PETROGRAPHICAL AND ELECTRON MICROPROBE ANALYSIS FOR DETECTION OF BA-RICH FELDSPAR IN MAFIC ROCKS FROM THE EL CHILAR ACCRETIONARY COMPLEX, CENTRAL MEXICO

Three samples studied belong to the Chilar Complex, an Early Mesozoic? Accretionary complex that is exposed in Toliman, Queretaro State, north of Mexico City. Mafic rocks form dikes whithing highly sheared chert blocks that form tectonically emplaced blocks, embedded in a sedimentary matrix.

Previous analysis with X-ray fluorescence and ICPM indicate anomalous barium enrichment in two samples of the mafic rocks. The goal of this study was to identify the minerals which show the Ba anomaly. By petrological analysis was possible to identify the susceptible minerals of contain Ba and with microprobe confirmed the location of minerals o zones of them with such anomaly.

Petrographical analysis were realized with a Leica DMLP and Olympus DP20 microscope camera. Microprobe analysis were carried out in a JEOL, JXA-8900/R microprobe. This instrument has two spectrometers WDS and EDS system. Thus using both techniques we confirm the location of minerals and zones of them with Ba anomaly.

One of the samples contains albite and dolomite or ankerite, the last appears in two varieties: euhedral pure crystals, and irregular or massive. Plagioclase shows elongated inclusions of apatite with different orientations. There are also amphibolite, biotite, sphene, titanite and chlorite. Barium is present in low concentrations and spread in the majority of minerals (325 ppm).

However, two samples with barium enrichment (> 8000 ppm) show a very different mineralogy. They contain euhedral diopside and some biotite. The major mineral components are feldspar, chlorite and amphibolite, it contains pyroxene crystals that are very fractured and deformed. It was determined that barium is mostly concentrated in feldspar (Celsian), which may be related to conditions of high pressure metamorphism.