Geophysical characterization and modeling of a Neoproterozoic intrusive volcaniclastic lamprophyre in southernmost Brazil

Abstract

Neoproterozoic spessartitic lamprophyres are linked to the Lavras do Sul Shoshonitic Association in the Sul-Riograndense Shield, southernmost Brazil. Bodies of intrusive spessartitic lamprophyres occur in trachyandesites of this association in the Cerro Tupanci region. There, Neoproterozoic igneous and metamorphic complexes are covered by volcanic-sedimentary Ediacaran sequences of the Camaquã Basin that include trachyandesites, conglomerates, sandstones, and spessartitic lamprophyre dikes of which a spessartitic lapilli tuff N-NE oriented is the main occurrence. Gamma spectrometric and gravimetric data for the lapilli tuff allowed definition of its occurrence as a dike elongated N-S, bearing low values for gamma spectrometry and a strong negative gravimetric anomaly approximately 450 m long and 70 m wide. Two-dimensional modeling of this anomaly suggests that the dike is subvertical to vertical. Magnetometry did not provide significant data regarding the lamprophyre but did show a normal dipolar anomaly to the west, suggesting the presence of a non-outcropping cylindrical intrusion in the area.

KEYWORDS:
gamma spectrometry; gravimetry; magnetometry; geophysical modeling