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The geodynamic processes that occurred beneath the Bangong – Nujiang suture zone during the evolution of the Indus – Yarlung Neo-Tethys and Bangong – Nujiang Meso-Tethys are unclear. We conducted a study of Early Cretaceous andesitic rocks from the Nyima area, north – central Tibet, to investigate these processes. The studied rocks have zircon U – Pb ages of 110–104 Ma and negative to positive εHf(t) values (−12.1 to+10.3). These rocks have affinities with trachytes, and have high total alkali contents and Na2O/K2O ratios (5–125). They also have high Sr (281–1409 ppm), Y (21.0–28.5 ppm), and Yb (2.00–2.65 ppm) contents, and exhibit depletions in Nb, Ta, and Ti. Such features indicate these rocks are not adakites, but are similar to typical arc rocks. The geochemical and isotopic data suggest the studied rocks were formed by partial melting of amphibolite-facies oceanic lithosphere, with the addition of enriched sedimentary materials. Based on the regional crustal evolution, we suggest that the Nyima andesitic rocks formed by melting of Bangong – Nujiang oceanic lithosphere. As such, the Early Cretaceous igneous rocks along the central Bangong – Nujiang suture zone were not associated with northward subduction in the Indus – Yarlung Neo-Tethys. [ABSTRACT FROM AUTHOR]

[Display omitted] • Multi-stage intrusions from pre- to post-mineralization at Bairong are studied. • They formed at 14.1 to 11.6 Ma and derived from a cogenetic magma chamber in a post-collisional setting. • They are all oxidized and hydrous with comparable magmatic S contents, but have different Cl contents. • Fluid exsolution and buffering effect of magmatic anhydrite possibly result in S-Cl decoupling. • Oxidized and hydrous magma with effective fluid exsolution is the key factor for Bairong Cu-Mo mineralization. Volatiles are considered to play an important role in porphyry Cu mineralization, and high apatite S and Cl contents are often used to distinguish fertile magma systems from barren ones in the subduction zone. However, magmatic volatile evolution and its implications in post-collisional porphyry deposits remain enigmatic. To address this issue, we conducted zircon U-Pb dating and Hf isotope, and zircon and apatite chemical compositions of multi-stage magmatic rocks from the Bairong post-collisional porphyry Cu-Mo deposit in the Gangdese belt, Tibet. The magmatic rocks included pre-mineralized porphyritic monzogranite and biotite monzogranite, syn -mineralized monzogranite porphyry, and post-mineralized dacite porphyry that formed in 14.1–12.7 Ma, ca. 12.1 Ma, and 11.8–11.6 Ma, respectively. All these intrusions have depleted zircon Hf isotope (ε Hf (t) = 1.7–8.0, 3.7–9.0, 3.3–9.7 for pre-, syn-, and post-mineralized intrusions, respectively) together with the research results of contemporaneous magmatic rocks in the adjacent area, suggesting that the multi-stage intrusions at Bairong are derived from a cogenetic magma chamber in post-collisional extension setting. All the intrusions have similar zircon Ce N /Ce N *, Eu N /Eu N * ratios with average ΔFMQ values of 2.1 – 2.6, together with the high apatite Eu N /Eu N * ratios and low Ga concentrations, implying that their magmas are all oxidized. These intrusions have zircons (10,000 × Eu N /Eu N *) /Y all > 1 and Ce/Nd/Y mostly > 0.01, and obtain low average titanium-in-zircon temperatures of 659 to 714 °C, which indicate their magmas are hydrous. Apatite Cl contents decrease, combined with the drop of apatite X Cl /X OH and the surge of X F /X Cl ratios from pre- to syn -mineralized intrusions, possibly indicating that fluid exsolved from magma at the syn -mineralization stage. The apatite SO 3 contents in these different-stage intrusions, however, are almost unchanged. Combined with previous studies, the buffering effect of magmatic anhydrite maintains the stability of magma S content, which together with fluid exsolution leads to the S-Cl decoupling. Consequently, we conclude that the oxidized and hydrous magma with effective fluid exsolution is a critical control on the formation of Bairong porphyry Cu-Mo mineralization. Considering the wide variations of apatite Cl contents and the occurrences of S-Cl decoupling, simple application of high apatite SO 3 and Cl contents to distinguish the ore-forming and barren systems in the post-collisional porphyry system should be cautious. [ABSTRACT FROM AUTHOR]

The suggested Early Cretaceous timing of early formation of the Tibetan Plateau remains controversial. This study investigates this uncertainty by examining newly identified Early Cretaceous adakite-like magmatic rocks in the Rutog and Nyima areas of the northern–central Tibetan Plateau. Zircon U Pb dating indicates that these rocks formed at ca. 110 Ma and have negative to positive zircon ε Hf (t) values. They contain high concentrations of Sr and have high Sr/Y values, but contain low concentrations of Y and Yb and have low (La/Yb) N , suggesting they have an adakite affinity. They also have ε Nd (t) values that range from −4.5 to +1.3. These geochemical and isotopic data indicate that the adakite-like rocks probably formed from magmas generated by the partial melting of garnet-bearing amphibolite within a thickened region of juvenile and ancient crustal material. This magmatism also most likely occurred during the Lhasa–Qiangtang collisional event. Crustal thicknesses along the Bangong–Nujiang suture zone (BNSZ) were > 40 km in places at ca. 110 Ma, and most likely had a significant impact on the initial uplift of the Tibetan Plateau. In summary, the Early Cretaceous adakite-like magmatic rocks in the Rutog and Nyima areas suggest that the Tibetan Plateau started to form during the Early Cretaceous. • Early Cretaceous adakite-like rocks occur in the north-central Tibet. • Adakite-like rocks were derived from the thickened crust. • The Lhasa–Qiangtang collision happened before ca. 110 Ma. • The significant crustal thickening led to the early uplift in the Early Cretaceous. [ABSTRACT FROM AUTHOR]

The Early Paleozoic paleogeographic locations of the Tibetan terranes in Gondwana remain poorly constrained. This study reports petrology, zircon LA-ICP-MS U-Pb ages, whole-rock major and trace element compositions, and Sr-Nd-Hf isotopic data for the gneissic granites in the Amdo microcontinent, central Tibetan Plateau. LA-ICP-MS zircon U-Pb dating yielded Cambrian ages of ca. 500 Ma for the granites. A Cryogenian age of ca. 800 Ma was recognized from inherited zircons. Geochemical data indicate that these granites belong to high-K calc–alkaline I-type granitoids. They mostly exhibit negative values of zircon ε Hf (t) (−7.1 to +0.1) and whole-rock ε Nd (t) (−5.9), as well as old zircon Hf model ages (TC DM = 1718–1472 Ma) and whole-rock Nd model ages (TC DM = 1771 and 1777 Ma). These granites can be interpreted as resulting from partial melting of ancient crustal materials that have been modified by mantle-derived magmas. Integrating previous magmatic studies with the data presented in this contribution, we suggest that the Amdo microcontinent was linked to the Indian Gondwanan margin and recorded the arc-related magmatism along the proto-Tethyan margin. • Cambrian gneissic granites occur in the Amdo microcontinent, central Tibet. • A Cryogenian age of ca. 800 Ma is recognized from inherited zircons. • The granites are high-K calc–alkaline I-type granitoids. • The Amdo microcontinent has paleogeographic affinity to the Indian Gondwanan margin. [ABSTRACT FROM AUTHOR]

Magmatic rocks preserve a lot of information about the formation and evolution of continental crust, and they play an important role in understanding the growth and reworking of ancient continent. Cretaceous magmatic rocks occurring in the Lhasa terrane are a window to investigate magmatic processes and continental evolution of the Tibetan Plateau. In this study, we report zircon U Pb ages and Hf isotopic data, as well as whole-rock major and trace elemental compositions for Early Cretaceous magmatic rocks, including andesite and dacite from the Zenong Group, and associated diorite and granite in the Xinji area, north-central Tibetan Plateau. Zircon from andesite and dacite yielded weighted-mean 206Pb/238U ages of 120–108 Ma and variable zircon ε Hf (t) values between −12.3 and +2.2. Geochemical compositions suggested that they both displayed high-K calc-alkaline features. Andesite was probably formed by mixing between crust- and mantle-derived melts, whereas dacite was most likely generated by partial melting of ancient crust with input of mantle-derived material. Dioritic and granitic intrusions were intruded into the Precambrian strata and Zenong Group, respectively. Zircon from diorite gave a weighted-mean 206Pb/238U age of ca. 116 Ma and zircon ε Hf (t) values of −0.5 to +7.3. Diorite displayed medium-K calc-alkaline series signatures and was most likely formed by partial melting of mantle wedge peridotite accompanied by crustal assimilation. Granite yielded zircon weighted-mean 206Pb/238U ages of 107–104 Ma, and positive zircon ε Hf (t) values of +1.5 to +6.7, indicating a major source of juvenile crust. Granitic samples show irregular REE distribution with variable Ce anomalies, which were probably inherited from subducted pelagic sediments. The Early Cretaceous magmatic rocks in the Xinji area records distinct formation processes involved partial melting of mantle, re-melting of crust, and mixing between mantle and crust. Such processes, however, had a complicated source from mantle, ancient and juvenile continental crust, and recycled pelagic sediments, suggesting simultaneous crustal growth and reworking of the Lhasa basement during the Early Cretaceous. • Early Cretaceous magmatic rocks occur in the Xinji area, central Tibet. • Ce anomalies of granites were inherited from subducted pelagic sediments. • The Bangong-Nujiang Tethys Ocean experienced southward subduction. • Simultaneous growth and reworking occurred in the Lhasa basement. [ABSTRACT FROM AUTHOR]