Hangqiang Xie

Juvenile magmatism and crustal recycling at the end of the Neoarchean in Western Shandong Province, North China Craton: Evidence from SHRIMP zircon dating

Western Shangdong Province experienced major crustal growth as a result of 2.75 to 2.50 Ga tectonothermal events, different from other Archean areas in the North China Craton. Besides early Neoarchean tonalite-trondhjemite-granodiorite (TTG) and supracrustal assemblages, there are large volumes of late Neoarchean gabbroic, dioritic and granitic rocks in western Shandong. SHRIMP zircon dating of thirty-one samples of different rock types from late Neoarchean rocks yielded a narrow age range from 2560 to 2490 Ma. Based on these data and previously published results, the following conclusions can be drawn: 1) the Archean basement can be divided into three belts: a Late Neoarchean (2525-2490 Ma) crustally-derived granite belt in the northeast, an Early Neoarchean (2.75-2.60 Ga) rock belt in the center, and a Late Neoarchean (2550-2500 Ma) belt of juvenile rocks in the southwest; 2) the tectonic regime in western Shandong Province probably changed from compressional to extensional at around 2525 Ma (between 2530 and 2520 Ma); 3) crustal recycling with addition from the mantle occurred at the end of the Neoarchean; 4) the late Archean magmatic rocks were probably formed in an arc environment.

Is the Ordos Block Archean or Paleoproterozoic in age? Implications for the Precambrian evolution of the North China Craton

The Ordos Block is a large but poorly exposed crustal segment in the western part of the North China Craton. Little is known about its Precambrian basement rocks and their evolution because of an extensive cover by younger sedimentary rocks. We report SHRIMP zircon ages and Hf-in-zircon isotopic compositions for five samples recovered from drill holes that penetrated into the Ordos basement. Based on these data, two age groups of rocks can be distinguished: an early Paleoproterozoic group that is represented by one sample from the northern margin of the Ordos Block and a late (or middle-late) Paleoproterozoic group consisting of the remaining four samples. The early Paleoproterozoic sample contains zircon cores and metamorphic rims with ages of >2.4 Ga and >2.28 Ga, respectively. The cores have εHf(t) values of −8.8 to 6.2 and Hf crustal model ages of 2606 to 3221 Ma. The remaining rocks of late (or middle-late) Paleoproterozoic age contain zircons that may also show a core-rim structure, but with the cores having ages of ∼2.08 Ga and the metamorphic rims recording ages of ∼1.9 Ga. The zircon cores have εHf(t) values of −9.1 to 10.1, and Hf crustal model ages of 2039 to 3068 Ma. Combined with data from earlier studies, we draw the conclusion that late Paleoproterozoic metasedimentary rocks are widespread in the Ordos basement. There is no evidence from our samples of an Archean component, and this calls into question the prevailing view that the Ordos basement is an Archean cratonic block. The fact that the Ordos basement was involved in a widespread late Paleoproterozoic tectono-thermal event indicates that most current models for the tectonic evolution of the western portion of the North China Craton need to be revised.

SHRIMP zircon U-Pb dating of late Paleoproterozoic kondalites in the Daqing Mountains area on the North China Craton

The Daqing Mountains area comprises a typical occurrence of the Khondalite Belt in the Western Block of the North China Craton (NCC). In this area, both early and late Paleoproterozoic metasedimentary rocks have been identified in what was originally called the Upper Wula Mountains “Subgroup”. Six metasedimentary rock samples yielded SHRIMP U-Pb zircon ages of 2.56–2.04 Ga for detrital and 1.96–1.83 Ga for metamorphic zircons. Based on these data and previously published results, the following conclusions can be drawn: 1) The source region for the late Paleoproterozoic detrital sedimentary rocks is mainly 2.55–2.4 and 2.2–2.04 Ga in age, consistent with the early Precambrian geological history identified widely in the basement of the NCC. 2) The majority of sedimentary rocks of the khondalite series were deposited between 2.04 and 1.95 Ga, and then in a protracted period (1.96 and 1.83 Ga) underwent a complex history of amphibolite to granulite-facies metamorphism.

Widespread late Neoarchean reworking of Meso- to Paleoarchean continental crust in the Anshan-Benxi area, North China Craton, as documented by U-Pb-Nd-Hf-O isotopes

We present an integrated study of zircon U-Pb dating and O-Hf isotopic analyses combined with whole-rock geochemistry and Nd isotopic systematics on widespread late Neoarchean syenogranites (Qidashan Pluton) and other granitoids (granodiorite, quartz monzonite, monzogranite) in the Anshan-Benxi area, eastern North China Craton. All these rocks were emplaced at ca. 2.5 Ga according to SHRIMP zircon U-Pb dating and other indirect methods. The syenogranites are characterized by high SiO2 and K2O/Na2O and low CaO, FeOt, MgO, TiO2 and P2O5. However, they differ in trace element and REE compositions and can be roughly subdivided into two types. Type 1 syenogranite has strongly negative Eu anomalies and Ba depletion, with large variations of LREE to HREE differentiation; type 2 syenogranite has no negative Eu anomaly and Ba depletion. Other granitoids are also rich in K2O and show similar trace element and REE compositional features as the type 2 syenogranite. Most syenogranite samples have whole-rock εNd(t) values of –10.5 to –2.7 and tDM(Nd) ages of 2.96 to 3.90 Ga, whereas the other granitoids have εNd(t) values of –5.2 to –2.2 and tDM(Nd) ages of 2.95 to 3.19 Ga. Magmatic zircon from the syenogranites has εHf(t) values and Hf crustal model ages ranging mainly from –11.0 to +4.4 and 2.70 to 3.46 Ga, respectively; those from other granitoids have εHf(t) values and Hf crustal model ages of –16.0 to +2.4 and 2.81 to 3.72 Ga, respectively. All these rocks contain Meso- to Paleoarchean xenocrystic zircon grains with εHf(t) values and Hf crustal model ages ranging from –14.3 to +8.3 and 2.79 to 3.93 Ga, respectively. Decoupling of whole-rock Nd and Hf-in-zircon isotopes occurs in some samples. Most low-U magmatic zircon (U<200 ppm) from all granitoids has δ18O values in the range of +4 to +7.2 permil, whereas high-U magmatic zircon (U≥200 ppm) shows a larger variation with the lowest δ18O value being 1.6. The xenocrystic zircon grains have δ18O values in the normal range of Archean magmatic zircon. Combined with previous studies, we conclude that the 2.5 Ga granitoids formed by recycling of mature, old continental crust, including metasedimentary sources, probably in an extensional tectonic environment.

ZIRCON AGES AND METAMORPHIC EVOLUTION OF THE ARCHEAN ASSEGAAI-DE KRAALEN GRANITOID-GREENSTONE TERRAIN, SOUTHEASTERN KAAPVAAL CRATON

The Assegaai, De Kraalen and Witrivier greenstone belts in the southeast part of the Kaapvaal craton contain strongly deformed volcano-sedimentary successions affected by amphibolite- to granulite-facies metamorphism at ∼3.22 to 3.20 Ga. These greenstone belts consist predominantly of mafic-ultramafic volcanic successions intercalated with chert and BIF. Single zircon SHRIMP ages of 3222±8 and 3193±5 Ma for two granitoids intrusive into the Assegaai greenstone belt provide a minimum age for the succession. Calc-silicates and amphibolites of the De Kraalen and Witrivier greenstone belts record a high-pressure metamorphic event (M1) at ∼12 to 15 kbar and ∼600 °C and 800 °C, respectively. This high-pressure event was overprinted by a medium-pressure (7-8 kbar) amphibolite-facies event (M2A) that was recorded in all three greenstone belts, suggesting that the De Kraalen and Witrivier supracrustal rocks were buried to a deeper crustal level than the Assegaai rocks during crustal thickening. Decompression of the rocks along clockwise P-T paths possibly occurred during subsequent crustal extension at ∼3.22 to 3.20 Ga. The low geothermal gradient of this area (15-20 °C/km) contrasts with higher geothermal gradients reported for other Archean terrains worldwide and may support the view that crustal thickening processes related to convergence in a setting similar to subduction zones were operating in Paleoarchean times.

Formation Ages and Environments of Early Precambrian Banded Iron Formation in the North China Craton

The North China Craton (NCC) has had a long geological history back to ca. 3.8 Ga ago, but the most important tectonothermal event occurred at the end of the Neoarchean, the most important period of BIF formation. There are three ancient terranes (>2.6 Ga) in the NCC. Most BIFs are distributed along the western margin of the Eastern Ancient Terrane, accounting for about 89 % of the total identified BIF iron ore resources in the NCC. They are considered to have formed on a continental basement in terms of rock association of the BIF-bearing supracrustal sequences which were intruded by slightly younger crustally derived granites. Most BIFs in the NCC show positive Y anomalies, implying that Fe in the BIFs had an affinity to seawater; however, positive Eu anomalies in the BIFs may suggest that high-temperature fluids also played a role in their formation. It seems that the BIFs were deposited on a continental margin, or more likely a back-arc basin environment, and a stable environment was one of the key factors for the formation of large-scale BIFs. In this paper, we provide new BIF metallogenic prospects for the NCC, namely in the northeast of Anben, between Anben and eastern Hebei, and between eastern Hebei and western Shandong.