Patterns of phosphorus enrichment in alkali feldspar, South Mountain Batholith, Nova Scotia, Canada
Kontak, Daniel J.; Martin, Robert F.; Richard, Linda
European Journal of Mineralogy Volume 8 Number 4 (1996), p. 805 - 824
published: Jul 29, 1996
manuscript accepted: Feb 29, 1996
manuscript received: Feb 9, 1995
ArtNo. ESP147050804013, Price: 29.00 €
Abstract The 370 Ma peraluminous South Mountain batholith, Nova Scotia (eastern Canada), consists of the association granodiorite - monzogranite - leucomonzogranite - leucogranite, cut by granitic pegmatite; deuteric alteration and mineralization (Sn, W, U, F, base metals) locally affected the most evolved rock-types. Bulk analyses of the perthitic K-feldspar (Kfs) indicate a strong correlation between concentration of phosphorus and degree of fractionation (e.g., increasing Rb). The Kfs in pegmatites may even contain in excess of 1.0 wt.% P2O5. Electronmicroprobe analyses indicate that 1) P is structurally bound in both K-rich and Na-rich phases (berlinite substitution: Al + P = 2Si), 2) its concentration is somewhat erratic, even on a local scale (< 100 μm), and 3) it generally favours the K-rich phase over the albite in a perthitic intergrowth, although this may reflect late loss of P from the albite. X-ray-diffraction data confirm orthoclase as the dominant polymorph of Kfs. Highly ordered microcline does occur, but only in deformed or hydrothermally altered rocks, e.g., near the East Kemptville Sn deposit. The scarcity of triclinic Kfs is consistent with rapid cooling accompanying uplift. Subsidiary factors that also could have impeded the inversion to microcline are the peraluminous bulk-compositions and the presence of structurally bound P. The appearance of secondary phosphates in mineralized centres (e.g., East Kemtpville) is consistent with local fluid-mediated release of structurally bound P from orthoclase upon its conversion to microcline.