Generation of high-Mg diorites and associated iron mineralization within an intracontinental setting: Insights from ore-barren and ore-bearing intrusions in the eastern North China Craton

Abstract

High-Mg andesitic rocks (HMAs) are commonly generated through slab-mantle interactions in arc settings. In this study, however, we report the high-Mg diorites in the Zibo and Laiwu areas of the eastern North China Craton (NCC), which were particularly produced during lithospheric thinning within an intracontinental setting. Some of the rocks generated large-scale Fe skarn deposits (e.g., the Zibo quartz monzonite and the Laiwu Kuangshan monzonite), whereas the others are ore-barren (e.g., the Zibo gabbroic diorite and the Laiwu Jinniushan and Jiaoyu monzodiorites), providing an excellent case to reveal how the non-arc HMAs were formed and the crucial factors controlling the Fe mineralization. All the rocks are characterized by high Mg# values (51–84) at andesitic compositions (52.5–65.0 wt%) and show arc-like geochemistry (e.g., enriched in LREEs and LILEs and depleted in HREEs and HFSEs), of which the strongly mineralized intrusions are more felsic with higher Mg# values (69–84) and stronger fractionation between LREEs and HREEs. Zircon U-Pb dating indicates that these rocks were emplaced at the early Cretaceous (130–132 Ma), coeval with the peak thinning of the lithosphere in the eastern NCC. All the rocks have transitional Sr-Nd-Pb isotopic compositions (0.704726 < (87Sr/86Sr)i < 0.707507, −16.0 < εNd(t) < −5.4, 16.334 < (206Pb/204Pb)i < 17.629, 15.205 < (207Pb/204Pb)i < 15.447, 36.306 < (208Pb/204Pb)i < 37.754) between the local lithospheric mantle and the ancient crust. Combined with their high Mg# and arc-like geochemistry, they were most likely derived from an anciently metasomatized and recently hydrated subcontinental lithospheric mantle (SCLM) with various crustal contamination. Magma mixing modelling suggests that in the Zibo area little crustal materials were involved into the gabbroic diorite, but ~20% middle-upper crustal materials were assimilated into the monzodiorite and quartz monzonite during magma ascent. In the Laiwu area, lower crustal materials (at least ~30%) were significantly involved into the Jinniushan, Jiaoyu and Kuangshan intrusions through magma mixing at the base of the lower crust. Sedimentary rocks (mainly carbonates interbedded with coal-bearing shales, sandstones and evaporitic rocks) were particularly assimilated into the strongly mineralized intrusions in both the Zibo and Laiwu areas. Such assimilation probably contributed to promote fluid exsolution and metal extraction from the melts, since the strongly mineralized intrusions have the lowest H2O contents (2.4–3.2 wt% H2O), the highest Mg# values and the lowest oxygen fugacities (2.5 < ∆FMQ < 3.1). It is thus considered that the assimilation of sedimentary rocks is crucial for generating ore-forming fluids. P-T calculations indicate that all the high-Mg diorites were emplaced shallowly (mainly T < 760 °C and P < 140 MPa). In combination with the co-development of bimodal volcanics, metamorphic core complexes and rift basins in the eastern NCC during the early Cretaceous, extreme lithospheric extension should have occurred in the study region. Such intracontinental extension was likely caused by the asthenospheric upwelling induced by the stagnant Paleo-Pacific Plate in the transition zone beneath the eastern Asian continent. The geodynamic regime is different from those of the typical arc settings.