The Orestes Melt Zone (OMZ) is a massive contact melt zone (∼20 m thick by several kilometers long), located in the McMurdo Dry Valleys of Antarctica. The OMZ formed at shallow crustal depths by melting of the A-type Orestes Granite owing to intrusion of the underlying, doleritic Basement Sill. The OMZ can be divided broadly into two melting facies. The upper melting facies is distal from the contact and formed by melting at low temperature and water-saturated, or near water-saturated, conditions. The lower melting facies is proximal to the contact and formed by melting at high temperature and water-undersaturated conditions. Separate melting reactions occurred in both of the melting facies, resulting in distinct textures and melt compositions. Melting in the distal facies generated melts with compositions that plot near a predicted eutectic composition. Melting in the proximal facies was accomplished in part by replacement reactions in restitic feldspars. These reactions resulted in the development of plagioclase mantles on both restitic plagioclase and K-feldspar, and melt compositions that diverged from predicted minimum melt along an unexpected path, towards enrichment in orthoclase component. Thermal modeling indicates that this melt zone was active for a minimum of ∼150 years, with a contact temperature of ∼900 °C. Upon cooling, recrystallization generated ocellar textures around restitic quartz, as well as faceted albite as a late-stage product. Observations of the OMZ, combined with thermal modeling, provide new insights into the origin of rapakivi and albite granites. This study has implications for the origin of these two associated granite types in other geological settings.

中文翻译：

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南极麦克默多干旱谷的Orestes熔体带：接触熔体带中的空间分布熔体，对Rapakivi和Albite花岗岩的形成有影响

奥雷斯特斯熔岩带（OMZ）是一个巨大的接触熔岩带（厚约20 m，长几公里），位于南极的麦克默多干旱谷。OMZ是由于下伏的白云质基底基岩侵入而在浅地壳深度通过熔化A型Orestes花岗岩而形成的。OMZ大致可分为两个熔融相。上部熔融相远离接触，并且是在低温和水饱和或接近水饱和的条件下熔融形成的。较低的熔融相接近于接触，是在高温和水不饱和条件下熔融形成的。在两个熔化相中都发生了单独的熔化反应，从而导致了明显的织构和熔化成分。远端相中的熔体产生的熔体的成分接近预期的共晶成分。近端相的融化部分是通过再结晶长石中的置换反应完成的。这些反应导致在斜生斜长石和钾长石上都出现了斜长石披风，熔体组成从预计的最小熔体沿着意外的路径偏离，从而富集原正石酶组分。热模型表明，该熔融区至少在约150年内处于活动状态，接触温度为约900°C。冷却后，再结晶会在方晶石英周围产生晶胞纹理，并在后期产品上生成刻面钠长石。OMZ的观测结果与热模型相结合，提供了对rapakivi和钠长石花岗岩成因的新见解。