PALEONTOLOGICAL EVIDENCES FOR A LATE JURASSIC AGE OF THE GUATEMALA OPHIOLITES

CHIARI MARCO ¹, DUMITRICA PAULIAN ², MARRONI MICHELE ³, PANDOLFI LUCA ³, PRINCIPI GIANFRANCO ⁴

presenter's e-mail: marroni@dst.unipi.it

1 - Istituto di Geoscienze e Georisorse, CNR, Italy
2 - Institut de Géologie et Paléontologie, Université de Lausanne, Switzerland
3 - Dipartimento di Scienze della Terra, Università di Pisa & Istituto di Geoscienze e Georisorse, CNR, Italy
4 - Dipartimento di Scienze della Terra, Università di Firenze & Istituto di Geoscienze e Georisorse, CNR, Italy

In Guatemala, the boundary between the Maja block of the North American plate and the Chortis block from the Caribbean plate is represented by east-west trending, left-lateral strike slip fault zones. The main fault zone crops out along the Motagua Suture Zone (MSZ), where slices of HP/LT ophiolites, known as El Tambor Group, occur. In the northern side of the MSZ, these HP/LT ophiolites are represented by serpentinites where blocks of eclogites and jadeitites have been found. In the southern side also minor blueschists rocks have been found as slices associated to serpentinites, eclogites and jadeitites.
Owing to the occurrence of pervasive deformation and the associated HP/LT metamorphism, the age of the ophiolite sequence from the MSZ is still unknown. Generally they are interpreted as remnants of the oceanic lithosphere of Cretaceous age, but geodynamic reconstructions where these ophiolites are regarded as derived from a Jurassic oceanic basin are also proposed.
Recent investigations on the sequence cropping out near Puente del Rio Grande transect (southern side of the MSZ) point out to a Late Jurassic age of the ophiolites from El Tambor Group. In this transect a complete sequence consisting of metabasalts showing a transition to metacherts and micaschists has been recognized. This sequence is deformed under blueschists P/T conditions, but the primary lithological features are still preserved. In the metacherts, boudins of less deformed cherts have been sampled for paleontogical investigations. The examined sample contains very poorly preserved radiolarians. The assemblage comprises the following taxa: Dactyliodiscus sp. 1 , Mirifusus sp., Podobursa sp. The presence of Dactyliodiscus sp. 1 (nondescript species common in the Oxfordian of Eastern Carpathians) could indicate a Late Jurassic age for the sample.
On the whole, this finding represents a valuable tool for the geodynamic reconstructions of the evolution of the Caribbean area.

ACCEPTED as Poster Presentation
in session: "G20.11 - Caribbean plate tectonics"