Shiwen Xie

Formation and Evolution of Archean Continental Crust of the North China Craton

The North China Craton (NCC) has had a long geological history back to ca. 3.8 Ga ago. In the Anshan area, northeastern part of the craton, three distinct complexes with ages of 3.8–3.1 Ga (Baijiafen, Dongshan, and Shengousi) have been identified, along with widespread 3.1–2.5 Ga rocks of different origins and ages. In eastern Hebei Province, abundant 3.88–3.4 Ga detrital zircons were obtained from metasedimentary rocks of the Caozhuang Complex, and the oldest rock identified is a 3.4 Ga gneissic quartz diorite. The oldest zircons that may originally have been derived from the NCC are 4.1–3.9 Ga grains in Paleozoic volcano-sedimentary rocks in the northern Qinling Orogenic Belt bordering the NCC in the south. 3.0–2.8 Ga rocks occur in Anshan, eastern Hebei, eastern Shandong, and Lushan. ca. 2.7 Ga rocks of igneous origin are exposed in eight areas of the NCC, but ~2.7 Ga supracrustal rocks have so far only been identified in western Shandong. ca. 2.5 Ga intrusive and supracrustal rocks and associated regional metamorphism occur in almost all Archean areas of the NCC. Banded iron formations contain the most important ore deposit of the Archean in the NCC and mainly formed during the late Neoarchean. Ancient crustal records obtained from deep crust beneath the NCC are similar to those in the exposed areas, with the oldest ca. 3.6 Ga rock enclaves occurring in Xinyang near the southern margin of the NCC. This synthesis is based on the compilation of a large database of zircon ages as well as whole-rock Nd isotopic and Hf-in-zircon isotopic data in order to understand the formation and evolution of the early Precambrian basement of the NCC. Considering the craton as an entity, there is a continuous age record from 3.8 to 1.8 Ga, and two tectono-thermal events are most significant in the late Neoarchean to the earliest Paleoproterozoic and late Paleoproterozoic history, with age peaks at ~2.52 and ~1.85 Ga, respectively. Whole-rock Nd and Hf-in-zircon isotopic data show similar features, documenting the addition of juvenile material to the continental crust at 3.8–3.55, 3.45, 3.35–3.3, 2.9, and 2.85–2.5 Ga with the late Mesoarchean to early Neoarchean being the most important period. Crustal recycling began as early as 3.8 Ga and continued until 3.25 Ga and appears to have played a more important role than juvenile additions between 3.25 and 2.90 Ga. After outlining the general geological history of the NNC basement, we discuss several issues relating to Archean crust formation and evolution and arrive at the following major conclusions: (1) Similar to several other cratons, the late Mesoarchean to early Neoarchean was the most important period for rapid production of continental crust, and the most intensive and widespread tectono-thermal event occurred at the end of the Neoarchean. (2) In our new tectonic model, we define and outline three ancient terranes containing abundant 3.8–2.6 Ga rocks, namely the Eastern Ancient Terrane, Southern Ancient Terrane, and Central Ancient Terrane. (3) Vertical magmatic growth is seen as the main mechanism of crust formation prior to the Mesoarchean. We favor a multi-island arc model related to subduction/collision and amalgamation of different ancient terranes in the late Neoarchean. (4) The NCC may already have been a large crustal unit as a result of cratonic stabilization at the end of the late Neoarchean, probably due to magmatic underplating.

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.

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.