PETROLOGY AND GEOCHEMISTRY OF RECENT BEACH SANDS, TAMAULIPAS, MEXICO

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.