GENESIS OF ULTRAPOTASSIC QUARTZ PORPHYRY IN THE NORTHERN ARABIAN-NUBIAN SHIELD
B.A. Litvinovsky *,**, N.S. Karmanov *, and E. Vapnik **
* Geological Institute, Siberian Branch of the RAS, 6a ul. Sakh'yanovoi, Ulan Ude, 670047, Russia
** Department of Geological and Environmental Sciences, Ben-Gurion University of the Negev,
POB 653, Beer Sheva 84105, Israel
Keywords: Ultrapotassic rhyolite, bimodal suite, quartz porphyry dikes, disequilibrium crystallization
Pages: 1103-1127
Abstract
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
|
