Origin, ascent and oblique emplacement of magmas in a thickened crust: An example from the Cretaceous Fangshan adakitic pluton, Beijing

The Fangshan pluton in the Western Hills of Beijing records a history of emplacement from primarily vertical (ascent) to oblique injection along a detachment fault. Thus, the pluton provides a classic example answering the “room” problem. This pluton and the Nanjiao apophyse were produced by Cretaceous magmatism that formed the regionally extrusive East China Mesozoic igneous province. The Fangshan pluton includes two intrusive phases, a 136-Ma monzodiorite phase intruded by a 131-Ma quartz monzodiorite phase. Rocks from both phases have chondrite-normalized REE patterns enriched in LREE and depleted in HREE. They have primitive mantle-normalized trace element patterns with distinct negative Nb and Ti anomalies and high Sr (1067–1348 ppm) and low Y (8–17 ppm) with high Sr/Y ratios (71–137). These features are characteristic of adakites and suggest that the magmas were derived by partial melting of the lower part of a thickened continental crust. The pluton has magmatic foliations and lineation, indicative of the intrusion of the earlier magmas slightly obliquely from SE to NW. The magmatic fabrics have enclave ratios Rfmag + Rp of 3.0–3.5. Asymmetric solid-state deformational fabrics and strains (Rs) for the northwestern (3.5–4.0) and southeastern parts (1.5–2.0) again suggest a later oblique injection of magmas from SE to NW. Both the Fangshan pluton and Nanjiao apophyse suggest that magmas migrated most probably along the pre-existing detachment fault. Such an emplacement is also supported by the aeromagnetic polarization data and strong deformation of the Beiling syncline. Thus, magma emplacement in the Fangshan pluton probably involved creation of space by both faulting and wallrock strain.