#Dalmatian jasper full
Late Cretaceous peralkaline rhyolites (69.4 ± 0.4 Ma - 2σ analytical, 1.9 Ma full external uncertainty, 40Ar/ 39Ar sanidine single crystal laser dating) from Lake Chad are the oldest lavas of the "Cameroon Hot Line". Thus, Sr was capable of entering the structure of alkali-amphiboles. Sr-enrichment of the rock samples, coupled with only minor negative Eu anomaly (Eu/Eu*: 0.52-0.78), further suggests that early-stage separation of plagioclases has not significantly contributed to the magma evolution. both fluids-melts interactions and/or post-magmatic high-temperature fluid imprint could be involved.
Thus, dianite could form in a complex magmatic-hydrothermal system, where i.e. M-type curves were recognized La-Nd and Er-Lu segments whilst W-type prevails in the Gd-Ho segment of the chondrite-normalized REE patterns. perpotassic and peralkaline nature of dianite, as well as the presence of rare composite M- and W-type tetrad effect. Whole-rock major and trace element data indicate i.e. Accessory, hydrothermal phases of dianite comprise barite and sphalerite, which both indicate S-bearing fluid influx. The origin of this amphibole is likely related to the crystallization from highly-evolved Na-, Si-rich peralkaline magma at the higher oxidation conditions, although post-magmatic formation from hydrothermal fluids cannot be excluded. Felted/fine-fibrous fabrics of the potassic-magnesio-arfvedsonite along with (K, Na) ↔ NH4⁺/H3O⁺ substitution revealed by Raman bands at 24 cm⁻¹, suggest it crystallized in a water-rich environment. The most abundant type of amphibole species recognized in dianite is represented by amphibole (IV) potassic-magnesio-arfvedsonite that not only envelops abundant anhedral K-feldspars, but also appears to be associated with clustered and nest-like micaceous aggregates. Later, as a result of a dissolution-precipitation process, they were partially replaced by potassic-richterite. Magnesiohastingsite-pargasite minerals possibly crystallized at relatively low oxidation conditions, from Ca- and Al-rich melt contaminated by adjacent skarn-type deposits.
Magnesio-hastingsite and pargasite – magnesio-hastingsite species form intergrowths and/or individual crystals with a tabular-like habit, and are partially replaced by minor (III) colourless potassic-richterite marked by slight Sr impurity (up to 0.42 wt.% of SrO). Dianite represents a unique amphibole–rich rock of ambiguous origin that has been reported from the alkaline Murun Complex in eastern Siberia (Russia).
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