Western Gangdese Research Articles

Ore–forming potential of the Wubaduolai monzogranite porphyry, western Gangdese copper belt, Tibet: Insights from geochronology and petrogeochemistry

The Wubaduolai deposit is a newly discovered porphyry copper deposit in the Zhunuo-Beimulang ore-concentrated area of the Gangdese belt. The monzogranite porphyry is the causative intrusion in the deposit with a zircon U-Pb age of ∼16 Ma. Monzogranites have high Sr (332–673 ppm) and Sr/Y ratios (35–69), typical of adakite affinity. They have initial 87Sr/86Sr ratios of 0.7083–0.7093 and εNd(t) values between −9.2 and −7.1. The zircon εHf(t) values range from −6.45 to −2.09 and zircon δ18O values have a narrow range of 7.4 to 8.1 ‰ for the porphyry. These geochemical characteristics indicate that the parental magma was derived from juvenile Tibetan lower crust, with contributions from Indian plate-released fluids and metasomatized mantle-derived mafic magmas. The porphyry shows high values of log w(Fe2O3/FeO) (>0) and V/Sc (mostly > 12) in whole rock and high values of △FMQ (4.13–5.79), 10000 × (δEu)/Y(>4), and low Dy/Yb (<0.3) in zircons, suggesting that the magma was oxidized and hydrous. The oxidized and hydrous parental magma is consistent with those evaluated for the Zhunuo and Beimulang deposits. The three-component mixing model result shows that compared with the ore-related porphyries in the Zhunuo and Beimulang deposits, the Wubaduolai intrusion has higher proportion of Indian plate-released fluids and mafic magmas and lower proportion of juvenile Tibetan lower crust. Therefore, we suggest that although the causative intrusions have similar oxygen fugacity and water content, the source has more Indian continental crust and mantle-derived materials and fewer juvenile Tibetan lower crust materials at Wubaduolai, implying that the Wubaduolai monzogranite porphyry has lower potential for mineralization.

Sluggish rise of the western Gangdese mountains after India-Eurasia collision

With the most prominent topography on Earth, the Tibetan Plateau has profound influences on the hydrologic cycle and climate dynamics in Asia. However, the regional uplift history of the Tibetan Plateau remains highly debated. Here, we use europium anomalies and LREE/HREE ratio in detrital zircon, combined with whole-rock La/YbN ratio, to constrain the crustal thickness and topographic evolution of the Gangdese mountains in southern Tibet. Europium anomaly, controlled by pressure-sensitive minerals plagioclase and garnet, has been shown to correlate with crustal thickness. Our results reveal contrasting crustal thickening histories of the eastern (east of ∼88°E) and western (west of ∼88°E) parts of the Gangdese. Prior to the India-Eurasia collision (∼60–55 Ma), the crust of the eastern Gangdese thickened continuously from ∼40 km to nearly 60 km, while the western Gangdese maintained a mildly thickened crust of ∼50 km. Both parts underwent substantial thinning (to 40–45 km) immediately before the India-Eurasia collision, but the eastern Gangdese re-thickened rapidly afterwards. In the western Gangdese, post-collisional thickening was delayed until ∼40–20 Ma, which resulted in a mild elevation of ∼2000 m for most areas of southern Tibet until the late Oligocene. This diachronous uplift of the Gangdese after collision may have resulted from more mantle-derived magma additions in the eastern Gangdese before collision, which could rheologically weaken its lithosphere. We suggest that the diachronous uplift of the Gangdese after the India-Eurasia collision may have dominated the moisture transfer pattern across southern Tibet through the late Eocene and intensified the South Asian monsoon through the Oligocene-Miocene boundary (∼25–20 Ma).

Late Oligocene to Pleistocene thermo-tectonic evolution of the Karakoram Fault Zone: New insights from basement and detrital apatite fission track thermochronology

The Karakoram Fault Zone displays distinctive topographic and tectonic characteristics, with intricate river drainage systems. Although there are extensive thermochronological datasets for the western Tibetan Plateau, limited investigations have been carried out to uncover the low-temperature thermal history of the Karakoram Fault Zone, which connects the Lhasa terrane with the Pamir. In this contribution, apatite fission track (AFT) thermochronology was conducted on 13 basement and 4 conglomerate samples collected from the western Gangdese batholith and the Ayi Mountain Range, on either side of the Karakoram fault. Inverse thermal history modeling results reveal that the western Gangdese batholith experienced two stages of rapid basement cooling at 27–23 Ma (late Oligocene-early Miocene) and 16–4 Ma (middle Miocene-Pliocene). The adjacent Ayi Mountain Range experienced cooling in the late Miocene (∼8 Ma) and during the Pliocene to Pleistocene (4–1 Ma). We suggest that tectonic forces associated with rollback and lateral break-off of the Indian continental lithosphere, and the dextral strike-slip activity of the Karakoram fault were responsible for the first cooling phase. The second accelerated cooling phase was due to the deep fragmentation of the Indian slab, the activity of the Karakoram normal fault, and associated ∼E-W extension. Additionally, since the late Oligocene, the Indus River incision has likely also contributed to the exhumation of the western Tibetan Plateau. Subsequently, the western Gangdese batholith reached high elevations, resulting in the reversal of the Yarlung River. The dextral-normal motion of the Karakoram fault, which is still active today, was the main cause of the Pliocene to Pleistocene accelerated cooling detected in the Ayi Mountain Range.

Exhumation and preservation of the Bangbule Pb-Zn-Cu deposit in the western Gangdese metallogenic Belt, Tibet: Constraints from fission track thermochronology and fault gouge dating

The Bangbule Pb-Zn-Cu deposit is a large-scale skarn deposit that was recently discovered in the western Gangdese Metallogenic Belt. Previous studies have focused mainly on ore genesis and ore-forming fluid, while exhumation and preservation after mineralization have not been the focus. We report fission track ages (ZFT: 43 ± 3 to 52 ± 3 Ma; AFT: 24 ± 2 to 32 ± 3 Ma, MTL: 12.2 ± 1.9 to 13.0 ± 1.6 μm) and the timing of fault activation (44.3 ± 0.9 Ma), revealing the processes of cooling/exhumation and we evaluate the preservation of the deposit. The Bangbule deposit experienced two rapid exhumation events since the Eocene (52–24 Ma and 12 Ma to present) and a slower stage during the late Oligocene to middle Miocene (24–12 Ma). The exhumation thickness (1.6–3.1 km) after mineralization was similar to the metallogenic depth (peak: 1.5–2.5 km). The volcanic rocks of the Dianzhong Formation partially protect the deposit, making mineral exploration of the Bangbule deposit feasible. Since the Oligocene, due to climate change and deep subducted slab tearing, the exhumation thickness in the western Gangdese (∼0.6–3.1 km) has generally been lower than that in the middle-eastern part (∼3–8 km). The lower degree of exhumation may be one of the reasons why less large-scale deposits have been discovered in the western Gangdese.

Deciphering differential exhumation in the Gangdese orogen in southern Tibet using exposed porphyry alteration systems and geomorphic analysis

Topography and exhumation vary strongly in the Tibetan Plateau, reflecting different geodynamics and surface processes. The Gangdese orogen in southern Tibet is characterized by west-to-east variations in topography and occurrences of the Miocene porphyry copper deposits. We integrated remote sensing based hydrothermal alteration mapping and geomorphological analysis to detect patterns of differential exhumation since the Miocene. High abundances of propylitic and potassic alteration of the eastern Miocene granitoids are identified, indicating &amp;gt;2 km of exhumation since the Miocene. The western Gangdese has intensive phyllic and argillic alteration representing ∼1 km of exhumation. Geomorphic analyses also show E-W changes that most likely resulted from differential rock uplift histories. Channel steepness index values indicate the eastern Gangdese has channel gradients between 211 m1.02 and 457 m1.02, compared to values between 30 m1.02 and 160 m1.02 in the west. Linear inversion of tributaries of the Yarlung river shows a pulse of rock uplift between 25 Ma and 7 Ma with a maximum rate of 110 m/ m.y. in the east. In contrast, the western Gangdese exhibits slower and persistent rock uplift rates between 20 m/m.y. and 40 m/m.y. since 20 Ma. The integrated results record differential exhumation and rock uplift since the Miocene. We invoke the geometric variations of the subducted Indian lithosphere to interpret the E-W differential exhumation since the Miocene. Steeper subduction of the Indian lithosphere beneath the eastern Gangdese induced mantle upwelling, triggering rapid rock uplift and deep exhumation. Conversely, the shallower subduction of the Indian slab in the west generated slow and prolonged rock uplift with less exhumation. Our study refines the exhumation process of the Gangdese since the Miocene.

Geochronology and petrogeochemistry of Miocene porphyries from the Beimulang deposit, western Gangdese copper belt

The Beimulang deposit, a newly discovered large porphyry copper deposit, containing 1.3 Mt @ 0.38% Cu, prominently features mineralized orebodies and hydrothermal alteration in the monzogranite porphyry. Zircon U-Pb dating reveals ages of 14.4–14.0 Ma for the monzogranite porphyry and 11.2–11.0 Ma for the granite porphyry. A consistent Re-Os age of 14.1 ± 0.4 Ma from molybdenite suggests a possible link between mineralization and the monzogranite porphyry. The Beimulang Miocene porphyries are rich in SiO2 (70.01–72.81 wt%) and Sr (217–864 ppm) and have high Sr/Y ratios (26–139), which were originated from partial melting of the subduction-modified Tibetan juvenile lower crust. The Beimulang Miocene porphyries show high log w(Fe2O3/FeO) values (>0) in whole rock and high δEu values (mostly > 0.3), high ΔFMQ (4.32–6.43) in zircons, suggesting a high oxidation state. The Beimulang Miocene porphyries show high V/Sc ratios in whole rock and high 10,000 × (δEu)/Y, high (Ce/Nd)/Y, low Dy/Yb ratios (i.e., mostly > 13, > 4, > 0.02 and < 0.3, respectively) in zircons, indicating they are notably hydrated. The granite porphyry has lower εNd (t) (0.70849 – 0.70996) values and higher (87Sr/86Sr)i (-9.3 – -7.8) in whole rock, lower εHf (t) (-5.25 – -2.70) and higher U/Yb (6.0–18.3), δ18O (6.42–7.57‰) in zircon than the monzogranite porphyry (εNd(t) = -5.52 – -4.45; (87Sr/86Sr)i = 0.70744–0.70818; εHf (t) = -3.24 – -0.92; U/Yb = 2.62–8.96; δ18O = 6.10–7.53‰), indicating that the granite porphyry contains fewer juvenile materials compared to the monzogranite porphyry. The high-Mg dioritic rocks in the Beimulang-Zhunuo area reveal significant correlations with monzogranite porphyry rather than granite porphyry in terms of Sr, Sr/Y, Dy/Yb, δEu and SiO2 contents. Taking into account the contemporaneous presence of high-Mg dioritic rocks with the monzogranite porphyry, which are not contemporaneous with the granite porphyry at Beimulang, it can be inferred that these diorite magmas played a significant role in forming the Miocene monzogranite porphyry rather than granite porphyry. Accordingly, the low fertility of the granite porphyry is possibly due to the limited contribution of juvenile melts and lack of the dioritic magmas.

Petrogenesis and tectonic environment of the Middle Permian Luerma igneous complex in the southern Lhasa subterrane, Tibet: Evidence from geochemistry; Rb–Sr, Sm–Nd, and Lu–Hf isotopes; and zircon U–Pb geochronology

Due to the lack of corresponding magmatic records, the timing of the opening of the Yarlung–Tsangpo Neo‐Tethyan Ocean remains controversial. The Middle Permian Luerma igneous complex, which is situated in the western Gangdese magmatic belt of the southern Lhasa subterrane, western Tibet, can be divided into four or more rock types, including foid gabbro, amphibole gabbro, monzogabbro, and pyroxene‐bearing monzodiorite. Through zircon U–Pb dating using laser ablation–inductively coupled plasma–mass spectrometry, the age of the Luerma igneous complex is determined to be ~261 Ma. The complex is rich in titanium, iron, magnesium, and total alkalis, with medium to high levels of calcium, potassium and aluminium, as well as low levels of silicon. Therefore, it is classified as a shoshonite–latite rock series. The complex is relatively enriched in light rare earth elements and high‐field‐strength elements, without obvious Eu and Ce anomalies. These rocks exhibit relatively low (87Sr/86Sr)i values, high εNd(t) values, and high εHf(t) values. The geochemical and isotopic characteristics indicate that the magmas are derived from partial melting of the upper mantle in an intraplate tectonic environment. The new data presented here provide supplementary evidence for the initial rifting of the Lhasa terrane from the northern margin of Gondwana during the Middle Permian. These data also confirm that the opening of the Yarlung–Tsangpo Neo‐Tethyan Ocean occurred in the Middle Permian.

Subduction-related Late Triassic Luerma porphyry copper deposit, western Gangdese, Tibet, China: Evidence from geology, geochemistry, and geochronology

Due to the difference of lithospheric structure between the eastern and western segment of Gangdese metallogenic belt, it is generally believed that porphyry copper deposits are mainly developed in the east. However, the newly discovered of Luerma porphyry copper deposit in the western segment of Gangdese provides an excellent research object for us to supplement this understanding. The molybdenite re-Os isotope dating of two samples from quartz veinlet ores (ca. 213.1 ± 5.2 Ma and ca. 212.0 ± 3.7 Ma), combined with the LA-ICP-MS zircon U-Pb dating of three ore-bearing quartz monzonite porphyry samples (ca. 212.7 ± 0.9 Ma, 213.1 ± 1.1 Ma, and ca. 211.8 ± 1.8 Ma), indicates a new generation of Late Triassic Luerma copper porphyry-type mineralization events in the western Gangdese magmatic belt. The Luerma porphyry is a strongly peraluminous shoshonite series rock, is rich in silicon total alkalis, potassium and aluminum, has low contents of calcium, magnesium, and iron. It is enriched in light rare earth elements and large ion lithophile elements, relatively depleted in heavy rare earth elements and high field strength elements, and has negative Eu anomalies and no Ce anomalies. The porphyry exhibits relatively low (87Sr/86Sr)i values (0.705617 on average), high εNd(t) values (0.63 on average), high εHf(t) values (11.98 on average), and young two-stage Hf model ages. The Luerma ore-related porphyries are most likely generated dominantly from partial melting of juvenile crust that formed by metasomatized mantle during the subduction of the NeoTethys Ocean to the Lhasa terrane and underwent crystal fractionation during the process. The deposit is currently the oldest and westernmost porphyry deposit in the Gangdese porphyry copper belt, providing evidence that porphyry copper mineralization in this belt began in at least the Late Triassic.

Identifying potential porphyry copper mineralization at the Zhu’nuo ore-cluster district in western Gangdese, southern Tibet: Insights from shortwave infrared (SWIR) spectrometry and geochemical anomalies

The Zhu’nuo ore-cluster district in the western Gangdese belt, southern Tibet, is characterized by a significant Cu geochemical anomaly, where the Zhu’nuo super-large porphyry Cu deposit (PCD) was discovered by stream sediment geochemical method. However, due to the extended coverage of volcanic rocks of the Linzizong group and the large area of ore-forming element anomalies, it is quite difficult to locate the concealed PCDs only by geochemical method. Therefore, the application of new technical methods is urgently needed to guide the exploration breakthroughs in the Zhu’nuo district. Recently, our studies have shown that the wavelength variation of diagnostic absorption peaks of alteration minerals measured by shortwave infrared (SWIR) spectrometry is closely related to the mineralization temperature in the Zhu’nuo PCD, which can be footprints to trace the hydrothermal and/or mineralization (H/M) centers of porphyry ore systems. Hence, we carried out SWIR studies in the Luobuzhen-Cimabanshuo areas with high Cu geochemical anomaly. In this study, sixteen alteration minerals were identified by SWIR, consisting mainly of white mica, chlorite, epidote, and kaolinite. Among them, the white mica and chlorite were widely distributed in the study area, and the Al-OH wavelength position of white mica (∼2200 nm; Pos2200) and Fe-OH wavelength position of chlorite (∼2250 nm; Pos2250) varied between 2191–2219 nm and 2239–2260 nm, respectively. Moreover, there is an increasing trend of Pos2200 from porphyry center to periphery, whereas Pos2250 shows the opposite tendency. When combined with geochemical anomalies of stream sediments and geological characteristics and SWIR features in the Zhu’nuo PCD in study area, three potential H/M centers, namely Cimabanshuo, South Luobuzhen and Ya’nuri, were determined, which are dominated by short wave anomalies (Pos2200, 2191–2206 nm; Pos2350, 2243–2250 nm). Significantly, we showed that the Dongshibu-Hongshan-Luobuzhen may belong to the same porphyry system by SWIR. And SWIR implies that the true center is located in South Luobuzhen (H/M II), which has great prospecting potential in its depth, rather than the previously recognized Dongshibu Cu prospect delineated by stream sediment data. We found that the Dongshibu Cu prospect does not show any alteration information associated with porphyry mineralization as reflected by SWIR, consistent with the barren Cu results from the late drilling validation. This decoupling may be caused by the long-term migration and weathering of stream sediment samples, during which the obtained geochemical anomaly do not represent their in-situ information. Therefore, caution must be taken when using stream sediment data to delineate prospecting targets. Instead, SWIR is a potentially powerful vectoring tool to aid exploration for porphyry systems, especially in the western regions where the level of prospecting work is extremely low.

Late Cretaceous post-orogenic delamination in the western Gangdese arc: Evidence from geochronology, petrology, geochemistry, and Sr–Nd–Hf isotopes of intermediate–acidic igneous rocks

Late Mesozoic uplift of the Gangdese arc occurred prior to the India–Asia collision and was important in the evolution of the Tibetan plateau, but the geodynamic mechanisms that drove this process are unclear. In this paper, we report geochronological, petrological, geochemical, and Sr–Nd–Hf isotopic data for intermediate–acidic igneous rocks from the western Gangdese arc. These igneous rocks were emplaced at 87–67 Ma and are characterized by enrichments in light rare earth and large-ion lithophile elements, depletions in high-field-strength elements, high ( 87 Sr/ 86 Sr) i ratios (0.7074–0.7116), and low ε Nd (t) (−7.5 to −4.1) and ε Hf (t) values (−16 to 0.8). These magmas were most likely derived by high-temperature (960–1200 °C) partial melting of the thickened crustal materials, with minor mantle input, related to post-orogenic lithospheric delamination. The increasing ε Nd (t) and ε Hf (t) values and decreasing initial Sr isotope ratios from the ca. 87 Ma diorites to ca. 67 Ma diorites and rhyolites indicate an increasing mantle contribution as delamination proceeded. Estimated pressure conditions for the high-Sr/Y gabbroic diorites indicates the magmas were derived from crust at pressures of up to 19 kbar (i.e., a depth of ~66 km). Therefore, the western Gangdese arc had been thickened during the Late Cretaceous, possibly due to the flat-slab subduction of Neo-Tethyan oceanic lithosphere and coeval collision of the Qiangtang–Lhasa terranes. • Intermediate–acidic rocks in the western Gangdese arc emplaced from 87 to 67 Ma. • Geochemical indices show they formed dominantly by melting of crustal material. • They formed at 960–1200 °C by melting of thickened crust with minor mantle input. • Nd-Hf-Sr isotopes indicate more mantle involvement following ongoing delamination. • Western Gangdese arc has been thickened during the Late Cretaceous.

Petrogenesis of the Late Jurassic Granodiorite and Its Implications for Tectonomagmatic Evolution in the Nuocang District, Western Gangdeses

The Gangdese magmatic rocks of the southern Lhasa terrane, are generally thought to be an important window to witness the formation and evolution of the Neo-Tethys oceanic opening, subduction, and closure, and India-Eurasian continental collision. We investigated a new occurrence of granodiorite in the Nuocang district of western Gangdese, southern Lhasa terrane, and conducted a series of analyses on their petrology, chronology, and geochemistry. The Nuocang granodiorites have the zircon U-Pb ages of 151–154 Ma, which suggest that Late Jurassic granitoids are present in the western Gangdese of southern Lhasa terrane. They are relatively high in SiO2, Al2O3, low K2O, Na2O, and Sr/Y ratios, enrichments of LILE and LREE, and depletion of HFSE, with the positive correlation between Rb and Th, and negative correlations between SiO2 and P2O5, Rb, and Y, showing the features of I-type granites. The relatively high (87Sr/86Sr)i values from 0.712231 to 0.712619, low εNd(t) values from −9.56 to −8.99, together with the negative εHf(t) values from −10.8 to −5.0 (mean value −8.9) suggested that the Nuocang granodiorites probably sourced from the partial melting of the ancient Lhasa terrane, with parts of mantle materials involving in. Combined with the previous geochronology and geochemical data of Mesozoic magmas in the Gangdese belt, as well as the Late Jurassic granodiorite, in this paper, we propose that the Nuocang granodiorites formed in a continental margin arc environment triggered by the northward subduction of Neo−Tethys oceanic crust.

Petrogenesis of Eocene Wangdui adakitic pluton in the western Gangdese belt, southern Tibet: implications for crustal thickening

AbstractLower-crust-derived adakitic rocks in the Gangdese belt provide important constraints on the timing of Tibetan crustal thickening and on the relative contributions of magmatic and tectonic processes. Here we present geochronological and geochemical data for the Wangdui porphyritic monzogranites in the western Gangdese belt. Zircon U–Pb dating yields emplacement ages of 46–44 Ma. All samples have high Sr (321–599 ppm), low Yb (0.76–1.33 ppm) and Y (10.6–18.3 ppm) contents, with high La/Yb (51.1–72.3) and Sr/Y (21.0–51.4) ratios, indicating adakitic affinities. The low MgO (0.97–1.76 wt %), Cr (7.49–53.6 ppm) and Ni (4.75–29.1 ppm) contents, as well as high 87Sr/86Sr(i) (0.7143–0.7145), low ϵNd(t) (−10.4 to −9.8) and zircon ϵHf(t) (−17.7 to 0.4) values, suggest that the Wangdui pluton most likely originated from partial melting of the thickened ancient lower crust. In combination with previously published data, despite the east–west-trending heterogeneity of crustal composition in the Gangdese belt, the La/Yb ratios of magmatic rocks reveal that both western and eastern segments experienced remarkable crustal thickening in the Eocene. However, in contrast to the thickened juvenile lower crust in the eastern segment formed by the underplating of mantle-derived magmas, tectonic shortening plays a more crucial role in thickening of the ancient basement in western Gangdese. In fact, such Eocene-thickened ancient lower-crust-derived adakitic rocks are widely distributed in the central Himalayan–Tibetan orogen. This, together with the extensive development of fold–thrust belts, suggests that tectonic shortening might be the main mechanism accounting for the crustal thickening associated with the India–Asia collision.

Rapid Exploration Using pXRF Combined with Geological Connotation Method (GCM): A Case Study of the Nuocang Cu Polymetallic District, Tibet

The Nuocang Cu polymetallic district is located in western Gangdese, close to the south of the Luobadui–Milashan fault zone. A large number of metal deposits with the potential to be prospected, such as Chagele, Zhalong, Longgen, and Sangmola, have been found near this district. To further isolate the target prospecting area, we used a portable X-ray fluorescence spectrometer (pXRF) to conduct an in situ 1:10,000 soil pedogeochemical survey. The results show that the use of a pXRF combined with the geological connotation method (GCM) can accurately delineate the anomalies related to mineralization and highlight “weak” and “small” anomalies. It was also shown to effectively shorten the working cycle and ensure the continuity and timeliness of field work. Through sizing tests, the analysis of −10~+60 mesh soil samples achieved the best anomaly delineation effect. By studying the supra-ore, near-ore, sub-ore halo, and Th/U, the degree of denudation and the oxidation-reduction environment of the deposit were judged to be moderate. Ultimately, depending on the target area delineated by the pXRF, six Cu-Pb-Zn orebodies were uncapped by five exploratory trenches, which proved the potentiality of the Cu-Pb-Zn polymetallic epithermal deposit controlled by minor faults in Central Nuocang.

In situ Sr-O isotopic and elemental compositions of apatite and zircon from Pengcuolin granodiorites: Implications for Jurassic metallogenic variation in the southern tibet

The Gangdese belt is among the most significant magmatic and metallogenic belts in the world. The multistage magmatism that generated the deposits in the Gangdese is a major subject of scientific interest and valuable economic resource. However, the metallogenic variability within the Jurassic arc system is still controversial. In this paper, we present a study of granodiorites in the Pengcuolin to investigate the magma source and crystallisation process in the western Gangdese belt. This work includes detailed descriptions of mineral textural features, major and trace elements of bulk rock and apatite, trace element of zircons and in situ Sr isotope of apatite and O isotope of zircon. The granodiorites yielded zircon U-Pb ages of 169 ± 0.7 Ma and 165 ± 0.8 Ma, and all samples are depleted in high-field-strength elements (HFSEs) and enriched in large-ion lithophile elements (LILEs). In situ Sr isotopic compositions of apatites exhibit low 87Sr/86Sr ratios (0.7036–0.7057). The zircon δ18O values range from 4.29‰ to 6.04‰. In addition, the granodiorites lack Eu anomalies. These results indicate that the Pengcuolin magmas were derived from asthenospheric mantle that was metasomatised by slab-derived fluids following intrusion into the active continental margin in the Middle Jurassic. By comparing previously published data for Jurassic arc rocks with the results of this study, we find apparent differences between the western segment (zircon Ce4+/Ce3+ and Eu/Eu* ratios: 40–2220 and 0.07–1.49, respectively; TTi-Zr: 587–804 °C; S and Cl contents in magma: 7–443 ppm and 0.030–1.264 wt%, respectively) and the eastern segment (zircon Ce4+/Ce3+ and Eu/Eu* ratios: 3–1380 and 0.08–0.66, respectively; TTi-Zr: 547–1053 °C; S and Cl contents in magma: 7–138 ppm and 0.006–0.093 wt%, respectively). The eastern segment is characterised by low water and volatile contents and a reduced oxidation state. In contrast, the western segment exhibits a more highly oxidised state with higher water and volatile contents, which provide ideal conditions for the formation of porphyry deposits.

Geology and factors controlling the formation of the newly discovered Beimulang porphyry Cu deposit in the western Gangdese, southern Tibet

The porphyry Cu deposits in the Gangdese metallogenic belt were mainly formed in the eastern portion. The Beimulang is a newly discovered deposit formed in post-collisional setting in the western part of the belt and adjacent to the super-large Zhunuo porphyry Cu deposit. There are three epochs of magmatism at Beimulang, including pre-mineralization quartz porphyry (PQP; 49.7 ± 0.4 Ma), inter-mineralization monzogranite porphyry (IMGP; 14.8 ± 0.2 Ma), monzogranite (IMG; 14.1 ± 0.2 Ma), and late-mineralization granite porphyry (LGP; 11.7 ± 0.1 Ma). The various degrees of alteration and sulfide occurred at the IMG(P) and LGP indicate that there are at least two episodes of fluid exsolution events associated with different intrusions spanning ∼ 3 m.y. at Beimulang, which are unlike other Gangdese porphyry Cu deposits showing single fluid exsolution around only one porphyry at Miocene (e.g., Qulong, Zhunuo, Dabu). The PQP is characterized by high SiO2 (75.6–81.4 wt%) and K2O (2.1–4.3 wt%) concentrations, high Nb/Ta (8.4–11.3) ratios, and low Mg# (22–43), Ni (1.5–2.7 ppm) contents. They have high (87Sr/86Sr)i (0.705098–0.707767) ratios and low εNd(t) (-4.9 to −1.9) values. These geochemical characteristics indicate that they were probably derived from a juvenile basaltic lower crust during the ongoing Indian-Asian collision. The IMGP and IMG show similar petrography and geochemical characteristics, such as high SiO2 (65.0–69.8 wt%), K2O (3.6–4.3 wt%), Sr (550–809 ppm) contents, high Sr/Y (79–89) ratios, and low Mg# (43–48), Th/Yb, and Nb/Yb values, combined with high (87Sr/86Sr)i (0.707254–0.708017) and low εNd(t) (-7.9 to −6.0), implying a similar source probably originated in the subduction-modified lower crust metasomatized by Neo-Tethyan oceanic slab-derived fluids. The LGP displays trends produced by fractionation of magma with a composition similar to the IMG(P), but wall rock assimilation plays a significant role during magmatic evolution. Zircons from the IMG(P) show higher Eu/Eu* (>0.3), 10000*(Eu/Eu*)/Y (>4), and (Ce/Nd)/Y (>0.04) ratios than those of the PQP, indicating the fertile magmas were more oxidized and hydrous. The high magmatic oxidation state for the IMG(P) is also supported by using the zircon (ΔFMQ = +4.1 to + 6.1), and amphibole (ΔFMQ =+2.6 to + 3.4) oxybarometers. Their high magmatic water contents are supported by evaluations using amphibole (∼3.5 wt%) and plagioclase (∼6.7 wt%) compositions, respectively. However, the LGP has higher magmatic oxygen fugacity (Eu/Eu* >0.3, ΔFMQ = +4.2 to + 7.3) caused probably by auto-oxidation of the magmas, but lower magmatic water contents (Sr/Y = 31 ± 15; V/Sc = 6 ± 4) caused presumably by previous fluid exsolution and/or fractionation of hydrous silicate minerals during magma ascent than those of the IMG(P). Therefore, the highly hydrous and oxidized magmas with juvenile material are favorable to form porphyry Cu deposits in Tibetan orogen. On the contrary, magmatic evolution can probably reduce the water content of magma chamber, which is not beneficial to porphyry Cu mineralization.

Constraints on the rollback of the Neo-Tethyan oceanic plate: Geochronology and geochemistry of volcanic rocks from the Dianzhong Formation, western Gangdese belt (Tibetan Plateau)

In this contribution, we report new U-Pb SHRIMP zircon ages and geochemical data for volcanic rocks from the Dianzhong Formation. The unit represents the lower part of the Linzizong volcanic succession located in the Shiquanhe area, western part of the Gangdese belt. Zircon U-Pb dating of three trachytes yields ²⁰⁶Pb/²³⁸U crystallization ages of 71.5±0.6 Ma, 70.8±1.0 Ma and 68.9±1.0 Ma. Whole rock major and trace element analyses indicate a main trend of calc-alkalic to high-potassic calc-alkalic, as well as an enrichment in large ion lithophile elements (LILEs), depletion in high field strength elements (HFSEs), with negative Nb, Ta and Ti anomalies. In addition, these volcanic rocks are enriched in LREE and depleted in HREE without a clear Eu anomaly. These geochemical features are similar to those of active continental margin volcanic rocks. In combination with previously published zircon U-Pb ages and geochemical data of the Dianzhong Formation, we suggest that the Linzizong volcanism started as early as the late Cretaceous (∼71 Ma) and the volcanic rocks of the Dianzhong Formation in the Shiquanhe area are the product of rollback of the Neo-Tethyan oceanic plate.

Heterogeneous mantle associated with asthenosphere and Indian slab metasomatism: Constraints on fertilization of porphyry Cu mineralization in Tibetan orogen

Previous models related to ultrapotassic mantle and post-collisional porphyry Cu mineralization have been challenged by the newly identified ultrapotassic lamprophyres (14.31 ± 0.61 Ma), diorite porphyries (14.82 ± 0.21 Ma) and high Sr/Y monzogranite porphyries (14.19 ± 0.23 Ma) from Dabu deposit in the Gangdese, Tibet. We found that the continental crust and lithospheric mantle beneath Gangdese were highly heterogeneous. The western Gangdese (80°E to 89°E) are mainly characterized by enriched lithospheric mantle with εNd(t) ≤ -10 and 87Sr/86Sr ≥ 0.71, and relatively normal crustal thickness and maturation (∼55–65 km), whereas eastern Gangdese (89°E to 95°E) are dominated by relatively depleted lithospheric mantle with εNd(t) ≥ -10 and 87Sr/86Sr ≤ 0.71, and much more thickened and mature crust (∼62–75 km). Based on three-component modeling, the geodynamic mechanism in the western Gangdese is subducted Indian slab metasomatism induced by its roll-back and subsequent break-off, whereas the eastern Gangdese is upwelling asthenosphere metasomatism resulted from convective delamination of thickened lithospheric mantle root. The lamprophyres at Dabu with εNd(t) values of −11.34 to −10.59 represent the very rare and well-preserved ancient lithospheric mantle without significant metasomatism and modification. Instead of previous model that focused on connection with Indian continental plate, we prefer to the generation of porphyry Cu deposits in Tibetan orogen were optimal alignments and combinations of multiple magmatic and dynamic processes, involving metasomatism of subducted Neo-Tethyan slab, metamorphic dehydration and thickening of lower crust, long-lived melting-assimilation-storage-homogenization (MASH) hot zone in the lower crust, and further fertilization of multiple magma chambers (∼30 km, ∼10 km, 1–6 km) in the upper crust.

Zircon U-Pb Age of Granitoids from the Mamu Pb-Zn Skarn Mineralized Area in Western Gangdese and Its Geological Significance

Zircon U-Pb Age of Granitoids from the Mamu Pb-Zn Skarn Mineralized Area in Western Gangdese and Its Geological Significance

Zircon U–Pb geochronology, Hf isotope composition, and petrochemical characteristics of Paleocene granitoids in the western Gangdese Belt, Tibet

Abstract —The research team studied the petrology, whole-rock geochemistry, zircon U–Pb age, and stable isotopic characteristics of the Rongguo Longba and Garongcuo granites of the Nuocang area to understand better the impact of Neo-Tethys ocean subduction and In-dia–Eurasia continental collision on Paleocene tectonomagmatic processes along the southern margin of the Gangdese Belt. The Rongguo Longba granite and Garongcuo granite porphyry formed at 61.86 and 62.17 Ma, respectively. The Nuocang granitoids are characterized by (1) high SiO2, NaO2, and Al2O3 contents and low FeOtot, MgO, and TiO2 contents; (2) LREE and LILE enrichment and HREE and HFSE (Nb, P, and Ti) depletion; and (3) obvious negative Eu anomalies. These features indicate that the Nuocang granites are of the high-K calc-alkaline and peraluminous granite types. Furthermore, their zircon Hf isotope characteristics suggest that the magma source region has an ancient crystalline basement. The basaltic andesitic crystal tuff is the product of garnet–peridotite partial melting and crust contamination from rising magma emplacement.

A Downgoing Indian Lithosphere Control on Along-Strike Variability of Porphyry Mineralization in the Gangdese Belt of Southern Tibet

Abstract The E-trending Gangdese porphyry copper belt in southern Tibet is a classic example of porphyry mineralization in a continental collision zone. New zircon U-Pb geochronological, zircon Hf-O, and bulk-rock Sr-Nd isotope data for the Miocene mineralizing intrusions from the Qulong, Zhunuo, Jiru, Chongjiang, and Lakange porphyry copper deposits and Eocene igneous rocks from the western Gangdese belt, together with literature data, show that both Paleocene-Eocene igneous rocks and Miocene granitoids exhibit coupled along-arc isotopic variations, characterized by bulk-rock ɛNd(t) and zircon ɛHf(t) values increasing from ~84° to ~92°E and then decreasing toward ~95°E. These are interpreted to reflect increasing contributions of subducted Indian continental materials from ~92° to ~84°E and from ~92° to ~95°E, respectively. The Miocene mineralizing intrusions were derived from subduction-modified Tibetan lower crust represented isotopically by the Paleocene-Eocene intrusions, with contributions from Indian plate-released fluids and mafic melts derived from mantle metasomatized by subducted Indian continental materials. Involvement of isotopically ancient Indian continental materials increased from east (Qulong) to west (Zhunuo), which is interpreted to reflect an increasingly shallower angle of the downgoing Indian slab from east to west, consistent with geophysical imaging. Exploration of Gangdese Miocene porphyry copper deposits should focus on the Paleocene-Eocene arc where the subarc mantle was mainly enriched by fluids from the subducted Neo-Tethyan oceanic slab. Neodymium-Hf isotope data for mineralizing igneous rocks from porphyry copper deposits globally show no obvious correlations with Cu endowment. Although Nd-Hf isotopes are useful for imaging lithospheric architecture through time, caution must be taken when using Nd-Hf isotopes to evaluate the potential endowment of porphyry copper deposits, because other factors such as tectonic setting, crustal thickening, magma differentiation, fluid exsolution, and ore-forming processes all play roles in determining Cu endowments and grades.