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Alkali feldspar microtextures from Pogari formation granite pebbles (Middle Bosnia)

Several reactions on minerals of the exotic reddish I-type granite pebbles of Cretaceous Pogari formation (Middle Bosnia) were observed using polarizing microscope: sericitization of plagioclase and chloritization of biotite (with epidote as a possible by-product), myrmekite, „plagioclase overgrowth“, „intergranular plagioclase“, exsolution and „patch“ perthites. These reactions reflect the cooling history of a parent magma. Some of them might have been interconnected („cyclic“ reactions) and could have formed simultaneously. The Al–Si ordering, which controls the change in symmetry in K-feldspars ((K, Na)AlSi3O8) from sanidine (Sa ; c.c. 2/m, s.g. C2/m), through orthoclase (Or ; c.c. 2/m, s.g. C2/m) to microcline (Mc ; c.c. , s.g. C ), and microstructures provide evidence for the thermal history of feldspars. There are three important processes that occur in feldspar crystals during cooling: (1) the homogenous composition exsolutes into K- and Na-rich lamellae and lenses at high temperature (below solvus) ; (2) the C2/m symmetry degrades to C ; (3) twinning occurs in both Na- and K-rich lamellae. The last two processes are simultaneous. During subsolidus, coherent exsolution and coarsening leads to finescale, regular, lamellar „strain-controlled“ crypto- or fine micro-perthitic intergrowths, with lamellae typically ≤ lµm thick and coherent or semicoherent interfaces between albite (Ab)-rich and K-feldspar (Or)-rich phases. Because of the difference in cell dimensions between the exsolved phases, elastic strain (coherency strain energy) develops at the surfaces of perthite lamellae. Dislocations (possible fluid pathways) can also nucleate on the straight albite lamellae and the intergrowths are then semicoherent. Reduction of total free energy by release of coherency strain during dissolution-reprecipitation reactions (related to micropores and turbidity), termed „unzipping“, leads to growth of incoherent subgrain mosaics of albite and microcline. These „deuteric perthites“ („patch“ perthite), up to 103 times coarser than strain-controlled intergrowths, are incoherent and have irregular morphologies. Differences in composition between K-rich host grain and Na-rich „patches“ (P.P.) are visible in optical microphotograph and SEM backscatter electron (BSE) image. These images reveal alkali feldspar microtextures: primary exsolution perthites (E.P.), with some lamellae partly coarsened, and irregular Na-rich „patch“ perthite (P.P.) in host K-rich feldspar, indicating the „unzipping“ process that took place during subsolidus, deuteric phase of granitic magma cooling, when alkali feldspars react readily with magmatic fluids.

alkali feldspars; deuteric microtextures; SEM with EDS

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