Nódulos calcossilicatados zonados em mármores da região de Alvito, Alentejo (Sul de Portugal) Zoned calc-silicate nodules in marbles from Alvito region, Alentejo (Southern Portugal)
Calc-silicate nodules have developed metasomatic concentric zones in the Lower Cambrian metalimestones of the contact metamorphic aureole of Late Carboniferous diorites in the Alvito region, Alentejo (Southern Portugal). The zonation pattern includes: a core, characterized by a fine-grained granoblastic mosaic of diopsidic clinopyroxene, plagioclase (An 98-96), and retrograde prehnite, a garnet/vesuvianite zone consisting mainly of medium-grained poiciloblastic grossular and/or vesuvianite with
... or vesuvianite with clinopyroxene, plagioclase, and wollastonite inclusions, and a wollastonite zone consisting of wollastonite, diopside and prehnite. A prehnite zone is observed in only one nodule and contains prehnite, fluorapophyllite, quartz, hedembergitic clinopyroxene and feldspar. The mineralogical and chemical data indicate that these nodules may have formed accordingly the following stages: (i) metasomatic zones started to grow due to inter-diffusion resulting from the metamorphism of two adjacent contrasting rock types, probably thin sheet intrusions or thin levels of metavolcanics of intermediate composition in metacarbonate rocks; an early stage would be characterized by a clinopyroxene core and a wollastonite rim; (ii) garnet and vesuvianite grew after the metamorphism peak at the expense of clinopyroxene, wollastonite, and plagioclase; (iii) fluorapophyllite and prehnite define a final hydrothermal stage due to the influx of aqueous fluids enriched in fluor and potassium; (iv) the nodules are interpreted as a result of boudinage in layers or intrusions in the limestones, during the first Variscan tectonic phase (D 1), and latter softly deformed by the second Variscan phase (D 2).