The petrogenesis of granitoid rocks unusually rich in apatite in the Western Tatra Mts. (S-Poland, Western Carpathians)

Abstract

In the bottom part of the tongue-shaped, layered granitoid intrusion, exposed in the Western Tatra Mts., apatite-rich granitic rocks occur as pseudo-layers and pockets between I-type hybrid mafic precursors and homogeneous S-type felsic granitoids. The apatite-rich rocks are peraluminous (ASI = 1.12–1.61), with P2O5 contents ranging from 0.05 to 3.41 wt.% (<7.5 vol.% apatite), shoshonitic to high-K calc-alkaline. Apatite is present as long-prismatic zoned crystals (Ap1) and as large xenomorphic unzoned crystals (Ap2). Ap1 apatite and biotite represent an early cumulate. Feldspar and Ap2 textural relations may reflect the interaction of the crystal faces of both minerals and support a model based on local saturation of (P, Ca, F) versus (K, Na, Al, Si, Ba) in the border zones. Chondrite-normalized REE patterns for the apatite rocks and for pure apatite suggest apatite was a main REE carrier in these rocks. Minerals characteristics and the whole rock chemistry suggest both reduced S-type and I-type magma influenced the apatite-rich rocks. The field observations, mineral and rock chemistry as well as mass-balance calculations point out that the presence of apatite-rich rocks may be linked to the continuous mixing of felsic and mafic magmas, creating unique phosphorus- and aluminium-rich magma portions. Formation of these rocks was initially dominated by the complex flowage-controlled and to some extent also gravity-driven separation of early-formed zoned minerals and, subsequently, by local saturation in the border zones of growing feldspar and apatite crystals. Slow diffusion in the phosphorus-rich magma pockets favoured the local saturation and simultaneous crystallization of apatite and feldspars in a crystal-ladden melt.