FLUORAPATITE AND BIOTITE CHEMISTRY IN ORTHOPYROXENE-BEARING MIGMATITES: EXAMPLES FROM CALIFORNIA AND SOUTHERN INDIA

Migmatites from Cone Peak, California and the Satnur-Sangama Road, Southern Karnataka, India consist of coarser grained orthopyroxene-bearing leucosomes with subordinate biotite in finer grained hornblende-biotite bearing hosts with sporadic clino and orthopyroxene. The dehydration index (modal abundances of anhydrous (cpx + opx +gt) to hydrous (hb + bi) Fe-Mg silicates ) is 2 to 80 x's higher in leucosomes than in adjacent hosts. Leucosomes at Cone-Peak are tonalitic and the boundary with the host is marked by a biotite enriched zone. Leucosomes from the Sangama Road are granitic with discontinuous zones depleted in Fe-bearing silicates and enriched in magnetite and ilmenite at their margins. At both localities fluorapatites are coarser grained and recrystallized in the leucosomes compared to the hosts. Electron microprobe analyses indicate that fluorapatite from Cone Peak typically contains between 0.5 and 1.0 wt% total REE with no detectable Y. Fluorapatite from Sangama road contains up to 0.5 wt% Y2O3 with (Y + total REE) typically between 0.6 and 1.4 wt %. Inclusions of monazite occur sporadically in fluorapatite in the hosts and leucosomes at both localities. Fluorapatie from Cone Peak leucosomes have higher Cl (average 0.4 wt %) than fluorapatite from the host or biotite zones (average 0.25wt %). Biotites from Cone Peak leucosomes are nearly identical to biotites from the host and biotite zones (average TiO2 5.7 vs. 5.6 wt%, FeO 23.4 vs. 23.8 wt%, F 0.06 vs. 0.12 wt%, Cl 0.2 vs. 0.2 wt %). Biotites from Sangama Road leucosomes have lower average TiO2 (3.5 vs. 4.2 wt %), FeO (11.4 vs. 13.1 wt %), Cl (0.11 vs. 0.16 wt %) and higher F (1.2 vs. 1.0 wt %) than biotites from the host, while average Cl in fluorapatites from the leucosomes are slightly lower (0.28 vs. 0.33 wt %). The leucosomes appear to have formed by dehydration partial melting of the hosts perhaps triggered by the fluid influx. Monazite inclusions formed by the reaction of fluorapatite with a fluid during or after the solidification of the leucosomes. At Cone Peak fluids in the leucosomes were more Cl rich than in the adjacent host while the opposite was true for the migmatites along the Sangama Road.