In southern Israel and on the Sinai Peninsula, ultrapotassic quartz porphyries (UPQP) with 6.5–10 wt.% K2O and 0.1–2.5 wt.% Na2O were found in a bimodal dike suite that formed at the final stage of the Pan-African orogeny. The suite is made up mainly of quartz porphyry (4–5.5 wt.% K2O); mafic rocks amount to < 5%. The UPQP form rare dikes or patches in quartz porphyry dikes. These are typical igneous rocks with microgranophyric and spherulitic matrices. There is no mineralogical evidence for the gain of K at the postmagmatic stage. Evidence for low-temperature adularization found in some ultrapotassic rhyolites from other world areas has not been revealed either. Alkali-feldspar phenocrysts in the UPQP and quartz porphyries have high contents of orthoclase (≥85−90%). Study of melt inclusions in quartz phenocrysts in both the UPQP and ordinary quartz porphyries showed that the phenocrysts crystallized from magmas of quartz porphyry composition that contained 2–3 wt.% H2O, up to ∼1% F, and 0.1–0.15% Cl. Scanning electron microscope studies showed that many alkali quartz porphyry dikes have a heterogeneous matrix. Rounded and oval segregations (0.3–15 mm across) with microgranophyric and spherulitic textures amount to ∼50%. They are enriched in K2O and are compositionally similar to typical UPQP. In the microcrystalline aggregates hosting these segregations Na dominates over K.
To account for the UPQP generation, a model is proposed for the disequilibrium crystallization of silicic magma during its flow along fractures in cooled country rocks. The magma was of alkali rhyolite composition and contained 5–7% phenocrysts. At the early stages of the matrix crystallization, spherulitic and microgranophyric segregations formed. They were enriched in K, which is typical of a haplogranite system at the beginning of crystallization. Since the system remained closed for major components, the portion of Na in the residual melt increased. The disequilibrium crystallization conditions inhibited a chemical interaction between early and late phases. During the “magma mash” movement, partial separation of the solid and residual liquid phases might have occurred. The likely separation processes were filter pressing, side-wall crystallization, and separation of the liquid and solid phases above the “rigid percolation threshold”. The UPQP resulted from the crystallization of a mixture of early microcrystalline segregations and minor residual melt.
