Crust/mantle interaction during the construction of an extensional magmatic dome: Middle to Late Jurassic plutonic complex from western Liaoning, North China Craton
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Differentiating magmatic doming and low-angle normal faulting remains critical for fully understanding the thermal, mechanical and chemical evolution of continental landmasses under extension. This zircon U–Pb dating and geochemical study documents two Middle to Late Jurassic batholiths (Lüshan and Haitangshan) from the Yiwulüshan range of western Liaoning, North China Craton. They consist of a variety of lithologies including gabbro, diorite, granodiorite, monzogranite, together with microgranular magmatic enclaves (MME) and mafic dykes. Synthesizing petrologic, elemental, whole-rock Sr–Nd and zircon Hf isotopic data leads to the characterization of multiple mafic and felsic end-members and their concomitant interaction in building the magmatic dome. A subduction-related metasomatized lithospheric mantle source is fingerprinted by the gabbroic to dioritic rocks with enriched large ion lithophile elements, depleted high field strength elements and heterogeneous isotopic compositions (87Sr/86Sri = 0.70541 to 0.70577, εNd(t) = − 1.78 to − 5.54 and zircon εHf(t) = − 6.0 to 8.1). One felsic magma end-member of ancient mafic lower crustal parentage is discernable from adakitic granites with high Sr/Y and evolved isotopic composition (87Sr/86Sri = 0.70533 to 0.70792, εNd(t) = − 18.8 to − 21.7, zircon εHf(t) = − 18.5 to − 28.8), whereas another felsic magma end-member of newly underplated crustal heritage manifests itself from some monzogranites with non-adakitic elemental affinity and juvenile isotopic composition (87Sr/86Sri = 0.70429 to 0.70587, εNd(t) = − 4.47 to − 5.87, zircon εHf(t) = 4.3 to 1.3). Hybridization processes between mantle-derived mafic magma and ancient crustal-derived felsic magma result in the formation of MME-bearing granodiorites with intermediate isotopic signatures (87Sr/86Sri = 0.70491 to 0.70499, εNd(t) = − 15.3 to − 15.8, zircon εHf(t) = − 12.7 to − 17.4). Subsequent fractional crystallization of the hybridized magmas endows the differentiated monzogranites with low Sr/Y and highly evolved isotopes (87Sr/86Sri = 0.70496 to 0.70605, εNd(t) = − 16.0 to − 18.7, zircon εHf(t) = − 14.3 to − 21.5). Apart from distinguishing Middle–Late Jurassic extensional magmatic doming from Early Cretaceous detachment faulting, this complex mafic–felsic magma association encapsulates a multi-level crust/mantle interaction leading to lithospheric thinning and concomitant crustal architectural reorganization in the Yanshan belt during the Late Mesozoic. Near-synchronization of a two-stage extensional pattern in the Yanshan belt and even across NE continental Asia accords well with gravitational collapse and convective removal of lithospheric mantle within an evolved post-collisional to within-plate extensional regime.
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