Geologia USP. Série Científica (Jun 2009)
Geology and Geochronology of Ponte Nova Mafic-Ultramafic Massif (SP-MG)
Abstract
The Ponte Nova alkaline mafi c-ultramafi c massif lies in the northern sector of the Serra do Mar Province, close to theeastern part of the Mantiqueira mountain range, SE Brazil. The massif is exposed in two different areas separated by basementrocks: the larger (~5.5 km2) is elliptical in shape and has a wide variety of rock types; the smaller (~1 km2) is irregular,petrographically less complex, and occurs south of the former. It is mainly composed of an alkaline gabbro associationgenerated by successive magmatic pulses. The more abundant rocks are melagabbros and ultramafi c cumulates (e.g., olivineclinopyroxenites and olivine-bearing melagabbros) found in the lower parts of the massif, together with porphyritic,equigranular and banded gabbros and monzogabbros in the upper partions. A magmatic breccia and a suite of dykes (lamprophyresto tephriphonolites) cut the intrusion. K/Ar geochronological data for the different rock types yielded an averageLate Cretaceous age of 87.6 Ma which is comparable to the age of other alkaline intrusions present in the northern sectorof the referred province. The difference in ages makes it diffi cult to sustain the hypothesis of a genetical association withthe Trindade mantle plume.
