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Plagioclase Zoning as a Record of Magma Evolution
Author(s) -
NIU Zhijian,
LIU Yue,
DI Yongjun
Publication year - 2014
Publication title -
acta geologica sinica ‐ english edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.444
H-Index - 61
eISSN - 1755-6724
pISSN - 1000-9515
DOI - 10.1111/1755-6724.12382_18
Subject(s) - beijing , china , chinese academy of sciences , mineral resource classification , geology , earth science , geography , geochemistry , archaeology
(e.g. plagioclase, amphibole, clinopyroxene or biotite) can play a valuable role in indicating the evolutionary history of magma mixing processes (Kuscu, 2001). Most zoning studies in magmatic systems carried out to date have addressed plagioclase feldspar because the chemical variations in this mineral can readily be inferred from its optical properties in polarized light using an optical microscope (Hibbard, 1981). The formation of complex chemical zoning in plagioclase depends on the change of crystallization conditions (e.g. pressure, temperature and volatile content of the magma) (Lofgren, 1974; Tsuchiyama, 1985; Di, et al, 2003). Thus when crystals grow in the magma chamber, their grain-scale inhomogeneity, they may contain a record of the processes during that growth. Growing crystals record different environments and crystallization conditions (e.g. mixing of magmas, degassing, assimilation). “Normal” zoning in plagioclase is the direct result of magma cooling. “Reverse” zoning is less common and the zonation genetics are interpreted differently by different authors. For example, temperature and pressure effects have been proposed to result from crystals settling into deeper, hotter magma (Anderson et al. 2000) or from reheating due to magma recharge (Ruprecht and Wörner, 2007). It has also been argued that crystallization of calcic plagioclase is caused by assimilation limestone during magmatic eruption. By contrast, changes in temperature, pressure and H2O content will affect the anorthite component (An) content but will cause little change in minor elements (e.g. Fe, Sr, Ba, etc.). This is because the bulk compositional change of the host magma would have a stronger impact on Sr, Mg and Fe in the plagioclase than the effect of changing partition coefficients due to changing An in the crystal (Ginibre and Wörner , 2007). The relationships between the concentration of minor elements in feldspar and the An can be used to determine the formation mechanisms of reverse zoning pattern. If the content of minor elements varies with change in An NIU Zhijian, LIU Yue and DI Yongjun, 2014. Plagioclase Zoning as a Record of Magma Evolution. Acta Geologica Sinica (English Edition), 88(supp. 2): 1477-1478.