Collision-related genesis of the Sharang porphyry molybdenum deposit,Tibet: Evidence from zircon U–Pb ages,Re–Os ages and Lu–Hf isotopes

Sharang is a low-fluorine, calc-alkaline porphyry Mo deposit hosted mainly in a granite porphyry of a multi-stage plutonic complex in the northern Gangdese metallogenic belt, largely with stockwork and ribbon-textured mineralization. The observed age estimates suggest that the formation of the magmatic host complex (52.9–51.6 Ma) and the ore deposit itself (52.3 Ma) occurred during the main stage of the India–Asia collision. The host rocks are characterized by lower zircon ε_Hf(t) values than those of the pre-ore and post-ore rocks. This suggests that the Lhasa terrane basement might play an important role in the formation of Sharang ore-forming intrusions. In view of the framework of magmatic–metallogenic events we suggest that slab roll-back may have induced melting of juvenile crust and ancient continental complexes during the India–Asia collision. This proposal focuses exploration for additional molybdenum deposits on the collision zone.     

Combined U-Pb,Lu-Hf,Sm-Nd and Ar-Ar multichronometric dating on the Bailang eclogite constrains the closure timing of the Paleo-Tethys Ocean in the Lhasa terrane,Tibet

The Lhasa terrane, the main tectonic component of the Himalayan–Tibetan orogen, has received much attention as it records the entire history of the orogeny. The occurrence of Permian to Triassic high-pressure eclogites has a significant bearing on the understanding of the Paleo-Tethys subduction and plate suturing processes in this area. An eclogite from the Bailang, eastern Lhasa terrane, was investigated with a combined metamorphic P–T and U–Pb, Lu–Hf, Sm–Nd and Ar–Ar multichronometric approach. Pseudosection modeling combined with thermobarometric calculations indicate that the Bailang eclogite equilibrated at peak P–T conditions of ~ 2.6 GPa and 465–503 °C, which is much lower than those of Sumdo and Jilang eclogites in this area. Garnet–whole rock–omphacite Lu–Hf and Sm–Nd ages of 238.1 ± 3.6 Ma and 230.0 ± 4.7 Ma were obtained on the same sample, which are largely consistent with the corresponding U–Pb age of 227.4 ± 6.4 Ma for the metamorphic zircons within uncertainty. The peak metamorphic temperature of the sample is lower than the Lu–Hf and Sm–Nd closure temperatures in garnet. This, combined with the core-to-rim decrease in Mn and HREE concentrations, the slightly U-shaped Sm zonation across garnet and the exclusive occurrence of omphacite inclusion in garnet rim, are consistent with the Lu–Hf system skewing to the age of the garnet core and the Sm–Nd system favoring the rim age. The Sm–Nd age was thus interpreted as the age of eclogite-facies metamorphism and the Lu–Hf age likely pre-dated the eclogite-facies metamorphism. ⁴⁰Ar/³⁹Ar dating of hornblende from the eclogite yielded ages about 200 Ma, which is interpreted as a cooling age and is probably indicative of the time of exhumation to the middle crust. The difference of peak eclogite-facies metamorphic conditions and the distinct metamorphic ages for the Bailang eclogite (~ 2.6 GPa and ~ 480 °C; ca. 230 Ma), the Sumdo eclogite (~ 3.4 GPa and ~ 650 °C; ca. 262 Ma) and Jiang eclogite (~ 3.6 GPa and ~ 750 °C; ca. 261 Ma) in the same (ultra)-high-pressure belt indicate that this region likely comprises different slices that had distinct P–T histories and underwent (U)HP metamorphism at different times. The initiation of the opening the Paleo-Tethys Ocean in the Lhasa terrane could trace back to the early Permian. The ultimate closure of the Paleo-Tethys Ocean in the Lhasa terrane was no earlier than ca. 230 Ma.     

Metallogeny of the northeastern Gangdese Pb–Zn–Ag–Fe–Mo–W polymetallic belt in the Lhasa terrane,southern Tibet

The northeastern Gangdese Pb–Zn–Ag–Fe–Mo–W polymetallic belt (NGPB), characterized by skarn and porphyry deposits, is one of the most important metallogenic belts in the Himalaya–Tibetan continental orogenic system. This belt extends for nearly four hundred kilometers along the Luobadui–Milashan Fault in the central Lhasa subterrane, and contains more than 10 large ore deposits with high potential for development. Three major types of mineralization system have been identified: skarn Fe systems, skarn/breccia Pb–Zn–Ag systems, and porphyry/skarn Mo–Cu–W systems. In this study, we conducted a whole-rock geochemical, U–Pb zircon geochronological, and in situ zircon Hf isotopic study of ore-forming rocks in the NGPB, specifically the Jiangga, Jiaduopule, and Rema skarn Fe deposits, and the Yaguila Pb–Zn–Ag deposit. Although some of these deposits (porphyry Mo systems) formed during the post-collisional stage (21–14 Ma), the majority (these three systems) developed during the main (‘soft collision’) stage of the India–Asia continental collision (65–50 Ma). The skarn Fe deposits are commonly associated with granodiorites, monzogranites, and granites, and formed between 65 and 50 Ma. The ore-forming intrusions of the Pb–Zn–Ag deposits are characterized by granite, quartz porphyry, and granite porphyry, which developed in the interval of 65–55 Ma. The ore-forming porphyries in the Sharang Mo deposit, formed at 53 Ma. The rocks from Fe deposits are metaluminous, and have relatively lower SiO₂, and higher CaO, MgO, FeO contents than the intrusions associated with Mo and Pb–Zn–Ag mineralization, while the Pb–Zn–Ag deposits are peraluminous, and have high SiO₂ and high total alkali concentrations. They all exhibit moderately fractionated REE patterns characterized by lower contents of heavy REE relative to light REE, and they are enriched in large-ion lithophile elements and relatively depleted in high-field-strength elements. Ore-forming granites from Fe deposits display ⁸⁷Sr/⁸⁶Sr_(i) = 0.7054–0.7074 and ε_Nd(t) = − 4.7 to + 1.3, whereas rocks from the Yaguila Pb–Zn–Ag deposit have ⁸⁷Sr/⁸⁶Sr_(i) = 0.7266–0.7281 and ε_Nd(t) = − 13.5 to − 13.3. In situ Lu–Hf isotopic analyses of zircons from Fe deposits show that ε_Hf(t) values range from − 7.3 to + 6.6, with T_DM(Hf)^C model ages of 712 to 1589 Ma, and Yaguila Pb–Zn–Ag deposit has ε_Hf(t) values from − 13.9 to − 1.3 with T_DM(Hf)^C model ages of 1216 to 2016 Ma. Combined with existing data from the Sharang Mo deposit, we conclude that the ore-forming intrusions associated with the skarn Fe and porphyry Mo deposits were derived from partial melting of metasomatized lithospheric mantle and rejuvenated lower crust beneath the central Lhasa subterrane, respectively. Melting of the ancient continental material was critical for the development of the Pb–Zn–Ag system. Therefore, it is likely that the source rocks play an important role in determining the metal endowment of intrusions formed during the initial stage of the India–Asia continental collision.     

Rapid Eocene erosion,sedimentation and burial in the eastern Himalayan syntaxis and its geodynamic significance

The lower Bomi Group of the eastern Himalayan syntaxis comprises a lithological package of sedimentary and igneous rocks that have been metamorphosed to upper amphibolite-facies conditions. The lower Bomi Group is bounded to the south by the Indus–Yarlung Suture and to the north by unmetamorphosed Paleozoic sediments of the Lhasa terrane. We report U–Pb zircon dating, geochemistry and petrography of gneiss, migmatite, mica schist and marble from the lower Bomi Group and explore their geological implications for the tectonic evolution of the eastern Himalaya. Zircons from the lower Bomi Group are composite. The inherited magmatic zircon cores display ²⁰⁶Pb/²³⁸U ages from ~ 74 Ma to ~ 41.5 Ma, indicating a probable source from the Gangdese magmatic arc. The metamorphic overgrowth zircons yielded ²⁰⁶Pb/²³⁸U ages ranging from ~ 38 Ma to ~ 23 Ma, that overlap the anatexis time (~ 37 Ma) recorded in the leucosome of the migmatites. Our data indicate that the lower Bomi Group do not represent Precambrian basement of the Lhasa terrane. Instead, the lower Bomi Group may represent sedimentary and igneous rocks of the residual forearc basin, similar to the Tsojiangding Group in the Xigaze area, derived from denudation of the hanging wall rocks during the India–Asia continental collision. We propose that following the Indian–Asian collision, the forearc basin was subducted, together with Himalayan lithologies from the Indian continental slab. The minimum age of detrital magmatic zircons from the supracrustal rocks is ~ 41.5 Ma and their metamorphism had happened at ~ 37 Ma. The short time interval (< 5 Ma) suggests that the tectonic processes associated with the eastern Himalayan syntaxis, encompassing uplift and erosion of the Gangdese terrane, followed by deposition, imbrication and subduction of the forearc basin, were extremely rapid during the Late Eocene.     

Metamorphism and tectonic evolution of the Lhasa terrane,Central Tibet

The Lhasa terrane in southern Tibet is composed of Precambrian crystalline basement, Paleozoic to Mesozoic sedimentary strata and Paleozoic to Cenozoic magmatic rocks. This terrane has long been accepted as the last crustal block to be accreted with Eurasia prior to its collision with the northward drifting Indian continent in the Cenozoic. Thus, the Lhasa terrane is the key for revealing the origin and evolutionary history of the Himalayan–Tibetan orogen. Although previous models on the tectonic development of the orogen have much evidence from the Lhasa terrane, the metamorphic history of this terrane was rarely considered. This paper provides an overview of the temporal and spatial characteristics of metamorphism in the Lhasa terrane based mostly on the recent results from our group, and evaluates the geodynamic settings and tectonic significance. The Lhasa terrane experienced multistage metamorphism, including the Neoproterozoic and Late Paleozoic HP metamorphism in the oceanic subduction realm, the Early Paleozoic and Early Mesozoic MP metamorphism in the continent–continent collisional zone, the Late Cretaceous HT/MP metamorphism in the mid-oceanic ridge subduction zone, and two stages of Cenozoic MP metamorphism in the thickened crust above the continental subduction zone. These metamorphic and associated magmatic events reveal that the Lhasa terrane experienced a complex tectonic evolution from the Neoproterozoic to Cenozoic. The main conclusions arising from our synthesis are as follows: (1) The Lhasa block consists of the North and South Lhasa terranes, separated by the Paleo-Tethys Ocean and the subsequent Late Paleozoic suture zone. (2) The crystalline basement of the North Lhasa terrane includes Neoproterozoic oceanic crustal rocks, representing probably the remnants of the Mozambique Ocean derived from the break-up of the Rodinia supercontinent. (3) The oceanic crustal basement of North Lhasa witnessed a Late Cryogenian (~ 650 Ma) HP metamorphism and an Early Paleozoic (~ 485 Ma) MP metamorphism in the subduction realm associated with the closure of the Mozambique Ocean and the final amalgamation of Eastern and Western Gondwana, suggesting that the North Lhasa terrane might have been partly derived from the northern segment of the East African Orogen. (4) The northern margin of Indian continent, including the North and South Lhasa, and Qiangtang terranes, experienced Early Paleozoic magmatism, indicating an Andean-type orogeny that resulted from the subduction of the Proto-Tethys Ocean after the final amalgamation of Gondwana. (5) The Lhasa and Qiangtang terranes witnessed Middle Paleozoic (~ 360 Ma) magmatism, suggesting an Andean-type orogeny derived from the subduction of the Paleo-Tethys Ocean. (6) The closure of Paleo-Tethys Ocean between the North and South Lhasa terranes and subsequent terrane collision resulted in the formation of Late Permian (~ 260 Ma) HP metamorphic belt and Triassic (220 Ma) MP metamorphic belt. (7) The South Lhasa terrane experienced Late Cretaceous (~ 90 Ma) Andean-type orogeny, characterized by the regional HT/MP metamorphism and coeval intrusion of the voluminous Gangdese batholith during the northward subduction of the Neo-Tethyan Ocean. (8) During the Early Cenozoic (55–45 Ma), the continent–continent collisional orogeny has led to the thickened crust of the South Lhasa terrane experiencing MP amphibolite-facies metamorphism and syn-collisional magmatism. (9) Following the continuous continent convergence, the South Lhasa terrane also experienced MP metamorphism during Late Eocene (40–30 Ma). (10) During Mesozoic and Cenozoic, two different stages of paired metamorphic belts were formed in the oceanic or continental subduction zones and the middle and lower crust of the hanging wall of the subduction zone. The tectonic imprints from the Lhasa terrane provide excellent examples for understanding metamorphic processes and geodynamics at convergent plate boundaries.     

Uplift and cooling magnetisation record in the Bamble and Telemark terranes,Sveconorwegian orogenic belt,SE Norway,and the Grenville-Sveconorwegian loop

The ~ 1100 Ma Sveconorwegian orogenic belt comprises allochthonous terranes juxtaposed by major fault zones and emplaced against, and onto, the south-western margin of the Fennoscandian Shield. To resolve the magnetic signature acquired during post-orogenic uplift and cooling and evaluate wider correlations with the contemporaneous Grenville belt of North America, this study reports a regional palaeomagnetic study on a range of rock types from sectors of the medium-high metamorphic grade Bamble terrane (48 sites and 390 cores) and the adjoining medium-low grade Telemark terrane (33 and 240 cores) juxtaposed by an orogen-parallel (Porsgrunn- Kristiansand) fault zone with major vertical displacement. Magnetite and ilmeno-hematites are magnetic carriers with the latter more significant in the higher metamorphic grades. Magnetic intensities are stronger in the higher-grade terrane presumably due to the growth of metamorphic ferromagnets, but are an order lower than values predicted for the lower continental crust and indicate that an additional mechanism is responsible for high magnetisations in deep crust. Anisotropy of magnetic susceptibility (AMS) largely reflects the NE–SW tectonic grain of the last stage of Sveconorwegian ductile deformation. The magnetisation record is filtered by excluding magnetisations possibly acquired during regional Mesozoic dyke emplacement, development of the Permo-Carboniferous Oslo Rift and Late Proterozoic magmatism. The remaining record is a dual polarity signature summarised by mean poles at 31.9°N, 50.9°E, (N = 191 components) in the Bamble terrane and at 34.2°N, 58.9°E (N = 151 components) in the Telemark terrane. However these means are non-Fisherian and embrace arcuate distributions of magnetic components acquired during protracted exhumation cooling of the orogen with the best-defined parts comprising clockwise trajectories correlating with each another but indicating that cooling in Telemark was more protracted; in each case directions of more shallow NW-direction tend to be derived from lower unblocking temperature components. The geochronological evidence indicates that regional temperatures had fallen to permit acquisition of magnetisation by ~ 950–900 Ma and the two swathes define the younger limb of a clockwise (Grenville-Sveconorwegian) APW loop embracing the approximate interval 940–850 Ma; the outward path of this loop (~ 1020–940 Ma) is probably at present recorded only in dyke swarms from the Finnish sector of the shield. Correlation of APW between Laurentia and Fennoscandia confirms that the two shields broke apart shortly after culmination of the Sveconorwegian orogeny when Fennoscandia rotated rapidly clockwise into a secondary configuration adjacent to the eastern margin of Laurentia; the Grenville and Sveconorwegian orogenic frontal zones formed in alignment were reoriented at a high angle to one another in a coupling that appears to have persisted during most of the remainder of Neoproterozoic times.     

Thermal evolution of the Hengshan extensional dome in central South China and its tectonic implications: New insights into low-angle detachment formation

The Hengshan massif is an exhumed, mid-crustal, plutonic–metamorphic dome formed during Cretaceous crustal extension in the Jiangnan orogenic belt, central South China. Multiple thermochronometers (mica ⁴⁰Ar/³⁹Ar, apatite fission track and zircon (U–Th)/He) are applied to its footwall along a slip-parallel transect to quantify its thermal history and cooling rate, and the slip magnitude, rate, initial geometry and kinematic evolution of the low-angle Hengshan detachment fault. Our thermochronological data, in conjunction with previous ages, indicate that (1) footwall rocks cooled from ~ 700 °C to ~ 60 °C in less than 60 Myr (136–80 Ma) at variable rates ranging from ~ 50 °C/Myr to ~ 13 °C/Myr, (2) the Hengshan detachment fault accommodated ~ 8–12 km of total slip at variable slip rates from 0.14 to 1 mm/yr during tectonic exhumation, (3) the footwall has been tilted ~ 26°–50° to the east since slip began, indicating that the low-angle Hengshan detachment fault initiated at a steep dip and was passively rotated to a more gentle orientation during subsequent normal slip. This study provides compelling evidence supporting that the low-angle detachment fault in the extensional dome can be generated by the reactivation and passive rotation of an initially steep reverse fault during normal slip. In addition, our thermochronological data constrain the time of extension in the Hengshan dome between 136 and 80 Ma, which implies that the back-arc extension within South China associated with the rollback of the Paleo-Pacific slab might have lasted until at least 80 Ma.     

Genesis of the giant Zijinshan epithermal Cu-Au and Luoboling porphyry Cu–Mo deposits in the Zijinshan ore district,Fujian Province,SE China: A multi-isotope and trace element investigation

The Zijinshan ore district occurs as one of the largest porphyry-epithermal Cu–Au–Mo ore systems in South China, including the giant Zijinshan epithermal Cu–Au deposit and the large Luoboling porphyry Cu–Mo deposit. The mineralization is intimately related to Late Mesozoic large-scale tectono-magmatic and hydrothermal events. The Cu–Au–Mo mineralization occurs around intermediate-felsic volcanic rocks and hypabyssal porphyry intrusions. In this study, we summarize previously available Re–Os isotopes, zircon U–Pb age and trace elements, and Sr–Nd–Pb isotope data, and present new Pb–S and Re–Os isotope data and zircon trace elements data for ore-related granitoids from the Zijinshan high-sulfidation epithermal Cu–Au deposit and the Luoboling porphyry Cu–Mo deposit, in an attempt to explore the relationship between the two ore systems for a better understanding of their geneses. The ore-bearing porphyritic dacite from the Zijinshan deposit shows a zircon U-Pb age of 108–106 Ma and has higher zircon Ce⁴⁺/Ce³⁺ ratios (92–1568, average 609) but lower Ti-in-zircon temperatures (588–753 °C, average 666 °C) when compared with the barren intrusions in the Zijinshan ore district. Relative to the Zijinshan porphyritic dacite, the ore-bearing granodiorite porphyry from the Luoboling deposit show a slightly younger zircon U–Pb age of 103 Ma, but has similar or even higher zircon Ce⁴⁺/Ce³⁺ ratios (213–2621, average 786) and similar Ti-in-zircon temperatures (595–752 °C, average 675 °C). These data suggest that the ore-bearing magmatic rocks crystallized from relatively oxidized and hydrous magmas. Combined with the high rhenium contents (78.6–451 ppm) of molybdenites, the Pb and S isotopic compositions of magmatic feldspars and sulfides suggest that the porphyry and ore-forming materials in the Luoboling Cu–Mo deposit mainly originated from an enriched mantle source. In contrast, the ore-bearing porphyritic dacite in the Zijinshan Cu–Au deposit might be derived from crustal materials mixing with the Cathaysia enriched mantle. The fact that the Zijinshan Cu–Au deposit and the Luoboling Cu–Mo deposit show different origin of ore-forming materials and slightly different metallogenic timing indicates that these two deposits may have been formed from two separate magmatic-hydrothermal systems. Crustal materials might provide the dominant Cu and Au in the Zijinshan epithermal deposit. Cu and Au show vertical zoning and different fertility because the gold transports at low oxygen fugacity and precipitates during the decreasing of temperature, pressure and changing of pH conditions. It is suggested that there is a large Cu–Mo potential for the deeper part of the Zijinshan epithermal Cu–Au deposit, where further deep drilling and exploration are encouraged.     

Thermal history of the giant Qulong Cu–Mo deposit,Gangdese metallogenic belt,Tibet: Constraints on magmatic–hydrothermal evolution and exhumation

A complete thermal history for the Qulong porphyry Cu–Mo deposit, Tibet is presented. Zircon U–Pb geochronology indicates that the mineralization at Qulong resulted from brecciation-veining events associated with the emplacement of a series of intermediate-felsic intrusions. Combined with previously published ages, our results reveal a whole intrusive history of the Qulong composite pluton. Causative porphyries were emplaced at ~ 16.0 Ma as revealed by ⁴⁰Ar–³⁹Ar dating of hydrothermal biotite (15.7 ± 0.2 Ma) and sericite (15.7 ± 0.2 Ma). Zircon and apatite (U–Th)/He (ZHe and AHe) dating of Qulong revealed that both followed similar, monotonic thermal trajectories from 900 °C (U–Pb ages: 17.5–15.9 Ma) to 200 °C (ZHe: 15.7–14.0 Ma), and that the causative porphyries experienced faster cooling at a maximum rate of greater than 200 °C/myr. The Qulong deposit was exhumed between 13.6 Ma and 12.4 Ma (AHe) at an estimated rate of 0.16–0.24 mm/y, which is consistent with previous estimates for other Gangdese Miocene porphyry deposits. Our AHe thermochronology results suggest that neither the Gangdese thrust system, nor the Yadong–Gulu graben affected or accelerated exhumation at the Qulong deposit.     

Early Mesozoic metamorphism and tectonic significance of the eastern segment of the Lhasa terrane,south Tibet

The metamorphic belt in the Basongco area, the eastern segment of Lhasa terrane, south Tibet, occurs as the tectonic blocks in Paleozoic sedimentary rocks. The Basongco metamorphic rocks are mainly composed of paragneiss and schist, with minor marble and orthogneiss, and considered previously to be the Precambrian basement of the Lhasa terrane. This study shows that the Basongco metamorphic belt experienced medium-pressure amphibolite-facies metamorphism under the conditions of T = 640–705 °C and P = 6.0–8.0 kbar. The inherited detrital zircon of the metasedimentary rocks yielded widely variable ²⁰⁶Pb/²³⁸U ages ranging from 3105 Ma to 500 Ma, with two main age populations at 1150 Ma and 580 Ma. The magmatic cores of zircons from the orthogneiss constrain the protolith age as ca. 203 Ma. The metamorphic zircons from all rocks yielded the consistent metamorphic ages of 192–204 Ma. The magmatic cores of zircons in the orthogneiss yielded old Hf model ages (T_DM2 = 1.5–2.1 Ga). The magmatic zircons from the mylonitized granite yielded a crystallization age of ca. 198 Ma. These results indicate that the high-grade metamorphic rocks from the Basongco area were formed at early Jurassic and associated with coeval magmatism derived from the thickening crust. The Basongco metamorphic belt, together with the western and coeval Sumdo and Nyainqentanglha metamorphic belts, formed a 400-km-long tectonic unit, indicating that the central segment of the Lhasa terrane experienced the late Paleozoic to early Mesozoic collisional orogeny.     

Duration of high-pressure metamorphism and cooling during the intraplate Petermann Orogeny

The Ediacaran–Cambrian Petermann Orogeny, central Australia, is an exceptional example of intraplate orogenesis. It involved sub-eclogite facies metamorphism and extreme basin inversion during the exhumation of Musgrave Province basement from beneath the formerly contiguous Centralian Superbasin. Sensitive High Resolution Ion Microprobe (SHRIMP) U–Pb geochronology of zircon, titanite and rutile, along with Ti-in-zircon thermometry from meta-igneous samples, have been used to determine the timing and duration of high-pressure metamorphism and subsequent cooling associated with this orogenic event. Peak metamorphic temperatures of 720–760 °C were attained at 544 ± 7 Ma (U–Pb zircon). Subsequent cooling to 600–660 °C by ~ 521 Ma occurred at a rate of ~ 2.6–7.0 °C Myr^− 1, as recorded by the closure of Pb diffusion in titanite. Further cooling to 585–560 °C by 498–472 Ma occurred at a rate of 0.9–4.8 °C Myr^− 1, as recorded by Pb closure in rutile. The duration of tectonism was long-lived (> 40 Myr) across the central and western parts of the orogenic system, and deformation occurred in a comparatively warm and weak portion of crust, characterised by regional thermal gradients of 17–26 °C km^− 1. This proposed duration of tectonism is much longer than that permitted by a shear heating mechanism, which requires an exceptionally short duration of tectonism, and additionally, an overall cold lithosphere characterised by geothermal gradients of ~ 9 °C km^− 1.     

Cenozoic volcanism and tectonic evolution of the Tibetan plateau

Cenozoic volcanism on the Tibetan plateau, which shows systematic variations in space and time, is the volcanic response to the India–Asia continental collision. The volcanism gradually changed from Na-rich + K-rich to potassic–ultrapotassic + adakitic compositions along with the India–Asia collision shifting from contact-collision (i.e. “soft collision” or “syn-collision”) to all-sided collision (i.e. “hard collision”). The sodium-rich and potasium-rich lavas with ages of 65–40 Ma distribute mainly in the Lhasa terrane of southern Tibet and subordinately in the Qiangtang terrane of central Tibet. The widespread potassic–ultrapotassic lavas and subordinate adakites were generated from ~ 45 to 26 Ma in the Qiangtang terrane of central Tibet. Subsequent post-collisional volcanism migrated southwards, producing ultrapotassic and adakitic lavas coevally between ~ 26 and 8 Ma in the Lhasa terrane. Then potassic and minor adakitic volcanism was renewed to the north and has become extensive and semicontinuous since ~ 20 Ma in the western Qiangtang and Songpan–Ganze terranes. Such spatial–temporal variations provide important constraints on the geodynamic processes that evolved at depth to form the Tibetan plateau. These processes involve roll-back and break-off of the subducted Neo-Tethyan slab followed by removal of the thickened Lhasa lithospheric root, and consequently northward underthrusting of the Indian lithosphere. The Tibetan plateau is suggested to have risen diachronously from south to north. Whereas the southern part of the plateau may have been created and maintained since the late-Oligocene, the northern plateau would have not attained its present-day elevation and size until the mid-Miocene when the lower part of the western Qiangtang and Songpan–Ganze lithospheres began to founder and detach owing to the persistently northward push of the underthrust Indian lithosphere.     

Geochronology and geochemistry of Late Cretaceous igneous intrusions and Mo–Cu–(W) mineralization in the southern Yidun Arc,SW China: Implications for metallogenesis and geodynamic setting

The Sanjiang Tethyan Metallogenic Domain (STMD) is an important part of the Tethyan giant metallogenic belt. The Yidun Arc is a part of the STMD in the eastern Tibetan Plateau. Recently, four newly discovered Mo–Cu–(W) ore deposits related to granitic intrusions were found distributed along the north-south strike in the southern Yidun Arc, which are identified as the Xiuwacu, Relin, Hongshan, and Tongchanggou deposits herein. These four deposits formed along high-angle north-northwest or north-west strike-slip faults, with vein-type and porphyry-type Mo–Cu mineralization developed in the intrusions. Molybdenite Re–Os and zircon U–Pb dating together with zircon Hf isotopes and whole-rock geochemistry of the intrusions were studied to discern the relationship between mineralization and magmatism, metallogenesis, and tectonic settings. Molybdenite from skarn-type mineralization at the Hongshan deposit has a Re–Os isochron age of 81.2 ± 2.6 Ma (MSWD = 1.3, n = 5) consistent with previously published zircon U–Pb ages and Re–Os ages of porphyry-type Mo mineralization. These results indicate that the Hongshan is a Late Cretaceous porphyry-skarn Cu–Mo deposit. Zircon U–Pb ages of the granitic intrusions in the Xiuwacu, Relin, and Tongchanggou deposits varying from ~ 87.4 Ma to ~ 82.7 Ma. Combined with published molybdenite Re–Os age spectrum (~ 85 Ma to ~ 81.2 Ma), it is proposed that the Mo–Cu–(W) mineralization in the Shangri-La region is spatially, temporally, and probably genetically related to the Late Cretaceous granitic intrusions. The Relin, Hongshan, and Tongchanggou intrusions have high SiO₂ (65.2–70.0 wt.%), Sr (363–905 ppm), Sr/Y (22–72), and La/Yb (37–69) ratios, and low Y (11.6–17.0 ppm) and Yb (0.97–1.59 ppm), which displayed adakitic affinities. Their low MgO (0.66–1.44 wt.%), Mg# (25–46), variable negative zircon ε_Hf(t) values (− 7.9 to − 2.3), and Proterozoic two-stages Hf model ages (T_DM2 = 1.13–1.62 Ga) suggest that they were probably dominantly derived from partial melting of thickened lower continental crust. According to the tectonic evolution of the Bangong Meso-Tethys Ocean during the Late Mesozoic, the Late Cretaceous igneous event and mineralization in the Yidun Arc likely formed under a late- or post-collision extensional environment, probably related to the collision between the Lhasa and Qiangtang terranes during the Late Cretaceous.     

Paleomagnetic data bearing on the Mesozoic deformation of the Qiangtang Block: Implications for the evolution of the Paleo- and Meso-Tethys

Paleomagnetism has played an important role in quantifying the Mesozoic evolution of “Proto-Tibet”. In this paper, we present new paleomagnetic data from five Middle-Upper Jurassic sedimentary sequences (Quemo Co, Buqu, Xiali, Suowa and Xueshan Fms.) of the eastern North Qiangtang Terrane (QT) at Yanshiping (33.6°N, 92.1°E). The new paleomagnetic results form a large dataset (99 sites, 1702 samples) and reveal a paleopole at 79.1°N/306.9°E (dp = 3.9°, dm = 6.3°) for the Quemo Co Fm., at 68.9°N/313.8°E (dp = 2.1°, dm = 3.7°) for the Buqu Fm., at 66.1°N/332.1°E (dp = 2.7°, dm = 4.6°) for the Xiali Fm., at 72.4°N/318.6°E (dp = 3.9°, dm = 6.7°) for the Suowa Fm., and at 76.9°N/301.1°E (dp = 7.9°, dm = 13.2°) for the Xueshan Fm. These results indicate clockwise (CW) rotations of ~ 19.8 ± 9.4° between ~ 171.2 and 161.7 Ma and counterclockwise (CCW) rotations of ~ 15.4 ± 13.4° between ~ 161.7 and < 157.2 Ma for Yanshiping. We attribute the change in rotation sense at approximately ~ 161.7 Ma to the initial collision of the Lhasa and Qiangtang terranes. Using this and other paleomagnetic data from the Lhasa, Qiangtang and Tarim terranes, as well as other geological evidence (e.g., tectonism-related sedimentary sequences, volcanism, and HP metamorphism), we propose a new conceptual evolution model for the Mesozoic QT and Tethyan Oceans. The Longmo Co-Shuanghu oceanic slab was subducted before 248 Ma, followed by continental collision of the North-South Qiangtang subterranes between ~ 245 and 237 Ma. The Qiangtang Terrane experienced post-collisional exhumation between ~ 237 and 230 Ma during subduction of the Jinsha oceanic slab. The collision of the Qiangtang and Songpan-Ganzi terranes occurred between ~ 230 and 225 Ma. The QT experienced post-collisional relaxation from ~ 225 to ~ 200 Ma, followed by subsidence and extension-related exhumation between ~ 200 and 162 Ma in association with subduction of the Bangong-Nujiang oceanic slab. Finally, these events were followed by the scissor-like diachronous collisions of the Lhasa and Qiangtang terranes between ~ 162 Ma and the mid-Cretaceous.     

Cenozoic thermo-tectonic evolution of the Gangdese batholith constrained by low-temperature thermochronology

Gangdese batholith in the southern Lhasa block is a key location for exploring the Tibetan Plateau uplift and exhumation history. We present the new low-temperature thermochronological data from two north–south traverses in the central Gangdese batholith to reveal their cooling histories and corresponding controls. Zircon fission track ages show prominent clusters ranging from 23.7 to 51.6 Ma, apatite fission track ages from 9.4 to 36.9 Ma, apatite (U–Th)/He ages between 9.5 and 12.3 Ma, and one zircon (U–Th)/He age around 77.8 Ma. These new data and thermal modeling, in combination with the regional geological data, suggest that the distinct parts of Gangdese batholith underwent different cooling histories resulted from various dynamic mechanisms. The Late Eocene–Early Oligocene exhumation of northern Gangdese batholith, coeval with the magmatic gap, might be triggered by crust thickening followed by the breakoff of Neotethyan slab, while this stage of exhumation in southern Gangdese batholith cannot be clearly elucidated probably because the most of plutonic rocks with the information of this cooling event were eroded away. Since then, the northern Gangdese batholith experienced a slow and stable exhumation, while the southern Gangdese batholith underwent two more stages of exhumation. The Late Oligocene–Early Miocene rapid cooling might be a response to denudation caused by the Gangdese Thrust or related to the regional uplift and exhumation in extensional background. By the early Miocene, the rapid exhumation was associated with localized river incision or intensification of Asian monsoon, or north–south normal fault.     

Mapping crustal structure beneath southern Tibet: Seismic evidence for continental crustal underthrusting

Receiver function imaging along a temporary seismic array (ANTILOPE-2) reveals detailed information of the underthrusting of the Indian crust in southern Tibet. The Moho dips northward from ~ 50 km to 80 km beneath the Himalaya terrane, and locally reaches ~ 85 km beneath the Indus–Yalung suture. It remains at ~ 80 km depth across the Lhasa terrane, and shallows to ~ 70 km depth under the Qiangtang terrane. An intra-crustal interface at ~ 60 km beneath the Lhasa terrane can be clearly followed southward through the Main Himalaya Thrust and connects the Main Boundary Thrust at the surface, which represents the border of the Indian crust that is underthrusting until south of the Bangong–Nujiang Suture. A mid-crustal low velocity zone is observed at depths of 14–30 km beneath the Lhasa and Himalaya terranes probably formed by partial melt and/or aqueous fluids.     

Geology and genesis of the post-collisional porphyry–skarn deposit at Bangpu,Tibet

Bangpu deposit in Tibet is a large but poorly studied Mo-rich (~ 0.089 wt.%), and Cu-poor (~ 0.32 wt.%) porphyry deposit that formed in a post-collisional tectonic setting. The deposit is located in the Gangdese porphyry copper belt (GPCB), and formed at the same time (~ 15.32 Ma) as other deposits within the belt (12 ~ 18 Ma), although it is located further to the north and has a different ore assemblage (Mo–Pb–Zn–Cu) compared to other porphyry deposits (Cu–Mo) in this belt. Two distinct mineralization events have been identified in the Bangpu deposit which are porphyry Mo–(Cu) and skarn Pb–Zn mineralization. Porphyry Mo–(Cu) mineralization in the deposit is generally associated with a mid-Miocene porphyritic monzogranite rock, whereas skarn Pb–Zn mineralization is hosted by lower Permian limestone–clastic sequences. Coprecipitated pyrite and sphalerite from the Bangpu skarn yield a Rb–Sr isochron age of 13.9 ± 0.9 Ma. In addition, the account of garnet decreases and the account of both calcite and other carbonate minerals increases with distance from the porphyritic monzogranite, suggesting that the two distinct phases of mineralization in this deposit are part of the same metallogenic event.Four main magmatic units are associated with the Bangpu deposit, namely a Paleogene biotite monzogranite, and Miocene porphyritic monzogranite, diabase, and fine-grained diorite units. These units have zircon U–Pb ages of 62.24 ± 0.32, 14.63 ± 0.25, 14.46 ± 0.38, and 13.24 ± 0.04 Ma, respectively. Zircons from porphyritic monzogranite yield ε_Hf(t) values of 2.2–8.7, with an average of 5.4, whereas the associated diabase has a similar ε_Hf(t) value averaging at 4.7. The geochemistry of the Miocene intrusions at Bangpu suggests that they were derived from different sources. The porphyritic monzogranite has relatively higher heavy rare earth element (HREE) concentrations than do other ore-bearing porphyries in the GPCB and plots closer to the amphibolite lithofacies field in Y–Zr/Sm and Y–Sm/Yb diagrams. The Bangpu diabase contains high contents of MgO (> 7.92 wt.%), FeOt (> 8.03 wt.%) but low K₂O (< 0.22 wt.%) contents and with little fractionation of the rare earth elements (REEs), yielding shallow slopes on chondrite-normalized variation diagrams. These data indicate that the mineralized porphyritic monzogranite was generated by partial melting of a thickened ancient lower crust with some mantle components, whereas the diabase intrusion was directly derived from melting of upwelling asthenospheric mantle. An ancient lower crustal source for ore-forming porphyritic monzogranite explains why the Bangpu deposit is Mo-rich and Cu-poor rather than the Cu–Mo association in other porphyry deposits in the GPCB because Mo is dominantly from the ancient crust.The Bangpu deposit has alteration zonation, ranging from an inner zone of biotite alteration through silicified and phyllic alteration zones to an outer propylitic alteration zone, similar to typical porphyry deposits. Some distinct differences are also present, for example, K-feldspar alteration at Bangpu is so dispersed that a distinct zone of K-feldspar alteration has not been identified. Hypogene mineralization at Bangpu is characterized by the early-stage precipitation of chalcopyrite during biotite alteration and the late-stage deposition of molybdenite during silicification. Fluid inclusion microthermometry indicates a change in ore-forming fluids from high-temperature (320 °C–550 °C) and high-salinity (17 wt.%–67.2 wt.%) fluids to low-temperature (213 °C–450 °C) and low-salinity (7.3 wt.%–11.6 wt.%) fluids. The deposit has lower δD_V-SMOW (− 107.1‰ to − 185.8‰) values compared with other porphyry deposits in the GPCB, suggesting that the Bangpu deposit formed in a shallower setting and is associated with a more open system than is the case for other deposits in this belt. Sulfides at Bangpu yield δ³⁴S_V-CDT values of − 2.3‰ to 0.3‰, indicative of mantle-derived S implying that coeval mantle-derived mafic magma (e.g., diabase) simultaneously supplied S and Cu to the porphyry system at Bangpu. In comparison, the Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb = 18.79–19.28, ²⁰⁷Pb/²⁰⁴Pb = 15.64–15.93, ²⁰⁸Pb/²⁰⁴Pb = 39.16–40.45) of sulfides show that other metals (e.g., Mo, Pb, Zn) were likely derived mainly from an ancient crustal source. Therefore, the formation of the Bangpu deposit can be explained by a two-stage model involving (1) the partial melting of an ancient lower crust triggered by invasion of asthenospheric mantle-derived mafic melts that provide heat and metal Cu and (2) the formation of the Bangpu porphyry Mo–Cu system, formed by magmatic differentiation in the overriding crust in a post-collisional setting.     

Preservation and exhumation history of the Harizha-Halongxiuma mining area in the East Kunlun Range,Northeastern Tibetan Plateau,China

The extent to which ore bodies are preserved in orogenic belts remains a poorly understood area of ore deposit research. Using zircon and apatite fission track analysis together with apatite (U-Th)/He dating we constrained the erosion history of the ore bodies in the Harizha–Halongxiuma mining area of the East Kunlun Range, Northern Tibetan Plateau, China. Apatite fission-track ages range from 114 ± 8 to 87 ± 6 Ma, with mean track lengths varying from 11.4 ± 1.9 to 12.9 ± 2.0 μm. Zircon fission-track ages vary from 205 ± 14 to 142 ± 7 Ma. In addition, apatite (U–Th)/He dating yielded ages of 60–56 Ma. The thermal history of Jiapigou was modelled based on the apatite fission-track data, including ages and track lengths, with constraints of zircon fission-track ages and (U-Th)/He ages. The exhumation history of the Harizha–Halongxiuma mining area was reconstructed with these age data, revealing that since the early Mesozoic the area has undergone three cooling stages: (1) rapid cooling from 175 ± 30 Ma to 100 ± 10 Ma with a cooling rate and inferred exhumation of 2.0 ± 0.8 °C/Myr and 4.3 ± 1.7 km, respectively; (2) a relatively stable stage from 100 ± 10 Ma to 40 ± 10 Ma with a cooling rate and inferred exhumation of 0.3 ± 0.1 °C/Myr and 0.5 ± 0.2 km, respectively; and (3) rapid cooling since 40 ± 10 Ma with a cooling rate and inferred exhumation of 1.2 ± 0.6 °C/Myr and 1.4 ± 0.4 km, respectively. This exhumation history is consistent with the subduction process of Pacific plate and the strike slip movements of Dunmi fault. The total exhumation after main mineralization is calculated to be 7.6 ± 3.2 km, suggesting that ore bodies in the Harizha–Halongxiuma mining area remain partially preserved.     

Geology,geochronology, and Hf isotope geochemistry of the Longtougang skarn and hydrothermal vein Cu–Zn deposit,North Wuyi area,southeastern China

The recently discovered Longtougang skarn and hydrothermal vein Cu–Zn deposit is located in the North Wuyi area, southeastern China. The intrusions in the ore district comprise several small porphyritic biotite monzonite, porphyritic monzonite, and porphyritic granite plutons and dikes. The mineralization is zoned from a lower zone of Cu-rich veins and Cu–Zn skarns to an upper zone of banded Zn–Pb mineralization in massive epidote altered rocks. The deposit is associated with skarn, potassic, epidote, greisen, siliceous, and carbonate alteration. Molybdenite from the Cu-rich veins yielded a Re–Os isochron age of 153.6 ± 3.9 Ma, which is consistent with U–Pb zircon ages of 154.0 ± 1.3 Ma for porphyritic monzonite, 154.0 ± 0.8 Ma for porphyritic biotite monzonite, and 152.0 ± 0.8 Ma for porphyritic granite. Geological observations suggest that the Cu mineralization is genetically related to the porphyritic biotite monzonite and porphyritic monzonite. All the zircons from intrusive rocks in the ore district are characterized by ε_Hf(t) values between − 13.41 and − 4.38 and Hf model ages (T_DM2) between 2054 and 1482 Ma, reflecting magmas derived mainly from a Proterozoic crustal source. Molybdenite grains from the deposit have Re values of 14.6–27.7 ppm, indicative of a mixed mantle–crust source. The porphyry–skarn abundant Cu and hydrothermal vein type Pb–Zn–Ag deposits in the North Wuyi area are related to the Late Jurassic porphyritic granites and Early Cretaceous volcanism, respectively. The Late Jurassic mineralization-related granites were derived from the crustal anatexis with some mantle input, which was triggered by asthenospheric upwelling induced by slab tearing during oblique subduction of the paleo-Pacific plate beneath the South China block, and the Early Cretaceous mineralization-related granitoids mainly from crust material formed within a series of NNE-trending basins during margin-parallel movement of the plate.     

Geology and genesis of the giant Beiya porphyry–skarn gold deposit,northwestern Yangtze Block,China

The Beiya ore deposit is located in the northwestern Yangtze Block, to the southeast of the Tibetan Plateau, SW China. The deposit is hosted by a porphyritic monzogranitic stock that is cross-cut by a porphyritic granite and later lamprophyre dikes. The whole-rock geochemistry of the porphyritic monzogranite and granite intrusions is both potassic and adakite-like, as evidenced by high K₂O/Na₂O (2.2 to 24.8), Sr/Y (53.2 to 143.2), and (La/Yb)_N (4.9 to 28.9) ratios. Both intrusions have comparable zircon U–Pb ages of ca. 36 Ma and εHf(t) values of − 6.8 to + 2.7. Zircons within these intrusions have Hf isotope crustal model ages with a prominent peak at ca. 840 Ma, and both of the intrusions have similar Sr–Nd–Pb isotopic compositions that are comparable to the compositions of amphibolite xenoliths hosted by potassic felsic intrusions in western Yunnan. The contemporaneous lamprophyre dikes show Nb–Ta depletion, enriched (⁸⁷Sr/⁸⁶Sr)_i and εNd(t), and extremely low Nb/U ratios (1.6–3.6), suggesting that these dikes were formed from magmas generated by partial melting of a metasomatized subcontinental lithospheric mantle (SCLM). The geochemistry of the porphyritic intrusions and the lamprophyre dikes suggests that the Beiya porphyries formed as a result of partial melting of a thickened and K-rich region of the lower crust, triggered by melting of metasomatized SCLM. The ca. 840 Ma U–Pb ages and εHf(t) values (− 6.8 to + 2.7) of xenocrystic zircons within the porphyritic intrusions suggest that these zircons were produced in a continental arc setting at ca. 840 Ma. The peak Hf model age of the zircons crystallized from the intrusions and the U–Pb ages of the xenocrystic zircons within the intrusions suggest that these porphyritic intrusions formed from magmas sourced from a juvenile crust that formed at ca. 840 Ma. This juvenile crust is most likely the source for the metals within the porphyry–skarn deposits in the study area, as the SCLM-derived lamprophyre dikes in this area are barren.Massive Fe–Au orebodies (~ 99 million metric tons at an average grade of 2.61 g/t Au) within the study area are generally located within the skarn-altered boundary of the porphyritic monzogranite stock and along the faults in the surrounding Triassic carbonates. The Fe–Au orebodies are spatially and genetically associated with skarn comprising garnet and diopside. Petrographic observations show that the massive Fe–Au orebodies mainly consist of hematite and magnetite with disseminated pyrite that hosts native gold and electrum.The porphyritic granite contains porphyry-style mineralization in the form of disseminated and veinlet-hosted pyrite and chalcopyrite. Pyrite-hosted lattice-bound gold is present within both the massive Fe–Au and the porphyry-type mineralization in the study area, and is present at concentrations up to 10 ppm Au (as determined by in situ LA-ICP-MS analysis). Subsequent weathering altered the primary magnetite–hematite–sulfide assemblage in the Fe–Au orebody into a magnetite–limonite assemblage, and generated laterite-type mineralization in which gold is hosted by limonite.