Search

The Gantaohe Group, located in the Zanhuang area, contains typical North China Craton Paleoproterozoic strata, and is one of the best-preserved examples of these rocks. It is therefore a significant area for deciphering evolutionary process in the trans-North China Orogen. In this paper, we present petrologic, detrital and volcanic zircon U–Pb ages and Lu–Hf isotopes from three formations of the Gantaohe Group. The zircons from a conglomerate at the base of the Gantaohe Group yielded a young population of ca. 2.1 Ga. Dacite from the upper section of the Nansizhang Formation yielded zircon 207 Pb/ 206 Pb ages of 2090 ± 14 Ma. Thus, the initial deposition of the Gantaohe Group occurred at ∼2.1 Ga. The Group underwent greenschist to low amphibolite facies metamorphism, identical to regional 1.87–1.85 Ga metamorphism in Zanhuang Complex. Therefore, we propose that the Gantaohe Group was deposited in the period between 2.1 Ga and 1.9 Ga. Sandstones in the Gantaohe Group are dominated by Q (monocrystalline and polycrystalline quartz) and F (K-feldspar and plagioclase) with only minor lithic fragments, suggesting that detrital components were mainly derived from a cratonic source. Detrital zircons from sandstones exhibit a dominant population of 2.55–2.5 Ga grains with minor contributions of ∼2.7 Ga and ∼2.1 Ga grains, which further indicate they were dominantly sourced from ∼2.5 Ga TTG gneisses, late Archean Zanhuang and late Archean to Paleoproterozoic Guandu Groups, with subordinately input from ∼2.7 Ga TTG gneisses and the Paleoproterozoic Xuting granite. Given that the sandstone clastic components chiefly comprise quartz and feldspar, there is a lack of volcanic lithics, and the major detrital zircons are 400 Ma older than the initial deposition age, we propose that the Gantaohe Group was deposited in a rift-related setting far from any back-arc basin at ∼2.1 Ga or 1880–1810 Ma syn-collisional processes. A few 2.5–2.7 Ga zircons have positive ε Hf ( t ) values identical with the contemporaneous depleted mantle and the 207 Pb/ 206 Pb ages are similar to their single stage model ages ( T DM1 ), which suggests some degree of 2.5–2.7 Ga crustal growth. Most ∼2.5 zircons, however, show 207 Pb/ 206 Pb ages that are much younger than the Hf single stage model ages, which indicates intensive crustal reworking in the late Neoarchean. The zircons collectively present two stage model ages ranging from 2.6 to 2.9 Ga. Therefore, we suggest that the 2.6–2.9 Ga periods represents a time of intensive crustal growth in the Zanhuang Complex. [ABSTRACT FROM AUTHOR]

Abstract: SHRIMP zircon U–Pb dating of the Huangjinshan granite porphyry in the Wutai area, North China Craton (NCC) yielded concordant ages, with a weighted mean 207Pb/206Pb age of 2137±9Ma, which indicates that the granite was emplaced in the middle Paleoproterozoic. The granite porphyry is enriched in SiO2, REEs (except Eu), Zr, Nb, Ga, Y and Zn, and depleted in MgO, CaO, Al2O3, Sr, Ba, V, Cr and Ni, with high FeO*/MgO ratios. With the zircon saturation temperatures ranging from 856 to 895°C and a lack on detected inherited zircon, this indicates it is a ‘hot’ A-type granite, of the type emplaced in anorogenic or extensional settings with high heat flow. Zircons in the granite porphyry have ε Hf(t) values ranging from −1.72 to +1.84, and Hf model T DM1 ages of 2429–2554Ma. The Hf isotopes indicate source of the granites was pre-Neoarchean continental crust mixed with mafic rocks derived from depleted mantle, or directly from the partial melting of ca. 2.5Ga crust. Given that there are no syn-plutonic gabbro enclaves in the Huangjinshan granite porphyry, it is preferred that it originated mainly from partial melting of ca. 2.5Ga upper crust without mixing with mantle-derived magma. 2.2–2.0Ga volcanic and plutonic rocks, which exhibit bimodal magma characteristics emplacing in rifting setting, are abundant in the central NCC. The results presented here are further evidence for that 2.2–2.0Ga is an important stage of Paleoproterozoic rifting in the central NCC. [Copyright &y& Elsevier]

The Gantaohe Group is an important early Precambrian unit in the Trans-North China Orogen, North China Craton, and is mainly composed of greenschist-facies metabasalt, meta-sandstone and dolomitic marble. We report whole-rock geochemical compositions and SHRIMP zircon ages as well as LA-ICP-MS Hf-in-zircon isotopeic analyses for metabasalts from the Gantaohe Group. SHRIMP dating yielded a weighted mean Pb/Pb age of 2087±16 Ma (MSWD=1.3) for magmatic zircons, but there are also abundant ca. 2.5 Ga inherited zircon xenocrysts. The magmatic zircons shows a large ɛ( t) variation in ɛ( t) from −7.17 to +0.45, suggesting an isotopically highly heterogeneous source for the metabasalt. Chemically all samples show no distinct Zr or Hf anomalies, and some samples show no Nd or Ta anomalies in a primitive mantle-normalized trace element variation diagram, and their whole-rock ɛ( t) values range from 4.0 to −0.8. We suggest that the basalt is formed by partial melting of a depleted mantle source, followed by significant crustal contamination. Field observations, the presence of abundant inherited zircon, as well as isotope and trace elements geochemistry support formation of the Gantaohe Group on top of a continental basement. These data and the regional geology lead us to conclude that the Trans-North China Orogen constituted an intracontinental rift during the Paleoproterozoic that was connected to the Eastern Block since the end of the Archean. [ABSTRACT FROM AUTHOR]

Zircon U-Pb ages (SHRIMP and LA-ICPMS) and Lu-Hf isotope data (LA-ICPMS) are presented for two granite and two quartzite pebbles from the basal conglomerates of the Sijizhuang Formation in the Hutuo Group from the Wutai Mountains area in the North China Craton. These two granite pebbles give zircon Pb/Pb ages of 2513±8 Ma and 2527±8 Ma respectively, which are consistent with the emplacement ages of the Wangjiahui grey granite and Guangmingsi or Shifo granite in the Wutai Mountains. Detrital zircons from those two quartzite pebbles are mostly 2550-2490 Ma old with lesser number of 2800-2550 Ma grains, which is similar to the ages of detrital zircons from quartzites in the Gaofan Subgroup of the Neoarchean Wutai Group. Thus, the pebbles in the Hutuo Group basal conglomerates were derived locally from Wutai Mountains Neoarchean sources. Zircons from the Sijizhuang Formation conglomerate granite and quartzite pebbles mostly have positive ɛ( t) values, a minority with ɛ( t) values like model depleted mantle (DM) of the same age, but with most showing DM model ages 200-100 Ma. This indicates that most of the source materials were derived from the mantle within the previous 200 million years, whereas some are derived from 2550-2510 Ma juvenile crustal additions. This additional evidence suggests that in the North China Craton there was important initial polycyclic crustal formation and cratonization in the late Neoarchaean, prior to superimposed Palaeoproterozoic orogenic cycles. [ABSTRACT FROM AUTHOR]