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New U–Th–Pb chemical data from both electron probe microanalysis and U–Pb laser ablation-inductively coupled plasma mass spectrometry on monazite inclusions in giant andalusite porphyroblasts indicate that the lowermost subdivision of the Rocky Cape Group in northwest Tasmania underwent metamorphism at ca 1100 Ma. The Proterozoic Rocky Cape Group is a ∼10 km-thick succession of quartz arenite, siltstone and pelite. These sediments were deposited between ca 1450 and ca 1010 Ma. Metamorphism within the Rocky Cape Group ranges from the prehnite–pumpellyite to upper greenschist facies, locally reaching garnet grade. In the lowermost formation of the Rocky Cape Group, andalusite-bearing mineral assemblages in the Pedder River Siltstone formed prior to or synchronously with monazite development. The andalusite-bearing mineral assemblage indicates relatively low-pressure conditions of <200 MPa, consistent with the stratigraphic thickness of the Rocky Cape Group (∼10 km). However, the presence of the intermediate-temperature biotite–andalusite mineral assemblages (500 °C) in the study area suggests the contribution of additional heat sources. Garnet porphyroblasts were observed ∼2 km away. Similar-age medium-grade metamorphism and magmatism at about 1100 Ma have been observed regionally in the South Tasman Rise and East Tasman Plateau, and in areas that are thought to have been contiguous with northwestern Tasmania such as Dronning Maud Land, central Arizona (Yavapai–Mazatzal) and Belt–Purcell Basin in the southwestern and northwestern Laurentian crustal sequences. The pressure and temperature relationships, mineral associations and regional data suggest additional heat flow was present through the lower part of the Rocky Cape Group. The higher geothermal gradient could be attributed to burial during basin development and deposition of the upper Rocky Cape Group or associated magmatism on the margin of Laurentia during assembly of Rodinia. Low-pressure, intermediate-temperature mineral associations occur in the Rocky Cape Group. Heat sources are unrelated to nearby Devonian granite. Robust ages reveal the first discovery of 1100 Ma metamorphism in Tasmania. These provide data for a missing link in Rodinian paleogeographic reconstructions. [ABSTRACT FROM AUTHOR]

In this article, dumortierite from Croatia is described for the first time. Dumortierite formed in a pegmatite dyke cutting through Cretaceous two-mica leucogranite of the magmatic-metamorphic complex of Mt. Moslavačka Gora. The pegmatite dyke shows a magmatic mineral association of coarse-grained quartz, orthoclase, microcline and albite, less abundant muscovite, biotite, pinkish andalusite and blue-coloured prismatic dumortierite I crystals. Subsequent alteration by titanium-rich hydrothermal fluids led to partial replacement of dumortierite I and andalusite by secondary fibrous to acicular purple dumortierite II enriched in Mg and Ti. During temperature decrease perthite developed in feldspars and at a still later stage, sericite partially replaced not only feldspars but also andalusite and both types of dumortierite along grain boundaries and cracks. Final alteration at very low temperatures caused formation of clay minerals at the expense of feldspars. According to mineral chemical analyses, the feldspars are represented by albite and K-feldspar with a low albite component. Biotite corresponds to annite and its subhedral shape and chemical composition point to magmatic crystallisation from a peraluminous melt derived from a crustal source. Coarse muscovite flakes contain 1.31-1.48 wt.% FeO and 0.56-0.70 wt.% TiO2. Their Na/(Na+K) ratios (0.08-0.09) prove a magmatic origin, whereas lower ratios in sericite (0.04-0.06) indicate formation during retrogression. Magmatic muscovite is in textural equilibrium with andalusite, also implying an igneous origin for the latter, which belongs to the S3 textural type of andalusite in felsic igneous rocks. Electron microprobe analyses clearly show a strong positive correlation between Si tetrahedral deficiency (3-Si) and the sum of Al+Ti, (R²= 0.85) in both types of dumortierite, implying Al replacement by Ti. However, Al replacement by Ti is not restricted to Al in the octahedral position, as generally accepted, but most probably also in the tetrahedral position. Distinct pleochroic colours in dumortierite are usually explained by the [Fe/(Fe+Ti)]x100 factor, but according to this study, elevated Mg contents stabilize red to violet coloured dumortierite at higher [Fe/(Fe+Ti)] x100 factors than those previously suggested. Dumortierite-bearing pegmatite and host two-mica leucogranite show strong chemical similarities in their major, minor and trace element contents. Both rock types have a strong peraluminous character (ASI = 1.6 in pegmatite vs 1.8 in leucogranite), low CaO/Na2O ratios (0.11 vs 0.14), high Rb/Ba (74.5 vs 16.4) and Rb/Sr ratios (78.4 vs 43.3) as well as relatively high Al2O3/TiO2 ratios (261 vs 210). For the leucogranitic melt these characteristics indicate derivation from a pelitic source and low melting rates at relatively low temperatures. With respect to the field relationships and the chemical similarities, formation of the pegmatitic melt by fractional crystallisation during solidification of the two-mica leucogranite is inferred. Based on the mineralogical composition, the dumortierite-bearing pegmatite from Mt. Moslavačka Gora may be a member of the abyssal pegmatite class and the AB-BBe subclass. However, its formation by fractional crystallisation from a granitic melt argues against this interpretation, as all other dumortierite-bearing granitic pegmatites occur in high-grade metamorphic host rocks and are thought to be products of anatectic melting of country rocks. Therefore, the investigated pegmatite is quite unique and not fully comparable with any previously described dumortierite-bearing pegmatite worldwide. [ABSTRACT FROM AUTHOR]

Detailed microstructural, microfabric and chemical analyses of felsic granulite within and around a ductile shear zone at the Plešovice quarry in the southern Bohemian Massif revealed deformation-induced, retrograde transformation of kyanite to andalusite. Although andalusite is absent outside the shear zone, its fraction among Al 2 SiO 5 polymorphs significantly increases up to ∼59% inside the shear zone, while the fraction of kyanite significantly decreases from ∼98% outside the shear zone down to ∼22% inside the shear zone. Three examples of andalusite and the relict kyanite pair suggest a common topotaxial relationship such that andalusite (010) is parallel to the precursory kyanite (100), while andalusite [100] and [001] are parallel to the precursory kyanite [010] and [001], respectively, indicating that the growth of andalusite was crystallographically controlled by the precursory kyanite. The felsic granulite in the ductile shear zone at the Plešovice quarry has been repeatedly subjected to brittle shearing accompanied by pseudotachylite formation and plastic shearing with mylonitization, suggesting development of the ductile shear zone at the brittle–plastic transition zone. Mylonitization of the felsic granulite was promoted by deformation-induced K-feldspar replacement by myrmekite at temperatures of 390–440 °C assuming a pressure of 250 MPa. • Deformation-induced, retrograde transformation of kyanite to andalusite revealed. • Andalusite growth was crystallographically controlled by the precursory kyanite. • Transformation occurred at temperatures of 390–440 °C assuming a pressure of 250 MPa. [ABSTRACT FROM AUTHOR]