Apatite geochemical and Sr-Nd isotopic constraints on the source and petrogenesis of alkaline volcanic-plutonic ring complex

Alkaline rocks commonly occur as volcanic-plutonic ring complexes, but the petrogenetic mechanism for the coexistence between volcanic and plutonic rocks remains controversial. The Early Cretaceous Houshihushan alkaline ring complex in the Yanshan Fold and Thrust Belt, China, contains arfvedsonite granites surrounded by syenites, trachytes and alkali rhyolites. In situ trace elements and Sr-Nd isotopic compositions of apatites from these rocks are presented to constrain the petrogenetic link between coeval alkaline volcanic and plutonic rocks. Petrographic and geochemical features suggest that apatites in the trachyte crystallized from the melt at an early stage. Their initial 87Sr/86Sr ratios (0.7048–0.7052) and εNd(t) values (−21.9 to −17.6) well constrain the Sr-Nd isotopic compositions of the lithospheric mantle source for the trachyte, better than those of whole-rock (0.7114 and −18.6, respectively). In contrast, apatites in the syenites crystallized from host melt throughout the whole history of magma evolution. Their geochemical features and variable initial 87Sr/86Sr ratios (0.7084–0.7175) and εNd (t) values (−16.0 to −12.7) suggest that the syenites were formed by the cumulation of their parental magma, coupled with gradually mixed by felsic magma with a high initial 87Sr/86Sr ratio. However, a few and small-size apatite grains crystallized from parent magma of the arfvedsonite granites and alkali rhyolites suggest that in situ geochemistry of apatites is not feasible to constrain the petrogenetic process of high-silica alkaline rocks. Combined with whole-rock data, the geochemical features and Sr-Nd isotopic compositions of apatites indicate a crystal mush model to interpret the genetic link between syenites and arfvedsonite granites and alkali rhyolites. The early accumulations of mixing mafic and felsic magmas formed syenites at the margin, while the residual silicic melts solidified as arfvedsonite granites at the core. After the exposure of the intrusive rocks by rapid exhumation, the injection of high-temperature mantle-derived magma into the crystal mush promoted the extraction of the residual melt from the magma chamber. The mantle-derived magmas and residual melts directly erupted to the surface to form trachyte and rhyolite in the middle, eventually forming the alkaline ring complex.