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<p>H<sub>2</sub>O-undersaturated experiments and phase-equilibrium modelling demonstrate that orthopyroxene is the main ferromagnesian mineral formed during high-temperature crystallisation of most S-type and many I-type granitic (s.l.) magmas. In most examples, at any pressure relevant to crustal magmatism, orthopyroxene will be stable to <i>T</i> &gt; 935&#xa0;°C, in typical calc-alkaline metaluminous or peraluminous granitic magmas. Once such magmas cool to between 800 and 750&#xa0;°C, almost all the ferromagnesian component will have crystalised as pyroxene. On further cooling, orthopyroxene will react to biotite within a narrow temperature band (~ 30&#xa0;°C). Textures in granitic rocks attest to this, although the significance of the textures produced by this reaction can be overlooked. Pristine orthopyroxene is rarely found in fully crystallised granitic rocks because such preservation would require solidification at <i>P</i> &lt; 1 kbar. During crystallisation at higher <i>P</i>, biotite-quartz aggregates replace orthopyroxene, resulting in a variety of diagnostic textures. Due to the normal, high-temperature, perisolidus, textural modifications in granitic rocks, the biotite-quartz pseudomorphs after early-formed orthopyroxene may be disregarded or simply described as ‘mafic clots’. As the presence of orthopyroxene has important implications for the temperatures and H<sub>2</sub>O contents of granitic magmas, we present an atlas of textures to assist in recognising these pseudomorphs. An important implication of the presence of orthopyroxene near the liquidi of many granitic magmas is that they formed at <i>T</i> &gt; 800&#xa0;°C, through fluid-absent or H<sub>2</sub>O-undersaturated melting of biotite-bearing or biotite + hornblende-bearing assemblages. Complete supersolidus reaction of orthopyroxene to biotite would be prevented by significant melt loss from the system. Thus, the rarity of orthopyroxene in granitic rocks indicates that fractional crystallization of granitic magmas at <i>T</i> &gt; 750&#xa0;°C is uncommon and that the loss of a silicic volcanic liquid fraction is also generally unlikely.</p>

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Most ordinary granitic magmas are born as charnockites or become charnockites during magma ascent: evidence for the prior existence of orthopyroxene and implications of high magma temperatures and inefficient crystal-melt separation

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  • 发表日期：

    2026年5月7日

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  • John D. Clemens,
  • Gary Stevens

摘要

H2O-undersaturated experiments and phase-equilibrium modelling demonstrate that orthopyroxene is the main ferromagnesian mineral formed during high-temperature crystallisation of most S-type and many I-type granitic (s.l.) magmas. In most examples, at any pressure relevant to crustal magmatism, orthopyroxene will be stable to T > 935 °C, in typical calc-alkaline metaluminous or peraluminous granitic magmas. Once such magmas cool to between 800 and 750 °C, almost all the ferromagnesian component will have crystalised as pyroxene. On further cooling, orthopyroxene will react to biotite within a narrow temperature band (~ 30 °C). Textures in granitic rocks attest to this, although the significance of the textures produced by this reaction can be overlooked. Pristine orthopyroxene is rarely found in fully crystallised granitic rocks because such preservation would require solidification at P < 1 kbar. During crystallisation at higher P, biotite-quartz aggregates replace orthopyroxene, resulting in a variety of diagnostic textures. Due to the normal, high-temperature, perisolidus, textural modifications in granitic rocks, the biotite-quartz pseudomorphs after early-formed orthopyroxene may be disregarded or simply described as ‘mafic clots’. As the presence of orthopyroxene has important implications for the temperatures and H2O contents of granitic magmas, we present an atlas of textures to assist in recognising these pseudomorphs. An important implication of the presence of orthopyroxene near the liquidi of many granitic magmas is that they formed at T > 800 °C, through fluid-absent or H2O-undersaturated melting of biotite-bearing or biotite + hornblende-bearing assemblages. Complete supersolidus reaction of orthopyroxene to biotite would be prevented by significant melt loss from the system. Thus, the rarity of orthopyroxene in granitic rocks indicates that fractional crystallization of granitic magmas at T > 750 °C is uncommon and that the loss of a silicic volcanic liquid fraction is also generally unlikely.