Xiao-Fang He

Mesozoic felsic volcanic rocks from the North China craton: Intraplate magmatism associated with craton destruction

The North China craton preserves the history of crustal growth and craton formation during the early Precambrian, followed by extensive lithospheric thinning in the Mesozoic associated with large-scale magmatism and metallogeny. The timing and petrogenesis of the voluminous Mesozoic magmatic rocks are important in understanding the mechanism of craton destruction. Here, we investigate a suite of volcanic rocks including basalt and trachybasalt, basaltic andesite and andesite, dacite and trachydacite, and rhyolite from the Yanshan belt in the northern part of the North China craton and provide evidence for intraplate magmatism along a paleosuture. We present bulk chemistry, zircon U-Pb geochronology and rare earth element data, and Lu-Hf isotopes from the volcanic suite and attempt to constrain the timing of magmatism and source characteristics. The zircon U-Pb data show two age peaks at 175−165 Ma and 155−145 Ma. Geochemically, the rocks are calc-alkaline with arc-related features. The andesites show adakitic affinities with high Sr contents (up to 1140 ppm), high Sr/Y (up to 76.2) and La N /Yb N ratios (up to 21.7), lack of a negative Eu anomaly, extreme depletion in Y and Yb, and relatively low MgO contents (1.3−4.4 wt%), indicating that they were likely derived from the partial melting of thickened lower continental crust. The Zr/Ba ratios indicate interaction between lower crust and primitive magma, possibly through magma underplating at the crust-mantle boundary. However, there was no arc-related tectonic setting in the study area during the Jurassic, which indicates that the elemental and isotopic compositions are possibly inherited from the basement rocks generated during the Paleoproterozoic subduction-accretion-collision process in the North China craton, subsequent to the 2.5 Ga cratonization event. Our zircon Hf isotope data also confirm the incorporation of Paleoproterozoic reworked material. The magmatism and craton destruction along a paleosuture were induced by mantle upwelling through far-field tectonics of the Pacific plate subduction. Our study presents a case where source components have strongly influenced the geochemical imprint of the magmas, with intraplate volcanics preserving a continental arc magmatic signature.

Crust–mantle interaction and craton destruction: evidence from Late Mesozoic plutons in the North China Craton

Late Mesozoic granitoids, widely distributed in the Yanshan Belt of the North China Craton (NCC), are important markers of cratonic destruction and lithospheric thinning. We report new petrological, geochemical, zircon U–Pb and Lu–Hf isotopic data on two late Mesozoic granitic plutons from the central NCC near the Trans-North China Orogen. We report zircon U–Pb emplacement ages of 162 – 156 Ma and 132 Ma from the Siganding and Fangshan plutons respectively, indicating Late Jurassic–Early Cretaceous magmatism. The presence of a dusty zone, and discontinuous zoning, in plagioclase phenocrysts and the occurrence of mafic microgranular enclaves in both granitoid plutons suggest mingling and mixing of felsic and basic magmas. The rocks investigated in this study show high Na2O + K2O and high Mg# (Mg# = Mg/(Fe + Mg)) and are metaluminous, with enrichment in light rare earth elements and large ion lithophile elements (Rb, Ba, U, and Sr) and depletion in high strength field elements (Th, U, Nb, Ta, P, and Ti). They also display negative εHf(t) values and high Sr/Y ratios, comparable with adakitic rocks, suggesting that the felsic magmas of these intrusions were probably derived from the partial melting of the Palaeoproterozoic (two-stage zircon Hf-depleted model ages between 2.5 and 1.9 Ga) thickened lower crust of the NCC, whereas the mafic magma was probably derived from subcontinental lithospheric mantle. We correlate the magmatism with lithospheric thinning beneath the NCC and associated crust–mantle interaction. It is inferred that lithospheric thinning was of limited extent in Late Jurassic (162 – 156 Ma) times. The magma flare-up at around 130 Ma was produced by large-scale melting of the enriched subcontinental lithospheric mantle with development of extensive intracontinental adakitic magmatic rocks derived from thickened ancient lower continental crust with variable involvement of subcontinental lithospheric mantle. Supplementary material: Mineral assemblages and abundance, representative electron microprobe analyses, LA-ICP-MS zircon U–Pb age data and LA-MC-ICP-MS Lu–Hf isotopic data are available at https://doi.org/10.6084/m9.figshare.c.3815308

Magnetite-apatite deposit from Sri Lanka: Implications on Kiruna-type mineralization associated with ultramafic intrusion and mantle metasomatism

Abstract Kiruna-type iron oxide–apatite associations occur in a variety of rock types and their origin has remained controversial. Most of the Kiruna-type deposits are associated with intermediate to felsic rocks, and in rare cases with ultramafic rocks. Here we investigate the Seruwila iron oxide–apatite deposit at the contact between the Highland and Vijayan complexes that has been defined as the “eastern suture” in Sri Lanka, which formed during the late Neoproterozoic assembly of the Gondwana supercontinent. The ore deposit is hosted in an ultramafic intrusion and comprises massive and disseminated mineralization. The ore-bearing rocks are mainly composed of low-Ti magnetite and chlor-fluorapatite. Our petrological and geochemical studies suggest a magmatic–hydrothermal model for the mineralization wherein: (1) the Cl-rich magmatic–hydrothermal fluid scavenged iron and P from the ultramafic magma, transported iron to shallower levels in the crust and deposited along the suture zone to form the massive type magnetite and apatite; and (2) the cooling of the hydrothermal fluids resulted in the growth of disseminated magnetite and the precipitation of sulfide minerals, followed by a calcic metasomatism (scapolitization and actinolitization). This model is in conformity with the genetic relation between Kiruna-type deposits and iron oxide–copper–gold (IOCG) deposits. We also report LA-ICP-MS zircon U–Pb ages from the host ultramafic intrusion suggesting its emplacement at ca. 530 Ma, which is younger than the regional high-grade metamorphism associated with the collisional assembly of the crustal blocks in Sri Lanka at ca. 540 Ma. By analogy with the common occurrence of Kiruna-type deposits in extensional tectonic settings, and the geochemical features of the studied rocks including low silica, high Mg, Fe, Ca with high field strength elements (HFSEs such as Nb, Ta, Zr, Hf, Ti) depletion and strong LREE and F enrichment, we theorize that the ultramafic magmatism occurred in a post-collisional extensional setting derived from a volatile- and LREE-rich metasomatized lithospheric mantle.