Evaluation of multiple emplacement mechanisms: the Huichizi granite pluton, Qinling orogenic belt, central China

Three end-member granite emplacement mechanisms, host-rock ductile shortening (HRDS), host-rock rigid displacement (HRRD) and magmatic absorption (MA), are identified in this study of the Huichizi pluton. The internal fabrics are mainly flattening types (ν=0.4–0.9) and strain is high (Es=1.0–0.6) near the boundaries and low (Es=0.7–0.30) toward the central parts of the pluton. In the 1.5-km-wide contact aureole, extension (ε) is measured in syn-emplacement deformed veins and the calculated average shortening normal to the sides and tips of the pluton are 1.14 km and 0.74 km, respectively. From this we conclude that the HRDS produced only about 20% space for the present volume of the pluton; the regional HRRD, derived from the uplift by about 8 km of the Qinling group in a flower structure, contributed 19∼32% space. MS, including stoping and material transfer processes, yielded another 37% space. Of this, stoping contributed a minimum of 1% space, estimated from the surface area of 2.8 km2 covered by xenoliths. Transfer processes provided about 36% of the needed space, deduced from the average 64/36 ratio of hybridization of mantle-derived to crustal- (Qinling group) derived magma. The three mechanisms may act nearly simultaneously as a multiple emplacement mechanism; this situation may apply to many other plutons as well.