Lilin Du

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.

Provenance of the Paleoproterozoic Hutuo Group basal conglomerates and Neoarchean crustal growth in the Wutai Mountains, North China Craton: Evidence from granite and quartzite pebble zircon U-Pb ages and Hf isotopes

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 207Pb/206Pb 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 ɛHf(t) values, a minority with ɛHf(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.