Relaciones genéticas entre las carbonatitas y las rocas nefélinicas del Complejo El Picacho, Sierra de Tamaulipas, NE de México

Abstract

El Picacho is an annular Tertiary magmatic complex with a surface of 9 km² • It is situated in the Eastern part of the Sierra de Tamaulipas, 70 km Southeast of Ciudad Victoria. The complex ha an elliptic shape and is bordered by Lower Cretaceous limestones of the Tamaulipas Superior Fonnation. El Picacho complex consists mainly of alkaline gabbros surrounded irregularly by granites and nepheline syenites. In its central pmt a semicircular body of nephelinitic rocks (nepheline syenites, ijolites and melteigites, among other varieties)  cuts the gabbroic  rocks, forming a fenitization aureole. The geochemical data indicate a differentiation process in the nephelinic rocks from melteigites and ijolites to nepheline syenites.

At least two magmatic pulses compose this unit. The gabbros at the contact and the nepheline syenites from the first pulse were later fenitized by alkaline fluids, coming from the ijolitic magma from the second pulse. The alkaline metasomatism has a sodic character.

Cutting the nephelinic rocks in the centre of the complex and the gabbros outside of the central body, two Th-Y rich carbonatite dykes were recognized. The first is a coarse grained soevite and consists of calcite, bands of iron oxides, pyrite, siderite, quartz, chalcedony, barite and bastniisite-(La,Ce). The other dyke is alvikitic and is composed of calcite, ankerite, pyrite, plagioclase, ilmenorutile, siderite, fluorapatite, barite, chalcedony and at least three REE-minerals (Th-bastnasite, Y-Th-britholite and cheralite).

C and O isotopic determinations point out to a mantle origin of the carbonatites. However, they are fractionated and reequilibrated with hydrothermal solutions at different degrees. Interstitial calcites from the nepheline syenites have isotopic values similar to those of the mantle and are even more primitive than !hose of the carbonatites of El Picacho. The values of δ³C PDB range from -4.029 to -6.133 ‰, while δ¹⁸0 SMOW vary from 13.020 to 19.530 ‰. We postulate a process of liquid immiscibility with the separation of a carbonate melt from an initially C0² -rich silicate magma. The soevites and the alvikites originated from the first magma, while the nephelinitic rocks represen! the product from the silicate magma. In the same way, a separation of a phosphorous enriched melt from the carbonatitic magma could be responsi ble for the origin of the apatite rich dykes in the centre of El Picacho.