Phase equilibrium modelling of a conformable sequence of supracristal lithologies from the Bushmanland Subprovince of the Namaqua–Natal Metamorphic Complex (South Africa) reveals a disparity of some 60–70°C in estimated peak metamorphic temperature. Aluminous metapelites were equilibrated at ~770–790°C, whereas two‐pyroxene granulite and garnet–orthopyroxene–biotite gneiss record distinctly higher conditions of ~830–850°C. Semi‐pelite and Mg–Al‐rich gneisses yield poorly constrained estimates that span the range derived from other lithologies. All samples record peak pressure of ~5–6 kbar, and followed a roughly isobaric heating path from andalusite‐bearing greenschist/lower amphibolite facies conditions through a tight clockwise loop at near‐peak conditions, followed by near‐isobaric cooling. The disparity in peak temperatures appears to be robust, as the low‐variance assemblages in all samples reflect well‐known melting reactions that only occur over narrow temperature intervals. The stable coexistence of both products and reactants of these melting reactions indicates that they did not go to completion before metamorphism waned. Calculated pressure–enthalpy diagrams show that the melting reactions are strongly endothermic and therefore buffer temperature while heat is consumed by melting. Because the respective reactions occur at distinct P–T conditions and have different reactant assemblages, individual lithologies are thermally buffered at different temperatures and to different degrees, depending on the occurrence and abundance of reactant minerals. Our calculations show that all lithologies received essentially the same suprasolidus heat budget of 19 ± 1 kJ/mol, which led to the manifestation of lower peak temperatures in the more fertile and strongly buffered aluminous metapelites compared with more refractory rock types. If little to no thermal communication is assumed, this implies that lithology exerts a first‐order control over the heating path and the peak temperature that can be attained for a specific heat budget. Our results caution that the metamorphic conditions derived from pelitic granulites should not be assumed or extrapolated to larger sections of an orogenic crust that consist of other, more refractory lithologies.

中文翻译：

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共存的粒状岩相岩性中记录的看似完全不同的温度

来自纳马夸－纳塔尔变质复合体（南非）布什曼兰省的超结晶岩性顺应序列的相平衡模型显示，估计的峰值变质温度相差约60–70°C。铝变质岩在约770-790°C时达到平衡，而二py花岗岩和石榴石-邻位比邻苯二酚-黑云母片麻岩记录了约830-850°C的较高条件。半珍珠岩和富含Mg-Al的片麻岩产生的约束估计较差，其范围涵盖了从其他岩性获得的范围。所有样品都记录了约5-6 kbar的峰值压力，并遵循了从含红柱石的绿片岩/较低角闪石相状态大致等压的加热路径，并在近峰条件下通过紧密的顺时针环路进行了随后的等压冷却。峰值温度之间的差异似乎很强，因为所有样品中的低方差组合反映了众所周知的熔化反应，该反应仅在狭窄的温度间隔内发生。这些熔融反应的产物和反应物的稳定共存表明它们在变质减弱之前尚未完全完成。计算得出的压力-焓线图表明，熔融反应强烈地吸热，因此缓冲温度，而熔融消耗热量。因为各自的反应发生在不同的地方 计算得出的压力-焓线图表明，熔融反应强烈地吸热，因此缓冲温度，而熔融消耗热量。因为各自的反应发生在不同的地方 计算得出的压力-焓线图表明，熔融反应强烈地吸热，因此缓冲温度，而熔融消耗热量。因为各自的反应发生在不同的地方P – T由于反应物矿物的存在和丰富，在不同的条件和不同的反应物组合下，各个岩性在不同的温度和不同的程度被热缓冲。我们的计算结果表明，所有岩性都获得了基本相同的超固相线热收支，即19±1 kJ / mol，这导致与更难熔的岩石类型相比，在更肥沃且缓冲性强的铝质变质岩中出现较低的峰值温度。如果假设几乎没有热传导，则意味着岩性对加热路径和特定热预算所能达到的峰值温度进行一级控制。