The Cerro Chopo basaltic cone (Costa Rica): An unusual completely reversed graded pyroclastic cone with abundant low vesiculated cannonball juvenile fragments

Abstract

Cerro Chopo is a partially dissected, asymmetric, isolated Pleistocene pyroclastic cone, located in front of the Cordillera de Guanacaste, in northern Costa Rica. The cone consists of ~ 0.09 km 3 of basaltic tephra, as well as ~ 0.14 km 3 of lateral lava flows. Tephras are tholeiitic, high-alumina, olivine basalts, and represent minor degrees (≤ 5%) of crystal fractionation. Major and trace element compositions are consistent with minor fractionation from a mixture of E-MORB and OIB magmas. The cone walls consist of alternating coarser- and finer grained well-sorted beds, containing continuous spectra from breadcrust to smooth surface cannonball bombs, but also less frequent cylindric fragments and broken clasts. Cerro Chopo is unique compared to other typical scoria cones because it contains ubiquitous reversely-graded layers, scarcity of scorias, and instead a wide range of dense (1.54–2.49 g/cm 3) poorly vesicular (5–40 vol.%) juvenile clast morphologies, including abundant cannonball juvenile bombs and lapilli. These are bombs with concentric layers surrounding vesiculated, dense and lapillistone cores and are interpreted to have repeatedly recycled through the vent. The cannonball bombs and lapilli have been described in a few scoria cones but are much less abundant than in Chopo. The reverse graded sequences are interpreted to have resulted from decreasing explosivity at the vent, in addition to local failure of tephra on slopes as the consequence of grain flows. Elsewhere on the Earth, most of the poorly-vesiculated spherical bombs, particularly cannonball, accretionary, composite, and core bombs, and its equivalent lapilli size (pelletal, spinning droplets and ellipsoidal lapilli), are all related to mafic to ultramafic, low-viscosity magmas.