Multiple magmatism in an evolving suprasubduction zone mantle wedge: The case of the composite mafic–ultramafic complex of Gaositai, North China Craton

Abstract

The suprasubduction zone mantle wedge of active convergent margins is impregnated by melts and fluids leading to the formation of a variety of magmatic and metasomatic rock suites. Here we investigate a composite mafic–ultramafic intrusion at Gaositai, in the northern margin of the North China Craton (NCC). The hornblende gabbro–serpentinite–dunite–pyroxenite–gabbro–diorite suite surrounded by hornblendites of this complex has long been considered to represent an “Alaskan-type” zoned pluton. We present petrologic, mineral chemical, geochemical and zircon U–Pb and Lu–Hf data from the various rock types from Gaositai including hornblende gabbro, serpentinite, dunite, pyroxenite, diorite and the basement hornblendite which reveal the case of multiple melt generation and melt–peridotite interaction. Our new mineral chemical data from the mafic–ultramafic suite exclude an “Alaskan-type” affinity, and the bulk geochemical features are consistent with subduction-related magmatism with enrichment of LILE (K, Rb, and Ba) and LREE (La and Ce), and depletion of HFSE (Nb, Ta, Zr, and Hf) and HREE. Zircon U–Pb geochronology reveals that the hornblendites surrounding the Gaositai complex are nearly 2billionyears older than the intrusive complex and yield early Paleoproterozoic emplacement ages (2433–2460Ma), followed by late Paleoproterozoic metamorphism (1897Ma). The serpentinites trace the history of a long-lived and replenished ancient sub-continental lithospheric mantle with the oldest zircon population dated as 2479Ma and 1896Ma, closely corresponding with the ages obtained from the basement rock, followed by Neoproterozoic and Phanerozoic zircon growth. The oldest member in the Gaositai composite intrusion is the dunite that yields emplacement age of 755Ma, followed by pyroxenite formed through the interaction of slab melt and wedge mantle peridotite at 401Ma. All the rock suites also carry multiple population of younger zircons ranging in age from Paleozoic to Mesozoic, suggesting continuous mantle metasomatism through melts and fluids associated with prolonged subduction, which is also substantiated by the pervasive hydration of all the ultramafic units. Zircon Lu–Hf isotopic data from the basement rock (hornblendite) on concordant grains yield εHf(t) values in the range of −23.8 to −5.2 with TDM of 1979–2424Ma and TDMC between 2754 and 2899Ma, suggesting Mesoarchean to Neoarchean reworked and juvenile sources. Concordant grains in the serpentinite also display a large range of εHf(t) values (−5.0 to 5.8) suggesting multiple sources, whereas the concordant magmatic zircon grains in the dunite have a tight range of εHf(t) values between −2.3 and 0.1 indicating primitive source. Those from the pyroxenite are characterized by highly negative εHf(t) values of −21.5 to −18.6 suggesting reworked ancient components. The diverse ages, lack of typical geochemical imprints, and magmas derived from multiple sources including Mesoarchean to Neoarchean reworked and primitive components within the same mafic–ultramafic complex exclude an “Alaskan-type” affinity and suggest multiple magmatism in an evolving and metasomatized suprasubduction zone mantle wedge. We correlate the tectonics with the prolonged subduction regime of the Paleo-Asian Ocean with melt–peridotite interaction and geologic history spanning through compression to extension.