PETROLOGY AND GEOCHEMISTRY OF RECENT BEACH SANDS, TAMAULIPAS, MEXICO

THAYER, Paul A., Earth Sciences, Univ of North Carolina at Wilmington, 601 South College Road, Wilmington, NC 28403-3297, Thayer@uncw.edu.

One hundred ten sand samples were collected from the high-tide beach at 0.6-km intervals from the mouth of the Rio Grande River southward for 66 km.  The sands are dominantly lower fine-grained (Mz=2.65f), very well sorted (s_I=0.31) subarkose and sublitharenite composed of monocrystalline and polycrystalline quartz, K-feldspar, plagioclase, rock fragments, heavy minerals, and skeletal grains.  Rock fragments include chert, carbonate, siliceous volcanic, sandstone, siltstone, metamorphic, and plutonic types.  Average major element composition/1s for 35 representative samples is (weight percent): SiO₂, 86.9/2.3; TiO₂, 0.55/0.48; Al₂O₃, 4.94/0.38; Fe₂O₃, 1.22/0.99; FeO, 0.32/0.24; MnO, 0.04/0.03; MgO, 0.29/0.07; CaO, 2.62/0.62; Na₂O, 1.24/0.12; K₂O, 1.35/0.18; P₂O₅, 0.06/0.02; H ₂O+, 0.06/0.03; CO₂, 1.35/0.22. Based on tectonic discrimination plots [SiO₂ vs. K₂O/Na₂O, and log (K₂O/Na₂O) vs. log (SiO₂/Al₂O₃)] the setting of the source terrane reflects a passive continental margin.  CaO and the normative F/Q+F ratio decrease irregularly southward and most likely result from carbonate rock fragment and feldspar grain breakage and abrasion due to wave action.  The normative Q/Q+F ratio shows a slight irregular southward increase.