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The timing of the Lhasa–Qiangtang collision following the closure of the Bangong–Nujiang Tethys Ocean has not been well constrained. An integrated study of whole-rock geochemistry and zircon U–Pb–Hf isotopes was carried out for Early Cretaceous quartz diorite-porphyrites and granites from the Yilashan and Amdo areas, northern Tibet. LA–ICP–MS zircon U–Pb dating reveal that the Yilashan and Amdo granitoids were emplaced at ~121–110 Ma. These granitic rocks display selective enrichment of light rare earth elements, large ion lithophile elements (e.g., Rb, U) and Th, but depletion of Sr and high field strength elements (e.g., Nb, Ta, Ti) compared to its neighboring elements. These new data, combined with regional geological setting, show that these igneous rocks were formed under a geodynamic setting of the Lhasa and Qiangtang (–Amdo) collision with oceanic slab breakoff and asthenospheric upwelling. The BNTO had been closed at ~121–110 Ma in the study area. Yilashan-Amdo granitoids roughly yield high (87Sr/86Sr)i ratios and obvious negative εNd(t) and zircon εHf(t) values along with old Nd TDM and zircon Hf TDM2 ages. Together with their variable U–Pb ages, these features indicate a Precambrian "hidden" crustal source beneath the northern Lhasa and Amdo terranes. The YLSS S-type granophyres were derived from partial melting of Paleoproterozoic lower crustal metagraywackes, whereas the YLSZ quartz diorite–porphyrites and the Amdo I- and A-type granites were mainly derived from partial melting of Paleo–Mesoproterozoic lower crustal mafic rocks with a certain amount of addition of mantle-derived melts. Minor amounts of the materials originated from the Amdo orthogneisses may also be involved in the formation of the YLSZ quartz diorite–porphyrites and the Amdo I-type granites. In addition, the Yilashan ophiolite was intruded by the ~112–108 Ma granophyric and quartz diorite–porphyritic intrusions before its final emplacement into the surrounding strata. [ABSTRACT FROM AUTHOR]

Post-collisional K-rich magmatic rocks of the Qinghai–Tibet Plateau provide important information for understanding the continental collision and uplift of this plateau during the Cenozoic. However, the source of these K-rich rocks is still debated, and limited investigations have been conducted in the western Kunlun Mountains (WKM) in the northwestern margin of the plateau. In this paper, we present geochemical, geochronological, and Sr–Nd–Hf isotope data for Quanshuigou post-collisional K-rich igneous rocks from the WKM. Geochronological results show that these rocks were emplaced during two stages: (1) ca. 5.3 Ma, as an intrusive suite of granite porphyries; and (2) ca. 4.8 Ma, as a suite of shoshonites. These rocks are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), and depleted in heavy rare earth elements (HREEs) and high-field-strength elements (HFSEs). Compositions of the granite porphyries classify these rocks as high-K calc-alkaline peraluminous A-type granites. Combining the negative zircon εHf(t) values (−6.9 to −1.1) with Hf crustal model ages (TDM2) of 1.16–1.53 Ga and negative εNd(t) values (−5.31 to −4.96) with two-stage Nd model ages (TDM2) of 1.22–1.46 Ga, these signatures suggest that the granite porphyries most probably originated from partial melting of thickened lower crust with limited input of mantle-derived magmas. In contrast, the shoshonites have low SiO2 contents (50.84–53.94 wt.%), relatively high Mg# values (50–57), and εNd (t) values ranging from −6.18 to −5.34, with single-stage Nd model ages (TDM1) of 0.97–1.02 Ga. Our analyses show that the parental magma of the shoshonites probably formed by partial melting of EMII-type lithospheric mantle, with the addition of subducted and recycled oceanic sediments. Hence, we conclude that these granite porphyries and shoshonites formed in a post-collisional, extension-related geodynamic setting and that the melting was probably triggered by motion on the Altyn Tagh strike-slip fault system. [ABSTRACT FROM AUTHOR]

Large‐scale Cenozoic magmatic rocks from the interplay between the Indian and Eurasian plate are exposed in the Yulong porphyry copper belt in the northern Jinshajiang–Ailaoshan domain. Alkali‐rich magmas along the Yulong porphyry copper belt can reveal the tectono‐magmatic processes in the Sanjiang region. In this study, we present new zircon U–Pb–Hf isotopes and whole rock geochemistry of Cenozoic granitoids from the Zhalaga area in the northern Yulong porphyry copper belt. The Zircon U–Pb dating results show that the Zhalaga granitic porphyry crystallized at ca. 42–38 Ma. These porphyry deposits are depleted in Nb, Ta, Sr, and Ti enriched in alkaline and rare earth elements (REEs), and exhibit high zircon saturation temperatures, that strongly indicate A‐type affinity. These data and the generally positive ∊Hf(t) values (2.0–4.5) suggest the magmas originated from a hybrid of partial melting of subduction‐modified lithospheric mantle, possibly triggered by upwelling of the asthenospheric mantle. Geochronological and geochemical data of the current and previous studies distinguish three magmatic phases during the Cenozoic in the Jinshajiang–Ailaoshan region: (1) ca. 62–48 Ma; (2) ca. 44–30 Ma; and (3) ca. 28–16 Ma. The strong collision between the Indian and Eurasian plates produced relatively fast convergence rates during the first episode (ca. 62–48 Ma), whereas the subsequent right‐lateral strike‐slip faulting in the Jinshajiang fault zone initiated at ca. 43 Ma is associated with the relatively low India–Eurasia convergence rates during ca. 44–30 Ma. These significantly impacted the nature and spatial distribution of the magmatism and the large‐scale metallogeny during the Cenozoic in the Sanjiang region. We suggest that the Zhalaga alkali‐rich magmas occurred in a transition period from involving soft to hard collisional settings. This remarkable example demonstrates that alkali‐rich magmas with A‐type affinity are also generated in an orogenic tectonic setting. [ABSTRACT FROM AUTHOR]

New zircon U–Pb ages, whole‐rock geochemistry and zircon Hf isotopes from the Habo porphyry Western Yunnan, China, were determined to provide constraints on the timing of uplift of the Eastern Tibetan Plateau. The intrusive rocks consist of shoshonitic porphyry (syenite porphyry and monzonite porphyry). Zircon laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) U–Pb dating indicates coeval emplacement ages of ~35 Ma. The porphyries have alkaline affinities, enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) (e.g., Rb, Th, U, Pb), with depletion of high field strength elements (HFSEs) (e.g., Nb, Ti, Ta) and weak Eu anomalies. They display uniform Lu–Hf isotopic compositions with negative zircon εHf(t) values ranging from ‐3.9 to ‐0.6. The chemical characteristics of the syenite porphyries indicated that they most likely originated from the lower crust, with mantle‐derived material involved in their generation. Geochemically, the monzonite porphyries are similar to the syenite porphyries; however, the lower MgO contents suggest that they were produced by different degrees of partial melting of the same lower crust source. Combined with the geochemical and isotopic data in this paper, imply that the alkali‐rich porphyries of the Habo polymetallic deposit were derived from the partial melting of lower crust, enriched by mantle magma, formed in a conversion stage from stress extrusion (a strike‐slip shear process) to local stress relaxation (a strike‐slip pull‐apart process) at the Ailaoshao tectonic zone. [ABSTRACT FROM AUTHOR]

Abstract This study reports on compositional, whole-rock Sr Nd isotopic, and zircon U Pb geochronological data for dolerite dykes in the Daba and Dongbo ultramafic massifs of the southwest Yarlung–Zangbo Suture Zone (YZSZ), SW Tibet. The 121 ± 2 Ma dolerite dykes from the Daba peridotite exhibit primitive mantle (PM)-normalised multi-element patterns with (La/Yb) N = 0.43–0.72 and negative Nb anomalies. They have high initial 87Sr/86Sr ratios (87Sr/86Sr (i) = 0.707197 to 0.707879) and high ε Nd (t) values (+7.4 to +7.9). The 125 ± 2 Ma dolerite intrusions within the Dongbo peridotite show PM-normalised trace element profiles [(La/Yb) N = 0.65–0.84] that are characterised by apparent negative Nb anomalies and moderate negative Ti (±Y) anomalies. They also have high 87Sr/86Sr (i) ratios (0.706108–0.706793) and high ε Nd (t) values (+7.8 to +8.2). Semi-quantitative Sm/Yb vs. La/Sm modelling demonstrates that the parental magmas of the investigated dykes were derived from 10%–20% (cumulative) melting of a (broad) mantle source region that had a spinel-bearing normal mid-ocean ridge basalt (N-MORB)-like lherzolitic composition. The geochemical and isotopic data indicate that the composition of the inferred mantle source was influenced by minor inputs of subducted crustal material. The petrogenesis of the Daba and Dongbo massifs can be linked to the upwelling of an asthenospheric source that caused continental rifting and subsequent seafloor spreading that was followed by the subduction initiation adjacent to a passive margin during the Early Cretaceous (~130–120 Ma). The results provide a more detailed and perhaps more elegant hypothesis for the tectonomagmatic evolution of the southwestern YZSZ "ophiolitic" peridotites after their accretion beneath a Neo-Tethyan marginal basin. Highlights • Dating of dolerite intrusions within the Daba and Dongbo peridotites at ~121 Ma and ~125 Ma, respectively • Genesis of dike-filling magmas by 10–20% melting of a spinel-bearing mantle source modified by minor slab input • Dolerite intrusion occurred at the onset of subduction adjacent to a rifted continental margin. [ABSTRACT FROM AUTHOR]