Origin of Tibetan post-collisional high-K adakitic granites: Anatexis of intermediate to felsic arc rocks

Abstract

Potassium-rich adakitic rocks have been used to infer high-pressure (HP) melting of thickened or foundered mafic lower crust in post-collisional settings. However, their origin remains debated because of their potassic rather than sodic nature. We address this debate by investigating the source of post-collisional Oligocene high-K adakitic granites in southern Tibet, which are widely attributed to melting of overthickened mafic lower crust. Our new data provide evidence for the generation of these high-K adakitic granites by anatexis of exposed migmatitic intermediate to felsic arc orthogneisses. These granites contain high-Th/U Oligocene magmatic zircons (30–22 Ma), and inherited zircons (66–48 Ma) with the same εHf(t) (0 to +12) as the coeval migmatites. The migmatites have in turn low-Th/U metamorphic Oligocene zircon rims around 66–48 Ma magmatic zircon cores recording the anatectic event at 29–25 Ma. Phase equilibrium modeling yields an anatectic temperature of ~740 °C and pressure (P) of ~0.9 GPa and shows that garnet is stable at P > 0.7 GPa in the melt-present field. The results indicate that high-K adakitic magmas are derived from melting of older intermediate to felsic arc rocks at intermediate pressures, in the garnet stability field, without any involvement of HP melting of metabasaltic rocks. We propose that hybridization between such purely crustal magmas and subcontinental lithospheric mantle–derived shoshonitic magmas results in the hybridized post-collisional Oligocene–Miocene mafic microgranular enclave–bearing potassic adakitic granitic rocks in southern Tibet.

Interplay between oceanic subduction and continental collision in building continental crust

“…2g–i ). This is because: (1) the formation of volumetrically insignificant post-collisional hornblende gabbros may indicate a diminished role of fractional crystallization, (2) the early appearance of plagioclase followed by hornblende in hornblende gabbro 35 also suggests damp fractionation, (3) evidence for Miocene migmatization of syn-collisional dioritic gneisses accompanied by post-collisional granitic rocks with xenocrysts from the dioritic gneisses, collectively recording anatexis 36 , and (4) more enriched radiogenic isotope compositions compared to the pre- and syn-collisional samples, along with high Cr and Ni contents, and Mg# (Fig. 2g ) in some Miocene intermediate-to-felsic samples, indicate hybridization between melts derived from the pre-existing Gangdese crust and ultrapotassic magmas from the Gangdese lithospheric mantle metasomatized by subducted Indian continental material 12 .…”

Continental Crustal Growth Processes Recorded in the Gangdese Batholith, Southern Tibet

“…Continued growth of the Gangdese crust from 70 to 10 Ma is recorded by gabbroic to intermediate rocks, which show either positive zircon ε Hf (t) ) originating from depleted mantle and/or melting of juvenile crust or negative zircon ε Hf (t) (Figure 6a,b) from assimilation of enriched continental lithospheric mantle (Yang et al 2015, Ma et al 2017). This period is also characterized by significant crustal reworking marked by the high-K calc-alkaline felsic rocks that dominate this period (Figure 5h-j) and are primarily produced by melting of the preexisting, juvenile middle and lower arc crust (Yi et al 2022. Despite the large uncertainties in the vertical dimension, the amount of crustal growth and reworking at 70-10 Ma can be evaluated using the outcrop areas of different lithologies, as exemplified by the Idaho Batholith in the northern US Cordillera (Gaschnig et al 2017).…”

“…This is indicated by the presence of ∼85 Ma zircons in ultramafic xenoliths and of 51−45 Ma zircons in dioritic and felsic granulite xenoliths entrained in Miocene ultrapotassic rocks, both of which experienced similar granulite-facies metamorphism at P-T conditions of 15−26 kbar (50−85 km) and 850-1,100°C during 17−13 Ma (Chan et al 2009, Wang et al 2016b. Except for the sporadic intrusions of 20 ± 5 Ma felsic rocks into the middle to upper crust, other exposed middle crustal rocks include the 65−38 Ma dioritic to felsic orthogneisses and paragneisses, which experienced metamorphism and melting at 9−12 kbar (30−40 km) and 710−760°C (Zhang et al 2015, Yi et al 2022. Volumetrically minor Miocene ultrapotassic rocks from the metasomatized mantle lithosphere (Sun et al 2018) (Figure 9g) are an important component that modifies the compositional architecture throughout the crust (Zhu et al 2022) as they are strongly enriched in incompatible trace elements and have evolved isotopic composition.…”

K‐Rich Adakite‐Like Rocks in Central Tibet: Fractional Crystallization of a Hydrous, Alkaline Primitive Melt

“…They commonly contain higher K 2 O contents and K 2 O/Na 2 O ratios than adakites, and we refer to them as K‐rich adakite‐like rocks (KARs) in this paper. A number of models have been proposed for their formation, including melting of K‐rich basalts in thickened, subducted, or foundered continental crust (e.g., Chen et al., 2013; Chung et al., 2003; Hou et al., 2004; Q. Wang et al., 2005, 2008), hybridization of crust‐derived adakitic magmas with mantle‐derived shoshonitic melts (e.g., Hou et al., 2004; R. Wang et al., 2018; X. Wang et al., 2019; Yang et al., 2015), fractional crystallization of medium‐K basaltic melts represented by mafic microgranular enclaves (Lu et al., 2015), and melting of intermediate to felsic arc rocks in the stability field of garnet (Yi et al., 2022). Most of the models essentially link the KARs with the melting of pre‐existing crustal rocks, and limited attention has been paid to the possibility that they result from fractional crystallization of mantle‐derived, high‐K basaltic melts.…”

“…Wang et al, 2018;X. Wang et al, 2019;Yang et al, 2015), fractional crystallization of medium-K basaltic melts represented by mafic microgranular enclaves (Lu et al, 2015), and melting of intermediate to felsic arc rocks in the stability field of garnet (Yi et al, 2022). Most of the models essentially link the KARs with the melting of pre-existing crustal rocks, and limited attention has been paid to the possibility that they result from fractional crystallization of mantle-derived, high-K basaltic melts.…”