Shujuan Jiao

Application of the two-feldspar geothermometer to ultrahigh-temperature (UHT) rocks in the Khondalite belt, North China craton and its implications

Abstract The Paleoproterozoic Khondalite belt in the North China craton preserves evidence for ultrahightemperature (UHT) crustal metamorphism associated with the collision of the Yinshan and Ordos Blocks. Here we apply two-feldspar geothermometry to UHT granulites from two localities newly reported in this study (Tuguishan and Xuwujia) and another two localities from previous studies (Dajing/ Tuguiwula and Dongpo) in the Khondalite belt. The presence of abundant perthite/mesoperthite in these rocks reflects post-peak slow cooling. The minimum estimated peak metamorphic temperatures are 832-998, 819-952, 844-1037, and 966-1019 °C computed at 8 kbar for the Dajing/Tuguiwula, Dongpo, Tuguishan, and Xuwujia areas, respectively. These results confirm the previous report of extreme metamorphism at Dajing/Tuguiwula and Dongpo, and reveal similar conditions in the new localities reported here, suggesting that UHT metamorphism is widespread in the Khondalite belt of the North China craton. Our study demonstrates that UHT metamorphism can be recognized using the two-feldspar geothermometry in rocks that do not possess other key UHT assemblages

Paleoproterozoic Granulites in the North China Craton and Their Geological Implications

It is well known that there are widespread Paleoproterozoic high-grade rocks, mainly garnet amphibolites and granulites, in the North China Craton. The granulites could be classified into two types, the high-pressure granulites and (ultra-)high-temperature granulites. The high-pressure granulites mainly distribute in the Huai’an-Xuanhua, Hengshan, Fuping, Zanhuang, Qianlishan–Helanshan, and Jiaobei regions (the central and east of the craton), while the (ultra-)high-temperature granulites distribute mainly in the Liangcheng–Jining–Zhuozi and the Daqingshan regions (west of the northern margin of the craton). Nevertheless, (ultra-)high-temperature granulites have been also discovered in northeast Hebei, north Henan, Shandong and Liaoning provinces, and the Korean peninsula, and some drilling cores from under the Ordos basin, though some of them might be formed in the Late Archean. The Paleoproterozoic high-grade metamorphism has partly overprinted on the Late Archean low- to high-grade rocks. Both types of granulites have recorded clockwise P-T paths, which might indicate a process similar to those happened in the Phanerozoic orogenic belts. In the past 20–30 years, several different tectonic models have been introduced to interpret the environments of the Paleoproterozoic granulites in the North China Craton. However, there are several things need to be clarified before any conclusion, e.g., (1) the distribution of the different types of high-grade rocks; (2) the relationship between the high-pressure granulites and the (ultra-)high-temperature granulites; and (3) the exhumation rate and genetic mechanism of these rocks. These Paleoproterozoic high-grade rocks in the North China Craton are more complex than any high-grade rocks in the Phanerozoic orogens, and await further study.

In situ determination of hafnium isotopes from rutile using LA-MC-ICP-MS

The hafnium isotopic analysis using laser ablation has been widely conducted on Hf-rich minerals (zircon/baddeleyite/calzirtite/eudialyte), however, little work has been reported on Hf-poor (<100 ppm) minerals. This work presents a detailed procedure of in situ hafnium isotopic analysis from rutile using laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). The rutile U-Pb dating reference material JDX shows homogeneous hafnium isotopic ratios, with 176Hf/177Hf=0.281795±0.000015 (2SD, n=33) and 176Lu/177Hf=0.000018±0.000004 (2SD, n=17) that suggest the possibility of using JDX as a new reference material hafnium isotopic measurement. We also measure hafnium isotopic compositions of other rutile U-Pb dating reference material (R10, Sugluk-4 and PCA-S207) and the 176Hf/177Hf values are similar to previously reported results, which confirms that we can acquire accurate and precise hafnium isotopic compositions using our developed analytical protocol. We analyzed hafnium isotopic compositions and U-Pb ages of rutile in high-temperature and ultrahigh-temperature granulites from various terrains of the Khondalite Belt from the North China Craton, combined with zircon results in the same area, suggesting that the metamorphic evolution history of the granulite is much more complicated than previously thought.