Paleoproterozoic UHT metamorphism with isobaric cooling (IBC) followed by decompression–heating in the Khondalite Belt (North China Craton): New evidence from two sapphirine formation processes

Abstract

Interpretation of reaction microstructures may provide constraints on the P-T path followed by rocks, with implications for the geodynamic evolution. Sapphirine generally occurs in diverse microstructures in ultrahigh-temperature (UHT) Mg-Al-rich granulites. Understanding multi-stage sapphirine formation processes and the resultant P-T path may provide insights into the cause of UHT metamorphism, which is otherwise under broad debate. Here, we investigate samples of UHT granulite containing two distinct types of sapphirine from the Dongpo locality in the Khondalite Belt, North China Craton, with the aim of understanding the processes of sapphirine formation and the metamorphic evolution of the host rocks. Petrographic observations show that early sapphirine, which occurs as coronas on spinel and as single porphyroblasts, formed together with biotite, sillimanite, and inclusion-rich garnet.Late symplectitic sapphirine along with fine-grained plagioclase and spinel plus plagioclase symplectites, formed by consumption of sillimanite, biotite, and garnet.Three pseudosections based on the bulk compositions of microdomains inferred to reflect spatially restricted equilibrium suggest that the rocks record near isobaric cooling (IBC) from ~980 to 830ºC at ~0.9 GPa for early sapphirine formation, and decompression and heating to ≤0.7 GPa and ~900ºC for late sapphirine formation.Our study in combination with other metamorphic P-T and age information reveals the common occurrence of IBC paths and long duration (c. 1.93 to 1.86 Ga) regional UHT metamorphism in the Khondalite Belt, North China Craton. Locally, this is followed by decompression-heating paths at c. 1.86 Ga. The Palaeoproterozoic UHT metamorphism with long-lived IBC path in the Khondalite Belt, North China Craton supports large hot orogen model in the amalgamation of this part in the supercontinent Nuna. K E Y W O R D S Khondalite Belt, North China Craton, phase equilibria modelling, sapphirine, ultrahigh temperature 358 |

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

“…The post‐ T max cooling (M 3 ) path is constrained mainly by the final mineral assemblage (Grt + Cpx + Opx + Amp + Pl + Ilm + Qz in sample 17AZ03‐1.3), which produces a near‐IBC path at 8–9 kbar from ~950 to ~830°C (solidus; Figure 7d). The P–T path is similar to the results of previous studies in the Daqingshan and Jining areas (7 and 4 in Figure 14a, respectively; Jiao & Guo, 2020; Li & Wei, 2016, 2018). Thus, a clockwise P–T path, involving heating decompression, followed by later near‐IBC processes, is defined from these UHT mafic granulite samples.…”

“…Existing work suggests that UHT granulites from the eastern part of the KB also record a later decompression stage with cooling (Li & Wei, 2018; Liu et al, 2021) or heating (Jiao & Guo, 2020), which may be related to the overprinting of ~1.85 Ga granulite facies or UHT metamorphism in the eastern part of the KB (Guo et al, 2012; Peng et al, 2014). By combining our findings with existing petrochronology studies in the KB (e.g., Huang et al, 2019; Jiao, Fitzsimons, et al, 2020; Jiao, Guo, et al, 2020; Li & Wei, 2018; Yin et al, 2014, 2015), a possible geological scenario for the KB includes the following: (1) the protoliths of mafic or pelitic granulites were thickened at lower crustal levels (~45 km depth) at ~ 1.96–1.95 Ga due to collision between the Yinshan and Ordos blocks (e.g., Zhao et al, 2012; Zhao & Zhai, 2013); (2) the granulites were exhumed to middle crustal levels (~25 km depth) at ~1.92 Ga, mantle‐derived mafic magmatism (e.g., mantle upwelling) or radiogenic heating led to UHT metamorphism (e.g., Huang et al, 2019; Peng et al, 2010; Zhao, 2009; Zou et al, 2022); (3) the UHT granulites from the western part of the KB (Alxa and Helanshan area; Gou et al, 2018, 2019; Zou et al, 2022; this study) experienced a prolonged cooling process over ~110 million years (Ma) (~1.92–1.81 Ga) at middle crustal levels (~25 km deep), whereas the UHT granulites from the eastern part of the KB (Daqingshan and Jining area) were exhumed to shallower depths during cooling (Jining area; Li & Wei, 2018; Liu et al, 2021) or heating (Daqingshan area; Jiao & Guo, 2020; Jiao, Fitzsimons, et al, 2020; Jiao, Guo, et al, 2020) after ~1.85 Ga.…”

The Himalayan Collisional Orogeny: A Metamorphic Perspective

“…The genetic background of UHT metamorphism is essential for understanding the thermal structure of orogenic belts and petrogenesis of granites (Stevens et al, 2007;Harley, 2008Harley, , 2016Clark et al, 2011;Kelsey and Hand, 2015;Cipar et al, 2020;Zheng and Chen, 2021). UHT metamorphism is formed chiefly during ancient continental collisions, such as the UHT in the Precambrian granulite terranes, the Khondalite belt in the North China Craton, and the collisional orogen between east and west Gondwana (Brown, 2007;Harley, 2008;Kelsey et al, 2008;Kelsey and Hand, 2015;Jiao and Guo, 2020), but much less in the Phanerozoic. In recent years, UHT metamorphism in the India-Asia collisional belt has been increasingly reported (Chen et al, 2021;Wang J M et al, 2021;Wu et al, 2022).…”

Effects of Fe3+ in sillimanite on mineral stabilities and parageneses in ultrahigh‐temperature metapelites

“…Ferric sillimanite from metapelites in amphibolite facies, 'normal granulite' facies and UHT metamorphism shows average Fe 2 O 3 contents of 0.3 AE 0.2, 0.9 AE 0.5 and 1.4 AE 0.3 wt%, respectively (Figure 3a; Grew, 1980;Sengupta et al, 1991;Sarkar et al, 2003; (Brandt et al, 2007;Grew et al, 2006;Harley et al, 1990;Jiao et al, 2015Jiao et al, , 2017Jiao & Guo, 2020;Kelsey et al, 2003;Korhonen et al, 2011Korhonen et al, , 2012Korhonen et al, , 2013Korhonen et al, , 2014Li et al, 2019;Li & Wei, 2016, 2018Liu et al, 2012;Mitchell et al, 2014;Pownall et al, 2014;Sajeev et al, 2006;Shimizu et al, 2013, b;Tsunogae et al, 2011;Wang et al, 2020;Yang et al, 2014) Korhonen & Stout, 2004;Santosh et al, 2007;Shimizu, Tsunogae, Santosh, Liu, & Li, 2013;Wang et al, 2020), which implies that Fe 2 O 3 in sillimanite (Fe 2 O 3 [Sil]) increases with higher metamorphic grades. Ferric sillimanite contains Fe 2 O 3 contents of 0.4-2.6 wt% in gneissic xenoliths from basaltic rocks and of 0.8-1.3 wt % in peraluminous granitoids (Figure 3a;Grew, 1980;Sassi et al, 2004).…”

“…HMg, high MgO type; LMgHO, low MgO and high oxygen fugacity type; LMgLO, low MgO and low oxygen fugacity type. Data are collected from literature (Brandt et al, 2007; Grew et al, 2006; Harley et al, 1990; Jiao et al, 2015, 2017; Jiao & Guo, 2020; Kelsey et al, 2003; Korhonen et al, 2011, 2012, 2013, 2014; Li et al, 2019; Li & Wei, 2016, 2018; Liu et al, 2012; Mitchell et al, 2014; Pownall et al, 2014; Sajeev et al, 2006; Shimizu et al, 2013, b; Tsunogae et al, 2011; Wang et al, 2020; Yang et al, 2014)…”