Abstract:
The study of volcanism in the ocean is fundamental to
better understand the dynamics of the Earth mantle and plate
tectonics. However, our understanding of this volcanism is
limited by difficulties to access the roots of ocean volcanoes
and the ocean floor. Recent results in Panama shown that
exposed accreted volcanoes can provide another way of
understanding volcanism in the oceans [1].
Petit spot volcanoes found on the subducting plate off
Japan are considered to reflect volcanism in response to plate
flexure [2]. However, petit spot volcanoes are very small and
it remains unclear if they are a common feature on the ocean
floor; documenting new occurrences of petit spot volcanoes is
a key multidisciplinary issue. We recognize here ancient petit
spot-like volcanoes accreted in Costa Rica based on new
geochemical, geological, 40Ar-39Ar, and biochronologic data.
Petit spot-like volcanoes acccreted in Costa Rica consists
of tectonic stacks of volcano-sedimentary material that
includes vesiculated pillow lavas, volcanic breccias and thick
radiolarite beds. Igneous sills compositionnally similar to the
lavas are common in the radiolarite beds. Major and trace
element contents of the igneous rocks indicate an alkalic,
moderately fractionated composition, and support very low
degrees of partial melting in the garnet stability field.
Normalized trace element patterns are very similar to those of
petit spot volcanoes in Japan, and distinct from those of
typical OIB, MORB and off-axis seamounts.
Step-heating 40Ar-39Ar dating on co-magmatic amphiboles
gave two ~175 Ma ages of formation for the petit spot-like
volcanoes in Costa Rica. Tectonostratigraphic and
biochronologic data clearly document a ~110 Ma age of
accretion, and indicate that the volcanoes did not formed close
to a subduction zone or a mid-ocean ridge. Therefore, we
propose that petit spot-like volcanoes may represent a
ubiquitous feature on the ocean floor, which can form far from
mid-ocean ridges and subduction zones. Possibly, petit spotlike
volcanoes exposed in Costa Rica reflect tectonicallyinduced
leaking of melts pre-existing at the base of the
lithosphere.
