Peridotite-derived detrital pyropes versus high-pressure felsic granulite-derived pyrope-almandine garnets from the Lower Triassic deposits of the NE foreland of the Bohemian Massif (S Poland, Central Europe)

摘要

Numerous pyrope grains (group I; Prp62–72Alm14–24Grs(<1)4–14Uv(1)2–14Sps<1–1Adr<1–2) have been found in the heavy mineral assemblages from the Older Buntsandstein arenites (Lower Triassic) cropping out in S Poland (NE foreland of the Bohemian Massif). This is the first find of the abundant detrital pyropes outside, but near to, the Bohemian Massif, and the first from the Lower Triassic deposits. In the heavy mineral assemblages from the younger wackes and marly limestones (Older Buntsandstein and Röt deposits), from the same area, Ca-low pyrope-almandines of subgroups IVA (Prp28–52Alm44–68Grs1–4Uv<1Sps1–2Adr<1) and IVB (Prp27–50Alm41–65Grs>5–15Uv<1Sps<1–2Adr<1–1) are the most numerous. The main aims of this study were to determine the primary and secondary source rocks for the garnets, and to identify a cause for the change of the garnet types in the older and younger deposits. Pyropes contained in the Bohemian Massif peridotites, presently exposed in the Moldanubian Zone, and the Kutná Hora Unit, show most comparable chemical compositions to group I pyropes. Some group I pyropes have no chemical counterparts within the comparative materials, which suggests that some other pyrope-bearing peridotites may have been exposed within the Bohemian Massif, but destroyed in the past. Garnets contained in the high-pressure felsic granulites known from several subunits of the Moldanubian Zone show similar chemical compositions to the garnets from subgroup IVA. The group I grains reveal features pointing to (possibly multiple) reworking and likely longer transport distance, while the group IV grains reveal signs of not advanced abrasion and possible short transport distance. However, no older or coeval sedimentary rocks with the similar heavy mineral assemblages, nor the garnet groups, are recognized within the Bohemian Massif or the adjacent areas, which suggests that the garnets may have been recycled from deposits largely destroyed in the past. The deposition of group IV garnets was a normal occurrence for a long time span. In contrast, the deposition of the detrital pyropes was an unusual event. These pyropes must have been redeposited from pyrope-bearing ‘horizons’ present in Carboniferous-Lower Triassic sedimentary rocks. Their availability was limited to the time span necessary for erosion of these horizons.