The Fangmayu Alaskan-type ultramafic intrusion: Implications for Paleoproterozoic assembly of the North China Craton

Abstract

Alaskan-type ultramafic intrusions constitute a rare suite of rocks formed in subduction-related settings and provide important insights into mantle source and geodynamic processes associated with convergent plate margins. The North China Craton (NCC) witnessed subduction-accretion-collision events in the Paleoproterozoic prior to the final cratonization, and preserve important imprints of magmatic and metasomatic processes associated with oceanic plate subduction along the margins of the amalgamated crustal blocks and within the collisional orogens. Here we investigate a suite of serpentinized dunite, serpentinite, olivine orthopyroxenite, and anthophyllite glimmerite from the Fangmayu ultramafic complex along the eastern margin of the Trans-North China Orogen (TNCO) adjacent to the Eastern Block of the NCC. We present petrology, mineral chemistry, zircon U-Pb geochronology, REE and Lu-Hf isotopes, and whole rock geochemistry including REE and PGE data with a view to investigate the magmatic and metasomatic processes in this ancient subduction zone. The lithological distribution in the Fangmayu intrusion shows concentric zoning with dunite in the core, mantled by peridotite and pyroxenite, similar to that of Alaskan-type intrusions. Mineral chemistry including spinel composition shows distinct characteristics of stratiform chromite associated with Alaskan-type ultramafic intrusions formed in an arc-related subduction environment. Phase diagram computations show that the olivine (XMg = 0.92–0.93) and orthopyroxene (XMg = 0.9–0.91) in the olivine orthopyroxenite formed at high temperature conditions (∼1200 °C), and the antigorite (XMg = 0.92–0.98) chemistry shows that serpentinization occurred at much lower temperature (∼570 °C). Zircon U-Pb data from the ultramafic suite define two distinct groups of ages: 2.3 to 2.0 Ga and ca. 1.84 Ga corresponding to magma crystallization within suprasubduction zone mantle associated with subduction, and metamorphism during continental collision and final cratonization of the NCC. The REE patterns of zircons distinguish dominant population of magmatic grains and a minor group of metasomatic grains. Zircon Lu-Hf isotope data exhibit both positive and negative εHf(t) values in the range of −8.5 to 4.4 with TDM of 2434–2714 Ma, suggesting both enriched and depleted components in the magma source. Geochemical data including PGE characteristics suggest hydrous melting of depleted mantle wedge under high temperatures and fluid pressure in a subduction-related setting. Our study on the Paleoproterozoic Alaskan-type intrusion offers insights into arc-continent and continent-continent collision within a long-lived subduction system in the North China Craton.