Paleoproterozoic ultrahigh‐temperature metamorphism in the Alxa Block, the Khondalite Belt, North China Craton: Petrology and phase equilibria of quartz‐absent corundum‐bearing pelitic granulites

Abstract

An Al-rich, quartz-absent corundum-bearing pelitic granulite is reported for the first time at the Diebusige complex in the Alxa Block in the western part of the Khondalite Belt (KB), North China Craton (NCC). A detailed petrographic study shows that a quartz-present domain occurs in porphyroblastic garnet cores from the corundum-bearing pelitic granulite, with a prograde metamorphic mineral assemblage (M 1 ) of garnet + plagioclase + quartz + biotite + spinel + rutile AE melt, and a quartz-absent domain occurs in the matrix of this rock, with a preserved peak mineral assemblage (M 2 ) of garnet + corundum + spinel + biotite + plagioclase + K-feldspar + rutile + melt and a retrograde metamorphic mineral assemblage (M 3 ) of garnet + sillimanite + corundum + biotite + plagioclase + K-feldspar AE rutile.Phase equilibrium modelling and Zr-in-rutile and ternary feldspar thermometry all reveal similar peak conditions of 890-940 C at 7.5-9.8 kbar, which indicate a high geothermal gradient of $110 C/kbar and ultrahigh-temperature (UHT) conditions. Combining this information with the detailed analysis of metamorphic zircons, we obtain the metamorphic evolution of the corundumbearing pelitic granulite. Specifically, the corundum-bearing pelitic granulite experienced granulite-facies metamorphism at $1950 Ma. Then, this rock was slowly uplifted with heating and partial melting until it achieved UHT conditions. Finally, it cooled to the solidus at $1830 Ma. Thus, we propose that the UHT metamorphic conditions of a corundum-bearing pelitic granulite from the Diebusige complex of the Alxa Block may be the result of long-term slow uplift with heating under a high geothermal gradient.

Paleoproterozoic ultrahigh‐temperature mafic granulites with a high‐pressure prograde path from the Alxa Block: Implications on the tectonic evolution of the Khondalite Belt, North China Craton

“…Therefore, these UHT rocks from the Alxa area and eastern part of the KB should experience a consistent evolution from HP granulite facies to UHT stage. Based on zircon U–Pb geochronology, we interpret the mafic granulites in this study to have undergone post‐ T max cooling at ~1.81 Ga, which is similar to the ages (1.8–1.83 Ga) recorded from a UHT pelitic granulite and an HP mafic granulite in the Alxa area (Zou et al, 2022, 2024). Combined with the cooling age (~1.7 Ga) inferred from apatite in this study, we propose that the UHT rocks in the Alxa area (Figure 14b) underwent decompression between ~1.95 and ~1.91 Ga and subsequently cooled from peak temperatures (~950°C) between ~1.91 and ~1.7 Ga at a rate of ~1.5–2.5°C/Ma in the middle‐upper crustal (<25 km deep).…”

“…Therefore, we interpret the highest temperature (946°C) as the metamorphic T max , whereas the lower temperatures (~824–900°C) are likely indicative of the pre‐ T max decompression or post‐ T max cooling stage. Combining with the results of phase equilibrium modelling and Ti‐in‐amphibole and REE‐in‐Grt–Cpx geothermometers, we suggest that the T max for the mafic granulite samples is ~880–950°C (~8–9 kbar), which is consistent with a nearby UHT pelitic granulite (17AZ04‐1) (9 in Figure 14a; Zou et al, 2022).…”

“…(a) The P–T diagram showing the P–T paths proposed for the HP/MP and UHT granulites in the Khondalite Belt, NCC. (1) The Alxa area UHT mafic granulite (this study), (2) the Helanshan HP pelitic granulite (Zhou et al, 2010), (3) the Qianlishan HP pelitic granulite (Yin et al, 2014), (4) the Jining UHT pelitic granulite (Li & Wei, 2018), (5) the Helanshan UHT pelitic granulite (Gou et al, 2018), (6) the Wulashan pelitic granulite (Cai et al, 2017), (7) the Daqingshan UHT pelitic granulite (Jiao & Guo, 2020), (8) the Jining UHT pelitic granulite (Liu et al, 2021), (9) the Alxa corundum‐bearing UHT pelitic granulite (Zou et al, 2022), (10) the Jining UHT pelitic granulite (Santosh et al, 2009) and (11) the Jining UHT pelitic granulite (Santosh et al, 2012). The boundary lines for metamorphic facies are modified from Brown (2007).…”

“…(b) The compiled T-t evolution of rocks in the KB. The dataset is from Qianlishan-Helanshan terrane (Yin et al, 2009(Yin et al, , 2011(Yin et al, , 2014(Yin et al, , 2015, Daqingshan-Ulashan terrane (Liu et al, 2017), Jining-Liangcheng terrane (Huang et al, 2019;Liu et al, 2012;Santosh et al, 2007a, b) and Alxa area (Zou et al, 2022;this study).…”

“…In addition, HP pelitic granulites with the representative mineral assemblage garnet + kyanite + K‐feldspar, preserving a clockwise P – T path characterized by an ITD segment, have been reported from the Qianlishan–Helanshan terrane (Figure 1b; Yin et al, 2014; Yin et al, 2015; Zhou et al, 2010). Zircon and monazite record the HP and UHT metamorphic ages of ~1.95 Ga and ~1.93–1.91 Ga, respectively (e.g., Gou et al, 2019; Huang et al, 2019; Li & Wei, 2018; Peng et al, 2014; Yin et al, 2009, 2011), and record the retrograde or overprinting metamorphic (granulite facies or UHT) ages of ~1.85 Ga (Guo et al, 2012; Jiao, Fitzsimons, et al, 2020; Jiao, Guo, et al, 2020; Peng et al, 2014; Zou et al, 2022).…”