Crystallization and Solidification of Poikilitic and Granular Rocks in the Ultramafic Sequence of the Xinjie Layered Intrusion (SW China): Constraints from Complex Growth Zoning of Clinopyroxene and Spatial Variation of Dihedral Angles

Abstract

AbstractCrystallization and solidification pathways in crustal magma chambers control the chemical and textural evolution of igneous rocks. However, deciphering these codes from completely solidified cumulates is challenging and requires a full understanding of growth processes of minerals and textures of rocks. In this study, we collected samples from a ca. 380-m-thick drill core intersecting the ultramafic sequence of the Xinjie layered intrusion in SW China, which is composed of wehrlite with poikilitic clinopyroxene (Cpx), and clinopyroxenite and olivine clinopyroxenite with euhedral to subhedral Cpx grains. Using micro-X-ray fluorescence and electron probe micro-analyzer mapping techniques, we found that the Cpx grains in clinopyroxenite and olivine clinopyroxenite are characterized by sector-zoned, Cr-rich cores, whereas the Cpx oikocrysts in wehrlite only show Cr-rich cores without sector zoning. The sector-zoned cores of the Cpx grains are widespread in clinopyroxenite and olivine clinopyroxenite and consist of Si–Mg-rich hourglass sectors and Cr–Al–Ti-rich prism sectors, indicating rapid growth of the Cpx grains due to moderate degrees of undercooling, which may be induced by fast cooling during initial emplacement of Cpx-saturated magma into cold country rocks. The sector-zoned Cpx grains thus likely constructed the initial crystal framework in the magma chamber. In contrast, the absence of sector zoning in Cr-rich cores of the Cpx oikocrysts indicates that an olivine-saturated magma may have intruded a high-temperature, Cpx-laden crystal mush so that magma undercooling was significantly reduced or eliminated and no sector zoning developed in the Cpx oikocrysts. Such a process may also result in the spatial variation of the medians of Cpx–Cpx–plagioclase dihedral angles (Θccp) in clinopyroxenite, which shows a significant reduction from an original impingement angle of ~60° in the middle and upper parts to ~40° in the basal part of the sequence. The reduction of Θccp could be attributed to melt-present textural equilibration in the Cpx-laden mush that was sustained by additional heat supplied by massive olivine-saturated melt input in the ~40-m-thick basal part. Here, we propose that the ultramafic sequence of the Xinjie layered intrusion may have formed by the intrusion of primitive, olivine-saturated magma into an early-formed Cpx-laden mush in a manner of sill-like sheets. This study provides new perspectives on the early history of a magma chamber and emphasizes the role of sill intrusion of primitive magmas into crystal mush in the formation of ultramafic sequences in layered intrusions, which has been relatively overlooked in previous studies.