Unusually dumbbell-shaped Guposhan–Huashan twin granite plutons in Nanling Range of south China: Discussion on their incremental emplacement and growth mechanism

Abstract

The Guposhan–Huashan twin plutons crop out as an unusually dumbbell-shaped batholith within the Middle Mesozoic Central Nanling Range granitic belt in south China. The batholith was controlled by the east–west-striking Yishan–Quannan deep crustal fault zone, and spatially associated with two regional anticlinoria. Both plutons show concentric zonation patterns and are composed of two emplacement sequences separated by an approximately 9m.y. magmatic lull, the 160–163-Ma Niumiao–Lisong–Wanggao sequence and the 148–151-Ma Xinlu–Huamei sequence, based on field mapping and observations, geochronology, and geochemistry. In a proposed incremental emplacement and growth model, the early magmas exploited physical discontinuities such as the contacts between rocks with contrasting competence and densities, as well as saddles of regional-scale anticlinoria. Assisted by dominant vertical heat flow and uplifting of the roofs, the Niumiao–Lisong–Wanggao-sequence magmas were subsequently emplaced in an over-accretion style as distinguishable increments during an approximately 3m.y. span. Gravimetric data, observations of field relationships, as well as analysis of finite strain and country rock rheology confirm that the plutons became thickened considerably due to largely vertical assembling and growth, and most of their space was created by vertical displacement of their country rocks. Interestingly the coeval twin plutons show contrasting development and growth modes during their assembling and thickening process. The Guposhan pluton was built on an outward building mode with a “reverse-zoning” pattern while the Huashan pluton on an inward building mode with a “normal-zoning” pattern. We propose that the contrasting modes largely resulted from their divergent thermal evolution path. For example, the incremental inward building mode helps retain thermal energy and maintain a high temperature in older units, which facilitates injection of subsequent, younger increment units of the same sequence within the older host units. The independent, newer sequence of Xinlu–Huamei magmas, however, were controlled by shallow brittle faults developed in the older sequence units and emplaced as stocks and dykes by a dyke-propagation mechanism.