Geochemistry and mineralogy of neoproterozoic exotic minerals-bearing post-collisional A-type granites of Jebel Qutn intrusion, Arabian shield, Saudi Arabia

The Jabal Qutn intrusion (JQI) is a Late Ediacaran post-collisional granitic intrusion in the northeastern part of the Arabian Shield, the western extension of the Neoproterozoic Arabian-Nubian Shield (ANS). The study area is dominated by Neoproterozoic rocks including sedimentary units of the Murdama group, granodiorites, and A-type granites. The latter form a ring complex that consists of an inner core of alkali feldspar granite and an incomplete outer zone of syenogranite with gradational contacts between them. The Qutn granites are mainly composed of feldspars, micas and quartz and contain a large variety of accessory phases including zircon, allanite, thorite, topaz, titanite, waimirite-(Y), bastnäsite, Nb-Ta oxides (columbite, qitianlingite and nioboaeschynite), wolframite, pyrochroite, apatite, and gold alloys (Au contents of 60–80 wt%). Rare mineral species such as qitianlingite and nioboaeschynite-(Y) are recorded for the first time in the whole ANS. In case of waimirite-(Y) the study area represents the third reported occurrence in the world. The granites are highly evolved (SiO$_2$ = 72.69–76.65 wt%), and exhibit almost identical chondrite-normalized rare-earth elements patterns with slight enrichment in LREE compared to HREE [(La/Lu)$_n$ = 1.24–2.92] and prominent negative Eu anomalies (Eu/Eu* = 0.04–0.13). ... mehrThey show geochemical characteristics of anorogenic granites and continuous compositional trends for both major and trace elements, indicating a common source, i.e. partial melting of juvenile crust, followed by extensive fractional crystallization of feldspars, apatite, and Fe-Ti oxides associated with minor crustal contamination. The granites were emplaced at shallow depths (< 6 km) with their estimated crystallization temperatures ranging between ca. 780 and 850 °C. The late formation of F-rich minerals (topaz, fluorite, waimirite-(Y)) implies a F-rich composition of the parental magma with strong late-magmatic F- and REE-enrichment. Textural characteristics provide evidence for a strong metasomatic alteration in the late-magmatic stage that mainly occurred in the apical and marginal parts of the alkali feldspar granite, with the hydrothermal solutions being concentrated in the upper part of the magma chamber before complete crystallization of granitic magma