Liu Dunyi

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 combination with previous studies, the main conclusions are as follows: (1) protolith of the khondalite series 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.

Zircon SHRIMP U-Pb dating of meta-diorite from the basement of the Songliao Basin and its geological significance

The basement of the Songliao Basin mainly contains low-grade metamorphic rocks and granites. It has been long disputed whether the basin has Precambrian metamorphic basement. This is a report of zircon SHRIMP U-Pb dating results of a meta-diorite sample, which was taken from the Si-5 drilling hole in the southern portion of the Songliao Basin. The SHRIMP analyses indicate that the meta-diorite with a weighted mean 207Pb/206Pb age of 1839±7 Ma (2σ, n = 8) was emplaced during Paleo-Proterozoic time. Additionally, the meta-diorite has old Nd model ages (TDM1: 2999Ma; TDM2: 2849Ma). These data suggest that the southern part of the Songliao Basin do possess Precambrian basement.

SHRIMP zircon U-Pb dating of the Gangou granitoids, Central Tianshan Mountains, Northwest China and tectonic significances

Two types of granitoids, highly deformed augen granitoids and undeformed fine-grained granitoids, are widely distributed in Gangou area, Central Tianshan Mountains, Northwest China. The augen granitoids are high-K calc-alkaline characterized by high K2O, Rb, Y and Th, and low Sr and Sr/Y. They also have high contents of large ion lithophile elements (LILE), such as Ba, Rb, K and Th, and relatively low contents of high field strength elements (HFSE), such as Zr, Y and Nb. The fine-grained granitoids are calc-alkaline characterized by high Sr, low Y and HREE, similar to adakitic rocks, and with Na2O/K2O<2 (1.76–1.91), high ISr (0.70689–0.70981), and negative εNd(t) (−2.4–−5.3). High-precision SHRIMP U-Pb zircon dating results indicate that the Gangou augen granitiods were formed at 428 ±10 Ma, and the fine-grained granitoids were emplaced at 361–368 Ma. These geochemical and U-Pb zircon data have significant implications for the timing of closure of the Mishigou-Gangou Ocean and the evolution of the Central Tianshan orogenic belt.

First report of zircon SHRIMP U-Pb dating from the Dufek granite in the Sør Rondane Mountains, East Antarctica

We report here the first zircon SHRIMP U-Pb chronological result yielding age of (619±7) Ma (11 spots) for the Dufek granite, the largest one of the Early Paleozoic granitoids in the Sør Rondane Mountains, East Antarctica. The analyzed zircon crystals are euhedral, have zonal patterns and Th/U ratios of 0.22∼1.04, indicating magmatic genesis. The single zircon crystal has the 206Pb/238U age of 664 Ma to 600 Ma with the analyzed spots being from the core parts and mantle-rim part of the zircon grains. The result of 619 Ma analysis shows beyond doubt that the Dufek granite formed during 620 Ma is much older by at least 70 Ma than that of the previously widely believed (500∼550 Ma) obtained by Rb-Sr, Sm-Nd and K-Ar isotopic analyses of the Sør Rondane Mountains Pan-African granitoids. Previous chemical characteristics and isotopic data supported that the Dufek granite has chemical affinity with A-type granites under within-plate setting. The U-Pb age of 620 Ma for magmatic zircons in the Dufek granite therefore may represent the age of earlier melt crystallization that occurred at the Late Neo-Proterozoic during the extensional tectonic setting during/after Gondwanaland formation.

Eastern Ancient Terrane of the North China Craton

Based on the spatial distribution of ancient rocks and zircons, three ancient terranee older than ca. 2.6 Ga have recently been identified in the North China Craton, namely the Eastern, Southern, and Central Ancient Terranee. The Eastern Ancient Terrane is the best studied and understood of the three ancient terranes. It has a long geological history back to ca. 3.8 Ga ago and includes the areas of Anshan-Benxi, eastern Hebei, eastern Shandong and western Shandong. In Anshan-Benxi, several different types of 3.8 Ga rocks were discovered together with 3.1–3.7 Ga rocks, whereas 2.9–3.0 Ga K-rich granites and 2.5 Ga syenogranite occur on larger scales. In eastern Hebei, 3.0–3.4 Ga rocks and older detrital and xenocrystic zircons were identified. In eastern Shandong, there are a large volumes of 2.7 Ga and 2.9 Ga rocks. In western Shandong, early Neoarchean (2.6–2.7 Ga) intrusive and supracrustal rocks are widely distributed. Whole-rock Nd and zircon Hf isotope data suggest that both mantle additions and crustal recycling played important roles within the Eastern Ancient Terrane during almost every tectono-magmatic event. Most BIFs in the North China Craton are late Neoarchean in age and are distributed on continental crust along the western margin of the Eastern Ancient Terrane, probably suggesting that a stable environment was one of the key factors for the formation of large-scale BIFs.

Zircon SHRIMP age of Mesoarchaean meta-argilloarenaceous rock in the Anshan area and its geological significance

In order to better understand the early continental evolution of the Anshan area, one of the typical Precambrian distribution areas of the North China Craton, the geochronology and REE composition of the zircons from the meta-argillo-arenaceous rock occurred as enclave in 3.1 Ga Lishan trondhjemite are studied by using SHRIMP II ion microprobe. It is indicated that the Paleoarchaean is a very important continental formation period in the Anshan area and 3.2 Ga can be regarded as the boundary between the Paleoarchaean and Mesoarchaean.