THE LOWER CAMBRIAN ALKALINE VOLCANISM FROM CABORCA BLOCK: GEOCHEMISTRY AND ITS RELATION WITH THE SOUTHERN LAURENTIA MARGIN RIFTING PROCESS

Mafic-ultramafic volcanism from the Caborca region is represented by discrete volcanic flows in the lower unit (Unit 1) of the Puerto Blanco Formation. This formation is mainly accepted as the base of the Cambrian for the region. The Neoproterozoic-Cambrian boundary is constrained by biostratigraphy studies. The volcanic event located in the Puerto Blanco Fm. is represented by metabasalts and picrobasalts flows, tuffaceous conglomerates, agglomerates, mafic tuffs, mafic lapillistones and ultramafic sill bodies intruding the lower La Cienega Fm. among other sedimentary rocks representing miogeoclinal prism deposits. The sequence is a homoclinal succession of Neoproterozoic and Lower Cambrian strata resting over a metamorphic basement known as the Bamori Complex.

Geochemistry results show an anorogenic alkaline volcanism with high TiO2N values (2.98 – 7.00%) and primary signatures. The magmatism was located in the mafic-ultramafic boundary. This characteristic is difficult to determine due to the low-grade metamorphism on the rocks. EMPA analysis show a green schist facies mineral association which among with the petrographic studies, reveals an important hydrothermal alteration and metasomatism in the rocks. The alteration observed on the rocks has prevented to obtain a precise crystallization age. Multielement diagrams show parallel spectrums with OIB patterns. Petrography and geochemistry analysis from the volcaniclastic units proved that the pyroclastic material is strictly related to the volcanic flows. This units represents the collapse and partial erosion of cinder cones and strombolian activity.

Trough tectonic setting diagram an alkaline intraplate volcanism was identified. Considering the position of the Caborca Block, an aborted rift or aulacogen is considered as a possible explanation for this event. The magmatism may be related to the southern Laurentia margin rifting process which can be tracked from Alaska and among the North America western margin. The Caborca Block lower Cambrian magmatism could represent a volcanic event close to the rift that developed on the southern margin of west Laurentia.