PHENGITE-HOSTED LILE ENRICHMENT IN ECLOGITE AND RELATED ROCKS - IMPLICATIONS FOR FLUID-MEDIATED MASS-TRANSFER IN SUBDUCTION ZONES AND ARC MAGMA GENESIS
Ss. Sorensen et al., PHENGITE-HOSTED LILE ENRICHMENT IN ECLOGITE AND RELATED ROCKS - IMPLICATIONS FOR FLUID-MEDIATED MASS-TRANSFER IN SUBDUCTION ZONES AND ARC MAGMA GENESIS, Journal of Petrology, 38(1), 1997, pp. 3-34
Geochemical differences between island arc basalts (IAB) and ocean-flo
or basalts (mid-ocean ridge basalts; MORB) suggest that the large-ion
lithophile elements (LILE) K, Ba, Rb and Cs are probably mobilized in
subduction zone fluids and melts. This study documents LILE enrichment
of eclogite, amphibolite, and epidote +/- garnet blueschist tectonic
blocks and related rocks from melanges of two subduction complexes. Th
e samples are from six localities of the Francisan Complex, California
, and related terranes of Oregon and Baja California, and from the Sam
ana Metamorphic Complex, Samana Peninsula, Dominican Republic. Most Fr
anciscan blocks are MORB-like in their contents of rare earth elements
(REE) and high field strength elements (HFSE); in contrast, most Sama
na blocks show an IAB signature of these elements. The whole-rock K2O
contents of both groups range from 1 to 3 wt %; K, Ba, Rb and Cs are a
ll strongly intercorrelated. Many blocks display K/Ba similar to metas
omatized transitions zones and rinds at their outer margins. Some tran
sition zones and rinds are enriched in LILE compared with host blocks;
others are relatively depleted in these elements. Some LILE-rich bloc
ks contain 'early' coarse-grained muscovite that is aligned in the fol
iation defined by coarse-grained omphacite or amphibole grains. Others
display 'late' muscovite in veins and as a partial replacement of gar
net; many contain both textural types. The muscovite is phengite that
contains similar to 3.25-3.55 Si per 11 oxygens, and similar to 0.25-0
.50 Mg per 11 oxygens. Lower-Si phengite has a significant paragonite
component: Na per 11 oxygens ranges to similar to 0.12. Ba contents of
phengite range to over 1 wt % (0.027 per 11 oxygens). Ba in phengite
does not covary strongly with either Na or K. Ba contents phengite inc
rease from some blocks to their transition zones or rinds, or from blo
cks to their veins. Averaged K/Ba ratios for phengite and host samples
define an array which describes other subsamples of the block and oth
er analyzed block. Phengite carries essentially all of the LILE in oth
erwise mafic eclogite, amphibolite, and garnet blueschist blocks that
are enriched in these elements compared with MORB. It evidently tracks
a distinctive type of LILE metasomatism that attends both high-land r
etrograde subduction zone metamorphism. An obvious source for the LILE
is a fluid in equilibrium with metasedimentary rocks. High-grade semi
pelitic schists from subduction complexes and subductable sediment dis
play LILE values that resemble those seen in the most LILE-rich blocks
. Modeling of Ba and Ti suggests that 1-40 wt % of phengite added to M
ORB can produce their observed LILE enrichment. Thus, the release of L
ILE from such rocks to fluids or melts in very high-T and -P parts of
subduction zones probably depends critically on the stability and solu
bility relations of phengite, which is thought to be stable at pressur
es as high as 95-110 kbar at T = 750-1050 degrees C.