Periodic Mixing of Magmas Recorded by Oscillatory Zoning of the Clinopyroxene Macrocrysts from an Ultrapotassic Lamprophyre Dyke

Abstract

Abstract Ultrapotassic rocks are volumetrically minor, but widely distributed in different geological settings. Extensive studies have concerned mantle melting processes that generated these rocks. However, crustal processes that they may have involved are poorly known. In this paper, we describe complex oscillatory zoning patterns of clinopyroxene (Cpx) macrocrysts from an ultrapotassic lamprophyre dyke in the Kyrgyz North Tianshan orogen. These macrocrysts commonly have a corroded or patchy-zoned core surrounded by a mantle with distinct oscillatory zoning, which is, in turn, surrounded by a euhedral rim. The oscillatory zoning of the mantle is composed of alternating coarse and fine layers with a clear resorption surface, or closely packed layers with a straight or wavy boundary in back-scattered electron images. High-amplitude oscillation of Mg#, Ti, Al, Cr and Sr across the layers of the mantle is attributed to magma mixing. Low-amplitude, high-frequency oscillation of Mg# across the closely packed layers was probably developed as a result of kinetic effects or crystal movement under thermal and chemical gradients. In addition, cryptic sector zoning of some macrocrysts clearly shows a Si- and Mg-rich hourglass sector and an Al- and Ti-rich prism sector. The sector zoning indicates crystallization of these macrocrysts under low degrees of undercooling, and the presence of concentric Cr-rich and Cr-poor layers within the same grain indicates that the growth process was disrupted by multiple magma recharging events. The cores of the macrocrysts have Mg# with three distinctive ranges: <84–90 (Core I), 74–84 (Core II) and 60–70 (Core III). The mantles have Mg# ranging from 64 to 90 without a distinct gap. The rims have a narrow range of Mg# from 76 to 80. The cores and mantles with high Mg# (≥85) have variable La/Yb from 1·8 to 5·0 and Dy/Yb from 2·3 to 4·6. The macrocrysts overall have variable 87Sr/86Sr from 0·7072 to 0·7084. Highly variable trace elements and 87Sr/86Sr within a single grain indicate that both primary and evolved magmas with different compositions were periodically recharged into the crustal magma reservoirs. Modelling results reveal that the melts in equilibrium with the Cpx macrocrysts may have been derived from the magma reservoirs at three different depths equivalent to crystallization pressures of ∼5·4, ∼3·3 and ∼1·6 kbar, respectively, making up a transcrustal magmatic system. The Cpx-laden melts in deep magma reservoirs may have been frequently transported to shallower reservoirs. Magma mixing in the shallower reservoirs led to heterogeneous magmas with different cooling rates and chemical compositions. Early crystallized Cpx crystals were overprinted with diverse zoning patterns during overgrowth and accumulation. Thus, the complex zoning patterns and compositions of the Cpx macrocrysts have important implications for a transcrustal magmatic system in the formation of ultrapotassic rocks.