Late Neoarchean geodynamic regime of the northeastern North China Craton: Constraints from metamorphosed volcanic rocks of the Anshan-Benxi greenstone belt

Abstract

The Anshan-Benxi greenstone belt in the northeastern North China Craton preserves ∼ 3.88 Ga to ∼ 2.5 Ga lithologies that exhibit based secular trends in their geological signatures and hosts large amounts of valuable Neoarchean banded iron formations. However, whether plume or subduction mechanism responsible for those iron deposits remains confusing. This contribution focuses on the metamorphosed volcanic rocks and aims to constrain their geodynamic regime, based on temporal and spatial distributions, petrogenesises and tectonic characteristics. The late Neoarchean metamorphosed volcanic rocks of the Anshan-Benxi greenstone belt are prominently exposed in the Waitoushan-Gongchangling-Benxi area and consist of plagioclase amphibolites, amphibole/mica plagioclase gneisses and amphibole/mica two-feldspar gneisses. All 107 metamorphosed volcanic rock samples may be geochemically and petrographically classified as tholeiites, tholeiitic to calc-alkaline basalts, andesite, and dacite-rhyolite. The tholeiites are characterized by approximately flat REE patterns with negligible Eu anomalies and show negative Nb anomalies on primitive normalized multiple element diagrams, consistent with a back-arc basin basalt origin. The tholeiitic to calc-alkaline basalts have slightly heavy REE-depleted patterns and obvious negative Nb anomalies, which were likely produced by partial melting of mantle peridotites that were metasomatized by subducted slab-released fluids and melts. They may be comparable to island arc basalts. The andesites exhibit notably high MgO, Mg# and transition metal element contents, low FeOT/MgO ratios, fractionated REE patterns and evident negative Nb, Ta and Ti anomalies, which are akin to Phanerozoic high magnesian andesites and are melts of mantle peridotites that were previously metasomatized by subducted slab melts/fluids. The dacite-rhyolites have the highest SiO2 and lowest MgO, Cr and Ni contents and were derived from intracrustal remelting.The zircon U-Pb dating results reveal that the arc tholeiitic to calc-alkaline basalt-andesite-dacite-rhyolite assemblages erupted at ∼ 2.57 Ga – ∼2.52 Ga and back-arc basin basalt lithological association at ∼ 2.55–2.52 Ga. Furthermore, both the arc basalt-andesite-dacite-rhyolite and back-arc basin basalt associations are predominantly exposed within the Waitoushan-Gongchangling-Benxi range of the northeastern Anshan-Benxi greenstone belt. Therefore, we suggest that the Waitoushan-Gongchangling-Benxi area represents a rear-arc to back-arc zone, which was subsequently subjected to consistent NE-SW compression (σ1) and top-to-northeast kinematic features during back arc closure and underwent up to high amphibolite facies metamorphism. However, the Anshan area in the southwestern range of the Anshan-Benxi greenstone belt preserves large amounts of thick-bedded metasedimentary rocks instead of back-arc and arc volcanic rocks, which indicates a stable depositional environment in a typical back arc basin depositional zone. The supracrustal rocks within the back arc basin depositional domain underwent NE-SW compression (σ1) and top-to-southwest kinematic features and mostly experienced greenschist facies metamorphism.These crust-mantle interaction features inferred by metamorphosed volcanic rocks in the Anshan-Benxi greenstone belt, spatial and temporal variations in the lithological assemblages, together with the style of structural deformations, indicate a late Neoarchaean geodynamic evolution from SW-dipping subduction to arc-back-arc development and collisions between ancient continental blocks and arcs.