TROPICS

3.2.2 Composition of lavas. There is an along
arc geochemical trend from Nicaragua to
Costa Rica to western Panama, with
increasing SiO 2, Na 2O, K 2O, large ion
lithophile elements (e.g Ba, Rb, Th and U)
and light rare earth elements. Examples of
this trend are shown in Figure 10a. This
trend is distinct from the well known NW to
SE decrease in subducted sediment tracers
(e.g. Ba/La) extending from Nicaragua into
NW Costa Rica.
Figure 10a. Data on arc lavas less than 5 Ma
(mostly from active volcanoes) compiled by
Kelemen et al 2003a, from GeoRoc, literature
data, and unpublished data from PI’s Carr,
Vogel, and their colleagues. CC: Range of
estimates for contintal crust compiled by
Kelemen (1995). Red arrow, estimate by Rudnick
& Gao (2003, 2007).
On the right side of Figure 10a is the
range of estimated compositions for bulk
continental crust (Mg# ~ 0.5). Clearly, lavas
with similar Mg# in the northwest, (distance
> 500km) are distinct from continental crust
in their major and trace element
compositions, while there is a marked
overlap between continental compositions
and lavas in the 500-300 km range. There is a
substantial gap in data (about 390-450 km) in
the Talamanca Mountains, where
compositions similar to continental crust
might be expected to be most common,
because the intermediate plutonic rocks and
lavas there are little studied.
These scatterplots (Figure 10a) provide
an honest view of regional variability, but
tend to obscure the covariance in the data,
diminish the data in clusters and overemphasize
outliers. Thus, the histograms in
Figure 10b supplement Figure 10a. The
regional data are divided into two distinct
groups, with the southeastern group
extending to Irazu volcano, and the
northwestern group from there to the border with Nicaragua. Clearly, lava compositions
similar to continental crust are well represented in the southern group (300-500 km) and
poorly represented in the northern group (500-700 km). In turn, the lavas of the northern
group are more similar to continental crust than lavas of the Izu-Bonin-Marianas island
arc in the western Pacific.
The question arises, whether the igneous rocks in southern Costa Rica and Panama
that resemble continental crust are a recent phenomenon, or have been erupted
throughout the lifetime of the arc. Plank et al. (2002) found that from the Miocene to the
C-16
TPI 6838742