Crystallization of anatectic melts in high-temperature metamorphic
terrains releases volatile-rich magmas that can be transported into
adjacent lithologies. This study addresses the variations in the oxygen,
boron and hydrogen isotopic composition of aplite-pegmatite dikes that
formed during the crystallization of anatectic melts in regional
high-temperature metamorphism on the island of Naxos, Greece, and
propagated upward into the overlying sequences of metamorphic schist.
The transport distance of these dikes was increased through a
significant horizontal component of travel that was imposed by
contemporaneous low-angle extensional shearing. Laser fluorination
oxygen isotope analyses of quartz, tourmaline, garnet, and biotite
mineral separates from the aplite-pegmatite dikes show a progressive
rise in delta(18)O values with increasing distance from the core. Oxygen
isotope fractionations among quartz, tourmaline, and garnet show
temperature variations from > 700degreesC down to similar to400degreesC.
This range is considered to reflect isotopic fractionation beginning
with crystallization at high temperatures in water-undersaturated
conditions and then evolving through lower temperature crystallization
and retrograde sub-solidus exchange. Two processes are examined for the
cause of the progressive increase in delta(18)O values: (1)
heterogeneous delta(18)O sources and (2) fluid-rock exchange between the
aplite/pegmatite magmas and their host rock. Although the former process
cannot be ruled out, there is as yet no evidence in the exposed
sequences on Naxos for the presence of a suitable high delta(18)O magma
source. In contrast, a tendency for the delta(18)O of quartz in the
aplite/pegmatite dikes to approach that of the quartz in the metamorphic
rock suggests that fluid-rock exchange with the host rock may
potentially be an important process. Advection of fluid into the magma
is examined based on Darcian fluid flow into an initially
water-undersaturated buoyantly propagating aplitic dike magma. It is
shown that such advective flow could only account for part of the
O-18-enrichment, unless it were amplified by repeated injection of magma
pulses, fluid recycling, and deformation-assisted post-crystallization
exchange. The mechanism is, however, adequate to account for hydrogen
isotope equilibration between dike and host rock. In contrast,
variations in the delta(11)B values of tourmalines suggest that
B-11/B-10 fractionation during crystallization and/or magma degassing
was the major control of boron geochemistry rather than fluid-rock
interaction and that the boron isotopic system was decoupled from that
of oxygen. Copyright (C) 2003 Elsevier Ltd.