Phase equilibria modelling and zircon age dating of pelitic granulites in Zhaojiayao, from the Jining Group of the Khondalite Belt, North China Craton

Abstract

The Jining Group occurs as the eastern segment of the Khondalite Belt, North China Craton and is dominated by a series of granulite facies rocks involving ‘normal’ pelitic granulites recording peak temperatures of ~850 °C and ultrahigh‐temperature (UHT) pelitic granulites recording peak temperatures of 950–1100 °C. The P–T paths and ages of these two types of granulites are controversial. Three pelitic granulite samples in the Jining Group comprising two sillimanite–garnet gneiss samples (J1208 and J1210) and one spinel–garnet gneiss sample (J1303) were collected from Zhaojiayao, where ‘normal’ pelitic granulites occur, for determination of their metamorphic evolution and ages. Samples J1208 and J1210 are interpreted to record cooling paths from the Tmax stages with P–T conditions respectively of ~870–890 °C/7–8 kbar and >840 °C/>7.5 kbar constrained from the stability fields of the observed mineral assemblages and the isopleths of plagioclase, garnet and biotite compositions in pseudosections. Sample J1303 is interpreted to record pre‐Tmax decompression from the kyanite‐stability fields to the Tmax stage of 950–1020 °C/8–9 kbar and a post‐Tmax cooling path revealed mainly from the stability field of the observed mineral assemblage, the plagioclase zoning and the biotite composition isopleth in pseudosections. The post‐Tmax cooling stage can be divided into suprasolidus and subsolidus stages. The suprasolidus cooling may not result in an equilibrium state at the solidus in a rock. Therefore, different minerals may record different P–T conditions along the cooling path; the inferred maximum temperature is commonly higher than the solidus as well as different solidi being recorded for different samples from the same outcrop but experiencing different degrees of melt loss. Plagioclase compositions, especially its zoning in plagioclase‐rich granulites, are predicted to be useful for recording the higher temperature conditions of a granulite's thermal history. The three samples studied seem to record the temperature range covering those of the ‘normal’ and UHT pelitic granulites in the Jining Group, suggesting that UHT conditions may be reached in ‘normal’ granulites without diagnostic UHT indicators. LA‐ICP‐MS zircon U–Pb data provide a continuous trend of concordant 207Pb/206Pb ages from 1.89 to 1.79 Ga for sample J1210, and from 1.94 to 1.80 Ga for sample J1303. These continuous and long age spectrums are interpreted to represent a slow cooling and exhumation process corresponding to the post‐Tmax cooling P–T paths recorded by the pelitic granulites, which may have followed the exhumation of deeply buried rocks in a thickened crust region resulted from a collision event at c. 1.95 Ga as suggested by the pre‐Tmax decompression P–T path.

Ultrahigh‐temperature metamorphism in the Helanshan complex of the Khondalite Belt, North China Craton: Petrology and phase equilibria of spinel‐bearing pelitic granulites

“…By comparing the X gr of garnet in the sample with calculated isopleths of X gr in the pseudosection (Figures b and b), the peak metamorphic conditions are estimated to have been 960–1,030°C and 6.3–7.3 kbar (Figures b and b), under a highly elevated geothermal gradient of ~150°C/kbar. These P–T conditions are comparable to the results constrained by phase diagram calculations for other pelitic UHT granulites in Khondalite Belt, NCC, which include: (a) the peak metamorphic conditions of 910–980°C and ∼7–9 kbar for the sapphirine‐bearing pelitic granulites in Daqingshan complex (Guo et al., ); (b) the peak metamorphic conditions of 950–1,020°C and 8–9 kbar for the spinel‐bearing pelitic granulites from Zhaojiayao of the Jining complex (Li & Wei, ); and (c) the peak metamorphic conditions of ∼1,030°C and 6.5–7.5 kbar for the spinel‐bearing pelitic granulite from Heling'er of the Jining complex (Liu et al., ). If we select the solution model of Elkins and Grove (), the temperatures derived from phase equilibria modelling are close to those derived from the ternary feldspar thermometry (>940°C) in this study, which are similar to those for sapphirine‐bearing UHT granulites by the ternary feldspar thermometry using the same solution model (Santosh, Tsunogae, et al., ).…”

“…1.95–1.94 Ga (Figure b) and experienced the slow close‐to‐isobaric cooling during the period of c . 1.94–1.83 Ga (Gou, Zhang, & Wang, ; Jiao et al., ; Li & Wei, ; Lobjoie et al., ; Zhang et al., ). Thus, we infer that the UHT pelitic granulites were formed in a two‐sided hot collision between the Yinshan and Ordos Terranes at c .…”

“…Except for the UHT metamorphism of c . 1.94–1.92 Ga (Guo et al., ; Li & Wei, , ; Lobjoie et al., ; Santosh, Wilde, et al., ; Santosh et al., ), the UHT metamorphic rocks with ages of c . 1.88–1.85 Ga were also reported (Jiao et al., ; Yang et al., ), which is consistent with that multiple mafic magmatic events that occurred in the region (Liu et al., ; Lobjoie et al., ; Peng et al., ).…”

“…In this interpretation, younger metamorphic ages in the range c . 1.94–1.83 Ga (Dong et al., ; Jiao et al., ; Wan et al., ; Yin et al., ; Zhao et al., ) are probably related to slow close‐to‐isobaric cooling following decompression from the peak pressure conditions (Dong et al., ; Li & Wei, ; Yin et al., , ; Zhao et al., ). Thus, the peak pressure metamorphism should occur at c .…”

Anatectic origin and geological significance of the Neoarchean ~2.5 Ga charnockite in the Jining area, northern North China Craton

“…Jining–Hohhot–Zhuozi was then reworked by a second stage of metamorphism at ~1.95 Ga. Peak metamorphism of the khondalite series belt occurred at 1,950–1,920 Ma, with conditions of 830°C and 0.90–1.10 GPa (Wang et al, ; Cai, Liu, Liu, Liu, et al, ; Li & WeiI, ; Cai et al, ). Previous estimates of metamorphic constraints yielded 875°C–975°C and 0.7–0.9 GPa, and 750°C –1,000°C and 0.8–1.2 GPa, for charnockites and granulites, respectively.…”