Incremental Emplacement of the Late Jurassic Midcrustal, Lopolith‐Like Qitianling Pluton, South China, Revealed by AMS and Bouguer Gravity Data

Abstract

AbstractModes of magma emplacement may provide key information on geodynamic‐tectonic settings of magmatism, when the threshold of magma volume and duration of emplacement process is crossed, brittle/ductile crustal deformation behavior, and volumes and rates of magma transfer in the crust. The majority of upper‐crustal granitic intrusions are incrementally emplaced as laccoliths or sills, while midcrustal or deeper‐level granitic intrusions appear to be predominantly emplaced as sills or lopoliths, but relatively few examples have been studied. We present a case study on the Jurassic Qitianling pluton from South China, which was constructed in the upper part of the middle crust (at a depth of ~10–15 km). To characterize its emplacement mechanism and evolution, we have combined field and microscopic observations, Anisotropy of Magnetic Susceptibility measurements and Bouguer gravity anomaly modeling. The main findings are (1) the generally undeformed granite‐country rock contacts with a narrow thermal aureole, (2) irregularly wavy and gradational contacts between the main granite facies, (3) dominant subhorizontal magnetic fabrics in the granites with a scattered magnetic foliation, (4) rare occurrence of dykes, and (5) a subcircular surface and lopolith‐like pluton geometry. Accordingly, we suggest that the Qitianling pluton forms an ~25‐km wide and ~5‐km thick lopolith that was incrementally emplaced during a period of tectonic quiescence, most likely by under‐accretion of multiple magma sheets deflected from magma pulses that ascended through pipe‐like channels. The mineralogical and compositional variation of the pluton may be the result of in situ fractionation, fractionation at deeper crustal level, or variation in source‐derived melt compositions.