Paper No. 1
Presentation Time: 08:30-18:30
PETROGENESIS OF ORDOVICIAN MAGMATIC ROCKS IN THE SOUTHERN CHIAPAS MASSIF AND RELATIONS WITH THE EARLY PALEOZOIC MAGMATIC BELTS OF NORTHWESTERN GONDWANA
The recent discovery of an Early Ordovician S-type granite in the southwestern part of the Chiapas Massif Complex added a new perspective to the Paleozoic history of the Maya block because no rocks of such age had been previously reported in this region. New geologic mapping west of Motozintla, Chiapas, revealed pelitic to psammitic metasedimentary successions (Jocote unit) intruded by granitoids and metabasites. This unit is unconformably underlain by the newly defined Candelaria unit, composed of deformed calcsilicate rocks and interlayered folded amphibolites. The Candelaria unit is probably the oldest succession of the southern Maya block. U-Pb LA-ICPMS geochronology yielded Early Ordovician (ca. 470 Ma) ages both in granitoids and metabasites, and Late Ordovician (ca. 450 Ma) ages in granitoids. Major and trace element geochemistry, as well as Nd and Sr isotope data suggest that folded amphibolites of the Candelaria unit may be mantle derived and related to rifts whereas Early Ordovician bimodal magmatism is more differentiated and reflects crustal contamination and volcanic-arc chemical signatures. A granitic stock (Motozintla pluton) intruded the area in the Late Ordovician. The geochemistry and petrology of the Motozintla pluton indicates less crustal contamination and mixed signatures of either volcanic arc or within plate setting. Comparison of our new data with published data from the Rabinal Suite and the Altos Cuchumatanes in Guatemala suggests the existence of an Ordovician magmatic belt in the southern Maya block from Chiapas to Guatemala. Similar but somewhat younger granites occur also in the Maya Mountains of Belize suggesting that magmatism switched in the Silurian from the southern Chiapas-Guatemala belt towards the Maya Mountains but kept its ambiguous chemical signatures, which seem to be typical for the Early Paleozoic magmatism in the southern Maya block.