SIGNIFICANCE OF SLAB-DERIVED PARTIAL MELTS AND AQUEOUS FLUIDS FOR THEGENESIS OF THOLEIITIC AND CALC-ALKALINE ISLAND-ARC BASALTS - EVIDENCEFROM MT ARAYAT, PHILIPPINES
M. Bau et U. Knittel, SIGNIFICANCE OF SLAB-DERIVED PARTIAL MELTS AND AQUEOUS FLUIDS FOR THEGENESIS OF THOLEIITIC AND CALC-ALKALINE ISLAND-ARC BASALTS - EVIDENCEFROM MT ARAYAT, PHILIPPINES, Chemical geology, 105(4), 1993, pp. 233-251
Mt. Arayat, an inactive stratovolcano, is located behind the volcanic
front of the Bataan segment of the Taiwan-Luzon Arc in the Central Val
ley of Luzon, the Philippines. Volcanism in this arc-segment results f
rom subduction of South China Sea Basin crust along the Manila Trench.
Eruption products of Mt. Arayat comprise pyroclastics and porphyritic
basalt and andesite lavas of high-K calc-alkaline affinity. Compared
to typical oceanic island-arc volcanics, the basalts display slightly
elevated abundances of Ta and Nb relative to Zr and Hf, and of Ta, Nb,
Zr, Hf and light rare-earth elements (LREE's) relative to Y and heavy
rare-earths (HREE's). Their REE distribution suggests mixing between
high-HREE low-LREE/HREE and low-HREE high-LREE/HREE components, the la
tter suggesting the presence of residual garnet in the source region o
f Mt. Arayat basalts. Enrichment of LREE's relative to HREE's is typic
al of calc-alkaline and high-K calc-alkaline island-arc basalts, but i
s usually missing or small in island-arc tholeiites. LREE enrichment i
s often attributed to a metasomatizing aqueous fluid generated during
dehydration of the subducted slab. For this model to be applicable, ex
tremely low abundances in the fluid of agents like F-, OH- and/or CO32
- which form hard complexes with the REE's, are a prerequisite. Due to
the temperature dependence of the Eu3+/Eu2+ redox potential, divalent
Eu predominates in aqueous fluids of low complex-forming capacity in
the subduction zone environment whereas the other REE's (with the poss
ible exception of Yb) are still trivalent. A LREE-enriched aqueous flu
id generated by slab dehydration should therfore display a positive Eu
anomaly. However, LREE-enriched island-arc basalts generally do not s
how positive Eu anomalies, implying that this LREE enrichment is unlik
ely to result from REE transfer by an aqueous fluid. Instead, a model
is favoured which assumes contributions from both slab-derived aqueous
fluids and partial melts. In the source region for island-arc tholeii
tes the mantle wedge is only affected by a LILE-enriched aqueous fluid
, whereas the source region for calc-alkaline and high-K calc-alkaline
basalts is additionally metasomatized by small volumes of melt genera
ted by small degrees of partial melting of the subducted slab with gar
net as a residual phase. These LREE-enriched partial melts may react w
ith the depleted peridotite of the mantle wedge to produce domains of
refertilized garnet-peridotite. Subsequent partial melting produces ca
lc-alkaline melts whose trace-element compositions display signatures
of both depleted peridotite and more fertile garnet-peridotite.