Enclaves from the A‐type granite of the Mégantic Complex, White Mountain Magma Series: Clues to granite magmagenesis

Abstract

The subsolvus biotite‐hornblende‐granite at Mégantic contains mafic to leucocratic enclaves. The groundmass of leucocratic enclaves is microgranitic; that of mafic enclaves is meladioritic. Apatite is commonly acicular in the enclaves, suggesting quenching of mafic magmas commingled with the granitic host. Mafic enclaves contain mafic clots (hornblende cores, biotite rims), some with relic cores of viAl‐Cr‐rich diopsidic augite similar to phenocrysts in associated basaltic dykes. The clots are interpreted to be pseudomorphs after mafic phenocrysts. The enclaves also contain granitic microxenoliths, xenocrysts of quartz with hornblende reaction rims, partially resorbed oligoclase and perthite megacrysts and biotite xenocrysts. The xenocrysts record hybridization between a mafic enclave magma and a granitic magma and/or rock. Enclaves define mixing lines on many variation diagrams. However, Rb, Sr, Ba, Zr, Nb, Y, REE, Zn, Pb, and F do not fit this pattern and appear to have been redistributed by residual granitic melts or exsolved volatiles. The data suggest that the granite is anatectic, heat being provided by crystallization of associated basalts. Mixing calculations suggest that as little as 5% hybridization with basalt or hawaiite can account for the Sr isotopic heterogeneity of the granites.