Abstract
The assembly and long-term evolution of the Eastern Block of the North China Craton are poorly constrained. Here we use bulk rock geochronological and geochemical data from mafic meta-igneous rocks (hornblendites, amphibolites and a metagabbro) of the Liaohe Group to reconstruct the Neoarchean to Paleoproterozoic history of the Jiao-Liao-Ji Belt, located between the Longgang and Nangrim blocks that together form the Eastern Block of the North China Craton. The mafic/ultramafic meta-igneous rocks have intrusive or tectonic contacts with the Liaoji granitic rocks (~2.2–2.0 Ga), which form the basement of the Jiao-Liao-Ji Belt. The major and trace element data indicate that the protoliths had calc-alkaline composition and formed along an active continental margin subduction zone. The mafic rocks form a whole-rock 176Lu/177Hf isochron with an age of 2.25 ± 0.31 Ga, overlapping with UPb zircon ages for mafic and granitic rocks from the Jiao-Liao-Ji Belt and consistent with being the emplacement age of the mafic protoliths along the active continental margin. In contrast, the whole-rock 147Sm/144Nd isochron age of 2.83 ± 0.18 Ga is likely to reflect the average age of the lithospheric mantle source from which the mafic/ultramafic protoliths were extracted. Together with geological evidence, we propose that the southwestern portion of the Longgang Block was an active continental margin since at least the early Paleoproteorozic. Literature age data from metamorphic zircons show that peak granulite metamorphism took place at ~1.96–1.88 Ga, resulting from the collisional event that fused the Longgang and Nangrim blocks into the Eastern Block of the North China Craton. Our bulk-rock 207Pb/206Pb age of 1824 ± 19 Ma and our 87Rb/86Sr age of 1671 ± 58 Ma reflect retrograde (cooling) stages during the exhumation of the Jiao-Liao-Ji Belt after the orogenesis.
Highlights
Archean to Palaeoproterozoic terranes record protracted periods of crustal formation, lithospheric stabilization, subsequent orogenesis and early history of plate tectonics
Positive linear correlations of the measured isotopic composition on all isotope correlation diagrams in these presumed Paleoproterozoic rocks point to radiogenic ingrowth having formed these arrays, in particular since the linear arrays on all isotope correlation diagrams have positive slopes
We propose that overthickening of the lithosphere during the collisional event is likely to have resulted in lithospheric destabilization, resulting in lithospheric mantle detachment/delamination (Li and Zhao, 2007)
Summary
Archean to Palaeoproterozoic terranes record protracted periods of crustal formation, lithospheric stabilization, subsequent orogenesis and early history of plate tectonics. The inevitably poly-deformed nature of the oldest cratonic regions on Earth requires multiple geochemical and geochronological tools in order to unravel the complete and complex history of lithospheric formation, stabilization, crustal differentiation and subsequent reworking through subduction and orogenesis. ⁎ Corresponding author at: GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstr. This leaves significant portions of the geologic record, those involving mafic rocks, underrepresented and effectively unsampled. Mafic and ultramafic rocks represent significant portions of the Precambrian record and offer links between the early crust, the mantle (both lithospheric and asthenospheric) from which it is derived, and its subsequent amalgamation into cratons Mafic and ultramafic rocks represent significant portions of the Precambrian record and offer links between the early crust, the mantle (both lithospheric and asthenospheric) from which it is derived, and its subsequent amalgamation into cratons (e.g. Manikyamba and Khanna, 2007; Hawkesworth et al, 2010; Naeraa et al, 2012; Zhao and Zhai, 2013)
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