GEOCHEMISTRY AND METAMORPHISM OF THE PALEOZOIC METASEDIMENTARY BASEMENT OF THE SIERRA MADRE ORIENTAL, NE MEXICO. POSSIBLE PATHS FROM THEIR DEPOSITIONAL ENVIRONMENT?

We present depositional conditions and possible protholits for Late Paleozoic metasediment in Mexico that were related to the Laurentia-Gondwana collision in Carboniferous time. The study aims to reconstruct the depositional and metamorphic evolution of the Granjeno Schist in NE Mexico to get a better control on the timing of subduction and collision processes involving these supercontinents. Remnants of the Mexican Paleozoic continental configuration are present in the Granjeno Schist, the metamorphic basement of the Sierra Madre Oriental in NE Mexico.

The Granjeno Schist comprises intercalations of metamorphic rocks with both sedimentary and volcanic protoliths. The unit was metamorphosed under sub to greenschist facies (250-345°C) during Mississippian.

The geochemical composition of the metasedimentary rocks is in accordance with iron shale, wacke and quartz arenite protoliths. Our data suggest that the metasedimentary units represent a variety of clastic sediments derived from mixed felsic basic sources compositions (e. g., Ti/Nb 200-400). Trace element characteristics point to a continental island arc or active continental margin setting (e. g., Th/Sc and Zr/Sc ratios of 5-8 and 0.3-0.5).

The metavolcanic rocks are associated with ocean-island basalt or mid-ocean ridge basalts due to the immobile trace element ratios Zr/Nb and Y/Nb in the ranges 4.91-8.06 and 0.74-1 for the IOB and >9.2 and >1.25 for the MORB, respectively.

Detrital zircon ages reveal that the major sources are Grenvillian rocks. Such rocks can be found in the ca. 1 Ga Oaxaquia Complex in NE Mexico (Novillo Gneiss). Hence, short transport can be assumed. Maximum depositional ages are Neoproterozoic, Silurian and Devonian. Indicating that the volcanosedimentary deposition probably took place during Devonian.

We suggest that Oaxaquia was situated between Laurentia and Gondwana during collision in the Carboniferous. Zircon data indicate that the Granjeno Schist was deposited before the collision of Laurentia and Gondwana. The presence of oceanic rocks and serpentinite lenses intercalated with tuff and active continental margin sedimentary rocks necessitates a near-continental environment, such as a back-arc basin. We present the first evidence of a subduction zone predating the collision of Laurentia and Gondwana.