Oligocene high-MgO alkali basalts in central Tibet: implications for magma–mush mixing and mantle processes

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Yue Qi, Qiang Wang, Gang-Jian Wei, Xiu-Zheng Zhang, Wei Dan, Zong-Yong Yang, Lu-Lu Hao, Wan-Long Hu
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引用次数: 0

Abstract

High-MgO (> 9 wt.%) basaltic rocks can be primary magmas and used to constrain the geochemistry and temperature of the mantle. However, high MgO contents can also result from mixing between evolved melts and antecrysts or xenocrysts, and thus the whole-rock composition might not represent the solidified equivalents of primary magma. Whether such mixing with crystals can result in erroneous interpretations of mantle processes remains unclear. This study presents a petrological and geochemical investigation of the post-collision high-MgO (> 9 wt.%) Lugu volcanic rocks in the southern Qiangtang terrane, central Tibet. The Lugu volcanic rocks comprise porphyritic and intersertal alkali basalts. Zircon U–Pb ages and 40Ar/39Ar dating suggest that the two types of alkali basalts were erupted at ca. 29 Ma. Based on detailed petrographic observations and geochemical analysis, the porphyritic alkali basalts may represent near-primary melts, which are characterised by low SiO2 contents (40.9–45.1 wt.%), high CaO/Al2O3 ratios (1.1–1.5), and arc-like trace element patterns. We suggest these basalts were derived by partial melting of enriched garnet peridotite (> 3GPa) in the presence of H2O and CO2. These geochemical features are different from those of the ca. 30 Ma (ultra)-potassic rocks in the Qiangtang terrane, indicating that heterogeneous lithospheric mantle existed beneath the Qiangtang terrane during the Oligocene. In contrast, although the intersertal alkali basalts have high MgO contents (> 9 wt.%), evidence from mineral chemistry indicates that whole-rock compositions of the intersertal alkali basalts represent mixtures of evolved residual melts and cumulate crystals. They were the product of polybaric fractional crystallisation and subsequent mixing of crystals and residual melts in a magmatic plumbing system. Furthermore, when intersertal alkali basalts are assumed to be primary melts, they would have been derived by partial melting of shallow (~2.5 GPa) CO2-poor pyroxenite or peridotite. These conditions are different from interpretations of the nature of mantle source and melting conditions for porphyritic alkali basalts. Our results highlight that the interpretation of petrogenetic processes should be preceded by detailed mineralogical investigations.
西藏中部渐新世高氧化镁碱性玄武岩:对岩浆-岩浆混合和地幔过程的影响
高氧化镁(> 9 wt.%)玄武岩可能是原生岩浆,并可用于制约地幔的地球化学和温度。然而,高氧化镁含量也可能是由于演化熔体与前晶或异晶之间的混合造成的,因此整个岩石的成分可能并不代表原生岩浆的凝固等同物。这种与晶体的混合是否会导致对地幔过程的错误解释,目前仍不清楚。本研究对西藏中部羌塘地层南部的碰撞后高氧化镁(> 9 wt.%)鲁古火山岩进行了岩石学和地球化学研究。泸沽火山岩由斑岩和岩间碱性玄武岩组成。锆石U-Pb年龄和40Ar/39Ar年代测定表明,这两种类型的碱性玄武岩喷发于约29Ma。根据详细的岩石学观察和地球化学分析,斑状碱性玄武岩可能代表近原生熔融体,其特征是二氧化硅含量低(40.9-45.1 wt.%)、氧化钙/氧化铝比率高(1.1-1.5)以及类似弧形的微量元素模式。我们认为这些玄武岩是由富集的石榴石橄榄岩(> 3GPa)在 H2O 和 CO2 的作用下部分熔融而形成的。这些地球化学特征不同于约这些地球化学特征与羌塘地层中的约30Ma(超)钾盐岩不同,表明在渐新世时期,羌塘地层下存在着异质岩石圈地幔。相比之下,虽然岩间碱性玄武岩的氧化镁含量较高(> 9 wt.%),但矿物化学证据表明,岩间碱性玄武岩的全岩成分是演化残余熔体和积晶的混合物。它们是多巴分段结晶以及随后在岩浆管道系统中晶体和残余熔体混合的产物。此外,如果假定深成岩间碱性玄武岩是原生熔体,那么它们应该是由浅(~2.5 GPa)CO2贫乏的辉长岩或橄榄岩部分熔化而成的。这些条件与对斑状碱性玄武岩的地幔源性质和熔融条件的解释不同。我们的研究结果突出表明,在解释岩石成因过程之前,应先进行详细的矿物学研究。
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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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