Metamorphic evolution of the Rio de la Plata Craton in the Cinco Cerros area, Buenos Aires province, Argentina.

HANS-JOACHIM MASSONNE, JORGE DRISTAS AND JUAN CRUZ MARTINEZ

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2012 vol. 38 p. 57 - 70

0895-9811

A metapelite and an interlayered granite were studied from the Cinco Cerros area ca. 65 km WNWof thecity of Mar del Plata. Garnet in these samples is slightly zoned with core and rim compositions ofpyr17(gro þ andr)6spes1.5alm75.5 and pyr13.5(gro þ andr)5.5spes2alm80, respectively, in the metapelite.Corresponding compositions in the granite are pyr15(gro þ andr)3.5spes3.5alm78 andpyr11(gro þ andr)3.5spes4.5alm81. We used the PERPLE_X computer software package to calculate PeTpseudosections. From the pseudosection of the metapelite PeT conditions of 6.7 kbar and 670 C werederived for an early metamorphic stage. Subsequently, a pressure release occurred at decreasingtemperatures. The final metamorphic PeT conditions recorded by the studied rock are 4.5 kbar and600 C compatible with the absence of cordierite, staurolite, and an Al2SiO5-phase. Garnet in the granitoidcrystallized between 715 and 690 C at a pressure around 7.7 kbar.UeThePb age dating with the electron microprobe was performed. 16 analyses of monazite in themetapelite formed three clusters resulting in ages of I: 2073  11.4 (1s) Ma, II: 1913  11.0 (1s) Ma, andIII: 1805  20.8 (1s) Ma. Thus, the Paleoproterozoic metamorphic event can be related to the Trans-Amazonian cycle and was followed by slow cooling.As our study area is close to the margin of the Río de la Plata Craton, where abundant magmatic arcderivedplutonic rocks are outcropping, we interpret the derived PeT data as follows: A heating event(not recorded by the studied rocks) resulted from magmas that intruded during the Trans-Amazoniancycle. This event was followed by slow exhumation, probably caused by erosion, accompanied by thermalrelaxation.
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