Allanite lies within epidote group that contain a significant amount of rare-earth elements. The mineral occurs mainly in metamorphosed clay-rich sediments and felsic igneous rocks. Is it possible to justify the source of allanite mineral grains from stream sediments?
The allanite grain in stream sediments is most likely to be derived from igneous rocks. It could be from a metaigneous source.
Have you ever identify allanite present in a paragneiss, i.e. formed by metaphorphism?
Xue-Ming Yang
Dr Yang....I have already found differences in zircon and monazite from igneous and metamorphic origin and was wondering if someone found the differences with respect to allanite.
Best regards
I wonder how a zircon/ monazite can form during metamorphism.
Their source is definitely either a precursor igneous/ sedimentary rock
Dear Latha
The question is whether the metamorphic process is able to affect the mineral chemistry of the two minerals or not. This depends on the blocking temperature of these minerals. We have got some comparative points, for example, Igneous zircon has characteristically higher Th/U ratios (>0.1) that usually do not overlap with Th/U ratios of metamorphic zircons (0.01–0.1) (Williams and Claesson 1987; Vavra et al. 1996; Hoskin and Black 2000; Hartmann and Santos 2004). The characteristic HfO2/ZrO2 ratio in zircon of granites ranges from 0.01 to 0.03 (Hoskin et al. 2000). Also thorite inclusions in monazite and apatite inclusions in zircon characterize the igneous origin. Zircon grains from a range of plutonic rocks (diorite–monzonite–granite sequence) contain inclusions of apatite and mafic phases (Jennings et al. 2011).
In general, igneous zircons could be distinguished from metamorphic zircons by their growth zoning patterns. The igneous zircon is characterized by oscillatory (concentric, euhedral) and sector (hourglass, fir tree) zoning (Pidgeon 1992; Hanchar and Miller 1993; Vavra et al. 1999; Hoskin 2000; Corfu et al. 2003), whereas the metamorphic zircon has no oscillatory zoning and tends to have poorly defined internal zoning and may show nongeometric patchy zoning (Pidgeon 1992; Hanchar and Miller 1993; Hoskin and Black 2000). Igneous zircon also tends to form euhedral crystals or euhedral overgrowths on preexisting grains. In contrast, metamorphic zircon morphology is highly variable and typically has rounded cores and/or rims (Pidgeon 1992; Hanchar and Miller 1993), but euhedral shapes are also possible, especially in very fluid-rich systems, such as in amphibolite facies mica schists or migmatites (Chiarenzelli and McLelland 1993)
Best regards
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