Magma chamber-related transition from magmatic to solid-state fabrics within the Late Triassic granitic Dushan pluton (North China)

Abstract

SUMMARY Penetrative petro-fabrics in plutons, particularly late-stage solid-state fabrics, are usually considered to record regional tectonism. However, these fabrics are fundamentally ambiguous to correlate with magma-chamber processes or regional tectonics. Understanding the processes of fabric formation requires a proper understanding of pluton formation and emplacement mechanism. In this paper, we provide a case study of a transition from magma chamber-related magmatic to solid-state fabrics within the Late Triassic granitic Dushan pluton (North China) based on a multidisciplinary investigation of its emplacement mechanism, including structural geology, isotope chronology, AMS data measurement, and gravity modeling. The Dushan pluton varies in a southwest direction from a megascopically isotropic monzogranite in the northeast to an arc-shaped gneissic monzogranite with locally mylonitic zones along the southwest margin. The compatible outward dipping mesoscopic and magnetic susceptibility foliations define a dome-like roof with a NE-SW long axis. Considering the absence of similar penetrative fabrics in the country rocks, a continuum of magma chamber-related fabric variations within the pluton, from magmatic to solid-state conditions, probably formed during pluton emplacement. Gravity modeling suggests that the Dushan pluton has a NE-SW trending tongue-like shape with a northeast positioned root. We propose a model in which the Dushan pluton was emplaced through a northeast feeder zone, beginning probably as a sill then evolving by inflation via several magma batches. Later magma batches inflated the country rocks to form the final pluton. Magma inflation deformed the former semi-solid magma, forming a curved gneissic to mylonitic foliation along the southwest margin of the pluton. Several multidisciplinary studies show that the North China Craton (NCC) experienced a Late Triassic regional crustal extension attending the Dushan pluton emplacement. Both mesoscopic and magnetic susceptibility fabrics of the pluton could be more related to magmatic process rather than regional tectonics, implying that such fabrics within plutons need to be used with caution for inferring regional tectonics.