Hf isotopes of the 3.8 Ga zircons in eastern Hebei Province, China: Implications for early crustal evolution of the North China Craton

Zircon U-Pb dating indicates that the fuchsite quartzite in eastern Hebei Province was derived from weathering and erosion of the 3.6-3.8 Ga granitic rocks. In-situ zircon Hf analyses show that the Lu-Hf isotopic system remained closed during later thermal disturbances. Zircons with concordant ages have Hf isotopic model ages of about 3.8 Ga, suggesting a recycling of this ancient crust. The ～3.8 Ga zircons have similar Hf isotopic compositions to those of chondrite, indicating that their source rocks (granitic rocks) were derived from partial melting of the juvenile crust which originated from a mantle without significant crust-mantle differentiation. Therefore, it is proposed that there was no large-scale crustal growth before ～3.8 Ga in eastern Hebei Province. Considering zircon Hf isotopic data from other areas, it is concluded that the most ancient crust in the North China Craton probably formed at about 4.0 Ga, and the possibility to find crust older than 4.0 Ga is very limited.

Palaeoproterozoic Khondalite Belt in the western North China Craton: New evidence from SHRIMP dating and Hf isotope composition of zircons from metamorphic rocks in the Bayan Ul-Helan Mountains area

Zircom U-Pb age and Hf isotope analyses were made on gneissic granite and garnet-mica two-feldspar gneiss from the Helanshan Group in the Bayan Ul-Helan Mountains area, the western block of the North China Craton (NCC). Zircons from the gneissic granite commonly show core-mantle-rim structures, with magmatic core, metamorphic mantle and rim having ages of 2323±20 Ma, 1923±28 Ma and 1856±12 Ma, respectively. The core, mantle and rim show similar Hf isotope compositions, with single-stage depleted mantle model ages (TDM1) of 2455 to 2655 Ma (19 analyses). Most of the detrital zircons from the garnet-mica two-feldspar paragneiss have a concentrated U-Pb age distribution, with a weighted mean 207Pb/206Pb age of 1978±17 Ma. A few detrital zircons are older (2871 to 2469 Ma). The age for metamorphic overgrown rim was not determined because of strong Pb loss due to their high U content. The zircons show large variation in Hf isotope composition, with TDM1 ages of 1999 to 3047 Ma. In com- bination with previous studies, the main conclusions are as follows: (1) protolith of the khondalite se- ries in the Helanshan Group formed during Palaeoproterozoic rather than the Archaean as previously considered; (2) The results lend support to the contention that there is a huge Palaeoproterozoic Khondalite (metasedimentary) Belt between the Yinshan Mountains Block and the Ordos Block in the Western Block of NCC; (3) The widely-distributed bodies of early Palaeoproterozoic orthogneisses in the Khondalite Belt might be one of the important sources for detritus material in the khondalite series; (4) Collision between the Yinshan Block, the Ordos Block and the Eastern Block occurred in the same tectonothermal event of late Palaeoproterozoic, resulting in the final assembly of the NCC.

In situ U-Pb dating of titanite by LA-ICPMS

Titanite is an ideal mineral for U-Pb isotopic dating because of its relatively high U,Th and Pb contents.Here,we developed a technique for U-Pb dating of titanite using the 193 nm ArF laser-ablation system and Agilent 7500a Q-ICP-MS.Standards of titanite (BLR-1 and OLT-1) and zircon (91500 and GJ-1) were dated using single spot and line raster scan analytical methods.To check the matrix effect,titanite (BLR-1) and zircon (91500) standards were analyzed as the external standards.The weighted mean 206 Pb/238 U ages of OLT-1 titanite are 1015±5 Ma (2,n=24) and 1017±6 Ma (2,n=24) by single spot and line raster scan analyses,respectively,using BLR-1 titanite as the external standard.These ages are consistent with its reference age of about 1014 Ma.However,using 91500 zircon as the external standard,the weighted mean 206 Pb/238 U ages are 917±4 Ma (2,n=24) and 927±5 Ma (2,n=24) for BLR-1 titanite,and 891±4 Ma (2,n=24) and 901±5 Ma (2,n=24) for OLT-1 titanite by single spot and line raster scan analyses,respectively.It is evident that these ages are ～12% younger than their reference values.Our results reveal that significant matrix effect does exist between different kinds of minerals during LA-ICPMS U-Pb age determination,whereas it is insignificant between same minerals.Therefore,same mineral must be used as the external standard for fractionation corrections during in situ LA-ICPMS U-Pb age analysis.In addition,we determined U-Pb ages for titanites from the Early Cretaceous Fangshan pluton,which indicates a rapid cooling history of this pluton.

Hf isotopes of zircon megacrysts from the Cenozoic basalts in eastern China

Cenozoic basalts are widely distributed in east- ern China, and some of them contain zircon megacrysts which are considered to be constituent mineral of the sub- continental lithospheric mantle (SCLM) and petrogenetically related to mantle metasomatism induced by addition of crustal materials. Using the Laser Ablation Multiple Collec- tor Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICPMS), zircon megacrysts from the Cenozoic ba- salts at Changle in Shandong, Mingxi in Fujian, and Penglai in Hainan provinces have been used for Hf isotopic analyses. The data indicate that there is no significant deviation for the different zircon grains in each locale, except those from Penglai. The obtained 176Hf /177Hf ratios are 0.28302 — 0.28308 for Changle, 0.28297—0.28300 for Mingxi, and 0.28288—0.28293 for Penglai, with corresponding εHf values of 8.7—10.8, 7.0—7.9, and 3.9—5.7, respectively. These data display that there existed some regional heterogeneity, but the Hf model ages clustere in the Phanerozoic. Therefore, it is inferred that metasomatism of the lithospheric mantle be- neath eastern China took place in the Phanerozoic, most probably in the Mesozoic-Cenozoic. However, the formation time of the lithospheric mantle is not clearly constrained based on the present Hf isotopic data.

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/177 Hf 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.

Zircon Hf isotope composition of metamorphic eclogite from Xindian,Dabie Terrain

Hf isotope measurement has been carried out for UHP metamorphic eclogite from Xindian by using LA-MC-ICP-MS technique. The result indicates that metamorphic growth zircon has high 176Hf/177Hf (0.282544―0.282612) and low 176Lu/177Hf (0.000004―0.000211) ratio,inherited and recrystallized proto-lith zircon has low 176Hf/177Hf (0.282266―0.282466) and high 176Lu/177Hf (0.000090―0.002144) composi-tions. The low 176Lu/177Hf of growth zircon comes from its decreasing of Lu and increasing of Hf during UHP process. The high 176Hf/177Hf deduced from high radiogenic 176Hf,which was produced from long-term evolution of high Lu/Hf ratio minerals. Partial recrystallization of protolith zircon would not cause reworking of Lu/Hf isotope in zircon. Compared to U-Pb,zircon Lu-Hf system has better stability. The initial Hf isotope composition of metamorphic growth zircon may represent the Hf isotope compo-sition of whole rock system at the same time. The initial εHf of +3.0 for metamorphic precursor formation of Xindian eclogite indicates that the source material mainly derived from weak depleted mantle or mixing of depleted mantle with old crust.