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.

中文翻译：

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周期性混合记录的火山岩的超钾化灯生化岩中的斜辉石宏结晶的岩浆的周期性混合。

摘要

超钾质岩的体积较小，但广泛分布在不同的地质环境中。广泛的研究涉及产生这些岩石的地幔融化过程。但是，他们可能涉及的地壳过程却鲜为人知。在本文中，我们描述了吉尔吉斯斯坦北天山造山带中超钾化煌斑岩脉中的斜辉石（Cpx）大晶体的复杂振荡分区模式。这些大晶体通常具有被腐蚀的或斑片状的核，被地幔围绕，并具有明显的振荡带，而该地带又被一整面的边缘所包围。地幔的振荡区带由交替的粗糙层和精细层组成，具有清晰的吸收表面，或者由紧密堆积的层组成，在反向散射电子图像中边界呈直线或波浪形边界。Mg＃，Ti，穿过地幔各层的Al，Cr和Sr归因于岩浆混合。在热和化学梯度下，由于动力学效应或晶体运动，Mg＃在紧密堆积的层上发生了低幅，高频振荡。另外，一些大晶体的隐秘区划清楚地显示出富硅和镁的沙漏区以及富铝和钛的棱镜区。扇形区划表明这些大晶体在低度过冷度下结晶，并且同一晶粒内存在同心的富铬层和贫铬层表明该生长过程被多次岩浆充注事件所破坏。大晶体的核心具有三个不同的Mg＃范围：<84–90（核心I），74–84（核心II）和60–70（核心III）。地幔的Mg＃范围从64到90，没有明显的差距。轮辋的Mg＃范围从76到80不等。Mg＃高（≥85）的岩心和地幔的La / Yb范围从1·8到5·0，Dy / Yb范围从2·3到4·六 大晶整体具有可变性⁸⁷ SR / ⁸⁶ 0·7072 Sr与0·7084。微量元素和⁸⁷ Sr / ⁸⁶单一颗粒中的Sr表示，具有不同成分的原生岩浆和演化岩浆都被定期充入地壳岩浆储层中。模拟结果表明，与Cpx大晶体平衡的熔体可能来自于三个不同深度的岩浆储层，分别相当于〜5·4，〜3·3和〜1·6 kbar的结晶压力，构成了一个经地壳岩浆系统。深部岩浆储层中富含Cpx的熔体可能经常被运往较浅的储层。浅层储层中的岩浆混合导致了冷却速率和化学成分不同的非均质岩浆。早期结晶的Cpx晶体在过度生长和积累过程中以不同的分区模式叠印。因此，