ARTÍCULO

Ordovician-Silurian rift-passive margin on the Mexican margin of the Rheic Ocean overlain by Carboniferous-Permian periarc rocks: Evidence from the eastern Acatlán Complex, southern Mexico

Morales-Gámez Miguel, Keppie J. Duncan, Norman Marc
Tectonophysics 461 (2008) 291-310, 2008.

ABSTRACT:

In the Xayacatlán area of the Acatlán Complex in southern Mexico, several, N–S, fault-bounded packages of rocks occur (from west to east): (i) greenschist facies, interbedded psammites and pelites (Huerta Unit): the youngest concordant detrital zircon in the western part of the unit is 455±4 Ma; (ii) greenschist-amphibolite facies arkoses and psammites (Amate Unit): the youngest concordant detrital zircon gave a 206Pb/238U age of 902±14 Ma, and older concordant ages of ~900–1300 Ma; (iii) low grade clastic rocks and marbles (Permian Tecomate Formation); and (iv) greenschist facies psammites, pelites and thin tholeiitic mafic slices (Salada Unit): the youngest concordant detrital zircon yielded a 206Pb/238U age of 352±3 Ma and older concordant ages of ~434–485 Ma, 511–630 Ma, and 920–1200 Ma. The eastern part of the Huerta Unit is cut by pegmatite that yielded an almost concordant 206Pb/238U zircon age of 464±4 Ma (a mean age of the four almost concordant young zircons is ~485.5 Ma) interpreted as the time of intrusion, and inherited zircons with concordant ages of ~530–580 Ma and ~910–1200 Ma. These data suggest that the Huerta Unit ranges in age from Early (possibly older) to Late Ordovician. The Amate Unit is cut by granitic dikes, in which the youngest concordant 206Pb/238U zircon ages are 452±6 Ma and 447±3 Ma interpreted as the time of intrusion, and older inherited zircon ages with concordant 206Pb/238U ages in the range of 950–1250 Ma. The intrusive ages are similar to the 442±1 Ma concordant U–Pb age for mafic dikes that also cut the Amate Unit. These data constrain the depositional age of the Amate Unit between ~900 Ma and latest Ordovician. The age of the Salada Unit is probably Carboniferous because it is structurally overlain by the Lower–Middle Permian Tecomate Unit. The preponderance of 510–630 Ma and 900–1250 Ma ages in both the detrital and inherited ages in some of the dated samples suggests a source in the adjacent and subjacent Maya and Oaxacan Complex, respectively, although a xenocrystic host rock source cannot be ruled out for the intrusive rocks. The igneous intrusions have been interpreted as part of a 480–440 Ma bimodal rift-related suite. Their synchroneity with deposition of the Huerta Unit (and possibly also the Amate Unit) suggests a rift-passive margin environment of deposition on the southern margin of the Rheic Ocean. On the other hand, the Salada Unit and the Tecomate Formation appear to be synchronous with a Permo-Triassic magmatic arc that developed on the western margin of Pangea.