We evaluate the controlling factors of hydrothermal wolframite and scheelite precipitation in the quartz vein-type Jiaoxi tungsten deposit situated in the western part of the Lhasa terrane (Tibet, ...China) using texture, major and trace element mineral geochemistry, and sulfur stable isotope geochemistry. Pyrite and chalcopyrite that are intergrown with Fe-enriched wolframite and siderite, have distinct in situ S isotope compositions (δ34SV-CDT) of -31.38 to +1.77 per mille, and +2.07 to +2. per mille0;, respectively. Major and trace element contents and in situ S isotope compositions of pyrite and chalcopyrite indicate that the hydrothermal evolution involved fluid-fluid mixing and greisenization. We report evidence for an early magmatic fluid, which is characterized by the enrichment of W, Mn, Zr, Ti, Sc, and Sn and depletion of Fe. This magmatic fluid was diluted by meteoric water and interacted with biotite monzogranite porphyry to leach Fe, Mg, and Zn into the system to form wolframites with variable Fe/(Fe+Mn) ratios ranging between -013;0.84. The late Fe-enriched magmatic fluid released from the muscovite granite mixed with meteoric water that leached minor Fe and S from shale to form late shale-hosted wolframite with a Fe/(Fe+Mn) mass ratio of >0.75 and coeval siderite and sulfides. This study highlights that multiple Fe sources were present in the system, including muscovite granite-released Fe through fluid exsolution, biotite monzogranite porphyry-released Fe during greisenization, and minor Fe released from the shale as a result of meteoric water leaching.
Display omitted
•The Xiongcun district is a unique area where OPCD and RPCD coexist.•A combination of apatite and zircon geochemistry can be used to track petrogenesis.•The mineralizing magmas of No. ...I and No. II deposit have similar fO2.•Apatite can be used as an indicator mineral for mineral exploration.
The Xiongcun district, a unique area where oxidized porphyry copper-gold deposits (OPCD) and reduced porphyry copper-gold deposits (RPCD) coexist, is located in the central part of the Gangdese porphyry copper belt (GPCB) in Tibet, China, and hosts the only known mineralization related to the Neo-Tethyan oceanic subduction. Apatite and zircon major and trace elements, in situ oxygen isotopes in zircon from fertile quartz diorite porphyry (QDP) of the No. I deposit, fertile hornblende quartz diorite porphyry (HQDP) of the No. II deposit, and barren hornblende quartz diorite porphyry with coarse quartz eyes (QQDP) in the Xiongcun district were determined by EPMA, LA-ICP-MS, and SHRIMP II ion microprobe to investigate the petrogenesis, magma oxidation state, and mineralization differences between the No. I and No. II deposits, as well as the potential of apatite as a metallogenic indicator. The results from this study show that a combination of REE, Ti, oxygen isotopes, and the Th/U ratios in zircon and REE as well as Sr in apatite can be used to track the magma compositions and crystallization histories in porphyry systems. Multiple indicators, including the apatite SO3 content, apatite Mn oxybarometer, and zircon Ce4+/Ce3+ ratio, indicate that (1) the parental magmas of fertile QDP and HQDP containing Cu and Au mineralization were more reduced than that of barren QQDP; (2) despite barren QQDP having higher oxygen fugacity than QDP and HQDP, the lower Cu and water contents in the magma result in a lower potential for ore formation; and (3) the similar oxidation states of QDP and HQDP show that the mineralization differences between RPCD (No. I deposit) and OPCD (No. II deposit) were not caused by the oxidation states of the magmas but were likely caused by the different processes of hydrothermal fluid evolution. Apatite grains from fertile porphyries (QDP and HQDP) have much higher Cl/F ratios than those from barren porphyry (QQDP), indicating that the Cl/F ratio of apatite can be used as a proxy for porphyry Cu-Au mineralization related to the Jurassic porphyry intrusions in the Gangdese porphyry copper belt.
Display omitted
•Early Permian arc magmatism was identified in the NLT.•Geochemical signatures of Permian-Triassic magmatic rocks in the Lhasa Terrane.•Collision between NLT and SLT was followed by ...subduction of the Neo-Tethys.•Subduction initiation of the Neo-Tethys is a response to Cimmeria-Eurasia collision.
Collision-induced subduction transference may have triggered subduction of the Neo-Tethys oceanic lithosphere. To verify this proposition, we present an integrated geochemical and geochronological studies of new and published data of the Permian-Triassic magmatism in the Lhasa Terrane. The newly identified Chongni mafic intrusions (gabbros and diorites) in the North Lhasa Terrane (NLT) were emplaced during the Early Permian (ca. 275 Ma). The mafic rocks have (87Sr/86Sr)t ratios between 0.7037 and 0.7042, and εNd(t) values between + 5.2 and + 5.8. The zircons of the Chongni mafic rock samples have εHf(t) values from + 9.1 to + 12.9. The isotopic data in conjunction with trace element features suggest that the Chongni mafic intrusions have been derived mainly by partial melting of a metasomatized mantle wedge source. The newly identified Quxu granodiorites and hornblende gabbros in the South Lhasa Terrane (SLT) were emplaced in the Late Triassic (ca. 228–203 Ma). The granodiorites are characterized by (87Sr/86Sr)t ratios of 0.7036–0.7037 and εNd(t) values of +6.6 to +6.7, while the hornblende gabbro samples have (87Sr/86Sr)t ratios of 0.7033–0.7035 and εNd(t) values of +7.3 to +7.4. The zircons of the granodiorite and hornblende gabbro samples revealed εHf(t) values of +10.3 to +14.0 and +10.8 to +16.4, respectively. The isotopic data in combination with trace element signatures suggest that the granodiorites were derived from a juvenile basaltic lower crust with a minor input of mantle components, while the hornblende gabbros are interpreted to have formed during partial melting of a formerly metasomatized depleted mantle wedge source. The Early Permian and Late Triassic magmatic rocks consistently show arc-related signatures, which probably record the northward subduction of the Sumdo Paleo-Tethys and the Neo-Tethys oceanic slabs, respectively. This evidence, in combination with that of other studies on the Permian-Triassic magmatism across the Lhasa Terrane, indicates that arc magmatism within the NLT switched to collisional magmatism at ca. 260 Ma due to the collision of oceanic islands/plateaus and, further on, the collision of the SLT with the NLT. Furthermore, the change from ca. 265–255 Ma alkaline magmatism to ca. 240–200 Ma calc‐alkaline magmatism within the SLT may reflect the geodynamic transition from lithospheric extension to Andean-style orogenesis. Therefore, we propose that subduction initiation of the Neo-Tethys oceanic lithosphere in the Tibetan Plateau resulted from the southward transference of the Sumdo Paleo-Tethys subduction zone, involving the collision of oceanic islands/plateaus and, subsequently, the collision of the SLT with the NLT. Additionally, the geological records of the Tethyan orogenic system suggest that the subduction initiation of the Neo-Tethys oceanic lithosphere is a uniform response to the collision between Cimmeria and Eurasia.
•The Xiongcun district present two middle Jurassic mineralizing episodes separated by 10m.y.•The porphyry mineralization in the Xiongcun district formed in island arc setting.•Four major metallogenic ...events existed in the Gangdese porphyry copper belt.
Two large copper–gold porphyry deposits (No.I and No.II) were discovered in the last decade within the Xiongcun district, which is located along the south margin of the Gangdese porphyry copper belt. These two deposits are hosted by two porphyries that were emplaced into the Early Jurassic volcano-sedimentary rock sequences of the Xiongcun Formation. The porphyries are both quartz diorite bodies, with one of these distinguished by its large quartz eyes. Our new molybdenite Re–Os and zircon U–Pb dates for No.I and No.II deposits and host intrusions reveal the presence of two Middle Jurassic ore-forming episodes in the Xiongcun district. The No.II deposit is related to the 181–175Ma quartz diorite porphyry and formed ca. 172.6±2.1Ma. The No.I deposit is related to the 167–161Ma quartz diorite porphyry with large quartz eyes and formed ca. 161.5±2.7Ma. Geochronological and geochemical features of the Jurassic ore-bearing porphyries in the Xiongcun district indicate that the porphyry copper–gold mineralization formed in an island arc setting, which is related to the northward subduction of the Neo-Tethys oceanic plate. These observations, in combination with geochronological data reported at Gangdese porphyry copper belt, indicated four major metallogenic events existed in the Gangdese porphyry copper belt at 172–161Ma, 65–48Ma, 40–23Ma, and 20–12Ma. We have identified two mineralization episodes comprise Middle Jurassic metallogenic event (172–161Ma) in the Xiongcun district, which provides new constrains for the Middle Jurassic metallogenic event in the Gangdese porphyry copper belt.
Jurassic sandstones in the Xiongcun porphyry copper–gold district, southern Lhasa subterrane, Tibet, China were analysed for petrography, major oxides and trace elements, as well as detrital zircon ...U–Pb and Hf isotopes, to infer their depositional age, provenance, intensity of source-rock palaeo-weathering and depositional tectonic setting. This new information provides important evidence to constrain the tectonic evolution of the southern Lhasa subterrane during the Late Triassic – Jurassic period. The sandstones are exposed in the lower and upper sections of the Xiongcun Formation. Their average modal abundance (Q21F11L68) classifies them as lithic arenite, which is also supported by geochemical studies. The high chemical index of alteration values (77.19–85.36, mean 79.96) and chemical index of weathering values (86.19–95.59, mean 89.98) of the sandstones imply moderate to intensive weathering of the source rock. Discrimination diagrams based on modal abundance, geochemistry and certain elemental ratios indicate that felsic and intermediate igneous rocks constitute the source rocks, probably with a magmatic arc provenance. The detrital zircon ages (161–243 Ma) and εHf(t) values (+10.5 to +16.2) further constrain the sandstone provenance as subduction-related Triassic–Jurassic felsic and intermediate igneous rocks from the southern Lhasa subterrane. A tectonic discrimination method based on geochemical data of the sandstones, as well as detrital zircon ages from sandstones, reveals that the sandstones were most likely deposited in an oceanic island-arc setting. These results support the hypothesis that the tectonic background of the southern Lhasa subterrane was an oceanic island-arc setting, rather than a continental island-arc setting, during the Late Triassic – Jurassic period.
Abstract
Continental crust has long been considered too buoyant to be subducted beneath another continent, although geophysical evidence in collision zones predict continental crust subduction. This ...is particularly significant where upper continental crust is detached allowing the lower continental crust to subduct, albeit the mechanism of such subduction and recycling of the upper continental crust remain poorly understood. Here, we investigate Paleocene S-type magmatic and volcanic rocks from the Linzizong volcanic succession in the southern Lhasa block of Tibet. These rocks exhibit highly enriched
87
Sr/
86
Sr,
207
Pb/
206
Pb and
208
Pb/
206
Pb together with depleted
143
Nd/
144
Nd isotope ratios. The geochemical and isotopic features of these rocks are consistent with those of modern upper continental crust. We conclude that these Paleocene S-type volcanic and magmatic rocks originated from the melting of the upper continental crust from microcontinent subduction during the late stage of India–Asia convergence.
Early Mesozoic (Late Triassic to Middle Jurassic) igneous rocks in the southern Lhasa subterrane, Tibet, record important information on the early tectono-magmatic evolution of the Neo-Tethys Ocean. ...This paper presents petrological, geochronological, and geochemical data for the Xiongcun Formation ignimbrites, to constrain the petrogenesis and tectonic setting of these rocks in the southern Lhasa subterrane. The Xiongcun Formation is dominated by andesitic and dacitic rocks. These rocks yield Early Jurassic (195–176 Ma) zircon UPb ages and display typical arc-like geochemical signatures, being enriched in large-ion lithophile elements (e.g., Rb, Ba, and U) and depleted in high-field-strength elements (e.g., Nb, Ta, and Ti). Sr–Nd–Pb–Hf isotopic compositions are highly depleted. These chemical and isotopic data suggest that the Xiongcun Formation ignimbrites probably formed in an intra-oceanic island arc setting related to the northward subduction of the Neo-Tethys oceanic slab. The parental magmas were derived from a depleted mantle source modified by fluids released from the Neo-Tethys oceanic slab. Combined with previously reported data, we suggest that the volcanic rocks in the Early Jurassic Xiongcun and Yeba formations of the southern Lhasa subterrane were generated by a single subduction system, but developed on oceanic and continental crust, respectively.
Display omitted
•Xiongcun Formation (XF) volcanism occurred during the Early Jurassic period.•XF ignimbrites were generated by the partial melting of a depleted mantle source.•XF ignimbrites formed in an intra-oceanic island arc setting.•XF and Yeba Formation volcanism were likely generated by a single subduction system.
The southern Lhasa subterrane is the leading edge of the Tibetan Plateau and preserves important records of the Neo-Tethyan oceanic subduction and India–Asia collision. However, the early tectonic ...evolutionary history of the Neo-Tethyan oceanic subduction remains unclear. Here we present results from a systematic study on the early Mesozoic (Middle Triassic–Late Jurassic) magmatic and sedimentary rocks in the southern Lhasa subterrane. These rocks are distributed zonally along the southern and northern parts of the subterrane, and have been classified into the north and south belts. The magmatic rocks in the north belt show a back-arc basin affinity, with accreted bimodal volcanic sequence and sediments from central and southern Lhasa subterranes, whereas the magmatic rocks in the south belt reveal typical magmatic arc characteristics. The sediments were derived from southern Lhasa subterrane, without the input of clastic components from the central Lhasa subterrane. Based on geochronological, geochemical, and sedimentary records from both belts, we conclude that the early Mesozoic magmatism was triggered by the northward subduction of the Neo-Tethyan oceanic slab, and its initial subduction likely occurred before the Middle Triassic (ca. 240 Ma). During the Early–Middle Jurassic (ca. 193–165 Ma), a short-lived episode of back-arc extension, we attribute to roll-back of the subducting Neo-Tethyan slab, resulted in the single magmatic arc that existed during the early stages (ca. 240–194 Ma) and evolved gradually into an arc–back-arc system. The north belt represents a back-arc basin setting associated with a continental arc, whereas an intra-oceanic and continental (island) arc simultaneously developed in the south belt, and both these belts were possibly generated under the same subduction system.
•The northward subduction of Neo-Tethys occurred since Middle Triassic.•The single magmatic arc evolved into an arc–back arc system at ca. 193 Ma.•Continental and oceanic arc magmatism during same subduction system.
The Early Mesozoic (ca. 240–185 Ma) magmatic rocks in the central Lhasa subterrane (CL) provide crucial insight into the geodynamics of the Early Mesozoic orogenic assembly in the Lhasa Terrane. Here ...we present an integrated geochemical and geochronological study combining new and published data of the Early Mesozoic magmatism in the CL to reveal the geodynamic processes and magmatic responses involved in this orogeny. The here studied A-type granodiorites from the Luoza batholith display zircon U-Pb ages of 199 Ma, and zircon saturation temperatures of 833–857 °C. In addition, zircons have highly enriched radiogenic Hf isotope compositions (εHf(t) = −20.1 to −4.3). The Luoza granodiorite samples have high (87Sr/86Sr)t ratios between 0.7174 and 0.7209, and negative εNd(t) values between −9.5 and − 9.1. We propose the granodiorites were generated via partial melting of ancient basement rocks with minor inputs of basaltic melts (~10%). The here studied monzogranites and syenogranites from the Menba batholith are normal calc-alkaline and highly fractionated I-type granites that yield zircon U-Pb ages from 197 to 184 Ma and 187 Ma, respectively. The zircon grains of both rock types have relatively higher εHf(t) values (−13.8 to +0.5) than those of the Luoza granodiorites. The Menba monzogranites have (87Sr/86Sr)t ratios between 0.7062 and 0.7081, and εNd(t) values between −6.9 and − 3.4. The calculated εNd(t) values for the Menba syenogranites range from −4.5 to −4.0. The isotope data set indicates that the Menba granitoids were generated via mixing of ancient crustal melts and mantle-derived magmas (~50%). The high-temperature and low-pressure conditions indicated by the Early Jurassic Luoza A-type granitoids are geodynamically linked to a post-collisional setting. This evidence, in combination with Early Mesozoic magmatism identified within the CL, clearly displays a transitional conversion from ca. 240–210 Ma S-type-dominated to ca. 210–185 Ma I-type-dominated magmatism. This distinct change in magmatism reflects the geodynamic change from a syn-collisional to a post-collisional setting, involving the breakoff of the Sumdo Paleo-Tethys oceanic slab at ca. 210 Ma. The Early Mesozoic magmatic belt within the CL may record the orogenic assembly between the central and southern Lhasa subterranes, which represents the important evidence for closure of the Sumdo Paleo-Tethys Ocean. Consequently, the Early Mesozoic magmatic belt within the CL reveals that the Sumdo Paleo-Tethys Ocean extended at least from the Luoza area, central Lhasa Terrane, to the easternmost end of the Lhasa Terrane.
•Early Jurassic (ca. 200–185 Ma) A-type and I-type granitoids in the CL.•The different sources and petrogenesis of Early Jurassic granitoids in the CL.•Early Jurassic granitoids in the CL were emplaced under a post-collisional setting.•Early Mesozoic magmatism in the CL records a post-subduction tectonic evolution.•Post-subduction tectonic events are used to trace the scale of extinct ocean basin.
The Lhasa terrane is widely regarded as an integral unit that separated from Gondwana in the Carboniferous or late Triassic, but this terrane may more reasonably consist of two subterranes that ...separated from Gondwana at different times. Here, we describe newly identified early Carboniferous (355–344 Ma) basic and intermediate intrusive rocks in the Xiongcun area, southern Tibet. The basic rocks that yield negative whole‐rock εNd(t) values (−5.04 to −2.94) and zircon εHf(t) values (−9.2 to −1.7), combined with the chemical compositions, indicate that the magmas were derived by the partial melting of an enriched lithospheric mantle in a back‐arc extensional setting. The intermediate rocks yield εNd(t) and εHf(t) values of −5.78 to −5.36 and −9.2 to −4.5, respectively, which suggests that they were likely produced via crust‐mantle mixing. By combining our results with those of previous studies on the Lhasa terrane, we consider that a broad back‐arc rift system, including northern and southern branches, developed along the northern margin of Gondwana under the southward subduction of the Paleo‐Tethys oceanic lithosphere during the early Carboniferous. The northern branch evolved into the Sumdo Paleo‐Tethys Ocean, which led to the separation of the North Lhasa terrane from northern Gondwana during the late Carboniferous. The subsequent northward subduction of the Sumdo Paleo‐Tethys oceanic lithosphere beneath the North Lhasa terrane during the early to middle Permian may have triggered the southern branch to further evolve into the Neo‐Tethys Ocean and eventually may have resulted in the South Lhasa terrane separating from Gondwana.
Key Points
The Lhasa terrane records early Carboniferous back‐arc rifting‐related magmatism
The initial rifting of the Lhasa terrane from Gondwana began in the early Carboniferous
The Lhasa terrane contains two subterranes separated from Gondwana at different times
PDF
