ALBERT

Description: Detailed petrological study of mid-oceanic ridge basalt-derived high-pressure amphibolite blocks from a fragment of the Caribbean subduction channel (La Corea serpentinite-matrix melange, eastern Cuba) has revealed contrasted zoning patterns of garnet porphyroblasts, including well-defined complex oscillatory prograde-retrograde concentric zoning in one sample. Calculated pressure-temperature (P-T) conditions for this sample using mineral inclusion assemblages and isochemical P-T projections reveal large P-T recurrences best explained by large-scale convective movement of the tectonic block in a serpentinitic subduction channel. The P-T conditions attending garnet growth followed an overall counterclockwise path as a consequence of continued refrigeration of the subduction channel during ongoing underflow after its onset ca. 120 Ma. These findings constitute the first report of large-scale convective circulation of deeply subducted material in the subduction channel, and are consistent with the thermomechanical behavior of the channel predicted by numerical models.

Description: The Rabinal Granite is a peraluminous S-type composite pluton formed upon partial melting of a metasedimentary source region that fringes the southernmost North America plate in central Guatemala. It is therefore considered, together with the intruded metasedimentary sequences, to be part of the continental basement of the Maya block. This leucocratic K-feldspar–plagioclase–quartz–muscovite ± biotite granite shows increasing deformation along its southern margin, where it is cut across by the dextral, Late Cretaceous, top-to-the-NE Baja Verapaz shear zone. Although it has been recently dated at 562–453 Ma (isotope dilution–thermal ionization mass spectrometry), the new data presented here, including laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb and 40 Ar- 39 Ar geochronology and electron-probe mineral chemistry, allow us to more precisely establish the timing of intrusion and metamorphic overprinting of the Rabinal Granite. The zircons dated by LA-ICP-MS indicate a crystallization age of 471 +3 / –5 Ma (Early Ordovician), as well as abundant inherited cores with Pan-African and Mesoproterozoic dates. Laser total fusion Ar-Ar analyses of magmatic low-silica muscovite (Si = 6.2–6.4 atoms per formula unit) indicate cooling following magmatic crystallization during the mid-late Paleozoic and variable extents of resetting of Ordovician micas during Cretaceous metamorphism and deformation. The pressure-temperature ( P - T ) conditions of the inferred Ordovician metamorphism that produced partial melting of the metasedimentary source of the Rabinal Granite and the ascent and crystallization of the granitic melt are uncertain, but a clockwise P - T -time path with maximum P and T of 〈8 kbar and 750 °C, respectively, is proposed. A second thermal event is recognized in recrystallized high-silica muscovite (Si up to = 6.8 atoms per formula unit) formed at peak P and T of ~8.5 kbar and ~300 °C, respectively. This second event, dated at 70.1 ± 0.6 Ma by means of laser total fusion 40 Ar- 39 Ar analyses on high-Si muscovite grains, is interpreted to be the result of subduction and accretion of the basement of the Maya block during the latest Cretaceous, likely in a transpressional tectonic regime related to the lateral collision of the Maya block with the Pacific (Farallon)–derived Caribbean arc. This finding represents the first direct evidence for latest Cretaceous subduction of the metamorphic Paleozoic basement of the Maya block, north of the Baja Verapaz shear zone.