Petrogenesis of Late Cretaceous Jiangla'angzong I‐Type Granite in Central Lhasa Terrane, Tibet, China: Constraints from Whole‐Rock Geochemistry, Zircon U‐Pb Geochronology, and Sr‐Nd‐Pb‐Hf Isotopes

Abstract

The Jiangla'angzong granite in the northern part of the Central Lhasa Terrane is composed of syenogranite and adamellite. LA‐ICP‐MS zircon U‐Pb analyses suggest that syenogranite has a weighted mean 206Pb/238U age of 86±1 Ma (mean square weighted deviation=0.37), which is in accordance with the muscovite Ar‐Ar age (85±1 Ma) of Cu‐Au ore‐bearing skarns and the zircon U‐Pb age (84±1 Ma) of adamellite. This suggests that the Jiangla'angzong magmatism and Cu–Au mineralization events took place during the Late Cretaceous. The granite contains hornblende, biotite, and pyroxene, and does not contain Al‐bearing minerals, such as muscovite, cordierite, and garnet. It has high contents of SiO2 (65.10–70.91wt%), K2O (3.44–5.17wt%), and total K2O+Na2O (7.13–8.15wt%), and moderate contents of A12O3 (14.14–16.45wt%) and CaO (2.33–4.11wt%), with a Reitman index (σ43) of 2.18 to 2.33, and A/CNK values of 0.88 to 1.02. The P2O5 contents show a negative correlation with SiO2, whereas Pb contents show a positive correlation with SiO2. Th and Y contents are relatively low and show a negative correlation with the Rb contents. These characteristics suggest that the Jiangla'angzong granite is a high K calc–alkaline metaluminous I–type granite. It is enriched in light rare earth elements (LREE) and large ion lithofile elements (LILE), and depleted in heavy rare earth elements (HREE) and high field strength elements (HFSE), with LREE/HREE ratios of 11.7 to 18.1. The granite has negative Eu anomalies of 0.58 to 0.94 without obvious Ce anomalies (δCe=1.00–1.04). The relatively low initial 87Sr/86Sr ratios of 0.7106 to 0.7179, positive εHf(t) values of 1.0 to 4.1, and two‐stage Hf model ages (TDM2) ranging from 889 Ma to 1082 Ma, These geochemical features indicate that the granite derived from a juvenile crust. The (143Nd/144Nd)t values from the Jiangla'angzong granite range from 0.5121 to 0.5123, its εNd(t) values range from –10.17 to –6.10, its (206Pb /204Pb)t values range from 18.683 to 18.746, its (207Pb /204Pb)t values range from 15.695 to 15.700, and its (208Pb /204Pb)t values range from 39.012 to 39.071. These data indicate that the granite was formed by melting of the upper crust with the addition of some mantle materials. We propose that the Jiangla'angzong granite was formed during the post‐collision extension of the Qiangtang and Lhasa terranes.

Formation of Late Cretaceoushigh‐Mggranitoid porphyry in central Lhasa, Tibet: Implications for crustal thickening prior to India–Asia collision

“…However, the majority of the zircon ε Hf ( t ) values of the Sebuta biotite granite porphyry are positive (Table 2). On age versus ε Hf ( t ) plot (Figure 13a), most data points, similar to other Late Cretaceous intermediate‐acid igneous rocks in the central Lhasa block (Cao, Zhang, Santosh, et al, 2019; Hou et al, 2015; Liu, Li, Huang, et al, 2018; Wang et al, 2019), lie between the depleted mantle evolution curve and the chondrite evolution curve. This indicates that the Sebuta biotite granite porphyry and many other coeval intermediate‐acid igneous rocks in the central Lhasa terrane were most likely derived from a juvenile crust probably with minor ancient crust.…”

“…Previous studies show that both slab rollback and break‐off of the southward Bangong–Nujiang Ocean lithosphere were accompanied with the generation of mafic magmas (Chen et al, 2014, 2017; Huang, Zhao, Zhu, et al, 2017; Li et al, 2018; Liu, Li, et al, 2018). Importantly, these basaltic magmas were denser than most crustal rocks, and therefore can accumulate at the crust/mantle boundary and formed underplated juvenile lower crust beneath the northern‐central Lhasa block (Cao, Zhang, Santosh, et al, 2019; Hao et al, 2016; Li et al, 2011; Richards, 2003).…”

“…Since the eclogitized lower crust is denser than the underlying mantle lithosphere, at ca. 89 Ma, the thickened lowermost crust, together with lithospheric mantle, delaminated and subsequently sank into the underlying asthenosphere due to gravitational instability (Cao, Zhang, Santosh, et al, 2019; Liu, Li, et al, 2018; Liu, Wang, et al, 2018; Sun, Hu, Zhu, et al, 2015; Yu et al, 2011). Afterwards, the subsequent upwelling of hotter asthenospheric mantle caused partial melting of the thickened lower crust.…”

“…Afterwards, the subsequent upwelling of hotter asthenospheric mantle caused partial melting of the thickened lower crust. In this case, melts generated by partial melting of the delaminated lower crust interacted with the overlying peridotite during ascent and formed the Sebuta and other high‐Mg # magmatic rocks from elsewhere along the central‐northern Lhasa terrane (Ma, Wang, Li, et al, 2013; Wang et al, 2014, 2019; Sun, Hu, Zhu, et al, 2015; Liu, Li, et al, 2018; Liu, Wang, et al, 2018; Cao, Zhang, Santosh, et al, 2019; Luo et al, 2019), whereas melts generated by partial melting of lower crust that have not yet delaminated generated the low‐Mg # adakitic rocks, such as Azhang adakitic rocks (Sun, Hu, Zhu, et al, 2015; Figure 16). It is noteworthy that some primary melts would be deprived of adakitic signature owing to the contamination of crustal materials and fractional crystallization of ore‐forming or/and accessory minerals (Chen, Xu, et al, 2015; Wang et al, 2014).…”

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

“…At present, research on the LS has mainly focused on the characteristics of island arc magmatism and the evolution of sedimentary basins since the Late Triassic, while research on the Palaeozoic is relatively lacking (Han et al, 2021; Lin et al, 2022; Liu et al, 2018; Liu, Li, Huang, Zhang, Lü, et al, 2019; Liu, Li, Li, Li, Ma, et al, 2022; Liu, Zhang, Huang, Li, Lü, et al, 2019). Compared with those on the Mesozoic and Cenozoic, geological surveys and research on the Palaeozoic are quite limited, and there are great differences in the understanding of the structural attributes of these rocks (Peng et al, 2000; Zhang et al, 2021).…”