The Origin and Evolution of the Houshihushan Alkaline Ring Complex in the Yanshan Orogenic Belt

The Houshihushan complex is an alkaline ring complex associated with a collapsed caldera, consisting of a circular screen of alkaline volcanic rocks and post-collapse resurgent intrusions including a ring dyke of porphyritic quartz syenite, a central composite hypabyssal intrusion of nested stocks of drusy alkali-feldspar granite and porphyritic alkali-feldspar granite, and cone sheets of quartz syenite porphyry and granite porphyry. Zircon LA-ICPMS U-Pb analyses yields mean ²⁰⁶Pb/²³⁸U ages of 119±3Ma for porphyritic quartz syenite, 121±2Ma for quartz syenite and 121±2Ma for granite porphyry, respectively. Volcanic rocks of the Houshihushan Ring Complex (HRC) have similar ages to those of the intrusive rocks, confirming it as a volcanic-intrusive complex. Porphyritic quartz syenites have high contents of Na₂O+K₂O (10.0%-10.5%) and K₂O (5.21%-5.42%) with positive Eu anomalies (Eu/Eu^*=1.05-1.40). Alkali-feldspar granites and porphyries are characterized by enriched Na₂O+K₂O, FeO^tot/MgO, Ga/Al, Zr, Nb and REE (except for Eu) and low abundance of Al₂O₃, CaO, MgO, Ba, Sr and Eu, indicative of A-type granitic rocks. The porphyries can be classified as aluminous A-type granites, and show high zircon saturation temperatures (880-901℃). All the A-type granites of the HRC posses negative ε_Nd(t) values from -13.9 to -12.2. Porphyritic quartz syenite magmas were derived from partial melting of intermediate to mafic granulites and gneisses in the lower crust that mixed with enriched mantle-derived basaltic magma, with subsequent differentiation of clinopyroxene. Alkali-feldspar granite magmas were produced by mixing of mantle-derived basaltic magmas with upper crustal felsic melts, with fractionation of feldspars. The petrogenetic processes of porphyritic magmas involved partial melting of quartzfeldspathic rocks at shallow crust depths coupled with differentiation of feldspars. We suggest that development of the HRC involved the following four-stage sequence: (1) massive alkaline laves and pyroclastics erupted explosively; (2) the subsided caldera formed because of loss of magma from an underlying magma chamber which reduced magma pressure and facilitated collapse of the roof of the magma chamber along near-vertical ring faults. Magma intruded passively up the opening ring-faults to form the ring dyke of porphyritic quartz syenite during caldera collapse; (3) the high-level magma chamer became overpressured, and hot peralkline A-type granite magma was emplaced as the central stock of porphyritic alkali-feldspar granite. The overlying crust was fractured to generate cone fractures that provided space for the ascent of felsic melts to form cone sheets of quartz syenite porphyry; (4) the chamber resurged and a cogenetic pluton was emplaced as the nested stock of drusy alkali-feldspar granite. Build-up of magma overpressure within the central source chamber imparted upward force to fracture the host rock and form new conical fractures. These fractures were filled with magma to form cone sheets of granite porphyry. The Houshihushan alkaline ring complex formed over a brief time period in an extensional setting related to destruction of the eastern North China Craton during Early Cretaceous, possibly associated with subduction of paleo-Pacific plate.