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The tectonic transition from oceanic subduction to continental collision is a fundamental process during orogenesis, yet the nature of such transition and associated deep geodynamic processes for the Tibetan Plateau are still controversial. The long‐lived magmatism of the Gangdese Belt of southern Tibet provides an excellent opportunity to study the tectonic transition from subduction of the Neo‐Tethys oceanic lithosphere to the continental collision between Indian and Asian plates. We present zircon U–Pb–Hf isotopes, whole‐rock geochemistry, and Sr–Nd isotopes for the intrusions from Maizhokunggar, southern Lhasa Terrane. The zircon U–Pb dating indicates that two episodes of magmatism occurred in the Maizhokunggar region, ca. 70 Ma diorites and ca. 60 Ma granitoids. The ca. 70 Ma diorites are medium‐K calc‐alkaline and characterized by pronounced high‐field‐strength element depletion and light rare earth element enrichment, similar to arc‐type rocks. They have homogeneous (87Sr/86Sr)i ratios (0.70378–0.70424) and εNd(t) values (+2.6 to +3.4), and εHf(t) (+5.9 to +13.0) with relatively young depleted mantle model ages (307–762 Ma), suggesting that they were generated from the juvenile mafic lower crust induced by the underplating of basaltic magmas from depleted mantle wedge. The ca. 60 Ma granitoids, Kajiacun (KJC) granodiorites, and Qulongxue (QLX) and Bangdacun (BDC) monzogranites are high‐K series and metaluminous to weakly peraluminous. The KJC granodiorites with high Sr/Y ratios (25–43) are adakitic and were likely derived from partial melting of a thickened lower crust. The BDC monzogranites have more evolved Sr–Nd–Hf isotopes and older Nd–Hf model ages than the QLX monzogranites. These geochemical variations were mainly controlled by the fractionation crystallization with assimilation of ancient crustal materials. We propose that at ca. 70 Ma, the subduction of the Neo‐Tethys oceanic slab triggered the partial melting of the juvenile lower crust and formed the diorites with arc affinity, and then the initial collision (>60 Ma) between the Indian and Asian plates caused locally thickened lower crust, resulting in adakitic granodiorites. Our study sheds light on the lithospheric scale magma evolution from the deep mantle to the shallow crust during the critical geodynamic transition to collision, and early uplift. [ABSTRACT FROM AUTHOR]

Here we report the first set of thallium (Tl) isotope data in alkaline rocks from the North China Craton to constrain the nature of recycled materials in the metasomatized subcontinental lithospheric mantle. Samples from the Hekanzi and Saima alkaline complexes display Tl isotope compositions (ε205Tl) identical to the present‐day upper mantle and continental crust, suggesting that neither the recycled low‐temperature altered oceanic crust nor the pelagic sediments were involved in their sources. The volcanic rocks from the Liaodong‐Jinan region, whose source was previously proposed to contain recycled low‐temperature altered oceanic crust, also display modern mantle‐like Tl isotope composition, suggesting a significant Tl‐loss in the recycled oceanic crust during the subduction due to the dehydration process. Our Tl isotope data provide complementary constraints to the Sr‐Nd‐Hf‐O isotopes on the nature of metasomatic agent in the subcontinental lithospheric mantle. Plain Language Summary: Recycling of crustal materials into Earth's mantle by subduction causes mantle heterogeneities. The sources of some alkaline rocks contain recycled crustal materials. Pelagic sediments, low‐temperature altered oceanic crust, and continental crust have sharp differences in thallium isotope compositions. Combined with Sr‐Nd‐Hf‐O isotopes, whole‐rock Tl isotopes place a better constraint on the metasomatic sources. Key Points: Thallium isotope compositions of the alkaline rocks were analyzed to track crustal recycling in the subcontinental lithospheric mantleThallium isotope suggested that low‐temperature altered oceanic crust and pelagic sediments were not involved in the investigated sourcesThe O‐Nd‐Hf‐Tl isotope data for the Liaodong‐Jinan volcanics reveal a significant Tl‐loss in the subducted slab during dehydration process [ABSTRACT FROM AUTHOR]

Calc‐alkaline granitoid intrusions with abundant mafic microgranular enclaves (MMEs) are widespread in the East Kunlun orogenic belt (EKOB). Here, we present petrological observations, zircon U–Pb ages, whole‐rock geochemical, and Sr–Nd–Hf isotopic data for quartz diorites and the MMEs in the Asiha gold ore deposit in the EKOB, to constrain their petrogenesis and tectonic setting. The laser ablation inductively coupled plasma mass spectrometer 206Pb–238U ages of zircons indicate that the host quartz diorites and their MMEs from the Asiha gold deposit crystallized at 228.4 ± 1.7 Ma and 229.0 ± 1.7 Ma, respectively, which is similar to the gold mineralization age. Whole‐rock geochemical data indicate that the quartz diorites are I‐type granites. The MMEs have εHf(t) values of −7.6 to −0.2, −2.1 on average, whereas those of the quartz diorites range from −3.2 to −0.8, with a mean value of −2.1, which is identical within analytical uncertainty. Moreover, the host quartz diorites and their MMEs have relatively homogeneous Sr–Nd isotopic compositions, with high initial 87Sr/86Sr ratios and low εNd(t) values ranging from 0.70914 to 0.70963 and from −5.9 to −5.5, respectively. In summary, the MMEs and host quartz diorites have similar geochemical characteristics, indistinguishable Sr–Nd–Hf isotopic composition, and almost simultaneous crystallization age. Geochemical evidence shows that they formed due to the mixing of crust‐ and mantle‐derived magmas. Combined with our new results and published geological, geochronological, and geochemical data, this paper proposes that the EKOB had evolved into a post‐collisional environment at about 230–228 Ma, and the host quartz diorites and their MMEs from the Asiha complex formed in the transitional stage from the syn‐collisional to the post‐collisional setting. [ABSTRACT FROM AUTHOR]