Experimental Constraints on the Storage Conditions and Differentiation of High-Ti Basalts from the Panzhihua and Hongge Layered Intrusions, SW China

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS Journal of Petrology Pub Date : 2024-07-10 DOI:10.1093/petrology/egae078
Dachuan Wang, Tong Hou, Roman Botcharnikov, Sarah Haselbach, Florian Pohl, Renat R Almeev, Andreas Klügel, Meng Wang, Jingyi Qin, Zhaochong Zhang, Francois Holtz
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Abstract

High-Ti basalts are commonly believed to represent parental magmas leading to the formation of mafic-ultramafic layered intrusions, such as Late Permian Panzhihua and Hongge in the Emeishan Large Igneous Province (SW China). Consequently, elucidation of the crystallisation and crustal differentiation of high-Ti basalts is critical for our understanding of the petrogenesis of these layered intrusions and the associated oxide ore mineralisation. Here, we present the results of crystallisation experiments carried out in internally heated pressure vessels using a primitive high-Ti basaltic composition. The experiments were conducted at 100 and 300 MPa, in the temperature interval of 950-1200 °C and with water activities (aH2O) from 0 to 1. The oxygen fugacity (fO2) was controlled and varied from FMQ - 1 to FMQ + 3.3 log units (FMQ corresponds to fayalite-quartz-magnetite buffer). The main mineral phases are olivine, clinopyroxene and plagioclase, accompanied by Cr-Fe-Ti-oxides, orthopyroxene, apatite and amphibole, depending on the conditions. Redox conditions primarily influence the stability fields of Cr-Fe-Ti oxides. Clinopyroxene, orthopyroxene and amphibole are pressure-dependent and have larger stability fields under high pressure conditions. The olivine→orthopyroxene and olivine→amphibole peritectic reactions are observed. Comparisons of phase equilibria between this study and experiments conducted with parental magma of Skaergaard layered intrusion demonstrate the effect of bulk system composition. For instance, ilmenite crystallisation is determined by intrinsic parameters such as fO2 but also by additional compositional parameters (e.g., melt Ti, Fe, Al and Mg content). Although COMAGMAT and MELTS modelling results generally reproduce the crystallisation sequence, only the stability field of clinopyroxene and its composition are perfectly modelled. The comparison of experimental results with the rocks from the lowest units of Panzhihua and Hongge layered intrusions are applied to constrain storage conditions in the magma reservoirs. Compared to Hongge, we conclude that the Panzhihua magma chamber was probably located at a shallower depth (~3-6 km), that magma crystallisation started at lower temperatures (~1125-1100 °C), higher fO2 (~FMQ + 1 to FMQ + 2), and that its initial melt H2O content was lower (~0.5-1 wt.%).
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中国西南攀枝花和红格层状侵入体高钛玄武岩储集条件和分异的实验约束
高钛玄武岩通常被认为是导致岩浆-超岩浆层状侵入体形成的母岩,如中国西南部峨眉山大火成岩省的晚二叠世攀枝花和红格。因此,阐明高钛玄武岩的结晶和地壳分异对于我们了解这些层状侵入体的成岩过程以及相关的氧化矿成矿过程至关重要。在此,我们介绍使用原始高钛玄武岩成分在内部加热的压力容器中进行的结晶实验结果。实验在 100 和 300 兆帕,950-1200 °C的温度区间内进行,水活度(aH2O)从 0 到 1。氧富集度(fO2)在 FMQ - 1 到 FMQ + 3.3 对数单位(FMQ 相当于辉绿岩-石英-磁铁矿缓冲)之间变化。主要矿物相为橄榄石、倩辉石和斜长石,根据条件不同,还有铬-铁-钛-氧化物、正长石、磷灰石和闪石。氧化还原条件主要影响铬-铁-钛氧化物的稳定性。霞石、正长石和闪石与压力有关,在高压条件下稳定场较大。观察到橄榄石→正辉石和橄榄石→闪石的包晶反应。这项研究与斯卡尔加德层状侵入体母岩浆实验之间的相平衡比较表明了体系成分的影响。例如,钛铁矿的结晶是由 fO2 等固有参数决定的,但也受其他成分参数(如熔体中的钛、铁、铝和镁含量)的影响。虽然 COMAGMAT 和 MELTS 建模结果总体上再现了结晶顺序,但只有霞石的稳定场及其成分是完美建模的。将实验结果与攀枝花和红格层状侵入体最低单元的岩石进行对比,用于约束岩浆储层的储集条件。与红格相比,我们得出结论:攀枝花岩浆腔可能位于较浅的深度(约3-6千米),岩浆结晶开始于较低的温度(约1125-1100 °C)、较高的fO2(约FMQ + 1至FMQ + 2),其初始熔体H2O含量较低(约0.5-1 wt.%)。
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来源期刊
Journal of Petrology
Journal of Petrology 地学-地球化学与地球物理
CiteScore
6.90
自引率
12.80%
发文量
117
审稿时长
12 months
期刊介绍: The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.
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