as pure re-melting of older arc rocks or fractional crystallization of mantle-derived melts.

A crystal mush origin for the high Sr/Y plutons: Insights from the Nyemo Igneous Complex in the Gangdese arc, southern Tibet

High Sr/Y plutons generated in continental arc resemble geochemically the Archean trondhjemite-tonalite-granodiorite suite and are widely used to interpret processes of intermediate-felsic crust generation during the Archean. However, this geochemical signature could be overinterpreted without detailed petrographic observation. Here we carry out a study of detailed field and petrographic observations, geochronology , and geochemistry for the Nyemo Igneous Complex (NIC) in the central Gangdese arc, southern Tibet . Our results show that the NIC is composed of gabbros , monzogabbros, monzodiorites, monzonites , and monzogranites that were formed at 91–89 Ma. Petrographic observations and geochemical data suggest that the diverse lithologies of the NIC can be ascribed to a cogenetic magma fractionation sequence defined by olivine → orthopyroxene + clinopyroxene + apatite + Fe-Ti oxides → plagioclase + biotite → K-feldspar → amphibole . The early crystallization of biotite and relatively late crystallization of amphibole indicates that the primary melts of the NIC have low water contents. The monzodiorites and monzogranites from the NIC have high Sr (412–1026 ppm), low Y (4.30–25.4 ppm), and thus high Sr/Y ratios (16–140). The orthocumulate texture defined by euhedral phenocrysts of normally zoned plagioclase and subhedral biotite indicates these high Sr/Y rocks are unlikely pure melts and instead represent fossil plagioclase-rich magma mush. We emphasize that the high Sr/Y signature does not necessarily reflect the pressures of magma generation involved of garnet/amphibole as residual or fractional phases. Late Cretaceous arc magmatic rocks in the eastern and central Gangdese arc show different crystallization sequences, requiring the presence of contemporaneous primary melts with distinct H 2 O contents along the Gangdese arc, which probably were caused by variable water input from the Neo-Tethyan oceanic lithosphere . • Nyemo Igneous Complex (NIC) in the Gangdese arc represents a fractionation sequence. • The primary melts of the Nyemo Igneous Complex have low water contents. • High Sr/Y plutons in the NIC represent fossil plagioclase-rich magma mush. • Arc magma evolution is controlled by the water input from the oceanic lithosphere.