STRUCTURAL ANALYSIS OF MORB PILLOW BASALTS AND CLASTIC ROCKS OF THE SOUTHWESTERN COSOLTEPEC FORMATION, ACATLÁN COMPLEX, SOUTHERN MEXICO: A REMNANT OF THE RHEIC OCEAN

The Acatlán Complex of southern Mexico's Mixteco terrane constitutes the largest inlier of Paleozoic rocks in the country and has been interpreted as either a vestige of the Iapetus or Rheic oceans. Critical tests for such a linkage are provided by the depositional age of its principle sedimentary unit, the Cosoltepec Formation, and the geochemical affinity of the mafic volcanic rocks with which this formation is locally associated. Near Olinalá, interbedded greenschist facies phyllite, psammite, and mafic volcanic rocks (±pillows) contain abundant polydeformed vein quartz. These rocks record three phases of deformation: (i), D1: tight to isoclinal folds (F1) in quartz veins with an axial planar solution cleavage (S1); (ii) D2: tight to isoclinal folds (F2) in quartz veins and S1, again with an axial planar solution cleavage (S2); and (iii) D3: chevron and kink folds (F3) with highly variable plunge that deform a composite S1/S2 foliation to produce kink bands. The three phases of folding generally plunge NE or SW (F2 and F3 locally plunging NW or SE), while S1 and S2, dip to the NW and SE. Each of these phases match those described from the Cosoltepec Formation east of Acatlán where D2 and D3 have been shown to be no older than Permian in age. Initial major and trace element analysis suggests the mafic volcanic rocks are of MORB composition, consistent with deposition of the Cosoltepec Formation within a continental rise prism above oceanic lithosphere. Detrital zircon studies of the Cosoltepec Formation have yielded minimum depositional ages that range from Late Ordovician to Devonian, precluding a link to the Iapetus Ocean and supporting, instead, a link to the Rheic Ocean.