Print this pageCathodoluminescence of Quartz as Provenance and Recycling Indicator: The Cambrian MesóN Group, NW Argentina
Quartz, the most common mineral in sandstones, contains source rock information that few provenance studies take into account. The cathodoluminescence (CL) of detrital quartz grains can be used to gain provenance information. The CL color spectra characteristics depend on defects in the crystal lattice. As a result, quartz grains of different formation conditions develop different CL colors. We applied the technique on detrital quartz grains from sedimentary rocks in NW Argentina. There, the Lower to Middle Cambrian Mesón Group overlies the Ediacarian to Lower Cambrian Puncoviscana Fm. with an angular unconformity. Therefore, the Puncoviscana Fm. could be a potential source for the Mesón Group. To elucidate possible recycling and to reveal the dominating source rock types of the Mesón Group we analyzed the CL color spectra of more than 300 detrital quartz grains from 17 sandstones and conglomerates from the Puncoviscana Fm. (77 grains) and the Mesón Group (226 grains). The rocks from the Puncoviscana Fm. are turbiditic graywackes with matrix contents exceeding 50 % and a framework mode with 70-90 % quartz and < 30 % feldspar. The Mesón Group is composed of shallow-marine fine- to middle-grained quartz-cemented sandstones and some granule-sized conglomerates. Both are dominated by quartz (> 85 %) in the framework mode. The CL results of quartz from the Puncoviscana Fm. reveal red and violet luminescing quartz (mean value: 35 %), as well as blue (45 %) and brown (20 %) quartz grains in varying proportions, indicating input from volcanic, plutonic and magmatic source rocks to different degrees. In the Mesón Group, blue as well as red and violet luminescing quartz grains dominate (55 and 45 % respectively), pointing to plutonic and volcanic sources and an absence of input from metamorphic source rocks. Most analyzed quartz grains of the Puncoviscana Fm., but only few of the Mesón Group, contain Fe3+, as revealed by a CL emission band at ca. 700 nm. Together with the large difference in metamorphic quartz grain content, this points to the absence of recycled quartz grains of the Puncoviscana Fm. in the Mesón Group. Instead the results imply that the detrital quartz of volcanic origin derived from exposed magmatic rocks in the source areas - at least during deposition of the Mesón Group. Hence, the large input of volcanic quartz grains into the Mesón Group may be due to regional source areas dominated by volcanic rocks in Early to Middle Cambrian time.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009