Archean Continental Crustal Accretion and Banded Iron Formations, Southeastern North China Craton

Abstract

The Huoqiu Complex (HQC) in the southeastern margin of the North China Craton (NCC) is dominated by Neoarchean TTG gneisses, amphibolites and voluminous metasediments. Geochemical characteristics of TTG gneisses such as high Sr/Y and (La/Yb)N with steep REE patterns and trace element modeling suggest that they are generated by partial melting of hydrous meta-basalts (amphibolites) at the base of a thickened mafic continental crust, leaving a rutile-bearing eclogite residue. LA-MC-ICPMS U–Pb age data from magmatic zircon grains show protolith emplacement ages of 2.76–2.71 Ga. Hf isotopic compositions of zircon grains from the amphibolite and TTG gneiss show e Hf(t) values of 2.4–15.5 and −3.0 to 1.5, respectively. The t DM2(Hf) model ages of the TTG gneisses range from 2.87 to 3.14 Ga, and are identical to the t DM1(Hf) ages of amphibolites (2.84–3.16 Ga) within analytical uncertainty, refining that the TTG gneisses formed by partial melting of amphibolite, and attest to large-scale remelting of the Mesoarchean continental rocks during early Neoarchean. The NCC represents one of only a few cratonic nuclei with a geological history extending back to the Eoarchean. However, extensive ca. 2.5 Ga crustal reworking has destroyed a considerable portion of the pre-existing crustal record, hindering the investigation of tectonothermal evolution prior to 2.5 Ga. The HQC represents rare material that survived the ca. 2.5 Ga tectonothermal events relatively intact, thus preserving valuable information on prior crustal growth. In situ detrital zircon U–Pb dating and Hf isotope analyses were conducted on three schists from three drilling holes cutting cross the basement of the HQC in order to decipher the nature (such as episodic vs. continuous, juvenile vs. reworked) and tectonic setting of continental crust formation and preservation, and to place the results into a broader geodynamic context. In combination with published data, the concordant age spectra of all detrital zircons (n = 204) yield 207Pb/206Pb ages of 2343–3997 Ma that cluster into two principal age populations with peaks at 3015 and 2755 Ma. The ca. 3.01 and 2.75 Ga zircons with positive e Hf(t) values plot close to the depleted mantle evolutional line and their U–Pb ages are similar to or only slightly younger than the tDM1 model ages, hence indicating at least two predominant episodes of juvenile continental crustal accretions (3.01 and 2.75 Ga). The older episode finds only a minor correspondence in other cratons, suggesting little juvenile crustal growth occurred globally at a time of subdued mantle-derived magmatism. By contrast, the younger episode is coincident with a global rise in superplume activity in the Neoarchean. According to the geochemical and geochronological data, the 3.01 Ga juvenile crust was likely generated in an island-arc subduction system, whereas the 2.75 Ga crustal rocks were probably formed during magmatic underplating and subsequent partial melting of lower crustal mafic rocks. Consequently, they record a tectonic transition from a compressive to an extensional setting on the southeastern margin of the NCC between 3.01 and 2.75 Ga. This sequence of the events heralds a shift, from a mixture of net crustal growth and crustal reworking during multiple short-lived magmatic pulses, to fragmentation and dispersal of the early continental nucleus within 250 Ma. The Huoqiu iron deposits with a typical banded iron formation (BIF) are hosted by Precambrian high-grade metamorphic rocks, mostly occurring as metasedimentary rocks. From three BIF samples 88 detrital zircons have been collected, they display clear oscillatory zoning, high Th/U ratio, and low to variable luminescence, indicating magmatic origin. There are two peaks of 2753 Ma and 2970 Ma in concordant 207Pb/206Pb ages distribution plot, which is well consistent with the protolith ages of regional wall rocks (e.g., TTG gneiss and amphibolite) in this region. These appearances in association with the relatively good linear correlations between Al2O3 and TiO2 for BIF and their wall rocks, suggest that at least minor terrigenous clastic sediments contributed to BIF deposition, which thus constrained the upper limited age of BIF deposition at 2.75 Ga. In addition, no detrital zircons record recently reported 2.71 Ga magmatic event in this region and neighboring blocks, probably constrained the lower limited age of BIF deposition at 2.71 Ga. The flysch rhythmic structure occurrence in BIF-bearing strata, abundant carbonate minerals such as primary breunnerite association with magnetite, and no volcanic record present within BIF ore body or adjacent rocks inferred that the Huoqiu BIF deposited in continental marginal sea or back-arc basin environment with little contributions from submarine volcanic hydrothermal fluids, thus belonged to Superior-type. They show similar REE patterns to seawater, however, the positive Eu anomalies and reduced Y/Ho ratios relative to seawater indicates a possible mixture of hydrothermal fluids. Thus, their material sources were dominated by seawater, with minor contributions from the volcanic hydrothermal fluids and terrigenous sediments.