Retrograde metamorphism of ultrahigh‐temperature granulites from the khondalite belt in Inner Mongolia, North China Craton: evidence from aluminous orthopyroxenes

Abstract

Orthopyroxene-sillimanite-sapphirine-spinel-quartz granulites have been reported from the khondalite belt in Inner Mongolia, North China Craton (NCC), which record ultrahigh-temperature (UHT) metamorphism. These rocks record metamorphic temperatures up to 10508C and pressures up to 10 kbar during the peak metamorphism, as indicated by diagnostic UHT mineral assemblages such as equilibrium orthopyroxene þ sillimanite þ quartz, sapphirine þ quartz and low-Zn-spinel þ quartz. To well constrain the retrograde P-T conditions, we investigate orthopyroxenes in different textural settings in these UHT rocks and the results reveal marked variations in alumina contents, indicating a multistage formation. The core of coarse orthopyroxene porphyroblasts and some of the grains associated with sillimanite and quartz have the highest alumina content of up to 7.61-8.05 wt%. Rims of coarse orthopyroxene porphyroblasts and those occurring as coronas surrounding garnet grains have intermediate alumina value ranging from 6.21 to 7.61 wt%. Fine grained orthopyroxenes which form symplectite with cordierite have the lowest alumina content between 5.52 and 6.15 wt%. Based on the X Al -X Mg isopleths, orthopyroxenes in different textural associations and textures indicate formation under varying temperature and pressure conditions, ranging from 9508C, 10.5 kbar to 8408C, 8.5 kbar. These results are in accordance with the P-T estimates from previous studies on these rocks using Ti-in-zircon geothermometry and pseudosection analysis. The reaction textures and orthopyroxene chemistry suggest an initial stage of isobaric cooling followed by isothermal decompression, and a final stage of cooling. Combined with the available age data which suggest the timing of the UHT event as 1.92 Ga, and the previous models on P-T evolution of the khondalites, our results suggest a complex exhumation of the UHT granulites within the Inner Mongolia Suture Zone associated with the final assembly of the NCC within the Columbia supercontinent during the Palaeoproterozoic.

Paleoproterozoic granulites from Heling'er: Implications for regional ultrahigh-temperature metamorphism in the North China Craton

“…Thus, Harley (2008) suggested that the assemblage of spinel + quartz should not be used as a diagnostic assemblage for UHT metamorphism and the composition of spinel should be checked carefully especially for Zn, Cr, V, Ti, Ni and Fe 3+ . As an indicative assemblage, spinel + quartz was reported in many cases where other diagnostic UHT assemblages are present, such as from the UHT rocks in Madurai Block of southern India (Tsunogae and Santosh, 2006), the Highland Complex in Sri Lanka (Sajeev and Osanai, 2004), Tuguiwula in the North China Craton Liu et al, 2008Liu et al, , 2010Liu et al, , 2011Santosh et al, 2007aSantosh et al, , 2009), among other examples. These spinels are characterized by low concentration of ZnO (b2.5 wt.%) and Cr 2 O 3 (b0.63 wt.%), and low X Fe 3+ value (Fe 3+ /Fe 3+ +Fe 2+ b 0.15) and could therefore stabilize under UHT conditions.…”

Short-lived high-temperature prograde and retrograde metamorphism in Shaerqin sapphirine-bearing metapelites from the Daqingshan terrane, North China Craton

“…1; e.g. Guo et al, 2012;Liu et al, 2010Liu et al, , 2011Santosh et al, 2006Santosh et al, , 2007aSantosh et al, ,b, 2009aSantosh et al, ,b, 2012Shimizu et al, 2013;Tsunogae et al, 2011). This implies that the Khondalite Belt might be a Paleoproterozoic hot orogen.…”

On ultrahigh temperature crustal metamorphism: Phase equilibria, trace element thermometry, bulk composition, heat sources, timescales and tectonic settings

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