20-06-22 14:50 发布者： 浏览次数：次
Subduction zones are important site of mass and energy exchange between the mantle and crust, among which melts/fluids derived from subducted crustal materials could strongly modify the geochemical characteristics and result in prominent mantle heterogeneity. The eastern North China Craton (NCC) is one of the best localities worldwide to investigate mantle metasomatism related to subduction. The lithospheric mantle beneath the eastern NCC experienced multiple episodes of metasomatism events in the Phanerozoic. Among these, the Triassic continental subduction-collision and Jurassic subduction of the Paleo-Pacific plate led to strongly modified chemical and physical characteristics of the lithospheric mantle (e.g., Wu et al., 2019; Zhao et al., 2013). This eventually induced the thinning of NCC’s lithosphere during the Early Cretaceous, accompanied by amounts of mafic magmatism and nearly concurrent giant gold mineralization (Zhu et al., 2015). Though the mechanism for the destruction of NCC is currently well-constrained, the extents and degrees of mantle metasomatism related to subduction remained controversial.
Fig. 1. Simplified geological map of the North China Craton showing the distribution of the Early Cretaceous mafic rocks in the Dabie and Sulu orogens, Luxi regions, Jiaodong Peninsula, and Liaodong Peninsula. Locations are labelled with formation ages, 87Sr/86Sr(t) and εNd(t) values.
To address this question, the student Xiang Wang in China University of Geosciences (Wuhan), under the guidance of Professor. Zaicong Wang, and with other cooperators conducted systematic study on lamprophyres in the Jiaodong Peninsula and combined with previously reported mafic rocks in adjacent regions (Fig. 1).
The consistent trace element patterns and Sr-Nd-Pb isotopic compositions of these Early Cretaceous lamprophyres (130 Ma) mimic those of Triassic ultrahigh-pressure metamorphic rocks from the Yangtze Craton. Because inherited Neoproterozoic zircons in the lamprophyres were exotic materials of eastern NCC, these geochemical features are interpreted to reflect an isotopically enriched mantle source influenced by subducted continental crustal materials from Yangtze Craton (Fig. 2). The lamprophyres also share similar arc-like trace element patterns and Sr-Nd isotopic compositions with other, almost coeval, lamprophyre dykes, basalts, gabbros and other mafic rocks in adjacent regions. This variable but marked enrichment indicates extensive and widespread mantle metasomatism by components derived from recycled crustal rocks over a large area of the lithospheric mantle beneath the eastern NCC (Fig. 2).
Fig. 2. Initial 87Sr/86Sr(t) vs. εNd(t) diagram for lamprophyre dykes and coeval IAB- and OIB-type mafic rocks from the eastern NCC.
However, the subduction-related mantle metasomatism displays spatial heterogeneity related to the distance from the subduction zone. In the eastern NCC, the εNd(t) values of coeval mafic rocks increase gradually with the increasing distance from Dabie-Sulu orogenic belt to the NCC's interior (Figs. 1, 3). This is consistent with the previously reported isotopic variations in Luxi areas (Yang et al., 2012a; Yang et al., 2012b), which likely reflects the waning spatial influence of the subducted Yangtze continental crust. Due to a left-lateral strike-slip motion along the large Tanlu fault during the Mesozoic, the Sulu orogen was displaced northward by about 500 km with respect to the Dabie orogen (Fig. 1). This process may also have dragged parts of the terranes from beneath the Luxi area to north, explaining the striking geochemical and isotopic similarity of mafic rocks in the Jiaodong Peninsula and southern Luxi area, and similar spatial variation of Nd isotope for two regions (Figs. 1, 3). Therefore, the overall increase in εNd(t) values from the southeastern edge to the interior of the eastern NCC forms a consistent pattern that probably reflects the waning influence of material from the subducted continental crust of Yangtze Craton added to the SCLM rather than regional crustal detachment, though ancient crustal detachment may partly occur in some regions (Huang et al., 2012).
This already enriched lithospheric mantle was subsequently metasomatized by fluids from subducting Paleo-Pacific plate, which promoted the hydration of mantle and provided the high water contents of the Early Cretaceous mafic rocks. After these two-stages of metasomatism, both related to subduction, but from different slabs, variable degrees of melting of the heterogeneously metasomatized lithospheric mantle during extension in the Early Cretaceous resulted in concurrent, but variable mafic rocks inheriting similar trace element and radiogenic isotope characteristics.
Fig. 3. Nd isotopic variation with time for the sub-continental lithospheric mantle since Paleozoic in the eastern NCC. The spatial distribution of Late Triassic to Early Cretaceous mafic rocks is given in Fig. 1.
The study was financially supported by the Chinese National Key Research and Development Program (2016YFC0600103) and benefited from the long-term and in-depth studies on cratonic evolution of NCC conducted by Chinese scientists.
Paper information: Wang, X. et al., 2020. Early cretaceous lamprophyre dyke swarms in Jiaodong Peninsula, eastern North China Craton, and implications for mantle metasomatism related to subduction. Lithos, 368-369: 105593. https://doi.org/10.1016/j.lithos.2020.105593
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